Chemical extractability of copper added to soils at two different concentrations in a batch experiment

Abstract

The bioavailability of Cu in soil is dependent on the desorption into the soil solution of Cu from the surfaces of soil colloid materials (viz. organic matter, the clay fraction, Fe and Mn oxides). Six samples of natural soils of the Typic Eutrochrepts type were used in this work to determine Cu extraction by using 0.1M Na2EDTA and 1M NH4OAc solutions. The soil samples were spiked with 3 or 1.5 g Cu kg-1 soil and analysed in batch tests. Most of the Cu added was sorbed within 24 h. Both single-step extractions with 0.5M Na2EDTA, 0.5M Ca(NO3)2 and 1M NH4OAc solutions, and sequential extractions, were performed. The proportions of copper extracted by the NH4OAc solution were found to be higher than those extracted by the Na2EDTA and Ca(NO3)2 solutions. The amount of Cu EDTA formed exhibited a high, negative correlation with OC, Fed and Mnd at an added copper concentration of 3 g kg-1. There were no significant differences between the mean percentage of copper extracted by the Na2EDTA solution from spiked and natural soils. There were, however, significant differences between the amount of copper extracted by the NH4OAc solution at both added Cu concentrations and that extracted from natural soils. Sequential extraction was highly efficient at both Cu concentrations. Copper was found to be primarily associated to carbonates. The correlations between non-extractable Cu (residual fraction) and organic carbon, Fed and Mnd (crystalline oxides) were highly significant at the higher Cu added concentration (3 g kg-1). On the other hand, that between residual Cu (non-extractable Cu) and pH was negative and highly significant at the lower Cu added concentration (1.5 g kg-1). There were no significant differences between the mean percentages of copper sequentially extracted and copper extracted by the Na2EDTA, NH4OAc, Ca(NO3)2 solutions at the two added Cu concentrations (3 g kg-1 and 1.5 g kg-1).