New insights into the dynamics of adsorption equilibria of humic matter as revealed by radiotracer studies

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ScienceDirect Geochimica et Cosmochimica Acta 133 (2014) 362–371 www.elsevier.com/locate/gca

New insights into the dynamics of adsorption equilibria of humic matter as revealed by radiotracer studies Holger Lippold ⇑, Johanna Lippmann-Pipke Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, (Research Site Leipzig), Permoserstr. 15, 04318 Leipzig, Germany Received 6 September 2013; accepted in revised form 4 March 2014; available online 14 March 2014

Abstract The mobility of contaminants in the subsurface hydrosphere can be governed by their interaction with aquatic humic substances, which may act as carriers. For modelling migration processes, retardation of humic molecules at mineral surfaces must be considered. There is, however, a lack of clarity concerning the reversibility of adsorption of these natural polyelectrolytes. In this work, evidence was provided that a dynamic adsorption equilibrium exists. For this purpose, adsorption of humic substances (purified Aldrich humic acid and an aquatic fulvic acid) onto kaolinite was examined in tracer exchange studies by means of 14C-labelled humic material. In addition, the kinetics of adsorption and desorption were investigated in batch experiments. Attaining the equilibrium state of adsorption took considerably longer for the humic acid than for the fulvic acid (24 h and 4 h, respectively). In desorption experiments, initiated by diluting the supernatant at constant pH, no net release was observed for both substances within a time frame of 4 weeks. However, when introducing radiolabelled humic or fulvic acid as a tracer into pre-equilibrated adsorption systems in the state of surface saturation, quantitative exchange was found to take place. This indicates that adsorption of humic matter is in fact a reversible process, albeit an exchange time of more than 4 weeks was required for both humic materials. Models for humic-bound contaminant transport (presuming dynamic equilibria) are thus applicable under appropriate conditions. Ó 2014 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Interaction with dissolved organic matter (DOM) has been recognised to be decisive for the mobility of organic and inorganic contaminants with low affinity for the aqueous phase, i.e., hydrophobic organic compounds that are sparingly soluble or higher-valent metals that are strongly adsorbed to mineral surfaces. Complexation with humic and fulvic acids, major constituents of DOM, can dominate the species distribution of toxic or radioactive metals (Dearlove et al., 1991; Choppin, 1992; Kim et al., 1992), and hydrophobic domains of DOM are a favourable

⇑ Corresponding author. Tel.: +49 3412352623.

E-mail address: [email protected] (H. Lippold). http://dx.doi.org/10.1016/j.gca.2014.03.003 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved.

micro-environment for non-polar organic pollutants (Gauthier et al., 1987; Chin et al., 1997; Tanaka et al., 1997). Owing to these amphiphilic properties, humic matter is also subject to a solid-liquid distribution in geochemical systems. Migration of bound contaminants is thus essentially determined by the adsorption behaviour of humic matter. For a quantitative assessment of the mobility of contaminants in flow systems, reactive transport models were developed that take retardation of humic carriers into account (Johnson et al., 1995; Knabner et al., 1996; Jordan et al., 1997; Lu¨hrmann et al., 1998; Tatalovich et al., 2000; Bryan et al., 2005; Kim and Kim, 2007). An outline of the various concepts is given in Lippold and Lippmann-Pipke (2009). All of these approaches are based on the advection– reaction–dispersion equation. For solid–liquid distribution, a dynamic exchange between adsorbed and dissolved state

