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Several methods have been suggested for removing excess fluoride in water. The process of removal of excess fluoride from water is described as defluoridation. One of such is adsorption. Present study has been conducted to investigate the efficacy of various low cost adsorbents like sludge of water treatment plant, sewage treatment plant, alum impregnated sludge of sewage treatment plant as defluoridating adsorbent material. The experiment has followed batch process. The effect of pH, contact time, adsorbent dose, fluoride concentration on removal efficiency of fluoride has been studied in detail…42%, 57.5% and 78% removal have been achieved at equilibrium time for wtp sludge, sewage sludge and alum impregnated sewage sludge respectively.
The adsorption kinetics have been found to follow first order rate mechanism as well as Freundlich and Langmuir isotherm model for sewage and alum impregnated sewage sludge and Bet isotherm model for wtp sludge. The optimum pH range was found to be 6.50-7.5 and adsorption equilibrium was found to remain from a range of 270-300 minutes after conducting the experiment with different initial concentrations.
Keywords: Defluoridation, Adsorption, Isotherm, Equilibrium. Wtp- water treatment plant, Stp- sewage treatment plant.
Buddhadeb Bhattacherjee. Executive Engineer- Kolkata Metropolitan Water & Sanitation Authority, Kolkata;Delhi University-INDIA.
Dr. Goutam Banerjee, Professor-Environ Engineering Division, Department of Civil Engineering, BHU, UP; Delhi University, Delhi-India.
Dr.Shibapriya Mukherjee, Professor-, Department of Civil Engineering, Jadovpur University, WB; Delhi University, Delhi-India
The presence of excess fluoride in drinking water has become a potential cause of agony to hundreds of million people the world over as it initiates the debilitating disease fluorosis. The problem of excessive fluoride in potable water and the consequence of fluorosis is major concern not only in India but also across the globe. Defluoridation has become a widely practiced process worldwide due to the problem of fluorosis caused by the presence of F- in drinking waters. Methods such as adsorption, precipitation, ion exchange and membrane processes have been employed for F- removal. Considerable work has been done all over the world on treatment of Fluorosis. Fluorosis is a problem of public health importance. Perhaps the magnitude of the problem and the possible methods of preventing it were not appreciated till recently (1986) when the Government of India launched a ‘Technology Mission on Safe Drinking Water'. One of the thrust areas of this Mission is to control and prevent fluorosis. In the Present study an effort has been made to assess the suitability of sludge of sewage treatment plant as bio adsorbents, the same impregnated with alum as alum impregnated bio adsorbents and wtp sludge - all as inexpensive adsorbents to effectively remediate fluoride-contaminated water.
The present investigation was aimed to remove fluoride by using sludge of water treatment plant, sludge of sewage treatment plant and sewage sludge impregnated by 0.09% Alum by batch adsorption method.
The bio-sludge has been collected from a trickling filter sewage treatment plant located at the southern part of the district of Howrah in West -Bengal; India. The alum to a requisite quantity was then mixed to prepare the adsorbent. The sludge of water treatment plant has been collected from water treatment plant at Serampore in the district of Hooghly in W-Bengal; India.
Instruments like spectrophotometer (model-uv 2100, for use at 570µm. — wave length providing a light path. of at least 1 cm.), digital pH meter, hot plate, oven with3000c thermostat and1000c heater, Electrical furnace with temperature range 10000C to 12000 C, Digital Nephero Turbidity meter (Model no 132), Jar test apparatus with rotational speed 400 rpm and 4nos stirrer, measuring balance, micron filter paper and other allied common apparatus.
Preparation of solution
Dissolving anhydrous sodium fluoride i.e. NaF, of 2.21gm in 1 liter distilled water stock solution of 1000µgF mililitre-1 is prepared .The test fluoride solution of 10mgF litre-1 was prepared from stock solution by appropriate dilution.
Now 500ml distilled water is taken in each of 4nos similar type of jars. The fluoride concentration of each jar is maintained by adding the requisite quantity of standard solution to it. The Filled up jars were again injected with different dose of adsorbing material and then were allowed of being stirred for a stipulated period of time.
The stirring operation was performed at room temperature, 29±20c. Several sets of test data were obtained to study the effects of several variables such as adsorbent dose, initial fluoride concentration, pH, particle size etc. Adsorption experiment was carried out by agitating the adsorbent mixed in distilled water of 500ml with the help of stirrer with a varying speed from high to low(100rpm for 2minitues and 30 rpm for 10 minitues) followed by a detention of 30 minitues as done in case of a zar test.
