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Being a recognized carcinogen, hexavalent chromium is hazardous to both human and environmental health. Thus, it is imperative to regulate and oversee their levels in a variety of industries, including textiles, dyes, pigments, and metal finishing. This study strives to reduce Cr(VI) in wastewater by using capacitive deionization in conjunction with an activated carbon-based electrode and a continuous electrochemical reactor (CER). Activated carbon derived from rubberwood sawdust demonstrated excellent properties, including a high surface area of 1157 m2 g−1. The electrical conductivity and mechanical stability of the electrode were enhanced by the incorporation of synthesized expanded graphite (EG) into the AC. Key parameters were optimized via systematic batch electroreduction experiments with an optimal response surface design. The efficacy of the fabricated CER was proved when it successfully reduced Cr(VI) in a 5 mg L−1 solution within 15 min under optimized conditions, in contrast to the considerably longer durations anticipated by conventional methods. Validation of these findings was done by treating industrial wastewater of 30 mg L−1 in the CER. The electroreduction of Cr(VI) followed the Langmuir isotherm with a maximum capacity of 13.491 mg g−1 and pseudo-second-order kinetics. These results indicate that the combined use of the modified AC electrode and CER holds potential as a sustainable and economical approach to effectively eliminate Cr(VI) from wastewater. [ABSTRACT FROM AUTHOR]

The quality of water is significantly impacted by the presence of Cr6+ and Ni2+ ions. This study investigates the effectiveness of a flow-by porous graphite electrode cell in removing these contaminants from simulated industrial wastewater. We explore the impact of various factors on the removal process, demonstrating the method's potential for efficient removal. The initial concentration of nickel and chromium ions (20 to 80 mg/l and 20 to 100 mg/l, respectively), the feed flow rate (0.28 to 1.11 ml/s), current density (0.2 to 2.25 mA/cm2) and pH all influence the removal rate and efficiency. A higher feed flow rate negatively affects the removal efficiency of both Ni2+ and Cr6+ ions. Nickel removal efficiency decreased by 34.9% at 20 ppm and 26% at 80 ppm, representing the highest and lowest reductions in efficiency, respectively. Chromium removal efficiency decreased by 19% at 100 ppm and 6.5% at 50 ppm, indicating the highest and lowest reductions in efficiency, respectively, under the same flow rate change. Under optimal conditions, the removal efficiency for Ni2+ was 99.47% after 15 min of operation at a current density of 1.96 mA/cm2, a flow rate of 0.28 ml/s, and a pH of 8 and the removal efficiency for Cr6+ was 99.97% after 10 min of operation at a current density of 2.25 mA/cm2, a flow rate of 0.28 ml/s, and a pH of 2. The flow-through porous electrode system achieves efficient heavy metal removal with operating costs of 0.24 USD/m3 for nickel and 0.38 USD/m3 for chromium at optimal conditions. [ABSTRACT FROM AUTHOR]

Wastewater containing a mixture of heavy metals, a byproduct of chemical, petrochemical, and refinery activities driven by urbanization and industrial expansion, poses significant environmental threats. Analyzing such wastewater through adsorbate-adsorbent experiments yields extensive datasets. However, traditional methodologies like the Box–Behnken design (BBD) within the response surface methodology (RSM) struggle with managing large datasets and capturing the complex, nonlinear relationships inherent in such experimental data. To address these challenges, ML techniques have emerged as promising tools for accurately predicting the removal percentage of heavy metals from wastewater. In this study, we utilized tree-based regression models—specifically decision tree regression (DTR), random forest regression (RFR), and extra tree regression (ETR)—to forecast the efficiency of gooseberry seed powder in removing chromium (Cr(VI)) from wastewater. Additionally, we employed an ML-based Nelder–Mead optimization approach to identify the optimal values for key features (initial Cr(VI) concentration, pH, and Indian gooseberry powder dosage) which maximized the Cr(VI) removal percentage. Our experimental results reveal that the ETR model achieved an impressive R 2 score of 0.99, demonstrating a low error rate in predicting the Cr(VI) removal percentage. Furthermore, we used DTR-Nelder–Mead, RFR-Nelder–Mead, and ETR-Nelder–Mead optimization approaches on a synthesized dataset of 2000 instances while varying the initial Cr(VI) concentration, pH, and Indian gooseberry powder dosage. The analysis determined that the DTR-Nelder–Mead and RFR-Nelder–Mead approaches yielded the highest Cr(VI) removal percentages of 78.21% and 78.107% at an initial concentration of 95.55 mg/L, respectively, a pH level of four, and an adsorbent dosage of 8 g/L of gooseberry seed powder. Furthermore, the ETR-Nelder–Mead approach obtained the maximum Cr(VI) removal percentage of 85.11% at an initial concentration of 99.25 mg/L, a pH level of 4.97, and an adsorbent dosage of 9.62 g/L of gooseberry seed powder. These results reported an increase in the Cr(VI) removal percentage ranging from 4.66% to 11.56% more than the Cr(VI) removal percentage obtained by experimentation. These findings underscore the efficacy of tree-based regression models and ML-based Nelder–Mead optimization in elucidating chromium removal processes from wastewater, offering valuable insights into effective treatment strategies. [ABSTRACT FROM AUTHOR]

Electrocoagulation has demonstrated its efficacy in treating industrial effluents by effectively removing pollutants, particularly metallic contaminants. The electrochemical processes occurring at aluminum electrodes have shown excellent performance in this regard. In this study, electrocoagulation experiments were conducted on an industrial effluent originating from an electroplating bath situated in Casablanca, Morocco. The primary objective was to eliminate chromium in the effluent and reuse the processed water for other applications within our facility. To achieve this goal, we systematically optimized various operational parameters influencing the electrocoagulation efficiency, including electrical voltage, electrode material, stirring speed, and electrode spacing. Additionally, we assessed their impact on pH, conductivity, and chromium concentration. The experiments were conducted in a well-mixed reactor using an industrial solution containing a high concentration of chromium, specifically 1 g/l of Cr (VI). The chromium removal efficiency was evaluated under specific operational conditions, including the use of aluminum electrodes, regulated voltages of 6 volts and 12 volts, an optimal stirring speed of 600 rpm, and an electrode spacing of 2 cm. [ABSTRACT FROM AUTHOR]

The present study draws attention to arsenic (V) (As(V)), chromium (III) (Cr(III)), and chromium (VI) (Cr(VI)) heavy metal pollutants, which have negative effects on aquatic ecosystems and indirectly pose a serious threat to the health of living things in the environment. Homopolymer and copolymer hydrogels were synthesized by the UV polymerization technique using 2-acrylamido-2-methyl-1-propansulfonic acid sodium salt (AMPS) and 3-acrlylamidopropil-trimethyl ammonium chloride monomers, and a crosslinker aimed to improve the swelling properties and metal sorption of the prepared hydrogels. The swelling balance values of hydrogels were determined by the gravimetric method in deionized water and different pH values, and their structural characterizations were carried out by Fourier transform infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA) techniques. Sorption efficiencies and sorption capacities of the synthesized hydrogels were determined using aqueous solutions containing As(V), Cr(III), and Cr(VI) heavy metal ions at 25 °C. It was determined that there was a significant increase in p(AMPS) (AM1)-based hydrogel's balance swelling value in deionized water compared to p(AP1) and p(AMPS-co-APTMACl) (AMAP1)-based hydrogels. The lowest equilibrium swelling value was reached in the AMAP1 hydrogel, and the main reason for this result was thought to be due to the closure of the active functional groups of the increased cross-links between AMPS and APTMACl monomer. When sorption results were examined, it was observed that the highest heavy metal ion removal for AM1, AP1, and AMAP1 was in As(V) metal. The highest metal ion holding capacity of AM1, AP1, and AMAP1 was found to be 494 mg/g, 2336 mg/g, and 1439 mg/g for As(V), 323 mg/g, 111 mg/g, and 276 mg/g for Cr(III), and 303 mg/g, 723 mg/g, and 554 mg/g for Cr(VI), respectively. Considering the swelling and sorption results, it is thought that the synthesized hydrogels will provide a significant advantage in the removal of heavy metal ions, which have negative effects on environmental pollution and live health. [ABSTRACT FROM AUTHOR]

