Effect of the presence of inorganic ions and operational parameters on free cyanide degradation by ultraviolet C activation of persulfate in synthetic mining wastewater
Graphical abstract
Introduction
The mining sector has had economic growth in many developing countries such as Colombia, Australia, Mongolia, among others, due to the increase in the gold and silver prices and the dynamics of investment flows from international mining companies (Ericsson and Löf, 2019, Joven-Quintero et al., 2020). Cyanidation is one of the most efficient methods to extract and recover gold and silver from ore (Johnson, 2015). In addition to its economic feasibility, cyanidation is widely used because it requires neither a complex infrastructure to be carried out nor a high degree of technical knowledge. However, cyanidation is a highly toxic method that affects the surrounding environment. It is estimated that around 18% of CN− world production is destined for gold mining exploitation (Guerrero, 2005).
Cyanidation is a leaching process where CN− is used to dissolve the gold or silver from the ore to form metal complexes. This complexation is typically carried out at strong alkaline conditions (pH > 10) to avoid the formation of HCN from CN− (Johnson, 2015, Kuyucak and Akcil, 2013). Elsner’s reactions (Eqs. (1), (2)) summarize the cyanidation process (Kuyucak and Akcil, 2013).
After forming the aurocyanurate complex, zinc is added to the solution to precipitate and recover the gold and silver from it (Logsdon et al., 1999). In this process, CN− also binds with other metals present in the ore (Fe, Co, Ni, Pt, Cd, Zn, Cu, Cr, among others), forming different cyanocomplexes that can have high toxicity (Johnson, 2015, Johnson et al., 2008). Eqs. (3), (4) show the overall reactions for the formation of [Fe(CN)6]3− and [Co(CN)6]3−, which are the most recalcitrant cyanocomplexes that can be found in mining wastewater (Osathaphan et al., 2014).
Unlike gold, other metals in the ore do not precipitate with the zinc added. At these conditions, the wastewater that contains a high concentration of cyanocomplexes and CN− is moved to open-air deposits to decompose them by sunlight exposure (via photolysis) and then be discarded. However, CN− and weak cyanocomplexes are not eliminated easily. The slow photolysis at uncontrolled environmental conditions leads to the release of CN− from these metal cyanocomplexes (e.g., Eq. (5)) (Johnson et al., 2008), and of CN− into CNO− which is also toxic (Eqs. (6), (7)) (Dzombak et al., 2006, Kuyucak and Akcil, 2013, Tian et al., 2016). Therefore, the open-air storage of cyanidation wastes without any additional treatment generates toxic emissions, representing a risk for surrounding ecosystems and water bodies (González-Ipia et al., 2020, Johnson et al., 2008, Joven-Quintero et al., 2020, Kuyucak and Akcil, 2013).
One of the biggest concerns of CN−, besides being a potent water pollutant, is that it is the most toxic molecule among all cyanide-based compounds (96-h LC50 range for cyanide: 0.05–0.18 mg L−1 (Dzombak et al., 2006, Ritcey, 2005)). The effects of this substance in humans and animals are well-known (Dzombak et al., 2006). Its high toxicity relies on inhibiting several enzymes such as succinyl dehydrogenase, cytochrome oxidase, and others, leading to the interruption of oxygen delivery to cells, blocking the adenosine triphosphate (ATP) production, and inducing cellular hypoxia (Ram, 2010). Additionally, aquatic species are greatly susceptible to small concentrations of CN−, for instance, concentrations of around 5.0 to 7.2 µg L−1 can reduce fish swimming ability and inhibit reproduction (Center for Human Rights and Environment, 2011). This high toxicity and the threat that it represents to humans and aquatic life makes it imperative to treat cyanide-polluted water.
Alkaline chlorination, precipitation with metals, adsorption, biological degradation, and membrane separation are some of the so-called conventional methods for treating wastewater contaminated with free cyanide (Kuyucak and Akcil, 2013, Rajesh et al., 2009, Young et al., 1995). However, these methods present limitations such as toxic by-products generation (e.g., cyanogenic gases), low degradation rate, the use of other polluting substances, and low elimination efficiency, among others (Kuyucak and Akcil, 2013, Rajesh et al., 2009, Young et al., 1995). Thus, investigations about alternative treatments to deal with free cyanide are warranted.
Advanced oxidation processes (AOPs) are an interesting alternative to address the problem of aqueous wastes containing cyanide derived from the mining industry. These AOPs are typically based on the in situ generation of highly reactive species (such as hydroxyl radical HO•, with high redox potential which facilitates the complete conversion of contaminants and their transformation into less or even non-toxic products (Deng and Zhao, 2015, Kim et al., 2016, Oturan and Aaron, 2014). One such promising AOPs for treating water contaminants utilizes the persulfate anion (S2O82−, PS) (Chen et al., 2018, Matzek and Carter, 2016, Moussavi et al., 2016). In this process, the peroxide bond in S2O82− undergoes cleavage forming sulfate radicals (SO4•−, E° = 2.6–3.1 V), which are several orders of magnitude more stable than HO• (half-life: SO4•−: 30–40 μs; HO•: 20 ns) (Ghanbari et al., 2016), and possess a higher oxidation potential than its precursor S2O82− (E° = 2.01 V). Therefore, the activation of PS allows for significantly reduced reaction times and an increase in the efficiency of the removal of contaminants.
