Effects Of Multi-soil Components Interactions On Copper Distribution In Long Time Scale

Heavy metal contamination of soils is one of the most serious environmental problems in China.Copper is one of the most common heavy metals in soils and can be toxic at elevated concentrations.Soil components(organic matter,clay minerals,and microorganisms)and retention time can govern the adsorption,fixation and distribution of copper.This study evaluated the interactions of soil components and aging on the distribution of exogenous copper.Three typical Chinese soils(Red soil,Cinnamon soil,and Black soil)were collected from Hunan,Henan and Heilongjiang Provinces.Soils were incubated with rice straw(RS)and engineered bacteria(Pseudomonas putida X4/p IME)in the presence of exogenous copper for 24 months,and soil samples were collected every 4 months.Some relevant soil physicochemical properties and reactive soil components were determined by chemical analysis and modern instrumental analysis,such as C NEXAFS,SR-FTIR and so forth.Sequential extraction was employed to obtain the distribution of Cu species in soils and the mobility factors of Cu were calculated.The relationships between soil properties and Cu fractions were analyzed with stepwise multiple linear regression.The main results are shown as follows:(1)The effect of rice straw and engineered bacteria on the concentration and composition of soil organic carbon.Rice straw and rice straw + engineered bacteria increased the concentration of soil organic carbon(SOC),humic substance(HS-OC)and dissolved organic carbon(DOC)in three soils.The concentration of dissolved organic nitrogen(DON)was decreased by 4.1-7.2,0.6-1.3 and 1.6-7.9 times in Red soil,Cinnamon soil and Black soil,respectively.The main types of soil organic carbon were aliphatic C,carboxyl C and O-alkyl C,which were accounted for 69.1%-86.6,79.6%-88.5% and 87.3%-90.3% in Red,Cinnamon and Black soil,respectively.The ratio of O-alkyl C to aliphatic C was increased with the application of rice straw in three soils.In addition,rice straw decreased the aromaticity of Red soil,while enhanced that of Black soil.The C/O values of humic acid(HA)in Black soil increased during the 16-month incubation,and the C/H values decreased at the last 8 months,indicating that the humic acid in Black soil became more aliphatic.(2)Concentration of iron oxides and the relationship with organic carbon.Concentration of DCB-extractable Fe was the largest in Red soil,and amorphous Fe was highest in the Black soil.Amorphous Fe reduced with the application of rice straw and rice straw + engineered bacteria in the three soils.In addition,amorphous Fe decreased with time in the 20-month incubation,and it increased during the rest 4 months.The Fe_p/Fe_d values increased with time,suggesting that aing enhanced the the complexation of iron oxides with organic matter.Soil total organic carbon,humic substance and dissolved organic carbon had a negative relationship with amorphous Fe in three soils(P < 0.01).The reasons were as follows: 1)Copper's competitive adsorption on organic matter;2)Amorphpus Fe coprecipitated with Cu.In the three soils,the spatial distribution of aliphatic C with Clay-OH or Fe-O had the highest relation coefficients.The spatial distribution between organic carbon and iron oxides were accelerated by rice straw,and the opposite results were found in the Cinnamon soil.The coefficients between organic carbon and minerals(clay or iron oxides)were increased with time in Red and Cinnamon soils,and vice versa in Black soil.(3)The dynamic changes for the forms and distribution of Cu were elucidated.Copper is maily existed as specifically adsorbed,humic acid bound,and Fe-Mn Oxides bound forms,which were accountes for 73.7%-89.7% of the total Cu in Red soil with different treatments.In Cinnamon soil,the sum of carbonate Cu and humic acid bound Cu was 81.8% in the fist 4-month incubation.The total concentration of carbonate Cu,humic acid bound Cu and Fe-Mn oxides bound Cu in Cinnamon soil was 83.6%-90.5% during the rest incubation.In Black soil,the humic acid bound Cu ranged from 55.0% to 73.9% which was relatively higher than other Cu forms.In all the three soils,the water-soluble Cu decreased,while exchangeable Cu and specifically adsorbed/carbonated Cu decreased with addition of rice straw and rice straw + engineered bacteria.(4)The dynamic change of Cu mobility,and the best efficincey as well as sustain time of amendments on reducing Cu mobilty in soils were observed.The mobility factors were lowest in Black soil(6.4-9.2)because of high organic carbon and clay content.Cu mobilty factors decreased with time in the 20-month incubation,however they increased thereafter in all the three soils.The reduction rates became similar after 16 months and 24 months in Red and Cinnamon soils,respectively.In the Black soil,the effect of RS and RS+EB on cutting down Cu mobility could sustain for more than 24 months.The reason of the above result may be that the reduction of humic substance,amorphous Fe and the increase of iron oxides complexed on the organic matter.The best efficiency of RS to reduce Cu mobilty were observed in Red,Cinnamon and Black soil in the 12-month,12-month and 8-month incubation,respectively.The engineered bacteria were only worked more efficiently in Red soil in the first 8-month incubation.(5)The major factors controlling Cu forms in differernt soils were investigated.Based on the step-wise linear regression analysis between soil properties and Cu forms,we found that organic carbon plays a more important role in shaping the distribution of relatively mobile Cu and iron oxides could be more critical in stabilizing Cu during the first-year incubation.Iron oxides are important for Red soil and Cinnamon soil,while organic matter is crucial for Black soil during the second-year incubation.Different measures should be taken into account regarding the content of organic matter and iron oxides depending on soil types for the risk assessment and remediation of Cu-contaminated soils.