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[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]

The growing concern of water pollution is a critical issue stemming from industrialization and urbanization. One of the specific concerns within this broader problem is the toxicity associated with chromium (Cr), especially in its Cr (VI) form. Transition metal carbides/nitrides (MXenes) are attractive materials for the treatment of water due to their unique properties such as layered structure, high surface area, conductivity, flexibility, scalable manufacture, and surface functions. Adsorption and photocatalysis reactions are the two promising methods for the removal of Cr (VI) by using MXenes. Still, most of the previous reviews were limited to the single application area. Hence, this review covers recent developments in MXene-based composites, highlighting their dual role as both adsorbents and photocatalysts in the removal of Cr (VI). MXene-based composites are found to be effective in both adsorption and photodegradation of Cr (VI). Most MXene-based composites have demonstrated exceptional removal efficiency for Cr (VI), achieving impressive adsorption capacities ranging from 100 to 1500 mg g−1 and degradation percentages between 80% and 100% in a relatively short period. The active functional groups present on the surface of MXene have a viable impact on the adsorption and photodegradation performance. The mechanism of Cr (VI) removal is explained, with MXenes playing a key role in electrostatic attraction for adsorption and as co-catalysts in photocatalysis. However, MXene-based composites have limitations such as instability, competition with co-existing ions, and regeneration challenges. Further research is needed to address these limitations. Additionally, MXene-based composites hold promise for addressing water contamination, heavy metal removal, hydrogen production, energy storage, gas sensing, and biomedical applications. [ABSTRACT FROM AUTHOR]

Fenton-conditioning is commonly used to improve dewatering ability for municipal biological sludge, however, its application in industries is scarce. In this study, biochar (FT-BC) was successfully synthesized from a Fenton-conditioned landfill leachate biological sludge under oxygen-limited. As compared to the corresponding blank and poly ferric-pretreated biochars (BC and PF-BC), moderate Fenton conditioning of the sludge could enable good removal performance for Cr (Ⅵ) by FT-BC. It was found that the oxygen central free radicals (OCFRs) on the biochar surface was intensively promoted due to Fenton electrophilic addition of ·OH onto the oxygen-containing functional groups in biomass. The amounts of OCFRs correlated positively well with the removal efficiency, indicating these persistent free radicals (PFRs)would mainly responsible for the reductive immobilization of Cr(VI)on the FT-BC surface. This study is expected to provide a new method for reclamation of industrial biological sludges with poor agglomeration by introducing simple Fenton pre-conditioning. [Display omitted] • Fenton-conditioning of landfill leachate biological sludge led to preparation of biochar with excellent Cr (Ⅵ) removal. • Electrophilic addition of ·OH to the oxygen-containing functional groups in biomass favored the sludge-based biochar. • Oxygen central free radicals were linearly responsible for the reductive immobilization of Cr(VI) on the biochar surface. [ABSTRACT FROM AUTHOR]

