Improving remineralization and manganese-removal of soft waters using a mixed CaCO3/MgO contactor

https://doi.org/10.1016/j.jwpe.2022.102995Get rights and content

Highlights

  • Adding 5 % MgO (by weight) to a calcite contactor improved Mn removal.

  • A feed water with 5 mg Mn/L was reduced to below 20 μg/L.

  • Long-term alkalinity/pH correction was improved by adding MgO to calcite.

  • Mixed MgO/CaCO3 can mineralize and remove Mn simultaneously.

Abstract

The aim of this study was to investigate the role of MgO on the long-term operation of a mixed media contactor. Specifically, the simultaneous removal of aqueous manganese and the remineralization of soft waters using pure calcite are limited by a remineralization breakthrough after 600 h of operation in the presence of high Mn concentrations (5 mg Mn/L). The introduction of as little as 5 % (m/m) MgO was able to improve the Mn removal kinetic, while maintaining remineralization objectives over 720 h of operation. After treating elevated concentrations of Mn, both media exhibited a Mn-coated layer which contributed to limiting the mass transfer from the media core to the liquid phase. X-ray photoelectron spectroscopy (XPS) identified this superficial layer as 10 % Mn oxides (MnOx) on MgO media compared to 1.4 % MnOx on calcite, suggesting the MgO acts as the preferred reaction surface during Mn removal. Therefore, it is postulated that in the presence of MgO, Mn removal is impacted by high pH conditions (pH = 10.5) which introduces a significant precipitation mechanism as Mn2+ is oxidized. 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. MgO is therefore thought to contribute to a complex sorption-coprecipitation process in the operation of a mixed media contactor.

Introduction

Drinking water with low mineral content or which has been softened during a treatment process often requires remineralization in order to avoid corrosion. Also called water conditioning or stabilization, the remineralization process typically aims to achieve bicarbonate equilibrium as well as to increase pH and alkalinity. At the municipal level, various post-treatment options are available, including the direct dosage of chemicals, blending of hard water with a soft effluent, and mineral dissolution [1]. For residential and small-scale installations, the application of a passive mineral dissolution step at the point-of-entry (POE) is preferred [2]. As such, calcite (CaCO3 or limestone) contactors are a common residential POE technology, due to the wide availability of the media, simple design and capacity to treat soft waters without requiring continuous chemical addition [1]. Due to increased pollution of aquatic environments, there is also a demand for improved and novel treatment techniques which are simultaneously capable of removing contaminants (organic, inorganic or pharmaceutical). Among these, the removal of heavy metals is of particular interest due to their associated health risks, and the fact that they may naturally occur at high concentrations depending on the water source.

The benefit of a CaCO3 based treatment is two-fold; the mineral provides calcium (Ca2+) hardness and bicarbonate alkalinity (Eq. (1)), as well as having been found effective in reducing aqueous metal concentrations by sorption/ion exchange (Eq. (2)).CaCO3s+CO2g+2H2OlCaaq2++2HCO3aqCaCO3s+Meaq2+MeCO3s+Caaq2+

Several researchers have studied the advantage of CaCO3 surface sorption of divalent metals (Me2+) in both wastewater and drinking water applications and have found the media to be especially effective for manganese (Mn) removal [3], [4], [5]. In a drinking water context, the occurrence of Mn causes aesthetic and operational issues and Mn removal is thus a common treatment objective [6]. As of 2019, Canada has introduced a health-based regulation of Mn (0.12 mg/L) due to increasing evidence of manganese neurotoxicity, especially for infants and children ([7], [8]; Health [9]). Currently, POE catalytic filtration with intermittent regeneration or POE cationic ion exchange (IX) coupled with point-of-use reverse osmosis are two of the most common domestic installations to remove Mn [10]. In an effort to address the shortcomings of these methods (such as the risk of Mn leaching and the high consumption of salts and production of spent brine), we previously proposed an alternative Mn removal method using a novel back-washable hollow fiber nanofiltration (HFNF) membrane [11]. The soft permeate which is produced by the HFNF membrane process requires remineralization, and it was determined that coupling the treatment with a calcite contactor polishing step increased hardness as well as reduced remnant levels of Mn which were not completely rejected by the membrane [12]. Further work led to the conclusion that although effective for the simultaneous remineralization and removal of Mn from soft waters, the lifespan of the pure calcite contactor was limited by the formation of a solid Mn-coating on the media surface. While this coating improved the Mn-removal capacity of the media due to the autocatalytic nature of the adsorption/oxidation of dissolved manganese by MnOx, it also caused a significant decrease in remineralization after 600 h of operation when treating high concentrations of influent manganese (5 mg Mn/L) [5]. The performance of a contactor can be improved by acidification at the inlet (i.e. injection of CO2) [1]; however, the initial aim of implementing a mineral dissolution step was to simplify the treatment chain and to minimize operational costs. Hence, it becomes interesting to investigate whether the contactor media can be adapted to extend the remineralization capacity in the presence of dissolved divalent metals such as Mn without adding an acidification step.

