Adsorption and desorption of natural organic matter on iron oxide: mechanisms and models

Abstract

The adsorption and desorption mechanisms of natural organic matter (NOM) on mineral surfaces are not completely understood because of the heterogeneity and complexity of NOM and adsorbent surfaces. This study was undertaken to elucidate the interaction mechanisms between NOM and iron oxide surfaces and to develop a predictive model for NOM adsorption and desorption. Results indicated that ligand exchange between carboxyl/ hydroxyl functional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, especially under acidic or slightly acidic pH conditions. This conclusion was supported by the measurements of heat of adsorption (microcalorimetry), FTIR and I3C NMR analysis, and competitive adsorption between NOM and some specifically adsorbed anions. A modified Langmuir model was proposed in which a surface excess-dependent affinity parameter was defined to account for a decreasing adsorption affinity with surface coverage due to the heterogeneity of NOM and adsorbent surfaces. With three adjustable parameters, the model is capable of describing a variety of adsorption isotherms. A hysteresis coefficient, h, was used to describe the hysteretic effect of adsorption reactions that, at h = 0, the reaction is completely reversible, whereas at h = 1, the reaction is completely irreversible. Fitted values of h for NOM desorption on iron oxide surfaces ranged from 0.72 to 0.92, suggesting that the adsorbed NOM was very difficult to be desorbed at a given pH and ionic composition. Our results imply that a better mechanistic understanding of the interaction between NOM and oxide surfaces is needed to improve our predictive capabilities in NOM transport and cotransport of contaminants associated with NOM or iron oxides.

Adsorption of arsenate on soils. Part 1: Laboratory batch experiments using 16 Chinese soils with different physiochemical properties

“…A similar phenomenon was observed for phosphate adsorption (Holford et al, 1974). For phosphate adsorption on soils with high extractable P, the Langmuir two-surface equation would be the same as the Langmuir one-surface equation because the high-energy surface (Fontes et al, 1992;Gu et al, 1994). In contrast, for soils with relatively low contents of OM, DOC or extractable P, both the high-energy and low-energy surface adsorption sites would be available for arsenate adsorption.…”

Effects of calcium ions on surface characteristics and adsorptive properties of hydrous manganese dioxide

“…Similar trends were observed at pH 5.5 and 8.5, and more HA could be removed as the pH increased. The mechanisms of humic substance adsorption onto metal oxides generally include anion exchange (electrostatic interaction), ligand exchange (surface complexation), hydrophobic interaction, entropic effects, hydrogen bonding, and cation bridging [13]. In the absence of Ca 2+ , δMnO 2 adsorbs HA mainly through the reactions: where L − and HL respectively represent the acidic and neutral forms of HA ligands, and ≡MnOHOH − stands for the hydrolyzed ≡Mn-O − .…”

Contrasting Roles Of Natural Organic Matter On Colloidal Stabilization And Flocculation In Freshwaters

“…Fulvic acids are chemically heterogeneous colloids with potential interactions due to non-DLVO effects (Grasso 2002) such as hydratation effects, hydrogen bonding or non polar interactions. Since the hydrodynamic diameter of FA is around 1-2 nm , the interaction of a FA with a hematite particle (d h =75 nm) corresponded to their adsorption (Sposito 1984;Gu et al 1994), resulting primarily in the modification of the colloidal properties, including the surface potential.…”

“…Therefore, the interaction of the HS with colloids larger than 10-20 nm corresponds to their adsorption (Sposito 1984;Gu et al 1994) and results primarily in a modification of the surface properties of the colloid (surface potential and dielectric constant). Indeed, inorganic colloids in contact with HS tend to have a similar negative surface charge (Davis and Gloor 1981;Tipping and Higgins 1982;Beckett and Le 1990;Tombacz et al 1999), irrespective of their intrinsic chemical nature.…”