Adsorption Of Fulvic Acid Research Articles

Ligand and Charge Distribution (LCD) model for the description of fulvic acid adsorption to goethite

The LCD model (Ligand and Charge Distribution) has recently been proposed to describe the adsorption of humic substances to oxides, in which the CD-MUSIC model and the NICA model for ion binding to respectively oxides and humic substances are integrated. In this paper, the LCD model is improved by applying the ADAPT model (ADsorption and AdaPTation) to calculate the equilibrium distribution of the humic substances based on the change of the average chemical state of the particles. The improved LCD model is applied to calculate the adsorption of fulvic acid (Strichen) to goethite, in which it is assumed that the carboxylic type of groups of fulvic acid can form innersphere complexes with the surface sites. The charge of the carboxylic groups in the innersphere complexes is distributed between the 0- and d-plane, whereas the charge of the other carboxylic and phenolic groups is located in the d-plane. The average distribution of the carboxylic and phenolic groups among their various chemical states (carboxylic groups: innersphere complex, protonated and deprotonated; phenolic groups: protonated and deprotonated) depends on pH, ionic strength and loading, and are the outcome of the model. The calculation shows that the LCD model can describe sufficiently the effects of pH, ionic strength and loading on the adsorption of fulvic acid, using one adjustable parameter ( log K ˜ S , 1 ) . The model calculations indicate that the chemical complexation between fulvic acid and goethite is the main driving force of the adsorption, while the electrostatic repulsion between the particles and the surface is the major limiting factor for further adsorption.

Effect of Accelerated Aging of MSWI Bottom Ash on the Leaching Mechanisms of Copper and Molybdenum

The effect of accelerated aging of Municipal Solid Waste Incinerator (MSWI) bottom ash on the leaching of Cu and Mo was studied using a "multisurface" modeling approach, based on surface complexation to iron/aluminum (hydr)oxides, mineral dissolution/precipitation, and metal complexation by humic substances. A novel experimental method allowed us to identify that the solid/liquid partitioning of fulvic acids (FA) quantitatively explains the observed beneficial effect of accelerated aging on the leaching of Cu. Our results suggestthat iron/aluminum (hydr)oxides are the major reactive surfaces that retain fulvic acid in the bottom ash matrix, of which the aluminum (hydr)oxides were found to increase after aging. A new modeling approach, based on the surface complexation of FA on iron/aluminum (hydr)oxides is developed to describe the pH-dependent leaching of FA from MSWI bottom ash. Accelerated aging results in enhanced adsorption of FA to (neoformed) iron/aluminum (hydr)oxides, leading to a significant decrease in the leaching of FA and associated Cu. Accelerated aging was also found to reduce the leaching of Mo, which is also attributed to enhanced adsorption to (neoformed) iron/aluminum (hydr)oxides. These findings provide important new insights that may help to improve accelerated aging technology.

Adsorption mechanism of fulvic acid onto freeze dried poly(hydroxo aluminum) intercalated bentonites

Two poly(hydroxo aluminum) intercalated Wyoming bentonites were prepared starting from two different aluminum pillaring solutions. The sorption mechanism of Laurentian fulvic acid (FA) onto these poly(hydroxo aluminum) intercalated bentonites was investigated at different pH values and at different ionic strengths (NaCl or CaCl2). Three mechanisms contribute to the FA adsorption, depending on the pH and the nature (NaCl or CaCl2) and ionic strength of the background electrolyte. In the presence of NaCl the FA sorption onto poly(hydroxo aluminum) intercalated bentonites can be mainly ascribed to ligand exchange between the amphoteric poly(hydroxo aluminum) hydroxyl groups and the deprotonated carboxylic groups of the FA. The FA adsorption due to ligand exchange reactions decreases with increasing pH. In presence of Ca2+ ions the FA adsorption is enhanced, compared to the presence of Na+, due to Ca2+ bridging between negatively charged groups on the FA molecules and the poly(hydroxo aluminum) intercalated bentonite. The FA adsorption due to Ca2+ bridging increases with increasing pH. A third mechanism is enhanced FA adsorption ascribed to FA-Ca-FA bridging and was detected from the FA adsorption in presence of Ca2+ at the zero point of charge of the poly(hydroxo aluminum) bentonite (pH 5).

