Fe and Cu in humic acid extracts modify bacterial inactivation pathways during solar disinfection and photo-Fenton processes in water

Abstract

The literature suggests a dual role for humic acids involving both agonistic and antagonistic processes in the photo-chemical cycle in water. In this work, the use of Fe- and Cu-containing humic acid (HA) extracted from coal waste was evaluated for the enhancement of solar-based disinfection processes, namely solar light (hv), hv/H2O2, and hv/H2O2/Fe2+ (photo-Fenton) processes. The differences in the enhancement of each process are reported herein. The degree of bacterial inactivation during solar disinfection under solar and visible light in the presence of the HAs was negatively correlated with the presence of metals, and was attributed to 3DOM* and 1O2-mediated events. However, the presence of metals greatly enhanced disinfection when H2O2 was added by inducing a photo-Fenton process, and significantly reduced the inactivation times (>50%) due to homogeneous Fe- (Fenton) and Cu-based (Fenton-like)-assisted inactivation pathways. The addition of small quantities of Fe to HA-containing samples further enhanced the inactivation due to the ample generation of highly photo-active Fe-HA complexes. The result was a further ∼60% reduction in exposure time to achieve a 7-log bacterial reduction, as well as the degradation of the HA itself as a sacrificial electron donor, via an Fe-mediated ligand to metal charge transfer process. This work reveals new pathways in disinfection assisted by metal-containing HAs, and demonstrates their efficiency in reducing microbial loads with simultaneous elimination of the additive.