THE INTERACTION OF SULFATE IONS WITH IRON SURFACES

Data were obtained which show the adsorption of sulfate ions from aqueous solutions onto Fe surfaces under various conditions. The amount of sulfate adsorbed was determined by using radioactive tracer techniques. An air- free system was used. An adsorption isotherm showing the adsorption of sulfate ions onto Fe surfaces at room temperature and pH 7 indicated that at maximum adsorption 0.35 of the surface was covered by sulfate ions. Isotherms were obtained for mixed solutions of sulfate and chromate, and sulfate and chloride. The chromate ion competed strongly with the sulfate ion when present in the solution in equal or greater concentration. Low concentrations of chloride ion increased the amount of sulfate ion adsorbed; high concentrations completely prevented adsorption. The effect of pH on the adsorption of sulfate and hydroxyl ion was determined. Highly polished coupons had greater activity than coupons less highly polished. Activity determinations indicated that adsorption increased up to 2 hr, then leveled off, and finally decreased until it reached a constant value after 14 hr. Washing had no effect on coupons exposed 14 hr or more. Ions from coupons of shorter exposure were removed by washing. The adsorption of sulfate on the oxide covering the surface was substantiated by data showing an increase in the amount of activity with increase in the amount of oxide on the surface.