Sorption of cadmium on humic acid: Mechanistic and kinetic studies with atomic force microscopy and X-ray absorption fine structure spectroscopy

Abstract

. 2001. Sorption of cadmium on humic acid: Mechanistic and kinetic studies with atomic force microscopy and X-ray absorption fine structure spectroscopy. Can. J. Soil Sci. 81: 337-348. Humic acids (HA) constitute a major fraction of the organic matter in soils and sediments. Little is known about the kinetics and mechanisms of Cd sorption by HAs, especially those pertaining to the surface features and structure of the Cd-humate complexes. We investigated Cd sorption by HA using the conventional batch method, N 2 -BET method, atomic force microscopy (AFM), and extended X-ray absorption fine structure (EXAFS) spectroscopy to gain a better understanding of the kinetics and mechanisms. Cadmium sorption by HA can be described by the parabolic diffusion equation. The increase in the apparent diffusion coefficient with an increase in the initial Cd concentration was less pronounced at higher initial Cd concentrations, apparently due to an enhancement in aggregation of the Cd-humate complexes. The HA before and after Cd sorption at Cd concentrations ≤ 10 -6 M in different reaction times was spheroidal in shape. The Cd-humate complexes formed at 10 -4 and 10 -3 M Cd gradually changed to resemble the shape of a bean with increasing reaction time, thereby substantially decreasing their specific surface. Cadmium ions raveled the spheroidal HA particles into a bean-like structure by coordinating to six O atoms from the functional groups of HA, with a Cd-O distance of 2.297(2) Å. The Cd-O bond strength decreased with increasing initial Cd concentration.Key words: Cadmium, humate complexes, conformation, atomic force microscopy, EXAFS Liu, C., Frenkel, A. I., Vairavamurthy, A. et Huang, P. M. 2001. Étude de la mécanique et de la cinétique de la sorption du cadmium par l'acide humique au moyen de la microscopie à force atomique et de la spectroscopie des structures fines par absorption des rayons X. Can. J. Soil Sci. 81: 337-348. Les acides humiques (AH) constituent une bonne partie de la matière organique présente dans le sol et les sédiments. On sait peu de choses sur la cinétique et la mécanique de la sorption du Cd par les AH, surtout en relation avec les propriétés de la surface et la structure des complexes Cd-humate. Les auteurs ont étudié la sorption du Cd par les AH selon la méthode par lots traditionnelle, la technique N 2 -BET, la microscopie à force atomique (MFA) et une variante de la spectroscopie des structures fines par absorption des rayons X (EXAFS) dans l'espoir de mieux comprendre la cinétique et la mécanique du phénomène. L'équation de diffusion parabolique décrit la sorption du cadmium par les AH. La hausse du coefficient de diffusion apparent associée à l'augmentation de la concentration initiale de Cd est moins importante quand la concentration initiale de Cd est plus élevée. On le doit apparemment à une meilleure agrégation des complexes Cd-humate. Les AH présentent une forme sphéroïdale avant et après la sorption du Cd (concentration ≤ 10-6 M) à divers temps de réaction. Les complexes Cd-humate qui voi...

Fate of cadmium in the rhizosphere of Arabidopsis halleri grown in a contaminated dredged sediment

“…Piou et al (2009) also suggested an important role of organic matter in the control of the seasonal dynamics of Cd. The binding of Cd to COOH/OH groups of organic matter is consistent with previous findings by Liu et al (2001) and Fulda et al (2013). Karlsson and Skyllberg (2007) identified a mixture of O-and S-containing ligands for Cd in a peat soil.…”

Extended X-ray Absorption Fine Structure Spectroscopy Evidence for the Complexation of Cadmium by Reduced Sulfur Groups in Natural Organic Matter

“…To our knowledge there are only two published EXAFS studies on the binding of Cd to organic matter in soils and waters. In one of the studies, Liu et al (17) reported Cd to be coordinated by six O atoms in a soil humic acid. Data were, however, not modeled in k-space, and therefore, the contribution from other ligands such as sulfur cannot with certainty be ruled out.…”

Cadmium Isotope Fractionation during Complexation with Humic Acid

“…First, the formation of Cd­(II)–carboxyl complexes occurred under acidic and neutral conditions because of the larger deprotonation of carboxylic groups compared with phenolic groups at low pH values (Figure a and Figures S3a,S4a). At low Cd concentrations (10 –6 and 10 –5 mol L –1 ), carboxylic groups complexed at least 80% of total Cd, while at a higher Cd concentration (10 –4 mol L –1 ), Cd complexation was limited to 60% due to (i) site saturation, (ii) charge neutralization of HA, which led to less favorable electrostatic attractions, and (iii) conformational changes of HA that limited the site availability for Cd complexation , (Figure a and Figures S3a,S4a). Although phenolic groups act as HASs for cations, PhO–Cd complexation was limited by the protonation of phenolic groups at pH levels between 2 and 7.…”

“…2a, S3a, and S4a). At low Cd concentrations (10 -6 and 10 -5 mol L -1 ), carboxylic groups complexed at least 80% of total Cd, while at a higher Cd concentration (10 -4 mol L -1 ), Cd complexation was limited to 60% due to (i) site saturation, (ii) charge neutralization of HA which led to less favorable electrostatic attractions, and (iii) conformational changes of HA that limited the site availability for Cd complexation 62,63 (Fig. 2a, S3a, S4a).…”