School of Environment and Energy, South China University of Technology, Guangzhou, 510006, China; Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore.
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore, 117585, Singapore.
Water Res. 2020 Apr 15;173:115590. doi: 10.1016/j.watres.2020.115590. Epub 2020 Feb 6.
Electrolytes are commonly employed as draw solutes in forward osmosis (FO). This work demonstrates that electrostatic interactions play a key role in ion transport in the FO process. The difference in diffusivity between the constituent ions of the draw electrolyte significantly impact the forward transport of the feed ions. Draw electrolyte composed of low-diffusivity cations and high diffusivity anions promoted forward transport of the feed anions and retarded that of the feed cation, and vice versa. The effects were remarkable even for the most commonly used draw electrolytes (NaCl or MgCl), where the forward flux of NO and NO was found to increase by a few folds and that of NH was reduced by similar magnitudes than that observed in a nonelectrolyte draw solute (glucose) system. More profound increase/reduction (up to 10 times) was observed for draw electrolytes composed of highly asymmetric cations and anions. An analytical model is developed by considering the electrostatic interaction between the draw and the feed ions, to predict its effect on the forward transport of the feed ions. The normalized diffusivity difference (θD) between the constituent ions of the draw electrolyte is found as a key factor that determines the transport behaviors of the feed ions. These results may have important implications in enhancing our understanding of bidirectional ion transport in FO. The findings may also be useful in the design and development of FO processes for enhanced removal of charged pollutants via draw solute selection and formulation.
电解质通常被用作正向渗透（FO）中的汲取溶质。本工作表明，静电相互作用在 FO 过程中的离子传输中起着关键作用。汲取电解质中组成离子的扩散率差异显著影响进料离子的正向传输。由低扩散率阳离子和高扩散率阴离子组成的汲取电解质促进了进料阴离子的正向传输，而阻碍了进料阳离子的正向传输，反之亦然。即使对于最常用的汲取电解质（NaCl 或 MgCl），这种影响也是显著的，其中 NO 和 NO 的正向通量增加了几个数量级，而 NH 的正向通量则减少了与在非电解质汲取溶质（葡萄糖）体系中观察到的相似数量级。对于由高度不对称阳离子和阴离子组成的汲取电解质，观察到更深远的增加/减少（高达 10 倍）。通过考虑汲取和进料离子之间的静电相互作用，开发了一个分析模型来预测其对进料离子正向传输的影响。发现汲取电解质中组成离子的归一化扩散率差异（θD）是决定进料离子传输行为的关键因素。这些结果可能对增强我们对 FO 中双向离子传输的理解具有重要意义。这些发现对于通过汲取溶质选择和配方设计来增强 FO 过程以去除带电污染物也可能是有用的。
