Redox alloy media provide one approach.

By Dr. Lars Guenter Beholz

Until recently, methods for removing chlorine from water have remained relatively unchanged. The two primary methods have been activated carbon for use in potable water chlorine removal applications, and sodium bisulfite and other S02-bearing chemicals for use in larger flow chlorine removal applications.

Although these methods work well, each has drawbacks. Non-catalytic carbons inherently have a relatively short life span, and chemical treatments ultimately pose disposal problems. An option that has emerged from two different water treatment arenas involves the electrochemical reduction of chlorine through redox chemistry.

Redox Explained

Reactions in which atoms undergo changes in oxidation number are called oxidation-reduction or "redox" reactions. In general, these reactions involve the transfer of electrons from one atom or molecule to another. The atom or molecule that loses the electrons is oxidized and is the reducing agent. The atom or molecule that gains the electrons is reduced and is the oxidizing agent because it had to retrieve electrons from something else.

The simplest way to keep track of all these terms is to remember that because an electron carries a negative charge, the overall charge of the species is reduced if it gains an electron. An amusing mnemonic device is: "LEO the lion goes GER." In this, "LEO" is "Losing Electrons is Oxidation," and "GER" is "Gaining Electrons is Reduction."

This process may be illustrated with a common laboratory zinc/copper (Zn/Cu) voltaic cell. In this illustration, one beaker contains a strip of metallic copper immersed in 1 molar (M) copper sulfate solution and the other beaker contains a strip of zinc immersed in 1 M zinc sulfate solution.

The metal strips are connected with a wire, allowing for electron flow. The salt bridge allows the passage of a charge through the wire in the form of electrons, to be balanced by the passage of a charge in the opposite direction in the form of ions.

Because of the difference in electrical potential between the two metals, the electrons flow spontaneously and are available to do external work. This work may be powering a light bulb or reducing chlorine to chloride.

The Role of Media

Dissimilar metal "redox alloy media" (RAM) most closely resemble this cell. In fact, these media are composed entirely of high-purity zinc and copper. Because the metals are alloyed, there's no need for the wire as with the voltaic cell. The solutions and salt bridge are replaced by water (the electrolyte solution) passing through the media.

The most significant difference between RAM treatment of water and the voltaic cell is that in the voltaic cell, there's no more power available once all the free ions and electrons have moved to their final destination. In systems where water is constantly flowing past the metal alloy, the ability of electrons to flow continues for a very long time.

In the dechlorination of water, chlorine is reduced to chloride by addition of electrons. These electrons may be donated by a variety of sources, so reduction of chlorine occurs naturally over a long period of time. The key to efficient chlorine removal for water treatment purposes thus lies in the ability to speed up the reduction process. Most commonly, this is achieved through a redox medium such as a RAM or catalytic carbon.

Use of these media have a long history. Copper-zinc alloys and couples have been used in oxidation-reduction, reductive coupling, cyclopropane synthesis, cyclization and aromatization. Catalytic carbons have been used in oxidation-reduction, halogenation, dehydrohalogenation, polymerization, isomerization, oxydehydration and cracking.

As different as RAM and catalytic carbons are, they have a lot in common:

First, they both behave catalytically. That is, they behave as if they're not permanently altered by the reaction.

Second, they provide, directly or indirectly, a source of electrons, thus allowing quick and efficient reduction of chlorine.

Third, the rate of reaction involving either medium is surface-area dependent.

Fifth, both media are recyclable or regenerable.

How It Works

Redox chemistry can become quite complex, and isolation of products and intermediates is often difficult. Therefore, most reactions presented are hypothetical, strongly suggested by experiment but hard m prove as absolute.

In general, these reactions are considered good if they explain all available experimental data. Conversion of chlorine to chloride by RAM is thought to occur as follows:

You can even imagine each of the above reactions involving hypochlorous acid as occurring in two steps, adding a proton m hypochlorous acid followed by reduction accounting for all products. For me last reaction above; this may be represented as follows:

Evidence supporting this mechanism includes the

Conversion of chloramines to chloride and ammonia is thought to occur in a similar fashion but at a slower rate. This slower rate could be due to the more covalent bond between N and Cl in the amine (more equal and stronger sharing of bonding electrons) and to ammonia being a poorer leaving group (an ammonia/water solution is less stable than pure water or hydroxide).

Catalytic conversion of hypochlorous acid or chloramines

Because the activity of both RAM and catalytic carbons

You might surmise from this that reducing the number of potential catalytic sites would have the opposite effect. For catalytic carbons, adsorbed contaminants eventually do occlude catalytic sites, rendering the carbon useless. This occurs slowly, allowing the bed life of catalytic carbons to exceed their granular activated carbon predecessors by two to four times.

RAM, Carbon Similarities

For RAM, the major threat to medium activity is large amounts of hydrocarbons that quickly coat and insulate the redox alloy from the surrounding water. Removal of chlorine from brine has also been reported to lead to problems with medium caking. However, these situations occur very rarely, allowing dissimilar metal redox alloy media to enjoy bed lives far in excess of 20 times those of granular activated carbon.

Both media can be restored - carbon by reactivation, RAM by washing. Used RAM may also be sold as a secondary metal alloy to recover approximately 20 percent of initial cost (based on current media price and commodity market value)

The redox properties of both RAM and catalytic carbons allow for a variety of water purification chemistry in addition to chlorine removal. For example, catalytic carbon manufacturers have promoted their products' abilities to effectively remove hydrogen sulfide and hydrogen peroxides from water. Catalytic carbons also retain the ability to remove all compounds that granular activated carbons remove.

RAM manufacturers have promoted their products' abilities to remove H2S and heavy metals, as well as inhibit scale and corrosion. These RAM are also able to significantly reduce the number of bacteria in water, partially through a significant lowering of oxidation/reduction potential.

No matter what the mechanism or media, redox chemistry as a means of chlorine removal is definitely a winner. This is particularly evident in light of the more than 100 percent growth in RAM use in the POU/POE market last year. As redox products are developed and improved further, conversion of chlorine to chloride via redox processes will become a popular chlorine removal method.