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Pyrite oxidation in a sediment sample of an open‐cut brown coal mine: mineral formation, buffering of acidity and modeling of cations and sulfate
Author(s) -
Ludwig Bernard,
Khanna Partap,
Balkenhol Ralf,
Friedrich Günther,
Dohrmann Rainer
Publication year - 1999
Publication title -
journal of plant nutrition and soil science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.644
H-Index - 87
eISSN - 1522-2624
pISSN - 1436-8730
DOI - 10.1002/(sici)1522-2624(199910)162:5<499::aid-jpln499>3.0.co;2-4
Subject(s) - pyrite , chemistry , dissolution , redox , sorption , sulfate , inorganic chemistry , silicate , ion exchange , sulfur , mineralogy , adsorption , ion , organic chemistry
Secondary reactions occurring in pyrite‒containing sediments under aerobic conditions are complex and are not fully understood. Objectives were to (i) study the formation of secondary minerals using x‒ray diffraction (XRD) and ion activity product (IAP) calculations; (ii) to obtain a budget of acidity producing and consuming processes; and (iii) to study the performance of a chemical equilibrium model (including kinetic reactions) using sequential batch experiments with varying input solutions on samples of different pyrite oxidation states. A sediment sample from the open pit coal mine Garzweiler, Germany, was oxidised in the laboratory to obtain four different pyrite oxidation states. Sequential batch experiments were carried out using H 2 O, 100 mM CaCl 2 and 10 mM NaOH as input solutions. A coupled equilibria model was used to describe the experiments. The model (PHREEQC) included inorganic complexation, redox reactions, precipitation/dissolution of sparingly soluble salts, multiple cation exchange and pyrite oxidation using a simple input function. IAP calculations and XRD showed the formation of large amounts of gypsum with increasing pyrite oxidation and for the highly oxidised sample also the formation of hydroniumjarosite. The budget of acidity consuming processes followed the order (i) release of Fe(III) into the solution phase (51% of produced acidity); (ii) Al release into solution and exchangeable phases (probably mainly due to silicate weathering, 22% of produced acidity); and (iii) CEC reducing processes (11% of produced acidity). Modeling of the sequential equilibration experiments with water and CaCl 2 gave satisfactory agreements between modeled and measured pH and sorption values, indicating that the main processes governing pH and ion sorption were quite well understood. However, model results of the alkaline additions at larger pyrite oxidation states differed considerably from the experimental results.