Gold Mining Wastewater Research Articles

Evaluating the Potential of Maize (Zea Mays) Cobs for Mercury Removal from Small-Scale Gold Mining Wastewater

Evaluating the Potential of Maize (Zea Mays) Cobs for Mercury Removal from Small-Scale Gold Mining Wastewater

A systematic evaluation of alkaliphilic microbial consortia from a soda lake for the biodegradation of cyanide-rich wastewater

Alkaliphilic microorganisms are one option for the treatment of cyanide-polluted wastewater. This study reports the degradation of cyanide from simulated gold mine wastewater using alkaliphilic microbial consortia harvested from a soda lake, Lake Chitu, Ethiopia. A novel aerobic-anoxic integrated treatment setup was established for the treatment process. Colorimetry was used to measure residual cyanide concentration, and 16S rRNA amplicon gene sequencing was used to study microbial diversity. This treatment system was able to degrade 97.49 ± 0.12% of 200 mg/L sodium cyanide. However, changes were observed (p < 0.05) when the established consortia were stressed with heavy metals. About 28 % of the initial inoculum persisted until the end of the treatment days. Twenty-eight bacterial phyla were identified, with Firmicutes, Proteobacteria, and Bacteroidota being the most abundant. At the end of the treatment process, Alkalibacterium (74.43%), Exaguobacterium (6.6%), and Halomonas (3.89%) were dominant. These findings indicate that alkaliphilic microbial consortiums from Ethiopian Rift Valley soda lakes are effective for the treatment of cyanide-polluted wastewater.

Comparative Analysis of Locally Sourced Adsorbents for Iponda and Idominasi, Abandoned Gold Mine Wastewater Bioremediation in Osun State, Nigeria

This study investigates and compares the effects of Neem leaf (NL) and coconut husk (CCH) adsorbents in reducing the levels of heavy metal concentration and elemental content in wastewater from two specific abandoned gold mining sites, namely Iponda (Iponda wastewater- IPWW) (longitude 4°43'18''E, Latitude 7°43'57''N ) and Idominasi (Idominasi wastewater -IDWW) (longitude 4°42'0''E, Latitude 7°40'59''N). Preparation of locally sourced adsorbents, bioremediation process through adsorption and physico-chemical analysis of the treated wastewater samples were carried out. Adsorbents were prepared from washed, air-dried neem leaves, grinded to 40 microns size to facilitate penetration of the adsorbate. In addition, washed and airdried CCH was carbonized in an electric muffle furnace at between 300 0C and 350 0C over a period of 30 minutes, and then granulated and sieved with a mesh size of 40 µm to ensure uniform size. Measured dosages of adsorbent (0.5 g, 0.75 g, and 1 g) were added to a 50 ml volume of wastewater and the mixture was agitated at different speeds ranging from 30 rpm to 120 rpm. The treated wastewater samples were then subjected to various physico-chemical analytical tests to determine the effect of the adsorbents on the reduction of the heavy metal concentration and elemental content of the wasted water samples. The results of this study demonstrate that both NL and CCH adsorbents have a significant impact on the wastewater from the two abandoned gold mining sites, IPWW and IDWW. This impact is reflected in the reduction of heavy metal and other element concentrations in the wastewater. Furthermore, it can be concluded that the bioremediation process is more effective when utilizing NL compared to carbonized CCH. Locally sourced cheap non-edible biomass materials hitherto considered as wastes have proved to be highly effective and efficient when used as adsorbents in the treatment of wastewater from abandoned gold mining sites. This is evident from the results obtained from this study.

Recovering and reusing water, H2SO4, nickel and cobalt from gold mining wastewater using air-gap membrane distillation and solvent extraction

