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Comparative efficiency of three advanced oxidation processes for thiosalts oxidation in mine-impacted water
Minerals Engineering 152 (2020) 106349
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Comparative efficiency of three advanced oxidation processes for thiosalts oxidation in mine-impacted water
T
Mélinda Gervais, Jennifer Dubuc, Marc Paquin, Carolina Gonzalez-Merchan, Thomas Genty, ⁎ Carmen M. Neculita Research Institute on Mines and Environment (RIME), University of Québec in Abitibi-Témiscamingue (UQAT), 445 Boul. de l’Université, Rouyn-Noranda, QC J9X5E4, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Thiosalts Mine effluents Advanced oxidation processes
Grinding and flotation of sulfide-containing ores produce metastable, partially oxidized sulfur compounds known as thiosalts, which are notorious contributors to receiving water bodies’ toxicity by producing delayed acidity. Thus, the continuous optimization of thiosalts oxidation into sulfate is required before treated mine water is discharged into natural streams, as thiosalts oxidation slows down at low temperatures and in the absence of UV. With this aim, the present study evaluated the treatment of thiosalts at initial concentrations of 100 mg/L, 290 mg/L and 630 mg/L using three advanced oxidation processes (AOPs): (1) wet ferrates Fe(VI), 65 mg/L; (2) ozone microbubbles (O3-microbubbles), 75 g/h sparging rate; and (3) hydrogen peroxide (H2O2), 2:1 (H2O2:thiosalts) molar ratio, at 8 °C and 22 °C. All treatability tests were performed on synthetic effluents, while H2O2 efficiency was also assessed with a real effluent, a leachate of non-desulfurized tailings collected at a flotation plant. Results showed that O3-microbubbles gave better efficiency (99%) than wet Fe(VI) (82%) after 2 h of thiosulfate treatment. Similar trends were observed with synthetic and real effluents for treatment with H2O2. Efficient removal of thiosalts in the real effluent required several days of reaction at 8 °C (98%), whereas efficient removal at 22 °C (90%) was reached after 1 h of treatment. All three AOPs tested proved promising for thiosalts removal. The O3-microbubbles showed the best efficiency in terms of thiosalts removal and treatment time. Moreover, H2O2 allowed for better thiosulfate removal than wet Fe(VI), but proved inefficient at oxidizing other intermediate S species and required longer treatment time.
1. Introduction One common environmental issue related to sulfidic (S2−) ore mining is acid mine drainage (AMD), often characterized by high concentrations of sulfates (SO42−), iron (Fe) and other metallic elements, in addition to low pH and high acidity. The acid generation process is relatively well known but still requires further study, mainly for sulfur (S) oxidation, which plays an important role in the acidification of the mining effluents (CANMET, 1999; Amos et al., 2015; Nordstrom et al., 2015). Although the complete oxidation of S generates SO42− in an alkaline medium (e.g. in the presence of a stabilizing agent), this oxidation also involves thiosalts formation (Forsberg, 2011). Thiosalts are metastable, partially oxidized S compounds. The most frequent thiosalts in mine effluents are thiosulfate (S2O32−), trithionate (S3O62−) and tetrathionate (S4O62−) (Miranda-Trevino et al., 2013; Range and Howboldt, 2019). Although the thiosalt concentrations in final effluent discharged
⁎
from Canadian mines are generally below concentrations that cause direct toxicity to aquatic species (Kuyucak and Yaschyshyn, 2007), previous studies have shown sublethal and acute toxicity effects in different aquatic organisms (Schwartz et al., 2006). A survey of 36 Canadian mining facilities revealed that 44% of sites with thiosalts in their effluents were affected by aquatic toxicity (CANMET, 1999). The toxicity of mine effluents that contain thiosalts is caused by the pH drop induced by the oxidation reaction of S (Kuyucak and Yaschyshyn, 2007). This acidity can detrimentally impact the receiving environment. Consequently, the acidified lakes can have pH values below 5 (Miranda-Trevino et al., 2013). The acidification may also be linked to the presence of acidogenic metals (especially Al, Fe and Mn) and lead to an impoverishment of the ecosystem by eliminating the species that are most intolerant to acidity. The delayed acidity caused by thiosalts, in addition to the risk of metal contamination, can make the watercourse receptor toxic to aquatic species. To protect water-receiving environments, the treatment of thiosalts is required.
Corresponding author. E-mail address: [email protected] (C.M. Neculita).
https://doi.org/10.1016/j.mineng.2020.106349 Received 28 October 2019; Received in revised form 16 March 2020; Accepted 18 March 2020 Available online 30 March 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
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The formation of thiosalts is related to the S2− concentration, crushing, flotation, pH, water temperature, residence time in the mill and in the flotation circuit, the pulp stirring speed, particle size, pulp density and air access during flotation (CANMET, 1999; MirandaTrevino et al., 2013; Range and Howboldt, 2019). The addition of neutralizing agents (e.g. lime) during AMD treatment also involves thiosalts formation and sulfuric acid (H2SO4) generation. In fact, thiosalts are sensitive to environmental conditions such as pH, temperature, redox potential (Eh), as well as the presence of bacteria and UV radiation. Thiosalts species vary according to these conditions. The S2O32− is known to be stable at neutral to alkaline pH, unlike major polythionates (S3O62− and S4O62−) that exhibit greater stability under acidic to neutral conditions (Vongporm, 2008; Miranda-Trevino et al., 2013). In fact, at low pH, S2O32− may undergo disproportionation and form SO2 and SO32− or be converted to S4O62− (Vongporm, 2008). In the mine context, pyrite and pyrrhotite are important iron sulfide precursors to the formation of thiosalts, including S2O32−, in tailings storage facilities (TSF). The S2O32− is often selected as representative because it is the dominant thiosalt present in mine water (MirandaTrevino et al., 2013; Range and Howboldt, 2019). Indeed, the extreme concentrations of thiosalts recently reported, especially after freezing/ thaw (FT) cycles, in leachates from column tests of weathered (3–5 years) sulfidic tailings (27.9% pyrrhotite), consisted mainly of S2O32−: 0.17–7.2 g/L vs 0.19–10 g/L (S2O32−), and 0.0085–2.2 g/L vs 0.0074–2.6 g/L (S4O62−), at ambient and FT conditions, respectively (Schudel et al., 2019). The tailings were sampled at the Raglan Mine, where prior to their discharge in TSF, tailings are thickened, filterpressed, deposited by “dry stacking” (water content around 15%), then leveled and left exposed to cold climatic conditions (typical polar, semiarid, with a mean annual air temperature of around −10.3 °C) (Schudel et al., 2019). The degradation pathway of thiosalts is influenced by the pH, acidity or alkalinity, temperature and catalysts (Vongporm, 2008). These conditions have an impact on stability, oxidation rate and oxidation products. Previous findings showed that the natural oxidation of thiosalts occurs extremely slowly in the absence of catalysts, strong oxidants, UV radiation and temperatures below 20 °C (Kuyucak, 2014). In natural environments, these factors are often uncontrolled. Thus, in the more northerly regions where temperatures fluctuate from very cold in winter to hot in summer, it is expected that in winter the oxidation will be very slow, or even stopped, whereas in summer oxidation is faster. For example, warm temperatures and the exposure of the UV sunrays accelerate the transformation of thiosalts into SO42−; however, the thiosalts remain partially oxidized. As a result, the thiosalts management methods must be optimized to prevent possible adverse effects on the receiving aquatic environment and treatment plants. Given that neutralizing agents are ineffective for treating thiosalts, advanced oxidation processes (AOPs) represent an environmentally friendly alternative for their efficient removal. For example, previous studies showed that ferrates Fe(VI), ozone microbubbles (O3-microbubbles) and peroxide (H2O2, alone or activated by a Fe catalyzer as Fenton’s reagent) contributed to the removal efficiency (> 98%) of complex contaminants such as cyanides and derivatives, including thiocyanates and ammonia in mine effluents (Forsberg, 2011; Sharma, 2011; Gonzalez-Merchan et al., 2016a, 2016b; Wahlström et al., 2017; Ryskie et al., 2020). As for S2O32−, overall oxidation reactions by Fe(VI), O3-microbubbles and H2O2 are shown in Eq. (1) (Johnson and Read, 1996), Eqs. (2) and (3) (Tan and Rolia, 1985), respectively. 3S2O32− + 4FeO42− + 14H+ → 4Fe3+ + 6SO32− + 7H2O 2S2O32−
