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Rice husk (RH) is one of the widespread agricultural waste materials in the world. At present, most of the husks are either burned out or thrown away as waste, causing serious environmental pollution. This study prepared the hydroxyapatite-modified carbonized rice husk (HAP@ CRH) nanocomposites from rice husk (RH) with modifications, aiming to explore the feasibility of preparing heavy metal ion sorbents. The preparation process affecting its adsorption properties on Cr(VI) ions was systematically investigated, which was optimized by carbonizing RH at 600°C for 2.0 h to produce CRH and then depositing hydroxyapatite particles on the surface of CRH with a Ca2+/CRH ratio of 0.2 g/g. The physiochemical properties of samples were characterized. After modifications to be HAP@CRH, the surface presented a honeycomb-like structure as its porosity increased. The Brunauer--Emmett--Teller specific surface area increased from 1.06 m2/g (RH) to 193.64 m2+/g (CRH) and 183.91 m2+/g (HAP@CRH) respectively, the total pore volume increased from almost zero to 0.089 and 0.202 cm³/g, respectively. The results demonstrated the effective adsorption property of HAP@CRH for Cr(VI) ions as the removal rate of 98.57% and the adsorption amount of 5.91 mg/g were achieved within 24.0 h with adsorbent dosages of 5.0 g/L for initial Cr(VI) concentration of 30 mg/L at 30°C and pH 2.0. This study is expected to promote the utilization of wasted rice husk biomass and provide a new economic and feasible adsorption material for wastewater treatment so as to achieve the purpose of treating waste with waste. [ABSTRACT FROM AUTHOR]

In present study, nitrate anion containing CdCr layered double hydroxides (LDH) was synthesized, and characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis-differential scanning calorimetry (TGA-DSC) and energy-dispersive X-ray-scanning electron microscopy techniques (EDX-SEM). The characteristic diffraction peaks in the XRD pattern of CdCr-NO3 LDH at 2θ values of 15.10 and 26.02° correspond to (003) and (006) planes (hkl), respectively of crystal system of the LDH, revealing layered structure of material with basal (d) spacing of d003: 0.59 nm and d006: 0.34 nm, containing intercalated water and nitrate anions. Stretching vibrations in FT-IR spectrum of the LDH at 3,378.01 and 1,355.51 cm-1 confirmed the intercalation of water and nitrate ions, respectively, in between hydrotalcite-like layers. EDX study confirmed that CdCr-NO3 LDH contained 20.07%, 72.75% and 7.18% of O, Cd(II) and Cr(III), respectively. SEM image revealed that nanostructured material is porous and layered in nature. TGA-DSC confirmed the thermal stability of material at below 509.44°C. CdCr-NO3 LDH was investigated as an efficient adsorbent for capturing of Cr(VI) ions form industrial effluents. The adsorption was optimized by factorial design approach. Maximum response of CdCr-NO3 LDH for capturing/adsorption of Cr(VI) ions was achieved at optimum concentration of Cr(VI) ions: 10 mg L-1, pH: 4.0, adsorbent amount: 40 mg and shaking time: 30 min at 25°C. Method worked well on real wastewater samples. [ABSTRACT FROM AUTHOR]

In the present study, nanostructured β-Cd(OH)2 adsorbent was synthesized, characterized by Fourier-transform infrared spectroscopy, X-ray diffraction and scanning electron microscopy analysis, and applied for Cr(VI) ions capturing (adsorption) from environmental aqueous samples. The central composite design of 18 adsorption experiments was employed for multivariate sorption optimization. Maximum adsorption (%) of Cr(VI) ions was calculated and found to be 98.5% with relative standard deviation (RSD) ≤ 3.5 at optimum concentration 15 mg L–1, pH 4.0, adsorbent dosage 50 mg, shaking time 20 min and shaking speed 120 rpm at 25°C. Langmuir, Freundlich and Dubinin–Radushkevich isotherms fitted well to adsorption data with correlation coefficient (R²) of 0.993, 0.982 and 0.994, respectively. Mono-layered (Qm) and multi-layered (Kf) capacities of β-Cd(OH)2 adsorbent for Cr(VI) ions retention were calculated and found to be 202.02 ± 2.0 and 4.95 ± 2.5 mg g–1, respectively. Sorption energy was calculated and found to be 8.45 ± 2.0 kJ mol–1, indicated chemisorption or ion exchange mechanism for Cr(VI) ions adsorption onto β-Cd(OH)2 adsorbent. [ABSTRACT FROM AUTHOR]

