HUMIC ACIDS EXTRACTED FROM SOIL AND SEWAGE SLUDGE: GENERAL CHARACTERISTICS AND MECHANISMS OF COPPER(II) BINDING

STEPHEN ALLEN BOYD, Purdue University

Abstract

Humic acids (HA) extracted from sludge, soil and sludge-amended soil were characterized by elemental and functional group analyses and visible and infrared spectroscopy. Humic acid extracted from a Chalmers soil appeared to be similar to an average HA (Schnitzer, 1978). With respect to the average HA and Chalmers HA, the sludge HA appeared to be a more aliphatic, N-enriched polymer with decreased amounts of carboxyls and phenolic hydroxyls. Proteinaceous materials associated with the sludge HA appeared to be the major source of N enrichment. Sludge organic matter was apparently recovered in HA extracted from soil 2 and 14 months after sludge application. The association of sludge-borne proteinaceous and aliphatic materials with the sludge and sludge-amended soil HA fractions was indicated by increased amide and C-H stretch IR absorptions, as well as the increased N contents and decreased C:H ratios. The observation of proteinaceous materials associated with HA extracted from sludge-amended soil as long as 14 months after application suggested these proteinaceous materials were resistant to decomposition, possibly as a result of their incorporation into the HA structure. Electron spin resonance spectroscopy was used to study the mechanism of Cu('2+) binding by humic acids extracted from soil and sewage sludge. The ESR data presented showed that the addition of pyridine (py) and 1,10-phenanthroline (phen) to the soil and sludge Cu-HA complexes resulted in the substitution of coordinated aquo ligands by the nitrogen donor ligands without displacement of HA oxygen donor ligands also coordinated to the Cu('2+) center. The addition of a more basic nitrogen donor, ethylenediamine (en), resulted in the displacement from cu('2+) of both coordinated aquo ligands and HA oxygen donors, forming Cu(en)(,2)('2+). The g(,(PARLL)) and A(,(PARLL)) values obtained for the py and phen adducts revealed that there were two HA oxygen donors coordinated equatorially to the Cu('2+) center in the original soil and sludge Cu-HA complexes. The characterization of the adducts with phen and en was consistent with the formation of a Cu('2+) chelate in the original Cu-HA complexes where the two equatorial Cu-HA oxygen bonds would occupy cis positions. The crystal field strength of soil and sludge HA oxygen donor ligands coordinated to the Cu('2+) center was stronger than that of H(,2)O. In the sludge Cu('2+)-HA complex, the Cu('2+) ion also appeared to form axial bonds with HA nitrogen donor ligands originating from proteinaceous materials associated with the sludge HA fraction. Characterization of an adduct with glycylglycine showed that the dipeptide formed coordinate bonds with Cu('2+) bound to sludge HA. Infrared spectroscopy was used to identify the mode by which carboxylates of a soil HA complexed Cu('2+) and Fe('3+). Carboxylic acid groups of HA were converted to carboxylates (CO(,2)('-)) by forming coordinate bonds with Cu('2+) and Fe('3+). The separation ((DELTA)) of the antisymmetrical, (nu)(,a)(CO(,2)('-)), and symmetrical, (nu)(,s)(CO(,2)('-)), stretching vibrations of CO(,2)('-), was obtained from the IR spectra of the metal-HA complexes. The values of (DELTA) for the metal-HA complexes were significantly larger than the corresponding reference value of (DELTA) for free CO(,2)('-), demonstrating that CO(,2)('-) formed a unidentate complex (i.e., a single M-O bond) with Cu('2+) and Fe('3+). The formation of bidentate or bridging complexes between CO(,2)('-) of HA and Cu('2+) or Fe('3+) was not observed.

Degree

Ph.D.

Subject Area

Agronomy

COinS