Effect And Mechanism Of Immobilizationlead Contaminated Soil By Phosphates With/without Organic Acids

Lead is a kind of heavy metal pollutants in the soil environment, and the remediation of heavy metal contaminated soils has become one of the global most concerned environmental issues. In this study, we investigated the effect of phosphates and oxalic acid on soils adsorption-desorption lead. We also investigated the oxalic acid,the P:Pb molar ratio, incubation time and the kinds of phoaphates, which affected lead bioavalibility and lead forms by the methods of deionized water or Ca C l2, Community Bureau of Reference(BCR) sequential extraction and toxicity characteristic leaching procedure(TC LP) methods. Additionally, the effects of organic acid on immobilization of lead in water by five types of phosphate(PR,APR,MPR,HAP,Fe PO4) was also studied by simulated experiment, and the mechanism was investigated by XRD, SEM/EDS. etc. The main results were summarized as following:1. In batch experiment, the results shows that phosphates and oxalic acid promotes the adsorption lead on soils. The dominant mechanism by which metals are immobilized is by exchanging H+, Ca2+, Mg2+ and K+, etc; and by precipitation of OH-, CO32- and PO43-, etc. It is also immobilization by organic complexation and specific adsorption on Fe-Mn oxides. The XRD results shows that the pyromorphite(Pb5(PO4)3Cl) was generated after Pb2+ adsorption on soils.2. The concentration of water extractable, exchangeable and TC LP-Pb all decreased with increasing incubation time and P:Pb molar ratio for both soils. For red soil, the concentration of water-extractable Pb after 120 days was reduced by 100% when soils were amended with APR, HAP and HAP+OA, and the TCLP-Pb was <5 mg L-1 for the red soil at P:Pb molar ratio 4.0. At the beginning of incubations, the co ncentration of water extractable and exchangeable Pb increased with application of oxalic acid in the yellow-brown soil. Compared with CK, the content of water extractable and exchangeable Pb reduced 84.2% and 58.1% on 49 days, respectively. The TCLP-Pb was <1.55 mg L-1 at all treatments applied to the yellow-brown soil after 480 days. Results of sequentialiv extraction procedure showed that the acid fraction and reducible fraction take a great proportion in lead contaminated soils. Oxalic acid activated phosphates, and so mixing insoluble phosphates with oxalic acid may be a useful strategy to improve their effectiveness in reducing Pb bioavailability.3. Compared with CK, the content of Pb in the shoots and root of C hinese cabbage were reduced with the addition of phosphates. High P:Pb molar ratio of KH2PO4 significantly increased the content of soil available phosphorus, which may bring the risk to environment. However, the application of lower P:Pb molar ratio of KH2PO4 and higher P:Pb molar ratio APR significantly reduced the Pb bioavailability. The concentration of water extractable, exchangeable and TC LP-Pb all significantly decreased with the addition of four phosphates, and the effect of phosphates on water extractable Pb was: APR>PR>HAP>KH2PO4>CK. However, the effect of phosphates on exchangeable and TCLP-Pb was: APR>KH2PO4>HAP>PR>CK. The fraction acid extractable and reducible are mainly speciation in anthropogenic lead contaminated red soil, which was 28%-38% and 52%-58%, respectively. The sequential extraction test also indicated that APR was most effective in Pb immobilization for red soil.4. The effect of phosphate on immobilization of lead in the order of HAP>MPR>APR>Fe PO4>PR. The results showed that three kinds of organic acids(5 mmol L-1) had promoting effect on the immobilization of Pb2+ by PR and Fe PO4. The mechanism of organic acids promoted the phosphates immobilization of Pb2+, which can be attributed to two kinds of reasons: for one thing, the organic acids may play a significant role in phosphate dissolution, which promote the efficiency of immobilization of heavy metals by phosphate; for another thing, adsorption of organic acids increase the negative surface charge of PR and Fe PO4, and increase Pb2+ adsorption by ternary complex formation among phosphates, organic acid and heavy metal. The presence of citric acid and acetic acid decreased Pb2+ adsorption on APR、MPR and HAP. But oxalic acids had strongly promoting effect on the immobilization of Pb2+ by all phosphates. However, pyromorphite phase was confirmed in the XRD pattern of MPR and HAP after adsorption of Pb2+ in the presence of oxalic acid.This might be attributed to the fact that MPR and HAP immobilization of aqueous Pb2+ mainly through precipitation as sparingly soluble pyromorphite, when there is sufficient phosphate in the system. The results shows that low LMWOAs(acetic acid, oxalic acid and citric acid) concentrations(<0.1 or 1.0 mmol L-1) make no difference on Pb(II) adsorption by APR, MPR and HAP, while high citric acid concentrations(>1.0 mmol L-1) inhibited Pb(II) adsorption. Conversely, high acetic acid and oxalic acid concentrations make no difference on Pb(II) adsorption by APR, MPR and HAP. The rate-determining step of Pb- immobilization reaction is apatite dissolution, and the dissolution rates of apatite increased with decreasing p H.