Mechanism study of synergistic effect of organic and inorganic foulants in membrane distillation

In this paper, we investigated the mechanism of membrane fouling during the membrane distillation (MD) process by using humic acid (HA) and calcium carbonate (CaCO3) as the model organic and inorganic foulants respectively. The aggregation induced crystallization theory was introduced to analyze the fouling process. Field emission scanning electron microscopy (FESEM) images and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and variation of foulants on membrane surfaces that treated different feeds at different concentration factors. The interface free energies of the membranes surfaces were calculated based on contact angles and surface tensions between the testing solutions and the membrane surfaces. The results indicated that the aggregations of HA onto the hydrophobic polyvinylidene fluoride (PVDF) membrane surface altered membrane surface properties, which reduced critical nucleus radius (R⁎) and facilitated the heterogeneous nucleation of CaCO3. Furthermore, the presence of calcium ion enhanced the aggregation of HA, which further promoted the crystallization of CaCO3.