Sorption selectivity in natural organic matter probed with fully deuterium-exchanged and carbonyl-13C-labeled benzophenone and 1H-13C NMR spectroscopy.

Specific functional-group or domain interactions of fully deuterium-exchanged, carbonyl-(13)C-labeled benzophenone and different types of natural organic matter (NOM) were investigated through two-dimensional (1)H-(13)C heteronuclear correlation NMR spectroscopy. The sorbents included Beulah-Zap lignite, type II kerogen (IL-6), Pahokee peat, Amherst humic acid, and a polystyrene-poly(vinylmethyl ether) (PS-PVME) blend. PS-PVME consists of PS and PVME chains that are mixed on a scale of <5 nm. The NOM sorbents all consist predominantly of a mixed aromatic-alkyl or aromatic-O-alkyl matrix that is homogeneous on the 3 nm scale, as evidenced by fast equilibration of aromatic and alkyl (1)H magnetization. In addition, Beulah lignite and IL-6 kerogen exhibit small fractions of distinct polymethylene (CH2)n domains, and Pahokee peat contains significant fractions of polar and nonpolar alkyl domains. Benzophenone-((13)C═O)-d10 shows proximity to both aromatic rings and alkyl segments in all samples but preferentially interacts with aromatic rings in PS-PVME and Beulah lignite, possibly due to π-π electron donor-acceptor interactions. The data for IL-6 kerogen are also compatible with preferential location of benzophenone near the alkyl-substituted edges of aromatic rings, while in Pahokee peat, clear signatures of benzophenone affinity to both aromatic-rich and nonpolar alkyl domains have been detected. Amherst humic acid shows evidence of some affinity to polar alkyl segments but which is weaker than that to aromatic rings. Our results indicate that specific interactions of the sorbate and the presence of domains in the sorbent influence the magnitude and selectivity of sorption.

Influence of molecular structure and adsorbent properties on sorption of organic compounds to a temperature series of wood chars.

Chars from wildfires and soil amendments (biochars) are strong adsorbents that can impact the fate of organic compounds in soil, yet the effects of solute and adsorbent properties on sorption are poorly understood. We studied sorption of benzene, naphthalene, and 1,4-dinitrobenzene from water to a series of wood chars made anaerobically at different heat treatment temperatures (HTT) from 300 to 700 °C, and to graphite as a nonporous, unfunctionalized reference adsorbent. Peak suppression in the NMR spectrum by sorption of the paramagnetic relaxation probe TEMPO indicated that only a small fraction of char C atoms lie near sorption sites. Sorption intensity for all solutes maximized with the 500 °C char, but failed to trend regularly with N2 or CO2 surface area, micropore volume, mesopore volume, H/C ratio, O/C ratio, aromatic fused ring size, or HTT. A model relating sorption intensity to a weighted sum of microporosity and mesoporosity was more successful. Sorption isotherm linearity declined progressively with carbonization of the char. Application of a thermodynamic model incorporating solvent-water and char-graphite partition coefficients permitted for the first time quantification of steric (size exclusion in pores) and π-π electron donor-acceptor (EDA) free energy contributions, relative to benzene. Steric hindrance for naphthalene increases exponentially from 9 to 16 kJ/mol (∼ 1.6-2.9 log units of sorption coefficient) with the fraction of porosity in small micropores. π-π EDA interactions of dinitrobenzene contribute -17 to -19 kJ/mol (3-3.4 log units of sorption coefficient) to sorption on graphite, but less on chars. π-π EDA interaction of naphthalene on graphite is small (-2 to 2 kJ/mol). The results show that sorption is a complex function of char properties and solute molecular structure, and not very predictable on the basis of readily determined char properties.

Sorption selectivity in natural organic matter studied with nitroxyl paramagnetic relaxation probes.

Sorption site selectivity and mechanism in natural organic matter (NOM) were addressed spectroscopically by the sorption of paramagnetic nitroxyl compounds (spin probes) of different polarity, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and HTEMPO (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl). The sorbents were Pahokee peat, Beulah-Zap lignite, and a polystyrene-poly(vinyl methyl ether) (PS-PVME) polymer blend representing the mixed aliphatic-aromatic, polar-nonpolar character of NOM. Nuclear-electron spin interaction serves as an efficient relaxation pathway, resulting in attenuation of the (13)C-CP/TOSS NMR signal for (13)C nuclei in proximity to the N-O· group (r(-6) dependence). In the natural solids the spin probes sorbed more specifically (greater isotherm nonlinearity) and had lower rotational mobility (broader electron paramagnetic resonance signals) than in PS-PVME. Titration with spin probe indicated almost no selectivity for the different carbon functional groups of PS-PVME, and little to no selectivity for the different carbon moieties of Pahokee and Beulah, including aromatic, alkyl, O-alkyl, di-O-alkyl, and O-methyl. In any case, sorption site selectivity of spin probes to NOM was always weaker than partition selectivity found in model solvent-water (toluene, hexadecane, anisole, octanol) and cellulose-water systems. The results indicate little or no preferential sorption in NOM based on functional group chemistry or putative microdomain character, but rather are consistent with the filling of pores whose walls have an average chemical environment reflecting the bulk chemical composition of the solid. This work demonstrates for the first time the use of paramagnetic probes to study sorption specificity.

