Oxidative removal of aqueous steroid estrogens by manganese oxides

This study investigated the oxidative removal of steroid estrogens from water by synthetic manganese oxide (MnO2) and the factors influencing the reactions. Using 1 × 10−5 M MnO2 at pH 4, estrone (E1), 17β-estradiol (E2), estriol (E3) and 17α-ethinylestradiol (EE2), all at 4 × 10−6 M, were rapidly removed within 220 min, indicating the effectiveness of MnO2 as an oxidizing agent towards estrogens. E2 removal increased with decreasing pH over the tested range of 4–8, due most likely to increased oxidizing power of MnO2 and a cleaner reactive surface in acidic solutions. Coexisting metal ions of 0.01 M (Cu(II), Zn(II), Fe(III) and Mn(II)) and Mn(II) released from MnO2 reduction competed with E2 for reactive sites leading to reduced E2 removal. Observed differential suppression on E2 removal may be related to different speciations of metals, as suggested by the MINTEQ calculations, and hence their different adsorptivities on MnO2. By suppressing the metal effect, humic acid substantially enhanced E2 removal. This was attributed to complexation of humic acid with metal ions. With 0.01 M ZnCl2 in solutions containing 1 mg l−1 humic acid, the binding of humic acid for Zn(II) was determined at 251 mmol g−1. An in vitro assay using human breast carcinoma MCF-7 cells indicated a near elimination of estrogenic activities without secondary risk of estrogen solutions treated with MnO2. Synthetic MnO2 is therefore a promising chemical agent under optimized conditions for estrogen removal from water. Metal chelators recalcitrant to MnO2 oxidation may be properly used to further enhance the MnO2 performance.