There are different ways to activate PS to obtain SO4•−: heat, transition metals, electrolysis, nanocarbons, radiolysis, UV radiation, and alkaline pH (Wacławek et al., 2017). The last two are considered in the present work due to their easy operation. In the first route, sulfate radicals can be formed by activating PS via ultraviolet C light (UVC), as shown in Eq. (8).
In the second route, PS can be decomposed at alkaline pH to form SO4•− and superoxide anion (O2•−) as shown in Eq. (9) (Furman et al., 2010, Ike et al., 2018). This is convenient for treating cyanide compounds in wastewater with high pH values. Moreover, it should be mentioned that at this alkaline pH, SO4•− can react with hydroxyls to form HO• (Eqs. (10), (11)) (Liang and Su, 2009), which introduces the possibility of using two different powerful radicals for the degradation of contaminants.
The UVC/S2O82− technology has been recently reported in the treatment of cyanide and cyanocomplexes (Moussavi et al., 2016, Samir Fernando Castilla-Acevedo et al., 2020), which are not eliminated by conventional systems or other AOPs (Osathaphan et al., 2014, Osathaphan et al., 2013). However, a significant limitation in applying AOPs to real industrial wastewater is the presence of inorganic ions because they can negatively interfere with the elimination of contaminants, reducing the treatment efficiency (Cardoso et al., 2016, Yawalkar et al., 2001). Since knowledge of the effect of inorganic ions on CN− degradation is lacking, this research systematically studies the effect of the presence of three common ion species, CO32−, HPO42−, and NO3−, and operational parameters on CN− elimination by UVC/PS.
In this work, the effect of light and oxidant at alkaline conditions were studied separately in the degradation of an aqueous solution containing 50 mg L−1 CN−. We also systematically studied the influence of S2O82− concentration and of dissolved oxygen (DO) on the UVC/PS system performance. As a result, a CN− degradation mechanism by UVC/PS was proposed. To further expand the application of UVC/PS to realistic wastewater, we also investigated the presence of inorganic ions at different concentrations as inhibitors or promoters in the elimination of CN−. The kinetic analysis and electrical efficiency per order (EEO) were also calculated as indicators of efficiency and indicators for the feasibility of the process.
Section snippets
Reagents
All commercial reagents were treated for 1 h at 80 °C to eliminate the possible presence of water. All parent solutions of PS and CN− were prepared prior to experiments by dissolving the required amount of salts in deionized water type II at pH 11 previously adjusted with 10 M NaOH solution. NaCN (98% purity, AppliChem Panreac) was used to prepare 50 mg L−1 of CN− as the contaminant solution. Sodium persulfate (Na2S2O8 at 98% purity, AppliChem Panreac) was used as the oxidizing agent. To study
Photolysis of free cyanide and activation of persulfate in alkaline media
Initially, the influence of the ultraviolet light and the alkaline activation of persulfate in alkaline media (PS/base) were studied separately on the degradation of 50 mg L−1 CN− at pH 11 for 60 min. Fig. 2 presents the time evolution of both the photolysis process and the activation of PS in alkaline media (in the absence of light), indicating that these processes individually only eliminated 8 and 11%, respectively, of the CN− initially present.
Studies carried out by Buck et al. reported
Conclusions
The present research demonstrated that alkaline activation of persulfate (PS) or ultraviolet C (UVC), when employed individually, were not viable for treating wastewater contaminated with CN−. However, the combination of UVC and PS under alkaline pH presented a high synergistic action and efficiency in the elimination of CN−. Moreover, it was concluded that the activation of persulfate by UVC light at alkaline conditions was an efficient process to degrade free cyanide in the presence of
CRediT authorship contribution statement
Valentina Satizabal-Gómez: Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Manuel Alejandro Collazos-Botero: Methodology, Investigation, Writing - review & editing, Visualization, Software. Efraím A. Serna-Galvis: Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Ricardo A. Torres-Palma: Formal analysis, Investigation, Writing - original draft, Writing - review & editing. Juan J. Bravo-Suárez: Formal analysis,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The Authors of this work are thankful to the Chemical Engineering Program of Universidad del Valle and COLCIENCIAS (Young Researcher Grant 2018-2019) in Colombia for financial support. The authors also thank the Chemical Engineering Process laboratory (LAPIQ) at Universidad del Valle. The authors from GIRAB thank Universidad de Antioquia (UdeA) for the support provided to their research group through “Programa de Sostenibilidad”. E.A. Serna-Galvis thanks the doctoral scholarship provided by
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