# Assessment of the Sorptive Fractionation of Organic Matter at the Goethite-Water Interface

Marawit Tesfa, Aline Dia, Khalil Hanna, Wei Cheng, Mathieu Pédrot, Rémi Marsac

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Marawit Tesfa, Aline Dia, Khalil Hanna, Wei Cheng, Mathieu Pédrot, et al. Assessment of the Sorptive Fractionation of Organic Matter at the Goethite-Water Interface. Environmental Research, 2025, 276, pp.121505. 10.1016/j.envres.2025.121505. insu-05016554

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Assessment of the Sorptive Fractionation of Organic Matter at the Goethite-Water Interface

Marawit TESFA, Aline DIA, Khalil HANNA, Wei CHENG, Mathieu PEDROT, Rémi MARSAC

PII: S0013-9351(25)00756-X

DOI: https://doi.org/10.1016/j.envres.2025.121505

Reference: YENRS 121505

To appear in: *Environmental Research*

Received Date: 26 September 2024

Revised Date: 28 March 2025

Accepted Date: 29 March 2025

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Please cite this article as: TESFA, M., DIA, A., HANNA, K., CHENG, W., PEDROT, M., MARSAC, R., Assessment of the Sorptive Fractionation of Organic Matter at the Goethite-Water Interface, *Environmental Research*, https://doi.org/10.1016/j.envres.2025.121505.

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© 2025 Published by Elsevier Inc.
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# Assessment of the Sorptive Fractionation of Organic Matter at the Goethite-Water Interface

Marawit TESFA¹,²*, Aline DIA¹, Khalil HANNA³, Wei CHENG⁴, Mathieu PEDROT¹, Rémi MARSAC⁵*

1 Univ Rennes, CNRS, Géosciences Rennes - UMR 6118, F-35000, Rennes, France

2 Subatech – UMR 6457, 4 Rue Alfred Kastler, BP 20722, 44307, Nantes Cedex 3, France

3 Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR - UMR6226, F-35000, Rennes, France

4 College of Resources and Environmental Science, South-Central Minzu University, Wuhan, China

5 Université Paris Cité, Institut de physique du globe de Paris, CNRS, F-75005 Paris, France

<footnote>* Corresponding authors: marawittesfa.research@yahoo.com and remi.marsac@cnrs.fr</footnote>
16 Abstract

The carboxylic (COOH) and phenolic (PhOH) functional groups of natural organic matter (NOM) are the major binding sites for environmental compounds such as minerals and contaminants. This study focusses on the sorptive fractionation of a humic acid onto goethite (α-FeOOH) at various pH, [NOM], [NaCl], and under static (batch) and dynamic flow (column) conditions. During batch experiments at low pH and [NOM], adsorption behaviors of COOH- and PhOH-rich compounds were similar. At low [NaCl] and high pH or [NOM], the adsorption of NOM decreased, with a preferential adsorption of PhOH over COOH-rich compounds. This can be ascribed to the surface site saturation and competition between COOH, PhOH and OH⁻ for surface binding, combined with electrostatic effects. Higher [NaCl], which was previously shown to favor NOM-NOM interactions at mineral surface, led to increased adsorption of COOH, while the adsorption of PhOH was not affected. Under flow-through conditions, multiple-binding-site mechanism was shown to control the fractionation of PhOH- and COOH-rich compounds. These results provide insights into the molecular interactions between the NOM and soil components, which control the fate and behavior of contaminants and nutrients in the environment.

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Keywords: Organic matter, Organo-mineral assemblages, Adsorption, Fractionation, Carboxylic and phenolic functional groups.

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36 Highlights

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• Fractionation between COOH- and PhOH-rich compounds varies with pH, [NaCl], [NOM] and water flow.

• No fractionation was observed when adsorption is optimal (low pH and [NOM])

• Adsorption of PhOH-rich compounds is favored at high pH and high [NOM] because they are less affected by goethite surface charge decrease.

• Partial neutralization of COOH-rich compounds at high [Na⁺] favored their adsorption over PhOH ones.

• Under flow-through conditions, multiple-binding-site mechanism kinetically favored PhOH-rich compounds.

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Graphical abstract

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# 1. Introduction

Investigating the composition of natural organic matter (NOM) and its reactivity towards mineral surfaces is key in understanding the biogeochemical cycle of carbon (C) and assessing the stability and flux of C in the context of climate change.<sup>1–5</sup> In soil and water environments, strong NOM-mineral interactions, resulting in the formation of organo-mineral complexes, protects NOM from degradation by microbial and root enzymes.<sup>6,7</sup> This stabilization phenomena evidently impacts the NOM turnover and its residence time in various compartments (e.g. water, soil, sediments), and thus the C cycle.<sup>8,9</sup> The transport and transfer of poorly soluble elements such as iron (Fe) between environmental compartments is often controlled by the formation of Fe-oxide-NOM organo-mineral colloids.<sup>10–12</sup> As natural organo-mineral assemblages can bind various elements and contaminants (e.g. Cu, As, radionuclides), these assemblages also play a key role on the biogeochemical cycle of many other elements.<sup>5,12–14</sup>

