Adsorption of Humic Substances onto Kaolin Clay Related to Their Structural Features

of specific interactions between metal oxide surface hydroxyl groups or adsorbed water and the oxygen of Eleven well characterized humic substances (HSs) were adsorbed adsorbed carboxyl or hydroxyl groups of organic acids from aqueous solution onto a Na-kaolin clay. The adsorption affinity (KL ), maximum adsorption capacity (b ), a coefficient of desorption including HA (Parfitt, 1977; Yost et al., 1990; Biber and hysteresis (H ), and the concentration of irreversibly adsorbed HS Stumm, 1994). Ligand exchange is highly affected by (IHS) were derived from adsorption-desorption isotherms. These pathe pH value of the adjacent solution. As a rule, adsorprameters were correlated with structural features of the HS. The tion of HS onto metal oxide surfaces increases with adsorption affinity was shown to correlate directly with the aromaticity decreasing pH value, passing a maximum at pH 4.3 of the HS and inversely with their polarity, expressed as the O/C to 4.7, corresponding to pKa values of most abundant atomic ratio. A dependency between polarity and maximum adsorpcarboxylic acids (Davis, 1981; Perdue, 1985; Murphy, tion capacity was not confirmed. The parameters b, H, and IHS expose 1990). The pH value determines the protonation state close correlation with the molecular weight (MW) and the partial negative charge of HS (Z ) at the operating pH value. The following of the sorbate as well as of the surface hydroxyl groups. quantitative relationship was obtained: b 715 0.06 MW As a result, the surface complexation via ligand ex529 Z (r 0.92). It allows a selection of HS with respect to the change becomes less favorable as soon as the pH value largest content of organic matter in HS-kaolin clay complexes. Among exceeds the point of zero net surface charge (pHzpc ) the HS studied the high molecular weight materials enriched with Csimply because of increasing electrostatic repulsion beand H-substituted aromatics, such as coal and peat humic acids (HAs), tween the surface and the anionic humic ligands. Noneare shown to be the most preferential materials for preparing stable theless, significant HS adsorption can be still observed HS-clay complexes. at these high pH values, for example, about 30% of the maximum adsorption in a hematite system at pH 9 (Vermeer et al., 1998a), and about 37% in a kaolin clay I of HS with clays take place in various system (Kretzschmar et al., 1997). For pure polycarboxenvironmental compartments, such as soils, sediylic aromatic acids, Evanko and Dzombak (1998) rements, and aquifers. They may affect the extractability ported no adsorption onto iron oxide surfaces at pH of humus and the rate of its decomposition. Adsorption pHzpc. However, polyhydroxybenzenes particularly with of HS onto minerals is also important in relation to hydroxyl groups in ortho-position could still attack elecspeciation and mobility of contaminants. Modern remetrophilic central metal ions of oxide surfaces at pH diation strategies apply cationic or anionic surfactants pHzpc. The authors addressed this effect to the formation immobilized onto mineral surfaces of aquifer materials of chelate surface complexes supported by hydroxyl for retardation of hydrophobic organic compounds groups in ortho-position. (HOC) in ground water flows (Wagner et al., 1994; Hydrophobic adsorption may be considered as a secHunter et al., 1996). An environmental friendly alternaond mechanism contributing to HS binding onto mineral tive to synthetic surfactants are naturally occuring matesurfaces. It becomes more favorable at low pH values, rials. Thus, covering mineral and sediment surfaces with when hydroxyl and carboxyl groups of HS are protonHS, that are natural surfactants, is of particular interest. ated. However, this mechanism cannot be distinguished Adsorption of HS onto mineral surfaces has been from the electrostatic attraction at pH pHzpc. At higher intensively investigated over the decades (Davis, 1982; pH values, hydrophobic adsorption can still occur in Baham and Sposito, 1994; Vermeer, 1998a,b; Specht et al., 2000). Despite that, the mechanisms governing the case it outweighs electrostatic repulsion (Lyklema, adsorption of HS are still not well understood. 1986). Similar to nonionogenic homopolymers (Day et Ligand exchange (carboxyl and hydroxyl groups of al., 1994), this process will become the more important, the HS versus surface hydroxyl groups of the minerals) the higher the molecular weight of HS is. As a consehas been frequently discussed as one mechanism for quence, fractionation of polydisperse polymers is to be HS binding (Tipping, 1981; Spark, 1997; Totsche, 1998). expected. That is, the high molecular weight HS may Several authors have provided spectroscopic evidence sorb preferentially (Davis, 1981; Jardine, 1989; Baham and Sposito, 1994; Kaiser et al., 1997). Vermeer and Koopal showed that bigger HA molecules displace faster G.U. Balcke, F.-D. Kopinke, Dep. of Remediation Res., Centre for Environm. Res., Leipzig-Halle GmbH, Permoserstr. 15, D-04318 Leipzig., Germany; N.A. Kulikova, Dep. of Soil Science and I.V. PermiAbbreviations: b, maximum sorption capacity; FA, fulvic acid; HA, nova, Dep. of Chemistry, Lomonosov Moscow State Univ., Leninskie humic acid; H, desorption hysteresis coefficient; HOC, hydrophobic Gory, 119899 Moscow, Russia; S. Hesse, F.H. Frimmel, Univ. of Karlsorganic compound; HS, humic substance; IHS, concentration of irreruhe, Engler-Bunte Inst., Engler-Bunte-Ring 1, D-76131 Karlsruhe, versibly adsorbed HS; KL, adsorption coefficient; MW, weight averGermany. Received 30 Aug. 2001. *Corresponding author (balcke@ aged molecular weight; OC, organic C; P, error probability; QSAR, hdg.ufz.de). quantitative structure activity relationship; SEC, size-exclusion chromatography; TC, total C; zpc, zero point of surface charge Published in Soil Sci. Soc. Am. J. 66:1805–1812 (2002).