Effects of Flooding and Ferrhydrite on Copper Fractionation in Paddy Soil

Abstract

Abstract Chemical fraction plays an influential role in solubility and potential bioavailability of heavy metals in soils. Seasonally flooding and iron reduction are significant environmental processes of paddy soil. They may influence the fractionation of cooper (Cu) in paddy soil. In this study, an anaerobic incubation experiment was conducted to simulate the anaerobic environment of flooded paddy soil. The fractionations of native and spiked Cu in paddy soil were determined using a modified sequential extraction procedure (SEP) method. The effects of flooding time on Cu fractions were determined. The effects of ferrihydrite on Cu fractions were also investigated to understand the interaction between Cu contamination and iron reduction process. The results showed that the native Cu in soil were most in the residual and organic matter fractions, but the more available fractions, such as, the exchangeable and carbonates fractions occupied only a little proportion. After flooding, the native Cu in soil released from organic matter fraction and crystalloid ferric oxides fraction, and transferred to the easy extracted fractions, such as, amorphous ferric oxides, carbonates and exchangeable fractions. The spiked Cu was easily absorbed by carbonates and organic matter in soil, but after flooding, the two fractions transferred into ferric oxides fraction, especially amorphous ferric oxides fraction. Similar phenomena were observed after adding ferrihydrite in soil except for the disposal of added 400 mg/kg Cu. The process of iron reduction was inhibited significantly by Cu contamination and the inhibition of Cu contamination to iron reduction decreased after adding ferrihydrite. The transformation of Cu fractions had significant relation to the process of iron anaerobic reduction in paddy soil.