2. 河北科技大学环境科学与工程学院, 石家庄 050018;
3. 放射性废物处理北京市重点实验室, 清华大学, 北京 100084
2. School of Environment Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018;
3. Beijing Key Laboratory of Radioactive Waste Treatment, INET, Tsinghua University, Beijing 100084
膜分离技术由于出水水质高、设备简单易操作、能耗相对较低、适应性强等特点,在水处理领域获得越来越多的关注(王建龙和刘海洋,2013).目前应用于水处理领域的几种膜分离技术.其中微滤(microfiltration,MF)、超滤(ultrafiltration,UF)、纳滤(nanofiltration,NF)和反渗透(reverse osmosis,RO)由机械压力驱动传质过程,是水和废水处理的常规技术.其他膜技术,如温度差驱动的膜蒸馏技术(membrane distillation,MD),电场驱动的电渗析技术(electro-dialysis,ED),一些由化学反应驱动的膜吸收技术(membrane absorption,MA)等也成为水处理领域的新型技术.正渗透(forward osmosis,FO)是一种由渗透压(浓度差)驱动的新型膜技术.可用于海水脱盐、废水处理等方面.
FO膜是一种渗透膜.名义孔径在1 nm以下,用于截留溶解性离子和盐类等物质,与RO相当(Zhao et al.,2012).但与RO相比,FO无需外加机械压力,具有低压操作、低膜污染、高截留的优点(Chung et al.,2012;Zhao et al.,2012),近年来在水处理领域受到较多关注.
2 FO原理(Basic principle of FO)FO膜是一种选择性渗透膜,膜的一侧是低渗透压的待处理水,另一侧是高渗透压的汲取液,水分子透过FO膜从低渗透压侧扩散到高渗透压侧,从而实现水与杂质的分离(图 1).该过程的驱动力是膜两侧溶液的渗透压差,不需外界提供压力(严滨等,2014;邹士洋等,2008;钟铭,2012).
FO已被用于含盐废水(Phuntsho et al.,2013;Alnaizy et al.,2013;Razmjou et al.,2013;Chanukya et al.,2013;Li et al.,2013;Sairam et al.,2011;Tan and Ng,2010;Choi et al.,2009;McGinnis and Elimelech,2007;McCutcheon et al.,2006;McCutcheon et al.,2005)、含盐地下水(Phuntsho et al.,2014)、盐湖水(Zhao et al.,2012)和海水(Ling and Chung,2011)的脱盐.大多数为实验室规模的小试研究,汲取液采用难挥发性(NaCl,Na2SO4,MgSO4等)或挥发性(NH3/CO2和NH4HCO3)盐溶液.其中Zhao等进行的盐湖水脱盐,回收率达到70%(Zhao et al.,2012).McGinnis等采用中试规模的FO处理高盐水(TDS>70,000 ppm),回收率达到60%,与蒸发浓缩技术相当,出水水质达标(美国宾州地表水排放标准TDS < 500 mg·L-1,氯化物 < 250 mg·L-1,钡 < 10 mg·L-1钡,锶 < 10 mg·L-1)(McGinnis et al.,2013).
2.1.2 城市污水处理Li等采用实验室规模FO处理模拟城市径流,发现FO能保持较高的渗透水通量和截留率,实现稳定运行,微量金属离子、磷、硝酸盐和总氮的截留率可分别达到98%~100%、97%~100%、52%~94%和65%~85%,能够克服传统膜技术能耗高等缺陷(Li et al.,2014).Linares等也利用FO处理模拟城市污水,进行实验室规模的研究,多数微量金属离子的截留率均高于99%,COD和磷的截留率将近99%,氨和总氮的截留率可分别达到67%~68%和56%~59%(Linares et al.,2013).FO作为城市污水的一种处理方法,实现其稳定、连续和长期运行是重要目标.
2.1.3 污水深度处理Cath等利用生活污水处理厂的二级和三级处理单元的出水和被污染的地表水作为待处理水,通过FO技术来制备饮用水,其对有机化合物的截留率较高(双氯芬酸>99%,二甲苯氧庚酸>80%,萘普生>90%,水杨酸>72%).在放大规模的中试试验中,FO对氨、硝酸盐和紫外(UV)吸收类化合物的截留率分别达到74%、78%和85%(Cath et al.,2010);Yangali-Quintanilla等也采用二级出水(沙特阿拉伯的吉达鲁韦斯污水处理厂)作为待处理水,间接稀释红海的海水,能量消耗为1.5 kWh·m-3,低于RO工艺的能耗(2.5~4 kWh·m-3)(Yangali-Quintanilla et al.,2011).
