Effects of Calcination Temperature of Naturally Occurring ...

165 Tanzania Journal of Engineering and Technology (Tanz. J. Engrg. Technol.), Vol. 39 (No. 2), Dec. 2020

Vol. 39(2), pp. 165-176, Dec. 2020ISSN 1821-536X (print)ISSN 2619-8789 (electronic)

Tanzania Journal of Engineering and TechnologyCopyright © 2020 College of Engineering andTechnology, University of Dar es Salaam

Full Length Research Paper

Effects of Calcination Temperature of Naturally Occurring Absorbents onDrinking Water Defluoridation

Brenda A. Mndolwa1 and Felix W. Mtalo2

1College of Earth Sciences and Engineering, University of Dodoma, Tanzania2Department of Water Resources Engineering, University of Dar es Salaam, Tanzania

Corresponding author: [email protected]

ABSTRACT

Currently, in Tanzania, fluoride removal from drinking water is treated mostly usingthe bone char method. The method has poor acceptability in some religiouscommunities and also causes water quality deterioration in taste and odour if thebones are not properly prepared. The use of local natural adsorbents as analternative is feasible with limitations of high levels of other impurities in treatedwater. Locally available gypsum, magnesite and bauxite were converted toadsorbents through calcination. The study was conducted to determine the removalefficiency, best calcination temperature and composite ratio of the three adsorbentsfor the removal of fluoride from natural drinking water with fluoride concentrationas high as 16.7 mg/L. The adsorbent materials were calcined at differenttemperatures ranging between 3500C and 6000C. Batch experiments were performedand samples were collected at different contact time intervals of 2 minutes to 60minutes, and residual fluoride was determined. Bauxite had the highest fluorideremoval efficiency compared to gypsum and magnesite. The best calcinationtemperatures were 3500C, 4000C, 6000C for gypsum, bauxite and magnesite,respectively. The best calcination temperatures were used to prepare composites atdifferent ratios of 1:2:3, 2:3:1 and 3:2:1, bauxite: gypsum: magnesite respectively.All the ratios gave low sulphate and iron as impurities within the recommendedstandards. The composites lowered fluoride concentration level to 1.53 mg/L, 2.07mg/L, 2.60 mg/L for 1:2:3, 2:3:1, 3:2:1 ratios, respectively. In conclusion the studyreveals that, it is possible for composites made of adsorbent calcinated at differentoptimum temperatures to give good results in fluoride removal from drinking water,as well as standard pH, iron and sulphate values in treated water.

Keywords: Fluoride, Bauxite, gypsum, magnesite, composite ratio, adsorbent.

INTRODUCTION

Fluoride is the pollutant that is persistentand cannot be degraded and has the abilityto accumulate in the soil, plants, animalsand also in human beings body (Tomarand Kumar, 2013). In different parts of theworld, contamination of drinking water

with excess fluoride content has beendescribed as one of the public healthproblem (Lavecchia et al., 2012). Waterwith excess fluoride and other chemicaland biological contents above therecommended drinking standards is notsuitable for domestic use (Rao, 2003).Moderate amount of 1 mg/L of fluoride is

Brenda A. Mndolwa and Felix W. Mtalo

Tanzania Journal of Engineering and Technology (Tanz. J. Engrg. Technol.), Vol. 39 (No. 2), Dec. 2020 166

of importance in human dental health bypreventing dental caries, but too lowfluoride concentration is serious especiallyin children health growth in developmentof their tooth enamel since it is consideredto be less developed at the childhood stage(Owusu-Agyeman et al., 2018). Fluorideplays a major role in bone and dentalmineralization, but when it is in excess itcan cause severe and serious health effectssuch as dental fluorosis, skeletal fluorosisand long term damage of thyroid, liver,kidney and brain (Lavecchia et al., 2012).

Fluoride in water is unable to bind withcations such as aluminium and ironbecause of their low reactivity in naturalwater, hence fluoride occurs as a free ion.Fluoride can occur naturally or due tohuman activities. Naturally, its occurrenceis mainly related to geogenic processes,which govern its concentration ingroundwater. It is affected through thecontact with the sedimentary carbonates(Sivasankar et al., 2016). Moreover, theaquifer characteristics can also contributeto presence of fluoride contamination intogroundwater after a prolonged contact withaquifer fluoride rich minerals. In aquifersof volcanic formation, mineral dissolutionis inhibited due to the low temperature,high altitude and high transmissivityfeatures of the aquifer hence resulting inlow fluoride concentration. For the case ofsedimentary formation aquifers they arecharacterised by low altitude and hightemperature which facilitates dissolution ofminerals and allow precipitation of CaF2

hence leading to high fluorideconcentration in groundwater (Shen et al.,2016)