H. Lippold, J. Lippmann-Pipke / Geochimica et Cosmochimica Acta 133 (2014) 362–371

is presumed. To ensure a steady local equilibrium under flow conditions, reaction rates of adsorption and desorption must be high enough. Non-equilibrium conditions in case of slow kinetics can be considered in some of the above models. Nevertheless, reversibility of adsorption remains a prerequisite. Humic compounds, however, are highly charged polyelectrolytes with a multitude of reactive groups. It is thus questionable whether the requirement of full reversibility is actually met for these substances. Adsorption of natural organic matter has been a topic of many studies for several decades. Besides retardation/ mobilisation of contaminants, the process is also of great importance for the stability of soil aggregates and particulate matter in natural waters, since the physical and chemical surface characteristics of solids and colloids are fundamentally changed by the formation of humic coatings (Tate and Theng, 1980; Tipping and Higgins, 1982; Gibbs, 1983). The acidity and chelating capacity of humic substances can accelerate the weathering process of minerals (Schnitzer and Kodama, 1976; Singer and Navrot, 1976; Tan, 1980). Complexation with released metals in turn influences the sorption behaviour of humic molecules. Ligand exchange (i.e., reaction of humic hydroxyl or carboxyl groups with surface hydroxyl groups, involving release of H2O or OH) has been identified to be the dominating mechanism of adsorption of humic substances onto mineral surfaces (Parfitt et al., 1977; Tipping, 1981; Davis, 1982; Jardine et al., 1989; Murphy et al., 1992; Gu et al., 1994, 1995; Schlautman and Morgan, 1994; Weigand and Totsche, 1998). This was evidenced by FTIR and NMR spectra of adsorbates, negative enthalpies of adsorption, and increases in pH. This specific chemisorption is accompanied by non-specific electrostatic interaction, which is the main reason for the influence of pH, controlling the protonation of humics and surfaces. The fact that adsorption still occurs at pH values above the point of zero charge (PZC) of minerals, where both surface and humic colloids are negatively charged, gave rise to the conclusion that additional mechanisms are operative. Van-der-Waals attraction (Schlautman and Morgan, 1994; Zhou et al., 1994; Niitsu et al., 1997) and hydrogen bonding (Parfitt et al., 1977; Murphy et al., 1990) were suggested. Furthermore, the presence of persistently bound metals can affect sorption substantially. Besides blocking effects, formation of metal bridges may be important, as was pointed out by Theng (1976) and Sposito (1984). From calorimetric studies, Jardine et al. (1989) as well as Baham and Sposito (1994) found that adsorption of DOM can be entropy-driven. They recognised the hydrophobic effect (Tanford, 1980) as a relevant mechanism, i.e., “repulsion” from water due to a gain in entropy on removal of highly structured water around non-polar molecular moieties. This conclusion is corroborated by the fact that adsorption of DOM – as a multi-component system – was found to involve fractionation effects, where hydrophobic constituents are enriched at mineral surfaces (Jardine et al., 1989; Dunnivant et al., 1992; Gu et al., 1995; Kaiser and Zech, 1997; Ussiri and Johnson, 2004). In some respect, this is in accordance with numerous studies indicating that adsorption of larger humic molecules is preferred over that

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of smaller ones (Davis and Gloor, 1981; Tomaic and Zutic, 1988; Ochs et al., 1994; Wang et al., 1997; Meier et al., 1999; Namjesnik-Dejanovic et al., 2000; Zhou et al., 2001; Hur and Schlautman, 2003), since the density of hydrophilic acidic groups is higher in smaller molecules. Preferential adsorption of larger molecules is, however, typical of neutral polymers as well (Cohen Stuart et al., 1980). The selectivity towards high-molecular-weight fractions of DOM is consistent with an obvious preference for aromatic structures, which are more abundant in larger molecules (Dunnivant et al., 1992; McKnight et al., 1992; Gu et al., 1995; Wang et al., 1997; Meier et al., 1999; NamjesnikDejanovic et al., 2000; Zhou et al., 2001; Feng et al., 2006). There are also systems where fractionation patterns deviate from these general trends (Davis and Gloor, 1981; Zhou et al., 2001; Hur and Schlautman, 2003; Wang and Xing, 2005; Reiller et al., 2006). This may be attributed to the fact that hydrophobic interaction is more or less important, depending on the type of mineral. Furthermore, humic materials differ in the characteristics of their molecular weight fractions. Adsorptive fractionation was shown to be a major reason for tailing of breakthrough curves in transport studies (Seders-Dietrich et al., 2013). In view of these polydispersity effects, mechanistic modelling of DOM adsorption is per se limited to “averaging” approaches. In many studies, the Langmuir equation was found suitable for the parameterisation of adsorption isotherms, being aware that the basic assumptions of this simple model are not actually valid in this case (Parfitt et al., 1977; Tipping, 1981; Jardine et al., 1992; Murphy et al., 1992; Day et al., 1994; Schlautman and Morgan, 1994; Wang et al., 1997; Zhou et al., 2001; Hur and Schlautman, 2003). Several authors considered the mechanism of adsorption in more detail. Vermeer et al. (1998) calculated adsorption profiles of humic-like model polyelectrolytes ab initio based on the self-consistent field (SCF) theory of Scheutjens and Fleer (1979), where the equilibrium distribution of polymer segments within a lattice structure is determined from all interactions, iteratively minimising the Gibbs free energy of the system. Lenhart and Honeyman (1999) included adsorption of humic acid in an electrostatic surface complexation model (SCM). Here, humic acid was represented as a set of monoprotic acids with different pKa values. More acidic and less acidic components were assumed to be adsorbed by ligand exchange and by electrostatic attraction, respectively. A more sophisticated model (ADAPT) was developed by Weng et al. (2007). They employed the NICA model to describe ligand exchange reactions of humic functional groups with a continuous affinity distribution, and used the Donnan model to describe electrostatic interactions. Equilibrium adsorption models such as the above rely on reversibility. However, in addition to fractionation processes, binding of DOM to mineral surfaces is characterised by hysteresis effects, i.e., deviations between adsorption and desorption isotherms. In spite of its importance for modelling transport processes, desorption of humic matter has received much less attention than adsorption. Results from existing studies provide a diverse picture. In desorption experiments initiated by diluting the

