Electrochemical accessibility of porous submicron MnO2 spheres as active electrode materials for electrochemical capacitors

Impact of media coating on simultaneous manganese removal and remineralization of soft water via calcite contactor

The aim of this study was to investigate the negative impact of a newly-formed manganese (Mn)-layer on calcite dissolution in the long-term operation of a calcite contactor. Simultaneous removal of Mn and remineralization of soft water in an up-flow calcite contactor was conducted and led to a progressive loading of Mn into the calcite matrix. The calcite contactor demonstrated high Mn removal; however, the hardness release decreased from 32 to 20 mg CaCO₃ L^-1 after 600 h of operation on a high Mn concentration (5 mg L^-1) feed. For an elevated Mn concentration (i.e. 5 mg Mn L^-1) in the feed water, the coated layer was mainly composed of Mn which inhibits the mass transfer from the calcite core to the liquid phase. The superficial layer was identified as 5.2% Mn oxides (MnOx) by X-ray photoelectron spectroscopy (XPS). Therefore, it is postulated that Mn removal starts with an ion exchange sorption reaction between soluble Mn²⁺ from aqueous phase and Ca²⁺ from the CaCO₃ matrix which is followed by a slow recrystallization of MnCO₃ into MnO₂. On the other hand, when the Mn content in the feed water was lower (i.e. 0.5 mg Mn L^-1), a considerably lower amount of MnOx was detected on the coated media. For all the examined conditions, the formation of this coating improved Mn removal due to the autocatalytic nature of the adsorption/oxidation of dissolved manganese by MnOx. A mechanistic model based on calcite dissolution and the progressive formation of a MnO₂ layer was implemented in PHREEQC software to predict the reduction in hardness release expected in long-term operation. The model was calibrated with experimental data and resulted in realistic breakthrough curves. In order to accurately predict the pH of the effluent stream, a slow-rate recrystallization of MnCO₃ into MnO₂ was implemented (compared to the fast precipitation of MnO₂ or the absence of MnO₂ formation).

Adsorption of micropollutants present in surface waters onto polymeric resins: Impact of resin type and water matrix on performance

The occurrence of micropollutants in water resources is raising substantial concerns, worldwide. These pollutants may have adverse impacts on the aquatic ecosystem and human health. Even though activated carbon is commonly used as an adsorbent to remove micropollutants from water, its low removal of hydrophilic components, energy-intensive regeneration procedure and slow adsorption can impair its applicability. Polymeric resins have been suggested as an effective alternative adsorbent due to their high porosity and accessible adsorption sites, significant adsorption concentration and stable chemical properties. In this work, we evaluated the performance of five commercially available polymeric resins (including two ion exchange resins) for the removal of nine selected micropollutants in water. More specifically, we investigated the effect of polymeric resin type and concentration, contact time and water matrix on the removal efficiency of five pharmaceuticals, two pesticides and two endocrine disruptors of high current concern (diclofenac, sulfamethoxazole, fluoxetine, caffeine, carbamazepine, 17-β estradiol, norethindrone, atrazine and desesthylatrazine). Results presented herein indicated that two hydrophobic polymeric resins can effectively adsorb over 80% of the targeted micropollutants within 30 min when the resin concentration was higher than 2.5 mL L^-1. The adsorption data were well described with the Freundlich isotherm and the pseudo-second order kinetic model very well described the kinetic process of the selected micropollutants onto the polymeric resins. Moreover, we observed that increasing the synthetic water temperature from 4 to 22 °C led to a marginally higher micropollutant uptake and the presence of natural organic matter had no noted impact on the efficiency of the resins in removing the tested micropollutants when the resin dosage was 5.4 mL L^-1. On the basis of these promising results, we conclude that polymeric resins are a promising alternative to activated carbon for micropollutants sorption in drinking water treatment.

Manganese dioxide core–shell nanostructure to achieve excellent cycling stability for asymmetric supercapacitor applications

This study present a facile and low-cost method to prepare core–shell nano-structured β-MnO₂@δ-MnO₂, in which β-MnO₂@nano-wires act as the cores to form 3D networks and δ-MnO₂ as the shells.