The use of ozone and high silica zeolites to enhance refractory compounds removal

This work investigates the removal of organic pollutants from water through a three-phase process combining adsorption onto hydrophobic zeolites and in situ oxidation by ozone gas. Zeolites are aluminosilicates with various crystalline structures – and especially different pore sizes - which offer a great selectivity, profitable to organic pollutants separation in a complex matrix like natural water. They are highly resistant to chemical agents as acids, bases or oxidising agents such as ozone. Moreover, high silica zeolites have a high adsorption capacity for organics. Ozone is known to be a powerful oxidising agent able to react with various organic compounds. Its action – either direct or indirect - leads to the decomposition of organics into smaller molecules that are generally biodegradable. Previous studies have shown that organics adsorbed onto zeolites could be oxidised by ozonated water faster than in bulk water because of a micropore concentration effect (Fujita et al, 2004 and Sagehashi et al, 2005). In the gas phase, Monneyron et al (2003) showed that high silica zeolites could catalyse ozone decomposition into radical species and that adsorption properties were not significantly modified after exposition to ozone. Hence it is expected that zeolites saturated with organics could undergo regeneration by ozone without degradation of their structures or decrease of their adsorption capacities. The present work showed that high silica zeolites could efficiently adsorb nitrobenzene from water although their capacity stayed beyond that of activated carbon, except at low concentrations. The adsorption capacity depended on the zeolite structure and the Si/Al ratio. Adsorption isotherms could be well described by Langmuir or Freundlich models. As regards the three phase coupled process, the adsorbent could be efficiently regenerated during an ozonation step consisting in bubbling ozone through a suspension of saturated zeolite in a nitrobenzene solution at equilibrium. The initial adsorption capacity was quickly recovered and, continuing the treatment, the adsorption capacity of the zeolite was even increased. This may be due to the cleaning of zeolites pores by ozone (Pic et al, 2005). Yet traces of template molecules could probably remain from the zeolite synthesis process. Until now the catalytic effect of the zeolites has not been evidenced in the liquid phase. Therefore future work will focus on the conditions in which the oxidation kinetics can be enhanced in the presence of zeolites through a concentration effect, and to what extent

Adsorption of Nitrobenzene from Water onto High Silica Zeolites and Regeneration by Ozone

This work investigates the removal of nitrobenzene (NB), a model pollutant from water, by combining adsorption onto zeolites and regeneration with ozone. The adsorption equilibrium isotherms of NB onto zeolites enabled the best adsorbent to be selected and zeolites with a high Si/Al ratio were the most efficient. The adsorption capacity depended on the Si/Al ratio and on the pore size. In a sequential process coupling adsorption and oxidation by ozone, NB was completely removed from water and the initial adsorption capacity of the zeolite was totally restored. Although no catalytic effect was noticed, the adsorption produced locally high concentrations, thus enhancing the oxidation rate for NB

Oxidation of nitrobenzene by ozone in the presence of faujasite zeolite in a continuous flow gas–liquid–solid reactor

This work investigates the oxidation of nitrobenzene (NB) by ozone in the presence of faujasite zeolite. Experiments were carried out in a gas–liquid–solid reactor were ozone transfer and NB oxidation took place at the same time. Three configurations of the reactor were compared: empty, filled with inert glass beads and filled with faujasite pellets. First, ozone transfer coefficient (kLa) and decomposition rate constant (kC) were determined for each configuration. In presence of solid, kLa was 2.0 to 2.6 times higher and kC was 5.0 to 6.4 times higher compared to the empty reactor. Then, the various configurations were evaluated in terms of NB removal and chemical oxygen demand (COD) decrease. The faujasite reactor showed higher removal of NB and decrease of COD compared to other configurations under the same conditions suggesting that the faujasite increases the oxidation rate of NB. Oxidation of NB in presence of faujasite also proved to be limited by the transfer of ozone from the gas to the liquid phase

Hydrodynamic and sedimentary processes of tidal bores: Arcins Channel, Garonne River in August-September-October 2015