2.1.4 特殊类型废水FO还被用于染料废水、太空废水、含油废水、含氯废水、垃圾渗滤液等特殊类型废水处理的研究中.Ge等对染料(酸橙8)废水进行浓缩处理,浓缩倍数达3(Ge et al.,2012);同样,Zhao等也对染料(活性艳红K-2BP)废水进行处理,截留率基本为100%(Zhao et al.,2015).此外,FO被应用到太空废水的回用,废水中所含的尿素被完全截留,废水的回收率达95%以上,(Cath et al.,2005a;Cath et al.,2005b).Hickenbottom等采用FO对原油和天然气开采时产生的钻井污泥及废水进行处理,水回收率为80%以上(Hickenbottom et al.,2013);Zhang等利用FO处理含油废水,废水中的油分子、NaCl和醋酸被截留,水回收率可达90%(Zhang et al.,2014).Kong等利用FO处理含氯废水中的九种卤乙酸,截留率达73.8%(Kong et al.,2014).Cath等提到Osmotek(现HTI公司)利用中试规模FO处理垃圾渗滤液,平均水回收率达91.9%,多数有害物的截留率高于99.0%,最终出水污染物排放量达到美国污染物排放系统规定的标准(Cath et al.,2006).
3 影响FO运行的因素(Affecting factors on FO performance) 3.1 膜FO膜是由支撑层和活性层构成的非对称性结构.支撑层使FO膜拥有较好的机械强度,结构相对疏松.而活性层相对于支撑层要薄,结构致密,透水性和截留性好,在FO分离过程中起关键作用(钟铭,2012).理想的FO膜需要满足以下要求:①高水通量和高盐截留量;②低浓差极化;③较强的耐酸碱性(Chung et al.,2012).
3.1.1 膜材料(制备与改性)最早的商业化的FO膜是美国HTI公司生产的三醋酸纤维素膜/醋酸纤维素膜(cellulose triacetate/cellulose acetate,CTA/CA).该纤维素膜有亲水性强,高水通量,低膜污染和高机械强度,耐氯等优点(McCutcheon et al.,2005;Mi and Elimelech,2008),但是易水解、耐酸碱性差(pH 3~8)(许春玲,2013;Lutchmiah et al.,2014).后来生产的复合薄膜(thin-film composite,TFC),材质是聚酰胺,该膜克服了前者的缺点,在pH为2~11都有较好的渗透性和稳定性,同时具有很好的耐压性(Lutchmiah et al.,2014).疏水性的支撑层提高了TFC膜的水通量,减小了内部浓差极化(McCutcheon and Elimelech,2008).
近年来研究者针对不同需求制备FO膜.采用相转化法制备CTA/CA膜.研究者发现影响制备的条件主要为环境湿度、凝胶浴温度、热处理温度(李丽丽和王铎,2012)、填充剂和退火温度(Sairam et al.,2011).也有研究者采用界面聚合法制备TFC膜,基膜为聚醚砜或聚砜,以间苯二胺(MPD)、均苯三甲酰氯(TMC)等作溶剂,在基膜上进行聚合得到复合膜(Yip et al.,2010;许春玲,2013).樊晋琼等通过超声将不同量的TiO2颗粒分散(约5 min)在水相或有机相中,采用界面聚合制备了TiO2/聚酰胺复合膜,膜的水通量是未添加TiO2膜的2倍,截盐率可以达到99.9%(樊晋琼等,2012).
此外,一些研究者为了缓解FO膜污染或提高FO截留效果对现有FO膜进行化学改性.Nguyen等采用两性离子氨基酸(L-DOPA)对CTA膜支撑层表面进行修饰,发现修饰后的膜具有更强的亲水性和防垢性,其污染程度要比未修饰膜低30%(Nguyen et al.,2013).Castrillón等用伯胺和聚乙二醇对TFC膜进行表面修饰,修饰后的膜防垢性同样增强,修饰后和未修饰的膜的水通量降低量分别是7.2%±2%和15.7%±5.3%(Castrillóna et al.,2014).Wang等通过对二苄氯和聚苯并咪唑的交联实现了对FO膜孔径的控制,使FO对氯化钠的截留率高达99.5%(Wang et al.,2007;Wang et al.,2009).
