C.M. González-García

Analysis of the adsorption isotherms of a non-ionic surfactant from aqueous solution onto activated carbons

Abstract The adsorption isotherms at 20°C onto four activated carbons of the non-ionic surfactant Triton X-100 from aqueous solution have been studied over a wide concentration range. The adsorption was explained using one or a combination of two Langmuir equations, depending on the equilibrium concentration range studied. A description of the adsorbed layer was constructed on the basis of the information obtained from the isotherms and assuming the model of van Oss et al. for the interfacial interaction between adsorbent and adsorbate. The results indicate that there are at least two kinds of interactions, the first related to a direct interaction between the activated carbon surface and adsorbate molecules, and the second mainly due to the interaction between surfactant molecules at the adsorbent–solution interface leading to the formation of interfacial aggregates.

Nitrogen adsorption isotherms on carbonaceous materials. Comparison of BET and Langmuir surface areas

Abstract This study compares the Brunauer–Emmett–Teller (BET) and Langmuir equations when applied to N2 adsorption isotherms at 77 K on various carbon blacks and activated carbons. Adsorbent samples varied greatly in the degree of development of their surface area and porosity. The activated carbons were more microporous and less mesoporous than the carbon blacks. The equations were applied up to p/p0=0.20, 0.30, and 0.40. The values of the lineal correlation coefficient as a rule were above 0.990. From the derived BET (SBET) and Langmuir (SL) surface areas, the conversion factor (α) of SL to SBET was calculated. The α mean value (ᾱ) and the standard deviation coefficient (σ) were also obtained. The N2 isotherms fit better to the Langmuir equation for the activated carbons and to the BET equation for the carbon blacks. The factor α was markedly higher than the unit for all carbons. It increased significantly with increasing p/p0 range. The α values were closer for each series of carbons, and larger for the carbon blacks than for the activated carbons; in spite of the fact that the carbon blacks were more mesoporous. As shown by the ᾱ increments, the dependence of ᾱ on the p/p0 range was less strong for the activated carbons for the two narrowest p/p0 ranges. The σ values indicated that the influence of the type of carbon on ᾱ was greater for the carbon blacks when fitting up to p/p0=0.30 or 0.40.

REMOVAL OF AN IONIC SURFACTANT FROM WASTEWATER BY CARBON BLACKS ADSORPTION

The adsorption of an ionic surfactant, sodium dodecyl sulfate (SDS), from aqueous solution onto six carbon blacks with different surface characteristics has been studied in various ranges of low concentrations. The process can be related to types of interactions that are always the same since the adsorption in these systems depends mainly on the porosity of the samples but not on the chemical composition of the surface. However, adsorption progresses follows two processes: the first one related to the adsorption of SDS into the narrowest pores with a suitable width, and the second one related to adsorption of SDS onto a more open surface.

Influence of porosity on the adsorption enthalpies of a non-ionic surfactant onto carbonaceous materials

The adsorption of a non-ionic surfactant (Triton X-100 (TX-100)) from aqueous solutions onto a set of carbon blacks and activated carbons has been compared by a calorimetric study. Different behaviours in the progress of the adsorption enthalpy as a function of surface coverage are obtained. However, irrespective of the nature of the carbon support, the process can be related to the same types of interactions since the evolution of adsorption enthalpy in these systems depends mainly on the porosity of the samples but not on the chemistry surface composition. The results at low coverage, indicate two kinds, at least, of adsorption behaviour: the first related to the adsorption of TX-100 in the narrowest pores with a suitable width, and the second on more open surfaces. In the narrowest micropores, the adsorption enthalpy can be twice that for a non-microporous surface as can be expected for slit-shaped micropores accommodating only one molecule. At higher coverage, the retention is due to interactions between surfactant molecules at the adsorbent-solution interface leading to the formation of interfacial aggregates.