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NORMAN HACKERMAN AND SARAH JANE STEPHENS
VOl. 58
THE ADSORPTION OF SULFATE IONS FROM AQUEOUS SOLUTIONS BY IRON SURFACES BY NORMAN HACKERMAN AND SARAH JANE STEPHENS^ Department of Chemistry, University of Texas, Austin, Texas Received March 6,I964
The quantity of sulfate ions sorbed by iron surfaces was determined directly with S 3 6 0 r - - in an air-free system a t room temperature. It was studied as a function of time sulfate ion concentration, surface Preparation and H. Uptake was determined also from mixed solutions of sulfate and chloride, and sulfate and chromate. Isotherms of trmly held sulfate on iron showed that adsorption approached a limiting value equivalent to about 0.35 monolayer. The presence of chromate ions in solution reduced the amount of sulfate adsorbed, as did large concentrations of chloride ion. Small concentrations of the latter increased the sulfate pick-u Up to pH 10 hydroxyl ions showed no noticeable effect on the sulfate isotherm but above that the quantity of sulfate axsorbed declined appreciably. There was removal of hydroxyl ion at all pH values studied, Le., from 7 t o 12.
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Introduction ~ t e e 1 . l ~A portion of the chromate ions which Ionic adsorption on metals has been detected in- became attached under a variety of conditions were directly by the effect on the electrocapillary curve.2 firmly sorbed. Because of the possibility of reducRadiotracers were’used by Paneth3who determined tion of the chromate it was not proved conclusively the specific surface area of lead with thorium B. that this was a true sorption process. For inThis isotope was also used by von H e v e ~ y , to ~ , ~ stance, with steel a t high enough chromate conshow that exchange occurs between a metal and centration to cause passivation a definite, reproduciits ions in solution; about 1000 atomic layers were ble amount of chromate was picked up and was said to be involved. Exchange between silver and uniformly spread over the surface. At lower radiosilver in solution was demonstrated by Rolline concentration corrosion occurred and some chroand by Coffin and Tingley.’ These studies also mate reduction must have occurred. I n the latter showed that gold and platinum acquired activity cases, however, the pick-up was much greater, Radioautographs in this solution. There was no proof that this was although less reproducible. exchange rather than adsorption, but exchange with showed that the uncorroded areas (cathodes) acquired more activity and also that a considerably non-common ions is possible. Radiosodium ions have been sorbed from aque- greater amount was associated with the reaction ous solution onto glass, aluminum, steel, silver and products ah the local anodes. Sulfur-35 in sulfate offered a means of determinplatinum.* Exchange of cobalt tracer from solution with a number of metals both in the absence ing anion sorption on iron. The activity acquired and in the presence of air was reported by Simnad by the sorbate should be that due to adsorption and Ruder.g They concluded that the amount alone, since the possibility of reduction of the sulfate of radioactivity acquired was the result of electro- ion under the conditions used was small. The podeposition by local cell action. Adsorption iso- tential differences between local anodes and caththerms have been obtained for radiocobalt on odes are too small to give electrochemical reduchydrous ferric oxide, and the effects of pH, ion tion, and the possibility of direct reaction in the concentration and quantity of sorbate also were essentially neutral sodium sulfate solutions is nil. The radiotracer has a sufficiently high activity and determined. lo The work cited so far deals solely with cations a long enough half life t o make it convenient to in solution and metals or metal oxides. Attempts handle and measure. Competitive effects between to study the adsorption of chloride ion on chromium this and other anions were studied by noting the were made by Marshall” using NaCla6. The effect of the “cold” foreign anion on the sorption results were equivocal because of the low activity of sulfate. It was necessary to work in an air-free system of the tracer and because of contamination by sulfur-35. A more successful study involved the because iron corrodes to a noticeable extent in uptake of tagged chromate on chromium12 and on these salt solutions in the presence of air. Even if the last traces of oxygen were not removed the (1) du Pont Fellow, University of Texas, 1951-1952. procedure described below reduced it to a point (2) N. I
