Characteristics Of Zn²⁺ And Cd²⁺ Adsorption In Vermiculite-aqueous Solutions And Adsorption Theories

As a new technology developed in recent years with characters of low energy consumption and low operation and maintenance cost, constructed wetlands (CW) used for treatment of wastewaters containing heavy metal pollutants have received great attention in fields of environmental science and ecology. Compared with that of conventional treatment processes, however, the application of CW techniques has been limited to certain areas mainly due to their relatively low treatment efficiency, large land use area, low capacity to resist hydraulic, organic and heavy metal pollutant loads and particularly instability to seasonal changes. There are thus so far no standardized guidelines and handbooks for design of CW processes that can be used for treatment of different types of wastewaters. As an effective solution to above mentioned problems, introduction of an adsorption buffer unit using materials with high adsorption capacity into a CW system can improve its treatment efficiency as well as enhance its sustainability. For selection of proper adsorbents and design of the buffer unit for removal of metal ions it needs to know the quantitative relationship between the amount of wastewater to be treated, the metal ion concentration in the wastewater, the adsorption capacity of the adsorbent and the adsorbent quantity to be used for reducing the metal ion concentration to a stipulated discharge standard. This involves mechanisms of ion adsorption in liquid/solid systems.All traditional adsorption isotherms, when being applied to describe the ion adsorption in liquid/solid systems, define the equilibrium ion adsorption density qe as a single function of the ion concentration in bulk solution Ce while all classical kinetic adsorption models deal with only the relationship between adsorption density q and contacting time t. The fundamental ground of classical models is that it implies a "dynamic equilibrium" between liquid and solid phases and therefore the equilibrium adsorption density qe only depends on the equilibrium concentration in bulk solution Ce, irrespective of the adsorption process history. The theory of thermodynamics applied to support the qe-Ce relation is that at the equilibrium state the chemical potential of the ions in the solid phase should be equal to that in the liquid phase. The main problem associated with classical adsorption isotherms and kinetic models is the instability of their constant parameters. The adsorbent concentration effect (a phenomenon of decline of traditionally defined adsorption isotherms with increasing adsorbent concentration) has been found to be in most cases responsible for the parameter inconstancy problem. Limited by their defined functions, classical models cannot be directly used for prediction of ion adsorption for a given adsorption system with known initial ion concentration Co and adsorbent concentration Wo. Improvement of existing liquid/solid adsorption theories and establishment of quantitative relationships with adsorption as functions of Co and W0 are therefore of both theoretical and practical significances not only for scientific research in the field of environmental interface chemistry but also for use of CW technology in wastewater treatment engineering practices.Designed experiments were thus carried out to investigate the adsorption characteristic of Zn²⁺and Cd²⁺in vermiculite-aqueous solution systems in the range of initial ion concentration 25-500 mg/L and adsorbent concentration 10-150 g/L under different conditions. The main objective was to test the adsorbent effect, analyze the basic relationship among adsorption components as well as their chemical potentials, and establish proper prediction models for practical use. Main results obtained from this study are summarized as follows:1. Accounted for by its nontoxic nature, low cost and high cation adsorption capacity, the natural vermiculite was proved to be a fine wetland filler for removal of Zn²⁺and Cd²⁺from wastewaters. Ion exchange was found to be the main mechanism for Zn²⁺and Cd²⁺adsorption in the tested system. Adsorption competition between K+ Zn²⁺and Cd²⁺was observed in mixed metal adsorption systems. The negative effect of decrease in solution pH on Zn²⁺and Cd²⁺adsorptions was found to be significant only in the pH range below 3.5, showing that Zn²⁺and Cd²⁺adsorption could be significantly depressed when high amounts of H+ ions were present in the sample solution. Temperature had a positive effect on Zn²⁺and Cd²⁺adsorption but its influence was statistically insignificant in the tested range between 15 and 45℃.2. There were obviously effects of adsorbent concentration (Wo) on the traditional adsorption isotherms (i. e., qe-Ce curves). In both Zn²⁺and Cd²⁺sample series the qe-Ce curves declined apparently with increasing Wo and the traditionally defined equilibrium constants also varied significantly at different W0 levels, showing clearly that, unlike that defined by classical models, qe is not a single function of Ce but a function of Ce and Wo. Due to presence of significant variation in adsorbent concentrations in tested samples, both Langmuir and Freundlich equations cannot be used to describe the combined data obtained from the present sample series.3. The equilibrium adsorption density qe is the difference between C0/W0 (the ratio of initial ion concentration Co to adsorbent concentration W0) and Ce/W0 (the ratio of equilibrium ion concentration in liquid phase to adsorbent concentration). Repeated tests indicate that these three ion/adsorbent ratios are closely related with unique values in the tested range. The observed phenomenon indicates that the intensity factor in liquid/solid ion adsorption systems is not Ce but Ce/W0 in the liquid phase and Q/Wo in the solid phase. The argument to support this intensity factor concept is that it is the relative level of ion quantity to adsorbent quantity that determines the direction and the rate of ion adsorption reactions.4. Based on an assumption that the equilibrium state of a liquid/solid ion adsorption system is determined by four mutually related essential components:ions in liquid phase A, ions in solid phase B, uncovered adsorption sites Wu and covered adsorption sites Wc, an ion adsorption reaction model for an ideal system containing a single ionic species was proposed as: A+Wu======B+Wc which defines a new equilibrium coefficient as In accordance with the reaction model, the condition for ion adsorption to reach its equilibrium defined by the chemical potentials of the components will be5. Based on the four adsorption components model, the following model is established for prediction of equilibrium adsorption, The proposed model fit well the experimental data obtained from the examined samples with satisfactory prediction accuracy.6. Based on the four adsorption components model, a new kinetic equation is further presented as Results from the kinetic experiment indicate that the above defined parameters remain nearly constant in the tested range, showing that given Co and Wo, the presented equation can be used to describe the kinetic ion adsorption process for Zn²⁺and Cd²⁺adsorptions in vermiculite-aqueous solutions.The proposed equilibrium and kinetic adsorption models are of high values both in theory and practice for design of wastewater treatment processes using adsorption techniques.