Dechlorination of Chlorophenols Using Magnesium-Palladium Bimetallic System

Abstract

Ninety-four percent removal of 10 mg L −1 of pentachlorophenol (PCP) was achieved by treatment with 154.5 mM Mg 0 and 0.063 mM K 2 PdCl 6 in the presence of 175 mM acetic acid in 1 h reaction time. Dechlorination of PCP was found to be sequential and phenol was identified as the end product along with accumulation of trace concentrations of tetra-and trichlorophenols. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) revealed that palladium in its metallic form (Pd 0 ) produced by reduction of Pd 4+ , was spatially separated from magnesium granules when acid was included in the reaction. These colloidal palladium particles generated active reductive species of hydrogen and dechlorinated chlorophenols. In the absence of acid, the efficiency of dechlorination of PCP by Pd/Mg 0 system was very low and chief mechanism of removal of the compound was through sorption onto solid surfaces. Thus, it was important to include acid in the system to: (a) facilitate corrosion of Mg 0 and reduction of Pd 4+ to Pd 0 , (b) provision of protons to produce H 2 , (c) retard formation of insoluble oxides and hydroxides that may deposit on the magnesium granules and sorb PCP and its partially dechlorinated products and. Application of 154.5 mM Mg 0 /0.063 mM K 2 PdCl 6 on PCP, 2,4,5-trichlorophenol (TCP) and 2-chlorophenol (MCP) with organic chloride equivalence showed that the rate and extent of removal increased with decrease in number of chlorine atoms on phenol.

Effects of solvent, pH, salts and resin fatty acids on the dechlorination of pentachlorophenol using magnesium–silver and magnesium–palladium bimetallic systems

“…5). Since the amount of acid dosed within this period, was much less (approximately 20 and 70 mM for pH 6.0 and 5.5, respectively), a very low sustained rate of magnesium corrosion and evolution of hydrogen along with a uniformly distributed colloidal suspension of Pd 0 particles were observed [18]. The uniform suspension of colloidal Pd 0 particles provided very high surface area onto which hydrogen was efficiently intercalated.…”

“…Total volume of acid required for all the experiments was <10% of the reaction volume and hence dilution effect was neglected. In the case of pH-uncontrolled experiments, the reaction was commenced by the direct addition of 175 mM acetic acid [17,18].…”

“…In our recent publications we reported that the efficient dechlorination of PCP by Mg/Ag and Mg/Pd systems through nucleophilic substitution required the addition of acid [17,18]. However in these studies the required amount of acid was added in one step at the beginning of the reaction and thus pH of the reaction phase was not maintained during the course of reaction.…”

“…However, both the bimetallic systems failed to remove PCP to any appreciable level at the concentrations of bimetals used in previous investigations [17,18]. We therefore decided to study the effect of major constituents of pulp bleaching effluent such as salinity (sodium chloride and sodium sulfate) and co-pollutants such as chlorinated and non-chlorinated resin and fatty acids (RFAs) on PCP dechlorination by Mg/Pd and Mg/Ag systems.…”

“…In the last decade, extensive research has been carried out on the application of bimetallic systems to dechlorinate chlorinated organic compounds [14][15][16][17][18]. Bimetallic systems make use of two metals, one in the zero-valent form (with a negative reduction potential like Mg +2 /Mg 0 , Fe +2 /Fe 0 , etc.)…”