Evaluation of the effectiveness of water treatment residuals and FeSO4 on decreasing As availability of As-contaminated soils

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Arsenic is one of the most hazardous metalloid, ubiquitously present in the environment. Because amorphous ferric and aluminum hydroxides can affect mobility and availability of arsenic, both As(III) and As(V) could be held on amorphous ferric and aluminum hydroxides strongly. In the study, a waste by-product of the drinking water treatment process, namely drinking water treatment residuals (WTRs) were evaluated for their ability on decreasing As availability with or without ferrous sulfate (FeSO4). After adding WTRs (5-20% (w/w) ) or ferrous sulfate to As(V)-spiked Chengchung soil (Cf soil) and Guandu naturally As-contained soil (Gd soil), three extraction methods including deionized water extraction, iron oxide-impregnated filter paper and NaH2PO4 extraction were used to evaluate the effect of WTRs on arsenic mobility and availability. WTRs have very huge adsorbing capacity to As, and the ability of decreasing As availability is almost the same during longer incubation times. And also, in the two As-contaminated soils after adding WTRs, no matter which extraction method was used, the extractable arsenic decreased with increasing WTRs contents. WTRs not only showed greatest affinity for As, but also reduced As availability of As-contaminated soils effectively by adding them into soils. In addition, the results of FeSO4 treatment to As(V)-spiked Cf soil show that extractable arsenic by deionized water extraction method is decreased about 87%, and that by other extraction methods only decreased 22-30%. Furthermore, extraction rate of Cf soil mixed with 10% (w/w) WTRs and FeSO4 is similar to Cf soil mixed with 20% (w/w) WTRs. It indicated that FeSO4 could effectively fix As into immobile forms and lower soluble As content in soil. But the effect of FeSO4 on naturally As-contained Gd soil is not distinct. In spite of the extraction rate of FeSO4 treatment was not obviously different from the control Gd soil, in the results of mixing 10% (w/w) WTRs with FeSO4 into Gd soil, all the extractable arsenic of three extraction methods are lower than only use FeSO4 or 10% (w/w) WTRs. The result of this study showed that adding WTRs into As-contaminated soils could immobilize arsenic and decrease its availability, and additionally mixing FeSO4 and WTRs into As-contaminated soils could enhance the effect.