Effect Of Accompamny Anions In Potassium Fertilizers On Phytoavailability Of Pb And Cd In Soils And Its Mechanisms

Through the study of fraction changes and mobilization of Cd and Pb co-existing with various ions in the soil, this paper is to theoretically illustrate the ensence of reaction between ions and soil, and to improve the understanding on the soil surface's chemical properties as well as increase the capacity on conditioning the behavior of Cd and Pb in the soil;in addition, this paper will provide the theoretically basis for practical remediation of Cd and Pb-contaminated soil by amendment and fertilizers. Accompany anions (.Cl^-, NO3_<sup>-, SO₄^<sup>2- H₂PO₄^-) in different potassium fertilizers could affect soil surface characteristics, which results in alteration of sorption-desorption process and speciation changes of Cd and Pb, and finally influences the phytoavailability of heavy metals. This research systematically studied the sorption-desorption process, fraction changes and the availability of Cd and Pb for plants in red latersol and yellow brown soil with potassium fertilizer application, using the methods such as chamber incubation, isotherm method, and pot trials. This study aims at providing a scientific suggestion for controlling the phytoavailability of Cd and Pb by potassium fertilization in the soil, Main results and new progress were summarized as following:1. All potassium fertilizer application decreased available Cd and Pb more or less after incubating 30 days of Cd and Pb-contaminated two soils, and the effect of K₂SO₄, KH₂PO₄ treatment were more significiant. In particular, KH₂PO₄ always took the effect on reducing the availability of Cd and Pb. However, the influence of KH₂PO₄ on the compound polluted soil was weaker than that on the single polluted soil. Such as in Yellow brown Soil, reducing percent of availability of Cd or Pb in the single polluted soil were 29.63%, 33.73% respectively, while those were 12.53%, 27.91% in the compound polluted soil. In general, the available Pb and Cd of red latersol were higher than that of yellow brown soil at the same contaminate level, for more Cd and Pb were absorbed in red latersol.2. Using isotherm method, adsorption-desorption of Pb by red latersol and yellow brown soil were measured under four anion ions (Cl", NO₃^-, SO₄^2-, H₂PO₄^-) to compare the effect of different accompany anions. The results showed that amount of adsorption increased with increasing concentration of Pb, the relationship could well be described by Freundlich isotherms equation X=AC^B. Freundlich metal retention parameters A, indicated effect orders of anion ions was H₂PO₄^-> SO₄^2-> Cl^-NO₃ in red latersol. However, effect of anion ions on Pb absorption by red latersol was higher than that of yellow brown soil. H₂PO₄ could significiantly enhance Pb²⁺ and Cd²⁺ adsorption, SO₄^2- also has similar effects. Desorption ratio of Pb were all from 0.06% to 1%, which indicated that Pb was mainly specifically absored by two soils;But desorption ratio of Cd was <3% in red latersol and that was from 1.4% to 19.2% in yellow brown soil, which meaned that Cd was specifically absored by red latersol and there was partially non-specific adsorption in yellow brown soil. Concluding from sorption-desorption, available Pb or Cd in soils could be greatly reduced by H2PO4 , effect of SO42" was relatively weaker than H2PO4\ and Cl\ NO3" had few influence.3. On the Cd slightly contaminated soil, rape was planted for three seasons and four potassium fertilizers were applied in every season. In the first season KCl could not promote plant uptake of Cd, and in the second, third seasons KCl increased Cd concentration of plant, which meant that only its higher application rate could promote plant uptake of Cd. The result was smiliar to plant uptake of Pb. KH2PO4 always decrease plant uptake of Pb or Cd. When co-existing with Cd, Pb could enhance its harms. However, Cd has not the same effect for Pb.4. Speciation analysis indicated SO42' and H2PO4' could reduce exchangeable and carbonate-binding Pb and increase Fe-Mn oxide, organic-binding and residual Pb. KH2PO4 could increase soil pH and insoluble Pb, which is one of major reasons of its reducing bioavailability of Pb.5. Further experiment was implemented for exploring the effects of anions on Pb bioavailability in situ. Plant's dry biomass of two seasons after KH2PO4 application was the highest among all treatments in red latersol with lower available P. In red latersol KCl application increased Pb concentration of soil solution, that up to 106.9% with the highest KCl rate.The similar results was obtained in yellow brown soil. K+ and SO42' could also lead to the increase of soluble Pb. However, after KHiPO4 addition there was positive correlation between Pb concentration uptake of shoot and root and Pb in the soil solution for the second season's plants in the two sample soils. Moreover there is significiant negative relationship between Pb concentration and P content in plant shoot of two seasons in red latersol, and in yellow brown soil only in the second season this relation showed in 1% significiant level.In summary, the effect of four accompany anions in different potassium on Pb, Cd availability of soil could be in the sequence: H2PO4"> SO42> Cl^NCV. H2PO4" could significiantly promote the absorption of Pb and Cd by two soil and increase Fe-Mn oxide, organic-binding and residual Pb and Cd, which resulted in reducing available Pb and Cd of soil, so H2PO4" could steadily decrease the bioavailability of Pb and Cd. KH2PO4 could be a good potassium form for remediation of contaminated soil by heavy metal. In order to guarantee remediation effect, KH2PO4 application doses should be increased in heavy metal combined contaminated soil. The specific absorbed availability of SO42' by soil was weaker than that of H2PO4", thereby theeffect of SO42" on Pb and Cd absorption and Pb and Cd availability change was weaker than that of H2PO4\ Though K2SO4 could reduce Pb and Cd availability, its influence was easily changed by other environmental factors. Consequently K2SO4 is an addition to control the availability of Pb and Cd in soil only under the certain conditions. Cl'and NO3" had small effect on soil charge, thus they had weak impact with Pb, Cd absorption and could not alter available Pb and Cd in soil. However Cl" could strongly complex with metal cation, and the complexion may be also available for plant. As the result CT could increase plant uptake of Pb and Cd to some extent. Thereby K.C1 fertilizer wa*s combined with hyperaccumulators for remediation of Pb and Cd-contaminated soil.