Enhanced electrokinetic remediation of lead- and cadmium-contaminated paddy soil by composite electrolyte of sodium chloride and citric acid

Abstract

The aim of this study was to inquire about the removal efficiencies of lead and cadmium in paddy soil by a composite electrolyte of sodium chloride and citric acid under electrokinetic remediation. The experiment was operated in a plexiglass tank, which was divided into one soil column (length × width × height = 20 cm × 5 cm × 5 cm) and two electrode chambers (length × width × height = 6 cm × 5 cm × 5 cm). In this paper, the composite electrolyte (sodium chloride + citric acid) which combined the merits of two different chemicals was investigated for electrokinetic remediation of lead- and cadmium-contaminated paddy soil under a voltage gradient of 1.5 V/cm during a 20-day treatment. The total concentrations of lead and cadmium in the initial soil were 940.83 and 4.51 mg/kg, respectively. As sodium chloride was solubilized in solution, a number of Na+, Cl−, OH−, and H+ ions were generated that could dissolve some of Pb and Cd in soil to form Cd–Cl and Pb–Cl. When the lead- and cadmium-contaminated paddy soil was implemented with 0.1 M sodium chloride, the total removal efficiencies of lead and cadmium were 23.10 and 27.94%, respectively, which were little higher than those implemented with deionized water. Citric acid was not only used to control pH, but it can also combine with metals to form soluble M-citrate. When sodium chloride was mixed with citric acid in electrokinetic (EK) remediation, high redox potential was obtained that forced most of the metals to migrate out from soil. The overall removal efficiencies of lead and cadmium were increased from 56.85 and 62.26% with single citric acid electrolyte to 80.37 and 90.86% with composite electrolyte of sodium chloride and citric acid, respectively. Eventually, the residual concentrations of lead and cadmium in the soil were only 184.70 and 0.41mg/kg, respectively, which met the demand of agricultural production and human health requirements. Citric acid + sodium chloride treatment poses less environmental risk than inorganic acid (HCl, HNO3, and H2SO4). There is good synergistic effect of sodium chloride and citric acid during the EK remediation process. Thus, citric acid + sodium chloride is considered as an effective electrolyte to remove lead and cadmium from paddy soil.