Abstract: Natural and human activities
are identified as the main reason for fluoride in groundwater. It is a major
problem throughout the world, posing a serious threat to human health. Several
treatments for the removal of fluoride are widely used in the adsorption
process for satisfactory results, especially with minerals-based adsorbents and
/ or surface. In this review, a comprehensive list of different chromosomes has
been compiled and their adsorption capacities under various conditions, it is
presented with the literature for the removal of fluoride, such as pH, basic
fluoride concentration, temperature, contact time, adsorbent surfaces. There is
some summery on the main advancements in formulating new adsorbents that have
so far been tested for fluoride removal. The literature review shows that many
adsorbents have shown good potential on Removal of fluoride. I mainly focused
on fly ash and its adsorption ability.
Keywords: fly ash, grapheme oxide, fluoride, groundwater, adsorption, pH,
time, temperature, research
Hardness and fluoride
concentration in ground water is a major water quality parameter which is
getting more attention of lots of responsible authorities and people. Recent
researches were founded that over concentration of fluoride and hardness in
water causing some disastrous health problems to society. Number of countries
as well as srilanka suffered due this problem in last few decades. Most of
those countries fulfill their water demand by using ground water. Therefore
removal of fluoride and hardness from ground water is very important
requirement due to above mentioned reasons.
Hardness and fluoride
Hardness is a
parameter of water that causes some problems to people who are using ground
water. Cations such as calcium (Ca²+) and magnesium (Mg²+) are the main reasons
for the hardness in water. There are few other cations which are also causing
hardness, but they are hardly considered (C.K. Geethamani, 2013). Total sum of
cations in water had been defined as the hardness water 1, 2. Total hardness
can be determined by using following equation;
Hardness = c (Mg2+) + c
Water can be
categorized using numerical value of hardness such as soft or hard. According
to Vesikirja, the hardness is classified by the following. The hardness is
divided in five different groups and the corresponding hardness values in ppm (Dinesh Mohan,
Very soft 0 – 37.5 ppm
Soft 37.5 – 87.5 ppm
Moderately hard 87.5– 175 ppm
Hard 175- 375 ppm
Very hard over 375 ppm
The geological nature
of environment can be determined by using hardness in groundwater. In srilanka
the soil is mostly basic and this leads to the water to be hard. People are
living in dry zone in srilanka experienced high hardness in water (Dinesh Mohan,
Fluoride is a water
contaminant which creates various human health problems. World health organization
stated that small concentration of fluoride is good for dental health but they also recommended any concentration above 1.5 mg/l is a threat
to human health.
Fluoride is widely distributed in the
geological environment and generally released into the groundwater by slow
dissolution of fluorine-containing rocks (Dinesh Mohan, 2015). Exceeding the recommended fluoride
concentration is increase the risk of some diseases such as osteoporosis,
arthritis, kidney diseases, brittle bones, cancer, infertility, brain damage,
Alzheimer syndrome, and thyroid disorder (Amit
Bhatnagara)Number of methods of
removing hardness and fluoride from ground water had been discussed by
researchers. They were tried few methods such as nanofiltration using different
kinds of materials. Also the precipitation and coagulation processes
with iron (III) (Amit Bhatnagara), activated alumina,
alum sludge (Andrew J. Frierdich a, 2017)and calcium had been
widely investigated. Other than that, ion exchange, reverse osmosis and
electrodialysis have also been studied for the removal of additional amounts of
fluoride from groundwater. But considering the cost, complicatedness of process
and secondary pollution are main shortcomings of those methods. Therefore
scientists are considering about some cheap, environment friendly and simplify
methods for defluoridation. As a result of above facts, recently the adsorption
process is the most popular method of defluoridation. Results of these methods seem to be
satisfactory and more attractive method for the removal of fluoride in terms of
cost, simplicity of design and operation (Helle U. Sø a, 2016).
Modified fly ash has a potential to
remove hardness and fluoride in water. However, modified fly ash alone is
ineffective to be used in water treatment due to their nano and micro levels of
particle size distribution and aggregation, which has reduced its adsorption
capacity and stability in water. Therefore, anchoring of modified fly ash in
suitable matrixes is most important to increase their stability in water and
enhances reactive performance. Graphene
oxide has a large number of micro-pores on the surface and it has been used to
impregnate different nano materials in the past. Modified fly ash impregnated
graphene oxide seems to be a good combination to remove toxic constituents in
groundwater (G.G. Hollmana, 2016). Hence, the study
focuses to impregnate modified fly ash in graphene oxide matrix and evaluates
the properties and performance in removal of hardness and fluoride in
groundwater (Widi Astuti 1 +, 2012).