In this paper, two flower-like magnesium silicate nanomaterials were prepared. These and another two commercial magnesium silicate materials were characterized using a scanning electron microscope, the N2 adsorption–desorption method, and other methods. The structure–activity relationship between the adsorption performance of these four magnesium silicate materials and their specific surface area, pore size distribution, and pore structure was compared. The results showed that the 3-FMS modified by sodium dodecyl sulfonate (SDS) had the largest specific surface area and pore size, the best adsorption performance, and the largest experimental equilibrium adsorption capacity (qe,exp) for Co2+, reaching 190.01 mg/g, and Cr3+, reaching 208.89 mg/g. The adsorption behavior of the four materials for Co2+ and Cr3+ both fitted the pseudo-second-order kinetic model and Langmuir adsorption model, indicating that chemical monolayer uniform adsorption was the dominant adsorption process. Among them, the theoretical adsorption capacity (qm) of 3-FMS was the highest, reaching 207.62 mg/g for Co2+ and 230.85 mg/g for Cr3+. Through further research, it was found that the four materials mainly removed Co2+ and Cr3+ through electrostatic adsorption, surface metal ions (Mg2+), and acidic groups (-OH and -SO3H) exchanging with ions in solution. The adsorption performance of two self-made flower-like magnesium silicate materials for Co2+ and Cr3+ was superior to that of two commercial magnesium silicates. [ABSTRACT FROM AUTHOR]

Removal of hexavalent chromium from synthetic wastewater was performed by employing vegetable-tanned buffing dust, a byproduct of tanneries. Different analysis techniques such as SEM, BET, and FTIR were used to characterize the vegetable-tanned buffing dust. The experiment on adsorption was carried out in a batch manner. Investigations were conducted into the various operating parameters, such as the dose of adsorbent, contact period, and initial Cr (VI) concentration. At a dose of 1g/100 mL and a contact period of 280 minutes, the maximum adsorption capacity was noted. The Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich adsorption isotherms were examined at various concentrations. With a maximum adsorption efficiency of 14.14 mg/g, the Langmuir isotherm model was found to be the most well-fitting for the current investigation. Kinetic analysis was performed for pseudo first and second order; the pseudo-second-order model demonstrated the most optimal fit with a regression coefficient (R2) of 0.9936. [ABSTRACT FROM AUTHOR]

Aims: Heavy metals are significant environmental pollutants and toxic to life and chromium (Cr) (VI) is one of them being discharged in the environment due to many human activities. The leather industry uses Cr(VI) salt in the tanning process, which is discharged untreated and becomes a source of many diseases. The use of microbes to remove metals is a cost-effective and clean method. The present study aims to isolate local and native microbes for Cr(VI) removal from tannery wastewater and enhance their capacity to bioremediate the tannery wastewater. Further, efficiency in free and immobilized forms was also checked. Methodology and results: Microbes were isolated from a local tannery wastewater outlet and after many rounds of minimum inhibitory concentration, and concentration of 500 µg/mL was found to be that concentration at which microbes could survive, above which they died. The sequencing of 16S rRNA and its analysis showed that it was closely related to Staphylococcus saprophyticus and in the given study, it was named B6. At 37 °C, pH 7.5 and 120 h of incubation, it removed 77% Cr(VI) from the reaction mixture. B6 was exposed to UV to obtain mutant. Exposure of 15 min to a UV lamp gave mutant MB6, which showed a removal capacity of 77% after 72 h only. Cr(VI) removal capacity of the mutant was then analyzed in the free and attached form where coal and sodium alginate were used as solid surfaces. Mutants immobilized on coal showed 91% Cr(VI) removal after 96 h, while sodium alginate showed 58% Cr(VI) removal in 120 h, thus showing coal as a more effective surface for adsorption. Conclusion, significance and impact of study: Our present study shows the use of cheap and environmentally friendly methods to remediate tannery wastewater, which is a big problem in a country like Pakistan. Pakistan is the second largest producer of leather but lacks a wastewater treatment facility. So, this method offers in-situ wastewater treatment, which can be further enhanced in different ways. [ABSTRACT FROM AUTHOR]

To address the water pollution problem caused by hexavalent chromium, this study took advantage of a multi-defect adsorption mechanism and constructed a multiple defect-containing type-II heterojunction material, CuS/Cv-CNNs. This material has simultaneous sulfur vacancies, copper vacancies, and carbon vacancies, which were used to accelerate the removal of chromium from wastewater. Attributed to the formation of electron-rich structures and free-moving ions by the construction of multiple defect sites, the photogenerated carriers generated by CuS/Cv-CNNs-70 were efficiently separated, and a high amount of physical Cr(VI) adsorption was achieved by electrostatic attraction and ion exchange, consistent with a Freundlich isothermal adsorption model. Studies on the adsorption thermodynamics demonstrate that CuS/Cv-CNNs-70 prefers to adsorb Cr(VI) when temperatures are raised. In addition, the removal of Cr(VI) (120 mg L−1) by CuS/Cv-CNNs-70 reached 97.10% after 60 minutes through the synergistic effect of adsorption and visible-light photocatalysis. This finding indicates that the synergistic effect between the multilocation defects stimulated the combined potential of adsorption and photocatalysis of CuS/Cv-CNNs-70 to realize the full utilization of the defects. In conclusion, this study confirms the prominent role of a multiple defect mechanism in wastewater treatment and environmental protection. [ABSTRACT FROM AUTHOR]

In this study, we developed a novel nanocomposite, polyurethane foam impregnated with zero-valent iron nanoparticles (PU@nZVI), for the effective removal of chromium(VI) from various water sources. The characterization of nanocomposite (PU@nZVI) was performed by XRD, SEM–EDS, TEM and FT-IR techniques. Using the response surface methodology, we optimized the removal conditions, achieving an optimal pH of 2 and a dose of 0.5 g/L. The PU@nZVI demonstrated an excellent maximum adsorption capacity of 600.0 mg/g for Cr6+. The adsorption kinetics and isotherms were best described by the pseudo-second-order model and the Freundlich isotherm, respectively. Significantly, the nanocomposite removed 99.98% of Cr6+ from tap water, 96.81% from industrial effluent, and 94.57% from treated sewage wastewater. Furthermore, the PU@nZVI maintained its efficiency over five adsorption–desorption cycles, highlighting its reusability. These results suggest that the PU@nZVI nanocomposite is a highly efficient and sustainable option for chromium(VI) removal in water treatment applications. [ABSTRACT FROM AUTHOR]

The present work studied electroplating wastewater treatment by the electro-oxidation (EO) method through a new Ti/TiO 2 -GO-SnO 2 electrode in a once-through continuous mode. The Ti/TiO 2 -GO-SnO 2 electrode was synthesized using an anodic oxidation technique and characterized by electrochemical characterizaftion. Response Surface Methodology (RSM) employing a Box-Behnken Design (BBD) model was utilized to optimize pH (3−7), current density (j = 16.72–50.16 mA/cm2), and time (t = 30–130 min) for removal of electroplating effluents and energy consumption. At the optimum condition, the values of the responses R 1 (% Cr(VI) removal), R 2 (% Zn removal), R 3 (% COD removal), and R 4 (energy consumption) were found to be 97.55%, 95.66%, 87.32%, and 14.97 kWh/m3, respectively. The effect of reactive species scavengers on removing electroplating effluents has been studied. The kinetics and cost analysis were also carried out under optimal conditions. Once-through continuous experiments were carried out at different flow rates to determine the viability of the process at industrial or pilot scale applications. The synthesized electrode proved highly reusable, maintaining durability even after sixty cycles of usage. Toxicity analysis showed that EO has the potential to detoxify electroplating effluents. The observed results confirm that the EO with Ti/TiO 2 -GO-SnO 2 electrode is low-cost and effective for removing electroplating effluents. [Display omitted] [ABSTRACT FROM AUTHOR]

This study aimed to synthesize magnesium oxide (MgO) using a colloidal starch method for two primary purposes: the removal of chromium (III) ions from synthetic wastewater and the subsequent use of the chromium-containing material as synthetic inorganic pigments (SIPs) in commercial paints. The synthesis used to obtain the oxide (St-MgO) is a promising method for using plants, such as cassava, as green fuels due to their abundance, low cost, and non-toxicity. With this, the oxide showed greater porosity and alkalinity, compared to commercial magnesium oxide (Cm-MgO). The MgO samples were subjected to structural characterization using XRD and FTIR, surface area and pore volume study by B.E.T. and SEM, and chemical composition by ICP-OES and thermogravimetric analysis (TGA). The crystalline periclase phase was identified for both samples, but the brucite phase was shown to be a secondary phase for the commercial sample. After the removal of chromium ions, the brucite crystalline phase became the majority phase for the samples, regardless of the concentration of ions removed. The pigments were characterized by color measurements and discussed in terms of colorimetric parameters using the CIELab method and electron spectroscopy (VIS-NIR). This study also evaluated the colorimetric stability of green pigments in aggressive environments (acidic and alkaline) over a 240 h exposure period, demonstrating minimal color difference. This study aims to develop materials for the decontamination of wastewater containing chromium and its reuse as a synthetic inorganic pigment, using an innovative and sustainable synthesis method. [ABSTRACT FROM AUTHOR]

