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High-resolution mass spectrometry for molybdenum speciation in sulfidic waters
Talanta 209 (2020) 120585
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High-resolution mass spectrometry for molybdenum speciation in sulfidic waters
T
Duc Huy Danga,∗, Wei Wangb, R. Douglas Evansb,c a
School of the Environment and Chemistry Department, Trent University, Peterborough, ON, Canada School of the Environment, Trent University, Peterborough, ON, Canada c Water Quality Centre, Trent University, Peterborough, ON, Canada b
ARTICLE INFO
ABSTRACT
Keywords: ESI-HRMS Molybdate Sulfide Reactions
High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdenum in interaction with sulfide and chloride. While reactions between molybdate and sulfide lead to creation of four conventional thiomolybdate species (MoO3S2−, MoO2S22−, MoOS32−, MoS42−), chemical formula assignment of recorded mass spectra confirmed the presence of intermediate thiomolybdate compounds (MoO3S22−, MoO2S32−, MoOS42−, MoS52−, MoS62−) and thio-chloro-molybdate compounds (MoO3Cl−, MoO2SCl−, MoOS2Cl−, MoOS3Cl−). Two of the intermediate thiomolybdate compounds were previously suggested theoretically, and this study provides analytical support for the existence of these compounds. Fast ESIHRMS analysis has allowed us to conduct a highly-resolved short-term kinetic study of these reactions, and we suggest that the reactivity between molybdate and sulfide is more complex than previously thought, particularly during the first 24 h of interaction. Also, the solution composition will impact reaction pathways, and different outcomes found in the literature may arise from choices of ionic strength and pH adjusting agents in previous studies . The occurrence of the thio-chloro-molybdate species detected in this study should be implemented in future Mo speciation models to better assess Mo reactivity in sulfidic waters and reducing environments.
1. Introduction Molybdenum is essential for life on Earth with important enzymatic implications ([1] and references therein). Indeed, this transition element could have been an important factor for life evolution during oxygenation of the Proterozoic ocean when dissolved Fe became scarce and Mo concentrations increased [2]. Also, molybdenum is an important paleo-proxy for Earth's oxygenation history due to its unique bimodal geochemical characteristics [2]; Mo is highly conservative in well-oxygenated surface waters as MoO42− [3–5], having low reactivity toward particles (except for slow co-precipitation with Mn oxide [6]). However, Mo becomes particle-reactive thiomolybdate in sulfidic conditions [7–9]. In euxinic environments, Mo reactivity toward particulate organic matter or sulfide minerals (e.g., pyrite) is enhanced [9,10]. It has been demonstrated by lab experiments that molybdate (MoO42−) is sequentially converted by sulfide to thiomolybdates (i.e., in order of MoO3S2−, MoO2S22−, MoOS32−, MoS42−) within the time frame of minutes, hours and days [7,11]. The interactions between molybdenum ions and sulfide ligands could lead to a high diversity in structural Mo–S complexes because close energies of the S 3p and Mo 4d orbitals
∗
provide low-energy exchange pathways [12]. Moreover, addition of a S atom by a S ligand could induce Mo reduction and potentially Mo–Mo dimerization [9,12]. In the natural environment, despite important changes in Mo speciation due to interactions with sulfur species, analytical tools able to detect such complex Mo–S reactivity are scarce. UV–Visible spectroscopy has been successfully used to assess the sequential sulfidation of molybdate anions to form four thiomolybdate species (MoO4-xSx2-, x = 1 to 4) [7]. However, this analytical tool suffers from the inconveniences of requiring high concentrations of analytes, as well as serious spectral interferences. On the other hand, Ion-Pair chromatography (IPC) or HPLC methods have been developed to resolve molybdate and four thiomolybdate species in 10–60 min, with UV–Visible or ICP-MS detectors [11,13,14]. This technique is time consuming and affected by potential matrix effects. At this point in time, all of these techniques can detect and identify only molybdate and the four traditional thiomolybdates (MoO4-xSx2, n = 1 to 4). In fact, Vorlicek et al. [9] conducted an experiment mixing sulfide and polysulfide with trithiomolybdate (MoOS32−) and noticed a deficit between total Mo concentration and the sum of UV–Vis detected Mo species
Corresponding author. E-mail address: [email protected] (D.H. Dang).
https://doi.org/10.1016/j.talanta.2019.120585 Received 19 September 2019; Received in revised form 11 November 2019; Accepted 19 November 2019 Available online 22 November 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
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(MoOS32− + MoS42−); they have attributed this phenomenon to the ligand-induced reduction of Mo via intermediate species (supposed to be MoOS42−, MoOS52−). This phenomenon seemed to be less significant for MoS42− than for MoOS32− [9]. Other thiomolybdate species (i.e., MoO3S2−, MoO2S22−) which are putatively involved in the early stage of the reaction (after minutes to hours after mixing of molybdate and sulfide) were not studied due to the lack of commercial salts or to synthesis methods, and to being considered unstable [7,11]. To the best of our knowledge, thus far no analytical tool provides good identification of such intermediates at environmental concentrations. In term of a methodology to quantify Mo redox species (Mo(V) and Mo (VI)), Wang et al. [15] have proposed a specific Mo(V)-tartrate complex which could be extracted on a XAD-7HP resin, confirming the occurrence of Mo(V) in hypoxic seawater. On the other hand, geochemical modeling could be also used to predict chemical speciation of Mo: for example the distribution between molybdate and the four conventional thiomolybdate species in the Black Sea water column [7]. Alltogether, these studies illustrate a requirement for a suitable analytical approach that is able to assess Mo–S complexes under natural conditions. Molybdenum can be detected by Electrospray Ionization-High Resolution Mass Spectrometry (ESI-HRMS), recognized by its specific isotopic pattern; seven Mo isotopes from 92Mo to 100Mo with 98Mo being the most abundant [16–18]. Previous experiments on oxomolybdate speciation have been carried out at high concentration (1 mM), which were conducive to polymerization [19]. ESI-HRMS has an advantage of high accuracy and resolution in the masses detected, which can allow molecule identification. Also, the time required for ESI-HRMS analyses is usually shorter (one to a few minutes) than IPC or HPLC techniques (typically 10 min to hours [11,13,14]), which is useful for undertaking high resolution kinetic studies. We have previously developed an ESI-HRMS method to study the chemical speciation of Mo in the presence of halides (e.g., F, Cl, Br), and found the formation of the chloro- and fluoro-molybdate complexes in lab experiments and natural waters [20]. This method demonstrated the applicability of the ESI-HRMS techniques to study chemical speciation of Mo in surface environments. Moreover, this investigation aims to provide an adaptation of this ESI-HRMS method to study the chemical speciation of Mo in reducing environments. We emphasize the interactions between molybdate ions at low concentration (i.e., 20 μM Mo) and sulfide, and critical early stages of the reaction (minute to hour scales) where intermediate thiomolybdate compounds may be involved.
