Dalibor M. Stanković

Degradation of triketone herbicides, mesotrione and sulcotrione, using advanced oxidation processes

Degradation of two triketone herbicides, mesotrione and sulcotrione, was studied using four different advanced oxidation processes (AOPs): ozonization, dielectric barrier discharge (DBD reactor), photocatalysis and Fenton reagent, in order to find differences in mechanism of degradation. Degradation products were identified by high performance liquid chromatography (HPLC-DAD) and UHPLC-Orbitrap-MS analyses. A simple mechanism of degradation for different AOP was proposed. Thirteen products were identified during all degradations for both pesticides. It was assumed that the oxidation mechanisms in the all four technologies were not based only on the production and use of the hydroxyl radical, but they also included other kinds of oxidation mechanisms specific for each technology. Similarity was observed between degradation mechanism of ozonation and DBD. The greatest difference in the products was found in Fenton degradation which included the opening of benzene ring. When degraded with same AOP pesticides gave at the end of treatment the same products. Global toxicity and COD value of samples was determined after all degradations. Real water sample was used to study influence of organic matter on pesticide degradation. These results could lead to accurate estimates of the overall effects of triketone herbicides on environmental ecosystems and also contributed to the development of improved removal processes.

The use of graphene nanoribbons as efficient electrochemical sensing material for nitrite determination

In this work new designed, highly sensitive electrochemical method is developed for the determination of nitrites in tap water using glassy carbon electrode modified with graphene nanoribbons (GNs/GCE). Graphene nanoribbons (GNs) have been newly synthetized and aligned to the surface of glassy carbon electrode (GCE) and exhibited excellent electrocatalytic activity for nitrite oxidation with a very high peak currents. Studies about electrochemical behavior and optimization of the most important experimental conditions were done using cyclic voltammetry (CV), while quantitative studies were done with amperometric detection. Nitrite provides a well-defined, oxidation peak at +0.9V (vs. Ag/AgCl, 3.0M KCl) in Britton-Robinson buffer solution (BRBS) at pH 3. The influence of most possible interferent ions has been examined and was found to be negligible. Under optimized experimental conditions in BRBS at pH 3 linear calibration curves were obtained in the range from 0.5 to 105µM with the detection limit of 0.22µM. Reproducibility of ten replicate measurements of 1µM of nitrite was estimated to be 1.9%. Proposed method and constructed sensor is successfully applied for the determination of nitrite present in tap water samples without any pretreatment. This developed method represents inexpensive analytical alternative approach compared to other analytical methods.

Application of non-thermal plasma reactor and Fenton reaction for degradation of ibuprofen.

Pharmaceutical compounds have been detected frequently in surface and ground water. Advanced Oxidation Processes (AOPs) were reported as very efficient for removal of various organic compounds. Nevertheless, due to incomplete degradation, toxic intermediates can induce more severe effects than the parent compound. Therefore, toxicity studies are necessary for the evaluation of possible uses of AOPs. In this study the effectiveness and capacity for environmental application of three different AOPs were estimated. They were applied and evaluated for removal of ibuprofen from water solutions. Therefore, two treatments were performed in a non-thermal plasma reactor with dielectric barrier discharge with and without a homogenous catalyst (Fe(2+)). The third treatment was the Fenton reaction. The degradation rate of ibuprofen was measured by HPLC-DAD and the main degradation products were identified using LC-MS TOF. Twelve degradation products were identified, and there were differences according to the various treatments applied. Toxicity effects were determined with two bioassays: Vibrio fischeri and Artemia salina. The efficiency of AOPs was demonstrated for all treatments, where after 15 min degradation percentage was over 80% accompanied by opening of the aromatic ring. In the treatment with homogenous catalyst degradation reached 99%. V. fischeri toxicity test has shown greater sensitivity to ibuprofen solution after the Fenton treatment in comparison to A. salina.

