Artisanal and small-scale gold mining (ASGM) is the leading global source of anthropogenic mercury (Hg) release to the environment. Top-down mercury reduction efforts have had limited results, but a ...bottom-up embrace of cyanide (CN) processing could eventually displace mercury amalgamation for gold recovery. However, ASGM transitions to cyanidation nearly always include an overlap phase, with mercury amalgamation then cyanidation being used sequentially. This paper uses a transdisciplinary approach that combines natural and social sciences to develop a holistic picture of why mercury and cyanide converge in gold processing and potential impacts that may be worse than either practice in isolation. We show that socio-economic factors drive the comingling of mercury and cyanide practices in ASGM as much or more so than technical factors. The resultant Hg-CN complexes have been implicated in increasing the mobility of mercury, compared to elemental mercury used in Hg-only processing. To support future inquiry, we identify key knowledge gaps including the role of Hg-CN complexes in mercury oxidation, transport, and fate, and possible links to mercury methylation. The global extent and increase of mercury and cyanide processing in ASGM underscores the importance of further research. The immediacy of the problem also demands interim policy responses while research advances, though ultimately, the well-documented struggles of mercury reduction efforts in ASGM temper optimism about policy responses to the mercury-cyanide transition.
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•Cyanide processing is common and increasing in artisanal and small-scale gold mining.•Mercury and cyanide processing often overlap during transitions.•Creation of mercury-cyanide complexes increases mercury mobility and toxicity.•There are knowledge gaps for the complexes fate, transport, and mercury methylation.•Policy responses are needed and must consider socio-economic and technical aspects.
Key Takeaways
In 2020, AWWA Water Science published a topical collection on potable reuse that aimed to address questions raised by increasing demand for water reuse.
Topics in the collection ...included source water management, pathogen detection and treatment, communications and outreach, and socioeconomics of potable reuse.
In an interview, the collection's guest editors discuss the lessons they have learned and the future challenges for potable reuse regulation and research.
Improving the feasibility and adoption of potable reuse will require additional research to ensure that it continues to be a safe and reliable water supply.
Gold cyanidation facilities in the Arequipa Region of Peru are challenged by the availability and quality of water for processing in an arid environment. The facilities reuse decant water which ...recycles residual cyanide but also undesirable constituents. To understand the impact of intensive water recycling on cyanide and metals concentrations, we collected barren water, decant water, and tailings samples from six gold cyanidation facilities with ore capacities of 10–430 tons per day. Processing facilities in Arequipa recycle all effluents, with decant waters making up 58 ± 11 % of process waters. Decant water contained non-target metals: copper (394 ± 161 mg/L), iron (59 ± 34 mg/L), and zinc (74 ± 42 mg/L). In addition, decant water mean free and complexed cyanide concentrations were 534 ± 129 mg/L and 805 ± 297 mg/L, respectively. Complexed cyanide concentrations remained more constant than free cyanide concentrations with 786 ± 299 mg/L for barren water and 805 ± 297 mg/L for decant water. Cyanide mass balances showed between 21 % and 42 % of unaccounted free cyanide from the start of gold cyanidation and discharge to the tailings storage facility (TSF). Free cyanide estimated losses due to volatilization were 0.8 kg and 2.5 kg of hydrogen cyanide per ton of ore processed at barren water pH of 10.1 and 9.7. Together these results indicate two acute hazards: 1) volatilization of free cyanide during processing and 2) loading and retention of cyanides and metals into TSFs. This study elucidates the extent of uncontrolled vapor phase cyanide release during gold processing operation and contaminant concentrations in the tailings storage facilities. The data highlights the need for improvement oversight, accountability, and regulation of gold processing facilities practicing intensive recycling and zero discharge.
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•Gold cyanidation is a major industry in the Arequipa region but is largely uncharacterized.•Water reuse without discharge leads to buildup of components in tailings and waters.•Mass balance of cyanide revealed a cyanide deficit during process operation.•Tailings were found to contain more cyanide and metals than previously thought.•Exposure risk through volatilization/dust warrant further research or policy response.
► The NF investigated exhibited significant less flux decline compared to a conventional RO membrane. ► NF testing revealed high selection for trace organic chemicals. ► NF exhibited similar permeate ...quality than conventional RO membranes. ► An economic analysis suggested significant cost savings by using a low-pressure NF membrane.