Chromium is the main constituent of the tanning salts used by tanneries for processing skins into leather, and considered as a pollutant in the effluents that this industry discharge. The present study aimed to evaluate the removal of chromium and other pollutants in effluents from the tanning industry applying the natural zeolite Neonite. Neonite is a commercially available zeolite, which was acquired from NEONITE S.A. Neonite was characterized through FTIR and XRD, identifying clinoptilolite as its main component. Treatment was applied to the samples of real effluents from the tanning stage or chromium bath (CB) and a composite effluent (CE), an experimental design was carried out for each effluent. The designs correspond to 32 factorial designs, the variables tested were the Neonite dosage (NE) measured in g/L, stirring time (StT) expressed in minutes and the pH of the sample, and the response variable was the residual chromium content (RCC). Statistical analysis was performed through Rsoftware version 4.2.3 and included Box-Cox transformation and ANOVA to determine the main effects and the behavior of the response variable in relation to the applied model and its optimization by response surface. In CB, a removal above 95% was obtained for RCC, with optimization values at 32 g/L of NE and pH 7. In the CE, a removal of more than 98% was obtained for RCC and values higher than 60% for BOD, COD, TSS and Fats, being the optimal conditions 4.73 g/L of NE and 12.17 minutes of StT. These results demonstrate that the application of Neonite for the removal of chromium and other pollutants from tanning industry effluents has a great potential. [ABSTRACT FROM AUTHOR]

The tannery wastewater from the tanning stage (TWT) comprises organic and Cr pollutants, which can adversely affect aquatic life and have carcinogenic effects. In this study, we investigated the performance of a Fenton-like process using commercial Nano-scale zero-valent iron (nZVI) for the simultaneous removal of Cr and organic matter from real TWT. We used an experimental design to select the principal operating parameters. A Plackett–Burman design identified variables for Cr-total and COD removal, followed by a central composite design (CC-D) to determine optimal variable levels. Finally, the response surface methodology (RSM) was used to find the optimum concentration of individual variables influencing Cr-total removal. Additionally, the effect of the leather-related, co-existing substances that influenced the efficiency of the process and the possibility of recycling nZVI were explored. The inclusion of nZVI was significantly more effective at removing both Cr-total and COD (97.3% ± 5.7% and 73.9% ± 9.1%, respectively), whereas the traditional Fenton process achieved lower removal rates (55.6% ± 10.0% for Cr-total and 34.8% ± 10.9% for COD). The optimal conditions for the Fenton-like process were nZVI/H2O2 = 1.05 w/w, and pH = 2.93. We obtained the best results during the first 5 min of the reaction, which increased after 48 h of agitation and subsequent neutralization. According to the results of four consecutive cycles, nZVI exhibited high reusability (97%) without compromising its adsorption potency. XPS analysis confirmed Cr removal through the adsorption mechanism on the nZVI surface. Hence, a Fenton-like process based on nZVI can be used as a promising alternative for treating organic and Cr wastewater. [ABSTRACT FROM AUTHOR]

This study investigated the adsorption performance of biochar produced from different types of urban biowaste material viz., sugarcane bagasse (SB), brinjal stem (BS), and citrus peel (CP) for removal of heavy metal ions (Pb, Cu, Cr, and Cd) from aqueous solution. The effects of biowaste material, dosage of biochar, solution pH, and initial concentration of heavy metal ions and isotherm models were performed to understand the possible adsorption mechanisms. The results showed that the biochar derived from BS and SB removes Cu (99.94%), Cr (99.57%), and Cd (99.77%) whereas biochar derived from CP removes Pb (99.59%) and Cu (99.90%) more efficiently from the aqueous solution. Biochar derived from BS showed maximum adsorption capacity for Cu (246.31 mg g−1), Pb (183.15 mg g−1), and Cr (71.89 mg g−1) while the biochar derived from CP showed highest for Cd (15.46 mg g−1). Moreover, biochar derived from BS and SB has more polar functional groups and less hydrophobicity than the biochar derived from CP. This study reveals that solution pH and biochar doses play a major role in removal of heavy metal ions from aqueous solution. The results of Langmuir model fitted well for Pb and Cu while the Freundlich model for Cr and Cd. Our study concludes that the biochar derived from different biowaste materials adsorbs heavy metal ions majorly through surface complexation and precipitation processes. The results of this study will be very useful in selecting the effective urban biowaste material for making biochar for heavy metal removal from the aqueous environment. Highlights: Sugarcane bagasse, brinjal stem, and citrus peel from metropolitan areas have potential for production of biochar. Biochar derived from SB and BS adsorb more Cu, Cr, and Cd ions. Biochar adsorption performance varies with different pH levels and biowaste materials. Langmuir isotherm model was well fitted for Pb and Cu while Freundlich model for Cr and Cd ions. Biochars mostly adsorb heavy metal ions by surface complexation and precipitation. [ABSTRACT FROM AUTHOR]

Biochar has gained popularity as green adsorbent when dealing with hexavalent chromium ions in the water environment. Herein, we prepared cotton stalk biochar (CSB) at distinct pyrolysis temperatures, i.e., 300 °C, 400 °C, and 500 °C (denoted as CSB300, CSB400, and CSB500, respectively), using cotton stalks as biomass, and we investigated its impact on Cr (VI) removal. The characterization results based on SEM–EDS, BET, CHN, XRD, and FTIR analysis showed that the surface area, surface morphology, and elemental and functional group composition of CSB were considerably influenced by pyrolysis temperature. Despite having the lowest surface area, CSB500 presented superior adsorption performance. Batch sorption experiments with Cr (VI) solutions (20 mg/L) showed the optimal removal conditions to be pH (2), contact time (2 h), and biochar dosage (3 g/L). Adsorption kinetics and isotherms were well defined by the Pseudo second order and Freundlich model. The thermodynamic studies showed the spontaneous and endothermic nature of the sorption process. The potential Cr (VI) removal mechanism mainly involved electrostatic attraction followed by Cr (VI) reduction to Cr (III) and complexation. Regeneration studies showed that the reusability aspect of the biochar could be improved, while the selected interfering ions had an insignificant effect on CSB500 adsorption capacity. In general, the current study's findings may provide mechanistic insights into the fate, mobility, and removal of Cr (VI) species in aquatic environment using biochar materials to wide range of environmental scholars. [ABSTRACT FROM AUTHOR]

The contamination of chromium (Cr(VI)) in groundwater threatens public health because of the industrial development being historically less supervised. To remove Cr(VI) from groundwater and other surface waters, a new FeCl3-modified magnetic biochar (MBPH550) was prepared from a waste peanut hull. The magnetic potential was to facilitate the separation of absorbent from the water phase. The removal efficiency, adsorption kinetics and isotherm models, and adsorption mechanism of Cr(VI) on MBPH550 were investigated. MBPH550 had a large specific surface of 243.23 m2/g and saturation magnetization of 5.65 emu/g. MBPH550 achieved a removal efficiency of Cr(VI) up to 92.2% ± 2.0%. Low pH favored the adsorption of Cr(VI). At pH 6.0, the adsorption of Cr(VI) on MBPH550 fits the Elovich kinetic model and Freundlich isothermal adsorption model, with an equilibrium adsorption capacity of 6.64 mg/g. In addition to the direct functional group coordination and precipitation on the surface of MBPH550, Cr(VI) was also reduced to Cr3+, Cr(OH)3, Cr2O3, and FeCr2O4, which were all deposited on the surface of MBPH550. MBPH550 could be a widely applied efficient adsorbent for removing Cr(VI) from wastewater, groundwater, and leachate of Cr-contaminated soil. [ABSTRACT FROM AUTHOR]

