Insight into the purification of arsenic-contaminated acid water by metal–organic framework MIL-53(Fe) with sulfite: The generation and effect of Fe(IV)

Abstract

Iron based metal–organic frameworks (Fe-MOFs) have garnered international interest as a novel and promising category of adsorbent materials for the elimination of arsenic from neutral waters. Nevertheless, their practical implementation is constrained by their inadequate adsorption capacity for aqueous arsenite (As(III)), a prevalent arsenic species found in acidic environments such as acid mine drainage. Herein, a novel oxidation-adsorption process to effectively purify As(III)-contaminated water is constructed by combining the MIL-53(Fe) of high acid resistance with sulfite (S(IV)). Notably, the adsorption capacity of the MIL-53(Fe)/S(IV) process for As(III) at pH 3.5 reached 86.4 mg g−1, surpassing the values achieved by MIL-53(Fe) alone by 21.6 times for As(III) and 1.62 times for As(V) (oxidized As(III)). During the oxidation process, MIL-53(Fe) effectively activated S(IV), leading to the formation of the crucial intermediate SO5•−, which then underwent two distinct pathways to respectively generate Fe(IV) and SO4•−, accounting for rapid oxidation of As(III). In the adsorption process, the S(IV) activation (i.e., complexation and electron transfer) on the MIL-53(Fe) surface created additional adsorption sites by dredging the pores and altering the electron cloud distribution within the adsorbent. Furthermore, As(V) trended to form monodentate complexes with surface Fe (e.g., FeOAs bond) in the MIL-53(Fe)/S(IV) process. Notably, this oxidative removal process demonstrated reliable performance in complex water matrices and practical arsenic-contaminated waters. Overall, this study presents a promising strategy to enhance the efficacy of Fe-MOFs for the removal of As(III).