3.1.2 膜性质目前已知对FO运行效果有影响的膜性质主要为表面电荷(Zheng et al.,2015)、亲疏水性(马艳杰,2012)、粗糙度(Gu et al.,2013)、活性层及支撑层的厚度、孔隙率、弯曲度和孔结构(Jung et al.,2011;Zhao et al.,2012)等.FO膜表面往往带负电荷(Jin et al.,2012;Zheng et al.,2015),会与被截留的污染物产生静电(排斥或吸引)作用,从而影响污染物的去除(Zheng et al.,2015).疏水性相对较强的膜会使疏水的物质在膜表面沉积,形成污染层(马艳杰,2012).膜表面形貌(粗糙度)也会影响污染物与膜之间的作用力,从而对膜污染产生影响(Mi and Elimelech,2010;Parida and Ng,2013;Gu et al.,2013).而活性层及支撑层的厚度、孔隙率、弯曲度和孔结构则更多地影响内外部浓差极化,详见本文4.1节.
3.1.3 膜方向FO膜是非对称性膜,支撑层疏松,活性层致密.水处理过程中根据活性层朝向分为两种运行模式:活性层朝向原水(active layer facing the feed solution,AL-FS)和活性层朝向汲取液(active layer facing the draw solution,AL-DS)(Zhao et al.,2011;许春玲,2013;Phuntsho et al.,2013;Parida and Ng,2013;Zhao et al.,2014;Lutchmiah et al.,2014).膜方向对水通量、截留率和膜污染也有较大影响.
多数学者认为AL-FS比AL-DS模式具有优势.研究者在FO截留微量硼、砷(Jin et al.,2012)、药物(卡马西平,磺胺甲恶唑)(Xie et al.,2012)、卤乙酸(HAA)(Kong et al.,2014)、二级出水中所含有机物(腐殖酸,生物聚合物,小分子酸等)(Parida and Ng,2013)的研究中发现AL-DS模式下,水通量虽然高,但会发生严重的内部浓差极化(Jin et al.,2012; Xie et al.,2012)和膜污染(Parida and Ng,2013),因此降低污染物的去除率(Xie et al.,2012;Kim et al.,2012;Kong et al.,2014).
也有学者认为AL-DS模式更好.Zheng等采用FO对水中四环素进行去除的过程中发现当pH为7~8时,四环素带负电,由于FO膜支撑层所带的负电荷要高于活性层,在AL-DS模式下增强了膜与四环素之间的排斥作用,提高了其截留率(Zheng et al.,2015).
此外,Zhao等认为膜方向的选择决定于废水的成分,当处理高污染的废水(废水回用,膜生物反应器和食物蛋白的浓缩)或含盐量较高的水(海水脱盐和盐水浓缩)时,膜方向应选用AL-FS模式,可以减少膜污染,实现稳定和高水通量的运行;反之,则采用AL-DS模式(Zhao et al.,2011).
3.2 汲取液汲取液产生FO的推动力,对FO效率具有直接影响.汲取液再生一直是限制FO广泛应用的关键问题之一.汲取液的选取应该满足以下要求:高于原水的渗透压、易于再生、低返混扩散性、安全无毒、成本低、抗生物污损等(钟铭,2012;Zhao et al.,2012;Ge et al.,2013;Linares et al.,2014).
3.2.1 汲取液种类(1)无机盐类
质量分数小,水溶性强的无机盐,可以产生高的渗透压,使FO具有较高的水通量.如KHCO3(Achilli et al.,2010),NaHCO3(Achilli et al.,2010),KCl(Phuntsho et al.,2011;Phuntsho et al.,2013),NaNO3和KNO3(Phuntsho et al.,2011),NaCl(Gray et al.,2006;Wang et al.,2011;Zhang et al.,2014)等.
质量分数相对较高的无机盐,如NH4H2PO4、(NH4)2HPO4、Ca(NO3)2、(NH4)2SO4等,往往具有溶质返混通量较低的优势(Phuntsho et al.,2011).
从降低汲取液再生成本角度,一些无机盐具有自身特性.如MgSO4和Na2SO4,由于SO42-可以被NF膜截留,因此可采用NF替代RO进行再生(Sairam et al.,2011;Zhao et al.,2012;Tan and Ng,2010).NH4HCO3用作汲取液,通过适当加热,可生成氨气和二氧化碳(继续回用),从而得到较纯净的水,与现有的膜技术相比,能耗节省72%~85%(McCutcheon et al.,2005;McCutcheon et al.,2006;McGinnis and Elimelech,2007).CuSO4作为汲取液时,回收时采用与Ba(OH)2发生置换反应,生成Cu(OH)2和BaSO4沉淀的方法,此过程无能量消耗,而且生成的Cu(OH)2还可以通过与H2SO4反应得到CuSO4,继续作为汲取液重复使用(Alnaizy et al.,2013).某些含氮、磷、钾等无机盐是化肥中所含的主要成分,因此,被稀释后的汲取液可以直接进行农业灌溉,降低了回收再生的能耗费用(Phuntsho et al.,2011).另外,海水作为汲取液,被稀释的海水在进一步进行海水淡化时,能耗和成本大大降低(Bamaga et al.,2011;Yangali-Quintanilla et al.,2011).
