Degradation of carbofuran by using ozone, UV radiation and advanced oxidation processes.

The degradation of carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yl methylcarbamate), a frequently used carbamate derivative pesticide that is considered a priority pollutant, is carried out in batch reactors by means of single oxidants: ozone, UV radiation and Fenton's reagent; and by the advanced oxidation processes (AOPs) constituted by combinations of ozone plus UV radiation, UV radiation plus H(2)O(2), and UV radiation plus Fenton's reagent (photo-Fenton system). For all these reactions, the apparent pseudo-first-order rate constants are evaluated in order to compare the efficiency of each process. In addition and by means of a competition kinetic model, the rate constants for the reaction of carbofuran with ozone and with hydroxyl radicals are also determined. The improvement in the decomposition levels of carbofuran reached by the combined processes in relation to the single oxidants, due to the generation of the very reactive hydroxyl radicals, is also established in every process. For the oxidant concentrations applied, the most effective process in removing carbofuran was the photo-Fenton system.     

Color, TOC and AOX removals from pulp mill effluent by advanced oxidation processes: a comparative study

Pulp mill effluent containing toxic chemicals was treated by different advanced oxidation processes (AOPs) consisting of treatments by hydrogen peroxide, Fenton's reagent (H2O2/Fe2+), UV, UV/H2O2, photo-Fenton (UV/H2O2/Fe2+), ozonation and peroxone (ozone/H2O2) in laboratory-scale reactors for color, total organic carbon (TOC) and adsorbable organic halogens (AOX) removals from the pulp mill effluent. Effects of some operating parameters such as the initial pH, oxidant and catalyst concentrations on TOC, color, AOX removals were investigated. Almost every method used resulted in some degree of color removal from the pulp mill effluent. However, the Fenton's reagent utilizing H2O2/Fe2+ resulted in the highest color, TOC and AOX removals under acidic conditions when compared with the other AOPs tested. Approximately, 88% TOC, 85% color and 89% AOX removals were obtained by the Fenton's reagent at pH 5 within 30 min. Photo-Fenton process yielded comparable TOC (85%), color (82%) and AOX (93%) removals within 5 min due to oxidations by UV light in addition to the Fenton's reagent. Fast oxidation reactions by the photo-Fenton treatment makes this approach more favorable as compared to the others used.     

Photodegradation of direct yellow-12 using UV/H2O2/Fe2+

A detailed investigation of photodegradation of direct yellow-12 (DY12) using UV/H(2)O(2)/Fe(2+) has been carried out in a photochemical reactor. Experiments studied degradation as a function of concentration, decolorization and reduction in chemical oxygen demand (COD). The effect of operating parameters, such as UV, pH, amount of Fenton's reagent (H(2)O(2) and FeSO(4)), and amount of DY12 dye has also been determined. It has been observed that simultaneous utilization of UV irradiation with Fenton's reagent increases the degradation rate of DY12 dye. The dye quickly losses its color and there is an appreciable decrease in COD value, indicating that the dissolved organic have been oxidized. The kinetics of degradation of the dye in dilute aqueous solutions follows pseudo-first order kinetics. Final products detected at the end of the reaction include NO(3)(-), NO(2)(-), N(2)O, NO(2), SO(2), CO(2) and CO. Results indicate that dye degradation is dependent upon pH, UV-intensity, concentration of Fenton's reagent and dye. Acidic pH has been found to be more suitable in comparison to neutral and alkaline. The optimum concentration of Fenton's reagent (H(2)O(2)/Fe(2+)) was found as 1500/500 mg l(-1) for 50 mg l(-1) DY12 dye in water at pH 4. The results indicate that the treatment of DY12 dye wastewater with UV/Fe(2+)/H(2)O(2) system is efficient.     

The mechanism and applicability of in situ oxidation of trichloroethylene with Fenton's reagent

