The top layer of natural rapid sand filtration was found to effectively oxidise arsenite (As(III)) in groundwater treatment. However, the oxidation pathway has not yet been identified. The aim of this study was to investigate whether naturally formed manganese oxide (MnO), present on filter grains, could abiotically be responsible for As(III) oxidation in the top of a rapid sand filter. For this purpose As(III) oxidation with two MnO containing powders was investigated in aerobic water containing manganese(II) (Mn(II)), iron(II) (Fe(II)) and/or iron(III) (Fe(III)). The first MnO powder was a very pure - commercially available - natural MnO powder. The second originated from a filter sand coating, produced over 22 years in a rapid filter during aeration and filtration. Jar test experiments showed that both powders oxidised As(III). However, when applying the MnO in aerated, raw groundwater, As(III) removal was not enhanced compared to aeration alone. It was found that the presence of Fe(II)) and Mn(II) inhibited As(III) oxidation, as Fe(II) and Mn(II) adsorption and oxidation were preferred over As(III) on the MnO surface (at pH 7). Therefore it is concluded that just because MnO is present in a filter bed, it does not necessarily mean that MnO will be available to oxidise As(III). However, unlike Fe(II), the addition of Fe(III) did not hinder As(III) oxidation on the MnO surface; resulting in subsequent effective As(V) removal by the flocculating hydrous ferric oxides.
Worldwide limestone filtration is used in many treatment plants for the conditioning and (re)mineralization of drinking water to increase concentrations of Ca^ ' and HCO3 , pH and saturation index, thereby improving the quality of the water regarding corrosion control, buffering and taste.Typical applications include (very) soft groundwater with (very) low alkalinity and desalinated water.In Norway, some plants use a product made of ground natural limestone, called micronized CaC03 slurry (MCCS), which is dosed as slurry of fine particles (1-2 nm) into the raw water. In this study the potential of MCCS as an alternative to limestone filtration was investigated. Experiments were performed to determine the dissolution kinetics of MCCS and other CaCO3-products, including natural limestone grains and two precipitated CaC03 powders. As expected from theory, the dissolution kinetics are strongly influenced by the particle size of the CaC03 and the driving force towards the chemical equilibrium. However, all CaCO3-products needed substantial detention times (30 min and more) to dissolve completely. It is concluded that MCCS is generally not a feasible alternative for limestone filtration as a stand-alone option for the conditioning and (re)minerallzation of drinking water. Applications of MCCS are limited and should either be found in combinations with coaguiation/filtration or with other conditioning and (re)mineraiization methods.
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