Study On Fouling Mechanisms Of Nanofiltration Membrane Induced By Complex Micropollutants In Surface Water

As an effective and cheap pretreatment technology,ultrafiltration has been widely used in commercial applications.According to statistics,its filtrate is more than 100 million m~3·day^-1.It is found that there still exists trace amount(0.6-3 mg·L^-1)of residual small-sized natural organics and almost completely penetrated heavy metals and hardness ions in ultrafiltration filtrate.Wherein,the preserved humic acid components after ultrafiltration filtration（PHACUF）are precursor of disinfection by-products,which is a potential risk for water quality and human health.Subsequently,in advanced treatment processes,PHACUF-induce nanofiltration membrane fouling,particularly in the presence of abundant divalent cations like Ca²⁺and Mg²⁺,is usually selectively neglected.Meanwhile,there also exist trace level pharmaceutically active compounds（PhACs）in natural surface water,which are difficult to be effectively removed by the existing activated sludge technology,and their continuously input into the environment produces a variety of complex toxicological effects.Therefore,it is of great significance to further explore the interaction between these composite micro pollutants and the resulting NF membrane fouling behaviors as well as influence mechanisms,so as to continuously promote the reuse of wastewater treated by membrane method,especially to strengthen the cascade and safe utilization of reclaimed water.The content of this paper mainly includes the following two aspects:（1）Based on the in-depth study of the properties of PHACUF,the fouling mechanisms of PHACUF on NF membrane with polyamide and polyethersulfone as functional layer materials under different Mg²⁺/Ca²⁺concentrations were comparably illustrated from macro and micro levels.PHACUF takes up only 22.2%but contributes 26.9%of raw humic acid DOC content and zeta potential,respectively,which means that the former has a higher charge density.Among the five fractions of PHACUF,the hydrophobic acid and hydrophobic neutral substance are the two components with the highest proportion that separately occupied 33.8%and 27.0%.There exists unneglectable standard blocking for polyethersulfone membrane when permeating bare PHACUF solution.However,with the addition of divalent cations,the main fouling mechanism changed to intermediate blocking.According to the isothermal titration calorimetry analysis,the enthalpy change and binding constant of Mg²⁺/Ca²⁺with PHACUF were-0.932k J·mol^-1,1.74×10~3 L·mol^-1 and-0.589 k J·mol^-1,2.83×10~3 L·mol^-1,respectively,it could be induced that fewer Mg²⁺ions could fully link with the carboxylic groups of PHACUF molecules,leading to larger and more dense flocs by the“bridging”effect and the aggravated nanofiltration membrane fouling with more compact layer structure,especially for more hydrophobic polyethersulfone membrane.It could be inferred that in Mg²⁺/Ca²⁺environment,PHACUF-induced nanofiltration membrane fouling was mainly formed and constituted by hydrophobic acids and neutral fractions with much higher both hydrophobicity and content percentage,whether for the membrane-foulant interaction in the early stage or the foulant-foulant interaction in subsequent stage.（2）Based on the results above,for the selected pharmaceutically active compounds（PhACs）that are difficult to be removed in natural surface water,the removal ratio of all target PhACs under multiple influent matrices（small-sized natural organics and divalent cations）circumstance by self-made NF membrane is about 94%.Static adsorption rates of the pristine NF membrane for all PhACs are less than 6%,while the adsorption ratio of selected PhACs under two kinds of background matrices（simulated/natural small-sized organics and divalent cations）are all higher than 50%,except for negatively charged sulfamethoxazole.In the background matrices,the interaction of solute-solute was greater than that of solute-membrane.Under the simulated/natural micro pollutant circumstance,due to the negative charge property,the formed fouling layer acted as an extra separation layer for positively charged PhACs.Meanwhile,the foulant changed the inherent characteristics of NF membrane functional layer with reduced positive charge and increased hydrophobicity.The enlarged electrostatic repulsion and weakened hydrophobic interaction between sulfamethoxazole and NF membrane increased its removal ratio.However,self-made NF membrane only have cake layer enhanced concentration polarization effect for neutral PhACs.The removal mechanisms of self-made NF membrane separation layer to selected target PhACs follows:electrostatic effect>adsorption effect>steric effect.