Highly efficient removal of thallium(I) from wastewater via hypochlorite catalytic oxidation coupled with adsorption by hydrochar coated nickel ferrite composite

Abstract

In this study, tannery wastewater was used as carbon source to hydrothermally synthesize magnetic carbon-coated nickel ferrite composite (NiFe2O4@C), which was employed as a catalyst for thallium (Tl) oxidation by hypochlorite and simultaneously as an adsorbent for Tl removal from wastewater. Compared with NiFe2O4@C adsorption or hypochlorite oxidation alone, the combination of NiFe2O4@C and hypochlorite substantially enhanced the rate and efficiency of Tl(I) removal. In addition, this process was highly effective for Tl(I) removal over a wide pH range (6–12). The maximum Tl(I) removal capacity was 1699 mg/g at pH 10, which is the highest one reported so far. Electron spin resonance spectra suggested the formation of hypochlorite-based free radicals induced by the NiFe2O4@C composite, which enhanced the Tl(I) oxidation and removal. Oxidation-induced surface precipitation and surface complexation were found to be the main Tl(I) removal mechanisms. Consecutive cyclic regeneration tests implied robust regeneration and reuse performance of the composite. Moreover, it was effective for Tl(I) removal from real industrial wastewater. Therefore, the hypochlorite catalytic oxidation coupled with adsorption by the magnetic NiFe2O4@C composite is a promising technique for Tl(I) removal from wastewater. This hybrid process also has great potential for the removal of other pollutants.