An investigation of the adsorption of silver from aqueous solution
The work presented in this thesis is concerned with the adsorption of silver from aqueous solution, on soda-glass, polyethylene and polypropylene. The silver concentrations used, ranged from 0.5 to 100 mg dm-3, and were investigated at a series of temperatures. The silver was present as AnalaR silver nitrate in doubly deionised water, and the adsorbed silver was detected as silver -110m using a Nal(Tl) well-type crystal to count the gamma activity. The study indicates that silver adsorption on glass is extremely pH sensitive, increasing with increasing pH, and reaching a maximum value at pH 11.5. At pH values greater than 11.5 silver adsorption is significantly reduced. The pH dependence was shown to be important at silver concentrations ranging from 0.5 to 100 mg dm-3. The presence of added cations, other than the hydrogen ion, also inhibit the adsorption of silver. The pH work was extended to include desorption under acid conditions, and the results have been used to formulate a set of guidelines, in an attempt to improve existing analytical technique when handling trace concentrations of silver. A contact-time study was carried out at various temperatures and concentrations of silver, to establish a tentative equilibrium time for the adsorption process. A kinetic analysis has been applied, from which specific rates for the forward and reverse processes have been assigned. The effect of pH and concentration on these rates has also been considered. The temperature dependence of silver adsorption was investigated, and a series of isotherms were produced at 281, 293, 298 and 303 K. The isotherms indicate a positive temperature coefficient for the adsorption of silver on glass. A thermodynamic treatment was applied, and a heat of adsorption calculated. A possible mechanism for the adsorption of silver on soda-glass has been proposed. The work was extended to investigate the adsorption of silver on substrates apart from glass, namely, polyethylene and polypropylene. The results have been compared to those obtained for glass.