Simultaneous Immobilization Of Cadmium,Lead,and Arsenic In Soils By Zeolite-Supported Nanoscale Zero-Valent Iron And The Associated Mechanisms

Heavy metals are severe environment pollutants because of their imperceptibility,persistence,and non-degradation properties,which have been dangerous to ecosystem and human health and become the key issue around the world.According to the survey of Chinese bulletin on soil pollution in 2014,the total exceed ratio of the contaminated sites reached 16.1%,especially the combined pollution of Cd,Pb and As.Since the availability and mobility of cationic metals（Cd and Pb）and anionic metalloid（As）change oppositely when adjusting soil physicochemical properties,it is hard to simultaneously immobilize these three pollutants by conventional amendment.Therefore,the development of a low-cost,high-efficient,stable,and safe amendment to in-situ change the migration and transformation process of Cd,Pb,and As in soil is imminent,which will be beneficial for reducing the enrichment of heavy metals in soil-crop system.As a kind of low-cost natural clay mineral,zeolite is a series of hydrated aluminosilicates,which can improve the soil physicochemical property and structure and adsorb the cationic metals.With the high specific surface area and good anionic adsorption capacity,nanoscale zero-valent iron（NZVI）can remove the environmental pollutants by redox reactions.Thus,the combination of zeolite and NZVI will make it possible to simultaneously immobilize the Cd,Pb,and As in soil.In this study,compared with zeolite,we（1）synthesized the zeolite-supported NZVI（Z-NZVI）and explore its adsorption capacities and properties for the coexisting Cd,Pb,and As though batch experiments;（2）conducted the soil incubation experiments to investigate the effects and durable effectiveness of different dosages of Z-NZVI for heavy metals immobilization in naturally contaminated acid red soil and alkaline Chao soil,as well as the changes of soil physicochemical properties;（3）utilized the magnetic property of Z-NZVI to separate it from different systems after remediation,then used the advanced characterization methods to elaborate on the immobilization mechanisms;（4）analyzed the effects of different dosages and incubation periods of Z-NZVI on soil microbial community structure and functional genes expression;（5）conducted the pot experiments to evaluate the impacts of Z-NZVI on the uptake of heavy metals by crops and their physiological and biochemical indexes after heavy metals immobilization.The main results are listed as follows,which provide the important theoretical reference for the potential application of the novel material Z-NZVI in the multi-metal contaminated farmland soil:（1）The research for the adsorption of Cd（Ⅱ）,Pb（Ⅱ）,and As（Ⅲ）by Z-NZVI in aqueous solutions.We simplified the liquid phase reduction process to synthesize the Z-NZVI and characterized its surface components.Batch experiments were conducted to investigate the adsorption characteristics of single and or mixed Cd（Ⅱ）,Pb（Ⅱ）,and As（Ⅲ）onto Z-NZVI,in which the mass concentration of adsorbent was 0.3g/L.The SEM showed that the NZVI evenly dispersed onto the framework of zeolite and the surface of zeolite became more porous and rough.FTIR spectra revealed that there were large amount of Si-OH,Si-O-Fe,Fe-OH,and Fe=O surface groups distributed on the Z-NZVI,which provided the active sites for heavy metals adsorption.The maximum isothermal adsorption capacity of Z-NZVI was 11.52 mg As（Ⅲ）g^-1,48.63 mg Cd（Ⅱ）g^-1,and 85.37 mg Pb（Ⅱ）g^-1at p H 6 and 10 h contact time,respectively,much higher than that of zeolite(0 mg As（Ⅲ）·g^-1,22.88 mg Cd（Ⅱ）·g^-1,32.23 mg Pb（Ⅱ）·g^-1).The competitiveness ranking for Z-NZVI of these three pollutants was Pb（Ⅱ）>As（Ⅲ）>Cd（Ⅱ）.The synergy and competition among the heavy metals were concurrent for the adsorption sites due to the formation of multilayer complexes onto Z-NZVI.（2）The effects of Z-NZVI on soil physicochemical properties and the available concentrations of heavy metals.The naturally contaminated farmland soils（acid red soil and alkaline Chao soil）were treated with 0,10,30 g kg^-1zeolite and Z-NZVI.After 1-180 d of incubation,the dynamic changes in soil physicochemical properties,heavy metal availabilities and their speciation distribution with different amendments were determined.The 10-30 g kg^-1zeolite treatments had no significant effect on soil physicochemical properties,but 30 g kg^-1Z-NZVI significantly increased the p H of both soils（P<0.01）.Meanwhile,30 g kg^-1Z-NZVI significantly decreased the EC of acid red soil and reduced the DOC of both soils at initial phase of incubation（P<0.01）.With the advantages of p H-adjusting and adsorption capacities,Z-NZVI immobilized DOC and mitigated its decomposition to maintain the higher p H and lower EC in soils,which was beneficial for the decrease in mobility of heavy metals.30 g kg^-1Z-NZVI amendment significantly decreased the available metal concentrations of Cd,Pb,and As by 10.2-96.8%in acid red soil and alkaline Chao soil after 180 d of incubation（P<0.05）.The results of the different fractions of heavy metals showed that Z-NZVI shifted part of these pollutants from acid-soluble fraction to reducible and residual fractions.Overall,the Z-NZVI application could simultaneously immobilize As,Cd,and Pb in contaminated soils during the incubation period,and the immobilization efficiency and stabilization were higher in alkaline Chao soil than acid red soil.