Roumen Zlatev

Corrosion and scaling at Cerro Prieto geothermal field

Purpose – The aim of this work is to study the corrosion and scaling factors, mechanisms and processes affecting the materials, equipment and installations of the Cerro Prieto geothermal field (GTF).Design/methodology/approach – The physicochemical characteristics of the geothermal well and fluids were analysed, recorded and related to the corrosion and scaling phenomena.Findings – The high temperature and salinity of the steam‐brine mixture and the presence of hydrogen sulphide and carbon dioxide impart a severe level of corrosivity.Originality/value – Corrosion and scaling control assure an efficient production regime, provide for the durability of the GTF engineering materials and equipment and contribute to environmental quality.

Anticorrosion behavior of conversion coatings obtained from unbuffered cerium salts solutions on AA6061-T6

Abstract The anticorrosive properties of cerium based conversion coatings deposited on AA6061-T6 alloy by immersion in unbuffered cerium chloride and cerium nitrate solutions in the presence of hydrogen peroxide were investigated and characterized by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 0.5 mol/L NaCl aqueous solution. The microstructure and chemical composition of the protective films were examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). It was found that the best corrosion protection was afforded by the samples treated during 600 s in cerium chloride solution at pH values ∼5.5-4, showing higher amounts of cerium and polarization resistance values greater than 10 Ω m2. Moreover, an ennoblement of the corrosion potential and decreasing of the cathodic and anodic currents were obtained compared with the cerium nitrate solutions application. This behavior was attributed to the influence of the deposition parameters such as type of the salt anion, i.e., chelating effect and chaotropic characteristics, pH fluctuations in the conversion solution and deposition time.

Some Clinical Applications of the Electrochemical Biosensors

Electrochemical biosensing, due to its sensitivity and specificity, combined with the low-cost and operation convenience of the equipment, is considered as a promising point-of-care approach in clinical analysis. This review presents the basic principles of operation, the current status, and the trends in the development and the clinical implementation of some selected electrochemical biosensors. These include: electrochemical glucose biosensors successfully applied in diabetes management, and electrochemical biosensors for cholinesterases and trypsin activities determination. The latter, although less common, demonstrate the potential of improving the existing clinical methods in the diagnostics and the treatment of neurotoxic, neurological, and pancreatic diseases.

Lipase Production Through Solid-State Fermentation using Agro-Industrial Residues as Substrates and Newly Isolated Fungal Strains

ABSTRACT Extracellular lipase production by Penicillium chrysogenum, Trichoderma harzianum and Aspergillus flavus was carried out through solid state fermentation using agro-industrial residues as substrates. For all three strains, the growth temperature was 29±1 °C, and 65% w (g/gds) moisture content. The effect of three factors on lipase production rate was investigated: initial pH (6.0 and 7.0), time of fermentation (72 h, 96 h and 120 h), and type of mixed substrate (wheat bran-olive oil, and wheat bran-castor oil cake). The process was optimized applying a mixed level factorial design. Fermentation time and pH were found to have positive effects on lipase production and secretion rates. However, the time effect was larger than initial pH. Type of substrate demonstrated minor effective importance than the other two factors, and Aspergillus flavus showed the larger lipase production among the three strains. Results indicated that the three fungal strains were able to grow and produce lipase in both culture mediums. The maximum lipase activity achieved was 121.35 U/gds by Aspergillus flavus, which was five and nine times the lipase produced by Trichoderma harzianum and P. chrysogenum respectively, at the same conditions. An initial neutral pH and 96 h of fermentation time were the optimum conditions for lipase production by Aspergillus flavus.

Effect of NO2 - in the Corrosion Behavior of Cerium Based Conversion Coatings on AA6061-T6

addition to the cerium chloride and hydrogen peroxide solution improved the corrosion resistance and coating morphology. Electrochemical measurements were performed by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) in 0.01 M NaCl aqueous solution and the coating morphology was analyzed by Atomic Force Microscopy (AFM).

In vitro Actinomyces israelii biofilm development on IUD copper surfaces.

BACKGROUND Female pelvic actinomycosis may involve fallopian tubes, ovaries, uterus and bladder. This condition is often associated with the use of intrauterine contraceptive devices (IUDs), vaginal pessaries and/or tampons. The predominant causative agent of human actinomycosis is Actinomyces israelii, which has been found on copper IUDs retrieved from patients. STUDY DESIGN In this work, a biofilm of A. israelii was developed in vitro on copper surfaces immersed in a simulated uterine fluid under anaerobic conditions. The biofilm was characterized using scanning electron microscopy (SEM), energy dispersive X-ray and atomic force microscopy. RESULTS The capacity of A. israelii to develop a biofilm over copper surfaces in synthetic media was demonstrated. SEM micrographies illustrate the exopolysaccharides production and bacterial distribution. CONCLUSION A. israelii was able to attach and grow in synthetic intrauterine media and to present on the copper surface is likely due to the production of biofilm.

Leptospirillum ferrooxidans based Fe2+ sensor.

