Classification of grasses in the Mpumalanga coalfields region and assessment of their suitability to increase the pH of acid mine drainage, for potential use in passive acid mine drainage remediation systems

Abstract

Grass samples collected from various locations in the Mpumalanga coalfields region, (South Africa), were classified and characterised using various analytical techniques to determine their physical and chemical composition. The suitability of the grasses for use in acid mine drainage (AMD) remediation was evaluated by adding a defined portion of these grasses to both synthetically prepared and real AMD collected from an AMD received dam (RD) source in the Mpumalanga coalfields region, (South Africa). This was to establish whether the grass addition was able to achieve an increase in pH, and if the effect was more notable for a particular grass type. Results showed that grass addition to AMD resulted in an increase in pH (or decrease in the hydrogen ion (H⁺) concentration of synthetically prepared AMD. The addition of different grass types produced varying results, and the Hyparrhenia hirta (G12 and G13) and Chrysopogon zizanioides (G15) formally known as Vetiveria zinanioldes grass types produced a greater overall percentage decrease in H⁺ concentration. Multiple grass additions sustained the overall high percentage decrease in H⁺ concentration for the best-performing grass types and improved the overall percentage decrease in H⁺ concentration of the more poorly performing Eragrostis curvula (G6), and Hyparrhenia filipendula (G5) grass types tested in synthetic AMD. The notable changes observed in cation and anion concentrations of the grass samples after contact with AMD would suggest that a cation-anion exchange reaction did occur. An exchange of the H⁺ ions in the AMD and the inorganic cations (M⁺) associated with anions present in the grass, could be responsible for the increase in pH observed in synthetic AMD after grass addition. The addition of grass to the RD AMD did not achieve any significant or sustained decrease in H⁺ concentration, which could be attributed to the more complex matrix and higher mineral acidity of the RD AMD which would require alternate experimental conditions to achieve a decrease in H⁺ concentration. Variances in structural and physical composition between the different grass types were not consistent, and it was not possible to attribute the decrease in H⁺ concentration in AMD to any compositional parameter. Using locally sourced lignocellulosic materials in passive remediation systems is vital to the successful design of a cost-effective and sustainable, remediation system. This work is important as it explores the suitability of grass types that are readily available from the immediate vicinity of the Mpumalanga coalfields, South Africa to increase the pH of contacted AMD at ambient temperature.