The urgent need to address the severe environmental risk posed by chromium-contaminated industrial wastewater necessitates the development of eco-friendly cleanup methodologies. Utilizing the Ficus benghalensis plant extracts, the present study aims to develop green zinc oxide nanoparticles for the removal of Cr metal ions from wastewater. The leaves of Ficus benghalensis , often known as the banyan tree, were used to extract a solution for synthesizing ZnO NPs. These nanoparticles were developed with the goal of efficiently eliminating chromium (Cr) from industrial effluents. Batch studies were carried out to assess the efficiency of these synthesized ZnO NPs in treating leather industrial effluent, with aiming for optimal chromium removal. This involved measuring the nanoparticles' capacity to adsorb Cr ions from wastewater samples by comparing chromium levels before and after treatment. Removal efficiency for Cr was estimated through the batches such as optimization of pH, contact time, initial Cr concentration and sorbent dose of ZnO NPs were of the batches. These synthesized ZnO NPs were found to be successful in lowering chromium levels in wastewater to meet permissible limit. The nanoparticles exhibited their highest absorption capacity, reaching 94 % (46 mg/g) at pH 4, with a contact time of 7 hours with the optimum sorbent dose of 0.6 g/L. Hence, the excellent adsorption capabilities of these nanoparticles, together with their environmentally benign manufacturing technique, provide a long-term and efficient solution for chromium-contaminated wastewater treatment. Its novel nature has the potential to significantly improve the safety and cleanliness of water ecosystems, protecting the both i.e. human health and the environment. [Display omitted] • The study synthesizes eco-friendly ZnO nanoparticles from Ficus benghalensis plant extracts. • Techniques such as SEM, EDX, FTIR, UV-Visible spectroscopy, and XRD were used to analyze the synthesized nanoparticles. • Batch experiments evaluated ZnO nanoparticles' effectiveness in adsorbing chromium ions from wastewater. • Optimal conditions for chromium sorption (94 %) were found as pH range of 3–4, contact time of 7 hrs, at a conc. of 46 mg/g. • Nanoparticles offer efficient solution for treating chromium-contaminated wastewater to protect water bodies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Poly(pyrogallol-tetraethylene pentamine) chelating resin was first synthesized as an efficient Cr(VI)-adsorbent. • Hazardous Cr(VI) is converted to nontoxic Cr(III) via adsorption-reduction synergy. • Saturated adsorption amount followed Langmuir model up to 714.29 mg g−1. • Almost total elimination of Cr(VI) in low concentration of solutions (<180 mg L-1). Facile fabrication of the ultra-high-performance adsorbent can effectively ameliorate the Cr(VI)-pollution elimination in sewage control. Herein, a simple synthesis strategy is proposed to tap a versatile chelating resin poly(pyrogallol-tetraethylene pentamine) (PPTA) with respect to Cr(VI) removal from solution. Multiple changing factors which affect the adsorption behavior of PPTA are explored sequentially, such as initial pH, adsorbate concentration, adsorbent dosage, temperature, foreign ions, etc. The microstructure and functional mechanism of synthetic adsorbent are investigated systematically by means of various characterizations including TEM, EDS, FT-IR, XPS, etc. Consequently, the as-prepared PPTA-3 microsphere by reactant ratio of 1: 1 represents a brilliant synergistic adsorption and reduction result for Cr(VI) by the drastic electrostatic interaction of –NH 3 + and –OH 2 + groups, including satisfactory removal efficiency which closes to 100 % in low concentration, favorable specificity for the influence from coexistent ions (Mo(VI), Mn(VII), Cl-, Cr(III), etc), and passable recyclability. Following the surpassingly fitting with Langmuir isotherm model, its maximum capacity reaches 714.29 mg g−1 at 30 °C. The removal performance is essentially in agreement with the pseudo-second-order kinetics, simultaneously, suffers the rate-limiting impact depending on intra-particle diffusion process. In brief, this newly developed chelating resin presents an effective means with regard to the Cr(VI)-wastewater treatment or other uses in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • CrO 4 2− was almost completely removed from alkaline Cr(VI) contaminated wastewater. • CrO 4 2− was transformed into chromium ferrite (Cr 0.5-x Fe 2.5+x O 4) by one-step process. • Chromium ferrite was formed by reaction of amorphous Cr 0.25 Fe 0.75 (OH) 3 with Fe2+. • Increasing temperature and pH could promote the formation of chromium ferrite. Current technologies for removal of Cr(VI) are generally fit for acidic wastewater. In this study, a new ferrite process for removal and recycling of Cr(VI) from alkaline wastewater to produce the valuable chromium ferrite has been developed. The results show that this new ferrite method is a one-step process which can be divided into two successive reactions including Cr(VI) reduction to form coprecipitation (Cr 0.25 Fe 0.75 (OH) 3) and subsequently magnetic conversion of Cr 0.25 Fe 0.75 (OH) 3 induced by Fe2+ under the same alkaline condition. The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation. Increasing temperature and pH can enhance the interaction of Fe2+ with Cr 0.25 Fe 0.75 (OH) 3 and further promote the formation of chromium ferrite, while suppressing the generation of nonmagnetic by-product goethite. Almost pure chromium ferrite is formed under proposed optimum conditions (Fe/Cr = 7:1, 65 °C and pH of 9) with Cr(VI) removal ratio around 100%. The Cr(VI) remained in the filtrate can be reduced to 0.01 mg/L which is much lower than the limits concentration for surface water (≤0.05 mg/L). The chromium ferrite product whose molecular formula can be expressed as Cr 0.5-x Fe 2.5+x O 4 (where 0 ≤ x < 0.5) presents good magnetic properties and has the potential to be recycled as a useful material. [ABSTRACT FROM AUTHOR]

The recycling of industrial solid by-products such as red mud (RM) has become an urgent priority, due to their large quantities and lack of reutilization methods can lead to resource wastage. In this work, RM was employed to fabricate green hydrochar (HC) to prepare zero-valent iron (ZVI) modified carbonous materials, and conventional iron salts (IS, FeCl 3) was applied as comparison, fabricated HC labeled as RM/HC and IS/HC, respectively. The physicochemical properties of these HC were comprehensively characterized. Further, hexavalent chromium (Cr(VI)) removal performance was assessed (375.66 and 337.19 mg/g for RM/HC and IS/HC, respectively). The influence of dosage and initial pH were evaluated, while isotherms, kinetics, and thermodynamics analysis were also conducted, to mimic the surface interactions. The stability and recyclability of adsorbents also verified, while the practical feasibility was assessed by bok choy-planting experiment. This work revealed that RM can be used as a high value and green fabricant for HC the effective removal of chromium contaminants from the wastewater. [Display omitted] • RM/HC and IS/HC were synthesized using red mud (RM) and FeCl 3 as iron source. • The capacity of Cr(VI) reached 375.6 and 337.1 mg/g for RM/HC and IS/HC, respectively. • Cr(VI) removal mechanisms were involved with chemical reduction and adsorption. • The removal efficiency retained over 80% for RM/HC after 5 times recycling. • Leaching testing and pot experiments were used to analysis practicality. [ABSTRACT FROM AUTHOR]

