Electrochlorination Fenton-like and Fenton processes for different categories of phosphonates oxidation to orthophosphate: Performance and mechanism characteristics

Considering the potential eutrophication risk of phosphonate and scarcity of phosphorus resources, the oxidation of organically-bound phosphorus to orthophosphate (ortho-P) is an important prerequisite for high-value phosphorus recovery. This study investigated efficiency and mechanism of different categories of phosphonates oxidation to ortho-P by classical Fenton and practical electrochlorination Fenton-like processes. The target phosphonates included two categories: nitrogen-free phosphonates (NF-PPs), i.e. 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and 1-Hydroxyethane-1,1-diphosphonic acid (HEDP), and aminophosphonates (A-PPs), i.e. Nitrilotris(methylene phosphonic acid) (NTMP) and diethylenetriamine penta(methylene phosphonic acid) (DTPMP). The results showed that ortho-P conversion for both NF-PPs and A-PPs by classical Fenton process were below 35 %, even though the content of Fe(II) and H₂O₂ were high enough to neglect the influence of phosphonate-Fe(II) complexation on Fenton reaction initiation. While electrochlorination Fenton-like process greatly enhanced ortho-P conversion of HEDP, NTMP and DTPMP. Potent oxidation relied on generation of HClO, Fe^IVO²⁺, and ¹O₂ rather than HO•. In particular, PBTC failed to decompose effectively by electrochlorination Fenton-like process possibly due to chlorine resistance. The non-N-element group was inferred to serve as a decisive role in phosphonates decomposition. This study provided the first systematic description of oxidation rules and mechanism characteristics of diverse phosphonates under classical Fenton and electrochlorination Fenton-like treatment.