Phosphate recovery using activated sludge cyanophycin: Adsorption mechanism and utilization as nitrogen-phosphorus fertilizer

Recovery of biopolymers is an important way of resource recovery in wastewater treatment plants. However, their recycling and application pathways need to be further developed. This study explored the application potential of cyanophycin granule polypeptide (CGP) extracted from activated sludge in recycling phosphate from wastewater and used for nitrogen-phosphorus fertilizer. The results showed that the adsorption behavior of phosphate by CGP was well described by Elovich and Langmuir-Freundlich models, which reflects the uneven energy distribution on the surface of CGP and belongs to multimolecular layer adsorption. The maximal adsorption capacity of CGP to phosphate was 53.739 mg P/g, and the effects of anions and macromolecules on adsorption are negligible. The guanidyls in CGP were involved in binding phosphates, and the adsorption energies of CGP-HPO42- and CGP-H2PO4- calculated by DFT are -0.645 and -0.179 eV, respectively. Moreover, CGP absorbed phosphate significantly promoted the germination and growth of Brassica napus, and altered communities of rhizosphere rare species. Transcriptomic results of Brassica napus showed that nitrate uptake and arginine synthesis were down-regulated and arginine metabolism was up-regulated, which may be attributed to the degradation of CGP to arginine and ammonium in soil and uptake by plant. Nitrogen and phosphate released by CGP degradation promoted plant hormone signal transduction, thereby accelerating cell enlargement and plant growth. These results indicate that CGP can effectively recover phosphate and be utilized as a nitrogen-phosphorus fertilizer.