腐殖酸聚集体对膜蒸馏过程膜污染的作用机理

Effect of aggregated humic acid on membrane fouling in direct contact membrane distillation

膜污染是膜蒸馏过程应用于工业水处理中遇到的主要问题之一。选取水体中具有代表性的有机物(腐殖酸)、微溶无机盐(碳酸钙)作为典型污染物,研究有机腐殖酸聚集体对于膜蒸馏过程膜污染进程的影响规律,探讨天然有机物-无机微溶盐混合水体中腐殖酸聚集体对于无机盐结晶过程的控制机理。结果表明:膜蒸馏通量的衰减大致可分为由滤饼层的形成造成的不可恢复部分以及由浓差极化、膜孔'半润湿'而造成的可部分恢复的通量降低。Ca2+通过加速腐殖酸分子的聚集过程,使表面负电性降低的腐殖酸聚集体率先吸附在聚偏氟乙烯中空纤维膜内表面,形成有机基质污染层;碳酸钙在有机腐殖酸聚集体的诱导下在膜内表面异相成核,最终成长为稳定的晶体。腐殖酸聚集体为无机盐晶体在疏水性膜内表面的生长提供了异相成核的基础。可通过控制污染水体中有机物的含量控制微溶碳酸钙在膜内表面成核及生长,实现控制其在膜内表面附着进而诱发疏水膜发生亲水化的过程。

Membrane fouling is one of the main obstacles for application of membrane distillation（MD）process.In this work,the most familiar organic solute,humic acid（HA）,and inorganic slightly soluble salt,calcium carbonate（CaCO3）,were selected as contaminants to study membrane fouling during MD process.The properties of feed solution during MD process,such as zeta potential and particle size distribution,were measured and used for illustrating the variation of feed solution properties.The variation of membrane surface morphology and contaminant chemical composition during MD process were characterized by Field emission scanning electron microscopy（FESEM）and X-ray photoelectron spectroscopy（XPS）.Severe flux decline was observed for hybrid feed solution during DCMD,and could be roughly attributed into three reasons：（1）A considerable amount of HA aggregates aroused by concentration polarization in the feed decreased diffusion coefficient i.e.relative low mass transfer coefficient,of H2O molecules in boundary layer;（2）Fouling layer as filtration cake on membrane,consisting of deposits of HA aggregates and CaCO3,made mass transport resistance gradual increase;（3）As though all through DCMD manipulations any wetting was not detected（according to the permeate conductivity variations）,long-term DCMD operation may also induce pore wetting gradually especially in the pores located on membrane surface.At the time surface pores were filled with the feed,pores located inside membrane may still wonderfully dry,and this ＂semi-leakage＂ is of recoverability when process was shutdown.The effects of divalent metal counter ion Ca^2＋ of HA and its aggregation foulant on precipitation of CaCO3 onto membrane surface were studied.The results showed that Ca^2＋ could accelerate HA aggregation,HA aggregates with lower electronegativity adsorb preferentially onto hydrophobic poly（vinylidene fluoride）membrane surface and form organic contaminant layer,and CaCO3 could nucleate heterogeneously on the inner surface of the organic contaminant layer on hydrophobic membrane and crystal grows,and so modify crystal morphology.