This report summarizes results obtained as part of a larger effort to demonstrate the applicability of electrolytic procedures for the direct anodic (oxidative) degradation of toxic organic wastes. We refer to this process as "electrochemical incineration" (ECI) because the ultimate degradation products, e.g., carbon dioxide, are equivalent to those achieved by thermal incineration processes. In this work, the ECI of 4-chlorophenol is achieved in an aqueous medium using a platinum anode coated with a quaternary metal oxide film containing Ti, Ru, Sn, and Sb oxides. The electrode is stable and active when used with a solid Nafion membrane without the addition of soluble supporting electrolyte. Liquid chromatography (LC), including reverse phase and ion exchange chromatography, is coupled with electrospray mass spectrometry (ES-MS) and used, along with gas chromatography-mass spectrometry (GC-MS) and measurements of pH, chemical oxygen demand (COD), and total organic carbon (TOC), to study the reaction and identify the intermediate products from the ECI of 4-chlorophenol. Twenty-six intermediate products are identified and reported. The most abundant of these products are benzoquinone, 4-chlorocatechol, maleic acid, succinic acid, malonic acid, and the inorganic anions chloride, chlorate, and perchlorate. After 24 h of ECI, a solution that initially contained 108 ppm 4-chlorophenol yields only 1 ppm TOC with 98% of the original chlorine remaining in the specified inorganic forms. LC-ES-MS and direct infusion ES-MS detection limits are between 80 ppb and 4 ppm for these intermediate products. Elemental analysis of the electrolyzed solutions by inductively coupled plasma mass spectrometry ICP-MS showed that only trace amounts (<25 ppb) of the metallic elements comprising the metal oxide film were present in the solution.
Pneumatically assisted electrospray mass spectrometry (ES-MS) is used in the negative ion mode for aqueous metal (M) solutions in an excess of hydrochloric or nitric acid, where the major anion X = Cl- or NO3-. A collision energy of approximately 20 eV removes anion-solvent clusters for most elements and leaves negative complex ions of the general form (Mn+Xn+1)-. Complexation with anions prevents charge reduction reactions at least to n = 3, even in cases where the third ionization energy of M greatly exceeds the first ionization energy of the solvent. These negative ions thus preserve the charge state of the metal cation from the solution and allow identification of both cations and anions in a single set of electrospray conditions. Cations such as Fe3+ or Cu2+ that have a lower oxidation state in solution produce a distribution of negative ions, each with a single negative charge overall; e.g., an Fe3+ solution produces both Fe(III)X4- and Fe(III)X3-. This distribution of FeIII and FeII species is attributed to electrochemical reduction of Fe3+ at the negatively charged ES needle. "Native" anions such as perrhenate or molybdate produce singly charged analogues such as ReO4- or HMoO4-. Metal-EDTA complexes are seen as M(III)Y- or M(II)HY-. The sensitivity for these "native" anions is suppressed by competition with the excess chloride or nitrate used to produce the metal-containing complex ions.
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