Abstract: Natural and human activitiesare identified as the main reason for fluoride in groundwater. It is a majorproblem throughout the world, posing a serious threat to human health. Severaltreatments for the removal of fluoride are widely used in the adsorptionprocess for satisfactory results, especially with minerals-based adsorbents and/ or surface.
In this review, a comprehensive list of different chromosomes hasbeen compiled and their adsorption capacities under various conditions, it ispresented with the literature for the removal of fluoride, such as pH, basicfluoride concentration, temperature, contact time, adsorbent surfaces. There issome summery on the main advancements in formulating new adsorbents that haveso far been tested for fluoride removal. The literature review shows that manyadsorbents have shown good potential on Removal of fluoride. I mainly focusedon fly ash and its adsorption ability.Keywords: fly ash, grapheme oxide, fluoride, groundwater, adsorption, pH,time, temperature, research 1. IntroductionHardness and fluorideconcentration in ground water is a major water quality parameter which isgetting more attention of lots of responsible authorities and people. Recentresearches were founded that over concentration of fluoride and hardness inwater causing some disastrous health problems to society. Number of countriesas well as srilanka suffered due this problem in last few decades.
Most ofthose countries fulfill their water demand by using ground water. Thereforeremoval of fluoride and hardness from ground water is very importantrequirement due to above mentioned reasons.1.1 Hardness and fluoride1.1.1 HardnessHardness is aparameter of water that causes some problems to people who are using groundwater. Cations such as calcium (Ca²+) and magnesium (Mg²+) are the main reasonsfor the hardness in water. There are few other cations which are also causinghardness, but they are hardly considered (C.
K. Geethamani, 2013). Total sum ofcations in water had been defined as the hardness water 1, 2. Total hardnesscan be determined by using following equation;Hardness = c (Mg2+) + c(Ca2+) mg/lWater can becategorized using numerical value of hardness such as soft or hard.
Accordingto Vesikirja, the hardness is classified by the following. The hardness isdivided in five different groups and the corresponding hardness values in ppm (Dinesh Mohan, 2015).Classification Scale;· 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 ppmThe geological natureof environment can be determined by using hardness in groundwater. In srilankathe soil is mostly basic and this leads to the water to be hard. People areliving in dry zone in srilanka experienced high hardness in water (Dinesh Mohan, 2015). 1.
1.2 Fluoride Fluoride is a watercontaminant which creates various human health problems. World health organizationstated that small concentration of fluoride is good for dental health but they also recommended any concentration above 1.5 mg/l is a threatto human health.
Fluoride is widely distributed in thegeological environment and generally released into the groundwater by slowdissolution of fluorine-containing rocks (Dinesh Mohan, 2015). Exceeding the recommended fluorideconcentration 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 ofremoving hardness and fluoride from ground water had been discussed byresearchers. They were tried few methods such as nanofiltration using differentkinds of materials. Also the precipitation and coagulation processeswith iron (III) (Amit Bhatnagara), activated alumina,alum sludge (Andrew J.
Frierdich a, 2017)and calcium had beenwidely investigated. Other than that, ion exchange, reverse osmosis andelectrodialysis have also been studied for the removal of additional amounts offluoride from groundwater. But considering the cost, complicatedness of processand secondary pollution are main shortcomings of those methods. Thereforescientists are considering about some cheap, environment friendly and simplifymethods for defluoridation.
As a result of above facts, recently the adsorptionprocess is the most popular method of defluoridation. Results of these methods seem to besatisfactory and more attractive method for the removal of fluoride in terms ofcost, simplicity of design and operation (Helle U. Sø a, 2016). Modified fly ash has a potential toremove hardness and fluoride in water.
However, modified fly ash alone isineffective to be used in water treatment due to their nano and micro levels ofparticle size distribution and aggregation, which has reduced its adsorptioncapacity and stability in water. Therefore, anchoring of modified fly ash insuitable matrixes is most important to increase their stability in water andenhances reactive performance. Grapheneoxide has a large number of micro-pores on the surface and it has been used toimpregnate different nano materials in the past. Modified fly ash impregnatedgraphene oxide seems to be a good combination to remove toxic constituents ingroundwater (G.G. Hollmana, 2016). Hence, the studyfocuses to impregnate modified fly ash in graphene oxide matrix and evaluatesthe properties and performance in removal of hardness and fluoride ingroundwater (Widi Astuti 1 +, 2012). Fly ash is a compositionof some oxides such as Al2O3, SiO2, and unburned carbon.
