The tools developed for acid mine drainage (AMD) prediction were proven unsuccessful to predict the geochemical behavior of mine waste rocks having a significant chemical sorption capacity, which delays the onset of contaminated neutral drainage (CND). The present work was performed in order to test a new approach of water quality prediction, by using a chelating agent solution (0.03 M EDTA, or ethylenediaminetetraacetic acid) in kinetic testing used for the prediction of the geochemical behavior of geologic material. The hypothesis underlying the proposed approach is that the EDTA solution should chelate the metals as soon as they are released by sulfide oxidation, inhibiting their sorption or secondary precipitation, and therefore reproduce a worst-case scenario where very low metal attenuation mechanisms are present in the drainage waters. Fresh and weathered waste rocks from the Lac Tio mine (Rio tinto, Iron and Titanium), which are known to generate Ni-CND at the field scale, were submitted to small-scale humidity cells in control tests (using deionized water) and using an EDTA solution. Results show that EDTA effectively prevents the metals to be sorbed or to precipitate as secondary minerals, therefore enabling to bypass the delay associated with metal sorption in the prediction of water quality from these materials. This work shows that the use of a chelating agent solution is a promising novel approach of water quality prediction and provides general guidelines to be used in further studies, which will help both practitioners and regulators to plan more efficient management and disposal strategies of mine wastes.
The prediction of water quality upon mine drainage is of utmost importance for the mining industry due to increasingly restrictive regulations and higher social awareness. Many mine water quality prediction tools such as static and kinetic testing were developed for acid mine drainage (AMD) prediction in the past 30 years (e.g., Sobek et al. 1978; Lawrence and Scheske 1997; Paktunc 1999; Blowes et al. 2003; Benzaazoua et al. 2004; Villeneuve et al. 2009; Plante et al. 2012; Bouzahzah et al. 2014). However, some mine wastes may generate circumneutral drainage waters with metal concentrations above the regulatory levels or that may affect the downstream environment; this phenomenon is called contaminated neutral drainage, or contaminated neutral drainage (CND) (Nicholson 2004; Heikkinen et al. 2009; Plante et al. 2011a, b). Some of the prediction tools developed for AMD appear inappropriate for low reactivity materials, such as low sulfide mine wastes, which may generate CND (Bussière et al. 2011; Plante et al. 2011a, b; Demers et al. 2013). Different approaches were tested for taking into consideration the sorption potential when using kinetic testing for CND prediction, such as batch sorption tests and kinetic testing after artificial saturation of the waste rocks (Éthier et al. 2010; Plante et al. 2010, 2011a; Demers et al. 2011; Éthier 2011). The present study exposes a new laboratory approach for the long-term prediction of drainage quality. This novel approach consists of a modified kinetic test in which the deionized water is replaced by a chelating agent solution. The main hypothesis behind the approach is that the chelating agent should inhibit geochemical reactions of the metals generated by sulfide oxidation (such as sorption and secondary mineral precipitation) and the dissolution of other minerals. Therefore, this approach should enable a better prediction of the quality of mine drainage, particularly for low reactivity mine wastes with a potential for CND generation, by generating maximum release rates from the beginning that stand on the conservative side for long-term prediction. Indeed, leaching with a chelating agent solution may represent a worst-case scenario where all the metals released from geochemical reactions (e.g., sulfide oxidation, acid neutralization) are flushed out of the material. The test simulates a situation where the material does not participate in any phenomena that may reduce the metal concentrations in the leachates (ex.: sorption, ionic exchange). This is particularly relevant considering the current trend from legislators worldwide to lower the acceptable metal concentrations in mine drainage.
The main objective of the present study is to investigate the geochemical behavior of mine wastes in kinetic testing using a chelating agent solution, with emphasis on the metal concentrations in the leachates, the rate of sulfide oxidation, the dissolution of acid-neutralizing minerals, and the dissolution of previously generated secondary minerals. This work should help us understand the leaching and attenuation mechanisms that will (hopefully) lead to better predictions. The hypothesis behind the study is that the chelating agent will chelate the metals as soon as they are released in the drainage waters and competitively form soluble and stable metal-chelate complexes, which will prevent these metals from participating in other geochemical reactions, such as sorption and secondary precipitation, while having a negligible effect on sulfide oxidation and on the geochemical behavior of other minerals.