A 50mlsamples was siphoned out from 1cm below the surface for residual fluoride estimation. Turbidity of supernatant water is examined in turbidity meter. If turbidity was found beyond the acceptable limit, the same water was again filtrated through Whatmanfilter paper no 42 (micron filter paper) which brings down the turbidity level to its acceptable limit. The filtrate was analysed for residual fluoride concentration by SPADNS method using a spectrophotometer following standard method. The fluoride level of pervious and latter sample gives the quantity of fluoride removed. To keep pH in desired value 0.1N HCL or 0.1N NaOH is to be added as per requirement.
The reduction of fluoride is also recorded by varying different parameters like pH, adsorbent doses, concentrations of fluoride, temperatures. The adsorbent materials were
kept in hot air oven and then dried materials were grinded like powder and passed through different sieves of varying sizes.The basic equation which describes the fluoride removal phenomenon is given as follows.
E= (C0 -Cf)/C0 x100 ————————-(1)
WhereC0 is initial concentration, Cf is residual or final concentration at equilibrium time and
E is the removal efficiency
In each case the optimum doses are recorded to find out the optimum removal. Optimum dose is very common term frequently used for this type of study. The minimum dose applied for maximum removal is generally considered as optimum dose. However for the sake of convenience of this work, a value from a range of optimum values of a parameter has been chosen as optimum one where more than one such value is available.
In batch adsorption studies, a predetermined amount of adsorbent is mixed with a sample, stirred for a given contact time and subsequently separated by filtration. Powdered adsorbent are more suitable for batch type of sorption study.
Results and discussions
The physico-chemical properties of raw adsorbent attribute to a great extent to control the phenomenon of removal and therefore same is presented in table I.
Description of parameter
mg/kg dry weight.
mg/kg dry weight
Below detectable limit.
after 18hrs at hot air oven
Table 1: Physico-chemical properties of wtp sludge
In both the tables the most important properties have only been described. The physical parameters have a great role on adsorption kinetics, whereas chemical properties determine the acceptability of the adsorbent-materials in respect to treatment of water. Presence of any material beyond acceptable limit is not desirable and hence requires suitable technology to bring down the percentage presence of those materials within Prescribed standards.
Description of parameter
Total organic carbon.
mg/kg dry weight.
mg/kg dry weight.
mg/kg dry weight
mg/kg dry weight.
Below detectable limit.
mg/kg dry weight.
after 18hrs at hot air oven
Table II Physico chemical properties of bio sludge.
As alum is a good coagulant, an attempt has been made to produce a unique bio adsorbent by impregnating bio sludge with alum (0.09% for the present study).
Impregnation ratio is an important factor In activation of adsorbent in sorption. The impregnation ratio is defined as,
Weight of the active agent
Impregnation ratio (I.R) = ———————————
Weight of the raw adsorbing material
In the present study 4mg/l alum has been mixed with 45gm of adsorbent .Putting the value in above formula impregnation ratio comes with a very low numerical figure and so the impregnation has been expressed here as
Percentage of raw adsorbent; as in this case it comes to 0.09% of raw adsorbent.
The removal efficiency of fluoride has been found out against each parameter. The standard solution i.e. 10mgF litre-1 was considered as base concentration for experiments with various parameters. The removal efficiency is a dependent variable where parameter under influence of which reduction of fluoride takes place is a variable of independent in nature. Duration of time to reach to optimum removal is termed as equilibrium time or equilibration time, as beyond that span of time uptake does not increase and at this point of time the rate of adsorption becomes equal to the rate of desorption or the solute concentration becomes saturated.
The effect of various parameters on removal efficiency has been found out and is described as follows.
Effect of adsorbent dose
alum impregnated adsorbent
alum impregnated adsorbent
TableIII- Co relational matrix of the materials with adsorbent dose in gm/l.
A co- relational matrix of the materials with different adsorbent doses is presented in Table-III for verifying the degree of suitability of such experiment. The removal efficiency is taken as dependent variable based on adsorbent doses as independent variables. The following observations were noticed when relevant data were plotted in an adsorbent dose vs. removal efficiency curve.