+ 5O3 + 2H2O →
4SO42−
+
+ 3.5O2 + 4H
S2O32− + 4H2O2 + 2OH− → 2SO42− + 5H2O
complex, involve several intermediate reactions and vary among authors (Johnson and Read, 1996; Tan and Rolia, 1985). Mechanisms will not be discussed further as they are out of the scope of this paper. Previous findings have demonstrated that AOPs, such as H2O2, are the most promising approach to the treatment of thiosalts (MirandaTrevino et al., 2013; Kuyucak, 2014). In this context, the objective of the present study was to evaluate the comparative efficiency of three AOPs in thiosalts treatment from synthetic and real mine effluents: (1) wet Fe(VI), (2) O3-microbubbles and (3) H2O2. 2. Materials and methods 2.1. Preparation of synthetic effluents and sampling of real effluents The efficiency of the three AOPs was evaluated on synthetic effluents containing only S2O32− prepared with Na2S2O3·5H2O, grade American Chemical Society (ACS), and with a real effluent R4. For treatment with wet Fe(VI), the effluent contained 100 mg/L of S2O32− in distilled water. This concentration was selected based on previous studies that evaluated SCN− oxidation by Fe(VI) (Gonzalez-Merchan et al., 2016a). For O3-microbubbles treatment, 300 L of synthetic effluent were prepared using tap water to obtain a maximum concentration of 500 mg/L of SO42− after treatment, or 290 mg/L of S2O32−. These concentrations, which are below the acute lethality levels for rainbow trout (Oncorhynchus mykiss) and daphnia (Daphnia magna) according to standard toxicity tests, were selected to simplify effluent discharge following treatment as specified by the Government of Quebec’s Directive 019, environmental regulations pertaining to the mining industry (MDDELCC, 2012). For H2O2 treatment, the synthetic effluent was prepared with 1 g/L S2O32− in distilled water, as surrogate concentration for the real effluent. The real effluent R4, containing 630 mg/L S2O32−, is a leachate from a column essay containing nondesulfurized tailings sampled at the 11th cycle of a flotation plant. 2.2. Sampling and analysis of contaminated and treated effluents The physicochemical composition of real mine effluent R4 is presented in Table 1. The main physicochemical parameters of all effluents were measured before and after each treatability testing. They included pH, redox potential (Eh), electrical conductivity, as well as dissolved Fe by inductively coupled plasma - atomic emission spectroscopy (ICP-AES; CEAEQ, 2014) (sample filtrated on 0.45 μm). For treatability tests with wet Fe(VI), efficiency was assessed by measuring total S by ICP-AES as well as S2O32− and SO42− concentrations by ion chromatography (IC; CEAEQ, 2014). Efficiencies for O3-microbubbles and H2O2 treatments were determined by comparing total S obtained by ICP-AES with S species (S2O32−, SO32− and SO42−) measured by IC to better evaluate recovered S, as indicated in Table 2. An additional preparation step was performed for the samples that contained partially oxidized S because accurate measurement of S by Table 1 Characterization of real mine effluent R4, before treatment. Parameter pH Eh Acidity Alkalinity S-S2O32− S-SO32− S-SO42− Sa S
(1) (2) (3)
The mechanisms of thiosalts oxidation by the three AOPs are
a
2
Units
mV mg/L CaCO3 mg/L S by IC
mg/L ICP-AES
Total sulfur (S) = Sum of the sulfur from the thiosalts and sulfates.
R4 8.1 354 0 252 638 1.2 297 936 956
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Table 2 Conditions of treatability testing performed.
Table 3 Ferrates doses and thiosalt concentrations (mg/L) after treatment by Fe(VI).
Treatability test
Effluent type
Type of measure for S species
Fe(VI) doses
S-S2O32−
S-SO42−
Oxidation efficiency (%)
Fe(VI)
Synthetic
O3-microbubbles H2O2
Synthetic Real effluent R4
Total S measured by ICP-AES S2O32−/SO42− measured by IC S species measured by IC vs. total S measured by ICP-AES
0 55 65 75 85 95
76 23 14 16 15 18
<1 53 63 60 61 58
<1 69 82 78 80 76
ICP-AES requires S to be completely oxidized. To fully oxidize S, the sample was boiled for 5 min with H2O2 at a 2:1 M ratio. The pH and Eh were measured using electrodes (Orion 915 BNWP double junction Ag/ Ag Cl and Orbisint CPS 12D Pt, respectively). The electrical conductivity was measured with a VWR® Traceable® Expanded Range Conductivity Meter equipped with an epoxy probe.
2.4. Data processing Efficiency of thiosalts oxidation was calculated with the Eq. (5).
Efficiency(%) = [(C i − C f )/C i] × 100
(5)
where Ci and Cf are the concentrations before and after each treatability test, respectively.
2.3. Treatability testing 2.3.1. Fe(VI) The wet Fe(VI) were synthesized following a protocol based on Thompson et al. (1951), Ciampi and Daly (2009) and GonzalezMerchan et al. (2016a). The preparation required a source of Fe3+, an appropriate oxidant and strongly alkaline conditions to ensure Fe(VI) stability Eq. (4).
3. Results and discussion 3.1. Treatability tests with Fe(VI)
2.3.2. O3-microbubbles The treatability tests with O3-microbubbles were performed with the Absolute Ozone Magnum 200 O3 generator. This system transforms medical grade O2 into O3, which is distributed in the effluent by the microbubble technology with a NikuniKTM32N brand pump along with the OHR MX-F15 (Original Hydrodynamic Reaction Technology) static mixer. The pump produces an output of 75 g/h when operated at 30 PSI and when the O2 flow is of 8 L/min (Ryskie et al., 2020). The ozonation was carried out on the synthetic effluent of 290 mg/L S2O32− for a period of 3 h using the ozonation pilot with the same pump operating parameters used by Ryskie et al. (2020). The pH of the effluent was not adjusted before the O3 treatment, the goal being to correlate the oxidation of the thiosalts with a drop in pH.
3.1.1. pH influence on S species evolution The total initial S concentration measured by ICP-AES in the synthetic effluent was 76 mg/L. The concentration of S-S2O32− decreased (from 76 to 14 mg/L) after treatment, while the concentration of SSO42− increased (from < 1 to 63 mg/L) at a Fe(VI) dose of 65 mg/L, representing 82% of S2O32− removal efficiency (Table 3). No significant increase in removal efficiency at greater values of Fe(VI) dose were observed; this finding could be explained by the high pH values reached after addition of the wet Fe(VI) (Fig. 1a) due to their preperation process, which requires strong oxidant and alkaline conditions (Ciampi and Daly, 2009; Gonzalez-Merchan et al., 2018). Indeed, the S2O32− show greater stability at neutral to alkaline pH values (Miranda-Trevino et al., 2013). In addition, Fe(VI) have a greater oxidizing power in acidic media (Eh = 2.20 V) than in alkaline media (Eh = 0.70 V) (Sharma, 2011), which suggests that the alkalinity of the solution resulting from addition of the Fe(VI) inhibits the Fe(VI) from reacting to their full potential. Our findings agree with these statements as the pH of the solutions at every Fe(VI) dose reached values near 13 (Fig. 1a). As shown in Fig. 1b and c, Eh and conductivity values after treatment also increased with the Fe(VI) doses, implying that the solution became more oxidizing as the amount of added Fe(VI) increased. This observation also suggests that the Fe(VI) could not react to their expected potential due to the pH being too alkaline, promoting their stability rather than their reactivity. Unreacted Fe(VI), along with any remaining bleach from the preparation process, could have contributed to the ascending trend of Eh and conductivity values without actually enabling complete S2O32− oxidation.
2.3.3. H2O2 The synthetic and real effluent R4 were treated using 35% H2O2 with a H2O2:S2O32− molar ratio of 2:1. This ratio was reported as optimal for S2O32− oxidation (Kuyucak, 2014). The effect of temperature was tested at 8 °C (simulating spring and fall seasons) and at 22 °C (simulating summer season) for the synthetic and real effluent treated with H2O2 as well as raw effluent R4. The samples were stirred with magnetic stirring bars (Stirrer bar C2 VWR to level 3 of velocity) for 1 h and then stored at either 8 °C or 22 °C. Eh and pH were monitored for 14 days, while the sulfur species were monitored at 1 h, 7 days and 14 days.
3.1.2. Dissolved Fe Dissolved Fe concentrations after treatment increased with the Fe (VI) doses. The pH and Eh values showed no significant variation within the 2 h of treatment while a slight decrease in conductivity was noted, which could be related to the precipitation of Fe(III). In fact, as previously reported, Fe(VI) as a strong oxidizer generates Fe(III) after reduction and acts as a flocculent-coagulant, efficiently removing contaminants (Jiang and Lloyd, 2002; Sharma, 2011; Goodwill et al., 2016). The Fe(III) precipitation resulting from the Fe(VI) reduction could explain S2O32− removal despite enhanced stability of Fe(VI) and S2O32− in basic media.