tightened, wrapped with aluminum foil and kept shaken. After 5, 30 min, 1, 2, 3, 4, 6, 8, 20, 29, 45, 53 and 72 h, aliquots were collected with syringe and needle via the rubber septum and directly injected into 2 mL vials (with screw cap and septum, Agilent) containing 1 mL of N2purged methanol (HPLC-grade, BDH). 2.2. Instrumentation Mass spectra were collected using an Orbitrap Q Exactive (Thermo Fisher Scientific) in infusion mode at the Water Quality Center (Trent University). Negative ions were examined at a resolution of 140,000, using a full MS scan over the 150–350 m/z range. The S-lens level was set at 50. Injection flow rate was set to 15 μL min−1 and auxiliary gas at 0 (arbitrary units: arb) while other ESI-HRMS parameters were subjected to optimization: spray voltage from 1.5 to 4 kV, capillary temperature from 180 to 380 °C, sheath gas pressure from 2 to 22 arb. Prior to ESI-HRMS data acquisition, a routine mass calibration was carried out with Pierce™ LTQ ESI Positive/Negative Ion Calibration Solution (88322/88324, Thermo Fisher Scientific) using the recommended manufacturer parameters (sheath gas = 12 arb, auxiliary gas = 0 arb, spray voltage = 4 kV, capillary temperature = 320 °C). Mass spectra were acquired in duplicate with two microscans lasting 1 min corresponding to 60–80 scans to reach 100% of the Automatic Gain Control (AGC) target. The maximum injection time was set to 10 ms. The pH values were measured on an Accumet AB250 m using a pH combination electrode (Fisher Scientific). 2.3. Data processing Mass spectra were processed using Thermo Xcalibur Qual Browser software after an automatic processing defining mass tolerance of 10 ppm and mass precision at four decimals. For each compound, the elemental composition was assigned and checked by the isotope simulation module of the software. In addition, the spectrum list was exported to be processed by Winnow, a program that scans the mass spectrum list for the presence of target elements or ions based on their isotope patterns (mass differences and intensity ratios between isotopologues [21]). 3. Results and discussion 3.1. Mass spectra, peak assignment and compound identification
2. Experimental section
An example of the mass spectrum of solution B at 29 h is given in Fig. 1. In summary, 16 compounds containing Mo, Cl and S were detected by the Winnow program (Table 1). These compounds were assigned by (i) the difference between the measured and theoretical masses of the composition (ΔM less than 10 ppm, Table 1) and (ii) their specific isotopic patterns. Note that the isotopic pattern of a heteroatomic compound is a combination of individual isotopic patterns of the elements composing the compound. Lists of peaks (i.e., isotopologues) detected for HMoOS5 and HMoOClS2 are given as examples in Fig. 1. For both compounds, O isotopes were not all detected due to the low abundance of 17O and 18O. Their isotopic patterns are then a combination of Mo, S and Cl isotopes. For HMoOS5 (named γ5), only 13/14 isotopologues were detected (Fig. 1), probably due to the non-resolved 92 Mo34S32S4 and 94Mo32S5 (respective abundance of 13.3 and 37.5% of the most abundant 98Mo32S5); their respective masses are of 270.7657 and 270.7682. Concerning HMoOClS2, when both Cl and S atoms are present, only peaks containing heavy Cl (37Cl) are found but not the heavy S (34S), except for the most abundant 98Mo peak (Fig. 1). In fact, the respective differences between these heavy isotopes and the most abundant 35Cl and 32S are only 1.9970 (37Cl–35Cl) and 1.9958 (34S–32S), making the peaks containing 37Cl and 34S not well-resolved. Also, relative to the most abundant 35Cl and 32S, 37Cl is more abundant than 34S (24% and 4%, respectively). Therefore, the isotopologue containing
2.1. Chemicals and sample preparation A stock solution of molybdate (10 mM) was prepared in a Teflon bottle by dissolving reagent ( (NH4)6Mo7O24·4H2O, (99% min, Acros organic) in 18.2 MΩ water. A 50 mM sulfide solution (Na2S·9H2O, 98% +, Acros organic) was prepared in a Schott glass bottle in an N2-filled glovebox. Phosphate buffer stock solutions (Na2HPO4, NaH2PO4, Baker) were prepared at 0.2 M and readjusted for pH 7 with HNO3 (double distilled trace-metal grade, BHD Aristar plus/DuoPUR Milestone) and NH4OH (Fisher). Molybdate solutions for use in speciation measurements, at a final concentration of 20 μM, were mixed with sulfide (0.2 mM; S/Mo ratio of 10, similar to previous studies: [11,13]). Phosphate buffer (final concentration of 2 mM) was added to prevent pH drift, as observed by Lohmayer et al. [11]. All solutions were prepared in a glovebox filled with N2 and in 100 mL Schott bottles with screw caps having a silicone O-ring and a butyl rubber stopper. In N2-purged 18.2 MΩ water containing molybdate, sulfide and phosphate buffer, three configurations were set up containing: no ionic buffer (solution A), 1 mM NaCl (solution B), or 1 mM NaNO3 (solution C). Time zero was defined as the moment when sulfide was added in solutions as the last reactant. The bottles were then withdrawn from the glovebox, with caps firmly 2
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Fig. 1. Mass spectrum of the solution B collected at 29 h (upper panel) with zooms on isolated peaks of molybdenum compounds (middle and lower panels). List of isotopologues detected in β2 (HMoOClS2) and γ5 (HMOOS5) are given in framed legend. 37
Cl is easier to detect than that with 34S. Although the full isotope pattern is not detected for these compounds, the high accuracy in the molecular masses and the simultaneous detection of 37Cl and 34S peaks with the most abundant 98Mo (Fig. 1) are best support for tentative formula assignment. These 16 compounds are categorized in three main groups (α, β and
γ), and named as αi, βi, γi with i being the number of S atoms. The α group includes the molybdate, conventional thiomolybdate species (MoO4-xSx2-, x = 1 to 4) and two additional MoS52− and MoS62− species showing a line of sequential S addition reactions (up to 6 S: α1 to α6) on the molybdate anion (α0). Similarly, the β group encompasses a series of S addition reactions (up to 3 S: β1 to β3) on MoO3Cl (β0)
Table 1 Summary of Mo species detected in this study with corresponding theoretical and measured masses and the difference between them ( M (ppm) = (Mmeasured/ Mtheoretical 1) × 106 ). n = number of observations. n.d. stands for not detected. The note * indicates compounds that were protonated during ESI process, see text for more details. Compound
α0 α1 α2 α3 α4 α5 α6 β0 β1 β2 β3 γ2 γ3 γ4 γ5 γ6
Formula
HMoO4 HMoO3S HMoO2S2 HMoOS3 HMoS4 HMoS5 HMoS6 MoO3Cl MoO2ClS MoOClS2 MoClS3 HMoO3S2 HMoO2S3 HMoOS4 HMoOS5 HMoOS6
Measured mass
162.894 178.871 194.848 210.825 226.802 258.774 290.746 180.860 196.837 212.814 228.791 210.843 226.820 242.797 274.769 306.741
Error (2 sd)
0.001 0.002 0.001 0.001 0.002 0.002 0.001 0.002 0.001 0.002 0.001 0.001 0.001 0.002 0.002
n
52 52 52 49 33 22 2 12 46 50 5 9 44 46 49 28
Theoretical monoisotopic exact mass
162.8923 178.8695 194.8467 210.8238 226.801 258.773 290.7451 180.8585 196.8356 212.8128 228.7899 210.8416 226.8187 242.7959 274.768 306.74
ΔM (ppm)
10.4 8.4 6.7 5.7 4.4 3.9 3.1 8.3 7.1 5.6 4.8 6.6 5.7 4.5 3.6 3.2
3
Time of first appearance (hours)
Note
Solution A
Solution B
Solution C
Solution D
0.08 0.08 0.08 4 6 n.d.
0.08 0.08 0.08 1 2 n.d.
0.08 0.08 0.08 0.08 0.5 45
0.08 0.08 1 29 n.d. n.d.
* * * * * * *
0.5 0.08 n.d. 8 0.08 0.08 0.08 0.5
0.08 0.08 20 29 0.08 0.08 0.08 0.1
0.08 0.08 n.d. 2 0.08 0.08 0.08 0.08
0.08 0.5 n.d. n.d. 0.08 1 0.08 n.d.
