Understanding colloidal FeSx formation from iron phosphate precipitation sludge for optimal phosphorus recovery

•Phosphate can be recovered from synthetic FePO4 via S2− addition.•Much improved settling of the colloidal FeSx particles was obtained at pH 4.•The isoelectric point of colloidal FeSx particles was found to be at pH 4.•An efficient method for P recovery from chemical iron-based sludge is demonstrated.•The method proposed is a step toward the sustainable use of P as a resource.

The use of sulfide to form iron sulfide precipitates is an attractive option for separation and recovery of phosphorus and ferric iron from ferric phosphate sludge generated in wastewater treatment. The key factors affecting the simultaneous generation and separation of iron sulfide precipitates and phosphate solution from ferric phosphate sludge have so far not been thoroughly investigated. This study therefore focuses on the recovery of phosphorus from synthetic sludge by controlled sulfide addition under different operating conditions. The factors that affect the phosphorus recovery, as well as the optimal process conditions to achieve an effective solid–liquid separation, were investigated. The separation of the FeSx particles is a significant challenge due to the colloidal nature of the particles formed. Faster separation and higher phosphorus recovery was achieved when operating at pH 4 with dosing times of at least 1 h. At this pH, phosphorus recovery of 70 ± 6% was reached at the stoichiometric S/Fe molar ratio of 1.5, increasing to over 90% recovery at a S/Fe molar ratio of 2.5. Zeta potential results confirmed the colloidal nature of the iron sulfide precipitate, with the isoelectric point around pH 4, explaining the fast separation of the FeSx particles at this pH.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (48 K)Download as PowerPoint slide