Adsorption Of Chromium (VI) From Wastewater Using Natural And Modified Akadama Clay

In aqueous systems, chromium（VI） is usually discharged from the industries ofleather tanning, electroplating, production of steel and alloys, etc. Exceed Cr（VI） iscarcinogenic and mutagenic to living organisms and seriously threaten human health.Various technologies such as chemical precipitation, ion exchange, membranetechnologies, adsorption, etc are available for Cr （VI） removal. Nowadays, adsorptionhas become the most versatile and widely used technology for its operation simplicity,availability, and recyclability. Among numerous adosrbents, mineral materials arecompetitive and concerned by public.In this study, a volcanic clay originated in Japan-Akadama clay, with goodproperties of porosity, permeability, and affinity, was used for Cr （VI） removal fromaqueous solution. Batch experiments were carried out to investigate the effect ofcontact time, initial pH, and adsorbent dose on Cr （VI） adsorption. Results showedthat Cr （VI） adsorption on natural Akadama clay reached equilibrium in180min. Theoptimum pH was2. The maximum adsorption capacity was4.29mg g-1at an initialconcentration of50.0mg L-1and adsorbent dosage of5g L-1. The equilibrium datafitted Freundlich isotherm better than Langmuir isotherm, and they were wellexplained by pseudo-second-order kinetic model. Adsorption mechanism analysisproved that electrostatic adsorption dominated the removal process. Results from thisstudy demonstrated that natural Akadama clay had the potential to be an efficientadsorbent for Cr （VI） removal compared to other natural mineral adsorbents.In order to improve the adsorption capacity of Cr （VI） and widen the adsorptionconditions, HCl-modified Akadama clay （CMAC） was developed as an adsorbent forthe treatment of Cr （VI） contaminated wastewater. The influence of pH, coexistingions etc. were evaluated by batch experiments. Wide pH ranges of2¹1were foundto be suitable for Cr （VI） adsorption onto CMAC. The maximum experimentaladsorption capacity of7.47mg g-1was obtained. The kinetic data fittedpseudo-second-order model better than pseudo-first-order model, indicating the Cr（VI）adsorption was mainly a chemical process. The Cr （VI） adsorption process well fittedthe Freundlich isotherm model and Dubinin-Radushkevich isotherm model ratherthan the Langmuir isotherm model, revealing that heterogeneous adsorption occurred onto CMAC. CMAC had a high selectivity for Cr （VI） ions in presence of coexistingions.Inorganic salts were also used to modify the original Akadama clay. Among Al3+,Fe³⁺, Ca²⁺, Mg²⁺, and Mn²⁺, Fe³⁺performed the best modification effect on Akadamaclay, and the adsorption capacity of Cr （VI） onto Fe-modified Akadama clay （FMAC）was greatly enhanced. A factorial design methodology was applied to evaluate theimportance of parameters and to optimize the adsorption process. Results revealedthat initial concentration is most important for the adsorption capacity, followed byadsorbent dose, and contact time, but pH of the solution had no effect on theadsorption of Cr （VI）. The maximum experimental adsorption capacity of Cr （VI）onto FMAC has been found to be22.74mg g-1at a dose of1.0g L-1with initial Cr（VI） concentration of400.0mg L-1. The experimental data was found to followpseudo-second order model and Freundlich isotherm model. Thermodynamic studiesindicated that the adsorption reaction was spontaneous and endothermic in nature.Under the same conditons, natural Akadama clay, CMAC, and FeAC wereapplied to treat Cr（VI） containing waster discharged from leather tannery. Resultsrevealed that the Cr（VI） removal efficiencies were23.1%,38.9%, and94.5%,respectively. FeAC had the optimum adsorption efficiency.In conclusion, natural Akadama clay, CMAC, and FMAC could be effectivelyused as adsorbents for Cr （VI） removal from wastewater. Among them, FMAC,presenting the best performance based on its simple operation, low cost, wide pHconditions, and high adsorption capacity, could be considered in practical application.