Removal efficiency increases with the increase in adsorbent dose till 50mg/L-for wtp and bio- sludge, whereas for alum impregnated bio sludge it is 45 mg/L; thereafter no further removal is noticed. From the R2 value (as it approaches unity) it can be concluded that when it satisfy simple linear relationship for wtp and bio sludge; it maintains logarithmic relationship for alum impregnated bio sludge. At 50mg/l of adsorbent the maximum fluoride removal is obtained and beyond that no further removal was noticed and therefore 50mg/l has been considered as optimum dose for wtp and bio sludge whereas 45mg/l is taken optimum dose for alum impregnated bio sludge.. The maximum removal was observed 42%for wtp sludge, 57.5% for bio sludge and 78% for alum impregnated bio sludge.
Effect of initial concentration of adsorbate
Initial concentration vs. removal efficiency curve reveals the observations as discussed hereunder.
The slope of the line clearly indicates a decreasing linear trend of removal efficiency with increase in initial concentration. It follows in each of the case a first order equation representing reversal linear relation ship. The equations are as follows:-
i. Y= (-) 1.5X+70.5. for bio adsorbents. ii) Y= (-) 1.4X+56.443 for wtp sludge. iii)Y= (-) 1.2862X+90.331 for alum impregnated bio sludge..
Where y is removal percentage and X stands for fluoride concentration of solution.
Effect of pH
When relevant data are plotted in a curve for removal efficiency vs. pH all three adsorbents obey a polynomial relationship. Removal efficiency for wtp sludge shows maximum removal of 55% at pH=2, and decreases with increase in pH. For bio-adsorbent maximum removal goes high to 68% at pH=2, and decreases with increase in pH.For alum impregnated bio-adsorbent, removal efficiency shows maximum removal of 82% at pH=2. and decreases with increase in pH
Optimal removal range has however been considered as 6.5-7.5 i.e. the neutral pH range and therefore no artificial adjustment is required to keep the pH of fluoridated water in the said range as the pH of all the adsorbents lye in that range. The optimal removal values have been chosen therefore as those values which are achieved in the natural pH range.42%, 57.5%and 78% removal are achieved for water treatment sludge, bio adsorbent and alum impregnated bio adsorbents respectively. .
Effect of particle size and temperature
The other important parameters against which removal efficiency have been verified are contact time, particle size and temperature. Temperature has not much effect on it whereas there is a sharp rise in removal efficiency following a linear relationship with a value of R2 as0.973,0.941and 0.9658 for wtp sludge, bio- adsorbent and alum impregnated bio-adsorbent respectively as particle size grows smaller from 1.4mm to 600µ.
Effect of contact time
With the increase in contact time the removal increases for bio adsorbent , wtp sludge and also for alum impregnated bio adsorbent(observations as stated are understood when data are plotted on removal efficiencyvs.pH curve.) .No further uptake is noticed beyond 300 minitues for first two adsorbents and 270 minitues for alum impregnated bio adsorbent. The time in minitues as mentioned are considered as equilibrium time, for respective case. Moreover when removal for first two adsorbent follow linear equation as perceived from R2 value, removal for alum impregnated adsorbent is closer to a logarithmic relationship.
The kinetics of adsorption of fluoride has been studied by estimating the effect of contact time on % removal of Fluoride. The rate of adsorption of fluoride obeys
i.. first order reaction for wtp sludge. The linear equation as obtained is Y=0.1X +10.75.
ii. first order reaction for stp sludge. The linear equation as obtained is Y=0.1X +23.14.
iii. Logarithmic relationship rather than simple linear equation for alum impregnated bio adsorbent.
The logarithmic relationship as established is as follows:
Where Y is the % removal and X is contact time in minitues.
When q is the amount of fluoride sorption at any point of time and qe is the amount of sorbate at equilibrium time, then q/qe =U (t) is called the fractional attainment. A relationship between U (t) and contact time can be expressed in the following form
Ln [1-U (t)] =k't
Where k' is overall rate constant in time-1 unit. The sorption of fluoride from liquid phase to solid phase can be considered as a reversible reaction with equilibrium being established between two phases. A simple first order reaction kinetic model is generally used to establish the rates of reaction between two phases. If U (t) is fractional attainment of equilibrium between the two phases, Ln [1-U (t)] versus time is plotted using the sorption kinetic data. A near straight line plot is observed for all concentrations indicating that reaction can be approximated to first order reversible kinetics. Theoretically when time equates to zero the value of [1-U (t)] equates to unity. This indicates the initial high rate of fluoride uptake during the first few minutes. Equilibrium time of contact is
i.300minitues for wtp sludge and bio adsorbent with a removal of 42% and 57.5% respectively.
ii..270minitues for alum impregnated bio sludge and 78% removal is observed at that time.
