The mercury problem in artisanal and small scale gold mining

Application of mercury in the concentration of gold

Part 111

Another adverse impact on the environment due to Artisanal and Small scale Gold Mining (ASGM) is deforestation. In Peru, for example, it was shown that from 2006 to 2009 alone approximately 20 km2 of Amazon forest was cleared each year for ASGM. Because this deforestation and mining is unregulated and strongly correlated with rising gold prices, it is unfortunately likely to continue to damage one of the most biodiverse ecosystems in the world.

  Challenges and Opportunities for Chemists

The problem of mercury use and emissions in ASGM is profound. It is also a problem that has existed for many decades and therefore one which has resisted many well‐intentioned interventions by governments, environmental advocates and humanitarian service organizations. Stemming mercury pollution in ASGM is not as simple as legally restricting or banning the use of mercury, as such measures have already been implemented in areas with the highest levels of mercury use, with little success. Banning mercury use may also hinder engagement and education of miners working outside the law, so in these cases mercury use continues unabated. Poor and ineffective enforcement of legislated mercury controls, lack of direct engagement and support of artisanal miners, the high market value of gold, and a strong black market for mercury trade all but ensure that mercury use in ASGM is likely to continue for years. In taking measures to address the mercury problem in ASGM, one must keep in mind that these miners live in some of the poorest areas of the world and have few, if any, options for other employment and income. Mining for gold is their livelihood and a means to support their family. This consideration is important because while there are an estimated 10–19 million ASGM workers at any time, 80–100 million people are thought to be directly dependent on the associated income. Policies, technologies and service work aimed at lowering the burden of mercury emissions on health and the environment should therefore consider how such measures support the miners.

  The chemistry community can provide important technological advances that may help overcome some aspects of the mercury problem in ASGM. Some specific opportunities and challenges are discussed next to help encourage contributions from the chemistry research community, with an aim to spur eventual collaboration with environmental scientists, public health advocates and even field work with ASGM miners and their supporters. For each of these needs, it must be clear that any potential solution will be easier to implement if it is extremely low in cost, scalable, easy to transport to remote locations, operates with intermittent or no central power supply, requires little or no training for operation, and provides immediate and obvious benefit to miners. Only then, will uptake of any technological solution be realistic.

 Low-cost and portable mercury monitoring

 The mercury emissions from ASGM pollute air, water and soil. In many cases, this pollution is a threat to food and water supplies. It is therefore important to have real-time, portable, and cost‐effective monitoring of mercury levels-especially in air and water. While portable atomic absorption spectrometers and hand-held X-ray fluorescence instruments have been used to monitor mercury pollution at mining sites  (including ASGM locales), these instruments typically cost thousands of dollars. It would be useful for ASGM communities to have access to low-cost, low-maintenance sensors for rapid measurement of mercury levels in air and water. Such technologies could potentially help the miners, other users of contaminated waterways, and local authorities limit exposure to mercury, provided the sensors require minimal training and are portable or even disposable. As mercury can be interconverted between various oxidation states with various ligands that impact mobility and toxicity, the ability to assess speciation is also important. Additionally, such technologies could be coordinated with information campaigns on the dangers of mercury exposure.

 Low-cost point of care diagnostics for mercury exposure

 Like mercury analysis in the environment, clinical analysis of mercury exposure typically relies on atomic absorption spectrometers. To administer care and medical advice to miners in remote locations, portable and disposable mercury diagnostics might be valuable. In developing such technologies, chemists might consider ways in which breath, saliva, hair or urine might be analyzed in a rapid and cost-effective fashion. These technologies could also be used by the miners to help self-monitor exposure to mercury.

New strategies for tailings processing

 The mercury-laden tailings in ASGM are one of the more challenging problems for remediation. After milling mercury with ore, microbeads of mercury are dispersed in the fine sand and water and can be carried far from the mine when discharged in water courses. Many thousands of tonnes of tailings are generated each year in ASGM and removing mercury and recovering residual gold are critical problems in tailing processing. In some cases, the tailings are processed in large vats of aqueous sodium cyanide, which solubilizes gold for eventual recovery on a sorbent or through precipitation with zinc. Unfortunately, cyanide also complexes with mercury in the tailings, facilitating its transport into the environment through wastewater. Mechanical separation of the mercury from the sand and soil in the tailings, for instance on a shaker table, may be useful in recovering mercury and preventing its release in the environment, but such strategies have limited uptake at present. Simple and rapid chemical remediation of mercury in tailings (and the recovery of gold) is an outstanding problem in ASGM. Any potential solution must be able to process large volumes of tailings (tonnes), operate on a shorter timescale than cyanide leaching techniques, and facilitate gold recovery at a level that incentivizes uptake. Ideally, the technique should also be environmentally innocuous and only generate discharges that are recyclable or biodegradable. 

 Soil remediation

 Like the tailings problem, remediating mercury-contaminated soil is a large-scale problem in ASGM areas. Keeping in mind the limited financial resources of ASGM communities and the governments in these jurisdictions, any strategy for soil remediation must require low‐capital outlay and simple protocols for remediation. Extensive excavation and capping, off-site disposal, and washing or thermal treatment of soil (all measures taken in wealthy nations) are likely impractical solutions for soil remediation in ASGM. Instead, there is a need for the development of soil amendments that can trap and immobilize diverse forms of mercury. These soil additives must be very inexpensive and scalable to have any chance of impact on contaminated areas spanning several square kilometers. These so-called “in situ remediation” techniques must also sequester the mercury in a way that prevents not only leaching into ground water, but also methylation by bacteria and subsequent bioaccumulation. In an effort to address this need, a recent study by our laboratory introduced a mercury sorbent made from recycled cooking oils and sulfur that immobilized floured mercury metal in soil. The material, prepared by inverse vulcanization of extremely low-cost feedstocks, can be milled with mercury-contaminated soil to convert the metal to the non-toxic and highly insoluble metacinnabar. Notably, the polymer changes from brown to black as it reacts with the mercury. This is useful because the mercury flour is typically not distinguishable from most soil.  While additional field tests are required to ascertain whether the scale and efficiency of this mercury immobilization method can have on ASGM, it is notable that this study explicitly considers cost, scale and ease of operation required for use in ASGM.

 Another potential strategy in remediation and reclamation of mercury‐contaminated soil in ASGM areas is through phytoremediation. In this technique, plants are used to extract mercury from soil. Improving the efficiency of mercury uptake in the plant and a plan for the fate of mercury‐contaminated biomass are still major (biochemistry and chemistry) issues to be resolved for impact in ASGM. Nevertheless, this strategy is promising in that it is relatively low‐tech and low‐cost in its deployment and may therefore be an important measure to take in remediating mercury‐contaminated soil at ASGM sites.