A tidal bore is a compressive wave of tidal origin, propagating upstream as the tidal flow turns to rising. It might be observed when a macro-tidal flood flow enters a funnel shaped river mouth with shallow waters. The occurrence of tidal bores has a significant impact on the natural systems, the bore propagation being associated with intense sediment scouring and suspension of bed materials. The tidal bore of the Garonne River was extensively investigated in the Arcins channel in 2010, 2012 and 2013, typically over one to two days. Herein new field measurements were repeated systematically at the same site on 29 August-1 September 2015 and on 27 October 2015. The nature of the observations was comprehensive, encompassing hydrodynamics and turbulence, sedimentology and transport. The aim of the study was to comprehend the temporal evolution of hydrodynamics and sediment processes in the Garonne River estuarine zone during a spring tide period. Instantaneous velocity measurements were performed continuously at high-frequency (200 Hz) prior to, during and after each afternoon tidal bore. Instantaneous sediment concentration and suspended sediment flux data were derived from careful calibration of acoustic backscatter and checked against water sample concentration. Between 29 August and 1 September, the sediment material characteristics showed some temporal trend: (a) the bed material granulometry data showed a progressively broader grain size distribution associated with some increase in median sediment size; and (b) the apparent yield stress and effective viscosity of bed materials increased over the first four days of tidal bore occurrence. The tidal bore occurrence had a marked effect on the velocity and suspended sediment field, including a rapid flow deceleration and flow reversal during the bore passage. The turbulent Reynolds stress data indicated large shear stresses, together with large and rapid fluctuations, during the bore passage and the early flood tide. A turbulent event analysis was further conducted in the highly-unsteady rapidly varied tidal bore flow. The suspended sediment concentration (SSC) data indicated a gradual increase in initial mean SSC estimate prior to the bore from 29 August to 1 September 2015, and very large SSCs during the passage of the tidal bore front and early flood tide. A comparison between suspended sediment flux data showed very significant suspended sediment flux on 29 August 2015, i.e. on the first day of tidal bore occurrence, with a decreasing magnitude over the next three days. The data suggested a two-stage bed scour process: at each tidal bore event, surface erosion occurred initially, in the form of stripping; the first stage was followed by delayed mass erosion, occurring about 5-15 minutes after the tidal bore. The sediment flux data yielded a mass transport of about 500 tonnes of sediments per second in the 70 m wide channel, in average for the first hour of flood tide. The present work culminates a 5-year research project at the same site, showing a progressive siltation of the channel, particularly during the last three years

Unsteady turbulence in a tidal bore: field measurements in the Garonne River in October 2013

A tidal bore is an unsteady rapidly-varied free-surface flow generated by the rapid rise in water elevation during the early flood tide, when the tidal range exceeds 4.5 to 6 m and the channel bathymetry amplifies the flood tidal wave. This study describes a detailed field investigation conducted in the Garonne River (France). The tidal bore was undular on 19 October 2013 and the bore front was followed by some well-defined whelps. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage. Altogether the investigation characterised some unusual turbulence transient in a large river system

Field Measurements in the tidal bore of the Garonne River after a recent flood

A tidal bore is a natural phenomenon associated with the rising flood tide. Composed of surface waves, it may occur in estuaries and propagate up rivers. The present study was conducted in the Garonne River (France) in the Arcins channel. Using an ADV unit and further recording equipments, experimental data are collected during a flat undular bore with a bore Froude number close to unity. Velocity analysis and sediment characterisation revel a slight rise in water elevation starting about 70 s prior to the front and a flow reversal about 50 s after the bore front. The turbulent transport of suspended sediment is presented in term of mass flux per unit area highlighting a negative (upriver) sediment mass transfer

Turbulence and turbulent events in tidal bores: field observations

A tidal bore is a compressive wave of tidal origin, propagating upstream as the tidal flow turns to rising when a macro-tidal flood flow enters a funnel shaped river mouth with shallow waters. New field measurements were conducted in the Garonne River tidal bore at Arcins in 2015. Instantaneous velocity measurements were performed continuously at high-frequency prior to, during and after each bore. The bore occurrence had a marked effect on the velocity and turbulent Reynolds stress field, with large and rapid fluctuations, during the bore passage and the early flood tide. A turbulent event analysis was conducted in the highly unsteady rapidly-varied tidal bore flow. The method detects bursting events by comparing the absolute value of an instantaneous turbulent flux with its standard deviation. This analysis, based upon basic concepts, was extended to the rapidly varied, highly-unsteady tidal bore flood flow motion. The turbulent event data analysis showed relatively close results for all studies and all fluxes. A very large majority of turbulent events had a duration less than 0.01 s, with on average 20 turbulent events per second. During all studies, the event duration showed some tidal trend, with longer turbulent events immediately after the tidal bore passage, occurring simultaneously with sediment erosion processes

Field Measurements of Unsteady Turbulence in a Tidal Bore: the Garonne River in October 2013

A tidal bore is an unsteady rapidly-varied open channel flow generated by the swift advance of the early flood tide in a funnel-shaped river estuary when the tidal range exceeds 4.5 to 6 m. This contribution presents a detailed field investigation conducted on the tidal bore of the Garonne River (France). The bore was undular and the bore's leading edge was followed by well-defined secondary waves, or whelps. The instantaneous ADV velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage. The investigation characterized some unusual transient turbulence caused by the bore propagation in a large river system, and the results suggested the advection of large-scale eddies in the wake of the bore front. The present study highlighted the need for detailed field measurements with fine temporal resolution, to characterize the highly unsteady rapidly-varied nature of tidal bore flows

Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision

A detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred. The measured bore, within the Arcins Channel of the Garonne River ( France), was undular in nature and was followed by well-defined secondary wave motion. Due to the local river geometry a collision between the Arcins channel tidal bore and the bore which formed within the main Garonne River channel was observed about 800m upstream of the sampling site. This bore collision generated a transient standing wave with a black water mixing zone. Following this collision the bore from the main Garonne River channel propagated 'backward' to the downstream end of the Arcins channel. Velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage, including during the 'backward' bore propagation. Large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion, consistent with the murky appearance of the flood tide waters. Copyright (C) 2015 John Wiley & Sons, Ltd