Towards a better hydraulic cleaning strategy for ultrafiltration membrane fouling by humic acid: Effect of backwash water composition

As a routine measurement to alleviate membrane fouling, hydraulic cleaning is of great significance for the steady operation of ultrafiltration (UF) systems in water treatment processes. In this work, a comparative study was performed to investigate the effects of the composition of backwash water on the hydraulic cleaning performance of UF membranes fouled by humic acid (HA). Various types of backwash water, including UF permeate, Milli-Q water, NaCl solution, CaCl₂ solution and HA solution, were compared in terms of hydraulically irreversible fouling index, total surface tension and residual HA. The results indicated that Milli-Q water backwash was superior to UF permeate backwash in cleaning HA-fouled membranes, and the backwash water containing Na⁺ or HA outperformed Milli-Q water in alleviating HA fouling. On the contrary, the presence of Ca^2 + in backwash water significantly decreased the backwash efficiency. Moreover, Ca^2 + played an important role in foulant removal, and the residual HA content closely related to the residual Ca^2 + content. Mechanism analysis suggested that the backwash process may involve fouling layer swelling, ion exchange, electric double layer release and competitive complexation. Ion exchange and competitive complexation played significant roles in the efficient hydraulic cleaning associated with Na⁺ and HA, respectively.     

Fatty acid fouling of forward osmosis membrane: Effects of pH, calcium, membrane orientation, initial permeate flux and foulant composition

Octanoic acid (OA) was selected to represent fatty acids in effluent organic matter (EOM). The effects of feed solution (FS) properties, membrane orientation and initial permeate flux on OA fouling in forward osmosis (FO) were investigated. The undissociated OA formed a cake layer quickly and caused the water flux to decline significantly in the initial 0.5 hr at unadjusted pH 3.56; while the fully dissociated OA behaved as an anionic surfactant and promoted the water permeation at an elevated pH of 9.00. Moreover, except at the initial stage, the sudden decline of water flux (meaning the occurrence of severe membrane fouling) occurred in two conditions: 1. 0.5 mmol/L Ca^2 +, active layer facing draw solution (AL-DS) and 1.5 mol/L NaCl (DS); 2. No Ca^2 +, active layer-facing FS (AL-FS) and 4 mol/L NaCl (DS). This demonstrated that cake layer compaction or pore blocking occurred only when enough foulants were absorbed into the membrane surface, and the water permeation was high enough to compact the deposit inside the porous substrate. Furthermore, bovine serum albumin (BSA) was selected as a co-foulant. The water flux of both co-foulants was between the fluxes obtained separately for the two foulants at pH 3.56, and larger than the two values at pH 9.00. This manifested that, at pH 3.56, BSA alleviated the effect of the cake layer caused by OA, and OA enhanced BSA fouling simultaneously; while at pH 9.00, the mutual effects of OA and BSA eased the membrane fouling.     

XDLVO理论解析不同离子条件下海藻酸钠微滤膜污染

应用extended Derjaguin-Laudau-Verwey-Overbeek(XDLVO)理论定量解析海藻酸钠微滤膜污染中界面微距作用力,探讨不同离子条件对膜污染影响的主控机制.结果表明,对于亲水性膜表面,范德华力促进膜污染,极性作用力阻碍膜污染;而对于疏水性膜表面,范德华力减缓膜污染,极性作用力加速膜污染.海藻酸钠与微滤膜接触时双电层作用力对膜污染贡献相对微弱.离子强度及钙离子对海藻酸钠微滤膜污染的影响主要是通过改变极性作用力来实现的.较高的离子强度会降低极性作用力的排斥性或升高极性作用力的吸引性,从而加重海藻酸钠微滤膜污染.虽然钙离子使出水通量急速下降,但钙离子的存在使得物理清洗效率提高.不同离子条件下初始阶段及后期阶段污染趋势K分别与膜-海藻酸钠粘附自由能及海藻酸钠-海藻酸钠粘聚自由能具有良好的线性相关性,表明XDLVO理论适用于描述不同离子条件下海藻酸钠微滤膜污染行为.     

