| With the development of the economy and urbanization,the amount and type of pollutants increasing,which led to serious water pollution,especially the increasingly eutrophication.As the main limiting factor of eutrophication,phosphorus with relatively low concentration can cause serious eutrophication.However,the phosphorus removal efficiency of conventional wastewater treatment technology is insufficient,a large amount of phosphorus content in the secondary effluent still exist.so it is necessary to take wastewater advanced treatment technology to strengthen the phosphorus removal.Constructed wetland,as a kind of sewage ecological treatment system with synthetic action of plant,substrate and microbial,is widely used,especially in the treatment of the secondary effluent,because of its less investment cost,good decontamination performance,convenient operation and management and remarkable environmental benefits.The removal of phosphorus in constructed wetland mainly depend on the absorption of plants,the assimilation of microorganisms and the retention and precipitation of substrates.The effect of the substrate plays a vital role in phosphorus removal.The ability of phosphorus adsorption directly affect the purification of phosphorus.so the modification of substrate has become a focus for enhancement of phosphorus removal in constructed wetland.From the perspective of substrate modification,the study has been undertaken to screen the Mg-containing substrates with high phosphorus adsorption ability for constructed wetland and to explored the characteristic of phosphorus adsorption,followed by the feasibility analysis of phosphorus removal in Mg-modified constructed wetland.The study also evaluated the performance of phosphorus removal in magnesia/magnesite modified CWs and the effect of magnesium on plants growth and microbial communities in the system,and illuminated the mechanism of phosphorus removal.For the disadvantage presented in magnesia modified CW,a series of modifications,such as aeration and acidic substrate addition,were introduced to optimize this system and to evaluate the feasibility from the performance of pollutants removal,plant and microbial growth and nitric oxide release.The main results are as followed:1.The results of isothermal adsorption experiment showed that,the adsorption of magnesia and magnesite on phosphorus was suitable for the Langmuir and Freundlich isothermal adsorption equation,and the maximum theoretical adsorption capacity was 11.20 mg/g and 0.89 mg/g,respectively.The phosphorus adsorption by dolomite and talc were only described by Freundlich isothermal.With the increase of time,the adsorption rate of magnesia and talc showed an increasing trend,then reached saturation after a period of time.The adsorption rate of magnesite was significantly increased after 60 min,while the phosphorus adsorption rate of dolomite presented a slow slope during the whole experiment.Phosphorus adsorption kinetics experiments show that the pseudo second-order kinetics was observed to fit the adsorption processes of magnesia,magnesite,dolomite and talc.The rate constant k was 0.05 min-1,0.002 min-1,0.09 min-1,0.19 min-1,respectively.The adsorption capacity of magnesia,magnesite,dolomite and talc was increased with the increasing of initial concentration.Because the adsorption is the exothermic process,the low temperature was more favorable to phosphorus adsorption on magnesia,magnesite,dolomite and talc.With the increase of pH,the adsorption capacity of dolomite and talc decreased,while the adsorption capacity of magnesia and magnesite was increased.2.Results showed that the average TP removal efficiency of magnesia group(MA-CW)reached 93.3%,which was 17.5%and 18.6%higher than that of magnesite group(ME-CW)and control group(QZ-CW),respectively.P fractionation analysis showed that the proportion of phosphorus sequestration by substrate in MA-CW was 90.4%,while it was only 65.1%and 58.8%in ME-CW and QZ-CW,respectively.Phosphorus assimilation by plant in ME-CW accounted for 9.52%,higher than other groups,which was attributed to the stimulated plant growth caused by Mg addition.The average total chlorophyll content of plants in ME-CW was 4.61 mg g-1 FW during the study period and the plant growth rate was superior to other groups,whereas in MA-CW,excessive alkaline(pH = 10.29)caused by MgO dissolution depressed plant growth.Mg addition also elevated the absolute abundance of(PAOs),which followed the order of MA-CW(1.76±0.03×107 copies g-1)>ME-CW(1.35±0.02×107 copies g-1)>QZ-CW(4.48±0.13×106 copies g-1).Characterization of magnesia and magnesite before and after phosphate adsorption was compared and the results indicated that phosphorus adsorption by magnesium mainly relied on co-precipitation of Mg3(PO4)2 and MgHPO4,as well as ligand-exchange on the surface.3.Peat group showed good potential for enhancement of contaminant purification,plant and microbial growth and reduction of nitrous oxide release.The addition of peat provided a sufficient carbon source for the denitrification process,so the removal of nitrate,nitrite and total nitrogen was superior to that in magnesia group.Intermittent aeration provided aerobic environment,which needed by the process of ammonia nitrogen removal.The oxidation from nitrite to nitrate facilitated by enough oxygen reduced the accumulation of nitrite.However,compared to the peat group,aeration group could not fundamentally achieve total nitrogen removal;In addition,both peat and aeration groups could accomplish phosphorus removal effectively.The total chlorophyll content,fresh biomass and variation of antioxidant enzyme activity in peat group showed that,under alkaline condition peat was more favorable to plant growth.The activity of plant antioxidant enzyme in peat group showed a high growth trend as a respond for adversity in system.The abundance of functional genes has improved.Compared to other groups,the release flux of nitrous oxide in peat group is 4.97 mg N m-2 h-1,which had an advantage in reducing the release flux of nitrous oxide. |
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