Fly ash is a composition
of some oxides such as Al2O3, SiO2, and unburned carbon. That is the reason behind adsorption ability
of fly ash. Low capacity may occured by high crystallinity of Al2O3 and SiO2 as
well as the presence of the unburned carbon. The effect of fly ash
crystallinity for Pb(II) adsorption has been notified (dissananyake,
2009) (Blaney, 2007) .
Previous researches regarding fly ash and other industrial waste
The industrial waste residue, generated during the manufacture of
aluminium sulphate (alum) from kaolin by the sulphuric acid process was used as
defluoridating media by Nigussie et al.. According to the research, fluoride
adsorption remains constant within pH 3-8. When the pH value is increasing the
ability of fluoride adsorption was decreased. Within the pH range of 3 to 8
positive and neutral ions are presented on the surface of the absorbent, that
is the reason for better adsorption in
that range of pH. The research data shows that D-R model is well describing the
adsorbent when the concentration 332.5 mg/g. presence of bicarbonate is
considerably affected to the performance of the adsorbent (G.G.
Hollmana, A two-step process for the synthesis of zeolites from coal fly ash,
HCL activated red mud is another aluminium industrial waste.
Removal of fluoride also studied for this composition too. Original form of red
mud is not activated as HCL activated red mud. At the pH value of 5.5 and after
2h hours of time, the peak adsorption was recorded. Behaviour of Chemical
nature and metal oxides are the basis of this fluoride removal. Results were
interpreted in terms of pH values.
0.331mmol/g was the result obtained from the Langmuir model (Dinesh Mohan,
Tor et al. also studied the suitability of granular red mud (GRM)
as a adsorbent of fluoride from water. The maximum fluoride removal (0.644
mg/g) was achieved at pH 4.7. Equilibrium was obtained after 6 hours. The
capacities of the breakthrough and exhaustion points were found to decrease
with increase in the flow rate. Sorption capacity of the column method was higher
than its batch samples for Thomas model (Amit Bhatnagara).
Investigation on Adsorption
of fluoride on waste carbon slurry (a fertilizer industry waste) was showed that, Maximum
adsorption capacity (4.861 mg/g)
of fluoride on carbon slurry was observed at 15 mg/L initial fluoride concentration using 1.0 g/L adsorbent dose.there
was no specified pattern obtained with the pH. Desorption was achieved under alkaline
conditions (pH 11.6) from exhausted carbon
slurry (Andrew J. Frierdich a, 2017).
C¸ inarli et al was studied the ability of coal mining waste for
remove fluoride from groundwater. The maximum adsorbent capacity of the
material was 15.67mg/g according to the Langmuir model. Acidic conditions were
favorable for the defluoridation. Optimum pH was 3.5. Waste mud, original waste mud (o-WM),
acid-activated (a-WM) and precipitated waste mud (p-WM) tested for check the
ability of fluoride removal. Maximum fluoride sorption was observed from p-WM by
Langmuir model with the value of 27.2mg/g. between 0-40 0c of
temperature the adsorbent ability was increased.
The p-WM was found to be capable for at least five times for
further adsorption process without
regeneration (kokshela, 2016).
Spent bleaching earth (SBE) was tested and founded that maximum
adsorbent observed at the concentration of7.752 mg/g. pH and anions are
adversely affected on the capability of adsorbent.
Alum sludge is one of the waste products generated during the
manufacture of alum from bauxite by the sulphuric acid process and mainly
consists of oxides of aluminium and titanium with small amounts of undecomposed
silicates. Each of these oxides is known to possess adsorption and ion exchange
properties. To take advantage of these properties of alum sludge, Sujana et al.
