Scientists have discovered a sustainable way to recover toxic metals and cyanide from wastewater associated with mining, using biochar made from waste sawdust. Biochar is a waste product of the timber industry, which would otherwise end up in landfills, and has been shown to be highly effective.
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Acid Mine Drainage and Cyanide
Mining activity usually occurs deep underground beneath the water table, so mine shafts need to be continuously pumped out to prevent flooding.
When mining operations are abandoned, the excavated spaces fill with water. Toxic metals such as copper, zinc, iron, nickel, lead, cadmium, and mercury, together with sulfur, may leach into the water from the surrounding rocks.
The water becomes acidified due to oxidation, which is often accelerated by chemical interactions with certain bacteria and micro-organisms. It is a pollutant for aquatic life in streams and rivers, and particularly hazardous for indigenous people using soil for agriculture or streams for drinking water.
In gold mining, cyanide is used to leach gold from rocks, and this also contaminates the water.
Wastewater from mining is referred to as acid mine drainage (AMD). Contaminated water from waste rock piles, spoil heaps, and crushed rock, is called acid rock drainage (ARD), but AMD is generally used as an umbrella term for both.
Environmental Hazardous Effects of Tailings
AMD is disposed of in pond and dam systems known as tailings. Tailings are notorious for failing events, creating sinkholes and landslides, which then contaminate the environment. Long-term toxic effects of dam failing accidents depend on how quickly remedial actions are taken, the quantity of material involved, what the characteristics of the material are, and the rate of discharge.
The Samarco iron tailing dam failure in 2015 was one of the largest ever reported failings worldwide. It killed 19 people and contaminated 650 km of rivers, all the way to the Atlantic Ocean, when 25 million m3 of waste residues slid down a mountainside in Minas Gerais, Brazil.
Gold Mining Cyanide Tailings
AMD from gold mining contains heavy metals, but also cyanide, an organic compound with potentially deadly effects.
Cyanide is used in two ways; The first is heap leaching, whereby cyanide solution is sprayed over heaps of crushed ore, which dissolves the gold, and is collected on pads underneath. This process is repeated until all the gold has been collected.
The second way is tank leaching, where cyanide is sprayed into tanks containing ore and collected in a more controlled way. The tailings are still highly toxic because although cyanide breaks down rapidly in surface water, it can remain in groundwater for a very long time. Cyanide tailing accidents have resulted in major fish kills, contaminated drinking water, and agricultural land.
In 2014, 500,000 gallons of cyanide spilled from ponds at Proyecto Magistral mine in Mexico after heavy rainfall. In 2000, a tailings dam failed in Romania, killing fish and poisoning water supplies as far as 250 miles away downriver.
Because gold mining is particularly hazardous, scientists have started researching alternative ways to deal with gold mining wastewater.
Biochar Method for Removing Heavy Metals and Cyanide
A process has been developed using low-cost biochar made from sawdust, with some effective end results.
Biochar is charcoal produced by pyrolysis or thermal decomposition of biomass in the absence of oxygen. It has many uses, from improving soil fertility and agricultural yields, to carbon sequestration as a possible way of mitigating climate change.
Biochar decreases the concentration of cyanide ions and other heavy metals in tailings wastewater.
Sawdust waste, usually destined for landfills, has the potential for creating methane, a potent greenhouse gas, or the potential to cause fires. By using this timber industry waste product as a possible solution for gold mining waste, a more sustainable win-win situation is achieved.
Scientists tested the biochar with wastewater containing cyanide, chromium, iron, zinc, nickel, lead, manganese, and copper, with the pH maintained at a constant rate of pH 7 through the addition of carbon dioxide. Higher pH levels were shown to slow down the absorption process.
The results showed biochar as a bio absorbent in water treatment with an average yield of 73%.
The smaller the biochar particle size, the larger the number of contaminants removed. The higher the surface area, the greater the absorption rate. Saturation was attributed to clogging of biochar pores during the bio absorption process and reached after 14 hours.
Alternative Methods for Gold Mining and Cyanide Removal
An alternative way to extract gold from ore is the possibility of using thiosulphate instead of cyanide, as a low-cost, non-toxic method. However, it is not without issues and a lack of funding has halted investigative research.
A number of other reagents have been proposed after lobbying by indigenous communities and environmental NGOs calling for safer less toxic chemicals to be used for leaching.
Some alternatives suggested include ammonia, sodium sulfide, iodine, bromine, and natural organic acids, but most of these lack research to be considered strong contenders to replace cyanide at the current time. Funding is needed either from governments or private investors, with an interest in making them commercially viable.
This currently leaves cyanide extraction as the most popular extraction method, and with it, the hazardous threats to the environment, aquatic, and human life. This less-than-ideal situation may provide an opportunity for biochar to step in and be developed further as a commercial water treatment for use in the mining industry.
References and Further Reading
Potential to remove heavy metals and cyanide from gold mining wastewater using biochar (01.20.2022) Manyuchi.M.M, Sukdeo.N, Stinner.D. In Elsevier Physics and Chemistry of the Earth (pdf) Journal parts A/B/C, published in Science Direct online https://www.sciencedirect.com/science/article/pii/S1474706522000055
The Environmental impacts of one of the largest tailing dam failures worldwide.(09.06.2017) Hatje.V, Pedreira.R.M.A, De Rezende et al. in Nature online https://www.nature.com/articles/s41598-017-11143-x
Cyanide Use in Gold Mining (2019) in Earthworks online https://earthworks.org/issues/cyanide/
Thiosulphate as an alternative to cyanide for gold processing – issues and impediments (18.07.2013) Muir.D, Aylemore.MG in Taylor & Francis online https://www.tandfonline.com/doi/abs/10.1179/037195504225004661
Alternatives to cyanide in the gold mining industry: what prospects for the future? (05.25.2005) Hilson.G, Monhemius.A.J, in Journal of Cleaner Production Elsevier (pdf) published in Science Direct. https://www.sciencedirect.com/science/article/pii/S0959652605000636