Kuban State University, 149 Stavropolskaya st., 350040 Krasnodar, Russia.
Int J Mol Sci. 2019 Jul 23;20(14):3593. doi: 10.3390/ijms20143593.
Electrodialysis (ED) with ion-exchange membranes is a promising method for the extraction of phosphates from municipal and other wastewater in order to obtain cheap mineral fertilizers. Phosphorus is transported through an anion-exchange membrane (AEM) by anions of phosphoric acid. However, which phosphoric acid anions carry the phosphorus in the membrane and the boundary solution, that is, the mechanism of phosphorus transport, is not yet clear. Some authors report an unexpectedly low current efficiency of this process and high energy consumption. In this paper, we report the partial currents of HPO, HPO, and PO through Neosepta AMX and Fujifilm AEM Type X membranes, as well as the partial currents of HPO and H ions through a depleted diffusion layer of a 0.02 M NaHPO feed solution measured as functions of the applied potential difference across the membrane under study. It was shown that the fraction of the current transported by anions through AEMs depend on the total current density/potential difference. This was due to the fact that the pH of the internal solution in the membrane increases with the growing current due to the increasing concentration polarization (a lower electrolyte concentration at the membrane surface leads to higher pH shift in the membrane). The HPO ions contributed to the charge transfer even when a low current passed through the membrane; with an increasing current, the contribution of the HPO ions grew, and when the current was about 2.5 ( was the theoretical limiting current density), the PO ions started to carry the charge through the membrane. However, in the feed solution, the pH was 4.6 and only HPO ions were present. When HPO ions entered the membrane, a part of them transformed into doubly and triply charged anions; the H ions were released in this transformation and returned to the depleted diffusion layer. Thus, the phosphorus total flux, (equal to the sum of the fluxes of all phosphorus-bearing species) was limited by the HPO transport from the bulk of feed solution to the membrane surface. The value of was close to / ( is the Faraday constant). A slight excess of over / was observed, which is due to the electroconvection and exaltation effects. The visualization showed that electroconvection in the studied systems was essentially weaker than in systems with strong electrolytes, such as NaCl.
电渗析（ED）与离子交换膜是从城市污水和其他废水中提取磷酸盐以获得廉价矿物肥料的一种很有前途的方法。磷通过阴离子交换膜（AEM）由磷酸的阴离子运输。然而，哪种磷酸阴离子在膜中和边界溶液中携带磷，即磷传递的机制尚不清楚。一些作者报道说，该过程的电流效率异常低，能耗高。在本文中，我们报告了通过 Neosepta AMX 和 Fujifilm AEM Type X 膜传递的 HPO、HPO 和 PO 的部分电流，以及通过耗尽的 0.02 M NaHPO 进料溶液的扩散层测量的 HPO 和 H 离子的部分电流作为研究膜的施加电位差的函数。结果表明，通过 AEM 传递的阴离子电流部分取决于总电流密度/电位差。这是由于由于浓度极化的增加（膜表面处电解质浓度降低导致膜中 pH 变化更大），内部溶液的 pH 值随电流的增加而增加。当电流通过膜时，即使电流很小，HPO 离子也有助于电荷转移；随着电流的增加，HPO 离子的贡献增加，当电流约为 2.5 时（是理论极限电流密度），PO 离子开始通过膜传递电荷。然而，在进料溶液中，pH 值为 4.6，仅存在 HPO 离子。当 HPO 离子进入膜时，一部分转化为双电荷和三电荷阴离子；在这种转化中释放出 H 离子，并返回耗尽的扩散层。因此，磷总通量，（等于所有含磷物种通量的总和）受 HPO 从进料溶液主体向膜表面的传输限制。的值接近/（是法拉第常数）。观察到的值略高于/，这是由于电对流和激发效应。可视化表明，在研究的体系中，电对流本质上弱于具有强电解质（例如 NaCl）的体系。