Gold mining industry has attracted significant attention for its impact on the environment, encouraging a responsive interest in recovering valuable materials from gold mining wastewater. This study, aligned with the principles of the circular economy, aimed to explore the extraction of water, sulfuric acid, nickel, and cobalt from gold wastewater, employing air gap membrane distillation (AGMD), non-dispersive solvent extraction, and conventional solvent extraction. In the AGMD process, it was demonstrated that 69 % of the wastewater could be reclaimed in the form of high-quality distilled (6.6 ± 1.8 L/m2h). The reclaimed water played a pivotal role in the subsequent non-dispersive extraction stage, acting as a re-extraction agent. Hollow membrane contactors were employed in this stage, achieving a recovery rate of over 95 % for sulfuric acid from the concentrate, with high selectivity over iron (βFe = 488) and magnesium (βMg = 8011;). The acid-depleted stream was then subjected to conventional solvent extraction, wherein Cyanex 272 proved highly effective in extracting nickel, although with limited success in extracting cobalt. The previously used acid current was reintroduced as a re-extractant, demonstrating an 89.3 % re-extraction efficiency. In summary, this study conclusively demonstrated the feasibility of reintroducing water, sulfuric acid, and nickel into the economic cycle through the applied methods. The recovered materials were carefully examined for their potential applications, not only within the gold mining but also across various industrial sectors. This multifaceted approach aligns with the circular economy's objectives, fostering the efficient and sustainable reintroduction of recovered materials into economic processes.

Weibull Technique for Evaluation of Swelling: Composite Graphite Resin Electrode for Electrochemical Treatment of Gold Mining Wastewaters

This paper evaluated the swelling of graphite resin electrodes developed for utilization in the electrochemical treatment of gold mining wastewater. Graphite-resin electrodes were developed from used dry cells and resin using non-heat treatment processes (segregation). The Microstructure of the electrode was determined using a scanning electron microscope (Carl Zeiss Smart Evo 10) to ascertain the composition of the electrode. The swelling property of the electrodes was measured using the standard method through a combination of gold mining wastewater and chloride salt solutions. Effects of operational factors (particle size, percentage binder and compressive “compacting” pressure) on the swelling of the electrodes were monitored and evaluated statistically (using analysis of variance). Weibull probability distribution (2 and 3 parameters) was applied to the swelling through Microsoft Excel Solver and Moment Likelihood Method to ascertain the usefulness of the electrode in environmental pollution control through computation of reliability. The study revealed that the swelling was in the range of 1.48 % to 2.24 %, particle size (F5,20 =196.48, p = 2.76 x 10-16), percentage binder (F4,12 =181.58, p = 1.27 x 10-10), and compressive pressure (F3,12 = 106.69, p = 6.43 x 10-9) were significant factors that influence swelling of graphite-resin electrode at 95 % confidence level. the values of α and β for 2-parameters Weibull distribution are 63.162 and 15.098, and 1.265 and 10.089 for MSE and MLM methods, respectively. The Table shows that the values of α, β and θ for 3-parameters Weibull distribution are 3.679, 8.097 and 0.168, and 4.350, 7.165 and 0.198 for MSE and MLM methods, respectively. It was concluded that particle size and compacting pressure are significant factors that had an effect on the swelling of graphite resin electrodes for treatment water and wastewater.

Hybrid membrane distillation and ion exchange process for resources recovery from mining wastewater

This work aims to achieve a process intensification through the hybridization of direct contact membrane distillation (DCMD) and ion exchange (IX) to increase resource recovery (water, H2SO4, Copper (Cu), Nickel (Ni), and Cobalt (Co)) from a gold mining wastewater. An increase in maximum recovery rate was achieved by the DCMD in the hybrid configurations – from 34 to 52 %. Recovery of Cu, Ni and Co is best achieved by NH4OH (4 mol/L), however with considerable carryover. Conversely, selective recovery can be achieved by different eluting agents H2SO4 0.1 mol/L to recover Co (~80 %), H2SO4 1 mol/L to recover Ni (~47 %), and NH4OH 4 mol/L to recover Cu (~26 %), with a lower carryover of other ions (<14 %). Scale-up calculations showed that the Amberlyst A26 mass required for the hybrid IX (batch) is 2.4× lower than the Dowex M4195, which resulted from the exchange capacity and selectivity. Conversely, the estimated Amberlyst A26 mass required for the hybrid IX (fixed-bed) was higher due to the higher initial concentration of H₂SO₄. Lastly, CapEX ranged from 0.2 to 5.6 M US$, and OpEX from 2.61 to 11.42 US$/m3. Considering earnings with the recovered resources the hybrid fixed-bed configurations are a clear choice due to the lowest payback times (<4 years).

