Characterization of the porous structure of different humic fractions

Colloids and Surfaces A: Physicochem. Eng. Aspects 256 (2005) 129–135

Characterization of the porous structure of different humic fractions R.A. Alvarez-Puebla a, ∗ , P.J.G. Goulet a , J.J. Garrido b a

Materials and Surfaces Science Group, School of Physical Sciences, University of Windsor, Windsor, Ont., Canada N9B 3P4 b Department of Applied Chemistry, Universidad Publica de Navarra, Campus Arrosad´ıa, E-31006 Pamplona, Spain Received 1 May 2004; accepted 29 December 2004 Available online 1 February 2005

Abstract Humic substances (HS) are the most important source of organic carbon in the environment. Their colloidal character and high surface functionality make them excellent adsorbents, possessing a superior capacity for the retention of ionic and molecular pollutants, and for facilitating the processes of mobilization/immobilization of these in the environment. However, the characterization of their microstructural properties is essential to the understanding of these processes. In this work, the porous texture of two humic substances, and their fractions (fulvic acid, FA; brown and grey humic acids, BHA and GHA; and humin, HU), have been studied using both experimental and computational methods. It has been determined that the fulvic acids have the largest apparent surface area, and the most narrow microporosity distribution. The coincidence of the maxima in the micro, meso and macropore distributions, among the different humic fractions, points toward a textural similarity between them, independent of their origin. The results obtained from the application of quantum computational methods corroborate the experimental findings, and thus show potential for even greater application towards this type of surface chemistry. © 2004 Elsevier B.V. All rights reserved. Keywords: Humic substance; Gas adsorption; Mercury porosimetry; Porosity distributions; Computational chemistry

1. Introduction Humic substances (HS) are the most important source of organic carbon in both terrestrial and aquatic environments [1], and play a key role in nature. They contribute to the growth of plants, are responsible for the structure and physical–chemical properties of soil, and are involved in the majority of surface phenomena that occur in soil [2]. Humic substances can be fractionated into fulvic acids (FA), brown humic acids (BHA), grey humic acids (GHA), and humin (HU), as a function of their solubility at different pH values. FA are soluble at any pH; BHA and GHA are soluble from pH 2 and pH 7 upwards, respectively; and HU are insoluble at any pH [2]. The solubility of these fractions is closely related to molecular mass, structural branching complexity, molecular polarity and chemical composition [1]. Additionally, humic substances show a strong tendency for interaction ∗

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0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.12.062

with metal cations due to both their colloidal character, and large number of surface functional groups [3,4]. Because of this, they play an important role in determining the mobilization/immobilization behaviour of metals in the environment. As well, they show strong retention of atmospheric gases such as O2 , N2 , and CO2 , making them available to microorganisms and plants, and also for biomineralization [2,5–9]. Thus, it is clear that the characterization of the microstructural properties of HS is essential to understanding the retention processes that take place in soil. The microstructural properties of HS have been extensively studied by means of microscopy [10–13], light scattering measurements [14–17], and computational techniques [18–20]. However, there are few papers on the characterization of the porosity of HS using gas adsorption [21–23]. Gas adsorption methods, applied to the characterization of the microstructural properties of solids, present important advantages over microscopy and light scattering, including, most importantly, that they provide information about texture and porosity distribution. In this paper, we present a study of the

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porous texture of two HS and their fractions, examined using the techniques of N2 (77 K) and CO2 (273 K) gas adsorption, as well as mercury porosimetry. The experimental results are supported by those from theoretical modelling that confirm the information about the porous structure of these materials, and also suggest the potential for predicting their behaviour in the environment.

2. Materials and methods 2.1. Humic samples The materials used in this study were two commercial humic substances from Fluka and Acros Organics. Both HS were fractionated using the methods proposed by the International Humic Substances Society [2,24]. The HS and their different fractions were purified using a 10,000 Da dialysis membrane (SpectraPor Membrane Tubing, Spectrum Industries, Inc. LA), first with 0.5 M HCl, then with water, until the total disappearance of chloride ions, as verified by the addition of AgNO3 . The dialyzed samples were adjusted to a pH of 5 by adding 2 M HCl, freeze-dried in a Corning desiccator which was attached to a −50 ◦ C cold trap coil and vacuum pumped to a pressure of 0.02 mmHg. Samples were then lyophilized for 48 h. and stored under vacuum. All of the reactants used for fractionation and purification were of analytical grade, and MilliQ-distilled water was used throughout. 2.2. Experimental techniques C, H, N, and S contents were determined using an elemental analyser CHNS EA1108 by Carlo Erba. UV–vis absorption spectra were recorded with a Perkin-Elmer Lambda 3B spectrophotometer from 200 to 900 nm. He density was measured with a He picnometer (AccuPic 1330, Micromeritics). The picnometer was calibrated before every measurement and 50 results were co-added for every