Materials and methods
The materials tested are waste rocks from the Tio mine (Rio Tinto Iron and Titanium), located near Havre-Saint-Pierre in Québec (Canada), an iron and titanium open-pit mine exploiting a hemo-ilmenite-rich ore since 1950. Some of the Tio mine waste rock piles are known to generate sporadic Ni-CND. However, humidity cell leachates on these waste rocks generate Ni concentrations orders of magnitude below the concentrations observed in situ (Plante et al. 2011a, b), and many leachates had Ni concentrations close to or below the detection limit (0.004 mg l−1) of the ICP used. On the other hand, field-scale tests (90–100 t waste rocks) at the mine site generate Ni concentrations similar to those observed at the full scale (Bussière et al. 2011; Demers et al. 2011) but only on waste rocks that were weathered during 25 years prior to their installation in the field cells (Bussière et al. 2011). The Tio mine waste rocks were chosen because their mineralogical composition and geochemical behavior were extensively studied in previous works done in the laboratory and in the field (Plante et al. 2010, a, b; Demers et al. 2011, 2013; Bussière et al. 2011) involving kinetic testing and sorption studies for drainage quality prediction. Indeed, it was previously demonstrated that Ni in the Tio mine waste rock drainage is generated by oxidation of the less than 1 % sulfide minerals content (mainly Ni-bearing pyrite) within the wastes and that calcic plagioclase (close to labradorite composition) provides acid neutralization and pH buffering of the leachates (Bussière et al. 2011; Plante et al. 2011a, b). It was also demonstrated that the Tio mine waste rocks generate CND after approximately 15 to 25 years of field weathering and that the delay was caused by Ni sorption onto some of the waste rock mineral surfaces (Plante et al. 2010; Demers et al. 2011, 2013). Ni sorption within the Tio mine waste rocks is mainly associated with oxyhydroxides generated by pyrite oxidation (Plante et al. 2010). More details on the geochemical behavior of the Tio mine waste rocks can be found in previous studies (Plante et al. 2010; 2011a, b, 2014; Bussière et al. 2011; Demers et al. 2011, 2013).
Chelating agent selection
The chelating agent chosen for the purpose of this study was the disodium salt of ethylenediaminetetraacetic acid or Na2EDTA. This chelating agent is widely known for its strong chelating properties over a wide range of cations, and these properties are used in waste and soil decontamination (e.g., Andrade et al. 2007; Evangelou et al. 2007; Tsang et al. 2007; Karlfeldt Fedje et al. 2010; Jagetiya and Sharma 2013; Jelusic and Lestan 2014), as well as in processes involving reductive dissolution of iron oxides (e.g., Panias et al. 1996; Bridge and Johnson 1998; Hallberg et al. 2011; Johnson 2012). EDTA was chosen for its strong chelating properties, its reductive properties (in order to avoid an exaggerated increase in sulfide oxidation), and its widespread use in other fields involving soil or geologic materials.
The waste rocks selected for the present study were chosen in order to compare a fresh (produced approximately 1 month prior to sampling) and weathered (approximately 25 years of field weathering) material with high hemo-ilmenite content (>50 wt%). Working on fresh and weathered waste rocks enable (1) to test the ability of the approach to simulate the long-term release rates and (2) to verify the impact of the EDTA solution on the already weathered surfaces, particularly secondary precipitates and previously sorbed species. The fine fractions (<500 μm) of the samples were chosen because they were submitted to kinetic testing and sorption studies in previous investigations (Plante et al. 2011a, b). These fractions were prepared by sieving without grinding the samples for surface preservation purposes. Working on the fine fraction is also relevant for mine drainage prediction purposes because the fine-grained fraction of waste rocks comprise the greatest surface area and are, therefore, the most reactive (e.g., Price and Kwong 1997).
Physical, chemical, and mineralogical characterization methods
The samples were characterized for their physical, chemical, and mineralogical properties. The specific gravity (Gs) of the samples was determined with a Micromeritics Helium Pycnometer following ASTM standard D4892 (ASTM Standard D4892 2014). The grain size distributions were measured with a Microtrac S3500 laser grain size analyzer (Merkus 2009). The specific surface areas (S.S.A.) of the samples were determined with a Micromeritics Specific Surface Analyzer. The chemical composition was determined by a Perkin Elmer Optima 3100 ICP-AES following a multi-acid digestion (HNO3-Br2-HF-HCl) of 500 mg of a pulverized aliquot. The sulfur and carbon contents were analyzed by an ELTRA CS-2000 induction furnace coupled with an infrared analyzer. The sulfate sulfur (Ssulfate) composition was determined by ICP-AES following a 40 % HCl extraction. The sulfide sulfur (Ssulfide) was determined by the difference of the total sulfur (Stotal), by induction furnace and sulfate sulfur (Ssulfide = Stotal − Ssulfate). The mineralogical composition was estimated using a combination of X-ray diffraction (XRD), optical microscope (OM), and scanning electron microscope coupled with an energy-dispersive spectrometer (SEM-EDS). The XRD spectrometer was a Bruker AXS Advance D8 with a copper cathode, acquired at a rate of 0.02°s−1 between 2Θ values of 5° and 70°. DiffracPlus EVA software (v. 9.0) was used to identify the mineral species, and the quantitative mineralogical compositions were evaluated using the TOPAS software (v. 2.1) with a Rietveld (Rietveld 1993) refinement. The SEM-EDS system used was a Hitachi S-3500N equipped with an Oxford X-Max 20 mm2 silicon drift detector (SDD). The SEM observations were performed with 15-mm working distance and 100-μA current at 20 kV.
