Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance
Introduction
Although arsenic is not particularly abundant in the earth's crust, it is a widely distributed element that is highly toxic (i.e., as inorganic As(III)) [1], [2]. These properties and the solubility and reactivity of arsenic compounds, make leaching and pollution of natural waters by arsenic a matter of worldwide concern. This situation has provoked significant research activity directed at the development of speciation and detection methods, and of efficient removal techniques.
Speciation and quantification of arsenic are difficult because the concentrations of interest in water are typically at μg/L levels, which are of the same order of magnitude as the detection limits of many of the most relevant techniques [3]. Among these techniques, electrochemical methods can be useful for both speciation and detection of arsenic at μg/L levels [1], [2], [4].
The removal of arsenic species can be accomplished via various methods, including adsorption, precipitation, coagulation, and membrane separation [5], [6]. In most of these methods, the efficiency towards As(III) removal is significantly less than for As(V), which makes it necessary to increase the pH of the solution to pre-oxidize As(III) species. In the case of adsorption, however, apparently both As(III) and As(V) can be removed under appropriate conditions [6].
Recently, electrosorption on porous carbons has been proposed as a possible technique for arsenic removal from water [7]. Adsorption from solution by activated carbons is strongly dependent on the chemical nature of the adsorptive (i.e., molecular structure, size, charge, etc.), the pH of the solution, ionic strength, porosity and surface chemistry of the carbon material [8]. Thus, the surface charge of the porous carbon relative to that of the adsorptive can have a strong influence on the adsorption process. In these cases, the adsorption properties of the porous carbon may be modified via the application of an electric potential, and the adsorption or desorption of charged species may be achieved by changing the polarity of the applied potential [7]. This type of experiment has been performed with arsenic species in solution at conditions similar to those in natural waters with positive results [7].
The preceding motivated our interest in exploring the electrochemical behavior of arsenic species in porous carbon electrodes. However, the application of conventional electrochemical techniques, like cyclic voltammetry, to porous carbons is complicated by the characteristically large contribution of the double layer charge of these materials. This impedes the direct observation of faradaic processes, especially at low concentrations. A complementary approach used in the current work to help circumvent this problem, is the application of an electrochemical quartz crystal microbalance (EQCM) to monitor changes of the electrode mass with ng sensitivity, in addition to the detection of charge-transfer reactions as with conventional electrochemical techniques. The objective of the current study is to investigate the electrochemical behavior of arsenic species with a porous carbon through the use of the EQCM.
Section snippets
Materials
A powdered commercial activated carbon was selected (carbon black T-10157 from Cabot Corp.) to serve as an electrode. The porous texture of this carbon was characterized by gas adsorption (N2 @77K and CO2 @ 273K) with an Autosorb-6 apparatus (Quantachrome Corp.). The N2 adsorption isotherms for this carbon are type I, although with a wide knee (i.e., wide micropore size distribution), and a positive slope at relative pressures greater than 0.2, which is indicative of the presence of
Response of the Pt electrode
In Fig. 1 are presented data obtained during a steady voltammetric cycle for the EQCM bare Pt electrode: (a) the CV data at 50 mV/s; (b) the corresponding EQCM mass data; and (c) the derivative of the EQCM mass data, or the massogram [14]. Each voltammetric cycle begins at 0 V, ramps anodically to +0.8 V, then cathodically from +0.8 to −0.7 V, and then completes the cycle anodically from −0.7 to 0 V. The data shown are for the fifth cycle, except for the anodic portion from −0.7 to 0 V, which is the
Conclusions
The interactions of arsenic species with platinum and porous carbon electrodes have been investigated and compared using EQCM and cyclic voltammetry in alkaline solutions. In the case of the bare Pt electrode, the processes associated with the adsorption/desorption of oxygen and hydrogen can be well differentiated with the EQCM. In the presence of arsenic, reduction/deposition of As, as well as electrocatalyzed oxidation/desorption by Pt can be distinguished with the EQCM. These features are
Acknowledgements
This work was partially supported by grant number 5 P42 ES013660 from the U.S. National Institute of Environmental Health Sciences (NIEHS), NIH, and by the Generalitat Valenciana (RED ARVIV/2007/076) and Ministerio de Educación y Ciencia (Project CTQ2006-08958/PPQ). The authors also wish to acknowledge the following: E. Morallon to the Generalitat Valenciana for a travel grant (BEST/2007/038); J.M. Calo for support from the Programa de Ayuda para Investigadores Senior, 2006, from the
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