The Fixation Of Heavy Metals By Halloysite Depolymer And Its Application In Soil Remediation

The heavy metal pollution of soil is becoming more and more serious due to human industrial production activities.Affected by soil composition and environmental geochemical processes,soil heavy metal pollution is characterized by complexity,diversity and regionalization.Secondary pollution often occurs in the process of selective remediation of heavy metals.Therefore,finding an efficient and stable remediation technology to improve the quality of heavy metal contaminated soil has become the top priority of soil pollution remediation.As the important components of soil,clay minerals are widely present in soil and have good compatibility with soil.Remediation of heavy metal contaminated soil using clay minerals can not only effectively control the biotoxicity of soil heavy metals,but also effectively reduce the disturbance of its own remediation processes on the soil environment.Halloysite（Hal）is found to be a ubiquitous natural nano-clay material.In the natural environment,halloysite is a coiled multi-walled tubular structure,whose inner surface is composed of abundant Al-OH active groups,and its outer surface is composed of Si-O-Si inert groups.By introducing exogenous silicon to depolymerize and transform the halloysite,the halloysite tube can be opened layer by layer and the active groups hidden on its inner surface can be exposed,thus greatly improving its ability to fix heavy metals.In addition,the introduction of essential elements for plants growth such as Ca and Mg to induce the depolymerization and transformation of halloysite can not only further improve the fixation performance of heavy metals,but also promote the growth of crops and achieve the effect of increasing yield and income when these materials are used in soil remediation.Therefore,the study of depolymerization and transformation of halloysite and the development of functional halloysite passivation materials with high efficiency will have a good engineering application prospect in the field of heavy metal contaminated soil remediation.The main research results of this paper are as follows:（1）Halloysite is depolymerized and transformed under the action of exogenous silicon,and then the multi-layer coiled nanotube of halloysite can be opened to expose the highly active hydroxyl groups on the inner surface of halloysite.Finally,halloysite is generated.Specifically,exogenous silicon preferentially bind with the inner-surface active group,reorganizes to form a two-dimensional cross-linked channel after opening the halloysite tube,and initially forms dickite layered structure,with each two layers following the TO-TO cross-arrangement.When exogenous silicon continues to participate in the reaction,the primary product continues to recombine with aluminum hydroxyl group under the effect of hydrogen bonding,rolling into small spheres layer by layer,and finally completing the transformation from halloysite to dickite.Through the adsorption experiments of heavy metals(Pb²⁺,Cd²⁺and Cr³⁺),it is found that the exposure of hydroxyl groups on the inner surface of the tube can greatly promote the fixation of heavy metals.The fixation capacity for heavy metal(Pb²⁺,Cd²⁺and Cr³⁺)was increased by 8.3 times,5.8 times and 6.9 times,respectively.（2）Based on the above research,during the process of halloysite demineralization,halloysite can be recombined to form sodium gobbinsite through controlling the magnesium during depolymerization and polymerization.When the aluminum content in the solution is relatively low,the aluminum atom in the halloysite aluminum-oxygen tetrahedron is isomorphically replaced by Mg,the skeleton oxygen atom is recombined,and the magnesium ion is replaced to occupy the T position.The abundant Al-O-（Si-O）1,2-Al sequence was generated,which was transformed into a molecular sieve skeleton in situ through further dealumination,desilication,substitution and recrystalization processes,and finally produced high-efficiency sodium gobbinsite.By inducing the structural reorganization of the halloysite,not only the highly active aluminum hydroxyl group（Al-OH）on the inner surface of the halloysite tube can be activated,but also the structural properties of the zeolite skeleton can be obtained.And therefore the ability to fix the heavy metal（Cd）has also been significantly iimproved.The fixation capacity for heavy metal(Cd²⁺)was increased by 8.2 times.The adsorption process is accompanied by the formation of phillipsite,otavite,garranite and gmelinite.（3）Due to its high hydration energy,charge density,lower hydration radius and hydroxide solubility product,the introduction of calcium,during the zeolization of halloysite,can induce the depolymerization and transformation of halloysite into more efficient calcium-based zeolites.The redepolymerization and crystallization of halloysite is promoted by the induction of calcium ions and the zeolite skeleton is formed through the support of calcium ions between the siloxane groups and aluminum oxide groups.The crystal units present a six-membered ring arrangement.During the formation of calcium-based zeolite,the abundant aluminum hydroxyl groups on the inner surface are exposed.After forming calcium-based zeolite,these aluminum hydroxyl groups are abundantly present between the outer surface of layered calcium-based zeolite and the skeleton.In the interaction process between calcium-based zeolite and heavy metals,the fixation ability of heavy metals is significantly improved through lattice replacement and hydroxyl complexation with heavy metals.The fixation capacity for heavy metal(Pb²⁺and Cd²⁺)was increased by 14.2 times and 10.8 times,respectively.（4）Taking the self-developed new passivating material halloysite depolymer as the starting point,halloysite depolymer（calcium-based zeolite）with better performance was selected as passivating remediation material for passivating remediation of heavy metal(Pb²⁺and Cd²⁺)contaminated soil.Tessier method was used to explore the changes in available states of heavy metals during the remediation process.16S r RNA high-throughput sequencing technology was used to analyze the composition of microbial community and metabolic pathways in soil after calcium zeolite remediation,to further clarify the synergistic effects of depolymerization and transformation technology on the growth of heavy metals,microorganisms,and plants during the remediation process.Pot experiment verified the effect of repaired materials on vegetable growth,which also further clarify the synergistic effect of depolymerization transformation technology on heavy metals,microorganisms and plant growth in the process of remediation.The study provides a theoretical basis for the research and development of clay mineral passivation materials and a potential engineering application value for the promotion of materials in the field of contaminated soil remediation.