A practical soil washing method for use in a Cd-contaminated paddy field, with simple on-site wastewater treatment

• We developed a simple method for restoration of paddy fields by soil washing.
• NaOH, MgO, and CaCO3 were used to treat washing wastewater containing various metals.
• In an on-site experiment, the target pH was between 8 and 9, controlled using MgO.
• A field experiment revealed that all metals could be effectively removed.
• The reduced metal concentrations were lower than the effluent standards in Japan.

Heavy metal contamination in rice paddies is a serious problem in monsoon Asia, and these fields require appropriate restoration measures. Although soil washing is a promising remediation technology, high cost for the treatment on soil washing leachate (wastewater) is one of the critical problems. This study sought to develop a simple method for the restoration of paddy fields by soil washing, with simplified wastewater treatment. Ferric chloride solution (FeCl3) was used as a washing chemical to extract Cd from a soil, which produced the wastewater containing Cd and other metals. Three alkali materials (NaOH, MgO, and CaCO3) were tested to treat the wastewater and determined MgO is optimal. In an on-site experiment, the target pH for wastewater treatment was controlled between 8 and 9 by using MgO. All metals in the wastewater could be effectively removed, reaching levels substantially lower than those permitted by Japanese standards. The treated wastewater could be discharged to agricultural canal. Therefore, our novel simplified method effectively removed heavy metals from the wastewater produced by on-site soil washing and contribute drive down the cost.