Dinuclear copper(II) octaazamacrocyclic complex in a PVC coated GCE and graphite as a voltammetric sensor for determination of gallic acid and antioxidant capacity of wine samples

A novel efficient differential pulse voltammetric (DPV) method for determination gallic acid (GA) was developed by using an electrochemical sensor based on [Cu2tpmc](ClO4)4 immobilized in PVC matrix and coated on graphite (CGE) or classy carbon rod (CGCE). The proposed method is based on the gallic acid oxidation process at formed [Cu2tpmcGA](3+) complex at the electrode surface. The complexation was explored by molecular modeling and DFT calculations. Voltammograms for both sensors, recorded in a HNO3 as a supporting electrolyte at pH 2 and measured in 2.5×10(-7) to 1.0×10(-4) M of GA, resulted with two linear calibration curves (for higher and lower GA concentration range). The detection limit at CGE was 1.48×10(-7) M, while at CGCE was 4.6×10(-6) M. CGE was successfully applied for the determination of the antioxidant capacity based on GA equivalents for white, rosé and red wine samples.

Sensitive electrochemical determination of yohimbine in primary bark of natural aphrodisiacs using boron-doped diamond electrode

For the first time, a simple and sensitive analytical method for the direct determination of yohimbine is presented using differential pulse voltammetry with a boron-doped diamond electrode. Two irreversible oxidation peaks, a distinct one at +0.80 and a second poorly-defined one at +1.65 V, were observed when cyclic voltammetry was carried out in Britton–Robinson buffer solution at pH 7 (vs. Ag/AgCl). With optimized differential pulse voltammetric parameters (pulse amplitude 100 mV, pulse time 25 ms, step potential 5 mV and scan rate 10 mV s−1), the current response of yohimbine at +0.80 V was linearly proportional to the concentration in the range from 0.25 to 90.9 μmol L−1 with a low detection limit of 0.13 μmol L−1 (0.046 mg L−1) and a good repeatability (relative standard deviation of 2.5% at 18.4 μmol L−1 for n = 6). The practical applicability of the developed method was demonstrated by the assessment of the total content of yohimbine in extracts of the primary bark of natural aphrodisiacs such as Pausinystalia yohimbe and Rauvolfia serpentina with recoveries in the range of 92–97%. The proposed electrochemical procedure represents an inexpensive and effective analytical alternative for the quality control analysis of products containing yohimbine and other biologically and structurally related alkaloids used as natural dietary supplements.

Sensitive voltammetric determination of thymol in essential oil of Carum copticum seeds using boron-doped diamond electrode.

Essential oil of Carum copticum seeds, obtained from a local shop, was extracted and content of thymol was analyzed using square-wave voltammetry at boron-doped diamond electrode. The effect of various parameters, such as pH of supporting electrolyte and square-wave voltammetric parameters (modulation amplitude and frequency), was examined. In Britton-Robinson buffer solution (pH 4), thymol provided a single and oval-shaped irreversible oxidation peak at +1.13 V versus silver/silver chloride potassium electrode (3M). Under optimal experimental conditions, a plot of peak height against concentration of thymol was found to be linear over the range of 4 to 100μM consisting of two linear ranges: from 4 to 20μM (R(2)=0.9964) and from 20 to 100μM (R(2)=0.9993). The effect of potential interferences such as p-cymene and γ-terpinene (major components in essential oil of C. copticum seeds) was evaluated. Thus, the proposed method displays a sufficient selectivity toward thymol with a detection limit of 3.9μM, and it was successfully applied for the determination of thymol in essential oil of C. copticum seeds. The Prussian blue method was used for validation of the proposed electroanalytical method.