蛋白质及无机离子对纳滤膜去除PFOS的影响

以全氟辛烷磺酸（PFOS）为目标去除物,选取牛血清蛋白（BSA）为典型蛋白质类有机物,考察了BSA及其浓度,以及BSA与无机离子共存时,离子强度、离子种类对聚酰胺纳滤膜去除水中PFOS的影响.研究发现,原液中存在BSA时,PFOS的去除率有显著提高,而且BSA浓度越高,PFOS的去除率越高;当BSA与无机离子共存时,离子强度越大,PFOS去除率越高.这可能是因为BSA不但会吸附一部分PFOS,还会造成膜污染,BSA浓度越大,膜污染越严重,膜的筛分能力越强,且膜面与PFOS之间的静电排斥力越大,从而提高了PFOS的截留率.而无机离子的存在减小了BSA分子之间及与膜面之间的静电排斥力,使BSA污染层更加厚实,进一步增强了膜的筛分能力.此外,Ca²⁺提高PFOS去除率的能力优于Na⁺.     

基于膜特征参数变化的蛋白质超滤过程膜污染研究

为进一步研究多种蛋白质体系超滤过程的膜污染机制,采用切割相对分子质量为50×103的聚醚砜(polyethersulfone,PES)超滤膜,对溶菌酶(lysozyme,LYS)、牛血清蛋白(bovine serum albumin,BSA)、LYS+BSA等3种不同蛋白质溶液的超滤过程进行了研究.运用接触角仪、场发射扫描电镜(field emission scanning electron microscope,FESEM)、原子力显微镜(atomic force microscope,AFM)测定了不同污染阶段膜特征参数的变化.结果表明,超滤膜通量变化明显呈现3个阶段:初期(约0~5min)衰减迅速、中期(约5~60 min)衰减缓慢、后期(约60~120 min)趋于稳定;整个超滤过程中,LYS污染膜的通量衰减幅度最大,LYS+BSA次之,BSA最小.膜特征参数变化表明:LYS对膜的初期污染主要以膜孔窄化为主,中期污染由膜孔堵塞和膜孔窄化共同控制;BSA初期膜污染以膜孔堵塞为主,中期污染以膜孔窄化为主;滤饼层过滤是BSA、LYS后期膜污染的主要机制.LYS+BSA二元混合溶液中的LYS对膜污染的产生起主导作用.     

阳离子强度对鼠李糖脂作用下三氯生在底泥-水相中分配的影响

探讨了阳离子强度(Na~+/Ca~(2+))对鼠李糖脂(RL)胶束形态的影响,并据此阐述了阳离子强度对RL作用下三氯生(TCS)在底泥-水相中分配的影响规律.结果表明,阳离子对TCS的表观溶解度和其在底泥-水中的分配无明显作用,但能通过与RL分子间产生的盐桥作用影响RL的胶束聚集程度及溶液的稳定性,使水相中的RL浓度发生变化,进而导致RL作用下TCS的表观溶解度及其在底泥-水相中分配的差异.随着阳离子强度的增加,RL胶束粒径在Na~+溶液中先减小后增大,而在Ca~(2+)溶液中持续减小;RL溶液的zeta电位在Na+/Ca~(2+)溶液中逐渐减小,其溶液的稳定度下降.相比Na~+,Ca~(2+)与RL分子间的盐桥作用更强.低浓度Na~+(50 mmol·L~(-1))可促进RL胶束的形成,提高其对TCS的增溶能力,促进TCS从底泥迁移至水相,提高了TCS在水相的分配比例(K*d值从64.16 L·kg~(-1)降低到40.81 L·kg~(-1)(30 mmol·L~(-1)Na~+));但高浓度Na~+(50 mmol·L~(-1))会因盐析作用降低TCS的表观溶解度,并抑制其在RL作用下向水相的迁移.而Ca~(2+)在很低浓度下(1 mmol·L~(-1))就能与RL分子产生强的盐桥作用而生成紧密的胶束聚集体,致使RL沉淀,大大降低RL对TCS的增溶作用,反而提高TCS在底泥相的分配比例(K*d值从64.16 L·kg~(-1)提高到4987.76 L·kg~(-1)(30 mmol·L~(-1)Ca~(2+))).     

中亚热带常绿阔叶林湿沉降过程中盐基离子变化特征

基于野外实验和室内实验相结合的方法,于2012年9月至2013年8月对四面山常绿阔叶林大气降水、穿透水、地表枯透水、土壤渗滤液进行了持续一年的森林不同层次盐基离子季节动态变化特征的研究.结果表明,四面山大气降水全年pH平均值为4.90,最大值为5.14,大气降水明显偏酸性;土壤层和林冠层能使降雨的p H值有所升高,其中土壤层对p H值的调升幅度最大,其次为森林冠层;森林冠层只对Na~+有一定的截留作用,降雨能够淋溶森林冠层的Ca~(2+)、Mg~(2+)、K~+;研究结果也表明,枯枝落叶的降解是导致各盐基离子质量浓度在枯透水中增加的主要原因;枯透水下渗的过程淋溶了森林土壤中的Ca~(2+)、Mg~(2+)、Na~+.四面山大气降水经过森林生态系统后Ca~(~(2+))、Mg~(2+)、Na~+、K~+的总截留率分别为33.82%、-7.06%、74.36%、42.87%,森林生态系统对Ca~(2+)、Na~+、K~+表现出了截留作用,其中对Na~+截留率最大.     

