1. Introduction
Geochemical processes occurring in natural groundwater grant to these samples interesting physical-chemical features such as the presence of bicarbonate, fluorine, and colloidal iron, the latter in form of (hydr)oxides as goethite (α-FeOOH) and lepidocrocite (γ-Fe-OOH) . Natural abiotic photoinduced processes involving such species and dissolved organic matter (DOM) could lead to the formation of reactive oxygen species (ROS) such as hydroxyl radicals (OH) (E°= 2.7 V vs. NHE) which would be responsible for organic pollutants removal in sunlight irradiated surface and well waters . However, the concentration of solar light photoinduced OH radicals in surface waters is low (~10-17 mol L-1) , so their effect on the degradation of contaminants would be negligible. For this reason, it is necessary finding strategies to enhance natural abiotic photoinduced processes in waters aiming to the removal of chemical and microbiological pollution, thus, altering as little as possible the water environment. The addition of hydrogen peroxide (H2O2) to natural groundwater samples has been explored for our group exhibiting a promising result to achieve the bacteria inactivation and the removal of organic pollutants .
Goethite is a visible light-absorbing iron (hydr)oxide with apparent promising photocatalytic activity removing both bacteria and organic pollutants from water . However, despite these interesting photocatalytic features, some authors have argued that goethite exhibits high electron-hole recombination since its conduction band potential is too positive (+0.24 V vs NHE at pH 7.0) compared for example with that of molecular oxygen (-0.33 V vs NHE) avoiding an efficient electron transfer and increasing thus charge carriers recombination .
Interestingly, Du et al., reported for the first time that photocatalytic activities of several iron (hydr)oxides such as goethite and lepidocrocite at concentrations of 0.5 g L-1 were strongly enhanced by the presence of fluorine amounts ranging between 8.5-85 mg L-1. Authors claimed that fluorine could alter the surface properties of iron (hydr)oxides. In previous studies of our group, it was found that both 2,4-D (one of the herbicides most widely used around the world and frequently found in drinking water samples ) degradation and bacteria inactivation in natural well waters by the addition of H2O2 (10 mg L-1) under simulated sunlight irradiation was strongly enhanced by the presence of natural amounts of fluoride (0.1-1.2 mg L-1) . Since fluoride and goethite are often found in groundwater samples, their mutual contact in these waters may induce “natural” fine-tuning leading to the generation of “natural” fluoridated iron (hydr)oxides with probably enhanced photocatalytic activities. In addition, the natural presence of other anions in these waters such as bicarbonates may have an important effect since these can react with OH radicals yielding a less oxidative carbonate radical (CO3-•) (E°= 1.78 V vs NHE) would be also able to participate in the degradation of organic pollutants . These outcomings demonstrate that the chemical matrix of groundwater could exert an important role in the removal of organic pollutants or bacteria from natural waters photo-induced by goethite/H2O2 system or pristine goethite.
In this sense, photo-induced removal of 2,4-D by goethite (α-FeOOH) at natural concentrations found in well waters (0.3 mg L-1) by addition of H2O2 at neutral pH and simultaneous presence of fluoride and bicarbonate, in natural amounts, was evaluated. These results would demonstrate that only the addition of hydrogen peroxide would be necessary to carry out photo-Fenton-like and photocatalytic processes for effective removal of organic contaminants in natural waters with the presence of these species and how fluoride and bicarbonate ions would affect its performance. This fact may avoid the use of iron salts, photocatalysts or complexing agents in natural groundwater samples as commonly reported. The effect of the fluoride and bicarbonates concentrations often found in groundwater samples on 2,4-D removal at pH 6.9 under simulated sunlight irradiation and simultaneous presence of H2O2 and goethite was carried out by using a response surface model (RSM) of type screening factorial design 22.