The Effect And Mechanism Of Immobilization On Soil Lead By Phosphate Rocks And Its Activated Products

Lead in soil will percolate into ground water or enter the food chain by plant absorbing, instead of being decomposed by living organisms or degraded through chemical methods, hence doing severe harm to the health of human beings and animals and the ecology environment. In this study, changes of Pb forms in yellow cinnamon soil, latosol and meadow soil after being added with phosphorus rocks (PRs) were determined and the effects of PRs addition amount, incubation time and kinds of PRs on Pb forms in the soils were studied, to study the immobilization mechanism of activated and inactivated PRs to Pb under different pH conditions using the means of X-ray diffraction (XRD), X ray photoelectron spectrum (XPS), Fourier Transform Infrared (FTIR) and scanning electron microscope with X-ray energy dispersive spectroscopy (SEM/EDS), etc. The main results are described below.1) For both the yellow cinnamon soil and the latosol after being added with Zhongxiang PR for2days, the exchangeable Pb content decreased effectively while the residue Pb content increased, which was positively correlated with the PR addition amount. Compared with the control experiment, the exchangeable Pb content in yellow cinnamon soil decreased from22.5%to54.4%, while the decreasing range of exchangeable Pb content in latosol was61.7%-72.8%, with the increase of the PRs amount from0mg P/kg,50mg P/kg,500mg P/kg to2000mg P/kg; when the exogenous Pb content was at200mg/kg, the exchangeable and residue Pb contents in the two soils showed a similar result to the condition that the soils were not Pb pollution, and the exchangeable Pb content decreased from86.0%to23.9%for yellow cinnamon soil,90.2%-20.9%for latosol, reaching the optimum immobilization effect with the PR addition amount of2000mg P/kg. After the contaminated meadow soil was added with Zhongxiang PR for2days, its residue Pb content tended to rise, and the reduction range of its exchangeable Pb content was only0.7%-1.3%.2) The yellow cinnamon soil and the latosol polluted by exogenous lead were set as light pollution (Y2, L2) and heavy pollution (Y4, L4) soils respectively. After they were added with Zhongxiang PR of2000mg P/kg, as time went by, their exchangeable Pb contents increased slightly first and then tended to be stable, while the residue Pb contents revealed contrary trend; the increasing range of the residue Pb content in meadow soil was1.3%-14.5%.3) After four kinds of PRs were added into the yellow cinnamon soil and the latosol, the exchangeable Pb content in the soils was reduced remarkably and the residue Pb content increased. The immobilization effects can be summarized as follows:Baokang PR (BPR) was the best, Zhognxiang PR (ZPR) was not better than Nanzhang PR (NPR), and Kaiyang PR (KPR) was inferior to others. Although KPR contains the most amount of total and available phosphorus, it showed the worst effects to immobilize Pb in soils, which proved that the total and available phosphorus contents in PRs were not the determinants in the immobilization process, and most probably for the reason that high specific surface area of PRs could absorb Pb2+4) Different amount activated PRs (APRs) was added into the yellow cinnamon soil, the changes of exchangeable Pb contents were:with low APRs addition amount, the effect of activated Baokang PR (ABPR) was the best and activated Kaiyang PR (AKPR) was the worst; with medium APR addition amount, the effect of activated Nanzhang PR (ANPR) was the best while AKPR was worse than others; when the APR addition amount was high, little exchangeable Pb was detected. Whatever APR amount was applied to the latosol, ABPR showed the best immobilization effect among all the APRs. After different kinds and amount APRs were added to the two soils, some of the residue Pb content was reduced. The exchangeable Pb content in both the soils decreased remarkably after being added with four APRs, and the effects was much better than that with PR, which demonstrated that PRs could release more available phosphorus and provide more opportunities to react with Pb after activated by oxalic.5) Reacting with200mg Pb/L Pb solution with different pH, both the PRs and the PRs activated by oxalic (APRs) could absorb Pb effectively with the pH of3.0-5.0; and when pH≥3, the Pb adsorption rate was over90%. With the solution pH deceased, more phosphorus was dissolved but pH had a little effect on Pb adsorption. XRD analysis showed that clearly defined fluorapatite and calcite peaks could be observed in all samples. The formation of weddellite or whewellite was observed in all samples after treatment with oxalic. The presence of cerussite was detected in all raw PRs reacting with the Pb solution, whereas the peaks of pyromorphite were observed in all APRs. XPS surface analysis confirmed that the Pb removal was more efficient with APRs than PRs by comparing the end BE of Pb4f(135.05-135.55eV). The SEM/EDS indicated that the tested sample of raw PR powders contained large agglomerate crystals containing a significant amount of Ca, P, O, F and C, which proved the existence of fluorapatite; after PR was treated with oxalic acid, the original cluster crystal structures were changed to amorphous; a mass was formed after raw PR reacted with the Pb solution; after the oxalic acid-treated PR reacted with the Pb solution, anomalous block shaped particles and Pb was observed. The FT-IR spectra showed that after PR was activated by oxalic acid, the absorption patterns of CO32-disappeared, peak intensity of PO43-decreased greatly, and corresponding characteristic absorption patterns of C2O42-began to appear. It suggested that CO32-played a key role in the reaction between PR and Pb solution, while PO43-determined the reaction between APR and lead solution. APRs were recommended as an adsorbent to immobilize Pb in soils and remove Pb from aqueous solution because of the low solubility of pyromorphite.