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supernatant after an adsorption step, hardly any desorption of humic material was observed for iron oxide, aluminium oxide or clay mineral surfaces over extended time periods (Murphy et al., 1992; Gu et al., 1994, 1995; Zhou et al., 1994; Avena and Koopal, 1998; Joo et al., 2008), with no difference between hydrophilic and hydrophobic DOM fractions (Gu et al., 1995). Accordingly, elution of DOM in column experiments was found to be incomplete (Dunnivant et al., 1992; Weigand and Totsche, 1998). In contrast, Baham and Sposito (1994) reported that DOM adsorption onto clay minerals was fully reversible. Little or no hysteresis was also observed for sand surfaces, where adsorption of DOM is comparatively weak (Weigand and Totsche, 1998; Joo et al., 2008). For describing adsorption/desorption isotherms, Gu et al. (1994) derived a kinetic Langmuir model with a hysteresis coefficient allowing for completely reversible up to completely irreversible adsorption processes. Contrary to desorption upon dilution, humic substances are readily desorbed by increasing the pH value (Davis, 1980; Wang et al., 1997; Avena and Koopal, 1998; Vermeer et al., 1998). This might be ascribed to the induced decrease in affinity, but in principle, the starting situation is the same in both cases: an excess of adsorbed material, referring to the adsorption isotherm at the respective conditions. In consideration of this ambiguous state of knowledge, the objective of the present study was to obtain an improved understanding of the reversible/irreversible character of adsorption of humic matter onto mineral surfaces. For this purpose, advantage was taken of the principle of tracer exchange, using humic and fulvic acids that were partly radiolabelled with 14C. In case of reversibility, a dynamic equilibrium exists, i.e., a permanent run of adsorption and desorption at equal rates. Such an exchange can be detected by introducing a radiotracer into pre-equilibrated systems where all adsorption sites are occupied. Additional time-dependent studies were conducted to investigate the kinetics of adsorption and desorption. Kaolinite was chosen as an adsorbent, motivated by the fact that contradictory findings on reversibility were reported for clay minerals (Murphy et al., 1992; Baham and Sposito, 1994). A natural aquatic fulvic acid and purified Aldrich humic acid were used as humic components. Even though the provenance of the coal-derived Aldrich material is not very close to environmental systems, it has often been shown that its behaviour is in accordance with other natural humic acids (e.g., Kim et al., 1990; Reiller et al., 2002; Hur and Schlautman, 2003). For the purpose of comparison, tracer exchange experiments were also performed with the REE terbium(III), using the radioisotope 160Tb as a tracer.

