Effect of acetate, citrate, and lactate incorporation on distribution of cadmium and copper chemical forms in soil

Abstract

Some organic acids of low molecular weight, generated through breakdown of giant wastes or root exudation, increase the solubilization of heavy metals, such as cadmium (Cd) and copper (Cu) through complexation reactions and may affect the distribution of their chemical forms in soil. By using natural soil and soil spiked with Cd or Cu, the effect of acetate, citrate, and lactate incorporation on the solubilization of these metals and the distribution of their chemical forms was assessed. To this end, four concentration levels of organic acids were incorporated to soil: 0, 0.4, 0.8, and 1.6 mM at constant ionic strength in 0.02 M potassium chloride (KCI) at pH 5.5 at a soil to solution ratio of 1:20. After 24 h equilibrium, the supernatant was separated and used to determine the concentration of organic acid not retained by soil through the technique of Zone Capillary Electrophoresis and the metal concentration through inductively coupled plasma-mass spectrometry (ICP-MS). The residue was fractionated to determine Cd and Cu chemical forms through a sequential extraction method of five steps. Results showed that acetate and lactate retention is higher in non-sterilized soils which would indicate that anion retention would partly correspond to consumption by microorganisms rather than to anion adsorption by soil particles. It was also found that a higher metal content has a negative effect on acetate retention. The presence of lactate and citrate increases the solubility of both metals, and treatment with these acids in some cases affect the distribution of their chemical forms. Acetate incorporation increased the amount of Cu associated to organic matter, and the presence of citrate affected the fractions of exchangeable Cu, carbonate, and associated to manganese oxides. The fraction of both metals associated to iron oxides was the least affected by the incorporation of organic acid anions.