Simultaneous Adsorption Immobilization Of Cadmium And Arsenic In Water And Soil By Sepiolite-modified Nanoscale Zero-valent Iron And The Related Mechanisms

Arsenic and cadmium are seriously polluted in water and soil,which have a great influence on the quality of human health agricultural products and the safety of ecological environment,due to the huge differences in their occurrence forms and chemical behaviors,it is often difficult to take into accounts of both arsenic and cadmium in the remediation of water or soil.Nano zero valent iron（nZVI）is the commonly used material for arsenic adsorption,which has strong adsorption capacity for arsenic but is easy to agglomeration and oxidize;Sepiolite is alkaline and porous clay mineral,which is a commonly used cadmium adsorption material.Therefore,this paper is based on the properties of the above two materials,synthesize the sepiolite supported nano zero-valent iron（SEP-nZVI）,by carrying out batch adsorption experiment,soil incubation experiment and pot experiment,combined with spectral characterization and other studies,studied and clarified the adsorption capacity of arsenic and cadmium in single and co-contaminated water or soil,and the adsorption mechanism.Study the effects of SEP-nZVI application on the absorption of arsenic and cadmium by crops（water spinach）and investigate the soil microbial community structure and diversity.The main results are listed as follows:（1）The adsorption effect and mechanism of SEP-nZVI on single As（III）,As（V）and Cd（II）in water were studied.The preparation of SEP-nZVI was characterized by spectral analysis and other methods.It was found that nZVI was dispersed in the surface and pores of sepiolite.At pH 7,the adsorption of SEP-nZVI on As（III）,As（V）and Cd（II）maximum adsorption capacities of the three ions were 230.29,155.47 and 11.37 mg·g^-1,respectively.The adsorption binding force of the three ions was As（V）>As（III）>Cd（II）,and the binding strength of adsorption was As（III）>As（V）>Cd（II）.The changes of mineral phases and valence states of newly formed As（III）,As（V）and Cd（II）on SEP-nZVI surface were studied by XRD,XPS and other characterization methods,and the mechanism of adsorption was also discussed.The results show that SEP-nZVI adsorbed arsenic and cadmium ions through electrostatic interaction on the surface,and then carried out ion exchange,complexation and precipitation reaction with Fe-O/Fe-OH on the surface of SEP-nZVI,immobilization at the interface of the modified material,Fe₄（As O₄）₂O₃,Fe As O₄,Cd O,Cd Fe₂O₄ and so on are made into more stable mineral phases.42.67%As（III）was oxidized to As（V）on SEP-nZVI surface.（2）The adsorption effect and mechanism of SEP-nZVI on co-combined As（III）-Cd（II）in water.Results showed that As（III）significantly promoted the adsorption of Cd（II）,and the maximum adsorption capacity of Cd（II）was 27.79 mg·g^-1.The addition of Cd（II）had little effect on the adsorption of As（Ⅲ）at low concentration.As（Ⅲ）promoted the adsorption of Cd by electrostatic adsorption or Fe-As bridging.As-Cd is immobilization on SEP-nZVI surface by electrostatic adsorption,REDOX,coprecipitation,complexation and formation of B-type ternary complexes.SEP-nZVI adsorption of arsenic and cadmium existed two processes:synergistic（formation of terpolymer）and competitive（competition of Fe-O complex site）.（3）The effects of SEP-nZVI addition on co-contaminated soil physicochemical properties,bioavailability and fraction of As and Cd were analyzed under different water conditions.The results showed that after 36 days of cultivation under four supplemental levels of SEP-nZVI（0,1%,3%,5%）,waterlogging（CF）,alternating dry and wet（AWD）,and 70%field capacity（70%WHC）,the soil pH was increased under three soil moisture conditions when the supplemental level of SEP-nZVI was 3%and 5%.The increase of 0.6～1.0 units of CF treatment,0.5～0.7 units in AWD treatment,0.6 units under 70%WHC;The change of soil pH under 1%was similarly to that of the CK.At three soil moisture conditions,the addition of SEP-nZVI can significantly reduce the content of available As and Cd in soil,by 45%-80%and 5%-45%,respectively,and promote the transformation of arsenic and cadmium in acid extraction fraction,oxidation fraction and reduced fraction to more stable residue fraction.The soil pH was negatively correlated with available arsenic and cadmium content(r_pH-As=-0.711**,r_pH-Cd=-0.929**,CF for example),while the content changes of available arsenic and available cadmium were positively correlated(r _Cd-As=0.668**).These results indicate that SEP-nZVI can increase pH and immobilization co-contaminated arsenic and cadmium in soil.（4）Application of SEP-nZVI could reduce the absorption of arsenic and cadmium by water spinach.Conduct pot experiment,0,0.1%,0.25,0.5%,0.75%SEP and SEP-nZVI were added to As-Cd co-contaminated soil,water spinach was planted.The biomass and contents of arsenic and cadmium in soil and water spinach were analyzed.The results showed that adding sepiolite could increase the biomass of water spinach,and the yield increased by 28.67%compared with CK.The biomass of water spinach increased when adding 0.1%SEP-nZVI,but decreased when SEP-nZVI adding increased.With the increase of SEP-nZVI dosage,arsenic and cadmium contents in water spinach decreased.Arsenic contents in ground and underground parts decreased by 42.84%-63.23%and 8.95%-79.39%,and cadmium contents in ground and underground parts decreased by 0%-58.52%and 10.70%-60.85%,respectively.（5）The addition of SEP-nZVI also affected the microbial community structure and diversity in the arsenic and cadmium co-contaminated soil at three soil moisture conditions.1%SEP-nZVI treatment had no significant effect on soil community abundance and diversity index.3%and 5%SEP-nZVI treatments have a negative effect on Alpha diversity of soil microbial community,and the level of microbial evolution was sensitive.Soil physicochemical properties and available arsenic and cadmium were significantly correlated with the abundance of some specific dominant phyla（Actinomyces,Firmicutes）（p<0.05）,but had no significant effect on most other phyla.SEP-nZVI,incubation time and soil moisture were the reasons for the difference of soil microbial community structure,among them,3%and 5%SEP-nZVI have a greater impact on change the difference of soil microbial community.Therefore,compared with sepiolite,the sepiolite supported nano zero-valent iron（SEP-nZVI,iron:sepiolite=1:3）synthesized in this study can significantly improve the adsorption of arsenic and cadmium in aqueous solution under single or compound conditions.The results of soil incubation test and microbial analysis show that the addition level of SEP-nZVI is 1%,which can effectively immobilization arsenic and cadmium in co-contaminated soil,and has no effect on the microbial community structure in soil.The results of pot experiment showed that in the premise of adjusting the proportion of sepiolite and iron,adding SEP-nZVI could increase the biomass of crops and reduce the contents of arsenic and cadmium in plants,that is,it could effectively reduce the absorption of arsenic and cadmium by crops.