2Fe(NO3)3 + 3NaOCl + 2Na2FeO4 + 3NaCl + 6NaNO3 + 5H2O
10NaOH
→ (4)
For the evaluation of Fe(VI) efficiency in the treatment of thiosalts, 300 mL of synthetic effluent with 100 mg/L S2O32− were placed in five 1 L beakers of a jar tester, while a sixth beaker was used as a control. The tested Fe(VI) doses ranged from 55 to 95 mg/L Fe(VI). The reaction time was 2 h, and the samples were stirred at 20 RPM at room temperature (22 °C). Samples of the supernatant were collected immediately at the end of the reaction, filtered through a 0.45 μm nylon filter and all physicochemical parameters mentioned above were analyzed for post-treatment characterization.
3
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Fig. 1. pH (a), Eh (b) and conductivity (c) of treated effluents with Fe(VI).
during the first 2 h of treatment. In addition, previous research for the removal of ammonia nitrogen (N-NH3) by O3-microbubbles revealed that O3 consumption was 10 times higher at pH 7 than at pH 9 and 11 (Ryskie et al., 2020), which suggests that the optimal pH for treatment by O3-microbubbles could be around neutrality. Consistently, the highest thiosulfate removal efficiencies were observed during the first hour of treatment, where initial pH was also near neutrality. In 3 h, the initial S-S2O32− (167.9 mg/L) was entirely transformed into S-SO42−, whose concentration increased from 11.2 to 212.3 mg/L (Table 4), indicating 111% recovery of total initial S (due to ± 15% error on measurements by IC). The final pH reached 2.5, which is lower than the required minimal criterium (pH 6) for discharge, according to Canada’s federal regulations (MDMER, 2018). Incomplete S recovery after 1 h suggests that undetected, partially oxidized sulfur species still present could have been fully oxidized after 2 h of treatment, allowing for complete S recovery. Major polythionates included S3O62− and S4O62− (Miranda-Trevino et al., 2013), which were undetectable by the IC used in the present study. The initial Eh (438 mV) was within the Eh boundaries generally obtained for surface water. After 1 h, the Eh decreased to 297 mV, and increased to 444 mV then to 1026 mV after 2 h and 3 h, respectively. The lower Eh values observed during the first 2 h of treatment could be due to the maximal O3 consumption at neutral values (Ryskie et al., 2020), where accumulation in the solution is hindered. The high Eh values obtained after 2 h and 3 h indicate that the medium was strongly oxidizing due to an O3 excess, generating more %OH (Khuntia et al., 2015).
3.2. Treatability tests with O3-microbubbles 3.2.1. pH influence on S species evolution The total initial S (S-S2O32−, S-SO32− and S-SO42−) measured by IC in the synthetic effluent was 190.9 mg/L (Table 4). The measure of total S by ICP-AES differed negligibly (< 15%) from that by IC, so total S recovery was based on the measure of total initial S by IC. S-S2O32− and S-SO32− species decreased as treatment time increased, while S-SO42− increased (Table 4). Overall, the pH dropped from 7.30 to 2.54 after treatment due to the generation of SO42− accompanied by acidity when S2O32− were completely oxidized. After 1 h of treatment, thiosalts concentration decreased to 155.8 mg/L, representing 82% of the total initial S, while 2 h of treatment gave 99% total recovered S in the form of S-SO42−. At the same time, the pH dropped from 7.3 to 5.2 after 1 h, then to 2.8 after 2 h of treatment (Table 4). The pH decreased rapidly (2.1–2.4 units/h) during the first 2 h of treatment, whereas the pH variation was minor (0.28 units/h) during the last hour of treatment. Previous findings showed that the production of hydroxyl radicals (% OH) in the presence of O3 is greater at higher pH values (Khuntia et al., 2015). These %OH have very high oxidation potentials and could explain the rapid pH drop caused by the oxidation of S2O32− into SO42− Table 4 Characterization of synthetic effluents after treatment. Time
(h)
0
1
2
3
pH Eh Acidity Alkalinity S-S2O32− S-SO32− S-SO42− S total IC S-Recovered (IC)
– (mV) (mg CaCO3/L)
7.3 438 16 41 167.9 11.8 11.2 191 > 99
5.23 297 27 3 94.9 28 32.9 156 82
2.82 444 130 <1 1.6 22.6 167 191 > 99
2.54 1026 203 <1 0 0 212 212 111
(mg/L)
(%)
3.3. Treatability tests with H2O2 3.3.1. Preliminary testing Preliminary treatability tests were performed on a synthetic effluent of 1039 mg/L S-S2O32− at 8 °C and 22 °C, before treatment of the real effluent. At 8 °C, S-S2O32− decreased to 73 mg/L in 14 days, 4
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representing 93% removal efficiency, while S-SO42− increased from < 1 to 610 mg/L in the same time. The pH, which was at 6.19 before treatment, dropped to 3.54 after 14 days. These results indicate that thiosalts oxidation occurred, but that the oxidation was incomplete, as demonstrated by a lack of recovered sulfur in the form of S-SO42−. At 22 °C, S-S2O32− decreased to 151 mg/L in 14 days, representing 85% removal effiency, while S-SO42− increased from < 1 to 1408 mg/L for the same treatment time. Meanwhile, the initial pH of 6.09 dropped to a final value of 2.43. Significant evaporation was observed for the sample at 22 °C, causing measures of S-S2O32− and S-SO42− concentrations by IC to be overestimated. However, the trend of the results showed that H2O2 could be an efficient thiosalts treatment method. In this context, treatability tests with H2O2 on real effluent R4 were performed with closed-lid jars.
H2O2 as an oxidizing agent alone and that it could decompose to S2O32− at pH values between 3 and 8 at ambient temperature (Miranda-Trevino et al., 2013), thus explaining the increase in SS2O32− during treatment. H2O2 instability in aqueous solutions as a function of time could also explain the incomplete oxidation of thiosalts. Due to slow oxidation rates with thiosalts, H2O2 could decompose before reacting with the S intermediates (Miranda-Trevino et al., 2013). Hence, the addition of a peroxide stabilizer such as sodium silicate (Charron et al., 2006) or catalyst such as ferric sulfate (MirandaTrevino et al., 2013) could improve thiosalt removal efficiency with H2O2. 3.3.2.2. pH and alkalinity. After 14 days of continuous aeration at 8 °C, the alkalinity of the control sample remained stable, consistent with the unchanged S species concentrations. At 22 °C, the alkalinity of the control sample decreased (from 218 to 129 mg CaCO3/L), likely due to the correlation between generated SO42− and acidity. The pH of effluent R4 treated at 8 °C slightly decreased (0.53 units) after 14 days (Fig. 2), while the alkalinity decreased from 284 to 223 mg CaCO3/L. The alkaline conditions, which allow the buffering of the generated protons (H+), could account for the pH stability at near neutral conditions. The loss in alkalinity was explained by the generated acidity, when thiosalts were oxidized into SO42−. The sample treated at 22 °C was more reactive, as the kinetics of treatment were accelerated with temperature. At treatment day 7, the pH reached a lightly acidic value of 6.07, decreased to 5.4 the next day and reached a final value of 3.16 after 14 days. The absence of alkalinity was suspected after 9 days of treatment (pH < 4.5) and confirmed with a measurement of alkalinity of < 1 mg CaCO3/L after 14 days of treatment (Table 5). This complete loss of alkalinity indicates that the acidity generated (estimated by the decrease in pH) exceeded the neutralizing potential of the effluent. The generated acidity was causally related to thiosalts oxidation into SO42−, considering that the effluent does not contain any other significant source of acidity from acidogenic metals (Al, Fe and Mn concentrations were of 0.01, 0.03 and 0.416 mg/L, respectively).
