Dechlorination of Chlorophenols Using Magnesium-Palladium Bimetallic System

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ABSTRACT
94% removal of PCP (10 mg l-1 initial concentration) was achieved by using 3.75 g l-1 Mgo and 25 mg l-1 K2PdCl6 in presence of 175 mM acetic acid in 1h 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. Pd/Mgo reaction carried out in the absence of acid revealed that dechlorination of PCP was insignificant and chief
mechanism of PCP removal was sorption on to solid surfaces. Thus, it is important to include acid in the system, to serve triple purposes of [1] provision of protons to produce H2, [2] retardation of formation of insoluble oxides and hydroxides that may sorb PCP and its partially dechlorinated products and, [3] reduction of Pd+4 to Pdo, especially while treating small concentrations of PCP.

Experiments using pre-coated and instantaneously prepared Pd/Mgo bimetal particles revealed that 94% of initial PCP was dechlorinated in 48 h using instantaneously prepared bimetal particles, while only 51% of initial PCP could be dechlorinated using ex-situ prepared (i.e Pdo predeposited on Mgo) bimetal particles under identical reaction conditions. While negligible sorption was noticed in case of the former, sorption of PCP in case of the latter was
significant.

Application of the optimized concentrations of Pd/Mgo on 10 mg l-1 initial concentrations of 2,3,4,6 tetrachlorophenol (TeCP), 2,4,5 trichlorophenol (TCP), 2,4 dichlorophenol (DCP) and, 2 chlorophenol (MCP) under identical reaction conditions as PCP revealed that extents of removal were quite close to each other for all these chlorophenols. However, reactions carried out with PCP, 2,4,5 TCP, 2,6 DCP and 2 CP with chlorine atom equivalence showed that the rate of removal increased with decrease in number of chlorine atoms on phenol (Figure 1). This essentially proves that Pd/Mgo system is capable of cleaving C-Cl bonds irrespective of the bond energy.

Repetition of the protocol outlined by Morales et al (J. Hazard. Mater. B, 90, 97–108, 2002) for PCP dechlorination using Pd/Mgo system revealed that 20% of initial PCP was found unreacted in the supernatant and 10% was sorbed onto residual solid surfaces at the end of 48h of reaction (Figure 2).

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