Anion Interactions with Macromolecule-Carboxyl-Bound Cations

Abstract

The interaction of anions with cations bound to cation-exchange materials is a little-understood branch of chemistry. The chemistry of such systems is important as a basis for the understanding of many problems in such diverse branches of science as biology, soil chemistry, heterogeneous catalysis, and colloid science. The aim of this thesis was to study the chemistry of the interaction of macromolecule-carboxyl-bound cations with anions. The most outstanding result of this work was the discovery that some anions could form complexes with cations on macromolecule carboxyl groups, even when the corresponding cation-anion salt was water-soluble. Examples of such interaction are: (a) Molybdate with macromolecule-carboxyl-bound potassium, magnesium, cobalt and zinc. (b) Iodide with macromolecule-carboxyl-bound aluminium. (c) Iodate with macromolecule-carboxyl-bound potassium, magnesium, cobalt, copper, zinc, aluminium and iron. (d) Chloride with macromolecule-carboxyl-bound aluminium and iron. (e) Sulphate with macromolecule-carboxyl-bound sodium, potassium, calcium, cobalt, copper, zinc, aluminium and iron. Further, it was found that equilibrium between the macromolecule-carboxyl-bound cations and the anion in solution was attained only slowly. This necessitated the use of a specially-designed apparatus to ascert in when equilibrium had been reached. Most of the naturally-occurring macromolecular compounds to which this work was related had other functional groups together with carboxyl groups. As these functional groups could have interfered with observations on the interactions of anions with macromolecule-carboxyl-bound cations, it was necessary to obtain a suitable model compound for use in these experiments. The weak cation-exchange resin, Zeocarb 226, was chosen as it was of high molecular weight and unifunctional in carboxyl groups. A range of cations were separately adsorbed on to Zeocarb 226 at pH 5 to give a range of Zeocarb 226-cation systems. These systems were used in the studies on anion adsorption by macromolecule-carboxyl-bound cations. The adsorption of anions by Zeocarb 226-cation systems was studied in an apparatus that allowed solution in equilibrium with the Zeocarb 226-cation system, to continuously circulate over the Zeocarb 226-cation system. Provision was made, in the apparatus, for monitoring the solution with a Geiger counter. The anions studied were added to the circulating system tagged with a radioactive tracer so that the adsorption process could be followed. Phosphate adsorption by Zeocarb 226-cation systems was extensively studied because of the importance of phosphate in soil fertility. Phosphate was not removed from solution at pH 2-3, pH 5-6, or pH 8-9 in detectable amounts by the following Zeocarb 226-bound cations: hydrogen, sodium, potassium, calcium, magnesium, manganese, cobaltous, cupric or zinc. Phosphate was removed from solution by Zeocarb 226-titanic, Zeocarb 226-ferric and Zeocarb 226-aluminium systems at all the pH ranges mentioned above. The Zeocarb 226-bound cations that removed phosphate from solution form water-insoluble salts with phosphate, so that a precipitation mechanism for the removal of phosphate from solution was possible. However, considerable evidence was found to suggest that such a mechanism was not in operation and that the adsorption of anions by Zeocarb 226-cation systems was probably due to an adsorption mechanism. In an experiment to determine the importance of macromolecule-carboxyl-bound cations in the retention of phosphate by the soil, it was found that a Patua loam soil was able to release 0.3% of its total phosphorus to an equal weight of Zeocarb 226 (hydrogen form) in 48 hours. A further experiment showed that alumina was able to retain only two and one half times more phosphate at its adsorption maximum than was Zeocarb 226-aluminium at its adsorption maximum. Both these results suggest that macromolecule-carboxyl-bound cations are of some importance in the retention of phosphate by the soil. An attempt was made to determine whether the soil phosphorus fractionation proceedure of Chang and Jackson could be applied to macromolecule-carboxyl-bound phosphate. It was found that all of the phosphate could be removed from Zeocarb 226-ferric and Zeocarb 226-aluminium systems by the combined effect of the ammonium chloride, ammonium fluoride and sodium hydroxide extractants of the Chang and Jackson method. However, all phosphate bonded to aluminium on Zeocarb 226 was not removed by the extractant for aluminium-bonded phosphorus and all phosphate bonded to ferric iron on Zeocarb 226 was not removed by the extractant for iron-bonded phosphorus. Hence this procedure cannot be used, as described by Change and Jackson, to differentiate between iron-bonded and aluminium-bonded phosphate on Zeocarb 226. Normal soil conditions are subject to changes in ionic strength, and such changes were applied to Zeocarb 226-cation-phosphate systems. It was found that increase of ionic strength in solution had a depressing