Electrochemical Monitoring of Methylparathion Degradation in an Acid Aqueous Medium in Presence of Cu(II)

Abstract

A study was undertaken to determine the effect of Cu(II) in degradation of methylparathion (o,o-dimethyl o, 4-nitrophenyl phosphoriotioate) in acid medium. Initial electrochemical characterization of Cu(II) and methylparathion was done in an aqueous medium at a pH range of 2–7. Cu(II) was studied in the presence of different anions and it was observed that its electroactivity depends on pH and is independent of the anion used. Methylparathion had two reduction signals at pH ≤ 6 and only one at pH > 6. The pesticide's transformation kinetic was then studied in the presence of Cu(II) in acid buffered aqueous medium at pH values of 2, 4, and 7. Paranitrophenol appeared as the only electroactive product at all three pH values. The reaction was first order and had k values of 5.2 × 10^?3 s^?1 at pH 2, 5.5 × 10^?3 s^?1 at pH 4 and 9.0 × 10^?3 s^?1 at pH 7. It is concluded that the principal degradation pathway of methylparathion in acid medium is a Cu(II) catalyzed hydrolysis reaction.     

Effect of Fe (III) on acid degradation of methylparathion

A study was undertaken to determine the transformation kinetic of methylparathion (O, O, -dimethyl O-4 nitrophenylphosphorotioate) in the presence of Fe(III) between pH 2 and 7. The Fe(III) was not electroactive under the conditions used in this study, and polarographic signals were exhibited by methylparathion and main degradation product only. Data suggest that hydrolysis of methylparathion in an acid medium is catalyzed by Fe(III) and the pesticide did not degrade in this medium without this cation. Methylparathion degradation was observed at all the pHs studied and was independent of the predominant chemical form of Fe(III) in the aqueous medium. The reaction was first-order with pH-dependent rate constant (k) values ranging from 3.3 x 10(- 3) h(- 1) to 7.0 x 10(- 3) h(- 1). The k values increased as pH decreased, suggesting that Fe(III) acted as an electrophile in the reaction mechanism.     

Photochemical degradation of methylparathion in the presence of humic acid

Photochemical degradation of methylparathion (O,O,-dimethyl O-4 nitrophenylphosphorothioate) in the presence of humic acid between pH 2 and 7 was monitored by differential pulse polarography. Humic acid was not electro-active under the experimental conditions used in this study. Only the pesticide and its main degradation product at pH 2 exhibited polarographic signals. Photolysis of methylparathion in acid media was sensitized by humic acid since the pesticide did not degrade in the absence of this compound. Methylparathion degradation in the presence of humic acid was observed at each of the studied pHs. The reaction was first-order with rate constant values ranging from 2 x 10(-3) to 6.3 x 10(-3) min(-1).     

Effect of Fe (III) on acid degradation of methylparathion

A study was undertaken to determine the transformation kinetic of methylparathion (O, O, -dimethyl O-4 nitrophenylphosphorotioate) in the presence of Fe(III) between pH 2 and 7. The Fe(III) was not electroactive under the conditions used in this study, and polarographic signals were exhibited by methylparathion and main degradation product only. Data suggest that hydrolysis of methylparathion in an acid medium is catalyzed by Fe(III) and the pesticide did not degrade in this medium without this cation. Methylparathion degradation was observed at all the pHs studied and was independent of the predominant chemical form of Fe(III) in the aqueous medium. The reaction was first-order with pH-dependent rate constant (k) values ranging from 3.3 × 10^{? 3} h^{? 1} to 7.0 × 10^{? 3} h^{? 1}. The k values increased as pH decreased, suggesting that Fe(III) acted as an electrophile in the reaction mechanism.     

Photochemical degradation of methylparathion in the presence of humic acid

Photochemical degradation of methylparathion (O,O,-dimethyl O-4 nitrophenylphosphorothioate) in the presence of humic acid between pH 2 and 7 was monitored by differential pulse polarography. Humic acid was not electro-active under the experimental conditions used in this study. Only the pesticide and its main degradation product at pH 2 exhibited polarographic signals. Photolysis of methylparathion in acid media was sensitized by humic acid since the pesticide did not degrade in the absence of this compound. Methylparathion degradation in the presence of humic acid was observed at each of the studied pHs. The reaction was first-order with rate constant values ranging from 2 × 10^?3 to 6.3 × 10^?3 min^?1.     