3.3.2. Real effluent 3.3.2.1. Temperature influence on S species evolution. The measure of total initial S (S-S2O32−, S-SO32− and S-SO42−) by IC in effluent R4 was 936 mg/L of S, 638 mg/L of which were in the form of S-S2O32−. The control sample at 8 °C did not undergo important variation of the SS2O32−. After 1 h and 7 days of treatment, S-SO42− concentrations increased to 302 mg/L and 301 mg/L, respectively. After 14 days, the SSO42− concentration slightly increased to 304 mg/L (Table 5). For the control sample at 22 °C, the S-S2O32− concentration decreased to 108 mg/L, while the S-SO42− concentration increased to 26 mg/L after 14 days (Table 5). These results indicate that S2O32− can be oxidized slowly at room temperature and in the absence of an oxidizing agent, generating other intermediate S species that oxidize at a slower rate in the same conditions. The S intermediates generated were not detected by the IC. The sample treated at 8 °C showed a rapid drop of S-S2O32− after 1 h of treatment (from 638 to 103 mg/L). After 14 days, the S-S2O32− decreased to 10 mg/L, whereas S-SO42− increased from 297 to 440 mg/ L. Between days 7 and 14, S species varied insignificantly, indicating that oxidation was incomplete and slow at 8 °C (Table 5). These observations agree with previous findings: S2O32− and major polythionates (S3O62− and S4O62−) are mostly stable at low temperatures (Miranda-Trevino et al., 2013). The sample treated at 22 °C showed that the S-S2O32− decreased rapidly after 1 h of treatment (from 634 to 3.5 mg/L). Although SSO42− increased (from 302 to 695 mg/L) while S-S2O32− decreased (from 638 to 64 mg/L) after 14 days of treatment, the balance of S compounds showed that S2O32− was not completely oxidized. In fact, between 1 h and 14 days of treatment, S-S2O32− increased (from 3.5 to 64 mg/L) (Table 5). Therefore, complete oxidation of S2O32− could not be confirmed after 14 days because approximately 20% of the initial S was not recovered in the form of S-SO42. Previous findings consistently showed that especially S3O62− has low reactivity in the presence of
4. Conclusion The results of the present study showed that thiosalts could be efficiently oxidized into SO42− using the three AOPs [wet Fe(VI), O3microbubbles and H2O2]. After 2 h of treatment with 65 mg/L of wet Fe (VI), 82% of thiosalts were oxidized, but with a strong alkaline pH (~12), the sample required further treatment. A pH adjustment during treatment might improve thiosalt removal by enhancing Fe(VI) reactivity and eliminate the polishing step. Treatment with O3-microbubbles (sparging rate of 75 g/h) allowed for 99% of thiosalts oxidation into SO42− after 2 h and entailed very acidic final pH (~2.5), which would also require further treatment. A continuous pH adjustment
Table 5 Evolution of S species measured by IC for 14 days. Time
1h
Temperature Molar ratio of
7 days
8 °C H2O2:S2O32−
pH Alk. (mg CaCO3/L) S-S2O32− (mg/L) S-SO32− (mg/L) S-SO42− (mg/L) S total IC (mg/L) S2O32− removal (%) Recovered S (%)
22 °C
8 °C
22 °C
9 days
14 days
22 °C
8 °C
22 °C
Cont.
2:1
Cont.
2:1
Cont.
2:1
Cont.
2:1
2:1
Cont.
2:1
Cont.
2:1
7.82 216 638 1.3 302 942 0 > 99
8.22 284 103 23.6 383 509 84 54
8.11 218 650 1.4 302 952 −2 > 99
8.59 252 3.5 5.9 423 433 99 45
8.21 N/A 650 2.6 301 953 −2 > 99
8.30 N/A 6.1 7.3 438 451 96 48
7.93 N/A 627 2.8 304 934 2 98
6.07 N/A 22.7 6.5 544 573 96 60
4.36 N/A 26.3 2.2 766 795 96 83
7.96 212 613 2.1 304 919 4 98
7.69 223 9.7 5.6 440 455 98 48
7.84 129 542 7.4 327 876 15 92
3.16 <1 64.3 1.8 695 761 90 80
Cont.: Control; Alk: Alkalinity. 5
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Fig. 2. pH (a) and Eh (b) evolution for the treated effluents with H2O2.
during treatment could eliminate the polishing step, accelerate O3 consumption and S2O32− oxidation. Treatability tests at 8 °C and 22 °C with H2O2 using a H2O2:S2O32− molar ratio of 2:1 confirmed the crucial role of temperature in thiosalts treatment. Low temperatures in the presence of H2O2 favored intermediate S species stability but the S2O32− was only partially oxidized. Higher temperatures accelerated thiosalts oxidation by H2O2 but the complete oxidation of S3O62− and S4O62− could not be confirmed. The H2O2 removed 96% of thiosalts, but the 7-day reaction time is deemed considerably longer than wet Fe (VI) and O3-microbubbles treatment. Although all three AOPs tested seem promising for thiosalts treatment, O3-microbubbles showed the greatest efficiency and in the shortest time. Further research should focus on the upscaling effects and the techno-economical assessment of thiosalts treatment by O3-microbubbles to complete the performance evaluation and document the potential applicability of this water treatment process to the mining industry.
Work performed to thiosalts consortium. Job no: 601838, Report: MMSL 99-055 (CR), 125 p. Centre d’Expertise en Analyse Environnementale du Québec (CEAEQ), 2014. Détermination des anions: méthode par chromatographie ionique, MA. 300-Ions 1.3, Rév. 3 (in French), MDDELCC, QC, Canada. Charron, I., Couvert, A., Laplanche, A., Renner, C., Patria, L., Requieme, B., 2006. Treatment of odorous sulphur compounds by chemical scrubbing with hydrogen peroxide – stabilisation of the scrubbing solution. Environ. Sci. Technol. 24, 7881–7885. Ciampi, L.E., Daly, L.J., 2009. Methods of synthesizing a ferrate oxidant and its use in ballast Water. Patent US7476342B2. Forsberg, E., 2011. Study on thiosalts formation in alkaline sulphidic ore slurries under anaerobic conditions and methods for minimizing treatment cost in the mining industry. A case study at Boliden Mineral AB. MSc thesis, Engineering Technology, Industrial and Management Engineering, Lulea University of Technology, Sweden, 113p. Gonzalez-Merchan, C., Genty, T., Paquin, M., Gervais, M., Bussière, B., Potvin, R., Neculita, C.M., 2018. Influence of ferric iron source on ferrate’s performance and residual contamination during the treatment of gold mine effluents. Miner. Eng. 127, 61–66. Gonzalez-Merchan, C., Genty, T., Bussière, B., Potvin, R., Paquin, M., Benhammadi, M., Neculita, C.M., 2016a. Ferrates performance in thiocyanates and ammonia degradation in gold mine effluents. Miner. Eng. 95, 124–130. Gonzalez-Merchan, C., Genty, T., Bussière, B., Potvin, R., Paquin, M., Benhammadi, M., Neculita, C.M., 2016b. Influence of contaminant to hydrogen peroxide to catalyzer molar ratio in the advanced oxidation of thiocyanates and ammonia nitrogen using Fenton-based processes. J. Environ. Chem. Eng. 4 (4), 4129–4136. Goodwill, J.E., Jiang, Y., Reckhow, D.A., Tobiason, J.E., 2016. Laboratory assessment of ferrate for drinking water treatment. J. Am. Water Works Assoc. 108 (3), 164–174. Jiang, J.Q., Lloyd, B., 2002. Progress in the development and use of ferrate(VI) salt as an oxidant and coagulant for water and wastewater treatment. Water Res. 36, 1397–1408. Johnson, M.D., Read, J.F., 1996. Kinetics and mechanism of the ferrate oxidation of thiosulfate and other sulfur-containing species. Inorg. Chem. 35, 6795–6799. Khuntia, S., Majumder, S.K., Ghosh, P., 2015. Quantitative prediction of generation of hydroxyl radicals from ozone microbubbles. Chem. Eng. Res. Des. 98, 231–239. Kuyucak, N., 2014. Management of thiosalts in mill effluents by chemical oxidation or buffering in the lime neutralization process. Miner. Eng. 60, 44–60. Kuyucak, N., Yaschyshyn, D., 2007. Managing thiosalts in mill effluents studies conducted at the Kidd Metallurgical site. In: The Proceedings of Mining and the Environment IV Conference, Sudbury, Ontario, Canada, October 19-27, 2007. Metal and Diamond Mining Effluent Regulation (MMER), 2018. SOR/2002-222 [http:// laws-lois.justice.gc.ca]. Ministère du Développement Durable, de l’Environnement et des Parcs (MDDELCC), 2012. Directive 019 sur l’industrie minière. http://www.mddefp.gouv.qc.ca/milieu_ ind/directive019/directive019.pdf. Miranda-Trevino, J.C., Pappoe, M., Hawboldt, K., Bottaro, C., 2013. The importance of thiosalts speciation: review of analytical methods, kinetics, and treatment. Crit. Rev. Environ. Sci. Technol. 43, 2013–2070. Nordstrom, D.K., Blowes, D.W., Ptacek, C.J., 2015. Hydrogeochemistry and microbiology of mine drainage: an update. Appl. Geochem. 3, 3–16. Range, B.M.K., Hawboldt, K.A., 2019. Review: reemoval of thiosalt/sulfate from mining effluents by adsorption and ion exchange. Miner. Process. Extr. M. 40, 79–86. Ryskie, S., Gonzalez-Merchan, C., Neculita, C.M., Genty, T., 2020. Efficiency of ozone microbubbles for ammonia removal from mine effluents. Miner. Eng. 145, 106071. Schudel, G., Plante, B., Bussière, B., McBeth, J., Dufour, G., 2019. The effect of arctic conditions on the geochemical behaviour of sulphidic tailings. Tailings and Mine Waste, Vancouver, BC, Canada, November 17–20. Schwartz, M., Vigneault, B., McGeer, J., 2006. Assessing the potential toxicity of thiosalts in the context of the metal mining effluent regulation. Presentation made at the 33rd Aquatic Toxicity Workshop, Jasper, AB.
