Relative oxidation power (C12=1.0)
Oxidative potential (V)
Hydroxyl radical Atomic oxygen (singlet) Ozone
Hydrogen peroxide Perhydroxyl radical Permanganate Hypobromous acid Chlorine dioxide Hypochlorous acid Hypoiodous acid Chlorine Bromine Iodine
3.03 2.80 2.42 2.07 1.78 1.70 1.68 1.59 1.57 1.49 1.45 1.36 1.09 0.54
pesticide industry are chlorine and hydrogen peroxide (H2O2). However, the use of chlorine may create objectionable chlororganics such as chloromethanes and chlorophenols in the wastewater. When organic pollutant concentrations are very high, the use of chemical oxidation may be too expensive because of the high chemical dosages and long retention time required.
At least nine United States pesticide manufacturers use chemical oxidation to treat wastewater . In these systems, more than 98% of cyanide, phenol, and pesticides are removed; COD and other organics are reduced considerably. Some plants use chemical oxidation to reduce toxic compounds from the wastewater to make the streams more suitable for subsequent biological treatment.
Reynolds et al.  conducted a comprehensive review of aqueous ozonation of five groups of pesticides: chlorinated hydrocarbons, organophosphorus compounds, phenoxyalkyl acid derivatives, organonitrogen compounds, and phenolic compounds. Generally, chlorinated compounds were more resistant to ozonation than the other groups. With the exception of a few pesticides, most of the compounds in the four other groups could achieve complete destruction upon ozonation. The presence of bicarbonate ions could decrease reaction rates by acting as free radical scavengers. Contact times and pH were important parameters. Atrazine destruction by ozonation was evaluated in a bench-scale study in the presence of manganese . Mncatalyzed ozonation was enhanced in the presence of a small amounts of humic substances (1 mg/L as DOC).
A newer development in chemical oxidation is the combination of ultraviolet (UV) irradiation with H2O2 and/or ozone (O3) oxidation. This combination generates hydroxyl radical, which is a stronger oxidant than ozone or H2O2. The UV light also increases the reactivities of the compounds to be oxidized by exciting the electrons of the molecules to higher energy levels . As a result, lower chemical dosages and much higher reaction rates than other oxidation methods can be realized. When adequate chemical dosages and reaction times are provided, pesticides and other organic compounds can be oxidized to carbon dioxide, inorganic salts, and water . Beltran et al.  evaluated atrazine removal in bubble reactors by treating three surface waters with ozone, ozone in combination with H2O2 or UV radiation. Surface water with low alkalinity and high pH resulted in the highest atrazine removal, and ozonation combined with H2O2 or UV radiation led to higher atrazine removal and higher intermediates formation as compared to single ozonation or UV radiation.
The UV/O3 process has been shown to be effective in destroying many pesticides in water . Pilot tests conducted in California on synthetic pesticide wastewaters demonstrated that 15 mg/L each of organic phosphorous, organic chlorine, and carbamate pesticides can be UV-oxidized to nondetectable concentrations . Figure 12 shows a UV/oxidation process flow diagram with the option of feeding both O3 and H2O2. The combination of O3 and H2O2 without UV can also generate the powerful hydroxyl radicals and can result in catalyzed oxidation of organics .
The UV/O3 process was investigated as a pretreatment step to biological treatment by measuring biodegradability (BOD5/COD), toxicity (ED50), and mineralization efficiency of treated pesticide-containing wastewater . The investigator found that after treatment of an industrial pesticide wastewater by the UV/O3 process for one hour, COD was reduced by only 6.2% and TOC by merely 2.4%. However, the value of BOD5/COD increased significantly so that the wastewater was easily biodegradable (BOD5/COD>0.4)
and the toxicity obviously declined (EC50 reduction>50%). The UV light intensity used was 3.0 mW/cm2 and O3 supply rate was 400 g/m3/hour. The investigator concluded that using UV/O3 as pretreatment for a biological unit is an economical approach to treating industrial wastewaters containing xenobiotic organics as most part of the mineralization work is done by the biological unit rather than photolytic ozonation.
Balmer and Sulzberger  found that the kinetics of atrazine degradation by hydroxyl radicals in photo-Fenton systems were controlled by iron speciation, which further depended upon pH and oxalate concentration. Nguyen and Zahir  found that the photodecomposition of the herbicide methyl viologen with UV light was a hemolytic process leading to the formation of methyl pyridinium radicals, which then underwent photolysis at a much faster rate, producing environmentally benign byproducts. In a separate study, Lu  investigated the photocatalytic oxidation of the insecticide propoxur, in the presence of TiO2 supported on activated carbon. Photodegradation of the insecticide followed a pseudo-first-order kinetics described by the Langmuir-Hinshelwood equation. Photocatalytic oxidation of the fungicide metalaxyl in aqueous suspensions containing TiO2 was explained in terms of the Langmuir-Hinshelwood kinetic model .
Was this article helpful?