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.