Anoxic conditions are beneficial for abiotic diclofenac removal from water with manganese oxide (MnO2)

Abstract

This is the first study examining pharmaceutical removal under anoxic conditions with MnO₂. This study compares the abiotic removal of seven pharmaceuticals with reactive MnO₂ particles in the presence of oxygen (oxic conditions) and in the absence of oxygen (anoxic conditions). Due to the novelty of pharmaceutical removal under anoxic conditions, the influence of phosphate buffer, pH, and MnO₂ morphologies is also examined. Results show that over 90% of diclofenac is removed under anoxic conditions. Additionally, we found that (1) anoxic conditions are beneficial for diclofenac removal with MnO₂, (2) phosphate buffer affects the pharmaceutical removal efficiencies, (3) higher pharmaceutical removal is obtained at acidic pH compared to that at neutral or alkaline conditions, and (4) amorphous MnO₂ removes pharmaceuticals better than crystalline MnO₂. The pharmaceutical molecular structure and properties, MnO₂ properties especially reactive sites of the MnO₂ surface, are important for degradation kinetics. This study provides a fundamental basis towards understanding pharmaceutical degradation with MnO₂ under anoxic conditions, and development of a cost-effective, sustainable technology for removal of pharmaceuticals from water.

Keywords: Abiotic pharmaceutical removal; Anoxic conditions; Manganese oxide; MnO2 morphologies; MnO2 reactivity mechanism; pH effects.

Fig. 1

Pharmaceutical removal with MnO₂ in demiwater with pharmaceutical mixture (a), demiwater with only diclofenac solution (b), phosphate buffer with only diclofenac solution under oxic conditions (black square) and anoxic conditions (black up-pointing triangle) (c). Experimental conditions: [MnO₂]₀ = 7 mM, [pharmaceutical]₀ = 1 mg L⁻¹, pH ~ 8.5. In phosphate buffer with diclofenac solution, [phosphate] = 50 mM, [ionic strength] = 0.1 M. Error bars are standard deviations determined

Fig. 2

The effects of phosphate on diclofenac removal with MnO₂ under oxic and anoxic conditions. Solid lines are processes under both oxic and anoxic conditions; dashed lines are the processes only under oxic conditions. (1) MnO₂ removes diclofenac via oxidation and produces Mn(II) (Forrez et al. ; Huguet et al. 2013); (2) Mn(III) comes from MnO₂ synthesis process (Remucal and Ginder-Vogel 2014); (3) Mn(II) is oxidized to Mn(III) by O₂; (4) Mn(III) from MnO₂ was stabilized by Mn₃(PO₄)₂ formed via Eq. 1 (Jin et al. 2014); (5) Mn(III) oxidizes diclofenac and produces Mn(II)