Volcanic activities are one of the naturalsources of fluoride contribution to theenvironment, whereby varieties of gasescontaining HF, NH4F, SiF4, (NH4)2SiF6,NaSiF6, K2SiF6 and KBF4 and few otherminor constituents such as sulphate can beemitted when volcanic activities takesplace (Borgnino et al., 2013). Also, a

significant amount of soluble compoundsincluding fluoride are being released intowater when the fresh-erupted tephra gets incontact with water resulting to thecontribution of fluoride concentration inwater (Ayris and Delmelle, 2012).

Fluoride pollutants can be present in air,dental products, food and beverages, soilas well as in water (Fawell et al., 2006).As a result of human activities, in theprocess of manufacturing differentproducts, the industries can releasefluorine to the environment either in formof gas such as HF, SiF4, F2, and H2SiF4 orin a form of particulate matters that isCaF2, NaF, and Na2SiF6 (Ozsvath, 2015).

The problems of excessive fluorideamounts in drinking water are highlyendemic and hence they have encouragedand motivated more researchers to exploredifferent methods and materials that can beused to solve the problem (Rao, 2003).Over years, as a result of different researchconducted in different places worldwide,there have been development of techniquesfor removal of fluoride (Al-Hawamdeh etal., 2013).

Fluoride removal techniques includeNalgonda technique, reverse osmosis andnano-filtration, electro-coagulationtechnique and precipitation. Similarlyusing different adsorption materials suchas activated alumina, bone char, clay andaluminium have been proven to beeffective. However, most of them appearedto be neither sustainable nor effective inremote rural regions of developingcountries mainly due to their high cost anddependence on skilled personnel formaintenance (Cherukumilli et al., 2017).

These techniques face setbacks whereassome are not cost effective and others areless expensive but less effective with lowquality of the treated water as well as poorsorption capacity (Al-Hawamdeh et al.,2013; Thole and Mtalo, 2012). Each

Effects of Calcination Temperature of Naturally Occurring Absorbents on Drinking Water Defluoridation


technique has its benefits andshortcomings that limit its use. Forinstance, membrane separation processwhich includes reverse osmosis and nano-filtration, are very efficient but it suffersfrom high energy consumption andmembrane fouling, hence making itexpensive, whereas chemical precipitationtend to produce large amount of sludge(Cherukumilli et al., 2017). Among thesetbacks of Nalgonda method is theformation of sludge, high chemical dosage,requires a technical personnel to operateand it is not suitable for water with highfluoride content greater than 10 mg/L,(Wambu et al., 2014). The use of bonechar, interferes with some taboos andreligious beliefs of some societies hence itis not accepted globally and when notproperly prepared produces low waterquality with taste and odour of rotten meatto drinking water. Bauxite and clayincreases water turbidity, colour as well assome of the residual ions (Thole andMtalo, 2012; Feenstra and Erkel, 2007).Despite activated alumina method beingpopular, it has some limitations such as itis not effective when the total dissolvedsolids in water exceeds 1500 mg/L. It isalso pH selective hence it works betterunder certain pH levels and when it is leftout of operation for 2 or 3 days it providesroom for microorganism growth(Shrivastava and Vani, 2009).

There is no single method of treatingdrinking water which meets all thestandards. Therefore, there is a need forobtaining knowledge on the removal offluoride using resourceful techniquesunder ideal conditions using differentnaturally occurring materials. The currentresearch aims at improving the fluorideremoval process in drinking water usingthe naturally occurring rock materialsusing bauxite, gypsum and magnesiteobtained in Tanzania. The scope of thework was the calcination of bauxite,gypsum and magnesite at temperature of

350 to 6000C to improve their adsorbentcapability.

MATERIALS AND METHODS

Boulders of magnesite, gypsum andbauxite sourced from Chambogo,Makanya and Lushoto, respectively, werecrushed and milled separately and thensieved through standard sieves of ≥ 0.075mm. The materials were then calcinated ina furnace at different temperatures of350oC, 400oC, 450oC, 500oC, 5500oC and600oC for duration of 2 hours. Thesamples were then removed from thefurnace and left to cool to roomtemperature.

Different dosages of 1 to 10 grams ofbauxite, gypsum and magnesite wereweighed separately using a weighingbalance, and then placed in the beakercontaining 100 mL of 16.7 mg/L fluoridecontaminated water obtained from Maji yaChai river in Arusha. Samples werecollected at location with coordinates -3.37111 S and 36.8964 E, using thepolyethylene sampling bottles, then wastransported to the laboratory and stored atroom temperature of 25oC as furtheranalysis was on going.

