Effects of added paramagnetic ions on the 13C CP MAS NMR spectrum of a de-ashed soil

A de-ashed soil was reacted with a series of paramagnetic (Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Pr3+, Eu3+) and non-paramagnetic (Na+, Ca2+, Zn2+) ions. Amendment with non-paramagnetic ions did not effect the NMR properties of the soil organic matter (SOM), whereas a number of NMR properties including signal intensity, broadness and relaxation rate constants (T1 rhoH, T-1 H, T1C) were effected by the paramagnetic cations. Amendment with lanthanide cations (Pr3+, Eu3+) resulted in the selective loss of mainly carbonyl and carbohydrate signal in the C-13 CP/MAS NMR spectrum, but did not effect any of the relaxation rate constants, T1 rhoH, T-1 H or T1C. Amendment with Co2+ and Ni2+ resulted in similar signal losses as well as decreases in T1H and T1C (but not T1 rhoH). Amendment with Mn2+, Fe3+ and Cu2+ resulted in larger signal losses and decreases in T1 rhoH, T1H and T1C. The selective loss of carbonyl and carbohydrate signal in the lanthanide amended samples suggested that the majority of metal binding sites were uronic acid-type structures, Also, since most of the carbonyl signal was not lost, the majority of carbonyl groups were not carboxylate groups associated with metal binding, and may be ester or amide functionalities. Samples amended with Mn2+, Fe3+ and Cu2+ were not characterised by a single value for T1 rhoH, indicating the presence of domains separated by at least 3-10 nm. Proton spin relaxation editing (PSRE) subspectra for the slowly relaxing domains (long T1 rhoH) contained sharp resonances attributable to polymethylene, cellulose and lignin structures, whereas PSRE subspectra for the rapidly relaxing domains (short T1 rhoH) contained much broader resonances consistent with more amorphous material. All samples showed multiple T1H behaviour. However, differences between PSRE subspectra were greatest for samples amended with paramagnetic transition metal cations. The slowly relaxing PSRE subspectra for these samples were dominated by a polymethylene resonance at 33 ppm, indicating the presence of highly hydrophobic domains, at least 10-30 nm across, which contained few metal binding sites. (C) 1999 Elsevier Science B.V. All rights reserved.