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摘要
Recent studies have shown that n-C7-precipitated asphaltenes adsorb onto nanoparticles to produce isotherms that are significantly influenced by the dispersed states of both the adsorbate and the adsorbent. In the present work, we investigate this behavior further by determining the adsorption of asphaltene and resin fractions isolated from four different sources onto kaolinite using the depletion method in toluene. Treated conventionally (amount adsorbed, Gamma, versus equilibrium bulk concentration, c(e)), adsorption isotherms for fixed initial concentrations (c(0)) of CS and C7 asphaltenes and variable kaolinite mass (m(s)) are found to be Type I as classified by IUPAC, whereas under the same experimental conditions CS C7 resins exhibit Type III behavior. By fixing m(s) and varying c(0), however, Type II isotherms are produced by the resins. All of the adsorption results for the same fraction type were found to be very similar, irrespective of the source. The Types I and III isotherms are described very well by the thermodynamic solid liquid equilibrium (SLE) model of Montoya et al. (Energy Fuels 2014, 28, 4963-4975) based on the association theory of Talu and Meunier (AIChE J. 1996, 42, 809-819). Individual isotherms (F versus c(e)) are well-fitted by a shifted Langmuir equation for asphaltenes and by a general Freundlich (power law) relationship for resins. The SLE results verify that in toluene solution the adsorption behavior is complicated by concentration-dependent nanoaggregation of asphaltene species, whereas resin-resin interactions are weaker, but accompanied by adsorbent particle aggregation. On the other hand, when the adsorption data for each fraction type is replotted in terms of the ratio of the experimental parameter c(0)/m(s), as originally done by Wang et al. (Colloids Surfaces A: Physicochem. Eng. Aspects 2016, 504, 280-286), each set of data merges to a single isotherm which is reasonably well-approximated by a Langmuir-type relationship (we term this a "pseudo-Langmuir equation"), which allows the maximum adsorption to be determined for the different adsorbate/adsorbent systems. The average maximum adsorbed amounts calculated in this way for each of the component types are very similar, being slightly larger for C7A compared with CSA, with the values for the C5-C7R fractions being generally lower and more variable, possibly reflecting some source dependence.
