Iron oxide adsorption is a

water treatment Water treatment is any process that improves the Water quality, quality of water to make it appropriate for a specific end-use. The end use may be drinking water, drinking, industrial water supply, irrigation, river flow maintenance, water recrea ...

process that is used to remove

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...

from

drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...

. Arsenic is a common natural contaminant of

well A well is an excavation or structure created in the ground by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...

water and is highly

carcinogenic A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...

Iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...

adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...

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 Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter m ...

out of the water stream. The addition of

ferric chloride Iron(III) chloride is the inorganic compound with the formula . Also called ferric chloride, it is a common compound of iron in the +3 oxidation state. The anhydrous compound is a crystalline solid with a melting point of 307.6 °C. The colo ...

, FeCl₃, to well water immediately after the well at the influent to the treatment plant creates ferric hydroxide, Fe(OH)₃, and

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

, 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 Sodium hypochlorite (commonly known in a dilute solution as bleach) is an Inorganic chemistry, inorganic chemical compound with the chemical formula, formula NaOCl (or NaClO), comprising a sodium cation () and a hypochlorite anion (or ). It may ...

, NaOCl, at the well head. NaOCl is usually added for

disinfection A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...

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 Anthracite, also known as hard coal, and black coal, is a hard, compact variety of coal that has a submetallic luster. It has the highest carbon content, the fewest impurities, and the highest energy density of all types of coal and is the hig ...

, iron-manganese oxidizing sand, and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

sand over support

gravel Gravel is a loose aggregation of rock fragments. Gravel occurs naturally throughout the world as a result of sedimentary and erosive geologic processes; it is also produced in large quantities commercially as crushed stone. Gravel is classifi ...

.{{citation , title = Chemistry of Water Treatment, date = 1998 , author = Faust, Aly , publisher = CRC Press

References

Water treatment