Evolution of the fulvic acid fractions during co-composting of olive oil mill wastewater sludge and tree cuttings

Fulvic acids (FAs) were isolated by a conventional procedure from two mixtures of the sludge residue obtained from olive oil mill wastewater (OMW) evaporated in open-air pond and tree cuttings (TC) at different stages of the co-composting process. The FAs were analyzed for elemental (C, H, N, S, O) and acidic functional group (carboxylic and phenolic) composition, and by ultraviolet/visible, Fourier transform infrared and fluorescence spectroscopies. At the initial stage of composting, FAs from the OMW sludge–TC mixtures were characterized by a prevalent aliphatic character, large contents of C, S-containing groups, proteinaceous materials and polysaccharide components, extended molecular heterogeneity, small O and acidic functional group contents, and small degrees of aromatic ring polycondensation, polymerization and humification. As composting proceeded, C, H and S contents, C/N ratio, and aliphaticity decreased, whereas N, O, COOH and phenolic OH contents, C/H and O/C ratios, and aromaticity increased. These results suggested that, with increasing the composting time, the chemical and structural properties of the FA components of the two OMW sludge–TC mixtures approached the characteristics typical of native soil FAs. Thus, co-composting of OMW sludge mixed with TC may represent a suitable treatment for enhancing the quality of organic matter in these materials when used as soil amendments.