选取汲取液时,还需考虑待处理水的成分.当待处理水中含有结垢的先驱物(Ba2+、Ca2+、Mg2+、SO42-和CO32-)时,MgCl2因不易结垢被认为是最好的汲取液(Achilli et al.,2010).
(2)有机类
小分子有机类汲取液,易挥发回收.如Stone等采用一种由叔胺、二氧化碳和水的混合物作,称为可变极性溶剂(Switchable polarity solvents,SPS)作为汲取液.能够产生较高的渗透压(>13 Osm·kg-1).该汲取液通过鼓入CO2和氮气,适当的加热便可回收(Stone et al.,2013).还有研究者选用甲醚作为汲取液,将其放置在室温下,便可以挥发,实现分离,基本无能量的消耗(Sato et al.,2014).
大分子有机类汲取液,具有低溶质返混通量等特点.如2-甲基咪唑基类化合物(Yen et al.,2010),两性离子(甘氨酸、脯氨酸、甜菜碱)(Lutchmiah et al.,2014),EDTA钠盐(Hau et al.,2014),复杂化合物Na4[Co(C6H4O7)2]·2H2O(Na-Co-CA)(Cui et al.,2014),木质素磺酸钠(NaLS)(Duan et al.,2014),磷腈钠盐和锂盐(Stone et al.,2013)等可以产生高的渗透压.
聚合高分子电解质使FO具有高水通量,高盐截留率,基本无溶质返混现象,也可作FO汲取液.聚丙烯酰胺(PAM)(Zhao et al.,2015),聚合水凝胶(Li et al.,2013),胶质溶液(Gadelha et al.,2014),可产生较稳定的渗透压和水通量,溶质返混通量要远远低于其他汲取液.一些热敏性和水溶性较强的聚合高分子电解质,在45℃,2 bar条件下通过热UF过程便可回收(Ou et al.,2013).
(3)纳米材料
超亲水性的纳米颗粒,直径大约为5 nm,可产生较高的渗透压,可用于FO系统(Ling and Chung,2011).Na等合成了一种超强亲水性柠檬酸磁性纳米材料(cit-MNPs).该材料被作为一种适用的汲取液应用到FO中(Na et al.,2014).
3.2.2 汲取液浓度汲取液浓度影响水通量.主要原因是浓度升高导致其渗透压升高,膜两侧的渗透压差(πD-πF)变大,渗透驱动力变大,水通量升高(McCutcheon et al.,2006;Wang et al.,2009;Chanukya et al.,2013;Cui et al.,2014).有研究认为在一定的浓度范围内,水通量随着汲取液浓度的增加而增加,超过一定值后,水通量不再变化.如Cornelissen等采用ZnSO4作为汲取液时,浓度在0.5~2.3 mol·L-1范围内,水通量随浓度的增加而增加,超过2.3 mol·L-1,水通量基本不变(Cornelissen et al. 2008);除此之外,Hau等用EDTA作为汲取液时,当浓度超过1.0 mol·L-1,水通量维持不变(Hau et al.,2014),原因是水通量的升高加重了支撑层内稀释型内部浓差极化.
汲取液浓度对溶质返混通量的影响见解不一.有研究认为影响较小,基本可以忽略(Zhao et al.,2015).也有研究汲取液(EDTA钠盐)浓度较低时,溶质返混通量随着汲取液浓度的增加而增加;浓度较高时,溶质返混通量变化不明显(Hau et al.,2014).
汲取液浓度的增大可能增加溶液的粘度,从而增加泵的能耗(Phuntsho et al.,2013).
3.3 原水性质 3.3.1 原水组成根据原水中主要物质组成可将原水分为无机类和有机类.其中无机类主要是含盐水(Phuntsho et al.,2014;Phuntsho et al.,2013;Alnaizy et al.,2013;Razmjou et al.,2013;Chanukya et al.,2013;Li et al.,2013;Zhao et al.,2012;Sairam et al.,2011;Tan and Ng,2010;Choi et al.,2009;McGinnis and Elimelech,2007;McCutcheon et al.,2006;McCutcheon et al.,2005).有机类包括染料废水(Ge et al.,2012;Zhao et al.,2015)、太空废水(Cath et al.,2005a;Cath et al.,2005b)、含油废水(Zhang et al.,2014),含氯水(卤乙酸)(Kong et al.,2014)、城市污水(Linares et al.,2013),地表水中污染物(PhACs,TrOCs)(Xie et al.,2012;Alturki et al.,2013)等,详见本文2.1~2.4节应用部分.
