Oxidation
Conventional oxidants such as oxygen, chlorine, permanganate and ozone are widely used to oxidise arsenite to arsenate.
Atmospheric oxygen is readily available as an oxidising agent, however the kinetics of air oxidation of arsenic are very slow (taking weeks) and the reaction needs to be catalysed.
Chlorine is a rapid and effective oxidant, but may lead to reactions with organic matter, producing toxic trihalomethanes as a by-product. In developing countries, the quality of commercially available chlorine can be uncertain.
in Europe and the USA, ozone is widely used as an oxidising agent. It is effective in oxidising Fe and Mn and removes arsenic and other metals to values below detection limits.
Permanganate effectively oxidises arsenite, along with Fe and Mn. It is widely available in developing countries and relatively stable, with a long shelf life.
Further methods applying oxidation:
Solar Oxidation and Removal of Arsenic (SORAS)
SORAS is a simple water treatment method that uses sunlight and plastic bottles to reduce arsenic levels in drinking water. The process is based on photochemical oxidation of As(III) followed by precipitation or filtration of As(V) adsorbed on Fe(III)oxides. Groundwater polluted by arsenic often contains also iron as Fe(II) or Fe(III) used for the removal of arsenic. In such situations, SORAS reduces arsenic contents to approx. 50 – 80 %. The method is conceived for usage at household level to treat small quantities of drinking water at virtually no cost.
SORAS principle
Further Reading:
- Hug, S. J., Canonica, L., Wegelin, M., Gechter, D., von Gunten, U. (2001) Solar oxidation and removal of arsenic at circumneutral pH in iron containing waters. Environmental Science and Technology 35(10), 2114-2121. pdf
- Hug, S.J. (2001) An adapted water treatment option in Bangladesh: solar oxidation and removal of arsenic (SORAS). Environmental Sciences 8, 467-479. pdf
Subsurface Oxidation
This method is currently still being tested in field trials in Bangladesh. The principle is that aerated water is periodically injected into the subsurface via an existing tubewell, where it partially displaces existing iron-containing groundwater. The injected water oxidises adsorbed ferrous iron on the soil grains, resulting in the formation of ferric iron oxides which act as a surface for the adsorbance of ferrous iron and arsenic. When the flow is reversed, water with reduced iron and arsenic content is abstracted.
Further Reading:
- van Halem, D., Olivero, S., de Vet, W.W.J.M., Verberk, J.Q.J.C., Amy, J.L., van Dijk, J.C. (2010) Subsurface iron and arsenic removal for shallow tube well drinking water supply in rural Bangladesh. Water Research 44(19), 5761-5769.