Phosphate removal by spent low temperature shift catalyst through process optimization and mechanistic study

Abstract

Abstract Natural water bodies become eutrophic due to phosphate pollution from industrial and agricultural sectors. This research addresses waste disposal and water pollution at the same time by recycling spent low-temperature shift catalyst (LTSSC) from the fertilizer industry as a sustainable phosphate adsorbent. High surface area (247 ± 5 m 2 /g), mesoporous structure, and many metal oxide sites (CuO 49%, ZnO 32.2%, and Al 2 O₃ 13.9%) were shown by characterization using XRF, XRD, BET, SEM–EDX, FTIR, particle size analysis, and zeta potential. Under optimum conditions (contact time 1.64 h, dose 5 g/L, initial concentration 9.58 mg/L, stirring 278 rpm), Response Surface Methodology optimization achieved 92.95 ± 2.8% phosphate removal. Chemisorption via inner-sphere complexation is established by endothermic spontaneous thermodynamics (ΔH° = 18.42 ± 1.3 kJ/mol, ΔG° = − 7.56 ± 0.6 kJ/mol, ΔS° = 36.6 ± 2.5 J/mol·K), FTIR-detected M–O-P bonds, XRD-confirmed crystalline AlPO₄ formation, and Langmuir isotherm (qmax = 0.174 ± 0.005 mg/g). Over 15 cycles, regeneration using 0.1 M NaOH demonstrated > 75% capacity retention. Heavy metal leaching tests showed concentrations below WHO/USEPA limits. The treatment cost, according to techno-economic study, is $0.52/m 3 , which is less than that of commercial adsorbents. This study shows how to use a circular economy method to address two environmental concerns by turning hazardous industrial waste into usable phosphate adsorbent.