The mechanisms controlling NOM binding to minerals involve many processes because of the extreme heterogeneity and structure complexity of NOM, which can be conceptualized as a micellar-like (i.e. supramolecular) structure; able of disassembling, reassembling and reorganizing in response to changes in the chemical environment.<sup>2,5,15–17</sup> NOM fractionation at the mineral water interface is driven by functional groups, because some organic compounds preferentially adsorb onto a mineral phases.<sup>5,18–26</sup> Deciphering the mechanisms and parameters that control NOM sorptive fractionation remains a challenge albeit the extensive studies on the subject. Indeed, this can be investigated on the basis of several criteria with numerous techniques. For instance, fractionation between aromatic and aliphatic moieties, with a preferential adsorption of aromatic compounds, has been widely investigated using different spectroscopic techniques such as UV-vis spectrophotometry, diffuse reflectance infrared Fourier transformed spectroscopy, Fourier transformed infrared mass spectroscopy, solid and liquid nuclear magnetic resonance.<sup>6,20,27,28</sup> Other studies brought to light the preferential adsorption of high molecular weight (Mw) compounds using chromatography methods.<sup>22,29</sup> A recent kinetic study showed a preferential adsorption of aromatic, high Mw and carboxylic (COOH)-rich compounds, followed by selective adsorption of lignin-like, phenolic (PhOH)-rich compounds.<sup>18</sup>

In fact, the type and number of functional groups determine the specific interactions of NOM with minerals, especially COOH and less abundant acid-base functional groups, which will be referred only to as “PhOH” further in this study,<sup>2,18,27,30–35</sup> Hence, fractionation
according to the size or the hydrophobicity of NOM may be only a consequence of the binding extent and mechanisms (including chemical and electrostatic effects) of COOH and PhOH functional groups of NOM onto hydrophilic mineral surfaces.36,37 For instance, it has been shown previously that stronger binding of humic acid is related to the larger number of ligands per molecule and its corresponding charge, compared to fulvic acid.38 However, the effects of solution composition on the fractionation of NOM with respect to its COOH and PhOH groups has too rarely been investigated.22 In fact, investigation of the fractionation of COOH and PhOH groups relies on the accurate determination of their concentrations in the aqueous phase at environmentally relevant NOM concentrations. This is not straightforward because conventional analytical methods for the solution require large NOM concentrations, typically hundreds of mg L⁻¹,39 which is well above natural concentrations. More recently, UV-vis spectroscopic acid-base titration approaches allowed the quantification of these functional groups at environmentally relevant NOM concentrations.22,40,41 Based on this approach, following studies revealed the effect of surface loadings on NOM fractionation at mineral surfaces.22 However, other factors such as pH or ionic strength might also play a great role,¹⁵ as they were found to affect the adsorption of simple organic ligands.22,33,42,43

This study examines the NOM fractionation through the quantification of COOH and PhOH functional groups under various chemical and physical conditions, using Leonardite humic acid (LHA) as a model NOM and goethite (α-FeOOH) as a model mineral, which is commonly found in soils. Using UV-vis spectroscopic acid-base titration after filtration, the fraction of both COOH and PhOH groups adsorbed onto goethite as a function of pH, NOM and background electrolyte (NaCl) concentrations was determined. In addition, as water flow conditions often occurs in natural soils and sediments, which might not allow the establishment of chemical equilibria, the effect of dynamic flow was tested through column experiments using goethite coated sand (GCS). These fractionation results were compared to the adsorption of dissolved organic carbon (DOC) and chromophores, aiming to assess the underlying fractionation mechanisms.

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## 2. Materials & Methods

Reagents. All samples were prepared with ultrapure water using a MilliQ purification system. The NOM sample used in this study, LHA, reference code 1S104H, was purchased from the International Humic Substance Society (IHSS). The LHA sample COOH and PhOH quantities were evaluated by potentiometry in a previous study: 4.01±0.13 and 2.00±0.37 mol kgNOM⁻¹

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respectively.³⁹ Reagents such as, HCl, NaOH, NaCl, and the pH-buffers salts tris(hydroxymethyl)aminomethane (TRIS), 3-(N-morpholino)propanesulfonic acid (MOPS), 2-(N-morpholino)ethanesulfonic acid (MES) and acetic acid (AA) were purchased from Sigma Aldrich.

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**Solution preparation.** A goethite solution was prepared following the description further detailed elsewhere.⁴³,⁴⁴ Briefly, a volume of 500 mL of KOH at 2.5 M was added at a rate of 4 mL min⁻¹ to 120 mL of Fe(NO₃)₃·9H₂O at 0.5 M. The solution was stirred and purged with nitrogen before and during the synthesis process to prevent any interaction with atmospheric carbon dioxide. The mixture was maintained at 60°C for 98 h. After precipitation of particles, the solid phase was dialyzed for a duration of 2 weeks using Spectra/Por® Dialysis Tubing (MWCO: 1kD) with a daily change of water coupled with conductivity measurements. The water change was operated until the conductivity values were found to be below 5 µS cm⁻¹. The synthetic mineral was previously characterized⁴⁴ by X-ray powder diffraction (XRD) using a D8 Bruker diffractometer, the specific surface area was previously measured by Brunauer-Emmett-Teller (BET) and the pH of zero-point charge (pHzpc) determined by titration at 298 K in 0.01, 0.1 and 1 mol L⁻¹ of NaCl. The results indicated that the mineral phase was indeed goethite with a specific surface area of 81 m² g⁻¹ and a pHzpc value of 9.1.⁴⁴