A multi-method comparison of Atchafalaya Basin surface water organic matter samples.

Surface water organic matter (OM) was isolated from two distinct sites within the Atchafalaya Basin using a combination of XAD-8 and XAD-4 non-ionic macroporous resins and characterized by a suite of analytical methods, including elemental analysis, (13)C cross polarization magic angle spinning nuclear magnetic resonance, attenuated total reflectance Fourier transform infrared, luminescence spectroscopy including parallel factor analysis, and ultraviolet-visible spectroscopy. The major findings of the study are (i) despite the large differences in hydrology, optical properties, iron content, dissolved oxygen, and degree of human exploitation, the spectral and elemental signatures of the hydrophobic acids and transphilic acids fractions of the isolated OM for the different sites were remarkably similar; (ii) the luminescence characteristics of the four studied fractions provided information on the relative contributions from terrestrial and microbial input sources, as well as the degree of humification; and (iii) a detailed analysis of the total luminescence data led to a new dual excitation model based on quinone exciplexes for long wavelength emissions.

Studying organic matter molecular assemblage within a whole organic soil by nuclear magnetic resonance.

This work shows the applicability of two-dimensional (2D) (1)H-(13)C heteronuclear correlation (HETCOR) nuclear magnetic resonance (NMR) spectroscopy to the characterization of whole soils. A combination of different mixing times and cross polarization (CP) methods, namely Lee-Goldberg (LG)-CP and Ramp-CP are shown to afford, for the first time, intra- and inter- molecular connectivities, allowing for molecular assemblage information to be obtained on a whole soil. Our results show that, for the brackish marsh histosol under study, two isolated domains could be detected. The first domain consists of O-alkyl and aromatic moieties (lignocellulose material), while the second domain is comprised of alkyl type moieties (cuticular material). The role of these domains is discussed in terms of hydrophobic organic compound sorption within soil organic matter (SOM), including the possible effects of wetting and drying cycles.

Competitive sorption and desorption behavior for three fluoroquinolone antibiotics in a wastewater treatment wetland soil.

Significant amounts of pharmaceuticals are discharged into the environment through wastewater effluent. Sorption has been shown to be a significant aqueous removal pathway for many of these compounds. Competition between ciprofloxacin (CIP), ofloxacin (OFL) and norfloxacin (NOR) and their sorption to, and desorption from, a surrogate Louisiana wastewater treatment wetland soil were investigated to gain insight into the fate and transport of the pollutants within wastewater treatment wetlands. This study was undertaken in the context of a treatment wetland that continuously receives pharmaceuticals. Therefore it is important to understand the total capacity of this soil to sorb these compounds. Sorption to this treatment wetland soil was found to provide a major and potentially long-term removal pathway for these antibiotics from wastewater. LogK(F) values for all three compounds were between 4.09 and 3.90 for sorption and 4.24 and 4.05 microg(1-1/)(n)(cm(3))(1/)(n)g(-1) for desorption. The compounds were sorbed in amounts ranging from 60% to 90% for high and low loading, respectively. The majority of the compounds were sorbed to the soil within the first 20h, indicating that treatment wetland may not need long retention times (weeks to months) in order to remove these compounds. Sorption K(D) values for competition (20 ppm of each compound for 60 ppm of total fluoroquinolones) ranged from 2300 to 3800 cm(3)g(-1) which is between both the 20 (4300-5800 cm(3)g(-1)) and 60 (1300-3000 cm(3)g(-1)) ppm single compound K(D) values, indicating that there is competition between these three compound for sorption sites. Sorption and desorption data (single component and mixture) collectively provide the following evidence: (1) NOR and, to a lesser extent, CIP outcompete OFL for sorption sites, (2) OFL sorbes to its share of "quality" sorption sites, and (3) competition only occurs for lesser "quality" binding sites.