Phosphorus-induced Migration And Transformation Processes Of Heavy Metals On Water-root Interface

As one of the most popular soil remediation methods, chemicalimmobilization is a technology in which the additives can induce heavymetal ions stabilized and mineralized as less soluble precipitates, resultingin the reducing translocation and bioavailability of heavy metals.Phosphorous-bearing materials are typically viewed as one of the mostimportant chemical amendments, which can react with heavy metals suchas Pb to form insoluble metas-phosphate precipitates. Up to now, most ofworks on the phosphorous-bearing materials used as chemical stabilizersare focused on single metal. However, soils are often contaminated bycombined heavy metals, so it is more practical to applyphosphorous-bearing materials for immobilization of combined heavymetals in a contaminated soil. In addition, one of the evaluation indexes ofthe remediation effectiveness is phytoaviailablity of heavy metals. Theeffect of phosphorous-bearing materials on phytoaviailablity of heavymetals, especially transformation of heavy metals on the plant root surfaceneeds further studies.Hydroponic experiment was carried out by taking Hoagland solutionas an alternative medium of soil solution. Chinese cabbage (Brassicachinensis), the sensitive vegetable, was exposed to single Pb and mixedPb+Cu+Zn, and then solution was added with phosphate rock (PR)、triplesuperphosphate (TSP) or their combination (P+T). The objective of thisstudy was to determine transformation of Pb、 Cu、 Zn on water-rootinterface and their transport in the plant body. Meanwhile,Visual-MINTEQ was used to simulate the effect of phosphorus-containingsubstances on transformation and phytoavailability of the heavy metals and the underlying mechanism. The main results obtained are as follows:(1)Adding phosphorus-containing substances promoted P absorptionof Chinese cabbage and increased P contents in root and shoot of plant.And the increased amount in root was larger than in shoot. The increasedamount of P content in plant body was in positive relationship with thesolubility of phosphorus-containing substances, namely TSP>P+T>PR.Besides TSP, other phosphorus treatments had no effects on absorption ofPb in Chinese cabbage body and Pb content in culture medium, however,TSP treatment increased the absorption of Pb in Chinese cabbage root andshoot by25.3%and255%. The possible reason was that excessiveabsorption of P by plant leading to P-rich stimulation, which made theplant to release a large number of small molecule organic complexes.These complexes complexed Pb to enhance its ability of migration greatly,which leaded to more absorption of Pb by plant to stabilize P in the body.XRD analysis showed that some phosphate precipitation consisting ofchloropyromorphite [Pb10(PO4)6Cl2] and hydroxypyromorphite[Pb10(PO4)6(OH)2] existed in culture medium and plant root surface ofcontrol treatment. It showed that phosphorus-containing substance wasreleased by plant root under the stress of Pb to defense Pb intrusion, whichleaded to the same result of control、PR and P+T treatments. There weremore phosphate precipitates on plant root surface of P treatments, whichmeant exogenously added P induced more precipitation of Pb on the rootsurface.(2)When Chinese cabbage suffered the combined stress of Pb、Cuand Zn, the content of P in plant body got significant increase by addingphosphorus-containing substance and the increased amount in root waslarger than in shoot. The content of Pb and Cu in culture medium wasdecreased by34.7%-61.7%and13.6%-41.6%respectively after theaddition of phosphorus-containing substance, the content of Pb and Cu inplant shoot was also decreased by21.6%-44.9%and10.5%-30.0%respectively. However, the content of Pb and Cu in plant shoot was increased by5.9%-31.9%and5.2%-25.0%respectively. The possiblereason was accumulation of Pb and Cu induced by over-absorbed P. Due tothe phosphate precipitates formed by the reaction between Pb and P andthe adsorption of Cu, Pb and Cu transition from root to shoot were curbedand the contents of Pb and Cu in shoot were decreased. For Zn, takingcompetitive adsorption of heavy metals and the selective absorption ofplants into consideration, it tended to transport from root to shoot. Thecontent of Zn in shoot was increased by22.26%、12.18%and1.05%respectively for PR、TSP and P+T treatment. XRD analysis showed thatsome phosphate precipitation consisting of chloropyromorphite[Pb10(PO4)6Cl2] and hydroxypyromorphite [Pb10(PO4)6(OH)2] existed inculture medium and plant root surface of control treatment. It showed thatphosphorus-containing substance was released by plant root under thestress of Pb、Cu and Zn to defense Pb、Cu and Zn intrusion. However, theprocess mentioned above was not as strong as under singe Pb stress. Therewere more phosphate precipitates on plant root surface of P treatments,which do good to the decrease of heavy metal content in plant body.(3)Visual-MINTEQ model calculations showed that: For the exposureto Pb,95%Pb was precipitated as only Pb5(PO4)3Cl; For the exposure toPb+Cu+Zn,99.99%Pb was precipitated as only Pb5(PO4)3Cl, and part ofCu and Zn were precipitated as Cu3(PO4)2and Zn3(PO4)2:4H2O.Theseprocesses would impacted by plant. Heavy metals would be adsorbed byroot surface and inner plant, which leaded to the raction happened on rootsurface and inner plant instead of culture medium. In real experiments, forthe exposure to Pb,57.8%、55.2%and46.6%Pb were absorbed by plantafter the addition of PR、TSP and P+T; for the exposure to Pb+Cu+Zn,55.4%、64.2%and52%Pb,65.4%，64%and55%Cu,55%，46.4%and32.9%Zn were absorbed by plant after the addition of PR、TSP and P+T.Whether for single Pb stress or combined stress, the consumption of theH2PO4-weren’t beyond21.7%of the total amount. There were lots of H2PO4-left in the culture medium which would be absorbed by plant andcoused P-rich stimulation. Heavy metals on root surface and in culturemedium would then be absorbed by plant induced by this P-richstimulation.Through this research, it was found that the role of plant root in thereaction between plant roots、 phosphorus-containing substances andheavy metals was not just providing reaction site. Phosphorus-containingsubstances could induce the stabilization of heavy metal on root surfaceand also could by absorbed by plant root. Heavy metals would be forced tomigrate from root surface to plant in the form of "rich P stimulus" whentoo many P absorbed by plant root.What’s more, the results of hydroponicexperiment for exploring the effect of phosphorous-containing substanceson the absorption of heavy metal by plant were some different from soilexperiment.