Effect Of Ozonation-electrodynamic Treatment On The Accumulation And Transfer Of Heavy Metals Within Domestic Sludge In Maize Soil System

Recently,domestic sludge has been an important factor of environmental pollution in the process of urbanization in China.Research showed that domestic sludge,rich in organic and trace elements,could be used as fertilizer,which was a good way to settle pollution problem and realized resource reused.But domestic sludge contains some heavy-metal ions,which will be transferred into plants,then into human food chain during sludge used in agricultural production.Thus,it is necessary to remove the harmful heavy-metal ions before domestic sludge used as fertilizer.In this study,the sludge was treated by ozone and electricity,respectively,to release and to remove heavy-metal ions.Then the treated sludge is used as fertilizer for maize.The main results are as follows:(1)Ozone could effectively degraded the macromolecular organic matters(MOMs)in sludge into small soluble molecules,and crushed large sludge particles into small ones,then heavy-metal ions,such as Cu,Pb,and Cu,etc.,were released.With treating time increasing,the amounts of degraded MOMs and released heavy-metal ions were both increased.Considering the balance of cost and heavey-metal residues,the optimal time of ozone treating sludge is 30 min.(2)In addition,the sludge was treated by electricity of direct current(DC)in 48 V to separate heavy-metal ions,Cr,Ni,Cd,Pb,Zn,and Cu.But the maximum removing ratio of the six heavy-metal ions cannot be obtained at the same time under the same electricity conditions.Among them,Zn and Cu need relative long time,120 and 240 min respectively,to reach their maximum removing ratio of 45.68 and 27.87%;while Cr,Ni and Cd rapidly reach their maximum removing ratio of 49.23,26.14,and 17.22%,respectively;Pb in sludge needs 60 min to get the maximum removing ratio,61.65%.So,only three heavy-metal ions,Cr,Ni and Cd,reach the maximum removing ratio from sludge within 30 min,the other three need relative long times.(3)Zn is clustered in leaves and roots of maize plants,and less of that exists in kernel and cob.When the sludge is applied in the field,Zn content of soil was increased,then resulting in that in maize also increased.Zn content of sludge was decreased after treated by ozone and electricity,thus Zn content in maize,especially in kernel,was decreased too.In addition,Zn was slowly moved to the deep layer of soil.(4)Cu contents in maize reduced with the direction of root to kernel.But the contents were not influenced by the Cu content in sludge.In the experiment,both sludge and organic fertilizers increased the Cu contents in soil significantly,but the Cu content in maize did not change significantly.Thus,sludge as maize fertilizer could not influence the Cu content in maize.(5)The maize organs with the highest and the lowest Cr contents is root and kernel,respectively,and Cr contents in every organ increased significantly with that of the soil increasing.When the sludge is treated,the Cr contents in each organ could be effectively decreased.Several kinds of sludge and organic fertilizers all increased Cr contends in the soil,while the treated sludge had less effects than organic fertilizer did,which indicated that the treated sludge could be used as a safety agricultural fertilizer.(6)Ni is mainly distributed in the roots of maize,and a little amount of Ni exists in the kernel.And the Ni contents in maize were not affected by the types of sludge.The Ni content in the soil positively and significantly influenced that in maize plant.Sludge fertilizer increased the Ni content of soil,but sludge processing could relieve this problem.However,the Ni content in maize was still harmful to human healthy even the sludge was treated.In the soil,Ni usually moves to the deep layer.(7)Pb is clustered into the roots and leaves of maize,and that is the least in kernel.The more the Pb content in soil was,the more the Pb content in maize increased.The sludge without any treatments significantly increased the Pb content in soil,while organic and treated sludge had not any influence on Pb content in soil.That is,the treated sludge did not increase Pb content of maize when compared with that of organic fertilizer.Pb usually moves to the deep layer of soil.(8)Cd is mainly clustered in the roots and stem of maize,while it distributed the least in kernel.The Cd content in maize was significantly increased when the Cd in soil increased.Both sludge and organic fertilizers increased the Cd contents of soil,but the sludge increased more Pb content of soil than organic fertilizer.In the study,neither ozone nor electricity effectively improved Cd contents of sludge,then the Cd content of soil.Thus,sludge application increased the Cd content in maize.(9)The antagonistic effect of Ni and Zn assimilation by maize was detected in the study.While other heavy-metal ions were all positively correlation with each other.And all heavy-metal ions contents in maize were positively related with those in soil.(10)In each sludge treatment,Zn,Cu,and Cd contents all meet for the need of agricultural fertilizer.And Cr content was qualified after sludge was treated.But Ni content in maize was high(0.4 mg/kg),even the sludge fertilizer was treated.Pb content in sludge was not high,which could not contaminate soil and food.As Pb content,sludge is better than organic fertilizer.