A considerable amount of research over the past decade has been directed toward the development of plasma based Advanced Oxidation Processes (AOPs) for the degradation of pollutants in wastewater. Despite these efforts and the exceptional efficiencies reported in laboratory-based tests, the direct contact between a non-thermal plasma (NTP) and liquid introduces a number of challenges. Critically, the characteristics of the NTP, and therefore its chemical composition, are directly linked to the electrical properties of the contaminated water, which inevitably varies. Furthermore, the use of short-duration high voltage pulses to generate NTP is considered the most effective approach, yet the generation of such pulses to create uniform large volume NTP is particularly challenging.

Given the drawbacks and challenges associated with direct-contact NTP reactors, several recent studies have examined the use of indirect-contact NTP systems for water treatment [1,2]. In particular, the surface barrier discharge (SBD) configuration enables large volume NTP generation in humid air with modest power supply requirements. Unfortunately, a key drawback associated with the use of indirect-contact plasma liquid reactors is the reduced transport of chemical species from the discharge to the liquid. Highly reactive species produced by NTP generated in humid air, such as atomic oxygen and hydroxyl radicals, rapidly react beyond the plasma to form longer-lived and less reactive intermediaries, such as ozone and hydrogen peroxide. To enhance the efficiency of an indirect-contact NTP reactor the transport of longer-lived intermediary species to the liquid must be maximized. One particularly convenient method to achieve this is through the use of a falling film reactor [3], whereby a thin liquid film flows in close proximity to an NTP source. Such reactors have been used extensively in direct-contact NTP studies [4,5], but have not yet been used for the in-direct NTP treatment of contaminated water.

This contribution considers the hypothesis that the efficiency of an indirect-contact NTP wastewater treatment processes can rival that of direct-contact treatment process by combining a falling film reactor with a SBD, figure 1. To test the hypothesis the decolorization of indigo carmine dye was examined as a function of plasma operating parameters whilst the generation of chemical species was assessed in both the liquid and gas phase. Analysis of dye decolorization as a function of energy input revealed that complete decolorization of Indigo Carmine could be achieved, figure 2. However, more energy was used at higher discharge powers to obtain total decolorization. To explain the faster and more efficient decolorization of dye at lower power, FTIR was used to quantify gas phase chemical species. Under high power operation it was observed that the gas phase chemistry rapidly transitioned from being ozone dominated to NO_x dominated, attributed to process known as Ozone poisoning. It was shown that the shift in gas-phase chemistry is primarily responsible for the reduction in decolorization efficiency under high power operation.

Using optimized NTP parameters the decolorization results were compared against several other plasma-based AOP approaches and alternative AOP technologies, Figure 3. The comparison illustrates the G₅₀ (g/kWh) value (i.e., the energy consumed to decompose one gram of pollutant) against the time needed to reach 50 % decomposition (t₅₀). It is clear that the indirect-contact plasma system investigated in this contribution compares favorably with other plasma-based studies, including those using direct-contact systems. It is also clear that the approach offers a higher efficiency compared to alternative AOP technologies, such as UV, UV/ H₂O₂ [6], H₂O₂/photo Fenton, UV/TiO₂ and Fenton/photo Fenton [7].

In summary, the combination of a large-area SBD and falling film reactor provides a convenient means for wastewater remediation without many of the challenges associated with direct-contact plasma-liquid systems. Despite the limited mass-transport of highly reactive species to the liquid layer, the large flux of longer-lived species was found to be highly effective for the efficient degradation of Indigo Carmine.

[1] Kumar et al., Frontiers in Physics, vol. 10, 2022.

[2] Pavlovich et al., Journal of Physics D: Applied Physics, vol. 46, no. 14, p. 145202, 2013.

[3] Krupež et al., Journal of Physics D: Applied Physics, vol. 51, no. 17, p. 174003, 2018.

[4] Hama Aziz et al., Journal of Hazardous Materials, vol. 343, pp. 107-115, 2018.

[5] Kozakova et al., Plasma Processes and Polymers, vol. 15, no. 6, p. 1700178, 2018.

[6] Rodríguez et al., Environmental Engineering Science, vol. 24, no. 3, pp. 363-371, 2007.

[7] Palma-Goyes et al., Electrochimica Acta, vol. 140, pp. 427-433, 2014.