Sorption of aminonaphthalene and quinoline on amorphous silica

Abstract

The adsorption of quinoline and aminonaphthalene was investigated from aqueous solution on amorphous silica @io2). Amorphous Si02 was not a strong adsorbent of these compounds and quinoline was adsorbed more strongly than aminonaphthalene. The adsorption of both compounds varied with pH. A maximum in adsorption occurred near their respective pK,'s. Temperature effects were significant, suggesting enthalpy contributions in the range of H bonding. Both compounds appeared to adsorb via identical mechanisms that included H bonding and ion exchange. The greater basicity of quinoline combined with (1) the enhanced electron-donating properties of the heteroatom N and (2) the delocalized .Ir-bonded ring system were proposed to account for its stronger adsorption. Aminonaphthalene showed greater exclusion from the internal region of porous Si02, indicating that the compounds orient differently at the surface. The triple-layer adsorption model and three outer-sphere surface complexes were used to model the adsorption data. Good predictions of adsorption isotherms and the effects of ionic strength and electrolyte cation were obtained. Experimental ProceduresSorbates. 1-Aminonaphthalene and quinoline (99% purity, Aldrich Chemical Co.) were used without further purification for sorbate stock solutions. These stock solutions were spiked with 14C-labeled aminonaphthalene or quinoline (Sigma Chemical Co.) that had been purified by high-pressure liquid chromatography (HPLC). The radiochemical purity of the 14C-labeled compounds was >99% and their specific activity was approximately 10 mCi mmol-'. Selected physicochemical properties of the sorbate molecules are summarized in Table I.Porous and Amorphous SiOp Microporous, amorphous, Merck silica gel [Si02(ap), Aldrich] was prepared by first washing in acid (0.1 M HC1) and then repeatedly (-10 times) in distilled/deionized water until chloride free. The SiO,(ap) was then dried at 100 "C. The microporous silica gel was reported by the supplier to have a surface area of 675 m2 g-', a particle size of 212-500 pm, a pore size of 4.0 nm, and a pore volume of 0.68 cm3 g-'.Nonmicroporous, amorphous Si02 [Si02(ac)] was prepared by adding dry Aerosil200 (Degussa, Inc.) to 0.01 M NaCl to yield a suspension with a 1:lO solids to solution ratio. The Si02(ac) had a surface area of 183 m2 g-', a particle size of 0.012 pm, and no internal pore space.General Experimental Procedures. Sorption experiments were conducted in an incubator-shaker controlled to within 0.5 "C of the desired temperature, which was usually 25 "C. Preweighed 25-mL Corex centrifuge tubes, sealed with Teflon-lined silicon rubber septa, were used for all equilibrations. Sorption experiments were performed with a single sorbate concentration over a range in pH (sorption edges) and with variable sorbate concentration at a single pH (isotherms). Unless otherwise indicated, the electrolyte was 0.01 M NaCl and the titrants for controlling pH were 0.1 M NaOH and 0.1 M HC1. The sorbent concentration was 50 g of Si02 L-' (1:20 solids to...

The influence of temperature on the adsorption of mellitic acid onto goethite

“…For example, Zachara et al [17] found that the adsorption of aminonaphthalene and quinoline onto amorphous silica over the pH range from 3 to 8 decreased as temperature increased. This result is in accord with our study, since the silica surface will be negatively charged over this pH range.…”

“…This small enthalpy change is consistent with the notion that mellitate-substrate bonds are the result of electrostatic and hydrogen bonding interactions (outer-sphere complexation). Zachara et al [17] found that the adsorption of both quinoline and aminonaphthalene to amorphous silica were usually exothermic, but depended on the temperature interval and surface coverage. They also suggested that adsorption was non-specific because the enthalpies spanned the range usually associated with van der Waals and hydrogen bonding.…”

“…Generally current studies have indicated that an increase in temperature is accompanied by a decrease in adsorption [12,[17][18][19][20][21]. In some of these investigations [17,18,20,21] thermodynamic parameters were calculated, and adsorption was shown to be exothermic. Most of these studies, however, dealt with nonpolar adsorbates such as herbicides and pesticides, making it difficult to anticipate the effect of temperature in systems that contain polar molecules such as the benzene carboxylic acids.…”

Influence of mesoporosity on the sorption of 2,4-dichlorophenoxyacetic acid onto alumina and silica

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