Dehalogenation of 2-Chloronapthalene by Cast Iron

Abstract

Aqueous 2-chloronapthalene was contacted with cast iron in batch systems, resulting in an initial rapid increase in the sorbed 2-chloronapthalene concentration (C _s ) followed by a slow decline, and an initial rapid decline in the aqueous 2-chloronapthalene concentration (C _a ) followed by a slower decline. The initial rapid partitioning of 2-chloronapthalene to the solid phase was due to its adsorption on elemental carbon present on the cast iron surface, while the residual aqueous phase 2-chloronapthalene underwent reductive dehalogenation at a slower rate through interaction with the metallic iron surface. The overall rate of change of total 2-chloronapthalene concentration (C _T = C _s +C _a ) with time, i.e., (d/C _T }{{d}t}) could be described by the expression,−k ₁ ⋅M⋅ (C _a )ⁿ, where M is the concentration of cast iron. The values of k ₁ and n were determined to be 1.576 × 10⁻⁵ hr⁻¹ g⁻¹ iron L and 1.945 respectively. Equilibrium partitioning of 2-chloronapthalene between solid and aqueous phases could be described by a Freundlich isotherm, C _s = K⋅ [C _a ]^m, where m and K were determined to be 0.55 and 4.92 × 10⁻³ L g⁻¹. Considering K to be the ratio of the adsorption (k ₂) and desorption (k ₃) rate constants, expressions were developed for describing the evolution of C _s and C _a with time. Putting k ₃ = 1 hr⁻¹ in these expressions resulted in adequate model fit to the experimental data. Napthalene was identified as the major dehalogenation by-product, with greater than 99 percent of the naphthalene produced partitioning to carbon present on the cast iron surface. No competition between 2-chloronapthalene and naphthalene for adsorption on the carbon surface was observed, suggesting non-specific adsorption of these compounds restricted only by the physical size of the molecules and the available carbon surface area.