Sources and Sinks of Hydroxyl\nRadicals upon Irradiation of Natural\nWater Samples

Abstract

Hydroxyl radical formation rates, steady-state concentration,\nand overall scavenging rate constant were measured\nby irradiation of surface lake water samples from Piedmont\n(NW Italy) and nitrate-rich groundwater samples from\nMoldova (NE Romania). Dissolved organic matter (DOM)\nwas the main source and sink of ^•OH upon lake water\nirradiation, with [^•OH] being independent of DOM amount.\nWater oxidation by photoexcited DOM is a likely ^•OH\nsource in the presence of very low levels of nitrate and\ndissolved iron. Under different circumstances it is not possible\nto exclude other processes, e.g., DOM-enhanced photo-Fenton reactions. Under the hypotheses of no interaction and\nabsence of mutual screening of radiation, nitrate would\nprevail over DOM as ^•OH source for a NO₃^-/DOM ratio higher\nthan 3.3 × 10^-5 (mol NO₃^-) (mg C)^-1, DOM prevailing for\nlower values. Substantial DOM photolability was observed\nupon irradiation of nitrate-rich groundwater, mainly due\nto the elevated ^•OH generation rate. For the first time to our\nknowledge, evidence was also obtained of the photoformation of potentially toxic and/or mutagenic nitroaromatic\ncompounds upon irradiation of natural lake water and\ngroundwater samples, proportionally to the nitrate levels.