Competitive Adsorption of Heavy Metals in Humic Substances by a Simple Ligand Model

Abstract

Proton and metal ion binding to humic acids has been recognized as an important factor in controlling metal speciation and mobility in aqueous and soil environments. The binding and competition behavior of humics has not been fully described, however, due to the polydisperse mixtures of natural organic polyelectrolytes with different functional groups. The simplified discrete binding group type of model (Model A) was used in this study. Model A was applied to describe three single metals and their competitive binding on the humic acids. Model A considers only a single carboxyl group type and a single phenolic hydroxyl group type with a variable electrostatic interaction factor that is expressed as a polynomial equation. Metal binding experiments were performed using a batch‐type dialysis equilibration method, and mass and charge balance expressions were used to calculate individual components of the system. The fit of Model A suggested that the major binding site for the three metal ions tested (Cd, Zn, and Cu) was a bidentate chelate of the carboxyl groups. Model A described two‐ and three‐metal competition on the humic acids using the intrinsic stability constant for the single metal ion. The model used in this study adequately described the competition of two metals; however, it showed weaknesses when applied to more complex systems, especially when metals with substantially different binding characteristics were present. Model A presented a better prediction with adjusted Cu concentration (2:1:1 Cu/Cd/Zn).

Modeling Copper(II) Complexation in a Peat Soil Based on Spectroscopic Structural Information

“…The fact that a bidentate complex is needed to describe the adsorption is in accordance with EXAFS data for the peat soil used in this study, showing that Cu(II) forms bidentate complexes involving possible combinations of amine, carboxyl, or carbonyl functional groups in a fi ve-membered ring structure (Karlsson et al, 2006). It is also in agreement with modeling results presented by Jeong et al (2007) showing that bidentate binding to carboxyls dominated the complexation of Cu in humic acids, and with ISE data presented by Christl et al (2001) showing that Cu was bonded as both mono-and bidentate complexes in fulvic and humic acids.…”

Effects of selected chemical and physicochemical properties of humic acids from peat soils on their interaction mechanisms with copper ions at various pHs

“…In general, it has been reported that the carboxylic groups of humic acids are mainly responsible for the binding processes with metal ions (Jeong et al, 2007;Rahman et al, 2010). However, other results also indicate both the carboxylic and phenolic functional groups of humic acids are capable of forming complexes (Christl and Kretzschmar, 2007;Christl, 2012;Erdogan et al, 2007;Pehlivan and Arslan, 2006); as well as the possibility of binding metal ions to HA particles by other donor atoms, e.g., nitrogen (Jimoh, 2011;Silva and Oliveira, 2002).…”

Metal Cation Complexation with Natural Organic Matter in Aqueous Solutions: Molecular Dynamics Simulations and Potentials of Mean Force

“…[43][44][45] Carboxylic groups are the most important interaction sites for metal binding to these molecules, as they are for many other organic and bioorganic molecules. Experimental and theoretical studies of the hydration of metal ions [48][49][50][51][52][53] and carboxylic functional groups [54][55][56][57] show that carboxylic groups are sites of strong association of divalent ions with NOM 24,[58][59][60] and that increasing the cation charge density increases the tendency to form a contact ion pair. 44,47 Here we present a computational molecular dynamics (MD) study of the interaction of Na + , Ca 2+ , and Mg 2+ with the carboxylic groups of a TNB NOM fragment and with acetate anion that provides an improved quantitative understanding of the structure and energetics of these interactions.…”