Arsenic in Drinking Water
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Oxidation Filtration (Iron Removal)
Oxidation/filtration refers to precipitative processes that are designed to remove naturally occurring iron and manganese from water. The processes involve the oxidation of the soluble forms of iron and manganese to their insoluble forms and then removal by filtration.
In groundwater containing iron, both the iron and the arsenic are usually found in their reduced form (i.e., Fe(II) and As(III)). For optimum arsenic removal, it is important that the iron and arsenic both be oxidized at the same time. To accomplish this, a chemical oxidant is needed because air oxidation will only oxidize Fe(II) and not As(III).
Arsenic can be removed via two primary mechanisms: adsorption and coprecipitation. First, soluble iron [Fe(II)] and As(III) are oxidized. The As(V) then adsorbs onto the iron hydroxide precipitates that are ultimately filtered out of solution. The arsenic removal efficiency is strongly dependent on the initial iron and arsenic concentrations.
In general, the Fe:As mass ratio should be at least 20:1, which assumes 1 mg/Fe removes 50 Âµg/As. These conditions customarily result in an arsenic removal efficiency of 80-90%. Arsenic removals decrease with increasing pH. In addition, high levels of natural organic matter (NOM), orthophosophates, and silicates weaken arsenic removal efficiency by competing for sorption sites on iron hydroxide precipitates.
Iron removal plants have been determined to be effective at removing arsenic. Arsenic removal can occur in a traditional iron removal treatment plant with mixing and settling basins followed by granular media filters.
Oxidation/filtration can be used as a special case of pressurized granular-media filtration where the granular media catalyzes the oxidation and precipitation of iron and manganese. Manganese-oxide (MnOx) media, which include manganese greensand and pyrolucite, are commonly used in oxidation/filtration processes because of their unique adsorptive and catalytic capabilities. Greensand is manufactured by coating glauconite with manganese dioxide, while pyrolucite is a naturally mined ore composed of solid manganese dioxide. Greensand media has been shown to be capable of removing up to 80% of arsenic by oxidation/adsorption.
In oxidation/filtration processes, water is passed through a column of MnOx media which adsorbs and catalyzes the oxidation of the iron and manganese. The filtering capacity of the granular MnOx media then retains the precipitated iron, manganese, and arsenic until it is backwashed out of the column. Backwashing creates waste water and sludge, which the water system must properly dispose. Arsenic appears to be removed primarily by the iron precipitates as opposed to those of manganese. Water systems with low levels of influent iron (less than 1.5 mg/L or less than 20:1 ratio with arsenic) may want to consider adding ferric chloride prior to oxidation. It is generally recommended that green sand be preceded by a 12 inch anthracite cap to filter any precipitated iron particulates before the green sand.
In order for greensand to retain its adsorption and catalytic oxidation capabilities for iron and manganese removal, the media must be regenerated with permanganate or chlorine. Typically these oxidants are added ahead of the filter where they provided continuous oxidation of the contaminants as well as regeneration of the MnOx media. Arsenic adsorbs to the iron floc formed in this chemical oxidation step and is physically filtered from solution by the greensand. Any arsenic that is not oxidized is adsorbed onto the MnO2 surface of the greensand particles.