Abstract
Thiocyanate may have played as important a role as cyanide in the synthesis of several molecules. However, its concentration in the seas of the prebiotic Earth could have been very low. Thiocyanate was dissolved in two different seawaters: a) a composition that comes close to the seawater of the prebiotic Earth (seawater-B, Ca2+ and Cl−) and b) a seawater (seawater-A, Mg2+ and SO42−) that could be related to the seas of Mars and other moons in the solar system. In addition, forsterite-91 was a very common mineral on the prebiotic Earth and Mars. Two important results are reported in this work: 1) thiocyanate adsorbed onto forsterite-91 and 2) the amount of thiocyanate adsorbed, adsorption thermodynamic, and adsorption kinetic depend on the composition of the artificial seawater. For all experiments, the adsorption was thermodynamically favorable (ΔG < 0). The adsorption data fitted well in the Freundlich and Langmuir–Freundlich models. When dissolving thiocyanate in seawater 4.0-A-Gy and seawater 4.0-B-Gy, the adsorption of thiocyanate onto forsterite-91 was ruled by enthalpy and entropy, respectively. As shown by n values, the thiocyanate/foraterite-91 system is heterogeneous. For all kinetic data, the pseudo-first-order model presented the best fit. The constant rate for thiocyanate dissolved in seawater 4.0-A-Gy was twice that compared to thiocyanate dissolved in seawater 4.0-B-Gy or ultrapure-water. The interaction between thiocyanate and Fe2+ of forsterite-91 was with the nitrogen atom of thiocyanate. In the presence of thiocyanate, sulfate interacts with forsterite-91 as an inner-sphere surface complex, and without thiocyanate as an outer-sphere surface complex.
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Introduction
Since the biomolecules and precursors of molecules in prebiotic seas were at very low concentrations (Dowler and Ingmanson 1979; Stribling and Miller 1987; Zaia et al. 2008), the most important step in the molecular evolution was the their preconcentration, because without this step the formation of polymers or new molecules would not have occurred (Bernal 1951). The preconcentration of biomolecules or precursors of biomolecules could occur in the following ways: sorption, wetting/drying cycles, freezing/sublimation, and sorption/precipitation with minerals. However, in prebiotic chemistry experiments, sorption onto minerals is the most common method used for the pre-concentration of molecules or precursors of molecules (Lahav and Chang 1976; Lambert 2008; Zaia 2004, 2012).
Because of its olive-green color, in 1790 G. Werner gave olivine its name. Indeed, olivine is a group of minerals whose chemical composition is Fe1−xMgxSiO4. The Mg-rich olivine and Fe-rich olivine are named forsterite and fayalite, respectively. The name forsterite was suggested by A. Levy in 1824 to honor J.R. Forster (1739–1806), an English mineral collector and dealer. In 1840, F. Gemelin named the Fe-rich olivine as fayalite because of the Faial Island in the Azores. These names are related to the mole percentage of Fe and Mg in olivine; thus forsterite-91 is the same as fayalite-9 (King 2009).
Olivine was a widespread mineral on the prebiotic Earth, and it is also found on Mars, the Moon, and in meteorites and comets (Brownlee et al. 2006; Hazen et al. 2008; Mølholt et al. 2008; Gunnlaugsson et al. 2009; Clark et al. 2014; Zandanel et al. 2021). In hydrothermal vents, Fe2+ in olivine reduces H2O to produce Fe3+, H2, and hydrocarbons. Thus, olivine played an important role in this geochemical process, known as serpentinization (Hellevang 2008; Martin et al. 2008; Hellevang et al. 2011; Neubeck et al. 2011). Olivine has been also used for amino acid adsorption (Cruz-Hernández et al. 2022), molecule polymerization (Fuchida et al. 2017; Fox et al. 2019; Šponer et al. 2021), protection/synthesis of molecules due to: UV irradiation (dos Santos et al. 2016), γ-irradiation (Colín-García et al. 2010), meteorite impacts (Umeda et al. 2016), and hydrothermal vents (González-López et al. 2021). Furthermore, olivine is used in some industrial processes, such as: adsorption of pollutants (Westholm et al. 2014), as a catalyst for gas production (Nelson et al. 2018; Kuba et al. 2021), and in biomass tar removal (Meng et al. 2022).
Thiocyanate has been found in hydrothermal vents (Dowler and Ingmanson 1979) and interstellar medium (Halfen et al. 2009). In addition, studies simulating prebiotic environments have shown the synthesis of thiocyanate (Bartlett and Davis 1958; Mukhin 1974; Raulin and Toupance 1977). Thiocyanate has been used for the synthesis of amino acids (Steinman et al. 1968; Perezgasga et al. 2003), high energy phosphates (Keefe and Miller 1996), tertiary amines (Wagner and Ofial 2015; Kouznetsov and Galvis 2018), and guanidine (Zaia et al. 2004). Since thiocyanate is a pollutant, several studies in the literature describe its adsorption onto materials, with the aim of removing it from water bodies (Li et al. 2006, 2008; Aguirre et al. 2010; Azizitorghabeh et al. 2021). However, these experiments cannot be used to infer what could have happened on the prebiotic Earth.
In the current work, thiocyanate was dissolved in two different seawaters. The first artificial seawater, denominated artificial seawater 4.0-A Gy, was suggested by Zaia (2004), based on the work of Izawa et al. (2010). This seawater has high Mg2+ and SO42− concentrations. The other seawater, denominated artificial seawater 4.0-B Gy, has high Ca2+ and Cl− concentrations. This seawater was suggested by Samulewski et al. (2021) based on the work of Halevy and Bachan (2017). As explained in the Relevance to Prebiotic Chemistry section, these seawaters could be used as models for the prebiotic Earth, Mars, and moons of the Solar System.
Because thiocyanate and olivine (forsterite-91) were commons materials on the prebiotic Earth and the artificial seawater composition used resembles the prebiotic seas, the experiments presented in this work were performed under conditions that could be found on the prebiotic Earth.
In the present work, the thermodynamics and kinetics of adsorption of thiocyanate onto forsterite-91 were studied. Forsterite-91 was characterized using X-ray diffractometry and Raman and infrared spectroscopy. In addition, the interaction between thiocyanate and forsterite-91 was studied using several spectroscopic techniques, such as: Raman, infrared, and Mössbauer.