Using the magnetic stirrer at 100 rpm, thematerial and fluoride contaminated naturalwater was allowed to stay in contact whilestirring at different time intervals for thefirst 2, 4, 6 ,8 and 10 minutes and then for20, 30, 40, 50 and 60 minutes. At the endof each contact time, the water sample wasfiltered, then pH, fluoride concentrationand ionic impurities concentration, ironand sulphate parameters were analysed.This was performed at each calcinatedtemperature. The calcination temperaturesat which fluoride removal was the highestfor each adsorbent were then mixedtogether at the ratios of 1:2:3, 3:2:1 and2:3:1 as mass ratios, bauxite: gypsum:magnesite, respectively. The fluorideremoval efficiency by each adsorbent



bauxite, magnesite and gypsum,respectively was calculated using theequation.

………………….. (1)

Where R is removal efficiency, Cf and Ciare the final and initial fluorideconcentration in solution (mg/L)respectively. The removal capacity wasdetermined using the equation (2).

……………………….. (2)

Where qe is the amount of adsorbedfluoride at equilibrium (mg/g), Co and Cfrepresents the initial and final fluorideconcentration (mg/L), V is the volume ofsolution (L) and m the mass of adsorbentin grams (g).

The adsorption isotherm was studied byfitting the data with Langmuir andFreundlich isotherms as expressed inequations (3) and (4). In Langmuirisotherm adsorption is assumed to occur atspecific homogeneous sites where there isno additional adsorption to take place(Piccin et al., 2011).

………………… (3)

Where, qe is adsorption capacity (mg g-1),Ce is Equilibrium concentration (mg L-1),Qm is maximum monolayer adsorption(mol g-1) and Kl is Langmuir constant (Lmol-1).

The Freundlich isotherm assumes amultilayer adsorption, mostly used todefine adsorption characteristics forheterogeneous surfaces (Kebede et al.,2016).

……………………….. (4)

Where Q is adsorption Capacity (mg g-1),F is Freundlich constant (Lkg-1), C isEquilibrium concentration (mg L-1) and1/n is heterogeneity factor (dimensionless).

RESULTS AND DISCUSSION

The raw materials, which are bauxite,magnesite and gypsum, were collectedfrom Usambara Mountains in Lushotodistrict, Chambogo in Same district andMakanya in Same district, respectively.The materials contained some sulphite andmetal oxide as impurities (Table 1).Bauxite contained about 57.4% of Al2O3,gypsum contained 29.4% of CaO andmagnesite contained about 47.0% of MgO.These are the major compounds in the rawmaterials used production of absorbents.

Table 1: Analysis of the material elements (XRF) results in percentage

Material Constituent Al2O3 SiO2 CaO MgOBauxite 57.41 4.18 <0.01 <0.01Gypsum 2.07 8.98 29.36 1.27Magnesite 0.12 2.30 0.22 47.03

Material Calcination

When bauxite (Aluminium hydroxide),gypsum (calcium sulphate dihydrate) andmagnesite (magnesium carbonate) were

calcinated, the following reactions tookplace as observed in the equations (5), (6)and (7).

2Al(OH)3(s) Δ Al2O3(s) + 3H2O(g) ……………………...……………………... (5)CaSO4.2H2O(s) Δ CaO(s) + SO3(s) + 2H2O(g) …………………………..……..... (6)MgCO3(s) Δ MgO(s) + CO2(g) ………………………………………………….. (7)



Laboratory Analysis

The water quality parameters analysedfrom the raw water were found to bewithin WHO and TBS standards except forfluoride which was found to be muchhigher compared to the standards (Table2). Different conditions and factorsaffecting the fluoride removal were

observed and the studied factors such asparticle size, contact time, dosage of theadsorbent, and calcination temperaturewere observed. Likewise, the removalcapacity of each adsorbent wasdetermined, the best temperature for mediacalcination and the best composite ratiowas also determined through the analysis.

