Crystal morphology control of synthetic giniite for enhanced photo-Fenton activity against the emerging pollutant metronidazole

Metronidazole (MNZ) is a recalcitrant antibiotic with toxic and carcinogenic effects in aquatic environments. In this work, Fe₅(PO₄)₄(OH)₃·2H₂O (giniite) particles were synthesised with three different alkaline cations (Li⁺, Na⁺ and K⁺) and used as Fenton catalysts for MNZ removal. It is shown that the addition of different cations during the hydrothermal synthesis process promote different morphologies from asterisk-like to flower-like and branches-like, maintaining the crystalline structure of pure giniite. The photo-Fenton activity of these particles was then evaluated through the degradation of MNZ under sunlight radiation for 9 h. The results indicate that the alkaline cation has a predominant role in the photo-Fenton efficiency, as demonstrated by the superior degradation efficiencies of Na@giniite particles (91.2% and 72.5% with giniite concentration of 0.2 g L^-1 and 0.07 g L^-1, respectively), related with its high surface area (10.7 m² g^-1). Thus, it is demonstrated the suitability of Na@giniite particles as Fenton catalyst for MNZ removal from water.

Degradation of metronidazole by UV/chlorine treatment: Efficiency, mechanism, pathways and DBPs formation

Metronidazole (MET) is a widely used antibiotic but is recalcitrant in aquatic environment. This study investigated elimination of MET by UV/chlorine process systematically. The degradation of MET in the process well fitted pseudo first-order kinetics. Decreasing pH from 9 to 5 raised the rate constant from 0.0199 min^-1 to 0.1485 min^-1, possibly ascribed to change in species distribution and apparent quantum yields of radicals. Scavenging experiments indicated that both HO and Cl contributed to the degradation of MET, and that HO was the dominant species in the pH range studied. The second-order rate constant between Cl and MET was determined to be (5.64 ± 0.1) × 10⁹ M^-1 s^-1. Three products were identified by UPLC-Q-TOF MS and degradation pathway was thus proposed. Significant amounts of chlorinated disinfection by-products (DBPs) were produced and 1,1,1-TCP was the dominant (83.6%-92.3%) in the UV/chlorine process. The kinetic model developed fitted well with experimental results, and was used to examine the effects of typical water parameters, such as chorine dosage, pH, inorganic anions, NOM and real water matrix. Furthermore, removal efficiency of MET by the UV/chlorine process were assessed in terms of electrical energy per order (EE/O). The efficiency was about 0.43 kWh m^-3 order^-1, 0.54 kWh m^-3 order^-1, 0.57 kWh m^-3 order^-1, respectively, for the removal of MET in ultrapure water (UPW) and two types of real water samples, indicating that UV/chorine was a practical method for authentic drinking water treatment practices.

Removal of metronidazole by TiO2 and ZnO photocatalysis: a comprehensive comparison of process optimization and transformation products

The photodegradation of antibiotic metronidazole (MNZ) was systematically studied and compared by using aqueous suspensions of TiO₂ and ZnO catalysts under 100-W UV irradiation. The degradation conditions were optimized using the central composite design and response surface methodology. The optimal photodegradation conditions obtained were at pH 6.0 with 1.5 g L^-1 of TiO₂ (86.10% removal for 50 mg L^-1 MNZ) and at pH 9.5 with 0.5 g L^-1 of ZnO (60.32% removal for 30 mg L^-1 MNZ) after 60-min irradiation at 20 °C. The degradation efficiency in the presence of TiO₂ was higher than that of ZnO. The participation of active species such as hydroxyl radicals (OH·), holes (h⁺), and superoxide radicals (O₂^-·) during MNZ photodegradation over TiO₂ and ZnO catalysts was also examined. Experimental results showed that MNZ oxidation was mainly driven by the presence of holes and superoxide radicals. Totally, 10 major intermediates were detected in UV/TiO₂ and UV/ZnO photocatalysis of MNZ using LC-QTof/MS system, in which 5 same intermediates were found. The remaining different intermediates led to the variations of degradation pathways of both processes. Moreover, some bigger transformation products than the parent MNZ were detected.

