Simultaneous removal of organic micropollutants and inorganic heavy metals by nano-calcium peroxide induced Fenton-like treatment

Groundwater can be contaminated by both organic micropollutants and inorganic heavy metals and thus, it is essential to develop environmental-friendly and cost-effective technologies for the remediation of such multiple contaminants. Advanced nanomaterials, including nano-calcium peroxide (nano-CaO2), induced Fenton-like treatment has been recently developed to effectively oxidise and remediate various organic micropollutants. The Ca(OH)2 residues have the potential to further remove toxic heavy metals via precipitation, however, it has been rarely studied. To investigate the proposed feasibility and understand the mechanisms, an optimised pH-regulated chemical precipitation method was developed to synthesis the nano-CaO2 material and then catalysed by Fe(II) towards simultaneous removal of the model compounds of p-nitrophenol (PNP) and cadmium (Cd). The Electron Spin Resonance (ESR) measurements demonstrated that hydroxyl radicals (∙OH) and singlet oxygen (1O2) are two major reactive oxygen species that lead to 93 % removal of PNP under the initial concentration of 40 mg/L. Simultaneously, over 99 % of the Cd (initial concentration of 10 mg/L) was removed through the precipitation with Ca(OH)2 and/or co-precipitation with ferrite. Such best removal performances were achieved under the optimal dose ratio of nano-CaO2 and Fe(II) at 500 mg/L to 75 mg/L, respectively. The existence of sunlight illumination and competition ions, i.e. K+, Na+, Ca2+, Mg2+, SO42-, NO3–, and Cl−, showed negligible effect on the removal performance, which supported its feasibility for the treatment of both ground- and surface water. Increasing the pH to 9, the time to remove 99 % Cd would be shortened from 60 min to 30 min, however, the degradation of PNP would dramatically reduce from 93 % to 20 % with 180 min. The removal performance was not affected by a large range of anions and cations, such as Na+, K+, Ca2+, Mg2+, Fe3+, SO42−, Cl− and NO3– ions, however, the existence of HCO3– and Mn2+ should be taken into consideration during the application as they could lead to obvious impacts on the treatment. Overall, this study provided a new insight of removing organic micropollutants and inorganic heavy metals simultaneously from groundwater via mechanisms revealed ex-situ nano-remediation technique.