Potysz, and 3 co-authors, reported in late 2018 that historical Cu-Slags located at former industrial sites in Poland are exposed to weathering that may trigger release of metallic contaminants to the surrounding soils. However, the extent to which different metals are leached depends on the mineral composition of the slags and the conditions of their deposition. Historical slags are often crystalline and composed of phases with a variable response to weathering. Also, weathering of crystalline slags (CS) is different from that of amorphous slags (AS).
To understand how the slag phase composition triggers weathering and the ultimate release of metallic elements, the authors studied the leaching of historical crystalline and amorphous slag. A combined geochemical and mineralogical approach determined the weathering sequence(s) of slag components subjected to different conditions of specific slag disposal. In detail, experiments under:
i) various pH conditions in the range of 2–13 in a batch mode simulated slag deposition within a deep soil profile, whereas
ii) exposure to solutions of artificial root exudates (ARE) in a dynamic experimental system simulated slag deposition within topsoil.
The results demonstrated that crystalline phases in the order Leucite > Plagioclase > Pyroxene are preferentially dissolved in inorganic conditions of a pH ≤ 7, in contrast to glass, which is preferentially dissolved in alkaline conditions (pH > 7). Also, inorganic conditions better promote the dissolution of metal-bearing phases (sulphides and metals) when compared to silicates. In contrast, artificial root exudates do not show preferential dissolution of any phase and cause the strongest leaching in all of the analyzed conditions. The authors showed that reconstructing the susceptibility of complex crystalline slags to
weathering requires a specific approach including analyses and interpretation of all major components such as Ca, Mg and K, not merely a focus on Si and metals.
The authors concluded that they simulated weathering conditions typical for the sites of historical slag disposal, in particular, they examined a range of pH conditions occurring typically at different soil depths during slag-soil interaction. Historical slags are often complex and composed of several crystalline phases with different weathering potential. They showed that for such slags a special approach is required including analyses of all major elements in major phases. As an example, in their study, Ca tracked the weathering of plagioclase and Mg tracked that of pyroxene. Silicon was less useful as it could not be easily distinguished between input from various silicates and the glass. Their study also showed that different rates of weathering are expected in organic and inorganic conditions, with the former promoting severe dissolution of bulk slag, with the latter being dominated by preferential dissolution of Cu-bearing phases. Also, the different slags (crystalline and amorphous) dissolve at different rates, with crystalline slags being more susceptible to weathering
in acidic and neutral conditions whereas amorphous slags are more susceptible in alkaline conditions.
Altogether, the authors show that there is strong control of phase composition over the leaching of major slag components and underlines the importance of an extended analytical approach when the weathering potential of such slags is estimated. Based on their geochemical study, the weathering sequence of the slag components can be ranked as follows:
i) under ARE conditions: bulk slag,
ii) under inorganic acidic conditions: Cu-bearing phases > crystalline phases (leucite > plagioclase, wollastonite > pyroxene) > glass, and
iii) under alkaline conditions: Cu-bearing phases (sulfides > metallic Cu) > glass > crystalline phases.
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