3.3.2 原水浓度原水中盐浓度的升高导致原水侧渗透压升高,膜两侧的渗透压差(πD-πF)变小,渗透驱动力变小,水通量降低(McCutcheon et al.,2006;Li et al.,2013),截留率降低不明显(Kim et al.,2012;Cui et al.,2014).
3.4 运行条件 3.4.1 温度温度影响FO的水通量(McCutcheon and Elimelech,2006;Cornelissen et al.,2008;Chanukya et al.,2013;Xie et al.,2013;Wang et al.,2014)、溶质返混通量(Xie et al.,2013;Zhao et al.,2015)和膜污染(Zhao and Zou,2011).
温度升高使溶液的粘度降低,扩散和传质系数提高,减小浓差极化,提高水通量(McCutcheon and Elimelech,2006).温度升高,会使溶液渗透压升高,最终使水通量增加(Chanukya et al.,2013),见公式(1):
(1) |
式中,π是渗透压(bar),R是气体常数(8.314 J·mol-1·K-1),T、V、aW分别是是温度(K)、摩尔体积(18 mL·mol-1)和水的活度(Chanukya et al.,2013).
3.4.2 pHpH改变影响膜表面性质,进而影响FO的水通量和截留效果.研究者发现pH的改变会引起交联膜聚合结构构象和表面疏水性的改变,随着pH的增加,膜表面的电负性增强,使聚合物基体上可电离官能团之间的排斥作用增强,最终使平均孔径变大,渗透水通量增加;另外,随着pH增加,膜表面的亲水性会增强,有利于提高水通量(Xie et al.,2012;Hau et al.,2014).Xie等发现当pH高于5.8时,原水中模拟污染物磺胺甲恶唑呈电负性,pH升高使FO膜AL表面电负性增强,因此膜表面与污染物之间的斥力增强,从而提高了污染物的截留率(Xie et al. 2012).
pH改变影响原水中污染物化学形态,进而影响FO的截留率.如Kim等用FO截留B3+时,当pH升高,B3+与OH-结合生成B(OH)3,水合半径增大,更易被截留;当pH继续升高,B(OH)3水进一步水解为B(OH)4-,B(OH)4-与FO膜表面负电荷产生排斥作用,截留率进一步提高(Kim et al.,2012).Xie发现pH在3.5~7.5范围变化时,原水中的卡马西平呈电中性,因此不受膜表面电荷变化的影响,截留率也不受影响;但pH高于5.8时,呈现电负性的磺胺甲恶唑的截留率随pH升高而升高(Xie et al. 2012).
pH改变影响汲取液溶质化学形态,进而影响返混通量.Hau等利用EDTA钠盐作为汲取液时,当pH高于7时,EDTA4-本身会结合自由态的Na+,生成Na[EDTA]3-,降低汲取液的返混通量(Hau et al.,2014).
3.4.3 流速和流向原水和汲取液的流速升高,增大膜表面的水流剪切力,可以产生较快的渗透流稀释作用,提高传质系数,降低外部浓差极化,从而使FO水通量增加(Hau et al.,2014;Jung et al.,2011;Phuntsho et al.,2013;Hau et al.,2014).但有些研究者认为当流速在小范围改变时,并不足以引起传质及外部浓差极化的改变(Kim et al.,2012).
流向指原水流和汲取液流的相对方向,为顺流或逆流.但可能由于FO研究的规模都很小,流向对FO的影响并未体现出来(Jung et al.,2011;Phuntsho et al.,2013).
4 存在问题(Existing problems)虽然FO成为近年来的研究热点,但目前仍未得到广泛应用,浓差极化、膜污染、汲取液溶质返混,汲取液的后处理等问题亟待解决.
