Poly (sodium acrylate-acrylamide) hydrogels for enrichment and purification of microalgal biomass in an open system: performance optimization and mechanistic analysis

Super adsorbent polymer gels can be utilized in microalgal culture systems to concentrate and harvest microalgal biomass through water absorption. In open microalgal culture systems, however, bacteria and other non-algal impurities may affect the water absorption efficiency of the hydrogels and the quality of harvested microalgae. This study prepared and tested hydrogels synthesized with varying sodium acrylate (SA) and acrylamide (AM) ratios in open systems to evaluate their biomass harvesting efficacy. Results showed that when W_SA:W_AM = 10:0, the chlorophyll a (Chl-a) concentration in the harvested microalgal biomass increased by 417.9 %, the Chl-a/VSS ratio increased by 3.7 %, and the concentration of extracellular polymeric substances (EPS) decreased by 9.5 % compared to the pre-harvest period. Additionally, the number of bacteria adsorbed in the hydrogel particles also significantly increased. It indicates that the poly (sodium acrylate-acrylamide) (PSA-AM) hydrogel absorbed both water and non-algal impurities, achieving both concentration and purification of microalgal biomass. Mechanistic analysis revealed that the pore size and ratio of the PSA-AM hydrogel acted as a sieve, separating microalgal cells from other substances such as water, EPS, and bacterial cells. Given that EPS and bacterial cells are more hydrophilic compared to microalgal cells, it may explain that the hydrogel particles absorbed water while also capturing EPS and bacterial cells. Moreover, the PSA-AM hydrogel exhibited superior reusability. In conclusion, this study provides valuable data and a theoretical basis for the application of PSA-AM hydrogel in open microalgal culture systems, which could further promote the purification of PSA-AM in microalgal biomass utilization by optimizing the preparation of hydrogels.