Materials and Methods
Materials
All reagents used in the experiments were analytical grade PA. Ultrapure water was obtained from Simplicity Merck with 18.0 \(\mathrm{M\Omega cm}\) at 25 °C.
Forsterite-91
The natural olivine sand (forsterite-91) was a gift from Dr. Anna Neubeck, Department of Geological Sciences, Uppsala University, 752 36 Uppsala, Sweden. The original sample was gently ground in an agate mortar, and all materials passing through a 53 mesh sieve were saved in plastic vials for further analyses.
Artificial Seawater
Artificial seawater 4.0-A Gy (Gy = billion years ago) was prepared as described by Zaia (2012). The following salts were dissolved in 1.0 L of ultrapure water: \({\mathrm{Na}}_{2}{\mathrm{SO}}_{4}\) (0.271 g), \({\mathrm{MgCl}}_{2}.6{\mathrm{H}}_{2}\mathrm{O}\) (0.500 g), \({\mathrm{CaCl}}_{2}.2{\mathrm{H}}_{2}\mathrm{O}\) (2.50 g), \(\mathrm{KBr}\) (0.050 g), \({\mathrm{K}}_{2}{\mathrm{SO}}_{4}\) (0.400 g), and \({\mathrm{MgSO}}_{4}\) (15.00 g).
Artificial seawater 4.0-B Gy (Gy = billion years ago) was prepared as described by Samulewski et al. (2021). The following salts were dissolved in 1.0 L of ultrapure water, \({\mathrm{MgCl}}_{2}\) (0.950 g), \({\mathrm{CaCl}}_{2}.2{\mathrm{H}}_{2}\mathrm{O}\) (29.400 g), \(\mathrm{KCl}\) (1.490 g), and \(\mathrm{NaCl}\) (1.170 g).
Methods
Thiocyanate Quantification
Thiocyanate quantification was performed using the method described by Martins et al. (2005) with modifications. To 15 mL test tubes, 0.5 mL of thiocyanate solution, 1.0 mL of FeCl3.6H2O solution (1000 mg L−1) solubilized in 10% nitric acid, and 8.5 mL of ultrapure water were added. After 5 min, using a Thermo Spectronic Genesys spectrophotometer, the absorbance at 460 nm was read against the blank. Equations 1 and 2 were used for the calculation of the amount of thiocyanate adsorbed onto forsterite-91.
Effect of pH on the Adsorption of Thiocyanate onto Forsterite-91
Thiocyanate (760 mg L−1) was dissolved in ultrapure water, artificial seawater 4.0-A Gy, and artificial seawater 4.0-B Gy. Subsequently, approximately 70 mg of forsterite-91 and 10 mL of thiocyanate solutions were added to a conical tube (15 mL). The pH of the solutions was adjusted with sodium hydroxide (0.10 mol L−1) or hydrochloric acid (0.10 mol L−1) to 4.0, 5.0, 6.0, 7.0, 8.0, and 9.0. The suspensions were stirred for 24 h at 25 °C. The samples were centrifuged at 9,000 rpm for 10 min. The supernatant was used for thiocyanate determination.
Adsorption Isotherm
Forsterite-91 (10 mg) and 1.5 mL of the following thiocyanate solutions dissolved in ultrapure water or seawater 4.0-A Gy were added to Eppendorf tubes (2 mL): 2.0, 5.0, 10, 25, 50, 75 100, 150, 200, 250, and 300 mg L−1. For artificial seawater-B 4.0 Gy the following concentrations were used: 8, 20, 40, 100, 200, 300, 400, 600, 800, 1000, 1400, and 1600 mg L−1. Each isotherm was performed in quadruplicate. The pH of the solutions was adjusted to 7.00 with sodium hydroxide or hydrochloric acid 0.10 mol L−1. The suspensions were stirred for 24 h at 25, 30, 35, 40, and 45 °C. The samples were centrifuged at 9,000 rpm for 10 min. The supernatant was used for thiocyanate determination. The mineral was lyophilized and characterized.
In the current work, the results of thiocyanate adsorption onto forsterite-91 were fitted to Langmuir, Freundlich, and Langmuir–Freundlich non-linear isotherm models to verify which model presented the best adjustment.
The non-linear expression of the Langmuir isotherm model (Eq. 3) (Limousin et al. 2007; Foo and Hameed 2010):
where C (mg L−1) is the concentration of thiocyanate in solution after the equilibrium, Ө (mg g−1) is the concentration of thiocyanate adsorbed onto forsterite-91 (difference between initial thiocyanate concentration and the concentration after the equilibrium), b (mg g−1) is the theoretical limit of adsorbed thiocyanate onto forsterite-91, and keq (L mg−1) is the equilibrium constant (adsorbate-adsorbent).
The non-linear expression of the Freundlich isotherm model (Eq. 4) (Limousin et al. 2007; Foo and Hameed 2010):
where C (mg L−1) is the concentration of thiocyanate in solution after the equilibrium, Ө (mg g−1) is the concentration of thiocyanate adsorbed onto forsterite-91 (difference between initial thiocyanate concentration and the concentration after the equilibrium), and Kf and n are empirical constants.
The non-linear expression of the Langmuir–Freundlich isotherm model (Eq. 5) (Limousin et al. 2007; Foo and Hameed 2010):
where C (mg L−1) is the concentration of thiocyanate in solution after the equilibrium, Ө (mg g−1) is the concentration of thiocyanate adsorbed onto forsterite-91 (difference between initial thiocyanate concentration and the concentration after the equilibrium), b (mg g−1) is the theoretical limit of adsorbed thiocyanate onto forsterite-91, and K and n are empirical constants.
Kinetic Adsorption
Forsterite-91 (70 mg) plus 10 mL thiocyanate solutions (760 mg L−1) were added to a conical tube (15 mL). Thiocyanate was dissolved in ultrapure water, artificial seawater 4.0-A Gy, and artificial seawater 4.0-B Gy. The pH of the samples was adjusted to 7.0, using hydrochloric acid (0.1 mol L−1) or sodium hydroxide (0.1 mol L−1). The suspensions were stirred for 0.02, 0.17, 0.50, 1.00, 2.50, 5.00, 8.00, 12.0, 15.0, 20.0, 24.0, 48.0, 72.0, and 96.0 h, at 298 K. Next, the tubes were centrifuged at 6,000 rpm, and the supernatant was separated and the thiocyanate quantified as described by Martins et al. (2005). All experiments were performed in quadruplicate.
