# Remediation Opportunities for Arsenic-Contaminated Gold Mine Waste
## 3 Mitigating Arsenic-Contaminated Mine Waste
### Arsenic Remediation Techniques
#### 3.3.2 Biological  
Remediation techniques that are environmentally sustainable and cost-effective have been investigated to minimise the impacts of traditional techniques [59; 82]. Biological techniques are environmentally sustainable because they utilise biological processes, such as microorganisms and plants to remediate PTEs (Table 3) [82; 83; 84; 85]. Microorganisms, such as fungi _Penicillium_ sp. and sulphur- and iron-oxidising bacteria such as _Acidithiobacillus thiooxidants_, can induce bioleaching and reduce toxic elements in soil [86]. Plants can be used to remediate contaminated soil through physiochemical mechanisms, called phytoremediation [87]. There are many phytoremediation techniques (Table 3), with phytoextraction and phytostabilisation being the most studied [73; 88; 89; 88]. Phytoextraction and phytostabilisation are dependent on the plant species, soil microbes, and the associated PTEs [84; 90]. Phytoextraction aims to reduce the concentrations of PTEs in contaminated soil by plant uptake mechanisms and storing PTEs in the above ground biomass, which is then harvested and appropriately disposed of (Figure 3) [31; 90; 91; 90]. There are various disposal options for harvested biomass; some include landfill, compositing, incineration, and pyrolysis [90; 92]. Composing has concerns with leaching when toxic elements are still present in the final product; therefore, it is common to apply hydrated lime to reduce leachate [92; 93; 94]. Arsenic leaching occurs when soil pH is below 6.0, and the aim of the application of hydrated lime is to increase soil pH to mitigate arsenic leaching [92; 93; 94]. However, lime amendments have been shown to inhibit plant growth [93; 94; 95]. Pyrolysis and incineration results in ash biomass and is currently being investigated as a solution to harvested biomass post-phyoextraction [92]. Phytostabilisation aims to accumulate PTEs in plant roots to mitigate bioavailability, reduce mobility, and limit exposure within the ecosystem (Figure 3) [13; 96]. However, As is a non-essential element for plants and can induce plant toxicity (phytotoxicity) and prevent plant function [16; 97]. Phytotoxicity must be considered, and a plant's ability to survive As or multi-elemental contamination must be assessed to ensure a successful phytoremediation application. Phytotoxicity can prevent high biomass, which is required for phytoextraction and can delay overall plant growth [16; 87; 97]. To mitigate phytotoxicity, the identification of metallophytes (metal(loid) tolerant plants) and hyperaccumulators is crucial. Hyperaccumulators translocate more toxins from soil via the root to the shoot by 2 or 3 orders of magnitude compared to other plant species [98]. Currently, there is a lack of identified metallophytes and As hyperaccumulators [16; 87; 97]. Biological remediation techniques are considered to be low-cost compared to the traditional physical and chemical methods [13; 16; 97]. In Europe, landfill disposal is estimated to cost USD 88.6 per t of mine waste [99]. Globally, approximately USD 1.5 billion is spent annually on acid and metalliferous drainage (AMD) management of mine waste [100]. Management of AMD mine waste includes physical and chemical treatments such as layering and mixing of waste rock, encapsulation, backfill disposal, and barriers to prevent leaching into groundwater [100]. The more cost-effective biological remediation methods, however, are slow and require longer application time which may take years or decades to reduce the toxicity of sites to safe levels [16; 86; 97].