Fenton's reagent is the result of reaction between hydrogen peroxide (H(2)O(2)) and ferrous iron (Fe(2+)), producing the hydroxyl radical (-*OH). The hydroxyl radical is a strong oxidant capable of oxidizing various organic compounds. The mechanism of oxidizing trichloroethylene (TCE) in groundwater and soil slurries with Fenton's reagent and the feasibility of injecting Fenton's reagent into a sandy aquifer were examined with bench-scale soil column and batch experiment studies. Under batch experimental conditions and low pH values ( approximately 3), Fenton's reagent was able to oxidize 93-100% (by weight) of dissolved TCE in groundwater and 98-102% (by weight) of TCE in soil slurries. Hydrogen peroxide decomposed rapidly in the test soil medium in both batch and column experiments. Due to competition between H(2)O(2) and TCE for hydroxyl radicals in the aqueous solutions and soil slurries, the presence of TCE significantly decreased the degradation rate of H(2)O(2) and was preferentially degraded by hydroxyl radicals. In the batch experiments, Fenton's reagent was able to completely dechlorinate the aqueous-phase TCE with and without the presence of soil and no VOC intermediates or by-products were found in the oxidation process. In the soil column experiments, it was found that application of high concentrations of H(2)O(2) with addition of no Fe(2+) generated large quantities of gas in a short period of time, sparging about 70% of the dissolved TCE into the gaseous phase with little or no detectable oxidation taking place. Fenton's reagent completely oxidized the dissolved phase TCE in the soil column experiment when TCE and Fenton's regent were simultaneously fed into the column. The results of this study showed that the feasibility of injecting Fenton's reagent or H(2)O(2) as a Fenton-type oxidant into the subsurface is highly dependent on the soil oxidant demand (SOD), presence of sufficient quantities of ferrous iron in the application area, and the proximity of the injection area to the zone of high aqueous concentration of the target contaminant. Also, it was found that in situ application of H(2)O(2) could have a gas-sparging effect on the dissolved VOC in groundwater, requiring careful attention to the remedial system design.     

Alkydic resin wastewaters treatment by fenton and photo-Fenton processes

Advanced oxidation processes are an emerging option to treatment of the painting industry effluents. The aim of this study was to compare the effectiveness of the Fenton and photo-Fenton processes in chemical oxygen demand (COD), total organic carbon (TOC) and phenolic compounds removal from wastewaters generated during alkydic resins manufacture. The optimized treatment conditions are the following: pH 3.0, 15.15x10(-3)molL(-1) FeSO(4) and 0.30molL(-1) H(2)O(2) for a reaction time of 6h. photo-Fenton experiments were carried out in the presence of sunlight or artificial radiation and complementary additions of H(2)O(2) were made for all experiments. The best results were obtained with photo-Fenton process assisted with solar radiation, with reductions of 99.5 and 99.1% of COD and TOC levels, respectively. Fenton and photo-Fenton (with artificial irradiation) processes presented lower but not insignificant removals, within 60-80% reduction for both COD and TOC. In addition, an excellent removal (95%) of total phenols was obtained using photo-Fenton process assisted with artificial irradiation. This study demonstrated that the use of photo-Fenton process on alkydic resins wastewater treatment is very promising especially when solar light is used.     

Degradation of carbon tetrachloride by modified Fenton's reagent

The degradation of tetrachloromethane (carbon tetrachloride-CT) by modified Fenton's reagent (catalyzed hydrogen peroxide) was investigated using a range of hydrogen peroxide concentrations and 1 mM iron(III) catalyst. The documented reactive species in modified Fenton's reactions, hydroxyl radical (OH*), is not reactive with CT, yet CT degradation was observed in the Fenton's reactions and was confirmed by chloride generation. Because CT is not reactive with OH*, a reductive mechanism which may involve superoxide radical anion is proposed for CT degradation in modified Fenton's systems. Scavenging of reductants by excess chloroform prevented CT degradation, confirming a reductive mechanism. Similar to CT, three other oxidized aliphatic compounds, hexachloroethane, bromotrichloromethane, and tetranitromethane, were also degraded by modified Fenton's reagent. The results show that modified Fenton's reactions act through a reductive mechanism to degrade compounds that are not reactive with OH*, which broadens the scope of this process for hazardous waste treatment and remediation.     

Decontamination of soil washing wastewater using solar driven advanced oxidation processes

Decontamination of soil washing wastewater was performed using two different solar driven advanced oxidation processes (AOPs): the photo-Fenton reaction and the cobalt/peroxymonosulfate/ultraviolet (Co/PMS/UV) process. Complete sodium dodecyl sulphate (SDS), the surfactant agent used to enhance soil washing process, degradation was achieved when the Co/PMS/UV process was used. In the case of photo-Fenton reaction, almost complete SDS degradation was achieved after the use of almost four times the actual energy amount required by the Co/PMS/UV process. Initial reaction rate in the first 15 min (IR₁₅) was determined for each process in order to compare them. Highest IR₁₅ value was determined for the Co/PMS/UV process (0.011 mmol/min) followed by the photo-Fenton reaction (0.0072 mmol/min) and the dark Co/PMS and Fenton processes (IR₁₅ = 0.002 mmol/min in both cases). Organic matter depletion in the wastewater, as the sum of surfactant and total petroleum hydrocarbons present (measured as chemical oxygen demand, COD), was also determined for both solar driven processes. It was found that, for the case of COD, the highest removal (69%) was achieved when photo-Fenton reaction was used whereas Co/PMS/UV process yielded a slightly lower removal (51%). In both cases, organic matter removal achieved was over 50%, which can be consider proper for the coupling of the tested AOPs with conventional wastewater treatment processes such as biodegradation.     

