Efficient reduction of Cr(VI) under visible light irradiation using CuS nanostructures

Photoreduction of hexavalent chromium, Cr(VI), identified as carcinogenic and mutagenic element, to Cr(III), believed to be an essential element, using copper sulfide nanostructures (CuS NSs) as photocatalyst was investigated under visible light irradiation (λ > 420 nm). The CuS NSs were synthesized at low temperature using a wet chemical route and fully characterized using various techniques such as scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and UV–vis spectrophotometry. The photocatalytic reduction of Cr(VI) using visible light irradiation was assessed by UV–vis by following the decrease of the characteristic absorbance at ∼352 nm in the presence of CuS NSs. While CuS NSs promoted Cr(VI) full reduction within 60 min, addition of adipic acid or formic acid accelerated significantly the photocatalytic reduction process. Under optimized conditions, CuS NSs rapidly (12 min) reduced Cr(VI) in presence of 0.5 mM adipic acid. Similarly, Cu NSs were found to be efficient for Cr(VI) reduction dissolved in water from a local lake. Stability studies showed that the photocatalyst could be recycled up to four times without any apparent decrease of its catalytic performance.