Water pollution is a significant global issue among various environmental issues on the earth for the sustainability of life. It has drawn considerable attention from researchers due to the continuously increasing discharge of highly toxic pollutants, such as chromium(VI), lead(II), cadmium(II) etc. These toxic ions are primarily released in the water bodies from various effluents generated due to various chemical processes in the industries. Therefore, significant investments from different countries have been made for the emergent research toward the remediation of toxic ions from polluted water. In this review article, a summary of synthesis and comprehensive analysis of a recently reported wide range of lignocellulose-based adsorbents for removing chromium(VI) and cadmium(II) have been complied. This review also summarizes the research data in terms of various physicochemical parameters such as adsorption capacity, specificity and selective affinity. The manuscript outlines the optimized adsorption conditions, reaction mechanism, and different characterization techniques to analyze the adsorbent surface functionality, morphologyand thermodynamic parametershave also been summarized. The current and future efforts toward effectively developing agro-waste-based adsorbents for enhanced purification of contaminated water have also been included. [ABSTRACT FROM AUTHOR]

Exploring the ratio of metal centers to organic ligands and the amount of DMF are important to improve the stability and adsorption efficiency of MOF materials as adsorbents. In this work, MIL101(Fe)-Na2CO3 was successfully obtained by modification with formic acid, sodium carbonate, carbon nanotubes, and moieties. The adsorption efficiency of MIL-101(Fe) on Cr(VI) was greatly improved, and the removal efficiency was able to reach 100% in 20 min with a maximum adsorption capacity of 20 mg/g. The inhibition order of the competing anions for the removal of hexavalent chromium was as follows: Cl− < NO3− < SO42−. The analysis of the adsorption thermodynamic model found that the adsorption of MIL101(Fe)-Na2CO3 for Cr(VI) showed spontaneous heat-absorbing and entropy-increasing chemisorption behavior. When using NaOH as the eluent and HCl as the regeneration stabilizer, MIL-101(Fe)-Na2CO3 had good adsorption capacity in multiple cycles. [ABSTRACT FROM AUTHOR]

In this study, COD, turbidity, total nitrogen, phosphate, sulphate, oil-grease and chromium removals from wastewaters taken from a local tannery using the electrocoagulation process were examined and the most suitable parameters for the electrocoagulation system were tried to be determined. Soluble iron plate electrodes were used as electrodes in the experiments. As a result of the studies carried out, the most suitable initial pH in uncontrolled pH was determined as pH = 4 and for controlled pH as pH = 5. In studies, the most suitable current density for iron electrodes was determined as 1.2 mA cm−2. Even if the increase in current density increased the amount of dissolved Fe, it negatively affected the removal efficiency. In addition, the turbidity of the wastewater was removed in 30 minutes, phosphate in 20 minutes and chromium in 15 minutes by 100%. During 60 minutes of operation, while 62.91% of TN, 87.8% of oil-grease were removed, sulphate was removed at the rate of 54.78%. In the studies, 1st and 2nd order kinetic equations for COD removal were examined and it was determined that removal kinetic is more suitable for 2nd order kinetics. [ABSTRACT FROM AUTHOR]

This study primarily focused on the efficient transformation of low-priced blue coke powder into a high-capacity adsorbent and aimed to address the pollution issue of hexavalent chromium (Cr (VI))-laden wastewater and to facilitate the effective utilization of blue coke powder. A two-step method was utilized to fabricate a blue coke-based nitric acid-modified material (LCN), and the impact of nitric acid modification on the material's structure and its efficacy in treating Cr (VI)-contaminated wastewater was evaluated. Our experimental results illustrated that, under identical conditions, LCN exhibited superior performance for Cr (VI) treatment compared to the method employing only potassium hydroxide (LCK). The specific surface area and pore volume of LCN were 1.39 and 1.36 times greater than those of LCK, respectively. Further chemical composition analysis revealed that the functional group structure on the LCN surface was more conducive to Cr (VI) adsorption. The highest amount of Cr (VI) that LCN could bind was measured at 181.962 mg/g at 318 K. This was mostly due to chemisorption, which is dominated by redox reactions. The Cr (VI) removal process by LCN was identified to be a spontaneous, exothermic, and entropy-increasing process. Several tests on recycling and reuse showed that LCN is a stable and effective chromium-containing wastewater adsorbent, showing that it could be used in many situations. [ABSTRACT FROM AUTHOR]

This present study depicts the successful employment of fixed-bed column for total chromium removal from tannery wastewater in dynamic mode using sodium alginate-powdered marble beads (SA–Marble) as adsorbent. The SA–Marble composite beads prepared were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and Brunauer, Emmett and Teller (BET) method. The adsorption process performance of this bio-sorbent was examined in batches and columns for real effluent (tannery wastewater). After 90 min, the total chromium removal efficiency could be kept above 90% in the batch experiment. The adsorption kinetics fit better with the pseudo-second-order model, indicating the chemisorption process and the adsorption capacity of about 67.74 mg g−1 at 293 K (C0 = 7100 mg L−1) was obtained. Additionally, dynamic experiments indicate that the total chromium removal efficiency could be maintained above 90% after 120 min at 293 K and 60 min at 318 and 333 K; it's an endothermic but rapid process. The effects of two adsorption variables (Temperature and time) were investigated using central composite design (CCD), which is a subset of response surface methodology (total Cr, COD, sulfate, and total phosphorus percentage removal). This work paves a new avenue for synthesizing SA–Marble composite beads and provides an adsorption efficiency of total chromium removal from tannery wastewater. Highlights: Powder marble coated in sodium-alginate beads (SA–Marble) was prepared & characterized, A reactor with a fixed bed was employed for the dynamic removal of total Cr from tannery wastewater, A capacity of 67.74 mg g−1 at 293 K (C0=7100 mg L-1) allows 90% total Cr removal after 90mn, Pseudo-II-order model simulates well the total Cr endothermic rapid chemisorption process, COD, SO2-4, and TP good removal suggest its use as a successful tannery wastewater treatment. [ABSTRACT FROM AUTHOR]

The three-dimensional network and ample pore structure of novel hydrogel materials enable outstanding adsorption performance for pollutants such as methylene blue (MB) and Cr6+ ions in wastewater. In order to develop an environmentally friendly hydrogel with high adsorption performance and low cost, a type of carboxymethyl cellulose (CMC) composite hydrogel was synthesised with montmorillonite (MMT) via chain radical polymerization, which gives it great potential for application in the field of wastewater purification. A series of hydrogel samples were characterised through SEM, FTIR and nitrogen porosimetry analysis, indicating the successful intercalation of MMT nanosheets into the hydrogel crosslinking network. The mass ratio of CMC to MMT, the amounts of adsorbent, the initial concentration of wastes, pH, and the adsorption temperature were investigated and optimised for hydrogel adsorption performance. When the initial concentration of MB is 60 mg/L, pH is 7, the dosage of MB is 0.5 g/L, and the adsorption temperature is 30 °C, the hydrogel sample the highest adsorption capability for MB removal, with an adsorption amount of 112.9 mg/g. When the initial concentration of Cr6+ is 10 mg/L with a pH of 7, the highest adsorption capacity of the hydrogel for Cr6+ removal is 1.35 mg/g. The fitting results of the isothermal models, the kinetic models, internal particle diffusion models and the thermodynamics of the experimental data of the adsorbate adsorption process show that the adsorption of MB by hydrogel is a spontaneous segmented process of multi-layer physical and chemical adsorption. Additionally, the adsorption of Cr6+ ions by hydrogel is a spontaneous segmented process of multi-layer physical adsorption. [ABSTRACT FROM AUTHOR]

The wastewater generated during the electroplating of joint bearings for aircraft was collected in categories and treated by chemical precipitation and biochemical method. After being treated by the given process, the removal rates of total chromium, hexavalent chromium, and total zinc reached 76.7%, 100%, and 98.7%, respectively. The quality of effluent meets the demand of the Table 2 stipulated in GB 21900-2008 Emission Standard of Pollutants for Electroplating and the Grade III of GB 8978-1996 Integrated Wastewater Discharge Standard. [ABSTRACT FROM AUTHOR]

Layered double hydroxides (LDHs) with their unique structural chemistry create opportunities to be modified with polymers, making different nanocomposites. In the current research, a novel PET-PAN embedded with Mg-AI-LDH-PVA nanocomposite membrane was fabricated through electrospinning. SEM, EDX, FTIR, XRD, and AFM were carried out to investigate the structure and morphology of the nanocomposite membrane. The characterization of the optimized nanocomposite membrane showed a beadless, smooth structure with a nanofiber diameter of 695 nm. The water contact angle and tensile strength were 16° and 1.4 Mpa, respectively, showing an increase in the hydrophilicity and stability of the nanocomposite membrane by the addition of Mg-Al-LDH-PVA. To evaluate the adsorption performance of the nanocomposite membrane, operating parameters were achieved for Cr(VI) and methyl orange at pH 2.0 and pH 4.0, respectively, including contact time, adsorbate dose, and pollutant concentration. The adsorption data of the nanocomposite membrane showed the removal of 68% and 80% for Cr(VI) and methyl orange, respectively. The process of adsorption followed a Langmuir isotherm model that fit well and pseudo-2nd order kinetics with R2 values of 0.97 and 0.99, respectively. The recycling results showed the membrane's stability for up to five cycles. The developed membrane can be used for efficient removal of pollutants from wastewater. [ABSTRACT FROM AUTHOR]