examined its use as an adsorbent for the removal of fluoride from polluted
waters. The data indicate that treated alum sludge surface sites were
heterogeneous in nature and that fitted into a heterogeneous site binding
model. The Langmuir maximum sorption capacity for fluoride removal by alum
sludge was reported to be 5.394 mg/g. For an increase in temperature from 307
to 337 K, and with 20 mg/L of initial fluoride concentration, an adverse effect
was observed on the adsorption of fluoride. The adsorption decreased from 85 to
72% at pH 6.0 although the fluoride adsorption at a given temperature increased
with time. This was attributed to the escaping tendency of the molecules from
interface at high temperatures and thereby diminishing the extent of
adsorption. Fluoride removal was found maximum at pH 6.0, and found to decrease
sharply above that as a result of stronger competition from hydroxide ions on
adsorbent surface (dissanayake, 2000). Also, adsorption
was found less in the acidic range which was proposed to be a result of the
formation of weakly ionized hydrofluoric acid. Defluoridation with alum sludge
in presence of phosphate and silicate at higher concentrations (10–50 mg/L) had
an adverse effect on fluoride removal. The affinity sequence for anion
adsorption on treated alum sludge was in the following order phosphate ? silicate > sulphate > nitrate. The
potential of thermally activated titanium rich bauxite (TRB) for adsorptive
removal of excess fluoride from drinking water was examined by Das et al.
Thermal activation at moderate temperatures (300–450 ?C) increased the adsorption capacity of TRB. The fluoride uptake
increased with increasing pH and reached to a maximum at pH 5.5–6.5 and
thereafter decreased. The Langmuir maximum adsorption capacity for fluoride was
observed to be 3.7–4.13 mg/g. The presence of common interfering ions in water
did not greatly affect the uptake of fluoride from aqueous solution indicating
fluoride specific sorption behaviour of TRB (MasterPozzolith, 2017).
Nearly complete desorption of adsorbed fluoride from loaded bauxite
was achieved by treating with aqueous solutions of pH ? 11.1 (NaOH ?0.015 mol/dm3). Red mud was also modified
by AlC13 (MRMA) and by heat activation (MRMAH) and tested for the removal of
fluoride from water .The results showed that the adsorption capacities of MRMA
and MRMAH were 68.07 and 91.28 mg/g, respectively, which were much higher than
that of RM (13.46 mg/g). The Langmuir isotherm was the best-fit adsorption
isotherm model for the experimental data. The solution pH affected the removal
efficiency significantly, and the highest removal efficiency was achieved at pH
7–8 (N. Gandhi1*, 2012).
Chaturvedi et al. was investigated the ability of fly ash for
fluoride removing of water. The removal of fluoride was found favourable at low
concentration, high temperature and acidic pH. The Langmuir maximum sorption
capacity of fly ash for fluoride ranged from 20.0 to 20.3 mg/g.
Also it shows that
fluoride removal ability of fly ash is mostly depends on pH value.6 to 7 pH
values are more favourable for adsorption process. Geethmani studied the other
important parameter of the adsorption process such as effects of contact time,
concentration, dosage and temperature (C.K. Geethamani, 2013).
There were slight
increase of adsorption with temperature, agitation speed and concentration.
When refer the contact time, there was sudden adsorption within first 10
minutes. Then the adsorption rate slowed down. Active sites of the top surface
were finished in first 10 minutes and then it slow down due to time taken to
reach inner surface sites. It was the identified reason for the sudden
adsorption within first 10 minutes (Amit Bhatnagara).
of fluoride above 80% was removed from its 10 mg / nasal solution with an
equilibrium 120 minute equilibrium and 3 g / g of fly ash concentrate. pH
values ??are better than adsorption when it is a neutral solution. The
efficiency of the process was absorbed and the nature of the process showed
increased efficiency. Lopomor was described in more detail in the contour
model. Monocylation capacity for glutenous ash for fluoride is 10.86 mg / g (A. K. CHATURVEDI, 1989). The equilibrium
motion of the equilibrium data was shown as adsorption based on fluoride ion
fluoride ion. This process demonstrates that the process of circulation is
followed. The internal variable spread model shows that it is a multi-phase
adsorption process and is associated with an internal variation with another
internal mechanism. Real field touring water ashes indicate that flying ash is
not only a good adsorbent to remove fluoride from the water; Groundwater is
also a good adsorbent for removing other anions (P.D. Nemade*, 2002).
All those previous researches shown that
pH, contact time, temperature, composition, concentration and different models
are very important facts when this research is going on. Therefore I have to
consider about all these facts when I carrying my research. There are lots of
researches done considering the effect of fly ash on fluoride removing groundwater,
but hard to find any researches regarding graphene oxide involvement. Thus this
research is very important due to its background.
I would like to acknowledge
Department of civil engineering department of university of moratuwa and all
especially to the environmental engineering section and Dr. ashani ranathunga
for their valuable support to this paper. Also I would like to acknowledge all
the researchers and authors of for their previous researches and books which
are really helped me to create this review.
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