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A 250 mL NOM stock solution (1 g L⁻¹) was prepared by dispersing NOM powder (25 mg) in ultrapure water using NaOH. The pH was maintained at about 9 with the addition of NaOH (0.1 mol L⁻¹) to ensure the complete dissolution. All pH measurements were performed with HANNA, HI5221 pH meter. The pH of the NOM stock solution was monitored for 24 hours, with adjustments performed at regular intervals (3, 6 and 12 hours) after beginning the NOM dissolution process. The NOM stock solution was continuously mixed on a shaking table. Then, DOC concentration was determined using a Shimadzu TOC-L analyzer. The [DOC] will be used as a proxy to [NOM].

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The preparation of the buffer solutions followed the lines of a previous work.⁴⁵ Succinctly, buffer salt was dispersed in a total volume of 100 mL ultrapure water to achieve a final concentration of 1 mol L⁻¹. The pH of the buffer solution was adjusted in order to match the pKa, the pKa – 1, or the pKa + 1 values of the buffer salt upon acid (HCl, 1, 0.1 mol L⁻¹) or base (NaOH, 1, 0.1 mol L⁻¹) addition. Eleven buffer solutions, with pH ranging from 3.8 to 9, were therefore prepared with TRIS, MOPS and MES, as well as AA. Buffers were not used during the reaction of NOM with goethite in batch or column experiments, but they were used

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to adjust the pH of NOM solutions (i.e., after removal of goethite by filtration) to perform
spectrophotometric acid-base titrations (see below).

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**Batch adsorption experiments.** Adsorption experiments were conducted in 50 mL polypropylene tubes with 50 m² L⁻¹ of goethite for varying:

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(i) pH from 5 to 9.5 (pH-edge) for two different [DOC] (25 and 50 mg L⁻¹) in 0.01 M NaCl

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(ii) [DOC] from 25 mg L⁻¹ to 200 mg L⁻¹ (isotherm) at pH 5, 7 or 9 in 0.01 M NaCl

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(iii) ionic strength (0.01 M NaCl and 0.1 M NaCl) for isotherms at pH 5 and 7

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The pH values were adjusted using 0.1 M NaOH or HCl solution (no buffer was used).

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Samples were homogenized with a magnetitic stirrer and purged by N₂(g) bubbling throughout the adsorption experiment. After the initial pH adjustment, samples were left on a shaking table and pH was monitored and readjusted to desired value at 3, 6 and 12 hours after the start of the adsorption process. The pH of the samples was measured after equilibrium was reached (i.e. after 24 hours based on preliminary studies) and then the samples were centrifuged at 3000 g for 30 mins. Subsequently, the samples were filtered at 0.2 µm using cellulose acetate filters (Sartorius®). The filtrates of these samples were then titrated by UV-vis spectroscopy, detailed hereafter.

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**Breakthrough column experiments.** Breakthrough column experiments of LHA in GCS were performed by Cheng et al.⁴⁴ Previously published relevant results⁴⁴ (i.e. LHA breakthrough results on the basis of DOC and absorbance measurements) will be rediscussed here along with additional unpublished data (i.e. spectrophometric acid-base titration), collected during the same period within the same experiment conducted on NOM acid-base functional groups. The experimental procedure is detailed in Cheng et al.,⁴⁴ and is briefly described herebelow for clarity.

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Goethite was coated onto cleaned Fontainebleau quartz sand (100-150 µm), and characterized as previously detailed.⁴⁴,⁴⁶ The goethite content deposited on the sand surface (0.99 g/100 g) was measured by acid digestion. 15 g of dry GCS was packed into a glass chromatographic column of 1.6 cm internal diameter. This leads to a porous bed length of 4.7 cm. After packing, the column was wetted upward with a 0.01 M NaCl solution at pH 5 and at a rate of 0.5 mL min⁻¹. The pore volume (Vp) was estimated by the weight difference between the saturated and dry column. Breakthrough column experiments were conducted at constant flow rate and under water-saturated conditions. A 30 mg L⁻¹ NOM solution in 0.01 M NaCl at
pH 5 was injected into the column with a continuous flow at 0.5 mL min⁻¹. The effluent of the column was collected with a fraction collector and stored at 4°C for further solution analyses, as described below.

**Solution analysis.** Filtered solutions from batch experiments and effluent solutions from the column experiment were analyzed for [DOC] using a Shimadzu TOC-L analyzer. Absorbance of these samples was analyzed by UV-vis Shimadzu UV2600 spectrophotometer using a 1-cm-wide quartz cuvette and measurements were conducted within a range of wavelengths (λ) between 200 to 800 nm. The rates of DOC adsorption (%DOCads) and chromophores adsorption (%Chrads) were calculated using equations 1 and 2:

$$ \%DOC_{ads} = \frac{DOC_{raw} - DOC_{f}}{DOC_{raw}} \times 100 \quad (1) $$

$$ \%Chr_{ads} = \frac{UV@254_{raw} - UV@254_{f}}{UV@254_{raw}} \times 100 \quad (2) $$

where DOCraw and UV@254raw represent the DOC and UV absorbance measurement at 254 nm of NOM before adsorption, respectively, whereas DOCf and UV@254f are corresponding measured values after filtration. By repeating of some of the experiments three times, the error associated with %DOCads and %Chrads was estimated to be equal to ±5.