Removal of fulvic acids using the surfactant modified zeolite in a fixed-bed reactor

Modification of zeolite (clinoptilolite) surface with a quaternary ammonium, hexadecyl trimethyl ammonium (HDTMA), to improve the removal efficiency of fulvic acids (FA) in a zeolite fixed bed was investigated. The experiment consisted of modifying zeolite with HDTMA followed by adsorption and desorption of FA in the column. The effects of relevant parameters, such as HDTMA loading levels, FA flow rate and eluant concentration were examined, respectively. Optimization studies show that the surfactant modified zeolite (SMZ) bed with HDTMA loading of 120% of ECEC at a flow rate of 5 BV/h had the best performance and the volume of 23 BV of 30 vol% ethanol solution with the feed flow rate of 5 BV/h was sufficient for complete regeneration of SMZ and desorption of FA. Measurements of Zeta potential of SMZ indicate that a monolayer formation is the most viable packing that enables maximum removal of FA. FA adsorption on SMZ surfaces is largely due to the hydrophobic interaction and hydrogen bonding while FA desorption depends on hydrogen bonding. Partition and mass transfer also play an important role in the adsorption and desorption of FA.

Interactions of calcium and fulvic acid at the goethite-water interface

Interactions of calcium and fulvic acid (Strichen ) with the surface of goethite were studied with batch and titration experiments. The mutual influence of the interactions on the adsorption of fulvic acid, calcium ions and protons were examined. Adsorption of the fulvic acid to goethite decreased with increase in pH (pH range 3–11). Addition of Ca (1.0 mM) at intermediate and high pH significantly enhanced the adsorption of fulvic acid. Compared to the adsorption to pure goethite, the presence of fulvic acid enhanced the adsorption of Ca significantly. In comparison to the simple linear sum of Ca bound to fulvic acid and goethite, the interactions between goethite and fulvic acid led to a reduced adsorption of Ca at low pH and an enhanced adsorption at high pH. With the adsorption of fulvic acid, protons were released at low pH and coadsorbed at high pH. When Ca was added, fewer protons were released at low pH and fewer coadsorbed at high pH. The experimental results can be adequately described using a surface complexation model, the Ligand and Charge Distribution (LCD) model, in which the CD-MUSIC model for ion adsorption to mineral oxides and the NICA model for ion binding to humics are integrated. In the model calculations, adequate descriptions of the ternary system data (Ca-fulvic acid-goethite) were obtained with parameters derived from three binary systems (fulvic acid-goethite, Ca-goethite and Ca-fulvic acid) without further adjustment. The model calculations suggest that the interactions between Ca and fulvic acid at the surface of goethite are mainly due to the electrostatic effects.

Removal of fulvic acid from water electrochemically using active carbon fiber electrode

Humic acids (HA) are a group of widely existing natural organic compounds and potential contaminants to underground water reservoirs. Fulvic acid (FA) is a typical humic acid of relatively low molecular weight. Electrochemical removal of FA from water by active carbon fiber (ACF) electrodes was studied by using light scattering photometer (LSP), fluorescence spectroscopy and total organic carbon analyzer (TOC). The experiments showed that FA molecules aggregated and that the average particle diameter in FA solution increased from below 10 nm to hundreds of nanometers during the treatment process. When iron was added to the anode, the FA could be coagulated efficiently at the early stage of the treatment. The proposed mechanism of the removal process is: adsorption of FA on ACF surface --> aggregation of FA --> desorption of FA aggregates from ACF --> coagulation of FA aggregates by nFe(OH)(2).mFe(OH)(3) dissolved from anode. Experiments were also repeated using graphite and stainless-steel electrodes, and the results were compared with that of ACF electrodes. FA aggregation was not observed in these experiments and most FA was not removed from the solution. At the end of this paper, FA samples from Huai River sediment were successfully treated using the ACF electrode.

