Modeling the adsorption of weak organic acids on goethite : the ligand and charge distribution model

A detailed study is presented in which the CD-MUSIC modeling approach is used in a new modeling approach that can describe the binding of large organic molecules by metal (hydr)oxides taking the full speciation of the adsorbed molecule into account. Batch equilibration experiments were performed using the iron (hydr)oxide goethite to determine the adsorption of a series of weak organic acids (e.g. lactic acid, oxalic acid, malonic acid, phthalic acid, citric acid, and fulvic acid). In order to develop the new modeling approach, the binding of weak organic acids with a well-defined structure and charging behavior is first described using the classical CD-MUSIC model approach. The adsorption can be described accurately with a limited number of surface species, which differ in the degree of protonation and in the number of inner sphere and outer sphere complexes formed between the reactive groups of the organic molecule and the oxide surface. For the formation of inner and outer sphere complexes, a constant distribution of charge over the solid/water interface is assumed. With the same charge distribution for inner and outer sphere complexes, the adsorption of FA is described with the CD-MUSIC model using a small set of discrete surface species. This approach is not fully satisfactory since it does not take the full speciation of the adsorbed weak organic acids into account as can be inferred from spectroscopic data. In order to take the full possible speciation of the adsorbed organic molecule into account, a new model concept was developed. In the ligand and charge distribution (LCD) model concept, the number of inner sphere, outer sphere and proton complexes of the reactive groups of one adsorbed organic molecule is calculated using the NICCA equation. From the resulting speciation of the adsorbed molecule, the main input parameters of the CD-MUSIC model are obtained. The new model concept is successfully tested on the adsorption and infrared data of benzenecarboxylic acid adsorption on goethite by Boily et al. (2000a,b). The LCD model was extended for the surface complexation of the phenolic groups in order to describe the previously determined data of FA adsorption by goethite. Simultaneously, the concentration, pH, and salt dependency of the FA adsorption are described well. Furthermore, the co-adsorption of protons upon the adsorption of FA by goethite is predicted accurately. The developed model approach offers new insights in the fundamental understanding of ion adsorption under natural conditions.</p

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. Copyright (C) 2003 Elsevier Science Ltd

Modeling the binding of benzenecarboxylates by goethite: The ligand and charge distribution model

A heterogeneous complexation model approach has been developed to describe the adsorption of large organic molecules by goethite taking the full speciation of the adsorbed molecules into account. The essence of the model is the calculation of the mean mode of an adsorbed organic molecule, defined by an overall affinity, charge distribution, and reaction stoichiometry. The calculation of the mean mode of the adsorbed organic molecule is based on the pH-dependent interaction of a functional group for binding a proton or forming an inner or outer sphere complex with specific surface sites. The distribution of the organic molecules over the solid and solution phases is calculated using the CD-MUSIC model with parameters obtained from the mean mode of the adsorbed molecule. The mean mode of the adsorbed molecule is calculated using the competitive Langmuir or NICCA equation in combination with an electrostatic model comprising charge distribution, taking the competitive binding of functional groups with protons or surface sites into account. The new model is applied to the adsorption data of a series of benzenecarboxylic acids. The model approach can describe simultaneously the concentration, pH, and salt dependency of the benzenecarboxylate adsorption. Furthermore, the model is applied to describe data obtained from IR spectroscopy. The new approach predicts the data reasonably well using only three adjustable parameters in case of the NICCA-CD-MUSIC approach. The average stoichiometry of the overall reaction is not an input parameter of the model but is an output parameter. In future work the new model will be applied to the adsorption of larger organic acids, e.g., fulvic aci

Efeito do pH na adsorção e dessorção de cádmio em Latossolos brasileiros Effect of pH on cadmium adsorption and desorption in Brazilian Oxisols

