Maddocks, GA 2009, 'Reactive in-situ covers for the remediation of mine waste rock using BAUXSOL lime and biosolids', PhD thesis, Southern Cross University, Lismore, NSW.
Copyright GA Maddocks 2009
Failure to design, construct and remediate waste rock and tailings storage facilities at mine sites leads to adverse environmental degradation and unforeseen financial costs. Typical closure options for these facilities include barrier or store and release covers. This thesis investigates the use of reactive in-situ covers as an alternative engineering design approach that involves mixing reagents (e.g. BauxsolTM) with waste rock to neutralise acidity and to immobilise major and minor metals. It was unknown whether this approach could be achieved at a field scale, whether the use of BauxsolTM would achieve its primary objectives, whether there would be adverse effects on the soil chemistry or whether there would be adverse ecotoxicological problems.
Four 400 m2 field trials were conducted at a mine site and included a Control; Bauxsol™ (25 kg / m2) plus biosolids (15 kg / m2); Lime (2.5 kg / m2) plus biosolids (15 kg / m2) and a fourth site that was encapsulated with 0.3 m of compacted clay and 0.1 m of topsoil. The results suggest that soil chemistry can be significantly improved by mixing Bauxsol™ with the top 0.5 m of the waste rock profile i.e., creating a reactive in situ cover. This was sufficient to create a root zone up to 1.6 m deep that had pH greater than 5 and lower concentrations of metals measured using a sequential extraction procedure. Treatment of the acid mine waste with Lime did not achieve marked improvement of soil conditions in soil layers below the amended zone. The capping treatment created a topsoil layer with higher pH, but the underlying mine soil remained unimproved. Leachate pH in the Control became increasingly acidic (pH 4.57 to pH 3.95). The addition of Lime and biosolids led to an initial increase in leachate pH, compared to the Control, however this decreased over the duration of the study (pH 5.37 to pH 4.89). In the Bauxsol™ and biosolids treatment leachate pH increased to 6.92 after the first rainfall event and continued to increase over the duration of the study to pH 7.4. After 24 months metal leachate concentrations (mg / L) in the lysimeters for Al, Cd, Cu, Mn and Zn were (Control: 32.6, 5.7, 12.7, 39.3, 121.8), (Bauxsol™: 0.07, 0.02, 0.07, 0.57, 0.23) and (Lime: 2.19, 1.19, 2.33, 3.6, 28.4). No leachate was available for collection from the Clay treatment indicating that this technique was functioning in terms of minimising the infiltration of water into the mine soil.
Ecotoxicological studies of major and minor metals in eucalypt leaves from the field trials and earthworm bioaccumulation studies were undertaken. The Bauxsol™, Ca(OH)2 and Clay treatments in the field trials allowed good tree growth of four eucalypt species, compared to the Control. There was no statistical difference in tree growth between the Bauxsol™, Lime or Clay treatments over the two years of monitoring. However the growth of one tree species was poor in the Bauxsol™ treatment.
Laboratory bioaccumulation assessments found that there was good motility and no mortality of the earthworm species E. fetida after 28 days exposure to metal loaded BauxsolTM. The bioaccumulation of metals in E. fetida and bioaccumulation factors were below reported toxicity thresholds to cause mortality and below reported bioaccumulation factors for moderately contaminated soils, indicating that the metals bound to the Bauxsol™ reagents are mostly non-bioavailable E. fetida. Analyses of the 20 % treatment at 28 days using a sequential extraction procedure showed that > 95 % of the metals are bound within the Fe / Mn oxide fractions. However, changes occurred in metal fractionation after exposure to E. fetida for Cd, Cr and Fe, Mn. The data also showed that the exchangeable (1M MgCl2) and the Toxicity Characteristic Leaching Procedure extractant are useful as indicators of metal bioavailability to E. fetida.