Efficient removal of arsenic and phosphate contaminants by diatomite-modified schwertmannite

Abstract

Schwertmannite (Sch) has been proved to be a promising and efficient adsorbent for the removal of various metal ions. However, how to further improve the adsorption capacity and stability of Sch to As(III), As(V) and P(V) is always a problem. In this study, we developed a method to synthesize various Schs with different initial ratios of SO42-:Fe(III). Our results show that Schs can be used to remove As(III), As(V) and P(V) in a wide concentration range of pH 3.0–7.0, and negligible difference can be observed in the adsorption capacities of various Schs towards As(III), As(V) and P(V), indicating that increasing initial concentrations of SO42- does not necessarily increase the adsorption capacity of Sch. In addition, the maximum adsorption capacities (Qmax) of Sch-0.75 towards As(V) and P(V) increases with decreasing pHs, whereas increase with increasing pHs towards As(III). The Qmax values of As(III), As(V) and P(V) are 80.5, 161.5 and 139.9 mg g−1 at pH 3.0, respectively, and are 214.5, 107.6 and 86.5 mg g−1 at pH 7.0, respectively, indicating that the strong dependence of Qmax on pH values. Besides, we also successfully developed a method to synthesize the diatomite-modified Sch (DMS). The synthesized DMS with the ratio of Fe(III) salts and diatomite of 4:1 (DMS-4) shows the maximum adsorption capacities of As(III), As(V) and P(V) of 59.81, 151.52 and 236.97 mg g−1 at pH 3.0, respectively, and of 282.49, 126.58 and 165.02 mg g−1 at pH 7.0, respectively. DMS-4 also shows high stability during repeated experiments, and the removal efficiencies of DMS-4 to As(III), As(V) and P(V) maintain 97.42 %, 88.39 % and 94.72 % after five cycles, whereas the removal efficiencies of Sch-0.75 rapidly decrease to 63.61 %, 31.12 % and 50.71 % towards As(III), As(V) and P(V) after five cycles. This study provides a new method to synthesize stable DMS with high adsorption capacities towards As(III), As(V) and P(V).