Effect of cobalt doping level of ferrites in enhancing sensitivity of analytical performances of carbon paste electrode for simultaneous determination of catechol and hydroquinone

This work presents the simultaneous determination of catechol (CC) and hydroquinone (HQ), employing a modified carbon paste electrode (CPE) with ferrite nanomaterial. Ferrite nanomaterial was doped with different amount of cobalt and this was investigated toward simultaneous oxidation of CC and HQ. It was shown that this modification strongly increases electrochemical characteristics of the CPE. Also, electrocatalytic activity of such materials strongly depends on the level of substituted Co in the ferrite nanoparticles. The modified electrodes, labeled as CoFerrite/CPE, showed two pairs of well-defined redox peaks for the electrochemical processes of catechol and hydroquinone. Involving of ferrite material in the structure of CPE, cause increase in the potentials differences between redox couples of the investigated compounds, accompanied with increases in peaks currents. Several important parameters were optimized and calibration curves, with limits of detection (LOD) of 0.15 and 0.3µM for catechol and hydroquinone, respectively, were constructed by employing amperometric detection. Effect of possible interfering compounds was also studied, and proposed method was successfully applied for CC and HQ quantification in real samples.

Microwave-hydrothermal method for the synthesis of composite materials for removal of arsenic from water

Composite material Zr-doped TiO2, suitable for the removal of arsenic from water, was synthetized with fast and simple microwave-hydrothermal method. Obtained material, Zr-TiO2, had uniform size and composition with zirconium ions incorporated into crystal structure of titanium dioxide. Synthetized composite material had large specific surface area and well-developed micropore and mesopore structure that was responsible for fast adsorption of As(III) and As(V) from water. The influence of pH on the adsorption capacity of arsenic was studied. The kinetics and isotherm experiments were also performed. The treatment of natural water sample containing high concentration of arsenic with composite material Zr-TiO2 was efficient. The concentration of arsenic was reduced to the value recommended by WHO.

Voltammetric determination of an antipsychotic agent trifluoperazine at a boron-doped diamond electrode in human urine

A simple and efficient procedure is described for electrochemical determination of trifluoperazine (TFP), a prominent compound in a large group of phenothiazine derivatives with potent physiological activity. This method is based on the electrochemical oxidation of TFP in Britton–Robinson buffer solution in pH 6 at a boron-doped diamond electrode. Cyclic voltammetry provided a four well defined oxidation peaks at +0.65, +0.83, +1.06 and +1.35 V (vs. Ag/AgCl/3 M KCl electrode). Differential pulse voltammetry was applied as a very sensitive analytical technique for the determination of micromolar amounts of TFP. Two oxidation peaks on higher potentials were chosen for quantification of TFP. Under optimized conditions, the analytical curves obtained were linear in the TFP concentration range of 1.0 to 37.0 μmol L−1, with a detection limit of 0.7 and 0.6 μmol L−1, respectively. The effect of interfering agents (common urinary compounds) appeared to be negligible confirming the favourable selectivity of the method. The proposed method was analytically applied by determination of trifluoperazine content in model human urine samples with good accuracy (recoveries varied from 97% to 104%). The developed approach could be beneficial in analysis of TFP in biological samples using a boron-doped diamond electrode as an up-to-date electrochemical sensor and could represent an inexpensive analytical alternative to separation methods.

Comparative Electrochemical Determination of Total Antioxidant Activity in Infant Formula with Breast Milk

The aim of this study is to investigate the significance of breast milk and infant formula in the prevention of oxidative stress, by electrochemical determination of the total antioxidant potential, demonstrating the relationship between the antioxidant capacity of milk and postnatal age. Human breast milk, commercial UHT milk, and infant formulas supplemented with prebiotics were used. Samples were diluted in phosphate buffer solution, and the total antioxidant activity was potentiometrically measured by the iodine/iodide redox couple with the Pt Fisher electrode as a working electrode and saturated calomel as a reference electrode. Cyclic voltammograms and differential pulse voltammograms were recorded with the glassy carbon electrode as the working electrode, an accessory platinum electrode, and an Ag/AgCl reference electrode. The potentiometric measurement indicates that human breast milk has the highest redox potential, while the commercial UHT milk has very low potential. Infant formulas also have high potential. The main advantage of electrochemical methods used to assess the total antioxidant activity of milk was that they directly monitored the electron-donating ability of the compounds and could be used for the quantitative analysis of the total antioxidants of different types of milk.