Bigot, M, Guterres, J, Rossato, L, Pudmenzky, A, Doley, D, Whittaker, M, Pillai-McGarry, U & Schmidt, S 2012, 'Novel metal-binding hydrogel particles alleviate soil toxicity and facilitate healthy plant establishment of the native metallophyte grass Astrebla lappacea in mine waste rock and tailings', in AB Fourie & M Tibbett (eds), Proceedings of the Seventh International Conference on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 533-549, https://doi.org/10.36487/ACG_rep/1208_46_Rossato_Bigot
Soil contaminants are potentially a major threat to human and ecosystem health and sustainable production of food and energy where mineral processing wastes are discharged into the environment. It is rarely possible to totally stabilise, render harmless or remove toxic metals from these wastes. Metallophytes (metal-tolerant plants) have evolved mechanisms to survive on many natural metal-rich soils and mining wastes. However, in extreme conditions, metal concentrations in soils/wastes often exceed even the metal-tolerance thresholds of metallophytes and the sites remain barren with high risks of contaminant leaching and dispersion into the environment via erosion. A novel soil amendment based on micron-size thiol functional cross-linked acrylamide polymer hydrogel particles (X3) binds toxic soluble metals irreversibly and significantly reduces their concentrations in the soil solution to below the phytotoxicity thresholds. X3 thoroughly mixed with the surface soil layer (top 50 mm) of toxic mine waste materials in pots in glasshouse conditions successfully reduced total soluble concentrations of major contaminants in waste rock (aluminium (Al), copper (Cu), zinc (Zn), cobalt (Co) and manganese (Mn)) and tailings (sodium (Na), sulphur (S), Zn, Mn, Co and cadmium (Cd)) by 90.3 to 98.7% in waste rock, and 88.6 to 96.4% in tailings immediately after application. Soil solution pH was significantly increased from 2.5–3.8 and from 7.1–7.7 in X3-amended waste rock and tailings, respectively. Salinity of the soil solutions (assessed via electrical conductivity (EC) measurements) decreased significantly from 12.4–3.5 and from 81–14 mS.cm-1 in X3-amended waste rock and tailings, respectively. After 61 days, the quality of the unamended bottom layer of the X3-treated pots was also improved and showed significant increase in pH and reduction of metal concentrations by 91.6 to 95.6% in waste rock and 64.9 to 84.1% in tailings, as well as reductions of EC in both mine substrates and reduction of saline element concentrations (Na and S) by 69 to 74% in the tailings particularly. Furthermore, the combination of X3 and metallophytes was more efficient at improving soil solution quality than X3 alone. The addition of X3 to the substrates increased substrate water retention and water availability to plants by up to 108% and 98% for waste rock and tailings respectively via a substantial reduction in salinity, while it significantly decreased substrate penetration resistance allowing easier root penetration in surface soil. Soil quality improvement by X3 allowed successful early establishment of the native metallophyte grass, Astrebla lappacea, on both waste rock and tailings where plants failed to establish otherwise. In X3-amended waste rock, plant establishment percentage (45%) was not significantly different from the amended sand control (41%). Although X3-amended tailings had lower establishment percentages (35%) than the amended sand control (61%), plants had established at a satisfactory percentage when compared to unamended tailings treatments (0%). No toxicity or deficiency symptoms were observed on leaves throughout the experiment. The X3 remediation technique promises to be very robust and applicable to a wide range of situations where decades of attempted plant establishment had previously been unsuccessful.
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