Isotherms are often engaged to study the performance of adsorption mechanism.
The sorption data for the removal of fluoride ions have been correlated with Freundlich and
Langmuir and Bet models.
Now the isotherms are discussed and presented with some notations so that their linearised interpretations can be well understood.
The linearised Freundlich adsorption isotherm, is commonly used in the form
Log (qe) = log K + 1/n log Ce,
where qe is the amount of metal ions adsorbed per unit weight of adsorbents (mg g-1), K
and 1/n are the Freundlich constants, if 1/n < 1, bond energies with surface density, if
1/n > 1, bond energy decreases with surface density and if 1/n = 1 all surface sites are equivalent. If the R2 value of Freundlich model approaches to unity denotes a very good linear relationship between log (Ce) and log (qe).Ce is the equilibrium concentration (mg/l). Linear plots of log qe(x/m) vs. log Ce at are applied to confirm the applicability of Freundlich models has been shown in relevant Figures.
Langmuir isotherm is based on the assumption that point of valence exists on the surface of
the adsorbent and that each of these sites are capable of adsorbing one molecule. Thus, the
adsorbed layer will be one molecule thick. Furthermore, it is assumed that all the adsorption sites have equal affinities for molecules of the adsorbate and that the presence of adsorbed molecules at one site will not affect the adsorption of molecules at an adjacent site.
The linear form of the Langmuir isotherm can be expressed as,
1/qe = (1/Qo) + (1/bQoCe).
where qe is the amount adsorbed (mg/g) and Ce is the equilibrium concentration of adsorb- ate in (mg/l), Qo and b are the Langmuir constants related to capacity and energy of adsorption respectively.
When 1/qe is plotted against 1/Ce, a straight line with slope 1/bQo is obtained which shows
that the adsorption follows the Langmuir isotherm .The Langmuir constants b and Qo are calculated from the slope and intercept with Y-axis. The essential characteristics of a Langmuir isotherm can be expressed in terms of dimensionless separation factor, and describe the type of isotherm defined by-
R =1/ (1 + bCo),
where b and Co are terms appearing in Langmuir isotherm.
BET isotherm is presented in the form as follows:
Where x=amount of solute adsorbed (mg), m=weight of adsorbent (mg), C=Concentration of solute solution (mg/l).
A, a, b, n, xm are constants
To interpret Bet in linearised form.C/Cs vs. x (Cs-C) graph is prepared. Plotting the data in the graph R2 value is obtained and its fitness is judged .
The data available from the test has been put to Langmuir as well as Freundlich model of Isotherm which are very popular Isotherms for adsorption studies. Results reveal its fitness to a great extent to both the Langmuir and Freudlich model .Exceptions noticed for Bet model where alum impregnated bio adsorbent is found misfit and wtp sludge shows comparative poor resuls with R2 value falling below 90%. .
The result shows that all the chosen materials can be used as Fluoride removal adsorbent. The maximum rate of removal is 78% for alum impregnated bio adsorbent whereas the rate goes down to 42% for wtp sludge and achieves 57.5% for stp- sludge. The equilibrium time is
i. 4.5hrsfor alum impregnated adsorbent when 65% of total removal takes place within 1.5hrs and within 3hrs around 88% removals takes place.
ii..5hrs for bio adsorbent .The maximum rate of removal is 57.5% though 60% of total removal takes place within 1.5hrs.
iii. 5hrs and the maximum rate of removal is 42% for wtp sludge. Like bio adsorbent 60% of total removal takes place within 1.5hrs.
While attaining fractional attainment, the first order reaction kinetic model has been used to
establish between rates of reaction of two phases i.e. solid and liquid phase. The result reveal R2value for wtp sludge and bio adsorbent are closer to unity than impregnated bio adsorbent which is 0.8258. This perhaps indicate the rate of reaction, for impregnated adsorbent, has not followed adsorption all along though the value is quite in support of adsorption mechanism and impregnation of alum might have led to partial precipitation without ignoring adsorption .
For attaining optimum removal, pH was not adjusted artificially in any way and therefore treatment with adsorbents became easy and less complex. Like all other low cost adsorbent, reactivation of used adsorbing material for defluoridation purpose does not prove to be cost effective and hence discarded.
.The salient features in support of using the alum impregnated sewage sludge are as follows:
- i. The removal rate is varying from medium to high(42% to 78%) depending on the adsorbing material.
