Removal of Arsenate from Aqueous Solution by Synthetic Siderite-Modified Biochar: Characteristics and Mechanisms

High arsenic groundwater has been observed throughout the world, which poses severe health impacts on human beings. It urgently needs effective adsorbents to lower arsenic concentration from the target groundwater for the purpose of drinking. In this study, siderite-modified biochar (SID-BC) was synthesized with the hydrothermal method as an adsorbent for arsenate (As(V)) removal from aqueous solution. Batch experiments were performed to investigate the effects of the mass ratio of siderite to biochar (i.e., 4:1, 4:2, 4:3, and 4:4), contact time, initial As(V) concentration, temperature, and solution pH on As(V) adsorption on the adsorbents. The highest adsorption of As(V) was observed on the SID-BC with the mass ratio of 4/3 (SID-BC-4-3), which may result from the high specific area of the adsorbent (8 times larger than the pristine biochar and 4 times larger than the pristine siderite). Adsorption kinetics revealed that As(V) adsorption was in line with the pseudo-second-order model. As(V) adsorption on SID-BC-4-3 was better fitted to Langmuir isotherm and D-R model, relative to Freundlich isotherm, with the maximum calculated adsorption capacity of 33.6 mg g(-1). Thermodynamic study showed that the adsorption reaction was an exothermic nature and a spontaneous process. Although Cl- and SO(4)2(-) showed negligible effects on As(V) adsorption, HCO3, CO32-, and PO43- had an evidently negative effect on the adsorption of As(V). The lowest As(V) adsorption was observed with the coexistence of PO43-. Results of SEM, XRD, FTIR, and XPS indicated that the adsorption was controlled by the chemical process via the complexation of As(V) with Fe-O and co-precipitation with Fe(III) hydroxide.