Reações de adsorção e dessorção de metais em solos são influenciadas por atributos de superfície dos colóides e pela composição da solução no meio. Este estudo avaliou o efeito do pH sobre a adsorção (Cd ads) e dessorção (Cd des) de Cd em amostras do horizonte A de l7 Latossolos do Brasil. Amostras de cada solo foram suspensas em Ca(NO3)2 5 mmol L-1 (pH ajustado para 4,5; 5,5; e 6,5; relação solo:solução 1:67) e colocadas para reagir com Cd(NO3)2 0,20 mmol L-1 (relação solo:solução final 1:100) por 72 h. Imediatamente após a retirada do sobrenadante para determinação do Cd ads, adicionaram-se, ao resíduo remanescente, 25 mL de Ca(NO3)2 5 mmol L-1 para realização da dessorção do Cd adsorvido nas amostras de solo. Foi verificado um aumento na Cd ads de 1,3; 1,7 e 2,2 vezes decorrente da elevação do pH de 4,5 para 5,5; de 5,5 para 6,5 e de 4,5 para 6,5, respectivamente. Isso corresponde a uma percentagem de Cd adsorvido de 27 % em pH 4,5; 35 % em pH 5,5 e 55 % em pH 6,5. O efeito dos atributos dos solos sobre a adsorção de Cd só foi evidenciado em pH 5,5 e 6,5 por meio das correlações entre o Cd ads e a matéria orgânica, área superficial específica (SE), CTC a pH 7,0 (CTC), teores de caulinita, hematita, Fe2O3 extraído pelo oxalato ácido de amônio e argila, dentre outros atributos. Todavia, apenas a CTC e o conteúdo de argila, em pH 5,5, e a SE, em pH 6,5, foram incluídos no modelo de predição de Cd ads obtidos por meio de análises de regressão múltipla. A adsorção em valores de pH mais elevados não propiciou redução no Cd des, o qual se situou em torno de 20 % para pH 4,5 e 40 % para pH 5,5 e 6,5. A baixa proporção de Cd adsorvido por estes Latossolos, principalmente em menores valores de pH, reforça a necessidade da adoção de critérios adequados quando do uso ou descarte de resíduos que contêm Cd em áreas agrícolas ou próximas a aqüíferos.<br>Adsorption and desorption reactions of metals in soils are influenced by the surface of the soil colloid attributes and solution composition. This study evaluated the effect of the pH on Cd adsorption (Cd ads) and desorption (Cd des) in l7 Brazilian Oxisol samples that differed in their chemical, physical and mineralogical attributes. Samples of each soil, suspended in 5 mmol L-1 Ca(NO3)2 (pH adjusted to 4.5; 5.5, and 6.5; ratio soil:solution 1:67) were placed to react with 0.20 mmol L-1 Cd(NO3)2 (final ratio soil:solution 1:100) for 72 h, after which they were centrifuged and the Cd concentration of the solution determined. Thereafter, 25 mL of 5 mmol L-1 Ca(NO3)2 were added to the remaining residue to desorb the Cd retained in the soil samples. An increase of the pH solution from 4.5 to 5.5, from 4.5 to 6.5 and from 5.5 to 6.5 caused a 1.3; 2.2 and 1.7-fold increase in the Cd adsorption, respectively. The mean percentage of Cd adsorbed (Cd%ads) was 27 (pH 4.5), 35 (pH 5.5) and 55% (pH 6.5). The effect of soil attributes on Cd ads was only evidenced at a pH of 5.5 and 6.5, by the correlations between Cd ads and the soil organic matter, specific superficial area (SSA), CEC at pH 7.0 (CEC), kaolinite, hematite, oxalate-and-DCB-Fe and clay contents. However, only CEC and clay content, at pH 5.5 and the SSA, at pH 6.5, were included in the model of Cd ads prediction, obtained through regression analyses. The adsorption in values of higher pH did not propitiate reduction in Cd des, which was around 20% for pH 4.5 and 40% for pH 5.5 and 6.5. The small proportions of Cd adsorbed by these Oxisols, mainly at lower pH values, which are an indication of high mobility and bioavailability, reinforces the need for the adoption of appropriate criteria to use or discard residues containing Cd in agricultural areas or close to aquifers