CrediT authorship contribution statement Mélinda Gervais: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Jennifer Dubuc: Methodology, Formal analysis, Visualization, Writing - review & editing. Marc Paquin: Methodology, Formal analysis, Writing - review & editing. Carolina Gonzalez-Merchan: Methodology, Formal analysis, Writing - review & editing. Thomas Genty: Supervision, Writing - review & editing, Project administration, Funding acquisition. Carmen M. Neculita: Conceptualization, Visualization, Supervision, Writing - review & editing, Project administration, Funding acquisition. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Research Chairs Program, industrial partners of RIME UQAT-Polytechnique (Agnico Eagle, Canadian Malartic Mine, Iamgold Corporation, Raglan MineGlencore, Newmont-Goldcorp, and Rio Tinto), CTRI (Technology Transfer Center for Industrial Waste), and Mabarex. References Amos, R.T., Blowes, D.W., Bailey, B.L., Sego, C.D., Smith, L., Ritchie, A.I.M., 2015. Wasterock hydrogeology and geochemistry. Appl. Geochem. 57, 140–156. CANMET, 1999. Characterization of thiosalts generation during milling of sulphide ores.
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potassium ferrate (VI). J. Chem. Soc. 73, 1379–1381. Vongporm, Y., 2008. Thiosalt Behaviour in Aqueous Media. MSc thesis. Memorial University, St. John’s, NF, Canada, pp. 168. Wahlström, M., et al. (15 authors), 2017. Water Conscious Mining (WASCIOUS), Report, Northern Council of Ministers, Denmark, 171p.
Sharma, V.K., 2011. Oxidation of inorganic contaminants by ferrates (VI, V, and IV)kinetics and mechanisms: a review. J. Environ. Manage. 92, 1051–1073. Tan, K.G., Rolia, E., 1985. Chemical oxidation methods for the treatment of thiosaltcontaining mill effluents. Can. Metall. Q. 24 (4), 303–310. Thompson, G.W., Ockerman, L.T., Schreyer, J.M., 1951. Preparation and purification of
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Comparative efficiency of three advanced oxidation processes for thiosalts oxidation in mine-impacted water
T
Mélinda Gervais, Jennifer Dubuc, Marc Paquin, Carolina Gonzalez-Merchan, Thomas Genty, ⁎ Carmen M. Neculita Research Institute on Mines and Environment (RIME), University of Québec in Abitibi-Témiscamingue (UQAT), 445 Boul. de l’Université, Rouyn-Noranda, QC J9X5E4, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Thiosalts Mine effluents Advanced oxidation processes
Grinding and flotation of sulfide-containing ores produce metastable, partially oxidized sulfur compounds known as thiosalts, which are notorious contributors to receiving water bodies’ toxicity by producing delayed acidity. Thus, the continuous optimization of thiosalts oxidation into sulfate is required before treated mine water is discharged into natural streams, as thiosalts oxidation slows down at low temperatures and in the absence of UV. With this aim, the present study evaluated the treatment of thiosalts at initial concentrations of 100 mg/L, 290 mg/L and 630 mg/L using three advanced oxidation processes (AOPs): (1) wet ferrates Fe(VI), 65 mg/L; (2) ozone microbubbles (O3-microbubbles), 75 g/h sparging rate; and (3) hydrogen peroxide (H2O2), 2:1 (H2O2:thiosalts) molar ratio, at 8 °C and 22 °C. All treatability tests were performed on synthetic effluents, while H2O2 efficiency was also assessed with a real effluent, a leachate of non-desulfurized tailings collected at a flotation plant. Results showed that O3-microbubbles gave better efficiency (99%) than wet Fe(VI) (82%) after 2 h of thiosulfate treatment. Similar trends were observed with synthetic and real effluents for treatment with H2O2. Efficient removal of thiosalts in the real effluent required several days of reaction at 8 °C (98%), whereas efficient removal at 22 °C (90%) was reached after 1 h of treatment. All three AOPs tested proved promising for thiosalts removal. The O3-microbubbles showed the best efficiency in terms of thiosalts removal and treatment time. Moreover, H2O2 allowed for better thiosulfate removal than wet Fe(VI), but proved inefficient at oxidizing other intermediate S species and required longer treatment time.
1. Introduction One common environmental issue related to sulfidic (S2−) ore mining is acid mine drainage (AMD), often characterized by high concentrations of sulfates (SO42−), iron (Fe) and other metallic elements, in addition to low pH and high acidity. The acid generation process is relatively well known but still requires further study, mainly for sulfur (S) oxidation, which plays an important role in the acidification of the mining effluents (CANMET, 1999; Amos et al., 2015; Nordstrom et al., 2015). Although the complete oxidation of S generates SO42− in an alkaline medium (e.g. in the presence of a stabilizing agent), this oxidation also involves thiosalts formation (Forsberg, 2011). Thiosalts are metastable, partially oxidized S compounds. The most frequent thiosalts in mine effluents are thiosulfate (S2O32−), trithionate (S3O62−) and tetrathionate (S4O62−) (Miranda-Trevino et al., 2013; Range and Howboldt, 2019). Although the thiosalt concentrations in final effluent discharged
⁎
from Canadian mines are generally below concentrations that cause direct toxicity to aquatic species (Kuyucak and Yaschyshyn, 2007), previous studies have shown sublethal and acute toxicity effects in different aquatic organisms (Schwartz et al., 2006). A survey of 36 Canadian mining facilities revealed that 44% of sites with thiosalts in their effluents were affected by aquatic toxicity (CANMET, 1999). The toxicity of mine effluents that contain thiosalts is caused by the pH drop induced by the oxidation reaction of S (Kuyucak and Yaschyshyn, 2007). This acidity can detrimentally impact the receiving environment. Consequently, the acidified lakes can have pH values below 5 (Miranda-Trevino et al., 2013). The acidification may also be linked to the presence of acidogenic metals (especially Al, Fe and Mn) and lead to an impoverishment of the ecosystem by eliminating the species that are most intolerant to acidity. The delayed acidity caused by thiosalts, in addition to the risk of metal contamination, can make the watercourse receptor toxic to aquatic species. To protect water-receiving environments, the treatment of thiosalts is required.
Corresponding author. E-mail address: [email protected] (C.M. Neculita).
https://doi.org/10.1016/j.mineng.2020.106349 Received 28 October 2019; Received in revised form 16 March 2020; Accepted 18 March 2020 Available online 30 March 2020 0892-6875/ © 2020 Elsevier Ltd. All rights reserved.