That is the reason behind adsorption abilityof fly ash. Low capacity may occured by high crystallinity of Al2O3 and SiO2 aswell as the presence of the unburned carbon. The effect of fly ashcrystallinity for Pb(II) adsorption has been notified (dissananyake, 2009) (Blaney, 2007) . 2.Previous researches regarding fly ash and other industrial waste The industrial waste residue, generated during the manufacture ofaluminium sulphate (alum) from kaolin by the sulphuric acid process was used asdefluoridating media by Nigussie et al.. According to the research, fluorideadsorption remains constant within pH 3-8.
When the pH value is increasing theability of fluoride adsorption was decreased. Within the pH range of 3 to 8positive and neutral ions are presented on the surface of the absorbent, thatis the reason for better adsorption inthat range of pH. The research data shows that D-R model is well describing theadsorbent when the concentration 332.5 mg/g. presence of bicarbonate isconsiderably affected to the performance of the adsorbent (G.G.
Hollmana, A two-step process for the synthesis of zeolites from coal fly ash, 1999). HCL activated red mud is another aluminium industrial waste.Removal of fluoride also studied for this composition too.
Original form of redmud is not activated as HCL activated red mud. At the pH value of 5.5 and after2h hours of time, the peak adsorption was recorded. Behaviour of Chemicalnature and metal oxides are the basis of this fluoride removal. Results wereinterpreted in terms of pH values. 0.331mmol/g was the result obtained from the Langmuir model (Dinesh Mohan, 2015).
Tor et al. also studied the suitability of granular red mud (GRM)as a adsorbent of fluoride from water. The maximum fluoride removal (0.
644mg/g) was achieved at pH 4.7. Equilibrium was obtained after 6 hours. Thecapacities of the breakthrough and exhaustion points were found to decreasewith increase in the flow rate. Sorption capacity of the column method was higherthan its batch samples for Thomas model (Amit Bhatnagara). Investigation on Adsorptionof fluoride on waste carbon slurry (a fertilizer industry waste) was showed that, Maximumadsorption 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.
therewas no specified pattern obtained with the pH. Desorption was achieved under alkalineconditions (pH 11.6) from exhausted carbonslurry (Andrew J. Frierdich a, 2017).
C¸ inarli et al was studied the ability of coal mining waste forremove fluoride from groundwater. The maximum adsorbent capacity of thematerial was 15.67mg/g according to the Langmuir model. Acidic conditions werefavorable 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 theability of fluoride removal. Maximum fluoride sorption was observed from p-WM byLangmuir model with the value of 27.
2mg/g. between 0-40 0c oftemperature the adsorbent ability was increased.The p-WM was found to be capable for at least five times forfurther adsorption process withoutregeneration (kokshela, 2016). Spent bleaching earth (SBE) was tested and founded that maximumadsorbent observed at the concentration of7.
752 mg/g. pH and anions areadversely affected on the capability of adsorbent.Alum sludge is one of the waste products generated during themanufacture of alum from bauxite by the sulphuric acid process and mainlyconsists of oxides of aluminium and titanium with small amounts of undecomposedsilicates. Each of these oxides is known to possess adsorption and ion exchangeproperties. To take advantage of these properties of alum sludge, Sujana et al.examined its use as an adsorbent for the removal of fluoride from pollutedwaters.
The data indicate that treated alum sludge surface sites wereheterogeneous in nature and that fitted into a heterogeneous site bindingmodel. The Langmuir maximum sorption capacity for fluoride removal by alumsludge was reported to be 5.394 mg/g.
For an increase in temperature from 307to 337 K, and with 20 mg/L of initial fluoride concentration, an adverse effectwas observed on the adsorption of fluoride. The adsorption decreased from 85 to72% at pH 6.0 although the fluoride adsorption at a given temperature increasedwith time. This was attributed to the escaping tendency of the molecules frominterface at high temperatures and thereby diminishing the extent ofadsorption. Fluoride removal was found maximum at pH 6.0, and found to decreasesharply above that as a result of stronger competition from hydroxide ions onadsorbent surface (dissanayake, 2000).
