R.C. Bansal

Adsorption of chromium by activated carbon from aqueous solution

Abstract Adsorption isotherms of Cr(III) and Cr(VI) ions on two samples of activated carbon fibres and two samples of granulated activated carbons from aqueous solutions in the concentration range 20–1000 mg/l have been studied. The adsorption isotherms have been determined after modifying the activated carbon surfaces by oxidation with nitric acid, ammonium persulphate, hydrogen peroxide and oxygen gas at 350°C and after degassing at different temperatures. The adsorption of Cr(III) ions increases on oxidation and decreases on degassing. On the other hand, the adsorption of Cr(VI) ions decreases on oxidation and increases on degassing. The increase of Cr(III) and the decrease of Cr(VI) on oxidation and the decrease of Cr(III) and the increase of Cr(VI) on degassing have been attributed to the fact that the oxidation of the carbon surface enhances the amount of acidic carbon–oxygen surface groups while degassing eliminates these surface groups. Thus while the presence of acidic surface groups enhances the adsorption of Cr(III) cations, it suppresses the adsorption of Cr(VI) anions.

Removal of copper from aqueous solutions by adsorption on activated carbons

The adsorption isotherms of Cu(II) ions from aqueous solutions in the concentration range 40–1000 mg l−1 on two samples of granulated and two samples of activated carbon fibres containing varying amounts of associated oxygen have been reported. The adsorption isotherms are type I of BET classification showing initially a rapid adsorption tending to be asymptotic at higher concentrations. The amounts of oxygen associated with the carbon surface has been enhanced by oxidation with nitric acid and ammonium persulphate in the solution phase and with oxygen gas at 350°C and decreased by degassing of the oxidized carbon samples at 400, 650 and 950°C. The adsorption of Cu(II) ions increases on oxidation and decreases on degassing. The increase in adsorption on oxidation depends on the nature of the oxidative treatment while the decrease in adsorption on degassing depends on the temperature of degassing. This has been attributed to the increase in the carbon–oxygen acidic surface groups on oxidation and their decrease on degassing. Suitable mechanisms consistent with the results have been proposed.

Studies in surface chemistry of carbon blacks. Part I. High temperature evacuations

Surface complexes on 15 samples of carbon blacks of different varieties were studied by evacuating at 1200°. The total oxygen evolved as CO2, CO and H2O was fairly close to the value obtained by ultimate analysis. The total hydrogen evolved as water vapour and free hydrogen was significantly less, by 30–40 per cent, than the hydrogen content obtained by ultimate analysis, showing that hydrogen was held much more firmly. The evolution of CO2 commenced at around 200–300° and finished at about 500–700° while that of CO commenced at approximately 500–700° and finished in the 1000–1200° temperature range. The liberation of free hydrogen commenced in the 500–700 range and continued even at 1200°. There was no correlation between total oxygen (or any part thereof) and total hydrogen (or any part thereof). The oxygen content was less than 1.5 per cent in furnace blacks, varied between 3 and 4 per cent in channel blacks and between 4 and 8 per cent in colour blacks. Broadly speaking, the furnace, channel and colour blacks contained 9, 25 and 30 mg oxygen/100 m2 of the surface. The channel blacks ranked with furnace blacks in having a lower value of oxygen disposed off as CO2-complex but with colour blacks in having a higher value of oxygen disposed as CO2-complex but with colour blacks in having a higher value of oxygen disposed as CO-complex per unit surface. Suitable explanations based on the history of formation of the carbon blacks have been offered.

Studies in surface chemistry of carbon blacks Part II. Surface acidity in relation to chemisorbed oxygen

Surface acidity of a series of samples of carbon blacks was determined by titrating with sodium hydroxide and barium hydroxide, and the value in each case was found to be almost exactly equivalent to the amount of carbon dioxide evolved. The acidity decreased on evacuation at gradually increasing temperatures, and the decrease at a particular temperature was equivalent to the amount of carbon dioxide eliminated at that temperature. These observations cannot be explained on the basis of carboxylic or lactone groups. It appears more likely that the acidity is due to the existence of an adsorbed layer of carbon dioxide held chemically or quasichemically on the surface giving rise to what may be termed a “CO2-complex”. The strength of the complex as an acid depends upon its concentration per unit surface. Treatment of carbon blacks with hydrogen at 170° was found to cause an appreciable increase in their acidity, probably due to the reduction of quinones to hydroquinones.

Effect of gasification on the porous characteristics of activated carbons from a semianthracite

Abstract Activated carbons with degrees of burn-off between 20% and 80% have been prepared from a semianthracite char by gasification in steam at 1123 K. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The pore size distribution has been calculated from t plots, and helium and mercury densities. The ungasified char has a narrow pore size distribution and mainly consists of very fine microporosity. The pores have constrictions that do not allow adsorption of nitrogen, but the microporosity can be measured by the adsorption of CO2 at 273 K. Smic and CO2 (D-R) surface areas of the ungasified char are about the same. The gasification of the char broadens the micropore distribution. The small microporosity calculated from t plots increases with burn-off between 20%–35%. Gasification to higher degrees of burn-off causes burning of the walls between the adjacent pores and eliminates pore constrictions. This results in widening the micropores, creating meso and macroporosity and decreasing microporosity. When the degree of gasification is 80%, the small microporosity disappears almost completely, although the total porosity has been increased almost three times. While the nitrogen surface area increases with burn-off, the CO2 surface area decreases at higher degrees of burn-off.