The samples were submitted to kinetic testing in weathering cells. The weathering cells are small-scale humidity cells (Cruz et al. 2001; Villeneuve et al. 2009; Plante et al. 2011a) that were frequently used in the past few years as an alternative to humidity cells when working on small sample quantities (<100 g) or when faster results are needed, since these tests accelerate the reaction rates with regard to the humidity cells (Hakkou et al. 2008; Villeneuve et al. 2009; Plante et al. 2011a; Bouzahzah et al. 2014). The weathering cell consists of a Buchner funnel with a 0.45-μm filter over which the sample is placed. The sample (typically 70 g) is leached twice a week (with 3 days between flushes) with 50 ml deionized water and left open to the atmosphere between flushes without control or monitoring of the degree of saturation of the materials. The sample thickness in weathering cells is relatively small (<1 cm) and the frequent wetting/drying of the sample generates oxidizing conditions ideal for studying the geochemical behavior of mine wastes.
Each sample was submitted to a control (fresh-control and weathered-control cells) and a modified (EDTA) weathering cell test (fresh-EDTA and weathered-EDTA cells). The modified weathering cell protocol is the same as the original one, but the deionized water is replaced by a 0.03 M EDTA solution. The EDTA is a chelating agent known to form stable complexes with various soluble elements. The EDTA concentration (0.03 M) was chosen as being able to chelate at least 10 times the mean total metal concentrations obtained in leachates from previous kinetic studies (Plante et al. 2011a) over the same materials. This EDTA concentration is also within the typical range of concentrations used in soil decontamination studies (e.g., Tsang et al. 2007; Vaxevanidou et al. 2008; Qiu et al. 2009; Jelusic and Lestan 2014). The pH values of the EDTA leaching solutions were adjusted after 15 flushes (using diluted NaOH) to be closer to those of the corresponding control cells (without EDTA). Thus, the pH of the fresh-EDTA cell was adjusted from a natural pH of approximately 5 to 8.0, while the pH of the weathered-EDTA cell was adjusted from a natural pH of approximately 5 to 6.5. The characteristics of each weathering cell tests are shown in Table 1, while Fig. 1 schematizes the weathering cell tests used.
A fifth weathering cell (fresh-Ni-sat) was performed in order to verify the capacity of EDTA to extract already sorbed Ni from the materials in the conditions of a weathering cell. In order to do so, the Ni sorption sites of the fresh material were first artificially saturated with a Ni solution (10 mg l−1 during the first 85 days, and 25 mg l−1 during days 85–288, see Table 1) using a procedure described in Plante et al. (2011a). When saturation was achieved, the material was flushed with a 0.3 M EDTA solution following the same procedure as previously described for the EDTA cells.
Leachates from the weathering cells are analyzed for pH, conductivity, sulfur, and metal concentrations. The metal concentrations in the leachates are analyzed by ICP-AES on filtered aliquot (<0.45 μm) acidified to 2 % HNO3 for preservation. Results from both the classic and protocol-modified weathering cells will be compared for the fresh and weathered waste rock samples in order to validate the EDTA approach to CND prediction.
Physical, chemical, and mineralogical characterizations
The physical characterization results are presented in Table 2. The Gs values are typical of the high hemo-ilmenite Tio mine waste rock samples (Plante et al. 2011a). The S.S.A. of both materials are of the same order, with the fresh having a slightly lower value (1.29 m2 g−1) than the weathered (1.61 m2 g−1). The grain size distributions show relatively coarse materials and are similar for both, with similar D10 (9.5–11.8 μm), D50 (93.0–111 μm), and D90 (236–238 μm) values.
The partial chemical characterization of the waste rock samples are shown in Table 2. The Fe and Ti contents confirm that the samples are rich in hemo-ilmenite (FeTiO3): if all Ti is present within hemo-ilmenite, the fresh and weathered samples respectively contain 46 and 41 wt% hemo-ilmenite. The Ctotal contents are low, with 0.141 wt% in the fresh sample and 0.050 wt% in the weathered sample. The Ca and Al contents are lower for the weathered sample. The Stotal content is higher in the fresh sample (0.832 wt%) than that in the weathered sample (0.339 wt%). The Ssulfate is also higher in the fresh sample (0.143 wt%) than that in the weathered sample (0.078 wt%). Consequently, the Ssulfide contents of the fresh and weathered samples are respectively 0.689 and 0.261 wt%. The Ni content, mostly associated with sulfide minerals (Plante et al. 2011a), is also higher in the fresh sample (0.074 wt%) than in the weathered sample (0.053 wt%). Likewise, the Co and Cu contents are lower in the weathered sample, while the Zn content is similar.
The mineralogical analyses (Table 2) confirmed that the samples are of similar composition, dominated by hemo-ilmenite (50–65 wt %), an intergrowth of hematite and ilmenite. The most abundant gangue mineral is a calcic plagioclase (15–25 %) close to labradorite ((Ca,Na)(Si,Al)4O8) in composition. The samples also contain pyroxenes (5–15 %), K-feldspar (<3–5 %), as well as traces of spinel, sulfides, and micas. The mineralogical investigations revealed that the sulfides are mainly pyrite (FeS2), with very small amounts of millerite (NiS) and chalcopyrite (CuFeS2). Some pyrite grains show Ni-enriched zones of up to 10 % Ni. More details about the mineralogical composition of the samples can be found in Plante et al. (2011a).