* * * * *
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Fig. 2. Reaction scheme explaining the reactivity between molybdate ion, chloride and sulfide summarized in three main lines α (blue zone), β, (green zone) and γ (red zone). The area delimited by full black line shows four conventional thiomolybdate species (MoO4-xSx2-, x = 1 to 4 [7,9,11]) while zone defined by dashed line shows possible reactions explaining the Mo deficit as MoOS32− loss as suggested by Vorlicek et al. [9]. The * indicate doubly charged compounds detected with an additional proton caught during electrospray, see text for more details. The framed legend represents the scheme with αi, βi, γi notations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
MoOS32− and sulfide/polysulfide; compounds γ4 and γ5 have been proposed in a reaction scheme to explain the rate of MoOS32− loss [9]. In addition, our mass spectra revealed additional compounds (γ2, γ3, γ6), potentially homologues to the reactions proposed by Vorlicek et al. [9]. However, it was previously assumed that sulfide (ΣHS−) alone could result only in a series of Mo(VI) thioanions but not Mo reduction [9]. However, in the experiment conducted by Vorlicek et al., where 3 mM ΣHS−, 70 mM NaCl, 1 mM borate buffer and 350 μM MoOS32− (Run N in Ref. [9]) were mixed at pH 8.5, ΣSo was calculated to be 0.636 mM and Mo deficit was approximately 30–50 μM (8–14% of total Mo). A similar phenomenon could happen as well in our solutions, explaining the occurrence of the γ series compounds (see further discussion below about the interaction between sulfide and nitrate). Also, compound γ′5 (similar to α′6) has been suggested to be a species produced by the molecular arrangement of γ5 prior to MoVI-to-MoIV reduction [9]. However, as γ5 and γ′5 are isomers (similar to α6 and α′6), they cannot be differentiated by mass spectrometry, but the S addition
which has been previously identified as a product of the reaction between molybdate and chloride [20]. The γ group comprises five nonconventional thio-(oxo)molybdate compounds; two of which have been suggested as intermediates during ligand-induced Mo reduction [9]. Also, it has to be noted that during solution-to-gas transition (electrospray), double charged species tend to catch a proton from the media to enhance their stability in the gas phase by decreasing their charge density; further details are discussed in Refs. [20,22]. These compounds are then detected with an additional H compared to the original aqueous species (noted by * in Table 1). A reaction scheme can be proposed then, based on the three main reaction lines (α, β and γ, Fig. 2). We propose that two additional compounds (α5 and α6) can complete the conventional reactions among the MoO4-xSx2- species, by S addition on the Mo]S double bonds, such reaction and functional group have been proposed by Coucouvanis [12]. Also, the γ compounds are the best support for the hypothesis of Vorlicek et al. [9] to explain the Mo deficit following the reaction of 4
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Fig. 3. Optimization of ESI-HRMS parameters for Mo speciation varying spray voltage, sheath gas pressure and probe temperature (a). The red zone shows the optimized value for highest intensity of Mo compounds. The ratio of α2/α0 was plotted against α1/α0 during optimization operations showing a linear variation in the intensity of these compounds. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reaction on γ5 to create γ6 would require a molecular rearrangement and probably a reduction MoVI to MoIV, which could be detected by HRMS (see Fig. 2). 3.2. Optimization of ESI parameters A previous investigation [20] has discussed the necessity of ESI parameter optimization to minimize differences in ionization efficiency of different compounds during the solution-to-gas transition. Flow rate and auxiliary gas were kept fixed at the previously optimized values (15 μL min−1 and 0 arb, respectively). Optimization of other parameters (spray voltage, sheath gas, probe temperature in order of optimization procedure) was conducted on a solution containing molybdate (20 μM) and sulfide (0.2 mM) after 2 h of reaction. Variations in the intensity of compounds α0, α1, α2, γ2 (the most abundant at the sampling time) as a function of spray voltage (1.5–4.5 kV), sheath gas (2–22 arb) and probe temperature (180–380 °C) are shown in Fig. 3a. The values where highest intensities of these compounds were recorded were selected as optima. The differences in ionization efficiency of these molybdate and thiomolybdate compounds apparently are not significant, as their intensities seemed to vary linearly with optimizing parameters (Fig. 3b). Note that this was not the case for HMoO4 (α0) and MoO3Cl (β0); their ionization efficiencies varied significantly during parameter optimization [20]. The difference in ionization efficiency of these two compounds was minimized only when spray voltage was above 4 kV, and sheath gas above 16 arb (probe temperature did not induce significant variations). This could be a consequence of the difference in charge density of MoO42− and MoO3Cl− in solution; when MoO42− passes into the gas phase, this compound is forced to take a H from the media. Similar to the molybdate ion, this additional step is observed for other doubly charged thiomolybdate species as well. For this reason, it is highly possible that other doubly charged thiomolybdates that we detected in this study (other than α0, α1, α2, γ2 observed during this optimization) would behave similarly. Also, when the parameters are optimized for doubly charged species, there is a difference (requiring a factor of correction) between the intensity of doubly charged and singly charged species (compounds with and without a * noted in Table 1, respectively), The intensities recorded for singly charged species (e.g., MoO3Cl (β0)) would be underestimated using the optimized parameters of this investigation. However, when all the samples were analyzed under fixed analytical conditions, it is a reasonable assumption that the same correction factor would exist to correct such differences in ionization efficiency. 3.3. Early stage of the molybdate-sulfide reaction The kinetic study (over a period of 3 days) was carried out on three solutions (final pH = 7.3 ± 0.1) containing either Cl− (Solution B), NO3−(Solution C), or none of them (Solution A). Temporal variations in the percentages of molybdate (α0) and the four conventional thiomolybdates (α1 to α4) are shown in Fig. 4. Proportions of the 14 compounds identified in the solutions are shown in Fig. S1, while the ratios within couples of compounds (i.e., product/reactant of each S-addition reaction) are given in Fig. 5. In general, results similar to those of previous studies [7,11] were observed: proportion of α0 dropped within a few minutes after adding sulfide leading to a production of α1, which reached its highest 5
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proportion after 1 h (Fig. 4a and b). Also, the percentage of α0 (MoO42−) was almost constant after 1h (14 ± 3, 8 ± 2 and 9 ± 2% in solutions A, B and C, respectively, Fig. 4a). A similar finding previously had been observed by Lohmayer et al. [11], whereas no MoO42− was detected by Helz and Erickson [7]. The proportions of α2, α3 and α4 kept increasing throughout the course of the experiment (Fig. 4 c,d,e). However, it appears that α3 (MoOS32−) was created much earlier (5 min, Fig. 4c) than in previous work (after hours, Erickson and Helz, (2000)), as was the case for γ3 (MoO2S32−) (Fig. 5c, f). Whereas the proportion of α2 kept increasing with time, the α3/α2 and γ3/α2 ratios showed maxima at 5 min then continuously decreased over the course of the experiments (Fig. 5b and c). As α3 and γ3 are both products of the S addition on α2, it is possible that the reactions creating α3 and γ3 occur faster than previously predicted, on a timeframe of minutes, leading to a slower appeance of α2. Once α3 was present in solution, S addition to α3 would produce α4 and γ4 (after an hour, Figs. 4e, 5d and 5g) which then slows production of α3 (Fig. 4d). A minor production of α5, γ5 and γ6 was detected (less than 10%, Fig. S1, 5e-i). Generally, once α3 was created in solution after 5 min, the creation of the γ series compounds seemed to be faster than that of the α compounds; the addition of a S to open the Mo]S double bond would be more favored than the S substitution on Mo–O- or Mo]O bonding. This observation is in agreement with the experiment conducted by Vorlicek et al. [9] where the ΣModeficit (assumed to be γ4 and/or δ5) was created before the creation of MoS42− (α4); the effect seemed to be more noticeable when polysulfide was added (Run O in Vorlicek et al., [9]. Erickson and Helz [7] have demonstrated that the reaction between sulfide and molybdate is a first order process, while the reaction between molybdate and sulfide/polysulfide seemed to be more complex than a single-step reaction, occurring mostly during the first 24h, [9]. The observations of our study support the quick creation of the γ line during the first 24 h of reaction, before the conventional α line can stabilize. 3.4. Effect of solution composition Previous studies have Mo-sulfide/polysulfide experiments conducted in different conditions, but most of them have been performed either without ionic strength control or with high NaCl concentrations (Table 2). In our study, we have conducted the experiments in three configurations: without ionic strength control (solution A), 1 mM NaCl (Solution B) and 1 mM NaNO3 (Solution C). Even though the kinetics of successive reactions between Mo and sulfur seems to be similar in these three solutions (Figs. 4 and 5), intensity of the production of thiomolybdates seemed to be different: Solutions C > B > A (i.e., nitrate > chloride > no ionic buffer). It has been shown by Luther III [23] that a two-electron transfer from any NOx compound (nitrate included) to sulfide is thermodynamically favored at any pH. This observation may explain the main difference between this study and previous ones concerning the formation of the line γ when we used only sulfide. Sulfide would create only the four conventional thiomolybdate coumpounds [7,9]. However, it is possible that nitrate in our solutions may have allowed a two-electron transfer to oxidize sulfide to elemental sulfur, which led to the formation of polysulfide. This hypothesis is supported by the comparison of the formation of γ compounds in solutions A, B and C; in solutions A and B (Figs. S1a–3, S1b-3) compounds γ4-6 increased with time and reach the maximum at ca. 10% only after 24 h, whereas in solution C, the percentage of these compounds γ were stable along the course of the experiment (Figs. S1c–3). Nitrate in solutions A, B would be at trace level, used as a pH adjustor (HCl was avoided due to the formation of MoO3Cl−, β0). In solution C with an excess of nitrate (N/S ratio = 5), the sulfide oxidation would be faster than in solution A and B. In solution B (1 mM NaCl), the β compounds were observed with up to 3 S and only two S substituted on Mo]O structure (β1 and β2). For
Fig. 4. Sequential creation of four conventional thiomolybdate species (α1 to α4) from molybdate (α0) in three solutions without ionic buffer (A: circle symbols) or with chloride (B: square symbols) and nitrate (C: diamond symbols) as ionic strength control. 6
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Fig. 5. Kinetics of the Mo-sulfide reactivity considering couples of compounds (product/reactant) in three solutions without (A: circle symbols) or with Cl (B: square symbols) and NO3− (C: diamond symbols) as ionic strength control.
