Urban wastes as sources of valuable chemicals for sustainable development: surfactants, dispersing polymers and polyelectrolytes of biological origin

Abstract

Following previous work indicating urban wastes to be sources of valuable products with remarkable surface activity properties, a new humic acid-like (HAL) substance (cHAL3) was isolated from green residues composted for 7 days. This material was characterized for its chemical composition by elemental analysis, nuclear magnetic resonance and infrared spectroscopy, and potentiometric titration of carboxylic and phenol groups, for its surface activity properties in aqueous solution at pH 7 and for its molecular weight. The results indicate that cHAL3 has good surface activity properties mostly due to molecules with molecular weight up to 1 kg/mol; it also contains larger ionic polymeric molecules or aggregates with molecular weight up to 267 kg/mol. In the attempt to obtain evidence for possible source–structure–properties relationships, cHAL3 was compared with other previously reported HAL substances (i.e. cHAL2) isolated from the same green residues before composting and cHAL isolated from a mix of kitchen and green residues composted for 15 days. The results show that cHAL3 and cHAL2 have slightly different chemical compositions but similar surface activity properties in aqueous solution at pH 7 (i.e. critical micellar concentration (cmc) 0.91–0.97 g/L and surface tension at the cmc (γ cmc ) 38 mN/m). By comparison, the cHAL biosurfactant is less polar and less hydrophilic and exhibits enhanced surface activity (i.e. cmc = 0.40 g/L and γ cmc = 36 mN/m). Thus, composting in the 0–7-day range does not appear to result in chemical composition changes in the isolated HAL substances, which can significantly affect surface tension properties. However, changing both the biomass nature and the composting time seems to affect significantly the surface activity and the content of ionic functional groups in the humic-like isolates. The results offer scope for a long-range process and product development study aiming to the valorization of biomass wastes as source of high-value chemical auxiliaries.