Effects Of Coexistence Of Dissolved Organic Matter And Nano Zinc Oxide On Clogging And Performance Of Constructed Wetland

With excellent light,electricity,magnetism and reaction sensitivity,Nano-zinc oxide(ZnO NPs)is widely used in the fields of medical science,automobiles,daily necessities and agriculture.However,its widespread use has led to a large number of ZnO NPs entering natural-water and soils along with atmospheric deposition,soil erosion,industrial wastewater and domestic sewage,which poses a threat to ecosystems and human health.At present,the existence of ZnO NPs has been detected in the influent and effluent of many sewage treatment plants at home and abroad.Constructed wetlands are usually used as advanced wastewater treatment processes,and the appearance of ZnO NPs in the effluent from sewage treatment plant means that they will inevitably flow into the constructed wetland sewage system.Dissolved organic matter(DOM)is widely present in wetlands and has various characteristics such as rich functional groups,huge molecular charge and hydrophilicity.It can be adsorbed to the surface of nanoparticles(NPs)to form a nano-scale coating and change the dispersion stability and mobility of NPs in the water environment,which may bring adverse effects on the performance and clogging of constructed wetlands.In this paper,humic acid(HA)was used as a representative substance of DOM to study the changes of particle size of ZnO NPs in the presence of different concentrations of HA.The changes of effluent quality and permeability coefficient of constructed wetlands are examined under the conditions of coexistence of DOM and ZnO NPs at different concentrations;the clogging materials are characterized and analyzed,and thus this paper reveals the mechanism of system performance and clogging effects of constructed wetland under the coexistence of ZnO NPs and DOM.The specific research contents and results of this study are as follows:(1)Changes in particle size of ZnO NPs in the presence of HADOM can affect the agglomeration and dispersion of ZnO NPs through complexation,adsorption,etc.The average particle size of ZnO NPs in ultrapure water was 560 nm,and it was increased to 627 nm at 1 mg/L HA.With the increase of the added HA concentration,the average particle size of ZnO NPs decreased gradually.When HA concentration was 25mg/L,the average particle size of ZnO NPs decreased to 360 nm.Low concentrations of HA(≤1 mg/L)could promote the agglomeration of ZnO NPs,while high concentrations(1–25 mg/L)of HA had the opposite effect.(2)Effects of coexistence of DOM and ZnO NPs on the treatment performance of constructed wetlandsWhen different concentrations of HA and 1 mg/L of ZnO NPs were added to the influent water,the COD showed an upward trend.Meanwhile,the average removal efficiency of COD in each experimental group decreased.Under the condition of 1 mg/L HA and Zn O,the removal efficiency of COD decreased most obviously(18.07%).Before adding ZnO NPs and HA,the removal efficiencies of ammonia nitrogen in each reactor were about 55.00%.After adding different concentrations of HA and ZnO NPs,the removal efficiency of ammonia nitrogen showed a decline of different degrees,in which the coexistence of 1mg/L HA and ZnO NPs had the most influence.After 10 days of operation,the removal efficiency of total phosphorus decreased to different degrees.Then,After another 10 days,the removal efficiencies of total phosphorus in each reactor began to improve,which might be because the accumulated extracellular polymer(EPS)reintroduced a large number of phosphorus adsorption sites.(3)Effects of coexistence of DOM and ZnO NPs on the clogging of constructed wetlandsThe reactor clogging was characterized by SEM-EDS,FT-IR and XPS,and EPS was extracted for component content analysis.The results showed that the contents of polysaccharide,protein and DNA in EPS had varying degrees of increase after adding different concentrations of HA and ZnO NPs into the influent water.Among them,1 mg/L HA and ZnO NPs stimulated microorganisms to produce more EPS,which might create stronger clogging for constructed wetlands.The permeability coefficient of the reactor with1mg/L HA and ZnO NPs decreased from the initial 1.26 cm/s to 0.41 cm/s,significantly higher than those of other reactors,which was consistent with the above experimental results.(4)Effects of coexistence of DOM and ZnO NPs on microorganisms in constructed wetlandsThe results of high-throughput sequencing showed that Proteobacteria(48.6%-62.1%-62.1%),Bacteroidetes(9.7%-18.2%-18.2%)and Chloroflexi(7.3%-15.9%)were the dominant bacteria in each reactor.Proteobacteria was the most important type of bacteria in the activated sludge system,and its change in the system could explain the difference removal efficiencies of COD and ammonia nitrogen to a certain extent.After adding different concentrations of HA and ZnO NPs,the activities of Nitrosomonas and [Desulfo bacterium]＿catecholicum＿group in the system were inhibited,which further explained the reason for the decreased denitrification ability of constructed wetland system.Meanwhile,the proportion of Thauera that could secrete EPS was the largest in CW2,reaching more than 22%,which might be one reason for the serious clogging in CW2.In this paper,the stability of 1 mg/L ZnO NPs(environmental background concentration)in different concentrations of DOM was investigated,and the effects of different concentrations of DOM and ZnO NPs on the clogging and system performance of constructed wetland ecosystems were investigated,which could provide theoretical support for the stable operation of the constructed wetland system.