Liquid-phase adsorptions solutions

Liquid-phase adsorption methods are widely used for quaUty control and specification purposes. The adsorption of iodine from potassium iodide solution is the standard ASTM method D1510-83 (2). The surface area is expressed as the iodine number whose units are milligrams of iodine adsorbed per gram of carbon. It is quite fortuitous that the values of iodine numbers turn out to be about the same as the values for surface areas in square meters per gram by nitrogen adsorption for nonporous carbon blacks. 

Carbowax 4000, gas-liquid phase adsorption contributes only Moderately to the retention of all solutes, decreasing rapidly with increasing liquid phase loading. At low phase loadings adsorption at the gas-liquid interface nay contribute between 3 and 42% to the retention of the solutes investigated... 

For instance, in liquid-phase adsorption, it has been established that the adsorption capacity of an activated carbon depends on the adsorbent s pore structure, ash content, functional groups [184-186], the nature of the adsorbate, its p//a, functional groups present, polarity, molecular weight, and size [187], and, finally, the solution conditions, such as pH, ionic strength, and the adsorbate concentration [188],... 

A novel and simple method for determination of micropore network connectivity of activated carbon using liquid phase adsorption is presented in this paper. The method is applied to three different commercial carbons with eight different liquid phase adsorptives as probes. The effect of the pore network connectivity on the prediction of multicomponent adsorption equilibria was also studied. For this purpose, the Ideal Adsorbed Solution Theory (lAST) was used in conjuction with the modified DR single component isotherm. The results of comparison with experimental data show that incorporation of the connectivity, and consideration of percolation processes associated with the different molecular sizes of the adsorptives in the mixture, can improve the performance of the lAST in predicting multicomponent adsorption equilibria. 

The process patterns found in liquid systems are more diverse and frequently much more complex than those in vapor-phase applications. In part, this arises from the greater number of factors that can influence adsorption from solution. The various permutations in which these factors can be joined confer a flexibility that makes liquid-phase adsorption adaptable to many diverse situations.1 2> 3... 

Adsorption from the liquid phase is used to remove organic components from aqueous wastes, colored impurities from sugar solutions and vegetable oils, and water from organic liquids. Adsorption can also be used to recover reaction products that are not easily separated by distillation or crystallization. Some of the same types of solids are used for both vapor-phase and liquid-phase adsorption, though often adsorbents with larger pores are preferred for use with liquids. 

Liquid phase adsorption is used mainly to bind turbid substances, to purify and decolor solutions, and to separate organic pollutants. Grained active carbon is employed as an adsorbent, mixed in a pulverized form with the liquid or as particles in a percolation process. 

IB. Liquid-Phase Adsorption. Adsorption of both the solvent and the solute occurs when a binary liquid solution is exposed to a solid adsorbent. What one observes is the preferred adsorption of the solute. While some texts consider correction of the results for the amount of solvent adsorbed, the fraction of the total that is adsorbed is usually assumed to be negligibly small. 

The classical thermodynamic approach has been applied to liquid phase adsorption by Larionov and Myers and by Minka and Myers. It was shown that for sorption of carbon tetrachloride-isooctane and benzene-carbon tetrachloride on aerosil the adsorbed solutions show approximately ideal behavior whereas adsorbed mixtures of benzene, ethyl acetate, and cyclohexane on activated carbon showed appreciable deviations from ideality. However, it is shown that the activity coefficients and hence the adsorption equilibrium data for the ternary systems may be successfully predicted, by classical methods, from data for the constituent binaries. 

Applications of liquid-phase adsorption include removal of organic compounds from water or organic solutions, colored impurities from organics, and various fermentation products from fermentor effluents. Separations include paraffins from aromatics and fructose from glucose using zeolites. 

Abuzaid and Nakhla, and Vidic et al. found that the adsorption of phenol by activated carbons from aqueous solutions in the presence of molecular oxygen in the test environment resulted in a threefold increase in the adsorption capacity of the carbon. This has been attributed to the oxygen induced polymerization reactions on the surface of the carbon. Juang et al. studied liquid-phase adsorption of eight phenohc compounds on a PAN-based activated carbon Fiber in the concentration range of 40 to 500 mg/L and observed that the chlorinated phenols showed better adsorption than methyl substituted phenols. Moreno-CastiUa et al. studied the adsorption of several phenols from aqueous solutions on activated carbons prepared from original and deminerahzed bituminous coal and found that the adsorption capacity depended upon the surface area and the porosity of the carbon, the solubility of the phenolic compound, and the hydrophobicity of the substituent. The adsorption was attributed to the electron donor-acceptor complexes formed between the basic sites on the surface of the carbon and the aromatic ring of the phenol. 

If fhe adsorbent and adsorbate are contacted long enough, equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by adsorption isotherms. Liquid-phase adsorption, in general, is a more complex phenomenon than gas-phase adsorption. For example, although one may envision monolayer coverage in liquid adsorption, the... 

Methods 3 and 4 are specific to liquid phase adsorption and especially effective when recovery of adsorbate is desirable. Desorption by alkaline solution is often used for recovery of organic acids adsorbed on... 

Liquid phase adsorption methods depend on the establishment of an equilibrium between adsorbed and unadsorbed solute molecules. Adsorption of solute on to the surface of a solid continues until it reaches a saturation point giving a clear plateau in the isotherm. As the isotherm usuaUy tends towards a limiting value, the limit has often been taken to correspond to the covering of the surface with a monolayer of solute. The equation derived for monolayer coverage is ... 

The following are some of the typical industrial applications for liquid-phase carbon adsorption. Generally liquid-phase carbon adsorbents are used to decolorize or purify liquids, solutions, and liquefiable materials such as waxes. Specific industrial applications include the decolorization of sugar syrups the removal of sulfurous, phenolic, and hydrocarbon contaminants from wastewater the purification of various aqueous solutions of acids, alkalies, amines, glycols, salts, gelatin, vinegar, fruit juices, pectin, glycerol, and alcoholic spirits dechlorination the removal of... 

Two other methods worth discussing are wet air oxidation and regeneration by steam. Wet oxidation may be defined as a process in which a substance in aqueous solution or suspension is oxidized by oxygen transferred from a gas phase in intimate contact with the liquid phase. The substance may be organic or inorganic in nature. In this broad definition, both the well known oxidation of ferrous salts to ferric salts by exposure of a solution to air at room temperature and the adsorption of oxygen by alkaline pyrogallol in the classical Orsat gas analysis would be considered wet oxidations. 

The support materials for the stationary phase can be relatively inactive supports, e.g. glass beads, or adsorbents similar to those used in LSC. It is important, however, that the support surface should not interact with the solute, as this can result in a mixed mechanism (partition and adsorption) rather than true partition. This complicates the chromatographic process and may give non-reproducible separations. For this reason, high loadings of liquid phase are required to cover the active sites when using high surface area porous adsorbents. 

Baviere et al. [41] determined the adsorption of C18 AOS onto kaolinite by agitating tubes containing 2 g of kaolinite per 10 g of surfactant solution for 4 h in a thermostat. Solids were separated from the liquid phase by centrifugation and the supernatant liquid titrated for sulfonate. The amount of AOS adsorbed is the difference between initial solution concentration and supernatant solution concentration at equilibrium. 

On the other hand, potential measurements at the free surface of purified water have shown50 that the value for a flowing surface differs by about 0.3 V from that for a quiescent surface, as a result of adsorption of surface-active residual impurities in the solution (probably also coming from the gas phase). Since emersed electrodes drag off the surface layer of the solution as they come out of the liquid phase, the liquid layer attached to emersed solid surfaces might also be contaminated. 

---