Kinetics and mechanism of the enhanced reductive degradation of CCl4 by elemental iron in the presence of ultrasound

Enhanced rates of sonolytic degradation of CCl4 in the presence of Fe-0 are demonstrated. In Ar-saturated solutions, the first-order rate constant for CCl4 degradation is k(US) = 0.107 min(-1), whereas in the presence of Ar and Fe-0, the apparent first-order rate constant is found to depend on the total surface area of elemental iron in the following fashion: k(obs) = (k(US) + k(Fe0)A(Fe0)) min(-1), where k(US) = 0.107 min(-1), k(Fe0) = 0.105 L m(-2) min(-1), and A(Fe0))= reactive surface area of Fe-0 in units of m(2) L-1. In the coupled ultrasound and iron system, the contribution to the overall degradation rate by direct reaction with Fe-0 results in an overall rate enhancement by a factor of 40. These enhancements are attributed (I)to the continuous cleaning and chemical activation of the Fe-0 surface by the combined chemical and physical effects of acoustic cavitation and (2) to accelerated mass transport rates of reactants to the Fe-0 surfaces. Additional kinetic enhancements are due to the production of H+ during the course of the reaction. Furthermore, the concentrations of the principal reaction intermediates, C2Cl6 and C2Cl4, are influenced substantially by the total available surface area of Fe-0.