Table 1 Processes to remove arsenic from groundwater (adapted from Ravenscroft et al., 2009)

Treatment of water

Oxidation and filtration

Oxidation of iron or manganese of which the oxyhydroxides adsorb arsenic prior to filtration or sedimentation.

Or;

Passive oxidation and sedimentation which requires high iron concentrations in the source water. Oxygen from the air mixes with the water causing iron to precipitate and arsenic to co-precipitate.

Or;

Slow sand filtration. Aeration causes oxidation and precipitation of iron. Arsenic may be removed by direct adsorption onto iron precipitate or oxidised by bacteria growing on filter.

Coagulation, co-precipitation and filtration

Addition of metal salts to water, producing flocs onto which arsenic is adsorbed.

Lime softening

Tends to be used at large treatment plants to raise pH. Uses precipitating mechanism to remove arsenic.

Adsorption processes

Relies on the adsorption of arsenic onto the surfaces of adsorbents such as activated alumina, synthetic iron hydroxides, iron oxide coated sands, greensand filtration, manganese oxides, titanium oxide, cerium oxide, biological adsorptive filtration and water hyacinth.

Zero-valent iron

Refers to the use of iron filings or nails. The process appears to work by either co-precipitation and adsorption on iron oxyhydroxde, or, when used with sulphate, by forming arsenopyrite.

Ion exchange

Ion exchange resins are only effective for charged ions and so only works well for arsenic in the form of arsenates (As(V)).

Membrane technologies

Membrane filtration relies on synthetic membranes containing billions of microscopic holes that act as selective barriers to the movement of molecules under the influence of a pressure gradient, such as reverse osmosis and nanofiltration.

Electrolytic methods

A complex form of ion exchange controlled by alternating electric currents in which ions migrate from the less to the more concentrated solution.

Phytofiltration

An emerging technology where plants are used to remove contaminants from water.

Arsenic removal in situ

Works by modifying the redox conditions in an aquifer, exploiting the capacity of iron oxyhydroxides to adsorb arsenic.