WaterSubject

Organic pollutants are often present in drinking water, groundwater, and domestic and industrial wastewaters. A water treatment based on the chemical oxidation of organic compounds by advanced oxidation processes (AOPs) that is useful for purifying drinking water, groundwater and for cleaning industrial wastewater has been reported recently. Several of these studies have focused on using these systems as a pre-treatment for biological systems when the dissolved organic matter is toxic, inhibitory or recalcitrant to microorganisms. The degradation and mineralization of organic pollutants in wastewater by AOPs is based on the generation of a very reactive free hydroxyl radical (OH*). This radical is highly reactive, non-selective and may be used to degrade a wide range of organic pollutants. It reacts with most organic compounds by forming to a double bond or by abstracting hydrogen atoms from organic molecules. The resulting organic radicals then react with oxygen, which leads to the complete mineralization of CO₂, H₂O and mineral acids. Fenton and Fenton-like systems (Fe⁺²/Fe⁺³/H₂O₂) are often used for industrial water treatment based on AOPs. The degradation rate of organic pollutants with Fenton reagents strongly depends on irradiation with ultraviolet (UV) light, and increases with increased UV irradiation intensity. The use of UV light results in a significant increase in the cost of industrial water treatment. Post-treatment requires the elimination of Fenton reagents as colloidal precipitates. The separation of colloidal precipitates requires the use of additional processes such as coagulation, sedimentation and filtration.

Nanoparticles of inorganic materials, such as metal oxides and semiconductors, have generated considerable attention due to their novel properties compared with their bulk materials. A number of reports have shown that iron oxide has special photochemical and catalytic properties that strongly depend on particle size. However, only a few studies of the catalytic activity of colloidal iron-based nanoparticles have been made.

Catalyst recovery is required following water treatment using AOPs. In order to avoid this step, nanocatalysts may be immobilized on inert surfaces, foams or nanofibers without a reduction in catalytic activity. Therefore, investigation of the effectiveness and applicability of new nanoparticle catalysts may be divided into the following main steps:

• Synthesis of nanoparticle catalysts and testing of their catalytic behavior.

• Immobilization of nanocatalysts on inert surfaces, foams or nanofibers, and investigation of their catalytic behavior.

• Investigation of the regeneration of the adsorbent and the nanocatalyst in successive catalytic runs.

The objective of this research work is to continue the efforts of the first step: to test the catalytic behavior of iron oxide-based nanocatalysts through the degradation and mineralization of organic pollutants in wastewater as a possible method of treating contaminated groundwater and industrial wastewater.

For this study, typical organic contaminants, such as ethylene glycol and phenol, were chosen as simulating pollutants. Ethylene glycol is used in large quantities as a car-cooling fluid or as an airplane and runway deicer. Large quantities of ethylene glycol have created environmental hazards, leading to the serious pollution of drinking water. Several types of industrial waste contain phenols; they are very harmful and highly toxic towards microorganisms. Many phenol compounds are used as solvents or reagents in industrial processes and are therefore very common contaminants in industrial wastewater and contaminated drinking water sources.

This study presents information about the catalytic properties of iron-based nanoparticles for the degradation of some organic pollutants in wastewater by AOPs. The main conclusions are as follows:

• Ethylene glycol and phenol are efficiently destroyed by the Fenton-like reaction with iron(3) oxide-based nanocatalysts in the presence of hydrogen peroxide without ultraviolet (UV) light or any radiation sources.

• Complete oxygenation of ethylene glycol with iron oxide-based nanoparticles exhibits first-order reaction kinetics.

• A strong effect of nanocatalyst concentration on the reaction rate coefficient was shown.

• The ethylene glycol degradation rate with the iron-based nanoparticles without UV light or visible radiation sources is about 2–4 times higher than the values reported in the literature for Fenton’s reagent/H₂O₂ and UV.

• A strong effect of catalyst concentration on the phenol destruction rate was shown.

• The phenol destruction with the iron oxide-based nanocatalysts may be simulated by exponential decay.

• The phenol destruction rate with the iron oxide-based nanoparticles is about 35 times higher than the values reported in the literature for Fenton’s reagent/H₂O₂ and UV.

• No effect of nanocatalyst aging was observed.