J. Restivo

Catalytic performance of heteroatom-modified carbon nanotubes in advanced oxidation processes

Abstract Multi-walled carbon nanotubes (CNTs) were submitted to chemical and thermal treatments in order to incorporate different heteroatoms on the surface. O-, S- and N-containing groups were successfully introduced onto the CNTs without significant changes of the textural properties. The catalytic activity of these heteroatom-modified CNTs was studied in two liquid phase oxidation processes: catalytic ozonation and catalytic wet air oxidation (CWAO), using oxalic acid and phenol as model compounds. In both cases, the presence of strongly acidic O-containing groups was found to decrease the catalytic activity of the CNTs. On the other hand, the introduction of S species (mainly sulfonic acids) enhanced the removal rate of the model compounds, particularly in the CWAO of phenol. Additional experiments were performed with a radical scavenger and sodium persulfate, in order to clarify the reaction mechanism. Nitrogen functionalities improve the catalytic performance of the original CNTs, regardless of the process or of the pollutant.

Catalytic ozonation of metolachlor under continuous operation using nanocarbon materials grown on a ceramic monolith.

The catalytic ozonation of the herbicide metolachlor (MTLC) was tested using carbon nanomaterials as catalysts. Multiwalled carbon nanotubes were used in semi-batch experiments and carbon nanofibres grown on a honeycomb cordierite monolith were tested in continuous experiments. The application of the carbon catalyst was shown to improve the mineralization degree of MTLC and to decrease the toxicity of the solution subject to ozonation. Degradation by-products were also followed in order to compare the two processes. The application of the carbon coated monolith to the continuous ozonation process was shown to have potential as it improved the TOC removal from 5% to 35% and decreased the inhibition of luminescent activity of Vibrio Fischeri from 25% to 12%.

Catalytic ozonation of oxalic acid using carbon nanofibres on macrostructured supports

Carbon nanofibres (CNFs) were grown on different macrostructured supports such as cordierite monoliths, carbon felts and sintered metal fibres. The resulting composites exhibited excellent resistance to attrition/corrosion and its porosity is mainly due to mesoporous structures. The CNF/structured materials were tested in the ozonation of oxalic acid in a conventional semi-batch reactor after being crushed to powder form, and in a newly designed reactor that may operate in semi-batch or continuous operation. The CNFs supported on the different structured materials exhibited high catalytic activity in the mineralization of oxalic acid.

Process design for wastewater treatment: catalytic ozonation of organic pollutants

Emerging micropollutants have been recently the target of interest for their potential harmful effects in the environment and their resistance to conventional water treatments. Catalytic ozonation is an advanced oxidation process consisting of the formation of highly reactive radicals from the decomposition of ozone promoted by a catalyst. Nanocarbon materials have been shown to be effective catalysts for this process, either in powder form or grown on the surface of a monolithic structure. In this work, carbon nanofibers grown on the surface of a cordierite honeycomb monolith are tested as catalyst for the ozonation of five selected micropollutants: atrazine (ATZ), bezafibrate, erythromycin, metolachlor, and nonylphenol. The process is tested both in laboratorial and real conditions. Later on, ATZ was selected as a target pollutant to further investigate the role of the catalytic material. It is shown that the inclusion of a catalyst improves the mineralization degree compared to single ozonation.