Chemical properties of metal--humic acid fractions of a sewage sludge--amended aridisol

Humic acid (HA) was extracted from a Domino soil (Xerollic Calciorthids) cropped with barley (Hordeum vulgare L.) and sorghum [Sorghum bicolor (L.) Moench] in experimental plots that received applications of sewage sludge for 4, 5, 6, or 7 yr at rates of 0,22.5, 45, or 90 Mg ha-1 yr-1. The HA samples were characterized chemically by elemental and functional group analyses, total trace metal content, infrared (IR), and electron spin resonance (ESR) spectroscopy. As compared to the nonamended Domino (Xerollic Paleorthids) soil HA, the highest sludge-amended soil HA was extracted in higher yields and had higher N and S contents, lower C/N molar ratios, higher total acidity and phenolic OH content, lower organic free radical content, and higher relative intensity of the amide I and amide II IR absorption bands. These differences are believed to be caused by the incorporation of components typical of the applied sludge organic fraction into the soil HA, i.e., proteinaceous materials, sulfur-containing compounds, and phenolic components. The HA also exhibited a well-defined selectivity in respect to trace metal adsorption/desorption. Copper, Ni, Zn, Fe, and Cr were adsorbed strongly onto readily available sites, or through exchange with more weakly adsorbed trace metal ions (Mn, V, Ti, and Mo). However, the soil HA tended to reach metal saturation at the highest sludge application rates. The ESR analysis showed the presence of indigenous Cu2+, Fe3+, and VO2+-HA complexes in all HA samples. These complexes had similar chemical and structural properties, and were concluded to represent the speciation of the metals in association with HA. In general, the impact of the organic constituents in fresh sludge on the soil HA appeared to be mitigated somewhat by the continual tendency of soil organic matter to attain equilibrium, whereas the metal-organic portion of the soil HA was significantly affected by sludge application.