bog “Kleiner Kranichsee” (near Carlsfeld, Germany). Purification was carried out according to the recommendations of the International Humic Substances Society (Aiken, 1985). Basic properties of the humic materials are specified in Table 1. Contents of C, H, N, S and O were measured with a Vario EL III elemental analyser (Elementar, Germany). Acidities were determined as described by Stevenson (1994) by means of an automatic titrator TitroLine alpha (Schott, Germany). Analyses of residual metal contents were conducted by ICP-OES, using a Spectroflame P/M instrument (Spectro, Germany). Kaolinite (KGa-1b) standard material was obtained from the Source Clays Repository of the Clay Minerals Society of America. It was used without further pretreatment. 2.2. Preparation of radiotracers Radiolabelling of humic substances was accomplished by an azo-coupling reaction with [14C]aniline (Mansel and Kupsch, 2007). For generating the reactive benzenediazonium compound, [14C]aniline hydrochloride (Biotrend, Germany) with an activity of 6 MBq was added to 400 lL of a solution of 0.1 M NaNO2 in 1 M HCl, placed in an ice bath. 50 lL of this mixture were then given to 1000 lL of a solution of 0.5 g L1 of humic or fulvic acid in borate buffer (also cooled to 0 °C), together with 80 lL of 0.1 M NaOH to maintain the pH value in a range between 8 and 9. This mixture was allowed to react for 30 min. In order to separate the radiolabelled humic compound from non-reacted radioactive material, HA was precipitated by acidification to pH < 1 with concentrated HClO4. After centrifugation, the supernatant was removed, and the precipitate was redissolved in 3 mL 10–2 M NaOH. This procedure was repeated for 10 times until the activity in the supernatant was negligible. Finally, the precipitate was dissolved in 0.1 M NaClO4/10–3 M NaOH. FA (which cannot be precipitated) was purified by ultrafiltration, using Microsep centrifuge filters (Pall, US) with a MWCO of 1 kDa. The retentate was rinsed with 1 mL 0.1 M NaClO4 for 10 times until the activity in the filtrate was insignificant. Specific activities obtained were about 1 MBq per mg of HA or FA. [160Tb]Tb(III) was produced by neutron activation of natural Tb(III) at the TRIGA Mark II reactor of the University of Mainz. 40 h of irradiation at a neutron flux of 7  1011 n cm2 s–1 yielded a specific activity of 2 MBq mg–1. After transformation into a perchlorate system by evaporating with concentrated HClO4, a stock solution in 0.1 M NaClO4 was prepared.

2. MATERIALS AND METHODS 2.3. Adsorption and desorption experiments 2.1. Materials Humic acid (HA) from Sigma–Aldrich (Germany) was purified by repeated precipitation and redissolution with 0.1 M HCl and 0.1 M NaOH/0.01 M NaF, respectively, followed by dialysis and lyophilisation. An aquatic fulvic acid (FA) was isolated from surface water collected on the raised

All experiments were conducted at room temperature. A suspension of kaolinite in 0.1 M NaClO4 (pH 4.7) was dispensed into 5 mL centrifuge tubes while stirring, and a small amount of 14C-labelled HA or FA in 0.1 M NaClO4 (adjusted to pH 4.7) was then added, yielding a total volume of 4 mL and a suspension concentration of 5 g L1

H. Lippold, J. Lippmann-Pipke / Geochimica et Cosmochimica Acta 133 (2014) 362–371 Table 1 Humic materials: elemental compositions, contents of acidic groups and contents of residual metals. Aldrich HA

Bog water FA

C H N S O

(wt.%)

53.2 4.3 0.8 3.7 38.0

53.0 4.3 0.6 0.6 41.5

Total acidity COOH groups Phenolic OH groups

(meq g1)

6.0 5.2 0.8

8.1 5.5 2.6

(mg g–1)

n.d. 0.6 n.d. 3.3 n.d. 3.2 n.d.

0.9 1.8 0.2 0.4 0.1 5.5 0.2

Al Ca Cu Fe Mg Si Zn

a

n.d. = not detected. a Taken as difference between total acidity and COOH content.

kaolinite. The concentration of the humic component was 150 mg L1 in adsorption experiments and 40 mg L1 in desorption experiments. After rotating end-over-end at 10 rpm for different equilibration times, the systems were centrifuged at 7000 rpm for 10 min, and an aliquot of the supernatant (3 mL) was removed. Adsorption was determined radioanalytically from the depletion relative to a reference solution. For desorption experiments, the removed part of the supernatant was replaced by 3 mL 0.1 M NaClO4 solution (pH 4.7), and the diluted systems were again rotated for different time periods (up to 4 weeks). After centrifugation, aliquots were analysed for changes in concentration. Analyses were carried out with a Tri-Carb 3110 TR Liquid Scintillation Analyser (Perkin Elmer, US), using Ultima Gold scintillation cocktail (Perkin Elmer, US). Wall adsorption was found to be insignificant. 2.4. Tracer exchange experiments An experimental procedure similar to that described above for adsorption studies was applied. Labelled and non-labelled substances were, however, separately introduced. A series of kaolinite suspensions was prepared with non-labelled HA, FA or Tb(III) at a range of concentrations (4 mg L–1 – 180 mg L1), covering an adsorption isotherm including the plateau region. The systems were completed by adding a small amount of the radiotracer (14C-labelled HA/FA or 160Tb). The concentration of the tracer solution was 0.1 mg L–1, and the added volume of 20 lL was negligible compared to the total volume of 4 mL, i.e., the composition of the systems was not significantly changed. Two different procedures were applied: In 1-step experiments, the radiotracer was instantaneously added together with the non-labelled substance. In 2-step experiments, the system was first allowed to equilibrate with the non-labelled substance for 48 h, and the radiotracer was introduced

