正渗透膜过程中临界通量的影响因素

临界通量是膜过程中一种重要的污染特性指标。采用阶梯汲取液浓度递增法测定不同污染物、架桥离子浓度及膜面流速对正渗透(FO)膜过程临界通量的影响。结果表明，海藻酸钠(SA)、腐殖酸(HA)及二氧化硅(SiO2)污染时FO膜临界通量值分别为29.32, 46.35和32.17 L/(m2?h)；随Ca2+浓度由0 mmol/L增大至10 mmol/L，SA污染下FO膜的临界通量由29.22 L/(m2?h)显著降低至9.48 L/(m2?h)，原因为Ca2+与SA分子中的羧基的螯合作用及Ca2+在膜?污染物之间的架桥作用；此外，当膜面流速从5 cm/s增至15 cm/s时，SA-Ca2+污染下的FO膜临界通量由9.48 L/(m2?h)提高至31.59 L/(m2?h)，表明改善膜表面湍动有利于提高临界通量，扩大操作通量范围。

Factors affecting the critical flux in a forward osmosis membrane process

Critical flux is an important concept in fouling control for membrane-based processes. Operation below the critical flux can maintain the membrane flux and reduce the maintenance cost which is associated with membrane cleaning and replacement in forward osmosis (FO) processes. In this research, the effects of foulant type, binding ions concentration, and cross-flow velocity on the critical flux in FO processes were investigated using a draw solution concentration stepping method. The results showed that the draw solution concentration stepping was feasible for the critical flux determination in FO processes. The thin-film composite (TFC) membranes exhibited a low critical flux for sodium alginate (SA) fouling with a value of 29.32 L/(m2?h), then followed nano-silica (SiO2) with a critical flux value of 32.17 L/(m2?h) and humic acid (HA) of 46.35 L/(m2?h). This indicated that the critical flux behavior in FO processes was dependent on the properties of both the membrane and foulants, including the membrane surface roughness, intermolecular adhesion of foulants, and the interaction between foulants and membrane. The atomic force microscopy (AFM) results revealed the deposition of foulants onto the ridge-and-valley structure of the membrane surface, leading to the deviation of water flux from the baseline with the increasing draw solution concentration. As the Ca2+ concentration increased from 0 mmol/L to 10 mmol/L, the critical flux for alginate fouling dramatically decreased from 29.22 L/(m2?h) to 9.48 L/(m2?h), which can be attributed to the interaction between alginate and membrane as well as the intermolecular aggregation of alginate and the interaction by Ca2+ binding. Moreover, the critical flux for SA?Ca2+ complexes fouling increased from 9.48 L/(m2?h) to 31.59 L/(m2?h) with the cross-flow velocity ranging from 5 cm/s to 15 cm/s, which indicated the improvement of the solution turbulence can enhance the critical flux, thereby expanding the operating ranges of flux.