Research Express@NCKU - Articles Digest

oil/solid organic-matter (SOM) content dictates the extent and mode of uptake of organic compounds by geologic materials. Normally, if natural solids contain more than 0.1-0.2 % of SOM and little charcoal-like substance or soot, adsorption of nonionic organic solutes from water by solid surfaces is insignificant compared to the concurrent solute partition into SOM; this is attributed to the strong competitive adsorption of water for usually polar mineral/inorganic surfaces. However, for solids containing little or no SOM, adsorption with mineral/inorganic surfaces becomes the major/dominant process, even if the overall uptake is weak. Although a significant advance has been achieved in modeling the uptake of organic ligands and ionizable solutes (e.g., carboxylic acids) by some hydrophilic minerals, the mechanism for relatively weak adsorption of nonionic organic solutes by these minerals remains to be elucidated. The latter subject has not been well attended in conventional adsorption studies, where such organic solutes or vapors normally condense under strong adsorptive forces onto solid surfaces (or the pore space), producing highly nonlinear isotherms with large exothermic heats. Knowledge on adsorption of nonionic solutes on hydrophilic minerals is needed for fate assessments of various environmental chemicals.

It is recognized that adsorption isotherms of low-polarity nonionic organic solutes (e.g., lindane, chlorobenzenes, and PAHs) from water on virtually organic-free hydrophilic minerals (e.g., silica and alumina) and low-organic-carbon clays (e.g., kaolinite and smectite) are essentially linear over a wide range of relative concentrations (C_e/S_w), where C_e is the equilibrium solute concentration and S_w is the solute water solubility. The adsorption data with organic-free silica and alumina suggest that in these solute-mineral systems water must be preferentially adsorbed, thus making the competitive solute adsorption weak and linear. The specific solute-mineral layout that yields linear solute adsorption, which resembles superficially the linear solute partition with SOM, has been a subject of interest to be further resolved, since the conventional adsorption isotherms are typically nonlinear and no significant solute partition with solid inorganic material is anticipated.

To characterize the linear adsorption phenomena in aqueous nonionic organic solute-mineral systems, the adsorption isotherms of some nonpolar organic solutes [1,2,3-trichlorobenzene (TCB), lindane (LIN), phenanthrene (PHN), and pyrene (PYN)] and polar nonionic solutes [1,3-dinitrobenzene (DNB) and 2,4-dinitrotoluene(DNT)] from single- and binary-solute solutions on hydrophilic silica and alumina were established. Toward this objective, the influences of temperature, ionic strength, and pH on adsorption were also determined. Table 1 lists the physicochemical properties of the test compounds. The representative adsorption isotherms of nonpolar PHN and polar DNT on silica and alumina at 25(±1) ˚C and 45(±1) ˚C are shown in Figures 1 and 2, in which the mass of adsorbed solute per unit weight of the solid (Q) is plotted against the equilibrium solute concentration in water (C_e). The isotherms are all practically linear and extend to high relative concentrations (C_e/S_w). Measured linear adsorption coefficients (K_d) of all solutes on silica and alumina at different temperatures and ionic strengths are listed in Table 2.

Table 1.Physicochemical properties of the organic compounds used in adsorption experiments^a

^a MW = molecular weight;ρ = density at room temperature (g/mL); mp = melting point (˚C); S_w = water solubility at 25˚C (µg/mL);∆

= molar heat of solution in water at 25˚C (kJ/mol); K_ow = octanol-water partition coefficient.^b NA = not available.

Figure 1. Adsorption isotherms of PHN from 0.01M CaCl₂ on silica (pH = ~ 5.25) and alumina (pH = ~ 6.28): ◆ 25˚C on silica; ▲ 25˚C on alumina; ◊ 45˚C on silica; ∆ 45˚C on alumina; * 25˚C on silica with PYR; ■ 25˚C on alumina with PYR.

Figure 2. Adsorption isotherms of DNT on alumina (pH = ~ 6.28): ▲ 25˚C in 0.01M CaCl₂; * 25˚C in 0.01M CaCl₂ with DNB; ◊ 25˚C in 0.01M CaCl₂ with PHN; ∆ 25˚C in 0.05M CaCl₂

Table 2.Adsorption coefficients of selected organic solutes on silica and alumina

Experimental data manifest that the linear adsorption of organic solutes (contaminants) exhibits low exothermic heats (i.e., the K_d values show small temperature coefficients) and practically no adsorptive competition. The solute-solid configuration and the adsorptive force consistent with these effects were hypothesized. For nonpolar solutes, the adsorption occurs presumably by London (dispersion) forces onto a water film above the mineral surface. For polar solutes, the adsorption is also assisted by polar-group interactions. The reduced adsorptive forces of solutes with hydrophilic minerals due to physical separation by the water film and the low fractions of the water-film surface covered by solutes offer a theoretical basis for linear solute adsorption, low exothermic heats, and no adsorptive competition.

The postulated adsorptive forces are supported by observations that ionic strength or pH poses no effect on the adsorption of nonpolar organic compounds on hydrophilic minerals while it exhibits a significant effect on the uptake of polar organic compounds. The model effectively consolidates the outstanding characteristics of nonionic compounds with natural hydrophilic minerals. The noncompetitive adsorption as illustrated has an important consequence to the transport and fate of individual contaminants in multi-contaminant natural aquatic environments.