The Removal Efficiency Of Cadmium And Its Microbial Mechanisms Effected By Zeolite Supported On LDHs Coating In Constructed Wetland

The pollution of heavy metal cadmium in water environment is becoming more and more serious,so effective control measures are urgently needed.Constructed wetlands have the advantages of simple process,perfect treatment effect,and low operation costs.But the cadmium adsorption capacity of natural substrate is limited and its surface is not conducive for microorganisms to adhesion.Therefore,it is urgent to develop new and efficient fillers which can effectively improve cadmium,and can enhance the adhesion performance of microorganisms on the surface of the natural substrate in constructed wetlands.Layered double hydroxide(LDHs)has a large specific surface area,better adsorption property.Based on the previous research results,zeolites were selected as the research object,and the LDHs coating was placed on the surface of the zeolites to study the effect and mechanism of cadmium removal by microorganism growing on modified zeolite.LDHs was synthesized by co-precipitation method under alkaline conditions and in situ coated on original zeolites to obtain four different types of LDHs coating(Zn Al-LDHs,Zn Fe-LDHs,Mg Al-LDHs and Mg Fe-LDHs),and which were load on the surface of original zeolite to get different LDHs coating modified zeolite.A small test system was constructed to simulate artificial wetland,and the microbial domestication experiment was carried out to investigate the influence of cadmium on the microbial growth on the surface of LDHs modified zeolite.Purification experiments of cadmium and detection of related microbial indicators was carried out to investigate the microbial action and mechanisms for cadmium by microorganism growing on LDHs modified zeolite.The main research results are as follows:The physical and chemical characteristics of zeolites before and after LDHs coating were analyzed,and the results showed that the LDHs coating was successfully loaded on the surface of original zeolites,and the apparent morphology and chemical composition of the zeolites were changed.The related microbial experiment results after the microorganism domestication experiment indicated that,in the cadmium existing environment,the activity,richness and diversity of the microbial community on the surface of zeolites was enhanced by LDHs coating,in other words,the LDHs coating can enhance the cadmium resistance of microorganisms,and Mg-LDHs coating was best.The results of purification experiments showed that the LDHs coating is conducive to improve the biological and non-biological cadmium removal,and the Mg Al-LDHs coating modified zeolites was the best.The results of cadmium content of microorganisms growing on filler showed that,the microbial community on Zn Fe-LDHs coating on zeolites showed the best extracellular cadmium removal,and the Mg Al-LDHs coating on zeolites showed the best intracellular cadmium removal.This indicates that the LDHs coating increases cadmium removal by microorganisms growing on the surface of filler.After the cadmium purification experiment,relevant microbial experiments showed that: the enzyme activity and the EPS content in the microbial community growing on the surface of LDHs coating modified zeolites are higher than that of the original zeolites,which confirmed that the microorganisms growing on the surface of modified zeolites have a better cadmium removal performance in the cadmium purification experiments;in addition,the composition of microbial communities growing on the surface of filler was analyzed,and found that,in the microbial communities growing on the surface of LDHs modified zeolites,Proteobacteria had the highest relative abundance at the phylum level,while the relative abundance of microorganisms with cadmium tolerance or removal ability was also higher at the genus level,such as Brevundimonas,Pseudomonas,Comamonas,etc.This indicated that the LDHs coating promoted the growth of cadmium-resistant bacteria in the microbial community.In conclusion,the LDHs coating can improve the diversity and richness of microbial community and enhance the cadmium removal effect.Different types of LDHs coatings modified zeolites have different cadmium removal by microorganisms,among which Mg Al-LDHs coatings modified zeolite showed the best,which is expected to be used in practical ecological restoration projects.