Sorbent Concentration Effect On Adsorption Phenomenon At Solid-liquid Interface

The adsorption phenomenon at solid-liquid interface is one of the most universal and important behaviors occurring in nature and one of the important research contents in environmental chemistry. It is because the adsorption-desorption behaviors of the pollutants at solid-liquid interface (such as soil-liquid interface) have a major influence on distribution, migration and transformation of the pollutants. In addition, the technology used in the environmental engineering project is mostly related to the adsorption process. Therefore, scientific knowledge of the pollutants adsorption-desorption behaviors is an important foundation for the research of environmental pollution formation mechanism and the exploration of the effective pollution preventive approaches.The adsorption thermodynamics and kinetics are the basic contents of solid-liquid interface adsorption studies. Earlier, it has been observed in the adsorption-desorption equilibrium studies that adsorption isotherms decline with the sorbent concentration (Cs) increases, i.e., the adsorption equilibrium constants vary with Cs. It was described as "solids effect" or "sorbent concentration effect"(Cs-effect). This seems to contradict the thermodynamic equilibrium theory. Many solids effect models have been proposed, such as the solute complexation model, the particle interaction model, the metastable-equilibrium adsorption theory, the flocculation model, the power function (Freundlich-like) model, and the four components adsorption model. But these models have not been widely accepted because the application scope is limited or the model parameters cannot be experimentally measured. The essential reason for Cs-effect still lacks reasonable or accepted explanation. The Cs-effect also occurs in adsorption kinetics at solid-liquid interfaces, where adsorption rate constants vary with Cs. The existing theoretical models cannot accurately describe or predict Cs-effect. This brought a series of problems in the development of solid-liquid interface adsorption theory and the technology application. First, the lack of scientific knowledge about the physical process nature of the adsorption phenomena will affect the model construction and hinder the theory development; second, the uncertainty of model parameters creates difficulties in environmental treatment project and the engineering design. In view of this, the Cs-effect phenomenon on the soil-liquid interface adsorption was studied. The related mechanism was discussed. The Cs-effect model (including thermodynamic and kinetic model) was constructed. These would enhance our scientific knowledge of the solid-liquid interface adsorption phenomena and provide the theory basis for the design of environmental treament engineering (such as wastewater treatment and soil remediation).The main research contents and conclusions are listed as follows:(1) The adsorption thermodynamics (isotherms) and kinetics on solid-liquid interface adsorption were studied. The model sorbents include kaolinite, the layered double hydroxides (LDHs) and EDTA intercalated nanocomposite (EDTA-LDH). The model adsorbates include Pb(II), Cu(II), Zn(II) and Hg(II). The results showed that there were obvious Cs-effect in adsorption thermodynamic and kinetic process.(2) The adsorption data were fitted with the classical isotherm equations (Langmuir and Freundlich equation) and the kinetic equations (pseudo-first-order and pseudo-second-order kinetic equation). It was found that the classical equations could adequately describe the adsorption phenomenon for a given Cs value but could not describe or predict Cs-effect. In other words, the adsorption equilibrium constants and the rate constants vary with Cs. The constants obtained at given Cs cannot predict the adsorption behavior at other Cs.(3) Combining with the analysis of the literature and the experimental results, the author believed that the Cs-effect was an objective phenomenon due to the sorbent particle-particle interactions (including collision, adhesion, electrostatic force and Van de Waals force etc.). In order to explain and describe the Cs-effect, a surface component activity (SCA) model was proposed. The SCA model suggests that the surface component (adsorption site or adsorbed solute) activity coefficients are not equal to unity but are the functions of Cs and the adsorption amount (Γ) respectively. This is because the interaction existed in the same component (the sorbent particles and the adsorbate molecules) makes a real adsorption system deviating from an ideal one. When the thermodynamic equilibrium law applied to the real adsorption system, it should use activity instead of concentration to correct as same as the solution system. As the classical adsorption models do not consider the same component interaction, they cannot describe or predict the Cs-effect in the real adsorption system.(4) Based on the SCA model, the Cs-dependent Langmuir equation, the Cs-dependent Freundlich equation and the Cs-dependent partition coefficient equation, denoted as Langmuir-SCA, Freundlich-SCA and the SCA-partition coefficient equation, were derived. The related equilibrium constants are independent of Cs. Therefore, the equilibrium constants obtained at given Cs can predict the adsorption equilibrium behavior at other Cs.(5) Based on the SCA model, the Cs-dependent first-order kinetic equation and the Cs-dependent second-order kinetic equation, denoted as SCA-first-order and SCA-second-order kinetic equation, were derived. The related rate constants are independent of Cs. Therefore, the rate constants obtained at given Cs can predict the adsorption kinetic behavior at other Cs.(6) The best supposed and optimized function of the adsorption sites activity coefficient (fH2Os) and Cs is exponential form, i. e., fH2Os=exp(-γCsα). The theoretical relationship between fH2Os o and adsorption parameters was derived. The value of fH2Os can be obtained by adsorption data. This lays the foundation for the practical application of the SCA model equation.(7) The SCA model equations were examined using the adsorption experimental data obtained in this paper and literature and compared with the existing Cs-effect model equations. The results showed that the SCA model equations can better describe the experimental results of Cs-effect.