Mechanisms And Control Strategies Of Fouling Propensity And Reverse Ion Diffusion Of Polyamide Forward Osmosis Composite Membrane

Water scarcity has become a global challenge. Membrane processes hold significant promise for addressing this global challenge with its advantages, including high purification efficiency, low energy consumption, and convenience in operation and administration. As an emerging osmosis-driven membrane processes, forward osmosis(FO) has been considered as an effective supplement for the conventional pressure-driven membrane processes with its particular advantages in treating some special source waters. FO has shown great potential applications in wastewater reclamation, and sea water and brackish water desalination.Membrane materials, the core of FO technology, directly determine further development and application of FO processes. In recent years, successful fabrication of polyamide thin-film composite(TFC) FO membranes, an important milestone of the development of FO processes, significantly improve the performances(higher water flux and rejection) of FO process. However, membrane fouling and reverse ion diffusion, which significantly decrease the purification efficacy and increase operation cost, are still inevitable problems in the real application of TFC FO membranes and become bottlenecks for the future development of TFC FO membranes. This research seeks to investigate the role of surface properties on TFC FO membrane performance. In particular, mechanisms of membrane fouling and reverse ion diffusion of TFC FO membranes will be elucidated. Additionally, some surface modification control strategies will be developed to control fouling and reverse ion diffusion, which are of significant importance for the future fabrication of high performance TFC FO membranes.The first section of this research investigated the influence of surface structure on the fouling propensity of TFC FO membranes. Specifically, we compared membranes fabricated through identical procedures except for the use of different solvents(n-methyl-2-pyrrolidinone, NMP and dimethylformamide, DMF) during phase separation. FO fouling experiments showed that the TFC membranes fabricated using NMP(NMP-TFC) had significantly less flux decline when compared to the membranes fabricated using DMF(DMF-TFC). Water flux was also more easily recovered through physical cleaning for the NMP-TFC membrane. To determine the fundamental cause of these differences in fouling propensity, the active and support layers of the membranes were extensively characterized for surface characteristics relevant to fouling behavior. Polyamide active layer surface roughness was found to dominate all other investigated factors in determining the fouling propensities of our membranes relative to each other. The support layers of the two membrane types were also characterized for their morphological properties, and the relation between support layer surface structure and polyamide active layer surface properties and ridge structure formation was discussed. Taken together, our findings indicated that support layer structure has a significant impact on the fouling propensity of the active layer. Thus, in design of support layer structures for high performance TFC FO membranes, it should not only focus on obtaining a support layer with a structure that minimizes internal concentration polarization, but also consider its impact on active layer fouling propensity.Based on realization of significant role of support layer surface properties on TFC FO membrane performance, in the second section, the active layer–support layer interface structure of TFC FO membrane was also investigated for its possible influence on transport properties. The polyamide active layer of TFC membranes is rich with nitrogen while the underlying polysulfone support layer is abundant with sulfur. In this research, we exploited this elemental contrast and present the application of two elemental analysis techniques — scanning transmission electron microscopy energy dispersive X-ray spectroscopy(STEM–EDX) and X-ray photoelectron spectroscopy(XPS) C60+ ion-beam sputtering — to reveal the chemical composition of the polyamide–polysulfone interface of TFC FO membranes. STEM–EDX elemental mappings revealed a clear structure of the active layer–support layer interface. Additionally, the XPS depth profiles suggested that a mixing layer, comprising both polyamide and polysulfone signatures, is present at the polyamide–polysulfone interface, implying a penetration of polyamide into the polysulfone surface pores. This research preliminary elucidated the correlation of the interfacial properties to TFC FO membrane transport properties and also highlighted the potential and robustness of these elemental analysis techniques in characterization of the nanostructure at the active layer–support layer interface of TFC FO membranes.The third section of this research presented an in situ surface modification on TFC FO membrane to achieve better antifouling properties. The membrane was fabricated and modified in situ, grafting an amine-terminated poly(ethylene glycol)(PEG), to dangling acyl chloride surface groups on the nascent polyamide active layer. Surface characterization by contact angle, ATR-FTIR, XPS, Zeta potential, and AFM, confirmed the successful grafting of PEG on the membrane surface. Improved fouling resistance of the modified membranes was demonstrated through dynamic fouling experiments, which showed a significantly lower flux decline for the modified membranes compared to pristine polyamide(14.3% ± 2.7% vs 2.8% ± 1.4%, respectively). AFM adhesion force measurements with a modified tips was used to model foulant–membrane interactions. The modified membranes exhibited weaker foulant–membrane interactions, further confirming its enhanced fouling resistance.In the fourth section of this research, we investigated the role of membrane surface chemistry and charge on reverse ion diffusion and initiated bidirectional ion diffusion in forward osmosis(FO). In particular, bidirectional diffusion of ammonium(NH4+) and sodium(Na+) was examined using FO membranes with different materials and surface charge characteristics. Using an ammonium bicarbonate(NH4HCO3) draw solution, we observed dramatically enhanced cation fluxes with sodium chloride feed solution compared to that with deionized water feed solution for TFC FO membrane. However, the bidirectional diffusion of cations did not change, regardless of the type of feed solution, for cellulose triacetate(CTA) FO membrane. We related this phenomenon to the membrane fixed surface charge by employing different feed solution p H to foster different protonation conditions for the carboxyl groups on the TFC FO membrane surface. Membrane surface modification was also carried out with the TFC membrane using ethylenediamine to alter carboxyl groups into amine groups. The modified TFC FO membrane, with less negatively charged groups, exhibited a significant decrease in the bidirectional diffusion of cations under the same conditions employed with the pristine TFC FO membrane. Based on our experimental observations, we proposed Donnan dialysis as a mechanism responsible for enhanced bidirectional diffusion of cations in TFC FO membranes.