Arsenic distribution in environment and its bioremediation: A review

enhance mutagenesis by other agents, apparently

(V) was first studied by Frank and Clifford (1986)

by inhibiting DNA repair (Li and Rossman 1989).

utilizing chlorine, monochloramine, and oxygen.

Arsenite can also cause aneuploidy. Unlike

Solid phase oxidants such as birnessite (δ-MnO2)

spindle poisons, arsenite does not inhibit spindle

have also proven to be very useful in the oxidation

fiber formation; instead, it deranges the spindle

of As (III) to As (V) at a optimum pH of about 6.5

apparatus, possibly by accelerating microtubule

(Manning et al. , 2002).

polymerization. Arsenite may act as carcinogen by

Coagulants

such

as

aluminium

alum,

Al2

causing DNA hypomethylation leading to aberrant

(SO4)3.18H2O, ferric chloride, and ferric sulfate are

gene expression (Zhao et al. , 1997). Chronic exposure

effective in removing As from water by utilizing the

to arsenite caused a progressive increase in CpG

electrostatic attachment of negatively charged ions

methylation within the p53 promoter, which would

to the flocs. As (III) occurs in non-ionized form, it is

be expected to block transcription of the p53 gene

not subject to significant removal. Oxidation of As

(Mass et al. , 1997). The p53 gene is an important

(III) to As (V) is thus required as a pretreatment for

tumor-suppressor gene whose protein product plays

efficient removal (Ahmed 2003). Other adsorption

an important role in cell cycle control, apoptosis, and

techniques such as activated alumina, activated

control of DNA repair.

carbon (Ghosh and Yuan 1987) and activated bauxite

Remediation of as contaminated soil and water

(Gupta and Chen 1978) are also proved to be efficient

technologies.

There have been increasing needs of removal of As

from contaminated soils and groundwater to limit

Ion exchange is a physico-chemical process by

the detrimental impacts of As on living beings.

which an ion in the liquid medium is exchanged

There are a plenty of remediation methods have

for saturated ion on the solid phase. Synthetic resin

been developed based on physical, chemical and

saturated with the preferred anion is most widely

biological approaches that may achieve complete or

used for exchangingAs anions in contaminated water

partial removal of As from soil and groundwater or

(Johnston and Heijnen 2001). Generally strong-base

the reduction of its bioavailabity in order to minimize

resins (SBA) and sulfate-selective resins are found to

toxicity.

be most efficient in As removal over a broader pH

range (USEPA 2000. The biggest disadvantage of

Conventional Methods

using the ion exchange process is the non-removal

of As (III) species because of its uncharged behavior

The most commonly used conventional methods

until pH of about 9 (pKa =9.22). This characteristic of

include oxidation, co-precipitation and adsorption,

As (III) anions necessitates a pre-oxidative step for

ion exchange and membrane techniques such as

the conversion of As (III) to As (V) prior to removal

filtration, reverse osmosis etc (Shen 1973; Cheng et

from water. The removal of As (V) is independent

al. , 1994; Kartinen and Martin 1995; Hering et al. ,

of the initial concentration and pH of the liquid

1996; Joshi and Chaudhuri 1996).

medium (Johnston and Heijnen 2001).

Most conventional treatment methods are effective in

Membrane

techniques

like

Reverse

osmosis

removing As in pentavalent form and hence include

(RO), Nano-filtration (NF), Ultra-filtration (UF),

an oxidation step as preteatment to convert arsenite

microfiltration (MF), membrane distillation (MD)

to arsenate. Arsenite can be oxidized by oxygen,

are proved to be capable of removing dissolved As

ozone, free chlorine, hypochlorite, permanganate,

from water with a high efficacy. Membranes are

hydrogen peroxide, and fulton’s reagent (Ahmed

typically synthetic materials with billions of pores

2003). Chemical oxidation of As (III) to As (V)

or microscopic holes that act as a selective barrier

is currently being used in most conventional

allowing the passage of certain constituents with the

treatments. The chemical oxidation of As (III) to As

rejection or exclusion of others in the water (Uddin

195