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subsequently. Different time periods for exchange (up to 4 weeks) were then admitted. Adsorbed or exchanged amounts were determined from the decrease in the concentration of the tracer in the supernatant. In order to correct the measured 14C decay count rates for colour quenching caused by humic material, equilibrium concentrations of HA and FA were first determined by means of UV–Vis spectrometry, using a Lambda 45 spectrophotometer (Perkin Elmer, US). Quenching as a function of HA/FA concentration was measured for a fixed 14C activity comparable to the experimental conditions. 3. RESULTS AND DISCUSSION 3.1. Kinetics of adsorption and desorption Adsorption of humic and fulvic acid onto kaolinite as a function of time is shown in Fig. 1. The chosen starting concentration of 150 mg L–1 is situated in the plateau region of the adsorption isotherms of HA and FA, where adsorbed amounts are at maximum (see Fig. 4). The time needed to reach this final state is substantially different for the two humic materials. For FA, equilibrium is attained within 4 h, whereas 24 h are required for HA. Similar results were obtained at lower concentrations. Presumably, this difference is a consequence of the higher average molecular weight of HA. It has been suggested in the literature that adsorption of humic matter is accompanied with intrinsic competition processes: Initially, smaller humic molecules are rapidly adsorbed, but subsequently, they are replaced by larger molecules in a comparatively slow process. Evidence was provided by means of size exclusion chromatography (Zhou et al., 2001; Hur and Schlautman, 2003), field-flow fractionation (Pitois et al., 2008) or UV–Vis spectrometry (Ochs et al., 1994; Joo et al., 2008; Pitois et al., 2008), showing directly or indirectly that the average molecular weight of humic compounds remaining in the supernatant slowly decreased with time. It should be noted that a displacement of small molecules was not actually detected in these studies. The occurrence of competitive exchange processes was, however, theoretically predicted on the basis of a kinetic multicomponent Langmuir approach (NOMADS model) by Van de Weerd et al. (1999). Since displacement is more complicated for larger molecules, this process may take more time in the case of HA. An alternative explanation was given by Vermeer et al. (1998), who pointed out that an electrostatic barrier is built up on adsorption of the negatively charged HA molecules. This reasoning applies, however, to FA as well, where adsorption was found to be more rapid. Fig. 2 shows the results of desorption experiments, initiated by diluting the supernatant after an adsorption step. A lower starting concentration (40 mg L–1) was chosen here, since the dilution step should comprise concentrations within the sloped part of the adsorption isotherm. The horizontal lines indicate the expected equilibrium concentrations, assigned to the residual total concentrations on the basis of the respective adsorption isotherm. As can be seen from the solution concentrations as a function of time, these target concentrations are not even approached during an

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Fig. 1. Adsorption of humic acid (a) and fulvic acid (b) onto kaolinite as a function of time (150 mg L1 HA or FA, 5 g L1 kaolinite, 0.1 M NaClO4, pH 4.7).

equilibration time of 672 h (4 weeks). For HA, within the limits of accuracy, no change in concentration was observed over the entire time span. For FA, even a minor decrease was detected, although no comparable trend was found in the adsorption kinetics (Fig. 1b). Obviously, the expected desorption process does not take place for both humic materials. This is in accordance with comparable studies published in the literature, where little or no desorption was observed upon dilution (Murphy et al., 1992; Gu et al., 1994, 1995; Zhou et al., 1994; Avena and Koopal, 1998; Joo et al., 2008). Several explanations have been proposed for hysteresis effects in the adsorption of humic substances. Zhou et al. (1994) and Joo et al. (2008) suggested that part of the humic molecules are irreversibly bound by chemical adsorption (ligand exchange), while others are reversibly bound by physical adsorption (electrostatic or hydrophobic interaction). Avena and Koopal (1998) considered the existence of a subsurface layer of adsorbed molecules, from where desorption is kinetically hindered. Multi-layer adsorption is, however, not observed for humic substances. The most plausible explanations for restricted reversibility were given by Gu et al. (1994, 1995). They argued that each humic molecule is bound by a multitude of sorption sites, and desorption requires simultaneous detachment.