Also, adsorptionwas found less in the acidic range which was proposed to be a result of theformation of weakly ionized hydrofluoric acid. Defluoridation with alum sludgein presence of phosphate and silicate at higher concentrations (10–50 mg/L) hadan adverse effect on fluoride removal. The affinity sequence for anionadsorption on treated alum sludge was in the following order phosphate ? silicate > sulphate > nitrate. Thepotential of thermally activated titanium rich bauxite (TRB) for adsorptiveremoval 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 uptakeincreased with increasing pH and reached to a maximum at pH 5.5–6.5 andthereafter decreased. The Langmuir maximum adsorption capacity for fluoride wasobserved to be 3.7–4.13 mg/g. The presence of common interfering ions in waterdid not greatly affect the uptake of fluoride from aqueous solution indicatingfluoride specific sorption behaviour of TRB (MasterPozzolith, 2017).Nearly complete desorption of adsorbed fluoride from loaded bauxitewas achieved by treating with aqueous solutions of pH ? 11.
1 (NaOH ?0.015 mol/dm3). Red mud was also modifiedby AlC13 (MRMA) and by heat activation (MRMAH) and tested for the removal offluoride from water .The results showed that the adsorption capacities of MRMAand MRMAH were 68.07 and 91.28 mg/g, respectively, which were much higher thanthat of RM (13.
46 mg/g). The Langmuir isotherm was the best-fit adsorptionisotherm model for the experimental data. The solution pH affected the removalefficiency significantly, and the highest removal efficiency was achieved at pH7–8 (N.
Gandhi1*, 2012). Chaturvedi et al. was investigated the ability of fly ash forfluoride removing of water. The removal of fluoride was found favourable at lowconcentration, high temperature and acidic pH. The Langmuir maximum sorptioncapacity of fly ash for fluoride ranged from 20.
0 to 20.3 mg/g.Also it shows thatfluoride removal ability of fly ash is mostly depends on pH value.6 to 7 pHvalues are more favourable for adsorption process. Geethmani studied the otherimportant parameter of the adsorption process such as effects of contact time,concentration, dosage and temperature (C.K. Geethamani, 2013).
There were slightincrease of adsorption with temperature, agitation speed and concentration.When refer the contact time, there was sudden adsorption within first 10minutes. Then the adsorption rate slowed down.
Active sites of the top surfacewere finished in first 10 minutes and then it slow down due to time taken toreach inner surface sites. It was the identified reason for the suddenadsorption within first 10 minutes (Amit Bhatnagara).The percentageof fluoride above 80% was removed from its 10 mg / nasal solution with anequilibrium 120 minute equilibrium and 3 g / g of fly ash concentrate. pHvalues ??are better than adsorption when it is a neutral solution.
Theefficiency of the process was absorbed and the nature of the process showedincreased efficiency. Lopomor was described in more detail in the contourmodel. Monocylation capacity for glutenous ash for fluoride is 10.86 mg / g (A.
K. CHATURVEDI, 1989). The equilibriummotion of the equilibrium data was shown as adsorption based on fluoride ionfluoride ion. This process demonstrates that the process of circulation isfollowed.
The internal variable spread model shows that it is a multi-phaseadsorption process and is associated with an internal variation with anotherinternal mechanism. Real field touring water ashes indicate that flying ash isnot only a good adsorbent to remove fluoride from the water; Groundwater isalso a good adsorbent for removing other anions (P.D. Nemade*, 2002).
3. ConclusionAll those previous researches shown thatpH, contact time, temperature, composition, concentration and different modelsare very important facts when this research is going on. Therefore I have toconsider about all these facts when I carrying my research. There are lots ofresearches done considering the effect of fly ash on fluoride removing groundwater,but hard to find any researches regarding graphene oxide involvement. Thus thisresearch is very important due to its background.AcknowledgementsI would like to acknowledgeDepartment of civil engineering department of university of moratuwa and allespecially to the environmental engineering section and Dr. ashani ranathungafor their valuable support to this paper.
Also I would like to acknowledge allthe researchers and authors of for their previous researches and books whichare really helped me to create this review. References MasterPozzolith. (2017). UK. Amit Bhatnagara, ?.
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