Evaluation of phosphate rock and activated phosphate rock for remediation of copper-contaminated soils

Abstract: Significant effort has been made to evaluate the effectiveness of phosphate minerals on in-situ remediation of contaminated soils. Phosphate minerals, such as phosphate rock (PR) and activated phosphate rock (APR), have been shown to effectively immobilize heavy metals in contaminated soils. The APR was originated from PR by oxalic acid. In order to compare the effectiveness of PR and APR on immobilizing the Cu-contaminated soils and to investigate the change of different form of Cu, the natural polluted soil from mining areas and an artificially polluted soil with exogenous Cu originated from garden soil were collected. The soils and materials were mixed and cultured at room temperature for 10 days, and the samples were air-dried, passing through a 0.150-mm sieve. Based on the three-stage extraction defined by the European Reference Materials Bureau (Community Bureau of Reference, BCR method), different mass fractions (0, 0.1%, 0.5%, 1%, 4%, 8%, respectively)of the phosphate rock（PR）and its activated products (APR) were added into the natural and artificially polluted soil. The forms of copper and its contents in polluted soil were analyzed with modified BCR sequential extraction procedures. The Cu extractions in soils were performed using different reagents, and the details of the experimental protocol were as follows: Step 1: A total of 20 mL of acetic acid was added to 1.0 g air-dried soil and shaken (200±10 rpm) overnight at (25±1)℃. The mixture was centrifuged at 3000×g for 20 min to separate the extraction from the residue. Step 2: A total of 20 mL of hydroxylammonium chloride, adjusted with nitric acid to pH=1.5, at (25±1)℃, was added to the residue from step 1 and the extraction performed as per Step 1. Step 3: The residue from step 2 was treated with 5 mL 30% hydrogen peroxide and was covered and digested for 1 h at room temperature with occasional manual shaking, then adjusted with nitric acid to pH=2. Then the residue was heated to (85±2)℃ for 2 h in a water bath and its volume reduced to <3mL (uncovered); a further 5mL 30% H2O2 was added and heated to (85±2)℃ for 1 h; then 25mL 1M ammonium acetate (adjusted with nitric acid to the pH 2) was added and shaken for 16 h at (25±1)℃. The extract was separated from the solid residue by centrifugation and decantation as per Step 1. Residual fraction: 1.0 g air-dried soil was digested with hydrochloric acid - nitric acid-perchloric acid; the total concentration of Cu was analyzed using atomic absorption spectrometers (Varian 240FAAS). The total content of Cu minus the total Cu of step 1, step2 and step 3 was considered the residual Cu fraction. The basic properties of the soils were determined, including pH, organic matter, cation exchange capacity, soluble phosphorus, total phosphorus, and total copper.. Powder X-ray diffraction (XRD) data of the PR and APR samples were collected on a diffraction meter (Brucker Advance D8 diffraction meter), using Cu Kα radiation (40 kV and 30 mA) between 10-55? with a scanning rate of 5?/min. All the data were analyzed with Excel, Origin 7.5, and SAS (v. 8).The results indicated that both PR and APR were effective at copper immobilization in both polluted soils. When the mass fraction of PR was 8%, the content of soluble copper in the polluted soils declined by 25.8% and 40.0%, the trend of oxidized and reduced state varied, and the content of residual copper increased by 77.1% and 41.3%, respectively. The activity of copper in the soils was significantly reduced. When activated PR by oxalic acid was added into the soils, the residue form of copper increased by 82.6% in the natural soil but only by 17.0% in the artificial soil. The soluble form of copper in the artificially contaminated soil increased slightly and in the natural soil varied little, but the effect on other forms of copper was not significant. The results would provide reference for the comprehensive utilization of phosphate rock to managing heavy-metal-polluted soils.