Evaluation of the effects of temperature intensity and duration on the sorption properties of mine tailings contaminated soils and the implication on fire-affected soils

Abstract

Soil heavy metal contamination that is caused by anthropogenic activities such as mining is a global concern and so it is necessary for a sustainable remediation strategy to be employed for the contaminated soil to be suitable for various purposes again. Sorption is a chemical process that is employed in most remediation strategies. The soil’s sorption capacity is influenced by several soil properties which are in turn affected by soil management including fire events. Wildfires could occur naturally, accidentally or deliberately and may last for a few hours to several days depending on the availability of combustible material, with consequences on soil temperature. As soils experience fire, their temperature could increase, causing changes in soil properties like pH, cation exchange capacity (CEC), electrical conductivity (EC) and organic matter (OM) are observed. These soil properties also influence soil heavy metal absorption capacity and consequently their mobility in the soil environment which is important in the design of most remediation strategies aimed at eliminating or reducing heavy metals mobility in contaminated soils. Some heavy metal remediation strategies make use of organic amendments such as sewage sludge which is also used to improve soil fertility. The use of sewage sludge to remediate heavy metals in soils exploits its ability to immobilize these contaminants in the soil. This study aimed to evaluate the efficiency of sewage sludge as a heavy metal immobiliser in heat affected mine tailings contaminated soils with a view of understanding how fire events may affect the efficiency of sewage sludge as a heavy metal remediation material for soils that have experienced temperature changes. It also aimed to determine how soil properties are affected by different temperatures and the duration of the heating, and the implication of these temperature changes on the bioavailability of heavy metals in contaminated soil. The mine tailings contaminated soils were collected around a gold mining environment and characterised for their basic physicochemical properties including texture, pH, OM, CEC, exchangeable bases, and heavy metal content. The soils were then heated at different temperatures (100, 200, 300, and 400°C) and for different periods of time (two, four, and six hours). A portion of the heated soil was kept aside as a control sample while the second portion was mixed with sewage sludge at ratios of 80:20 and 60:40 soil: sludge. The soil/sludge mixtures were kept in the open (under natural iv conditions) for six months to allow equilibration between the soil and the sludge. The pH, electrical conductivity, cation exchange capacity and organic matter content of the soil/sludge mixtures were again measured. A three step BCR sequential extraction procedure was applied to assess and study the changes in segregation of As, Cd, Co, Cr, Cu, Ni, Pb and Zn in the different soil geochemical fractions. The BCR extraction procedure was applied on the heated, unheated, and sludge amended soils, as well as on the heated soils to which sludge had not been added. The soil pH increased with increasing temperature and the duration of heating and at high temperatures, the soil organic matter was destroyed. At lower soil temperatures of below 200°C, there was a decrease in soil cation exchange capacity while at a temperature of 400°C, an increase was observed. An increase in soil electrical conductivity was observed when the soil temperature was raised above 200°C and when it was held at this temperature for long periods while the opposite pattern was observed at 100°C. Arsenic, Zn and Co had the highest concentrations in the tailings contaminated soil among the studied metals whereas Cd had the lowest. There was a decrease in the total concentration of the heavy metals after the soil experienced increased temperatures except for Cd and Cu where there were no significant impacts. Sludge addition was shown to be a good immobiliser for As, Cu and Pb in the heat affected tailings contaminated soils. For the success of any soil heavy metal remediation strategy that makes use of the sorption ability of the soil, information on whether the soil has experienced fire events is needed because the fire events affect soil OM and the mineralogical properties which play a pivotal role in the sorption capacity of soils.