Purification of storage brines from the preservation of table olives

The chemical oxidation of the wastewaters generated during storage of table olives in NaCl brines, prior to their manufacturing process, was studied. Ozone alone produced COD removals in the range 14-23%, and a higher average removal of 73% of the aromatic compounds. The additional presence of hydrogen peroxide and UV radiation increased these values to 39% for COD and 86% for aromatics. However, UV radiation alone only gave a removal of 9% for COD and 27% for aromatics, and the additional presence of 0.5M H(2)O(2) led to 13% for COD and 38% for aromatics, respectively. The Fenton's reagent oxidation achieved a COD removal of 24% for the higher concentrations of Fe(2+) and H(2)O(2). The most effective process was the combination O(3)/UV/H(2)O(2) with total removals of 65 and 92% for the COD and aromatics, respectively. The aerobic treatment of these effluents gave a 66% removal regardless of the initial biomass concentration used, and a rate constant of 0.19 per day was obtained for the process by using the Contois model. Finally, the aerobic treatment of the wastewaters previously ozonated alone, and ozonated with UV radiation, gave increases in the COD removal and a final rate constant of 0.44 per day. The enhancements were due to the chemical oxidations, these procedures being suitable technologies as pre-treatments to subsequent biological processes for the purification of these residues.     

A comparative study of ultrasonic cavitation and Fenton's reagent for bisphenol A degradation in deionised and natural waters

Bisphenol A (BPA), a xenobiotic that exhibits endocrine disrupting action can be found in surface water. Its complete elimination can be obtained by advanced oxidation processes, notably upon the application of ultrasonic waves. In order to evaluate the feature of ultrasound relevance and the involvement of the hydroxyl radical in the BPA sonochemical degradation, ultrasound action was compared to Fenton's reaction in the cases of deionised acidic water (pH 3) and natural water (pH 7.6, main ions concentration: Ca(2+)=486mgL(-1), Na(+)=9.1mgL(-1), Cl(-)=10mg L(-1), SO(4)(2-)=1187mgL(-1), HCO(3)(-)=402mgL(-1)). Ultrasound was performed at 300kHz and 80W. Fenton's process was operated using ferrous sulphate (100micromolL(-1)) and continuous H(2)O(2) addition at the rate as it is produced when sonication is applied in water in absence of substrate. Experiments carried out in deionised water show that both processes exhibit identical BPA elimination rate and identical primary intermediates. Main chemical pathways involve reactions with OH radical. Chemical oxygen demand (COD) and total organic carbon (TOC) analyses show that the Fenton's process is slightly more efficient than ultrasonic treatment for the removal of BPA by-products in the case of deionised water. Experiments conducted in natural water evidenced the inhibition of the Fenton process while the ultrasound action was not hampered.     

Degradation of sodium dodecyl sulphate in water using solar driven Fenton-like advanced oxidation processes

Synthetic wastewater samples containing a model surfactant were treated using two different Fenton-like advanced oxidation processes promoted by solar radiation; the photo-Fenton reaction and Co/PMS/UV processes. Comparison between the different experimental conditions was performed by means of the overall surfactant degradation achieved and by obtaining the initial rate in the first 15 min of reaction (IR15). It was found that, for dark Fenton reaction, the maximum surfactant degradation achieved was 14% under low iron and oxidant concentration. Increasing Fenton reagents by one magnitude order, surfactant degradation achieved 63% in 60 min. The use of solar radiation improved the reaction rate by 17% under same conditions and an additional increase of 12.5% was obtained by adjusting initial pH to 2. IR15 values for dark and irradiated Fenton reactions were 0.143 and 0.154 mmol/min, respectively, for similar reaction conditions and this value increased to 0.189 mmol/min when initial pH was adjusted. The use of the Co/PMS system allow us to determine an increase in the degradation rate, for low reaction conditions (1 mM of transition metal; 4 mM oxidant) similar to those used in dark Fenton reaction. Surfactant degradation increased from 3%, for Fenton reaction, to 44.5% in the case of Co/PMS. When solar irradiation was included in the experiments, under same reaction conditions described earlier, surfactant degradation up to 64% was achieved. By increasing Co/PMS reagent concentration by almost 9 times under irradiated conditions, almost complete (>99%) surfactant degradation was reached in 5 min. Comparing IR15 values for Co/PMS and Co/PMS/UV, it allow us to observe that the use of solar radiation increased the degradation rate in one magnitude order when compared with dark experiments and further increase of reagent concentration increased reaction rate twice.     