Effect of fulvic acid on the photochemical degradation of methylparathion

Photochemical degradation of methylparathion (O,O,–dimethyl O-4 nitrophenylphosphorothioate) in the presence of fulvic acid (FA) between pH 2 and 7 was studied by differential pulse polarography (DPP). Fulvic acid and its photoproducts were not electro-active under the experimental conditions used in this study, and only the pesticide exhibited polarographic signals. Photolysis of methylparathion in acid media was sensitized by fulvic acid since the pesticide did not degrade in the absence of this compound. Methylparathion degradation was observed at each of the studied pHs. The reaction was first-order with rate constant values ranging from 3.3 × 10^?3 to 8.8 × 10^?3min^?1.     

Degradation of octylphenol and nonylphenol by ozone - part I: direct reaction

This aqueous reaction between ozone and two alkylphenols (APs), namely octylphenol (OP) and nonylphenol (NP), has been investigated. Both compounds are important endocrine disrupting chemicals, which arise from the biodegradation of alkylphenol ethoxylates and are often found at relatively high concentrations in wastewater effluents. In this paper the results of an experimental study are presented which provide values for the reaction rate constants between molecular ozone and undissociated OP and NP, and overall reaction rate constants for the degradation of the two APs at pH values in the range of 7-9. The kinetic rate constants for OP and NP degradation by molecular ozone were 4.33(+/-0.18) x 10(4) and 3.90(+/-0.10) x 10(4) M(-1) s(-1), and the reaction stoichiometry was similar in both cases and equal to approximately 1.3:1 ([O3]:[AP]). The overall second order reaction rate constants for the two APs increased significantly with increasing pH, which is believed to be mainly due to the increasing influence of indirect radical reaction with increasing pH; this aspect is considered in more detail in a companion paper. A preliminary investigation of the reaction mechanism suggests that an initial product of ozonation is hydroxyl-alkyl phenol.     

Removal of methyl parathion from water by electrochemically generated Fenton's reagent

The electro-Fenton process was used to assess the degradation of methyl parathion (MP) in aqueous solutions. This oxidation process allows the production of hydroxyls radicals which react on the organic compounds, leading to their mineralization. Degradation experiments were performed either in perchloric, sulphuric, hydrochloric and nitric acid media under current controlled electrolysis conditions at different pH. The pH effect as well as the nature of the medium (i.e., the nature of the ions present in medium) on the degradation and mineralization efficiency were studied. The mineralization of the initial pollutant was investigated by total organic carbon measurements which show a complete mineralization at pH 3 in perchloric medium. The absolute rate constant of MP hydroxylation reaction was determined as (4.20+/-0.11)x10(9)M(-1)s(-1). Complete degradation of MP and its metabolites occur in less than 45min. Degradation reaction intermediates such as aromatic compounds, carboxylic acids and inorganic ions were identified and a mineralization pathway is proposed.     

Degradation of octylphenol and nonylphenol by ozone - part II: indirect reaction

The indirect reaction of octylphenol (OP) and nonylphenol (NP) with hydroxyl radicals (*OH) during ozonation was investigated at pH values ranging from 6 to 9. A parameter Rct, representing the ratio of the *OH-exposure to the ozone-exposure, was measured using a method involving a low concentration of p-chlorobenzoic acid as a *OH-probe compound during the ozonation. By assuming that Rct is a constant value at a given pH, the second order rate constants of the alkylphenol reaction with hydroxyl radicals were determined as 1.4(+/-0.2) x 10(10) and 1.1(+/-0.2) x 10(10) M(-1) s(-1) for OP and NP, respectively. The proportions of each alkylphenol degraded by direct molecular ozone reaction and indirect hydroxyl radical reaction were predicted at different pH values. The contribution of indirect *OH reactions with each AP was found to represent over 50% of the total degradation for pH approximately 7, and the contribution increases substantially with pH>7.     

