School of Civil and Environmental Engineering, Nanyang Technological University, N1-1B-35, 50 Nanyang Avenue, Singapore 639798, Singapore.
Water Res. 2012 May 1;46(7):2478-86. doi: 10.1016/j.watres.2012.02.024. Epub 2012 Feb 20.
Osmotically driven membrane processes, such as forward osmosis (FO) and pressure retarded osmosis (PRO), are attracting increasing interest in research and applications in environment and energy related fields. In this study, we systematically investigated the alginate fouling on an osmotic membrane during FO operation using four types of draw solutions (NaCl, MgCl(2), CaCl(2) and Ca(NO(3))(2)) to elucidate the relationships between reverse (from draw solution to feed solution) and forward (from feed solution to draw solution) solute diffusion, and membrane fouling. At the same water flux level (achieved by adjusting the draw solution concentration), the greatest reverse solute diffusion rate was observed for NaCl draw solution, followed by Ca(NO(3))(2) draw solution, and then CaCl(2) draw solution and MgCl(2) draw solution, the order of which was consistent with that of their solute permeability coefficients. Moreover, the reverse solute diffusion of draw solute (especially divalent cation) can change the feed solution chemistry and thus enhance membrane fouling by alginate, the extent of which is related to the rate of the reverse draw solute diffusion and its ability to interact with the foulant. The extent of fouling for the four types of draw solution followed an order of Ca(NO(3))(2) > CaCl(2) >> MgCl(2) > NaCl. On the other hand, the rate of forward diffusion of feed solute (e.g., Na(+)) was in turn promoted under severe membrane fouling in active layer facing draw solution orientation, which may be attributed to the fouling enhanced concentration polarization (pore clogging enhanced ICP and cake enhanced concentration polarization). The enhanced concentration polarization can lead to additional water flux reduction and is an important mechanism governing the water flux behavior during FO membrane fouling. Findings have significant implications for the draw solution selection and membrane fouling control in osmotically driven membrane processes.
渗透驱动膜过程，如正向渗透（FO）和压力延迟渗透（PRO），在环境和能源相关领域的研究和应用中引起了越来越多的关注。在这项研究中，我们使用四种汲取液（NaCl、MgCl2、CaCl2 和 Ca(NO3)2）系统地研究了 FO 操作过程中汲取液对渗透膜的海藻酸钠污染，以阐明反向（从汲取液到原料液）和正向（从原料液到汲取液）溶质扩散与膜污染之间的关系。在相同的水通量水平（通过调整汲取液浓度实现）下，观察到 NaCl 汲取液具有最大的反向溶质扩散速率，其次是 Ca(NO3)2 汲取液，然后是 CaCl2 汲取液和 MgCl2 汲取液，其顺序与溶质渗透系数的顺序一致。此外，汲取液中溶质的反向扩散（特别是二价阳离子）会改变原料液的化学性质，从而通过海藻酸钠增强膜污染，其程度与反向汲取液扩散速率及其与污染物相互作用的能力有关。四种汲取液的污染程度依次为 Ca(NO3)2>CaCl2>>MgCl2>NaCl。另一方面，在活性层面向汲取液的方向上，严重的膜污染会促进进料溶质（例如 Na+）的正向扩散，这可能归因于污染增强的浓差极化（孔堵塞增强 ICP 和滤饼增强浓差极化）。增强的浓差极化会导致额外的水通量减少，是控制 FO 膜污染过程中水通量行为的重要机制。这些发现对渗透驱动膜过程中汲取液的选择和膜污染控制具有重要意义。