Water quality of the weathering cells leachates
The pH and electrical conductivity results from the weathering cells are shown in Fig. 2a, b, respectively. The pH of the fresh-control leachates vary between 7.5 and 8.0, while the pH of the weathered-control leachates generally vary between 6.0 and 7.0. The pH of the EDTA cells are between 4.0 and 5.5 in the beginning, because the EDTA sodium salt used to make the solution is slightly acidic. After 15 leaches, it was decided to adjust the EDTA water pH close to that of the respective control cells using diluted NaOH: fresh-EDTA water was adjusted to pH 8.0, while weathered-EDTA water was adjusted to pH 6.5. This pH modification by NaOH is not expected to significantly affect the geochemical responses (sulfide oxidation, sorption). Afterwards, the pH of these leachates remained close to the adjusted values. The electrical conductivities (Fig. 2b) of the EDTA cells are significantly higher (4000–6000 μS cm−1) than that of the control cells (20–250 μS cm−1) because of the EDTA concentration in the leaching water. The electrical conductivity increases occurring around 55 days for the EDTA cells correspond to the pH adjustments with NaOH at the 15th flush.
The concentrations of elements associated with metal sulfide oxidation (sulfates and metals) are shown in Fig. 3a–f. The concentrations are generally higher in the first flushes and gradually decrease and stabilize after 2 months of testing. This behavior is typical of kinetic test results and is believed to be caused by rapid depletion of the finer (more reactive) particles, by leaching of the oxidation products generated prior to the test, and by passivation of the mineral surfaces, which decrease their reactivity (e.g., (Benzaazoua et al. 2004; Villeneuve et al. 2009)).
All element concentrations are higher in EDTA cells than in the corresponding control cell. The S (considered being entirely expressed as sulfates) concentrations (Fig. 3a) are considered to be generated by sulfide oxidation and are generally between 1 and 45 mg l−1 after 60 days. The fresh-EDTA cell renders higher S concentrations (around 25 mg l−1) than the corresponding control cells (generally between 1 and 10 mg l−1). The Fe concentrations (Fig. 3b) are significantly greater in EDTA cells (10–100 mg l−1) than in control cells (0.01–0.1 mg l−1). The fresh-EDTA cell releases higher Ni concentrations (around 30 mg l−1) than the other cells (generally between 1 and 10 mg l−1), and the Ni concentrations of the EDTA cells are higher than the corresponding control cells (Fig. 2c). Likewise, the Co, Cu, and Zn concentrations (Fig. 3d–f, respectively) follow the same patterns, with higher concentrations in the EDTA cells (typically between 0.5 and 10 mg l−1) than in control cells (generally between 0.001 and 0.1 mg l−1).
The concentrations of Al, Ca, Mg, and Si, associated with acid neutralization in the Tio waste rocks, are shown in Fig. 4. The Al concentrations (Fig. 4a) are greater by two orders of magnitude in the EDTA cells (5–100 mg l−1) than in the control cells (generally below 0.01 mg l−1). The Ca concentrations (Fig. 4b) are higher for the weathered-EDTA cell (steady decrease from 100 to 10 mg l−1) than for the weathered-control cell (steady decrease from 10 to 1 mg l−1). The Ca concentrations from the fresh-control cell are lower than the fresh-EDTA cell until approximately day 75, after which the Ca concentrations are lower in the fresh-EDTA cell than in the fresh-control cell. The Mg concentrations (Fig. 4c) are higher in the EDTA cells (10–100 mg l−1) than in the control cells (in the order of 0.5–1.0 mg l−1). Finally, the Si concentrations (Fig. 4d) are generally higher in the EDTA cell than in the control cell for a given material.
The main results of the fresh-Ni-sat cell are shown on Fig. 5. The pH of the leachates vary between 7.5 and 8.5 during the first 295 days (prior to EDTA leaching), after which the pH peaks around 9 in the first EDTA flush, before gradually stabilizing to pH 6.5, the pH of the EDTA solution. The concentrations of Fe are close to the detection limit before the EDTA leaching, while the Ni concentration gradually increases from 0.05 mg l–1 at the beginning of the test to values between 5 and 10 mg l–1 during the leaching with the 10 and 25 mg l–1 Ni solution. The EDTA leaching increased the Ni concentration to a peak of 200 mg l–1 and gradually decreased the Ni concentration to approximately 5–10 mg l–1 (Fig. 4b). The Fe concentration was below 0.05 mg l–1 for most of the leachates but increased with the EDTA leaching to values generally between 25 and 50 mg l–1 (Fig. 4b). The S concentrations remained around 10–25 mg l−1 throughout the water and EDTA leaching (Fig. 4b).
The cumulative mass leached out of the cells can be used in kinetic testing to calculate the elemental release rates from the samples. These cumulative masses are normalized to the total sample mass, allowing easier comparison between the materials. The cumulative and normalized masses of S, Fe, Ni, Ca, Al, and Mg for the control and EDTA cells are shown in Fig. 6a–f, respectively. The slopes of these data, once the concentrations are stabilized (after approximately day 20 for S and Ca, and at approximately day 50 for Fe, Al, Mg, and Ni), give access to the elemental release rates, in mg kg–1 day–1. Similarly, the release rates of Al, Mg, and Si were also calculated. All the calculated release rates are shown in Table 3.