Photocatalytic degradation of the herbicide pendimethalin using nanoparticles of BaTiO3/TiO2 prepared by gel to crystalline conversion method: a kinetic approach

Photocatalytic degradation of the herbicide, pendimethalin (PM) was investigated with BaTiO3/TiO2 UV light system in the presence of peroxide and persulphate species in aqueous medium. The nanoparticles of BaTiO3 and TiO2 were obtained by gel to crystallite conversion method. These photo catalysts are characterized by energy dispersive x-ray analysis (EDX), scanning electron microscope (SEM), x-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) adsorption isotherm and reflectance spectral studies. The quantum yields for TiO2 and BaTiO3 for the degradation reactions are 3.166 Einstein m(-2) s(-1) and 2.729 Einstein m(-2) s(-1) and catalytic efficiencies are 6.0444 x 10(-7) mg(-2)h(-1)L2 and 5.403 x 10(-7) mg(-2)h(-1)L2, respectively as calculated from experimental results. BaTiO3 exhibited comparable photocatalytic efficiency in the degradation of pendimethalin as the most widely used TiO2 photocatalyst. The persulphate played an important role in enhancing the rate of degradation of pendimethalin when compared to hydrogen peroxide. The degradation process of pendimethalin followed the first-order kinetics and it is in agreement with Langmuir-Hinshelwood model of surface mechanism. The reason for high stability of pendimethalin for UV-degradation even in the presence of catalyst and oxidizing agents were explored. The higher rate of degradation was observed in alkaline medium at pH 11. The degradation process was monitored by spectroscopic techniques such as ultra violet-visible (UV-Vis), infrared (IR) and gas chromatography mass spectroscopy (GC-MS). The major intermediate products identified were: N-propyl-2-nitro-6-amino-3, 4-xylidine, (2, 3-dimethyl-5-nitro-6-hydroxy amine) phenol and N-Propyl-3, 4-dimethyl-2, 6-dinitroaniline by GC-MS analysis and the probable reaction mechanism has been proposed based on these products.     

Rapid reductive dechlorination of atrazine by zero-valent iron under acidic conditions

The dechlorination of atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) via reaction with metallic iron under low-oxygen conditions was studied using reaction mixture pH values of 2.0, 3.0, and 3.8. The pH control was achieved through addition of sulfuric acid throughout the duration of the reaction. The lower the pH of the reaction mixture, the faster the degradation of atrazine. The surface area of the sulfuric acid-treated iron particles was 0.31 (+/- 0.01) m2 g-1 and the surface area normalized initial pseudo-first order rate constants (kSA, where rate = kSA x (surface area/l) x [Atrazine]) at pH values of 2.0, 3.0, and 3.8 were equal to, respectively, 3.0 (+/- 0.4) x 10(-3) min-1 m-2 l, 5 (+/- 3) x 10(-4) min-1 m-2 l, and 1 (+/- 1) x 10(-4) min-1 m-2 l. The observed products of the degradation reaction were dechlorinated atrazine (2-ethylamino-4-isopropylamino-1,3,5-triazine) and possibly hydroxyatrazine (2-ethylamino-4-isopropylamino-6-hydroxy-s-triazine). Triazine ring protonation may account, at least in part, for the observed effect of pH on atrazine dechlorination via metallic iron.     

Degradation of Acid Orange 7 by electrochemically generated (*)OH radicals in acidic aqueous medium using a boron-doped diamond or platinum anode: a mechanistic study

