Fenton Redox Chemistry: Arsenite Oxidation by Metallic Surfaces
S.C. Borges Freitas *, D. van Halem *, A.B.M. Badruzzaman**, W.G.J. van der Meer *,***
* Delft University of Technology, Faculty of Civil Engineering and Geosciences, Stevinweg 1, Delft, The Netherlands (e-mail: S.C.BorgesFreitas@TUDelft.nl)
** Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh *** Oasen Drinking Water Company, P. O. Box 122, 2800 AC Gouda, the Netherlands
Abstract: Pre-oxidation of As(III) is necessary in arsenic removal processes in order to increase its
efficiency. Therefore, the Fenton Redox Chemistry is defined by catalytic activation of H2O2 and
currently common used for its redox oxidative properties. In this study the effect of H2O2 production
catalysed by O2 in metallic surfaces (MS), such as metallic Cu/Zn alloys and Composite Iron Matrix
(CIM), for As(III) oxidation is under investigation. Preliminary results show that As(III) is partly oxidized only by the presence of O2 without addition of MS. However, with delayed addition of MS,
As(III) oxidation occurred but only due to ROS catalyzed by the metallic surfaces.
Keywords: As(III) oxidation; Fenton; Metallic Surfaces; Reactive Oxygen Species
Introduction
Arsenic is a component natural occurring but a toxic metalloid element of the earth’s crust and is widely distributed throughout the environment in the air, water and soil.
Redox chemistry determines the behaviour of arsenic in the environment. Inorganic arsenic, As(III), is highly toxic comparative to organic arsenic, As(V), and both co-exist in natural waters, in which around 80% is As(III). Contrary to As(V) anions, As(III) is the dominant specie and non-charged, therefore more difficult to remove from natural waters (Smedley and Kinniburgh 2002). Thus, pre-oxidation of As(III) to As(V) is necessary in arsenic removal processes in order to increase its efficiency.
Fenton chemistry is defined by catalytic activation of H2O2 and currently common
used for its redox oxidative properties. Therefore, the efficiency of Fenton reactions are directly dependent on O2 availability, H2O2 production and concentration, pH,
reaction time, metallic surfaces (MS) and also Fe(II)/H2O2 ratio (Hug and Leupin
2003, Lee, Lee et al. 2013).
Previous studies, demonstrated that Cu(II) catalyzes the production of reactive oxidative species (ROS) through a copper redox cycle mechanism. Others studied show the effects of surface chemistry such as ligand type and surface oxidation on ROS generated by the copper. Several studies have associated the toxic effects of copper to its ability in inducing ROS formation (Pourahmad, O'Brien et al. 2003, Bopp, Abicht et al. 2008, Sandrini, Bianchini et al. 2009). Composite Iron Matrix (CIM) can be applied for catalytic oxidation. The oxidation of iron is coupled with the oxidation of As(III) to the more readily removable arsenic(V) species (Hussam and Munir 2007, Neumann, Kaegi et al. 2013).
The principal is that H2O2 production is catalyzed by the presence of O2 in contact
with the metallic surfaces (MS). Thus, if there is enough H2O2 production, then
Figure 1. Scheme of As(III) oxidation by ROS catalyzed by metallic surfaces.
Therefore, the objective of the study is to investigate the effect of H2O2 production
catalysed by O2 in metallic surfaces (MS), such as metallic Cu/Zn alloys and
Composite Iron Matrix (CIM), for As(III) oxidation. Material and Methods
Batch experiments: Batch experiments were conducted using glass beakers and
natural groundwater. Initial conditions were 200 μmol As(III)/L and 8.3 μmol Fe(II)/L at pH 7.05±0.01. Oxidation of As(III) was measured using two different doses of metallic Cu/Zn alloys and CIM. Because of the co-presence of oxygen and iron(II), As(III) oxidation was monitored over time, during a total of 72h at stirring speed of 180 rpm.. Therefore, three different setups of experiments were prepared: (i) without MS; (ii) with immediate addition of MS; (iii) with delayed addition of MS.
Results and Conclusions
As(III) oxidation by O2 and ROS
Dissolved oxygen is a very important parameter for the catalyzed Fenton reactions. In the presence of dissolved oxygen, Fe(II) oxidizes, thus Figure 2(A) shows that As(III) is partly oxidized to As(V) only by the presence of O2 without addition of MS. Some
studies indicate that for occurrence of As(III) oxidation, reactive oxygen species (ROS), such as, H2O2, must be catalyzed, during the Fe(II) oxidation by O2. However,
with delayed addition of MS, e.g. Cu/Zn alloys, oxidation of As(III) by Fe(II) oxidation/precipitation occurred first. Subsequently, MS alloys were added and then As(III) oxidation occurred but only due to ROS formation by the metallic surfaces. The faster the catalyzed reactions occurred the more oxidation of As(III) - Figure 2(B).
Figure 2 - As(III) oxidation by: (A) Fe(II) and O2 without addition of MS; (B) O2 and ROS formation
Research is ongoing to understand the importance of Fenton reactions in the production of H2O2 for As(III) oxidation in the presence and absence of Fe(II) and
other groundwater elements under different metallic surfaces. References
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