Immobilization Mechanism of Four Types of Amendments on Cu and Cd in Polluted Paddy Soil

Abstract

Cu and Cd are common pollutants in the soil surrounding copper smelting enterprises. The regional characteristics of southern China results in a high risk of Cu and Cd reactivation in soil after immobilization with soil amendment. To clarify the degree of risk of secondary activation of Cu and Cd, four types of amendments, namely limestone (LS), maifanite (MF), biochar (BC), and iron modified biochar (Fe-BC), were used to study Cu and Cd fraction distribution in soil and soil colloids and the type and fractional changes of in-situ iron oxides. The results showed that the soil amendments were ranked by their immobilizing effect on soil Cu and Cd in the order LS, MF, Fe-BC, and BC; the exchangeable and carbonate-bound fractions of Cu in the soil reduced by 8.19% and 2.33%, and the readily reducible iron- and manganese-bound fractions of Cu increased by 8.00% and 2.69%, respectively, when treated with LS and MF. The risk of secondary activation of heavy metals was higher in soil treated with LS and MF than in soil treated with other amendments. The readily reducible iron- and manganese-bound fractions of Cu reduced by 2.21% and 5.90% and the organic-bound fractions of Cu increased by 4.75% and 3.48% when treated with BC and Fe-BC, respectively. This indicated that the immobilization effect tends to be stable. The exchangeable and carbonate-bound fractions of Cd in the soil decreased by 7.64%, 8.34%, 2.37%, and 6.73%, and the residual fractions increased by 8.27%, 9.18%, 5.73%, and 9.60% respectively, indicating that the amendment treatments resulted in better stability of Cd than Cu. The Cu and Cd contents of soil colloids were 489.92 mg ·kg-1 and 2.57 mg ·kg-1, respectively, which were considerably higher than those in soil (239.98 mg ·kg-1 and 1.93 mg ·kg-1, respectively). The amorphous iron oxide-bound Cu and Cd contents of soil colloids increased significantly with the application of each of the four amendment, which was the main reason and mechanism for the decrease in heavy metal bioavailability. With the extension of aging time, long-term immobilization can be achieved if the heavy metals are further transformed into crystalline iron oxide-bound and residual fractions.