Effects Of Salt And Biochar Addition On Migration And Transformation Of Heavy Metals In Mudflat Soil

With the rapid social and economic development,soil heavy metal pollution in China has become a growing problem.The behaviors and environmental risks of the heavy metals in the soil are generally affected by many factors,and the investigation on the relation between migration,transformation and bioavailability and soil characters is of great importance in risk assessment and control for the heavy metals in agricultural soils.As an important reserve of farmland resources,the tidal flat in the east coast of China can be used for agricultural production after maturation and improvement through a massive input of organic materials.Sewage sludge derived from domestic waste water treatment has been proved to be an economical and efficient soil modifier that can be used for rapid amendment of tidal flats.The application of sewage sludge can inevitably bring heavy metals into the tidal flats.Meanwhile,the profound changes of soil physiological properties,especially contents of salt and organic matter and physical structures of the soil,during the amendment process of the tidal flat may affect heavy metal migration and environmental behaviors.In order to explore the effects of the changes of salt content and organic matter in the process of soil improvement in tidal flat on the environmental behaviors of soil heavy metals,we conducted two leaching experiments with simulated soil columns:in first leaching experiment,the soil in the column was sampled from tidal flat amended by sewage sludge and designed four concentrations of NaCl solution were designed as leachants,0(blank control),0.025,0.05 and 0.1 mol L-1;and in second experiment with pure water as the leachant,the column was divided into two layers,then the bottom-layer(20-40 cm)was filled with unamended tidal flat soil treated with 0.5%,1%and 2%biochar and the top-layer was filled with sewage sludge containing heavy metals.the leachates were collected at the intervals of 48h during the experiments and soil was sampled at the end of the leaching experiment for the determination of heavy metals(including Cd,Cr,Cu,Mn,Ni,Pb,Zn).The fractions of heavy metals in soil samples were also analyzed.The following results were obtained:(1)With the increase of salt concentration of the leachant,total amount of Cd,Pb and Ni in the leachate decreased,in which the decrements of Cd and Pb were significant(p<0.05),while the accumulated Cu,Mn and Zn in the leachate significantly increased.Cr accumulated in the leachate was not affected by salt concentration.(2)The concentrations of Cd,Cr,Cu,Mn,Ni,Pb,Zn increased then decreased with leaching periods.Except for Pb,leaching process of heavy metals could be accelerated by elevated salt concentration in the leachant.(3)The accumulative changes of the heavy metals in the leachates showed two different tendency:S curve for Cd,Cr,Cu,Mn,Ni and Pb,and linear type for Zn.(4)With increased salt concentration in the leachant,total heavy metals in 0-20cm soil decreased,while those in 20-40 tended to increase and the decreasing trend was observed only in highest salt concentration treatment(5)The ratios of active state Cd,Cr,Cu,Pb and Zn in 0-20 cm and 20-40 cm soil layers after leaching were decreased by salt solutions of different concentrations.The ratios of active Mn and residual(RES)Cd,Cr,Cu,Pb and Zn were increased(6)With the increase of biochar dosage,the total amount of Cd,Cr,Cu,Mn,Ni and Zn in the 0-20 cm soil layer decreased.The total amount of Cd and Cu in the 20-30 cm soil layer increased.The total amount of Cu in the 30-40 cm soil layer decreased significantly.(7)Application of biochar reduced the proportion of active state Cd in the soil layer 0-20 cm,20-30 cm,20-40 cm after leaching and increased the proportion of active state Cr,Cu,Pb,Zn and residue state(RES)Cd.There was no obvious effect of biochar application on the morphology of Mn and Ni.