Persistence of CaCl₂ washing effect for amelioration of a Cd-contaminated paddy field soil

Soil washing is one of the methods used to remediate soil contaminated with heavy metals, and when the contaminated elements have been effectively removed the washed soil can be used for agriculture. Soil washing was conducted using 0.5 mol L^-1 CaCl₂ solution at pH 4 as an extracting agent to remediate a paddy field soil contaminated with Cd. Dolomite powder was applied to neutralize the soil to the original pH 6.2. After CaCl₂ washing, the content of Cd extractable in 0.1 mol L^-1 HCl decreased from 2.4 to 0.8 mg kg^-1. Subsequently, a pot experiment was carried out to evaluate the effect of soil washing on Cd concentration in polished rice (Cd_pr) for three successive years. Using the washed soil, Cd _pr was ≤ 0.2 mg kg^-1 with and without a treatment that simulates midseason drainage, whereas it was > 0.5 mg kg^-1 in the unwashed soil with the midseason drainage treatment. The reasons for low Cd _pr growth in the washed soil were the low content of exchangeable Cd in the soil and the resultant high soil pH (> 7). To evaluate the effect of soil pH on Cd_pr in the fourth year, we adjusted soil pH to 5 with H₂SO₄ before transplanting rice seedlings. The Cd _pr in the washed soil with the midseason drainage treatment increased to 0.47 mg kg^-1, whereas it was less than 0.2 mg kg^-1 under continuous flooding. Thus, high pH or whole season flooding are important to keep Cd_pr at ≤ 0.2 mg kg^-1 even after soil washing. With the application of dolomite and other ordinary fertilizers, soil properties were little affected by the present soil washing procedure because the difference in rice yield between the washed and unwashed plots was not significant within each year.