Utilizing wood ash to stimulate algal growth in mine waters from northern Sweden

Abstract: Acid rock drainage (ARD), caused by the oxidation of sulphide bearing minerals, continues to be an environmental impact of both closed and operational mines, leading to waters with low pH values and high metal concentrations (Park et al., 2019). A potential treatment method for ARD is controlled eutrophication, where algae growth is stimulated in the water and metals are accumulated within the algae cells or sorbed to the outside of the cell walls (Samal et al., 2020). The aim of this study is to determine if the addition of wood ash to acidic mine waters can increase the pH, stimulate algal growth, and lower metal concentrations in the water. Two types of wood ash, as well as KNO3 and KH2PO4 were utilized for this study, along with three water sample types: neutral (pH of 7.0) water from Åkerberg pit lake, acidic (pH of 5.7) water from Maurliden mine site (Maurliden East), and very acidic (pH of 2.3) water from Maurliden mine site (Maurliden West). Two experiments were performed, Experiment I (Exp. I) involved a one-time addition of wood ash and KNO3 to samples from each of the three sites, and Experiment II (Exp. II) involved a feeding style ash addition where ash was added every five days to samples from Åkerberg and Maurliden East. Exp. II also included one sample set where artificial nutrients only (KNO3 and KH2PO4) were added to Maurliden East samples. Samples were placed inside of a climate chamber to provide daily irradiation and pH, electrical conductivity (EC), and fluorescence signal measurements were taken daily. Additionally, absorbance samples and O2 saturation measurements were taken daily for Exp. II. At the end of each experiment, samples were sent to an accredited laboratory for elemental analysis and chl-a analysis.  Algal growth was observed in Exp. I in Åkerberg samples only, and in Exp. II in Åkerberg samples and in Maurliden East samples with artificial nutrient additions only. The algal growth rate was similar in both experiments; however, Exp. II yielded a higher concentration of chl-a than Exp. I. Precipitates were observed in all samples in both experiments, including in control samples. pH results cannot confirm the ability of the wood ash to increase the pH in the samples from Maurliden East or West, and metal concentration decreases in these samples are most likely due to precipitation reactions. Metal concentration decreases in Åkerberg samples could be due to precipitation reactions and/or algal uptake.