Modes of Interaction of Simazine with the Surface ... - ACS Publications

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Modes of Interaction of Simazine with the Surface of Amorphous Silica in Water. Part II: Adsorption at Temperatures Higher than Ambient Serena Esposito,† Filomena Sannino,‡ Marco Armandi,§ Barbara Bonelli,§ and Edoardo Garrone*,§ †

Department of Civil and Mechanical Engineering, Università degli Studi di Cassino e del Lazio Meridionale, Via G. Di Biasio 43, 03043 Cassino, Frosinone, Italy ‡ Department of Agriculture, Università di Napoli “Federico II”, Via Università 100, 80055 Portici, Naples, Italy § Department of Applied Science and Technology and INSTM Unit of Torino-Politecnico, Politecnico di Torino, Corso Duca degli Abruzzi 24, I-10129, Torino, Italy S Supporting Information *

ABSTRACT: The conclusions of a previous study (S. Esposito et al. J. Phys. Chem. C 2013, 117, 11203−11210) concerning room temperature adsorption of simazine (Sim) on amorphous silica in water have been checked against a set of experiments in the range 40° to 60 °C, where equilibrium conditions are more likely to be attained. Adsorbed amount as a function of pH has a complex behavior with temperature, confirming the presence of two types of protonated adsorbed species, respectively monomeric (SimH+) and dimeric (Sim2H+), the latter prevailing both at high temperatures and loadings. A simple model for adsorption involving proton transfer from the solid indicates that the pH value at which the uptake is maximum (pH*) is the half sum of the pKa’s of both the active silanol species and the protonated entity given rise, pH* = [pKa(1) + pKa(2)]/2. From this, it results that (i) the dimer Sim2 is more basic than the monomer Sim by 2 units of pKa; (ii) adsorbed simazine is more basic then the molecule in solution also by ca. 2 units in pKa; and (iii) the pKa of the silanol species involved is probably not ca. 4 as recently proposed, but more likely ca. 7, in agreement with old classical views. From the qualitative energetic point of view, the reaction Sim(aq) + SiOH → SiO−··· SimH+ is exothermic, the formation of the dimer from the monomer is endothermic (reaction SiO−···SimH+ + Sim(aq) → SiO−···Sim2H+), whereas the reaction 2 Sim(aq) + SiOH → SiO−···Sim2H+ is slightly exothermic. At 25 °C, the adsorbed monomer is irreversibly held, and the dimer only partially. The isotherm at 40° shows that adsorption of the dimer occurs almost reversibly, whereas equilibrium in the formation of the monomer is not completely reached. The isotherm at 60 °C shows instead that both species are formed under near-equilibrium conditions.

1. INTRODUCTION In the first part of this work1 the room temperature adsorption from aqueous solutions of Simazine (2-chloro-4,6 bis(ethylamino)-s-triazine), hereafter referred to as Sim (structure in Scheme 1), was studied on three amorphous silicas: one

all solids in a narrow pH interval around 5.5. This fact, together with pH changes along adsorption and IR spectroscopic data concerning dried samples, provided evidence that adsorption involves proton transfer from acidic SiOH species to N atoms at ethylamino chains, those at the ring being basically less basic. The populations of the two species appear to be similar. The proposed explanation was the occurrence of a two steps process, in which first a protonated molecule is formed (SimH+), onto which a second molecule may anchor, so forming a dimer (Sim2H+), held together by both sharing of the positive charge and hydrophobic interactions:

Scheme 1. Structure of Simazine [2-Chloro-4,6bis(ethylamino)-1,3,5-triazine]

commercial nonporous (Aerosil) and two homemade microporous. Although the process is heavily kinetically controlled at room temperature, two types of adsorbate species were evidenced, causing the presence of two regions in the pseudoisotherms (adsorbed amounts versus steady state concentration in solution). At 25 °C one species is irreversibly held, the other is partially reversible. Adsorption takes place on © 2013 American Chemical Society

Si−OH + Sim(aq) → Si−O−···SimH+

(1)

Si−O‐···SimH+ + Sim(aq) ↔ Si−O−···Sim2H+

(2)