sample. For the examination of the porous texture with pH, samples of 0.5000 ± 0.0005 g were studied using N2 (77 K) and CO2 (273 K) adsorption isotherms (ASAP 2010, Micromeritics) [17]. Hg density and intrusion porosimetry were performed with a Micromeritics Autopore II 9320 mercury porosimeter with a maximum 413 MPa injection pressure, assuming a contact angle of 140◦ and a mercury surface tension of 484 × 10−3 N m−1 . For both gas adsorption and mercury intrusion porosimetry, three analyses were performed. The absolute error was never more than 2.1%. All apparent surface areas were calculated by applying Dubinin–Radushkevich’s method (DR) to the N2 (77 K) and CO2 (273 K) adsorption isotherm results [25]. For calculation of micropore distributions, the Hovarth–Kawazoe’s method (HK) [26] was applied to the CO2 (273 K) adsorption isotherms. For the application of this method with GHA results, the parameters deduced for active carbon were employed as it is a material of similar chemical nature [27]. 2.3. Molecular modelling Molecular modelling was carried out with HyperChem 7 Software [28], and the model design was based upon the widely used TNB (Temple–Northeastern–Birmingham) model [29–32]. The TNB model was optimized using the molecular mechanics (MM) OPLS force field [33] with the Polak–Ribiere algorithm. The convergence limit was set at a maximum acceptable gradient of 0.042 kJ mol−1 nm−1 . Quenched Dynamic (QD) cycles, with 0.5 and 1 ps heat and run times, respectively, at 700 K, and a step size of 1 fs, were performed on the optimized structure in order to explore the conformational space [34]. The most stable local minima were re-optimized using the same conditions, and then optimized again using the PM3/TM semi-empirical method [35], with a convergence limit of 0.418 kJ mol−1 nm−1 [36]. HA were modelled in accordance with their elemental composition, number of acidic groups, and average molecular weight (Table 1), by polymerization of 23 modified-TNB

Table 1 Some physical and chemical properties of humic substances and their fractions Sample code

C (%)

H (%)

N (%)

S (%)

O (%)

Ash (%)

Mw a (g mol−1 )

ρ(He) b (g cm−3 )

ρ(Hg) c (g cm−3 )

Fluka HS HU GHA BHA FA

47.9 39.8 55.5 51.2 40.1

4.91 5.02 4.28 4.10 3.57

0.67 0.21 0.80 0.86 0.67

1.18 1.12 1.30 0.72 0.65

24.8 37.5 38.1 43.2 55.0

20.5 16.4

1.84 × 104 1.47 × 104 2.24 × 104 1.23 × 104 1.06 × 103

1.59 1.54 1.49 1.53 1.69

0.80 0.98 0.73 0.96 0.79

Acros Organics HS HU GHA BHA FA

46.6 42.8 59.9 52.3 43.9

4.30 6.53 4.45 3.95 3.75

0.55 0.18 0.50 0.73 0.58

0.43 2.05 3.08 2.01 2.13

25.8 28.6 32.1 41.0 49.6

22.3 19.8

1.63 × 104 1.34 × 104 2.02 × 104 1.14 × 104 9.07 × 104

1.61 1.38 1.26 1.46 1.68

0.86 1.05 0.61 1.01 0.70

a b c d

d d d

d d d

Average molecular weight estimated from Mw = 3.99ε280 + 490 according to [41]. Solid phase density measured with helium. Bulk density measured with mercury intrusion. Not detected.

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subunits (previously optimized with molecular mechanics, OPLS, and the PM3 semi-empirical method) with a peptidic bond between NH2 and COOH groups [3]. The polymeric structure was optimized using the OPLS force field, with the Polak–Ribiere algorithm. For the docking between the two HA molecules the system was subjected to cycles of molecular dynamics (MD) in the following conditions: 2 ps of warming up to 298 K, 3 ps of stabilization, and 10 ps of simulation, with a time step of 1 fs. Due to the fact that the system is too large to establish periodic boundary conditions, the effect of the solvent was considered by applying the Langevin equation for movement [37]. After every MD cycle, a geometric optimization was carried out on the system using the Polak–Ribiere algorithm, down to a gradient of 0.042 kJ mol−1 nm−1 .