Abstract A bioremedial approach was investigated on the removal of Cr(VI) from aqueous solution using a novel chromium reducing bacteria isolated from coalmine wastewater. Cr(VI) removal efficacy of the bacterium was determined in a series of batch studies under the influence of various parameters viz., pH (1–7), temperature (20–40 °C), initial metal concentration (1–150 mg/L), agitation speed (80–150 rpm) and substrate concentration (1–5 mg/L). Oxygen involvement in the removal process was determined by different incubation conditions. Substrate consumption and its resultant biomass generation were considered for determining the viability of the microbe under varied metal concentration. The microbial isolate survived in Cr(VI) tainted solution with initial concentration of 1–140 mg/L, among which maximum remediation was found in 60 mg/L Cr(VI) loaded solution. The bacterial species also survived in other metal solution viz., Fe(II), As(V), Cu(II), Pb(II), Zn(II), Mg(II), Mn(II) apart from Cr(VI). Multiple approaches were tested to facilitate understanding of the bacterial Cr(VI) removal mechanism. The bacteria accumulated metal ions in the exponential growth phase both on and within the cell. Underlying latent factors which governed the bacterial growth and its removal activity was determined with the classical Monod equation. The isolated bacterium also survived in the bimetallic solutions with significant removal of Cr(VI). The microbial species isolated from mining area was identified as Pseudomonas brenneri by 16s rRNA molecular characterization. Hence, the isolated novel bacterium illustrated promising involvement towards bio-treatment of Cr(VI) laden wastewater. Graphical abstract Image 1 Highlights • Cr(VI) reducing bacterium was isolated from coalmine area. • The isolate was identified as Pseudomonas brenneri by 16s rRNA characterization. • The incubation condition parameters were optimized. • Cr(VI) bioremediation efficiency was tested for both single and bimetallic solutions. • Cr(VI) bioremedial process was substantiated with Monod's equation. [ABSTRACT FROM AUTHOR]

Water contamination can be detrimental to the human health due to higher concentration of carcinogenic heavy metals such as chromium (Cr) in the wastewater. Many traditional methods are being employed in wastewater treatment plants for Cr removal to control the environmental impacts. Such methods include ion exchange, coagulation, membrane filtration, and chemical precipitation and microbial degradation. Recent advances in materials science and green chemistry have led to the development of nanomaterial that possess high specific surface areas and multiple functions, making them suitable for removing metals such as Cr from wastewater. Literature shows that the most efficient, effective, clean, and long-lasting approach for removing heavy metals from wastewater involves adsorbing heavy metals onto the surface of nanomaterial. This review assesses the removal methods of Cr from wastewater, advantages and disadvantages of using nanomaterial to remove Cr from wastewater and potential negative impacts on human health. The latest trends and developments in Cr removal strategies using nanomaterial adsorption are also explored in the present review. • Nanomaterials have great potential for efficient Cr removal from wastewater. • Present work presents major experimental conditions influencing Cr removal and recovery from wastewater. • Efficient nanomaterials on the basis of published publications have been found for effective chromium removal from wastewater. • Adsorption techniques are seen to be preferable to other methods because they ecofriendly, and sustainable. • The optimized chromium removal conditions used on this topic are compiled here. [ABSTRACT FROM AUTHOR]

Anaerobic fermentation is considered as a cost-effective way of biomass waste disposal. Chromium (Cr) is one of the heavy metals that often been blamed for unsatisfactory operation or failure of anaerobic fermentation. The impact of Cr (added as K 2 Cr 2 O 7 ) on mesophilic anaerobic fermentation of Phragmites australis straw and cow dung was demonstrated by investigating the biogas properties, process stability, substrate degradation and enzyme activities during the fermentation process. The results showed that 30, 100 and 500 mg/L Cr 6+ addition increased the cumulative biogas yields by up to 19.00%, 14.85% and 7.68% respectively, and brought forward the daily biogas yield peak. Meanwhile, the methane (CH 4 ) content in the 30 (52.47%) and 100 (40.57%) mg/L Cr 6+ -added groups were generally higher than the control group (37.70%). Higher pH values (close to pH 7) and lower oxidation-reduction potential (ORP) values in the Cr 6+ -added groups after the 15th day indicated the better process stability compared to the control group. Taking the whole fermentation process into account, the promoting effect of Cr 6+ addition on biogas yields was mainly attributable to better process stability, the enhanced degradation of lignin and hemicellulose, the transformation of intermediates into VFA, the higher coenzyme F 420 activities and the efficient generation of CH 4 . These results demonstrate that an appropriate addition of Cr 6+ could enhance the anaerobic fermentation which support the regulations utilizing of the Cr 6+ contaminated biowaste. [ABSTRACT FROM AUTHOR]

Porous matrix immobilization is considered as an effective approach to address the engineering challenges during practical application of nanoadsorbent. In this work, polyethersulfone (PES) beads with diameter of around 2.5 mm, in which nanosized amine-rich polymer spheres (APSs) with particle size of ∼400 nm are immobilized, are fabricated via phase inversion route. APSs-PES beads possess asymmetric and hierarchical structure with an exterior porous surface dense layer and numerous inner interpenetrating fingerlike channels, which promote the transportation of molecule and prevent the leaching of nanoadsorbents. The well distributed APSs with nitrogen-containing functional groups provide abundant adsorption sites. The cooperative attributing of the hierarchical structure and APSs demmonstrates enhanced Cr (VI) adsorption efficiency. The adsorption capacity of Cr (VI) is 243.9 mg/g at 298 K by Langmuir fitting. The effects of pH, temperature, the ratio of adsorbents and coexisting ionic are further studied in detail. The Cr (VI) removal mechanism is further explored by the combination of SEM, XPS, FTIR measurements and the adsorption parameters, further revealing the synergistic contribution of electrostatic attraction and reduction process. Therefore, this kind of adsorbents may show promising prospects on elimination of hazardous Cr (VI) in wastewater treatment. [ABSTRACT FROM AUTHOR]