Colloid formation in groundwater: effect of phosphate, manganese, silicate and dissolved organic matter on the dynamic heterogeneous oxidation of ferrous iron

Subsurface aeration is the in situ oxidation of Fe from groundwater that is used to make drinking water potable. When subsurface aeration is applied to an anaerobic groundwater system with pH>7, Fe(II) is oxidised heterogeneously. The heterogeneous oxidation of Fe(II) can result in the in situ formation of Fe colloids. To study this, the effect of substances commonly found in groundwater (e.g. PO 4, Mn, silicate and fulvic acid) on the heterogeneous oxidation process was measured. The heterogeneous oxidation of Fe(II) becomes retarded when PO 4, Mn, silicate or fulvic acid is present in the groundwater in addition to Fe(II). Phosphate and fulvic acid retarded the oxidation process most. The heterogeneous oxidation was described using a model with a homogeneous ( k 1) and an autocatalytic oxidation rate constant ( k′ 2). From the modelling it followed that the homogeneous oxidation rate constant was not affected or even slightly elevated whereas the autocatalytic oxidation rate constant decreased remarkably by the addition of PO 4, Mn, silicate or fulvic acid. From speciation calculations it followed that the decreased availability of the Fe(II) species can only explain a small part of the retarded autocatalytic oxidation process. Therefore exploratory calculations were performed to gain insight into whether the adsorption of PO 4 or fulvic acid could explain the retarded autocatalytic oxidation. These calculations showed that the adsorption of fulvic acid could explain the retarded autocatalytic oxidation process. In contrast the adsorption of PO 4 only partly explained the retarded autocatalytic oxidation process. In terms of colloid formation this study shows that the heterogeneous oxidation of Fe(II) in presence of PO 4, Mn, silicate or fulvic acid leads to the formation of Fe colloids.

The effect of cadmium on fulvic acid adsorption to Bacillus subtilis

The effects of Cd on the adsorption of an aquatic fulvic acid (FA) to the surface of Bacillus subtilis were investigated from pH 2.5 to 7.0, at fixed ionic strength (0.1 M NaClO 4) and at ambient temperature (∼22 °C). Cd (14 mg/l) had no effect on FA adsorption at pH<5 but increased FA adsorption at pH>6. The effects of Cd (0, 14 mg/l) on FA adsorption to B. subtilis were further examined as a function of initial FA concentration (0–45 mg C/l) at pH 6.5. FA adsorption isotherms also were measured at pH 6.5 as a function of dissolved Cd concentration (0–14 mg/l) at three initial FA concentrations (4, 8, 22 mg C/l). At all FA concentrations studied at pH 6.5, FA adsorption increased with increasing initial total Cd concentration. Under all studied conditions, preferential adsorption of high- to intermediate-molecular-weight FA components to B. subtilis resulted in a fractionation of the FA pool, with lower-molecular-weight components remaining in solution. At pH>6, Cd further enhanced the adsorption of high- to intermediate-molecular-weight FA components but did not significantly enhance the adsorption of lower-molecular-weight components. Hence, the overall process of adsorptive fractionation was not altered significantly by the presence of Cd. Overall, the results of this study (1) demonstrate that FA adsorption to bacterial surfaces can be altered by the presence of a metal cation, and (2) provide further evidence that microbe–metal–ligand interactions may significantly affect the mobility and fate of natural organic matter in the subsurface.

Modeling the binding of fulvic acid by goethite: the speciation of adsorbed FA molecules

Under natural conditions, the adsorption of ions at the solid–water interface may be strongly influenced by the adsorption of organic matter. In this paper, we describe the adsorption of fulvic acid (FA) by metal(hydr)oxide surfaces with a heterogeneous surface complexation model, the ligand and charge distribution (LCD) model. The model is a self-consistent combination of the nonideal competitive adsorption (NICA) equation and the CD-MUSIC model. The LCD model can describe simultaneously the concentration, pH, and salt dependency of the adsorption with a minimum of only three adjustable parameters. Furthermore, the model predicts the coadsorption of protons accurately for an extended range of conditions. Surface speciation calculations show that almost all hydroxyl groups of the adsorbed FA molecules are involved in outer sphere complexation reactions. The carboxylic groups of the adsorbed FA molecule form inner and outer sphere complexes. Furthermore, part of the carboxylate groups remain noncoordinated and deprotonated.