The geochemical results from the weathering cells conducted in this study suggest that the use of an EDTA solution instead of deionized water in kinetic testing by weathering cells has a significant impact on the geochemical responses of the waste rocks, increasing the elemental concentrations in the leachates (Figs. 3, 4, and 5), especially for metals with low solubility at circumneutral pH (such as Al, Fe, Cu; Cravotta 2008). The EDTA effectively chelates the metals as soon as they are released by the oxidation of sulfides or by the dissolution of neutralizing minerals in response to sulfide oxidation. However, EDTA may also increase mineral dissolution directly through metal-EDTA complex formation, extract metals from secondary phases that are already present within the materials prior to kinetic testing and extract metals that were sorbed at the mineral surfaces. The following will discuss these matters.
Thermodynamic equilibrium calculations using Vminteq version 3.0 (USEPA 1999) were performed on weathering cell data in order to estimate the proportions of the different EDTA species in solution in the range of pH values encountered in the present study. The results are shown in Fig. 7.
Thermodynamic modeling suggests that most metals were entirely chelated by EDTA. The calculations also suggest that the EDTA concentration used (0.03 M) was enough to chelate all metals in solution while most of the EDTA remained uncomplexed in solution (generally more than 50 % present as HEDTA3− and H2EDTA2−, depending on the equilibrium pH of the leachate), which confirms that the EDTA solution was adequate to meet the objectives of the procedure. Finally, the thermodynamical calculations suggest that no metallic secondary minerals (such as iron and aluminum oxihydroxides, nickel, as well as copper, zinc, and cobalt hydroxides) are oversaturated in the leachates from the EDTA cells, which supports the hypothesis that secondary mineral precipitation is avoided by the EDTA solution.
Effect of EDTA on the inflection point of the cumulative curves
The cumulative masses shown in Fig. 6 show an inflection point for F, Al, and Mg that corresponds to the transition between phases 1 and 2, after which the release rates decrease. The Ca, Ni, and S results do not show any inflection point, but a rather smooth flattening of the curve around 20–40 days, typical of the geochemical behavior of mine wastes submitted to kinetic testing, as mentioned earlier. Similar results were obtained on Si leaching (not shown). Thus, it appears that the increase in pH decreased the extraction of Fe, Al, and Mg by EDTA. These results suggest that a significant part of the Fe in the EDTA-cell leachates comes from other sources than sulfide oxidation. It is possible that a portion of the metals previously sorbed on the mineral surfaces desorb and are released by EDTA leaching, as will be demonstrated later on in the paper. Previous studies demonstrated that EDTA can release strongly sorbed metals and metals associated with oxides (Peters 1999; Lim et al. 2004; Kirpichtchikova et al. 2006; Tsang et al. 2007; Vaxevanidou et al. 2008; Qiu et al. 2009). As will be discussed below (microscopic observations, Figs. 8 and 9), iron oxihydroxides are present within the materials initially and are partially dissolved upon EDTA leaching. It has been long known that the dissolution of iron oxihydroxides is enhanced in acidic conditions and in the presence of ligands like EDTA (e.g., Ryan and Gschwend 1991; Bondietti et al. 1993; Scheckel and Sparks 2001; Brantley 2008), which is consistent with the results obtained in the present study. In addition, the extent of EDTA sorption onto oxihydroxide surfaces (a phenomena involved in desorption and dissolution mechanisms), generally decreases with an increase in pH (e.g., Bondietti et al. 1993), which is also consistent with the higher metal release rates obtained at lower pH values. Therefore, it appears that at least part of the Fe, Al, and Ni in the EDTA leachates comes from the dissolution of Fe oxihydroxides and Al hydroxide, and from the solubilization of Ni that was sorbed within the material initially. Other studies also observed a control of Ni concentrations in mine drainage by oxyhydroxides (Nicholson et al. 1999; Nicholson and Rinker 2000). Similar results were obtained for Al and Mg (Fig. 6e, f, respectively). It was previously demonstrated (Plante et al. 2011a, b) that secondary Al phases (and probably sorbed Al) are probably present within the waste rocks and therefore, the behavior of Al can be interpreted similarly as those of Ni and Fe. In addition, it was previously demonstrated (Plante et al. 2011a, b) that Mg in the Lac Tio waste rock leachates is mainly from silicate weathering (pyroxenes) and that Mg was also present in secondary precipitates. Therefore, its behavior under EDTA leaching can also be interpreted as those of Fe and Al, i.e., the increase in Mg dissolution is partially attributable to the dissolution of secondary Mg precipitates. However, microscopic evidence (discussed later on) does not show evidence of significant hemo-ilmenite dissolution upon EDTA leaching, and thus Mg in the leachates most probably comes from silicate weathering and dissolution of Mg bearing secondary phases.
