Effect of oxidant-to-substrate ratios on the degradation of MTBE with Fenton reagent.

Prior studies have shown the effectiveness of Fenton reagent (FR) for degrading low concentrations (1.0-2.0mg/L) of methyl tert-butyl ether (MTBE), similar to those found in contaminated groundwater. The present study investigates the effect of increasing FR doses on the extent of degradation and mineralization of a given initial MTBE concentration. The FR to MTBE molar ratio (FMMR) was the operating variable, and was investigated at values between 0.5:1 and 200:1. This approach provided sequential snapshots of the MTBE degradation process, which may help to improve the understanding of MTBE degradation with FR. The initial MTBE concentration (MTBE(0)) was approximately 22.7 microM ( approximately 2.0mg/L), and FR was used in a 1:1 molar ratio of ferrous iron (Fe(2+)) and hydrogen peroxide (H(2)O(2)). The concentrations of MTBE and the following main reaction byproducts: tert-butyl formate (TBF), tert-butyl alcohol (TBA), acetone and methyl acetate were determined from samples collected at specific intervals over a total reaction time of 1h. Total organic carbon (TOC) was monitored to determine MTBE mineralization, and the total concentration of tert-butyl compounds (tBC) was monitored due to the suspected toxicity associated with this functional group. The results showed that the minimum FMMR necessary for achieving complete MTBE degradation was 20:1, but at that FMMR, TOC and tBC reduction were only 45.6% and 24.9%, respectively. Complete MTBE mineralization was not achieved in any case, even at FMMRs as high as 200:1, where only 63.3% of mineralization was observed (although tBC reduction reached 99.6%, since traces of TBA were still detected). These results confirm the relative inability of FR to achieve complete mineralization of a target substrate, even those that are highly reactive with the hydroxyl radical.

Characterizing the passage of personal care products through wastewater treatment processes.

Wastewater treatment facilities use secondary treatment to stabilize the effect of discharged effluent on receiving waters by oxidizing biodegradable organic matter and reducing suspended solids and nutrients. The process was never specifically intended to remove trace quantities of xenobiotics, such as endocrine-disrupting compounds (EDCs) and pharmaceuticals and personal care products (PPCPs). Nevertheless, European studies performed at bench-scale or at small facilities have demonstrated that a critical minimum solids retention time (SRT) can achieve good reduction of many EDCs and pharmaceuticals. The objective of this study was to expand these findings to the removal performance for 20 PPCPs commonly found in the influent to full-scale facilities operating in the United States. The participating plants use SRT conditions ranging from 0.5 to 30 days and include facility capacities ranging from 19 000 m3/d (5 mgd) to greater than 1 136 000 m3/d (300 mgd). Two pilot membrane bioreactors were also included in the study. The 20 PPCPs were categorized into nine bin combinations of occurrence frequency and treatment reduction performance. While most compounds were well removed, galaxolide (a musk fragrance) occurred frequently and was resistant to removal. A minimum critical SRT, defined as the minimum SRT, needed to consistently demonstrate greater than 80% removal (SRT80), was compound-dependent, with most compounds removed at 5 to 15 days and a small group requiring longer SRTs. From limited data, no additional removal could be attributed to the use of membrane bioreactors, media filters, or longer hydraulic retention times. Reverse osmosis was effective in removing any remaining compounds.

Oxidation kinetics and effect of pH on the degradation of MTBE with Fenton reagent.