4.1 浓差极化 4.1.1 外部浓差极化和内部浓差极化外部浓差极化发生在FO膜外部,即活性层和支撑层的表面.可分为浓缩型外部浓差极化和稀释型外部浓差极化.当采用AL-FS模式时,原水中水分子通过FO膜时,溶质(污染物)被截留,在膜的活性层与原水界面区域溶质浓度越来越高,发生浓缩型外部浓差极化(图 2a),在膜的支撑层与汲取液界面区域溶质浓度会被水稀释;当采用AL-DS模式时,水分子通过FO膜进入汲取液,膜的活性层与汲取液界面区域汲取液被稀释,溶质浓度降低,发生稀释型外部浓差极化(图 2b),同时在膜的支撑层与原水界面区域发生浓缩型浓差极化(McCutcheon and Elimelech,2006;Jung et al.,2011;Zhao et al.,2012;Chanukya et al.,2013).内部浓差极化发生在FO的支撑层内部,分为浓缩型内部浓差极化和稀释型内部浓差极化(McCutcheon and Elimelech,2006;Gray et al.,2006;Jung et al.,2011).当AL-FS时,发生稀释型内部浓差极化(图 2a蓝色区域);当AL-DS时,发生浓缩型内部浓差极化(图 2b蓝色区域).与发生在支撑层内部浓差极化相比,发生在支撑层外部的浓差极化可以被忽略.
(1)外部浓差极化
2006年,McCutcheon等根据薄膜理论分析外部浓差极化(McCutcheon et al.,2006;McCutcheon and Elimelech,2006;Jung et al.,2011).浓缩型外部浓差极化仅发生在原水侧:
(2) |
稀释型外部浓差极化与浓缩型浓差极化相似,但是仅发生在汲取液侧:
(3) |
式中,πFeed,m和πDraw,b分别是原水一侧膜表面和主体溶液的渗透压;πDraw,m和πDraw,b分别是汲取液一侧膜表面和主体溶液的渗透压(bar);JW是水通量(L·m-2·h-1);κ是传质系数(m·s-1).传质系数κ与Sh密切相关,其中
(4) |
式中,D是溶质的扩散系数(m2·s-1);dh是水力直径(m);Sh舍伍德数由公式(5)(6)获得:
(5) |
(6) |
式中,Re是雷诺数,Sc是施密特数,L是管道长度(m).
(7) |
式中,L,H分别是矩形槽的长(m)和高(m).
水通量可以简化为:
(8) |
式中,A是水透过膜的渗透系数(m3·m-2·s-1·bar-1).
由公式(2)(3)(8),McCutcheon and Elimelech得到水通量可以表示为
(9) |
该公式既包含了浓缩型外部浓差极化,也包含了稀释型外部浓差极化(McCutcheon and Elimelech,2006).
随后,Zhao等对该公式进行了修正,得到
(10) |
但该模型并不包含内部浓差极化(McCutcheon and Elimelech,2006;Zhao et al.,2012).
(2)内部浓差极化
有学者采用溶液扩散理论对内部浓差极化进行研究,得到水通量公式(11)(Cath et al.,2006;Gray et al.,2006):
(11) |
式中,K为溶质在多孔支撑层内的阻力系数(s·m-1).而
(12) |
式中,t、τ、ε为支撑层的厚度(m)、弯曲度、孔隙率.但是公式(12)仅适用于水通量较小的情况,对FO膜而言,水通量相对较大,因此需要深入研究,分别讨论浓缩型和稀释型内部浓差极化.对于浓缩型内部浓差极化(AL-DS模式下)和稀释型内部浓差极化(AL-FS模式下),水通量的表达式分别如公式(13)和(14)(McCutcheon and Elimelech,2006;Cath et al.,2006;Gray et al.,2006; Zhao et al.,2012;Chanukya et al.,2013):
(13) |
(14) |
式中,B是溶质的渗透系数(m·s-1).
(3)内部浓差极化和外部浓差极化
为更好地表达浓差极化对水通量的影响,将以上内部浓差极化和外部浓差极化公式综合得到AL-DS和AL-FS模式下的浓差极化公式(15)(16)(McCutcheon and Elimelech,2006;Jung et al.,2011;Zhao et al.,2012):
AL-DS模式
(15) |
AL-FS模式
(16) |
两种类型外部浓差极化均降低膜两侧的渗透压,使水通量降低.外部浓差极化可以通过增大流速,加剧膜表面湍流程度,增大膜表面剪切力等方法,使膜表面溶液浓度与主体溶液浓度尽可能地达到均一来削减其影响(Jung et al.,2011;Zhao et al.,2012;钟铭,2012),也可以通过降低水通量,减小膜表面溶液浓度的变化,从而缓解外部浓差极化(李刚等,2010;Zhao et al.,2012).
