Abstract
The increasing industrial application of metal oxide Engineered Nano-Particles (ENPs) is likely to increase their environmental release to soils. While the potential of metal oxide ENPs as environmental toxicants has been shown, lack of suitable control treatments have compromised the power of many previous assessments. We evaluated the ecotoxicity of ENP (nano) forms of Zn and Cu oxides in two different soils by measuring their ability to inhibit bacterial growth. We could show a direct acute toxicity of nano-CuO acting on soil bacteria while the macroparticulate (bulk) form of CuO was not toxic. In comparison, CuSO4 was more toxic than either oxide form. Unlike Cu, all forms of Zn were toxic to soil bacteria, and the bulk-ZnO was more toxic than the nano-ZnO. The ZnSO4 addition was not consistently more toxic than the oxide forms. Consistently, we found a tight link between the dissolved concentration of metal in solution and the inhibition of bacterial growth. The inconsistent toxicological response between soils could be explained by different resulting concentrations of metals in soil solution. Our findings suggested that the principal mechanism of toxicity was dissolution of metal oxides and sulphates into a metal ion form known to be highly toxic to bacteria, and not a direct effect of nano-sized particles acting on bacteria. We propose that integrated efforts toward directly assessing bioavailable metal concentrations are more valuable than spending resources to reassess ecotoxicology of ENPs separately from general metal toxicity.
Highlights
Manufactured particles with at least one dimension between 1 and 100 nm [1] have been termed Engineered nanoparticles (ENPs)
This suggested that 50% of the bacterial growth was inhibited by about 3.4 mmol CuSO4 g21 soil in the mineral soils, and that about 19 mmol CuSO4 g21 resulted in a similar inhibition of bacterial growth in the organic soil
The ENP form of CuO produced a pronounced inhibition of bacterial growth in the mineral soil (Fig. 1A), resulting in a clear concentration response curve that could be well-described by a sigmoidal model (R2 = 0.98), and proved toxic to the bacterial community (log(EC50) = 1.5560.10)
Summary
Manufactured particles with at least one dimension between 1 and 100 nm [1] have been termed Engineered nanoparticles (ENPs). Metal oxide ENPs are receiving increasing attention in material science and nano-technology based industries for a large variety of applications, including catalysts, sensors and for their incorporation into commercial products [2]. Several studies have demonstrated the potential of metal oxide ENPs as environmental toxicants [7,8]. Lack of suitable control treatments have compromised the power of early assessments to determine and quantify the potential environmental impact in a useful context, i.e. whether the ENP property of the substance made it more toxic than it would have been in a generic (nonENP) form. To enable progress in our understanding of the ecotoxicology of ENPs, it is important to assess (i) whether the nano-form size that characterizes ENP per se increases the toxicity beyond other forms of the substance, and (ii) how the observed toxicity of the ENP compares to the well-known toxicity of ionic forms of heavy metals. While many studies have been conducted to determine if heavy metal ENPs can be toxic, there is a scarcity of studies that have investigated if the ENP form of heavy metal toxicants are more toxic than non-ENP forms of the same substance
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