Thermodynamics of Nitroaromatic Compound Adsorption from Water by Smectite Clay

Abstract

Nitroaromatic compounds enter the environment through their use as explosives, pesticides, solvents, and synthetic intermediates in the manufacturing of dyes, perfumes, and drugs. Recent studies have found that many nitroaromatic compounds are strongly retained by smectites, especially K+-saturated smectites. Sorption occurs when nitroaromatic compounds replace water associated with the clay and form complexes between K+ and -NO2 groups. This study seeks to further understand nitroaromatic-clay interactions from the viewpoint of energetics. Adsorption isotherms of 1,3-dinitrobenzene, 1,4-dinitrobenzene, and 1,3,5-trinitrobenzene from aqueous solution by K+- and Ca2+-saturated smectite (SWy-2) were measured at several temperatures between 4 degrees C and 37 degrees C to determine the molar differential adsorption enthalpies. Adsorption was found to be an exothermic process on both homoionic K+- and Ca2+-smectite. The smaller adsorption enthalpy on Ca-SWy-2 was consistent with its much smaller adsorption capacity for nitroaromatics compared to K-SWy-2. Our best estimate forthe enthalpy of 1,3,5-trinitrobenzene interactions with K-SWy-2 is -124 kJ/mol, which is referenced to gas-phase 1,3,5-trinitrobenzene, corrected forthe displacement of interlayer water, and can be directly compared with quantum chemical enthalpies from the literature. Our comparable estimates for 1,3- and 1,4-dinitrobenzene interaction enthalpies are near -90 kJ/mol. We conclude that our adsorption enthalpy results are consistent with the hypothesis that nitroaromatic compounds are sorbed strongly by K-smectites because they form inner- and/or outer-sphere complexes with K+ cations in clay interlayers. Indeed, the basal spacings of rewetted clay films in the presence of nitroaromatic compounds imply that water molecules cannot effectively compete with the adsorbed nitrobenzenes for reactive sites on K-SWy-2.

Linear Adsorption of Nonionic Organic Compounds from Water onto Hydrophilic Minerals: Silica and Alumina

“…Similarly, on the SA basis, the present PYR Kd of 12.0 with the alumina (SA ) 155 m 2 /g) is of the same order of magnitude as that (0.231) with R-alumina (SA ) 11.7 m 2 /g) observed by Mader et al (10). To the adsorption heat effect, the solute molar heats of adsorption are first obtained by application of the van't Hoff equation using the linear adsorption coefficients (i.e., Kd values) at two temperatures, i.e., where ∆H h ad is adsorption heat per mole of the solute (kJ/ mol), R is the gas constant (8.31 J/K-mol), and Kd,1 and Kd,2 (24), and 26.8 kJ/mol for DNB (25) at ∼25°C. The magnitudes and signs of ∆H h ad relative to -∆H h w manifest that adsorbed solutes do not form a condensed phase on mineral surfaces; if a condensed phase were formed, the ∆H h ad would be more exothermic than -∆H h w because the solute condensation heat (-∆H h w) is a component of the adsorption heat (26).…”

“…(23). The ∆H h w values of PHN and PYR are from May et al (24) and that of DNB is from Li et al (25). b NA ) not available.…”

Organic xenobiotics removal in constructed wetlands, with emphasis on the importance of the support matrix

“…The amphiprotic character of silanol and aluminol groups in clays surfaces is responsible for a pH-dependent surface charge in clays (however, water molecules associated with exchangeable cations and clay surfaces may obscure these charged adsorption sites; such effect is dependent on the hydration strength of the exchangeable ions [170][171][172][173][174][175]). Electrostatic interactions with the surface and mechanisms such as cation exchange, cation bridging with the surface, surface complexation, and hydrogen bonding seem to be involved in the capture of ionic and polar species from aqueous media [121][122][123][124][125]176].…”

Sorption of aromatic hydrocarbons onto montmorillonite as affected by norfloxacin

“…It has been recognized that clay minerals, especially those exchanged with weakly hydrated cations (K + , Cs + ), can strongly retain NACs [10][11][12]. Two mechanisms have been proposed: (1) n-electron-donor-acceptor (EDA) interaction of NACs (-electron acceptors) with lone electron pairs of siloxane oxygens (n-donors) [10,11] and (2) cation-polar interaction of nitro groups with exchangeable cations [12][13][14]. There is scientific debate on the concurrence and relative importance of these two mechanisms for sorption of NACs to clay minerals for a long time.…”