The fundamentals of the Fenton reagent-based degradation of low concentrations of methyl tert-butyl ether (MTBE) in batch reactors under initially anaerobic conditions are discussed in this work. The objective of the study was to quantitatively verify the feasibility of MTBE degradation with Fenton reagent under such conditions. The conclusions may be potentially helpful to further develop an effective in situ treatment of MTBE-contaminated groundwater. Initial MTBE concentrations ([MTBE](0)) of 11.4 and 22.7 microM (approximately 1.0 and 2.0 mg/L, respectively) were treated with Fenton reagent (FR) using a [FR](0):[MTBE](0) molar ratio of 10:1. FR was used in equimolar mixture of Fe(2+)and H(2)O(2) (i.e., [Fe(2+)](0):[H(2)O(2)](0)=1:1). This analysis considers the hydroxyl radicals (OH(*)) produced by FR as the main species responsible for the degradation processes. The effects of [MTBE](0) and pH on the oxidation kinetics were investigated. Under these conditions it was observed that: (i) MTBE was degraded at high extent (90-99%) after 1h of reaction time, (ii) MTBE mineralization was low in all cases and reached only 31.7% at the best conditions, and (iii) In all cases, most of MTBE degradation occurred during the initial 3-5 min of reaction. During this brief initial phase, MTBE transformation followed pseudo-first order kinetics, while the subsequent phase exhibited a sharp drop in degradation rate and had almost negligible contribution to the overall degradation. Experiments performed at acidic pH exhibited the best degradation results, while at neutral pH the degradation rates dropped significantly. Other parameters included in this analysis were: TOC reduction and total concentration of compounds containing the tert-butyl group in their structure (tBC). These compounds were analyzed because of the concerns related to their potential toxicity. Tert-butyl formate (TBF), Tert-butyl alcohol (TBA), acetone and methyl acetate were identified and quantified as major reaction intermediates.

Effect of oxygen in a thin-film rotating disk photocatalytic reactor.

A novel experimental procedure was developed to measure oxygen mass transfer during the oxygenation of water in a thin film of a rotating disk photocatalytic reactor (RDPR). The increase in dissolved oxygen (DO) of initially deaerated water was monitored with time in the reactor vessel at different disk angular velocities after exposure of the reactor to the atmosphere. Oxygenation was predominantly achieved by oxygen mass transport through the thin liquid film carried by the disk and to a much lesser extent by direct oxygenation of the water in the reactor vessel via a surface renewal mechanism. A mathematical model was developed to simulate the phenomenon considering both cases of presence and absence of oxygen mass transport limitations. In the latter case, the model considered that the amount of liquid carried by the disk was saturated with oxygen when returning to the reactor vessel. On the basis of the model and the experimental data, it was proven that mass-transfer limitations existed until the water in the reactor vessel became saturated with oxygen. Results obtained from the model were validated by an alternative analysis using dimensionless groups characteristic to the system. The study revealed that the mass-transfer coefficient increased linearly with disk angular velocity and thus disk Reynolds number. The results showed that oxygen mass-transfer limitations decreased with increasing disk angular velocity, mainly due to an increase in the overall mass-transfer coefficient. In the presence of UV radiation, the influence of oxygen on the photocatalytic oxidation of 4-chlorobenzoic acid was investigated in the RDPR operated in batch and continuous mode. The photocatalytic reactions occurred in a thin film of liquid carried by the disk in the presence of UV radiation and ST-B01 composite spherical ceramic (SiO2/Al2O3) balls coated with anatase TiO2 catalyst. It was found that the initial degradation rate followed Langmuir kinetics with respect to oxygen concentration in the gas phase. When the oxygen concentration in the gas phase surpassed that in air, the degradation rates did not improve significantly, suggesting that operation with air instead of oxygen is most probably a more realistic practical choice. Measurements of DO during the presence and absence of UV radiation suggested that the photocatalytic reactions were mainly oxygen concentration-limited rather than oxygen mass-transfer-limited.

Placenta previa casuística en el Hospital Gineco-Obstétrico Isidro Ayora

La etiología de la placenta previa es desconocida, se presume que la vascularización endometrial disminuida a nivel del fondo y cuerpo uterino, constituyen el factor causal. Hemos analizado 149 historias clínicas, concluyendo que en nuestro medio los factores de riesgo son la edad materna entre 23 a 32 años con una media de 30, multiparidad, teniendo mayor riesgo a una edad gestacional de 37 a 40 semanas con un pico a las 38 semanas, el diagnóstico ecográfico es una gran ayuda pero vemos que en nuestro medio la sospecha clínica puede ser el único examen con el que podemos contar preoperatoriamente en no pocas ocasiones; la presencia o ausencia de dolor no es una sintomatología que nos pueda ayudar a precisar el diagnóstico...