In the current work, the results of thiocyanate adsorption onto forsterite-91 were fitted to a non-linear pseudo-first-order model, non-linear pseudo-second-order model, non-linear Elovich model, and non-linear intra-particle diffusion model.
The non-linear expression of the pseudo-first-order model (Eq. 6) (Ho et al. 2000):
where k1 (min−1) is the pseudo-first-order constant, qe is the amount of thiocyanate adsorbed (mg g−1) at the equilibrium concentration, and qt is the amount adsorbed (mg g−1) at time t.
The non-linear expression of the pseudo-second-order model (Eq. 7) (Ho et al. 2000):
where k2 (g mg−1 min−1) is the pseudo-second-order constant, qe is the amount of thiocyanate adsorbed (mg g−1) at the equilibrium concentration, and qt is the amount adsorbed (mg g−1) at time t.
The non-linear expression of the Elovich kinetic model (Eq. 8) (Mezenner and Bensmaili 2009):
where qt is the sorption capacity at a time t (mg g−1), α is the initial sorption rate (mg g−1 min−1), and β is the desorption constant (g mg−1).
The non-linear intra-particle diffusion model (Eq. 9) (Mezenner and Bensmaili 2009):
where qt is the amount adsorbed (mg g−1) at time t, kd is the intra-particle diffusion rate constant (mg−1 min−1/2), and C is the intercept which is related to the magnitude of the diffusion resistance layer.
X-ray Diffractometry
The crystallographic characterization was performed by X-ray powder diffraction (XRD) in an X'Pert PRO MPD diffractometer from PANalytical, with CuKα radiation (λ = 1.540 Å) in Bragg–Brentano geometry from 5 to 90° (2θ). Rietveld refinement (PANalytical X'Pert HighScore Plus software version 2.2d) was used to quantify the crystalline phases and structural parameters (lattice parameters, unit cell volume, and preferred orientation), using a Pseudo Voigt as a peak form function. The crystallographic information cards used for the Rietveld refinement were 40,726-ICSD for Forsterite (Orthorhombic 62), 100,245-ICSD for Clinochlore (Triclinic 2), and 5133-ICSD for Willemseite (Monoclinic 15). The Forsterite composition was supposed to be Mg1.838 Fe0.156 Ni0.006 (Si O4) by complementary XFR results.
Fourier Transform Infrared Spectroscopy-FTIR
Forsterite-91 samples were analyzed in ATR-FTIR. The spectra were obtained with a resolution of 4 cm−1 in the range of 4000–400 cm−1 in Bruker-Vertex 70 spectrometer, equipped with an ATR accessory with a Ge Crystal 45°.
Raman Spectroscopy
Forsterite-91 samples were analyzed with a micro-Raman WITec alpha 300 s spectrophotometer, using an excitation wavelength of 532 nm, aperture of 10 µm, and exposure time of 10 min.
Mössbauer Spectroscopy
57Fe Mössbauer transmission spectra were obtained at RT, in a spectrometer operating with a 57Co(Rh) source, moved with constant acceleration. The equipment was calibrated from the spectrum of an a-Fe thin foil, measured at RT. The fits were performed considering a Lorentzian line shape and applying the minimum chi-square method.
Statistical Analysis
Comparisons among means were assessed using the Tukey test at a significance level of p < 0.05.
Results
Characterization of Forsterite-91
The X-ray diffractograms show the observed (measured), calculated (refined), and difference between measured and calculated curves (Fig. 1A). The GOF (goodness of fit) quality factor was 9, revealing a satisfactory overlap. The forsterite-91 cell parameters were a = 10.21 Å, b = 5.98 Å, and c = 4.75 Å. The volume was 291Å3 and the second forsterite phase exhibited a preferred orientation in [020] direction.
A X-ray diffractogram and simulated phases from Rietveld analysis. The forsterite appears duplicated and one of them seems to be texturized in [020] direction. B FT-IR spectrum of forsterite-91 solid, and C Raman spectrum of forsterite-91 solid
The FT-IR spectrum of forsterite-91 solid could be divided into three regions: a) from 1200 cm−1 to 700 cm−1, b) from 700 cm−1 to 450 cm−1, and c) from 450 cm−1 to 400 cm−1. The first region presented five bands at 1000 cm−1 (shoulder), 980 cm−1, 950 cm−1 (shoulder), 885 cm−1, and 838 cm−1 (shoulder). The second region presented five bands at 668 cm−1, 608 cm−1, 520 cm−1 (shoulder), 504 cm−1, and 470 cm−1. The third region presented a band at 417 cm−1 (Fig. 1B). The Raman spectrum of forsterite-91 presented five bands at 964 cm−1, 920 cm−1, 856 cm−1, 822 cm−1, and 234 cm−1 (Fig. 1C).
Effect of pH
The adsorption of thiocyanate dissolved in ultra-pure water, artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) onto forsterite-91 increased when the pH increased. However, the adsorption of thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) onto forsterite-91 decreased when the pH was higher than 7.0 (Fig. 2). For pH higher than 4.0, thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) presented the same adsorption onto forsterite-91 (p > 0.05) (Fig. 2). In addition, thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) presented the highest and lowest adsorption of thiocyanate onto forsterite-91, respectively (Fig. 2).
Effect of the pH on the adsorption of thiocyanate (760 mg L−1) onto forsterite-91 (70 mg). For each pH, means with different capital letters were statistically different from each other by Tukey test (p < 0,05). Thiocyanate was dissolved in ultra-pure water, artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations). Each value is mean of four experiments. The samples were stirred for 24 h at 25 °C. Artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) were prepared as described by Zaia (2012) and Samulewski et al. (2021), respectively
Adsorption Isotherm
For thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), nonlinear isotherm fits showed good R2/RMSE parameters for the Freundlich and Langmuir–Freundlich models (Table 1). However, when thiocyanate was dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), the three models presented a good fit, with slightly better values for the Langmuir model (Table 1).
For thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) the qmax values obtained from the Langmuir model were higher than qmax values obtained for thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) (Table 1). The qmax values obtained from the Langmuir model indicated that for thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the adsorption process is exothermic and for thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) the process is endothermic (Table 1).
All the n values, with the exception of thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) using the Langmuir–Freundlich model, were higher than 1. In addition, n values increased with an increase in the temperature (Table 1).
An essential feature of the Langmuir isotherm can be expressed by the separation factor (RL), as shown in Eq. 10:
All the RL values were lower than 1, indicating a process favorable to adsorption (Table 2). However, when the thiocyanate concentration decreased, the RL values increased, which indicated that at the low thiocyanate concentration, adsorption became less favorable. In contrast, at a high initial thiocyanate concentration the RL values were closer to 0, indicating that adsorption became more favorable (Table 2).
For better understanding of the adsorption process, the thermodynamic parameter was obtained. Gibbs free energy (ΔG) can be calculated using Eq. 11:
where, kD is the thermodynamic equilibrium constant (L g−1), and kD can be obtained by plotting qe/Ce versus qe and extrapolating qe to zero (Fig. S1). For thiocyanate dissolved in artificial seawater 4.0-A at 318 K, it was not possible to obtain the Keq value (Table 3), because the data did not fit in the curve (Fig. S1).
The thermodynamic parameters enthalpy (ΔH) and entropy (ΔS) can be determined according to Eq. 12:
where, R is the universal gas constant (8.314 J mol−1 K−1) and T is the temperature (K).
For all temperatures, the ΔG values were negative, indicating that the process was thermodynamically favorable (Table 3). For thiocyanate dissolved in ultra-pure water, the adsorption process onto forsterite-91 was favorable from the point of view of enthalpy and entropy (Table 3). However, for thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and 4.0-B Gy (high Ca2+ and Cl− concentrations), the adsorption onto forsterite-91 was ruled by enthalpy and entropy, respectively (Table 3). It should be noted that ΔS values were much higher when dissolving thiocyanate in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) than when dissolving it in ultra-pure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) (Table 3).
Kinetic Experiments
Among the kinetic models tested, the pseudo-first-order presented the best fit for the data (Table 4, Fig. S2). For the sample of thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the value of constant rate was more than twice as high as the constant rates obtained from the thiocyanate samples dissolved in ultrapure water or artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) (Table 4). In addition, the constant rate for thiocyanate dissolved in ultrapure water was slightly higher than the constant rate for thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) (Table 4). The qmax values (Table 4) obtained from the pseudo-first-order model presented good agreement with those obtained from the Langmuir isotherm model (Table 1). For thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) its adsorption onto forsterite-91 was less than half that when dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) (Tables 1 and 4, Fig. 2).
Spectroscopy
Infrared Spectroscopy
There was no difference among the FT-IR spectra of solid forsterite-91 and forsterite-91 mixed with ultrapure water or artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) at several different pHs (Figs. 3A, C and S3). However, for experiments performed with forsterite-91 plus artificial seawater 4.0-A (high Mg2+ and SO42− concentrations) and forsterite-91 plus thiocyanate dissolved in artificial seawater 4.0-A (high Mg2+ and SO42− concentrations), at pH 7.00, due to the presence of sulfate in this seawater, the FT-IR spectra presented a band at 1085 cm−1 and two shoulders at 1085 cm−1/1126 cm−1, respectively (Fig. 3B). In experiments performed in other pHs, in general, the FT-IR spectra of the samples of forsterite-91 plus artificial seawater 4.0-A (high Mg2+ and SO42− concentrations) presented a band at 1085 cm−1, while the samples with thiocyanate dissolved in artificial seawater 4.0-A (high Mg2+ and SO42− concentrations) presented two bands, one in the region of 1080 cm−1 and another in the region 1120 cm−1 (Fig. S3).
FT-IR spectra: A thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus ultrapure water and lyophilized sample of forsterite-91 plus thiocyanate dissolved ultrapure water, B thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus artificial seawater 4.0-A Gy and lyophilized sample of forsterite-91 plus thiocyanate dissolved artificial seawater 4.0-A Gy, and C thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus artificial seawater 4.0-B Gy and lyophilized sample of forsterite-91 plus thiocyanate dissolved artificial seawater 4.0-B Gy. The samples were stirred for 24 h at pH 7.00 Artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) were prepared as described by Zaia (2012) and Samulewski et al. (2021), respectively
Solid thiocyanate presented bands at 470/480 cm−1, 744 cm−1, and 2040 cm−1 that can be assigned to δ(NCS) bending, ν(CS) stretching, and ν(CN) stretching, respectively (Figs. 3 and S3) (Jones 1956; Baranyi et al. 1976). For all samples studied in the present work, after the adsorption of thiocyanate onto forsterite-91 the band at 2040 cm−1 shifted to 2070 cm−1 (Figs. 3 and S3). After the thiocyanate adsorption onto forsterite-91, the bands at 470/480 cm−1 and 744 cm−1 were not observed, because of the strong bands of forsterite-91 in the same region (Figs. 3 and S3).
Raman Spectroscopy
The Raman spectrum of solid thiocyanate presented two bands, one at 750 cm−1 and another at 2054 cm−1 (Figs. 4 and S4), due to ν(CS) stretching, and ν(CN) stretching, respectively (Baranyi et al. 1976). However, for all samples, these bands vanished after thiocyanate adsorbed onto forsterite-91 (Figs. 4 and S4). In addition, the characteristic bands of sulfate were not observed in the forsterite-91 plus artificial seawater 4.0-A Gy and forsterite-91 plus artificial seawater 4.0-A Gy plus thiocyanate (Figs. 4B and S4B).
Raman spectra: A thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus ultrapure water and lyophilized sample of forsterite-91 plus thiocyanate dissolved ultrapure water, B thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus artificial seawater 4.0-A Gy and lyophilized sample of forsterite-91 plus thiocyanate dissolved artificial seawater 4.0-A Gy, and C thiocyanate solid, forsterite-91 solid, lyophilized sample of forsterite-91 plus artificial seawater 4.0-B Gy and lyophilized sample of forsterite-91 plus thiocyanate dissolved artificial seawater 4.0-B Gy. The samples were stirred for 24 h at pH 7.00 Artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) were prepared as described by Zaia (2012) and Samulewski et al. (2021), respectively
Mössbauer Spectroscopy
All the Mössbauer spectra were fitted to a doublet since no other site contribution could be identified inspecting the experimental data (Fig. S5). The fitted hyperfine parameters are presented in Table 5. It was verified that the isomer shifts and quadrupole splitting values are typical of Fe2+ (Table 5).
