New Generation Material for the Removal of Arsenic from Water - Advanced Materials for Agriculture, Food and Environmental Safety - page 62

3.1 Introduction

Arsenic is omnipresent and its contamination of groundwater is a world-
wide issue. Groundwater contamination due to arsenic in the Bengal delta
region of Bangladesh and West Bengal, India, is considered one of the big-
gest natural calamities of the 21st century. Indication of arsenic poison-
ing was observed in the mid-1980s in Bangladesh and West Bengal [1, 2].
Similarly, in dif erent areas of Pakistan arsenic concentration was found to
be high in surface and groundwater. For instance, in the Sindh province of
Pakistan, 16-36% of the population have been exposed to arsenic contami-
nated water with over 10-50 μg/L. h is high level of arsenic (>10 μg/L) may
cause skin lesions and rhagades, and also damage mucous membranes, and
the digestive, respiratory, circulatory and nervous systems. Furthermore, it
is associated with skin, liver and lung cancers.

3.1.1 Properties of Arsenic [3-6]

Before going through a detailed study on the removal of arsenic from
water, we will elucidate some properties about this toxic atom. Arsenic is
a chemical element with symbol As and atomic number 33, and is ranked
20th in natural abundance, comprising about 0.00005% of the earth's
crust, 14th in the seawater, and 12th in the human body. h is metalloid
is a silver-grey brittle crystalline solid with atomic weight 74.9 g/mol,
specii c gravity 5.73 g/L, melting point 817°C (at 28 atm), boiling point
613°C, and vapor pressure 1 mm Hg at 372°C; the arsenic chemistry in
an aquatic system is more complicated because the element can be stable
in four oxidation states (−3, 0, +3 and +5) under dif erent redox condi-
tions. Soil corrosion and leakage contribute to 612×108 and 2380×108
g/year of arsenic, respectively, in dissolved and suspended forms in the
oceans. Arsenic(III) is a hard acid and preferentially complexes with
oxides and nitrogen. Conversely, arsenic(V) behaves like a sot acid,
forming complexes with sulphides [7]. Inorganic forms of arsenic most
ot en exist in water supplies. Arsenic is uniquely sensitive to mobilization
(pH 6.5-8.5) and under both oxidizing and reducing conditions among
heavy metalloids [8]. Two forms are common in natural waters: arsenite
and arsenate, referred to as arsenic(III) and arsenic(V). Pentavalent (+5)
or arsenate species are, and H 2 , while trivalent (+3) arsenites include
As(OH) 3 , AsO 2 O and . Pentavalent species predominate and are stable
in oxygen rich aerobic environments. Trivalent arsenites predominate in
moderately reducing anaerobic environments such as groundwater [9].