Identification of produced powerful radicals involved in the mineralization of bisphenol A using a novel UV-Na(2)S(2)O(8)/H(2)O(2)-Fe(II,III) two-stage oxidation process

A two-stage oxidation (UV-Na(2)S(2)O(8)/H(2)O(2)-Fe(II,III)) process was applied to mineralize bisphenol A (BPA) at pH(i) (initial pH) 7. We take advantage of the high oxidation potential of sulfate radicals and use persulfate as the 1st-stage oxidant to oxidize BPA to less complex compounds (stoichiometric ratio: [S(2)O(8)(2-)](0)/[BPA](0)=1). Afterwards, the traditional photo-Fenton process was used to mineralize those compounds to CO(2). To the best of our knowledge, this is the first attempt to utilize the two processes in conjunction for the complete degradation of BPA. During the 2nd-stage reaction, other oxidants (H(2)O(2) and Iron alone) were also employed to observe the extent of enhancement of photo-Fenton. Further, qualitative identification of both hydroxyl and sulfate radicals was performed to evaluate their dominance under different conditions. The BPA degradation in this UV/persulfate process formulated a pseudo-first-order kinetic model well, with a rate constant of approximately 0.038 min(-1) (25 degrees C), 0.057 min(-1) (35 degrees C), and 0.087 min(-1) (50 degrees C), respectively. The much lower activation energy (DeltaE = 26 kJ mol(-1)) was further calculated to clarify that the thermal-effect of an illuminated system differs from single heat-assisted systems described in other research. Final total organic carbon (TOC) removal levels of BPA by the use of such two-stage oxidation processes were 25-34%, 25%, and 87-91% for additional Fe(II,III) activation, H(2)O(2) promotion, and Fe(II,III)/H(2)O(2) promotions, respectively.     

Degradation and detoxification of formaline wastewater by advanced oxidation processes

In this study, advanced oxidation processes (AOPs) utilizing the combinations of UV/H(2)O(2), Fenton, photo-Fenton and the combination of Fenton/photo-Fenton reactions were investigated in lab-scale experiments for the degradation of formaline wastewater. The studied toxic chemicals were formaldehyde and methanol in mixture solution, so-called formalin, which is the embalming agent in mortuaries. The experimental results showed that the photo-Fenton process was the most effective treatment process among the studied AOPs. Pseudo-first-order degradation rate constants of formaldehyde and methanol were obtained from batch experimental data. In the combination of Fenton/photo-Fenton reactions, the results show that applying UV light at an early stage of the reaction might not be necessary for a speedy oxidation reaction of the Fenton process. With Fenton and photo-Fenton processes, mineralization of formaline wastewater can be achieved, as no residual TOC is detected in the effluent after the reaction period. It is suggested that Fenton and photo-Fenton processes are viable techniques for the formaline wastewater treatment as they were able to provide high degradation of formaldehyde and methanol with relatively low toxicity of the by-products in the effluent.     

Assessment of Fenton's reagent and ozonation as pre-treatments for increasing the biodegradability of aqueous diethanolamine solutions from an oil refinery gas sweetening process