Discussion
Characterization of Forsterite-91
The two forsterite-91 phases added together represent 91.3 percent of the weight of the sample. In addition to forsterite-91, two other phases were identified, Clinochlore and Willemseite, presenting respectively 7.3 and 1.3 percent of the weight (Fig. 1A).
Because some vibrational modes are infrared active and others are Raman active, these two analytical tools are complementary (Colthup et al. 1990). In addition, although they are not directly comparable, it is useful to use them to characterize minerals. For the forsterite-91 spectrum (Fig. 1B), the bands in the regions from 1200 cm−1 to 700 cm−1, from 700 cm−1 to 450 cm−1, and from 450 cm−1 to 400 cm−1 could be attributed to Si–O asymmetric stretching, asymmetric bending, and motions of divalent cations against the O sublattice, respectively (Hofmeister and Pitman 2007; Hamilton 2010).
The Raman peaks at 822 cm−1 and 856 cm−1 could be attributed to asymmetric SiO4 stretching (Fig. 1C). Furthermore, these peaks are characteristic of olivine, and their shifting is related to the ratios of Mg/(Mg + Fe). Thus, these peaks could be used to identify olivine rich in Mg or Fe (Kuebler et al. 2006). The Raman peaks at 234 cm−1 and 920 cm−1/964 cm−1 could be attributed to the lattice mode and internal stretching of the SiO4 tetrahedron, respectively (Kolesov and Geiger 2004; Kuebler et al. 2006; Weber et al. 2014).
Effect of pH
The forsterite surface changes according to the pH. For acidic pH (reaction 1) a SiO4-rich layer forms on the surface of the forsterite-91, and for alkaline pH (reaction 2) an Mg/Fe-rich layer forms on the surface of the forsterite-91 (Pokrovsky and Schott 2000). Since thiocyanate is negatively charged, as the pH of the samples increases, the forsterite surface becomes more exposed with Fe2+ ions (Mg/Fe-rich layer). Consequently, the adsorption of thiocyanate onto forsterite-91 increases.
As observed in this work, Zaia et al. (2020) also reported that thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) at neutral pH, presented higher adsorption onto ferrihydrite than thiocyanate dissolved in modified artificial seawater 4.0-A Gy (without sulfate). In addition, when dissolved in ultrapure water or KCl (0.1 mol L−1), at neutral pH, thiocyanate did not adsorb onto ferrihydrite (Zaia et al. 2020). However, when thiocyanate was dissolved in KCl (0.1 mol L−1) at neutral pH the adsorption on forsterite-91 was similar to when thiocyanate was dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) (data not shown). According to Vu and Moreau (2015), the adsorption of thiocyanate onto ferrihydrite occurred only at a very acidic pH, and a low sulfate concentration decreased its adsorption. Thus, when thiocyanate is dissolved in a salt solution, its adsorption onto forsterite or ferrihydrite is complex with Mg2+ and SO42− playing an important role.
Adsorption Isotherm
All isotherms were fitted using nonlinear models, because they produced more reliable results of isotherm parameters than linear fits (Kinniburgh 1986; Kumar 2007; Limousin et al. 2007; Foo and Hameed 2010).
The adsorption of thiocyanate onto several materials has been studied: hydrotalcite (Li et al. 2006, 2008; Aguirre et al. 2010; Wu et al. 2011), AgCl/hydrotalcite (Xie et al. 2013), activated carbon (Namasivayam and Sangeetha 2005; Namasivayam and Sureshkumar 2007; Aguirre et al. 2010), zeolite/diatomite (Aguirre et al. 2010), and ferrihydrite (Vu and Moreau 2015; Zaia et al. 2020). In general, the Langmuir isotherm model presented the best fit for the studies of thiocyanate adsorption onto several materials (Namasivayam and Sangeetha 2005; Li et al. 2006, 2008; Wu et al. 2011; Xie et al. 2013; Vu and Moreau 2015). These results are in agreement with those of thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) (Table 1). However, when thiocyanate was dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the Freundlich and Langmuir–Freundlich isotherm models presented the best fit (Table 1). It should be noted that, in other studies, the Freundlich isotherm model presented a better fit for thiocyanate adsorption than the Langmuir isotherm model (Namasivayam and Sangeetha 2005; Namasivayam and Sureshkumar 2007; Zaia et al. 2020).
A good fit using the Langmuir isotherm model suggests that a) the energy adsorption sites of the material are homogeneous, meaning they have same energies, b) each energy site has only one thiocyanate molecule, and c) there is no interaction among adsorbed thiocyanate molecules (Tien 1994). In contrast, a good fit using the Freundlich isotherm model suggests that a) the energy adsorption sites of the material are heterogeneous, meaning they have different energies, and c) there is interaction among adsorbed thiocyanate molecules (Tien 1994).
It is difficult to make comparisons among the data, since the conditions that the experiments were performed in are different (ionic force, materials). Some experiments were performed without control of ionic force (Namasivayam and Sangeetha 2005; Namasivayam and Sureshkumar 2007; Li et al. 2006, 2008; Wu et al. 2011; Xie et al. 2013) while others controlled ionic force (Vu and Moreau 2015; Zaia et al. 2020). When thiocyanate was dissolved in a high concentration of calcium chloride (artificial seawater 4.0-B Gy, Table 1) or a high concentration of sodium nitrate (Vu and Moreau 2015), the Langmuir isotherm model presented a better fit. It should be noted that Vu and Moreau (2015) adsorbed thiocyanate onto ferrihydrite and in the present work thiocyanate was adsorbed onto Forsterite-91 (artificial seawater 4.0-B Gy, Table 1). However, when thiocyanate was dissolved in artificial seawater with a high concentration of magnesium and sulphate, and adsorbed onto Forsterite-91 (artificial seawater 4.0-A Gy, Table 1) or ferrihydrite (Zaia et al. 2020), the Freundlich isotherm model presented the best fit. Thus, in the adsorption of thiocyanate, magnesium and sulfate play a different role to other ions. Zaia et al. (2020) suggested a mechanism for the adsorption of thiocyanate onto ferrihydrite based on the shell of ions.
