Removal of methylene blue and azo reactive dyes from aqueous solution and textile effluent via modified pulsed low-frequency ultrasound cavitation process.

Organic dyes in the aqueous solutions and textile effluents cause severe environmental pollution due to their carcinogenic and mutagenic nature. Ultrasound (US) cavitation is one of the promising advanced oxidation processes (AOP) to remove the organic dyes from the aqueous solutions and textile effluents. Nevertheless, the conventional low-frequency US cavitation process exhibits very low efficiency in the dye removal process and demands effective modification to improve its performance. In this investigation, a conventional pulsed low-frequency (22 ± 2 kHz) US cavitation process has been modified by varying the US power (50-150 W), initial solution pH (2-10), and O2 flow rate (1-4 L min-1) to enhance the decomposition of cationic methylene blue (MB) dye in an aqueous solution. The operation of the classic Haber-Weiss reaction, both in the forward and backward directions, and the ozone effect have been observed, for the first time, under the modified US cavitation process, as confirmed via the radical trapping experiments. Moreover, the hydrothermally synthesized hydrogen titanate (H2Ti3O7) nanotubes (HTN) have been utilized as sonocatalyst, for the first time, for 100% dye removal, with effective regeneration obtained via an in-situ thermal activation of persulfate (PS, S2O82-). The most optimum values of US power, initial solution pH, O2 flow rate, HTN, and PS concentrations for 100% MB decomposition are observed to be 150 W, 2, 2 L min-1, 0.3 g L-1, and 10 mM, respectively. The decomposition of industrial azo reactive dyes in an aqueous solution as well as in a textile effluent has also been demonstrated using a modified pulsed low-frequency US cavitation process involving the thermal activation of PS without the use of HTN, which justifies its suitability for a commercial application.

Autoclave and pulsed ultrasound cavitation based thermal activation of persulfate for regeneration of hydrogen titanate nanotubes as recyclable dye adsorbent.

In the dye removal application, regeneration of hydrogen titanate nanotubes (HTN, H2Ti3O7) has been achieved via thermal activation of persulfate anion (PS, S2O82-) by using the conventional hot plate technique which has limitations from the commercial perspective since it does not provide any precise control over the thermal generation process typically during the scale-up operation. To overcome this drawback, HTN have been synthesized via hydrothermal process which exhibit the methylene blue (MB) adsorption of 93% at the initial dye concentration and solution pH of 90 µM and 10 respectively. HTN have been regenerated via the thermal activation of PS by varying its initial concentration and regeneration temperature, within the range of 0.27-1 wt% and 40-80 °C, under the thermal conditions set by the autoclave and pulsed ultrasound (US) cavitation process. The results of recycling experiments suggest that the optimum values of initial PS concentration and temperature, for the regeneration of HTN under the autoclave conditions, are 1 wt% and 70 °C with the maximum MB adsorption of 92%, while, the corresponding values for the pulsed US cavitation process are 1 wt%, 80 °C, and 91% respectively. Thus, the regeneration and recycling of HTN have been successfully demonstrated by using the autoclave and pulsed US cavitation process. Under the optimum conditions, MB degradation involves the generation and attack of SO4•- for both the thermal generation techniques. The regeneration techniques developed here may be utilized in future during the scale-up operation and also for the regeneration of adsorbents besides HTN.