Removal of Cd(II) and Cr(VI) from Aqueous Solution by Peanut Shell Biochar-zero-valent Iron Composites: Performance, Effects and Mechanisms

Biochar-zero-valent iron composites are promising materials for the removal of heavy metals from aqueous solution, but further research is still required on the co-removal of multiple heavy metals. This paper presents the performance, effects and mechanisms of peanut shell biochar (BC)-supported zero-valent iron (ZVI) composites in the co-removal of Cd(II) and Cr(VI) from aqueous solution. Samples were subjected to the isotherm test, the kinetic test, the adsorption performance test and microscopic tests, and impact factors such as initial pH, reaction time, dosage and BC/ZVI mass ratios were considered. The results show that the removal efficiency of biochar-zero-valent iron composites for Cd(II) and Cr(VI) co-removal is significantly higher compared to individual applications of zero-valent iron and biochar. Under the condition of dosage of 0.2 g, initial pH = 4.0, and BC/ZVI mass ratio of 4:1, the removal efficiency of Cd(II) and Cr(VI) can reach 98.66% and 98.45%, respectively. In this case, the maximum adsorption capacity of Cd(II) and Cr(VI) can reach 20.44 mg/g and 3.70 mg/g. The removal of Cd(II) follows the pseudo-second-order and Freundlich model, whereas the removal of Cr(VI) is better fitted by pseudo-first order and Langmuir model. The co-removal process of Cd(II) and Cr(VI) can be divided two phase, and the mechanisms of Cd(II) and Cr(VI) are also remarkably distinct. The reduction, adsorption, complexation or co-precipitation are the dominant mechanisms for Cr(VI) removal, while the complexation of biochar and adsorption of the iron oxyhydroxide layer predominate for Cd(II) removal.