Sorption of cadmium(II) and copper(II) by soil humic acids: temperature effects and sorption heterogeneity

Abstract

Sorption by humic acids is known to modify the bioavailability and toxicity of metals in soils and aquatic systems. The sorption of cadmium(II) and copper(II) to two soil humic acids was measured at pH 6.0 using ion-selective electrode potentiometric titration at different temperatures. Sorption reactions were studied with all components in aqueous solution, or with the humates in suspension. Adsorption reactions were described using a multiple site-binding model, and a model assuming a continuous log-normal distribution of adsorption constants. Adsorption of Cu 2+ was more favourable than adsorption of Cd 2+ . The log-normal distribution model provided the closest fit to observations and allowed parameterisation of adsorption data using a mean adsorption constant (log K μ ). Sorption of Cd 2+ to dissolved humic acids increased slightly in extent and sorption affinity with increasing temperature, but the effect was small (log K μ 2.96-3.15). A slightly greater temperature effect occurred for sorption of Cd 2+ to solid-phase humic acids (log K μ 1.30-2.08). Sorption of copper(II) to both aqueous-and colloidal-phase humates showed more pronounced temperature dependence, with extent of sorption, and sorption affinity, increasing with increasing temperature (log K μ 3.4-4.9 in solution and 1.4-4.5 in suspension). The weaker adsorption of Cd 2+ than Cu 2+ , and smaller temperature effects for dissolved humates than suspended humates, suggested that the observed temperature effects had a kinetic, rather than thermodynamic, origin. For any metal-to-ligand ratio, free metal ion concentration, and by inference metal bioavailability, decreased with increasing temperature. The consistency of the data with kinetic rather than thermodynamic control of metal bioavailability suggests that equilibrium modelling approaches to estimating bioavailability may be insufficient.

Assessment of Humic and Fulvic Acid Sorbing Potential for Heavy Metals in Water

“…Recent studies [56,57] have provided evidence that adsorption isotherms actually aid in the development of a broader understanding of the adsorption mechanism. However, the majority of research studies concentrate on the adsorption of one [56][57][58], two [59][60][61], or even three metals [62]. In our case, the simultaneous sorption of 26 metals introduces a level of complexity that exponentially increases the number of variables and experimental permutations.…”

Lead Remediation of Contaminated Water by Charcoal, LA Red Clay, Spinach, and Mustard Green

“…Although new methods for removing lead from water are beginning to emerge, yet many of them have their shortcomings [28][29][30][31], they either introduce chemical to the water or affect the pH or salinity of the water. Therefore, this project examines phytoremediation option that does not use live-plant but uses supernatant extract of spinach and mustard green.…”

Immobilisation of Humic Substances Using Plasma Modification

“…The interaction of humic substances with metals could play an important role in removing these hazardous substances from the environment [4,[6][7][8][9][10]. Many studies therefore focus on the interaction of heavy metals in wastewater with HSs [3][4][5][8][9][10][11][12].…”

“…In recent times, specialists have been taking increased interest in humic substances (HSs) HSs are the major organic constituents of soils, natural waters, river, lake and sea sediments, peat, brown-black coals and other natural materials as a product of chemical and biological transformations of animal and plant residues [1][2][3][4][5][6][7]. The interaction of humic substances with metals could play an important role in removing these hazardous substances from the environment [4,[6][7][8][9][10].…”