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Cost effective method for
Removal of
Fluoride from Polluted water
By: Dr. Abida Begum
Water pollution is
a serious problem as almost 70% of India’s surface water resources and a growing
number of its groundwater reserves have been contaminated by biological, organic
and inorganic pollutants. Pollution of surface and groundwater resources occurs
through point and diffuse sources. Examples of point source pollution are
effluents from industries and from sewage-treatment plants. Typical examples of
diffuse pollution sources are agricultural runoffs due to application of
inorganic fertilizers and pesticides and natural contamination of groundwater by
fluoride, arsenic and dissolved salts due to geo-chemical activities. In pursuit
of measures to achieve sustainability in water management, the Centre for
Sustainable Technologies (CST) at the Indian Institute of Science (IISc),
Bangalore has begun to address treatment of fluoride-contaminated groundwater
for potable requirements. The fluorosis problem is severe in India as almost 80%
of the rural population depends on untreated groundwater for potable water
supplies.
Sources of pollution
Pollution
of surface and groundwater resources occurs through point and diffuse sources.
Examples of point source pollution are effluents from industries,
sewage-treatment plants and untreated domestic sewage. The main sources of
diffuse pollution may be anthropogenic activities, such as agricultural
applications of fertilizers and pesticides or of geo-chemical origin, such as
natural contamination of groundwater sources by fluoride, arsenic and dissolved
salts. Pollution from point sources can be controlled by disposal in engineered
facilities, treatment and recycling of waste materials. Minimizing application
of fertilizers and pesticides is a way to control pollution from agricultural
activities. Natural contamination of groundwater sources by fluoride, arsenic
and dissolved salts is dealt with by suitable treatment of extracted
groundwater.
Industrial
pollution
In case of industrial units,
effluent in most of the cases is discharged into pits, open ground, or open
unlined drains near the factories, thus allowing it to move to low lying
depressions resulting in groundwater pollution. The industries, which are
burgeoning at a fast rate,
produce about 55,000 million m3 of wastewater per day, out of which
68.5 million m3 is discharged into river and streams. Thus the
magnitude of damage caused to our water resources can be estimated from the fact
that about 70% of rivers and streams in India contain polluted water. The
incidence of surface and groundwater pollution is highest in urban areas where
large volumes of waste are concentrated and discharged into relatively small
areas. The groundwater contamination is detected only some time after the
subsurface contamination begins. Although the industrial sector accounts for
only 3% of the annual water withdrawals in India, its contribution to water
pollution, particularly in urban areas, is considerable.
Pollution from domestic activities
Inadequate treatment of human
and animal wastes contributes to the high incidence of water-related diseases in
the country. To date, only 14% of rural and 70% of urban inhabitants have access
to adequate sanitation facilities. Fluorine, a fairly common element of the
earth’s crust, is present in the form of fluorides in a number of minerals and
in many rocks. Excess fluoride in drinking water causes harmful effects such as
dental fluorosis and skeletal fluorosis. The permissible limit of fluoride level
is generally 1ppm. The high fluoride levels in drinking water and its impact on
human health in many parts of India have increased the importance of
defluoridation studies. The fluoride -bearing minerals or fluoride-rich minerals
in the rocks and soils are the cause of high fluoride content in the
groundwater, which is the main source of drinking-water in India.
A study was conducted to find a suitable low-cost
environmentally friendly method for the removal of fluoride in the groundwater
that is used by common man. A few natural materials such as pea-nut shell
carbon, Bombax malabaricum carbon, untreated charcoal, fly-ash from
bagasse and MgO2 maintained at PH 8.5 were used. The capacity of
fluoride removal by the individual materials was studied and accordingly five
columns were set up and studied for their defluoridation capacities.
Methodology
The fly-ash from bagasse has oxides of Si, Al, Fe,
Ti, Ca, MgSO3 and alkalies along with mixed oxides.
The pea nut shell and dried fruits of
Bombax
malabaricum were carbonized in the electrical conventional heating reactor
by two stages carbonization process in the range of 250-600 oC and
600-700 oC respectively. The materials were placed in closed
stainless steel vessel by maintaining inert conditions and pyrolysis was carried
out at 40 oC for 30 minutes followed by next stage to develop the
pore size structure.
Powdered untreated charcoal and MgO2
were used.
All the materials used for defluoridation were
treated with 0.5 M HNO3 for the removal of unwanted materials. The
acid washed product was thoroughly washed with hot distilled water to remove
acidity and chlorides. The products were finally dried. All chemicals were of
analytical reagent grade, and distilled water was used throughout. The standard
solutions were prepared 1mg/L, 5 mg/L 10mg/L 15mg/L and 20 mg/L using DDW (the
pH maintained was 8.5 HCO3-2.1 mg/L, Ca 1.2 mg/L, SO4 0.43
mg/L). Sorption experiments were conducted at 23oC in a column packed with
450 g of each material (Bombax malabaricum carbon, untreated charcoal,
fly-ash from bagasse and MgO2) The concentrations of fluoride in
water fed on the columns were 1, 5, 10, and 20 mg/l. 250 ml of a fluoride
solution was first poured into the column to moisten the natural material. Then
a 250-mL portion of the solution was placed in the top reservoir and the
draining rate was adjusted to ca 2.0 ml/hr. Each experiment with solution of a
given concentration was run with a fresh portion of each natural material. The
stopper was adjusted at a standard rate of 1.5 ml /min. The samples were
collected at intervals of 20, 40, 60, 80, 120 and 140 minutes. The fluoride
level in the effluent was monitored potentiometrically by using a
fluoride ion-selective electrode. Further 5 locations of ground water were
analyzed. From each location 3 samples were taken and tested for fluoride
concentration before and after treatment. The results showed that all the
samples were found to be within desirable limit of fluoride concentration of
Indian standards but for untreated charcoal where the fluoride concentration was
above permissible limit.
The results
showed that the sorption of fluoride increases with increasing concentration of
incoming solution. Pure water leaches fluoride for all the five materials may be
due to formation of fluoride complexes. When a solution with low fluoride
concentration (1 mg/l) is passed through the column, the fluoride level in the
effluent was nil in the case of peanut shell carbon, fly-ash and MgO2
at 80 minutes. However, after 120 minutes the effluent becomes completely free
of fluoride in all the columns.
It is remarkable that the
retention capacity of the natural materials increases with increasing fluoride
concentration in incoming solution. . For the 5, 10 and 20 mg/l F-
concentrations the respective drops in F- concentration are 27, 58
and 76.5 per cent. Complete retention of the fluoride by fly ash occurs after
100 minutes for the lower F-
concentrations (1, 5, and 10 mg/l) and after 120 minutes for the higher F-
concentrations. The rate of adsorption is controlled by the rate of diffusion of
fluoride in the inter-capillary pores of the particles. In the case of charcoal
treatment the retention of fluoride did not change significantly with time,
although there was some defluoridation at the initial stage. Strong sorptive
capacity of residual carbon in the natural materials used, may be due to the
fact that residual carbon samples showed signs of significant oxidation that has
led to highly porous and fragmented particle structures. Hence all the materials
used in the experiment are good sorpbents except untreated charcoal. Further,
ground water from five locations were analyzed. From each location 3 samples
were taken and tested for fluoride concentration before and after treatment.
The results revealed that after the treatment the samples were found to be
within desirable limit of fluoride concentration of Indian standards except in
the case of untreated charcoal the fluoride concentration was above permissible
limit. It is a preliminary experiment and it needs further research.
*Department of Chemistry, P.E.S
School of Engineering, Bangalore-560 012 <[email protected]> |
