Dimitrios Berk

Photo-removal of sulfamethoxazole (SMX) by photolytic and photocatalytic processes in a batch reactor under UV-C radiation (λmax=254 nm).

In this study, photolytic and photocatalytic removal of the antibiotic sulfamethoxazole (SMX) under UVC radiation (λ=254 nm) was investigated. The light intensity distribution inside the batch photoreactor was characterized by azoxybenzene actinometry. The intensity of incident radiation was found to be a strong function of position inside the reactor. 12 mg L(-1) of SMX was completely removed within 10 min of irradiation under UVC photolysis, compared to 30 min under TiO(2) photocatalysis. COD measurement was used as an indication of the mineralization efficiency of both processes and higher COD removal with photocatalysis was shown. After 6h of reaction with photolysis and photocatalysis, 24% and 87% removal of COD was observed, respectively. Two of the intermediate photo-products were identified as sulfanilic acid and 3-amino-5-methylisoxazole by direct comparison of the HPLC chromatograms of standards to those of treated solutions. Ecotoxicity of treated and untreated solutions of SMX towards Daphnia magna was also investigated. It was found that a 3:1 ratio of sample to standard freshwater and a high initial concentration of 60 mg L(-1) of SMX were used to obtain reliable and reproducible results. The photo-products formed during photocatalytic and photolytic processes were shown to be generally more toxic than the parent compound.

In situ characterization of nitrifying biofilm: minimizing biomass loss and preserving perspective.

Methods for characterizing nitrifying bacteria within biofilms are of key importance to understand and optimize the nitrification kinetics of attached growth treatment facilities. In this work, we propose an analytical protocol based upon environmental scanning electron microscopy (ESEM) and confocal laser scanning microscopy (CSLM) in combination with fluorescent in situ hybridization (FISH) to characterize the structure of nitrifying biofilm as it remains attached to the original reactor substratum. This protocol minimizes the loss of mass and distortion of in situ perspective commonly associated with traditionally applied microscopic techniques and thereby enables a more accurate estimation of the nitrifying biomass within biofilm attached to the substratum. The use of ESEM eliminates the destructive preparatory procedures associated with traditional scanning electron microscopy and thus the loss of mass and shrinking of the samples. ESEM is used in this study to evaluate the percent coverage of the substratum with biofilm and the biofilm thickness. CLSM-FISH is used to determine cell counts in the biofilm and to characterize the undisturbed substratum/biofilm interface. By hybridizing and analyzing the nitrifying biofilm using CLSM as it remains attached to the substratum, the loss of material and distortion of in situ perspective associated with the biofilm detachment process is minimized. Moreover, by conducting the CLSM analysis directly on the nitrifying biofilm as it remains attached to the substratum it is shown that cell counts at the substratum/biofilm interface differ significantly from that located above the interface.

Rapid and reliable quantification of biofilm weight and nitrogen content of biofilm attached to polystyrene beads

Increased popularity of attached-growth wastewater treatment systems (e.g. biological aerated filtration processes-BAF) has created the need for a rapid and reliable method of characterizing biofilms. In addition to the mass of the biofilm that may serve as a control parameter for attached-growth treatment systems, the nitrogen content of the biofilm is also of great interest with increasingly strict nitrogen removal guidelines. Existing methods that may be used to analyse biofilms in such processes involve complex sample preparation and microbiological expertise that limit their application in many biofilm wastewater treatment studies and at existing treatment facilities as a feasible method of monitoring the biofilm. This paper describes a simple technical procedure that enables biofilm samples attached to polystyrene beads to be characterized in terms of the biofilm mass and the nitrogen content of the biofilm. The proposed protocol incorporates an agitation procedure that demonstrates 99.9% removal of the biofilm from polystyrene beads; a modified TSS procedure that measures the removed biofilm mass; and subsequently a modified total Kjeldahl nitrogen (TKN) procedure that enables the nitrogen content of the biofilm to be measured directly on the filter. Moreover, this protocol allows numerous beads to be analysed with limited manipulation and without the loss of critical mass.

Kinetic analysis of attached growth nitrification in cold climates

The rate of nitrification within a laboratory-scale Biological Aerated Filtration treatment system at 4 degrees C was investigated during an exposure time of approximately four months (acclimatized experiments). In addition, shock experiments from 20 degrees C to 4 degrees C and from 4 degrees C to 20 degrees C were performed. The acclimatized experiments demonstrated that the exposure time the system remained at low temperature strongly affects the rates of nitrification. Nevertheless, the experiments showed that significant nitrification rates are maintained for up to 115 days at 4 degrees C. The rate of ammonia removal after an exposure time of 115 days at 4 degrees C was shown to be as high as 16% of the rate of removal observed at 20 degrees C. The 20 degrees C to 4 degrees C shock experiment demonstrated a 56% decrease in the rate of ammonia removal. On the other hand, the 4 degrees C to 20 degrees C shock experiment demonstrated an increase in the relative rates of ammonia removal of up to 300% when compared to rates of removal measured after 115 days at 4 degrees C. Thus, although the rates of nitrification have been shown to decrease significantly as a function of exposure time at 4 degrees C, the process has demonstrated important rates of ammonia removal at 4 degrees C for the approximate span of the North American winter.

