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)

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 center dot-, which then underwent two distinct pathways to respectively generate Fe(IV) and SO4 center dot-, 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., Fe-O-As 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).