Size fractionation upon adsorption of fulvic acid on goethite: equilibrium and kinetic studies

We examined adsorption equilibrium and kinetics of an aquatic fulvic acid (XAD-8 resin extract) onto goethite (α-FeOOH). Molecular weight distributions were determined using high-pressure size exclusion chromatography (HPSEC). Overall adsorption isotherms and those of the most abundant intermediate molecular weight (IMW) fraction (1250–3750 Da) fit the Langmuir adsorption equation, as is commonly observed for humic substances. However, this overall fit masked the non-Langmuir isotherm shape of high and low molecular weight (HMW, LMW, respectively) fractions. We observed preferential adsorption of HMW fractions at low pH and of IMW fractions at higher pH. We also observed preferential adsorption of components with higher absorbance normalized to moles C (ε 280), probably reflecting greater aromaticity. Over the first 6 h of adsorption experiments, adsorbed organic carbon increased and weight average molecular weight ( M w ) of the organic matter remaining in solution decreased, consistent with slower adsorption of higher molecular weight components. Observations of fractionation upon adsorption agreed well with a field study showing lower M w and lower ε 280 organic matter in deeper ground water relative to surface and shallow ground water.

Adsorption of fulvic acid on goethite

The adsorption of fulvic acid by goethite was determined experimentally as a function of concentration, pH, and ionic strength. The data were described with the CD-MUSIC model of Hiemstra and Van Riemsdijk (1996), which allows the distribution of charge of the bound fulvate molecule over a surface region. Simultaneously, the concentration, pH, and salt dependency of the binding of fulvic acid can be described. Using the same parameters, the basic charging behavior of the goethite in the absence of fulvic acid could be described well. The surface species used in the model indicate that inner sphere coordination of carboxylic groups of the fulvate molecule is important at low pH, whereas at high pH the outer sphere coordination with reactive groups of the fulvate molecule with high proton affinity is important.

Electrokinetic properties of spontaneously precipitated calcium carbonate polymorphs: the influence of organic substances

The influence of organic matter on the surface charge of calcium carbonate polymorphs in the nascent stage was investigated in this work. Precipitation was initiated from the solution containing calcium and carbonate ionic species and selected organic solutes (fulvic, citric and propionic acids). Ionic strength of the solution was adjusted by the addition of NaCl. Electrophoretic mobilities of the resulting suspensions were measured and electrokinetic ( ζ) potentials calculated. Precipitation from the blank solution resulted in the formation of positively charged vaterite. Fulvic acid was found to be the inhibitor of both nucleation and crystal growth. When present in the system from the beginning it completely inhibited nucleation, and momentarily stopped further precipitation when added to a suspension where vaterite seed was already present. ζ-Potential of such a suspension was negative, clearly indicating fulvic acid adsorption on the positively charged sites of vaterite surface, which disabled further crystal growth. Propionic acid had no significant influence either on the precipitation process or the surface charge. Citric acid caused the formation of another calcium carbonate polymorph-calcite. ζ-potential of the precipitate was positive but much lower than ζ-potential of vaterite formed from the blank solution. The difference in electrokinetic behaviour of those two precipitates was the result of citric acid adsorption on the positive sites of the newly formed nucleus, by which mechanism the crystal growth was induced to form calcite structure.

