Hydrogels as Forward Osmosis Desali-nation Draw Agents Feasible with Thermal-Pressure Dewatering

In reverse osmosis, water is moved across a membrane using an applied pressure. In contrast, forward osmosis<!–[if supportFields]> XE “forward osmosis” <![endif]–><!–[if supportFields]><![endif]–> does not typically require this pressure, though it can be used for added efficiency, and instead takes advantage of natural osmosis from a high to low concentration of solutes across a semi-permeable membrane. In the desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–> process of forward osmosis, molecules chosen as a draw agent are used to create a higher concentration than the water being desalinated. Thus water naturally flows across the membrane to dilute the draw agent. To finally reclaim the desalinated water, the draw agent is removed. This can be done in several ways, with distillation being the most common. Therefore, draw agents are designed to be removed with a minimal energy cost; effectively, a low distillation temperature. The work done by Li et al. (2011) is on the use of hydrogels—complex polymer chains with a high concentration of hydrophilic regions—as draw agents. Upon taking in water, the polymers also expand to create an additional drawing force to pull water across the membrane. Lastly, hydrogels can be externally stimulated to change hydrophilic regions to hydrophobic ones, thus expelling purified water from the hydrogel. In this study, Li et al. look into two possible external stimuli—hydraulic pressure and a combination of hydraulic pressure with low temperature thermal processing. It was found that the polymer poly(sodium acrylate)-co-poly(N<!–[if supportFields]> XE “nitrogen, N” <![endif]–><!–[if supportFields]><![endif]–><!–[if supportFields]> XE “nitrogen” <![endif]–><!–[if supportFields]><![endif]–>-isopropylacrylamide)—PSA-NIPAM for short—was the most effective when both water intake and dewatering were taken in account.—Erin Partlan
Li, D., Zhang, X., Yao, J., Simon, G., Wang, H., 2011. Stimuli-responsive polymer hydrogels as a new class of draw agent for forward osmosis<!–[if supportFields]> XE “forward osmosis” <![endif]–><!–[if supportFields]><![endif]–> desalination<!–[if supportFields]> XE “desalination” <![endif]–><!–[if supportFields]><![endif]–>. Chem. Commun. 47, 1710–1712.
Li et al. tested four different hydrogels to see the effect of ionic charge on water intake and thermal sensitivity in water expulsion. The flux<!–[if supportFields]> XE “flux” <![endif]–><!–[if supportFields]><![endif]–> of water across a membrane was measured over time, and it was found that the charged polymers affected a greater drawing force on the water. In all four cases, the flux of water decreased over time as expected as the hydrogels filled with water. This decrease in flux will occur in all batch-operated forward osmosis<!–[if supportFields]> XE “forward osmosis” <![endif]–><!–[if supportFields]><![endif]–> processes since it is dependent on the difference in solute concentration. For dewatering, Li et al. used hydrostatic pressure at varying temperatures and on hydrogels with varying water contents. It was found that dewatering at room temperature for short times (two minutes) was not effective as all hydrogels released less than 5% of their water, even when comparing hydrogels with 50% water content vs. 80% water content. However, when dewatering was performed for the same amount of time and pressure at an elevated temperature of 50 C, it was highly effective; the non-charged, thermally-sensitive polymer only released 5% of the 80% water content hydrogel at room temperature, but it released 75% at the elevated temperature. Other hydrogels did not experience as marked a change with warming, but there was some improvement; the charged, thermally-sensitive polymer released 3% of a 66.7% water content hydrogel at room temperature, compared to 17% at the elevated temperature. Overall, considering both the processes of forward osmosis and dewatering at an elevated temperature, PSA-NIPAM was found to be the most effective.
Li et al. suggest that this hydrogel can be effectively used with optimization of conditions. While the authors showed that thermal-pressure dewatering is feasible, they suggest that further research be conducted to determine the effects of other stimuli on the hydrogels—for example, using solar energy to stimulate the hydrogel using both heat and light.

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