S.O. Pehkonen

The Degradation of Organophosphorus Pesticides in Natural Waters: A Critical Review

Organophosphorus pesticides (OPs) have been widely used throughout the world since the decline in the use of organochlorine pesticides in the 1960s and 1970s. They are less persistent in the environment when compared with organochlorine pesticides and thus pose less long-term health risks to nontarget aquatic organisms and humans. However, in recent years several governmental agencies, including the USEPA, have started to reconsider the wide use of organophosphorus pesticides due to concern about their effects on the central nervous systems of humans, children in particular. This review discusses the fate of organophosphorus pesticides in the aquatic environment via processes such as adsorption, hydrolysis, oxidation, and photochemical degradation. Furthermore, the breakdown products of OPs are discussed, as new research has indicated that the products of degradation can be very harmful as well and because relatively little research has been carried out on comprehensive product identification. Recommended future research areas are highlighted.

The influence of ionic strength, nutrients and pH on bacterial adhesion to metals.

Bacteria-metal interactions in aqueous solutions are important in biofilm formation, biofouling and biocorrosion problems in the natural environment and engineered systems. In this study, the adhesion forces of two anaerobes (Desulfovibrio desulfuricans and Desulfovibrio singaporenus) and an aerobe (Pseudomonas sp.) to stainless steel 316 in various aqueous systems were quantified using atomic force microscopy (AFM) with a cell probe. Results show that the nutrient and ionic strength of the solutions influence the bacteria-metal interactions. The bacteria-metal adhesion force was reduced in the presence of the nutrients in the solution, because a trace organic film was formed and thus decreased the metal surface wettability. Stronger ionic strength in the solution results in a larger bacteria-metal adhesion force, which is due to the stronger electrostatic attraction force between the positively charged metal surface and negatively charged bacterial surface. Solution pH also influences the interaction between the bacterial cells and the metal surface; the bacteria-metal adhesion force reached its highest value when the pH of the solution was near the isoelectric point of the bacteria, i.e. at the zero point charge. The adhesion forces at pH 9 were higher than at pH 7 due to the increase in the attraction between Fe ions and negative carboxylate groups.

Kinetics and mechanisms of UV-photodegradation of chlorinated organics in the gas phase.

Over the last two decades, the application of photodegradation for the destruction of a wide spectrum of organic compounds in air has gained considerable interest in abating environmental pollution. This paper presents the results of a fundamental study conducted to evaluate the gas phase oxidation kinetics of volatile organic compounds (VOCs) with respect to different parameters pertinent to the operating conditions of air stripping and soil vapor extraction processes. Photodegradations of three chlorinated VOCs: chloroform, carbon tetrachloride (CTC) and trichloroethylene (TCE), were investigated in a semi-batch reactor using a low-pressure mercury UV lamp. The effects of different experimental parameters, such as the initial concentrations of the VOCs, the reaction medium, relative humidity, light intensity, temperature and the effect of mixture that may influence the kinetics of the gas phase photodegradation were evaluated. Mechanisms of photodegradation as supported by the experimental data are also proposed.

Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters

The degradation of methylmercury chloride by hydroxyl radicals (*OH) has been investigated using nitrate photolysis from 285 to 800 nm with a 450 W Xenon lamp as the (*OH source. The identified products are Hg(2+), Hg(0), CHCl(3) and formaldehyde. The second-order rate constant at pH of 5 at room temperature was determined to be 9.83(+-0.66)x10(9) M(-1) x s(-1)using benzoic acid as the *OH scavenger. The effects of chloride concentration and methylmercury speciation have also been investigated. A mechanism of the CH(3)HgCl-*OH reaction has been proposed. The calculated methylmercury degradation rates in natural waters using the above rate constant were comparable to the in situ photodegradation rates reported previously, indicating that degradation by (*)OH may be one of the important pathways of methylmercury degradation in sunlit surface waters.

Copper corrosion in mildly alkaline water with the disinfectant monochloramine

Abstract The corrosion of pure copper for a period of 2–30 days was investigated in simulated tap water containing monochloramine (normally at 4.0 mg/l) in the pH range of 7.6–8.4, using electrochemical impedance spectroscopy, potentiodynamic scan, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma and scanning electronic microscopy (SEM). The XPS spectra and SEM indicated that after 8 days of immersion, the copper surface was covered by both cupric and cuprous oxides. Diffusion in the oxide film was found having a significant effect on the corrosion process. The results showed that at pH 8.0 the polarization resistance ( R p ) increased with time during the first 6 days and reached relative stability after 8 days. An equivalent circuit model was proposed to fit the corrosion process, and the elements extracted from the model predicted a corrosion rate in the order of 0.1 μm/yr at pH of 8.0. The values of R p increased with higher pH for all 2, 4, 6, and 8-day immersions, but decreased at a higher monochloramine concentration of 20 mg/l. In the control experiment, R p had the largest value after a 30-day immersion time. Dissolved inorganic carbon (DIC) enhancess the dissolution of copper by a factor of 2, when the concentration of DIC increased from 0.59 to 4.0 mM. Increasing ionic strength from 0.005 to 0.02 M reduced R p , but the increase of R p with immersion time was faster at higher ionic strengths.

