(376aw) Mitigation of Bidirectional Solute Flux Via Membrane Surface Coating of Zwitterion Functionalized Carbon Nanotubes in Forward Osmosis Process

Osmotically driven membrane processes are being developed to address global freshwater shortage, wastewater reuse, and water-energy nexus. Among those processes, forward osmosis (FO) utilizes osmotic pressure gradient across a semi-permeable membrane to reclaim high-quality water for versatile applications, such as irrigation, food processing and life support. The major advantages of FO-based water treatment systems include reduced operating pressure, high rejection of undesired compounds, low fouling intensity of FO membranes, and less energy demand if energy-intensive draw regeneration can be properly addressed [1, 2].

A key challenge and also impediment for FO applications is the bidirectional solute flux, including reverse solute flux (RSF) and forward solute flux (FSF). RSF is defined as the cross-membrane diffusion of draw solutes to the concentrated feed solution, and can result in severe loss of draw solutes and gradual salinity build-up at the feed side, leading to reduced osmotic driving force, increased fouling propensity, and elevated operation cost due to periodical replenishment of draw solutes [3, 4]. Accumulation of the reversed draw solutes will require further treatment of the feed solution before it can be discharged to a natural water body. Thus, mitigation of solute accumulation at the feed side is important, and several approaches have been proposed and studied. However, most of these approaches, including integrated microfiltration or electrodialysis, can only reduce salinity accumulation in the feed solution, they cannot mitigate or reduce RSF effectively.

FSF (also known as membrane rejection) is defined as the cross-membrane diffusion of feed solute to the diluted draw, and can lead to undesirable contamination of final product water and enhanced fouling propensity in downstream draw solute regeneration process (e.g. reverse osmosis, RO). Previous study indicated that ~20% of the phosphorus in wastewater (the feed) could penetrate through FO membrane and accumulate in the final draw [5]. Similar to RSF, FSF is resulted from FO membrane failing to be a perfect barrier, and has yet to be effectively addressed by advanced membrane fabrication or modification.

Herein, mitigation of bidirectional solute flux in FO process has been comprehensively investigated via surface coating of zwitterion-functionalized carbon nanotubes (Z-CNT). If embedded in membrane structure, the Z-CNT was confirmed to enhance RO desalination performance, leading to elevated salt rejection and enhanced biofouling resistance [6]. Hence, the Z-CNT was further introduced into FO process to evaluate potential mitigation of bidirectional solute flux via surface deposition. Specific RSF and FSF with a unit of gram diffused solute per liter recovered water (g L^-1) were selected to enable better comparison with previous studies. With simple evaporation coating of 0.1 mg cm^-2 Z-CNT on active layer, the coated commercial FO membrane presented a consistent RSF of 0.28 ± 0.02 g L^-1 for four consecutive cycles (12 hours each with 1 mol L^-1 NaCl as the draw and deionized water as the feed), rendering a 56.3% reduction comparing to that of the original membrane (0.64 ± 0.01 g L^-1). Comparable water flux (p>0.10) was observed between the coated (12.17 ± 0.26 LMH) and the original (12.46 ± 0.73 LMH) FO membranes. Subsequently, various initial draw concentrations (0.25, 0.50, 0.75, and 1.00 mol L^-1 NaCl) were successively tested, rendering a very stable and mitigated specific RSF (~0.30 ± 0.01 g L^-1). The Z-CNT coated FO membrane also exhibited effective RSF mitigation over a wide selection of commonly used draw solutes, including (NH₄)₂HPO₄ (74.5% reduction), NH₄H₂PO₄ (83.8% reduction), NH₄Cl (70.8% reduction), and NH₄HCO₃ (61.9% reduction). Further investigation focused on assessing potential mitigation of specific FSF by replacing deionized water with synthetic wastewater as the feed and 1 mol L^-1 NaCl as the draw. The penetration efficiencies of contaminants across the Z-CNT coated FO membrane were significant reduced from 87.8% to 50.2% for NH₄⁺-N, 26.1% to 16.4% for NO₃^--N, 38.5% to 27.5% for NO2^--N, 60.9% to 26.4% for K⁺, 4.6% to 0% for Mg²⁺, 0% to 0% for Ca²⁺, 4.3% to 2.6% for SO₄^2--S, and 2.4% to 0% for PO₄^3--P. These results have collectively demonstrated that consistent mitigation of bidirectional solute flux in commercial FO membrane could be effective achieved with simple surface coating of Z-CNT and warrants further investigation on membrane fouling resistance by using real wastewater.

References

[1] Liu, Z., Bai, H., Lee, J. and Sun, D.D. (2011) A low-energy forward osmosis process to produce drinking water. Energ. Environ. Sci. 4(7), 2582-2585.

[2] Su, J., Zhang, S., Ling, M.M. and Chung, T.S. (2012) Forward osmosis: an emerging technology for sustainable supply of clean water. Clean Technol. Environ., 1-5.

[3] Achilli, A., Cath, T.Y. and Childress, A.E. (2010) Selection of inorganic-based draw solutions for forward osmosis applications. J. Membr. Sci. 364(1), 233-241.

[4] Boo, C., Lee, S., Elimelech, M., Meng, Z. and Hong, S. (2012) Colloidal fouling in forward osmosis: Role of reverse salt diffusion. J. Membr. Sci. 390–391, 277-284.

[5] Zou, S., Qin, M., Moreau, Y., He, Z. Nutrient-Energy-Water Recovery from Synthetic Sidestream Centrate Using a Microbial Electrolysis Cell - Forward Osmosis Hybrid System. J. Clean. Prod. 154, 16-25.

[6] Chan, W.F., Marand. E., Martin, S.M. Novel zwitterion functionalized carbon nanotube nanocomposite membranes for improved RO performance and surface anti-biofouling resistance, J. Memb. Sci. 509 (2016) 125–137.