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The formation of thiosalts is related to the S2− concentration, crushing, flotation, pH, water temperature, residence time in the mill and in the flotation circuit, the pulp stirring speed, particle size, pulp density and air access during flotation (CANMET, 1999; MirandaTrevino et al., 2013; Range and Howboldt, 2019). The addition of neutralizing agents (e.g. lime) during AMD treatment also involves thiosalts formation and sulfuric acid (H2SO4) generation. In fact, thiosalts are sensitive to environmental conditions such as pH, temperature, redox potential (Eh), as well as the presence of bacteria and UV radiation. Thiosalts species vary according to these conditions. The S2O32− is known to be stable at neutral to alkaline pH, unlike major polythionates (S3O62− and S4O62−) that exhibit greater stability under acidic to neutral conditions (Vongporm, 2008; Miranda-Trevino et al., 2013). In fact, at low pH, S2O32− may undergo disproportionation and form SO2 and SO32− or be converted to S4O62− (Vongporm, 2008). In the mine context, pyrite and pyrrhotite are important iron sulfide precursors to the formation of thiosalts, including S2O32−, in tailings storage facilities (TSF). The S2O32− is often selected as representative because it is the dominant thiosalt present in mine water (MirandaTrevino et al., 2013; Range and Howboldt, 2019). Indeed, the extreme concentrations of thiosalts recently reported, especially after freezing/ thaw (FT) cycles, in leachates from column tests of weathered (3–5 years) sulfidic tailings (27.9% pyrrhotite), consisted mainly of S2O32−: 0.17–7.2 g/L vs 0.19–10 g/L (S2O32−), and 0.0085–2.2 g/L vs 0.0074–2.6 g/L (S4O62−), at ambient and FT conditions, respectively (Schudel et al., 2019). The tailings were sampled at the Raglan Mine, where prior to their discharge in TSF, tailings are thickened, filterpressed, deposited by “dry stacking” (water content around 15%), then leveled and left exposed to cold climatic conditions (typical polar, semiarid, with a mean annual air temperature of around −10.3 °C) (Schudel et al., 2019). The degradation pathway of thiosalts is influenced by the pH, acidity or alkalinity, temperature and catalysts (Vongporm, 2008). These conditions have an impact on stability, oxidation rate and oxidation products. Previous findings showed that the natural oxidation of thiosalts occurs extremely slowly in the absence of catalysts, strong oxidants, UV radiation and temperatures below 20 °C (Kuyucak, 2014). In natural environments, these factors are often uncontrolled. Thus, in the more northerly regions where temperatures fluctuate from very cold in winter to hot in summer, it is expected that in winter the oxidation will be very slow, or even stopped, whereas in summer oxidation is faster. For example, warm temperatures and the exposure of the UV sunrays accelerate the transformation of thiosalts into SO42−; however, the thiosalts remain partially oxidized. As a result, the thiosalts management methods must be optimized to prevent possible adverse effects on the receiving aquatic environment and treatment plants. Given that neutralizing agents are ineffective for treating thiosalts, advanced oxidation processes (AOPs) represent an environmentally friendly alternative for their efficient removal. For example, previous studies showed that ferrates Fe(VI), ozone microbubbles (O3-microbubbles) and peroxide (H2O2, alone or activated by a Fe catalyzer as Fenton’s reagent) contributed to the removal efficiency (> 98%) of complex contaminants such as cyanides and derivatives, including thiocyanates and ammonia in mine effluents (Forsberg, 2011; Sharma, 2011; Gonzalez-Merchan et al., 2016a, 2016b; Wahlström et al., 2017; Ryskie et al., 2020). As for S2O32−, overall oxidation reactions by Fe(VI), O3-microbubbles and H2O2 are shown in Eq. (1) (Johnson and Read, 1996), Eqs. (2) and (3) (Tan and Rolia, 1985), respectively. 3S2O32− + 4FeO42− + 14H+ → 4Fe3+ + 6SO32− + 7H2O 2S2O32−
+ 5O3 + 2H2O →
4SO42−
+
+ 3.5O2 + 4H
S2O32− + 4H2O2 + 2OH− → 2SO42− + 5H2O
complex, involve several intermediate reactions and vary among authors (Johnson and Read, 1996; Tan and Rolia, 1985). Mechanisms will not be discussed further as they are out of the scope of this paper. Previous findings have demonstrated that AOPs, such as H2O2, are the most promising approach to the treatment of thiosalts (MirandaTrevino et al., 2013; Kuyucak, 2014). In this context, the objective of the present study was to evaluate the comparative efficiency of three AOPs in thiosalts treatment from synthetic and real mine effluents: (1) wet Fe(VI), (2) O3-microbubbles and (3) H2O2. 2. Materials and methods 2.1. Preparation of synthetic effluents and sampling of real effluents The efficiency of the three AOPs was evaluated on synthetic effluents containing only S2O32− prepared with Na2S2O3·5H2O, grade American Chemical Society (ACS), and with a real effluent R4. For treatment with wet Fe(VI), the effluent contained 100 mg/L of S2O32− in distilled water. This concentration was selected based on previous studies that evaluated SCN− oxidation by Fe(VI) (Gonzalez-Merchan et al., 2016a). For O3-microbubbles treatment, 300 L of synthetic effluent were prepared using tap water to obtain a maximum concentration of 500 mg/L of SO42− after treatment, or 290 mg/L of S2O32−. These concentrations, which are below the acute lethality levels for rainbow trout (Oncorhynchus mykiss) and daphnia (Daphnia magna) according to standard toxicity tests, were selected to simplify effluent discharge following treatment as specified by the Government of Quebec’s Directive 019, environmental regulations pertaining to the mining industry (MDDELCC, 2012). For H2O2 treatment, the synthetic effluent was prepared with 1 g/L S2O32− in distilled water, as surrogate concentration for the real effluent. The real effluent R4, containing 630 mg/L S2O32−, is a leachate from a column essay containing nondesulfurized tailings sampled at the 11th cycle of a flotation plant. 2.2. Sampling and analysis of contaminated and treated effluents The physicochemical composition of real mine effluent R4 is presented in Table 1. The main physicochemical parameters of all effluents were measured before and after each treatability testing. They included pH, redox potential (Eh), electrical conductivity, as well as dissolved Fe by inductively coupled plasma - atomic emission spectroscopy (ICP-AES; CEAEQ, 2014) (sample filtrated on 0.45 μm). For treatability tests with wet Fe(VI), efficiency was assessed by measuring total S by ICP-AES as well as S2O32− and SO42− concentrations by ion chromatography (IC; CEAEQ, 2014). Efficiencies for O3-microbubbles and H2O2 treatments were determined by comparing total S obtained by ICP-AES with S species (S2O32−, SO32− and SO42−) measured by IC to better evaluate recovered S, as indicated in Table 2. An additional preparation step was performed for the samples that contained partially oxidized S because accurate measurement of S by Table 1 Characterization of real mine effluent R4, before treatment. Parameter pH Eh Acidity Alkalinity S-S2O32− S-SO32− S-SO42− Sa S
(1) (2) (3)
The mechanisms of thiosalts oxidation by the three AOPs are
a
2
Units
mV mg/L CaCO3 mg/L S by IC
mg/L ICP-AES
Total sulfur (S) = Sum of the sulfur from the thiosalts and sulfates.
R4 8.1 354 0 252 638 1.2 297 936 956
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Table 2 Conditions of treatability testing performed.
Table 3 Ferrates doses and thiosalt concentrations (mg/L) after treatment by Fe(VI).
Treatability test
Effluent type
Type of measure for S species
Fe(VI) doses
S-S2O32−
S-SO42−
Oxidation efficiency (%)
Fe(VI)
Synthetic
O3-microbubbles H2O2
Synthetic Real effluent R4
Total S measured by ICP-AES S2O32−/SO42− measured by IC S species measured by IC vs. total S measured by ICP-AES
0 55 65 75 85 95
76 23 14 16 15 18
<1 53 63 60 61 58
<1 69 82 78 80 76
ICP-AES requires S to be completely oxidized. To fully oxidize S, the sample was boiled for 5 min with H2O2 at a 2:1 M ratio. The pH and Eh were measured using electrodes (Orion 915 BNWP double junction Ag/ Ag Cl and Orbisint CPS 12D Pt, respectively). The electrical conductivity was measured with a VWR® Traceable® Expanded Range Conductivity Meter equipped with an epoxy probe.
2.4. Data processing Efficiency of thiosalts oxidation was calculated with the Eq. (5).
Efficiency(%) = [(C i − C f )/C i] × 100
(5)
where Ci and Cf are the concentrations before and after each treatability test, respectively.
2.3. Treatability testing 2.3.1. Fe(VI) The wet Fe(VI) were synthesized following a protocol based on Thompson et al. (1951), Ciampi and Daly (2009) and GonzalezMerchan et al. (2016a). The preparation required a source of Fe3+, an appropriate oxidant and strongly alkaline conditions to ensure Fe(VI) stability Eq. (4).
3. Results and discussion 3.1. Treatability tests with Fe(VI)
2.3.2. O3-microbubbles The treatability tests with O3-microbubbles were performed with the Absolute Ozone Magnum 200 O3 generator. This system transforms medical grade O2 into O3, which is distributed in the effluent by the microbubble technology with a NikuniKTM32N brand pump along with the OHR MX-F15 (Original Hydrodynamic Reaction Technology) static mixer. The pump produces an output of 75 g/h when operated at 30 PSI and when the O2 flow is of 8 L/min (Ryskie et al., 2020). The ozonation was carried out on the synthetic effluent of 290 mg/L S2O32− for a period of 3 h using the ozonation pilot with the same pump operating parameters used by Ryskie et al. (2020). The pH of the effluent was not adjusted before the O3 treatment, the goal being to correlate the oxidation of the thiosalts with a drop in pH.
