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

Abstract

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 Mn 2+ from aqueous phase and Ca 2+ 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).

Application of Magnesium Oxide Media for Remineralization and Removal of Divalent Metals in Drinking Water Treatment: A Review

“…They justified this mechanism by the quantitative measurement of the molar amount of Mg 2+ released and the amount of Pb 2+ or Cd 2+ adsorbed in batch experiments, finding a linear relationship [79]. While we were not able to find any other work investigating metal removal by cation exchange for MgO, previous research has found that calcite effectively removes Me 2+ from solution by such a mechanism [76].…”

“…In a water treatment context, the interaction of dissolved metals with a mineral surface becomes interesting if it leads to the retention of the metal. Using calcite for the remineralization and simultaneous removal of manganese from synthetic feedwater, Pourahmad and co-workers found that the media was progressively loaded with a stable MnO x film [76]. The authors proposed that, upon initial removal as MnCO 3 through an ionexchange sorption reaction, the metal carbonate slowly recrystallizes as MnO 2 .…”

“…Since then, several workers have adapted treatment chains to include magnesium addition to the water conditioning process. However, this includes only a few who aimed to introduce magnesium hardness through the application of MgO media [10,76,99]. Both Hasson and Schwartz focused on Mg 2+ remineralization, the latter improving the dissolution process with either CO 2 or H 2 SO 4 injections at the inlet of MgO columns [10,99].…”

“…Both Hasson and Schwartz focused on Mg 2+ remineralization, the latter improving the dissolution process with either CO 2 or H 2 SO 4 injections at the inlet of MgO columns [10,99]. Working at a scale more appropriate for domestic application, Pourahmad and co-workers successfully adapted a pure calcite contactor to improve the long-term remineralization of drinking water by introducing a fraction of MgO into the column [76]. It appears that despite being a media well known to the industry, the investigation of MgO remineralization for drinking water purposes has been somewhat overlooked in the research field.…”

“…While not yet adapted to MgO media, Pourahmad and co-workers investigated CaCO 3 /MgO blends in drinking water treatment and successfully validated manganese removal and remineralization via PHREEQC for a pure calcite column [76]. Importantly, this work suggests that although media coating eventually passivates the grains in terms of hardness and alkalinity release, this same coating improves metal removal due to the autocatalytic sorption of manganese by MnO x [76]. This illustrates the need to further investigate the modeling of media evolution over time when aiming to maintain both hardness addition and metal removal objectives.…”