Person: SEMERCİ, NESLİHAN
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SEMERCİ
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NESLİHAN
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Publication Metadata only Modelling the relative toxicity of metals on respiration of nitrifiers using ion characteristics(TAYLOR & FRANCIS LTD, 2009) SEMERCİ, NESLİHAN; Sacan, M. T.; Cecen, F.; Erturk, M. D.; Semerci, N.The effects of eight transition metals were studied in a nitrifying system to investigate the relationship between the ionic characteristics of metals and their toxicity to nitrifiers. The cumulative oxygen consumption and the cumulative carbon dioxide production were monitored throughout each respirometric batch run to determine the toxicity of metals to nitrifiers. Several quantitative cationic-activity relationship (QCAR) models were developed on the basis of these different toxicity endpoints using quantum chemical descriptors. Descriptors were calculated with density functional theory (DFT) at the B3LYP/LANL2DZ level using the Gaussian 03W software. Additionally, the same descriptors were recalculated using another basis set, B3LYP/SDD, to test the impact of the basis set on prediction of toxicity. Of the calculated descriptors, mainly the gaseous phase descriptors explained significant variances in both toxicity endpoints. However, the energy of the polarized solute-solvent (EPSS) was the most important common descriptors in modelling labile toxicity. A combination of the aqueous phase energy of the highest occupied molecular orbital (EHOMO(aq)) and the maximum value for the energy of the lowest unoccupied molecular orbital of the most important metal species (ELUMOCmax) produced the best two-descriptor model for both pTO2 and pTCO2. The electron donor/acceptor ability of metals and the electron acceptor ability of metal species (ELUMOCmax) seemed to be important in explaining toxicity in aqueous media regardless of the measured endpoints for nitrifiers.Publication Metadata only Monitoring of population shifts in an enriched nitrifying system under gradually increased cadmium loading(ELSEVIER SCIENCE BV, 2008) ÇALLI, BARIŞ; Mertoglu, Bulent; Semerci, Neslihan; Guler, Nuray; Calli, Baris; Cecen, Ferhan; Saatc, Ahmet MeteThe changes in nitrifying bacterial population under cadmium loading were monitored and evaluated in a laboratory scale continuous-flow enriched nitrification system. For this purpose, the following molecular microbiological methods were used: slot-blot hybridization, denaturing gradient gel electrophoresis (DGGE), real-time PCR followed by melting curve analysis, cloning and sequence analysis. The initial cadmium concentration was incrementally increased from 1 to 10mg/l which led to a drop in ammonia removal efficiency from 99 to 10%. inhibition was recovered when cadmium loading was stopped. During the second application of cadmium. nitrifying population became more tolerant. Even at 15 mg/l Cd, only a minor inhibition was observed. To investigate the variations in ammonia and nitrite oxidizing bacteria populations in a period of 483 days, ammonia monooxygenase (amoA) and 16S rRNA genes-based molecular techniques were used. An obvious shift was experienced in the diversity of ammonia oxidizers after the first application of 10mg/l Cd. Metal-tolerant ammonia oxidizing species became dominant and the microbial diversity sharply shifted from Nitrosomonas and Nitrosococcus sp. to Nitrosospira sp. which were observed to tolerate higher cadmium loadings. This result indicated that the extent of nitrification inhibition was not only related to the metal concentration and quantity of microorganisms but also depended on the type of species. (C) 2008 Elsevier B.V. All rights reserved.Publication Metadata only Importance of cadmium speciation in nitrification inhibition(ELSEVIER, 2007) SEMERCİ, NESLİHAN; Semerci, Neslihan; Cecen, FerhanIn this study, the influence of Cd speciation on nitrification inhibition was investigated in batch suspended growth activated sludge systems which contain biomass enriched in terms of nitrifiers. For this purpose, parallel measurements of specific oxygen uptake rates (SOUR), ammonium utilization rates (q(NH4)-N) and Cd uptake were carried out. Cd speciation was adjusted with a strong complexing agent, ethylenediaminetetraacetic acid (EDTA). Free and biosorbed Cd concentrations were theoretically determined by using the MINEQL+ program and the Cd adsorption constant, whereas labile Cd was determined by voltammetric measurements. The presence of EDTA decreased nitrification inhibition by lowering the available Cd species and by preventing biosorption of Cd. Almost complete recovery from inhibition was attained by EDTA addition to nitrifying bacteria which were inhibited by Cd for a certain time. These results suggested that the sites sensitive to Cd were rather located on the surface of bacterial cell than inside. Nitrification inhibition depended on equilibrium concentrations of free (Cd2+), labile (Cd-volt) and biosorbed Cd (Cd-volt) and did not correlate with the total Cd. The measurement of labile metal by voltammetry in inhibition studies is a promising approach since it is easy to apply in practice. (C) 2007 Elsevier B.V. All rights reserved.Publication Metadata only Effect of continuous Cd feeding on the performance of a nitrification reactor(SPRINGER, 2009) SEMERCİ, NESLİHAN; Semerci, Neslihan; Cecen, FerhanThe inhibitory effect of Cd on nitrification was investigated in a continuous-flow system with enriched nitrifying bacteria. The maximum specific ammonium utilization rate and the half-saturation constant were found as 671 mg NH4-N/g VSS day and 0.48 mg/l, respectively. In the case of continuous Cd input at 1 and 2.5 mg/l, nitrification was inhibited by 30% and 47%, respectively. Inhibition ranged from 20% to 40% and no further increase in inhibition was exhibited in new runs except at 10 mg/l influent Cd. At 10 mg/l influent Cd, specific ammonium utilization and nitrate production rates were inhibited by 90%. On the contrary, a serious nitrite accumulation was not observed during this period. When Cd feeding was stopped, recovery from inhibition was observed after 37 day which was seen by the improvement in ammonium utilization and nitrate production rates. A shift in microbial population from the initial Nitrosomonas sp. to the Cd-tolerant Nitrosospira sp. was observed in the recovery period from severe Cd inhibition. After the domination of Nitrosospira species, redosing at 10 mg/l and then at 15 mg/l did not affect the performance as before.