A comparative study of the degradation of Acid Orange 7 (AO 7) aqueous solutions in acidic medium of pH 3.0 by electro-Fenton process using Pt or boron-doped diamond (BDD) anode was reported. The oxidative degradation of AO 7 by electrochemically generated hydroxyl radicals follows a pseudo-first order kinetic with a similar rate constant with BDD or Pt anode. The absolute rate constant of the AO 7 hydroxylation reaction was determined as (1.10+/-0.04)x10(10)M(-1)s(-1) by using the competition kinetic method. The comparative study of TOC measurements during electro-Fenton treatment showed a higher mineralization rate with BDD than Pt anode at the first hours of electrolysis because of the higher oxidizing power of this anode. The electro-Fenton degradation of AO 7 was followed by monitoring the formation and evolution of aromatic intermediates which are oxidized to aliphatic carboxylic acids before mineralization (transformation to CO(2) and inorganic ions, i.e. sulphate, nitrate and ammonium). The follow-up of the solution toxicity evolution shows the formation of intermediates more toxic than AO 7 and the connection between toxicity and aromaticity. A mineralization reaction pathway of AO 7 by electro-Fenton degradation involving all the intermediates identified was proposed.     

Investigation of solution chemistry effects on sorption behavior of Cu(II) on ZSM-5 zeolite

The sorption of Cu(II) from an aqueous solution using ZSM-5 zeolite was investigated by batch technique under ambient conditions. Sorption was investigated as a function of pH, ionic strength, foreign ions, humic substances, and temperature. The results indicate that the sorption of Cu(II) on ZSM-5 zeolite is strongly dependent on pH. Sorption is dependent on ionic strength at low pH, but independent of ionic strength at high pH values. The presence of humic/fluvic acid (HA/FA) enhances the sorption of Cu(II) on ZSM-5 zeolite at low pH values, and reduces Cu(II) sorption at high pH values. Sorption isotherms were well simulated by the Langmuir model. Thermodynamic parameters (i.e., deltaH0, deltaS0 and deltaG0) for the sorption of Cu(II) were determined from temperature-dependent sorption isotherms at 293.15, 313.15, and 333.15 K, respectively. Results indicate that the sorption process of Cu(II) on ZSM-5 zeolite is spontaneous and endothermic.     

Characterization of primary precipitate composition formed during co-removal of Cr(VI) with Cu(II) in synthetic wastewater

BACKGROUND, AIMS AND SCOPE: Hexavalent chromium [Cr(VI)] cannot react with either carbonate or hydroxide to form chromium precipitates. However, by using a precipitation technology to treat plating wastewater containing Cr(VI), Cu(II), Ni(II) and Zn(II), approximately 78% of Cr(VI) (initial 60 mg/L) was co-removed with the precipitation of Cu(II), Ni(II) and Zn(II) (each 150 mg/L) by dosing with Na2CO3 (Sun 2003). Direct precipitation by forming Cu(II)-Cr(VI) precipitates followed by adsorption of Cr(VI) onto freshly formed Cu-precipitates was subsequently found to be the main mechanism(s) involved in Cr(VI) co-removal with Cu(II) precipitation by dosing Na2CO3 stepwise to various pH values (Sun et al. 2003). This study was. carried out to further characterize the formation of primary precipitates during the early stages of copper precipitation and simultaneous removal of Cr(VI) with Cu(II). METHODS: Test metal-solutions were prepared with industrial grade chemicals: CuCl2 x 2H2O, Na2SO4 and K2Cr2207. NaCO3 was added drop-wise to synthetic metal-solution to progressively increase pH. For each pH increment, removal of soluble metals was detected by atomic absorption spectrophotometer (AAS) and surface morphology of precipitates was analyzed by scanning electron microscope (SEM). To further characterize the formation of primary precipitates, a series of MINEQL+ thermodynamic calculations/analyses and equilibrium calculations/ analyses were conducted. RESULTS AND DISCUSSION: MINEQL+ thermodynamic calculation indicated that, for a system containing 150 mg/L Cu(II) and 60 mg/L Cr(VI) with gradual Na2CO3 dosing, if any precipitates can be formed at pH 5.0 or lower, it should be in the form of CuCrO4. Comparison tests using systems containing the same equivalent of Cu(II) plus Cr(VI) and Cu(II) plus SO4(2-) showed that the precipitation occurred at a pH of around 5.0 in the Cu(II)-Cr(VI) system and around 6.0 in the Cu(II)-SO4(2-) system. The discrepancy of the precipitation was indeed caused by the formation of Cu-Cr precipitates. The initiation of copper removal at pH around 5.0 for the Cu-Cr co-removal test was not attributable to the formation of Cu-CO3 precipitates, instead, it was most likely through the formation of insoluble Cu-Cr precipitates, such as CuCrO4 and CuCrO4 x 2Cu(OH)2. Experimental tests, equilibrium calculations, MINEQL+ thermodynamic calculations and surface morphologies for systems using higher concentrations of Cu(II) and Cr(VI) further verified the most probable composition of primary precipitates is copper-chromate. CONCLUSION: In the Cu-Cr co-removal test with Na2O3 dosing to increase pH and induce metal precipitation, copper-chromate precipitates are the primary precipitates produced and contribute to the initial simultaneous removal of copper and chromium.     

