Spontaneous electron transfer, along with anion uptake by doping, form the basis of a new concept for the reduction of toxic Cr(VI) using an electronically conductive polymer, e.g., polypyrrole. Proof of concept experiments are described which show that aqueous sulfuric acid solutions containing 5–100 ppm Cr(VI) can be thus converted at ∼100% efficiency to the environmentally more tractable Cr(III) species at time intervals spanning several minutes. The reversibility of the polymer redox process enables the electrochemical recycling of polypyrrole for repeated treatment of Cr(VI). The key advantages relative to the chemical and electrochemical cleanup strategies currently employed include those related to material recyclability, selectivity, and efficiency at low (ppm) Cr(VI) levels.

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-

This study concerns the growth, redox behavior, and analysis of poly(pyrro1e chloride) films ranging in thickness from -0.5 to -1.5 pm. Changes in the electrolyte pH from radical-coupling reactions during the oxidation of pyrrole lead to estimates of the polymerization efficiency in 0.1 M KCI which vary from 82% to 92% depending on the charge consumed. These values are compared with the Coulombic efficiency computed earlier in this laboratory using electrochemical quartz crystal microgravimetry (EQCM). The EQCM technique was combined with coulometry and in situ pH and K-ion-selective elactrode measurements for reexamining the extent of permselectivity of polypyrrole with respect to the chloride ion in aqueous elcctrolyta. Chloride ions contribute -75-8596 to the transport of the total charge during the redox of polypyrrole in 0.1 M KC1. The residual charge is partitioned between H+ and K+ depending on the initial pH of the electrolyte, which was varied from -1 to -7 in this study. Potassium ion transport was further verified by X-ray photoelectron spectroscopy. The extent of permselectivity is also dependent on the nature of the electrolyte cation, the bulky tetraethylammonium ion showing the most ideal behavior in this regard. Redox cycling of polypyrrole in 0.1 M CsCl and 0.1 M HCl was accompanied by a m b l e cation movement especially at potentials below ~-3 0 0 mV (vs Ag/AgCl). Finally, the ion transport during the redox of poly(pyrro1e chloride) was found to be intrinsically asymmetric in two related respects: first, the redox charge monitored via chronocoulometry in the two switch directions, i.e., oxidized/reduced, was not identical. Second, the measured redox charge was only 27-3596 of that theoretically expected from the polymer doping level for the 0.1 M KCl electrolyte. These data underline the inadequacy of the chronocoulometry/voltammetry procedure for assay of the doping level in conducting polymers such as polypyrrole, especially for rather thick (>a few hundred nanometers) polymer films.

The anodic electrochemistry of two new soluble derivatives of polythiophene, namely, poly(3‐ethylmercaptothiophene) (PEMT) and poly[3, 4‐bis(ethylmercapto)thiophene] (PBEMT), is discussed in this article. For comparison with these systems, the corresponding behavior of polythiophene (PT), poly(3‐methylthiophene) (PMT), and poly(3‐hexylthiophene) (PHT) is also presented. In all five cases, the first anodic oxidation was reversible and was accompanied by reversible electrochromic behavior. When the polymers were driven to more positive potentials, there was immediate loss of electroactivity and concomitant color change to black. The threshold potential for this electrochemical deactivation varied from 1.38V (vs. SCE) for the case of PBEMT to 2.00V (vs. SCE) for PMT. The deactivation mechanism is discussed in detail for PEMT with the aid of data from voltammetry, coulometry, FTIR spectroscopy, and elemental analyses. An electrochemical cross‐linking/chain extension technique is discussed for PEMT as a means of growing smooth polymer films on electrode substrates starting from soluble oligomers in solution.

A self-doped electroactive copolymer, poly{ pyrrole-co-[3-(pyrrol-l-yl)propanesulphonate]}, in which the charge balancing counterion is covalently attached to the polymer chain, has been synthesized and cation controlled charge transport parameters have been evaluated.

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A laboratory experiment illustrating the principle and application of electrocoagulation is described using oil-water emulsions as the medium to be treated and iron as the anode. The destabilized oil droplets are shown to be separated from the aqueous phase via electrolysis and iron hydrooxide coagulant formation. This simple experiment is shown to afford opportunities for exploring concepts related to colloid chemistry, electrochemistry, corrosion, and analytical chemistry.