Furthermore, they explicated that hysteresis is an inevitable consequence of the fractionation processes that are typically involved in the adsorption of humic matter. Since the solid surface is enriched in high-affinity components, the composition of the supernatant is not unique in the adsorption and desorption phase of the experiment. This “pseudo hysteresis” was also quantitatively demonstrated by means of the NOMADS model by Van de Weerd et al. (1999). In view of the absence of any measurable release of HA or FA, however, a non-equilibrium situation appears to be most probable, i.e., desorption is kinetically hindered, and rates are below the limits of observability. 3.2. Equilibrium characteristics of adsorption of humic matter The above results suggest that binding of humic substances to kaolinite is nearly irreversible. This one-way situation has implications with respect to the equilibrium characteristics of the adsorption process. Models on solute transport rely on a permanent exchange between adsorbed and dissolved state, sufficiently rapid to keep up with the velocity of advection. However, if no desorption occurs, one may conclude that such a dynamic equilibrium cannot exist. Instead, the observed equilibrium may be a static one,

Fig. 2. Desorption of humic acid (a) and fulvic acid (b) from kaolinite as a function of time, initiated by dilution of the supernatant (see Section 2.3. for experimental details). The equilibrium lines indicate the corresponding equilibrium concentrations based on the adsorption isotherms.

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comparable to “adhesion”. For identifying the dynamic or static character of an equilibrium process, radiotracer experiments are uniquely suited, since radiolabelling offers the opportunity to distinguish between chemically identical species. In this way, it is possible to gain direct insight into the dynamics of equilibria that appear static to the eye. The experimental approach is schematically illustrated in Fig. 3. It is based on the state of surface saturation, where all available sorption sites are occupied. Macroscopically, this state is visible from the fact that an increase in concentration no longer results in an increase in adsorption (plateau region of the adsorption isotherm). In this situation, radiolabelled humic molecules (HS*) are added to the system. The behaviour of this tracer unveils the character of the equilibrium on a microscopic level. In case no adsorption is found for HS*, the equilibrium is identified to be a static one: The surface is blocked by non-labelled molecules (HS) that cannot be replaced, i.e., adsorption is irreversible. If, however, a decrease in the concentration of HS* in the supernatant is detected, an exchange must have taken place, since free binding sites are not present in the state of surface saturation. Consequently, a dynamic equilibrium of adsorption and desorption exists, and reversibility is thereby proven. In order to test the experimental setup, first studies were performed with a metal, terbium(III), where reversible adsorption may be presumed. The radioisotope 160Tb was used as a tracer. As described in Section 2.4., a small amount of it was instantaneously or subsequently introduced to systems containing non-radioactive Tb(III) at a range of concentrations. The experiments were not limited to the state of surface saturation but were set up to cover a complete adsorption isotherm. Equilibration times before and after tracer addition were 48 h. In Fig. 4a, adsorption isotherms of Tb(III) are shown for both experimental procedures – 1-step equilibration (which corresponds to a conventional adsorption experiment) and 2-step equilibration (where the tracer encounters the equilibrium state). At equilibrium concentrations higher than 60 mg L–1, adsorption reaches its plateau level, indicating complete occupation of available sorption sites. However, both isotherms coincide throughout the concentration