Selective electrochemical reduction of mercury(II) from a simulated traditional gold mining wastewater contaminated with cyanide and heavy metals

In traditional gold mining (TGM), the wastewater discharges are contaminated with high concentrations of cyanide and heavy metals. Understanding the effects of different cyanide species on the electrochemical behavior of mercury(II) is critical for its removal from TGM wastewater. Herein, we evaluated mercury electroreduction using a model solution simulating a TGM wastewater containing Fe(II), Cu(I), Zn(II), Hg(II), and CN– ions. According to speciation diagrams, the predominant Hg(II) species is Hg(CN)42–. Cyclic voltammetry was used to study the reduction processes from the simulated TGM wastewater solution and to select the potential for the potentiostatic deposition process of mercury. Scanning electron microscopy with energy-dispersive X-ray spectroscopy was used to study the morphology and elemental composition of the deposits. The free cyanide (CN–) concentration affects the electrochemical behavior of the anionic cyanide complex Hg(CN)42– in addition to the chemical stability of 316 stainless steel under open circuit conditions. Voltammetry and electrochemical impedance spectroscopy measurements show that the 316 stainless steel electrode becomes more resistive when exposed to the simulated TGM wastewater solution due to a passive surface oxide, while the passive layer on the titanium electrode inhibits Hg(II) reduction.Graphical

Preliminary phytotoxicity of Mercury in conventional gold mining wastewater on Typha latifolia and Pistia stratiotes

Small-scale mining, or conventional mining, is quite common in Indonesia. Almost all of these artisanal mining activities disregard environmental safety around the mining areas. The current widely used gold extraction method for gold exploitation is the amalgamation method, which utilizes mercury (Hg). The use of this method can result in environmental pollution from mercury. Based on the impact of mercury that can contaminate the environment, it is necessary to treat gold mining wastewater before disposing of it into the environment. One alternative that can be used to treat gold mining wastewater is to use plants (phytotreatment). The aims of study was to observe the tolerance of Hg to Typha latifolia and Pistia stratiotes plants, The methods was call the Range Finding Test (RFT). The study was conducted using synthetic Hg at concentrations of 0.3, 0.7, 1, 3, and 6 mg/L and a control (0 mg/L) in a batch reactor. The reactors were added with soil up to 25% as a growing medium. The physical observations of plants were carried out for 28 days. Typha latifolia and Pistia stratiotes could both grow and thrive at a Hg content of 6 mg/L, according to physical observations of the plants. In the first seven days, Pistia stratiotes plants exhibited a stress reaction in the form of yellowing and falling leaves. Both plants had vigorous, extended root development in all concentrations. Based on these findings, phytotreatment can be utilized as an alternate therapy to lower the Hg levels in gold mine wastewater.

Rethinking gold mining wastewater treatment with an integrated process of membrane distillation and membrane contactors for minimal waste discard and resource recovery

The generation of wastewater in the mining industry is followed by concerns about the environmental impacts derived from its incorrect disposal. This study intended to simultaneously treat and recover water for reuse, sulfuric acid, and copper from gold mining wastewater, from an innovative integrated system of membrane distillation (MD) and non-dispersive solvent extraction (NDSX). The performance of this system was compared when different polymeric membranes (namely polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE)) were tested. The PP membrane yielded better results in terms of distillate flux (0.67 ± 0.32 L/m2h), owing to its reduced thickness compared with the other membranes. However, the PTFE membrane demonstrated superior outcomes in terms of rejection efficiency, achieving values >99.2 %. The occurrence of fouling due to the organic phase was observed, requiring periodical cleaning protocols and the development of oleophobic membranes to reduce the fouling rate in MD. In the extraction and re-extraction processes the PTFE membrane outperformed in terms of extraction efficiency (recovery of 94.0 % for acid and 97.3 % for copper). The results were explained by its greater porosity and lower roughness, variables that directly impacted the mass transfer of sulfuric acid and copper across the membrane. The study, therefore, demonstrates potentiality in the recovery of high-quality water, sulfuric acid, and copper, clearing the path of technologies with the potential to lower the wastewater discarded from industrial facilities.