A novel electrochemical biosensor integrating the strictly autotrophic bacterial strain Leptospirillum ferrooxidans as a recognition element and a Clark type oxygen probe as a transducer was designed, metrologically and analytically characterized and applied for the specific Fe(2+) determination. The bacterial Fe(2+) oxidation involves O(2) consumption, thus the quantification was performed registering the decrease of the oxygen reduction current. The limit of detection was found to be 2.4 micromol L(-1) and the sensitivity of the determinations-3.94 nAL micromol(-1). The response time of the biosensor is 18s for Fe(2+) concentrations of 10(-5) to 10(-4) mol L(-1). The biosensor was applied as well for the indirect determination of Fe(2+) oxidizing species such as Cr(2)O(7)(2-), reaching a sensitivity of 2.47 nAL micromol(-1). The transducer characteristics were evaluated and optimized to obtain short response time and high sensitivity. The analytical performances of the biosensor subject of the present work were found to be similar to that of the At. ferrooxidans based one developed by the authors earlier, avoiding however the sulfur compounds interference, because of the substrate specificity of the applied bacterial strain.

Organophosphorus Pesticides Determination by Electrochemical Biosensors

According to the definition given by the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA, 2008), a pesticide is any substance or mixture of substances intended for preventing, destroying, repelling or mitigating any pest (insects, mice and other animals, unwanted plants, fungi, microorganisms such as bacteria and viruses, and prions). Considering their chemical structure, the pesticides are organophosphorus, carbamates, organochlorines, and pyrethroid ones (U. S. EPA, 2009). Currently, more than 30 % of the registered pesticides in the world market (Hill, 2003) and about 45 % of those registered with U. S. Environmental Protection Agency (EPA) (Roger & Dagnac., 2006) are organophosphorus (OP). The organophosphorus pesticides, because of their high toxicity, fast biodegradation, low bioaccumulation, and broad target spectrum are extensively used in the agricultural and veterinary practices for protection of field and fruit crops, and for parasites control in domestic animals. However, their intensive and indiscriminate application, as well as their high acute toxicity generated risks to man and his environment. The resulting public concern created a demand for the development of reliable, sensitive, simple and low-costing methods for their fast “ in field” detection. In this work are reviewed the principles of the emerging electrochemical biosensors based methods for organophosphorus pesticides determination during the last decades, as methods of choice for “in situ” and “on line” application. Two main analytical techniques are considered, involving respectively the direct enzyme transformation of the pesticide and its enzyme activity inhibition effect, both followed by the conversion of the signal produced by the interaction between the bioreceptor and the analyte, into electrical one. The advantages and the drawbacks of each of them are discussed. The recent trends in the development of electrochemical biosensors for OP pesticides quantification, including nanomaterials transducer modification and genetic engineering of the biological recognition element are revised. Special attention is paid to the electrochemical biosensors based methods application for OP pesticides residues detection in food and in the environment.

An amperometric microbial biosensor for the determination of vitamin B12

In this study we describe a sensitive amperometric microbial biosensor that is fast, economic, reliable, and can compete with the existing proposed methods for vitamin B12 determination. Taking advantage of the bacterial strain Tetrasphaera duodecadis which oxidizes vitamin B12 with oxygen consumption, we shaped a promising alternative tool for the direct and specific determination of vitamin B12 in different samples without pre-treatment. For this purpose, a vitamin B12 amperometric microbial biosensor was constructed based on one-step immobilization of the bacterium by filtration of a concentrated bacterial mass through a 0.15 μm pore size cellulose filter and fixed on a Clark-type oxygen probe, serving as a transducer, and exploiting the described processes. The results obtained indicate a sensitive capability with a linear sensor concentration range from 10−7 mol L−1 to 10−5 mol L−1 and the response time of about 700 seconds at 10−6 mol L−1. Furthermore, the Tetrasphaera duodecadis membrane attached to the Clark type oxygen probe has an estimated 1 month lifetime at room temperature. In addition, the developed prototype allowed the assessment of B12 status directly in samples prepared for that purpose. The results were well correlated with those obtained with commercial samples, thus demonstrating that the proposed microbial sensor offers an accurate and useful analytical tool that can be easily applied to prevent diseases caused by the lack of vitamin B12.

Rapid method for corrosion protection determination of VCI films

Purpose – To develop and test a rapid method for evaluation of the corrosion protection (CP) of carbon steel (CS) by vapour corrosion inhibitors (VCI) films.Design/methodology/approach – The determination of the CP by VCI on CSs is commonly carried out in a chamber applying neutral salt spray (NSS) and usually it takes many days. The common disadvantage of the various rapid methods created until now is the need of special laboratory equipment making their application complicated and inconvenient for field tests. The method for CP measurement of VCI films on CS described in this study is based on measurement of the height of the anodic peak under galvanostatic condition applied earlier on other types of films and coatings. By means of a calibration plot: peak height (V) vs NSS protection time (s), CP of VCI films on CS specimens expressed in hours NSS is determined in a few seconds without using an NSS chamber. Measurements of CP by VCI films under field conditions are performed applying a hand held tester...