Adsorption at the muscovite-water interface

Abstract

This thesis describes an investigation into the different ways in which orthophosphate ions can be adsorbed onto the (001) surface of Muscovite. The major experimental problem arose from the need to keep the system free from contamination. In macro-chemistry this does not normally constitute a great problem, but it is of major important in surface chemistry where submicrogram amounts of adsorbate are involved. The chemistry of the solid-liquid interface was reviewed. The main part of the review deals with the theories describing the distribution of cations between the Stern and diffuse layers near the surface of a charged electrode and that of the mineral, bentonite. It is predicted from these theories that cations such as Aluminium which have large hydration energies, cannot occur in the Stern layer on the (001) surface of muscovite. Adsorption of orthophosphate ions onto the (001) surface of muscovite, which had been pretreated with 1M solutions of either aluminium chloride, ferric chloride, chromic chloride or calcium chloride, was studied. It was found that treatment with all of the above salts except calcium chloride enhanced the adsorption of orthophosphate ions relative to untreated muscovite. The concentration of orthophosphate on the muscovite surface was determined by using the ratio-isotope P-32. When complete, it appeared that this line of investigation would not uncover the mode of orthophosphate adsorption. Therefore, a more intensive study of the muscovite - aluminium chloride - orthophosphate system was undertaken. The analytical techniques available for the determination of aluminium were reviewed. It was decided that the fluorimetric method was the most suitable, and a discussion of the parameters affecting fluorescence is given. The fluorimetric technique was used to determine the amount of aluminium adsorbed by the (001) surface of muscovite after treatment with 1M aluminium chloride. In view of the theories proposed to explain adsorption, it was believed that this determination would greatly assist an understanding of the processes involved. A technique was developed to determine the amount of aluminium desorbed by 0.01N hydrochloric acid. It was found that less than 5 aluminium atoms per 50 Å2 of the muscovite surface were extracted by 0.01N hydrochloric acid. The adsorption of carrier free P-32 orthophosphate ions by muscovite, treated with 1M aluminium chloride solution, was studied Techniques were developed to avoid the precipitation of aluminium orthophosphate on the surface of the muscovite. The uniformity of adsorption of orthophosphate was detected by autoradiography. The value of the technique, lies in its ability to provide information concerning the orthophosphate distribution, without destroying the original sample. The adsorption of carrier free P-32 orthophosphate ions on muscovite, treated with aluminium hydroxide colloid, was studied. A quantitative comparison of the amount of carrier free orthophosphate ions adsorbed by untreated muscovite, aluminium chloride treated muscovite and aluminium hydroxide colloid treated muscovite was made. Adsorption ratios were obtained for the three systems. Suspensions of metallic orthophosphate colloidal particles were analysed by paper chromatography. Similarly, aluminium hydroxide colloidal particle, which had carrier free P-32 orthophosphate ions adsorbed on them, were analysed by paper chromatography. The radioactive components were detected by exposing Xray film to the beta particles emitted by the P-32 isotope. Not only were the suspensions divided into their component parts but a study of the stability of the systems at various pH values was also achieved. It was shown that aluminium orthophosphate particles were soluble in the chromatographic solvent at pH 1.5. The aluminium hydroxide colloid, onto which carrier free P-32 orthophosphate had been adsorbed, was very stable under the same conditions. It appears that orthophosphate ions are adsorbed very strongly by the positively charged aluminium hydroxide colloidal particles. The exact mode of adsorption is not apparent but it may have been either an ion exchange process, involving replacement of hydroxyl groups with orthophosphate ions. Or it may have been purely the result of electrostatic attraction. Adsorption by diffusion of the orthophosphate ions into the porous colloidal particles is another possibility. A qualitative study was made of the stability of the aluminium hydroxide colloidal particles at pH's ranging from 3.9 to zero. It was found that the colloid was stable for at least four weeks, even at pH zero. Since the analysis of the aluminium content of the colloid requires its total conversion to aluminium ions, it is obvious that hydrochloric acid of strength 1M or less (i.e. pH 0 or above), at room temperature, would be of no value in determining the amount of colloid adsorbed onto the surface of muscovite. Specific Conclusions It was found that orthophosphate ions can "be adsorbed onto the surface of muscovite in several ways. (1) The Bockris et al theory of specific adsorption predicts that large unhydrated anions such as the orthophosphate ion can be specifically adsorbed onto negatively charged surfaces, e.g. muscovite. (2) Orthophosphate ions can be adsorbed in the form of colloidal particles of metallic orthophosphates. (3) Orthophosphate ions can be adsorbed by a muscovite surface which has been treated with 1M aluminium chloride solution. It appears that the orthophosphate ions are adsorbed into a layer of aluminium hydroxide. (4) Orthophosphate ions are adsorbed onto the surface of aluminium hydroxide particles, which are themselves electrostatically adsorbed on the surface of muscovite. (5) The existence of an aluminium muscovite having aluminium ions in the cation exchange sites appears from the Bockris et al theory to be impossible. Therefore, the Rankin theory describing the adsorption of orthophosphate ions onto a positively charged aluminium muscovite surface is not tenable. It has been shown that this is true for orthophosphate adsorption on calcium chloride treated muscovite. Muscovite treated with 1M aluminium chloride solution and washed with water at pH 5-5, adsorbed an 0.01N hydrochloric acid extractable form of aluminium at an average surface concentration of less than five aluminium atoms per 50 Å2. Muscovite when treated with 0.01N hydrochloric acid decomposes readily at its edges and releases aluminium ions into solution. Adsorption of orthophosphate ions on muscovite does not occur by precipitation of calcium orthophosphate at pH ca, 5.5, or by any other means when muscovite is treated with calcium chloride. An aluminium hydroxide colloid suspension was stable at pH 0 and at pH 3 9 over a period of four weeks.