Metal Contamination Evaluation, Eco-toxicity And Microbial Remediation Of Paddy Soil Located In E-waste Recycling Area

Improper e-waste recycling will make them a contamination source of heavy metal and persistent toxic substances.This study focuses on heavy metals contamination,eco-toxicity and microbial remediation of paddy soils by e-waste recycling in Taizhou,Zhejiang province,China,where primitive e-waste processing activities has been done for more than 20 years.The main results obtained are as the follows:Improper e-waste recycling operations caused the accumulations of Cu,Zn and Cd in the paddy soils. Using the environmental quality standard for soils(China,Gradeâ…¡) as reference,soil samples of seven sites were contaminated with trace metals,among which,site G was heavily contaminated with Cd(6.37mg·kg^-1), and weakly with Cu(256.36 mg·kg^-1) and Zn(209.85 mg·kg^-1).The composite pollution index of the detected soil samples ranges from 0.63 to 15.53.When judged from the composite pollution index,four investigated areas were in warning situation,two were moderately contaminated and one was heavily contaminated.Zn appeared to be strongly bound in the residual fraction(69.51%to 75.85%),no matter the soil was metal contaminated or not.The residual fraction of Cu ranged from 49.76%to 58.58%.However,Cd and Cu in the flooded paddy soil are mainly bounded in the un-residual fraction,with fraction of 32.53%～63.40%and 25.86%～50.24%respectively.Compared with uncontaminated soils,the contaminated paddy soils have more metal present in the un-residual fraction.The above paddy soil samples were also tested for the possible eco-toxicological effects from the e-waste contamination.Compared with that of the control soil extraction treating Vicia fata root cells,the micronucleus rates of the site G and site F extraction treating group were higher,increasing by 2.6 fold and 1.7 fold,respectively.Low germination rates were observed in site C(50%) and site G(50%) soil extraction treated rice seeds.The shortest root length(0.2377cm) was detected in site G soil treated groups,only 37.57% of that of the control soil treated groups.The thousand kernel weight was significant negatively related with the Nemerow index.These indicated the accumulation of heavy metal might be the main reason for the reduction of thousand kernel weight in the contaminated soils.Soil enzymatic activities(acid phosphatase,catalase,peroxidase,urease,sulfate reductase and invertase) were investigated under field conditions in a flooded paddy soil located in site G,which was contaminated seriously by heavy metal.Most of the tested soil enzymatic activities were lower in the e-waste contaminated soil than that in the uncontaminated soil.The relationship between soil enzymatic activities and different metal fractions was characterized by regression models with R² values ranging from 35.5%to 76.6%.On the basis of stepwise regression analysis,it was reasonable to assume that the sulfate reductase was inhibited by the accumulation of exchangeable Zn and the residual Cd in paddy soil.Acid phosphatase was inhibited by accumulation of carbonate Cd.Urease was sensitive to the increasing of the residual Cd concentration. Although catalase and peroxidase was also sensitive to the accumulation of exchangeable Cu and residual Zn in paddy soil,respectively,the R² values of the regression model were very low.Thus our results indicated that the sulfate reductase,acid phosphatase and urease were more sensitive to metal stress than invertase, catalase and peroxidase in complex contaminated paddy soil and sulfate reductase can be a valuable indicator for evaluating toxic effect of complex pollutants on soil organisms.The effect of soil physiological groups on metal fractionation in the flooded paddy soil in site G was investigated.The stepwise regression results showed that the metal speciation would be changed by soil microorganism through the modifying of un-residual fraction,although the metal speciation kept relatively constant and distinctive in the rice growing season.Soil microorganisms had large contribution to the fractions of Zn bound in organic matter and Cd bound in Fe-Mn hydroxide.The organic Zn was significantly improved by the bacteria in soil,which explained 69.8%of the variance to the equation of organic Zn.While the Fe-Mn oxidizing Cd was only significantly related with the abundance of bacteria and sulfur oxidizing bacteria,with 64.7%and 21%of variability explained to the equation.But the effects on other fractions were relative weak.Several bacteria strains with multiple-metal resistant ability were isolated from the contaminated paddy soil and eight were examined for their metal mobilization effect on mine tailing.The result indicated the metal-resisting bacteria had mobilization effect on the metals in mine tailing,through the increasing of exchangeable fraction,carbonate fraction and organic fraction,while the Fe-Mn oxide fraction was decreased.The effect of bio-augment with native microorganisms was evaluated in the flooded paddy soil.Different physiological group had different effect on the bio-available metal in paddy soil.Pb was mobilized by the augnment of acid-producing fungi,but Cu,Cd and Zn were immobilizd at the same time.Cd was mobilized by the augument of metal-resistant bacteria,but Cu,Pb and Zn were immobilizd at the same time.All the four tested metals were immobilized by the augument of sulfate-reducing bacteria or ferric iron-reducing bacteria. These indicated when combined with flooding and bio-augmention with special soil microbial physiological group,the metals would be immobilizd in contaminated paddy soil.A highly cadmium-resistant Burkholderia fungorum strain,Cd-3254 was selected for research on metal resistance mechanism.The strain grew in slightly acidic medium containing up to 10 mmol·L^-1 CdCl₂.In growth medium amended with Cd(â…¡),strain Cd-3254 accumulated no cadmium intracellular cytoplasmic.But the strain can modify the bio-available cadmium concentration in medium through secreting extracellular polymeric substances(EPS),which was confirmed by the result of FT-IR and XRD.The FT-IR analysis suggested that cadmium binding sites on the bacterial cell surface are most likely carboxyl,hydroxyl and amino groups.XRD analysis showed complex crystal existing in the EPS gotten from the cadmium-amended medium.Therefore,our results indicated sequestration of cadmium by the EPS might be one potential mechanism for detoxifying cadmium in Burkholderia fungorum.