7
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[11]
[13] this study
This study reports on the identification of thiomolybdate and thiochloro-molybdate species, and the sequential creation of these species in a short-time kinetic experiment. The results support the existence of intermediate thiomolybdate species prior to the reduction of Mo(VI) to Mo(IV) induced by S ligands. We propose a reaction scheme with three major lines α, β and γ corresponding to different Mo reactivities. The output of this study indicates that future experiments conducted on Mosulfide reactions should pay special attention to ionic strength control and pH adjustment. While adding Cl will create the β line, excess of nitrate will foster the oxidation of sulfide to elemental sulfur and ultimately polysulfide, which will favor the creation of the γ line. The occurrence of the β line has important implications in estuarine and marine environments where Mo–Cl reactivity has not been considered in Mo speciation models. Future applications of this ESI-HRMS method on natural waters, combined with other analytical tools, would contribute to our understanding of the speciation of Mo in sulfidic waters.
None 2 mM phosphate None None, 1 mM NaCl, 1 mM NaNO3
NaOH, HCl HNO3, NH4OH
IPC-ICP-MS or HPLCUV RP-IPC-UV ESI-HRMS None None
0.07 M NaCl
10 mM borate
NaOH, HCl/HNO3
UV–Vis
[9]
[7]
No Mo deficit/reduction 1st order reactions NH4+ catalytic effect Mo deficit 1st order for long-term but not during the first 24h. pH drift in the course of the experiment Highest sulfidation of molybdate at pH 7 No pH drift UV–Vis 0.7 M NaCl
None
HCl, NaOH, NH4Cl
Reference Analytical technique
Acknowledgements This work was supported by a Canadian NSERC (Natural Sciences and Engineering Research Council) Collaborative Research and Development Grant to R.D.E. The authors wish to thank Dr. Naomi Stock for helpful discussions, Michael Doran for making the Winnow program publicly available and Dr. Hayla Evans for manuscript revision. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.talanta.2019.120585.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
9 days 3 days
60 days
sulfide sulfide 0.45 mM molybdate 0.02 mM molybdate
7.5 7.3
Sulfide, polysulfide 0.2 mM Molybdate
5–11
40 days Sulfide, polysulfide 0.04–0.35 mM MoOS32−, MoOS42-
8.2, 8.5
60 days sulfide 1 mM Molybdate
7.9–8.5
Ionic strength control Experiment duration
4. Conclusion
References
Sulfur species
pH
now, it is difficult to compare the absolute percentage between this β series and the rest of the compounds, as their intensities are underestimated using the current ESI parameters. However, they constituted a non-negligible fraction of Mo speciation ( greater than 10%, Figs. S1b–4). These conditions would be reflective of marine environments where interaction between molybdate, chloride and sulfide has not been considered in molybdenum (VI) speciation yet [7].
Declaration of interests
Molybdenum species
Table 2 Summary of the conditions of previous experiments conducted on molybdate-sulfide reactions.
pH buffer
pH readjustment
Observation about the Mo–S experiments
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[1] F. Bittner, R.R. Mendel, Cell biology of molybdenum, Biochim. Biophys. Acta 1763 (2006) 621–635, https://doi.org/10.1007/978-3-642-10613-2_6. [2] C. Scott, T.W. Lyons, A. Bekker, Y. Shen, S.W. Poulton, X. Chu, A.D. Anbar, Tracing the stepwise oxygenation of the Proterozoic ocean, Nature 452 (2008) 456–459, https://doi.org/10.1038/nature06811. [3] K.W. Bruland, M.C. Lohan, Controls of trace metals in seawater, in: K.K. Turekian, H.D. Holland (Eds.), Treatise on Geochemistry, Elsevier Ltd, 2003, pp. 23–47. [4] J. Gaillardet, J. Viers, B. Dupré, Trace elements in river waters, in: K.K. Turekian, H.D. Holland (Eds.), Treatise on Geochemistry, Elsevier Ltd, 2003, pp. 225–272. [5] Y. Sohrin, K.W. Bruland, Global status of trace elements in the ocean, Trends Anal. Chem. 30 (2011) 1291–1307, https://doi.org/10.1016/j.trac.2011.03.006. [6] K.K. Bertine, K.K. Turekian, Molybdenum in marine deposits, Geochem. Cosmochim. Acta 37 (1973) 1415–1434, https://doi.org/10.1016/0016-7037(73) 90080-X. [7] B.E. Erickson, G.R. Helz, Molybdenum(VI) speciation in sulfidic waters: stability and lability of thiomolybdates, Geochem. Cosmochim. Acta 64 (2000) 1149–1158, https://doi.org/10.1016/S0016-7037(99)00423-8. [8] N. Tribovillard, T.J. Algeo, T. Lyons, A. Riboulleau, Trace metals as paleoredox and paleoproductivity proxies: an update, Chem. Geol. 232 (2006) 12–32, https://doi. org/10.1016/j.chemgeo.2006.02.012. [9] T.P. Vorlicek, M.D. Kahn, Y. Kasuya, G.R. Helz, Capture of molybdenum in pyriteforming sediments: role of ligand-induced reduction by polysulfides, Geochem.
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[17] J. Le Quan Tuoi, E. Muller, Electrospray mass-spectrometry on A magnetic-sector instrument applied to heteropolyacids, Rapid Commun. Mass Spectrom. 8 (1994) 692–694, https://doi.org/10.1002/rcm.1290080905. [18] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von Nagy-Felsobuki, Electrospray mass spectral study of isopolyoxomolybdates, J. Chem. Soc. Trans. (1999) 311–321. [19] J.P. Gustafsson, Modelling molybdate and tungstate adsorption to ferrihydrite, Chem. Geol. 200 (2003) 105–115, https://doi.org/10.1016/S0009-2541(03) 00161-X. [20] D.H. Dang, D. Evans, Application of ESI-HRMS for molybdenum speciation in natural waters: investigation of molybdate-halide reactions, Talanta 179 (2018) 221–229. [21] M.C. Doran, K.L. LeBlanc, A computer program to simplify analysis of mass scan data of organometallic compounds from high-resolution mass spectrometers, Rapid Commun. Mass Spectrom. 30 (2016) 2561–2567, https://doi.org/10.1002/rcm. 7748. [22] M.J. Deery, O.W. Howarth, K.R. Jennings, Application of electrospray ionisation mass spectrometry to the study of dilute aqueous oligomeric anions and their reactions, J. Chem. Soc. Dalton Trans. (1997) 4783–4788. [23] G.W. Luther, The role of one- and two-electron transfer reactions in forming thermodynamically unstable intermediates as barriers in multi-electron redox reactions, Aquat. Geochem. 16 (2010) 395–420, https://doi.org/10.1007/s10498-0099082-3.