[Display omitted] Adsorption is a green technology of treating heavy metal-contaminated strong acid wastewaters for the recycling of heavy metal and reuse of strong acid. Herein, three amine polymers (APs) with different alkalinities and electron donating abilities were prepared to investigate the adsorption-reduction processes of Cr(VI). It was found that the removal of Cr(VI) was controlled by the concentration of –NRH+ on the surface of APs at pH > 2, which relies on the alkalinity of APs. However, the high concentration of NRH+ significantly facilitated the adsorption of Cr(VI) on the surface of APs and accelerated the mass transfer between Cr(VI) and APs at strong acid environment (pH ≤ 2). More importantly, the reduction of Cr(VI) was enhanced at pH ≤ 2, due to the high reduction potential of Cr(VI) (E ≥ 0.437). The ratio of reduction to adsorption (α) of Cr(VI) was above 0.70, and the proportion of Cr(III) bonding on Ph-AP excessed 67.6 %. Finally, a proton-enhanced mechanism of Cr(VI) removal was verified by analyzing FTIR and XPS spectra as well as constructing DFT model. This study provides a theoretical basis for the removal of Cr(VI) in the strong acid wastewater. [ABSTRACT FROM AUTHOR]

Chromium (Cr) is one of the hazardous heavy metals that is naturally carcinogenic and causes various health problems. Metallic nanoparticles such as silver and copper nanoparticles (Ag NPs and Cu NPs) have gained great attention because of their unique chemical, physical, and biological attributes, serving diverse and significant role in various useful and sustainable applications. In the present study, both of these NPs were synthesized by green method in which Azadirachta indica plant extract was used. These nanoparticles were characterized by using advanced instrumental techniques such as Fourier transmission infrared (FTIR), X-ray diffraction (XRD), scanning electron microscope attached with energy-dispersive spectroscopy (SEM-EDS), and elemental mapping. These environmentally friendly nanoparticles were utilized for the batch removal of Cr from the wastewater. For analysis of adsorption behaviour, a range of kinetic isotherm models (Freundlich, Temkin, Dubinin, and Langmuir) and kinetic models (pseudo-first-order and pseudo-second-order) were used for the Cu-NPs and Ag-NPs. Cu NPs exhibited the highest Cr removal efficiency (96%) within a contact time of 10–15 min, closely followed by Ag NPs which achieved a removal efficiency of 94% under the similar conditions. These optimal outcomes were observed at a sorbent dose of 0.5 g/L for Ag NPs and 0.7 g/L for Cu NPs. After effectively capturing Cr using these nanoparticles, the sorbates were examined through SEM-EDX analysis to observe how much Cr metal was attached to the nanoparticles, potentially for future use. The analysis found that Ag-NPs captured 18% of Cr, while Cu-NPs captured 12% from the aqueous solution. More precise experimental conditions are needed for higher Cr removal from wastewater and determination of the best conditions for industrial-level Cr reuse. Although nanomaterial exhibit high efficiency and selectivity for Cr removal and recovery from wastewater, more research is necessary to optimize their synthesis and performance for industrial-scale applications and develop efficient methods for Cr removal and recovery. [ABSTRACT FROM AUTHOR]

Due to their toxicity, Cr(VI) levels are subject to strict legislation and regulations in various industries and environmental contexts. Effective treatment technologies are also being developed to decrease the negative impacts on human health and the environment by removing Cr(VI) from water sources and wastewater. As a result, it would be interesting to model and optimize the Cr(VI) removal processes, especially those under neutral pH circumstances. Microbial fuel cells (MFCs) have the capacity to remove Cr(VI), but additional research is needed to enhance their usability, increase their efficacy, and address issues like scalability and maintaining stable operation. In this research work, ANFIS modeling and artificial ecosystem optimization (AEO) were used to maximize Cr(VI) removal efficiency and the power density of MFC. First, based on measured data, an ANFIS model is developed to simulate the MFC performance in terms of the Cu(II)/Cr(VI) ratio, substrate (sodium acetate) concentration (g/L), and external resistance Ω. Then, using artificial ecosystem optimization (AEO), the optimal values of these operating parameters, i.e., Cu(II)/Cr(VI) ratio, substrate concentration, and external resistance, are identified, corresponding to maximum Cr(VI) removal efficiency and power density. In the ANFIS modeling stage of power density, the coefficient-of-determination is enhanced to 0.9981 compared with 0.992 (by ANOVA), and the RMSE is decreased to 0.4863 compared with 16.486 (by ANOVA). This shows that the modeling phase was effective. In sum, the integration between ANFIS and AEO increased the power density and Cr(VI) removal efficiency by 19.14% and 15.14%, respectively, compared to the measured data. [ABSTRACT FROM AUTHOR]

Natural zeolite from Malang Regency is a raw material that can be used as an adsorbent for heavy metal waste. However, this natural zeolite needs activation and optimization to be utilized maximally as an adsorbent. This study was conducted for both physical and chemical activation of natural zeolite. Characterization results of XRD indicated that the natural zeolite possesses crystalline phases and is of the mordenite type. Based on the XRF results, the Si/Al ratio increased from 5.768 to 6.119 after activation. Based on the characterization results using the BET method with SAA analysis, the surface area of non-activated natural zeolite was determined to be 20.8045 m²/g, while activated natural zeolite had a surface area of 137.8196 m²/g. Results of the adsorption study in treating liquid laboratory waste using simulated Pb and Cr metal wastes demonstrated that activated natural zeolite performed more effectively as an adsorbent than non-activated natural zeolite. The adsorbent's optimum mass was 0.3 grams, resulting in an adsorption percentage of 97.43% for Pb and 97.56% for Cr, with a contact time of 30 minutes at pH 6. The adsorption kinetics of Pb and Cr metals were described using a pseudo-second-order rate equation. The adsorption of Pb and Cr was depicted using the Langmuir equation, indicating the monolayer formation on the homogenous adsorbent surface during the adsorption process. The activated natural zeolite has the potential to be employed as an independent adsorbent in the treatment of heavy metal waste. [ABSTRACT FROM AUTHOR]

The treatment efficiency of Chromium (Cr)-containing Printed Circuit Board (PCB) wastewater is significantly hampered by the limited physiological activity of microorganisms when activated sludge is applied. In this study, the biodegradation and electron transfer based on sulfur metabolism in the integrated (BESI®) process use sulfur as the electron acceptor to achieve sulfate reduction and sulfide oxidation, leading to efficient removal of Cr. The concentrations of total Cr and Cr(VI) in the effluent were reduced to 0.5 mg/L and 0.1 mg/L, respectively, from an initial range of 25–32 mg/L in the influent. The removal of Cr (ΔC(Cr(VI))) mainly occurred in the Sulfate Reduction (SR) reactor, which was significantly correlated with the generation of sulphide (ΔC(S2−)) (R2 = 0.9987). Meantime, analysis of the microbial community showed that Cr (VI) stress increased the diversity of the bacterial community in sludge. The presence of Clostridium (52.54% and 47.78%) in SR & Sulfide Oxidation (SO) reactor, along with the Synergistaceae (31.90%) and Trichococcus (26.59%) in aerobic reactor, might contribute to the gradient degradation of COD, resulting in a removal efficiency exceeding 80% when treating an influent with a concentration of 1000 mg/L. In addition, the main precipitation components in the SR reactor were identified by scanning electron microscope, indicating that Cr has been removed from wastewater as Cr(OH)3 precipitation. This study sheds light on the potential of using the BESI® process for the real PCB wastewater treatment. [ABSTRACT FROM AUTHOR]