Effect of coal preoxidation on the development of microporosity in activated carbons

Abstract Activated carbons have been prepared from a semianthracite preoxidised in air to different degrees. The activation has been carried out in steam at 850 °C to 50 ± 1% burnoff. The adsorption isotherms of nitrogen at 77 K and CO2 at 273 K have been determined. The adsorption isotherms indicate that the activated carbons obtained from non-oxidised coal has a poor porosity. The porosity increases with increase in the degree of coal preoxidation. The BET surface area increases from 160 m2g−1 in the case of activated carbon obtained from nonoxidised coal char to 847 m2g−1 for the activated carbon from oxidised coal char, t-plots obtained show that the degree of oxidation enhances both small and medium microporosity, the increase in small microporosity being much larger. The activated carbons obtained from preoxidised coal chars are largely microporous in character.

Characterization of activated carbons by the BET equation: an alternative approach

The adsorption isotherms of nitrogen at 77 K have been determined on two series of activated carbons and two samples of activated carbon fibres. The carbons were highly microporous and contained varying amounts of different forms of microporosity. Characterization of the carbons was undertaken using an alternative linear form of the BET equation and the results of the BET parameters obtained have been compared with those obtained from the usual classical linear form of the equation. The surface areas calculated from the two linear forms were similar while the C values were quite different. The C values obtained from the alternative linear form appeared to be more reasonable, being consistent with and related to the microporous character of these carbons. The alternative linear form gave BET plots which were more sensitive to deviations from linearity allowing the relative pressure range for the application of the BET equation to be determined without any ambiguity and more precisely.

Surface characteristics and surface behavior of polymer carbons—I: Associated oxygen and hydrogen

Eleven different chars prepared from four polymer precursors viz. polyfurfuryl alcohol (PF), polyvinylidene chloride (PVDC), polyvinyl chloride (PVC) and urea formaldehyde resin (UF) have been studied for their surface complexes by evacuating to 1200°C in steps. The amount of oxygen and hydrogen associated with different chars depend upon the nature of the polymer used and the history of their carbonization. The associated oxygen and hydrogen constitute different types of surface compounds which involve different bonds at different sites asociated with varying energy levels. The total oxygen evolved as CO2, CO and H2O corresponds to the value obtained by ultimate analysis while the total hydrogen evolved as water vapor and free hydrogen is ~ 25–30% less compared to ultimate analysis. In PVDC and PVC chars oxygen evolved as CO2 constitutes ~ 60% of total oxygen but for PF and UF chars oxygen associated as CO forms the major part of total oxygen contents.

Iodine adsorption method for measuring surface area of carbon blacks

Abstract Adsorption of iodine by carbon blacks from aqueous, chloroform and benzene solutions was studied and conditions for estimating surface areas of carbon blacks were standardized. It was found that the adsorption of iodine by carbon black from 0.3 N aqueous solution of iodine in potassium iodide (14 moles per mole of iodine) after 75 hr of contact as well as that from 0.3 N solution of iodine in chloroform or benzene after 15 or 20 days of contact yields ultimate value which is not exceeded on raising the concentration or prolonging the time of contact and can serve as index of surface area, irrespective of the nature of the carbon black. Adsorption was found to be mostly physical in nature.

Preparation of active carbons from coal: Part III: Activation of char

Active carbons with a burn-off of 52% have been prepared from four coals of different rank and origin after preoxidation to different degrees at 543 and 473 K, and further carbonization at 1123 K. The activation has been carried out with CO2 at 1123 K at two flow rates viz. 7 cm3 min−1 and 500 cm3 min−1. Active carbons have also been prepared from a preoxidized coal by activation to different degrees of burn-off between 10 and 80%. The effect of the degree of oxidation, the flow rate of the activating gas and the extent of burn-off on the porous structure development of active carbons has been examined. The active carbons prepared from unoxidized coal have poor textural characteristics (porosity, N2 and CO2 surface area). Nevertheless, the textural characteristics are enhanced as the degree of preoxidation of the coal is increased. The low flow rate of CO2 (activating gas) produces active carbons with a better microporous character. The degree of activation (the extent of burn-off) of the carbon determines the porous structure of the active carbon. At low degrees of burn-off (less than 50%) the product is largely microporous. At higher degrees of burn-off between 35–65% the product has a mixed porous structure and contains all types of pores. Active carbons with a given textural character can be obtained by controlling the degree of oxidation of coal and the degree of activation of the carbonized material.