CPMAS 13 C NMR characterization of humic acids from composted agricultural Saudi waste

CPMAS 13C NMR spectroscopy was used to characterize humic acid (HA) extracted from three types of composted agricultural Saudi waste of different origins, including agricultural crop plants (P), date palm fronds (D), and animal waste (A). The intensity of each region was estimated by the CPMAS spectrum indicating that HA (P) has a lower carbohydrate content than the other two samples. There was a greater presence of aromatic carbon substituted by oxygen or nitrogen in HA (A) than in HA (D), while HA (P) contained the highest content of aliphatic compounds. All samples exhibited strong peaks for a carbonyl of the carboxyl group. However, there was no absorbance for the carbonyl carbon in any of them. The lowest carboxyl group content among the samples was in HA (D) while HA (P), and HA (A) had very similar carboxyl contents. For all samples, the total aliphaticity was higher than the total aromaticity, with HA (P) having the highest total aliphaticity, and HA (A) having the lowest total aliphaticity. The composition and functional groups of the investigated samples did not exactly match those of any previously reported HA models, however, some similarity was noted between the Dragunov's model and HAs extracted from plant PEMs.