The Destruction of Volatile Organic Compounds

The major stationary sources in North America of VOCs (Chapter 3) are the evaporation of organic solvents, the manufacture of chemicals, and the petroleum industry and its storage activities. Wastewater effluent that is contaminated with VOCs, e.g., the water emanating from chemical or petrochemical plants, is commonly treated by a two-step process:

1. The VOCs are removed from the wastewater by air stripping. In this process, air is passed upward into a downward stream of the water, and the volatile materials are transferred from the liquid to the gas phase. This technique does not work well for compounds that are highly water soluble.

2. The resulting VOCs, now present in low concentration in a contained mass of humid air, are destroyed by a process of catalytic oxidation. For example, air heated to 300-500°C is passed for a short time over platinum or, depending upon the VOC, some other precious metal that is supported on alumina. The energy costs of this step are very high since it involves heating a large volume of humid air. Note that the outlet air from such processes contains hydrogen chloride, HCl, if the VOCs originally contained chlorine; this compound must be removed by scrubbing with a basic substance before the air is released into the atmosphere.

The removal of VOCs from gaseous emissions from industries usually operates by the same catalytic oxidation process; typically the concentration of VOCs in the air stream is thereby reduced by 95%. A primary heat exchanger recovers and reuses the VOCs' heat of combustion to warm incoming gases to the operating temperature.

The adsorption of compounds onto activated carbon (see Box 14-1) or onto synthetic carbonaceous adsorbents is a cost-effective technology used for the removal of low-level VOC concentrations from both liquid and vapor streams; it is also useful for nonvolatile organic compounds. These adsorbents can be easily regenerated by treatment with steam or by other thermal techniques as well as by solvents; the concentrated pollutants can be subsequently destroyed by catalytic oxidation.

Advanced Oxidation Methods for Water Purification

Conventional water purification methods often do not successfully deal with synthetic organic compounds such as chloroorganics that are dissolved at low concentrations; examples include the common groundwater pollutants trichloroethene and perchloroethene. The conventional method for the treatment of water containing such pollutants is adsorption of the chloroorganics onto activated carbon; this removes the compounds but doesn't destroy them. The wastewater from pulp-and-paper mills also contains organochlorines that are resistant to conventional treatments.

In order to cleanse water of these extra-stable organics, so-called advanced oxidation methods (AOMs) have been developed and deployed. The aim of these methods is to mineralize the pollutants, i.e., to convert them entirely to C02, H20, and mineral acids such as HC1. Most AOMs are ambient-temperature processes that use energy to produce highly reactive intermediates of high oxidizing or reducing potential, which then attack and destroy the target compounds. The majority of the AOMs involve the generation of significant amounts of the hydroxyl free radical, OH, which in aqueous solution is a very effective oxidizing agent, as it is in air (see Chapters 1-5). The hydroxyl radical can initiate the oxidation of a molecule by extraction of a hydrogen atom or addition to one atom of a multiple bond, as it does in air (Chapter 5); in water, as an additional alternative, it can also extract an electron from an anion.

Since the generation of OH in solution is a relatively expensive process, it is economical to use AOMs to treat only the components of the wastes that are resistant to the cheaper, conventional treatment processes. Thus, integrating an AOM with pretreatment of the wastewater by biological or other processes to first dispose of the easily oxidized materials is often appropriate.

Ultraviolet (UV) light is often used to initiate the production of hydroxyl radicals and thus to begin the oxidations. Commonly, hydrogen peroxide, H202, is added to the polluted water and UV light from a strong source in the 200-300-nm range is shone on the solution. The hydrogen peroxide absorbs the ultraviolet light (especially that closer to 200 nm than to 300 nm) and uses the energy obtained to split the O—O bond, resulting in the formation of two OH radicals:

Alternatively, and less commonly, ozone is produced and then photochemi-cally decomposed by UV light. The resulting oxygen atom reacts with water to efficiently produce OH via the intermediate production of hydrogen peroxide, which is photolyzed:

o3—>o2 + o o* + H20 H202-» 2 OH

A fraction of the oxygen atoms produced by ozone photolysis are electronically excited, and these react with water to directly produce hydroxyl radicals, as discussed in Chapter 1.

PROBLEM 14-15

Given that the enthalpies of formation for H202 and OH are, respectively, —136.3 and +39.0 kj mol-1, calculate the heat energy required to dissociate one mole of hydrogen peroxide into hydroxyl free radicals. What is the maximum wavelength of light that could bring about this transformation? [Hint: See Chapter I J. Given that light of 254-nm wavelength is usually used, and that all the energy of each photon that is in excess of that required to dissociate one molecule is lost as waste heat, calculate the maximum percentage of the input light energy that can be used for dissociation itself.

Hydroxyl radicals for wastewater treatment can also be efficiently produced without the use of UV light by combining hydrogen peroxide with ozone. The chemistry of the intermediate processes is complex, but the overall reaction between these two species is

H202 + 2 03-► 2 OH + 3 02

This ozonelH202 method is more cost-effective and easier to adapt to existing water treatment systems than is any other AOM system.

It is also possible to generate the hydroxyl radical electrolytically. In most such applications, a metal ion (such as Ag or

Ce3+) is first oxidized to a more positively charged ion (Ag2+ or Ce4+ in our examples) that will subsequently oxidize water to H+ and OH.

The biggest liability associated with advanced oxidation processes is that their action produces toxic chemical by-products. For example, in the ozone/ peroxide and peroxide/UV treatments of groundwater contaminated with trichloroethene and perchloroethene, the toxic intermediates trichloroacetic acid, CCI3COOH, and dichloroacetic acid, CHCl2COOH, are formed in about 1% yield.

Continue reading here: Photocatalytic Processes

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