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High-resolution mass spectrometry for molybdenum speciation in sulfidic waters
T
Duc Huy Danga,∗, Wei Wangb, R. Douglas Evansb,c a
School of the Environment and Chemistry Department, Trent University, Peterborough, ON, Canada School of the Environment, Trent University, Peterborough, ON, Canada c Water Quality Centre, Trent University, Peterborough, ON, Canada b
ARTICLE INFO
ABSTRACT
Keywords: ESI-HRMS Molybdate Sulfide Reactions
High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdenum in interaction with sulfide and chloride. While reactions between molybdate and sulfide lead to creation of four conventional thiomolybdate species (MoO3S2−, MoO2S22−, MoOS32−, MoS42−), chemical formula assignment of recorded mass spectra confirmed the presence of intermediate thiomolybdate compounds (MoO3S22−, MoO2S32−, MoOS42−, MoS52−, MoS62−) and thio-chloro-molybdate compounds (MoO3Cl−, MoO2SCl−, MoOS2Cl−, MoOS3Cl−). Two of the intermediate thiomolybdate compounds were previously suggested theoretically, and this study provides analytical support for the existence of these compounds. Fast ESIHRMS analysis has allowed us to conduct a highly-resolved short-term kinetic study of these reactions, and we suggest that the reactivity between molybdate and sulfide is more complex than previously thought, particularly during the first 24 h of interaction. Also, the solution composition will impact reaction pathways, and different outcomes found in the literature may arise from choices of ionic strength and pH adjusting agents in previous studies . The occurrence of the thio-chloro-molybdate species detected in this study should be implemented in future Mo speciation models to better assess Mo reactivity in sulfidic waters and reducing environments.
1. Introduction Molybdenum is essential for life on Earth with important enzymatic implications ([1] and references therein). Indeed, this transition element could have been an important factor for life evolution during oxygenation of the Proterozoic ocean when dissolved Fe became scarce and Mo concentrations increased [2]. Also, molybdenum is an important paleo-proxy for Earth's oxygenation history due to its unique bimodal geochemical characteristics [2]; Mo is highly conservative in well-oxygenated surface waters as MoO42− [3–5], having low reactivity toward particles (except for slow co-precipitation with Mn oxide [6]). However, Mo becomes particle-reactive thiomolybdate in sulfidic conditions [7–9]. In euxinic environments, Mo reactivity toward particulate organic matter or sulfide minerals (e.g., pyrite) is enhanced [9,10]. It has been demonstrated by lab experiments that molybdate (MoO42−) is sequentially converted by sulfide to thiomolybdates (i.e., in order of MoO3S2−, MoO2S22−, MoOS32−, MoS42−) within the time frame of minutes, hours and days [7,11]. The interactions between molybdenum ions and sulfide ligands could lead to a high diversity in structural Mo–S complexes because close energies of the S 3p and Mo 4d orbitals
∗
provide low-energy exchange pathways [12]. Moreover, addition of a S atom by a S ligand could induce Mo reduction and potentially Mo–Mo dimerization [9,12]. In the natural environment, despite important changes in Mo speciation due to interactions with sulfur species, analytical tools able to detect such complex Mo–S reactivity are scarce. UV–Visible spectroscopy has been successfully used to assess the sequential sulfidation of molybdate anions to form four thiomolybdate species (MoO4-xSx2-, x = 1 to 4) [7]. However, this analytical tool suffers from the inconveniences of requiring high concentrations of analytes, as well as serious spectral interferences. On the other hand, Ion-Pair chromatography (IPC) or HPLC methods have been developed to resolve molybdate and four thiomolybdate species in 10–60 min, with UV–Visible or ICP-MS detectors [11,13,14]. This technique is time consuming and affected by potential matrix effects. At this point in time, all of these techniques can detect and identify only molybdate and the four traditional thiomolybdates (MoO4-xSx2, n = 1 to 4). In fact, Vorlicek et al. [9] conducted an experiment mixing sulfide and polysulfide with trithiomolybdate (MoOS32−) and noticed a deficit between total Mo concentration and the sum of UV–Vis detected Mo species
Corresponding author. E-mail address: [email protected] (D.H. Dang).
https://doi.org/10.1016/j.talanta.2019.120585 Received 19 September 2019; Received in revised form 11 November 2019; Accepted 19 November 2019 Available online 22 November 2019 0039-9140/ © 2019 Elsevier B.V. All rights reserved.
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(MoOS32− + MoS42−); they have attributed this phenomenon to the ligand-induced reduction of Mo via intermediate species (supposed to be MoOS42−, MoOS52−). This phenomenon seemed to be less significant for MoS42− than for MoOS32− [9]. Other thiomolybdate species (i.e., MoO3S2−, MoO2S22−) which are putatively involved in the early stage of the reaction (after minutes to hours after mixing of molybdate and sulfide) were not studied due to the lack of commercial salts or to synthesis methods, and to being considered unstable [7,11]. To the best of our knowledge, thus far no analytical tool provides good identification of such intermediates at environmental concentrations. In term of a methodology to quantify Mo redox species (Mo(V) and Mo (VI)), Wang et al. [15] have proposed a specific Mo(V)-tartrate complex which could be extracted on a XAD-7HP resin, confirming the occurrence of Mo(V) in hypoxic seawater. On the other hand, geochemical modeling could be also used to predict chemical speciation of Mo: for example the distribution between molybdate and the four conventional thiomolybdate species in the Black Sea water column [7]. Alltogether, these studies illustrate a requirement for a suitable analytical approach that is able to assess Mo–S complexes under natural conditions. Molybdenum can be detected by Electrospray Ionization-High Resolution Mass Spectrometry (ESI-HRMS), recognized by its specific isotopic pattern; seven Mo isotopes from 92Mo to 100Mo with 98Mo being the most abundant [16–18]. Previous experiments on oxomolybdate speciation have been carried out at high concentration (1 mM), which were conducive to polymerization [19]. ESI-HRMS has an advantage of high accuracy and resolution in the masses detected, which can allow molecule identification. Also, the time required for ESI-HRMS analyses is usually shorter (one to a few minutes) than IPC or HPLC techniques (typically 10 min to hours [11,13,14]), which is useful for undertaking high resolution kinetic studies. We have previously developed an ESI-HRMS method to study the chemical speciation of Mo in the presence of halides (e.g., F, Cl, Br), and found the formation of the chloro- and fluoro-molybdate complexes in lab experiments and natural waters [20]. This method demonstrated the applicability of the ESI-HRMS techniques to study chemical speciation of Mo in surface environments. Moreover, this investigation aims to provide an adaptation of this ESI-HRMS method to study the chemical speciation of Mo in reducing environments. We emphasize the interactions between molybdate ions at low concentration (i.e., 20 μM Mo) and sulfide, and critical early stages of the reaction (minute to hour scales) where intermediate thiomolybdate compounds may be involved.
