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

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

Piezocatalytic degradation of pollutants in water: Importance of catalyst size, poling and excitation mode

Piezocatalysis is a promising area of research that would enable new advances in environmental catalytic processes independent of energy sources such as light or electricity. To shed more light on this field, theoretical and experimental studies were conducted using poled and unpoled BF-KBT-PT ceramics as catalysts in order to investigate the effect that piezocatalyst size, piezocatalyst poling/unpoling and agitation mode have on the degradation of a dye, Rhodamine B (RhB), in water. While an apparently contradictory trend in the theoretical and experimental results was observed in relation to piezocatalyst size, poling indeed had a significant effect on the degradation of RhB, indicating that a complex combination of different phenomena such as ‘top-to-bottom’ electric potential difference due to ‘bulk’ piezoelectric polarisation, nanoscale piezoelectric response and sonocatalysis may result in the overall catalytic degradation of RhB. However, the greatest contribution to the degradation of the dye would come from sonochemistry, as ultrasound in absence of a catalyst already achieved a remarkable degradation of RhB. This study therefore demonstrates the complexity of piezocatalysis, and why other phenomena besides bulk piezoelectric polarisation of catalysts must be taken into account in piezocatalysis research

Synergistic sono-adsorption and adsorption-enhanced sonochemical degradation of dyes in water by additive manufactured PVDF-based materials

The present study proposes the first mechanistic model accounting for the most meaningful physico-chemical phenomena taking place in liquid phase adsorption processes under ultrasound. Initially, this study was aimed at developing an easy-to-make and easy-to-recover piezocatalyst for the degradation of RhB in water by combining the high piezocatalytical performance of BaTiO3 with a compatible piezoelectric support such as PVDF, manufactured by a customised additive manufacturing – direct ink writing system with in-situ poling. However, initial results showed that the resulting PVDF-BaTiO3 composite slabs performed worse than BaTiO3 piezocatalysts on their own, and that poling did not have any effect on their performance (82% RhB removal after 2 h when using either poled or unpoled PVDF-BaTiO3 composite slabs compared to 92% RhB removal after 2 h in presence of BaTiO3 piezocatalysts). Further investigation with pure PVDF materials demonstrated that, instead of piezocatalysis, synergistic ultrasound-assisted adsorption and sonochemical degradation were taking place, enabling the removal of >95% of the dye within 40 min of ultrasound treatment in the presence of 4 g L–1 of additive manufactured PVDF slabs. The results of this study and their evaluation with the mechanistic model proposed for liquid phase adsorption under ultrasound suggest that the adsorption of RhB on additive manufactured PVDF slabs was enhanced by the structure, higher specific surface ratio and higher volume of mesopores achieved through the 3D-printing process, as well as the minimisation of film resistance to mass transport due to ultrasound. Moreover, adsorption on additive manufactured PVDF enhanced the sonochemical degradation of the dye due to its high concentration in the adsorbed phase. This study demonstrates that adsorption processes, especially in the presence of PVDF materials, may be significantly more important in piezocatalysis than what has been reported to date, to the point that the synergistic combination of sono-adsorption and sonochemical degradation in presence of additive-manufactured PVDF slabs may be enough to achieve high removal rates of dyes in water

Effect of frequency and power on the piezocatalytic and sonochemical degradation of dyes in water

For the very first time, the effect of frequency on the piezocatalytic degradation of dyes has been systematically evaluated. To achieve this, a combination of systems and experimental setups operating at different ultrasonic frequencies ranging from 20 kHz up to 1 MHz were used. In addition, the effect of ultrasonic power was investigated at a low ultrasonic frequency of 20 kHz and higher ultrasonic frequency of 576 kHz to shed more light into the controversial discussion surrounding the ‘true’ mechanisms behind piezocatalysis. The results revealed that mechanical effects derived from acoustic cavitation, predominant at lower ultrasonic frequencies (100 kHz), the chemical effects derived from acoustic cavitation were so remarkable, that it raised the question of whether a piezocatalyst is really necessary when the optimisation of frequency and power may be enough for sonochemistry to fully degrade organic pollutants at a fast rate (pseudo first-order degradation reaction rate constant up to 0.037 min−1)