Effect of EDTA on the release rates and cumulative masses leached
The S (as sulfate) release rate can be assumed to be the sulfide oxidation rate, because S is mostly generated by the oxidation of sulfide minerals, and it is believed to be entirely leached out of the cell as sulfate (as suggested by thermodynamical equilibrium calculations, not shown). Therefore, the results suggest that the sulfide oxidation rates are in the same order for all cells (1.54 × 10−5 to 4.25 × 10−5 mol kg−1 day−1) except for the fresh-EDTA (1.43 × 10−4 mol kg−1 d−1). One hypothesis that could explain this difference is surface passivation, i.e., rate inhibition by the buildup of coatings such as ferric hydroxide that restrict oxygen and product transfer rates to and from the fluid phase (Nicholson et al. 1989). Since EDTA inhibits most secondary mineral precipitation, the mineral surfaces do not get passivated anymore and therefore, the sulfide oxidation rate from the fresh-EDTA weathering cell is higher than the fresh-control cell. If this hypothesis is true, the fact that the sulfide oxidation rates from the weathered-control and weathered-EDTA cells are similar to the fresh-control cell suggests that the weathered sample surfaces were already passivated from previous field weathering. Thus, this would in turn imply that the EDTA does not totally remove the precipitates that are already passivating the sulfides in the weathered surfaces; it only inhibits the formation of new passivating precipitates.
The cumulative Fe leached out of the EDTA cells (Fig. 6b; 1600–2100 mg kg−1) is significantly greater than for the control cells (approximately 1–2 mg kg−1), which is consistent with the inhibition of Fe precipitation as secondary oxihydroxide precipitates by EDTA. The Fe release rates (Table 3) are similar for the EDTA cells (1.08–1.20 × 10−4 mol kg−1 day−1) and are significantly higher than the control cells (3.78–5.21 × 10−8 mol kg−1 day−1). These results are explained by the fact that EDTA inhibits the precipitation of secondary iron precipitates, which are oversaturated in the control cells according to thermodynamic calculations with Vminteq (not shown). The same behavior was also observed for Al, which is leached in low concentrations in the control cells, but at higher concentrations in the EDTA cells (Fig. 4a) for the same reasons as for Fe.
If the iron is most likely released by sulfide oxidation and is not precipitated in the EDTA cells, the Fe release rates from the EDTA cells are probably the same as their sulfide oxidation rates, since the major sulfide mineral is pyrite. Table 3 shows that the S and Fe release rates from the fresh-EDTA cell are similar, with 1.43 × 10−4 and 1.20 × 10−4 mol kg−1 day−1, respectively. However, the Fe release rate of the weathered-EDTA cell (1.08 × 10−4 mol kg−1 day−1) is approximately 1 order of magnitude higher than the S release rate (1.77 × 10−5 mol kg−1 day−1), probably due to the dissolution of secondary iron precipitates present in the weathered waste rock; this will be demonstrated by the optical and scanning electron microscopy observations discussed in the next section.
The cumulative Ni leached out of the EDTA cells (Fig. 6c; 115–300 mg kg−1) is significantly greater than for the control cells (approximately 1 mg kg−1), which is consistent with the inhibition of Ni sorption by EDTA. The total Ni leached out from the fresh-EDTA cell (300 mg kg−1) is almost three times higher than from the weathered-EDTA cell (115 mg kg−1), which is consistent with the higher Ni and Ssulfide content of the fresh material. The Ni release rates of the EDTA cells (1.30 × 10−5 mol kg−1 day−1 for the fresh and 2.46 × 10−6 for the weathered) are significantly higher than those of the control cells (1.19 × 10−8 and 4.32 × 10−7 mol kg−1 day−1), which shows that Ni sorption is inhibited with the use of the EDTA solution in kinetic testing. As will be demonstrated later on (microscopic investigations, Figs. 8 and 9), the EDTA does not remove all iron oxyhydroxides already present on the sulfide surfaces. Therefore, it appears that Ni sorption is most probably inhibited by EDTA complexation rather than because of the lack of oxyhydroxides.
The cumulative Ca (Fig. 6d) leached out of the EDTA cells are higher than the corresponding control cells for both materials (fresh 5900 vs 1900 mg k−1 for the EDTA and control cells, respectively; weathered 1600 vs 700 mg kg−1 for the EDTA and control cells, respectively). The main source of Ca in the Tio mine wastes is plagioclase dissolution (Plante et al. 2011b). These results suggest that EDTA promotes plagioclase dissolution. An increase in aluminosilicate dissolution upon leaching with EDTA was also observed in other studies (e.g., Blake and Walter 1999; Tsang et al. 2007; Brantley 2008). Similar observations are reported in the literature, which shows that the aluminosilicate dissolution is enhanced by chelating agents such as EDTA.
