Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance

doi:10.1016/j.electacta.2009.02.023

Electrochimica Acta

Volume 54, Issue 16, 30 June 2009, Pages 3996-4004

https://doi.org/10.1016/j.electacta.2009.02.023 Get rights and content

Abstract

The interactions of arsenic species with platinum and porous carbon electrodes were investigated with an electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry in alkaline solutions. It is shown that the redox reactions in arsenic-containing solutions, due to arsenic reduction/deposition, oxidation/desorption, and electrocatalyzed oxidation by Pt can be readily distinguished with the EQCM. This approach was used to show that the arsenic redox reactions on the carbon electrode are mechanistically similar to that on the bare Pt electrode. This could not be concluded with just classical cyclic voltammetry alone due to the obfuscation of the faradaic features by the large capacitative effects of the carbon double layer.

For the porous carbon electrode, a continual mass loss was always observed during potential cycling, with or without arsenic in the solution. This was attributed to electrogasification of the carbon. The apparent mass loss per cycle was observed to decrease with increasing arsenic concentration due to a net mass increase in adsorbed arsenic per cycle that increased with arsenic concentration, offsetting the carbon mass loss. Additional carbon adsorption sites involved in arsenic species interactions are created during electrogasification, thereby augmenting the net uptake of arsenic per cycle.

It is demonstrated that EQCM, and in particular the information given by the behavior of the time derivative of the mass vs. potential, or massogram, is very useful for distinguishing arsenic species interactions with carbon electrodes. It may also prove to be effective for investigating redox/adsorption/desorption behavior of other species in solution with carbon materials as well.

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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Arsenic species interactions with a porous carbon electrode as determined with an electrochemical quartz crystal microbalance

Abstract

Introduction

Section snippets

Materials

Response of the Pt electrode

Conclusions

Acknowledgements

Chemosphere

J. Hazard. Mat.

Carbon

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

J. Electroanal. Chem.

Electrochim. Acta

Water Res.

Surf. Coat. Tech.

Electrochim. Acta

Int. J. Electrochem. Sci.

Electrochim. Acta

Carbon

J. Power Sources

Carbon

Carbon