Electrochemical degradation of the antibiotic metronidazole in aqueous solution by the Ti/SnO2-Sb-Ce anode

Metronidazole (MNZ) is an antibiotic pollutant with a high occurrence in the ambient medium. In this study, the anode material Ti/SnO2-Sb-Ce prepared in the lab was employed to investigate the feasibility of the electrochemical process to treat antibiotic in wastewater. The result showed that metronidazole could be effectively removed using Ti/SnO2-Sb-Ce. The degradation efficiency of 88% was obtained under the current density 1.6 mA cm(-2), pH = 5.6 (not adjusted), electrolyte (Na2SO4) concentration of 0.2 M for electrolysis 2 h. The removal percentage was higher by 17% compared with the control when the bare Ti was applied. Meanwhile, the energy consumption on Ti/SnO2-Sb-Ce was about one-seventh of that on Ti. The characterization of the material was conducted by the thermal field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectrometer (EDS) and cyclic voltammetry (CV). The Ti/SnO2-Sb-Ce anode displayed compact, multi-porous morphology and good redox reversibility. The influencing factors such as current density, pH, concentration of Na2SO4, initial MNZ concentration were studied to obtain main factors and optimum conditions. In addition, a preliminary study on the mechanism of the electro-oxidation was carried out. The results demonstrate that chemisorbed oxygen has a dominant role in MNZ removal.

The photodegradation of metronidazole in the presence of coexisting pharmaceuticals

The objective of this study was to investigate if coexisting compounds could affect the fate of pharmaceuticals in surface water under solar irradiation. The degradation of metronidazole (MET) in the presence of different coexisting pharmaceuticals was investigated in batch experiments with exposure to sunlight. Tinidazole, which has a similar structure to MET, was employed as an analogue. The results indicated that the presence of an analogue with a similar photosensitive group to MET could inhibit the photodegradation of MET. In addition, the effect of coexisting pharmaceuticals with different absorption spectra on the degradation of MET was investigated. The results showed that the effect depended on the degree of overlapping absorption spectra between MET and the coexisting pharmaceuticals. The relationship between the degree of the influence and the ultraviolet absorption spectra of coexisting pharmaceuticals found in this study could give guidance in assessing the fate of pharmaceuticals in environmental water.

Gamma irradiation of pharmaceutical compounds, nitroimidazoles, as a new alternative for water treatment

The main objectives of this study were: (1) to investigate the decomposition and mineralization of nitroimidazoles (Metronidazole [MNZ], Dimetridazole [DMZ], and Tinidazole [TNZ]) in waste and drinking water using gamma irradiation; (2) to study the decomposition kinetics of these nitroimidazoles; and (3) to evaluate the efficacy of nitroimidazole removal using radical promoters and scavengers. The results obtained showed that nitroimidazole concentrations decreased with increasing absorbed dose. No differences in irradiation kinetic constant were detected for any nitroimidazole studied (0.0014-0.0017 Gy(-1)). The decomposition yield was higher under acidic conditions than in neutral and alkaline media. Results obtained showed that, at appropriate concentrations, H(2)O(2) accelerates MNZ degradation by generating additional HO(); however, when the dosage of H(2)O(2) exceeds the optimal concentration, the efficacy of MNZ degradation is reduced. The presence of t-BuOH (HO() radical scavenger) and thiourea (HO(), H() and e(aq)(-) scavenger) reduced the MNZ irradiation rate, indicating that degradation of this pollutant can take place via two pathways: oxidation by HO() radicals and reduction by e(aq)(-) and H(). MNZ removal rate was slightly lower in subterranean and surface waters than in ultrapure water and was markedly lower in wastewater. Regardless of the water chemical composition, MNZ gamma irradiation can achieve i) a decrease in the concentration of dissolved organic carbon, and ii) a reduction in the toxicity of the system with higher gamma absorbed dose.

Degradation of the pharmaceutical metronidazole via UV, Fenton and photo-Fenton processes

Degradation rates and removal efficiencies of Metronidazole using UV, UV/H2O2, H2O2/Fe2+, and UV/H2O2/Fe2+ were studied in de-ionized water. The four different oxidation processes were compared for the removal kinetics of the antimicrobial pharmaceutical Metronidazole. It was found that the degradation of Metronidazole by UV and UV/H2O2 exhibited pseudo-first order reaction kinetics. By applying H2O2/Fe2+, and UV/H2O2/Fe2+ the degradation kinetics followed a second order behavior. The quantum yields for direct photolysis, measured at 254 nm and 200-400 nm, were 0.0033 and 0.0080 mol E(-1), respectively. Increasing the concentrations of hydrogen peroxide promoted the oxidation rate by UV/ H2O2. Adding more ferrous ions enhanced the oxidation rate for the H2O2/Fe2+ and UV/H2O2/Fe2+ processes. The major advantages and disadvantages of each process and the complexity of comparing the various advanced oxidation processes on an equal basis are discussed.

