| Biochar is an excellent material used to immobilize heavy metals in soil.The change in heavy metals fractions caused by various environmental factors is an important factor in determining the long-term effect of biochar applied to the remediation of heavy metals contaminated soil.Common water management practices in paddy soil,such as flooding,draining,and alternate drying and wetting,can change the heavy metals fractions by affecting the physicochemical properties of soil and biochar after applying biochar to the soil.However,the current findings regarding the effect of moisture on the availability of soil heavy metals and the properties of biochar are not quite the same,particularly in the heavy metals co-contaminated soil.Therefore,in this study,to investigate the effects of humidification,flooding,alternate drying and wetting,and draining conditions on the physicochemical properties of soil and biochar and understand the cause of changes in the forms of heavy metals,the soil with biochar was used to simulate the water changes during the growth cycle of early maturing rice through incubation experiment.It is expected to provide data references and technical support for the water management on the co-contaminated paddy soil with biochar,and provide guidance for the long-term practice of biochar remediation of heavy metals.The main results are as follows:(1)With humidity(75% Water holding capacity)as the normal moisture condition,the addition of biochar increased the soil pH,dissolve organic matter(DOC)and amorphous iron oxide(Feo)content,and transform the acid soluble fraction of Pb,Cd,and Cu to a more stable fractions like reducible and residual fractions.As a result,the heavy metals leaching toxicity and bioavailability are reduced,biochar has a positive effect on heavy metals remediation.In addition,the acid soluble content of Cd and Cu is extremely negatively correlated with pH,DOC and Feo of the soil.Biochar has the best immobilization effect on Cd,and the fastest immobilization speed,followed by Cu,Pb,and Zn.(2)Flooding and two types of dry-wet alternation can not only reduce the acid soluble content and TCLP extraction content of Pb,Cd and Cu in the soil after biochar addition,but also have a significant effect on the reducible and residue contents of Pb and Cd.In particular,the effect of flooding on the continuous transformation of heavy metals in the soil to a stable fraction after biochar addition is the best,and it is proportional to the incubation time,followed by the alternate dry and wet treatments of3 d flooding 1d drying and 3d flooding 5d drying.On the contrary,the content of the acid soluble and TCLP extraction of the four metals under 30% WHC condition increased,which reduced the remediation effect of biochar on heavy metals.(3)Under flooding conditions,the acid soluble content of Pb,Cd,Cu,and Zn is positively correlated with soil pH,soluble organic carbon,and amorphous iron oxide,with Cd having the strongest correlation.Especially there is a positive correlation between pH,soluble organic carbon and amorphous iron oxide.(4)The dry-wet alternation not only destroys the physical structure of biochar,but it also improves the types of oxygen-containing functional groups that can be related to the adsorption performance of heavy metals and the O/C on the surface of biochar. |
