Ecotoxicity Assessment for Antimony Pollution in Contaminated Ecosystems

Abstract

<p>Aquatic pollution by antimony (Sb) from mining and industrial dispersion causes concern for both environment and public health. While the fate and transport of Sb has been relatively well studied in recent years, much less is understood about its ecotoxic effects in environmental systems, which are often assumed to be similar to arsenic (As). The aim of this thesis was to assess the spatiotemporal distribution and bioaccumulation of Sb (and co-occurring As) in Bakers Creek-Macleay River catchment of northern New South Wales (NSW), which has been strongly affected by over a century of gold (Au)-Sb mining and processing. Another aim was to understand the ecotoxicity of Sb using a suite of sensitivity assays including ecologically important soil bacterium and freshwater fish species, as well as species sensitivity distribution for freshwater risk management. A review of literature revealed that little is understood.</p><p> Sampling of the Macleay catchment system showed that water, sediment, detritus, and biota were enriched with Sb and As, although this enrichment decreased substantially downstream from the source of contamination. While bioaccumulation in the food webs exists and some of the greatest concentrations of Sb reported in biota were recorded, As showed greater uptake efficiency with bioaccumulation factors (BAFs) ~11× greater than those of Sb for both the stream autotrophs and heterotrophs, although total concentrations appeared comparable in detritus. Temporal changes in metalloid sources had little or no influence on trophic distribution of Sb (and As). Biomagnification of both metalloids through food webs was not observed; rather there was a biodiminution with increasing trophic level between autotrophs and carnivores. This field evaluation is the first evidence on the biogeochemical behaviour and ecological risk posed by co-contamination of Sb with As in aquatic ecosystems.</p><p> Understanding Sb pollution impacts on soil bacterial populations and their functions is essential, and this is very little explored for Sb. In the bacterial incubation experiment, it was demonstrated for the first time that even at low concentrations, both Sb(III) and Sb(V) reduced the survival of the important nitrogen-fixing bacterium, <i>Azospirillum brasilense</i> Sp7 and induce phenotypic variation in the surviving population. Bacterial toxicity was greater with Sb(III) than Sb(V). Exposure to Sb contaminated creek water from Bakers Creek of the Macleay catchment also reduced the population of <i>A. brasilense</i> Sp7 and stimulated development of mutant variants. Given the importance of these diazotrophic bacteria in nitrogen cycling in soils, this data provides evidence that Sb in soils can present a risk to healthy soil function.</p><p> Acute semi-static studies were performed to study the response of site-specific fish species, Australian bass, and Silver perch, to Sb(III) and Sb(V) by examining bioavailability and the effects of Sb on body ion regulation (Na, Ca, Mg, and K). The 96 h LC50s for Sb(III) were 13.55 and 18.03 mg L^-1 for Australian bass and Silver perch, respectively, and the LC50 for Sb(V) were 165.33 mg L^-1 and >258.7 mg L^-1, respectively. Relative median potency values derived from the LC50s demonstrated greater toxicity of Sb(III) than Sb(V) to both fish species, with fish species-specific response to exposure. The data further indicated that fish body Sb concentration was generally predictive of waterborne Sb acute toxicity, but had no effect on fish ionoregulatory homeostasis. The results also provide clear evidence that Sb risk assessment should be based on Sb provider compound.</p><p> The species sensitivity distributions constructed for Sb(III) using available freshwater species acute toxicity data sourced from temperate and tropical regional studies showed that PNEC of 156 μg L^-1 Sb(III) would be protective of most species in identified global freshwater systems, with 195 μg L^-1 Sb(III) estimated to be protective of temperate waters. The threshold estimate of 20 μg L^-1 Sb(III) was determined as an interim PNEC for the tropics using temperate data. Application of PNECs to ecological risk assessment of Sb(III) using risk quotients showed that the majority of identified global freshwaters where Sb(III) concentrations are recorded would fall into the low ecological risk category. While this work demonstrates the sensitivity distribution of freshwater species to Sb(III), it also highlights the limited Sb toxicity data, particularly Sb(V), on which to base accurate risk assessment and minimise extrapolation uncertainties.</p><p> In the last part of this project, a genomic screening of fish erythrocytes was conducted to elucidate Sb genotoxicity in aquatic ecosystems. Silver perch were exposed to sub-lethal and environmentally relevant Sb(III) or Sb(V) concentrations for 14 days. The DNA damage was assessed by the single cell gel electrophoresis (SCGE) and the clastogenic and/or aneugenic potential by the micronucleus (MN) test. Cytotoxicity of Sb was measured by calculating the ratio of polychromatic erythrocytes (PCEs) to normochromatic erythrocytes (NCE) in the circulatory blood. The SCGE assay showed that Sb(III) induced a significant non-dose-related increase in DNA damage at the 2^nd day of exposure. The erythrocytes were not sensitive to the clastogenic and/or aneugenic effects of Sb as calculated by the nonsignificant frequency of MN in the cells scored. The 1.8 mg L^-1 Sb(III) exposure was the only concentration at which a significant increase in the PCE/NCE ratio at the 2^nd day of exposure was induced. Both cytogenetic assays showed that Sb(V) presented no significant genotoxic effects at the exposure concentrations, nor altered the PCE/NCE cytotoxicity index. This work demonstrates the potential of Sb(III) to interfere with genetic material in aquatic species.</p><p> This is the first time a detailed spatiotemporal assessment of Sb and co-occurring As bioaccumulation in freshwater aquatic food webs has been undertaken, Sb toxicity examined in Australian indigenous fish species and ecologically important rhizosphere bacterium, and Sb species-specific risk assessment in identified global freshwater systems was conducted. This thesis contributes new knowledge on Sb ecotoxicology, and advances our understanding of bioaccumulation in aquatic systems. The work shows that Sb can bioaccumulate in biota, but does not transfer from trophic level to another or biomagnify, rather decreases between autotrophs and carnivores. Waterborne Sb is toxic to soil functional bacterium, and fish species with species-specific response to Sb oxidation states. The presence of Sb in a multicomponent ecosystem clearly poses an ecotoxic risk.</p>