Immobilization effects of phosphate rock activated by oxalic acid on exogenous lead in latosol

Phosphate rock has been widely and successfully used in immobilization of lead in contaminated soils. However, limited investigations are obtained on the lead form in the contaminated soils particularly immobilized by oxalic acid-activated phosphate rock. This study focused on the changes of exogenous lead (Pb) added into latosol with the application of phosphate rock (PR) and oxalic acid-activated phosphate rock (APR) by Tessier's sequential extraction procedure. The results indicated that the content of exchangeable Pb in latosol was decreased obviously with the increase of PR and APR application compared with CK (64.1 mg/kg), and that reduce to 0.1 mg/kg for PR treatment and less than the detection limit in APR treatments. The contents of NaAc extractable Pb in all treatments were higher than CK (27.2 mg/kg) except for APR3 treatment which was 24.5 mg/kg and reached the maximum in PR2 treatment (41.8 mg/kg). The Fe-Mn oxide-bound Pb contents in all treatments were higher than CK except for PR1 treatment (69.5 mg/kg), and reached the maximum 117.2 mg/kg in APR3 treatment. Organic compound-bound Pb contents with the addition of PR and APR were higher than CK (21.8 mg/kg) except for PR1 treatment (20.7 mg/kg), also reached the maximum 46.5 mg/kg in APR3 treatment. The residue fraction was increased to 60.6 mg/kg compared with CK treatment (44.2 mg/kg), and changed from 42.7 to 43.5 mg/kg for APR treatments which were not obviously lower than CK treatment (p<0.05). To sum up, with the addition of PR in the soil, the contents of exogenous exchangeable Pb were decreased obviously and the residual Pb contents augmented conversely in most treatments, showing that Pb was effectively immobilized and better fixation effect was obtained in APR. Meanwhile, the results of phosphorus fraction indicated that the released P ability was increased more for APR than that for PR. Although exogenous phosphorus was added into latosol, the released phosphorus is unlikely to pose a risk to environment except for adding more than 5 000 mg/kg APR. The X-ray diffraction (XRD) and scaning electron microscope (SEM) results also demonstrated that the released phosphorus ability of PR was increased after being activated by oxalic acid, thus APR was better in immobilizing latosol lead. This research provides a reference for the proper utilization of APR to immobilize lead in contaminated soils .