Industrial wastewater polluted with high concentrations of Cr is commonly discharged into water resources without proper treatment. This gives rise to the deterioration of water quality and imposes adverse effects on public health. Therefore, this study is aimed at removing Cr from electroplating wastewater using activated carbon produced from water hyacinth under a full factorial experimental design with three factors and three levels (pH,2,5 and 8, adsorbent dose 0.5,1and1.5 in 100 mL and contact time 30, 60 and120 min). A phosphoric acid solution of 37% was used to activate the carbon, which was then subjected to thermal decomposition for 15 min at 500 °C. The activated carbon was characterized by the presence of a high surface area (203.83 m2/g) of BET, cracking of adsorbent beads of SEM morphology, amorphous nature of XRD, and many functional groups of FTIR such as hydroxyl (3283 cm−1), alkane (2920 cm−1), nitrile (2114 cm−1) and aromatics (1613 cm−1). The minimum Cr adsorption performance of 15.6% was obtained whereas maximum removal of 90.4% was recorded at the experimental condition of pH 2, adsorbent dose of 1.5 g/100 mL, and contact time of 120 min at a fixed value of initial Cr concentration of 100 mg/L. Similarly, the maximum Cr removal from real electroplating wastewater was 81.2% at this optimum point. Langmuir's model best described the experimental value at R2 0.96 which implies the adsorption is chemically bonded, homogeneous, and monolayer. Pseudo-second-order model best fits with the experimental data with R2 value of 0.99. The adsorbent was regenerated for seven cycles and the removal efficiency decreased from 93.25% to 21.35%. Finally, this technology is promising to be scaled up to an industrial level. [ABSTRACT FROM AUTHOR]

This study gives a description of the formation of self-nitrogen doped activated carbon (NDAC) by a novel way of employing fish meal (mixture of Atherina hepseetus and Sardina pilchardus of 60% protein) as nitrogen dopant, ZnCl2 as impregnate agent, sawdust as carbon source and water with a mass ratio (2:1:1:12), which subjected to the hydrothermal process. The hydrothermal mixture was oven dried and carbonized under a flow of nitrogen for one h at 600, 700, and 800 °C. The characterization of NDAC was performed by using various analytical techniques analyses. The synthesized NDAC exhibited unique features such as microporous structure (1.84 ~ 2.01 nm), high surface area (437.51 ~ 680.86 m2/g), the volume of total pores (0.22 ~ 0.32 cm3/g) and nitrogen content (12.82 ~ 13.73%). Batch removal tests were achieved to investigate the impact of chromium ions starting concentration (100–400 mg/L), NDAC dose (0.5–2.5 g/L), pH and contact time (5–120 min). Such helpful characteristics of NDAC, particularly for NDAC600, were suitable to use as an excellent adsorbent for Cr6+ ions with a maximum adsorption capacity (Qm) (769.23 mg/g), and the highest chromium ions adsorption uptake (81.18%) was obtained at pH value 1.5 at room temperature. Both Halsey and Temkin models fitted the adsorption data quite reasonably. The uptake of toxic chromium ions is best represented with pseudo-second-order rate kinetics data. [ABSTRACT FROM AUTHOR]

Introduction-- Constructed Wetlands (CWs) are a recognized technology to treat industrial wastewater. Objective-- A pilot system of two horizontal subsurface flow CWs was used to remove Cr and Zn from industrial synthetic wastewater. Methodology-- The study was carried out at UA in Barranquilla, Colombia. Two containers of 0.375 m2 were filled with a gravel bed (~10 mm and 40% of porosity), and a 0.3 m water column. One container was planted with Cyperus odoratus L. and another without plants was used as a control. Results-- The removal efficiency of Cr and Zn was 93% and 96% in the CW planted, respectively, and 67% and 98% removal were obtained in the unplanted system with statistical differences (P< 0.05). The observed difference in biomass production (0.1 kg m2 and 0.6 kg/m2) could be related to seasonal weather that could have favored the growth of the plant. C. odoratus reached a Translocation Factor greater than 1.5 for Cr and Zn, which is greater than that, reported by others for Cyperus species. However, a Bioconcentration Factor > 13.6 for Zn and < 7.7 for Cr indicated that C. odoratus is an accumulator species for Cr and Zn. Sorption metal processes in gravel can be occurring due to the high removal efficiency of Zn in unplanted systems. Conclusions-- These results show that C. odoratus could be recommended for use in constructed wetlands technology due to fast-growing and absorption and translocation heavy metals capacity. [ABSTRACT FROM AUTHOR]

The heavy metals released from various industries are toxic to the environment even at minute concentration. Box-Behnken design is the Response Surface Methodology (RSM) tool used to model higher order response surfaces using lesser experimental trials than a normal factorial technique. The current study, involves the application of Box-Behnken experimental design in evaluating and optimizing the Hexavalent Chromium [Cr(VI)] removal using agricultural industrial wastes as adsorbents. The effect of process variables such as pH (2-9), adsorbent dosage (0.1-3g) and initial Cr(VI) concentration (50-100 mg/L) on the industrial wastes are evaluated. The saw palmeto extract exhibited maximum chromium removal at a pH of 2 with 1g biomass dosage and at an initial metal ion concentration of 100 ppm. Similarly, the maximum Cr(VI) removal efficiency using waste of black cumin extract was observed with 2g biomass dosage at an initial metal ion concentration of 60 ppm for a pH value 2. The maximum chromium removal using waste of ginger root extract was observed with 2g biomass dosage at an initial metal ion concentration of 60 ppm for a pH value 2. The RSM technique provided a mathematical model which helped in determining the interactions between various process variables which affected the removal efficiency of Cr(VI). [ABSTRACT FROM AUTHOR]

The utilization of biochar, derived from agricultural waste, has garnered attention as a valuable material for enhancing soil properties and serving as a substitute adsorbent for the elimination of hazardous heavy metals and organic contaminants from wastewater. In the present investigation, amide-modified biochar was synthesized via low-temperature pyrolysis of rice husk and was harnessed for the removal of Cr(VI) from wastewater. The resultant biochar was treated with 1-[3-(trimethoxysilyl) propyl] urea to incorporate an amide group. The amide-modified biochar was characterized by employing Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) techniques. During batch experiments, the effect of various parameters, such as adsorbent dosage, metal concentration, time duration, and pH, on Cr(VI) removal was investigated. The optimal conditions for achieving maximum adsorption of Cr(VI) were observed at a pH 2, an adsorbent time of 60 min, an adsorbent dosage of 2 g/L, and a metal concentration of 100 mg/L. The percent removal efficiency of 97% was recorded for the removal of Cr(VI) under optimal conditions using amide-modified biochar. Freundlich, Langmuir, and Temkin isotherm models were utilized to calculate the adsorption data and determine the optimal fitting model. It was found that the adsorption data fitted well with the Langmuir isotherm model. A kinetics study revealed that the Cr(VI) adsorption onto ABC followed a pseudo-second-order kinetic model. The findings of this study indicate that amide-functionalized biochar has the potential to serve as an economically viable substitute adsorbent for the efficient removal of Cr(VI) from wastewater. [ABSTRACT FROM AUTHOR]

Environmental application of three different talc deposits toward the removal of hexavalent chromium ions Cr(VI) from aqueous solution as simulated polluted water was investigated. Three talc samples (T1, T2 and T3) were used from Wadi Atshan area, Eastern desert, Egypt. The affecting parameters, such as pH, contact time, solution pH and the dosage, were studied during the progress of the adsorption process of chromium (IV). The results showed that a contact time of 65 min for T3, 80 min for T2, 85 min for T1 under optimum condition at pH 7 at 25 °C. The adsorption capacity for the removal of Cr(VI) using the three samples T1, T2 and T3 was evaluated to be 78%, 86% and 97%, respectively, under optimized conditions utilizing 1.0 g/L of the adsorbent. Characterization of the three talc samples was performed using XRF, XRD and FTIR analyses in order to assess the physicochemical properties of the adsorbents. This approach provided new class of adsorbent as highly efficient materials for Cr(VI) removal based on talc deposits which possess some privileges such as availability of the natural resources that makes the process to be low cost and simple. [ABSTRACT FROM AUTHOR]

