Remediation Of Cd-contaminated Soil By Modified Nanoscale Zero-valent Iron: Role Of Plant Root Exudates And Inner Mechanisms

Due to the harmful effects of soil heavy metal pollution on the soil ecosystem and human health,environmental problems caused by soil heavy metal pollution are increasingly becoming a global threat.Among these heavy metals,cadmium（Cd）is one of the most dangerous heavy metals because of its high toxicity and bioavailability.There have been many reports on the remediation of Cd contaminated soil.Among them,the application of nanoscale zero-valent iron（n ZVI）technology is also becoming mature.Its mechanism is mainly to fix heavy metals in the soil,and the effect is significant.On the other hand,phytoremediation is also widely welcomed by people due to its economic and environmental advantages.Therefore,the cooperative application of these two repair methods has gradually become the object of research by researchers.From previous studies,we found that when n ZVI cooperates with phytoremediation of soil heavy metal Cd,the effect of plant absorption is better than that of single phytoremediation,which indicates that it may be caused by the increase of the bioavailable form of Cd in the soil or the increase of the plant’s own absorption capacity.This result seems to contradict the single use of n ZVI to fix heavy metals in the soil.In response to this phenomenon,we have launched this experimental study,and hope to make a reasonable explanation for this phenomenon.In this study,the role of exogenous root exudates and microorganisms was investigated in the application of modified n ZVI for the remediation of Cd-contaminated soil.In this experiment,citric acid（CA）was used to simulate root exudates,which were then added to water and soil to simulate the pore water and rhizosphere environment.In detail,the experiment in water demonstrated that low concentration of CA facilitated Cd²⁺removal by n ZVI,while the high concentration achieved the opposite.Among them,CA can promote the adsorption of Cd not only by direct complexation with heavy metal ions,but also by indirect effect to promote the production of iron hydroxyl oxides which has excellent heavy metal adsorption properties.Additionally,the H⁺dissociated from CA posed a great influence on Cd²⁺removal.When the concentration of H⁺increased,there was a strong competition between H⁺and Cd²⁺,which reduced the stability of Cd²⁺complexes formed on the surface of oxided n ZVI.On the other hand,higher concentration of H⁺can also decompose iron oxides produced after the oxidation of modified n ZVI,thus reducing the adsorption capacity of Cd²⁺on the surface of the material.The situation in soil was similar to that in water,where low concentrations of CA contributed to the immobilization of Cd²⁺by n ZVI,while high concentrations promoted the desorption of Cd²⁺and the generation of CA–Cd complexes which facilitated the uptake of Cd by plants.For the soil environment in this experiment,the oxidation of n ZVI,the metabolism of soil microorganisms,and the transformation of heavy metal Cd form are all important factors affecting the physical and chemical properties of soil organic matter（OM）,p H,and cation exchange capacity（CEC）.Specifically,with the oxidation of n ZVI and the metabolism of microorganisms,the content of OM in the soil gradually decreases and the p H value gradually increases.At the same time,OM contains a large number of active functional groups（such as-COOH,-OH）that can ionize H⁺to make the soil p H go low.In addition,with the oxidation of n ZVI,iron oxides increase and p H rises,which facilitates the transition of Cd to a stable state.When the reaction is on the index CEC,the CEC value increases.Finally,high throughput sequencing technology was used to analyze the community composition and Beta diversity of the microbial community in Cd-contaminated soil added with CA and n ZVI.It was found that a high concentration of CA was not conducive to the growth of microorganisms,while carboxymethyl cellulose（CMC）had the effect of alleviating the biological toxicity of n ZVI.