Remediation of metal-contaminated paddy soils by chemical washing with FeCl3 and citric acid

Purpose Batch extraction experiments were conducted to investigate the chemical washing of metal-contaminated paddy soils using mixtures of ferric chloride (FeCl 3 ) and citric acid (CTA). Materials and methods The effects of the extractants (FeCl 3 , CTA, or FeCl 3  + CTA) and washing cycles (single-step or multistep washing) on the metal removal efficiency were studied. FeCl 3  + CTA increased metal removal efficiency to some extent compared to single FeCl 3 or CTA. When the liquid-to-solid ratio was 2:1 with three washes and two rinses, a mixture of 10 mmol L −1 FeCl 3 and 20 mmol L −1 CTA removed 78.9% Cd, 15.8% Cu, 34.0% Zn, and 18.1% Pb from a Typic Fe–accumuli–Stagnic Anthrosols. Using multistep washing instead of single-step increased the metal extraction effect. Moreover, the removal efficiency and speciation of metals in different soils and changes in soil properties in washed soils were also studied. Results and discussion The removal rate of Cd was higher than that of Zn, Cu, or Pb because of the higher proportions of acid-soluble forms of Cd in the soils. The removal of metals from different soils depended on the distribution of metal speciation and soil type. Soil pH, CEC, exchangeable Ca, and exchangeable Mg decreased significantly after soil washing. Conclusions We conclude that chemical washing of metal-contaminated paddy soils with mixtures of FeCl 3 and citric acid may be a feasible technique for soil remediation.