Department of Desalination and Water Treatment, Zuckerberg Institute for Water Research, Jacob Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus 84990, Israel.
Wolfson Department of Chemical Engineering, Technion - Israel Institute of Technology, Technion City, 32000, Haifa, Israel.
Water Res. 2019 Sep 15;161:381-391. doi: 10.1016/j.watres.2019.06.013. Epub 2019 Jun 6.
Calcium phosphate scaling is one of the main limitations in effluent desalination using membranes. This may be overcome by tailoring membranes with lower rejection of the scalant ions. In this study, we systematically examined the use of negatively and positively charged membranes, rejecting ions mainly based on Donnan exclusion, as a low-scaling alternative to dielectric-exclusion-dominated polyamide NF membranes for effluent desalination. The two charged membranes exhibited a lower calcium and especially phosphate rejection than the polyamide membrane. Consequently, the calcium phosphate supersaturation and then the propensity to scaling of the charged membranes were much lower than the polyamide membrane. This also allowed filtering at a much higher recovery ratio with the charged membranes. It was also found that, despite the fact that the charged membranes had an opposite fixed charge, their scaling behavior was similar. Apparently, although these membranes showed opposite selectivity towards scalant ions (phosphate and calcium) in single salt solutions, the rejection pattern in mixed salt solutions resulted in similar saturation indices, much lower than for polyamide membrane. The scale formed on all three membranes was identified as amorphous calcium phosphate (ACP), although its saturation index was lower than its solubility factor. This was explained by concentration polarization which increases the saturation index in the solution adjacent to the membrane surface. Tests in absence of permeate flux showed a much slower precipitation that took a few days compared with filtration conditions (few hours). In addition, under these conditions, the effect of the scaling on the membrane permeability was generally reduced and the scale contained crystalline calcium phosphate products, different from ACP. The results indicate that the ion rejection and resulting polarization next to the membrane surface plays a crucial role in scaling. Thus, tuning ion selectivity of NF membranes towards scalant ions presents a promising alternative for scaling mitigation during effluent desalination.
磷酸钙结垢是膜法淡化处理废水的主要限制因素之一。通过对膜进行修饰，降低结垢离子的排斥率，可克服这一问题。在这项研究中，我们系统地考察了使用带负电荷和正电荷的膜，主要基于道南排斥来排斥离子，作为一种替代介电排斥主导的聚酰胺纳滤膜的低结垢方法，用于废水淡化处理。与聚酰胺膜相比，这两种带电膜对钙和特别是磷酸盐的排斥率较低。因此，带电膜的磷酸钙过饱和度和结垢倾向要低得多。这也允许在更高的回收率下用带电膜进行过滤。研究还发现，尽管这两种带电膜的固定电荷相反，但它们的结垢行为相似。显然，尽管这些膜在单盐溶液中对结垢离子（磷酸盐和钙）表现出相反的选择性，但在混合盐溶液中的排斥模式导致相似的饱和指数，远低于聚酰胺膜。在所有三种膜上形成的垢均被鉴定为无定形磷酸钙（ACP），尽管其饱和指数低于其溶解度因子。这是由于浓差极化导致膜表面附近溶液的饱和指数增加所致。在没有渗透通量的情况下进行的测试表明，与过滤条件（数小时）相比，沉淀速度要慢得多，需要几天时间。此外，在这些条件下，结垢对膜渗透性的影响通常会降低，并且结垢中含有不同于 ACP 的结晶磷酸钙产物。结果表明，膜表面附近的离子排斥和由此产生的极化在结垢中起着至关重要的作用。因此，调整纳滤膜对结垢离子的离子选择性，为在废水淡化处理过程中减轻结垢提供了一种很有前景的方法。
