Abstract Forward osmosis membrane bioreactor (FOMBR) has a more intricate membrane fouling mechanism than MBR because of the special existence of the reverse diffusion of draw solution. The mechanisms of membrane fouling affected by reverse diffusion of draw solution and ion content in gel layer were investigated from thermodynamic perspective and molecular level in this study. Phase-contrast microscopy non-invasively observed that the molecular chain structure of the gel layer containing low content of reverse diffused solute (NaCl) was sparse, while the alginate molecular chains with high NaCl content were parallel and compact. Density functional theory (DFT) further simulated the formation mechanism of different alginate chain structures, that is, the reverse diffused solute shortened the distance between hydrogen bonds and reduced the interaction energy at the terminals of alginate chains. This proved that the NaCl trapped in alginate layer directly affected its structure. Heating experiments indicated that gel layer acted as a “chemical potential barrier” that prevented bound water from turning into free water. Accordingly, the “chemical potential barrier” described by Flory-Huggins theory was proposed, which was confirmed to account for the filtration of alginate gels affected by reverse diffusion of draw solution. To the best of our knowledge, this is the first membrane fouling study that explores the effect of reverse diffusion of draw solution on the “chemical potential barrier” of gel layer from perspective of molecular chain structure. This study investigated FOMBR fouling mechanism at molecular level and provided a new strategy for FOMBR system fouling quantification.

中文翻译：

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基于热力学机制对受驱动溶液反向扩散影响的正渗透膜污染的新分子水平见解

摘要 正渗透膜生物反应器（FOMBR）由于驱动液逆向扩散的特殊存在，比MBR具有更复杂的膜污染机理。本研究从热力学角度和分子水平研究了驱动溶液反向扩散和凝胶层离子含量对膜污染的影响机制。相衬显微镜无创观察到反扩散溶质（NaCl）含量低的凝胶层分子链结构稀疏，而NaCl含量高的海藻酸盐分子链平行且致密。密度泛函理论（DFT）进一步模拟了不同海藻酸盐链结构的形成机制，即，反向扩散的溶质缩短了氢键之间的距离，降低了藻酸盐链末端的相互作用能。这证明捕获在藻酸盐层中的NaCl直接影响了其结构。加热实验表明，凝胶层充当“化学势垒”，防止结合水变成自由水。因此，提出了 Flory-Huggins 理论描述的“化学势垒”，这被证实可以解释藻酸盐凝胶的过滤受吸引溶液的反向扩散影响。据我们所知，这是第一次从分子链结构的角度探讨驱动溶液反向扩散对凝胶层“化学势垒”的影响的膜污染研究。