The aim of this work was to evaluate the efficiency of three chemical oxidation processes for increasing the biodegradability of aqueous diethanolamine solutions (aqueous DEA solutions), to be used as pre-treatments before a biological process. The raw aqueous DEA solution, sourced from a sour gas sweetening plant at a Mexican oil refinery, was first characterized by standardized physico-chemical methods. Then experiments were conducted on diluted aqueous DEA solutions to test the effects of Fenton's reagent, ozone and ozone-hydrogen peroxide on the removal of some physicochemical parameters of these solutions. Lastly, biodegradability tests based on Dissolved Organic Carbon Die Away OECD301-A, were carried out on a dilution of the raw aqueous DEA solution and on the treated aqueous DEA solutions, produced by applying the best experimental conditions determined during the aforementioned oxidation tests. Experimental results showed that for aqueous DEA solutions treated with Fenton's reagent, the best degradation rate (70%) was obtained at pH 2.8, with Fe(2+) and H(2)O(2) at doses of 1000 and 10,000 mg/L respectively. In the ozone process, the best degradation (60%) was observed in aqueous DEA solution (100 mg COD/L), using 100 mg O(3)/L at pH 5. In the ozone-hydrogen peroxide process, no COD or DOC removals were observed. The diluted spent diethanolamine solution showed its greatest increase in biodegradability after a reaction period of 28 days when treated with Fenton's reagent, but after only 15 days in the case of ozonation.     

Feasibility study for the remediation of groundwater contaminated by organolead compounds

The aim of this research was to assess the effectiveness of chemical oxidation, Advanced Oxidation Processes (AOPs) and adsorption on granular activated carbon (GAC) for the ex situ remediation of a groundwater contaminated by organolead compounds, including tetraethyl lead (TEL), triethyl lead (TREL) and diethyl lead (DEL). The groundwater of concern was collected from the site of a former tetraalkyllead producing company in Trento (Italy), and showed an average total organic lead (TOL) content about 95.1 microg/L (TEL 0.5 microg/L, TREL 86.4 microg/L, DEL 8.3 microg/L). The main target of the study was to find out which method was more effective in reducing the pollutant content. For this purpose, several laboratory tests were performed, including chemical oxidation tests with different reactants (hydrogen peroxide, modified Fenton's reagent, potassium permanganate, activated potassium persulfate, oxygen and combinations of potassium permanganate and modified Fenton's reagent), AOPs with ozone, UV radiation and hydrogen peroxide and filtration on granular activated carbon. A combination of chemical and physical treatments was also tested, with GAC filtration followed by chemical oxidation. According to the results achieved, the treatments which showed the best remediation performances were: chemical oxidation with modified Fenton's reagent, AOPs with hydrogen peroxide and ozone (perozone), AOPs with hydrogen peroxide and UV radiation, and the combined treatment with activated carbon filtration followed by chemical oxidation with perozone. All these treatments showed a 90% TOL removal, with excellent removals of both TEL and TREL, and final DEL concentrations below 5 microg/L.     

Co-oxidation of p-hydroxybenzoic acid and atrazine by the Fenton's like system Fe(III)/H2O2

The system Fe(III)/H2O2 has been used to oxidise an aqueous solution of p-hydroxybenzoic acid (pHB) in the absence of light. In the process, typical operating variables such as reagent concentration exert a positive influence in the pHB degradation rate. Optimum pH has been found to be around 3. The kinetic study suggests that the mechanism involved in this system differs to some extent from that reported for the classic Fenton's chemistry in pure water. Thus, formation of a complex Fe(III)-pHB seems to be a key step to initiate the oxidising mechanism. Stoichiometric measurements of the H2O2 consumption per mole of pHB degraded indicate a possible reduction of complexed Fe(III). Simultaneous oxidation of pHB (and other similar compounds such as tyrosol (Ty) or p-coumaric acid (pCu)) and atrazine have shown a synergistic effect of the first substance to remove the pesticide.     

Monitoring of decolorization kinetics of Reactive Brilliant Blue X-BR by online spectrophotometric method in Fenton oxidation process

Online spectrophotometry method is employed to monitor simulated Reactive Brilliant Blue X-BR (RBB X-BR) in aqueous solution in Fenton oxidation process. The effects of initial dosage of FeSO(4) and H(2)O(2), pH value, initial concentration of dye and temperature have been studied. The results show that online spectrophotometric method is a quick, feasible and convenient technique to monitor color removal of RBB X-BR in Fenton process. The optimal dosage of H(2)O(2) and pH is 3.529mM and 3, respectively. The optimal dosage of Fe(2+) for color removal is 0.1618mM. The concentration of initial FeSO(4) against the reaction rate constant (k(ap)) for decolorizing is linear correlation as: k(ap)=0.1354[Fe(2+)](o) (R(2)>0.99). The apparent activation energies of reaction is 25.21kJmol(-1) (R(2)>0.99). The intrinsic reaction rate constant of OH with RBB X-BR in aqueous solution is 7.396x10(10)M(-1)s(-1). The molecule structure of RBB X-BR is decomposed and not mineralized by Fenton's reagent. The main intermediate products are 1,2-diacetylenzene and 2,5-diritrobenzoic acid. The probable mechanism of the decoloration of RBB X-BR is also discussed.     