Non-dispersive solvent extraction as an alternative for sulfuric acid and copper recycling from membrane distillation concentrate of gold mining wastewater

The wastewater from the pressure oxidation process is a complex stream enriched in sulfuric acid and dissolved metallic species, conventionally treated by neutralization, a technique that does not allow for by-product recovery. Previous studies focused on water recovery from the wastewater by membrane distillation and the concern related to concentrate production after water is recovered is now addressed by using hollow-fiber membrane contactors in non-dispersive solvent extraction processes intended for sulfuric acid and copper recycling. Tris(2-ethylhexyl)amine (TEHA), Aliquat 336, and Lix 84-I were used as extractants, the first two for acid recovery and the latter for copper. It was investigated the contribution of different organic phases flow rate (4–12 cm3/s), organic phase composition (5–15 wt%), and temperature (30–60 °C) on the performance of hollow fiber membrane contactors, with an in-depth disclosure of mass transfer phenomena limiting the extraction efficiency. For both acid and copper recovery, a reduction in viscosity forces prevails over the decrease in inertial forces, improving the flow regime. Even so, the predominant mechanism was the diffusive transport of both solutes into the fluid, across the membrane pores, rather than being carried along by the aqueous phase flow. The acid and copper recovery from membrane distillation concentrate achieved values greater than 91%, which represents an acid stream containing 5.86 g-H2SO4/L and an aqueous copper solution with a concentration of 269.0 mg/L, both at high purity. It was discussed different opportunities for their reuse within the gold ore processing circuit and challenges related to their external commercialization. Preliminary economic analysis suggested that the mineral acid would present slightly higher costs than currently practiced (0.54 US$/kgH2SO4; market prices: 0.03–0.11 US$/kgH2SO4), although the operational costs related to copper were comparable with market prices (11.58 US$/kgCu; market prices: 10.31 US$/kgCu). In a broad context, the opportunity for acid and sulfuric acid recycling has the potential to extend sustainability in the mining sector.

Insights into sustainability of engineered carbonaceous material-based technologies for advanced cyanide removal from wastewater

Cyanide (CN) is a serious concern in industrial and goldmine wastewater. Strict regulatory standards have been established by various agencies due to the detrimental effects that CN has on human health. Therefore, before discharge to water bodies or land, it is essential to create a sustainable model for the safe removal of CN. Carbon-based materials are well known for their adsorption and oxidation features, which can be conducive to CN removal. This paper reviews the relevant literature on the application of modified and unmodified carbon-based materials to CN removal in water; these materials include activated carbon (AC), graphene, graphene oxide (GO), and carbon nanotubes (CNTs). Moreover, CN removal mechanisms and photocatalytic removal of CN are comprehensively discussed, with a particular emphasis on modifying carbon-based materials. It has been observed that adding various elements to carbon-based materials improves their surface area, functional groups, CN adsorption capacity, and pore volume. Impacts of operational parameters, isotherm models, kinetics, and types of carbon-based materials are also outlined. This study provides insight into the real-scale applicability of carbon-based materials for CN removal from waters. Moreover, this review indicates that essential work on CN removal using carbon-based materials is still needed. Future research should focus on developing modified carbon-based materials to encourage multidisciplinary research. The most crucial gap in the literature is that the studies have been performed on a lab scale. Therefore, further pilot and real-scale applications should be conducted. Overall, the cost assessment, environmental effects, and human health risks of carbon-based materials should be studied in future research to achieve a realistic perspective on applicability on an industrial scale.

Membrane distillation and ion exchange combined process for mining wastewater treatment, water reuse, and byproducts recovery

