Arsenic Pollution in Groundwater

Long term exposure to arsenic (As) in drinking water has variety of health concerns including several types of cancer, cardiovascular disease, diabetes, and neurological effects (National Research Council, 1999). To protect people against long term arsenic protection EPA (Environmental Protection Agency) recently lowered the MCL (Maximum Concentration Limit) of arsenic from 50 µm/l to 10 µm/l. Many countries including Bangladesh, western India, Taiwan, Mongolia, Vietnam, Argentina, Chile, Mexico, Ghana and the United States have problem due to contamination of arsenic in drinking water. In all those locations the source for arsenic is primarily natural rather than anthropogenic or geothermal.

Arsenic as a metal:

Arsenic is naturally occurring element. Its atomic number 33, atomic weight is 74.92;. Chemically, arsenic is always present as compounds with oxygen, chlorine, sulpher, carbon and hydrogen on one hand and with lead, gold and iron on the other. Organic arsenic is generally fewer toxins than inorganic arsenic. Arsenic in element from is insoluble in water. It is soluble in oxidize from.
History of Arsenic pollution

Inorganic arsenic has been recognized as a human poison since a long times. Oral intake of arsenic has proved to be deadly on many occasions. But it had been used to treat skin ailments, worms and syphilis in the past. A century ago, incidence of skin cancer were observed due to use of arsenic medicine. It is still used in cattle fodder to enhance physical growth.

In general, most naturally occurring arsenic compounds are insoluble in water and adsorption of arsenic compounds to other soluble minerals or organic compounds plays major role in arsenic concentration and transport in ground water. Speciation of arsenic again controls the adsorption behavior.

Speciation of arsenic compounds:

Arsenic species generally present as arsenate [As(V)] or arsenite [As(III)] for Eh conditions prevalent in most ground water. Arsenic as metal rarely occurs in nature. Both As(V) and As(III) form protonated oxyanions in aqueous solutions and the degree of protonation depends on pH. The Eh-pH diagram for arsenic is shown in figure-1 with most dominant species under different Eh-Ph range. Those compounds under favorable conditions can also form methylated compounds such as methylarsonic acid [CH3AsO(OH)2], dimethylarsinous acid [(CH3)2AsO(OH)] etc.

Redox potential:

Natural geochemical and biological processes play critical role in controlling the fate and transformation of Ar in the subsurface. Arsenite is thermodynamically unstable in aerobic environments and should oxidize to As(V) though in nature the transformation occur slowly due to kinetic barrier to the reaction. Presence of other oxides such as FeO, Fe2O3, MnO2 and even clay minerals shown to be capable of oxidizing As(III). Oxidation of As(III) to As(V) by microorganisms using oxygen as terminal electron acceptor has also been reported.

Adsorption/ Desorption

Adsorption reactions are between arsenic and the mineral surfaces is the most important controlling factor for dissolved concentration of arsenic in ground water. Oxides of aluminum (Al), and manganese (Mn) are the dominant source or sink for As in aquifer sediments because of their chemistry, widespread occurrence, and tendency to coat other particles. When exposed to water, metal ions on the oxide surface complete their coordination shells with OH groups.
There are two widely accepted mechanisms for adsorption of solute by a solid surface.

outer sphere surface complexation- involves the electrostatic attraction between a charged surface and an oppositely charged ion in the solution
inner sphere surface complexations- involves the formation of a coordinative complex with the mineral surface.

Arsenic adsorbs by ligand exchange with OH- and OH2+ surface functional groups. Some of the reactions can be represented as:

SurfOH + H3AsO4=SurfH3AsO4+H2O
SurfOH+H2AsO4-=SurfHAsO2-+H2O
SurfOH+HAsO4-2=SurfAsO4-2+H2O
SurfOH+H2AsO3-=SurfHAsO3-+H2O
SurfOH+ H2AsO3-=SurfHAsO3-+H2O

Figure-2 represent process of hydrated surface generation.

Adsorption models for arsenic transport in groundwater:
Two types of models are generally developed to describe adsorption/desorption reactions of arsenic in ground water:

Emperical adsorption models

Langmuir Isotherm
Freundlich isotherm

2. Surface complexation models.

ARSENIC POLLUTION IN GROUND WATER