In this paper, a kind of zero valent FeNi alloy nano powder was coated on the surface of SrFe-MOF powder by a simple, efficient and pollution-free method, forming a core-shell type FeNi p @SrFe-MOF Magnetic powder (FSM). The FSM powder has a porous structure and a specific surface area of 9.16 ​m2/g. Under normal temperature and without changing the pH value of the wastewater, the FSM magnetic powder has an immediate and efficient removal effect on Congo Red (CR) dye wastewater. Furthermore, the powder has good cycling performance, with the CR removal rate remaining above 90% after five cycles. Also at room temperature and without changing the pH value of the wastewater, the FSM magnetic powder provides some immediate, efficient, and repeated use degradation of Cr(VI) wastewater. The FSM magnetic powder has good magnetic properties so that can be easily separated and recovered by magnetic field treatment, which solves the problem of efficient recovery of raw materials in wastewater treatment. FSM magnetic powder has broad application prospects for treating wastewater. In this paper, a kind of zero valent FeNi alloy nano powder was coated on the surface of SrFe-MOF powder by a simple, efficient and pollution-free method, forming a core-shell type FeNi p @SrFe-MOF Magnetic powder (FSM). Under normal temperature and without changing the pH value of the wastewater, the FSM magnetic powder has an immediate and efficient removal effect on Congo Red (CR) dye wastewater. Furthermore, the powder has good cycling performance, with the CR removal rate remaining above 90% after five cycles. Also at room temperature and without changing the pH value of the wastewater, the FSM magnetic powder provides some immediate, efficient, and repeated use degradation of Cr(VI) wastewater. The FSM magnetic powder has good magnetic properties so that can be easily separated and recovered by magnetic field treatment. [Display omitted] • FSM powder has an immediate and efficient removal effect on CR at room temperature and without changing the pH value of the wastewater. • FSM powder has some removal effect on Cr(VI) at room temperature and without changing the pH value of the wastewater. • FSM powder has good magnetic properties, which can be separated and recycled by magnetic field treatment. [ABSTRACT FROM AUTHOR]

Review papers help researchers understand the direction and flow of research and help beginners quickly acquire relevant knowledge. Therefore, the review paper should describe only accurate contents and should be written with only core and important matters. Recently, Othmani et al. (2022) reviewed more than 200 papers related to the adsorptive removal of three harmful pollutants: toxic organics (phenols), anionic heavy metal (Cr(VI)) and cationic heavy metal (Cd(II)) by agricultural waste materials. However, Cr(VI)-related section of this review is missing something important that should be provided to the readers of this journal. In fact, the removal mechanism of Cr(VI) by nonliving biomass including agricultural waste materials under acidic conditions is not a simple anion adsorption but a complex mechanism involving redox reaction called "adsorption-coupled reduction". Therefore, when interpreting the factors affecting Cr(VI) removal, it should be interpreted in terms of the redox reaction concept, not anion adsorption. • Reduction of Cr(VI) to Cr(III) by agricultural waste materials. • Complex mechanism called "adsorption-coupled reduction". • Interpretation of experimental data with reduction reaction concept. [ABSTRACT FROM AUTHOR]

In this study, hexavalent chromium Removal from aqueous environments was investigated by using polyaniline composites with some natural waste materials. Batch experiments were used, and some parameters such as contact time, pH and adsorption isotherms were determined for the best composite with the highest removal efficiency. Scanning Electron Microscope (SEM), Fourier Transform Infrared (FTIR) spectroscopy, and X-ray Diffraction (XRD) were used to characterize the composites. According to the results, the polyaniline/walnut shell charcoal/PEG composite outperformed other composites and showed the highest chromium removal efficiency of 79.22%. Polyaniline/walnut shell charcoal/PEG has a larger specific surface area of 9.291 (m2/gr) which leads to an increase in its removal efficiency. For this composite, the highest removal efficiency was obtained at the pH = 2 and 30 min contact time. The maximum calculated adsorption capacity was 500 mg/g. [Display omitted] • Polyaniline composites were synthesized with various additives. • Polyaniline coated on different materials was used for chromium removal. • Batch experiments were performed and various parameters were determined for the best composite. • Addition of polyethylene glycol improves chromium removal efficiency of the composite. • Adsorption decreases by increasing polyethylene glycol from 2 to 5 g/L. [ABSTRACT FROM AUTHOR]

An activated carbon fiber supported nanoscale zero-valent iron (ACF-nZVI) composite for Cr(VI) removal from groundwater was synthesized according to the liquid phase reduction method. The techniques of N 2 adsorption/desorption, FESEM, EDX, XRD and XPS were used to characterize the ACF-nZVI composite and the interaction between the ACF-nZVI composite and Cr(VI) ions. Batch experiments were conducted to evaluate the effects of several factors, including the amount of nZVI on activated carbon fiber (ACF), pH value, initial Cr(VI) concentration, and co-existing ions on Cr(VI) removal. The results indicate that presence of ACF can inhibit the aggregation of nanoscale zero-valent iron (nZVI) particles and increase its reactivity, and the Cr(VI) removal efficiency increases with increasing amounts of nZVI on ACF and a decrease in the initial Cr(VI) concentration. In acidic conditions, almost 100% of Cr(VI) in solution can be removed after 60 min of reaction, and the removal efficiency decreases with increasing initial pH values. The Cr(VI) removal is also dependent on the co-existing ions. Reusability experiments on ACF-nZVI demonstrate that the ACF-nZVI composite can keep a high reactivity after five successive reduction cycles. The removal mechanisms are proposed as a two-step interaction including the physical adsorption of Cr(VI) on the surface or inner layers of the ACF-nZVI composite and the subsequent reduction of Cr(VI) to Cr(III) by nZVI. [ABSTRACT FROM AUTHOR]

