Effect of cationic polyelectrolytes on the dissolution of magnetite in thioglycolic acid solutions

The influence of cationic polyelectrolytes, polyethyleneimine (PEI) and polyvinylbenzyltrimethylammonium chloride (PVBTA-Cl), on the kinetics of dissolution of an iron oxide (magnetite, Fe3O4) by thioglycolic acid (HTG), is presented. It was found that both polyelectrolytes accelerate the dissolution reaction at pH values below ca. 4, whereas an inhibition is observed at higher pH values. These findings are at variance with the results obtained in the case of anionic polyelectrolytes, in which the reaction is inhibited over the whole pH range, The effects of PEI concentration, of the addition of inert salt to the PEI-HTG-Fe3O4 system and of temperature on both systems were also studied. The main effects of PEI are the enhancement of the acidity of the oxide surface, and the decrease of the available surface sites for HTG complexation; the effect of PBVTA-Cl is more modest, due to the higher rigidity of the polyelectrolyte. Modeling of the experimental results in PEI media leads to a kinetic scheme similar to that describing the reaction in the absence of PEI, with a pK(a)(s), shifted by -2; this shift alters the pH dependence of the surface speciation, thus modifying the rate-pH profiles. For PVBTA-Cl, the shift derived from the data is -1.