Copper corrosion in distribution systems: evaluation of a homogeneous Cu2O film and a natural corrosion scale as corrosion inhibitors

The aim of this work is to assess the performance of different corrosion scales as effective copper corrosion inhibitors using synthetic waters and distribution system waters. The major objective of the study was to evaluate the stability of an artificially synthesized Cu2O film and naturally formed heterogeneous corrosion scales under the influence of a number of synthetic and real waters, using Electrochemical Impedance Spectroscopy (EIS). Cu2O coatings were synthesized by using a special flame aerosol deposition process. Based upon the EIS data, conclusions regarding the stability of the different scales under typical water quality conditions, (which are commonly encountered in water distribution systems), are drawn.

Self-assembled organic thin films on electroplated copper for prevention of corrosion

Self-assembled organic thin films of dodecanethiol (DT), mercaptobenzothiazole (MBT), benzotriazole (BTA), imidazole (IMD) and benzothiazole (BT) are formed by adsorption on the surface of copper thin film used in ultralarge-scale integrated circuits. The films are characterized by x-ray photoelectron spectroscopy. The inhibition of corrosion of these organic thin films is investigated in aerated 0.5 M H2SO4 solutions by electrochemical impedance spectroscopy and potentiodynamic polarization techniques. The presence of these films reduced corrosion by blocking the copper surface from the oxygen dissolved in the acid medium. The relative inhibition efficiencies of these inhibiting agents in preventing copper oxidation are found to be in the order of DT>MBT>BT>BTA>IMD. The effectiveness of the inhibitors increased with the temperature, concentration of the inhibitors, and duration of immersion in the solution. An adsorption model is proposed on the basis of variation of the impedance according to the inhibi...

Effect of Specific Water Quality Parameters on Copper Corrosion

Abstract The objective of this study is to clearly understand the effect of specific water quality parameters like pH, dissolved oxygen (DO), conductivity, and buffering capacity (attributable to d...

Biocorrosion Behavior of Titanium Oxide/Butoxide-Coated Stainless Steel

Stainless steel coated with multilayers of Ti oxide/butoxide was prepared via a layer-by-layer sol-gel deposition process. The successful buildup of the Ti oxide/butoxide coatings was ascertained by X-ray photoelectron spectroscopy and static water contact angle measurements. The differences in corrosion behavior between the Ti oxide/butoxide-coated and the uncoated coupons in a simulated seawater-based modified Baars medium inoculated with Desulfovibrio desulfuricans under anaerobic conditions were investigated by electrochemical analyses, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). Electrochemical analyses and SEM results revealed that the pristine, the hydroxylated, and the nitric acid-passivated coupons were all vulnerable to biocorrosion due to localized breakdown of the passive film under synergistic attack of Desulfovibrio desulfuricans and biogenic sulfide ions, leading to destructive pitting and crevice corrosion. However, the Ti oxide/butoxide-coated coupons exhibited desirable resistance in the biocorrosion environment and the coating remained relatively stable throughout the exposure period. EDX analysis revealed that the bioactive properties of the Ti oxide/butoxide coating arose from the concomitant deposition of calcium and phosphorous compounds. The increase in resistance of the Ti oxide/butoxide coatings with time was correlated with the increase in nucleation of calcium apatite in electrochemical studies.

Photocatalytic Inactivation of Bioaerosols by TiO2 Coated Membrane

Indoor air pollution by microbial contaminants is increasingly receiving attention as a public health problem. Under a suitable environment, such as in heating, ventilation and air conditioning (HVAC) system, airborne bacteria are able to proliferate and grow causing various allergies and illnesses. This can be particularly serious in tropical regions due to high relative humidity and warm temperatures all round the year. Application of photocatalysis using UV-A and TiO2 to inactivate air-borne bacteria is relatively new and systematic parametric study is required for the engineering design of a process based on this technology. This study investigates the effects of TiO2 mediated inactivation of various bacterial species in batch and continuous systems using different TiO2 loadings and radiation intensities. Gram-negative bacteria, E. coli and two Gram-positive bacteria, Microbacterium sp. and Bacillus subtilis were used for the inactivation studies. In both systems, inactivation rates of Gram-negative E. coli are higher than the Gram-positive Bacillus subtilis and Microbacterium sp. and the inactivation rates increased in presence of TiO2 for all bacteria. Depending on the type of bacteria, TiO2 loading and light intensity, an increase of 1.3-5.8 times in the inactivation rates was obtained from those in the absence of TiO2. The inactivation rates in the batch and continuous systems were reasonably comparable. Inactivation rates in the continuous system are somewhat higher than those in the batch system due to the unaccounted loss of bacteria via adsorption and settling on the reactor walls in the flow system. The study demonstrates an approach that can be used for the designing of large scale systems for the treatment of bioaerosol.