Research article 560 Environmental Geology 39 (6) April 2000 7 Q Springer-Verlag Received: 3 August 1998 7 Revised paper: 26 January 1999 7 Accepted: 23 February 1999 E. Puura 7 I. Neretnieks Department of Chemical Engineering and Technology, Royal Institute of Technology, Stockholm S-10044, Sweden E. Puura (Y) Institute of Geology, University of Tartu, Vanemuise 46, Tartu 51014, ESTONIA e-mail: epuura6math.ut.ee Atmospheric oxidation of the pyritic waste rock in Maardu, Estonia, 2: an assessment of aluminosilicate buffering potential E. Puura 7 I. Neretnieks Abstract The assessment of the aluminosilicate buffering potential during acid weathering of the Estonian alum shale is provided. It is found that the stoichiometric interaction between dissolved pyrite oxidation products and illite of the shale best describe the buffering process and are consistent with earlier field studies. The scheme includes in- congruent dissolution of illite with smectite and K- jarosite precipitating. This complex mechanism in- volves buffering of 8% of the acidity by K c and temporary precipitation of 25% of the acidity as K- jarosite. Dissolution proceeds at a low pH (1.5–3) until all pyrite in the shale particle is oxidised. Hence, if the total amount of illite present is larger than needed for stoichiometric interactions, only part of it is involved in a buffering process, neu- tralising a certain percentage of acidity. The next stage in shale weathering is the incongruent disso- lution of K-jarosite with the release of the precipi- tated acidity and the formation of ferric oxyhy- droxide. Key words Pyrite 7 Oxidation 7 Aluminosilicate 7 Buffering 7 pH Introduction Oxidation of pyrite in partially saturated dumps produces highly acidic pore water that dissolves aluminosilicates. Dissolution contributes to the neutralisation potential (NP) of the waste. In the widely used acid–base account- ing procedure of Sobek and others (1978) the NP is de- termined by measuring the acidity consumption of a sample in hydrochloric acid under boiling conditions, overestimating the NP. The alternative method of Lapak- ko (1994) accounts for the carbonate neutralisation ca- pacity only (Sherlock and others 1995). We believe that the combination of field evidence and modelling of hy- drochemical interactions makes it possible to better un- derstand the site-specific meaning of aluminosilicate NP. The solid phase composition of the Estonian alum shale is described in more detail in a companion paper (Puura and others 1999). The pyrite oxidation and acidity pro- duction model presented there did not account for alumi- nosilicate buffering capacity and formation of any sec- ondary solid phases other than gypsum and ferric oxyhy- droxide. The field study, however, indicated the precipi- tation of K-jarosite and a change of illite into smectite, contributing to the buffering of acidity. Solid solutions of jarosite have been recognised as being common secondary phases precipitating from acid mine drainage (Alpers and others 1989; Bigham and others 1996), from the acid pore-water in the weathering zone of partially saturated tailings (Blowes and others 1991), from groundwater fed by acid leachate (Herbert 1995), as well as from natural systems – acidic sulphate soils (van Bree- men 1973) and shale deposits eroded prior to contact with atmosphere (Pirrus 1982; Michel and van Everding- en 1987). The presence of K-jarosite is a certain indicator of local acidification, but also involvement of aluminosili- cates, as the only possible sources of K, in the buffering process. This paper aims to analyse the significance of illite disso- lution on dump behaviour through using the hydrochem- ical modelling code PHREEQC (Parkhurst 1995). Firstly, some theoretical considerations important in modelling are presented, including the definition of acidity for this system and the main pyrite oxidation reactions. Definition of acidity Above roughly pH 3, the oxidation of pyrite can be de- scribed with one overall reaction (Langmuir 1997): FeS 2 c3,75O 2 c3,5H 2 O]Fe(OH) 3 c2SO 4 2– c4H c (1)