Advanced ultrasound-assisted removal of organic pollutants in water: from piezocatalysis to sono-adsorption

Abstract

Piezocatalysis is a novel concept in the field of catalysis that aims to develop environmentally friendly catalytic processes independent of energy sources such as light and electricity by utilising prevalent mechanical vibrations like wind and tides. The present work aimed to fully understand the fundamentals of piezocatalysis by investigating the suggested mechanisms and rigorously implementing thorough control experiments to separate ‘true’ piezocatalytic activity from other phenomena that may also occur at the same time when using ultrasound. The first part of this work used theoretical and experimental approaches to investigate the concept of piezocatalysis. Potassium bismuth titanate-bismuth ferrite lead titanate (BF-KBT-PT) ceramics were used as catalysts to understand the effect of piezocatalyst size, poling/unpoling, and excitation mode on the degradation of Rhodamine B (RhB) in water. The results showed that whilst poling had a significant effect on the degradation of RhB, piezocatalysis is a more complex combination of different phenomena simultaneously contributing to the overall degradation of RhB.

The second part of this work investigated the effect of ultrasonic frequency and power on the piezocatalytic degradation of RhB. Different experimental set-ups with operating frequencies ranging of 20 kHz to 1 MHz and adjustable powers were used. The results revealed that, at lower ultrasonic frequencies (<100 kHz) and moderate acoustic powers, mechanical effects from acoustic cavitation had a positive effect on the piezocatalytical generation of radicals, enhancing the overall degradation of Rhodamine B. However, the sonochemical formation of radicals remained a significant contributor to the overall degradation. At higher frequencies (>100 kHz), though, the chemical effects from acoustic cavitation became so dominant that no piezocatalytical contribution to the degradation of RhB was noticed, leading to the question of whether piezocatalysts are necessary when optimising sonication parameters such as frequency and power can achieve fast degradation kinetic constant rates of 0.037 min−¹.

To further understand how piezocatalysis works, the third part of this study investigated the importance of the energy band theory mechanism by using three different piezocatalysts with varying energy band gaps and piezoelectric properties. Besides BF-KBT-PT, other materials such as zinc oxide and barium titanate were used to degrade RhB under the excitation of combined ultrasound and mechanical agitation. The results indicated that both energy band theory and screening charge effects may play important roles in the piezocatalytic contribution to the overall degradation process, as the piezocatalyst most likely to generate radicals via both mechanisms (poled BaTiO₃) achieved the best overall dye degradation.

Based on previous results, the final part of this study investigated the potential of PVDFcomposite materials for a more environmentally friendly removal of pollutants from water. A bulky and easy-to-recover piezocatalyst was developed using additive manufacturing. The results showed that PVDF-BaTiO₃ piezocatalysts behaved significantly different compared to BaTiO₃, indicating another ultrasound assisted phenomenon taking place. Additional experiments with non-piezoelectric PVDF revealed a possible contribution of sono-adsorption to the overall removal of RhB. In this context, a phenomenological model was developed, which for the first time accounted for the physico-chemical phenomena present during ultrasound-assisted adsorption. This study therefore provides insight on the occurrence of another concurrent phenomenon in piezocatalysis, in addition to demonstrating a new approach for additively manufacturing simple-to-recover PVDF-based catalysts.

In conclusion, this work provided evidence that piezocatalysis may indeed exist, but also that it is a far more complex process than what has initially been assumed in the literature. The importance of conducting thorough control experiments has been emphasized to better understand the role of other ultrasound-assisted phenomena simultaneously occurring during sonication for piezocatalysis.