Abstract
The synthesis of magnetic nanoparticles (MNPs) coated with hydrophilic poly-sodium-acrylate (PSA) ligands was studied to assess PSA-MNP complexes as draw solution (DS) solutes in forward osmosis (FO). For MNP-based DS, the surface modification and the size of the MNPs are two crucial factors to achieve a high osmolality. Superparamagnetic nanoparticles (NP) with functional groups attached may represent the ideal DS where chemical modifications of the NPs can be used in optimizing the DS osmolality and the magnetic properties allows for efficient recovery (DS re-concentration) using an external magnetic field. In this study MNPs with diameters of 4 nm have been prepared by controlled chemical co-precipitation of magnetite phase from aqueous solutions containing suitable salts of Fe2+ and Fe3+ under inert atmosphere and a pure magnetite phase could be verified by X-ray diffraction. Magnetic colloid suspensions containing PSA-coated MNPs with three different molar ratios of PSA:MNP = 1:1, 1:2 and 1:3 were prepared and assessed in terms of osmotic pressure, aggregation propensity and magnetization. Fourier Transform Infrared Spectroscopy (FTIR) confirmed the presence of PSA on coated MNPs and pristine PSA-MNPs with a molar ratio PSA:MNP = 1:1 exhibited an osmotic pressure of 30 bar. Molar ratios of PSA:MNP = 1:2 and 1:3 lead to the formation of less stabile magnetic colloid solutions, which led to the formation of aggregates with larger average hydrodynamic sizes and modest osmotic pressures (5.5 bar and 0.2 bar, respectively). After purification with ultrafiltration, the 1:1 nanoparticles exhibited an osmotic pressure of 9 bar with no aggregation and a sufficient magnetization of 25 emu/g to allow for DS regeneration using an external magnetic field. However, it was observed that the amount of PSA molecules attached to the MNPs decreased during DS recycling steps, leaving only strong chelate-bonded core-shell PSA as coating on the MNPs. This demonstrates the crucial role of MNP coating robustness in designing an efficient MNP-based DS for FO.
Highlights
Forward osmosis (FO) processes based on membrane technologies have garnered increased attention due to the great potential for lower energy techniques in wastewater treatment and desalination compared with a conventional reverse osmosis (RO) process [1]
We investigate the synthesis and characterisation of magnetic nanoparticles (MNPs) coated with hydrophilic poly-sodium acrylate (PSA), where the degree of ionization depends on the solution pH
In order to examine the stability of the PSA-coated MNPs and their technological applicability as draw solution (DS), we investigate the synthesis and characterisation of MNPs coated with PSA where three different molar ratios of PSA:MNP = 1:1, 1:2 and 1:3 were prepared by co-precipitation of Fe3+ and Fe2+ sulfates salts with hydrophilic PSA using ammonia in a one pot synthesis and purified with ultrafiltration (UF)
Summary
Forward osmosis (FO) processes based on membrane technologies have garnered increased attention due to the great potential for lower energy techniques in wastewater treatment and desalination compared with a conventional reverse osmosis (RO) process [1]. Based on the available literature, NaCl appears to be the most promising DS (approximately 40% of experiments), due to its high solubility, low cost and relatively high osmotic potential It has been used as a DS in concentrations from 0.3 M to 6 M, but is often used at 0.5 M, simulating the osmotic pressure of seawater and prompting the use of real seawater or RO brine as a DS [5]. An averaged water flux of 5.3 L/m2h and an osmotic pressure of 11.4 bar were obtained when using PSA-MNPs at very low DS concentration (0.078 %, wt %, ~ 1.3 g/L), where the polyelectrolyte chain remains extended. Others [7,17] report the use of (24–48 %, wt %) PSA draw solute (in the form of free polyelectrolyte osmotic agent) to generate the similar osmotic pressures (~11 bar). PSA-MNPs were characterized and assessed in FO using biomimetic aquaporin (AqP) flat sheet membrane
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