tightened, wrapped with aluminum foil and kept shaken. After 5, 30 min, 1, 2, 3, 4, 6, 8, 20, 29, 45, 53 and 72 h, aliquots were collected with syringe and needle via the rubber septum and directly injected into 2 mL vials (with screw cap and septum, Agilent) containing 1 mL of N2purged methanol (HPLC-grade, BDH). 2.2. Instrumentation Mass spectra were collected using an Orbitrap Q Exactive (Thermo Fisher Scientific) in infusion mode at the Water Quality Center (Trent University). Negative ions were examined at a resolution of 140,000, using a full MS scan over the 150–350 m/z range. The S-lens level was set at 50. Injection flow rate was set to 15 μL min−1 and auxiliary gas at 0 (arbitrary units: arb) while other ESI-HRMS parameters were subjected to optimization: spray voltage from 1.5 to 4 kV, capillary temperature from 180 to 380 °C, sheath gas pressure from 2 to 22 arb. Prior to ESI-HRMS data acquisition, a routine mass calibration was carried out with Pierce™ LTQ ESI Positive/Negative Ion Calibration Solution (88322/88324, Thermo Fisher Scientific) using the recommended manufacturer parameters (sheath gas = 12 arb, auxiliary gas = 0 arb, spray voltage = 4 kV, capillary temperature = 320 °C). Mass spectra were acquired in duplicate with two microscans lasting 1 min corresponding to 60–80 scans to reach 100% of the Automatic Gain Control (AGC) target. The maximum injection time was set to 10 ms. The pH values were measured on an Accumet AB250 m using a pH combination electrode (Fisher Scientific). 2.3. Data processing Mass spectra were processed using Thermo Xcalibur Qual Browser software after an automatic processing defining mass tolerance of 10 ppm and mass precision at four decimals. For each compound, the elemental composition was assigned and checked by the isotope simulation module of the software. In addition, the spectrum list was exported to be processed by Winnow, a program that scans the mass spectrum list for the presence of target elements or ions based on their isotope patterns (mass differences and intensity ratios between isotopologues [21]). 3. Results and discussion 3.1. Mass spectra, peak assignment and compound identification
2. Experimental section
An example of the mass spectrum of solution B at 29 h is given in Fig. 1. In summary, 16 compounds containing Mo, Cl and S were detected by the Winnow program (Table 1). These compounds were assigned by (i) the difference between the measured and theoretical masses of the composition (ΔM less than 10 ppm, Table 1) and (ii) their specific isotopic patterns. Note that the isotopic pattern of a heteroatomic compound is a combination of individual isotopic patterns of the elements composing the compound. Lists of peaks (i.e., isotopologues) detected for HMoOS5 and HMoOClS2 are given as examples in Fig. 1. For both compounds, O isotopes were not all detected due to the low abundance of 17O and 18O. Their isotopic patterns are then a combination of Mo, S and Cl isotopes. For HMoOS5 (named γ5), only 13/14 isotopologues were detected (Fig. 1), probably due to the non-resolved 92 Mo34S32S4 and 94Mo32S5 (respective abundance of 13.3 and 37.5% of the most abundant 98Mo32S5); their respective masses are of 270.7657 and 270.7682. Concerning HMoOClS2, when both Cl and S atoms are present, only peaks containing heavy Cl (37Cl) are found but not the heavy S (34S), except for the most abundant 98Mo peak (Fig. 1). In fact, the respective differences between these heavy isotopes and the most abundant 35Cl and 32S are only 1.9970 (37Cl–35Cl) and 1.9958 (34S–32S), making the peaks containing 37Cl and 34S not well-resolved. Also, relative to the most abundant 35Cl and 32S, 37Cl is more abundant than 34S (24% and 4%, respectively). Therefore, the isotopologue containing
2.1. Chemicals and sample preparation A stock solution of molybdate (10 mM) was prepared in a Teflon bottle by dissolving reagent ( (NH4)6Mo7O24·4H2O, (99% min, Acros organic) in 18.2 MΩ water. A 50 mM sulfide solution (Na2S·9H2O, 98% +, Acros organic) was prepared in a Schott glass bottle in an N2-filled glovebox. Phosphate buffer stock solutions (Na2HPO4, NaH2PO4, Baker) were prepared at 0.2 M and readjusted for pH 7 with HNO3 (double distilled trace-metal grade, BHD Aristar plus/DuoPUR Milestone) and NH4OH (Fisher). Molybdate solutions for use in speciation measurements, at a final concentration of 20 μM, were mixed with sulfide (0.2 mM; S/Mo ratio of 10, similar to previous studies: [11,13]). Phosphate buffer (final concentration of 2 mM) was added to prevent pH drift, as observed by Lohmayer et al. [11]. All solutions were prepared in a glovebox filled with N2 and in 100 mL Schott bottles with screw caps having a silicone O-ring and a butyl rubber stopper. In N2-purged 18.2 MΩ water containing molybdate, sulfide and phosphate buffer, three configurations were set up containing: no ionic buffer (solution A), 1 mM NaCl (solution B), or 1 mM NaNO3 (solution C). Time zero was defined as the moment when sulfide was added in solutions as the last reactant. The bottles were then withdrawn from the glovebox, with caps firmly 2
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Fig. 1. Mass spectrum of the solution B collected at 29 h (upper panel) with zooms on isolated peaks of molybdenum compounds (middle and lower panels). List of isotopologues detected in β2 (HMoOClS2) and γ5 (HMOOS5) are given in framed legend. 37
Cl is easier to detect than that with 34S. Although the full isotope pattern is not detected for these compounds, the high accuracy in the molecular masses and the simultaneous detection of 37Cl and 34S peaks with the most abundant 98Mo (Fig. 1) are best support for tentative formula assignment. These 16 compounds are categorized in three main groups (α, β and
γ), and named as αi, βi, γi with i being the number of S atoms. The α group includes the molybdate, conventional thiomolybdate species (MoO4-xSx2-, x = 1 to 4) and two additional MoS52− and MoS62− species showing a line of sequential S addition reactions (up to 6 S: α1 to α6) on the molybdate anion (α0). Similarly, the β group encompasses a series of S addition reactions (up to 3 S: β1 to β3) on MoO3Cl (β0)
Table 1 Summary of Mo species detected in this study with corresponding theoretical and measured masses and the difference between them ( M (ppm) = (Mmeasured/ Mtheoretical 1) × 106 ). n = number of observations. n.d. stands for not detected. The note * indicates compounds that were protonated during ESI process, see text for more details. Compound
α0 α1 α2 α3 α4 α5 α6 β0 β1 β2 β3 γ2 γ3 γ4 γ5 γ6
Formula
HMoO4 HMoO3S HMoO2S2 HMoOS3 HMoS4 HMoS5 HMoS6 MoO3Cl MoO2ClS MoOClS2 MoClS3 HMoO3S2 HMoO2S3 HMoOS4 HMoOS5 HMoOS6
Measured mass
162.894 178.871 194.848 210.825 226.802 258.774 290.746 180.860 196.837 212.814 228.791 210.843 226.820 242.797 274.769 306.741
Error (2 sd)
0.001 0.002 0.001 0.001 0.002 0.002 0.001 0.002 0.001 0.002 0.001 0.001 0.001 0.002 0.002
n
52 52 52 49 33 22 2 12 46 50 5 9 44 46 49 28
Theoretical monoisotopic exact mass
162.8923 178.8695 194.8467 210.8238 226.801 258.773 290.7451 180.8585 196.8356 212.8128 228.7899 210.8416 226.8187 242.7959 274.768 306.74
ΔM (ppm)
10.4 8.4 6.7 5.7 4.4 3.9 3.1 8.3 7.1 5.6 4.8 6.6 5.7 4.5 3.6 3.2
3
Time of first appearance (hours)
Note
Solution A
Solution B
Solution C
Solution D
0.08 0.08 0.08 4 6 n.d.
0.08 0.08 0.08 1 2 n.d.
0.08 0.08 0.08 0.08 0.5 45
0.08 0.08 1 29 n.d. n.d.
* * * * * * *
0.5 0.08 n.d. 8 0.08 0.08 0.08 0.5
0.08 0.08 20 29 0.08 0.08 0.08 0.1
0.08 0.08 n.d. 2 0.08 0.08 0.08 0.08
0.08 0.5 n.d. n.d. 0.08 1 0.08 n.d.
