No abstract

Smectite-type clays play a critical role in the trapping of fluids within soil and atmospheric nanoparticles. The hydrodynamics of cationsaturated smectite nanopores are well documented. However, little is known about the influence of small organic compounds. Here we investigate the effects of carbohydrates (glucose and cellobiose), representing an important class of organic compounds, on the hydration and nanopore structures of montmorillonite, a prototypical smectite. To achieve the same amount of adsorbed water, higher relative humidity was required in the presence of the adsorbed carbohydrates than with the clay alone. The decrease in the characteristic micropore water adsorption with the clay−carbohydrate aggregates implied that water adsorption was constrained within the clay nanopore regions. The presence of carbohydrates promoted water retention as a function of gradual decrease in moisture content. Moisture-dependent X-ray diffraction patterns determined that, relative to the mineral alone, greater nanopore sizes were preserved in the presence of the carbohydrates, despite severe dehydration that is expected to induce clay nanopore collapse. Fourier-transform infrared spectroscopy captured disruption in the population of exchangeable waters within the carbohydrate-populated clay nanopores, with some agreement with the measured water-desorption profiling. These new findings demonstrate that carbohydrates can restructure smectite interlayer nanopores and water trapping dynamics.

Beta-lactam antibiotics, which are used extensively in human and veterinary applications, are commonly detected in surface waters. To examine how the distinct structures of different generations of beta-lactam antibiotics can influence their persistence or degradation in environmental aqueous media, we examined the fate of two penams (amoxicillin and cloxacillin) and two cephems (cephalexin and ceftriaxone) at pH 5.0 and pH 7.0. By contrast to the lack of hydrolysis of the penam antibiotics at both pHs, we observed hydrolysis of cephalexin at pH 7.0 (t 1/2 = 12 d) and ceftriaxone at pH 5.0 (t 1/2 = 2.8 d). Using highperformance liquid chromatography coupled with a diode array detector or a high-resolution mass spectrometer, we were able to confirm thiotriazinone and 3-desacetyl cefotaxime as major hydrolysis products of ceftriaxone, and propose the hydrolytic cleavage of the benzene and cephem moieties from cephalexin. In addition, we studied the effects of smectite clay particles suspended in solutions without or with dissolved organic matter. The adsorption capacity of the clay was 4-to 9-fold higher at pH 7.0 than at pH 5.0. Subsequent X-ray diffraction analysis revealed that the antibiotic adsorption was not within the clay interlayer nanopores but occurred primarily on the external clay surfaces. The addition of dissolved organic matter interfered with the adsorption of a cephem antibiotic (ceftriaxone) on the clay, but the adsorption of a penam antibiotic (amoxicillin) remained unaffected. We employed molecular modeling simulations to probe the mechanisms of adsorption on the mineral surface. Our findings offer new insights on how the compound structures can dictate different fates of the beta-lactam class of antibiotics in environmental media.