Arsenic speciation of mine wastes for risk assessment

To assist mining companies in their attempts to rehabilitate land and waters used for, and impacted by, their activities, there is a need to study the occurrence, distribution, and speciation of metals and metalloids associated with mining. Furthermore, there is a need to provide credible estimations of the likely bioavailability of the specific forms of the metals/metalloids found to be present, to form the basis of a risk assessment of the potential of the metal/metalloid to cause toxicological damage. The work described in this thesis deals with arsenic speciation on solid and aqueous samples collected in areas associated with mining sites, and upon the ability of these samples to adsorb further arsenic, and to be adsorbed by naturally occurring iron minerals. It also extends the concept of risk assessment by undertaking work aimed at assessing bioavailability of species of arsenic shown to occur within the samples. Arsenic speciation in the solid phase has traditionally been difficult, and as a result different selective extraction techniques have been used to attempt to define various arsenic species. In this work such extractions have been done, and the results obtained compared with speciation using X-ray Absorption Fine Structure (XAFS) Spectroscopy applying the related XAFS techniques of X-ray Absorption Near Edge Structure (XANES) Spectroscopy and the Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy. The two analysis techniques are related, but they provide slightly different information about an elements (in this case arsenics) local coordination and chemical state. The oxidation states and the ratio of arsenic species present in powdered samples and their respective sequentially extracted fractions were determined. It is shown that X-ray spectroscopy is a far superior tool for defining arsenic speciation than is sequential extraction; whereas the former approach unequivocally defines actual chemical forms of arsenic, the latter method yields very misleading results regarding the actual arsenic species extracted in each step. There is effectively no correlation between results obtained by the two approaches. The only major drawbacks to X-ray spectroscopy approach are its cost and availability. XANES analysis of mine wastes indicates that As(V), comprising the arsenates of iron (28 %), calcium (25 %), and aluminium (23 %), is the dominant oxidation state in various mine waste samples including those dispersed as creek/river sediments. But mixed oxidation states of As(V), As(III) (consisting of sodium arsenite and arsenic sulfide), and As(-I) (consisting of arsenopyrite) were also observed to be present in some of the mine waste samples and their respective selective extraction fractions. The adsorption model of arsenic on dispersed creek/river sediments was also studied and ferrihydrite and goethite were identified as key iron oxyhydroxides for the adsorption of arsenic in creek sediments receiving tailings waters. XANES analysis revealed the presence of both As(V) and As(III) in the resulting sediment samples, with arsenic in the As(V) form dominant. The importance of bioavailability (BA) of a particular element is basic to any risk assessment, where BA refers to the ratio of the element absorbed compared to the amount ingested in the material in question. Traditionally, in the absence of specific BA data, this is assumed to be 100%. Experiments utilising the rat as an in-vivo model were undertaken in order to quantify the absolute bioavailability (ABA) of arsenic in mine waste samples (tailings and sediments); they showed that the ABA of contaminants varies between sites and the matrix of the sample. The total As(V) and As(III) ABA values for mine tailing and sediments samples fell in the range 6 to 32 %, with As(III) being invariably much more bioaccessible than As(V). It is clearly demonstrated that assuming 100% bioavailability of arsenic from soils and mine waste materials overestimates the risk associated with human exposure. Recent work has developed the concept of BA by use of an in-vitro physiologicallybased extraction test (PBET) to yield measurements of bioaccessibility (BAc). The PBET aims to simulate human uptake of the target element, and expresses BAc as uptake in terms of a percentage of available element. Assessment of BAc of some mine waste materials (tailing and sediments) dosed to animals to obtain ABA values was undertaken, and comparison made between BAc and ABA results. The BAc for the mine tailings samples ranged from 2.2 ± 0.1 to 11.5 ± 0.1 %, while river sediment samples were roughly 10 % of these values. When comparisons were made between the results obtained for bioavailability of arsenic in samples examined by the in vivo absolute bioavailability (ABA) method and the in vitro bioaccessibility (BAc) method, no correlation could be found in results obtained for any one of the tailing or sediment samples. Furthermore, the in vivo results indicate much higher values of ABA for As(III) than As(V), while the in vitro approach found that nearly all bioavailable arsenic is present as As(V); As(III) BA was negligible