Assessment of biochar, compost, and iron amendments to enhance the phytostabilisation of arsenic in gold mine waste by the Australian native plant species Poa labillardieri (Steud.)

Gold mine waste has elevated arsenic (As) concentrations that exceed the Australian national soil guidance values (100 mg/kg). In Australia, Victoria has reported As concentrations of 47,000 mg/kg, causing concern because As is toxic to living organisms; it is carcinogenic, has human health implications and damages ecosystems. Phytostabilisation is a sustainable alternative that is cost-effective, aims to immobilise As in the roots/rhizosphere and restore soil health. Australian natives were previously investigated for phytoremediation and results identified Poa labillardieri as a potential phytostabilising species. Soil amendments have soil ameliorating abilities and previous studies have shown that soil amendments enhance phytostabilisation. This study aimed to identify a soil amendment to optimise phytostabilisation using P. labillardieri. A greenhouse mesocosm experiment was conducted for 100 days and involved soil amendments, 5% biochar, 5% compost and 1% ferric oxide (Fe₂O₃) applied individually or in combination. The treatment combination of 5% biochar and 5% compost (BC), significantly (p ≤ 0.01) increased root As concentrations (657 ± 135 mg/kg) compared to the unamended control (317 ± 12 mg/kg). Shoot As concentrations ranged between 30 and 44 mg/kg for all treatments. The BC treatment increased soil-to-root bioaccumulation to 0.97, 1 being optimal for phytostabilisation. Soil As concentrations significantly decreased (677 ± 21.5 mg/kg) compared to the unamended control (781 ± 6.32 mg/kg). Additionally, removal of As from soil improved from 6.83 ± 5.93 to 50 ± 24 mg/m³ and in vitro bioaccessibility decreased from 47.0 ± 6.92 to 35.8 ± 0.72%. The BC treatment had greater root (0.01 ± 0.01 g), shoot (0.05 ± 0.02 g) and total biomass (0.06 ± 0.03 g), compared to the control (0.004 ± 0.002 g root biomass; 0.01 ± 0.01 g shoot biomass; 0.014 ± 0.012 g total biomass). The BC treatment shows potential to assist phytostabilisation using P. labillardieri; It is recommended that a field trial be conducted with the newly developed phytostabilisation technique involving P. labillardieri and soil amendment 5% biochar plus 5% compost to validate the results under natural climatic conditions.