After polystyrene@titanium dioxide (PS@TiO 2 ) composite with different size was calcined at designated temperature, TiO 2 hollow sphere with controllable size was obtained for high efficient photo-reduction of Cr(VI). The feature of the TiO 2 hollow sphere was investigated by SEM, TEM, XRD, UV–Vis, and photoluminescence. The photo-reduction of Cr(VI) were measured for the performance assessment of the TiO 2 hollow sphere, Cr(VI) was used as an electron acceptor. After irradiation for 2 h, the photo-reduction rate of Cr(VI) (pH = 2.82) for TiO 2 (450 nm) was 96%, which exhibited an increase of 5% and 8% compared with TiO 2 (370 nm) and TiO 2 (600 nm). The absorption edges of TiO 2 hollow sphere (450 nm) was largest with the increasing of hollow sphere size from 370 to 600 nm. The optimal hollow sphere size of TiO 2 was 450 nm for the photo-reduction of Cr(VI), because the light-harvesting efficiency (the best of absorption edge) and photo-generated electron-hole separation rate (the best of photo-reduction rate) of TiO 2 hollow sphere were controlled by its hollow sphere size. In addition, we find that the behavior of the hydrogen production was inhibited by the coexistence Cr(VI) solution. This study can improve our understanding of the mechanism for the activity enhancement by the optimal hollow sphere size of TiO 2 . [ABSTRACT FROM AUTHOR]

Cr(VI) pollution is a growing problem that causes the deterioration of the environment and human health. We report the development of an effective adsorbent for the removal of Cr(VI) from wastewater. N-doped cellulose-based hydrothermal carbon (N–CHC) was prepared via a two-step hydrothermal method. The morphology and structural properties of N–CHC were investigated by various techniques. N–CHC has many O and N groups, which are suitable for Cr(VI) adsorption and reduction. Intermittent adsorption experiments showed that N–CHC had an adsorption capacity of 151.05 mg/g for Cr(VI) at pH 2, indicating excellent adsorption performance. Kinetic and thermodynamic analyses indicates that the adsorption of Cr(VI) on N–CHC follows a monolayer uniform adsorption process, which is a spontaneous endothermic process dominated by chemical interaction and limited by diffusion within particles. In a multi-ion system (Pb2+, Cd2+, Mn7+, Cl−, and SO 4 2−), the selectivity of N–CHC toward Cr(VI) was 82.62%. In addition, N–CHC demonstrated excellent reuse performance over seven adsorption–desorption cycles; the Cr(VI) removal rate of N–CHC in 5–20 mg/L wastewater was >99.87%, confirming the potential of N–CHC for large-scale applications. C N/C-OR, pyridinic-N, and pyrrolic-N were found to play a critical role in the adsorption process. This study provides a new technology for Cr(VI) pollution control that could be utilized in large-scale production and other environmental applications. • An N-doped cellulose-based hydrothermal carbon was synthesized in two steps. • The synthesized N–CHC has high adsorption capacity for Cr(VI). • N–CHC has high Cr(VI) selective adsorption performance and reuse performance. • The removal rate of 5–20 mg/L Cr(VI) wastewater by N–CHC reach 99.87%. • C N/C-OR, pyridinic-N and pyrrolic-N play a key role for Cr(VI) removal. [ABSTRACT FROM AUTHOR]

In this study, Cr(VI) was removed sonocatalytically by the zinc oxide nanoparticle coated with polyethylene terephthalate (PET) fabricated through a facile co-precipitation method. The crystal structure, functional groups on the surface, morphology, surface composition and oxidation states of the nanomaterials were investigated by XRD, FTIR, SEM, EDX and XPS techniques. Environmental parameters including solution pH, catalyst dose, hexavalent chromium concentration, H 2 O 2 content, purging gases, organic compounds and type and anions strength on the sonotocatalytic removal of Cr(VI) were also investigated. Additionally, the contribution of each process, reusability, Cr(VI) reduction from actual water and electroplating wastewater were evaluated. Under the optimal conditions, [Cr(VI)] 0 =20 mg/L, nanocomposite loading=1.6 g/L and pH=5, 99.92% of Cr(VI) was removed within 60 min. By increaing, Cr(VI) concentration (5–50 mg/L), k obs decreased to values between 0.1498 and 0.0063 min−1 and the calculated electrical energy per order (E Eo) increased from 148.68 to 3535.24 kWh.m−3, respectively. The presence of purging gases, organic compounds and ionic strength negatively affected Cr(VI) reduction. Examination of radical scavengers showed that the most active radicals in Cr(VI) removal were O 2 • − and h+. The removal of the Cr(VI) using the US/ZnO-PET method (99.92%) was higher than that of the US/ZnO method (70.78%). The catalyst activity was well maintained up to eight consecutive cycles. In addition, the removal efficiency was approximately 72.23 and 68.55% for drinking water and real electroplating wastewater samples, respectively. The results of toxicity in the sonotocatalytic removal of Cr(VI) by Daphnia magna showed LC 50 and toxicity unit (TU) 48 h, which was equal to 81.46 and 1.227 vol percent. [Display omitted] • ZnO-PET nanocomposite was used at sonocatalytic removal of Cr(VI). • Structural characteristics of PET, ZnO and ZnO-PET were applied via XRD, FT-IR, SEM and EDX analysis. • ZnO-PET activity enhanced substantially when it was coupled with US irradiations. • Reusability and acute toxicity were investigated after treatment. • Sonocatalytic removal of Cr(VI) was surveyed by ZnO-PET nanocomposite in electroplating wastewater. [ABSTRACT FROM AUTHOR]