In this paper, we propose the combination of direct contact membrane distillation (DCMD) and ion exchange (IX) as an alternative for (i) generating reuse water and (ii) recovering byproducts from the DCMD concentrate by IX, from gold mining wastewater. The DCMD showed a steady flux (5.49 L/m2.h), ions rejection >99.7%, and a permeate recovery rate of 33.9%. The batch equilibrium results showed that the Amberlyst A26 had a strong affinity for H2SO4 (2897.9 mg/g). Conversely, Amberlite IRC747 and Dowex M4195 showed affinity towards Cu (83.4 and 103.1 mg/g), Ni (26.3 and 89.9 mg/g), and Co (6.5 and 24.4 mg/g). Batch kinetics showed that the rate-limiting step was intraparticle diffusion. Batch recovery of H2SO4 (1265 mg/L) from Amberlyst A26 could be achieved with H2O. Although the recovery of Cu (198.6 mg/L, 91.5%), Ni (62.4 mg/L, 30%), and Co (20.1 mg/L, 41.3%) from the Amberlite IRC747 could be achieved H2SO4 (1 mol/L), carryover of impurities was found. On the other hand, the recovery from the Dowex M4195 with NH4OH 4 mol/L was higher and carryover was lower. The fixed-bed tests showed that the ions breakthrough followed the same order as in the batch and they reached the breakpoint faster at higher feed velocity (8 BV/h compared to 4 BV/h). Elution from the Amberlyst A26 fixed bed achieved 7497.4 mg/L of H2SO4 with only 4.1% of carryover from other ions. The Dowex M4195 elution can achieve high selectivity when increasing the eluting agents' strength (H2SO4 0.1 and 1 mol/L, followed by NH4OH 4 mol/L) are used. By that, separated fractions of Cu, Ni, and Co could be recovered by each agent.

Electrochemical Corrosion Characteristics of 316L Stainless Steel in Simulated Coastal Gold Mine Wastewater under Different pH

Electrochemical Corrosion Characteristics of 316L Stainless Steel in Simulated Coastal Gold Mine Wastewater under Different pH

Versatile synthesis of activated carbon from coconut shells: a method for cyanide adsorption in artisanal and small-scale gold mining wastewater

Over the next years, Indonesia’s traditional gold mining technique (that employs mercury amalgamation) will be gradually replaced by cyanidation as a result of a Presidential Decree that has been recently issued. The cyanide-containing waste resulting from the processing tanks of this new method will be generally sedimented and flow directly into the surrounding small rivers. This study aimed to reduce the impact of cyanide contamination on the environment by using the simple activated carbon that can be produced by artisanal small-scale gold mining (ASGM). Water samples were taken from artificial wastewater as part of three replications. The highest recorded removal of cyanide from sewage was 98.43%, with an equilibrium reached at a fixed adsorbent dosage of 0.05 g over a contact time of 24 hours. Cyanide adsorption was also found to be pH-dependent, with the highest cyanide adsorption occurring at a pH of 8. The adsorption capacity for cyanide was estimated at 12.51 mg g&lt;sup&gt;-1&lt;/sup&gt; of the adsorbent, and it was considered to function based on a Langmuir isotherm model. The findings of this study confirm that the utilized glass wool in the assessed method can increase the yield of activated carbon, thereby offering a low-cost and effective adsorbent that can be used in order to remove cyanide from ASGM wastewater.

Characterization of Two Highly Arsenic-Resistant Caulobacteraceae Strains of Brevundimonas nasdae: Discovery of a New Arsenic Resistance Determinant.

Arsenic (As), distributed widely in the natural environment, is a toxic substance which can severely impair the normal functions in living cells. Research on the genetic determinants conferring functions in arsenic resistance and metabolism is of great importance for remediating arsenic-contaminated environments. Many organisms, including bacteria, have developed various strategies to tolerate arsenic, by either detoxifying this harmful element or utilizing it for energy generation. More and more new arsenic resistance (ars) determinants have been identified to be conferring resistance to diverse arsenic compounds and encoded in ars operons. There is a hazard in mobilizing arsenic during gold-mining activities due to gold- and arsenic-bearing minerals coexisting. In this study, we isolated 8 gold enrichment strains from the Zijin gold and copper mine (Longyan, Fujian Province, China) wastewater treatment site soil, at an altitude of 192 m. We identified two Brevundimonas nasdae strains, Au-Bre29 and Au-Bre30, among these eight strains, having a high minimum inhibitory concentration (MIC) for As(III). These two strains contained the same ars operons but displayed differences regarding secretion of extra-polymeric substances (EPS) upon arsenite (As(III)) stress. B. nasdae Au-Bre29 contained one extra plasmid but without harboring any additional ars genes compared to B. nasdae Au-Bre30. We optimized the growth conditions for strains Au-Bre29 and Au-Bre30. Au-Bre30 was able to tolerate both a lower pH and slightly higher concentrations of NaCl. We also identified folE, a folate synthesis gene, in the ars operon of these two strains. In most organisms, folate synthesis begins with a FolE (GTP-Cyclohydrolase I)-type enzyme, and the corresponding gene is typically designated folE (in bacteria) or gch1 (in mammals). Heterologous expression of folE, cloned from B. nasdae Au-Bre30, in the arsenic-hypersensitive strain Escherichia coli AW3110, conferred resistance to As(III), arsenate (As(V)), trivalent roxarsone (Rox(III)), pentavalent roxarsone (Rox(V)), trivalent antimonite (Sb(III)), and pentavalent antimonate (Sb(V)), indicating that folate biosynthesis is a target of arsenite toxicity and increased production of folate confers increased resistance to oxyanions. Genes encoding Acr3 and ArsH were shown to confer resistance to As(III), Rox(III), Sb(III), and Sb(V), and ArsH also conferred resistance to As(V). Acr3 did not confer resistance to As(V) and Rox(V), while ArsH did not confer resistance to Rox(V).

