Sequential oxidation and crystallization based on split-flow electrochemical oxidation with rapid filtration for...

Phosphonate oxidation can be achieved through multiple treatment pathways, yet the subsequent recovery of phosphorus from oxidation products remains a major challenge. In this study, an electrochemical oxidation (EO) process in a chloride-containing electrolyte was employed to rapidly convert ethylenediamine tetra(methylene phosphonic acid) (EDTMP) to orthophosphate (PO₄^3-). At a current density of 22.91 mA cm^-2 and a hydraulic retention time of 20 min, the EO process achieved 90% conversion of EDTMP to PO₄^3-. Implementing a split-flow anodic configuration produced a stable alkaline zone having a pH of 10-12 downstream of the cathode, enabling the in-situ crystallization of PO₄^3- with Ca²⁺ ions to form hydroxyapatite (HAP) colloids. Fine HAP particles were captured using a filtration column incorporating a graphite felt (GF) barrier and alumina (Al₂O₃) particles (AP) as the filtration media. This attachment allowed >90% phosphorus recovery within only 6 min at a split-flow ratio of 0.4. Most of the phosphorus-based products were captured on the GF and AP surfaces and the resulting phosphorous-enriched AP could be directly utilized as a slow-release fertilizer. This work demonstrates an efficient and scalable strategy for transforming phosphonate-laden wastewater into recoverable HAP colloids, offering new opportunities for sustainable phosphorus recovery and wastewater treatment.