Manganese is a common inorganic chemical element present in groundwater. Efficient removal of Mn from drinking water is important from an environmental and human health perspective. Basically, the presence of Mn in drinking water causes water supply pipes to clog because of the oxidation of Mn(II) to a precipitate (MnO2), resulting in a black color of water. Therefore, to meet quality standards for drinking water, a wide variety of physical and chemical methods, and/or their combinations, has been used to reduce Mn concentrations in polluted waters. Naturally formed MnOx in Mn removal filters is called birnessite, which is a common mineral in soils and sediments. Birnessite is quite ubiquitous and presents the most important naturally occurring reactant or catalyst in the transformation of many organic pollutants in natural environments. Consequently, numerous reports on the effect of various parameters (Mn2+, Mg2+, K+, pH, and anions) on the mineralogy and structure of birnessite have been published. Studies have shown that the structure and composition of MnOx and its environmentally-related reactivity are strongly dependent on, and are thus sensitive to, the solution’s chemical environment.
As of now, a MnOx filter used for simultaneous removal of ammonium, iron, and manganese from groundwater exists. Ideally, the MnOx forms a major component of birnessite. The existing MnOx filter depends on an oxidation–reduction reaction as reported. Considering the low dissolved oxygen (DO) concentration of groundwater, the influent of the filters typically undergoes an aeration treatment during water treatment. However, the effect of DO on the formation of MnOx in the filters has been ignored. To address this, the research group from the Xi’an University of Architecture and Technology in China led by Professor Tinglin Huang proposed to systematically characterize the mineral structure and reactivity of MnOx formed on the surface of the filter media with different influent DO concentrations during the start-up period. Their work is currently published in the research journal, Chemical Engineering Journal.
The research team explored the effects of dissolved oxygen on the formation of MnOx in manganese removal filters. In this endeavor, two identical filters were initialized by dosing potassium permanganate and manganese chloride with different influent DO concentrations. During the start-up process, the structural characteristics of the MnOx filter media in two filters were probed by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, Brunauer-Emmett-Teller specific surface area, and X-ray photoelectron spectroscopy.
Results of XRD analyses revealed that DO had great influence on the crystal structure of the samples. The authors reported that the morphologies of the MnOx changed from aggregated particles to a wrinkled shape in both filters. In addition, results of compositional analyses showed that the high DO condition was beneficial for the insertion of Ca, as well as the ion exchange of Ca and K, in the interlayer region of the MnOx. Also, it was observed that the MnOx that formed at high DO conditions possessed greater manganese removal activity than that formed at low DO conditions; this was related with the abundant lattice oxygen.
In summary, the study by Xi’an University of Architecture and Technology scientists probed the influence of dissolved oxygen on the crystal structure of MnOx samples. The researchers established that the vacancy defects played an important role in the removal of manganese at low DO conditions. Overall, the study plays an important role in optimizing the start-up of the MnOx filter and improves the understanding of the effect of DO on the formation of manganese removal activity and manganese removal mechanism. In fact, during an interview with Advances in Engineering, Professor Tinglin Huang, the lead author, pointed out that it was through their study that it was suggested that aqueous DO was a critical environmental factor controlling MnOx structure and reactivity in the environment. Generally, the study provides significant insights into future applications of the manganese removal techniques.
Ya Cheng, Tinglin Huang, Chenyang Liu, Shasha Zhang. Effects of dissolved oxygen on the start-up of manganese oxides filter for catalytic oxidative removal of manganese from groundwater. Chemical Engineering Journal, volume 371 (2019) page 88–95.