Self-driven phosphate enrichment by hydrogel beads for nutrient recovery

The enrichment of phosphate is necessary for high-efficiency nutrient recovery from wastewater through struvite precipitation. However, the majority of current nutrient enrichment processes focus on membrane-based technologies driven by external energy input. Here, the phosphate enrichment by negatively charged Poly(sodium acrylate) hydrogel beads as the self-driven dewatering agent under different conditions was investigated. The phosphate rejection decreased as its concentration increased but retained 56.9% even in 10 mmol/L PO₄³⁻ solution, which is well beyond the phosphate concentration in typical wastewater concentrates. Phosphate was concentrated 3.6 folds with a recovery of 70% using ∼1 wt% of hydrogel beads in 0.5 mmol/L phosphate solution. The effects of the pH, ionic strength of the nutrient stream, and the swelling ratio of hydrogels on the rejection of phosphate were investigated. In addition, the hydrogel beads removed 100% of nickel ions during the dewatering of the phosphate solution (0.5 mmol/L Ni²⁺ and 0.5 mmol/L PO₄³⁻), presenting an opportunity for simultaneous phosphate enrichment and purification during the pretreatment for nutrient recovery from wastewater. This study demonstrated that the spontaneous dewatering process utilizing ion-selective hydrogels is promising for nutrient enrichment to promote recovery efficiency.