内部浓差极化发生在支撑层内部,由公式(12)来看内部浓差极化与FO膜支撑层的弯曲度τ、厚度t、隙率ε、溶质的传质系数D有关,因此削减内部浓差极化,必须从膜制备和膜改性等方面考虑,使之成为无多孔支撑层结构的膜,使溶质分子无法渗透进入支撑层内部(钟铭,2012).
4.2 膜污染膜污染问题是几乎所有膜分离技术的重要问题(Mi and Elimelech,2008;Lee et al.,2010;刘彩虹,2013).膜污染使FO水通量下降(Holloway et al.,2007).但在某些条件下可以有限的提高目标污染物的截留率,如Hancock等采用FO对TrOC(医药、个人护肤品,增塑剂和阻燃剂)进行截留,发现由于膜污染,污染物的截留率>99%(Hancock et al.,2011);而对于微污染物,Linares等发现污染后的FO膜对亲水类中性化合物、疏水类中性化合物和离子态化合物的的截留率均高于未发生污染的膜(Linares et al.,2011).但膜污染过于严重时,会影响出水水质,甚至会增加能耗和处理成本(Lutchmiah et al.,2014).
FO膜污染几乎是可逆的,比RO过程中发生的膜污染程度轻.原因是FO操作时压力小,形成的污染层较疏松,通过简单的物理清洗就可以去除(Lee et al.,2010;Chun et al.,2015).但若长期应用到实际废水的过滤工艺中,也会产生不可逆的膜污染,需要通过选择合适的化学试剂进行清洗,来恢复其通量(Wang et al.,2015).
4.2.1 膜污染类型膜污染包括有机污染、无机污染和微生物污染.
更多FO研究针对的是有机污染.有机污染与分子内的粘附力有紧密关系,而且有机物之间的相互作用也可能影响膜污染的速度和程度.FO膜表面的有机污染是由化学作用和水力作用共同导致,化学作用主要是架桥,水力作用包括渗透拖曳力和表面剪切力(Mi and Elimelech,2008;Mi and Elimelech,2010).能够在膜表面形成有机污染的物质包括海藻酸钠(Lee et al.,2010;Liu and Mi,2012;刘彩虹,2013;Gu et al.,2013;Liu and Mi,2014;Kim et al.,2014),牛血清白蛋白(Lee et al.,2010;Arkhangelsky et al.,2012;刘彩虹,2013;Liu and Mi,2014),腐殖酸(Lee et al.,2010;刘彩虹,2013;Liu and Mi,2014),富里酸(刘彩虹,2013),溶菌酶(Gu et al.,2013)等.
导致膜表面无机污染的污染物主要为钙(Mi and Elimelech,2008;Mi and Elimelech,2010;Arkhangelsky et al.,2012;Parida and Ng,2013;刘彩虹,2013;Liu and Mi,2012;Liu and Mi,2014;Zhang et al.,2014)、硅(Lee et al.,2010;Arkhangelsky et al.,2012;Kim et al.,2014)等.钙离子除了直接在膜表面形成无机结垢污染,还能通过使有机污染物之间产生架桥作用,加重有机污染(Mi and Elimelech,2008;Parida and Ng,2013;刘彩虹,2013).硅纳米颗粒主要是在膜表面形成胶体污染(Lee et al.,2010).
生物污染主要由水中的微生物及其所分泌的胞外多聚物(EPS)导致(Lutchmiah et al.,2014),尤其是在FO-MBR中,原因是FO膜多呈疏水性膜,而生物聚合物中的蛋白质也多为疏水性物质,容易在膜表面沉积(马艳杰,2012).
4.2.2 膜污染的影响因素FO膜的亲疏水性(马艳杰,2012),膜表面电荷(Boo et al.,2012),粗糙度(Mi and Elimelech,2008;Mi and Elimelech,2010;Parida and Ng,2013;Gu et al.,2013),表面官能团(Arkhangelsky et al.,2012)对膜污染均有影响.对膜表面进行修饰(Nguyen et al.,2013;Castrillóna et al.,2014)或合成新复合薄膜(Emadzadeh et al.,2014),改变膜表面特性,可以使其防垢性增强.
原水中的污染物也影响膜污染.往往污染物-膜表面、污染物之间会产生协同作用,使污染加重,水通量下降(Arkhangelsky et al.,2012;Liu and Mi,2012;Parida and Ng,2013;Kim et al.,2014;Boo et al.,2012).