For the adsorption of thiocyanate onto several materials, the following qmax values were obtained: hydrotalcite from 96 to 109 mg g−1 (Li et al. 2006, 2008; Wu et al. 2011; Xie et al. 2013), agricultural solid waste from 8.6 to 16.2 mg g−1 (Namasivayam and Sangeetha 2005; Namasivayam and Sureshkumar 2007), and ferrihydrite from 130 to 538 mg g−1 (Vu and Moreau 2015; Zaia et al. 2020). The qmax values for the adsorption of thiocyanate onto Forsterite-91 were higher when dissolved in ultrapure water or artificial seawater 4.0-A (high Mg2+ and SO42− concentrations) than artificial seawater 4.0-B (high Ca2+ and Cl− concentrations) (Table 1). This result is in agreement with that obtained by Zaia et al. (2020). The adsorption of thiocyanate onto ferrihydrite was much higher than Forsterite-91 (Table 1) (Vu and Moreau 2015; Zaia et al. 2020), probably, because of the high surface area of ferrihydrite (≈ 200 m2 g−1) when compared to Forsterite-91 (≈ 0.4 m2 g−1) (Cornell and Schwertmann 2003; Kuba et al. 2021).
The n values have been related to the system heterogeneity; therefore, the larger the n values, the more heterogeneous the system. The system heterogeneity could be attributed to solid material or adsorbate (thiocyanate) or even a combination of both (Do 1998). In general, the n values in this work obtained from the Freundlich and Langmuir–Freundlich models were higher than 1 (Table 1). Several other authors also obtained n values higher than 1 for the adsorption of thiocyanate in several different materials (Namasivayam and Sangeetha 2005; Li et al. 2006; Namasivayam and Sureshkumar 2007; Zaia et al. 2020). For thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the system is more heterogeneous at high temperatures (Table 1). But when thiocyanate is dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), the heterogeneity of the system remains practically constant with an increase in the temperature (Table 1).
The separation factor (RL) indicates the shape of the isotherm, with lower values reflecting more favorable adsorption (Table 2). Zaia et al. (2020) also observed the same behavior for the adsorption of thiocyanate onto ferrihydrite. The RL values obtained concur with the free energy of Gibbs and Keq values (Table 3), meaning the adsorption of thiocyanate onto Forsterite-91 is a favorable process.
The ΔG values of this work are higher than those of Namasivayam and Sureshkumar (2007) and Xie et al. (2013), and lower than Namasivayam and Sangeetha (2005) (Table 3). As observed by Xie et al. (2013), as well as in the current work, the entropy ruled the adsorption of thiocyanate onto Forsterite-91 (artificial seawater 4.0-B Gy with high Ca2+ and Cl− concentrations) (Table 3). On the other hand, as observed by Namasivayam and Sangeetha (2005) and in the current work, the enthalpy ruled the adsorption of thiocyanate onto Forsterite-91 (artificial seawater 4.0-A Gy, with high Mg2+ and SO42− concentrations) (Table 3). For thiocyanate dissolved in ultrapure water or artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), the entropy values are positive (Table 3). However, when thiocyanate was dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the entropy value is negative (-4.71 J/mol K), this value is close to that obtained by Zaia et al. (2020) (-4.07 J/mol K) for the adsorption of thiocyanate onto ferrihydrite. The entropy values for thiocyanate dissolved in artificial seawaters 4.0-A Gy (high Mg2+ and SO42− concentrations) and 4.0-B Gy (high Ca2+ and Cl− concentrations) are very different from each other (Table 3). An explanation for this difference could be because thiocyanate is a large and polarizable ion, so it binds more readily to the larger and more polarizable Ca2+ than to the smaller and less polarizable Mg2+ (van der Vegt et al. 2016). Thus, when thiocyanate leaves the large cluster formed with Ca2+ to be adsorbed onto forsterite-91, a higher increase in the entropy occurs.
Kinetic Experiments
For the current work, the experimental kinetic data presented the best fit for the pseudo-first-order (Table 4). However, for other works, with one exception (Li et al. 2006), the pseudo-second-order model presented the best fit for the adsorption of thiocyanate onto several materials (Namasivayam and Sangeetha 2005; Namasivayam and Sureshkumar 2007; Li et al. 2008; Wu et al. 2011; Zaia et al. 2020). It should be noted that mostly on the adsorption kinetic data followed pseudo-second-order rate law (Revellame et al. 2020).
Because of the high polarizability of thiocyanate, the pseudo-first-order constant rate for thiocyanate dissolved in artificial seawater 4.0-A (high Mg2+ and SO42− concentrations) was higher than when it was dissolved in artificial seawater 4.0-B (high Ca2+ and Cl− concentrations) or ultrapure water (van der Vegt et al. 2016). Thiocyanate could form large and stable clusters with Ca2+ of artificial seawater 4.0-B and with water, which could slow down the adsorption rate.
Spectroscopy
Infrared Spectroscopy
In general, as shown by FT-IR spectra, sulfate from artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) interacts with forsterte-91 as an outer-sphere surface complex, because a band was observed in the 1085 cm−1 region (Figs. 3B and S3B) (Fukushi et al. 2013; Johnston and Chrysochoou 2016; Zaia et al. 2020). However, Peak et al. (1999) and Fukushi et al. (2013) observed that at low pH, sulfate interacts with iron oxides (goethite, ferrihydrite) as an inner-sphere surface complex and at high pH as an outer-sphere surface complex. Since at low pH the surface iron oxides became more positively charged the following mechanism was suggested (reaction 3) (Peak et al. 1999; Fukushi et al. 2013):
In the present work, even at low pH, the interaction between forsterite-91 and sulfate remains as an outer-sphere surface complex (Fig. S3B). However, we should remember that at acidic pH, the SiO4-rich layer (reaction 1) and at basic pH, the Mg/Fe-rich layer (reaction 2) of forsterite 91 are exposed, respectively (Pokrovsky and Schott 2000). The interaction between sulfate and Fe3+ (from goethite and ferrihydrite) was expected because they are a hard acid and hard base, respectively (Pearson 1963), but Fe2+ from forsterie-91 is a borderline acid (Pearson 1963).
