ARSENIC POLLUTION IN GROUND WATER
LIST OF CONTENTS
controlling arsenic concentration and transport
- Speciation of arsenic compounds:
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.
GEOCHEMISTRY OF ARSENIC
Factors controlling arsenic
concentration and transport: An understanding of factors controlling the distribution of arsenic
in ground water requires a knowledge of arsenic sources and of processes controlling arsenic mobility. Geochemistry of arsenic
is controlled by many factors including-
- Red-ox potential
- Adsorption/ desorption
- Arsenic speciation
and concentration of competing ions
- Biological transformation
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.
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 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.
are two widely accepted mechanisms for adsorption of solute by a solid surface.
Arsenic adsorbs by ligand exchange with OH- and OH2+
surface functional groups. Some of the reactions can be represented as:
- 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.
Figure-2 represent process of hydrated surface generation.
- SurfOH + H3AsO4=SurfH3AsO4+H2O
- SurfOH+ H2AsO3-=SurfHAsO3-+H2O
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
2. Surface complexation models.
- Langmuir Isotherm
- Freundlich isotherm
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