Abstract:

Objective Tetrakis(hydroxymethyl)phosphonium sulfate (THPS) flame-retardant finishing wastewater contains high levels of refractory organic phosphorus and coexisting carbon and nitrogen pollutants. The phosphorus in THPS exists in a stable organic-bound form, with the phosphorus atom shielded by four hydroxymethyl groups, rendering conventional chemical precipitation and biological treatment largely ineffective. This poses significant challenges to pollution control and low-carbon transformation in the textile industry under increasingly stringent discharge standards. This study systematically evaluates five advanced oxidation processes (AOPs) as pretreatment technologies for THPS finishing wastewater, aiming to identify optimal process routes for synergistic pollutant removal and carbon reduction.

Method Five AOPs, which are ozonation (O₃), alkali-activated persulfate (OH^-/PS), Fe²⁺-activated persulfate (Fe²⁺/PS), CaO₂-based Fenton-like oxidation (CaO₂-Fenton), and potassium ferrate (K₂FeO₄) oxidation, were investigated for treating real wastewater from a textile finishing plant (TP: 82.04 mg/L, $\mathrm{P}{\mathrm{O}}_{4}^{3-}$PO43-: 10.52 mg/L, NH₃-N: 302.99 mg/L, TN: 358.75 mg/L, TOC: 467.90 mg/L, COD:1 176 mg/L, pH=3.33). Experiments were conducted under optimal conditions at room temperature ((25±2) ℃), followed by chemical precipitation with CaCl₂ and MgCl₂ at pH=11, and the performance was comprehensively assessed based on TP, TOC, NH₃—N, and TN removal efficiency, operational cost, sludge production, and carbon emissions. A multi-criteria evaluation framework incorporating radar chart analysis was employed to provide a holistic comparison across all five processes.

Results All five AOPs exhibited limited direct mineralization of organic phosphorus to ortho-phosphate (PO₄^3-). The core mechanism was the partial cleavage of C-P bonds in THPS molecules, converting stable organic phosphorus into phosphate-group-bearing organic intermediates. These intermediates demonstrated significantly enhanced coordination capacity with Ca²⁺ and Mg²⁺ compared to intact THPS, as the exposed oxygen lone pairs of phosphate groups facilitate stronger complexation and more stable precipitation, forming a pre-oxidation activation-complexation precipitation synergistic mechanism. This explains why CaO₂-Fenton, despite generating the lowest aqueous PO₄^3- increment, achieved the highest TP removal rate (98.72%), while O₃, which produced the highest PO₄^3- increment, yielded a lower TP removal rate (70.52%) which free PO₄^3- is susceptible to competitive interference from coexisting ions in the high-strength wastewater matrix, limiting its precipitation efficiency. Among all combined processes, CaO₂-Fenton achieved the highest TP removal rate(98.72%), followed by K₂FeO₄ (72.33%), O₃ (70.52%), Fe²⁺/PS (56.29%), and OH^-/PS (53.64%). For organic carbon removal, O₃ exhibited the highest TOC removal rate (90.39%), while OH^-/PS and CaO₂-Fenton achieved 72.80% and 70.07%, respectively. K₂FeO₄ demonstrated the most effective nitrogen transformation, attributed to the direct oxidation capacity of Fe(VI) and the synergistic adsorption-coprecipitation of Fe(III) flocs. In terms of operational costs, OH^-/PS was the most economical (3.93 CNY/m³) with the lowest unit TOC removal cost (11.5 CNY/g), while CaO₂-Fenton achieved the lowest unit phosphorus removal cost (71.3 CNY/g). O₃ incurred the highest overall cost (12.30 CNY/m³) due to electricity consumption and generated direct carbon emissions of 5.26 kgCO₂/m³, whereas chemical-based processes with no direct electricity consumption demonstrated superior synergistic benefits for pollution reduction and carbon mitigation.

Conclusion AOPs combined with chemical precipitation are proven to provide effective pretreatment for THPS flame-retardant finishing wastewater via the pre-oxidation activation-complexation precipitation mechanism, which offers a new perspective for understanding organic phosphorus removal in complex industrial wastewater. Process selection should be guided by treatment objectives and cost constraints. CaO₂-Fenton is recommended for phosphorus-priority scenarios given its superior TP removal and lowest unit phosphorus removal cost, OH^-/PS is optimal for organic matter removal and low-carbon operation given its lowest overall operational cost, and K₂FeO₄ is preferred when stringent nitrogen discharge requirements apply. These findings provide scientific insight for low-carbon process selection in textile flame-retardant finishing wastewater treatment.

Key words: organic phosphorus wastewater, advanced oxidation process, pretreatment, oxidation-precipitation synergy, synergistic reduction of pollutant and carbon emission, wastewater treatment