With the increase of the azo pigment wastewater, it is necessary to seek an efficient and sustainable treatment method to address issues of damaging water ecosystems and human health. In this work, organic representing azo dye Acid Orange 7 (AO7), heavy metal representing hexavalent chromium (Cr(VI)), and inorganic representing ammonia nitrogen (NH 4 +-N) were selected to roughly simulate the azo pigment wastewater. The simultaneous decontamination of multi-target pollutants by 700 °C pyrolyzed peanut shell biochar (BC) with persulfate (PDS) was evaluated. The results showed that AO7, Cr(VI) and NH 4 +-N could finally reach 100%, 85% and 30% removal ratios separately in the BC/PDS/mixed pollutants system under certain basic conditions. Functional groups (hydroxyl groups (C–OH) and carboxylic ester/lactone groups (O–C=O)) were found by XPS as competing sites for adsorption and activation and were gradually consumed as the reaction proceeded. Combining a series of experiments results and EPR analysis, it was found that AO7 removal worked best and it relied on both the radical pathway (including SO 4 •−, •OH, O 2 −•, but not 1O 2) and adsorption. Cr(VI) was mainly adsorbed and reduced by BC surface to form Cr(OH) 3 and Cr 2 O 3 , and the remaining part could be reduced by O 2 −•, followed by •OH. NH 4 +-N was removed primarily by the radical same as AO7. Meanwhile, the three target pollutants have a co-competitive mechanism. Specifically, they competed for radicals and adsorption sites simultaneously, while the presence of AO7 and NH 4 +-N would consume the generated oxidizing radicals and further promote the removal of Cr(VI). The fixed-bed reactor simulated the continuous treatment of wastewater. Various anions (chloride (Cl−), nitrate (NO 3 −), carbonate (CO 3 2−), and hydrogen phosphate (HPO 4 2−)) interfered differently with the pollutant removal. These findings demonstrate a new dimension of BC potential for decontamination of azo pigment wastewater. [Display omitted] • C–OH and O–C=O on BC were competing sites for pollutants adsorption and PDS activation. • Competitive and synergistic mechanism existed among AO7, NH 4 +-N and Cr(VI). • AO7 and NH 4 +-N degradation were both dominated by SO 4.•−, •OH and O 2 −•. • Cr(VI) decontamination was dominated by adsorption, followed by O 2 −.•. • The fixed-bed reactor and anion interference were explored as supplementary applications. [ABSTRACT FROM AUTHOR]

The reactivity of zero valent iron (Fe 0 ) for removing Cr(VI) is self-inhibiting under neutral and alkaline conditions, due to the precipitation of ferrous hydroxide on the surface of Fe 0 . To overcome this difficulty, we incorporated a second metal (Co) into Fe 0 to form FeCo bimetallic nanoparticles (FeCo BNPs), which can achieve higher activity and significant improvement in the reaction kinetics for the removal of Cr(VI) compared with Fe 0 . The FeCo BNPs were synthesized by a hydrothermal reduction method without using any templates. The characterization analysis indicated that the products were highly uniform in large scale with 120–140 nm size in diameter. The obtained FeCo BNPs exhibited a remarkable removal ability for Cr(VI) in the pH range of 5.3–10.0. Especially, FeCo BNPs were able to reduce trace Cr(VI) (1.0 mg L −1 , pH = 7.5) down to about 0.025 mg L −1 within 1 h. XPS analysis confirmed that Cr(VI) was reduced to Cr(III) by FeCo BNPs, while Fe and Co was oxidized, implying a chemical reduction process. The enhanced removal of trace Cr(VI) could be originated from the introduction of Co, which not only served as a protecting agent against surface corrosion by galvanic cell effect, but also enhanced the efficient flow of electron transfer between iron and Cr(VI). All the results primarily imply that FeCo BNPs can be employed as high efficient material for wastewater treatment. [ABSTRACT FROM AUTHOR]

