pH effect on surfactant properties and supramolecular structure of humic substances obtained from sewage sludge composting

This work evaluates the surfactant properties of humic acids (HA) obtained from sewage sludge composts and the production of value-added products from the residue that is typically a disposal problem. To obtain the HA, aerobically digested sewage sludge was composted with either wooden shavings (HACOMP1) or tezontle (HACOMP2) as a bulking agent. HACOMP1 and HACOMP2 samples were evaluated for their effect on surface tension and compared with a commercial vermicompost sample (HALCOMP). The surface tension values for a concentration of 5000 mg/L, showed a decrease from 72 mN/m to 49 and 51 mN/m for HACOMP1 and HACOMP2 respectively, while the commercial vermicompost diminished only to 65 mN/m. The changes in surface tension could be related to the presence of functional groups, particularly COOH and a predominance of either aliphatic or aromatic chains. This fact was confirmed by FTIR analysis, COOH quantification, and the E₄/E₆ ratio. These results indicate that lower surface tension is due to the predominance of aliphatic chains, plus lower content of COOH groups and a higher E₄/E₆ ratio (short size molecule). Also, samples exhibit pH dependence, having a lower surface tension at pH values from 3 to 4. According to size exclusion chromatography (SEC) tests, this effect could be related to changes in the supramolecular structure of the humic acids, in such way that at this pH value there is an arrangement of molecules that decrease surface tension. Humic acids obtained from sewage sludge compost affect surface tension in a way similar to other humic acids obtained from soils and compost obtained from other organic compounds. Thus, humic acids can be used as surfactants, with the added advantage that their use also disposes of the hazardous waste that is the source of these compounds.