Received: October 31, 2013 Revised: November 28, 2013 Published: December 2, 2013 27047

dx.doi.org/10.1021/jp4107487 | J. Phys. Chem. C 2013, 117, 27047−27051

The Journal of Physical Chemistry C

Article

constant temperature, chosen between 40, 50, and 60 °C. In the second set, different volumes of solution (20 μmol L−1) were added to the silica sample at the different temperatures keeping constant the pH at 5.5, so obtaining an initial simazine concentration ranging from 0.50 to 20 μmol L−1. To check the reversibility of the processes, both adsorption and desorption runs were carried out, the latter contacting samples loaded with simazine with different volumes of pure water, and the amount of released simazine evaluated. 2.2. Simazine Evaluation. Simazine content was measured by means of an Agilent 1200 Series HPLC apparatus (Wilmington U.S.A.), equipped with a DAD array and ChemStation Agilent Software, using a Macharey-Nagel Nucleosil 100-5 C18 column (stainless steel 250 × 4 mm). The mobile phase, comprising a binary system of 65:35 acetonitrile:water, was pumped at 1 mL·min−1 flow in an isocratic mode. The detector was set at 220 nm and the injection volume was 20 μL. Quantitative determination was done on the basis of a calibration curve in the concentration range between 0.15 and 20 μmol/L.

Tentative structures for the adsorbed species are depicted in Scheme 2. Scheme 2. Possible Configurations for the Adsorbed Molecule: (a) Singly Protonated; (b) with a Second Molecule Interacting with the Species in Structure (a) via Hydrophobic Interactions and/or Sharing of the Proton

3. RESULTS AND DISCUSSION 3.1. Effect of pH on Adsorbed Amounts. The amount adsorbed in fixed conditions, when contacting 0.3 mg silica with 3.0 mL 13 μmol L−1 solution, markedly depends both on the solution pH and on temperature, as illustrated in Figure 1. The

The adsorption site probably requires, besides an acidic silanol, other peculiarities, because the maximum coverage attained is only ca. 4 molecules per 100 nm.2 To provide support to the interpretation advanced, the system simazine/water/silica has been studied in this second part of the work in presumably equilibrium conditions, by means of adsorption/desorption experiments from aqueous solutions at temperatures in the range 40°−60 °C on the non porous Aerosil 250 (Degussa) only. Two types of measurements have been run, i.e. concerning, respectively, the dependence of the uptake as a function of pH in a closed system and the adsorption/desorption data as a function of the concentration in solution.

2. EXPERIMENTAL SECTION 2.1. Adsorption/Desorption Experiments. A stock solution was prepared by dissolving 2 mg simazine in 500 mL doubly distilled water. The obtained concentration, 20 μmol L−1, is close to saturation at 25 °C (ca. 25 μmol L−1). Solubility increases with temperature, reaching, e.g., 84.30 μmol L−1 at 50 °C.2 All sorption experiments were carried out by adding 0.3 mg sorbent to 3.0 mL simazine solution in glass vials with Teflon caps at the desired temperature. After 24 h, the samples were centrifuged at 7000 rpm for 20 min, and the concentration in the supernatant solution, C, determined as detailed below. The amount of adsorbed simazine was calculated as the difference between the initial amount and that at the steady state. During adsorption, a slight increase in pH was observed,1 which was adjusted by the addition of 0.01 mM HCl solution. In the first set of experiments at constant simazine and solid amounts, the pH value was changed between 3.0 and 7.0 at a

Figure 1. Dependence of simazine uptake per unit surface area of Aerosil 250 as a function of the solution pH at different temperatures.

solid curve (black squares) refers to 25 °C, and coincides with the one already published:1 adsorption begins at ca. pH = 4, and ends at pH = 7, reaching a maximum at pH = 5.5. When T = 40 °C, the curve is much wider, extending from ca. 4 to 7.5, and larger adsorbed amounts are also observed. We ascribe this latter feature to the fact that kinetic (diffusional) constraints are partially overcome. The width seems to be due to the superposition of two peaks, one being that observed at 25 °C, the other one centered at ca. pH = 6.5. Indeed, at higher temperature the component observed at 25 °C disappears, and 27048



