Study On The Activating Factors And Mechanisms Of Heavy Metals In Rhizosphere Soil

Heavy metal pollution in soil has become a salient environmental problem in our country.Phytoextraction was advocated as a cost effective and environmentally-friendly technology,and the study of related mechanisms and technology has been a hot topic.Migration of heavy metals in soil-plant system and the improvement of phytoremediation efficiency are both controlled by heavy metal activation in rhizosphere soil.However,the relevant mechanisms were relatively deficient.In order to study these factors systematically,artificial Cd and Cd-Pb-Zn contaminated soils were used to study the activation of heavy metals in rhizosphere soil.Chemical sequential extraction methods and synchrotron based X-ray absorption fine structure(XAFS)spectroscopy were used to study the effect of soil pH on metal speciation in the solid phase of soil.Then,the thermodynamic model of Visual MINTEQ and the technology of square wave anodic stripping voltammetry were used to study the effect of pH,DOM and microorganism on the heavy metals speciation in the rhizosphere soil solution.Finally,the accumulation of heavy metals by different ecotype plants was compared and the activation mechanisms of heavy metals in the rhizosphere soil were investigated with various analytical data.Results may provide theoretical guidance for enhancing the efficiency of phytoremediation in situ field.Pot experiment was used to investigate different soil pH(pH4.0,pH5.5,pH7.0 and pH8.5)on the activation of heavy metals in rhizosphere.Sequential extraction experiment demonstrated the concentration of fuvic acid complexed Cd in rhizosphere soil increased due to the rhizosphere effect of Sedum alfredii Hance and Beta vulgaris Dracaenifolia under acidic conditions.Character of Cd XAFS showed that Cd was mainly bonded to gibbsite(inner bidentate ligand),montmorillonite minerals(inner sphere complexation,?XO·Cd+)and carbonate-precipitation in neutral and alkaline samples.While Cd transformed to complex with humic and montmorillonite(outer sphere complexation,?X2-·Cd2+)at acidic condition,which may contribute to the Cd activation in rhizosphere soil.After centrifugation of soil solution and measuring the main ions concentration in soil solution,the Visual MINTEQ model was used to calculate Cd speciation in soil solution.Results demonstrated that the dissolved Cd and proportion of Cd2+ in rhizospnere soil solutions increased with the pH decreasing.Furthermore,the calculation results had a good correlation with the measurement by ASV.There has a different activation mechanism of pH on Cd-Pb-Zn contaminated soil.Cd,Pb and Zn speciation in the rhizosphere soil changed and the concentration in soil solution increased dramatically with pH decreasing.However,free Zn2+ was the dominant species in soil solution,which occupied 47.8%-76.2%,while the Pb-DOM complexes were the principal Pb species,accounting for 85.5%-99.9%.Further to say,Zn,Cd and Pb concentration in the shoot of S.alfredii had a significantly negative correlation with soil pH.S.alfredii has the highest capability to uptake Cd,followed by Zn and Pb.However,the activation of Pb by pH regulation was inferior to that of Zn and Cd.Pot experiment was used to study different concentration and sources of DOM on the activation of heavy metals in rhizosphere soil.Results showed that the exogenous DOM addition facilitated the growth of S.alfredii,however,S.sarmentosum showed a distinct toxic sympom at T200.DOM derived from the chicken manure(DOMc)had the highest degree of Cd-DOM in rhizosphere soil and induced a highest concentration of Cd in soil solution,followed by DOMs and DOMr.Furthermore,because of the increasing fraction of Cd-DOM,the solvation of Cd in the soil matrix was remarkably favored.There had a higher fraction of Cd-DOM complexes in the rhizosphere of S.alfredii than that of S.sarmentosum,which activated soil Cd and favored the uptake by plants.It also demonstrated that not only free Cd2+,but also the Cd-DOM complexes had bioavailability to plants.The increased DOM concentration remarkably favored the solvation of Cd,Pb and Zn in soil matrix.Heavy metal accumulated in plant shoot increased with the DOM addition,and in the order DOMc>DOMs>DOMr(except T25).The proportion of Cd-DOM and Zn-DOM complexes increased with the DOM concentration and S.alfredii has a extraordinary hyperaccumulation ability of Cd and Zn,except for Pb.Moreover,another heavy metal activation mechanism was revealed that the sensitive effect of Hy fraction to Pb and Cd speciation in rhizosphere soil solution was important,despite the significant contribution of FA and HA.Compare the effects of sterilization or non-sterilization treatments on the activation of Cd in rhizosphere soil.Results showed that the PLFA concentrations of total fungal in rhizosphere soils of S.alfredii and S.sarmentosum at the non-sterilization treatments was significantly higher than the sterilization treatments.The PLFA concentrations of total fungal in rhizosphere soils of S.alfredii was significantly higher than S.sarmentosum.In addition,the accumulation of Cd by S.alfredii was significantly higher than that of S.sarmentosum.These indicated that the enhancement accumulation of Cd by S.alfredii may attribute to the raise of microbial activities and the changes of microbial community structure at non-sterilization treatment.