* * * * *
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Fig. 2. Reaction scheme explaining the reactivity between molybdate ion, chloride and sulfide summarized in three main lines α (blue zone), β, (green zone) and γ (red zone). The area delimited by full black line shows four conventional thiomolybdate species (MoO4-xSx2-, x = 1 to 4 [7,9,11]) while zone defined by dashed line shows possible reactions explaining the Mo deficit as MoOS32− loss as suggested by Vorlicek et al. [9]. The * indicate doubly charged compounds detected with an additional proton caught during electrospray, see text for more details. The framed legend represents the scheme with αi, βi, γi notations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
MoOS32− and sulfide/polysulfide; compounds γ4 and γ5 have been proposed in a reaction scheme to explain the rate of MoOS32− loss [9]. In addition, our mass spectra revealed additional compounds (γ2, γ3, γ6), potentially homologues to the reactions proposed by Vorlicek et al. [9]. However, it was previously assumed that sulfide (ΣHS−) alone could result only in a series of Mo(VI) thioanions but not Mo reduction [9]. However, in the experiment conducted by Vorlicek et al., where 3 mM ΣHS−, 70 mM NaCl, 1 mM borate buffer and 350 μM MoOS32− (Run N in Ref. [9]) were mixed at pH 8.5, ΣSo was calculated to be 0.636 mM and Mo deficit was approximately 30–50 μM (8–14% of total Mo). A similar phenomenon could happen as well in our solutions, explaining the occurrence of the γ series compounds (see further discussion below about the interaction between sulfide and nitrate). Also, compound γ′5 (similar to α′6) has been suggested to be a species produced by the molecular arrangement of γ5 prior to MoVI-to-MoIV reduction [9]. However, as γ5 and γ′5 are isomers (similar to α6 and α′6), they cannot be differentiated by mass spectrometry, but the S addition
which has been previously identified as a product of the reaction between molybdate and chloride [20]. The γ group comprises five nonconventional thio-(oxo)molybdate compounds; two of which have been suggested as intermediates during ligand-induced Mo reduction [9]. Also, it has to be noted that during solution-to-gas transition (electrospray), double charged species tend to catch a proton from the media to enhance their stability in the gas phase by decreasing their charge density; further details are discussed in Refs. [20,22]. These compounds are then detected with an additional H compared to the original aqueous species (noted by * in Table 1). A reaction scheme can be proposed then, based on the three main reaction lines (α, β and γ, Fig. 2). We propose that two additional compounds (α5 and α6) can complete the conventional reactions among the MoO4-xSx2- species, by S addition on the Mo]S double bonds, such reaction and functional group have been proposed by Coucouvanis [12]. Also, the γ compounds are the best support for the hypothesis of Vorlicek et al. [9] to explain the Mo deficit following the reaction of 4
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Fig. 3. Optimization of ESI-HRMS parameters for Mo speciation varying spray voltage, sheath gas pressure and probe temperature (a). The red zone shows the optimized value for highest intensity of Mo compounds. The ratio of α2/α0 was plotted against α1/α0 during optimization operations showing a linear variation in the intensity of these compounds. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reaction on γ5 to create γ6 would require a molecular rearrangement and probably a reduction MoVI to MoIV, which could be detected by HRMS (see Fig. 2). 3.2. Optimization of ESI parameters A previous investigation [20] has discussed the necessity of ESI parameter optimization to minimize differences in ionization efficiency of different compounds during the solution-to-gas transition. Flow rate and auxiliary gas were kept fixed at the previously optimized values (15 μL min−1 and 0 arb, respectively). Optimization of other parameters (spray voltage, sheath gas, probe temperature in order of optimization procedure) was conducted on a solution containing molybdate (20 μM) and sulfide (0.2 mM) after 2 h of reaction. Variations in the intensity of compounds α0, α1, α2, γ2 (the most abundant at the sampling time) as a function of spray voltage (1.5–4.5 kV), sheath gas (2–22 arb) and probe temperature (180–380 °C) are shown in Fig. 3a. The values where highest intensities of these compounds were recorded were selected as optima. The differences in ionization efficiency of these molybdate and thiomolybdate compounds apparently are not significant, as their intensities seemed to vary linearly with optimizing parameters (Fig. 3b). Note that this was not the case for HMoO4 (α0) and MoO3Cl (β0); their ionization efficiencies varied significantly during parameter optimization [20]. The difference in ionization efficiency of these two compounds was minimized only when spray voltage was above 4 kV, and sheath gas above 16 arb (probe temperature did not induce significant variations). This could be a consequence of the difference in charge density of MoO42− and MoO3Cl− in solution; when MoO42− passes into the gas phase, this compound is forced to take a H from the media. Similar to the molybdate ion, this additional step is observed for other doubly charged thiomolybdate species as well. For this reason, it is highly possible that other doubly charged thiomolybdates that we detected in this study (other than α0, α1, α2, γ2 observed during this optimization) would behave similarly. Also, when the parameters are optimized for doubly charged species, there is a difference (requiring a factor of correction) between the intensity of doubly charged and singly charged species (compounds with and without a * noted in Table 1, respectively), The intensities recorded for singly charged species (e.g., MoO3Cl (β0)) would be underestimated using the optimized parameters of this investigation. However, when all the samples were analyzed under fixed analytical conditions, it is a reasonable assumption that the same correction factor would exist to correct such differences in ionization efficiency. 3.3. Early stage of the molybdate-sulfide reaction The kinetic study (over a period of 3 days) was carried out on three solutions (final pH = 7.3 ± 0.1) containing either Cl− (Solution B), NO3−(Solution C), or none of them (Solution A). Temporal variations in the percentages of molybdate (α0) and the four conventional thiomolybdates (α1 to α4) are shown in Fig. 4. Proportions of the 14 compounds identified in the solutions are shown in Fig. S1, while the ratios within couples of compounds (i.e., product/reactant of each S-addition reaction) are given in Fig. 5. In general, results similar to those of previous studies [7,11] were observed: proportion of α0 dropped within a few minutes after adding sulfide leading to a production of α1, which reached its highest 5
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proportion after 1 h (Fig. 4a and b). Also, the percentage of α0 (MoO42−) was almost constant after 1h (14 ± 3, 8 ± 2 and 9 ± 2% in solutions A, B and C, respectively, Fig. 4a). A similar finding previously had been observed by Lohmayer et al. [11], whereas no MoO42− was detected by Helz and Erickson [7]. The proportions of α2, α3 and α4 kept increasing throughout the course of the experiment (Fig. 4 c,d,e). However, it appears that α3 (MoOS32−) was created much earlier (5 min, Fig. 4c) than in previous work (after hours, Erickson and Helz, (2000)), as was the case for γ3 (MoO2S32−) (Fig. 5c, f). Whereas the proportion of α2 kept increasing with time, the α3/α2 and γ3/α2 ratios showed maxima at 5 min then continuously decreased over the course of the experiments (Fig. 5b and c). As α3 and γ3 are both products of the S addition on α2, it is possible that the reactions creating α3 and γ3 occur faster than previously predicted, on a timeframe of minutes, leading to a slower appeance of α2. Once α3 was present in solution, S addition to α3 would produce α4 and γ4 (after an hour, Figs. 4e, 5d and 5g) which then slows production of α3 (Fig. 4d). A minor production of α5, γ5 and γ6 was detected (less than 10%, Fig. S1, 5e-i). Generally, once α3 was created in solution after 5 min, the creation of the γ series compounds seemed to be faster than that of the α compounds; the addition of a S to open the Mo]S double bond would be more favored than the S substitution on Mo–O- or Mo]O bonding. This observation is in agreement with the experiment conducted by Vorlicek et al. [9] where the ΣModeficit (assumed to be γ4 and/or δ5) was created before the creation of MoS42− (α4); the effect seemed to be more noticeable when polysulfide was added (Run O in Vorlicek et al., [9]. Erickson and Helz [7] have demonstrated that the reaction between sulfide and molybdate is a first order process, while the reaction between molybdate and sulfide/polysulfide seemed to be more complex than a single-step reaction, occurring mostly during the first 24h, [9]. The observations of our study support the quick creation of the γ line during the first 24 h of reaction, before the conventional α line can stabilize. 3.4. Effect of solution composition Previous studies have Mo-sulfide/polysulfide experiments conducted in different conditions, but most of them have been performed either without ionic strength control or with high NaCl concentrations (Table 2). In our study, we have conducted the experiments in three configurations: without ionic strength control (solution A), 1 mM NaCl (Solution B) and 1 mM NaNO3 (Solution C). Even though the kinetics of successive reactions between Mo and sulfur seems to be similar in these three solutions (Figs. 4 and 5), intensity of the production of thiomolybdates seemed to be different: Solutions C > B > A (i.e., nitrate > chloride > no ionic buffer). It has been shown by Luther III [23] that a two-electron transfer from any NOx compound (nitrate included) to sulfide is thermodynamically favored at any pH. This observation may explain the main difference between this study and previous ones concerning the formation of the line γ when we used only sulfide. Sulfide would create only the four conventional thiomolybdate coumpounds [7,9]. However, it is possible that nitrate in our solutions may have allowed a two-electron transfer to oxidize sulfide to elemental sulfur, which led to the formation of polysulfide. This hypothesis is supported by the comparison of the formation of γ compounds in solutions A, B and C; in solutions A and B (Figs. S1a–3, S1b-3) compounds γ4-6 increased with time and reach the maximum at ca. 10% only after 24 h, whereas in solution C, the percentage of these compounds γ were stable along the course of the experiment (Figs. S1c–3). Nitrate in solutions A, B would be at trace level, used as a pH adjustor (HCl was avoided due to the formation of MoO3Cl−, β0). In solution C with an excess of nitrate (N/S ratio = 5), the sulfide oxidation would be faster than in solution A and B. In solution B (1 mM NaCl), the β compounds were observed with up to 3 S and only two S substituted on Mo]O structure (β1 and β2). For
Fig. 4. Sequential creation of four conventional thiomolybdate species (α1 to α4) from molybdate (α0) in three solutions without ionic buffer (A: circle symbols) or with chloride (B: square symbols) and nitrate (C: diamond symbols) as ionic strength control. 6
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Fig. 5. Kinetics of the Mo-sulfide reactivity considering couples of compounds (product/reactant) in three solutions without (A: circle symbols) or with Cl (B: square symbols) and NO3− (C: diamond symbols) as ionic strength control.
