Effects of added paramagnetic ions on the [formula omitted] CP/MAS NMR spectrum of a de-ashed soil

Abstract

A de-ashed soil was reacted with a series of paramagnetic (Mn 2+, Fe 3+, Co 2+, Ni 2+, Cu 2+, Pr 3+, Eu 3+) and non-paramagnetic (Na +, Ca 2+, Zn 2+) 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 ( T 1ρ H, T 1 H, T 1 C) were effected by the paramagnetic cations. Amendment with lanthanide cations (Pr 3+, Eu 3+) resulted in the selective loss of mainly carbonyl and carbohydrate signal in the 13 C CP/MAS NMR spectrum, but did not effect any of the relaxation rate constants, T 1ρ H, T 1 H or T 1 C. Amendment with Co 2+ and Ni 2+ resulted in similar signal losses as well as decreases in T 1 H and T 1 C (but not T 1ρ H). Amendment with Mn 2+, Fe 3+ and Cu 2+ resulted in larger signal losses and decreases in T 1ρ H, T 1 H and T 1 C. 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 Mn 2+, Fe 3+ and Cu 2+ were not characterised by a single value for T 1ρ H, indicating the presence of domains separated by at least 3–10 nm. Proton spin relaxation editing (PSRE) subspectra for the slowly relaxing domains (long T 1ρ H) contained sharp resonances attributable to polymethylene, cellulose and lignin structures, whereas PSRE subspectra for the rapidly relaxing domains (short T 1ρ H) contained much broader resonances consistent with more amorphous material. All samples showed multiple T 1 H 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.