Complexation of lead and cadmium ions with humic acids from arctic peat soils.

Humic acids (HAs) have many significant environmental and geochemical functions in soils, bottom sediments, and aquatic environments. Their interaction with toxic heavy metal ions affects their transport and bioavailability. This study suggests that binding of heavy metal ions to HAs could potentially help to develop strategies for recovering metal-contaminated soils and groundwater. This study is aimed at investigation of sorption properties of HA preparations from peat soils based on determining the kinetic and thermodynamic parameters of the sorption processes of Cd and Pb ions. Based on a model experiment, the binding ability of HAs of Hemic Folic Cryic Histosol to Cd and Pb ions was revealed. It is shown that during the initial stage (first 20 min for Cd ions and 30 min for Pb ions) the kinetics of the process of sorption of metal ions by suspended HA preparations is better described by pseudo-second order equation. This indicates the chemisorption mechanism and limiting contribution of chemical reaction. The kinetics sorption process within the Boyd - Adamson - Myers model is described with high precision for both ions during the whole experiment. This indicates the intra-diffusion limitation of the sorption process. The thermodynamic characteristics (the limiting sorption capacity, constant of sorption equilibrium, Gibbs free energy change, entropy change and enthalpy change of sorption) of the sorption process of selected heavy metal ions have been calculated. It was found that the limiting specific sorption of Pb ions is almost an order of magnitude higher than that of Cd ions and amounts to 0.16-0.29 mmol/dm and 0.0078-0.034 mmol/dm, respectively. The sorption enthalpy variance of 48.4 kJ/mol for Cd ions and 22.6 kJ/mol for Pb ions indicate the endothermic nature of sorption on HA solid particles. It was shown that the limiting stage of sorption for Cd and Pb ions is ion diffusion to the sorbent, while chemisorption itself proceeds quite quickly. Values ΔS> -10 J/(mol ∙ K) indicate a dissociative sorption mechanism for both metals, i.e. non-sorbed ions in the solution are in a more ordered state than after sorption. The negative values of the Gibbs free energy change for Cd and Pb ions indicate that the interaction of ions with HA preparations from peat soils is a spontaneous process with a complex mechanism involving complexation and ion exchange processes.