For all pHs studied, in the presence of thiocyanate, sulfate interacts with forsterite-91 as an inner-sphere surface complex (bands at 1080 cm−1 and 1120 cm−1) (Figs. 3B and S3B). Thiocyanate did not have an effect on the adsorption of sulfate onto ferrihydrite (Zaia et al. 2020). However, high sulfate and Mg2+ concentrations from artificial seawater increased the adsorption of thiocyanate onto ferrihydrite (Zaia et al. 2020).
For all experiments, after thiocyanate adsorbed onto forsterite-91, the band at 2040 cm−1 due to ν(CN) stretching shifted to 2070 cm−1 (Figs. 3 and S3). This band (ν(CN) stretching) presented a small shift when thiocyanate adsorbed onto ferrihydrite or hydrotalcite (Wu et al. 2011; Vu and Moreau 2015). However, when dissolving thiocyanate in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), and adsorbing it onto ferrihydrite, this band (ν(CN) stretching) did not shift (Zaia et al. 2020). Thiocyanate can interact with metals through the nitrogen atom, when the metal is a soft acid and through the sulfur atom, when the metal is a hard acid. Since the Fe2+ is considered a borderline acid, the metal coordinates with the thiocyanate through the nitrogen atom (Pearson 1963). Furthermore, when thiocyanate interacts with metal through the nitrogen atom (M-NCS) and sulfur atom (M-SCN) the band due to ν(CN) stretching shifts in the region 2050 cm−1–2100 cm−1 and in the region 2085 cm−1–2130 cm−1, respectively (Baranyi et al. 1976; Azizitorghabeh et al. 2021). Thus, the shift of 30 cm−1 to a higher frequency (from 2040 cm−1 to 2070 cm−1) observed in this work indicates that the Fe2+ of forsterite-91 interacts with the nitrogen atom of thiocyanate.
Raman Spectroscopy
Thiocyanate and sulfate were detected on the surface of forsterite-91 using infrared spectroscopy (Figs. 3 and S3), but were not detected using Raman spectroscopy (Figs. 4 and S4). There are two possibilities for this discrepancy: 1) the amount of these compounds adsorbed onto forsterite-91 was too small to be detected by Raman spectroscopy or b) after the adsorption, these molecules are no longer active from the point of view of Raman spectroscopy.
Using Raman spectroscopy, the adsorption of sulfate has been observed in several iron/aluminum oxides (Wijnja and Schulthess 2000; Jubb et al. 2013) and fluorite (Jubb and Allen 2012). Usually, depending on the interaction between the sulfate and mineral, νSS(SO) stretching should be observed in the region 980 cm−1 and several peaks of νAS(SO) stretching should be observed in the region 1050 cm−1–1150 cm−1 (Wijnja and Schulthess 2000; Jubb and Allen 2012; Jubb et al. 2013). Since infrared spectroscopy data showed that sulfate interacts with forsterite-91 as an outer-sphere surface complex and in the presence of thiocyanate as an inner-sphere surface complex (Figs. 3B and S3B), and these bands are active in Raman spectroscopy, it is probable that due to the low surface area of forsterite-91 when compared to iron/aluminum oxides (Cornell and Schwertmann 2003; Kuba et al. 2021), the amount of sulfate adsorbed was too low to be detected by Raman spectroscopy.
The interaction between the metals and thiocyanate has been studied using Raman spectroscopy and band shifts, as these interactions have been observed in the bands at 750 cm−1 (ν(CS) stretching) and at 2054 cm−1 (ν(CN) stretching) (Kinnel and Standberg 1959; Barayi et al. 1976; Cao et al. 2002; Li and Gewirth 2003; Wang et al. 2019). Since infrared spectra showed the shift in the band due to (ν(CN) stretching (Figs. 3 and S3), this band is Raman active (Kinnel and Standberg 1959), and has been observed by other authors (Barayi et al. 1976; Cao et al. 2002; Li and Gewirth 2003; Wang et al. 2019). Thus, Raman spectroscopy probably does not have the sensitivity to detect the amount of thiocyanate adsorbed onto forsterite-91.
Mössbauer Spectroscopy
Considering the resolution of the Mössbauer technique, the ferric cation is virtually absent in all samples. The parameters isomer shift and quadrupole splitting obtained for pure forsterite-91 are consistent/similar to those reported by other authors for mineral samples of different origin (Thierry et al. 1981; Gismelssed et al. 2020). Both hyperfine parameters as found for all samples with thiocyanate revealed a minimal difference relative to the pristine samples or those without thiocyanate (Table 5), which means that the adsorption of thiocyanate had little effect, if any, on the valence or chemical neighborhood of the iron present in forsterite-91. Ultimately, only the iron cations at the forsterite-91 particle surface were affected by the adsorption, which may represent a fraction of iron too small to be probed by Mössbauer spectroscopy.
Relevance to Prebiotic Chemistry
As amino acids, purines, and pyrimidines are synthesized from cyanide, most researchers consider it as one the most important molecules on prebiotic Earth (Lowe et al. 1963; Ferris et al. 1978; Ferus et al. 2020). However, the role that thiocyanate could have played cannot be ruled out, since it could have played the same role as that of cyanide in the Strecker reaction, as well as a CN-source for the oxidative α-cyanation of tertiary amines (Perezgasga et al. 2003; Cleaves et al. 2014; Kouznetsov and Galvis 2018). In addition, methionine was obtained from the UV irradiation of the ammonium thiocyanate sample (Steinman et al. 1968). Thus, because thiocyanate contains sulfur, it opens the possibility for the synthesis of a larger variety of molecules than cyanide. However, both ions appeared, probably, at very low concentration in the seas of the prebiotic Earth (Dowler and Ingmanson 1979; Stribling and Miller 1987). Thus, for the synthesis of new molecules and polymers to occur, they need to be pre-concentrated.