In the present paper, eggshell and powdered marble, two carbonaceous materials, were used to remove Cr(III) ions from a real chrome tanning wastewater. The effects of initial effluent pH, adsorbent dose, contact time and temperature were studied. The maximum uptake of chromium ions was obtained at pH 5.0 with the dose 20 g L −1 and 12 g L −1 for eggshell and powdered marble respectively. Adsorption equilibrium was reached after 14 h contact time for eggshell and only after 30 min for powdered marble. Under these conditions, almost 99% Cr(III) was removed from chrome tanning wastewater having an initial concentration of chromium of 3.21 g L −1 . Kinetic data were satisfactorily described by a pseudo-second order chemical sorption model. The equilibrium rate constant was notably greater for powdered marble than for eggshell with 1.142·10 −3 (g mg −1 min −1 ) and 0.041·10 −3 (g mg −1 min −1 ) respectively. The adsorption isotherm were well described by a Langmuir model and showed that the interaction of chromium with the two adsorbents surface is a localized monolayer adsorption with a smaller energy constant for the powdered marble than for eggshell (0.020 (L mg −1 ) and 0.083 (L mg −1 ) respectively). The powdered marble was able to adsorb faster a large amount of Cr (III) in comparison to eggshell. The use of a standardized lettuce seed bioassay allowed evaluating a better effectiveness of the Cr adsorption on the powdered marble, removing up to 40% of the treated effluent toxicity than by eggshell 25%. The powdered marble could be considered as an effective, low cost carbonaceous material to be used for chromium removal from tanning wastewater. [ABSTRACT FROM AUTHOR]

Biochars derived from rice husk, the organic fraction of municipal solid wastes and sewage sludge, as well as a sandy loam soil, were used as adsorbents for As(V), Cr(III) and Cr(VI) removal from aqueous solutions. The kinetic study showed that sorption can be well described by the pseudo-second order kinetic model, while simulation of sorption isotherms gave better fit for the Freundlich model. The materials examined removed more than 95% of the initial Cr(III). However, removal rates for As(V) and Cr(VI) anions were significantly lower. Biochar derived from sewage sludge was efficient in removing 89% of Cr(VI) and 53% of As(V). Its ash high Fe2O3 content may have enhanced metal adsorption via precipitation. Soil was the most effective material for the removal of As(V), yet it could not strongly retain metal anions compared to biochars, as a significant amount of the adsorbed metal was released during desorption experiments. [Copyright &y& Elsevier]

The discharge of untreated tannery industrial wastewater into the environment has resulted in an adverse effect on the ecosystem and public health. Therefore, this work aimed to remove chromium ions from tannery wastewater through magnetite graphene oxide-nanocomposite (GO-Fe 3 O 4). The experimental design of the study was a full factorial 24 approach using pH, adsorbent dose, contact time, and initial chromium concentrations. The results of FTIR analysis revealed the presence of functional groups such as hydroxyl (3438 cm−1), alcohol (1230 cm−1), aromatic (1467 cm−1), ketone (1629 cm−1), and ether (1120 cm−1). Similarly, GO-Fe 3 O 4 acquired a high surface area of 296.2 m2/g whereas the XRD analysis showed the presence of predominant peaks which are attributed to the magnetite component. Moreover, the SEM image showed many ups and downs on the surface of the adsorbent. These cracks of morphology can create a conducive environment for the interaction of adsorbent and adsorbate. The maximum chromium removal of 95.9% was achieved at the optimum conditions of the initial chromium concentration of 40 mg/L, pH 4, adsorbent dose 1 g/100 mL, and contact time of 120 min whereas the removal of chromium from real tannery wastewater was found to be 90.3%. Based on the adsorption isotherm, the Langmuir model was the best fit for experimental data at R2 0.99, indicating homogeneous and monolayer adsorption. Finally, it can be concluded that GO-Fe 3 O 4 was effective for chromium removal, which is a promising technology to be scaled up at the industrial level for wastewater treatment. • Chromium is discharged with effluent from the tannery industry which is the major cause of water pollution. • Effective implementation of advanced wastewater treatment is very limited for chromium management. • GO-Fe 3 O 4 nanocomposites have emerged nanotechnology for chromium remediation. • In the Future, exploring the potential of GO-Fe 3 O 4 nanocomposites is highly encouraging. [ABSTRACT FROM AUTHOR]

Resin supported nanoscale zero-valent iron (R-nZVI) was synthesized by the borohydride reduction method. Batch experiments were conducted to evaluate the factors affecting Cr(VI) removal. It was found that nZVI loads, resin dose, pH value and initial concentration of Cr(VI) were all important factors. Scanning electron microscopy showed that the nZVI particles in R-nZVI became sphere after reacting with Cr(VI). This phenomenon was attributed to the co-precipitation of Cr(III) and Fe(III) on the surface of resin. X-ray diffraction pattern confirmed that Fe0 diminished after the reaction. At optimum conditions, the Cr(VI) removal efficiency was 84.4% when the initial concentration of Cr(VI) was 20.0 mg/L. Regeneration of R-nZVI and resin was possible. R-nZVI can also remove Cr(III) efficiently. However, the removal mechanisms of Cr(VI) (anion) and Cr(III) (cation) are different. The former is chemical reduction, while the latter is ion exchange at pH below 6.3 and precipitation at pH above 6.3. This study demonstrates that R-nZVI has the potential to become an effective agent for treating wastewater containing Cr(VI) and Cr(III). [Copyright &y& Elsevier]

Highlights: [•] A spent FCC catalyst was converted into a zeolitic mixture mainly containing A type zeolite (>75% w/w). [•] The product so obtained was used for Cr(III) removal from aqueous solutions. [•] The Cr(III) sorption produced amorphization of the zeolitic structure without changing the surface charge. [•] SEM observation indicated the arrangement of Cr(III) in the outer microsphere surface of the zeolite crystals. [•] Chromium replaces sodium cations countering the negative charge of A zeolite framework. [Copyright &y& Elsevier]

