Chemical remediation of Copper-contaminated soils using Calcium water treatment residue

Copper (Cu) contamination to agricultural soils is on the rise worldwide due to extensive use of Cu-containing fertilizers, fungicides, farm manures, and waste disposal in agricultural lands. In south Florida, Cu accumulation in soils under citrus production has been accelerated as a result of increased application of Cu-based fungicides for battling canker disease. In this study, laboratory analysis, greenhouse experiments, and field trials were conducted to investigate the status of soil Cu contamination in south Florida, the effectiveness of chemical remediation using lime-based water treatment residue (Ca-WTR) and the biogeochemical processes controlling Cu availability in Ca-WTR amended soils. Soil samples were collected from 18 representative commercial citrus groves in the Indian River area of South Florida and characterized for Cu chemistry and availability. Total Cu in the surface soils ranged from 4.74 (4 yrs grove) to 228 mg kg-1 (30 yrs grove), with approximately 50% of the soil samples having a total recoverable Cu concentration above or close to the critical Cu level of total Cu: 85 mg kg-1. On average, 48% of the total recoverable Cu was Mehlich-3 extractable, indicating high availability of the soil Cu. The largest proportion (25 to 58% of total Cu) of soil Cu was present as organically-bound. Both total recoverable Cu and Mehlich-3 extractable Cu were significantly correlated with this Cu fraction (P<0.001). Most of the Cu was accumulated in the surface soil layer (0-15 cm), but vertical leaching occurred in some soils with pH < 6.5. Two typical soils (a Spodosol and an Alfisol with a total recoverable Cu ~ 100 mg kg -1), which are dominant under citrus production in the Indian River area, were selected for remediation studies. The results from incubation studies indicated that amendment of Ca-WTR (pH 9.1, containing mainly CaCO3 and minor CaO) significantly raised soil pH. Water soluble and exchangeable Cu fraction decreased by 62% in the original and by 90% in the Cu-enriched soils (added with 400 mg Cu kg-1), whereas oxides-bound and residual Cu in the soils increased accordingly. Similar results were obtained from column leaching experiments. The cumulative amount of Cu in the leachates from 10 leaching events was reduced by 80% and 73%, respectively for Alfisol and Spodosol at the highest Ca-WTR amendment rates (20 g kg-1 for Alfisol and 100 g kg-1 for Spodosol). Greenhouse studies with ryegrass (Lolium perenne L.) and lettuce (Lactuca sativa L.) as indicator crop plants indicated that the growth of plants was inhibited in the slightly Cu-contaminated soils (~100 mg kg-1), but no plant survived in the severely Cucontaminated soils (added with 1000 mg kg-1). Amendment of Ca-WTR at 5-20 g kg-1 for the Alfisol (pH 5.45) and 5-100 g kg-1 for the Spodosol (pH 4.66) significantly reduced plant Cu concentrations and thus improved plant growth as evidenced by a significant increase in plant biomass yield (P<0.01).