Complexations in illite–fulvic acid–Cu 2+ systems

As part of an extended project to illustrate how heavy metals are complexed by natural aquatic particles, we conducted various experiments to study the adsorption of fulvic acid (FA) at aqueous illite surfaces and the complexation of heavy metal copper(II) in illite–FA bi-complexant systems. By analyzing batch adsorption and potentiometric titration data, we found that (i) the adsorption of FA by illite decreases with increases in pH values and its pH adsorption edge resembles those of SiO 2–FA and montmorillonite–FA systems described by other researchers, (ii) it is possible to effectively simulate the complexation of Cu 2+ ions in illite–FA bi-complexant systems by taking it to be an additive complexation of two mono-complexant systems (FA–Cu 2+ and illite–Cu 2+) and (iii) FA can inhibit the retention of heavy metals at solid surfaces by forming soluble complexes with metal ions. The above results and conclusions are supported by FT-IR analysis of various illite–FA–Cu 2+ systems.

The Effect of Humic Substances on Pb(II) Adsorption on Vermiculite

Abstract Adsorption of lead ion from aqueous solutions containing humic substances on vermiculite was investigated. Humic substances exhibited a strong influence on lead adsorption on vermiculite. The adsorption of humic acids on vermiculite plays an important role in lead ion distribution between the clay mineral and aqueous solution. There are remarkable differences in adsorption behaviors of humic acid itself among the humic acids taken from different origins. Adsorption behaviors of lead from aqueous solution containing fulvic acids were found to be less dependent on their origins, which may be attributable to negligible adsorption of fulvic acids on vermiculite.

Effet de la presence d'aluminium sur l'adsorption des acides fulviques naturels sur cap (Aluminium effect upon adsorption of natural fulvic acids onto PAC)

The removal of fulvic acids in natural water can be realised by several physico–chemical processes: activated carbon adsorption, coagulation–flocculation and oxidation. Activated carbon adsorption remains one of the most important processes used in water treatment. Many workers have demonstrated that the value of pH, the ionic composition, the temperature, the salt concentration and the type of carbon act upon the capacity of activated carbon to remove humic substances [Lee, M. C., Snoeyink, V. L. and Crittenden, J. C. (1981) Activated carbon adsorption of humic substances. JAWWA, 440–446; Randtke, S. J. and Jepsen, C. P. (1982) Effects of salts on actived carbon adsorption of fulvic acids. JAWWA 74, 84–93; Weber, Voice and Jodellah (1983)Adsorption of humic substances: The effect of heterogeneity and system characteristics. J. Am. Wat. Wks. Ass. 75, 612–619; Lafrance and Mazet (1985)Adsorption des acides humiques sur charbon activé en poudre. Influence du triphosphate de sodium. Wat. Res. 19, 1059–1064]. Whereas the association of activated carbon and coagulant–flocculent for the removal of organic matter is usually used in the case of an accidental or chronical pollution, only few workers have studied the mechanisms of elimination. The purpose of the present study was to evaluate the percent of removal of natural fulvic acids onto powdered activated carbon and in the presence of aluminium salts. Initially, one method of dosage of the fulvic acids has been chosen. The KMnO 4 oxidizability seemed to be the more efficient method because the pH and the presence of aluminium don't affect the results. Secondly, curves of the function of the order of introduction of the components, function of the pH values and function of the presence or not of aluminium salts have been drawn. Our results ( Fig. 9) show that the adsorption depends upon the values of the acidity constants (p K a) of the organic molecule, the order of introduction of the components, the PAC electrokinetic potential and the aluminium species (Al 3+, Al(OH) 2+, Al(OH) 2 +, Al(OH) 3, Al(OH) 4 −, …). The adsorption of the lone organic molecule was more important at low pH (80% of removal at pH 3) than at high pH (10% of removal at pH 8) and the removal decreased strongly between pH 4 and 6; this is certainly due to the formation of anionic substances (fulvates). The electrostatic interactions for a pH greater than 5.5 between PAC and aluminium salts are probably responsible for a modification in the adsorption of natural fulvic acids. If the pH is lower than 5.5 the adsorption of the organic molecule is increased because of a complexation between the fulvic acids and the aluminium ions. Many workers have demonstrated that aluminium strongly complexes with organic molecules [ Backes and Tipping (1987)Aluminium complexation by an aquatic humic fraction under acidic conditions. Wat. Res. 21(2) 211–216; Tipping, E., Backes, C. A. and Hurley, M. A. (1988) The complexation of protons, aluminium and calcium by aquatic humic substances: A model incorporating binding-site heterogeneity and macroionic effects. Wat. Res. 22(5) 597–611; Rakotonarivo, E., Tondre, C., Bottero, J. Y. and Mallevialle, J. (1989) Complexation de l'aluminium (III) polymérisé et hydrolysé par les ions salicylate. Etude cinétique et thermodynamique. Wat. Res. 23(9) 1137–1145; Sikora, F. J. and McBride, M. B. (1989) Aluminium complexation by catechol as determined by ultraviolet spectrophotometry. Environ. Sci. Technol. 23(3) 349–356; Thomas, F., Bottero, J. Y., Masion, A. and Genevrier, F. (1990) Mechanism of aluminium III hydrolysis with acetic acid and oxalic acid. Geochemistry of the earth's surface and of mineral formation. 2nd International Symposium, pp. 2–8. Aix en Provence; Cathalifaud, G., Ayele, J. and Mazet, M. (1997) Etude de la complexation des ions aluminium par des molécules organiques: Constantes et stoechiométrie des complexes. Application au traitement de potabilisation des eaux. Wat. Res. 31(4) 689–698]. The particular role of aluminium salts and pH in the adsorption of natural fulvic acids indicated that the carbon efficiency in removing humic substances can be significantly improved by the addition of aluminium salts in solution.