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range, even though the tracer – if subsequently added – is confronted with saturated surfaces. This clearly shows that there is a permanent exchange between adsorbed state and dissolved state. The radiotracer is involved in a dynamic equilibrium, and the reaction rates are high enough that the position of this equilibrium is adequately represented within the experimental time frame. Tracer exchange experiments with humic substances were carried out analogously. Here, the tracer was produced by radiolabelling with 14C. The stability of this label, which is essential for these investigations, was ascertained in ultrafiltration tests after different standing times. Furthermore, it is important that the labelling reaction does not show a pronounced selectivity towards any fractions with specific properties, and that the chemical modifications are minor enough to keep the behaviour of the labelled molecules unaffected. 14C is introduced as a phenyl group. The proportion of these non-humic phenyls is, however, less than 2 wt%. Significant implications in respect of sorption properties are thus unlikely. To prove this, comparative adsorption studies were performed using UV–Vis spectrometry and 14 C tracer analysis (Supplementary information, Appendix A, Fig. A.1.). Adsorption isotherms obtained were nearly coincident, indicating that there are no alterations in the sorption behaviour and that the labelling is as representative as data analysis based on UV–Vis absorption. The results of radiotracer experiments with humic acid are shown in Fig. 4b. Obviously, they are quite different from those found for Tb(III); the adsorption isotherms obtained in 1-step and 2-step procedures are not identical. In the latter case, the adsorption plateau is lowered by about one third. However, it is not shifted to zero, which is the most remarkable finding. This leads to the conclusion that an exchange takes place for humic molecules as well, notwithstanding their size and multiple bonding. Evidently, adsorption equilibria of humic matter are not static; there is a permanent movement onto the solid surface and back into the solution. On the other hand, the overall adsorption equilibrium is not quantitatively represented by the tracer. Strictly, the term “adsorption isotherm” is thus no longer applicable, since only apparent adsorbed amounts are displayed.

Fig. 3. Principle of radiotracer exchange experiments for probing the equilibrium characteristics of adsorption of humic substances (HS).

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(a)

(b)

Fig. 4. Isotherms of adsorption of terbium(III) (a) and humic acid (b) onto kaolinite in 0.1 M NaClO4, obtained with 160Tb and 14C-labelled HA as radiotracers, which were added at small amounts together with the non-radioactive substances (1-step experiment, open circles) or after an equilibration period (2-step experiment, full circles). Suspension concentrations for systems with Tb(III) and HA were 25 g L1 and 5 g L1, resulting in pH values of 4.4 and 4.7, respectively.

It seemed likely that different results would be obtained if the time for exchange was varied. This supposition was confirmed, as can be seen in Fig. 5. If the second equilibration period is reduced to 6 h, the plateau level (section B) is even more lowered. In contrast, if equilibration times are increased to 168 h (1 week) and 672 h (4 weeks), the plateau level is shifted to higher values. The more time the tracer is given, the more exchange takes place. Finally, the tracer distribution approaches the equilibrium state reflected by the adsorption isotherm for instantaneous tracer addition. Note that a maximum occurs in the region of incomplete surface coverage (section A in Fig. 5), most pronounced for short times of exchange. As long as there is no lack of free sites (linear part of the adsorption isotherm), solid–liquid distribution of the tracer equals that of the bulk of the humic material. Not before surface coverage is nearly complete, the kinetic hindrance comes into effect. The transition is not abrupt but extends over a certain concentration range that corresponds to the curved part of the adsorption isotherm (1-step experiment). The gradual change is not surprising, considering the macromolecular nature of humic matter. It reflects the decrease in the statistical occurrence of free surface spots that are large enough for direct access of a tracer molecule. Corresponding series of experiments were conducted with fulvic acid. The results were found to be similar (Supplementary information, Appendix A, Fig. A.2). In Fig. 6, the averaged data of all plateaus for HA and FA are plotted as a function of time. As in Figs. 4 and 5, adsorbed amounts are based on the measured decrease in the concentration of the tracer in the supernatant, scaled up to a decrease in the solution concentration of total HA or FA, which is, of course, an arbitrary transformation unless the tracer is in equilibrium. This state is indicated by the horizontal lines, showing the respective plateau values of the adsorption isotherms obtained in 1-step experiments. It can be seen that these values are asymptotically approached. After 4 weeks, solid–liquid distribution is almost adequately represented by the tracer. Obviously, the equilibrium is a dynamic one, but the exchange process is

so slow that it takes such a long time until the tracer is “fully involved”. For a quantitative comparison of the kinetics of adsorption and tracer exchange, first-order rate constants were calculated on the basis of an approach-to-equilibrium model, Eq. (1): d½HS ads  ¼ kð½HS eq ads  ½HS ads Þ dt