3. Results and discussion 3.1. Porous texture of HS and their fractions Elemental analyses of humic substances and their fractions (Table 1), show values in agreement with literature values for similar materials [24,38]. The solubility of all fractions increases with their percentage of oxygen, and decreases with their percentage of carbon. Table 1 shows that the only samples that produce ashes are HS and HU. This is in close agreement with data obtained from X-ray diffraction (XRD) measurements, that show small bands due to quartz, muscovite, and kaolinite [4]. Helium density (Table 1) varies from 1.26 to 1.69 g cm−3 , and the highest He densities are those of the FA, which are the most soluble, simple, and polar fractions. HU and HS show intermediate helium density values, and, in HU samples, the presence of mineral phases slightly increases the value of this parameter. The HS samples are a mixture of the humic fractions, and the dependence of helium density on the complexity of the fractions likely indicates that the simplest samples (i.e., FA) show more compact packing than the more complex ones (GHA). The porous texture of these humic samples was studied using adsorption isotherms of N2 at 77 K, and CO2 at 273 K. The molecular sizes of these two adsorbates are comparable (0.30 and 0.33 nm, respectively), but the samples adsorbed only CO2 , and this can be explained as being a result of the narrow microporosity of these materials. Whereas, CO2 can penetrate into the micropores of these materials, N2 cannot, and this is likely due to the diffusion restrictions imposed by its lower kinetic energy at 77 K [21,39]. Since the saturation pressure of CO2 is very high (2.61 × 104 mmHg), the CO2 adsorption isotherms (Fig. 1) show a low relative pressure interval (p/p0 < 0.035). These pressures correspond to the first stages of the adsorption process, where the adsorbate fills the smallest micropores accessible to it, and covers the walls of the larger ones [39]. The shapes of these isotherms are concave with respect to the relative pressure axis, but they

Fig. 1. Adsorption isotherm of CO2 at 273 K on HS and their fractions: (a) Fluka and (b) Acros Organics. (
) HS, () HU, () GHA, ( ) BHA and ( ) FA.

cannot be classified in accordance with IUPAC as they are incomplete. The apparent surface area of different humic samples increases as their molecular complexity decreases (FA > BHA > GHA), as shown in Table 2. This is probably due to increased intramolecular aggregation as macromolecule molecular mass increases (Table 1), and also to decreased polarity with increased structural complexity. HU consist of humic macromolecules that form particles with low apparent surface areas. These low apparent surface areas are consistent with the hypothesis that light fractions fill the pores and voids present in the 3D structure of humic materials, as reported previously [20,40]. The range of apparent surface area values is higher for Acros Organics samples than for Fluka samples, indicating higher apparent surface area heterogeneity for the former. The Dubinin parameter, D, is related to the average size of the microporosity distribution. This parameter increases with increase in the distribution interval, and the micropore size. Table 2 shows that the values for the D parameter vary from 0.10 to 0.20, indicating similar porosity distributions for the different samples. Noteworthy, however, is that FA present the lowest D values, and thus, the narrowest micropore distribution. The characteristic energy (E), calculated from the basic Polanyi–Dubinin theory, makes it possible to gain complementary information about micropore filling.

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Table 2 Results obtained by applying DR method to N2 (77 K) and CO2 (273 K) adsorption isotherms N2 (77 K) As

(m2 g−1 )

CO2 (273 K) nam (mmol g−1 )

As (m2 g−1 )

D

E (kJ mol−1 )

Fluka HS HU GHA BHA FA

<1 <1 <1 <1 <1

0.42 0.56 0.61 0.69 0.78

42.5 57.3 62.1 70.8 79.8

0.14 0.14 0.14 0.14 0.10

28.4 28.5 28.0 28.0 32.7

Acros Organics HS HU GHA BHA FA

<1 <1 <1 <1 <1

0.25 0.42 0.51 0.89 0.96

26.0 43.1 52.4 89.9 97.7

0.20 0.18 0.16 0.15 0.12

23.1 25.0 26.3 26.9 30.3

The characteristic energy (Table 2) is an energetic term that is a function of the adsorbate packing, and of the average size of the micropores of the solid. The values of E vary from 28 to 32 kJ mol−1 for Fluka samples, and from 23 to 33 kJ mol−1 for Acros Organics samples. These intervals are quite similar despite the chemical complexity of these substances. Moreover, the highest E values are observed for fulvic fractions, indicating that they have larger adsorbate–adsorbent interactions than heavier fractions. In summary, the FA fractions adsorb more CO2 , which indicates that they have a higher apparent surface area (As ). Additionally, FA have the highest characteristic energies, which indicates that they have the highest adsorbate–adsorbent interaction. The other fractions show significant decreases in the amount of adsorbed CO2 , which may be interpreted as a reduction in the apparent surface areas. The values of the D and E parameters do not show great variation between the various samples, as they all have similar micropore amounts, sizes, and distributions.