Unit-cell volume change as a metric of radiation damage in crystals of macromolecules

The use of third-generation synchrotron sources has led to renewed interest in the effect that ionizing radiation has on crystalline biological materials. Simple criteria have been sought to study the effects systematically. The unit-cell volume of protein crystals shows a linear increase with absorbed dose and has therefore been proposed to be such a measure. This paper demonstrates that the increase is sample dependent, and thus it might not be a useful indicator when comparing different samples. For individual samples, however, the increase can be used to quantify ambient temperature and dose-rate effects. In this study, highly absorbing cubic crystals of holoferritin have been used to accurately determine how cell volume changes with absorbed dose. The experiments show that, for this protein, a dose-rate effect exists and that trapped radicals can be mobilized at ca 180 K.

Investigation of possible free-radical scavengers and metrics for radiation damage in protein cryocrystallography

The introduction of highly intense wiggler and undulator beamlines has reintroduced the problem of X-ray radiation damage in protein crystals even at cryogenic temperatures. Several metrics for monitoring radiation damage are considered and unit-cell volume expansion is systematically investigated using crystals of three different types, but it is found to be too variable to be a useful metric. Radical scavengers of secondary radiation damage are investigated as possible mitigating agents. Styrene is found to be ineffective. A method of spectroscopically measuring the radiation damage with a microspectrophotometer was used and, in conjunction with crystallographic data, provided tentative but suggestive evidence for the efficacy of ascorbate as a free-radical scavenging agent in cryocooled hen egg-white lysozyme crystals.

Experiments testing the abatement of radiation damage in D-xylose isomerase crystals with cryogenic helium

Helium is a more efficient cryogen than nitrogen, and for macromolecular data collection at high-flux beamlines will deliver lower temperatures. An open-flow helium cryostat developed at the University of Toledo (the Pinkerton Device) has been used for macromolecular data collection. This device differs from standard commercial He cryostats by having a much narrower aperture providing a high velocity stream of He around the crystal that maximizes convective and conductive heat exchange between the crystal and the cryogen. This paper details a series of experiments conducted at the IMCA-CAT 17ID beamline using one crystal for each experimental condition to examine whether helium at 16 K provided better radiation-damage abatement compared with nitrogen at 100 K. These studies used matched high-quality crystals (0.94 A diffraction resolution) of D-xylose isomerase derived from the commercial material Gensweet SGI. Comparisons show that helium indeed abates the indicators of radiation damage, in this case resulting in longer crystal diffractive lifetimes. The overall trend suggests that crystals maintain order and that high-resolution data are less affected by increased radiation load when crystals are cooled with He rather than N(2). This is probably the result of a lower effective temperature at the crystal with concomitant reduction in free-radical diffusion. Other features, such as an apparent phase transition in macromolecular crystals at lower temperatures, require investigation to broaden the utility of He use.

The effect of gamma-radiation on nitroimidazole derivatives

The effect of gamma irradiation, in doses from 10 to 50 kGy on physical and chemical properties of metronidazole, ornidazole and tynidazole in solid state has been studied. Results of the measurements by the UV, IR, TLC, EPR, DSC methods revealed the presence of free radicals, products of decomposition, an increase in the melting point and a decrease or increase in the content of the substance studied. Microbiological assays proved that sterilization of the compounds studied was effective even with the smallest irradiation dose applied. The maximum tolerated dose and the safe sterilisation dose were determined.

Degradation kinetics of metronidazole in solution

The degradation kinetics of metronidazole in aqueous solutions of pH 3.1 to 9.9 under accelerated storage conditions were studied. The stability of metronidazole in solutions containing propylene glycol or polyethylene glycol 400 was also investigated. The reaction order for metronidazole in these aqueous and solvent systems followed pseudo-first-order degradation kinetics. The degradation rate of metronidazole was invariant under various total buffer concentrations at each specific pH within the investigated pH range. These results indicate that no general acid/base catalysis imposed by acetate, phosphate, and borate buffer species is responsible for the degradation of metronidazole. The catalytic rate constants for hydrogen ion, water, and hydroxyl ion for the degradation of metronidazole were 6.11 x 10(-5) M/s, 3.54 x 10(-8) L/s, and 4.10 x 10(-3) M/s, respectively. The pH-rate profile shows a pH-independent region of pH 3.9-6.6. Maximum stability of metronidazole was at pH 5.6 under zero total buffer species conditions. The ionic strength effect on metronidazole degradation in acetate and phosphate buffers followed the modified Debye-Huckel equation well. The Arrhenius plot showing the temperature dependence of metronidazole degradation indicates estimates of activation energy of 15.35 kcal/mol and a half-life of 963 h at room temperature in 0.1 M pH 3.1 acetate buffer solution (ionic strength = 0.5). Irradiation with UV light (254 nm) of the metronidazole solutions (pH 3.1 acetate buffer) accelerated degradation in comparison with light-protected samples. Incorporation of propylene glycol into the metronidazole solution at pH 3.1 increased stability; however, an adverse effect on the stability of metronidazole was seen when polyethylene glycol 400 solvent system was used.

