2.4.1 Coagulation/Flocculation
It involves the mixing of chemicals such as salts of iron (ferric chloride), aluminium
(alum) and manganese (manganese sulphate) into water to settle down the arsenic
through precipitation, co-precipitation or combination of both these processes.
Afterwards, the arsenic is separated from the solution by filtration as it gets adsorbed
onto the solid matrix (Wickramasinghe et al. 2004). In general, this approach
removes As
III as compared to As
V , and ferric chloride has been reported to be
more efficient than alum. The effectiveness of this method depends on the pH,
concentration of arsenic, arsenic speciation and presence of competing ions in the
water. However, it requires pretreatment process to oxidize As
III to As
V using
chlorine or permanganate ions since it causes partial removal of As
III from aqueous
solution (Ahmed 2001b). Nowadays, solar and microbial oxidation have also been
commonly utilized to convert As
III to As
V (Jain and Singh 2012).
2.4.2 Ion-Exchange Resins
It is a physio-chemical process involving exchange of ions between solid phase and
resin (solid) phase of similar charge. The exchanged ions are held electrostatically on
the surface of solid phase with ions of similar charge in a solution. The ion-exchange
media is usually packed into a column, and then water containing arsenic is allowed
to pass for removing the desired contaminant. The columns can be regenerated with
the brine solution for preparation of next cycle of removal (Ghurye et al. 1999).
2.4.3 Oxidation and Adsorption
Oxidation followed by adsorption is one of the conventional methods to treat
arsenic-contaminated water. Mostly, As
III is oxidized to As
V using several oxidants,
such as chlorine, potassium permanganate, ozone and hydrogen peroxide. Typically,
adsorption involves passing of contaminated water through a packed media to which
arsenic get attached via physical or chemical bonds. The common adsorbents that
have been reportedly employed in the field include granular ferric hydroxides,
activated alumina, iron coated sand and activated carbon with surface modifications
(Westerhoff et al. 2005; Hristovski et al. 2009).
2.4.4 Membrane Processes
In this technique, a semipermeable membrane has been utilized to separate the
arsenic from contaminated water. This membrane acts as a physical barrier to
some ions depending on their physio-chemical characteristics. The driving force
responsible for the movement of ions across the membrane is the generation of
potential difference between the two sides of membrane. The process involved for
40
A. Kumar et al.
It involves the mixing of chemicals such as salts of iron (ferric chloride), aluminium
(alum) and manganese (manganese sulphate) into water to settle down the arsenic
through precipitation, co-precipitation or combination of both these processes.
Afterwards, the arsenic is separated from the solution by filtration as it gets adsorbed
onto the solid matrix (Wickramasinghe et al. 2004). In general, this approach
removes As
III as compared to As
V , and ferric chloride has been reported to be
more efficient than alum. The effectiveness of this method depends on the pH,
concentration of arsenic, arsenic speciation and presence of competing ions in the
water. However, it requires pretreatment process to oxidize As
III to As
V using
chlorine or permanganate ions since it causes partial removal of As
III from aqueous
solution (Ahmed 2001b). Nowadays, solar and microbial oxidation have also been
commonly utilized to convert As
III to As
V (Jain and Singh 2012).
2.4.2 Ion-Exchange Resins
It is a physio-chemical process involving exchange of ions between solid phase and
resin (solid) phase of similar charge. The exchanged ions are held electrostatically on
the surface of solid phase with ions of similar charge in a solution. The ion-exchange
media is usually packed into a column, and then water containing arsenic is allowed
to pass for removing the desired contaminant. The columns can be regenerated with
the brine solution for preparation of next cycle of removal (Ghurye et al. 1999).
2.4.3 Oxidation and Adsorption
Oxidation followed by adsorption is one of the conventional methods to treat
arsenic-contaminated water. Mostly, As
III is oxidized to As
V using several oxidants,
such as chlorine, potassium permanganate, ozone and hydrogen peroxide. Typically,
adsorption involves passing of contaminated water through a packed media to which
arsenic get attached via physical or chemical bonds. The common adsorbents that
have been reportedly employed in the field include granular ferric hydroxides,
activated alumina, iron coated sand and activated carbon with surface modifications
(Westerhoff et al. 2005; Hristovski et al. 2009).
2.4.4 Membrane Processes
In this technique, a semipermeable membrane has been utilized to separate the
arsenic from contaminated water. This membrane acts as a physical barrier to
some ions depending on their physio-chemical characteristics. The driving force
responsible for the movement of ions across the membrane is the generation of
potential difference between the two sides of membrane. The process involved for
40
A. Kumar et al.