Enhancing solar disinfection of water in PET bottles by optimized in-situ formation of iron oxide films. From heterogeneous to homogeneous action modes with H2O2 vs. O2 – Part 1: Iron salts as oxide precursors

Abstract

Solar disinfection (SODIS) is a WHO-accepted intervention method for improving water sources in developing countries. Despite its effectiveness, the limitations of long exposure and bacterial regrowth risk demand further improvement of the practice. In this work, we have generated an iron oxide film on the inner surface of PET bottles used in SODIS, to generate further pathways of solar-mediated inactivation, namely a semiconductor mode of action and controlled iron leaching in the system, which both have demonstrated bactericidal capacity. More specifically, in this Part 1, the deposition process using Fe salts has been scrutinized, assessing the use of various homogeneous Fe precursors (FeCl3, FeSO4 and Fe2(SO4)3), amounts of iron (0.5–20 g/L) and deposition time (1–8 h) to find the delicate balance among deposition layer thickness and light penetration. At the optimal conditions (4 h deposition, 1 g/L FeCl3) SODIS was enhanced, reducing 60% the exposure time; a simple washing step brought a further reduction (70%), while eliminating regrowth in volumes from 330 up to 1500 mL reactors. A robust process and reactor was attained, able to reuse its precursor solution almost 10 times and the reactor in 5 consecutive tests, without the need for re-deposition. The modification also proved to be an invaluable iron source to fuel the photo-Fenton process, when H2O2 as an electron acceptor was added to the system. The improvement induced by the heterogeneous photo-Fenton process was around 80% compared to the SODIS/H2O2 process in plain PET bottles and exceeded 85% when compared to SODIS, while being durable to the high oxidative conditions. Finally, in the view of application in drinking water treatment, the process performed well in the lightly acidic region, due to the physicochemical implications of natural waters’ pH in iron cycling.