Microscopic investigation of the effect of EDTA on mineral surfaces
The effect of EDTA on the pyrite surfaces was investigated under the optical microscope and scanning electron microscope (SEM) on polished sections of the fresh and weathered samples. Polished sections enable relevant observations of the mineral surfaces when the sample is representative (Lindsay et al. 2009; Parbhakar-Fox et al. 2013). Figure 8 shows pyrite grains after dismantlement of the weathering cell tests on the fresh waste rocks sample when flushed with water (Fig. 8a–c) and with an EDTA solution (Fig. 8d, f). In both samples, only smaller pyrite grains (approximately <50 μm; see Fig. 8a, b) might show an iron oxyhydroxide rim, although some do not (Fig. 8d). Larger pyrite grains (approximately 50–500 μm) generally exhibit only rare, discontinued, and thinner rims (in the order of 2–3 μm), if any, as illustrated in Fig. 8c–f.
Figure 9 shows pyrite grains in the weathered sample after the weathering cell tests with water (a–d) and with the EDTA solution (e–h). In both samples, almost all pyrites show significantly thicker oxidation rims than in the fresh samples (approximately 10 to 50 μm thick; see Fig. 9a–c).
In the weathered-EDTA cell, the rims are generally thinner and discontinued (in the order of 2 to 10 μm; see Fig. 9e–g), probably as a result of their dissolution upon leaching with the EDTA solution. Also, the oxidation rims in the fresh and weathered samples flushed with EDTA are somewhat cloudy and display dissolution patterns, while they remain very clear in the samples flushed with water. Other studies also showed that EDTA can dissolve iron oxyhydroxides and release the metals bound to Fe or Mn oxides (Lim et al. 2004; Kirpichtchikova et al. 2006; Tsang et al. 2007; Vaxevanidou et al. 2008; Qiu et al. 2009). Furthermore, Qiu et al. (2009) stipulated that EDTA can readily dissolve poorly crystalline iron oxyhydroxides, while Vaxevanidou et al. (2008) believed that well crystallized oxyhydroxides were only partially dissolved by EDTA. In addition, other studies demonstrated that higher EDTA concentration in soil washing studies favors a higher extraction of oxides and hydroxides (Tandy et al. 2004; Di Palma and Ferrantelli 2005; Tsang et al. 2007). These observations are consistent with the results obtained in the present study, with iron oxyhydroxides in the fresh waste rocks being readily dissolved by an excess concentration of EDTA but that iron oxyhydroxides present in the weathered waste rocks were only partially dissolved.
Thus, a trade-off should be chosen so that the concentration of the chelating agent is in excess in order to chelate all metals considering a certain safety factor but that this safety factor needs to be low enough so that the dissolution of oxides and hydroxides is not needlessly exaggerated. The results presented here suggest that a chelating agent concentration of 10 times the expected total metals concentration is enough to effectively chelate all metals, but with a higher oxihydroxide dissolution than in typical field conditions. Therefore, the safety factor in chelating agent concentration should be within an order of magnitude or less.
It was previously demonstrated (Plante et al. 2011a, b) that hemo-ilmenite is weathered in the Lac Tio mine waste rocks upon weathering, by gradual conversion of the ilmenite (FeTiO3) to leucoxene or pseudorutile (Fe2Ti3O9), and then to rutile or anatase (TiO2), by the loss of iron atoms (Frost et al. 1983; Janßen et al. 2008; Nair et al. 2009). This phenomenon is easily recognizable under microscopic examinations, as it produces treillis-like structures in the hemo-ilmenite grains. However, the optical and electron microscopic observations herein (Figs. 8 and 9) do not show a significant difference on the presence of these treillis-like structures on the materials submitted to the control and EDTA cells. Therefore, Fe and Mg in the leachates are most probably coming from other sources than hemo-ilmenite weathering.
EDTA extraction of previously sorbed Ni onto waste rock surfaces
The effect of EDTA on sorbed Ni was verified through the fresh-Ni-sat cell test, by artificially saturating the sorption sites of the materials with a Ni solution, after which the materials were submitted to an EDTA cell. The cumulative S, Fe, and Ni (sorbed and extracted with EDTA) in the fresh-Ni-sat cell are shown in Fig. 10. The slopes of these plots are are shown in Table 3 for the water and EDTA leachings. The slopes were calculated between days 100–200 for the water leaching and during the last 15 flushes for the EDTA leaching, where the concentrations are stabilized (as well as the slopes of cumulative amounts). The S and Fe release rates obtained with water leaching in the fresh-Ni-sat cell (Table 3, 8.25 × 10−5 and 3.68 × 10−8 mol kg−1 day−1, respectively) are close to those of the fresh-control cell (Table 3, 4.25 × 10−5 and 3.78 × 10−8 mol kg−1 day−1, respectively). The S, Fe, and Ni release rates obtained with EDTA leaching of the fresh-Ni-sat cell (Table 3: 1.11 × 10−4, 1.08 × 10−4, and 3.72 × 10−5 mol kg−1 day−1, respectively) are similar to those of the fresh-EDTA cell (Table 3, 1.43 × 10-4, 1.20 × 10-4, and 1.30 × 10−5 mol kg−1 day−1, respectively). Figure 10a shows that more than 1500 mg kg−1 Ni was sorbed during the saturation phase of the fresh-Ni-sat cell (first 288 days) and that this Ni was completely removed during the EDTA leaching phase.
