Efficiency of chelating agents in retaining sludge-borne heavy metals in intensively applied agricultural soils

This paper presents an evaluation of different chelating agents for their effectiveness in removing Cu, Co and Zn in three distinctly different types of sludge-amended soils. Soil types (Luvisol, Arenosol and Vertisol) were each mixed with an anaerobically digested sludge at a 1:1 ratio followed by leaching with three types of chelating agents, namely: ethylenediamine tetra acetic acid, nitrilotriacetic acid and acetic acid. Aqua regia method was used to quantify pseudo total metal before and after treatment. Generally, chelating agents can be out competed by soil colloids in attracting cations. The efficiency of chelating agents was found to follow this order ethylenediamine tetra acetic acid < nitrilotriacetic = acetic acid in all the three metals, with ethylenediamine tetra acetic acid being the most effective chelating agent. More heavy metals were removed in Luvisol and Arenosol than in Vertisol implying that soils rich in clay fraction retain more cations than soils with minimal clay fraction. Similarly, copper responded positively to chelation than zinc and cobalt in Luvisols and Arenosols, although the results were not conclusive for Vertisols.