Exploration of the key points to maintain Ca-birnessite activity for ammonium oxidation through phenol impact

Abstract

Ca-birnessite (CBS) has outstanding catalysis for NH4+-N oxidation in groundwater, but its activity obviously drops in surface water. Exploring the reason behind this phenomenon is vital for the CBS application in water treatment. The residual micro-organic pollutants in surface water may be the main culprit for the activity deterioration. In this work, phenol was chosen as a typical organic pollutant, and the discrepancies of the CBS without and with phenol impact were explored meticulously, including NH4+-N removal performance and the CBS microstructure. Phenol showed significant damage to the CBS catalytic activity. The NH4+-N removal efficiency dropped almost two-thirds after the CBS reacted with phenol. The results of XRD, HRTEM, XPS, et al. showed that phenol could easily destroy the CBS microstructure and change its surface characteristics. Firstly, due to phenol and its intermediates adsorption, the manganese content in the CBS decreased from 13.6 % to 9.5 %, and the carbon content increased from 36.2 % to 51.3 %, which meant a decrease of the number of active sites and inhibition of the recycle between Mn(III) and Mn(II). Secondly, the phenol-treating CBS possessed a weak capacity for NH4+ adsorption and NO3− desorption, which was supported by the zeta potential increasing from −14 mV to −7 mV. Thirdly, the interlayer passage of phenol-treating CBS became narrow (from 0.52 nm to 0.42 nm), leading to the traffic block for NH4+ entrance and NO3− exit. These results demonstrated that it is essential to remove organic pollutants in advance before using the CBS to remove NH4+-N from water.