| With the continuous advancement of urbanization and industrialization of China,a series of industrial and agricultural production activities have caused much heavy metals entering into soils,which has led to increasingly serious contamination of heavy metal in soils of China,endangering the health of the general public and becoming an urgent environmental problem.At present,arsenic(As)and cadmium(Cd)compound contamination of arable land is widely distributed and needs remediation in China,especially in the red soil region of south China.In this study,indoor soil cultivation and pot experiment were carried out aiming to screen highly efficient passivation materials to remediate As and Cd contaminated red soils.For this purpose,original biochar and modified biochar,lime and red mud were used,focusing on the compound proportion,the dosage,the passivation effects,and corresponding impact on the availability and migration of arsenic and cadmium in red soil were also studied.And the effects of carbon-based composite materials(CCM)on the distribution of arsenic and cadmium in soil-crop system and the absorption of crops were illustrated.Based on the charateristics of CCM on the bioavailability of arsenic and cadmium in soils related future practical application methods were provided for risk management of As and Cd.(1)With biochar(modified)material as the core component,biochar(BC),modified biochar(MBC)and lime(SH),slag(LZ),red mud(RM)were applied singly or in combination,the passivation of arsenic and cadmium in red soil with different compound ratio was studied.The results showed that 1%BC,4%SH and 4%BC+4%SH could effectively reduce the content of water-soluble arsenic and cadmium(WSAs,WSCd)in soil under the treatment of biochar/lime alone or mixed application.Among them,4%BC+4%SH treatment had the best passivation effect on soil As and Cd,with passivation efficiency up to 55.27%and 76.39%respectively.And only 0.25%LZ,1%(MBC+LZ)and 4%(MBC+LZ)can effectively reduce the content of water-soluble arsenic,cadmium(WSAs,WSCd)in soil by single application and pairwise mixed application of cerium-manganese modified biochar,slag,and red mud.Among them,0.25%LZ had the best passivation effect on soil As and Cd,reaching 18.75%and 78.77%respectively.Along with the stabilization of soil As and Cd caused by single application or dual application of biochar/lime or cerium-manganese modified biochar/slag/red mud,soil As species could be transformed from non-specifically sorbed As and specifically sorbed As more easily used by plants into residual state,and soil Cd was transformed from acid extractable state which is more active into residual one.(2)Based on the self-developed cerium-manganese modified biochar material(MBC),and combined with the application of slag(LZ)and red mud(RM),the passivation performance of cerium-manganese modified biochar composite materials on arsenic and cadmium in red soil was investigated in order to determine the optimal dosage of application.The results showed that the water-soluble content of As and Cd could be significantly reduced by 16.07%-50.89%and 46.72%-77.38%respectively(P<0.05)with different ratio of MBC-LZ-RM,and the three groups of MBC-LZ-RM mixed with different dosage(0.5%-0.5%-0.5%,1%-1%-1%and 4%-4%-4%)had obviously stabilization effect,The mechanism of its stabilizing effect on soil As and Cd,are closely related to the transformation of soil As and Cd from the active state more easier to be absorbed into residual one.In comparison,a new carbon-based composite material prepared by mixing cerium-manganese modified biochar,slag and red mud with the optimal dose ratio of 4:1:1 could achieve the synchronous passivation of As and Cd in red soil successively.(3)When carbon-based composite material(MLR,meaning the ratio MBC:LZ:RM as 4:1:1)was applied alone or mixed with chemical fertilizer and organic manure(cow manure,pig manure),the total biomass of Water spinach was significantly improved,the yield increased by 3.14%-103.31%.At the same time,the content of total As and Cd in different parts of Water spinach were significantly decreased(P<0.05),the content of As and Cd in roots decreased by 2.06%-54.26%and 2.96%-55.62%,and the uptake in edible part was reduced by 1.97%-60.53%,16.24%-59.83%,respectively.The availability and mobility of As and Cd in soil were also significantly reduced by the application of new carbon-based composite material alone or in combination with fertilizer,and the content of available As and Cd in soil decreased by 2.39%-44.18%and 13.50%-61.58%respectively.In the meanwhile,As and Cd were transformed from the active state which could be easily absorbed by plants to residual one,and the migration ability of As and Cd in soil decreased significantly.With the application amount of MLR increased,the fixation effect of MLR was gradually improved.Compared with single application of MLR,the combined application of 0.5%MLR+chemical fertilizer and 0.5%MLR+pig manure had the most outstanding effect on the yield increase of Water spinach and the synchronous passivation of As and Cd.The biomass of roots,edible part of aboveground and total plants under the treatment of 0.5%MLR+chemical fertilizer and 0.5%MLR+pig manure increased by 34.14%,117.88%,100.75%and 52.10%,104.75%,93.98%respectively.At the same time,when 0.5%MLR+chemical fertilizer and 0.5%MLR+pig manure were applied together,As content in Water spinach roots decreased by 52.41%,47.93%,and that was reduced by 54.26%and 55.62%for shoots compared with the control.And the content of Cd in roots and shoots of cabbage was reduced by 50.00%,45.30%and 60.53%,59.83%.Moreover,the fixation efficiency of 0.5%MLR+chemical fertilizer and 0.5%MLR+pig manure on As in soil reached 44.18%,55.63%,and that was 39.30%,61.58%for soil Cd respectively,thus reducing the safety risk of heavy metals on soil environment and crop quality.(4)The application of novel carbon composite functional material(MLR)not only changed the distribution and morphology of heavy metals in soil-crop system,but also changed the activities of catalase,urease,phosphatase and sucrase.In this experiment,compared with the treatments of MLR,chemical fertilizer,organic fertilizer,MLR-chemical fertilizer and MLR-organic fertilizer,the catalase and urease activity of soil was significantly decreased by the single application of chemical fertilizer(P<0.05).Application of composite material(MLR),organic fertilizer and their combination can improve catalase activity.When MLR supplemental level was 0.25%and 0.5%,soil urease activity increased significantly after combined application of MLR with chemical fertilizer,and decreased with the material dosage used more,but all of them were higher than the control treatment.The effects of organic fertilizer alone,MLR alone and their combination on urease were not obvious.The activity of acid phosphatase decreased significantly when chemical fertilizer,organic fertilizer and MLR were applied alone or in pairs(P<0.05),with the exception of a very small number of treatments that lead to elevated acid phosphatase activity.For soil sucrase,the activity of sucrase decreased significantly when chemical fertilizer or organic fertilizer was applied alone(P<0.05),and the application of MLR significantly increased the activity of sucrase.When MLR was mixed with chemical fertilizer or organic fertilizer,the activity of soil sucrase increased.So the application of MLR significantly increased the activities of catalase and sucrase,but had no significant effect on soil phosphatase and urease.In conclusion,new carbon composite functional material prepared by mixing cerium-manganese modified biochar,slag and red mud at 4:1:1 is a new passivation material with good application potential and application prospect.It can not only effectively reduce the availability and migration of As and Cd in contaminated soil,but also contribute to the growth of crops planted on contaminated soil and achieve crop yield increase.At the same time,it can reduce the enrichment of As and Cd in vegetables,and ensure the quality and safety of agricultural products.The carbon composite functional material can play a stable role in the remediation of contaminated red soil,and its use in combination with agricultural measures(fertilization)will not affect the stabilization effect of this material,and can even strengthen the immobilization ability.In practical agriculture production activities in the future,abount 0.5%dosage of the new carbon-based composite functional material(MLR)is preferred to applicate in combination with chemical fertilizer or pig manure organic fertilizer. |
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