Dissolution kinetics of marcasite at acidic pH

Abstract

Marcasite (FeS2), which is a polymorph of pyrite and isostructural with arsenopyrite, is common in Acid Rock Drainage and Acid Mine Drainage (ARD & AMD) and its oxidative dissolution increases the acidity of water. The effects of pH (1–3), input dissolved oxygen (6.3 × 10−6 − 2.7 × 10−4 mol L−1), and temperature (25–70 °C) on marcasite dissolution were studied by means of stirred and non-stirred flow-through experiments. Under these experimental conditions, marcasite dissolution was characterized by a systematic deficit in released sulphur ( e.g. , Fe/S ratio > 0.5). Consequently, dissolution rates were only based on the release of iron at steady state. At this pH range, the pH effect on dissolution rates is negligible. However, an increase in the dissolved oxygen or in temperature results in an increase in the dissolution rate. A rise in temperature yields an apparent activation energy of 14.7 ± 8.5 kJ mol−1. Under these conditions an empirical rate law that accounts for the effects of pH, dissolved oxygen and temperature on marcasite dissolution is proposed as: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{R}\_{marcasite} = 10^{{-}5.9 {\pm} 1.5} \mathit{a}\_{O\_{2}(aq)}^{0.45 {\pm} 0.11} \mathit{a}\_{H^{+}}^{{-}0.02 {\pm} 0.06}e^{\frac{14.7 {\pm} 8.5}{\mathit{RT}}}\] \end{document} where R marcasite is the marcasite dissolution rate (mol m −2 s−1), R is the gas constant (kJ mol−1 K−1), T is the temperature (K), and a O2(aq) and a H + are the activities of dissolved oxygen and hydrogen ions in solution, respectively. At high oxygen concentrations ( e.g. , atmospheric concentration) the reaction rate appears to be independent of dissolved oxygen (DO), becoming constant when DO ≥ 1 × 10−4 M. This behaviour, consistent with a mechanism involving equilibrium adsorption–desorption for DO can be described by a non-linear rate dependence on DO concentration. X-ray photoelectron spectroscopic (XPS) analyses of the marcasite surface before and after dissolution showed an enrichment of the reacted marcasite surfaces in sulphur (polysulphides and sulphates). These products, which formed during marcasite dissolution, do not exert a passivation effect on long-term marcasite dissolution rate.