3.1.1. pH influence on S species evolution The total initial S concentration measured by ICP-AES in the synthetic effluent was 76 mg/L. The concentration of S-S2O32− decreased (from 76 to 14 mg/L) after treatment, while the concentration of SSO42− increased (from < 1 to 63 mg/L) at a Fe(VI) dose of 65 mg/L, representing 82% of S2O32− removal efficiency (Table 3). No significant increase in removal efficiency at greater values of Fe(VI) dose were observed; this finding could be explained by the high pH values reached after addition of the wet Fe(VI) (Fig. 1a) due to their preperation process, which requires strong oxidant and alkaline conditions (Ciampi and Daly, 2009; Gonzalez-Merchan et al., 2018). Indeed, the S2O32− show greater stability at neutral to alkaline pH values (Miranda-Trevino et al., 2013). In addition, Fe(VI) have a greater oxidizing power in acidic media (Eh = 2.20 V) than in alkaline media (Eh = 0.70 V) (Sharma, 2011), which suggests that the alkalinity of the solution resulting from addition of the Fe(VI) inhibits the Fe(VI) from reacting to their full potential. Our findings agree with these statements as the pH of the solutions at every Fe(VI) dose reached values near 13 (Fig. 1a). As shown in Fig. 1b and c, Eh and conductivity values after treatment also increased with the Fe(VI) doses, implying that the solution became more oxidizing as the amount of added Fe(VI) increased. This observation also suggests that the Fe(VI) could not react to their expected potential due to the pH being too alkaline, promoting their stability rather than their reactivity. Unreacted Fe(VI), along with any remaining bleach from the preparation process, could have contributed to the ascending trend of Eh and conductivity values without actually enabling complete S2O32− oxidation.
2.3.3. H2O2 The synthetic and real effluent R4 were treated using 35% H2O2 with a H2O2:S2O32− molar ratio of 2:1. This ratio was reported as optimal for S2O32− oxidation (Kuyucak, 2014). The effect of temperature was tested at 8 °C (simulating spring and fall seasons) and at 22 °C (simulating summer season) for the synthetic and real effluent treated with H2O2 as well as raw effluent R4. The samples were stirred with magnetic stirring bars (Stirrer bar C2 VWR to level 3 of velocity) for 1 h and then stored at either 8 °C or 22 °C. Eh and pH were monitored for 14 days, while the sulfur species were monitored at 1 h, 7 days and 14 days.
3.1.2. Dissolved Fe Dissolved Fe concentrations after treatment increased with the Fe (VI) doses. The pH and Eh values showed no significant variation within the 2 h of treatment while a slight decrease in conductivity was noted, which could be related to the precipitation of Fe(III). In fact, as previously reported, Fe(VI) as a strong oxidizer generates Fe(III) after reduction and acts as a flocculent-coagulant, efficiently removing contaminants (Jiang and Lloyd, 2002; Sharma, 2011; Goodwill et al., 2016). The Fe(III) precipitation resulting from the Fe(VI) reduction could explain S2O32− removal despite enhanced stability of Fe(VI) and S2O32− in basic media.
2Fe(NO3)3 + 3NaOCl + 2Na2FeO4 + 3NaCl + 6NaNO3 + 5H2O
10NaOH
→ (4)
For the evaluation of Fe(VI) efficiency in the treatment of thiosalts, 300 mL of synthetic effluent with 100 mg/L S2O32− were placed in five 1 L beakers of a jar tester, while a sixth beaker was used as a control. The tested Fe(VI) doses ranged from 55 to 95 mg/L Fe(VI). The reaction time was 2 h, and the samples were stirred at 20 RPM at room temperature (22 °C). Samples of the supernatant were collected immediately at the end of the reaction, filtered through a 0.45 μm nylon filter and all physicochemical parameters mentioned above were analyzed for post-treatment characterization.
3
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Fig. 1. pH (a), Eh (b) and conductivity (c) of treated effluents with Fe(VI).
during the first 2 h of treatment. In addition, previous research for the removal of ammonia nitrogen (N-NH3) by O3-microbubbles revealed that O3 consumption was 10 times higher at pH 7 than at pH 9 and 11 (Ryskie et al., 2020), which suggests that the optimal pH for treatment by O3-microbubbles could be around neutrality. Consistently, the highest thiosulfate removal efficiencies were observed during the first hour of treatment, where initial pH was also near neutrality. In 3 h, the initial S-S2O32− (167.9 mg/L) was entirely transformed into S-SO42−, whose concentration increased from 11.2 to 212.3 mg/L (Table 4), indicating 111% recovery of total initial S (due to ± 15% error on measurements by IC). The final pH reached 2.5, which is lower than the required minimal criterium (pH 6) for discharge, according to Canada’s federal regulations (MDMER, 2018). Incomplete S recovery after 1 h suggests that undetected, partially oxidized sulfur species still present could have been fully oxidized after 2 h of treatment, allowing for complete S recovery. Major polythionates included S3O62− and S4O62− (Miranda-Trevino et al., 2013), which were undetectable by the IC used in the present study. The initial Eh (438 mV) was within the Eh boundaries generally obtained for surface water. After 1 h, the Eh decreased to 297 mV, and increased to 444 mV then to 1026 mV after 2 h and 3 h, respectively. The lower Eh values observed during the first 2 h of treatment could be due to the maximal O3 consumption at neutral values (Ryskie et al., 2020), where accumulation in the solution is hindered. The high Eh values obtained after 2 h and 3 h indicate that the medium was strongly oxidizing due to an O3 excess, generating more %OH (Khuntia et al., 2015).
3.2. Treatability tests with O3-microbubbles 3.2.1. pH influence on S species evolution The total initial S (S-S2O32−, S-SO32− and S-SO42−) measured by IC in the synthetic effluent was 190.9 mg/L (Table 4). The measure of total S by ICP-AES differed negligibly (< 15%) from that by IC, so total S recovery was based on the measure of total initial S by IC. S-S2O32− and S-SO32− species decreased as treatment time increased, while S-SO42− increased (Table 4). Overall, the pH dropped from 7.30 to 2.54 after treatment due to the generation of SO42− accompanied by acidity when S2O32− were completely oxidized. After 1 h of treatment, thiosalts concentration decreased to 155.8 mg/L, representing 82% of the total initial S, while 2 h of treatment gave 99% total recovered S in the form of S-SO42−. At the same time, the pH dropped from 7.3 to 5.2 after 1 h, then to 2.8 after 2 h of treatment (Table 4). The pH decreased rapidly (2.1–2.4 units/h) during the first 2 h of treatment, whereas the pH variation was minor (0.28 units/h) during the last hour of treatment. Previous findings showed that the production of hydroxyl radicals (% OH) in the presence of O3 is greater at higher pH values (Khuntia et al., 2015). These %OH have very high oxidation potentials and could explain the rapid pH drop caused by the oxidation of S2O32− into SO42− Table 4 Characterization of synthetic effluents after treatment. Time
(h)
0
1
2
3
pH Eh Acidity Alkalinity S-S2O32− S-SO32− S-SO42− S total IC S-Recovered (IC)
– (mV) (mg CaCO3/L)
7.3 438 16 41 167.9 11.8 11.2 191 > 99
5.23 297 27 3 94.9 28 32.9 156 82
2.82 444 130 <1 1.6 22.6 167 191 > 99
2.54 1026 203 <1 0 0 212 212 111
(mg/L)
(%)
3.3. Treatability tests with H2O2 3.3.1. Preliminary testing Preliminary treatability tests were performed on a synthetic effluent of 1039 mg/L S-S2O32− at 8 °C and 22 °C, before treatment of the real effluent. At 8 °C, S-S2O32− decreased to 73 mg/L in 14 days, 4
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representing 93% removal efficiency, while S-SO42− increased from < 1 to 610 mg/L in the same time. The pH, which was at 6.19 before treatment, dropped to 3.54 after 14 days. These results indicate that thiosalts oxidation occurred, but that the oxidation was incomplete, as demonstrated by a lack of recovered sulfur in the form of S-SO42−. At 22 °C, S-S2O32− decreased to 151 mg/L in 14 days, representing 85% removal effiency, while S-SO42− increased from < 1 to 1408 mg/L for the same treatment time. Meanwhile, the initial pH of 6.09 dropped to a final value of 2.43. Significant evaporation was observed for the sample at 22 °C, causing measures of S-S2O32− and S-SO42− concentrations by IC to be overestimated. However, the trend of the results showed that H2O2 could be an efficient thiosalts treatment method. In this context, treatability tests with H2O2 on real effluent R4 were performed with closed-lid jars.
