微塑料对短流程膜工艺中膜污染的影响

Effects of Microplastics on Membrane Fouling During a Shortened Ultrafiltration Membrane Process

微塑料作为新型污染物越来越受到关注.随着微塑料在饮用水源地逐渐检出,亟需了解当前水处理工艺对其去除效能与机制.随着膜法饮用水处理技术的发展,短流程膜工艺以其占地面积小、去污效能高成为重要研究方向.因此,考察了微塑料对短流程膜工艺尤其膜污染的影响.结果表明,微塑料混凝前后,滤饼层始终是引起膜污染的关键诱因.超滤膜由于孔径小(d<0.1μm),微塑料(d<5 mm)本身不会引起严重膜污染.然而,与铁盐混凝后,由于絮体的存在使得滤饼层相对疏松,但随着混凝剂投量增加,小粒径微塑料容易进入絮体形成的网络空间,形成致密滤饼层,严重加剧膜污染.pH7.0时0.1 mmol·L^-1和0.9 mmol·L^-1FeCl3·6H2O水解絮体导致的膜比通量分别为0.82和0.76.然而,0.1 g小粒径微塑料(d<0.5 mm)分别与0.1 mmol·L^-1和0.9 mmol·L^-1FeCl3·6H2O混凝后导致的膜比通量分别降低至0.76和0.62.此外,水环境中微塑料多呈负电.与碱性环境相比,氯化铁水解絮体在酸性环境中呈正电且粒径较小,微塑料容易被絮体吸附、捕获,进而形成相对致密滤饼层,引起严重膜污染.pH 6.0和8.0时,0.1 g小粒径微塑料(d<0.5 mm)与0.3 mmol·L^-1FeCl3·6H2O混凝后膜比通量分别为0.55和0.79.

Microplastics have garnered much attention worldwide as a new emerging pollutant.As they are gradually detected in freshwaters,understanding how microplastics will behave during current drinking water treatment processes is urgently needed.In recent years,the shortened process with an ultrafiltration(UF)membrane has shown excellent performance because of its low land use and high water purification efficiency.In this work,the membrane performance induced by microplastics was investigated with a shortened UF membrane process.The results showed that membrane fouling was always induced by the cake layer before and after coagulating with microplastics.Owing to the small UF membrane pore size(d<0.1μm),slight membrane fouling was caused by microplastics(d<5 mm)alone.However,although the loose cake layer was formed because of the existence of flocs,the cyberspace formed by flocs was easily entered by small microplastics with increasing coagulant dosage.As a result,server membrane fouling was induced because of the formation of a dense cake layer.It was shown that the specific membrane flux induced by flocs alone was 0.82 and 0.76 in the presence of 0.1 mmol·L^-1 and 0.9 mmol·L^-1 FeCl3·6H2O,respectively.However,after coagulation the specific membrane fouling induced by the 0.1 g small microplastics(d<0.5 mm)was 0.76 and 0.62 with 0.1 mmol·L^-1 and 0.9 mmol·L^-1 FeCl 3·6H2O,respectively.In addition,microplastics were always negatively charged in water.In comparison with alkaline conditions,Fe-based flocs were positively charged under acidic conditions,which were also much smaller.Therefore,microplastics were more easily adsorbed by Fe-based flocs under acidic conditions,leading to severe membrane fouling because of the dense cake layer formed.After coagulating with 0.3 mmol·L^-1 FeCl3·6H2O,the specific membrane flux induced by 0.1 g small microplastics(d<0.5 mm)was 0.55 and 0.79 at pH 6.0 and 8.0,respectively.