In recent years, considerable interest has been given to using nanofiltration (NF) in lieu of reverse osmosis (RO) for water reclamation applications. This work first examined the operational and rejection performance of several NF membranes compared to a commonly employed RO (ESPA2) membrane at a water reclamation facility. Rejection performance of the NF membranes mainly differed for monovalent ions, however, operational performance characterized by specific flux and flux decline differed substantially among the membranes evaluated. Based on preliminary testing, a promising NF membrane (NF270) was selected for pilot-scale testing and compared to pilot- and full-scale operation of the ESPA2 membrane. While the ESPA2 membrane exhibited significant flux decline primarily due to second stage scale formation, the NF270 membrane exhibited minimal flux decline, presumably due to the partial passage of sparingly soluble salts. Additionally, the NF270 membrane operated at a specific flux double that of the ESPA2 membrane and exhibited minimal flux decline at elevated recovery (87–88%). The major limitation with the NF270 membrane is the poor rejection of nitrate and inability to meet the California TOC requirements of less than 0.5
mg
L
−1. NF270 membrane permeate TOC concentrations were only marginally greater than the ESPA2 membrane and averaged 0.62
mg
L
−1 during 1300
h of testing. The rejection of a wide range of trace organic chemicals was evaluated during ESPA2 and NF270 membrane pilot-scale testing. Analysis of feed and permeate samples collected during NF270 membrane testing revealed that although more compounds could be detected in NF270 membrane permeate samples compared to the ESPA2 membrane, concentrations with the exception of atenolol and TCEP were consistently less than 100
ng
L
−1, which is significantly below any toxicological threshold level. An economic analysis using information gained through pilot-scale testing and full-scale operation revealed that significant cost savings could be achieved by using a low-pressure NF, such as the NF270 membrane.
Some of the same unique physical and chemical properties that make per‐ and polyfluoroalkyl substances (PFAS) desirable for a wide range of commercial applications render them recalcitrant to many ...liquid treatment technologies. As developments in PFAS‐related toxicological studies increasingly suggest potential adverse human health effects, our industry has made great progress in the past several years on concentrating PFAS into small volume waste streams via adsorption and separation mechanisms. Coupled with residual PFAS‐containing commercial products that are being phased out, management of these concentrated waste streams presents an urgent need for the development and validation of destructive treatment technologies. Here, we field‐validate supercritical water oxidation to treat a concentrated waste stream of 12 perfluoroalkyl acids (PFAAs) with liquid and gaseous analysis, adhering to the recent Other Test Method 45 for stack emission sampling from the United States Environmental Protection Agency (USEPA) and USEPA Method 537.1, with quality control and quality assurance protocols from the Department of Defense/Department of Energy Quality Systems Manual 5.3. Results generated suggest greater than 99.999% destruction and removal efficiency of these 12 PFAAs after two ∼120‐min continuous flow trials, with an overall defluorination percentage of approximately 62.6%.
Per and polyfluoroalkyl substances (PFAS) are emerging and persistent organic pollutants that have been detected in many environmental media, humans, and wildlife. A common method to effectively ...remove PFAS from water is adsorption by activated carbon. Preliminary sorption experiments were conducted using five characterized Calgon Corporation coal‐based granular activated carbon (GAC; F100, F200, F816, F300, and F400), one coconut‐based GAC (CBC‐OLC 12 × 30), and one Jacobi Corporation coal‐based GAC (Omni‐G 12 × 40). Sorption of four representative PFAS onto each GAC was measured to select the most favorable carbon sources. F400 and CBC were chosen based on their performance in preliminary PFAS sorption experiments and contrasting properties. Freundlich and Langmuir isotherm models were developed for perfluorooctanoic acid (PFOA) and perfluorooctanoic sulfonate (PFOS) at an initial concentration of 1 mg/L. Sorption capacities were determined for PFOA and PFOS individually and in the mixture. Individual compounds showed higher sorption than when present in the mixture for both PFOA and PFOS. PFOS showed higher sorption than PFOA both individually and in the mixture and F400 showed higher sorption capacity than CBC. The presence of co‐contaminants (kerosene, trichloroethylene, and ethanol), and variations in groundwater conditions (pH, presence of
SO
4
−
2 anions, naturally occurring organic matter, and iron oxides) demonstrated limited impact on the sorption of PFAS onto GAC under the experimental conditions tested.