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only the latter component is left. Passing to 50° and to 60 °C, this component persists, though decreasing slightly in intensity. These observations are consistent with the interpretation advanced in the previous work, i.e. the presence of two protonated species, one reaching a maximum population at pH* = 5.5, the other with pH* = 6.5. Under the present circumstances, the increase in temperature markedly disfavors the former, and the latter to a much more limited extent. Thermodynamics suggests that the process of formation of the former species is definitely exothermic, and only slightly for the latter species. It thus appears feasible to assume that the former process is the formation of the protonated monomer SimH+ (reaction 1), yielding a species irreversibly held at 25 °C, and the latter is that of the protonated dimer Sim2H+. Note that the enthalpy change concerning the formation of Sim2H+ from its constituents Si−OH + 2Sim(aq) ↔ Si−O−···Sim2H+

to support such a view.6 Agreement on such view, however, is not universal.7 The limited population of sites for simazine adsorption requires that the most acidic species have to be chosen as constituent of sites. Even allowing for some difference in basicity for simazine in solution and when adsorbed on silica, i.e. some deviation from the value of 1.65, it is evident that a pKa value of 4.5 for the silanol species is not feasible. Instead, the “classical” value of 7 is acceptable, because this yields a pKa value for protonated simazine of ca. 4, a value not too far from that of the species in solution, corresponding to a moderate increase of the basicity of the molecule when adsorbed on the silica surface with respect to the solution. Basicity of a molecule is known to depend on the surroundings (e.g., solvent) so that it is not surprising that a simazine molecule interacting on one side with the highly hydroxylated silica surface, and on the other side with water may display acid/base properties somewhat different from the sister molecules in water environment. The same type of considerations holds for the dimer Sim2H+, for which pH* is 6.5. The silanol species being the same as for the monomer, a pKa value of 7 has to be assumed, which yields ca. 6.0 for the dimer of simazine acting as a proton donor. The dimer thus appears to be slightly more basic than the monomer. 3.2. Adsorption/Desorption Measurements. Figures 2−4 illustrate the dependence of the uptake per unit surface

(3)

is slightly exothermic, because the amount of Sim2H+ formed decreases with temperature. As a whole, the enthalpy content of the various configurations of the system is qualitatively described by Scheme 3. Scheme 3. Scheme of Enthalpy Levels for the Adsorbed Monomer SimH+ and Dimer Sim2H+ with Respect to the Isolated Species

Figure 2. Dependence of the simazine uptake at 25 °C per unit surface area of Aerosil 250 as a function of the equilibrium concentration in the liquid. Filled symbols: data obtained by increasing the starting solute concentration; open symbols: data obtained after adsorption by contacting the solid with pure water.

The process of formation of the dimer from monomer, as described in reaction 2, though endothermic, is spontaneous, and so appears to be entropy-driven. Prediction of entropy changes is not readily made a priori, as is often the case with species in solution: in the present case, the increase in order due to the localization of a second molecule is probably overbalanced by the disorder brought about by freeing water molecules coordinated to simazine. As detailed below, the formation of the species SimH+ (reaction 1) can be decomposed into four elementary steps under the assumption of equilibrium, from which it is inferred that the pH value at which maximum uptake is observed, pH*, equals the half sum of pKa(SiOH) and pKa(Sim), as described in detail in the Supporting Information: pH* = [pK a(SiOH) + pK a(Sim)]/2 3

A (adsorbed amount) on the concentration of simazine in the supernatant, measured as described above, for the three chosen temperatures. To deal with the highest possible loadings, at 25 °C the pH was kept constant at 5.5, whereas at 40° and 60 °C, pH was constant at 6.5. Filled symbols represent data obtained at increasing concentration; open symbols represent those obtained with decreasing concentration at the end of the adsorption run. Figure 2 shows indeed that at 25 °C the monomer is irreversibly held, whereas the dimer is partially reversible. The isotherms at 40 °C (Figure 3) show that adsorption/desorption data coincide, within the accuracy of the measurement, only as it concerns the second branch, i.e., the dimer. Superposition of adsorption/desorption is instead acceptable in the whole range for the 60 °C data, thus indicating that equilibrium is grossly attained for both species.