Oxidation of p-hydroxybenzoic acid by UV radiation and by TiO2/UV radiation: comparison and modelling of reaction kinetic

The phenolic compound p-hydroxybenzoic acid is very common in a great variety of agroindustrial wastewaters (olive oil and table olive industries, distilleries). The objective of this work was to study the photocatalytic activity of TiO2 towards the decomposition of p-hydroxybenzoic acid. In order to demonstrate the greater oxidizing power of the photocatalytic system and to quantify the additional levels of degradation attained, we performed experiments on the oxidation of p-hydroxybenzoic acid by UV radiation alone and by the TiO2/UV radiation combination. A kinetic model is applied for the photooxidation by UV radiation and by the TiO(2)/UV system. Experimental results indicated that the kinetics for both oxidation processes can be fitted well by a pseudo-first-order kinetic model. The second oxidation process can be explained in terms of the Langmuir-Hinshelwood kinetic model. The values of the adsorption equilibrium constant, K(pHB), and the second order kinetic rate constant, k(c), were 0.37 ppm(-1) and 6.99 ppm min(-1), respectively. Finally, a comparison between the kinetic rate constants for two oxidation systems reveals that the constants for the TiO2/UV system are clearly greater (between 220-435%) than those obtained in the direct UV photooxidation.     

Photo-Fenton discoloration of the azo dye X-3B over pillared bentonites containing iron

Both Fe pillared bentonite (Fe-B) and Al-Fe pillared bentonite (Al/Fe-B) were prepared and used as heterogeneous catalysts for the photo-Fenton discoloration of azo dye X-3B under UV irradiation. The catalysts were characterized by XRD, BET and TEM. The effects of solution pH, H(2)O(2) concentration, dye concentration and catalyst loading on the rate of discoloration were investigated in detail. The results indicate that the Fe-B and Al/Fe-B have high BET surface area (114.6 and 194.2 m(2)/g, respectively). Both the heterogeneous photo-Fenton processes employing the Fe-B or the Al/Fe-B as catalyst exhibit higher photo-catalytic activity compared to their corresponding homogeneous photo-Fenton process. The amount of Fe ions leached from the Al/Fe-B into the solution is much lower than that leached from the Fe-B during the reaction process.     

Photocatalytic degradation of Direct Red 23 dye using UV/TiO2: Effect of operational parameters

In this study, the photocatalytic degradation of Direct Red 23 (Scarlet F-4BS) was investigated in UV/TiO2 system. The effect of catalyst loading and pH on the reaction rate was ascertained and optimum conditions for maximum degradation were determined. The results obtained showed that acidic pH is proper for the photocatalytic removal of Direct Red 23. In addition, the effects of several cations (Cu2+, Al3+, Cr3+, and Sn4+) and anions (BiO3(-), SO4(2-), and CN(-)) and C2H5OH were examined in this photocatalytic process. On the order hand, three types of catalysts (Fe2O3, SnO2, and ZnO) were compared with TiO2. After 90 min reaction, the relative decomposition order established was UV/TiO2>UV/SnO2>UV/Fe2O3>UV/ZnO.     

Oxidation of dichlorvos with hydrogen peroxide using ferrous ion as catalyst.

This study examines how Fenton's reagent (Fe2+ and H2O2) decomposed dichlorvos insecticide. Results showed that dichlorvos decomposed in a two-stage reaction. The first stage is a Fe2+/H2O2 reaction in which dichlorvos swiftly decomposed. In the second stage, dichlorvos decomposed somewhat less rapidly, and it is a Fe3+/H2O2 reaction. The detection of ferrous ions also supports the theory of the two-stage reaction for the dichlorvos oxidation with Fenton's reagent. The dissolved oxygen of the solution decreased rapidly in the first stage reaction, but it slowly increased in the second stage with a zero-order kinetics. The Fenton system decomposed dichlorvos most rapidly when the initial pH in the solution is 3-4. In addition, increasing the concentration of hydrogen peroxide or ferrous ions can enhance the decomposition of dichlorvos. Consequently, the relationship of rate constant (kobs), [H2O2] and [Fe2+] at initial pH 3 is determined as kobs = 2.67 x 10(4)[H2O2]0.7[Fe2+]1.2.