Contaminant mobility and carbon sequestration downstream of the Ajka (Hungary) red mud spill: The effects of gypsum dosing

Abstract

A number of emergency pollution management measures were enacted after the accidental release of caustic bauxite processing residue that occurred in Ajka, western Hungary in October, 2010. These centred on acid and gypsum dosing to reduce pH and minimise mobility of oxyanion contaminants mobile at high pH. This study assessed the effectiveness of gypsum dosing on contaminant mobility and carbon sequestration through assessment of red mud and gypsum-affected fluvial sediments via elemental analysis and stable isotope analysis. There was a modest uptake of contaminants (notably As, Cr, and Mn) on secondary carbonate-dominated deposits in reaches subjected to gypsum dosing. C and O stable isotope ratios of carbonate precipitates formed as a result of gypsum dosing were used to quantify the importance of the neutralisation process in sequestering atmospheric carbon dioxide. This process was particularly pronounced at sites most affected by gypsum addition, where up to 36% of carbonate-C appears to be derived from atmospheric in-gassing of CO(2). The site is discussed as a large scale analogue for potential remedial approaches and carbon sequestration technologies that could be applied to red mud slurries and other hyperalkaline wastes. The results of this work have substantial implications for the aluminium production industry in which 3-4% of the direct CO(2) emissions may be offset by carbonate precipitation. Furthermore, carbonation by gypsum addition may be important for contaminant remediation, also providing a physical stabilisation strategy for the numerous historic stockpiles of red mud.

Acid transformation of bauxite residue: Conversion of its alkaline characteristics

“…8-10) [9,40,42,43]. Furthermore, with consumption of free OH -, Al(OH)4 -will precipitate as gibbsite (Al(OH)3) and possible dawsonite (Eqs.…”

Gypsum addition to soils contaminated by red mud: implications for aluminium, arsenic, molybdenum and vanadium solubility

“…The initial response to the accident was to dose affected rivers with weak acids and gypsum (up to 23,500 t: Rédey 2012) to neutralise the water, and (in some cases) to plough the red mud into the fields to prevent dust formation Gelencser et al 2011;Renforth et al 2012).…”

“…Acid dosing and gypsum addition to rivers were both effective in lowering pH values and metal(loid) concentrations in river waters downstream of the spill Mayes et al 2011;Renforth et al 2012). Lack of Ca 2+ in red mud leachate (Renforth et al 2012) limits the natural pH reduction mechanism (Equations 1 and 2).…”

“…Lack of Ca 2+ in red mud leachate (Renforth et al 2012) limits the natural pH reduction mechanism (Equations 1 and 2). Providing excess free Ca 2+ is therefore the main effect of gypsum addition.…”

“…Providing excess free Ca 2+ is therefore the main effect of gypsum addition. The reaction, which involves CO 2 in-gassing to form calcite with net OH -removal, can be rapid in high pH systems (Renforth et al 2012). …”