Accumulation and chemical fractionation of Cu in a paddy soil irrigated with Cu-rich wastewater

Abstract

The accumulation, chemical fractionation and availability of copper (Cu) to rice ( Oryza sativa) in a paddy soil previously irrigated with Cu-enriched wastewater were investigated. Chemical fractions of Cu in soil samples were partitioned using single and sequential extractions. The single extractant solutions used were water, ammonium acetate and EDTA for extraction of water-soluble, exchangeable and complexed Cu fractions in the soil. A three-step sequential extraction procedure (HAOc/NH 2OH.HCl/H 2O 2) was also used to fractionate Cu associated with soil solids into three fractions: weak acid-soluble, reducible Fe and Mn oxides bound and oxidisable organic matter bound. The residual fraction was calculated as the difference between total ( aqua regia extractable) Cu and the sum of the three sequential fractions. Copper accumulated mainly in the top layer (10 cm) of the soil profile after wastewater irrigation, increased almost seven-fold (158 mg kg −1) compared with non-irrigated soil (23 mg kg −1) from the same area, and was also elevated significantly in low layer (10–20 cm) below the soil surface. Concentrations of Cu in the water-soluble, exchangeable and complexed fractions increased from 0.02, 0.33 and 14.1 mg kg −1, respectively, in unpolluted soil to 0.12, 6.30 and 98.0 mg kg −1 in polluted soil. Irrigation also led to marked increases in the weak acid-soluble, reducible Fe and Mn oxide bound, oxidisable organic matter bound and residual fractions of Cu. In the contaminated paddy soil, the reducible Fe and Mn oxide bound fraction was the largest (36% of total Cu), followed by the oxidisable fraction, then the residual fraction, and the smallest was the acetic acid-soluble fraction. However, in the unpolluted soil, 53% of total Cu was present in the residual fraction followed by the reducible Fe and Mn oxide bound fraction (22% of total Cu). Accumulation of Cu in the bioavailable fractions resulted in a three-fold increase in Cu concentration (up to 38 mg kg −1) in rice plants in the heavily polluted (HP) soil compared with unpolluted soil. Plant yields decreased substantially due to Cu toxicity. Data indicate that the reducible Fe and Mn oxide bound fraction may be more important for controlling the mobility and bioavailability of Cu than the oxidisable organic matter bound fraction in paddy soils.