Adsorption of humic acid onto ultrafiltration membranes in the presence of protein and metal ions

Abstract

In this research, adsorption of humic acid (HA) from HA–BSA (bovine serum albumin) mixtures onto ultrafiltration membranes was elucidated in the presence of various metal ions. Three equilibrium adsorption isotherms i.e. Langmuir, Freundlich and Redlich-Peterson were tested to fit to the experimental sorption data. The Redlich-Peterson isotherm was found to well represent the measured adsorption data for all studied cases based on the higher coefficient of determination ( r 2). Influence of some factors on adsorption mechanism of humic acid including BSA addition, ionic strength, ionic charge and size of hydrated ion were investigated. Adsorption kinetic can be interpreted as two distinct regions (during filtration time) based on the intensity and relative stability of intermolecular interactions i.e. transient and steady state. At transition state electrostatic interactions between solutes and HA-macromolecules are more effective than interactions between membranes and HA. At steady state region intermolecular interactions lead to establishment of electrochemical bridges between solution's components and molecular aggregates form in the solution leads to superior adsorption on the membrane compared to transient state region. An increment in ionic strength and charge of ions in the solution improves the final sorption capacity of the membranes. However this initially decreases the sorption of HA on the membranes at transient state. Cation with lower charge density (e.g. Ba + 2 ) strongly attracts negatively charged humic acid molecules leading to higher sorption onto the membrane. There is less water molecules surrounding the cations with lower charge density. Cations with higher charge density (e.g. Mg + 2 ) attract more water molecules and therefore the cation's effective charge is shielded leading to less adsorption of HA-molecules.