Control of arsenic release from paddy soils using alginate encapsulated calcium peroxide

The mobilization of As in paddy soils is affected by iron redox cycles. In this regard, calcium peroxide (CaO₂) can be used as an alternative to maintaining oxidizing conditions by liberating oxygen under flooding environments. Nevertheless, the problem of increase in pH by CaO₂ dissolution remains unresolved. In this study, the encapsulation of CaO₂ using alginate is proposed. Encapsulated CaO₂ (CaO₂-b) using 1% sodium alginate was applied to As-contaminated soil to evaluate the ability of pH control and As mobility during flooding conditions. The pH increased rapidly from 6.8 to 9.0 in unencapsulated CaO₂ (CaO₂-p) within 1 day, while CaO₂-b increased slowly to 8.6 over 91 days. CaO₂ created an oxidizing condition in the soil by providing oxygen, thus effectively prevented the reductive dissolution of iron. The mobility of As decreased by 50% (CaO₂-p) and 83% (CaO₂-b) compared with that of the control soil. Furthermore, the As in pore water was three times lower than CaO₂-p because CaO₂-b released 1.8 times more Ca²⁺ to form Ca-As complexes than CaO₂-p. Consequently, the encapsulated CaO₂ reduced the negative effects of CaO₂ treatment on increasing pH of the soil and furnished a better environmental condition for inhibiting As mobility.