Speciation Analysis And Bioavailability Of Heavy Metals In Soil Of Maize Production Area In Tianjin

Tianjin,as an important maize-cultivated area in China,has a long history of sewage irrigation.As a region with wide planting and high yield of maize,the heavy metal accumulation of maize and soil in Beijing Sewage River irrigation area should be concerned,especially for the bioavailable metal which is easily absorbed by organisms.Therefore,it is necessary to evaluate the content,speciation,bioavailability and potentially release of metal in soil.In this study,several conventional extraction methods?total content:acid digestion,single step extraction:HNO₃ or Ca Cl₂,sequential extraction:modified BCR method?and diffusive gradient in thin film technique?DGT?were used to determine the metal content in soil.Based on the total amount,the risk assessment of metal enrichment was carried out.The available metal contents measured by several methods were compared with maize,and the relationship between rainfall and metal release was established using DGT induced sediment flux?DIFS?model and meteorological tillage data.The results are as follows:?1?The overall soil properties and metal concentration in samples were analyzed.In general,the sampling area was weakly alkaline,with p H range of 7.89-9.19.There was a significant negative correlation between soil p H and organic matter?r=-0.414,P<0.05?.There was a significant positive correlation between soil CEC and organic matter?r=0.331,P<0.05?.The heavy metals in maize did not exceed the food standard.Although the heavy metals in the soil all exceeded the background value and showed obvious enrichment,all the other elements were in a safe level except for four points of Cd exceeding the risk screening value.Based on the assessment of total contents,except for the slight pollution of Cd,only a few points reached the low-risk level,such as Zn in WQD and JHD,Pb in NHD,and the enrichment effect of other sites did not reach the warning line.Taking ecological risk index?E_r?as a reference,the assessment accuracy of Nemerow pollution index?NPI?was better than that of geo-accumulation index?GI?;?2?Different methods were used to extract the effective metal,the extraction efficiency and metal speciation of the extractant are analyzed,and the reliability of several methods to measure the metal effectiveness were compared.In the single extraction,the extraction rate of HNO₃ to Cd,Pb,Ni,Cu and Zn were higher than that of Ca Cl₂ by 7.09%,14.23%,2.66%,19.38%and 6.18%,respectively.The extraction efficiency of HNO₃ to the effective state was better than that of Ca Cl₂.In the sequential extraction,the average ratio of acid exchangeable?F1?and residual?F4?of Cd was 30.75%and 30.52%respectively,while the F1 of other elements were less than 3%and F4 were more than60%,showing that Cd in samples were relatively labile and the bioavailability of other four metals were deficient.Compared with chemical extraction method,DGT method was a reliable method for determining labile metal in soil.The fitting coefficients of DGT and maize were 0.92?Cd?,0.87?Pb?,0.88?Ni?,0.92?Cu?and 0.88?Zn?,respectively,which were better than the other methods;?3?The application of DIFS model revealed the metal dissociation mechanism of soil particles.Moreover,combined with rainfall data,the labile metal concentrations were quantified,which made the scale of labile metals produced by the soil more intuitive.Except for Cd and Ni,the supply capacity?R?of other elements in soil were lower than0.1,suggesting their release potential had little impact on ecological risk.Therefore,DIFS model was used to analyze Cd and Ni.The ratio of exchangeable state content to metal concentration in soil solution?K_d?was negatively correlated with the time required to establish equilibrium of concentration gradient?T_c?.However,in Cd,the correlation coefficient between K_d and T_c was-0.26,P value was 0.065,the correlation degree was low and not significant.In Ni,the correlation coefficient between K_d and T_c was-0.39,P value was 0.011,showing significant negative correlation.Among the two metal,k_-1 was more sensitive to T_c change than K_d.But in Ni,the relationship between k_-1 and T_C was closer.The R range of Cd in the study area was 0.24-0.65,and the R range of Ni was0.08-0.17.The supply capacity of Cd was significantly higher than that of Ni.The main factors affecting R were also different in the soils with diverse supply capacity.The annual release of labile Cd caused by precipitation in WQD,NHD,BDD and JHD were 0.47t?0.06t?1.09t and 0.53t,respectively.The annual release of labile Ni were 12.55t?1.74t?24.65t and 12.35t,respectively.The release of two elements were all highest in BDD area,which was greater than the sum of other three areas,indicating that the soil in this area has the strongest release potential of Cd and Ni.On the whole,the risk of total content in the collected soil was relatively small,but the monitoring of slightly polluted Cd should be strengthened,and some metals with potential enrichment risk should also be concerned.The speciation analysis showed that the mobility of Cd was higher,and DGT was a reliable method for the determination of effective metal.The release capacity of Cd was the highest,and the release amount of labile metal caused by rainfall indicated that we should pay attention to pollution of soil and surrounding water system by effective metal discharge,and provide a thinking for further research in the direction of bioavailable metal.