Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved. A New Way to Use Solid-State Carbon-13 Nuclear Magnetic Resonance Spectroscopy to Study the Sorption of Organic Compounds to Soil Organic Matter Ronald J. Smernik* ABSTRACT pothesized that the soil organic matter matrix consists of various components, or contains various domains with Several solid-state 13 C nuclear magnetic resonance (NMR) techniques different sorption properties. There is particular interest were used to characterize soil organic matter spiked with 13 C-labeled in the identity of the strongly sorbing components or do- organic compounds spanning a range of hydrophobicities (benzoic acid, benzophenone, naphthalene, phenanthrene, and palmitic acid). mains. Suggested candidates include black carbon (Chiou The chemical shifts of NMR resonances of the sorbed species were et al., 2000; Ran et al., 2002; Cornelissen and Gustafsson, shifted by up to 3 ppm relative to those of the neat compounds. 2004), kerogen (McGinley et al., 1993; Ran et al., 2003), Sorption also resulted in increased resonance linewidth for the com- long-chain hydrocarbon biopolymers (Mao et al., 2002; pounds containing a single 13 C label, indicating the presence of a range Chefetz, 2003; Khalaf et al., 2003), and nanometer-size of different chemical environments at the sites of sorption. On the voids (Xing and Pignatello, 1997; Gunasekara and Xing, other hand, sorption decreased the linewidth of the resonance of 2003). However, conclusive identification of strongly naphthalene, which was uniformly 13 C-labeled. This was attributed to sorbing organic matter phases has remained elusive, partly the removal of intermolecular 13 C– 13 C dipolar coupling. Heterogeneity due to the lack of techniques that can identify the loca- of the organic matter was demonstrated using the spectral editing tion of sorbed molecules in the heterogeneous organic technique proton spin relaxation editing (PSRE), which enabled the identification and quantification of charcoal-rich domains character- matter matrix. ized by rapid rates of proton spin–lattice relaxation in the static frame The solid-state 13 C NMR technique proton spin relax- (T 1 H), and humic domains characterized by slow rates of T 1 H relax- ation editing (PSRE) can detect, characterize, and quan- ation. Furthermore it was demonstrated that the sorbed 13 C-labeled tify chemically and physically distinct domains within molecules “inherit” the T 1 H “signature” of the organic matrix in their an intact organic matter matrix. The PSRE technique immediate vicinity. Thus PSRE on the spiked soils enabled evaluation has been used to identify charcoal (Smernik et al., 2000) of the relative affinity of the two domain types for the sorbate mole- and highly aliphatic (Preston and Newman, 1992, 1995; cules. The charcoal-rich domains were shown to have a twofold to Smernik and Oades, 1999) domains in soil organic mat- tenfold greater affinity for the organic compounds, with greater differ- ter, microbial- and plant-derived domains in sewage ences found for the more hydrophobic compounds. sludge (Smernik et al., 2003), and aromatic- and aliphatic- rich domains in kerogen (Petsch et al., 2001). This paper describes new ways in which solid-state 13 C T he bioavailability, toxicity, persistence, and trans- NMR spectroscopy can be used to investigate the sorp- port of organic molecules in soils are strongly af- tion of organic compounds to organic matter. By using fected by sorption to the solid phase. For hydrophobic, 13 C-labeled organic compounds, detailed information nonionic molecules, organic matter is the most impor- about the chemical of environment of sorbed molecules tant sorptive phase. Early studies used a partitioning can be determined directly, at environmentally relevant model to describe the sorption of organic molecules to concentrations. In particular, the application of PSRE soil organic matter (Chiou et al., 1979, 1983). However, to soil organic matter spiked with sorbed 13 C-labeled further investigations uncovered phenomena such as non- compounds offers a direct way to measure the affinity of linear sorption isotherms (Weber et al., 1992; McGinley different organic matter domains for the sorbed species. et al., 1993; Huang et al., 1997; Kohl and Rice, 1999; Chiou et al., 2000; Yuan and Xing, 2001), competitive sorption (McGinley et al., 1993), and desorption hysteresis (Be- MATERIALS AND METHODS noit et al., 1996; Huang and Weber, 1997; Lueking et Sample Preparation al., 2000; Yuan and Xing, 2001) that are inconsistent with a purely partitioning interaction. The soil used in this study was collected from the top 0 to 50 mm of a Xeric Epiaquert from Urrbrae, South Australia. Nonpartitioning sorption behavior is widely attrib- This soil has an organic carbon content of 32.7 g kg -1 , a large uted to the heterogeneity of organic matter (Weber et portion of which is believed to be natural charcoal (Smernik, al., 1992; Xing and Pignatello, 1997; Chiou et al., 2000; 2005). The soil was de-ashed using the procedure of Skjemstad Huang et al., 2003). A number of researchers have hy- et al. (1994) to isolate the organic matter. The organic carbon content of the HF-treated residue was 381.5 g kg -1 . The HF treatment consisted of nine successive treatments with 2% School of Earth and Environmental Sciences, The University of hydrofluoric acid solution. Each treatment involved shaking Adelaide, Waite Campus, Urrbrae, South Australia 5064, Australia. 3-g portions of soil in 50 mL of 2% hydrofluoric acid solution, Received 1 Oct. 2004. *Corresponding author (ronald.smernik@ adelaide.edu.au). end-over-end, for periods of 1 h (five times), 16 h (three times), Published in J. Environ. Qual. 34:1194–1204 (2005). Technical Reports: Organic Compounds in the Environment Abbreviations: CP, cross polarization; NMR, nuclear magnetic reso- nance; PSRE, proton spin relaxation editing; T 1 H, proton spin–lattice doi:10.2134/jeq2004.0371  ASA, CSSA, SSSA relaxation rate in the static frame; T 1 H, proton spin–lattice relaxation rate in the rotating frame. 677 S. Segoe Rd., Madison, WI 53711 USA 1194 Published online June 7, 2005