[Display omitted] • Separation of phenolics and residual waste can enhance their respective functions. • RSM biorefining improves phenolic recovery and residues' biosorption simultaneously. • Phenolic extraction improved Cr(VI) adsorption on residues by about 21%. • Phenolics with natural conjugational form shield adsorption capacity of residue. • Free form phenolics converted by extraction can improve multiple functions. Phenolic recovery from agricultural byproducts has been highlighted due to their health-promoting bioactivities. However, uncontrolled discard of residues after extraction process would induce environmental pollution and bioresource waste. In this study, biorefining of phenolic-rich rapeseed meal (RSM) and its defatted sample (dRSM) was attempted by holistic utilization of phenolic extract and residue separately. Phenolic removal could significantly improve residues' Cr(VI) adsorption capacities by about 21%, which presented extended physical surface and more released functional groups. Moreover, simulating raw material by remixing 3% separated phenolic extracts or main component sinapic acid therein with corresponding residues further improved about 12% adsorption efficiencies. These indicated that the different present forms of phenolics had opposite effects on Cr(VI) removal. While natural conjugational form inhibited hosts' biosorption, free form had enhanced functions for either extract or residue. Four optimal adsorption parameters (pH, adsorbent dosage, contact time and initial Cr(VI) concentration), three kinetic (pseudo-first order, pseudo-second order and intra-particle diffusion) models and two isotherms (Langmuir and Freundlich) were used to reveal the adsorption process. The maximum Cr(VI) adsorption capacity on residues could reach about 100 mg/g, which was superior to that of most biosorbents derived from agricultural byproducts, even some biochar. Together with the residues' advantages with everlasting capacity after 3 adsorption–desorption cycles and excellent abilities for adsorbing multiple co-existed metal ions (Cr(VI), Cd(II), Cu(II), Pb(II), Ni(II) and Zn(II)), phenolic recovery was first proved to be a new and sustainable strategy for modifying biosorbents from agricultural byproducts with zero waste. [ABSTRACT FROM AUTHOR]

Biochar adsorption has emerged as a favorable and environmentally friendly approach for removing metals such as chromium (Cr) from wastewater. However, the use of pristine biochar (PBC) is limited due to its finite adsorptive capacity, selectivity, and potential for secondary pollution. In this study, a novel bifunctionalized magnetic biochar (BMBC) was fabricated by incorporating cystamine as a ligand and glutaraldehyde as a crosslinker into alkali-treated magnetic biochar (MBC). This chemical modification introduced numerous amino groups and disulfide bonds onto the surfaces of BMBC. The biochar adsorbents' surface morphologies, crystal structures, and texture properties were characterized using SEM, XRD, and N2 adsorption-desorption techniques. The specific surface area was determined using the BET method. Furthermore, the surface functional groups and elemental compositions before and after adsorption were analyzed using FTIR and XPS, respectively. The results demonstrated higher Cr(VI) removal efficacy of BMBC (100%) than MBC (72.37%) and PBC (61.42%). Optimal conditions for Cr(VI) removal were observed at a solution pH of 2, a temperature of 50 °C, a reaction time of around 1440 min, and an initial adsorbate concentration of 300 mg/L. The sorption process followed a chemical mechanism and was controlled by monolayer adsorption, with a maximum adsorption capacity of 66.10 mg/g at 50 °C and a pH of 2, as indicated by the larger fitting values of the pseudo–second-order and Langmuir models. The positive ∆Ho and ∆So values and negative ∆G0 values suggested a spontaneous and endothermic Cr(VI) adsorption process with high randomness at the solid/liquid interface. The removal of Cr(VI) was attributed to the reduction of Cr(VI) into Cr(III) facilitated by the introduced amino acids, sulfur, and Fe(II), electrostatic interaction between Cr(VI) in the solution and positive charges on the adsorbent surface, and complexation with functional groups. The presence of co-existing cations such as Cu(II), Cd(II), Mn(II), and K(I) had little effect on Cr(VI) removal efficiency. At the same time, the co-existence of anions of Cl−, NO3−, SO42−, and HPO42− resulted in a 7.58% decrease in the Cr(VI) removal rate. After five consecutive adsorption/desorption cycles, BMBC maintained a high Cr(VI) removal rate of 61.12%. Overall, this novel BMBC derived from rice straw shows great promise as a biosorbent for treating Cr(VI) in wastewater. [ABSTRACT FROM AUTHOR]

Aims This study aimed to investigate the potential of Deinococcus wulumuqiensis R12 (D. wulumuqiensis R12) as a bioadsorbent for Cr(VI) removal from aqueous solutions. Methods and results Effects of various factors, including initial concentration of Cr(Ⅵ), pH, adsorbent dosage, and time were explored. The maximal Cr removal efficiency was achieved by adding D. wulumuqiensis R12 to the solution at pH 7.0 for 24 h, with an initial Cr concentration of 7 mg l−1. Characterization of bacterial cells showed that Cr was adsorbed to the surface of D. wulumuqiensis R12 by combining with functional groups, such as carboxyl and amino groups on the surface. Furthermore, D. wulumuqiensis R12 was able to keep its bioactivity in the presence of Cr and tolerate Cr concentrations as high as 60 mg l−1. Conclusions Deinococcus wulumuqiensis R12 demonstrates a comparatively high adsorption capacity for Cr(VI). Under the optimized conditions, the removal ratio reached 96.4% with 7 mg l−1 Cr(VI), and the maximal biosorption capacity was 2.65 mg g−1. More importantly, it was found that D. wulumuqiensis R12 still had strong metabolic activity and maintained its viability after adsorbing Cr(VI), which is beneficial for biosorbent stability and reuse. [ABSTRACT FROM AUTHOR]

The potential of pretreated sugarcane bagasse (SCB) as a low-cost and renewable source to yield activated carbon (AC) for chromate CrO42− removal from an aqueous solution has been investigated. Raw sugarcane bagasse was pretreated with H2SO4, H3PO4, HCl, HNO3, KOH, NaOH, or ZnCl2 before carbonization at 700 °C. Only pretreatments with H2SO4 and KOH yield clean AC powders, while the other powders still contain non-carbonaceous components. The point of zero charge for ACs obtained from SCB pretreated with H2SO4 and KOH is 7.71 and 2.62, respectively. Batch equilibrium studies show that the most effective conditions for chromate removal are a low pH (i.e., below 3) where >96% of the chromate is removed from the aqueous solution. [ABSTRACT FROM AUTHOR]

A novel pyrrolidinium based-task-specific ionic liquid was successfully synthesized and characterized to specific complexation of Cr(VI) ions in aqueous media. In fact, by the use of a powerful in-situ solvent formation microextraction technique, trace levels of Cr(VI) ions in some real water and wastewater samples were determined. The microextraction of Cr(VI) ions was carried out by complexation of Cr6+ ions with ionic liquid of (1-(2-(quinolin-8-yloxy)ethyl)pyrrolidinium chloride) abbreviated as [QPyr][Cl]. A complexation between [QPyr][Cl] as carrier with Cr(VI) ions as analyte in aqueous solution system was done to transfer of analyte to the non-aqueous phase and finally the determination of Cr(VI) ions in aqueous solution (before and after the above procedure) was performed by atomic absorption spectrometry. Analytical parameters affecting the microextraction system were optimized by using the response surface methodology (RSM). The resulting equation was used to draw response surface plots for prediction of the extractive behavior of the Cr(VI) ions and to find the optimal extraction conditions. At optimized conditions, the calibration curve was linear in the range 50–750 ng/L; the limit of detection of 5.5 ng/L and the relative standard deviation of 2.9% for seven replicate determinations were obtained. As a result, the technique was successfully being applied for the determination of Cr(VI) in some water and wastewater samples with satisfactory recoveries. [ABSTRACT FROM AUTHOR]

Surface chemical modification allows for the rational construction of biochar with desirable structures and functionalities for environment purification. Fruit-peel-derived adsorbing material has been well studied in the adsorption of heavy-metal removal due to its abundance and non-toxicity, but its precise mechanism in removing chromium-containing pollutants remains unclear. Herein, we explored the potential application of engineered biochar prepared from fruit waste via chemical modification to remove chromium (Cr) from an aqueous solution. By synthesizing two types of agricultural residue-derived adsorbents, including pomegranate peel adsorbent (PG) and its modified product, pomegranate-peel-derived biochar (PG-B), via chemical and thermal decomposition methods, we elucidated the adsorption property of Cr(VI) on the studied materials and identified the cation retention mechanism of the adsorption process. Batch experiments and varied characterizations demonstrated that superior activity was exhibited in PG-B, which can contribute to the porous surfaces caused by pyrolysis and effective active sites resulting from alkalization. The highest Cr(VI) adsorption capacity is obtained at pH 4, a dosage of 6.25 g L−1, and a contact time of 30 min. The maximum adsorption efficiency of 90.50% in a short period (30 min) was obtained on PG-B, while PG reached a removal performance of 78.01% at 60 min. The results from kinetic and isotherm models suggested that monolayer chemisorption dominated the adsorption process. The Langmuir maximum adsorption capacity is 16.23 mg g−1. This study shortened the adsorption equilibrium time of pomegranate-based biosorbents and presents positive significance in designing and optimizing waste fruit-peel-derived adsorption materials for water purification. [ABSTRACT FROM AUTHOR]