The dissolution of magnesium oxide (MgO) media introduces magnesium (Mg2+) hardness to the water. This is of particular interest as the World Health Organisation (WHO) recommends a baseline of 20 mg/L of calcium and 10 mg/L of magnesium in drinking water for the health benefit of the consumer. As such, several researchers have studied Mg2+ based remineralization using MgO [13], [14]. Notably, the MgO surface rapidly hydrates upon contact with water. The long-term addition of Mg2+ hardness and hydroxide (OH) alkalinity are therefore widely considered to be controlled by the hydration surface, or Mg(OH)2 [15]:MgOs+H2OlMgaq2++2OHaqMgaq2++2OHaqMgOH2s

MgO is also known to be an effective media in the removal of divalent metals when treating contaminated groundwater or acid mine drainage ([16], [17]; Tobias Stefan [18]). This is due to the oxidation of divalent metals at the high pH (up to 12) produced by the dissolution of MgO. Consequently, MgO is an appealing media choice to integrate with a calcite contactor. In fact, mixed CaCO3/MgO filters are already commercially available, with the recommendation of limiting the MgO fraction to 10–20 % due to its high reactivity. However, to the best of our knowledge, the application of a mixed CaCO3/MgO contactor with the dual objective of introducing Ca2+/Mg2+ hardness and reducing divalent metal concentrations has not yet been investigated. Table 1 provides a summary of the processes expected to contribute to remineralization and Mn removal in a mixed CaCO3/MgO contactor. More information on the mechanisms at play can be found in Szymoniak et al. [19].

The primary aim of this work is to improve the overall performance of a calcite contactor by blending MgO with calcite and to confirm the role of MgO in the Mn removal process by discriminating some of the mechanisms at play in a mixed media system (Table 1). The hypothesis is that the addition of a MgO fraction will increase the remineralization capacity of the filter and will help mitigate the impact of the solid Mn-coating which was previously determined to limit the long-term operation of a pure calcite contactor. Introducing a MgO fraction, therefore, aims to extend the operational longevity of the contactor with the following treatment targets: 1) introducing ≥30 mg CaCO3/L total hardness 2) reducing Mn concentrations to below the Health Canada recommended aesthetic objective of 0.02 mg Mn/L (Health [9]).

Section snippets

Media and synthetic feedwater

The synthetic feed water (SFW) was prepared by dilution of a stock solution (1000 mg Mn/L) into ultra-pure (Milli-Q™), resulting in a final Mn concentration ranging from 0.5 to 5.0 mg/L. The stock solution was prepared by dissolving powdered reagent grade (>99 % pure) MnSO4 (Fisher Scientific, NJ, USA) in Milli-Q™ water. Both the stock solution and SFW remained stable at a pH of ~6. The SFW was maintained at room temperature (23 ± 2 °C) in 60 L tanks, open to the atmosphere.

Experimental design

The experimental

Effect of MgO on the pH of the effluent and Mn removal kinetic

Our results confirm that the addition of only 5 % (w/w) MgO in the media blend increases the pH response significantly. Over 720 h of operation, the pure calcite column produced treated water with pH values between 8.6 and 8.8 [5] whereas the columns containing 5 % MgO produced effluents with a pH around 10.5 (Fig. 2).

In preliminary column tests, the addition of >5 % MgO at 2.27 m/h (EBCT = 20 min) produced an ‘over-corrected’ pH, thereby exceeding the Health Canada recommended maximal pH of

Conclusion

This work aimed to determine the role of MgO in a mixed media contactor with the hypothesis that the media improves remineralization and Mn-removal over long-term operation. The main findings are summarized as follows:

  • Over 720 h of operation, the mixed media contactor with a 5 % MgO ratio was shown to maintain effluent hardness levels of close to 30 mg CaCO3/L and to remove aqueous Mn concentrations to below 0.02 mg Mn/L.

  • The decrease in remineralization observed at ~600 h for pure CaCO3 was

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We would like to acknowledge the contribution of Julie Philibert, Mireille Blais, Tetiana Elyart and Yves Fontaine who provided technical assistance. This research was financially supported by the Natural Sciences and Engineering Research Council of Canada through its Discovery Grant program (RGPIN 2020 06409). Finally, the experiments benefited from access to the CREDEAU laboratories, a research infrastructure sponsored by the Canadian Foundation for Innovation.

References (26)

  • J.E. Tobiason et al.

    Manganese removal from drinking water sources

    Curr. Pollut. Rep.

    (2016)
  • M.F. Bouchard et al.

    Intellectual impairment in school-age children exposed to manganese from drinking water

    Environ. Health Perspect.

    (2011)
  • Health Canada

    Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Manganese. ((Catalogue no H144–39/2017E-PDF).)

    (2019)
  • View full text