(4) +

The pKa of simazine is close to 1.65, and pH* for SimH is 5.5. The pKa for silanol species in water has been traditionally assumed to be ca. 7.4 Recently, sophisticated measurements have suggested a bimodal distribution of acidic silanols into two families, with pKa ca. 4.5 and 8.5, respectively (the former being as acidic as acetic acid),5 and recent computational results seem 27049



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this is not so: on the one hand, full equilibrium is difficult to reach; on the other hand, the accuracy of measurements is intrinsically poor. Lastly, it has to be noted that, notwithstanding the popularity of Sim in agriculture, the nature of its water solutions has not been studied in detail. It is not known, e.g., whether the probable hydrophobic interactions yield dimeric species in solution: this is, e.g., the case of molecules with close structures like purine bases.9 This possibility, which we have in mind to investigate, would lead to different expressions for the above reactions 1 and 2 and the corresponding isotherm, which would involve the square root of the concentration. 3.3. Possible IR Evidence for the Structure of the Dimer. Two main mechanism of interaction (possibly occurring simultaneously) may hold together two simazine molecules in the Sim2H+ dimer, namely, sharing of the proton between secondary amino units of the two molecules, and the occurrence of hydrophobic interactions. Whereas the latter can be only brought into evidence by NMR measurements,10 the former may be in principle observed in the IR. For instance, ammonium species are known to form adducts with ammonia with formula N2H7+, where the transferred proton is shared between the two moieties, though unevenly. Such species are readily formed and studied by dosing gaseous ammonia on ammonium exchanged zeolites.11 The stretching mode of the H-donor N−H species, which suffers a marked bathochromic shift because of the interaction, is observed at ca. 2700 cm−1. We have examined carefully the IR spectra of loaded samples after drying, reported in the previous paper, and did not observe any band in this position. We are inclined, however, not to consider this as a negative piece of evidence. Indeed, the crystalline nature of zeolites allows the formation of an adduct with definite structure, which is not the case of amorphous silica in water. It is most probable that a number of slightly different arrangements of the two Sim molecules occur, to which would correspond slightly different interactions and shifts of the N−H band. The result would be the smearing out of the signal of perturbed N−H groups.


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CONCLUSIONS Measurements run in the range 40° to 60 °C on the one hand allow to approach more closely equilibrium conditions. On the other hand, a complex behavior is observed of the adsorbed amount as a function of pH with temperature, supporting the assignment proposed in the previous paper. The presence is confirmed of two types of protonated adsorbed species, respectively monomeric (SimH+) and dimeric (Sim2H+), the latter prevailing both at high temperatures and loadings. Applying a simple model for adsorption involving proton transfer from the solid, relating the pH value at which the uptake is maximum (pH*) to the pKa of both the silanol species and the protonated entity given rise, allows one to conclude that (i) the dimer is more basic than the monomer (the difference in pKa being ca. 2); (ii) adsorbed simazine is more basic then the molecule in solution by ca. 2 units in pKa; (iii) the pKa of the silanol species involved is probably not ca. 4 as recently proposed, but more likely ca. 7 in agreement with old views. From the energetic point of view, the formation of SimH+ is exothermic (reaction Sim(aq) + SiOH → SiO−···SimH+), that of the dimer from the monomer is endothermic (reaction SimH+ + Sim(aq) → SiO−···Sim2H+), whereas that of reaction 2 Sim(aq) + SiOH → SiO−···Sim2H+ is slightly exothermic.


In the case of truly equilibrium data, the isotherm corresponding to the stepwise adsorption of two molecules on the same site can be written as8 Na = NM[K1c + 2K 2c 2]/[1 + K1c + K 2c 2]

(5)

Na and NM being the adsorbed amount and the total adsorbed amount, respectively. K1 and K2 being the equilibrium constants of reaction 1 and 2, respectively, and it could be possible to extract quantitative thermodynamic information from the temperature dependence of both K1 and K2. Unfortunately, 27050



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ASSOCIATED CONTENT

S Supporting Information *

Detailed description of a model for the adsorption of a basic molecule on an acidic surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Mailing address: Department of Applied Science and Technology and INSTM Unit of Torino-Politecnico, C.so Duca degli Abruzzi 24, Politecnico di Torino, I-10129, Torino, Italy. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Prof. Michele Pansini of the Department of Civil and Mechanical Engineering, Università degli Studi di Cassino e del Lazio Meridionale for discussions. The authors thank Ms. Serena R. Bonavolontà for technical support.

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REFERENCES

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Modes of Interaction of Simazine with the Surface ... - ACS Publications

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