Recovery of phosphorus (P) from concentrated wastewater sources is key to creating a more sustainable phosphorus cycle. Using mineral-hydrogel composites containing calcium phosphate (CaP) and calcium silicate hydrate (CSH), here we show the synergistic performance of the two mineral seeds for phosphate (P) removal from a model P-rich solution containing concentrations ranging from 6.2 mg-P/L up to 50 mg-P/L. Mineral-hydrogel composites are formed by dripping a precursor solution of sodium alginate with variable amounts of ionic precursors of disodium hydrogen phosphate (P) and sodium silicate (Si). Using a model P-concentrated solution (0.6 mM CaCl2, 1.6 mM Na2HPO4, 30 mM NaCl, at initial pH 7), we tested the mineral-hydrogel composites for P-recovery by varying the amounts of and the ratio between the different ionic precursors. We found that mineral-hydrogel composites with 40 mM Si or 40 mM Si + 10 mM P mineral seeds performed best for P removal, recovering 114 and 128 mg P/g dry-hydrogel, respectively, after 1 reaction cycle of 6 hours. To further improve the performance, between reaction cycles the mineral-hydrogel composites were conditioned using CaCl2 and NaOH. To elucidate the mechanisms of the mineral-hydrogel composites during the P-recovery and conditioning process, using synchrotron-based wide angle x-ray scattering and ultra-small x-ray scattering, we examined mineral seed sizes and mineral phases in the composites along the reaction pathway. During the P-recovery process, the mineral seed underwent a phase transformation from CSH to CaP, and conditioning affected the phase transformation significantly. The conditioning process improved the P-recovery amount in the second cycle by 11% for both mineral seeds compared to mineral-hydrogel composites without the conditioning treatment. Overall, the 40 mM Si + 10 mM P mineral seeds recovered 14% more P than that of the 40 mM Si seeds after two cycles of reaction. This study presents a new environmentally sustainable way to recover P by utilizing mineral-hydrogel composites and their conditioning method between P-recovery operation cycles.

Advancing phosphate recovery through calcium silicate hydrate-hydrogel composite chemistry: Conditioning and phase transformation

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.