Abstract: In this study, natural Akadama clay was used for Cr (VI) removal from aqueous solution. Batch experiments were carried out to investigate the effect of contact time, initial pH, and adsorbent dose on Cr (VI) adsorption. Results showed that Cr (VI) adsorption on natural Akadama clay reached equilibrium in 180min. The Cr (VI) removal efficiency of 46.8% without pH adjustment increased to 73.8% at the optimum initial pH of 2. The maximum adsorption capacity was 4.29mgg−1 at an initial concentration of 50.0mgL−1 and adsorbent dosage of 5gL−1. The equilibrium data fitted Freundlich isotherm better than Langmuir isotherm, and they were well explained by pseudo-second-order kinetic model. Adsorption mechanism analysis proved that electrostatic adsorption dominated during the removal process. Results from this study demonstrate that natural Akadama clay has the potential to be an efficient adsorbent for Cr (VI) adsorption compared to other natural mineral adsorbents. [Copyright &y& Elsevier]

Results of a lab-scale experimental study are presented that aim to verify the use of ground shrimp shells, not otherwise conditioned, as an adsorbent material to remove chromium(III) from tannery wastewater. The obtained removal efficiency is found to be always over 90%, confirming the capacity of the tested materials to remove chromium(III). The adsorption process is well described by the Brauner–Emmett–Teller isotherm, indicating the existence of both weak and strong adsorption sites inside the shells. Kinetic tests allow to verify that the removal process takes place rapidly during the first 2 h and then tends to reach a plateau: the pseudo second-order model is found to be able to simulate with good approximation, the adsorption process. Analyses comparing the treatment process using shrimp shells vs. other chemical products frequently used for chromium removal, supported by microscopic observations, indicate that the efficiency obtained using shrimp shell is comparable to the efficiency obtained using sodium bicarbonate and sodium hydroxide, but the removal mechanism is different as the adsorption on the polysaccharidic matrix of the shells prevails over the precipitation of chromium salts. The skin tanned with recovered chromium is found to be of low quality; on the contrary satisfying tanning results are obtained using, as tanning agent, the sludge produced by the adsorption process without any further treatment, except for pH correction. [Copyright &y& Elsevier]

Abstract: Hexavalent chromium is more toxic and carcinogenic compared to its trivalent counterpart. Hexavalent chromium can be photocatalytically reduced to trivalent one using zinc oxide semiconductor and solar or UV radiation as light source. But an important ecological aspect is whether the net toxicity decreases after the photocatalytic process. In the present work, we have selected Anabaena flos-aquae as the model organism and examined whether the net toxicity to which it has been exposed in water decreases after photocatalytic reduction of hexavalent chromium using ZnO semiconductor as photocatalyst. We have studied the cumulative toxicities of both chromium (before and after treatment) and ZnO (both nano and micro) separately and as a suspension of ZnO in chromium. Quantitative cell surface roughness analysis was conducted using AFM to confirm the changes in cell surface properties. [Copyright &y& Elsevier]

Coir pith samples were chemically modified by grafting with acrylic acid for the removal of Cr(VI) from electroplating wastewater. The presence of acrylic acid on the coir pith surface was verified by a scanning electron microscope with an electron dispersive x-ray spectrometer (SEM/EDX), Fourier transform infrared spectroscopy (FTIR) and thermogravimetry (TG). The carbonyl groups (Che carboxylic acids (COOH) increased on the coir pith surface after grafting with acrylic acid. In addition, the thermal stability of the acrylic acid-grafted coir pith also improved. The optimum conditions for grafting the acrylic acid on the coir pith consisted of 2 M acrylic acid and 0.00125 M ceric ammonium nitrate (CAN, as an initiator). The maximum Cr(VI) removal (99.99 ± 0.07%) was obtained with the following conditions: a 1.3% (w/v) dosage of acrylic acid-grafted coir pith, a system pH of 2, a contact time of 22 h, a temperature of 30 °C, a particle size of <150 μm and an initial Cr(VI) of 1,171 mg l−1. At system pH of 2, Cr(VI) in the form can be adsorbed with acrylic acid-grafted coir pith via an electrostatic attraction. The adsorption isotherm of 2 M acrylic acid-grafted coir pith exhibited a good fit with the Langmuir isotherm. The maximum Cr(VI) adsorption capacity of the 2 M acrylic acid-grafted coir pith was 196.00 mg Cr(VI) g−1 adsorbent, whereas for coir pith without grafting, the maximum Cr(VI) removal was 165.00 mg Cr(VI) g−1 adsorbent. The adsorption capacity of the acrylic acid-grafted coir pith for Cr(VI) was higher compared to the original coir pith. This result was due to the enhancement of the carbonyl groups on the coir pith surface that may have involved the mechanism of chromium adsorption. The X-ray absorption near edged structure (XANES) and desorption studies suggested that most of the Cr(III) that presented on the acrylic acid-grafted coir pith was due to the Cr(VI) being reduced to Cr(III) on the adsorbent surface. FTIR confirmed the involvement of the carbonyl groups (Ce methoxy groups (OCH3) in the mechanism of chromium adsorption. Thermodynamic study, such as enthalpy (ΔH), free energy (ΔG) and entropy change (ΔS) indicated that the overall adsorption process was endothermic, spontaneous and randomness. In addition, the adsorption process was favored at high temperatures. [ABSTRACT FROM AUTHOR]