Swelling and aggregation of Leonardite upon pH change and Pb II binding: An AFM study

Abstract

Environmental context Natural organic materials, such as humic substances, play key roles in the binding and environmental fate of metals. We study the interaction of protons and metal ions with humic acids, and show changes to the mechanical properties of the particles and their capability to fix metal pollutants. The results will help refine current models of metal behaviour in the environment. Abstract The swelling and aggregation of Leonardite humic acid, due to acid-base and Pb II binding interactions, was studied through atomic force microscopy (AFM) tapping mode measurements and correlated with potentiometric experiments. These experiments allowed determination of parameters for the non-ideal competitive adsorption (NICA)-elastic polyelectrolyte network (EPN) model, which predicts size and electrostatic potential changes. AFM observations showed growth of agglomerates at low pH values. Height distribution analysis allowed discrimination of single particles from agglomerates. The size of individual particles increased slightly with pH increase. Agglomeration was evaluated through the dispersity, which increased at pH < 5, concomitant with a decrease of the electrostatic repulsion and an increase of protonated carboxylic groups, thus the agglomeration is attributed to both factors. In the presence of Pb II, agglomeration is observed to rise strongly with the increase in metal concentration, which is attributed to bridging of humic particles by Pb II ions. The AFM ex situ results suggest consistency between NICA-EPN predictions and experimental behaviour.