Title: Iron in Water and Processes for its Removal
This presentation provides an overview of the most common and accepted processes available for the removal of iron from water, their various advantages and disadvantages, and some practical advice to guide the choice of one process over another for a particular application.
Iron in water can be present as the divalent ferrous cation, a suspended solid (and possibly collidal) as trivalent ferric oxide/hydroxide, or as a zero valent molecular speices, often tied up in an organic matrics or with vartious chelating agents.
Ferrous iron can be removed by water softening IX resins but care must be taken to prevent oxidation before treamtent and also during regeneration.
Ferric iron is usually present as a suspended solid and can be relatively easy to filter or so small that most filtrations methods are problematic
Organic iron can be present as part of naturally occuring organic matter, various chelating chemicals or as part of a mineral complex
This paper explores the various forms of iron, unit processes for removal of iron, and a discussion of which processes are best suited for removal of the various forms
Iron comprises roughly 5 percent of the Earths crust and is widely distributed in both surface and groundwaters in nearly all geographic areas. Iron is a common building material and is widely used in water and wastewater treatment systems as well as in distribution piping. When conditions permit, iron corrodes or dissolves into the water it come in contact with. Iron is also a common additive in water treatment systems and in chemical processing and can react with other contaminants and/or precipitate. This results in a variety of chemical forms in which iron is found in water. The three most common forms are ferrous iron (usually present as a cation), ferric iron (usually present as a suspended or colloidal solid), and organic iron (sometimes present as an anion, or dissolved but uncharged.
Within these three broad categories there are numerous variations depending on how the iron got into the water and what other contaminants and bulk ions are also present. Although ferrous iron usually takes the form of a divalent cation, it can also be present as a zero valent species, uncharged but dissolved. We generally consider water softening resins to be effective to remove ferrous iron but removal of uncharged ferrous iron by cation exchange is poor. Ferric iron can take on several different forms depending on oxygen availability and can vary from a stable colloid that is not easily filtered, to large chucks of rust, to various dissolved species that may or may not be ionized. Organic iron might be part of an anionic complex resulting from contact with naturally occurring organics such as tannins, or zero valent as part of an organic chelator, or part of a larger inorganic complex such as ferric chloride.
This presentation discusses many of the more common types of water (groundwater, surface water, wastewater, condensate, etc.) that iron is found in, how the iron may have gotten there, and what form(s) the iron may be in. It also covers the basic theory of how the unit processes available to remove iron actually work. Knowledge about the type of water and how the iron got into it to start with provides important clues about the probable form of iron present. With this knowledge we can select an iron removal process that is appropriate for the type of iron that is present.
Peter Meyers, Technical Director for ResinTech, Inc., has a fascination with ion exchange that began in high school chemistry class and has never ended. During a career that now spans nearly a half-century, Mr. Meyers has participated in almost every aspect ion exchange — from engineering & process design to installation, start-up, & troubleshooting. His passion for the water treatment has made him one of the industry’s most prolific and sought-after speakers and authors. Peter has presented around the world on a wide range of ion exchange topics from demineralization, polishing, and softening to industrial process design and operation.