沙尘天气对兰州市PM10中主要水溶性离子的影响

利用在线监测仪器MARGA在兰州大学盘旋路校区对兰州市大气PM10中水溶性离子进行监测,监测期间(2011-04-01~2011-06-30)有15 d出现沙尘天气.兰州市PM10中主要水溶性离子物种为Ca2+、SO2-4和NO-3.扬沙天气期间NO-3和NH+4的浓度比非沙尘期间低,说明沙尘天气对当地人为源所排放污染物具有清除作用.沙尘天气期间,作为土壤污染源标识物的Mg2+、Na+和Ca2+离子都有明显增加,Na+和Mg2+相关系数为0.520,Na+和Ca2+相关系数为0.659,Mg2+和Ca2+相关系数为0.671,而非沙尘天气期间三者的相关系数并不高,Na+和Mg2+相关系数为0.065,Na+和Ca2+相关系数为0.131,Mg2+和Ca2+相关系数为0.163,说明沙尘天气期间三者之间具有相同的污染源,主要来自于土壤风沙尘,而非沙尘天气期间三者来源不同.Cl-的浓度在扬沙天气明显高于浮尘和非沙尘天气期间,说明外来的土壤风沙尘是Cl-的主要来源.     

Japanese Papilio butterflies puddle using Na+ detected by contact chemosensilla in the proboscis

Many butterflies acquire nutrients from non-nectar sources such as puddles. To better understand how male Papilio butterflies identify suitable sites for puddling, we used behavioral and electrophysiological methods to examine the responses of Japanese Papilio butterflies to Na⁺, K⁺, Ca²⁺, and Mg²⁺. Based on behavioral analyses, these butterflies preferred a 10-mM Na⁺ solution to K⁺, Ca²⁺, and Mg²⁺ solutions of the same concentration and among a tested range of 1?mM to 1?M NaCl. We also measured the ion concentrations of solutions sampled from puddling sites in the field. Na⁺ concentrations of the samples were up to 6?mM, slightly lower than that preferred by butterflies in the behavioral experiments. Butterflies that sipped the 10?mM Na⁺ solution from the experimental trays did not continue to puddle on the ground. Additionally, butterflies puddled at sites where the concentrations of K⁺, Ca²⁺, and/or Mg²⁺ were higher than that of Na⁺. This suggests that K⁺, Ca²⁺, and Mg²⁺ do not interfere with the detection of Na⁺ by the Papilio butterfly. Using an electrophysiological method, tip recordings, receptor neurons in contact chemosensilla inside the proboscis evoked regularly firing impulses to 1, 10, and 100?mM NaCl solutions but not to CaCl₂ or MgCl₂. The dose?Cresponse patterns to the NaCl solutions were different among the neurons, which were classified into three types. These results showed that Japanese Papilio butterflies puddle using Na⁺ detected by the contact chemosensilla in the proboscis, which measure its concentration.     

XDLVO理论解析钙离子对腐殖酸反渗透膜污染的影响机制

以XDLVO（extended Derjaguin-Landau-Verwey-Overbeek）理论定量解析腐殖酸反渗透（reverse osmosis,RO）膜污染过程中的界面相互作用,分析3种界面相互作用力在膜污染过程中的贡献,重点探究不同溶液pH条件下Ca2＋对腐殖酸RO膜污染的影响.结果表明,无论Ca2＋是否存在,对膜污染起促进作用的范德华力在pH 3时为主导作用力,而减缓膜污染的极性作用力在pH 7及pH 10时成为主控因素.静电力在所有测试条件下作用微弱,因此对膜污染贡献很小.Ca2＋主要通过改变极性作用力影响腐殖酸RO膜污染行为,且其影响随溶液pH值的降低而凸显,多数情况下Ca2＋会促进腐殖酸RO膜污染.界面自由能与膜污染程度相关性分析表明,XDLVO理论可以合理预测不同溶液条件下腐殖酸RO膜污染行为.