The development of soil liming strategies for irrigation with acid mine drainage

Abstract

Alternative mine water management and treatment strategies are becoming important due to limitations placed on natural water resources. This is because of the high cost and energy requirements associated with treating mine water. Irrigation with gypsiferous mine-influenced waters has been well demonstrated and is indeed successful on a commercial scale and it is expected that large-scale rollout is eminent. An alternative mine water management and treatment strategy is to irrigate with untreated acid mine water (AMD) on soil that has been treated with liming material. Hence, the aim of this study is the development of soil liming strategies for irrigation with acid mine drainage. This has been investigated by undertaking titrations to quantify the additional acidity generated by Fe, Al, and Mn present in AMD and how this buffers the neutralising potential of hydrated lime. The first hypothesis states that iron is expected to have the most significant buffering influence on the neutralising potential of hydrated lime compared to aluminium and manganese as its oxidation, hydrolysis and precipitation is strongly acidifying. This hypothesis was accepted. It was demonstrated that Fe has the potential to buffer the neutralisation potential of hydrated lime because of two acid generating influences, namely the ferric and ferrous iron buffers with each of them influencing the potential of hydrated lime below a neutral condition. The titration experiments also showed that 2.96 mmol of acidity can be generated per mmol of Mn in solution, 2.56 mmol of acidity can be generated per mmol of Fe in solution and 3,36 mmol of acidity can be generated per mmol of Al in solution. These quantifications form the basis for the development of soil liming strategies for irrigation with AMD because it enables a determination of the amount of liming material needed to neutralize the acidity of AMD to be applied to soil. Neutralization tests were also done for this study to determine the influence of the abovementioned metals on the potential of varying particle size fractions of calcite and dolomite to dissolve and neutralise AMD. The second hypothesis states that iron will limit the potential of larger particle sizes of agricultural limestone to neutralise AMD to a greater extent than aluminium and manganese as these larger particles are likely to be armoured by iron precipitates. This hypothesis is rejected. It was shown that the dissolution of the larger particle size fractions of calcite and dolomite is specifically influenced by Al precipitates and then by Fe precipitates during the neutralization of AMD. The former precipitate has a more prominent influence on restricting the potential of larger particle size fractions of calcite and dolomite to neutralize AMD. The latter precipitate further contributes to the armouring of the particles and a restriction in the dissolution of the medium and coarse agricultural limestone particle size fractions. The results of the neutralization tests show that neutralizing mine impacted water (AMD) is largely dependent on the particle size fraction of the liming material used. Decreasing the particle size fraction of calcite and dolomite increased their dissolution efficiency when used for neutralization. This facilitates the selection of an appropriate particle size fraction, applicable to neutralization of AMD for use as irrigation. Lastly, a soil column experiment was done to determine the potential of strategically liming soil with agricultural limestone to neutralise AMD to support a favourable rooting environment for crop growth. The final hypothesis for this study states that localising agricultural limestone within the top half of the soil profile improves the spatial contact with AMD and the potential for neutralization compared to incorporating limestone throughout the soil profile which reduces the spatial contact with AMD and the potential for neutralization. This hypothesis is rejected. It was shown that the placement position of the agricultural limestone within a soil profile does not have an influence on the neutralization of AMD to support a favourable rooting environment for crop growth. However, it is suggested that the type of agricultural limestone used in the soil profile, which can influence neutralization of AMD, is an important factor to consider. Therefore, it is recommended that liming materials with a slow dissolution efficacy be mixed throughout the soil profile. This is likely to increase the temporal contact between the liming material and AMD which contributes to a longer period to facilitate neutralization of the soil solution. The outcome of this study is used as a guideline for the development of soil liming strategies for irrigation with AMD as follows. 1) Calculate the total amount of acidity that Fe, Al, and Mn could generate as a neutral condition of the AMD is reached. 2) Utilise the appropriate size fraction of liming materials typically used in agricultural applications, which is calcite and dolomite. 3) Consider the influence of the dissolution efficacy of the agricultural limestone within a soil profile on neutralizing AMD. 4) Implement a monitoring program to track sulphate concentrations in the irrigation water as well as the soil moisture. 5) Incorporate an appropriate fertilizer management plan to address primary nutrient deficiency.