Toxicity assessment of metal mixtures to soil enzymes is influenced by metal dosing method

Metal oxides and annealed metals as alternatives to metal salts for fixed-ratio metal mixture ecotoxicity tests in soil

“…Compared to metal oxides and annealed metal complexes, nitrate salts were expected to produce higher toxicity. In a similar study, metal salts (after leaching) had similar extractable metal concentrations to metal oxides, in acidic soils, but while oxides only presented extractable Zn concentrations with metal nitrate salts, Zn was the most mobile but a larger proportion of other metals were also mobilized [30]. Annealed metal complexes, the least toxic, also presented mostly Zn and in one case Pb extractable concentrations but only a fraction of the concentrations observed for metal oxides.…”

“…The addition of iron increased precipitation by creating bonds between iron and the different metals but might be reducing the availability of the toxic metals. In fact, in a similar experiment, annealed metal complexes only had a fraction of the extractable metals compared to metal oxides and these were restricted to Zn and in one case to Pb [30]. If this is the case, it raises questions as to why toxicity is observed, for instance in smelter sites where minerals like franklinite (iron and zinc mineral-ZnFe 3+ 2 O 4 ) are the dominant form of zinc [32].…”

“…Annealed metal complexes, the least toxic, also presented mostly Zn and in one case Pb extractable concentrations but only a fraction of the concentrations observed for metal oxides. Also, Awuah et al [30] found that CaCl extractable concentrations for oxide and annealed metal dosing were soil dependant and higher in soils with lower pH, while metal salts had similar extractable concentrations independently of soil. The higher mobilization of metals in lower pH soils does not seem to correspond with toxic response except for the most acidic soil (3.22) where there appears to be a higher toxic response of oxides and annealed complexes compared to the remaining soils, especially for F. candida.…”

“…For annealed metal complexes, their toxicity is unknown for soil invertebrates, but this method was designed to incorporate metals in soil directly as a mixture, which is the result of a simulated smelting process, thereby increasing the realism of metal mixture dosing schemes for soil ecotoxicology. When tested using soil enzymes activity (ammonia monooxygenase and acid phosphatases activity), the toxicity of similar dosing methods was both soil and enzyme dependant and did not demonstrate a consistent trend [30].…”

“…[28,29]) but which are considered separately from non-nano oxides used in this study. Annealed metal complexes were selected as a dosing method simulating contamination resulting from a smelting operation and have not been previously tested in the scientific literature except for a similar study on the effects of these dosing methods to soil microbial processes [30]. In terms of their realism and environmental relevance, metal contaminated sites can present a variety of different chemical forms [7], but the annealed metal complexes closely resemble the minerals funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.…”