7
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[11]
[13] this study
This study reports on the identification of thiomolybdate and thiochloro-molybdate species, and the sequential creation of these species in a short-time kinetic experiment. The results support the existence of intermediate thiomolybdate species prior to the reduction of Mo(VI) to Mo(IV) induced by S ligands. We propose a reaction scheme with three major lines α, β and γ corresponding to different Mo reactivities. The output of this study indicates that future experiments conducted on Mosulfide reactions should pay special attention to ionic strength control and pH adjustment. While adding Cl will create the β line, excess of nitrate will foster the oxidation of sulfide to elemental sulfur and ultimately polysulfide, which will favor the creation of the γ line. The occurrence of the β line has important implications in estuarine and marine environments where Mo–Cl reactivity has not been considered in Mo speciation models. Future applications of this ESI-HRMS method on natural waters, combined with other analytical tools, would contribute to our understanding of the speciation of Mo in sulfidic waters.
None 2 mM phosphate None None, 1 mM NaCl, 1 mM NaNO3
NaOH, HCl HNO3, NH4OH
IPC-ICP-MS or HPLCUV RP-IPC-UV ESI-HRMS None None
0.07 M NaCl
10 mM borate
NaOH, HCl/HNO3
UV–Vis
[9]
[7]
No Mo deficit/reduction 1st order reactions NH4+ catalytic effect Mo deficit 1st order for long-term but not during the first 24h. pH drift in the course of the experiment Highest sulfidation of molybdate at pH 7 No pH drift UV–Vis 0.7 M NaCl
None
HCl, NaOH, NH4Cl
Reference Analytical technique
Acknowledgements This work was supported by a Canadian NSERC (Natural Sciences and Engineering Research Council) Collaborative Research and Development Grant to R.D.E. The authors wish to thank Dr. Naomi Stock for helpful discussions, Michael Doran for making the Winnow program publicly available and Dr. Hayla Evans for manuscript revision. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.talanta.2019.120585.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
9 days 3 days
60 days
sulfide sulfide 0.45 mM molybdate 0.02 mM molybdate
7.5 7.3
Sulfide, polysulfide 0.2 mM Molybdate
5–11
40 days Sulfide, polysulfide 0.04–0.35 mM MoOS32−, MoOS42-
8.2, 8.5
60 days sulfide 1 mM Molybdate
7.9–8.5
Ionic strength control Experiment duration
4. Conclusion
References
Sulfur species
pH
now, it is difficult to compare the absolute percentage between this β series and the rest of the compounds, as their intensities are underestimated using the current ESI parameters. However, they constituted a non-negligible fraction of Mo speciation ( greater than 10%, Figs. S1b–4). These conditions would be reflective of marine environments where interaction between molybdate, chloride and sulfide has not been considered in molybdenum (VI) speciation yet [7].
Declaration of interests
Molybdenum species
Table 2 Summary of the conditions of previous experiments conducted on molybdate-sulfide reactions.
pH buffer
pH readjustment
Observation about the Mo–S experiments
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[1] F. Bittner, R.R. Mendel, Cell biology of molybdenum, Biochim. Biophys. Acta 1763 (2006) 621–635, https://doi.org/10.1007/978-3-642-10613-2_6. [2] C. Scott, T.W. Lyons, A. Bekker, Y. Shen, S.W. Poulton, X. Chu, A.D. Anbar, Tracing the stepwise oxygenation of the Proterozoic ocean, Nature 452 (2008) 456–459, https://doi.org/10.1038/nature06811. [3] K.W. Bruland, M.C. Lohan, Controls of trace metals in seawater, in: K.K. Turekian, H.D. Holland (Eds.), Treatise on Geochemistry, Elsevier Ltd, 2003, pp. 23–47. [4] J. Gaillardet, J. Viers, B. Dupré, Trace elements in river waters, in: K.K. Turekian, H.D. Holland (Eds.), Treatise on Geochemistry, Elsevier Ltd, 2003, pp. 225–272. [5] Y. Sohrin, K.W. Bruland, Global status of trace elements in the ocean, Trends Anal. Chem. 30 (2011) 1291–1307, https://doi.org/10.1016/j.trac.2011.03.006. [6] K.K. Bertine, K.K. Turekian, Molybdenum in marine deposits, Geochem. Cosmochim. Acta 37 (1973) 1415–1434, https://doi.org/10.1016/0016-7037(73) 90080-X. [7] B.E. Erickson, G.R. Helz, Molybdenum(VI) speciation in sulfidic waters: stability and lability of thiomolybdates, Geochem. Cosmochim. Acta 64 (2000) 1149–1158, https://doi.org/10.1016/S0016-7037(99)00423-8. [8] N. Tribovillard, T.J. Algeo, T. Lyons, A. Riboulleau, Trace metals as paleoredox and paleoproductivity proxies: an update, Chem. Geol. 232 (2006) 12–32, https://doi. org/10.1016/j.chemgeo.2006.02.012. [9] T.P. Vorlicek, M.D. Kahn, Y. Kasuya, G.R. Helz, Capture of molybdenum in pyriteforming sediments: role of ligand-induced reduction by polysulfides, Geochem.
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[17] J. Le Quan Tuoi, E. Muller, Electrospray mass-spectrometry on A magnetic-sector instrument applied to heteropolyacids, Rapid Commun. Mass Spectrom. 8 (1994) 692–694, https://doi.org/10.1002/rcm.1290080905. [18] D.K. Walanda, R.C. Burns, G.A. Lawrance, E.I. von Nagy-Felsobuki, Electrospray mass spectral study of isopolyoxomolybdates, J. Chem. Soc. Trans. (1999) 311–321. [19] J.P. Gustafsson, Modelling molybdate and tungstate adsorption to ferrihydrite, Chem. Geol. 200 (2003) 105–115, https://doi.org/10.1016/S0009-2541(03) 00161-X. [20] D.H. Dang, D. Evans, Application of ESI-HRMS for molybdenum speciation in natural waters: investigation of molybdate-halide reactions, Talanta 179 (2018) 221–229. [21] M.C. Doran, K.L. LeBlanc, A computer program to simplify analysis of mass scan data of organometallic compounds from high-resolution mass spectrometers, Rapid Commun. Mass Spectrom. 30 (2016) 2561–2567, https://doi.org/10.1002/rcm. 7748. [22] M.J. Deery, O.W. Howarth, K.R. Jennings, Application of electrospray ionisation mass spectrometry to the study of dilute aqueous oligomeric anions and their reactions, J. Chem. Soc. Dalton Trans. (1997) 4783–4788. [23] G.W. Luther, The role of one- and two-electron transfer reactions in forming thermodynamically unstable intermediates as barriers in multi-electron redox reactions, Aquat. Geochem. 16 (2010) 395–420, https://doi.org/10.1007/s10498-0099082-3.
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