Reduction of Hexavalent Chromium in Aqueous Solutions by Polypyrrole: II. Thermodynamic, Kinetic, and Mechanistic Aspects

Abstract

This article elaborates on an earlier communication from this laboratory that described a new approach to the electrochemical remediation of Cr(VI) based on the use of a conducting polymer, namely, polypyrrole. The polymer film is shown to act as a catalyst for Cr(VI) reduction by shuttling reversibly between its two redox states. The thermodynamic and kinetic aspects of the process are described. The influence of solution pH, substrate [Cr(VI)] initial concentration, and the number of pyrrole units in the polymer film, on the extent of reduction of Cr(VT) is presented. Cyclic voltammetry experiments designed to probe the catalytic nature of the reduction process are described as are stability tests for the polypyrrole film electroactivity on extended contact with Cr(VI) containing solutions. InfroductionChromium occurs in two common oxidation states in nature, Cr(III) and Cr(VI). Because it is only weakly sorbed onto inorganic surfaces,1'2 Cr(VI) is notoriously mobile in nature. On the other hand, Cr(III) is readily precipitated or sorbed on a variety of inorganic and organic surfaces at near neutral pH."2 The properties of Cr(III) and Cr(VI) have been reviewed with respect to acute and chronic toxicity, dermal toxicity, systemic toxicity, toxicokinetics, cytotoxicity, genotoxicity, and carcinogencity.4 The hexavalent chromium compounds appear to be 10 to 100 times more toxic than their Cr(III) counterparts when both are administered by the oral route. The toxicology of chromi-

Remediation of chromium(VI) at polypyrrole-coated titanium

“…Scans of the solution were obtained every 10 min over a period of 3 h and show the removal of Cr(VI) from the solution. This is consistent with the reduction of Cr(VI) by PPy at other substrates [17][18][19][20][21][22]. The PPy was grown to a thickness of 0.5, 1.0 and 5.0 lm and similar UV-Vis experiments were performed in order to assess the effect of polymer thickness.…”

“…Investigated remediation techniques include electrochemical methods [8,9], the use of ferrous iron [7], photocatalysis utilising UV illumination of titanium [10,11] or zinc oxides [12] and a number of other interesting methods including the implementation of copper, hydrogen peroxide, hydrogen sulphide and methylated yeast biomass [13][14][15][16]. In the past number of years the use of conducting polymerbased materials in Cr(VI) remediation has arisen as an interesting area of research [17,18]. Typically, a substrate is coated with a layer of conducting polymer, which is then used to reduce the toxic Cr(VI) to Cr(III), Eq.…”

“…The PPy-modified platinum showed excellent cleanup efficiency in comparison to the uncoated mesh. Further development of this work, performed by Senthurchelvan et al [17], involved the investigation of a glassy-carbon substrate modified with PPy and provided further insight into the kinetics of the system and the effect of film thickness and Cr(VI) concentration. The catalytic properties of the film were investigated with cyclic voltammetry as was the stability of the film on exposure to Cr(VI).…”

Microfluidic Flow through Polyaniline Supported by Lamellar-Structured Graphene for Mass-Transfer-Enhanced Electrocatalytic Reduction of Hexavalent Chromium

“…14,42,43 As a result, further Cr(VI) reduction will be inhibited. Moreover, with increasing solution pH, the redox potential of Cr(VI) species will sharply decrease, which will challenge the reducing ability of the electrode.…”

“…Moreover, with increasing solution pH, the redox potential of Cr(VI) species will sharply decrease, which will challenge the reducing ability of the electrode. 42 For these reasons, the typical immersed parallel-plate electrode shows poor performance at neutral to alkaline conditions. Figure 2B shows the reduction kinetics using the LGS−PANI electrode at initial pH = 5.0.…”