effect, if any, on the amount of phosphate bound to Zeocarb 226-cation systems. However, evidence was obtained to show that the phosphate bound to Zeocarb 226-ferric and Zeocarb 226-aluminium systems was readily released to leaching water. It appeared that there were two forms of phosphate on these systems: (a) A form that was rapidly released to aqueous solution. (b) A form that was less rapidly released to aqueous solution. The series of experiments carried out on phosphate adsorption by macromolecule-carboxyl-bound cations suggest that phosphate is bound to sole extent in this form in the soil. Phosphate in this form is, in part, readily released to the soil solution and is thus important in soil fertility. Molybdate ions were removed from solution in detectable amounts by Zeocarb 226-bound potassium, magnesium, cobaltous, cupric, zinc, titanic, aluminium and ferric ions. Hydrogen, sodium, calcium and manganese bound to Zeocarb 226 were not able to remove detectable amounts of molybdate from solution. There was no observed retention of molybdate by any Zeocarb 226-cation system in alkaline solution. This result is consistent with the observed increase of available molybdenum when soils are limed. The divalent copper ion was found able to retain more molybdate than either ferric or aluminium ions, when these cations were bound to Zeocarb 226. Zeocarb 226-aluminium was the only system tested that was able to remove iodide from solution in detectable amounts. It is inferred in previous work on soil iodine, that soil iodine content and organic matter content are related. It is probable that this relationship is not due to the binding of iodide by macromolecule-carboxyl-bound cations. It is, however, possible that the relationship is due to binding of iodate by macromolecule-carboxyl-bound cations. Iodate was removed from solution by the following Zeocarb 226-bound cations: potassium, magnesium, cobaltous, cupric, zinc, aluminium and ferric. As iodate is the predominant form of iodine in the soil under oxidative conditions, and as it is retained by many more macromolecule-carboxyl-bound cations than is iodide, it is very likely that iodate retention by organic matter produces the relationship mentioned above. The adsorption of a further halide by Zeocarb 226-cation systems was tested. It was found that chloride was removed from solution by Zeocarb 226-aluminium and Zeocarb 226-ferric systems and not by any other Zeocarb 226-cation system tested. Once again it is of interest to note that a halide is removed from solution by very few Zeocarb 226-bound cations. The experiments devised to determine the adsorption of phosphate, molybdate, iodide, iodate and chloride by Zeocarb 226-cation systems all involved the detection of the hard beta-emitting radio-isotopes added as tracers. The adsorption process in these experiments were thus easily followed with the use of a Geiger-Muller tube. Similar adsorption processes using 35S-sulphate as the anion were found to be more difficult to follow. The soft beta-emitting 35S-sulphate was eventually determined in a scintillation counter using a special liquid scintillation fluid. Sulphate was removed from solution by Zeocarb 226-bound sodium, potassium, calcium, cobaltous, cupric, zinc, aluminium and ferric ions. Of the Zeocarb 226-bound cations tested only hydrogen, magnesium and manganese did not remove detectable amounts of sulphate from solution. Sulphate was the only anion tested that was removed from solution by the Zeocarb 226-calcium system. Most of the Zeocarb 226-cation systems retained larger amounts of sulphate at low pH values than at high pH values. Soil systems are also known to follow this adsorption pattern, hence macromolecule-carboxyl-bound cation sulphate adsorption follows the same pattern for sulphate adsorption as does soil. A series of experiments were carried out on anion adsorption by a wide range of Zeocarb 226-bound cations. The results from the adsorption of phosphate, molybdate, iodide, iodate, chloride and sulphate on the Zeocarb 226-cation systems and on Zeocarb 225-cation systems (Zeocarb 225 is a sulphonated cation-exchange resin), assisted in the development of a postulate to explain anion retention by macromolecule-carboxyl-bound cations. It was postulated that the anion adsorbed formed a long-lived ion-pair with the resin-bound cation. This postulate fitted well with the cation-anion systems that were known, from the literature, to form ion-pairs in aqueous solution. Ion adsorption pattern differences between Zeocarb 226-bound cations and the same Zeocarb 225-bound cations were explained by proposing that the electrostatic fields of the resins had an effect on the life of the ion-pair formed at the resin surface. Thus an observed anion adsorption by a given cation on Zeocarb 226 compared with no anion adsorption by the same cation on Zeocarb 225 was suggested to be due to the life of the ion-pair in the electrostatic field of Zeocarb 226 being of sufficient magnitude to be detected, while that in the electrostatic field of Zeocarb 225 was of insufficient magnitude for detection.