**Spectrophometric acid-base titration.** The acid-base properties of the NOM in the supernatant were characterized by spectroscopic acid-base titration, conducted as detailed in a previous study.⁴⁵ Succinctly, each filtered sample was divided into 13 subsamples of 4 mL. Out of these, 11 subsamples were adjusted to pH values ranging from pH 3.8 to 9 using an appropriate pH-buffer solution with a concentration of 10⁻³ mol L⁻¹. The two remaining subsamples were manually adjusted using HCl to pH 3 and 3.4. Subsequently, UV-vis absorbance measurements for each aliquot were recorded between 200-800 nm. Information concerning the acid-base properties of LHA after adsorption was obtained upon deconvolution of spectroscopic acid-base titration using a model described by equation 3, developed in Tesfa et al:⁴¹

$$ A_{\lambda,pH} = A_{0,\lambda} + A_{1,\lambda} \times f_{1,pH} + A_{2,\lambda} \times f_{2,pH} \quad (3) $$
The experimentally measured absorbance spectra dataset ($A_{\lambda,pH}$) is deconvoluted into three components: a proton-inert component and two others representing NOM major acid-base functional groups COOH and PhOH. The proton inert component ($A_{0,\lambda}$) is characterized by an absorbance spectrum varying as a function of wavelength only. The major acid-base functional groups are represented by their absorbance spectrum: $A_{1,\lambda}$ for COOH and $A_{2,\lambda}$ for PhOH (which depend only on $\lambda$) and their associated pH-dependent fractions ($f_{1,pH}$, $f_{2,pH}$). All parameters ($A_{0,\lambda}$, $A_{1,\lambda}$, $f_{1,pH}$, $A_{2,\lambda}$, $f_{2,pH}$) were optimized to best fit experimental $A_{\lambda,pH}$ values by an iterative equation-resolution method. Parameters' initialization was operated on the basis of chemical constraints. Further explanation on the optimization and initialization process, including the choice of the starting values, is detailed in Tesfa et al.⁴¹ As shown in Figure S1 (Appendix A in Supplementary Information (SI)), $A_{1,\lambda}$ and $A_{2,\lambda}$ spectra of raw LHA exhibit prominent peaks at $\lambda_{max}$ = 273 and 373 nm, respectively, which was also observed for LHA that has reacted with goethite. Subsequently, the percentage of COOH or PhOH groups adsorbed onto goethite was determined as follows:

$$ \%COOH_{ads} = \frac{A_{1,\lambda max,raw} - A_{1,\lambda max,f}}{A_{1,\lambda max,raw}} \times 100 \quad (4) $$

$$ \%PhOH_{ads} = \frac{A_{2,\lambda max,raw} - A_{2,\lambda max,f}}{A_{2,\lambda max,raw}} \times 100 \quad (5) $$

Where $A_{i,\lambda max,raw}$ and $A_{i,\lambda max,f}$ ($i$ = 1 or 2 for COOH and PhOH groups, respectively) are the specific absorbance of COOH and PhOH at their respective $\lambda_{max}$ (i.e. 273 and 373 nm) of LHA before adsorption ("raw") and after adsorption onto goethite and subsequent filtration ("f"). By repeating of some of the experiments three times, the error associated with $\%COOH_{ads}$ and $\%PhOH_{ads}$ was estimated equal to $\pm$5 and $\pm$10, respectively.
## 3. Results & Discussion

**Influence of solution pH on NOM adsorption processes.** The adsorption of NOM onto Fe-oxide surfaces is primarily controlled by the formation of surface complexes, involving typically ≡FeOH surface sites and COOH or PhOH functional groups.30,32–35 Surface complexation reactions may involve a ligand-exchange process between a surface -OH group and a deprotonated organic ligand (NOM-O⁻), in order to form a metal bonded surface complex (MB) as follows:

$$ \equiv FeOH + H^+ + NOMO^- \rightleftharpoons FeONOM + H_2O \quad (6) $$

Deprotonated organic ligands may also form hydrogen-bonded (HB) complexes (surface hydration shared ion pair) with surface -OH groups:32,33

