Black carbon evaluation in natural organic matter samples using recoupled long-range dipolar dephasing solid-state 13C NMR

There is considerable interest in quantitative determination of the contributions of black carbon to soil and sediment organic matter because of its potential influence on the transport and bioavailability of organic contaminants. In this study, recoupled long-range dipolar dephasing solid-state 13C NMR was used to evaluate the black carbon content differences in a Guanella Pass peat (GPP) and the IHSS leonardite organic matter samples. Activated carbon was used for comparison. This technique is based on long-range dipolar dephasing which is obtained by recoupling of long-range C–H dipolar interactions using two 1H 180° pulses per rotation period. Compared to standard dipolar dephasing, C–H recoupling greatly increases the dephasing efficiency. Direct 13C polarization was applied to prevent loss of the signals of interest because carbon atoms that are distant from protons cross-polarize poorly. Compared to the peat, slow dephasing was observed for the aromatic carbon in leonardite; and even slower dephasing was observed in activated carbon. At a dipolar dephasing time of 1 ms, almost all the aromatic-C signals of the peat have been dephased, while more than 1/4 of the aromatic carbon signal remains in the IHSS leonardite. Even after a dipolar dephasing time of 2 ms, the aromatic signal of activated carbon is still significant (20%). Ding and Rice [Ding, G., Rice, J.A., 2011. Effects of lipids on hydrophobic organic compound sorption/desorption hysteresis in natural organic matter. Chemosphere 84, 519–526.] used naphthalene and phenanthrene to examine the correlations between sorption/desorption hysteresis and carbon characteristics in the IHSS leonardite and the GPP. It demonstrated that during the sorption and desorption of polynuclear aromatic hydrocarbons to these natural organic matter samples, the hysteresis is positively correlated with the black carbon content in the peat and leonardite samples.