# Remediation Opportunities for Arsenic-Contaminated Gold Mine Waste
## 1 Introduction
### Gold Mining and Refining  
In 1848, the great gold era was born in California, following further discoveries primarily in North America, Australasia, and South Africa [18; 19]. The early 19 th century began with a yearly output of 10-20 t of gold, but by the mid-19 th century it was 180 t, and by the end of the century it reached 450 t. Gold oxides are rare, and gold is usually found in the form of a mineral and therefore requires extraction [20]. For example, arsenopyrite (FeAsS) and arsenian pyrite (Fe5\({}_{2}\)) host gold in the crystal structure, and extraction techniques are required to isolate gold [21; 22]. The most effective extraction operation, where approximately 95% of gold is recovered, is cyanidation of free milling ores such as quartz vein gold ores (Figure 1) [20]. However, free milling ores quickly became exhausted, and mining operations focused on refractory ores [23], which required new technology because cyanidation alone was not adequate for effective gold extraction [20]. Refractory ores are characterised by carbon, sulphide, and tellurium ores or a combination, sulphide being the most common (Figure 1) [20; 23]. A primary sulphide mineral that hosts gold is arsenopyrite; extraction is complex and requires additional treatments [24]. Additional treatments include fine grinding for semi-refractory ores and thermal treatments for refractory ores such as roasting and pyrolysis (Figure 1) [24]. Roasting was common because it was cost effective. Sulfphides are oxidised, and gold is rendered into a leachable state [20; 24]. However, this process creates arsenic emissions such as arsenic trioxide (As\({}_{2}\)O\({}_{3}\)) dust through precipitation of arsenic vapours [21]. Post-precipitation, arsenic trioxide dust is deposited on the surrounding environment, can be found in soil particle sizes as low as <20 \(\upmu\)m, and is highly bioavailable [21]. Relative bioavailability is the amount of a toxic substance that can enter the human/animal circulatory system relative to a reference compound, such as sodium arsenate [17; 25]. Bioavailability is vital in understanding and evaluating toxicity effects of arsenic-rich soil if it is consumed by a living organism [25].  
Future mining prospects are led by the worlds demand for new technology such as phones, computers, electric vehicles, and solar panels, all of which require metals. In 2017, there were approximately 7.7 billion mobile phone subscriptions, 48% of global households owned a computer, and global commitments to mitigating climate change focused on electric cars [26]. Advancements in electric technology and batteries heavily rely on mining of raw elements. For example, cobalt is an element mined to produce batteries, e.g., nickel-cobalt oxide batteries and lithium-cobalt oxide batteries [27]. Global climate change agreements could see cobalt demand increase 500% by 2025 for the production of 100 million electric cars by 2030 [26]. The majority of cobalt deposits were discovered in the Democratic Republic of the Congo (DRC), and a mining boom, resembling the gold rush, has developed [26,27]. Approximately 60% of global cobalt originates from the DRC, and cobalt is considered the \"oil\" of a modern-day low-carbon future [26]. There are numerous concerns regarding cobalt mines in the DRC, such as land degradation, humanitarian rights, human health, and pollution [27]. It is important to identify remediation techniques not only for legacy mine waste sites but also to provide sustainable solutions for the future mine waste that is to come because of our global demand for technology.