Application of the general rate law model to the sonolytic degradation of nonvolatile organic pollutants in aqueous media

Abstract

The pseudo-first and pseudo-second order equations have been the most commonly used models to characterize the sonolytic disappearance kinetics of nonvolatile pollutants in aqueous media. In this work, the general rate law model, i.e., pseudo-nth order kinetics equation, was applied for the first time to the sono-decomposition of different nonvolatile organic pollutants, naphthol blue black (NBB), furosemide (FSM), 4-isopropylphenol (4-IPP), and rhodamine B (RhB), in water. It was shown that the general rate law for a chemical reaction would apply to the kinetics of sonochemical decomposition. It is not feasible to set the order of ultrasonic pollutant degradation kinetics to pseudo-first or pseudo-second, as is typically used in numerous studies. The sonochemical oxidation reaction has a fractional order, the order is often non-integer, which frequently indicates a complex sonolytic decomposition reaction mechanism. The degradation mechanism of NBB and RhB does not change with the initial substrate concentration. They are ultrasonically degraded by hydroxyl radicals both in the bulk liquid solution and at the liquid/bubble interfacial layer. The destruction mechanism of FSM and 4-IPP changes as the initial contaminant concentration changes. At low initial substrate concentrations, these pollutants are oxidized mainly by reaction with hydroxyl radicals in the bulk liquid solution and at the interfacial shell of the cavitation bubbles. At high initial substrate concentrations, FSM and 4-IPP are degraded by thermal destruction in the liquid/bubble interfacial layer and by •OH radicals both in the bulk liquid solution and at the liquid/bubble interfacial layer. Additionally, the pseudo-nth order model predicts very well the sonolytic degradation at various sonication frequencies and intensities. The general rate law expression should be used to assess the real kinetics order of the sonolytic destruction process without any predetermined assumptions or constraints.