Effect of ionic condensation and interactions between humic substances on their mobility: An experimental and simulation study

• Ion binding to humic substances is assessed by Monte Carlo and zetametry.
• Mobility is derived via an all-ion MSA transport theory.
• Conductimetry shows small variation of mobility even for divalent counterions.

The effect of ionic strength and of the condensation of divalent ions on the mobility of humic substances have been addressed thanks to conductimetry experiments. The analysis of the experimental data were performed by the use of a Mean Spherical Approximation transport theory, which includes both hydrodynamic and electrostatic interaction effects between all the solutes. An input parameter in the transport theory is the extent of condensation of counterions, which was determined by the use of Monte-Carlo simulations and compared to the results of electrokinetic measurements. For the humic acid, the method gives reliable results and show that condensation leads to limited changes in the mobility of the humic acid for a ionic strength of 10 mM. However, for the smaller fulvic acid the extent of condensation is underestimated. The limitations of the method are discussed.

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