Advances in Ultrasound Technology for Environmental Remediation

Removal of toxic and organic conatminants has become a matter of considerable interest. If the untreated contaminant is released into environments, it is certain to cause significant environmental problems due to its accumulation in soil and water environments. Although biological method is generally used in the treatment of contaminants, this method is not perfect and continuous efforts to improve biological remediation are necessary. On the other hand, ultrasound technology could become an alternative method for waste management and environmental remediation. Contrary to conventional treatments, ultrasound technology may offer advantages, such as environmentally friendly, low costs, compact and others. The application of ultrasound technology for environmental remediation is still in developing stage but it is growing rapidly and holds a promising future as one of the leading “green” technologies for environmental remediation.

The application of ultrasonic technology has been receiving wide attention by the world in wastewater treatment and environmental remediation areas. The use of ultrasound technology is shown to be very promising for the degradation of persistent organic compounds in wastewater as it is proven to be an effective method for degrading organic effluent into less toxic compounds. The advantages of this technology include potential chemical-free and simultaneous oxidation, thermolysis, shear degradation, enhanced mass-transfer processes together etc. Overall, sonochemical oxidation uses ultrasound to produce cavitation phenomena, which is defined as the phenomena of the formation, growth and subsequent collapse of microbubbles, releasing large magnitude of energy, and induces localized extreme conditions. The sonochemical destruction of pollutants in aqueous phase generally involves several reaction pathways such as pyrolysis inside the bubble and hydroxyl radical-mediated reactions at the bubble–liquid interface and/or in the liquid bulk. This chapter mainly reviews the fundamental of ultrasound technology.

The use of ultrasound as one of the intensification technologies has undergone rapid development over the past decade. Among the many aspects in driving these developments, the increasing need to introduce environmentally friendly and clean technology, which is able to minimize contaminants at the source, is an important factor. Past studies show that ultrasound-assisted-chemical reactions have been carried out in many types of degradation reactions with high degradation rates and shorter reaction time as compared to conventional methods. Successful application of this technique to treat different types of halogenated hydrocarbons, pesticides, dyes, and other compounds has been widely reported in the literature. Many focus on addressing the drawbacks of onefold application of ultrasonic degradation by coupling with Fe²⁺, H₂O₂, Fenton reagents, photocatalysts, and others. This chapter summarizes the results obtained from laboratory-scale studies, illustrating the promise and practicality of ultrasound as an effective advanced oxidation technique in solving environmental problems.

The yield of sonication depends heavily on ultrasonic factors. Hence, the experimental conditions for ultrasound treatment must be carefully considered when a process is designed and controlled during the ultrasonic irradiation. Operating conditions such as applied ultrasound frequency, ultrasound intensity, liquid bulk temperature, initial pH of the solution, initial substrate concentration, and others affect ultrasound treatment performance in a positive or adverse way. As ultrasound alone is usually insufficient for total mineralization of organic compounds in the wastewater, the addition of various additives and combined or integrated treatments are of common interests for improving mineralization reaction and enhancing degradation efficiency of the pollutant as a whole. This chapter is a brief account of the main parameters influencing cavitation chemistry and ways to enhance the ultrasound treatment performance.

Despite ultrasound technique being one of the “green” technologies in environmental remediation and with many possible diverse field applications, there are hardly any physicochemical transformations carried out in industrial scale of operation due to the lack of unified design and scale-up strategies. Issues in scaling up of sonoreactors to meet industrial needs such as process efficiency and rates, energy conversion, high volume processes, and others present a considerable challenge toward further development of this technique. It is important to ensure that maximum efficiency can be attained in the design of industrial-scale sonoreactors due to the difficulty in replicating the exact reactor geometry and sonochemistry environment similar to laboratory-scale reactors as acoustic cavitation near ultrasonic transducers are relatively higher. Some design improvements to be investigated include transducer arrays and a larger exposed surface for ultrasound source, continuous flow reactor designs, and stirring during sonication. This chapter aims to identify some of the key issues in sonochemical processes for industrial-scale application and to update on some of the recent designs in sonochemical reactors.