REDUCTION OF SULPHUR DIOXIDE BY METHANE OVER TRANSITION METAL OXIDE CATALYSTS

The reduction of sulfur dioxide by methane for the production of elemental sulfur using two hydrodesulfuriz-ation catalysts was studied. Oxides of cobalt-molybdenum and molybdenum alone supported on γ-alumina were employed. The reactions were carried out in a quartz reactor at 650°C and 750°C and at molar feed ratios (SO2/CH4 varying between 0.50 to 2.50. The physicochemical stale of the catalysts was examined by SEM, BET and X-ray diffraction. Molybdenum oxide was converted to sulphide after the chemical reaction. The cobalt molybdenum catalyst was found to be the more active of the two; however, the molybdenum catalyst exhibited higher elemental sulphur yield (99%). Side reactions resulted in the formation of H2S, COS, CO, and elemental carbon. The production of these is minimized by operating at molar feed ratios greater than 1 and low temperature. The production of H2S is favoured over COS below 700°C.

Measurement of residence time distribution by laser absorption spectroscopy

The residence time distribution (RTD) was obtained in a tubular reactor with mean residence times of the order of milliseconds by using an infrared laser absorption technique. The reactor was operated at room conditions with a nitrogen or helium gas flow, the tracer used was methane. The fast response of the measuring device allowed a qualitative analysis of the micromixing in the flow. An axial dispersion model was used to describe the RTD of the reactor. It was found that the values for the dispersion coefficient were strongly influenced by the radial velocity profile in the reactor.

Nitrogen doping of graphene nanoflakes by thermal plasma as catalyst for oxygen reduction in Proton Exchange Membrane fuel cells

Proton Exchange Membrane fuel cells (PEMFCs) have two major hurdles to overcome before they may be commercially viable: cost and operating life. Platinum (Pt) catalyst represents the bulk of the PEMFC cost and is in finite supply, but functionalized carbon nanomaterials have been identified as potential replacements for Pt cathode catalyst. This works objective is to produce graphene nanoflakes (GNFs) and dope them with nitrogen in pyridinic sites through a second treatment step. An inductively-coupled thermal plasma (ICP) is used to dissociate methane at very high temperatures, with GNF nucleation commencing shortly after by way of rapid quenching. In a post-treatment, nitrogen doping occurs by manipulating plasma conditions and nitrogen precursor selection. Nitrogen doping up to 33.4 at.% has been demonstrated which, to our knowledge, more than doubles the largest reported amount.

Analysis and modelling of the isothermal and non-isothermal shrinking core model with non-linear kinetics

Abstract The shrinking core model including the effect of transient diffusion, non-linear kinetics and non-isothermal effects was solved using finite elements in space and finite differences in time. The solution for the first order isothermal case agreed with the pseudo steady state solution within the conditions defined by Bischoff. A high dimensionless bulk concentration, the solution differed from the pseudo steady state especially at high Damkohler number (Da > 10) where diffusion was dominating. In the non-linear kinetics case, the effect of the reaction order decreased with increasing Damkohler number. The critical parameters such as the dimensionless heat of reaction above which non-isothermal kinetic effects dominate were also determined.

Two-Dimensional Geometry Control of Graphene Nanoflakes Produced by Thermal Plasma for Catalyst Applications

The control of nanoparticle synthesis using thermal plasmas is difficult and often leads to problems of chemical and structural purity, and poor process robustness in terms of consistency of product from run to run. Good reactor design allowed to overcome these issues and to develop a new material based on graphene with a flake-like structure (labeled graphene nanoflakes, GNF) supporting nitrogen for catalytic applications, for example as platinum replacement in fuel cells. These structures showed not only to be active, but also stable in polymer electrolyte fuel cell operation. Characterization of these structures, in situ fuel cell studies and modeling analysis all indicate that achievement of stability relates on the crystalline two-dimensional graphene structure. This paper first reviews the basic aspects behind the structural objectives, describes the synthesis process design leading to this crystalline structure, and provides a two-dimensional analysis on the graphitic growth based on fundamental theory and CFD calculations. These calculations indicate that an independent control of the graphene structure thickness (number of atomic planes) and sheet lengths is possible in a thermal plasma reactor.

Nonintrusive fast response concentration measurement device for chemical reactors

A device is proposed by which the residence time distribution in a tubular chemical reactor can be measured with a response time on the order of hundreds of microseconds. This device works on the principle of infrared laser absorption spectroscopy. Because the light beam passes directly through the reactor cross section, there is no measurement cell thus making the technique nonintrusive. The device has been successfully tested with a tubular reactor with a gaseous flow at average residence times ranging from 0.04 to 0.7 s. The level of micromixing in the reactor can also be estimated due to the very fast response of the instrument.