Effect of radiation-induced reduction of nitroimidazoles on biologically active DNA

Radiation-chemical reductions have been carried out with several nitroimidazoles. Reduction of these drugs in the presence of single-stranded phi chi 174 DNA causes extensive lethal damage. However, relatively stable (end) products, do not contribute to the damage, although glyoxal is potentially toxic. This demonstrates that a short-lived intermediate in the reduction process is responsible. Further, the quantity of damage in the DNA depends on both dose (reduction)-rate and also the nature of the drug.

The radiation chemistry of some platinum-containing radiosensitizers and related compounds

Oxidation and reduction of cis- and trans-dichlorodiammine platinum II (cis- and trans-PDD), cis-dichlorobis(1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole-N3)-p latinum II (cis-Flap), and cis-dichlorobis(isopropylamine)-trans-dihydroxyplatinum IV (Chip) have been studied using pulse radiolysis. Spectra corresponding to platinum in various oxidation states have been observed and several rate constants have been obtained. Reduction of all the compounds, except cis-Flap, produces species of a lower oxidation state of platinum which subsequently have both chloride ligands replaced. Ultimately, these products disproportionate. In the case of cis-Flap, reduction occurred on the nitroimidazole ligand. This was verified by the absence of platinum metal after disproportionation. Oxidation of all four compounds consists of production of a higher oxidation state of platinum followed by replacement of chloride ligands and finally disproportionation of the products. Only cis-Flap and Chip could be reduced by oxidized DNA bases. The one-electron reduction potential of cis-Flap was found to be -370 +/- 10 mV. trans-Flap had almost the same value. It was not possible to measure the potentials of the other compounds since their ligands were replaced rapidly but it is estimated that the one-electron reduction potentials decrease in the order cis- or trans-Flap greater than Chip greater than cis-PDD greater than trans-PDD.

The oxygen dependence of the reduction of nitroimidazoles in a radiolytic model system

Radiation chemical reductions using eaq- and CO2- have been carried out in the presence of oxygen with metronidazole, p-nitroacetophenone, misonidazole and three other 2-nitroimidazoles. Low concentrations of oxygen were found to effectively inhibit the reduction of the first two compounds while much higher concentrations of oxygen were required for all of the 2-nitroimidazoles. These results parallel in vitro and in vivo experiments with metronidazole and misonidazole which also indicate that the reduction of the latter is significantly less inhibited by oxygen. Kinetic modelling of the radiochemical system suggests that the explanation for the differences lies in different reactions of the nitro radical anions; it appears that the anion derived from metronidazole undergoes disproportionation while that derived from misonidazole undergoes a unimolecular decay.

Radiation-induced reduction of nitroimidazole derivatives in aqueous solution

The radiation-induced reduction of N1-alkyl substituted 2- and 5-nitroimidazoles in aqueous solution containing sodium formate or 2-propanol was studied at pH 7.0 +/- 0.1 under deaerated conditions. Irrespective of 2- or 5-nitroimidazole, N1-unsubstituted nitroimidazoles (2-nitro-(1a), 4(5)-nitro-(2a), 2-methyl-4(5)-nitro (3a), and 4(5)-methyl-5(4)-nitro- (4a) imidazoles) were reduced stepwise to consume 6 electrons per molecule, whereas N1-alkyl substituted nitroimidazoles (1-methyl-2-nitro (5a) and 1-methyl-5-nitro (6a) imidazoles, misonidazole (7a), and metronidazole (8a] reacted with consumption of 4 electrons. In accord with the stoichiometry for the reduction of N1-unsubstituted nitroimidazoles, the formation of the amino derivatives of (1a)-(4a) was shown by HPLC or colour identification tests. 4-Electron-reduction products of N1-alkyl substituted 2-nitroimidazoles (5a) and (7a) were characterized by 13C and 1H n.m.r. and FDMS measurements, indicating that the hydroxyamino derivative of (5a) as a 4-electron-reduction product isomerizes to an oxime form. The formation of an analogous oxime-type product was also suggested for (7a) together with a product bearing the partially cleaved imidizole ring. The HPLC analysis showed that 4-electron-reduction products of N1-alkyl substituted 5-nitroimidazoles (6a) and (8a) are unstable relative to those of the corresponding 2-nitroimidazoles (5a) and (7a).