Abstract
The success of osmotically-driven membrane (OM) technology relies critically on high-performance membranes. Yet trade-off of membrane properties, often further complicated by the strongly non-linear dependence of OM performance on them, imposes important constraint on membrane performance. This work systematically characterized four typical commercial osmotic membranes in terms of intrinsic separation parameters, structure and surface properties. The osmotic separation performance and membrane scaling behavior of these membranes were evaluated to elucidate the interrelationship of these properties. Experimental results revealed that membranes with smaller structural parameter (S) and higher water/solute selectivity underwent lower internal concentration polarization (ICP) and exhibited higher forward osmosis (FO) efficiency (i.e., higher ratio of experimental water flux over theoretical water flux). Under the condition with low ICP, membrane water permeability (A) had dominant effect on water flux. In this case, the investigated thin film composite membrane (TFC, A = 2.56 L/(m2 h bar), S = 1.14 mm) achieved a water flux up to 82% higher than that of the asymmetric cellulose triacetate membrane (CTA-W(P), A = 1.06 L/(m2 h bar), S = 0.73 mm). In contrast, water flux became less dependent on the A value but was affected more by membrane structure under the condition with severe ICP, and the membrane exhibited lower FO efficiency. The ratio of water flux (Jv TFC/Jv CTA-W(P)) decreased to 0.55 when 0.5 M NaCl feed solution and 2 M NaCl draw solution were used. A framework was proposed to evaluate the governing factors under different conditions and to provide insights into the membrane optimization for targeted OM applications.
Highlights
Osmotically-driven membrane (OM) technologies, e.g., forward osmosis (FO) and pressure-retarded osmosis (PRO), have received much attention, largely fueled by the development of commercial FO membranes with high water flux in the last decade
The thin film composite (TFC) FO membrane in this study showed compromised mechanical strength, as a result of the higher porosity and macrovoids in the PSf layer, as well as the mesh support (Figure 2a)
In FO tests with more severe internal concentration polarization (ICP), an integral asymmetric cellulose triacetate (CTA) membrane (CTA-W(P)) with optimized structure of support layer showed higher water flux, despite of its lower water permeability coefficient
Summary
Osmotically-driven membrane (OM) technologies, e.g., forward osmosis (FO) and pressure-retarded osmosis (PRO), have received much attention, largely fueled by the development of commercial FO membranes with high water flux in the last decade. Water flux from FS to DS is driven by the osmotic pressure difference (∆π). Components in FS can be retained and concentrated, whereas water passing through the membrane can be recovered from the diluted DS by a re-concentration step if required. Niche applications using DSs without the need of regeneration (e.g., seawater [1], desalination brine [2], or fertilizer solutions [3]) are attractive due to the elimination of energy-intensive DS reconcentration step. OM and hybrid OM processes have been explored for diverse fields [4], such as wastewater reuse [5,6,7], desalination [8,9,10,11], osmotic membrane bioreactor [12], energy production [13,14,15], and food processing [16,17]
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