Two different seawaters were used in the experiments. The artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) was suggested by Samulewski et al. (2021) based on the work of Halevy and Bachan (2017). This seawater probably best represents the composition of salts present in the prebiotic Earth's ocean. The artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) was suggested by Zaia (2012) based on the work of Izawa et al. (2010). As there are some doubts about the existence of SO42− on the prebiotic Earth (Catling and Claire 2005), a seawater with high Mg2+ and SO42− concentrations could be a model for Mars as well as for other moons in the solar system (Tosca et al. 2011; Fox-Powell et al. 2016; Cockell et al. 2020). In addition, forsterite was a widespread mineral on the prebiotic Earth (Hazen et al. 2008), Mars (Mølholt et al. 2008; Gunnlaugsson et al. 2009; Clark et al. 2014), and other moons in the solar system (Clark et al. 2014; Zandanel et al. 2021). Therefore, the experiments presented in this work were performed under conditions that resemble those of the prebiotic Earth as well as Mars and other moons in the solar system.
In the current work, there are two important results: 1) dissolving thiocyanate in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), it adsorbed onto forsterite-91 and 2) the salts of artificial seawater have an effect on the amount of thiocyanate adsorbed, the adsorption thermodynamic, and the adsorption kinetic. Thiocyanate adsorption onto forsterite-91 reveals a great advantage of this molecule in relation to cyanide, since the latter does not adsorb onto several minerals, including forsterite-91 (see Table S1, supplementary material published by Samulewski et al. 2021). Although artificial seawater has been used in some studies (Tadayozzi et al. 2023; see Table S1, supplementary material), the role played by it in the adsorption of molecules has been neglected (Zaia 2012). For thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), the adsorption was ruled by enthalpy and entropy, respectively. In addition, dissolving thiocyanate in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) presented a higher adsorption and constant rate than dissolving it in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations). It should be noted that the thiocyanate adsorption onto ferrihydrite occurred only when the seawater contained Mg2+ and SO42− (Zaia et al. 2020). Therefore, since salts have an effect on the kinetics and thermodynamics of adsorption as well as on the amount adsorbed, it is recommended that for prebiotic chemistry experiments artificial seawater should be used.
Conclusion
X-ray diffractometry of the fosterite-91 sample showed that it contained about 91% of fosterite-91, 7.3% of Clinochlore, and 1.3% of Willemseite. FT-IR and Raman spectra presented characteristic peaks of forsterite-91.
The adsorption of thiocyanate onto forsterite-91 increased with the increase in pH. The highest adsorption of thiocyanate onto forsterite occurred when it was dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), and thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) presented the lowest adsorption.
For thiocyanate dissolved in ultrapure water or artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations), the adsorption data fitted well with the Freundlich and Langmuir–Freundlich models. However, for thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), the Langmuir model presented a slightly better fit than Freundlich and Langmuir–Freundlich models.
The qmax values obtained from the Langmuir and Langmuir–Freundlich models presented good agreement with the qmax values obtained from the pseudo-first-order model. The low qmax values obtained in this work when compared to others were probably due to the low surface area of forsterite-91.
In general, the n values from the Freundlich and Langmuir–Freundlich models were higher than 1, meaning that the thiocyanate (water/artificial seawater)/foraterite-91 system is heterogeneous.
All RL values were lower than 1, meaning the adsorption of thiocyanate onto forsterite-91 is a favorable process, confirmed by the negative values of Gibbs free energy. Dissolving thiocyanate in ultrapure water, the enthalpy and entropy values were favorable from the point of view of thermodynamics. However, for thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) and artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) the adsorption onto forsterite-91 was ruled by enthalpy and entropy, respectively. It is probable that when dissolving thiocyanate in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations), a large cluster formed with Ca2+, thus, when thiocyanate leaves the cluster to be adsorbed onto forsterite-91 a higher increase in the entropy occurs.
The pseudo-first-order model presented the best fit to the experimental kinetic data. The constant rate for thiocyanate dissolved in artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations) was twice that for thiocyanate dissolved in artificial seawater 4.0-B Gy (high Ca2+ and Cl− concentrations) or ultrapure water. The largest cluster formed by water and Ca2+ probably slowed down the adsorption rate of thiocyanate onto forsterite-91.
The FT-IR spectra showed thiocyanate and sulfate adsorbed onto forsterite-91. However, using Raman and Mössbauer spectroscopy, no changes were observed in these spectra. It is likely the low amount of thiocyanate adsorbed onto forsterite-91 was not enough to be detected by these techniques. Mössbauer spectra did not show Fe3+. The infrared spectra showed, for all experiments, that the band at 2040 cm−1 due to ν(CN) stretching shifted to 2070 cm−1, after the adsorption of thiocyanate onto forsterite-91. The interaction between thiocyanate and Fe2+ of forsterite-91 was with the nitrogen atom of thiocyanate. Thiocyanate has an effect on the adsorption of sulfate from artificial seawater 4.0-A Gy (high Mg2+ and SO42− concentrations). In the presence of thiocyanate, sulfate interacts with forsterite-91 as an inner-sphere surface complex, and without thiocyanate as an outer-sphere surface complex.
Data Availability
The data shown in this study is available from D.A.M.Z. and R.B.S. upon reasonable request.
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This research was supported by a grant from CNPq/Fundação Araucaria (project number: 46824, Agreement: 11/2017, Title: Paranaense Nucleus of Studies in Complex Oxides).
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Conceptualization: D.A.M.Z. and R.B.S.; methodology, G.W.F., A.U., F.F.I., A.P.J.; formal analysis, R.B.S., A.U., A.P.J., and D.A.M.Z.; resources, A.P.J.; data curation, G.W.F., A.U., F.F.I., and R.B.S.; writing—original draft preparation, G.W.F.; writing—review and editing, D.A.M.Z. and R.B.S.; visualization, R.B.S.; supervision, D.A.M.Z.; project administration, A.P.J.; funding acquisition, A.P.J.
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Ferreira, G.W., Samulewski, R.B., Ivashita, F.F. et al. Did Salts in Seawater Play an Important Role in the Adsorption of Molecules on Minerals in the Prebiotic Earth? The Case of the Adsorption of Thiocyanate onto Forsterite-91. Orig Life Evol Biosph 53, 127–156 (2023). https://doi.org/10.1007/s11084-023-09640-3
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DOI: https://doi.org/10.1007/s11084-023-09640-3