$$ \equiv FeOH + H^+ + NOMO^- \rightleftharpoons FeOH_2^- + ONOM \quad (7) $$

This reaction is pH-dependent because of the acid-base properties of NOM COOH and PhOH groups, as well as proton binding to the goethite surface. Note that as the pKₐ of PhOH-containing ligands is high, their surface binding under the protonated form can be expected.20 Acid-base reactions of the surface ≡FeO(H) groups also lead to the generation of a positive electrical surface charge at low pH (< pHzpc) and a negative one at high pH (> pHzpc). This electrostatic property allows the formation of an outer-sphere (OS) complexes (solvent-surface hydration-separated ion pair) where deprotonated organic ligands keep their first hydration sphere when locating at the beginning of the diffuse layer.33,43,46 Additional organic ligands accumulate within the diffuse layer surrounding the goethite surface as hydrated ions.33 Overall, both chemical reactions and electrostatic forces, which cannot be distinguished, contribute synergistically to preferential adsorption of NOM onto goethite surface at low pH, while limiting adsorption at high pH. The above discussed mechanisms can explain the present results. Figure 1a shows the percentage of NOM adsorbed onto goethite as a function of pH for [DOC] = 25 mg L⁻¹ in 0.01 M of NaCl. Values are calculated on the basis of both DOC analysis and UV absorbance measurements at 254 nm. NOM adsorption to goethite is strong, and decreases with increasing pH, e.g. from 90 to 65 % as determined by DOC analysis. Spectrophotometric acid-base titration of the filtrate allowed to separately determine carboxylic and phenolic group adsorption onto goethite. The impact of pH on the adsorption of both functional groups agrees with previous interpretations made on the basis of NOM overall adsorption, that is, an adsorption decrease with increasing pH (Figure 1b).
Molecular fractionation of NOM at goethite surface was evidenced by Han et al.⁴⁷ showing preferential adsorption of lignin-like, tannin and condensed aromatic compounds components as compared to carbohydrates and protein like components. Here, fractionation of NOM is, for instance, seen by comparing results obtained on the basis of DOC and UV absorbance at 254 nm analysis, which probe C and chromophores, respectively. The first (DOC) represents all NOM-constituting molecules while the second (UV absorbance at 254 nm) corresponds to a subset of molecules absorbing UV light (mainly aromatic compounds). Figure 1a shows that chromophores adsorption is higher than C adsorption throughout the investigated chemical conditions, with up to 15% of difference of adsorption between the two investigated parameters. Similar preferential adsorption of chromophores (i.e., aromatic compounds) has been frequently reported in the literature.⁶,¹⁸,²⁰,²²,²⁷,²⁸,⁴⁸ Under conditions where ligand binding is predominant, the chemical species driving adsorption are primarily COOH and PhOH functional groups. Because, COOH and PhOH groups might not be necessarily located on the same molecules and because their ratios differ according to the size and the composition of the NOM³⁵,³⁹ a fractionation between COOH- and PhOH-rich compounds can occur, as observed by Janot et al.²² at pH 6 at alumina surface in 0.1 M NaClO₄. Within the underlying experimental conditions ([DOC] = 25 mg L⁻¹, in 0.01 M NaCl) displayed in Figure 1b, an almost complete removal of NOM is observed as very little, or no COOH and PhOH groups were quantified in the solution for samples with pH < 7. Consequently, no conclusion regarding the fractionation of COOH- and PhOH-rich compounds can be drawn from these experiments. Conversely, at larger pH values, the percentage of PhOH groups adsorbed is stronger than that of COOH ones. These results confirm that NOM contains COOH- and PhOH-rich compounds with distinct adsorption behavior to goethite.
<figure><img src="image_4.png" /><figcaption>**Figure 1:** (a) Percentage of dissolved organic carbon (DOC) and chromophores (Chr) adsorbed onto goethite (50 m² L⁻¹) as a function of pH at low C (25 mg L⁻¹) and high C concentrations (50 mg L⁻¹) in 0.01 M NaCl. Corresponding percentage of COOH and PhOH groups adsorbed onto goethite at (b) low (25 mg L⁻¹) and (c) high [NOM] (50 mg L⁻¹).</figcaption></figure>

The pH<sub>ZPC</sub> is outlined by the dotted lines.
**Effects of surface site saturation.** For the lowest [DOC] investigated (25 mg L⁻¹) and [NaCl] (0.01 M), the difference in the percentage adsorbed is only very small, which might be attributed to the large number of available binding sites on goethite surface, at such relatively low surface loadings. Indeed, this set of experiments is conducted for an amount of C adsorbed much below 3 atoms per nm², which corresponds to the ≡FeOH surface site density generally considered for ligand binding at goethite surface.³²,³⁵,⁴⁴ An increase in NOM concentration drastically affects its adsorption at any pH investigated (5 ≤ pH ≤ 9.5). For [DOC] = 50 mg L⁻¹ (Figure 1a), NOM adsorption varied within a smaller range: from ~50% at pH 5 to ~25 % at pH 9.5. Such results can be explained by surface site saturation, which prevents further binding of organic ligand to the surface and is combined with progressive neutralization of the goethite surface by adsorption of negatively charged NOM, which in turn limits electrostatic attraction or even, induces repulsion between NOM and goethite surface.²⁰,²⁹,³⁵,⁴²,⁴⁹,⁵⁰ Adsorption studies of NOM onto goethite and other Fe-oxides also point out surface site saturation in these conditions, i.e. a plateau of adsorption reached at high pH and C loadings.²⁴,³⁰,³⁴,⁵¹

Surface site saturation has a measurable impact on the adsorption of both COOH and PhOH groups, where a clear fractionation is observed at pH > 5 (Figure 1c). Phenolic compounds are adsorbed at a somewhat constant rate throughout the experimental pH range (on average ~70 %) as opposed to carboxylic groups, whose adsorption decreases with pH (from approximately 60 to 20 %). This large difference in the percentage adsorbed between COOH and PhOH groups further confirms that the COOH-to-PhOH concentration ratio vary greatly between distinct NOM molecules. This also indicates that fractionation is less apparent with optimal adsorption conditions (i.e. low C loading and low pH) and becomes more apparent when adsorption is limited by (i) the amount of surface binding groups at high [DOC] loading or (ii) the effect of OH⁻, which both competes with ligands for metal bonding (MB), hydrogen-bonding (surface hydration shared ion pair; HB) and (iii) formation of an outer-sphere complexes (solvent-surface hydration-separated ion pair; OS). These reactions contribute to the development of negative goethite surface charge that affects more strongly the adsorption of negatively charged NOM molecules.