Mercury removal from gold mining wastewater using palm oil fuel ash (POFA)

Mercury is a harmful element that commonly accumulates in the environment through anthropogenic activities such as gold mining. This study aimed to examine the use of palm oil fuel ash (POFA) as an agricultural waste to remove mercury in gold mining wastewater. The technology applied in this study was adsorption and precipitation. POFA was first washed until neutral then mixed with gold mining effluent with time variation of 30, 60, 90 minutes. Next, precipitate it for 30 minutes. The adsorbent dose used in this study was 250 mg with 1 litre effluent. The maximum adsorption efficiency observed in this study was 96.77%. The utilization of POFA as an adsorbent reduced mercury levels in wastewater below the allowed released mercury level to the environment (0.0025 mg Hg/L). Results indicate that reducing mercury from gold mining wastewater by utilizing POFA was simple, effective, and low-cost to be implemented.

Potential to remove heavy metals and cyanide from gold mining wastewater using biochar

Gold processing results in the generation of tailings wastewater which contain cyanide (CN−) and heavy metals ions such as chromium (Cr3+), iron (Fe2+), zinc (Zn2+), nickel (Ni3+), lead (Pb2+), manganese (Mn2+) and copper (Cu2+). On the other hand, low cost-based bio adsorbents have received a lot of interest as a source of bio-based wastewater treatment for removal of wastewater contaminants. This study looks at the potential to remove contaminants from gold processing tailing dams using saw dust-based bio char. The bio char was produced through slow pyrolysis at 500 °C for 2 h. The bio char was then ground to particle sizes in the range of 0.5–2.0 mm before use as the bio adsorption media. The tailings wastewater was then passed through wastewater at various bio char loadings ranging from 10 to 50 mg/L at a pH of 7.0 for a period of 2–14 h. The wastewater was characterised using the APHA standard methods before and after bio adsorption. The CN, Cr, Fe, Zn, Ni, Pb, Mn and Cu ions concentration significantly reduced after the bio adsorption process with efficiency decreases as high as 75%, 78%, 74%, 87%, 82%, 70%, 85% and 90% respectively. Bio adsorbents can be successfully used to remove contaminants in gold tailings dam wastewater.

One-step recycling of mineral acid from concentrated gold mining wastewater by high-temperature liquid–liquid extraction

Liquid-liquid extraction (LLE) was investigated for mineral acid (H2SO4) recycling from membrane distillation (MD) concentrate treating gold mining wastewater. Five extractants (Alamine 336, Aliquat 336, Cyanex 923, TEHA, and Versatic Acid) were preferably diluted in kerosene or 1-decanol (30 vol%) in extraction experiments conducted at 60 °C. Cyanex 923 extracts the mineral acid mainly by solvation of the neutral species, whereas the mechanisms for amine-based extractants involve an ion-pair formation. An important outcome was the temperature contribution to extraction efficiency. The endothermic nature of extraction would be advantageous to acid recovery and the integration of MD with LLE considering that the MD concentrated is produced at ∼ 60 °C. If stripping is considered, Aliquat 336 (diluted in 1-decanol), Cyanex 923, and TEHA (diluted in 1-decanol) become the most appropriate organic phases for mineral acid recycling given the greater efficiency for acid recovery (>90%) after a single contact stage. The results for extraction efficiency, selectivity, stripping efficiency, and costs demonstrated that TEHA (30 vol% in 1-decanol) was the most cost-effective (US$ 0.57/kg) alternative for mineral acid recycling. It would be possible to recover 4.8 kg of mineral acid per cubic meter of MD concentrate as an acid solution (concentration: 25 mmol/L). Overall, LLE was a feasible alternative to enhance gold mining wastewater exploitation and MD concentrate management. From a broader perspective, acid recycling would imply environmental gains, economical savings given the lower requirements of neutralizing agents for mining wastewater treatment and residual sludge disposal, or additional incomes by its commercialization.