H2O2 as an oxidizing agent alone and that it could decompose to S2O32− at pH values between 3 and 8 at ambient temperature (Miranda-Trevino et al., 2013), thus explaining the increase in SS2O32− during treatment. H2O2 instability in aqueous solutions as a function of time could also explain the incomplete oxidation of thiosalts. Due to slow oxidation rates with thiosalts, H2O2 could decompose before reacting with the S intermediates (Miranda-Trevino et al., 2013). Hence, the addition of a peroxide stabilizer such as sodium silicate (Charron et al., 2006) or catalyst such as ferric sulfate (MirandaTrevino et al., 2013) could improve thiosalt removal efficiency with H2O2. 3.3.2.2. pH and alkalinity. After 14 days of continuous aeration at 8 °C, the alkalinity of the control sample remained stable, consistent with the unchanged S species concentrations. At 22 °C, the alkalinity of the control sample decreased (from 218 to 129 mg CaCO3/L), likely due to the correlation between generated SO42− and acidity. The pH of effluent R4 treated at 8 °C slightly decreased (0.53 units) after 14 days (Fig. 2), while the alkalinity decreased from 284 to 223 mg CaCO3/L. The alkaline conditions, which allow the buffering of the generated protons (H+), could account for the pH stability at near neutral conditions. The loss in alkalinity was explained by the generated acidity, when thiosalts were oxidized into SO42−. The sample treated at 22 °C was more reactive, as the kinetics of treatment were accelerated with temperature. At treatment day 7, the pH reached a lightly acidic value of 6.07, decreased to 5.4 the next day and reached a final value of 3.16 after 14 days. The absence of alkalinity was suspected after 9 days of treatment (pH < 4.5) and confirmed with a measurement of alkalinity of < 1 mg CaCO3/L after 14 days of treatment (Table 5). This complete loss of alkalinity indicates that the acidity generated (estimated by the decrease in pH) exceeded the neutralizing potential of the effluent. The generated acidity was causally related to thiosalts oxidation into SO42−, considering that the effluent does not contain any other significant source of acidity from acidogenic metals (Al, Fe and Mn concentrations were of 0.01, 0.03 and 0.416 mg/L, respectively).
3.3.2. Real effluent 3.3.2.1. Temperature influence on S species evolution. The measure of total initial S (S-S2O32−, S-SO32− and S-SO42−) by IC in effluent R4 was 936 mg/L of S, 638 mg/L of which were in the form of S-S2O32−. The control sample at 8 °C did not undergo important variation of the SS2O32−. After 1 h and 7 days of treatment, S-SO42− concentrations increased to 302 mg/L and 301 mg/L, respectively. After 14 days, the SSO42− concentration slightly increased to 304 mg/L (Table 5). For the control sample at 22 °C, the S-S2O32− concentration decreased to 108 mg/L, while the S-SO42− concentration increased to 26 mg/L after 14 days (Table 5). These results indicate that S2O32− can be oxidized slowly at room temperature and in the absence of an oxidizing agent, generating other intermediate S species that oxidize at a slower rate in the same conditions. The S intermediates generated were not detected by the IC. The sample treated at 8 °C showed a rapid drop of S-S2O32− after 1 h of treatment (from 638 to 103 mg/L). After 14 days, the S-S2O32− decreased to 10 mg/L, whereas S-SO42− increased from 297 to 440 mg/ L. Between days 7 and 14, S species varied insignificantly, indicating that oxidation was incomplete and slow at 8 °C (Table 5). These observations agree with previous findings: S2O32− and major polythionates (S3O62− and S4O62−) are mostly stable at low temperatures (Miranda-Trevino et al., 2013). The sample treated at 22 °C showed that the S-S2O32− decreased rapidly after 1 h of treatment (from 634 to 3.5 mg/L). Although SSO42− increased (from 302 to 695 mg/L) while S-S2O32− decreased (from 638 to 64 mg/L) after 14 days of treatment, the balance of S compounds showed that S2O32− was not completely oxidized. In fact, between 1 h and 14 days of treatment, S-S2O32− increased (from 3.5 to 64 mg/L) (Table 5). Therefore, complete oxidation of S2O32− could not be confirmed after 14 days because approximately 20% of the initial S was not recovered in the form of S-SO42. Previous findings consistently showed that especially S3O62− has low reactivity in the presence of
4. Conclusion The results of the present study showed that thiosalts could be efficiently oxidized into SO42− using the three AOPs [wet Fe(VI), O3microbubbles and H2O2]. After 2 h of treatment with 65 mg/L of wet Fe (VI), 82% of thiosalts were oxidized, but with a strong alkaline pH (~12), the sample required further treatment. A pH adjustment during treatment might improve thiosalt removal by enhancing Fe(VI) reactivity and eliminate the polishing step. Treatment with O3-microbubbles (sparging rate of 75 g/h) allowed for 99% of thiosalts oxidation into SO42− after 2 h and entailed very acidic final pH (~2.5), which would also require further treatment. A continuous pH adjustment
Table 5 Evolution of S species measured by IC for 14 days. Time
1h
Temperature Molar ratio of
7 days
8 °C H2O2:S2O32−
pH Alk. (mg CaCO3/L) S-S2O32− (mg/L) S-SO32− (mg/L) S-SO42− (mg/L) S total IC (mg/L) S2O32− removal (%) Recovered S (%)
22 °C
8 °C
22 °C
9 days
14 days
22 °C
8 °C
22 °C
Cont.
2:1
Cont.
2:1
Cont.
2:1
Cont.
2:1
2:1
Cont.
2:1
Cont.
2:1
7.82 216 638 1.3 302 942 0 > 99
8.22 284 103 23.6 383 509 84 54
8.11 218 650 1.4 302 952 −2 > 99
8.59 252 3.5 5.9 423 433 99 45
8.21 N/A 650 2.6 301 953 −2 > 99
8.30 N/A 6.1 7.3 438 451 96 48
7.93 N/A 627 2.8 304 934 2 98
6.07 N/A 22.7 6.5 544 573 96 60
4.36 N/A 26.3 2.2 766 795 96 83
7.96 212 613 2.1 304 919 4 98
7.69 223 9.7 5.6 440 455 98 48
7.84 129 542 7.4 327 876 15 92
3.16 <1 64.3 1.8 695 761 90 80
Cont.: Control; Alk: Alkalinity. 5
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Fig. 2. pH (a) and Eh (b) evolution for the treated effluents with H2O2.
during treatment could eliminate the polishing step, accelerate O3 consumption and S2O32− oxidation. Treatability tests at 8 °C and 22 °C with H2O2 using a H2O2:S2O32− molar ratio of 2:1 confirmed the crucial role of temperature in thiosalts treatment. Low temperatures in the presence of H2O2 favored intermediate S species stability but the S2O32− was only partially oxidized. Higher temperatures accelerated thiosalts oxidation by H2O2 but the complete oxidation of S3O62− and S4O62− could not be confirmed. The H2O2 removed 96% of thiosalts, but the 7-day reaction time is deemed considerably longer than wet Fe (VI) and O3-microbubbles treatment. Although all three AOPs tested seem promising for thiosalts treatment, O3-microbubbles showed the greatest efficiency and in the shortest time. Further research should focus on the upscaling effects and the techno-economical assessment of thiosalts treatment by O3-microbubbles to complete the performance evaluation and document the potential applicability of this water treatment process to the mining industry.
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CrediT authorship contribution statement Mélinda Gervais: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft. Jennifer Dubuc: Methodology, Formal analysis, Visualization, Writing - review & editing. Marc Paquin: Methodology, Formal analysis, Writing - review & editing. Carolina Gonzalez-Merchan: Methodology, Formal analysis, Writing - review & editing. Thomas Genty: Supervision, Writing - review & editing, Project administration, Funding acquisition. Carmen M. Neculita: Conceptualization, Visualization, Supervision, Writing - review & editing, Project administration, Funding acquisition. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Research Chairs Program, industrial partners of RIME UQAT-Polytechnique (Agnico Eagle, Canadian Malartic Mine, Iamgold Corporation, Raglan MineGlencore, Newmont-Goldcorp, and Rio Tinto), CTRI (Technology Transfer Center for Industrial Waste), and Mabarex. References Amos, R.T., Blowes, D.W., Bailey, B.L., Sego, C.D., Smith, L., Ritchie, A.I.M., 2015. Wasterock hydrogeology and geochemistry. Appl. Geochem. 57, 140–156. CANMET, 1999. Characterization of thiosalts generation during milling of sulphide ores.
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