Kinetics and Mechanism of the Enhanced Reductive Degradation of CCl_4 by Elemental Iron in the Presence of Ultrasound

Abstract

Enhanced rates of sonolytic degradation of CCl_4 in the presence of Fe^0 are demonstrated. In Ar-saturated solutions, the first-order rate constant for CCl_4 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_(Fe)^0A_(Fe)^0) min^(-1), where k^(US) = 0.107 min^(-1), k_(Fe)^0 = 0.105 L m_(-2) min^(-1), and A_(Fe)^0) = 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 (1) 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, C_2Cl_6 and C_2Cl_4, are influenced substantially by the total available surface area of Fe^0.