ð1Þ

[HS*]ads is the concentration of adsorbed tracer molecules at time t after addition in kinetic adsorption experiments or in 2-step exchange experiments, ½HS eq ads is the concentration of adsorbed tracer molecules at equilibrium. In exchange studies, this equilibrium state is represented by the 1-step experiments. The rate constants k are given in Table 2. From a mechanistic point of view, Eq. (1) is not actually adequate for the kinetics of adsorption, since the process was found to be reversible. This approach was chosen solely for the purpose of comparison. The rate constants for tracer exchange are two orders of magnitude lower than those for adsorption. However, in consideration of the desorption experiments, conducted over the same time period, it was not to be expected that a dynamic exchange (at measurable rates) would exist at all. In fact, both findings seem to be contradictory – if there is no desorption, there cannot be an exchange as well. It must be noted that the state of saturation does not necessarily mean that the whole surface is completely covered. For kaolinite, adsorption of humic matter concentrates on the edge faces of the crystallites because hydroxyl groups as binding sites are much less abundant on the basal faces (Sposito, 1984; Murphy et al., 1990; Brady et al., 1996). Surface saturation means occupation of all sites that are available under the given conditions. Nonetheless, considering the low concentration of labelled molecules, one might argue that they could occupy some residual sites while the bulk of the humic material is statically adsorbed. However, if such an independent adsorption process existed, it would be very improbable that the tracer distribution would aim at a final state which exactly

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Fig. 5. Tracer exchange experiments with

369

14

C-labelled humic acid, admitting different equilibration periods (conditions as in Fig. 4b).

Fig. 6. Mean plateau values from tracer exchange experiments with humic acid (a) and fulvic acid (b) as a function of equilibration time (experimental details as in Fig. 4b). The equilibrium lines denote the mean plateau values of the adsorption isotherms obtained in case of instantaneous tracer addition.

Table 2 Rate constants for adsorption and tracer exchange according to Eq. (1). HA Adsorption rate constant Exchange rate constant

(0.08 ± 0.01) h

FA 1

(0.004 ± 0.001) h1

(0.58 ± 0.15) h1 (0.003 ± 0.001) h1

represents the overall adsorption isotherm. Moreover, there would be no explanation for the slow kinetics. The apparent discrepancy between the results of desorption and exchange experiments can be resolved if the desorption data are reconsidered in terms of possible fractionation phenomena, as was discussed by Gu et al. (1995). If we imagine that the overall equilibrium distribution of humic material is determined by two hypothetical components – one that is adsorbed very weakly and another that is adsorbed very strongly, the equilibrium concentration will be almost entirely determined by the low-affinity fraction. In the dilution experiments, this fraction is removed for the most part. From this point of view, the equilibrium lines in Fig. 2 are meaningless. If the highaffinity component is adsorbed almost quantitatively, it is conceivable that the very minor change in its solution

concentration will not cause significant net desorption. Of course in reality there are more than two types of humic molecule, but nonetheless it is very probable that the observed behaviour is caused by fractionation, rather than slow desorption kinetics. 4. CONCLUSIONS The present radiotracer studies with 14C-labelled humic material provided evidence that adsorption equilibria of humic substances are characterised by a dynamic exchange between adsorbed and dissolved state. In view of the fact that no net desorption (at comparable rates) was observed in dilution experiments, this finding may be surprising. On the other hand, it is in line with kinetic adsorption studies suggesting a slow exchange of smaller molecules by larger molecules (e.g., Hur and Schlautman, 2003). Obviously, the absence of net desorption upon dilution is not indicative of a static equilibrium without any exchange. The different reaction kinetics observed in dilution and exchange experiments give rise to the assumption that the lacking net desorption is not caused by kinetic hindrance but by fractionation processes. Compared to the overall adsorption kinetics, tracer exchange was found to proceed very slowly. This in turn points to slow desorption kinetics, which was, however,

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not detectable in the dilution experiments. For mineral surfaces with lower affinity towards humic substances, adsorption equilibria are probably more dynamic, i.e., rates of exchange are likely to be higher. In conclusion, it may be inferred that transport modelling based on the advection–reaction–dispersion equation is applicable to humic substances just like to other solutes, as adsorption proved to be reversible. Naturally, the slow kinetics must be taken into account for specifying the conditions of a steady local equilibrium. In case of strongly adsorbing solid phases, flow velocities are required to be extremely low, since desorption must be possible within the time of residence. ACKNOWLEDGEMENTS This work was funded by the German Federal Ministry of Economics and Technology (BMWi), Project Ref. No. 02 E 10176. Technical support by the Johannes Gutenberg University of Mainz (Institute of Nuclear Chemistry) for the production of 160 Tb is gratefully acknowledged.

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