Iron oxide adsorption

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 usually a co-precipitation process followed by microfiltration.

Iron oxide adsorption filters the water through a granular medium containing ferric oxide. Ferric oxide has a high affinity for adsorbing dissolved metals such as arsenic. The iron oxide media eventually becomes saturated, and must be routinely backwashed and eventually replaced by new media.

Coprecipitation requires a pH adjustment to allow the formation of the precipitates. To that end, hydrochloric acid (HCl) is added at each wellhead by a 60 gpd solenoid diaphragm pump from a 950 gallon tank at each well site.

The addition of ferric chloride (FeCl3) to the well water immediately after the well at the influent to the treatment plant creates ferrous hydroxide (Fe[OH]3) and hydrochloric acid (HCl). Fe(OH)3 in water is a strong adsorbent of 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 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 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 with 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 to Fe(III) 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.