Synthesis of nano zero-valent metals from the leaching liquor of automobile shredder residue: A mechanism and potential applications for phenol degradation in water

• A leaching liquor of ASR was used to obtained nanoscale zero-valent metals (nZVMs).
• Phenol removal was increased with increases in the concentrations of nZVMs and H2O2.
• The highest removal of phenol was found at a pH of 3.0.
• The degradation of phenol was well fitted with pseudo-second-order kinetics.
• Possible mechanisms for the degradation of phenol by nZVMs were proposed.

Automobile shredder residue (ASR) (<0.25 mm) is a good source for the synthesis of nano zero-valent metals (nZVMs). The leaching of heavy metals was carried out with ASR using an ultrasonically assisted acid. nZVMs were prepared from the leaching liquor of ASR at a pH of 7 followed by the addition of a reducing agent. The degradation of phenol by the prepared nZVMs in an aqueous solution was examined to assess the possibility of applying nZVMs to remove phenol from wastewater. The prepared nZVMs were analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The effects of the nZVM dosage, the initial pH, and of different concentrations of phenol and H2O2 on the degradation of phenol were assessed. The phenol degradation rate was increased from 96.7% to 98.8% with an increase in the nZVM dose from 0.125 to 0.5 g/L. The highest phenol degradation rate was achieved at a pH of 3.0 with a removal efficiency of approximately 98.3%. The degradation efficiency of phenol was decreased from 99.0% to 91.4% with an increase in the phenol concentration from 5 to 40 mg/L; however, the degradation efficiency increased from 97.7% to 99.1% with an increase in the H2O2 concentration from 50 mM to 200 mM. XPS analyses found that various elements (Zn, Cu, Mn, Fe, Ni, Cr, Al and C) existed in the synthesized nZVMs. The degradation of phenol was well fitted with the pseudo-second-order kinetics. The pseudo-first-order and pseudo-second-order reaction rate constants are represented by kobs and k, respectively. Both increased with an increase in the amount of nZVM and the concentration of H2O2; however, the values of these constants were reduced as concentration of phenol increased. The value of kobs was reduced when the pH and the concentration of phenol increased. The possible mechanism for the degradation of phenol by nZVMs was the oxidation of phenol by hydroxyl radicals which were generated in the liquid medium during the reaction between H2O2 and nZVMs.

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