Metal oxide nanomaterials used to remediate heavy metal contaminated soils have strong effects on nutrient and trace element phytoavailability

Abstract

Engineered nanomaterials (ENMs) are increasingly utilised for the remediation of contaminated soils. In this study, contaminant (As, Pb), nutrient (N, P) and trace element (Cu, Fe, Mn, Zn) phytoavailability in three Australian soils contaminated (<10 years) with As (≈100 mg As kg−1 soil) or Pb (≈300 mg Pb kg−1 soil) was determined periodically. These soils were exposed to two ENMs (cerium dioxide – nCeO2 or titanium dioxide – nTiO2) (applied to soil at a concentration of 500 mg ENM kg−1 soil) with contaminant, nutrient and trace element phytoavailability assessed over a 260-day period. While As phytoavailability was not influenced by either ENM, the presence of nCeO2 in some soils increased Pb phytoavailability approximately two fold at the conclusion of the experiment (day 260), which was attributed to nCeO2 decreasing soil pH and/or outcompeting Pb2+ ions for sorption sites.More significantly, both ENMs significantly altered the phytoavailability of N, P and Zn across soils. In some instances >90% of the soil N was lost in the presence of both ENMs, while in some instances the phytoavailability of P and Zn was tripled and doubled respectively in the presence of ENMs.For N it was hypothesised that both ENMs altered the mineralisation of organic N and/or soil nitrification rates due to the catalytic and/or anti-microbial properties of the ENMs. For P, it was hypothesised that anti-microbial effects of both ENMs altered the activity of P-solubilising microbes. For Zn competition between the positively charged ENMs and Zn2+ ions was the most likely mechanism for altered Zn phytoavailability. This study suggested that while ENMs can potentially be effective as metal remediation agents when applied to soils, there are however a range of potentially deleterious ‘non-target’ effects on soil ecosystems that have not as yet been widely considered.