Proton–metal exchange processes in synthetic and natural polyelectrolyte solution systems

Proton–metal exchange processes that take place in polyelectrolyte solutions have been studied using previously reported potentiometric titration data for polyacrylic acid (PAA) and humic acid (HA) systems obtained at different degrees of polymer deprotonation, for various metals and at different added metal concentrations. It is shown that the extent of the exchange process, quantified by the parameter νexch, strongly depends on the way it has been determined, i.e. at constant polyelectrolyte characteristics or at constant pH, being significantly larger under constant pH conditions. In addition, it is found that the exchange process depends upon polyelectrolyte structural charge density, the degree of ionization, the type of metal and the total metal concentration in solution. For the experiments reported here, in general the larger exchange coefficients are found for PAA at the lowest reported degree of ionization (α=0.2) and low pH values, while the lower values correspond to HA at the highest studied degree of ionization (α=0.6) and high pH values. The ability of the heavy metal ions to induce H+ release increase following the order Ca≈Cd≈Ni<Pb≤Cu. The present analysis shows that counterion condensation theory of linear polyelectrolytes appropriately describes the experimental data, provided chemical bonding interactions are not too strong to disrupt the general polyelectrolytic behavior.