The effect of [DOC] is further shown in Figure 2a where three isotherms at different pH values (5, 7 and 9) in 0.01 M of NaCl are displayed. Adsorption behavior of NOM for all three isotherms is in excellent agreement with results from the pH-edge experiment (Figure 1a): the percentage of NOM adsorbed decreases with increasing pH and [DOC]. The adsorption rates of COOH and PhOH groups versus [DOC] are plotted in Figures 2b, 2c and 2d for each isotherm
331 experiments at pH 5, pH 7 and pH 9, respectively, in 0.01 M NaCl. Fractionation between
332 COOH and PhOH is not observed for the isotherm at pH 5. However, a noticeable fractionation
333 is observed at pH 7, and this fractionation becomes even more pronounced at pH 9. This
334 fractionation reveals a preferential adsorption of PhOH-rich compounds. Because they are pH
335 dependent, these results suggested that ligand binding reactions (which include both chemical
336 and electrostatic contributions) are key processes for the adsorption of NOM onto goethite
337 surfaces and subsequent fractionation. Especially, while the decrease with pH of COOH-rich
338 compounds adsorption is observed at any [DOC], the adsorption of PhOH-rich compounds
339 follows a bell-shape, which is observed in the pH-edges of phenolic compounds at the surface
340 of metal oxides.<sup>20,52,53</sup>

341 Collectively, these results show the larger adsorption of PhOH- as compared to COOH-
342 rich compounds. This may be explained by the larger pKₐ of the former. First, linear free
343 energy relationships have often been drawn between surface complexation reactions and pKₐ
344 of small organic ligand,<sup>54,55</sup> which explains the larger adsorption of small phenolic compounds
345 compared to carboxylic ones at high pH.<sup>20,33,52,53</sup> Second, the pKₐ values of COOH-rich
346 molecules are more negatively charged than PhOH ones thus, are more strongly affected by the
347 goethite surface charge, which decreases with the pH. Additionally, it should be kept in mind
348 that the site density, the COOH-to-PhOH ratio and the size of the molecule or of the
349 supramolecular assemblage, have an important impact on the adsorption of distinct NOM
350 compounds, as discussed in previous studies,<sup>36,37</sup> although the present results do not provide
351 direct information for these aspects. Finally, it can be suspected that adsorption of near
352 neutrally-charged PhOH-rich compounds is more favored than COOH-rich compounds through
353 NOM-NOM interactions, such as hydrophobic, van der Waals, H-bonds or π-π interactions.<sup>2,56</sup>

354 Interestingly, present results with LHA and goethite in 0.01 M NaCl are in opposition
355 to previous results obtained with purified Aldrich HA (PAHA) and alumina in 0.1 M NaClO₄,<sup>22</sup>
356 which showed preferential PhOH and COOH adsorption, respectively. This will be shown to
357 be an effect of background electrolyte concentration in the following section.
<figure><img src="image_5.png" /><figcaption>**Figure 2:** (a) Percentage of adsorbed NOM onto goethite (50 m² L⁻¹), determined on the basis of UV absorbance at 254 nm, as a function of initial [DOC] at various pH (5, 7 and 9) and NaCl concentrations (0.01 and 0.1 M). Corresponding percentage of COOH and PhOH functional groups adsorbed onto goethite in 0.01 M NaCl at (b) pH 5, (c) pH 7 and (d) pH 9, and in 0.1 M NaCl at (e) pH 5 and (f) pH 7.</figcaption></figure>
**Ionic strength effects on the structure of goethite-NOM-water interface.** The effects of background electrolyte concentration were further investigated at pH 5 and 7 in 0.1 M NaCl (Figures 2e and 2f, respectively). A significant increase in NOM adsorption to goethite is observed at both pH values when increasing [NaCl] from 0.01 to 0.1 M, in line with previous studies dedicated to NOM adsorption on metal-oxides.<sup>22,35,57,58</sup> However, this is generally not attributed to a process related to colloidal stability, nor it is due to a limitation of surface ligand binding processes because the opposite trend is generally observed for small organic ligands, i.e., adsorption is generally smaller at high ionic strength.<sup>33,43</sup> In fact, the increase of cation concentration affects the structure of NOM through a partial neutralization of its charge and, as a response, NOM can form larger supramolecular structures in aqueous solution or at mineral surfaces.<sup>15,51</sup> In addition, at high loadings, and with an increase in [NaCl], surface charge shielding by Na<sup>+</sup> cations may neutralize the charge of negatively charged NOM particles (i) promoting the adsorption of dissolved ones<sup>36</sup> and (ii) decreasing the electrostatic repulsion between the adsorbed and dissolved NOM. This allows the formation surface supramolecular structures (composed of multi-organic layers), expected to occur through NOM-NOM interactions, such as hydrophobic, van der Waals, H-bonds or π-π interactions.<sup>2,56</sup> For instance, Na<sup>+</sup> was shown to enhance the co-adsorption of two organic compounds at mineral surfaces interacting with each other through van der Waals interactions.<sup>43,46</sup>

