Copper phosphide and persulfate salt: A novel catalytic system for the degradation of aqueous phase micro-contaminants

The heterogeneous activation of sodium persulfate (SPS) using copper (I) phosphide (Cu3P) nanoparticles was tested in this work. The catalyst was synthesized via a two-step method involving preparation of Cu(OH)(2) and subsequent low temperature phosphidation and characterized with BET, XRD, TEM/HRTEM, SEM and XPS techniques. The XRD pattern showed the existence of hexagonal Cu3P phase with mean primary crystallite size of ca. 28 nm. The XP spectra indicated the presence of residual Cu(OH)(2) and Cu-3(PO4)(2) species on the catalyst surface, originating from the synthesis methods employed, which disappeared after exposure to reaction conditions. The activity of Cu3P was evaluated for the degradation of antibiotic sulfamethoxazole (SMX), which occurred in short reaction times. The effect of catalyst concentration (20-80 mg/L), SPS dosage (0.05-1 g/L) and SMX concentration (0.5-4.1 mg/L) on degradation was studied. Results obtained in ultrapure water (UPW) showed that SMX degradation increases with increasing catalyst content in the range 20-80 mg/L and SPS concentration in the range 0.05-0.5 g/L. In contrast, increasing SPS concentration at 1 g/L leads to lower reaction rates. Experiments were also conducted in bottled water, secondary treated wastewater, UPW spiked with bicarbonate or chloride ions, as well as UPW containing humic acid. With the use of t-butanol and methanol as radical quenching agents, SO4- was found to be the primary radical species responsible for SMX degradation. The simultaneous use of Cu3P and solar irradiation as persulfate activators resulted in a synergistic effect in experiments performed in UPW and wastewater. From a mechanistic point of view, Cu3P acts as an electron mediator or bridge to facilitate the electron transfer processes and this is accompanied by the intermediate formation of radicals. The catalyst remains intact, thus implying the catalytic nature of the process.