Membrane distillation and dispersive solvent extraction in a closed-loop process for water, sulfuric acid and copper recycling from gold mining wastewater

Membrane distillation (MD) and dispersive liquid–liquid extraction (d-LLE) were integrated for gold mining wastewater beneficiation and better use of natural resources. MD presented an average permeate flux of 5.51 ± 0.14 L/m2·h, without a significant flux decay (p < 0.001), and rejection efficiencies for acid and metallic species>98.7%. Five different extractants were studied for sulfuric acid recovery from MD concentrate. Considering the extraction and stripping efficiency, tris-2-ethylhexylamine (TEHA) was the most appropriate alternative for sulfuric acid extraction, requiring 2 stages of extraction and 1 of striping to achieve a recovery>95%. The endothermic nature of acid extraction contributed to the integration of d-LLE and MD, designating a proper destination for MD concentrate, a stream often forgotten while discussing membrane separation processes for wastewater treatment. Copper was then selectively recovered from the raffinate from acid extraction (βCu/Fe = 4,274, βCu/Al = 176, and βCu/Mg = 1,262) by Lix 84-I. A single contact stage ensured that 99.3% of copper was recovered. Fourier-transform infrared spectra demonstrated that the acid extraction mechanisms involve amine salts formation, whereas copper is extracted by anion exchange processes. The economic assessment was favorable for water (US$/m3 0.23) and copper (US$/kgCu 4.25) recovery, with prices lower than currently practiced for both commodities. For sulfuric acid, the operational expenditures corresponded to US$/kgH2SO4 0.423, slightly higher than its marketable price. Overall, the integrated system could be inserted in a perspective that departs from the linear focus of raw material utilization, prioritizing recycling and reuse processes.

Photochemical degradation of thiocyanate by sulfate radical-based advanced oxidation process using UVС KrCl-excilamp

The degradation of thiocyanates (SCN−) by UV-C-activated persulfate (PS) in the presence of ferric ion (Fe3+) was investigated. As a source of monochromatic far-UV-C irradiation (222 nm), mercury-free KrCl excimer lamp was used. Results showed that compared with direct photolysis, UVC/ PS and PS/ Fe3+, the combined UVC/ PS/ Fe3+ treatment had the highest initial reaction rate ω0 and removal efficiency. 99.99% conversion of thiocyanates (100 mg/L of initial concentration) was achieved in 40 min. The addition of Fe3+ in the UVC/ PS treatment was found to reduce energy consumption (calculated as amount of oxidized thiocyanates per consumed electrical energy) by 4.5 times, while only a 30% difference between direct photolysis and UVC/ PS was observed. The high efficiency of the UVC/ PS/ Fe3+ process revealed a synergistic effect (synergy index ƒ = 1.98). The effect of the initial SCN−, PS, and Fe3+ molar ratios and UV-C exposure time on SCN− removal in UVC/ PS/ Fe3+ was further investigated. It was found that at molar ratios [S2O82–]:[SCN−] = 3:1 and [S2O82–]:[Fe3+] = 1:0.1, effective decomposition of SCN− in a wide initial concentration range (from 50 to 500 mg/L or 0.86–8.6 mM) can be achieved. The strong role of •OH and SO4•− in the removal of SCN− was confirmed by the addition of radical scavengers. It was demonstrated that the presence of Cu2+ in simulated gold mine wastewater effluents neutralizes the inhibitory effects that S2O32– and NH4+ have on the degradation process.