Sorption and desorption of Cd(II) at the clay particle-water interface and its relevance to the distribution of metal ions in natural systems

Abstract

O estudo da sorção e dessorção de Cd(II) na interface de vários sorventes e soluções aquosas mostrou que a capacidade de troca depende do tipo de sorvente, do pH e do sistema tampão. Tampões como succinato e citrato, os quais são bons complexantes de Cd(II) suprimem completamente a incorporação dentro dos diferentes sorventes. Na presença do tampão Tris, a capacidade de troca observada decresce na seguinte ordem, γ-alumina > montmorilonita > caolinita ≈ sílica. A precipitação de Cd(II) torna-se importante na presença de bicarbonato aquoso na faixa de pH 5,5-7. As isotermas observadas para as diferentes argilas mostram saturação em concentração bastante altas de Cd(II). Isto sugere que em águas naturais, a sorção e distribuição de Cd(II) entre as fases geoquímicas dos sedimentos deve seguir uma relação linear. A cinética de dessorção de Cd(II) da superfície da montmorilonita, na presença do íon citrato, mostra que este é um ligante orgânico eficiente para remover o Cd(II) ligado em sítios facilmente acessíveis. O processo ocorre em duas etapas, com liberação inicial de aproximadamente 70% do Cd(II), seguido pela liberação lenta do metal de sítios com acesso controlado pela difusão.The sorption and desorption of Cd(II) that occurs at the interface between several sorbents and aqueous solutions has been investigated and it is shown that the observed exchange capacity depends on the type of sorbent, the pH and the buffering system. Buffers such as succinate and citrate, which are good complexants of Cd(II) completely supress its incorporation into the different sorbents. In the presence of Tris buffer the observed exchange capacity decreases in the order γ-alumina > montmorillonite > kaolinite ≈ silica, while precipitation is important in the presence of aqueous bicarbonate in the pH range 5.5-7. The sorption data fit with general purpose isotherms well, and the sorption isotherm shows saturation at rather high concentrations of Cd(II) in both clays. This suggests that under many experimental conditions in natural waters, the sorption and distribution of Cd(II) between geochemical phases of sediments should follow a linear relationship. The kinetics of desorption from montmorillonite, in the presence of citrate ion, is particularly important, showing that this efficient organic ligand removes all the Cd(II) bound to readily accessible sites in a fast step, while removal of Cd(II) from sites with diffusion-restricted access is much slower.

Distribution of hexavalent Cr species across the clay mineral surface–water interface

“…Indeed, equilibrium is attained in about 4 h. The somewhat slower kinetics is expected and consistent with observed results with other ions [12,15], where a biphasic kinetics is observed, a result consistent with the highly microporous structure of montmorillonite [19]. Thus, equilibrium adsorption measurements were done incubating the Cr(VI) and montmorillonite samples for 12 h and the experimental results for the adsorption isotherms are shown in Fig.…”

“…Thus, the solid-water interface, established between water and particles in natural waters, sediments, and soils, plays a commanding role in regulating the concentrations of most dissolved reactive trace elements in soil and natural-water systems and in the coupling of the various hydrogeochemical cycles. In this sense, model studies of the mechanism of distribution of ions such as CrO 2− 4 , HCrO − 4 , and Cr 3+ , which are environmentally important ionic water pollutants, is of interest, and laboratory models are essential for the understanding of the distribution and partitioning of metals ions in phases of sediments, soils or minerals in aqueous solutions [7,[10][11][12][13][14][15]. Thus, in the present work we describe a systematic study of the adsorption behavior of Cr(VI) on clays such as kaolinite and montmorillonite and hydrated oxides (alumina and silica gel), focusing both on the adsorption isotherms and on the species distribution at the surfaces of oxide, oxyhydroxide, and clay particles.…”

Authors’ response to comments on “Factors influencing the removal of divalent cations by hydroxyapatite”

“…• pH value of the solution was not mentioned in the manuscript [15][16][17], • initial pH values were neither measured nor adjusted, final pH values were measured [18,19], • initial and final pH values were measured [20,21], • initial pH values were adjusted, final pH values were not mentioned [13,[22][23][24][25][26], • initial pH values were adjusted, final pH values were measured [27][28][29][30], • initial and final pH values were adjusted [31], • the pH was controlled several times in the course of the reaction [32], • the pH was maintained constant throughout the equilibration time, by addition of acidic or alkaline solutions [33][34][35], • experiments were conducted at constant pH, using buffered solutions [36][37][38].…”

“…It was, for example, reported that adding either 0.1 mol/dm 3 NaOH or HCl was not successful in maintaining constant pH, which always tended to drop gradually, especially at high metal ion concentrations even after the addition of large quantities of alkaline solution [36]. On the other hand, if the pH is controlled by buffer solution, its' components can interfere with the reaction species in several ways, and observed sorption capacities are dependent on the applied buffering system [37,38]. In conclusion, all experimental techniques for obtaining sorption isotherm have pros and cons and, in our belief, it is the most important that authors are precise in describing experimental conditions.…”

Pré-concentração de Cr(III) e análise de especiação de cromo empregando montmorilonita saturada com íons potássio

“…Isso sugere que utilizar a MTK como uma forma de pré-concentração analítica de Cr(III) seria apropriado no que diz respeito ao processo de sorção, porém haveria uma limitação, pois a recuperação e, consequentemente, os estudos de pré-concentração não seriam satisfatórios. Uma possibilidade de melhorar o processo de dessorção do Cr(III) seria o emprego de agentes complexantes como citrato conforme citado na literatura, 35 o que, contudo, não foi avaliado no presente estudo. Por outro lado, a forma Cr(VI) não demonstrou interação com a MTK, sob as condições de pH estudadas, para as concentrações iniciais de 0,050 e 0,500 mg L -1 .…”