Phytoremediation of acid mine drainage (AMD) impacted soil

Research output: Book/ReportPh.D. thesisResearch

Acid mine drainage (AMD) is a type of mining wastewater generated from the oxidation of sulfidebearing minerals. Due to its acidity and, typically, high content of various metals and metalloids, AMD has immense environmental consequences and risks to human health. Depending on the local geology, AMD may also contain high concentrations of valuable rare earth elements (REE). Phytoremediation, a technique using plants to restore degraded lands, offers a sustainable and innovative alternative to traditional remediation methods with potential to minimise the negative impacts while taking advantage of potential value or resource-generating opportunities. The purpose of this PhD thesis was to investigate the phytoremediation potential of AMD impacted soil by establishing a continuum of research on the phytoremediation of AMD and AMD impacted environments, expanding knowledge on the geochemical impacts of AMD in soil, exploring metal and REE accumulation behaviour in AMD impacted soil-grown plants, and investigating metal and REE mobility in AMD impacted soil. Paper I reviewed the state-of-the-art of AMD, its impact on environmental systems, and its phytoremediation potential using novel techniques and approaches. The review showed that remediation of metal contaminants and soil acidity can be complemented by various value-generating opportunities, including energy crop production, extraction of critical elements (i.e. REE), carbon storage, co-cropping for agricultural production, and ecosystem restoration (biodiversity). Paper II investigated the geochemical impacts of AMD on soils sampled from four AMD impacted sites within major gold and coal mining regions of South Africa. The soils showed significant impact from AMD, including soil acidification (pH < 4), high sulfate contents (> 7000 mg kg-1), and arsenic contamination (23-360 mg kg-1 As). This study provided novel documentation of As contamination from AMD in one mining region, and showed unexpectedly low levels of REE in AMD impacted top soil, which was most likely due to loss from acid-induced leaching processes. Paper III assessed the phytoremediation potential of one of the sampled AMD impacted soils for the dual stabilisation of contaminants (arsenic, chromium, and zinc) and the extraction of REE. The study also investigated the effect of using a biochar amendment on the system’s potential. A 3-month pot experiment was conducted with Chyrosopogon zizanioides (vetiver grass) and Medicago truncatula (barrel clover) grown under mono-crop and co-crop in a growth chamber under climate controlled conditions using a moderately contaminated AMD impacted soil from a gold mining region in South Africa with a near-neutral pH. Soil pH, plant biomass, and plant and soil metal(loid) and REE contents were analysed at the end of the 3-month period. Additional calculations were made for translocation factor (TF) and bioconcentration factors (BCF), which are important phytoremediation indicator ratios. The results showed that co-cropping significantly lowered metal(loid) uptake by the M. truncatula roots by 32-80% and that the biochar amendment increased its shoot biomass by 40%. Translocation factors of metal(loid)s increased for both plant species under co-cropping. The plant uptake of REE also decreased under co-cropping and amendment by biochar and C. zizanioides exhibited the highest shoot extraction, which only accounted for 0.2% of the total soil REE contents. These results indicated that co-cropping with vetiver grass could potentially be used to improve metal(loid) tolerance of a secondary plant species by promoting metal(loid) exclusion and internal regulation. The results also supported that biochar can be used to improve immobilisation of select metal(loid)s likely through increased sorption and is an effective aid to phytostabilisation by altering soil chemical and physical properties for improved plant growth and metal(loid) tolerance. The phytoextraction of REE was deemed unsuitable for this AMD impacted soil based on the results; however, more studies were needed to assess the actual REE uptake potential of the select plant species in the studied soil medium. Ultimately, the co-cropping approach to phytostabilisation showed high potential for the simultaneous and safe cultivation of agricultural crops through phytoprotection as alternative forms of plant-based value. Paper IV investigated the mobility of REE and arsenic in AMD impacted soil under low pH conditions. Three acidic AMD impacted soils (pH 3.5-4.0) were packed into polyethylene columns and leached with a 0.001 M CaCl2 eluent at a rate of 0.5 ml/min. Eluate samples were collected at time intervals corresponding to L/S ratios from zero to 10. The study found that 30-98% of total soil REE contents was leached – the range was attributed to variation in REE speciation and the nature of AMD-soil interaction at the sites, while only 0.02-1.8% of arsenic was leached suggesting a low risk of arsenic mobility and bioavailability. The findings support the use of acidic AMD impacted media for the possible recovery of REE. In conclusion, this PhD thesis established new evidence of the significant impact of AMD on soil systems, demonstrated a higher phytostabilising potential of select metal(loid) under co-cropping and provided supporting evidence for leachability of REE in AMD impacted soil. Finally, these studies contributed to an advanced understanding of the geochemistry of AMD pollution under different environmental conditions as well as the phytoremediation potential of mining-impacted and marginal lands.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Number of pages200
Publication statusPublished - 2024

ID: 387032162