Paul et.al.

deposits(Anthony etal. ,2003).Asbondstooxygenand

Table 1. Major As mineral occurring in nature (Smedley

sulphur to produce a multitude of aqueous species

and Kinniburgh 2002)

and minerals like Arsenolite (As ₂ O ₃ ), Arsenopyrite

Mineral

Composition

Occurrence

(FeAsS), Realger (As S ), Orpiment (As S ) etc (O’Day

4 4

2 3

Native As

As

Hydrothermal veins

2006). Moreover, via biomethylation certain groups

Niccolite

NiAs

Vein deposits and norites

of microorganisms produce a large number of

Vein

deposits,

often

organoarsenic compounds (Stolz et al. , 2006), namely

associated

with

dimethylarsinate (DMA) and monomethylarsonate

Realgar

AsS

orpiment,

clays

and

(MMA), generated by replacing a hydroxyl (-OH)

limestones, also deposits

from hot springs

ligand by a methyl group (-CH ₃ ) in inorganic As (III)

and As (V) structures (Cullen and Reimer 1989).

Hydrothermal

veins,

Orpiment

As S

2 3

hot

springs,

volcanic

Redox potential (Eh) and pH are the main drivers

sublimation product

of As speciation in natural water, where the

H i g h - t e m p e r a t u r e

predominant chemical forms include oxyanions of

Cobaltite

CoAsS

deposits,

metamorphic

As (III) (H ₃ AsO ) - or H ₂ AsO -) and As (V) (H AsO

4 2

3

2

4-

rocks

or HAsO ₄ 2- ) (Suttigarn and Wang 2005; Rhine et al. ,

The most abundant As

2006). Generally, As (III) occurs in anoxic condition,

Arsenopyrite FeAsS

mineral,

dominantly

exhibiting greater toxicity and mobility than As (V)

mineral veins

which mainly occurs in aerobic conditions (Clifford

Tennantite

(Cu,Fe) ₁ 2

Hydrothermal veins

1990; Ehrlich 1996). Relative to the other oxyanion-

As4S ₁ 3

forming elements (e.g. Se, Sb, Mo, V, Cr, U, Re), As

Enargite

Cu ₃ AsS ₄

Hydrothermal veins

is the most problematic in the environment because

Secondary

mineral

of its relative mobility over a wide range of redox

formed by oxidation of

arsenopyrite, native

conditions (Smedley and Kinniburgh 2002).

Arsenolite

As ₂ O ₃

Arsenic and other As

Origin and sources of As

minerals

Secondary

mineral

Arsenic is a ubiquitous element in the environment. It

formed by oxidation of

ranks 20th in earth’s crust, 14th in sea water and 12th

Claudetite

As ₂ O ₃

realgar, arsenopyrite and

in the human body as an element (Woolson 1975). A

other As minerals

range of As compounds, both organic and inorganic,

Scorodite

FeAsO ₄ .2H ₂ O

Secondary mineral

are introduced into the environment through natural

and anthropogenic sources.

(Ni,Co)3

Annabergite

Secondary mineral

(AsO ) 2 .8H O

4

2

Natural Sources

Secondary

mineral,

Hoernesite

Mg ₃ (AsO ₄ ) 2 .8H ₂ O

smelter wastes

In nature, As occurs rarely in its elemental form

Conichalcite

CaCu(AsO ₄ )(OH)

Secondary mineral

and is widely distributed in a variety of minerals,

Oxidation product of

commonly as arsenide of iron, copper, lead, silver

P h a r m a c o - Fe ₃ (AsO ₄ )

arsenopyrite and other

siderite

and gold or as sulphide minerals, for example

2(OH) ₃ .5H O

2

As minerals

arsenopyrite (O’Day 2006). Weathering of these

The greatest concentrations of these minerals occur in

As bearing minerals, volcanic activity as well

mineralised areas and are found in close association

as mining waste resulted in occurrence of As in

with the transition metals as well as Cd, Pb, Ag, Au,

aquatic and terrestrial environment under both

Sb, P, W and Mo (Smedley and Kinniburgh 2002).

oxic and anoxic condition (Rhine et al. , 2006). A list

Oxidation and dissolution of the most common As

of some of the most common As minerals along

bearing minerals such as arsenian pyrite [Fe(AsS) ],

2

with their place of occurrence is given in Table 1.

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