Degradation of aqueous carbon tetrachloride by nanoscale zerovalent copper on a cation resin

Nanoscale zerovalent copper supported on a cation resin was successfully synthesized to enhance the removal of carbon tetrachloride (CCl(4)) from contaminated water. The use of the cation resin as a support prevents the reduction of surface area due to agglomeration of nanoscale zerovalent copper particles. Moreover, the cation resin recycles the copper ions resulting from the reaction between CCl(4) and Cu(0) by simultaneous ion exchange. The decline in the amount of CCl(4) in aqueous solution results from the combined effects of degradation by nanoscale zerovalent copper and sorption by the cation resin; thus the amount of CCl(4) both in aqueous solution and sorbed onto the resin were measured. The pseudo-first-order rate constant normalized by the surface-area and the mass concentration of nanoscale zerovalent copper (k(SA)) was 2.1+/-0.1 x 10(-2)lh(-1)m(-2), approximately twenty times that of commercial powdered zerovalent copper (0.04 mm). Due to the exchange between Cu(2+) and the strongly acidic ions (H(+) or Na(+)), the pH was between 3 and 4 in unbuffered solution and Cu(2+) at the concentration of less than 0.1 mg l(-1) was measured after the dechlorination reaction. In the above-ground application, resin as a support would facilitate the development of a process that could be designed for convenient emplacement and regeneration of porous reductive medium.     

Kinetics of heat-assisted persulfate oxidation of methyl tert-butyl ether (MTBE)

The kinetics of heat-assisted persulfate oxidation of methyl tert-butyl ether (MTBE) in aqueous solutions at various pH, temperature, oxidant concentration and ionic strength levels was studied. The MTBE degradation was found to follow a pseudo-first-order decay model. The pseudo-first-order rate constants of MTBE degradation by persulfate (31.5 mM) at pH 7.0 and ionic strength 0.11 M are approximately 0.13 x 10(-4), 0.48 x 10(-4), 2.4 x 10(-4) and 5.8 x 10(-4) S(-1) at 20, 30, 40 and 50 degrees C, respectively. Under the above reaction conditions, the reaction has an activation energy of 24.5 +/- 1.6 kcal/ mol and is influenced by temperature, oxidant concentration, pH and ionic strength. Raising the reaction temperature and persulfate concentration may significantly accelerate the MTBE degradation. However, increasing both pH (over the range of 2.5-11) and ionic strength (over the range of 0.11-0.53 M) will decrease the reaction rate. Reaction intermediates including tert-butyl formate, tert-butyl alcohol, acetone and methyl acetate were observed. These intermediate compounds were also degraded by persulfate under the experimental conditions. Additionally, MTBE degradation by persulfate in a groundwater was much slower than in phosphate-buffer solutions, most likely due to the presence of bicarbonate ions (radical scavengers) in the groundwater.     

Mineralization of C.I. Reactive Yellow 84 in aqueous solution by sonolytic ozonation