Contamination of water resources by industrial effluents containing heavy metal ions and management of solid waste from agricultural and food industries is a serious issue. This study presents the valorization of waste walnut shells as an effective and environment-friendly biosorbent for sequestrating Cr(VI) from aqueous media. The native walnut shell powder (NWP) was chemically modified with alkali (AWP) and citric acid (CWP) to obtain modified biosorbents with abundant availability of pores as active centers, as confirmed by BET analysis. During batch adsorption studies, the process parameters for Cr(VI) adsorption were optimized at pH 2.0. The adsorption data were fitted to isotherm and kinetic models to compute various adsorption parameters. The adsorption pattern of Cr(VI) was well explained by the Langmuir model suggesting the adsorbate monolayer formation on the surface of the biosorbents. The maximum adsorption capacity, qm, for Cr(VI) was achieved for CWP (75.26 mg/g), followed by AWP (69.56 mg/g) and NWP (64.82 mg/g). Treatment with sodium hydroxide and citric acid improved the adsorption efficiency of the biosorbent by 4.5 and 8.2%, respectively. The endothermic and spontaneous adsorption was observed to trail the pseudo-second-order kinetics under optimized process parameters. Thus, the chemically modified walnut shell powder can be an eco-friendly adsorbent for Cr(VI) from aqueous solutions. [ABSTRACT FROM AUTHOR]

Hexavalent chromium Cr(VI) is one of the most hazardous oxygen-containing anions to human health and the environment. Adsorption is considered to be an effective method for the removal of Cr(VI) from aqueous solutions. Based on an environmental perspective, we used renewable biomass cellulose as carbon source and chitosan as functional material to synthesize chitosan-coated magnetic carbon (MC@CS) material. The synthesized chitosan magnetic carbons were uniform in diameter (~ 20 nm) and contain a large number of abundant hydroxyl and amino functional groups on the surface, meanwhile owning excellent magnetic separation properties. The MC@CS exhibited high adsorption capacity (83.40 mg/g) at pH 3 and excellent cycling regeneration ability when applied to Cr(VI) removal in water, removal rate of Cr(VI) (10 mg/L) was still over 70% after 10 cycles. FT-IR and XPS spectra showed that electrostatic interaction and reduction with Cr(VI) are the main mechanisms of Cr(VI) removal by MC@CS nanomaterial. This work provides an environment-friendly adsorption material that could be reused for the removal of Cr(VI) in multiple cycles. [ABSTRACT FROM AUTHOR]

This study involved preparation and modification of Saccharum officinarium as adsorbent used for the removal of chromium (VI) ions in a batch process. The adsorbent was modified with oxalic acid for improved performance of the adsorbent by increasing the surface area of the adsorbent. Surface morphology of the adsorbents was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), while Fourier transform infrared (FT-IR) analysis was carried out before and after the adsorption of Cr (VI) ions to determine the participating functional group in the processes. The optimum adsorption was attained at pH 2 and contact time of 180 min with efficiency of adsorption of 56.7 and 92.6% onto RSO and MSO, respectively. The adsorption capacity increases with increase in initial metal ion concentration of the sorption mixture. The isotherms studies indicate that the experimental data were best fitted to Freundlich, Langmuir, and Sips models with R2 = 0.999 for adsorption of Cr (VI) ions onto raw S. officinarium (RSO) and modified S. officinarium (MSO). The maximum adsorption capacity obtained were 227.27 and 243.90 mg*g−1 while the adsorption energy obtained from D-R were found to be 3.460 and 6.325 kJ*mol−1 onto RSO and MSO, respectively. This revealed that the physiosorption process was favored in interaction of Cr (VI) ions onto both adsorbents. Separation factors obtained showed that the process is favored with increase in initial concentration of the adsorbate. Thermodynamic parameter values obtained showed that the sorption of Cr (VI) ions onto RSO and MSO is feasible, spontaneous, and endothermic in nature. The positive value of ΔS° indicates increase in disorderliness of the adsorption process. Kinetic data achieved at different initial concentrations of adsorbate have been analyzed, and the mechanism of the reaction was also studied by intra-particle and Bangham kinetic model. Each of the models was tested with R2 ˃ 0.9, where pseudo-second-order is the best-fitted model and Bangham mechanism only fitted with adsorption of Cr (VI) ions onto RSO. The reusability potential of RSO and MSO contributes to the economic values and reliability of the adsorbents for removal of Cr (VI) ions from aqueous solution. [ABSTRACT FROM AUTHOR]

BACKGROUND: The spent bleaching earth (SBE) is a solid waste produced during the refining of vegetable oil, with features of high residual oil and difficult disposal. At present, pollution control by the treatment of waste is a hot issue to attract more and more attention. This study used SBE as the raw materials and developed a new type of iron‐carbon composite (FexOy/SBE@C) by loading iron on spent bleaching earth carbon (SBE@C) to remove Cr(VI) in water. SBE@C was obtained by the pyrolysis of SBE. RESULTS: The behavior mechanism and factor analysis of FexOy/SBE@C for Cr(VI) removal in water were investigated for the first time. At first, one‐factor experiments in the preparation process were adopted to obtain FexOy/SBE@C with optimal performance. By characterization analysis of the optimal FexOy/SBE@C before and after Cr(VI) removal, it was found that the Cr(VI) removal mechanism was adsorption followed by reduction. Specifically, the adsorption kinetic and isotherm behaviors of FexOy/SBE@C for Cr(VI) belong to the pseudo‐second‐order kinetic model and Langmuir isotherm, respectively. Cr(VI) removal by FexOy/SBE@C is spontaneous (△G<0) and endothermic (△H>0). The presence of coexisting anions (i.e., BrO3− < SO42− < PO43−) and the increase of pH (only adsorption at pH>7) are not conductive to Cr(VI) removal by FexOy/SBE@C. Fortunately, N2 atmospheres can promote Cr(VI) removal (~100%) that is significantly higher than those in air atmosphere (63.34%) and natural conditions (82.25%). CONCLUSION: Overall, FexOy/SBE@C has the satisfactory Cr(VI) removal performance through physical adsorption and chemical reduction, and exhibits great potential application value in Cr(VI) elimination from water. © 2022 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

A novel plant cathode-sediment microbial fuel cell (P-SMFC) was constructed to treat Cr-containing wastewater, and the effects of the plants used, initial concentrations of Cr(VI) employed, and the external resistance on the treatment of wastewater and generation of electricity were investigated. The results showed that the system achieved the best performance when Acorus calamus was the cathode plant, the external resistance was 2000 Ω, and the initial Cr (VI) concentration of the overlying water of is 230 mg/L. A maximum power density of 40.16 mW/m2 was reached, and Cr (VI) and COD removal efficiencies in the overlying water were 99.94% and 98.21%, respectively. The closed-circuit installation promoted the attachment of many microorganisms to the cathode, anode and sediment, increased species abundance, and reduced species diversity. The P-SMFC is inexpensive to construct, it consumes no energy, and it can generate bioelectricity; it thus has great application development value as a chromium-containing wastewater treatment method. [ABSTRACT FROM AUTHOR]

A photolysis method was used to prepare a nanocomposite adsorbent (Chitosan-TiO2) and was tested for Cr(VI) removal from aqueous solution. The produce nanocomposite was investigated using, XRD, BET, FTIR, FESEM-EDX and TEM before and after Cr(VI) adsorption. The XRD results shows prepared anatase phase of TiO2 with 12 nm. According to BET measurements, the surface area of the TiO2/chitosan nanocomposite was lower and archived to 26 m²/g, while the TEM and FESEM images show a uniform distribution of TiO2 throughout the chitosan matrix. Adsorption and kinetic experiments were run in batch system under different conditions of pH, contact time, adsorbent dosage and temperature. Experimental Cr(VI) adsorption equilibrium and kinetics data fitted well to Langmuir model. The calculated Langmuir maximum adsorption capacity (qmax) value of nanocomposite was 488 mg/g. Moreover, the highest quantity of Cr(VI) uptake was achieved of pH = 2 and 45°C and TiO2 and CS-TiO2 had respective removal efficiencies of 94 and 87.5%. The thermodynamic parameters of Cr(VI) adsorption by nanocomposite affirm the spontaneous and endothermic nature of process. Chromium adsorption mechanism by CS-TiO2 nanocomposite were proposed and discussed. [ABSTRACT FROM AUTHOR]