Distribution characteristics and analytical protocols for remediation of fluorine-contaminated soils at a phosphate-gypsum waste landfill

To evaluate the fluorine contamination level of soils at a phosphate-gypsum waste landfill site, the fluorine distributions in the waste and soil, forms of fluorine-containing compounds in the soil and waste, and mobility and leachability of fluorine in the natural environment were investigated. It was hypothesized that some soils may be contaminated with fluorine released from the landfill. The fluorine content was chemically analyzed with incineration/distillation, alkaline fusion, aqua regia extraction, and sequential extraction procedures. The total fluorine concentrations in the phosphate-gypsum wastes ranged from 1787 to 4496 mg/kg. The soil below the landfill showed a total fluorine concentration of 332.7-776.0 mg/kg, which was approximately 3 to 4 times higher than that of the background soil (190.0-260.0 mg/kg). Moreover, the sequential fluorine extraction results showed that most of the fluorine content in the wastes and soils existed in the form of residues; thus, the extraction of fluorine in the natural environment may not be of significance. The X-ray diffraction and X-ray photoelectron spectroscopy results revealed that most fluorine compounds in the phosphate-gypsum waste landfill existed as CaF2, which has limited solubility in water. The soil below 0.5 m depth from the boundary between the phosphate wastes and soil contains up to approximately 60% residue; this indicates that the excavation of waste and soil till 0.5 m depth from the boundary may be crucial for decreasing the mobility of fluorine in the natural environment. Our results suggest that total concentrations and chemical forms of fluorine in the below soil may provide the excavation depth to minimize the removal amount of soil in the remediation procedure.