A process based on electrical discharge plasma was tested for the transformation of perfluorooctanoic acid (PFOA). The plasma-based process was adapted for two cases, high removal rate and high ...removal efficiency. During a 30 min treatment, the PFOA concentration in 1.4 L of aqueous solutions was reduced by 90% with the high rate process (76.5 W input power) and 25% with the high efficiency process (4.1 W input power). Both achieved remarkably high PFOA removal and defluorination efficiencies compared to leading alternative technologies. The high efficiency process was also used to treat groundwater containing PFOA and several cocontaminants including perfluorooctanesulfonate (PFOS), demonstrating that the process was not significantly affected by cocontaminants and that the process was capable of rapidly degrading PFOS. Preliminary investigation into the byproducts showed that only about 10% of PFOA and PFOS is converted into shorter-chain perfluoroalkyl acids (PFAAs). Investigation into the types of reactive species involved in primary reactions with PFOA showed that hydroxyl and superoxide radicals, which are typically the primary plasma-derived reactive species, play no significant role. Instead, scavenger experiments indicated that aqueous electrons account for a sizable fraction of the transformation, with free electrons and/or argon ions proposed to account for the remainder.
Per‐ and polyfluoroalkyl substances (PFAS), which are present in many waters, have detrimental impacts on human health and the environment. Reverse osmosis (RO) and nanofiltration (NF) have shown ...excellent PFAS separation performance in water treatment; however, these membrane systems do not destroy PFAS but produce concentrated residual streams that need to be managed. Complete destruction of PFAS in RO and NF concentrate streams is ideal, but long‐term sequestration strategies are also employed. Because no single technology is adequate for all situations, a range of processes are reviewed here that hold promise as components of treatment schemes for PFAS‐laden membrane system concentrates. Attention is also given to relevant concentration processes because it is beneficial to reduce concentrate volume prior to PFAS destruction or sequestration. Given the costs and challenges of managing PFAS in membrane concentrates, it is critical to evaluate both established and emerging technologies in selecting processes for immediate use and continued research.
Article Impact Statement
Readers will benefit from learning about established and emerging management approaches for per‐ and polyfluoroalkyl substances and the peculiarities of their application to reverse osmosis and nanofiltration concentrates.
Although more than 70% of the globe is covered with water, only a small portion is suitable for direct human use, making the scarcity of freshwater one of our plant's most serious challenges. In this ...context "desalination", defined as "the removal of salt from water", is one of the possible solutions for overcoming our planet's municipal and industrial thirst. By drawing upon the authoritative expertise of a remarkable team of international authors, this book aims to provide an encompassing and "multidisciplinary" introduction to various aspects of desalination. The forte of this publication is that it does not overtly focus on a particular sub-topic of desalination, but rather addresses the topic as a whole. In other words, the unique assortment of reader-friendly chapters is designed to strike a delicate balance between the technical and non-technical. The book is divided into five general sections:The first section presents an overview of water scarcity, followed by a review of integrated water management and the alternatives to desalination. The fundamentals of desalination are provided, including simple water chemistry;The second section covers the conventional technologies of today, including thermal and membrane desalination processes. The topics of pre- and post- treatment are given due credit, as no desalination plant can operate without them;The third section reviews the history of how desalination technologies originated, including a review of today's R&D activities and cutting edge research. The processes and engineering applied for membrane manufacturing are also presented;Section four is concerned with energy and environmental issues, including the application of renewable and nuclear energy, minimization of energy usage and the water-energy-nexus, brine management, and environmental impacts;Finally, section five covers the social and commercial issues, ranging from rural desalination, to the politics of desalination. Desalination costs and feasibility
Cyanide (CN−) from gold processing effluents must be removed to protect human health and the environment. Reducing the use of chemical reagents is desirable for small centralized and decentralized ...facilities. In this work, we aimed to optimize the use of ultraviolet (UV) radiation coupled with hydrogen peroxide (H2O2) to enhance the rate and extent of CN− removal in synthetic and actual gold processing effluents, from one centralized and one decentralized facility in southern Peru. Bench-scale studies conducted using H2O2 and ambient UV showed no significant effects on CN− destruction; however, experiments with higher UV intensity and H2O2 accelerated free CN− degradation. When a 1:1 stoichiometric ratio of CN−:H2O2 was tested, the highly concentrated effluent (1 g CN−/L) had a slower pseudo first-order rate constant (k = 0.0066 min−1) and took ~5 h longer to reach 99% destruction, compared with the low concentration effluent (100 mg CN−/L; k = 0.0306 min−1). Lastly, a TiO2 photocatalyst with low stoichiometric CN−:H2O2 ratios (1:0.1 and 1:0.2), in a compound parabolic solar concentrator, was tested to investigate the degradation of a high concentration effluent (1.28 g CN−/L). These results show a significant improvement to degradation rate within a 20 min period, advancing treatment options for mineral processing facilities.
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