h i g h l i g h t sFeS shows higher removal efficiency towards As(III) than As(V). In acidic solution FeS dissolve to S(-II) for As(V) reduction and As 2 S 3 formation. S(-II) react with As(V) to form S(0)-rich surface on FeS and inhibit As removal. S(II)-rich surface inhibit FeS dissolution and arsenic removal. As(V) reduction to As(III) by dosing Na 2 S prior to FeS improve As(V) removal. The utilization of arsenopyrite (FeAsS), the As ore mineral, produces strongly acidic wastewater with extremely high arsenic (As) concentrations. This study investigates the feasibility of using ferrous sulfide (FeS) as a sulfide [S(-II)] source to treat strongly acidic wastewater with high concentrations of arsenite [As(III)] and arsenate [As(V)]. The removal of As(III) by FeS was nearly 30% higher than that of As(V), and higher acidity and elevated FeS doses benefited their removal. At extremely high acidity of above 7 mol/L as H 2 SO 4 , the consumption of S(-II) by H 2 SO 4 inhibited As removal. At lower acidity of below 2 mol/L as H 2 SO 4 , elevated FeS doses up to 4 times the theoretical dose could achieve good As removal. SEM/EDS and XPS analysis indicated the formation of As 2 S 3 precipitates, and sulfur [S(0)] also formed in the As(V)-removing system. The tiny and negatively-charged S(0) particles tend to coat the FeS surface. The S(0)-enriched surface acts as a barrier to inhibit S(-II) detachment and As(V) penetration and inhibits As(V) removal thereafter. The reduction of As(V) to As(III) by Na 2 S, prior to dosing with FeS, is preferred to achieve rapid and favorable As(V) removal. The low-cost FeS provides available Fe(II) and S(-II) for As removal, and is practically valuable to treat acidic high-As wastewater.

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