Adsorption of Dinitrophenol Herbicides From Water by Montmorillonites

Abstract

The adsorption of two dinitrophenol herbicides, 4,6-dinitro-o-cresol (DNOC) and 4,6-dinitro-o-sec-butyl phenol (dinoseb), by two reference smectite clays (SWy-2 and SAz-1) was evaluated using a combination of sorption isotherms, Fourier transformation infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and molecular dynamic simulations. Clays were subject to saturation with various cations, and charge reduction. The DNOC adsorption decreased with increasing pH indicating that DNOC was primarily adsorbed as the neutral species. The FTIR spectra of DNOC-clay films showed that DNOC molecules are oriented parallel to the clay surface. Interlayer cations have a strong effect on adsorption depending largely on their hydration energies. Weakly hydrated cations, e.g. K+ and Cs+, resulted in greater sorption compared to more strongly hydrated cations such as Na+ or Ca2+. Lower hydration favors direct interactions of exchangeable cations with -NO2 groups of DNOC and manifests optimal interlayer spacings for adsorption. In the presence of sorbed DNOC, an interlayer spacing for K-SWy-2 of between 12 and 12.5 Å was maintained regardless of the presence of water. This d-spacing allowed DNOC molecules to interact simultaneously with the opposing clay layers thus minimizing contact of DNOC with water. The charge density of clays also affected sorption by controlling the size of adsorption domains. Accordingly, DNOC adsorption by low-charge clay (K-SWy-2) was much higher than by high-charge clay (K-SAz-1) and Li-charge reduction greatly enhanced dinoseb adsorption by K-SAz-1. Steric constraints were also evident from the observation that adsorption of DNOC, which contains a methyl substituent, was much greater than dinoseb, which contains a bulkier isobutyl group. Adsorption of DNOC by K-SAz-1 was not affected in the presence of dinoseb, whereas dinoseb adsorption was greatly reduced in the presence of DNOC.

Effect of copper on the adsorption of p-nitrophenol onto soils

“…Two mechanisms may be responsible for this phenomenon. First, the low charge of Jiangxi soil decreased the hydrated cations adsorbed thereby providing larger adsorption domains for p-nitrophenol, which was consistent with report of Sheng et al (2002). Second, Jiangxi soil has greater surface area as compared with Heilongjiang soil, which means that Jiangxi soil can provide more site-specific adsorption for p-nitrophenol.…”

“…In addition to the proposed EDA complex formation mechanism, smectite clays may also adsorb neutral organic molecules by hydrophobic interactions (Laird and Fleming, 1999). Adsorption of nitroaromatic compounds was also related to the types and hydration characteristics of exchangeable cations Sheng et al, 2002;Saltzman and Yariv, 1975). For weakly hydrated cations (e.g., K C and Cs C ), the combined size of the cations and associated water molecules does not occupy all of the available space.…”

Sorption of Denatonium to Kaolinite Clay from Water

“…Ca 2+ has an enthalpy of hydration of −1580 kJ/mol while that of K + is −314 kJ/mol. The impact of saturating cations and supporting electrolyte cations on contaminant sorption to clay minerals has been well documented in the literature and our results are generally consistent with previous studies in that sorption is enhanced in the presence of low hydration enthalpy ions and suppressed in the presence of higher hydration energy, divalent cations [31][32][33]. As observed in previous studies of contaminant sorption in the presence of the same cations (K + , Na + , Ca 2+ ), clay retention of contaminants with sodium ions present in the background electrolyte was intermediate [32].…”

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].…”