The change in the predominant adsorption mechanism highly affects the fractionation of organic molecules between the solution and goethite surface. Indeed, unlike results obtained at 0.01 M of NaCl, where only a slight or no fractionation between aromatic and aliphatic compounds is observed throughout the experimental [DOC] and pH conditions, a fractionation is observed in 0.1 M of NaCl (see Figure S2 in appendix B). The preferential adsorption of aromatic over aliphatic compounds is in line with the strong participation of aromatic π-systems in the interactions between organic molecules at mineral surfaces.<sup>56</sup>

The increase in [NaCl] from 0.01 to 0.1 M only slightly affects the adsorption of PhOH groups at pH 5 (Figures 2b, 2e) or pH 7 (Figures 2c, 2f). Oppositely, the adsorption of COOH groups significantly increases by ~50% for [DOC] > 50 mg L<sup>-1</sup> at both pH 5 (Figures 2b, 2e) and pH 7 (Figures 2c, 2f). The adsorption of COOH groups becomes larger than that of PhOH groups in 0.1 M of NaCl, as observed by with purified PAHA and alumina in 0.1 M NaClO<sub>4</sub>,<sup>22</sup> hence leading to an opposite fractionation trend to the one observed in 0.01 M of NaCl. These results can be explained by the greater negative charge of the COOH-rich compounds than the PhOH ones: addition of Na<sup>+</sup> might lead to a greater variation in the electrical charge of COOH-
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rich compounds, which see their adsorption rate vary to a greater extent. The small effect of [NaCl] increase on the adsorption PhOH-rich compounds further suggest that a fraction of these groups directly interact with surface ≡FeO(H) groups, although it is not possible to exclude that a fraction of PhOH groups also interacts in a second layer with already adsorbed NOM, as previously reported.¹⁸

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**Fractionation of NOM acid-base functional groups under flow-through conditions.** To check the sorptive fractionation of NOM while interacting with goethite surfaces under nonequilibrium conditions (i.e. closer to real-world conditions), we investigated the mobility of NOM components in GCS-packed column for [DOC] = 30 mg L⁻¹, pH = 5 and [NaCl] = 0.01 M. Data of column effluent measurements (DOC, UV254 and COOH and PhOH groups) are presented as relative concentration (C/C₀) *versus* the number of injected pore volumes (V/Vₚ). As already stated in section 2, this experiment was performed by Cheng et al.,⁴⁴ in which LHA breakthrough results on the basis of DOC and absorbance measurements were presented (repeated in Figure S3). These data will be rediscussed here along with additional unpublished data (i.e. spectrophotometric acid-base titration) (Figure 3).

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DOC values up to 6 mg/L together with absence of UV absorbance were observed in the outflow solution over the first breakthrough stage (≤ 25 PV). Such results suggest that aromatic components (UV absorbance at 254 nm) are primarily and preferentially adsorbed to goethite, as previously reported.¹⁸,⁴⁴ Indeed, a recent kinetic study¹⁸ provided evidences for a rapid primary phase adsorption of aromatic and polycyclic aromatic compounds of NOM to goethite, followed by lignin-like and aliphatic compounds. In the second stage of breakthrough (≥ 25 PV), UV absorbance at 254 nm breaks out and then follows the same trend as for DOC (Figure S3). This also falls in line with previous findings¹⁸,⁴⁴ where lower Mw of LHA aromatic components were shown to be first adsorbed on the goethite surface, and then replaced by the larger LHA components.

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As for batch experiments, additional spectroscopic analyses of the outflow solution revealed a fractionation between COOH- and PhOH-rich compounds (Figure 3). The breakthrough of COOH groups follows the trend of DOC and UV absorbance at 254 nm (i.e. aromatic components) relatively well. The COOH and PhOH groups break out at similar PV values (25 PV), then an extended tailing is observed for PhOH groups. This observation agrees with a recent kinetic study showing a preferential adsorption of COOH-rich compounds, followed by PhOH-rich ones.¹⁸ This suggests a kinetically limited adsorption of these groups and/or a diffusion-controlled process within mineral assemblages (i.e. GCS) and/or NOM-
covered minerals. This can be explained by the stronger electrostatic attraction between the positively charged goethite surface at pH 5 and the more negatively charged COOH-rich compounds. In addition, as stated above, PhOH and COOH groups may interact with various types of sites, e.g. uncoated goethite, NOM-covered goethite and unbound NOM. But, PhOH-rich compounds seem more affected by this surface heterogeneity. A such multi-binding site process may explain the occurrence of an extend tailing for the breakthrough curve of PhOH groups. This is also in agreement with previous findings<sup>20,44,59</sup> where co-binding of phenolic moieties of NOM with mineral-bound NOM and unbound NOM components may control the mobility of these species in heterogeneous environmental systems.