Adsorption of aquatic humic substances on colloidal-size aluminum oxide particles: Influence of solution chemistry

The adsorption of Suwannee River humic substances (HS) on colloidal-size aluminum oxide particles was examined as a function of solution chemistry. The amount of humic acid (HA) or fulvic acid (FA) adsorbed decreased with increasing pH for all solutions of constant ionic strength. In NaCl solutions at fixed pH values, the adsorption of HA and FA increased with increasing ionic strength. The presence of Ca 2+ enhanced the adsorption of HA but had little effect on FA. For identical solution conditions, the amount (by mass) of HA adsorbed to alumina was always greater than FA. Adsorption densities for both HA and FA showed good agreement with the Langmuir equation, and interpretations of adsorption processes were made from the model parameters. For FA, ligand exchange appears to be the dominant adsorption reaction for the conditions studied here. Ligand exchange is also a major adsorption reaction for HA; however, other reactions contribute to adsorption for some solution compositions. At high pH, cation and water bridging become increasingly important for HA adsorption with increasing amounts of Na + and Ca 2+, respectively. At low to neutral pH values, increases in these same two cations make hydrophobic bonding more effective. Calculations of HS carboxyl group densities in the adsorbed layer support the proposed adsorption reactions. From the adsorption data it appears that fewer than 3.3 HS-COO − groups per nm 2 can be bound directly as inner-sphere complexes by the alumina surface. We propose that the influence of aqueous chemistry on HS adsorption reactions, and therefore on the types of HS surface complexes formed, affects the formation and nature of organic coatings on mineral surfaces.

Effect of fulvic acid adsorption on the aggregation kinetics and structure of hematite particles

In this study, the effects of adsorbed fulvic acid, a naturally occurring organic acid, on the kinetics of hematite aggregation and on the resulting structure of hematite aggregates are investigated. A model based on the colloid stability theory, which also accounts for the structure of the aggregates formed, is used to describe the aggregation kinetics of these adsorbed particles. The study of aggregate structure shows that the fractal dimensions of hematite aggregates that are partially coated with fulvic acid molecules are higher than those obtained with no adsorbed fulvic acid. The scattering exponents obtained from static light-scattering experiments of these aggregates range from 2.83 ± 0.08 to 3.42 ± 0.1. The scattering exponents of greater than 3 indicate that the scattering is the result of an object that contains pores that are bounded by surfaces with a fractal structure and can be related only to surface fractal dimension. The high fractal dimensions are due to restructuring within the aggregates, which only occurred at low coverage by the organic acid. At high fulvic acid concentrations, restructuring is not observed and flocculation bridging occurs, resulting in more open structures, with scattering exponents ranging from 2.07 to 2.72.