In this work, eggshell powder was used as a low-cost adsorbent for the removal Cr(VI) and Co(II) ions from aqueous solutions as a function of contact time, adsorbent concentration, adsorbate concentration, pH and temperature. Eggshell powder was characterized by X-ray diffraction, scanning electron microscopy and N2 physisorption measurements. The kinetic studies showed that the experimental data of both solid–liquid systems studied were well adjusted to pseudo-second order kinetics and Langmuir isotherm models. The results showed that the metal ion adsorption equilibrium was attained after 300 min of contact time for the Cr(VI) ion and 180 min for the Co(II) ion. The Co(II) ion adsorption capacity of eggshell powder was found to be approximately four times greater than the Cr(VI) ion. From the Langmuir isotherms, the maximum adsorption capacities of 0.6815 and 2.2888 mg/g were found for Cr(VI) and Co(II) ion, respectively. The thermodynamic parameters were evaluated and showed that Cr(VI) and Co(II) ion adsorption onto eggshell powder were endothermic and spontaneous processes under the studied conditions. These findings demonstrate the important role of eggshell powder as a readily available and low-cost adsorbent for the efficient removal of Cr(VI) and Co(II) ions present in aqueous media. [ABSTRACT FROM AUTHOR]

The article focuses on sequestration of toxic Cr(VI) ions from industrial wastewater using waste biomass, and talks of alternative biotechnological innovation, biosorption, biosorbents such as fungi, bacteria, algae, and thermodynamics.

A new low-cost adsorbent such as sulphuric acid modifiedStrychnos potatorumseeds (SMSP) was employed for the removal of toxic Cr(VI) ions from aqueous solution in batch and column modes. Results showed that the removal of Cr(VI) ions could be effectively removed with the SMSP in batch operation. Adsorption of Cr(VI) ions onto SMSP followed the pseudo-first-order and Redlich–Peterson models in batch operation and Thomas model in column operation. Adsorption was more favourable at acidic pH of 2.0 with the maximum monolayer adsorption capacity of 202.7 mg/g. NaOH was a better desorbing agent with the recovery of 93.245% of Cr(VI) ions. Column studies were optimized with various parameters such as bed height (2–10 cm), Cr(VI) ions concentration (50–250 mg/L) and flow rate (5–25 mL/min). According to the fact that SMSP-based adsorption process could be a promising technology for the removal of Cr(VI) ions from wastewaters. [ABSTRACT FROM PUBLISHER]

Peppermint leaves (PML) have been explored as an effective and cheap adsorbent for removal of toxic Cr(VI) ions from aqueous solutions using batch system. Adsorption of Cr (VI) ions onto PML was found to be pH dependent and maximum removal of Cr(VI) ions was obtained at pH 2. It was also found that after 180 min of PML contact with chromium solution at the concentration of 0.3846 mmol/L, more than 95% of Cr(VI) ions can be removed. The equilibrium data were fitted with the Langmuir and Freundlich models. The adsorption kinetic data were best fitted with the pseudo-second-order. The activation energy Ea of the adsorption process was determined as 23 kJ mol-1, which may indicate a physisorption process. The Gibbs free energy, enthalpy, and entropy of the process were also determined, and their values revealed that the process is spontaneous and endothermic accompanied with randomness at the solid/solution interface. [ABSTRACT FROM AUTHOR]

In this study, an artificial neural network (ANN) based techniques is applied for the prediction of the percentage removal of Cr(VI) ions from aqueous solution using eight different natural biosorbents. The effects of operating parameters such as initial pH, initial Cr(VI) ion concentration, adsorbent dosages, and contact time are studied to optimize the conditions for maximum removal of Cr(VI) ions. The ANN with a single hidden layer trained with Leven- berg-Marquardt algorithm predicted the percentage removal of Cr(VI) ions from aqueous solution accurately. [ABSTRACT FROM AUTHOR]