Influence Of Solid Concentration On Adsorption At Solid-liquid Interfaces

The adsorption phenomenon at solid-liquid interface is one of the most universal behaviors occurring in nature and one of methods commonly used in wastewater treatment. The sorbent concentration (Cs) is an important factor in the adsorption behavior. It has been observed that the adsorption isotherms decline as Cs increases in numerous laboratories. This phenomenon is known as "sorbent concentration effect" or "solids effect"(Cs-effect). The classical thermodynamic models such as Langmuir and Freundlich isotherms cannot describe or predict the Cs-effect, because the adsorption equilibrium constants (or model parameters) vary with Cs. This seems to contradict the thermodynamic equilibrium theory. To describe and explain the Cs-effect, many sorbent concentration effect models have been proposed. But these models have not been widely accepted because the application scope is limited or the model parameters cannont be experimentally measured. Recently, our group proposed a new adsorption model, surface component activity (SCA) model. The SCA model suggests that the surface component (adsorption sites and adsorbed solute) activity coefficient is assumed to be the functions of Cs and the adsorption amount respectively rather than unity, as a result of the sobent particle-particle interactions. Three Cs-dependent adsorption isotherms (Langmuir-SCA, Freundlich-SCA and the SCA-partition coefficient equation) were derived based on the SCA model. At present, the universality of these SCA equations is to be tested yet and the effect of medium conditions on the activity coefficients of solid adsorption sites remains unclear. In view of this, this paper maked some researches on them, to improve our understanding of the adsorption phenomenon at solid-liquid interface and provide a fundamental basis of adsorption technology for further practical applications in wastewater treatment.(1) The adsorption experiments with kaolinite as the mineral adsorbent model and Zn(Ⅱ) and Cd(Ⅱ) as heavy metal pollutants (adsorbate) models on solid-liquid interface adsorption were conducted to study the effect of sorbent concentration on adsorption isotherm. The results showed that the adsorption isotherms declined significantly as the sorbent concentrations increased, i.e., there was obvious Cs-effect in adsorption thermodynamic process. The Zn(II) and Cd(II) adsorption isotherms could be adequately described using Langmuir and Freundlich models, respectively, for each given Cs. However, their parameters varied with Cs, i.e., the two classical models could not describe or predict the Cs-effect observed. The applicability of the Langmuir-SCA and Freundlich-SCA isotherms to fit the Cs-effect data was examined, and the results showed that they could accurately describe the adsorption behaviors of Zn(II) and Cd(II) on Kaolinite, respectively, which proved that the SCA model is applicable to the studied adsorption systems.(2) The adsorption experiments with chitosan beads as organic adsorbent model and methyl orange as organic dye pollutant (adsorbate) model on solid-liquid interface adsorption were conducted to study the effect of sorbent concentration on adsorption isotherm and the effect of medium conditions such as temperature, pH, and ionic strength on the Cs-effect. The adsorption isotherms of methyl orange on chitosan beads could be adequately described using Langmuir model for each given Cs. However, the classical models could not predict the Cs-effect observed, and its parameters varied with Cs. The applicability of the Langmuir-SCA isotherm to fit the Cs-effect data was examined, and the results showed that they could accurately describe the Cs-effect of methyl orange on chitosan beads, which proved that the SCA model is reasonable to the studied adsorption systems. Furthermore, adsorbent concentration could affect the distribution coefficient of methyl orange in solid-liquid phase obviously, and the SCA-partition coefficient could describe the results. The adsorption sites activity coefficient (fs/H2O) clearly decreased with increasing T (20-35℃) and pH (5-8), but no obvious change in fs/H2O was observed as CNaNO3varied in the range0.001-0.010mol·L-1.