Sonochemical degradation of p-nitrophenol in a parallel-plate near-field acoustical processor

The sonochemical degradation of p-nitrophenol (p-NP) in a near-field acoustical processor (NAP) is investigated. The pseudo-first-order rate constant, k, for p N P degradation increases proportionally from 1.00 x to 7.94 x 10^(-4) s^(-l) with increasing power to volume ratio (i.e., power density) over the range of 0.98-7.27 W/cm^3. An increase in the power-to-area ratio (i.e., sound intensity) results in an increase in k up to a maximum value of 8.60 x 10^(-4) s^(-1) a sound intensity of 1.2 W/cm^2. A mathematical model for a continuous-flow loop reactor configuration is required in order to extract k from the experimentally observed rate constant, k_(obs), which is a function of the relative volumes of reactor and reservoir. The nature of the cavitating gas (Ar, O_2) is found to influence the overall degradation rate and the resulting product distribution. The rate constant for p-NP degradation in the presence of pure O_2, k_(O_2), = 5.19 x 10^(-14) s^(-1), is lower than in the presence of pure Ar, k_(Ar) = 7.94 x 10^(-4) s^(-1). A 4:l (v/v) Ar/O_2 mixture yields the highest degradation rate, k_(Ar/O_2) = 1.20 x 10^(-3) s^(-1). Results of these experiments demonstrate the potential application of large-scale, high-power ultrasound to the remediation of hazardous compounds present at low concentrations. The NAP is a parallel-plate reactor that allows for a lower sound intensity but a higher acoustical power per unit volume than conventional probe-type reactors.