0.32 to 0.60 nm (Fig. 3a), in agreement with results obtained by applying the HK method. 3.3. Mercury intrusion porosimetry Fig. 4 shows the differential pore volume distributions obtained by means of mercury porosimetry for the various humic samples studied here. Between 10 and 104 nm, all

3.2. Hovarth–Kawazoe micropore distributions The Hovarth–Kawazoe (HK) method gives information about the distribution of micropores in a solid. Fig. 2 shows the differential microporosity distributions for the various humic samples studied in this work. All the samples show maxima at 0.34 and 0.42 nm, which are more defined for the FA and BHA samples. The heights of these maxima progressively decrease as follows: FA > BHA > GHA. On the other hand, FA samples show another maximum, shorter but wider, around 0.35–0.40 nm. This maximum is also present for the BHA and GHA samples, but shifted to lower diameter values. The absence of this maximum in HS plots supports the hypothesis that the light fractions (i.e. FA) block the pores of the heavier ones (BHA, GHA and HU). The comparison between experimental and theoretical results, obtained by applying computational techniques, is shown in Fig. 3. The computational model shows that micropore sizes vary from

Fig. 2. Differential micropore volume distributions of HS and their fractions, obtained by appying Hovarth–Kavazoe method to the adsorption isotherm of CO2 at 273 K: (a) Fluka and (b) Acros Organics. (
) HS, () HU, () GHA, ( ) BHA and ( ) FA.

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Fig. 3. (a) Two perspectives (van der Waals surfaces) of two aggregated monomer HS. Circles indicate the pores and their size. (b) Results obtained for two polymers of 23 units under the same conditions.

Fig. 4. Differential pore distributions obtained by means of mercury porosimetry: (a) Fluka and (b) Acros Organics. (
) HS, () HU, () GHA, ( ) BHA and ( ) FA.

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the samples show two maxima each, situated at 1170 and 3620 nm. These maxima are higher for FA, and decrease progressively with BHA and GHA, indicating that the volume of these macropores decrease as the size and complexity of the molecules increase. For HS and HU these maxima have intermediate values. FA, HS, and HU from Fluka, and FA and HS from Acros Organics show another maximum in this interval at 90 nm. Bearing in mind that this maxima is higher for FA, and that both HS and HU fractions can contain some FA percentage (bound to silicates for HU), it is likely that this maxima indicates the presence of a typical fulvic macropore. From 104 to 105 nm, the pore volume distributions show strong a maximum for GHA and HS samples that decreases as the size and complexity of the macromolecules decreases. This maximum can be assigned to the voids generated during the humic material’s aggregation process [20,40], as shown in Fig. 3b. The coincidence of the maxima among the fractions of the same series, and among the same fractions of different series, points toward a textural similarity common to all humic materials [12,16]. This is observed despite the fact that these materials have some plastic character, and can be deformed when pressure is applied during mercury intrusion. 3.4. Pore volume distributions Fig. 5 shows the pore volume distributions, the total pore volume (obtained by adding the volumes of micro, meso and macropores), and the total pore volume, VT , obtained by means of He, ρ(He) , and Hg, ρ(Hg) , density measurements (Table 1) in accordance with VT =

1 1 − ρ(Hg) ρ(He)

The total pore volume obtained by adding the volumes of micro, meso and macropores, and that obtained by means of He and Hg density measurements, show significant agreement despite their different theoretical bases. Micropore volumes were found to be quite low for all of the samples, in reference to the total pore volume. The

Fig. 5. Pore volume distributions, total pore volume (obtained by adding volumes of micro, meso and macropores), and total pore volume obtained by means of He and Hg density measurements. (a) Fluka, and (b) Acros Organics.

most microporous fractions are the simplest ones (FA). The mesopore volumes obtained by mercury porosimetry are in contrast with the complete lack of adsorption of N2 at 77 K. This is likely the result of the fact that the three dimensional structure of humic samples can be deformed at high pressures during mercury intrusion due to their plastic character. The macropore volume, including the interparticle volume, is the volume of mercury accumulated from pore lengths of 10–100 ␮m. For all of the samples, the volume of the macropores is higher than that of the micro or mesopores, which indicates that humic materials are, essentially, macroporous materials.

4. Conclusions The density of the packing of humic samples is a function of their structural complexity. The simplest fractions (FA) are dense and made up of small particles while the most complex ones (GHA) are large porous sponges. Accordingly, humic substances can be regarded as large sponges with voids filled with FA particles. They are solids with a very narrow microporosity where N2 cannot penetrate due to the diffusion restrictions imposed by its lower kinetic energy at 77 K. Fulvic acids have the largest apparent surface area, and the narrowest microporosity distribution of the fractions studied here. Coincidence of the maxima in the micro, meso, and macropore distributions, among the different humic fractions, points toward a textural similarity between them, independent of their origin. The results obtained applying computational chemistry methods corroborate those derived from experiment, and make this technique a tool that should be considered for wider application in surface chemistry.

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