A novel strategy for treating chromium complex wastewater: The combination of a Fenton-like reaction and adsorption using cobalt/iron-layered double hydroxide as catalyst and adsorbent

Advanced oxidation processes (AOPs) are promising technologies to treat wastewater containing heavy metal complexes, and the main challenge of treating wastewater containing chromium (Cr) complexes by these processes is the formation of more toxic and soluble hexavalent Cr (Cr(VI)). In this study, we proposed a combination process of a Fenton-like reaction and adsorption (Fenton-like reaction/adsorption) using cobalt/iron-layered double hydroxide (Co/Fe-LDH) as catalyst and adsorbent to treat Cr complex-containing wastewater. The results showed that the Cr(VI) formed during treatment process could be adsorbed by the Co/Fe-LDH immediately after its formation. The Co/Fe-LDH dosage was the key factor affecting the total Cr removal, and the initial pH of the solution was the key factor affecting the Co leakage from the material. Under suitable treatment conditions, the total Cr concentration could be reduced to less than 1.5 mg L−1. The superoxide free radical (O2−) is the main radical species in this process. During treatment, part of Cr(III) in Cr(III)-citrate is oxidised into Cr(VI), and the corresponding organic ligand is decomposed to H2O, CO2 or some intermediate products. Then, the released Cr(VI) and undecomposed Cr(III)-citrate are removed via the adsorption of Co/Fe-LDH. This study established a feasible strategy to treat wastewater containing Cr complexes by AOPs and avoid Cr(VI) accumulation.