Treatment of persistent organic pollutants in wastewater using hydrodynamic cavitation in synergy with advanced oxidation process

Persistent organic pollutants (POPs) are very tenacious wastewater contaminants. The consequences of their existence have been acknowledged for negatively affecting the ecosystem with specific impact upon endocrine disruption and hormonal diseases in humans. Their recalcitrance and circumvention of nearly all the known wastewater treatment procedures are also well documented. The reported successes of POPs treatment using various advanced technologies are not without setbacks such as low degradation efficiency, generation of toxic intermediates, massive sludge production, and high energy expenditure and operational cost. However, advanced oxidation processes (AOPs) have recently recorded successes in the treatment of POPs in wastewater. AOPs are technologies which involve the generation of OH radicals for the purpose of oxidising recalcitrant organic contaminants to their inert end products. This review provides information on the existence of POPs and their effects on humans. Besides, the merits and demerits of various advanced treatment technologies as well as the synergistic efficiency of combined AOPs in the treatment of wastewater containing POPs was reported. A concise review of recently published studies on successful treatment of POPs in wastewater using hydrodynamic cavitation technology in combination with other advanced oxidation processes is presented with the highlight of direction for future research focus

Chromia and chromium sulfide pillared clays differing in pillar density

Chromia pillared clays differing in pillar density were synthesized from the Li-form of montmorillonite calcined in a controlled manner. These materials were characterized by XRD, TGA, AAS, nitrogen adsorption and their thiophene hydrodesulfurization activity. It is found that clays varying in cation exchange capacity pillared with Cr-12 pillar precursor ions can provide a family of catalysts that differ in surface area, porous structure and concentration of active sites. Sulfidation of the heat-treated chromia pillared clay resulted in the formation of chromium sulfide pillared derivatives with high surface area and well-developed porous structure. The activities for thiophene hydrodesulfurization and consecutive butene hydrogenation over chromium sulfide pillared montmorillonite are highest on clays with the largest pillar concentration and correlated with the total surface area. However, the activity per number of pillars increased with decreasing pillar density. Thiophene HDS predominantly results in the formation of butane and butenes, irrespective of the pillar population. The rate of deactivation is somewhat larger for catalysts that have a larger lateral pillar spacing. The chromium sulfide pillared clays exhibit a relatively high stability in thiophene HDS compared to zeolitic catalysts which is most likely due to their low protonic acidit

The application of non-hydrothermally prepared stevensites as support for hydrodesulfurization catalysts

The stevensite-like materials containing Mg, Mg-Ni, and Mg-Co in the octahedral sheets were synthesized under non-hydrothermal conditions. The nature of divalent octahedral cations influences the surface area and pore volume of these materials. The thiophene HDS activities of stevensite-supported catalysts prepared by addition of Mo or W to the Ni-containing supports, especially to the Ni2+-exchanged stevensite, were superior in relation to those of their counterparts made by conventional co-impregnation of the Mg-stevensite with NiMo and NiW.The use of chelating agents, NTA and EN, affects beneficially the HDS activity of the catalyst which, however, is less obvious in the case of the CoMo catalysts. The positive role of the chelating agents is explained by weakening of the Ni(Co)-support interaction and by a change in the sulfidation sequence of Ni(Co) and Mo(W).The Ni2+- and Co2+-exchanged Mg-stevensite or that isomorphously substituted with these cations are promising candidates for application as supports for hydro-processing catalysts. The thiophene HDS activity of the NiMo and NiW stevensite-supported catalysts is comparable with that of the corresponding gamma-Al2O3 commercial one

Effect of high-temperature treatment on Fe/ZSM-5 prepared by chemical vapor deposition of FeCl3. I. Physicochemical characterization

The effect of severe (hydrothermal) treatment on Fe/ZSM-5 prepared by sublimation of FeCl3 is studied by a combination of high-resolution TEM, EXAFS, 57Fe Mössbauer spectroscopy, IR, UV–vis, nitrogen adsorption, 27Al NMR spectroscopy, and low-temperature nitrous oxide decomposition. The heterogeneous nature of Fe/ZSM-5 is stressed with a preponderance of iron oxide particles on the external zeolite surface. Additionally, neutral iron oxide nanoparticles and charge-compensating iron complexes are located in the micropores. Severe calcination at 973 K induces the growth and ordering of the iron oxide aggregates. Moreover, some of the occluded neutral iron oxide nanoparticles are transformed into charge-compensating iron complexes upon a protolysis reaction with the Brønsted protons. These effects are more pronounced in the case of steaming at 973 K, additionally resulting in the removal of Al from framework positions. Despite the low dispersion of iron oxide in Fe/ZSM-5, relatively low Fe---Fe coordination numbers were derived from the EXAFS data for Fe/ZSM-5; high-temperature treatments increased this number. This low value points to the disordered nature of the iron oxide aggregates rather than to the presence of an abundant fraction of binuclear iron clusters. Titration of sites active in nitrous oxide decomposition shows that their amount increases upon increasing severity of treatment of Fe/ZSM-5. Their number, however, remains very small (a few percent of the total iron) and appears to correlate to the amount of Fe2+ present after room temperature exposure to vacuum conditions. A comparison to a commercial HZSM-5 zeolite with a very low iron content is made. The catalytic performance of these materials is discussed in a companion paper (J. Catal. (2003))