(480a) Assessment of Temperature- and pH-Sensitive Hydrogels for Dewatering Dilute Algal Suspensions

In this study, temperature-sensitive poly-N-isopropylacrylamide (pNIPAAm) hydrogels and pH-sensitive poly-acrylic acid (PAA) hydrogels were investigated as reusable absorbents for dewatering dilute algal suspensions. When algal cultures were exposed to pNIPAAm hydrogels at room temperature, the gels absorbed a portion of the growth medium and swelled, while algal cells were not absorbed and stayed excluded from the gels. As a result, the concentration of suspended cells in the media increased during incubation of the culture with hydrogels. When the swollen pNIPAAm hydrogels were removed and incubated at 40 ºC, the absorbed medium was released due to collapse of the temperature-sensitive gels. After recovery of the cell-free medium, the de-swollen pNIPAAM could then be re-used to absorb more media from the culture and further increase cell concentrations. Based on these observations, cyclic swelling and deswelling was used to increase concentrations of cultures to process relevant values. Since swelling rates were initially high, cyclic swelling for 2h followed by deswelling was applied to repeatedly absorb media and concentrate the algal cultures. An increase in concentration from 1.12g/L to 98g/L was achieved in 8 cycles. While further removal of media to obtain thick algal pastes (>140g/L) is possible, recovery of gels from such concentrated slurries was observed to be difficult.

pH-sensitive PAA hydrogels were explored as an alternative to the temperature-sensitive pNIPAAm. PAA hydrogels absorbed media at normal growth pH (pH >7) and deswelled when the pH was reduced (pH <5). pH shifts over this range could be achieved by sparging the growth medium with CO₂-enriched air.

Overall, the results from these experiments demonstrate that stimuli-responsive gels could be an effective way to concentrate dilute algal cultures for further processing. An economic analysis of the hydrogel based dewatering process will also be presented.