Arsenic Removal Technologies
Oxidation Methods
Most
arsenic removal technologies are most effective at removing the
pentavalent form of arsenic (arsenate), since the trivalent form
(arsenite) is predominantly non-charged below pH 9.2. Therefore, many
treatment systems include an oxidation step to convert arsenite to
arsenate. Oxidation alone does not remove arsenic from solution, and
must be coupled with a removal process such as coagulation, adsorption
or ion exchange. more
Coagulation and Filtration Methods
By adding a coagulant such as alum, ferric chloride, or ferric sulfate to contaminated water, much of the arsenic can be removed. If arsenic is present as arsenite, the water should be oxidized first, using chlorine, permanganate, ozone, or other oxidants. After adding the coagulant, the water should be stirred, allowed to settle, and filtered for best results. Coagulation improves parameters such as turbidity and color, and can reduce levels of organic matter, bacteria, iron, manganese, and fluoride, depending on operating conditions. If the source water has high levels of phosphate or silicate, coagulation may be less effective. However, sulfate, carbonate, and chloride have little effect on removal rates. more
Adsorption Methods
Various solid materials, including iron and aluminum hydroxide flocs, have a strong affinity for dissolved arsenic. Arsenic is strongly attracted to sorption sites on the surfaces of these solids, and is effectively removed from solution. more
Ion Exchange Resins
Ion exchange resins are commercially produced, synthetic materials that
can remove some compounds from water. Most commonly they are used in water softening,
but some resins are very good at removing arsenic. These resins only remove arsenate, so if
the raw water contains arsenite, it should be oxidized first. Other compounds,
including sulfate, nitrate, nitrite, and chromate, are also removed to some degree by most arsenic
removal resins. more
Membrane Methods
These make use of synthetic membranes, which allow water through but reject larger molecules, including arsenic, chloride, sulfate, nitrate, and heavy metals. The membranes must be operated at high pressures, and usually require pretreatment of the raw water. Household level membrane units usually only treat about 10% of the water, resulting in a large waste stream. Municipal membrane units can achieve higher total recovery rates by using membranes in series. Currently available membranes are more expensive than other arsenic removal options, and are more appropriate in municipal settings, where very low arsenic levels are required. However, membrane technology is advancing rapidly, and it is conceivable that future generations of membranes could be used effectively in rural settings.
Further Reading:
- Towards a More Effective Operational Response: Arsenic Contamination of Groundwater in South and East Asian Countries. Volume II Technical Report, The World Bank and the Water and Sanitation Program of South and East Asia, 2004