Iron oxide adsorption
Encyclopedia
Iron oxide adsorption is a water treatment
process that is used to remove arsenic
from drinking water
. Arsenic is a common natural contaminant of well
water and is highly carcinogenic. Iron oxide
adsorption
treatment for arsenic in groundwater is a commonly practiced removal process which involves the chemical treatment of arsenic species such that they adsorb onto iron oxides and create larger particles
that may be filtered
out of the water stream.
The addition of ferric chloride, FeCl3, to well water immediately after the well at the influent to the treatment plant creates ferric hydroxide, Fe(OH)3, and hydrochloric acid, HCl.
Fe(OH)3 in water is a strong adsorbent of arsenate, As(V), provided that the pH is low. HCl lowers pH, assuring arsenic adsorption, and the disassociated chlorine oxidizes iron in solution from Fe+2 to Fe+3, which then may bond with hydroxide ions, OH-, thus creating more adsorbent.
This adjustment also lowers the pH of the well water, decreasing alkalinity and allowing more cationic species such Fe(+) or As(+) as to exist freely within the flow. Low pH also decreases the solubility of some iron and arsenic species as well as increasing the adsorptive reactivity of arsenate, As(V).
Additional oxidation of Fe+2 to Fe+3, also referred to as iron(II) and iron(III), is induced by the addition of sodium hypochlorite, NaOCl, at the well head. NaOCl is usually added for disinfection
although it may be used in this case towards the objectives of a distribution system free chlorine residual of 1 mg/L and the oxidation of aqueous As(III) to As(V), and aqueous iron Fe+2 to Fe+3, which will bond with hydroxide for further adsorption.
The filter media usually consists of anthracite, iron-manganese oxidizing sand, and garnet
sand over support gravel
.
Water treatment
Water treatment describes those processes used to make water more acceptable for a desired end-use. These can include use as drinking water, industrial processes, medical and many other uses. The goal of all water treatment process is to remove existing contaminants in the water, or reduce the...
process that is used to remove arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
from drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
. Arsenic is a common natural contaminant of well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
water and is highly carcinogenic. Iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
adsorption
Adsorption
Adsorption is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid...
treatment for arsenic in groundwater is a commonly practiced removal process which involves the chemical treatment of arsenic species such that they adsorb onto iron oxides and create larger particles
Flocculation
Flocculation, in the field of chemistry, is a process wherein colloids come out of suspension in the form of floc or flakes by the addition of a clarifying agent. The action differs from precipitation in that, prior to flocculation, colloids are merely suspended in a liquid and not actually...
that may be filtered
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
out of the water stream.
The addition of ferric chloride, FeCl3, to well water immediately after the well at the influent to the treatment plant creates ferric hydroxide, Fe(OH)3, and hydrochloric acid, HCl.
- 3H2O + FeCl3 → Fe(OH)3 + 3HCl
Fe(OH)3 in water is a strong adsorbent of arsenate, As(V), provided that the pH is low. HCl lowers pH, assuring arsenic adsorption, and the disassociated chlorine oxidizes iron in solution from Fe+2 to Fe+3, which then may bond with hydroxide ions, OH-, thus creating more adsorbent.
This adjustment also lowers the pH of the well water, decreasing alkalinity and allowing more cationic species such Fe(+) or As(+) as to exist freely within the flow. Low pH also decreases the solubility of some iron and arsenic species as well as increasing the adsorptive reactivity of arsenate, As(V).
Additional oxidation of Fe+2 to Fe+3, also referred to as iron(II) and iron(III), is induced by the addition of sodium hypochlorite, NaOCl, at the well head. NaOCl is usually added for disinfection
Disinfection
Disinfectants are substances that are applied to non-living objects to destroy microorganisms that are living on the objects. Disinfection does not necessarily kill all microorganisms, especially nonresistant bacterial spores; it is less effective than sterilisation, which is an extreme physical...
although it may be used in this case towards the objectives of a distribution system free chlorine residual of 1 mg/L and the oxidation of aqueous As(III) to As(V), and aqueous iron Fe+2 to Fe+3, which will bond with hydroxide for further adsorption.
The filter media usually consists of anthracite, iron-manganese oxidizing sand, and garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...
sand over support gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
.