| Human activities are the major contributors to heavy metal pollution in surface water.The high toxicity,food chain accumulation and non-biodegradability of heavy metal have become a serious threat to human and other organisms’ health.Constructed wetland(CW)is an important treatment technology for non-point source heavy metal surface water pollution due to their low operation and maintenance costs and eco-friendly features.Since the technology has been used for the treatment of heavy metal polluted water,the research theme has gradually shifted from the removal potential of various heavy metals and their geochemical transformation to the development and application of enhanced heavy metal removal methods.Therefore,the active development of enhanced C W remediation of heavy metal contaminated water and the study of its treatment efficacy and mechanism are of great practical significance for the promotion of CW technology in the broad spectrum of heavy metal water pollution remediation.In this study,we used bamboo biochar as substrate to construct BC-CWs(CWup:upper biochar,CWmid:middle biochar,and CWbot:bottom biochar in CWs)to comprehensively evaluate the performance of the BC-CWs system for cadmium removal under changing pH(from 5 to 9);to investigate the effects of biochar addition on the growth status and physiological response of CW plants,the migration and transformation pattern of cadmium in the substrate and its dominant physical and chemical processes,as well as the secretion of extracellular polymer substance(EPS)and microbial community characteristics;to clarify the mechanism of synergistic role of plant-substrate-microbial system for cadmium removal in BCCWs.This study provides theoretical guidance and technical support for the application of emerging biomass materials in non-point source biological water treatment technology and enhanced C W remediation of heavy metal polluted water bodies.The main research contents and conclusions are as follows.(1)This experiment was conducted to study the effect of different biochar location on the efficiency and stability of CW for cadmium removal by changing the influent pH.This experiment was divided into three stages.Under neutral influent conditions(pH 7),the average cadmium removal by BC-CWs reached 96.0-99.7%,which was significantly higher than that of gravel-based CW(CWcon;93.2-96.0%).Compared with pH 7,the removal effectiveness of CWs for cadmium was significantly lower under acidic influent conditions(pH 5),and BCCWs decreased more than CWcon for cadmium.The removal rates of cadmium were 85.2-91.3%for BC-CWs and 90.9%for CWcon.At alkaline influent(pH 9),the cadmium removal effectiveness of BC-CWs immediately recovered to more than 99%,which was significantly higher than that of the CWcon system.In addition,biochar addition caused a small increase in the initial 15 d effluent electrical conductivity(499.3-547.4 μs/cm)and increased acid neutralization capacity of the system compared to CWcon(500.8-507.0 μs/cm).the average effluent dissolved oxygen of BC-CWs(2.1-4.3 mg/L)was higher than that of CWcon(2.1-3.2 mg/L),which is consistent with the pattern of redox potential in the effluent.The comprehensive analysis showed that BC-CWs exhibited more stable physicochemical parameters and more efficient cadmium removal efficiency at varying influent pH compared with CWcon.(2)By monitoring the growth status and physiological response of plants,the fractionation and content of cadmium accumulation in the substrate,and the secretion of EPS and characteristics of microbial community,this experiment comprehensively investigated the effects of biochar on different components of CWs under cadmium stress.The results showed that the addition of biochar caused the accumulation of different fraction of Cd in the substrate at the beginning of the experiment,but BC-CWs exhibited a lower lipid peroxidation than CWcon.The cadmium retained in the substrate existed mainly in carbonate-bound and exchangeable fraction,and these biotoxicity-prone fraction of cadmium stimulated the secretion of microbial TB-EPS.As the experiment progressed,in stage II,biochar gradually improved the plant stress response to cadmium,but the mitigation process depended on the fraction of cadmium accumulated in the biochar substrate layer and its bioavailability and the extent to which plants used the nutrients in biochar;moreover,cadmium in the carbonate-bound fraction accumulated in biochar was more readily desorbed at this stage,followed by adsorption and ion exchange and adsorption with Fe/Mn(oxy)hydroxides and co-precipitation.Biochar can induce microorganisms to secrete large amounts of LB-EPS,thus alleviating the biological stress of microorganisms from the large amount of exchangeable cadmium in water.In phase III,biochar significantly enhanced plant photosynthesis due to its excellent growth-promoting properties,thus promoting plant growth and increasing its biomass,strengthening plant root networks to retain Cd through adsorption and uptake,and assisting plants to reduce Cd transfer to aboveground plant parts;in addition,biochar significantly enhanced Cd removal from water in the fraction of Fe/Mn(oxy)hydroxide binding and retention in the substrate.(3)Biochar complicated the microenvironment in the substrate layer by providing large amounts of nutrients and well-developed pore structures,enhancing DO transport and migration,and influencing the cadmium fraction in the rhizosphere microenvironment.biochar provides suitable microhabitats for microorganisms,reduces antagonism between microorganisms,promotes collaboration between microbial communities,improves community development and induces its evolution,and strengthens biosorption and bioaccumulation for Cd removal.In addition,biochar increased the secretion levels of indoleacetic acid and phytochelating peptides of microorganisms,alleviated the stress and migration of Cd in plants,gradually reduced the production of reactive oxygen species(main in the form of O2-and H2O2)in plants,promoted plant growth,and enhanced the removal of Cd through plant rhizosphere fixation.In conclusion,the removal of cadmium by biochar-based CW was mainly attributed to its rich pore structure and large amount of nutrients,which were able to promote the accumulation of Fe/Mn(oxy)hydroxide bound fraction of cadmium in the substrate and optimize the synergistic effect mainly by microbial community structure. |
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