Adsorption environmental applications

Adsorption, ion exchange, and catalysis share a great portion of environmental applications, as shown in the next section, and more extensively, in Chapter 2. Specifically, adsorption and catalysis are extensively used for the removal or destruction of air pollutants in gas streams as well as for purifying wastewaters or fresh water. Ion exchange has a special position among other techniques in the removal of heavy metals from wastewater. 

There ate many environmental applications of adsorption in practice and many others are being developed (Noble and Terry, 2004). Activated carbons and clays are frequently used for the removal of organic contaminants, such as phenol and aniline, both of which are prevalent in industry wastewaters and are known to have a significant negative impact on marine life and human health (IRIS, 1998 Dabrowski et al., 2005). Moreover, the adsorption on inexpensive and efficient solid supports has been considered a simple and economical viable method for the removal of dyes from water and wastewater (Forgacsa et al., 2004). Activated carbon, clays, coal, vermiculite, and other adsorbents have been used for this purpose. Specifically, adsorption can be employed in (Noble and Terry, 2004 Dabrowski, 2001) ... 

In general, large industrial fixed beds operate under near-adiabatic conditions, whereas small laboratory-scale fixed beds may approach isothermal operation (Ruthven, 1984). Especially, for most environmental applications, for catalytic, adsorption, and ion-exchange operations, the species to be removed are in such low concentrations that the operarion is nearly isothermal. Thus, the heat transfer to the external fixed-bed wall is often of minimal importance. 

Considering most environmental applications, for catalytic as well as for adsorption operations, the gas species to be removed are in such low concentrations (large excess of inerts) that the expansion factor is practically zero and the temperature is nearly constant throughout the reactor volume. 

In this section, the basic theory required for the analysis and interpretation of adsorption and ion-exchange kinetics in batch systems is presented. For this analysis, we consider the transient adsorption of a single solute from a dilute solution in a constant volume, well-mixed batch system, or equivalently, adsorption of a pure gas. Moreover, uniform spherical particles and isothermal conditions are assumed. Finally, diffusion coefficients are considered to be constant. Heat transfer has not been taken into account in the following analysis, since adsorption and ion exchange are not chemical reactions and occur principally with little evolution or uptake of heat. Furthermore, in environmental applications,... 

Fortunately, this is the case in many environmental applications where the gas species to be removed are in such low concentrations (large excess of inerts) that the expansion factor is practically zero. As pointed out in the introduction of this section, the basic principles of the analysis are also applicable in the case of adsorption of solutes from the gaseous phase. Again, for environmental applications, the concentration of solutes is so low that the pressure drop is only due to the flow of the gas. Here, the expansion factor has the same meaning, i.e. it measures the change of the volume of the gas phase, which is negligible in the case of low concentrations of the removed gas species. 

Why adsorption, ion exchange and heterogeneous catalysis in one book The basic similarity between these phenomena is that they all are heterogeneous fluid-solid operations. Second, they are all driven by diffusion in the solid phase. Thus, mass transfer and solid-phase diffusion, rate-limiting steps, and other related phenomena are common. Third, the many aspects of the operations design of some reactors are essentially the same or at least similar, for example, the hydraulic analysis and scale-up. Furthermore, they all have important environmental applications, and more specifically they are all applied in gas and/or water treatment. 

Environmental applications of functional mesostructures metal ion adsorption... 

In 2002, an interesting concept was proposed for coupling a C02-based supercritical extraction with air oxidation in order to remove and decompose pollutants from gases or liquids (134). An exemplary process scheme according to this preliminary concept is shown in Figure 5. Possible (future) environmental applications of such an integrated supercritical extraction-reaction system include treatment of liquid effluents, regeneration of catalysts and adsorption materials, and soil decontamination. 

This situation corresponds to the transfer of a dilute solute between phases. Many environmental applications of separations involving extraction, distillation, and adsorption fall into this category so it is not a hypothetical example. 

It is an understatement to say that adsorption is a diverse field. It impacts separation processes, materials science, catalysis, soil science, pharmaceutical products, environmental applications, and other widely different fields. A brief overview of those subjects, mainly oriented toward applications, is presented here. 

Cosolvents. The effect of organic solvents on the physical properties of water is a historically useful phenomenon and well defined for countless industrii and scientific applications. This phenomenon has always been ignored in the chemical models available for generi usage. Cosolvents affect the dielectric constant of the water, impacting activity coefficient calculations, hydration of species, and adsorption computations. Application of current chemical codes to environmental contamination investigations will require a capability for simulating the effect of a wide variety of miscible solvents on the aqueous system. As the currently available codes are refined, subroutines should be added that address this issue. 

Membrane-based separation processes are today finding widespread, and ever increasing use in the petrochemical, food and pharmaceutical industries, in biotechnology, and in a variety of environmental applications, including the treatment of contaminated air and water streams. The most direct advantages of membrane separation processes, over their more conventional counterparts (adsorption, absorption, distillation, etc.), are reported to be energy savings, and a reduction in the initial capital investment required. 

Analyzing the effect of surface chemistry one should not forget about the effects of porosity. In fact the pores are significant assets of activated carbons used in environmental applications. As mentioned above, pores smaller than 5 mn should be especially active in the adsorption process due to the possibility to accommodate water together with MM molecules and thus due to formation of microreactors for DMDS synthesis. The dependence of the amount of DMDS formed on the carbon on their volume of pores less than 5 nm is plotted in Fig. 33. A good linear agreement with slope equal to 1.02 was found for samples for which the saturation conditions were reached... 

This book intends to provide a comprehensive summary of environmental applications of activated carbons. I hope it will serve as a handbook or reference book. To understand the removal of contaminants and pollutants on activated carbons, the theoretical basis of adsorption... 

---