操作条件对膜污染也有影响,主要是膜方向、温度和流速.AL-DS模式比AL-FS模式更易受污染(Parida and Ng,2013;Phuntsho et al.,2013).温度升高会使膜结垢和清洗问题加剧(Zhao et al.,2011).温度的改变还会改变污染物的对流和扩散作用,从而影响膜污染(Kim et al.,2015).流速的提高可以增强膜表面的水力剪切作用,防止污染物在膜表面沉积(Lee et al.,2010;Arkhangelsky et al.,2012;Boo et al.,2013).
4.3 汲取液溶质返混FO过程中,由于膜两侧的浓度不同,汲取液中的溶质会通过FO膜进入原水中,这种现象被称为汲取液溶质返混.溶质返混使渗透压降低,引起膜污染,从而影响FO的稳定运行(Lee et al.,2010;Zou et al.,2011;Boo et al.,2012;She et al.,2012;Park et al.,2013).溶质返混现象在FO过程中是不可避免的(Yong et al.,2012;Kong et al.,2014;Kim et al.,2014;Gadelha et al.,2014;Hau et al.,2014;刘彩虹,2013),受到汲取液物化性质(扩散性,离子或分子尺寸,粘度)(Hancock and Cath,2009;Zou et al.,2011;Zhao et al.,2011),流速(Hancock and Cath,2009;Suh and Lee,2013),膜结构(Hancock and Cath,2009),内外浓差极化(Hancock and Cath,2009;Zhao et al.,2011)等因素的影响.
4.4 汲取液再生汲取液再生是影响FO技术能耗的关键.传统用RO再生能耗太高(Shaffer et al.,2015).目前,汲取液的再生方法包括:①直接利用,如灌溉(Phuntsho et al.,2011)、沙漠修复(Duan et al.,2014);②热分离(McCutcheon et al.,2005;McCutcheon et al.,2006;McGinnis and Elimelech,2007;McGinnis et al.,2013;Chanukya et al.,2013;Stone et al.,2013);③膜分离技术,如RO(Choi et al.,2009;Cath et al.,2010;Yangali-Quintanilla et al.,2011;Bamaga et al.,2011;Bowden et al.,2012)、NF(Tan and Ng,2010;Zhao et al.,2012;Su et al.,2012;Hau et al.,2014)、UF(Ling and Chung,2011;Ge et al.,2012;Gadelha et al.,2014)、MD(Yen et al.,2010;Wang et al.,2011;Ge et al.,2012;Zhang et al.,2014)、ED(Zhang et al.,2013);④化学反应沉淀(Alnaizy et al.,2013);⑤刺激响应及其相关的组合工艺等(Li et al.,2011;Li et al.,2013;Ou et al.,2013).
汲取液再生是影响FO技术能耗的关键.传统用RO再生能耗太高(Shaffer et al.,2015).目前,汲取液的再生方法包括:①直接利用,如灌溉(Phuntsho et al.,2011)、沙漠修复(Duan et al.,2014);②热分离(McCutcheon et al.,2005;McCutcheon et al.,2006;McGinnis and Elimelech,2007;McGinnis et al.,2013;Chanukya et al.,2013;Stone et al.,2013);③膜分离技术,如RO(Choi et al.,2009;Cath et al.,2010;Yangali-Quintanilla et al.,2011;Bamaga et al.,2011;Bowden et al.,2012)、NF(Tan and Ng,2010;Zhao et al.,2012;Su et al.,2012;Hau et al.,2014)、UF(Ling and Chung,2011;Ge et al.,2012;Gadelha et al.,2014)、MD(Yen et al.,2010;Wang et al.,2011;Ge et al.,2012;Zhang et al.,2014)、ED(Zhang et al.,2013);④化学反应沉淀(Alnaizy et al.,2013);⑤刺激响应及其相关的组合工艺等(Li et al.,2011;Li et al.,2013;Ou et al.,2013).
5 展望(Perspectives)以渗透压差作为驱动力的FO技术,在水处理领域引起了越来越多学者的关注(表 1).研究包括FO膜制备、FO运行、传质机理、不同类型水和废水处理中的应用等.在实验室规模研究基础上,FO技术在中试、实际应用规模的研究也逐渐开展.但仍然存在一些问题期待解决,如膜污染、汲取液再生、浓差极化、汲取液溶质返混等,使FO技术的广泛应用受到限制.未来的研究应针对这些问题在深入解析FO传质机理、膜污染过程、浓差极化过程等基础上,继续开发有效的膜污染控制手段、汲取液再生方法、高效的FO膜和汲取液.不仅进行实验室研究,还应进行实际应用规模的长期运行研究.随着科研工作的不断深入,低能耗、高回收率的FO技术一定能得到更广泛的应用前景.
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