Humic and Fulvic Acid Adsorption by Silicon and Aluminum Oxide Surfaces on Clay Minerals

AbstractNatural (untreated) clays are most probably amorphous on their outermost surface layers due to the continuous and natural process of partial dissolution and reprecipitation of the clay components at the solid‐aqueous interface. The pH‐dependent adsorption on Al and Si oxides, mordenite, kaolinite, and montmorillonite of a humic acid (HA) and a fulvic acid (FA) was described as occurring on Al and Si sites. The surfaces of the clay minerals were modeled as mixtures of amorphous Al and Si oxides. The results showed a strong adsorption of the organics by the Al sites on the Al oxide and kaolinite, and a weak adsorption of organics by the Si sites on the Si oxide, mordenite, and montmorillonite. At low pH values, the Si sites on the Si oxide, mordenite, and montmorillonite adsorbed FA; these latter observations strongly suggest that the adsorption of FA by the interplanar spaces of expanding clays is driven by forces that can be studied using amorphous Si oxide as the adsorbent. Multivalent cations will form organo‐metallic complexes that significantly increase adsorption, particularly on Si sites; exceptions were found with some FA‐metal complexes, which were attributed to the degree of complexation. The implication of these observations is that, in natural systems, the adsorption of aqueous compounds is highly dependent on the type of (amorphous) surface present at the outermost layer of the solid phase in contact with the liquid phase.

Effect of sodium chloride on interactions of fulvic acid and fulvate with montmorillonite

The interaction of montmorillonite and fulvic acid has been studied at various values of pH and 1 : 1 electrolyte concentrations. The fulvic acid was prepared from gallic acid by heterogeneous catalysis. The amounts adsorbed were determined from the decrease in solute concentration. The adsorption of fulvic acid on H-montmorillonite at pH of about 2.8 is higher than that of Na-fulvate on Na-montmorillonite at pH of about 7.5 in electrolyte-free systems. At low pH, a small amount of NaCl (10 or 20 mmol dm −3) had virtually the same effect on interaction as an amount of 200 or 500 mmol dm −3 at alkaline pH. The combined effect of the pH and NaCl was interpreted by calculating the surface charge, the potential, and the interaction energy of montmorillonite and humic substance particles by a simple model including the assumption of two widely different, intrinsic dissociation constants of the fulvic acid.

Adsorption of Humic Substances by Hydroxyaluminum‐ and Hydroxyaluminosilicate‐Montmorillonite Complexes

AbstractThe interaction between humic substances purified from an Andisol and montmorillonite (Mt) complexed with hydroxyaluminosilicate (HAS) or hydroxyaluminum (HyA) ions was studied. The HAS ions were formed from solutions containing HyA ions and orthosilicic acid at an Al concentration of 4 mM; Si/Al atomic ratios of 0, 0.25, 0.5, 0.75, and 1.0; and a NaOH/Al molar ratio of 2.0. The adsorption of humic substances by HAS‐Mt and HyA‐Mt complexes increased as pH decreased in the adsorption systems. At concentrations of 10 to 1000 mg humic substance L−1 and pH 5.5, the adsorption of humic acid by these complexes decreased with an increase in the Si/Al atomic ratio of hydroxy materials fixed in interlamellar spaces of Mt, whereas the adsorption of fulvic acid followed almost the opposite trend. When the adsorption data of humic substances by these complexes were evaluated using the Eadie‐Hofstee plot of the Langmuir adsorption equation, multiple linear relationships were obtained, indicating the presence of multiple populations of sites that have a differing affinity for humic substances. A ligand exchange reaction, in which humic substances displaced OH or OH2 groups from the coordination shells of structural Al on the edges and HAS or HyA polymers fixed on the edges, external planar surfaces, and/or the interlamellar spaces of Mt, may be involved in the adsorption processes. The results indicate that besides HyA ions, the reactivity of HAS ions in humus accumulation and the pedogenic implication in soil environments merit attention.