A previous study (Plante et al. 2010) suggests that Ni in the Lac Tio mine waste rocks is sorbed as Ni-Al layered double hydroxides (LDH). As demonstrated by Scheckel and Sparks (2001), the stability of Ni as Ni-Al LDH increases with time after sorption, as these convert into more stable forms. However, EDTA showed to be the most efficient extractant that Scheckel and Sparks (2001) used to remove even older, more stable Ni from aged Ni-Al LDH. The results obtained in the present study are consistent with those of Scheckel and Sparks (2001), as recently sorbed Ni is completely removed upon EDTA leachings. In addition, Fig. 10b shows the relationship between the cumulative Ni and Al removed from the EDTA cells and from the fresh-Ni-sat cell. The slopes (m) of the stabilized portion of the plots are 1.46, 0.56, and 0.08 for the fresh-Ni-sat cell, the fresh-EDTA, and the weathered-EDTA cells, respectively. These results show that the Ni/Al ratio removed from the fresh-EDTA and fresh-Ni-Sat cells are greater for the fresh samples, which suggest Ni is removed in a lesser extent in the weathered sample, an observation also consistent with the findings of Scheckel and Sparks (2001).
General guidelines on the use of chelating agents in kinetic testing for CND prediction
The results and observations obtained in the present study suggest that the use of EDTA in kinetic testing can effectively suppress secondary precipitation and metal sorption and avoids the delay in the development of contaminated drainage conditions in kinetic testing. They also suggest that sulfide oxidation and the subsequent dissolution of neutralizing minerals are slightly affected by the use of an EDTA solution as leaching media. However, EDTA also dissolves previously formed and more or less stable secondary oxyhydroxides, a phenomenon which is not encountered in natural geochemical conditions in waste rock piles. Nevertheless, these results show that the use of chelating agents in kinetic testing is a promising new approach for water quality prediction.
The methodology presented herein could serve as general guidelines when planning to use chelating agents in kinetic testing for metal-CND generating materials:
The use of chelating agent in kinetic testing should always be compared with a control test using water;
The concentration of the chelating agent concentration should be no more than 10 times the projected total metal concentrations, in order to ensure complete metal chelation without promoting adverse effects such as oxihydroxide dissolution. This subjectively chosen concentration could be adjusted along the way (for example, after the first 5 flushes), depending on the actual metal concentrations obtained;
An extensive characterization should be performed on the initial and final materials, in order to detect any significant changes in mineralogical composition that may have an effect on their geochemical behavior.
Perspectives on the use of chelating agents in kinetic testing
More work is needed in order to generalize the use of chelating agents in kinetic testing for metal-CND generating materials. EDTA is a relatively toxic and persistent chemical, which discourages its use, especially in field-scale experiments without proper EDTA recuperation and disposal. Therefore, future work will be directed to the use of less aggressive, less toxic, and more natural chelating agents which will effectively chelate the metals upon their release and suppress their sorption or precipitation as secondary minerals. The chelating agents to be used should also minimize the remobilization of previously formed secondary phases which would not dissolve in typical geochemical conditions in waste rock piles.
The aim of the present study was to test a novel methodology for the prediction of CND from mine wastes using a chelating agent (EDTA) as leaching media in a kinetic prediction test. The approach was tested on freshly produced and weathered waste rocks from the Lac Tio mine, which are known Ni-CND after 15–25 years of weathering in the field. The use of an EDTA solution in weathering cells enhances the geochemical responses by inhibition of sorption phenomena and of precipitation of secondary iron oxyhydroxides which may passivate mineral surfaces and trap other metals. The results from the present study show that this approach can be used to identify possible water contamination issues which would not appear in classic kinetic testing, but may appear after many years on the field (ex. when sorption phenomena within the wastes are being saturated over a few decades), and that need to be assessed further to determine whether or not those risks will be realized under the specific environmental conditions that will be present for the waste management scenario. The proposed EDTA method also enables to quantify the degree of attenuation of the dissolved metal concentrations by sorption phenomena and secondary precipitation and its evolution during kinetic testing. This study enables to propose general guidelines on the use of chelating agents in kinetic testing.
Further studies are needed in order to test other chelating agents, particularly those capable of chelating anions, and the effect of their concentrations on the water quality predictions. This approach should also be tested on other waste rocks as well as on tailing materials and be tested in larger-scale studies such as in laboratory humidity cells or column tests, as well as in field-scale prediction tests. Finally, the sorption capacities of mine wastes also need to be assessed further and taken into account in kinetic testing, particularly for near-neutral drainage conditions.
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Plante, B., Benzaazoua, M., Bussière, B. et al. Use of EDTA in modified kinetic testing for contaminated drainage prediction from waste rocks: case of the Lac Tio mine. Environ Sci Pollut Res 22, 7882–7896 (2015). https://doi.org/10.1007/s11356-015-4106-6
- Contaminated mine drainage
- Kinetic testing
- Prediction of the geochemical behavior of mine wastes
- Waste rock
- Contaminated neutral drainage (CND)