Abstract
The effect of silica on the immobilization reaction of boron by magnesium oxide was investigated by laboratory experiments. In the absence of silica, due to dissolution of the magnesium oxide, boron was removed from solutions by the precipitation of multiple magnesium borates. In the presence of silica, magnesium silica hydrate (M-S-H) was formed as a secondary mineral, which takes up boron. Here 11B magic-angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared spectrometer (FT-IR) data show that a part of the boron would be incorporated into M-S-H structures by isomorphic substitution of silicon. Another experiment where magnesium oxide and amorphous silica were reacted beforehand and boron was added later showed that the shorter the reaction time of the preceding reaction, the higher the sorption ratio of boron. That is, boron was incorporated into the M-S-H mainly by coprecipitation. The experiments in the study here show that the sorption of boron in the presence of silica is mainly due to the incorporation of boron during the formation of the M-S-H structure, which suggests that boron would not readily leach out, and that stable immobilization of boron can be expected.
Highlights
Immobilization technology is often used to control mobility and reduce toxicity in metal contaminated soils
In the first series of sorption experiments where boron was added to magnesium oxide, the boron sorption ratios were different with or without amorphous silica (Figure 1)
In the case of [B]ini = 9.25 mmol/L, the boron sorption ratio gradually decreased after 5 days while it remained constant at 44 % in the [B]ini = 0.46 mmol/L
Summary
Immobilization technology is often used to control mobility and reduce toxicity in metal contaminated soils. Immobilization is a widely proven useful method because of the speedy and simple applicability, relatively low cost, and small amount of waste generated compared with other methods such as sealing or excavation [1]. It offers a lower risk of dispersing polluting components as there is no need to move the originally contaminated soil. There is a clear need for knowing details of how hazardous elements are immobilized in the precipitated phases when making assessments of risks and long-term evaluations
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