| Chromium(Cr)is widely present in nature,which has strong migration ability and high toxicity.Cr in soil and water mainly exists in two oxidization states:highly toxic hexavalent Cr(Cr(Ⅵ))and less toxic trivalent Cr(Cr(Ⅲ)).Zero-valent iron(ZVI)is an excellent adsorption material,possessing strong reduction abilities and abundant surface reactivity sites.However,ZVI is prone to aggregation and passivation due to its magnetism,resulting in a decrease in its adsorption capacity and compromise electron transfer capacity with pollutants.Utilizing biochar as support material of ZVI to enhance its dispersibility and reduce its passivation.In this study,straw biomass was pyrolyzed at 800℃ to prepare biochar(BC)and supported ZVI(BC/ZVI)by wet reduction technique as repair amendments.Firstly,the mechanism of Cr(Ⅵ)removed by BC and BC/ZVI in humic acid(HA)and oxalic acid(OA)treatment in aqueous solution was explored.Secondly,combined with soil incubation experiments to set up three groups of treatments:(ⅰ)HA and OA;(ⅱ)different organic carbon types(glucose,glutamic acid,suberin);(ⅲ)different organic fertilizers(straw and chicken manure).Through the basic physical and chemical properties of soil,the graded extraction of Cr species,transformation of iron ions and iron minerals,and changes in microbial communities,the effects of exogenous organic matter on the remediation of Cr-contaminated soil by BC and BC/ZVI in practical applications though were discriminated.The main results are as follows:(1)Explored the mechanism of Cr(Ⅵ)removal from aqueous solutions by HA and OA treatments on BC and BC/ZVI.The adsorption and desorption experiments indicated that the removal of Cr(Ⅵ)by the two materials mainly involves in adsorption and reduction mechanisms.Compared with BC,the removal capacity of BC+HA to Cr(Ⅵ)was decreased by 2.5%,and the treatment with BC+OA increased by 91%.Compared with BC/ZVI,the removal capacity of BC/ZVI+HA and BC/ZVI+OA to Cr(Ⅵ)increased by 11.1%and 31.1%,respectively,mainly through reduction.HA inhibited the Cr(Ⅵ)reduction capacity of BC and BC/ZVI,while OA had a better synergistic promoting effect on the repair amendments.The EDS-mapping showed a uniform distribution of C,O,Fe,and Cr elements after adsorption,indicating that the immobilization of Cr may be enhanced by the formation of Fe-Cr binary complexes or Fe-Cr-DOM ternary complexes.XPS data showed that HA and OA could form Fe-COO bond on ZVI surface,which confirmed that there was a coprecipitation effect of FeDOM in the reaction process.(2)Explored the mechanism of Cr(Ⅵ)removal from the soil by HA and OA treatments on BC and BC/ZVI.The graded extraction results showed that the Cr(Ⅲ)content was the highest among all treatments,indicating that reduction was the main mechanism for Cr immobilization.Compared with all treatments,BC and BC/ZVI had relatively higher removal effect on available Cr in soil by HA treatment.Microbial diversity analysis showed that the abundance of Ensifer and Paenibacillus increased in BC+HA,BC+OA,and BC+HA+OA treatments.In addition,the abundance of Amoniphilus showed a significant increase in BC/ZVI+OA and BC/ZVI+HA+OA,while BC/ZVI+HA decreased the abundance of Lysinbacillus and increased the abundance of Ensifer.RDA and correlation analysis shown that amorphous iron oxides,complexed iron and Cr(Ⅲ)were negatively correlated with available Cr,indicating that reduction was the main immobilization mechanism,and iron minerals,especially complexed iron,were the main adsorption sites for soil Cr.(3)To explore the mechanism of Cr(Ⅵ)removal from soil by BC and BC/ZVI with different organic carbon treatments,i.e.sugar(glucose),protein(glutamic acid)and plant lipid(suberin)were set up.The results showed that soil EC increased by 15.5-382%and 94.3-404%in the BC and BC/ZVI treatment,respectively,by three organic carbon treatments.Graded extraction experiment showed that the content of precipitated Cr(Ⅲ)was in aggrement with the content of available Cr,which showed the rule of glutamic acid greater than glucose greater than suberin treatment,indicating that reduction was the main immobilization mechanism.Moreover,the addition of different components of organic carbon increased the content of Fe(Ⅱ)and Fe(Ⅲ)in BC and BC/ZVI treated soil,and promoted the release of iron minerals,but there was no significant difference for BC treated soil.Correlation analysis showed that BC and BC/ZVI treatments showed a negative correlation among soil Fe(Ⅱ),complexed iron,EC,and Cr(Ⅲ)and available Cr in soil by different organic carbon treatments.(4)To explore the mechanism of of Cr(Ⅵ)removal from soil by BC and BC/ZVI with different organic fertilizer,two treatments,i.e.straw organic fertilizer and chicken manure organic fertilizer were set up.The results showed that the soil DOC concentration in BC and BC/ZVI treatments increased by 169-210%with straw treatment and 53.8-69.8%with chicken manure treatment.The graded extraction experiment confirmed that the concentration of Cr(Ⅲ)was line with the content of available Cr,which showed the rule of straw greater than chicken manure treatment,indicating that reduction was the main immobilization mechanism.Straw addition improved the diversity of microbial communities;chicken manure increased the total amount of microbial species,and a new Cr-reducing bacteria TM7a was found.Both straw and chicken manure had no significant effect on the release of iron ions and iron minerals in soil by BC treatment.For BC/ZVI treatment,organic fertilizer addition had a significant effect on the concentration of iron ions in soil and promote the release of iron minerals.Compared with chicken manure,straw organic fertilizer was easier to combine with soil available Fe to form complexed iron,which was beneficial to decrease the available Cr.RDA and correlation analysis confirmed that DOC,complexed iron and Cr(Ⅲ)in soil treated with BC and BC/ZVI by different organic fertilizer treatments were negatively correlated with available Cr. |
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