（3）The adsorption and immobilization mechanisms of Z-NZVI for Cd,Pb,and As.The magnetic separation method was used to separate the Z-NZVI from multi-metal contaminated water and soils after reaction.The magnetic materials were characterized to investigate the changes in mineral phases and valence state of pollutants on Z-NZVI,and then the various mechanisms were identified.The results of XRD and XPS showed that B-type ternary complexation,heterogeneous co-precipitation,and redox reactions between Cd,Pb,As,Fe occurred concurrently,especially the formation of insoluble phases such as Cd₃（As O₄）₂,Pb Fe₂（As O₄）₂（OH）₂,and Pb⁰.First,heavy metals were rapidly adsorbed（Si-OH/Fe-OH/O-H substitution）and enriched onto the surface of Z-NZVI by aluminosilicates and iron oxyhydroxides.Second,specific redox of As（Ⅲ）,As（Ⅴ）and Pb（Ⅱ）occurred,which were depended on environmental p H and SEP.Meanwhile,the secondary mineralization of Z-NZVI provided a catalytic interface for bonded pollutants to form more stable heterogeneous minerals during the continuous amendment,which realized the encapsulation of heavy metals.（4）The impacts of Z-NZVI on soil microbial community structure and function.We extracted the total DNA and RNA after 1,15,and 180 d of incubation,respectively,and then performed the 16S r DNA high-throughput sequencing and real-time quantitative PCR.Combined with the bioinformatics analysis,the interactions among environmental factors,Z-NZVI,and indigenous microbes in two multi-metal contaminated soils were studied.During the throughout incubation period,the 10-30 g kg^-1Z-NZVI amendments had no significant effect on the abundance and diversity indexes of soil bacterial communities（P>0.05）,which ruled out the broad-spectrum microtoxicity of Z-NZVI.Compared with control,the transient and visible changes in the relative abundances of bacterial communities under Z-NZVI treatments were observed during 1-15 d of incubation,including the dominant genera such as the iron-oxidizing/sensitive,acidophil,denitrifying,and metal-resistant bacteria.However,the differences of community structures between treatments and control were ultimately eliminated with the extension of incubation time,since soil intrinsic environmental factors were the key drivers for the re-establishment of indigenous bacterial community.Due to the pollutants immobilization and electron donation by Z-NZVI,the relative expression levels of functional genes of indigenous bacteria for DNA replication and denitrification were promoted,which improved the total functional activity of microbes in multi-metal contaminated soils.（5）The effects of Z-NZVI on the uptake of heavy metals by crops and their antioxidant systems.We conducted the pot experiments to plant the high and low accumulated cultivars of Brassica chinensis in two multi-metal contaminated farmland soils（acid red soil and alkaline Chao soil）treated with 0,5,15 g kg^-1zeolite and Z-NZVI to investigate the subsequent changes in quality and physiological function of these crops.With the improvement of soil quality,5 g kg^-1Z-NZVI significantly increased the biomass of Brassica chinensis by 14.7-18.1%（P<0.05）and decrease the heavy metal contents in low accumulated cultivar to meet the standard of food safety.Z-NZVI significantly increased the Ca and Fe contents in Brassica chinensis（P<0.05）,which was beneficial for the detoxification mechanisms of heavy metals such as uptake suppression and cell wall binding.This finding was consistent with the result of the subcellular distribution that Z-NZVI decreased the proportion and content of heavy metal in organelle fraction,and then promoted the physiological function of Brassica chinensis.The modification of the nutrient contents in Brassica chinensis by 5 g kg^-1Z-NZVI increase the antioxidant enzyme activities and antioxidant contents,which alleviated the oxidative damage from heavy metals.However,15 g kg^-1Z-NZVI induced the excessive amounts of Fe in acid red soil,which aggravated the imbalance of antioxidant systems in.According to the comprehensive effects of Z-NZVI on the quality,physiological function,and health risk of Brassica chinensis,the combination of 5 g kg^-1Z-NZVI amendment and low accumulated cultivar was the best for safety production in multi-metal contaminated soils in this study,which was low-cost for only2336￥·ha^-1.Therefore,the synthesized Z-NZVI overcame the defects of zeolite and zero-valent iron themselves,which could simultaneously and steadily stabilize the Cd,Pb,and As in contaminated farmland soils through various adsorption and immobilization mechanisms during the incubation period.With the decreasing of the mobility and availability of heavy metals,Z-NZVI had no significant effect on soil microbial communities and promoted the microbial functional activities.Furthermore,the proper addition of Z-NZVI in soils would decrease the toxicities and contents of heavy metals in Brassica chinensis and improve the quality and physiological function of this crop,thus the safety in agriculture production came true.All the results from this study provide the scientific support and theoretical basis for the safe utilization of multi-metal contaminated farmland soil with Z-NZVI amendment,which shows the good potential application.