Adsorption Science & Technology

Manganese dioxide has been tested as an adsorbent for the removal of arsenic using radiometric techniques. Optimum conditions for maximum adsorption in terms of equilibrium time, nature and composition of electrolytes, shaking time, effect of foreign ions and other metal ions have been investigated. On the basis of these data, high separation factor values were obtained in the case of Co^II, Hf^IV and Te^IV. The adsorption data were also found to obey the Freundlich equation, the mean sorption energy of As on MnO₂ being 15.5 kJ/mol.

Nitrogen adsorption on metal (Ni, Cu, Zn) impregnated charcoal has been carried out at 77 K by the continuous flow method using a Quantasorb sorption system. It was observed that such metal impregnation did not contribute any extra surface area to the charcoal. The values of the surface area, micropore and total pore volumes determined from nitrogen adsorption follow the sequence Ni–charcoal < Cu–charcoal < Zn–charcoal < charcoal. Their behaviour is discussed in terms of the ionic radii of the metal ions concerned. The pore size distribution curves demonstrate the microporous nature of the charcoal, with the micropores contributing significantly to the total pore volume.

The adsorption of cationic dyestuffs (Basic Blue 69 and Basic Red 22) from aqueous solution by natural and activated clays has been investigated. The maximum adsorption capacity (q_max) of BB 69 and BR 22 on to natural clay was 390 mg/g and 365 mg/g, respectively. The natural clay was treated with H₂O₂ and HCl to enhance its adsorption capacity for dyestuffs. The efficiency of activation with H₂O₂ was greater by a factor of 23% and 13% in the case of BB 69 and BR 22, respectively, when compared with natural clay. The efficiency of activation with HCl was superior by 30% and 16% in the case of BB 69 and BR 22, respectively, relative to natural clay. The adsorption capacity has been correlated with the pore structure and the surface charge. The results obtained show that the adsorption capacity is independent of the S_BET values but depends on the surface charge.

Manganese/aluminium mixed oxide solids having the formula 0.2MnCO₃/Al₂O₃ were prepared by mechanical mixing of a known weight of finely powdered manganese carbonate and aluminium hydroxide. The solids obtained were treated with NaNO₃ (0.75–6 mol%) solution and dried at 110°C, then calcined in air at 500°C and 800°C for 6 h. The phases produced were identified by XRD analysis. The surface properties (S_BET, V_p and r̄) of the pure and doped solids were studied by using N₂ adsorption at – 196°C and their catalytic activities were determined by studying the oxidation of CO by O₂at 125–300°C.

The results obtained reveal that pure and doped mixed solids preheated in air at 500°C and 800°C consist of Mn₂O₃ (partridgite) and a poorly crystalline γ-alumina. Doping with sodium oxide at 500°C and 800°C resulted in a small decrease (14–19%) in the S_BET value of the treated solids. However, this treatment brought about a significant modification in the catalytic activity of the doped solids. Doping with 0.75% Na₂O at 500°C led to an increase of about 30–50% in the specific catalytic activity which was found to decrease on increasing the percentage of Na₂O above this limit, falling to values smaller than that measured for the undoped catalyst. Doping at 800°C led to a progressive decrease in the activity of the treated solid to an extent proportional to the amount of dopant present. The doping process at 500°C and 800°C did not modify the mechanism of the catalytic reaction but altered the number of catalytically-active sites contributing in the catalysis of CO oxidation by O₂ without changing their energetic nature.

The ability of sawdust to adsorb chromium(VI) from waste solutions has been studied in a batch reactor under forced convection conditions. Variables investigated were the rotational speed of the impeller, the initial concentration of chromium(VI) in solution and the weight and particle diameter of the sawdust. Increasing both the rotational speed of the impeller and the initial concentration of chromium(VI) in solution led to an increase in the volumetric mass transfer coefficient (k) according to the relationships k ∝ ω)^0.71 and k ∝ C^0.26, where ω is the rotational speed of the impeller (rev./min) and C₀ is the initial concentration of chromium(VI) in solution (mg/dm³). On the other hand, the volumetric mass transfer coefficient was found to decrease with increasing particle diameter of sawdust (d_p) according to the relationship k ∝ d_p^−0.87. The removal of chromium(VI) was also found to be significantly dependent on the weight of sawdust. Analysis of the equilibrium isotherm data has been undertaken using the Freundlich and Langmuir equations. The maximum adsorption capacity of sawdust for chromium(VI) was found to be 4.44 mg/g.

The adsorption of cobalt on lead dioxide from aqueous solutions has been optimized with respect to the shaking time, the amount of adsorbent, the pH and the concentration of adsorbate at 293 K. Under such optimum conditions, the adsorption of cobalt on lead dioxide has been studied at different temperatures and the process found to be endothermic with the distribution coefficient (K_D) increasing with increasing temperature. The data fitted very well to the Langmuir, Freundlich and Dubinin–adushkevich isotherms, allowing the corresponding constants to be calculated. Values of ΔH⁰ and ΔS⁰ were calculated from the slope and intercept of the plots of In K_D versus 1/T. The value of ΔG⁰, which decreased with increasing temperature, showed that the adsorption is more favourable at higher temperatures.

Samples of nickel on alumina (1.1–10.4 wt.% Ni) were prepared by impregnating alumina in hydrated nickel nitrate. The textural properties of the calcined catalysts have been determined from nitrogen adsorption at 77 K. Calcined catalysts were reduced prior to determination of the chemisorbed benzene at 308 K or chemisorbed hydrogen at 383 K. Hydrogenation of benzene was also determined at 473 K using the flow technique.

The surface area of NiO/Al₂O₃ catalysts decrease and their mean pore radii increase with an increase in NiO content. These changes are more pronounced for catalysts containing more than 5% metal. Chemisorption of benzene proceeds via the formation of π-bonds with nickel. The chemisorption of hydrogen indicated that the percentage dispersion decreases and the crystallite size increases with an increase in metal loading. Dispersion, crystallite size and metal surface are important factors in determining the activity of supported nickel catalysts for benzene hydrogenation.

Pure alumina, CuO/Al₂O₃ and ZnO/Al₂O₃ have been calcined in the temperature range 500–1000°C and the phase changes involved followed using DTA and XRD techniques. The surface properties of all the calcination products were determined from the adsorption of nitrogen at 77 K and the chemisorption of pyridine at 423 K. The catalytic conversion of isopropanol on all the thermal products was also investigated.

The surface areas of CuO/Al₂O₃ and ZnO/Al₂O₃ decreased while the pore radii increased with increasing calcination temperature from 500°C to 1000°C. CuAl₂O₄ forms above 800°C while ZnAl₂O₄ crystallizes at 600°C The acidity of the surface decreased when the calcination temperature was increased above 500°C. The conversion of isopropanol proceeds via dehydration of the alumina catalysts. CuO/Al₂O₃ and ZnO/Al₂O₃ surfaces catalyze both the dehydration and dehydrogenation of this alcohol.