The influence of neutralisation time lag on plant-available phosphorous in acid mine tailings

MSc (Environment Sciences), North-West University, Potchefstroom Campus

The main goal of vegetation establishment on tailings storage facilities is to stabilise their slopes to improve the retention and infiltration of water, thereby reducing the effects of wind and water erosion. The neutralisation of acidic soils with agricultural lime is a common practice but with acidic mine tailings (e.g. gold and coal tailings), acid is generated by other sources. Therefore, the neutralisation incubation time of these tailings vary greatly from natural soils because the acidity is caused by geochemical oxidation processes in the tailings material. The weathering and oxidation of minerals present within the tailings, most frequently pyrite (FeS2), produce sulphuric acid (H2SO4). These reactions may cause the tailings to have pH levels as low as 1.7. This extremely acidic environment accompanied by continuing oxidation reactions cause a major time lag in the neutralisation of the tailings material. Applying fertiliser directly after treating the material with the appropriate amount of lime, will have very little to no success as the optimum pH for these nutrients to be available for plant uptake is around 5.5 to 7.5, depending on the plant species. For example, preliminary studies have proven that gold tailings generally have a neutralisation incubation time of approximately six weeks, when the pH of the material increases from 3.0 to 6.0 over this period. This research focuses on the plant availability of phosphorus (P) in acid mine tailings. The aim of the study was to determine if a neutralisation incubation period of six weeks before fertiliser treatments would result in increased plant availability of P. Two different analytical methods were used to monitor P in the growth mediums (i.e. Olsen and Bray-1). The final results obtained from the pot trials showed an increase in the plant availability of P when superphosphate was applied after the neutralisation incubation period, compared to when lime and fertiliser were applied at the same time. Better germination rates were also obtained from the growth mediums where the Olsen method extracted higher concentrations of P (r + 0.789; p < 0.05). Additionally, it was found that the use of the Bray-1 method to extract P delivered inaccurate results in heavy limed tailings materials.

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