| Microplastics,as one of emerging pollutants in the water environment,have a significant impact on the migration and transformation of heavy metals,and the combined process of coagulation(CF)and ultrafiltration(UF)has been widely studied and applied due to its advantages of efficient purification and membrane pollution alleviating.Firstly,this work investigated the carrying of pristine polypropylene microplastics(PP-pris)and aged by acid(PP-acid)and alkali(PP-alka)for heavy metals Pb(Ⅱ)and Mn(Ⅱ)by the models of kinetics,isotherms and modern characterization techniques.The influences of different pH,co-existing ions,natural organic matter and turbidity particles on adsorption capacity were studied.Secondly,based on the above researches,the influences of the adsorption behavior of microplastics on removal of heavy metals,turbidity,humic acid and microplastics by CF-UF process were investigated.Finally,the effects of aging methods,concentration and adsorption behavior of microplastics on the morphology of flocs and the mechanisms of membrane pollution in CF-UF process were studied,and then the mechanisms of microplastics and carrying effects in CF-UF process were comprehensively evaluated.By studying the adsorption capacity of PP-pris microplastic for Pb(Ⅱ)and its effects on CF-UF process for removing pollutants,it was found that adsorption of PP-pris for Pb(Ⅱ)mainly was dominanted by interacteing with oxygen-containing functional groups of PP-pris,and affected by environmental factors such as pH,co-existing ions and natural organic matter.The samples of adsorption and un-adsorption was carried out by combined process of CF-UF,it was found that coagulation process itself presented high removal effect on Pb(Ⅱ)with low concentration of PP-pris.The removal of Pb(Ⅱ)was further improved because of the adsorption behavior of PP-pris for Pb(Ⅱ)with the high concentration of microplastics(more than 0.5 g/L).The density of flocs containing lots of PP-pris was relatively low,and the sedimentation performance decreased accordingly.The turbidity removal on the coagulation process were poor,and the turbidity of effluent water was about 15 times higher than that of raw water,but could be reduced less than 1.0 NTU after ultrafiltration;PP-pris may release dissolved organic matter,which performed strong inhibition effect on CF-UF process.As the concentration of PP-pris increased,the UV254 value of effluent water increased significantly.After 360 min of adsorption,the UV254 value with high concentration of PP-pris increased to 2.33-4.16 times of that of raw water.The removal of microplasticson the process of CF-UF was investigated based on weighing method,it was found that removal with low concentration PP-pris(0-0.5 g/L)by CF process was more than 80%,and the formation of flocs was inhibited by high concentration PPpris(0.5-2.0 g/L),which made it fading away for removing microplastics by CF-S process.However,the removal of microplastics by UF process in all experimental groups were more than 95%.Compared with PP-pris,PP-alka microplastics presented better adsorption effect on Mn(Ⅱ),which was mainly dominated by electronic exchange,complexation or coordination,and was also greatly affected by environmental factors such as pH,co-existing ions and natural organic matter.The samples of adsorption and un-adsorption was carried out by combined process of CF-UF,it was found that Mn(Ⅱ)existed as free ions and formed precipitation or hydroxyoxygen colloid difficultly.Due to steric hindrance effect of humic acid,Mn(Ⅱ)could not be captured by flocs through electrostatic forces or complexation.Although the adsorption capacity of PP-alka was better for Mn(Ⅱ)than PP-pris,the removal of Mn(Ⅱ)by CF-UF process was not improved significantly.Compared with PP-pris,low-concentration PP-alka exhibited certain promoting effect on turbidity removal on the process of coagulation.However,the microplastics of adsorption 360 min were more likely to combine with flocs,the content of low-density microplastics in flocs was increased and sedimentation performance decreased.Therefore,the adsorption behavior weakened the turbidity removal efficiency of cogulation process.Fortunately,ultrafiltration process could still remove more than 99%of turbidity particles.The removal of UV254 by CF-UF process decreased with the increasing of PPalka microplastics concentration,but was above 80%and better that the group containing PP-pris.The removal of microplastics was effective on the process of coagulation with concentration below 0.5 g/L,but it was difficult for flocs to settle with higher concentration,and the microplastic content in effluent increased,while the subsequent ultrafiltration removal effect could still be more than 95%.The effects of carrying of PP-pris and PP-alka on membrane fouling mechanisms of CF-UF process were investigated.It was found that low concentration of PP-pris(0-0.2 g/L)could promote the growth of flocs,which could be deposited on the membrane surface,alleviating membrane pollution and maintaining good permeability of ultrafiltration membrane.However,high concentration of PP-pris(0.2-2.0 g/L)released a large amount of dissolved organic matter in the process of water treatment,which inhibited the growth of flocs on the process of coagulation and caused more severe membrane resistance,thus reduced the drainability performance of the ultrafiltration membrane;With the different from PP-pris,the particle size of flocs increased gradually with the increasing of PP-alka concentration,but it was not conducive to form large flocs with the concentration of higher than 0.2 g/L.It was noteworthy that the average particle size(d50=404-458 μm)of flocs formed by PP-alka after adsorption in CF process at all concentration levels was larger than that of the experimental group without microplastics(d50=346 μm),which may be due to the relatively uniform distribution of PP-alka in the flocs.The flocs with larger particle size were formed in the effects of catching-sweeping and charge neutralization,and the anti-compressibility increased accordingly.Therefore,the cake layer formed by the flocs deposited on the membrane surface could play a significant role in intercepting pollutants,and further alleviating membrane pollution. |
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