Removal Of Disinfection Byproducts In Reclaimed Water By Nano-iron Composites:Efficiency,Application And Mechanisms

The high concentration of disinfection byproducts in reclaimed water affects the water supply safety of downstream drinking water plants and endangers human health.To solve this problem,Microcrystalline cellulose/nano-iron composites(MCC/NZVI)and fucoidan/nano-iron composites(FD/NZVI)were prepared by liquid phase reduction method.In this study,the removal efficiency and reaction kinetics of disinfection byproducts and their precursors by nano-iron composites in simulated water samples and municipal secondary effluent was studied.In addition,the effects of nano-iron composites on the removal of disinfection byproducts in municipal secondary effluent by soil and constructed wetlands was investigated when nano-iron composites were added into soil and constructed wetlands.The effects of interaction between soil organic matters and nano-iron composites on thihalomethanes(THMs)removal and the effects of nano-iron composites on plants in constructed wetlands were discussed.Furthermore,this study initially revealed the removal mechanism of disinfection by-products by nano-iron composites in complex environments by analysis of kinetic parameters of reaction kinetics and formation kinetics of degradation products.The main research conclusions are as follows:(1)MCC/NZVI could remove some of the disinfection byproduct precursors in water,which reduced the formation potential of THMs and dimethylnitrosamines(NDMA)by about 59%in simulated water samples,making the THMs and NDMA formation potential of municipal secondary effluent respectively decreased by 79.76%and 42%.MCC/NZVI could also remove some of the disinfection byproducts in water.The removal efficiency of THMs and NDMA in the simulated water sample could reach 62.6%and 29.3%,respectively,while the removal efficiency of THMs and NDMA in the municipal secondary effluent could be 74.5%and 42.6%.The removal of the disinfection byproducts and their precursors by MCC/NZVI were agreed with the first-order reaction kinetics,and its removal rates were faster than those of nanoscale zero valent iron(NZVI).What's more,the removal of disinfection byproducts and their precursors in simulated water samples was affected by MCC/NZVI dosage and the initial pH of water samples:When the precursors were removed,MCC/NZVI dosage of 2.5 g/L and pH of 11.0 were the best removal conditions;when removing disinfection byproducts,MCC/NZVI dosage of 2.5 g/L and pH of 5.0 were the best removal conditions.(2)There was a certain antagonism between the soil particles and the nano-iron composite,which inhibited the removal of CHCl3 in municipal secondary effluent by nano-iron composites and soil.When the FD/NZVI dosage was increased from 0.1 g/100 g soil to.0.4 g/100 g soil,the removal efficiency of CHCl3 in municipal secondary effluent by mixed soil system(soil mixed with FD/NZVI)decreased from 90.95%to 49.46%.The antagonism between soil particles and nano-iron composites slowed down the removal rate of CHCl3 and reduced the formation amount of CH2Cl2 produced in dehalogenation reaction of nano-iron.When the dosage of FD/NZVI was 0.4 g/100g soil,the reaction rate in mixed soil system(0.037 h-1)and the concentration of CH2Cl2(167.58 ?g/L)were only 11.9%and 55.9%,respectively,of those in mixed burning soil system(burning soil mixed with FD/NZVI)under the same conditions.(3)Nano-iron composites and constructed wetlands had a certain synergistic effect,which could enhance the removal efficiency of disinfection byproducts in municipal secondary effluent by both of them.When the dosage of FD/NZVI was 0.6 g/L,the removal efficiency of CHCl3 was 100%,which is 3.3 times the removal efficiency of constructed wetlands alone,and the reaction rate constant of CHCl3 was 0.22 h-1,which was 28 times as fast as CHCl3 removal rate in constructed wetland alone.The mutual reinforcement between the nano-iron composites and constructed wetlands also made the accumulated THMs degradation products(CH2Cl2 and CH2ClBr)to be further degraded.When the dosage of FD/NZVI was 0.6 g/L,the accumulated CH2Cl2(189.7 ?g/L)in constructed wetlands was gradually degraded to 30 ?g/L.In addition,the addition of FD/NZVI did not affect the uptake of THMs by plants in constructed wetlands,and the absorption of CHCl3,CHCl2Br,and CHBr2Cl by calamites was consistently around 300 ?g,240 ?g,and 35,?g.