<figure><img src="image_6.png" /><figcaption>**Figure 3:** Breakthrough curves of NOM, presented as relative concentration (C/C₀) versus the number of injected pore volumes (V/Vₚ), determined on the basis of acid-base titration (COOH and PhOH; data from the present work). Experimental conditions: [DOC] = 30 mg L⁻¹, pH = 5, [NaCl] = 0.01 M.</figcaption></figure>

## 4. Conclusions

In the present study, NOM fractionation at the goethite-water interface was investigated by quantifying COOH and PhOH functional groups, which have been widely shown to control NOM binding to minerals. The results confirmed that the adsorption of NOM varies with changes in chemical environment, i.e., adsorption decreases with increasing pH, NOM, and decreasing ionic strength. Optimal adsorption conditions (i.e., low pH and low [NOM]) showed no fractionation between COOH- and PhOH-rich compounds. In contrast, fractionation between COOH and PhOH occurred when NOM adsorption was limited due to (i) competition
with OH- for surface binding and (ii) competition between COOH- and PhOH-rich compounds at high surface loading (high [NOM]). At low [NaCl] and limited NOM adsorption, these processes contribute to the development of a negative goethite surface charge, which more strongly affects the adsorption of negatively charged NOM molecules, explaining the preferential adsorption of less negative PhOH-rich compounds (due to their high pKA) over COOH-ones. However, a drastic increase in COOH-rich compounds was observed with increasing IS, suggesting that a greater variation in their electric charge leads to a greater variation in their adsorption rate than in the case of PhOH-rich compounds, and potentially favoring the formation of surface supramolecular structures expected to occur through NOM-NOM interactions, such as hydrophobic, van der Waals, H-bond or π-π interactions. Experiments under flow conditions revealed further kinetically limited adsorption processes, and a multiple binding site mechanism was shown to control the fractionation of PhOH- and COOH-rich compounds.

This study provides, for the first time, a comprehensive understanding of the impacts of surface loading, pH, ionic strength and water flow on the fractionation of COOH- and PhOH-rich compounds of NOM at the goethite/water interface. Chemical parameters such as pH, surface site saturation, salt concentration and kinetics strongly affected the NOM fractionation through the involvement of different surface binding mechanisms and the formation of surface supramolecular assemblages. These results further illustrate the diversity of geochemical processes involved in the formation of organo-mineral assemblages, including competitive and cooperative effects between NOM constituents, and will provide important constraints for the understanding and modeling of the formation of these assemblages in natural soils and waters. Such models could be of special interest in the consideration of climate change or ecosystem sustainability, where the formation of organo-mineral assemblages is known to affect the C cycle, amongst other elements. However, many other environmental factors could affect the fractionation between NOM acid-base groups including (i) NOM and (ii) mineral composition, (iii) the presence of higher valence cations (e.g. Ca²⁺, Mg²⁺, Al³⁺, Fe³⁺) that influence the structure of organo-mineral assemblages via the formation of cation bridges,¹³,¹⁴ and/or (iv) major inorganic anions such as silicate that can compete with carboxylic and phenolic ligands for surface complexation.⁶⁰

Providing key insights onto the fractionation of organic matter acid-base functional groups at Fe-oxide-water interface is also highly valuable for the understanding and prediction of contaminants interactions with organo-mineral assemblages. Such assemblage formation occurs both as micro-meter scale particles in soils or suspended matter in water, as well as
nanometer scale colloidal particles in waters. In this context, NOM either (i) acts as a competitor for anionic contaminants binding to minerals such as phosphate, arsenate⁶¹ or other organic molecules,⁴⁴,⁶² or (ii) induces both competitive and cooperative effects for cations, which bind both to minerals and NOM acid-base groups.⁵,¹⁴,⁶³⁻⁶⁵ Modeling of the adsorption of contaminants to organo-mineral assemblages requires an accurate description of the distribution of COOH- and PhOH-rich compounds between the mineral surface and the solution, to determine the total amount of available surfaces (mineral and NOM) and residual aqueous (NOM) ion binding groups. As a consequence, the investigation of the main mechanisms controlling the fractionation between COOH- and PhOH-rich compounds at mineral surfaces may help in establishing more realistic geochemical models involving NOM and co-existing environmental compounds.

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Acknowledgments

This work was supported by the C-FACTOR project funded by ANR (project number ANR-18-CE01-0008; coordinator: R. Marsac). Through the support of the GeOHeLiS analytical platform of Rennes University, this publication is also supported by the European Union through the European Regional Development Fund (FEDER), the French Ministry of Higher Education and Research, the French Region of Brittany, and Rennes Metropole.

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Highlights

* Fractionation between COOH- and PhOH-rich compounds varies with pH, [NaCl], [NOM] and water flow.

* No fractionation was observed when adsorption is optimal (low pH and [NOM])

* Adsorption of PhOH-rich compounds is favored at high pH and high [NOM] because they are less affected by goethite surface charge decrease.

* Partial neutralization of COOH-rich compounds at high [Na+] favored their adsorption over PhOH ones.

* Under flow-through conditions, multiple-binding-site mechanism kinetically favored PhOH-rich compounds.
**Declaration of interests**

☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Marsac reports financial support was provided by ANR. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.