ADSOR,PTION OF MINERAL ACIDS
3625
is obtained by migniag K I = K 4 = low2,and Ka = lo4. This model agrees with their postulation that the species Pi(HPi)z- exists at concentrations higher than 0.1 M . Also, the slopes of their curves
agree with those of the theoretical model presented in Figure 8. It was not possible to obtain an adequate fit of the data by ignoring the formation of Pi(HPi)zand by assuming the formation of Pi(HPi-).
Kinetics of Adsorption of Mineral Acids on Alumina Lj. Ja&movi&,*J. Stevovib, and S. Veljkovib Iiastitute Boris Kidrich, Beograd, Yugoslavia (Received May 8, 1978)
Rates of adsorption of HeTeOa and H9P04 on alumina were examined in varying pH conditions a t 25" with the purpose of estimating the nature and the extent of reactions of anions with -A1(OH)g(HsO)2 sites. The experimental data were interpreted in t e r m of a multistage reaction, dominated by dissociative processes on the alumina. Two dissociation constants were introduced,in a reaction model, based on the reaction of surface ions with anions in solutions. The reaction was found to be of one-third order in relation to acids, suggesting loose surface complexes having free orientation of anions.
The adsorption of mineral acids on alumina is generally described in terms of the equivalent exchange of :queous electrolytes with OH- and H+ ions at the surface.',* Recently, it was shown that the equilibrium distribution of these ions, at the solid-solution interface, is dependent on the concentration of H+ ions in the The occurrence of a charged (k)or neutral surface has been attributed to the formation of metal-quo complexes of the type I5 +A -
B +OH -
[-Al(Hz0) (OH)O]-
+ HzO
The accompanylLngion?, A- and B+, may stay as counter ions outside the primary hydration shell of the surface I n some cases, A- can replace OH- inside the complex, while B+ can react with the negatively charged complex The final result is analogous t o the ion exchange of these ions. Existing data on the adsorption of acids do not take into consideration above double-layer characteristics of alumina. Kinetics of adsorption of electrolytes were not studied in details at low pHs, while some thermodynrtmic studies at high pHs support the above scheme3 It is, therefore, necessary to evaluate general lcinetice of adsorption of acids in varying pH and concentration conditions. In the present study, we have investigated these phenomena b7 : using X-16Te06 and H3P04. Their dissociation constants allow for a systematic study of the adsorption in a width region of pH values. Available data S ~ Q Wthat their kinetics of adsorption were poorly
defined, being inconsistent with standard ion exchange Our studies are aimed a t the evaluation of an adequate kinetic expression which should describe the influence of surface equilibria (at the alumina) on the rate of adsorption. Our results fit a, kinetic scheme which confirms that influence. Experimental Section Materials. The adsorbent used was aluminium oxide, Merck, Code No. 1078, particle size 100-200 mesh. After washing with double distilled water, the oxide was dried for 24 hr at 140". When necessary, the oxide was treated with 2.7 X IO-1 M HC1 for 20 hr at 25". After removal of acid, samples were washed with distilled water until the reaction was neutral and were dried as above. I n some experiments, oxide Code No. 1097, Brockmann type, was used. X-Ray analysis showed that the oxide was mainly amorphous. According to data given by the manufacturer, oxides No. 1078 and 1097 belong to the group "for column chromatography," which contains x and y aluminas, produced mainly from Ilydrargillite. Many geometric irregularities of oxide particles were (1) F. Umland, 2. Elektrochem. Der. Bunsenges. Phys. Chem., 60, 711 (1956). (2) K. C. Williams, J. L.Daniel, W. J. Thomson, and R . I. Kaplan, J . Phys. Chem., 69, 250 (1965). (3) 9. M. Ahmed, ibid., 73, 3546 (1969). (4) S. Takahashi, E. Shikata and H. Amano, J . A'ucl. Sei. Technol., 7 (3),130 (1970). ( 5 ) P. R. Sinha and A. K. Choudhury, J . Tndian Chem. Soc., 31, 311 (1954).
The Journal of Physical Chemistry, Vol. 76, N o . 84, 1972
3626
Figure 1. 1';irtirlrs of :\I2lh So. I l l i S X 40 (lieirhrrt, binocular mierosrape).
revealed by microscopic rxaminations, as shown in Figure 1. The bulk matrrial could br easily ground to very fino powdrr, indicating that the microstructure of thc original grl is prrsrrvcd inside the particles. It is probablr that many channrls brtween smaller aglomrratrs facilitntr the prnetration of liquids into the particlrs. Thrn most kinrtics studies could be based on a homogeneous reaction model (used in case of reactions occurring in ion exchangr resins).' Singular actions of some specific surface sites are thus masked by the behavior of bulk material. Both types of alumina, No. 1078 and 1097, had surface areas, as measured by N? adsorption (BET, sorptomctrr, Prrkin-Elmrr-Shell), in the range 95-115 mz/g. pHs of 10% suspensions in H 2 0 were found to be 4.1 ( K O . 1078) and 9.2 (No. 1097). According to data supplird by the manufacturer there are 0.2% CI- and 0.1% SO.?- in No. 1078, and 0.1% CI- and 0.1% SO.?- in K O . 1097. The sodium content was determinrd; samplrs No. 1078 and 1097 were first analyzed at the end of the standardization by H20, the determination of sodium bring done by radioactivation analysis. Sample No. 1078 has 0.07% Na, while No. 1097 has 0.4% Na. The treatment of the samples by 2.7 X lo-' Ai' HCI decreases the content of sodium in KO. 1097 to 0.15%; sample No. 1078 is practically unaffected (0.06% Na). Other impurities were determinrd spectroscopically: Fe (
