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Acid Mine Drainage (AMD) involves waters of low pH and high concentrations of sulphate and potentially toxic trace elements. Widely distributed, thallium is a highly toxic element found associated with coal deposits and the overlying strata and therefore is often found in elevated concentrations in AMD. Managing AMD typically involves using alkaline materials to raise the pH which can also promote the removal of many trace elements through precipitation and adsorption mechanisms. Cement kiln dust (CKD) is a waste product that has the potential to be an economic neutralent for treating AMD, however it may contain high concentrations of several trace metals including thallium. Thallium presents particular concern to the environment due to the low hydrolysis constant of Tl+ which means that it tends to remain in solution even in the neutralised AMD impacted waters. However, the geochemistry of thallium in AMD systems is poorly understood, with conflicting descriptions in the literature. The main objective of this study was to investigate the behaviour of thallium during the treatment of AMD at Stockton Coal Mine in order to gain a better understanding of the geochemistry of thallium in these systems. Neutralisation of the AMD samples with CKD showed that thallium mostly remained in the solution phase up to pH 8 (the highest pH studied). In contrast, most other trace metals including copper, zinc, cadmium and lead were almost entirely removed from solution over the same pH range. The CKD contained 7.2 gg-1 of thallium and the treatment of AMD samples with CKD increased the dissolved thallium concentration from 2 gL-1, in the untreated AMD, up to 10 gL-1. Below pH 6 the addition of CKD increased the concentration of thallium in solution, whilst at higher pH the thallium concentration appeared to plateau, and possibly even decreased slightly when the pH was greater than 7. To understand the geochemistry of thallium in AMD systems, thallium adsorption studies were also conducted. Hydrous aluminium oxide (10 mM) had no effect on solution concentrations of thallium while ferrihydrite (8.2 mM) was an effective adsorbent when the pH was sufficiently high. However, the position of the adsorption edge in the Tl+-ferrihydrite system was shown to be dependent on the concentration of thallium. The pH at which 50% of the Tl+ was adsorbed was approximately 6.8 and 8.3 when the total Tl+ concentration was 30 gL-1 and 9.6 mgL-1 respectively. Thallium(I) adsorption constants were determined from the experimental data using the Diffuse Layer Model and were able to describe the observed sorption behaviour. The data from this work also suggested that some of the reported Tl+ adsorption constants are too large by several log units. In general, the geochemistry and adsorption behaviour of Tl+ is consistent with the value of its hydrolysis constant. Investigation of the kinetics of adsorption suggested that the time required for chemical equilibrium in Tl+-ferrihydrite systems can differ significantly depending on the Tl+ concentration, with low concentration systems reaching equilibrium within minutes. This means that only adsorption constants derived from very low concentration experiments will represent adsorption equilibrium, consistent with some of the Tl+ concentrations and log Ks values in the literature, and explained the difference in the observations of Tl+ adsorption in this study. Modelling the geochemical changes during the neutralisation of AMD indicated that even the best case scenario, 90 % Tl+ removal would require a pH greater than 9. Modelling also indicated that the presence of Ca2+, Zn2+ and possibly Mg2+, at the levels detected in the AMD samples, could have a significant effect on Tl+ adsorption. These species have both a higher hydrolysis constant and solution concentration compared to Tl+ which means they effectively compete for the limited binding sites on ferrihydrite, shifting Tl+ adsorption to a more alkaline region. The modelling study also suggested that some of the more strongly binding species like Zn2+ will have a large effect on Tl+ sorption despite low concentrations. It also appeared that additional calcium, released through the dissolution of the neutralents, will not contribute significantly to the cation competition above that of the calcium already present in the AMD impacted waters. |
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