- ii. Corelational matrix of the materials used shows a positive correlation with adsorbent dose and hence experiment can be classified as moderate.
- iii. The availability of sludge in a treatment plant is abundant.
- iv. The reuse of sludge is a point of concerns for treatment plant management.
- v. It becomes a difficult task specially for any sewage treatment plant authority to provide sufficient space for storing sewage sludge within the plant campus when huge sludge is produced every day as disposal of sewage sludge is more problematic than sludge of water treatment plant. Therefore alter-native option of use of sludge as a defluorodating adsorbent is al-ways a low cost approach towards treatment for fluoride removal as well a good option towards its reuse.
- vi. Dried sewage sludge does not carry the risk of carrying pathogens.
- vii. Wtp sludge is generally free from
vii. Presence of organic carbon can be one possible reason responsible for high adsorption in case bio adsorbent.
vii. Apart from organic carbon, Presence of alum can be other possible reason responsible for high adsorption in case of alum impregnated bio adsorbent.
viii. .The reason of choosing alum as impregnating material is that it provides an initial purification from undesirable presence of elements by precipitations as well as by adsorptions.
All the selected adsorbing materials have been found to have the ability to emerge as a potential low cost defluorodating adsorbents in the coming days perhaps for all such reasons apart from the reasons unexplored,
Though defluoridation is the objective of the study, the main criteria is to provide safe drinking water and to make water potable a necessary measures need to be taken to bring down the presence of all obnoxious materials within prescribed limit which has not been discussed here.
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Information about this Article
Peer-review ratings (from 3 reviews, where a score of 100 represents the ‘average’ level):
Originality = 175.43, importance = 161.24, overall quality = 175.43
This Article was published on 9th May, 2009 at 19:54:04 and has been viewed 6241 times.
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The full citation for this Article is:|
PLACEHOLDER, P., Goutam, D. & Shibapriya, D. (2009). Comparative study on various unconventional low cost materials as defluoridating. PHILICA.COM Article number 161.
1 Peer review [reviewer #54666] added 14th July, 2009 at 17:52:22
It is an article exploring new areas to find out out defluoridating adsorbents.Perhaps it is the most unique idea to select all dicarded materials likedigested sludge which is nothing but a waste of water treat ment or sewage treatment plant .The disposal mange ment of those materials have always been a complex problem for the plant mangers. Therefore removal upto 57.5% by stp sludge and 42%bywtp sludge at equlibrium time is effective specially where fluoride contamination is notvery high and will mange to bring down the level within the limit for potable water.Moreover use of hybrid material by impregnating alum on to stp sludge is an unique concept with regards to defluoridation. The article will be a remarkable guideline for the researchers who are dealing with low cost adsorbents
Originality: 7, Importance: 5, Overall quality: 6
2 Peer review [reviewer #26828] added 23rd July, 2009 at 18:30:15
Comparative study always gives an understanding of the efficacy of one material over other.The paper has produced a comparative study among three adsorbents.Use of those materials as defluoridating adsorbents have so far been unkown, perhaps. As a thorough investigation through literature review as done by me never reveals the fact of using those material as adsorbents for defluoridation.Moreover use of impregnated materials has proved its potentiality as adsorbent.
Moreover the materials are mostly discarded ones.First order reverssible kinetic model has supported the adsorption characteristics of the materials. Alum impregnated sludge shows a good amount of removal of fluoride, followed by s.t.p.and w.t.p.sludge. As pathogens are removed when kept for around thirty days in open air under direct sun heat in Indian Scinario,The materials can emerge as very good low cost adsorbents. In totality it is a good paper showing a creative skill in opening up a new area for defluoridation. Thanks to the author.
Originality: 7, Importance: 6, Overall quality: 7
3 Peer review [reviewer #47336] added 30th September, 2011 at 15:30:47
A very interesting, and potentially important article.
An important practical question is that of the levels of fluoride and chloride ions left in the outflow of most commercial water filtration devices that are sold in stores and super-stores, such as those in the USA. Many such water filtration and purification desktop models utilize carbon clusters in graphite particles.
There are three practically important questions that I hope the authors would answer for us all?
1. How efficient are such filtration models in *defluoridation*, and how does their performance compare with that of the methods suggested by the authors in this article?
2. What levels of fluoride in drinking water cause the debilitating fluorosis disease?
3. What are the levels of fluoride left in water treated by the desktop (inexpensive) water filtration models that are now commercially available ?
Originality: 6, Importance: 7, Overall quality: 7