The operational parameters and mechanism of mineralization of C.I. Reactive Yellow 84 (RY84), one of the azo dyes, in aqueous solution were investigated using sonolytic ozonation (US/O(3) oxidation). Of the pseudo-first-order degradation rate constants of TOC reduction, 9.0 x 10(-4), 7.3 x 10(-3) and 1.8 x 10(-2)min(-1) were observed with US, O3, and a combination of US and O3, respectively. These results illustrate that ozonation combined with sonolysis for removal of TOC is more efficient than ozonation alone or ultrasonic irradiation alone without considering the operating costs. With the initial pH value at 10.0, the ozone dose at 4.5 g h(-1), the energy density of ultrasound at 176 W l(-1), and the initial concentration of RY84 at 100 mg l(-1), the extent of mineralization measured as TOC loss was maximized. The variation of the concentrations of related ions (oxalate, formate, acetate, NO3(-), NO2(-), NH4(+), Cl(-), and SO4(2-)) during the reaction process was monitored. Other organic intermediates detected by GC/MS were N-methyleneaniline, phthalic acid, 4-hydroxyphthalic acid, isocyanatobenzene, aniline, 4-iminocyclohexa-2,5-dien-1-one, butene diacid and urea. Based on these findings, a tentative degradation pathway was proposed.     

Kinetics of hydrolysis and cyclization of ethyl 2-(aminosulfonyl)benzoate to saccharin

The cyclization of ethyl 2-(aminosulfonyl)benzoate (ASB) to give saccharin was investigated in aqueous solutions at pH between 5.2 and 9.5 and in the temperature range of 296.2-334.2 K. The initial concentration of the reactant was varied between 1.45 x 10(-5) and 3.86 x 10(-4) M. Ultraviolet spectroscopy was used to obtain the kinetic data. The reaction is acid catalyzed and follows pseudo-first-order kinetics. The experimental rate constant, k(obs), increases with temperature and pH. Its dependence on the temperature and pH is well described by: k(obs) = k1 [OH-] = [(2.52 +/- 0.9) x 10(16) exp(-20.2 +/- 1 kcalmol(-1)/RT) s(-1)][OH-] A mechanism is proposed and the half-life of ethyl ASB is calculated.     

Adsorptive separation of cadmium from aqueous solutions and wastewaters by riverbed sand

Application of riverbed sand for the adsorptive separation of cadmium(II) from aqueous solutions has been investigated. Removal increased from 26.8 to 56.4% by decreasing the initial concentration of cadmium from 7.5 x 10(-5) to 1.0 x 10(-5)M at pH 6.5, 25 degrees C temperature, agitation speed of 100 rpm, 100 microm particle size and 1.0 x 10(-2) NaClO4 ionic strength. Process of separation is governed by first order rate kinetics. The value of rate constant of adsorption, k(ad), was found to be 2.30 x 10(-2)per min at 25 degrees C. Values of coefficient of mass transfer, beta L, were calculated and its value at 25 degrees C was found to be 1.92 x 10(-2)cm/s. Values of Langmuir constant were calculated. Values of thermodynamic parameters delta G0, delta H0 and delta S0 were also calculated and were recorded as -0.81 kcal/mol, -9.31 kcal/mol and -28.10 cal/mol at 25 degrees C. pH has been found to affect the removal of cadmium significantly and maximum removal, 58.4%, has been found at pH 8.5. Process can be used for treatment of cadmium(II) rich wastewaters.     

Photochemical behaviour of carbendazim in aqueous solution

To elucidate the photochemical behaviour of carbendazim (or MBC) in superficial waters, photolysis studies have been carried out in aqueous solutions at several pH using a UV light source (high pressure mercury arc lamp) or a solar light simulator (xenon arc lamp). The kinetics of photodecomposition of carbendazim was determined using HPLC-DAD and the identification of photoproducts was carried out with HPLC-MS (ESI negative and positive mode). According to the experimental results carbendazim is a rather stable molecule in the dark or in environmental conditions. The pH influence of the environmental medium on the photodegradation rate has been confirmed. The photochemical process can be considerably accelerated in alkaline solutions using HPK-quartz irradiation (quantum efficiency at pH 9 phi = 3.1 x 10(-3) degraded molecule per absorbed photon) while the photodegradation is not as efficient under a simulated sun irradiation (quantum efficiency in the suntest phi = 10(-4) at pH 7). Three photoproducts have been tentatively identified in pure water: 2-aminobenzimidazole, benzimidazole isocyanate and monocarbomethoxy-guanidine (issued from the cleavage of the benzimidazole ring). The last one seems very stable and could be accumulated in the environment.