Table 2: Analysis results of raw water

Parameter Amount in raw water WHO Standards TBS StandardsFluoride (mg/L) 16.7 1.5 1.5-4.0Chloride (mg/L) 29.9 250 200-800Nitrate (mg/L) 7.6 50 10-75pH 8.14 6.5-8.5 6.5-9.2Sulphate (mg/L) BDL 500 200-600

Effect of Dosage

Analysis was conducted to establishremoval efficiency within 10 minutesusing different concentration dosages.Different doses of 1 g to 10 g of adsorbentin 200 ml contaminated water with initialfluoride concentration of 16.7 mg/L wereused. Removal efficiency under differentdosage conditions was as shown in Figure1. Similar trend of removal efficiency wasobserved by (Patnaik et al., 2016). The

removal efficiency was observed toincrease from 36.5% to 87.7% withincreased adsorbent dosage from 1 to 10 g.The increase in fluoride removal efficiencywith increase in the adsorbent dose wasdue to the increase in surface area; hencemore active sites were available foradsorption of fluoride. However after adefinite adsorbent dose, the percentage ofremoval did not increase significantly andthat dose was considered as best dosewhich was 10 g.

Figure 1: Adsorbent dosage in relation with fluoride removal efficiency



Effect of Adsorbent Dosage on FluorideRemoval Capacity

Figure 2 shows that the removal capacitykept decreasing as the dosage increased atall the contact time. The same trend ofresult was observed by Patnaik et al.(2016). The adsorption process wasobserved to take place in the first 2minutes contact time of the reaction. At 10minutes contact time where the highestremoval capacity was observed, theequilibrium removal capacity decreasedfrom 1.13 mg/g to 0.27 mg/g, this was alsoobserved at the contact time of 2, 4, 6, and

8 minutes where removal capacitydecreased from 0.9 mg/g to 0.26 mg/g,0.98 mg/g to 0.27 mg/g, 1.06 mg/g to 0.27mg/g and 1.08 mg/g to 0.27 mg/grespectively. The removal capacity wasobserved to decrease to 0.27 mg/g offluoride as the dosage increased. That ismore fluoride ions were adsorbed as theadsorbent dose increases, hence adsorptioncapacity reached was 0.27 mg/g offluoride. After reaching 0.27 mg/g,fluoride removal seized. Desorption willbegin depending on initial concentrationand media volume/weight used.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 2 4 6 8 10 12

Rem

oval

cap

acit

y (m

g/g)

Dosage (grams)

2 minutes4 minutes6 minutes8 minutes10 minutes

Figure 2: Effect of adsorbent dosage on the fluoride removal capacity at differentcontact times

Figure 3 shows bauxite had highestremoval capacity in all calcinationtemperatures. Gypsum removalcapacity was high at lower calcinationtemperature of 350oC and poor athigher calcination temperature of600oC. Magnesite had highest capacityat higher temperatures of 600oC. Thiswas also reported by Singano (2000),that with magnesite fluoride removalwas high at higher calcination

temperatures. When magnesite iscalcinated the decomposition ofcarbonates takes place and formsmagnesium oxide MgO, which is theone responsible for fluoride removal.The decomposition of the carbonatesoccur starting at temperature of 400oCand hence proceeds rapidly abovetemperature of 500oC (Singano, 2000).Magnesite reaction on fluoride removalcan be seen in equation (8).

MgO(s) + H2O(l) + F-(aq) Mg(OH)F(aq) + OH-

(aq) ………………………………………….. (8)



Figure 3: Removal capacity of each adsorbent at different calcination temperatures

Despite the high removal capacity, bauxiteincreased the water turbidity and colour inthe treated water. Also, bauxite andgypsum lowered the pH value to 8.04 and8.06, respectively, while magnesite raisedpH to 8.62 as also reported by Thole et al.(2013). The rise of pH by magnesite wasalso reported by Singano (2000). Bauxiteand gypsum lowering pH can be explainedby the presence of sulphite in bauxite andgypsum.

The fluoride removal in drinking waterusing bauxite is through ion exchangeprocesses where fluoride ion with OH-

groups they exchange. Aluminium oxide(Al2O3) being in larger content in bauxiteas well as sulphite SO3

-, these are covalentoxides, hence when dissolve in water theybind the water molecules and releasesprotons which results to the lowering ofpH (Cherukumilli et al., 2017; Thole et al.,2013).

Magnesite is mainly MgCO3 compound,when magnesite is calcinated thedecomposition of carbonates takes placeand forms magnesium oxide MgO whichis the one responsible for fluoride removaland the cause of pH rise. Thedecomposition of the carbonates occurstarting at temperature of 4000C and henceproceeds rapidly above temperature of500oC (Singano, 2000).

Moreover, gypsum calcinated at lowtemperature of 150oC to 300oC gave betterperformance in fluoride removal thoughwith the highest residual number of otherions such as sulphate and iron, which arenot within the recommended standards.However, gypsum calcinated attemperatures above 300oC introduces lessionic contaminants in treated water (Tholeet al., 2011). Results in Table 3 prove thepresence of residual sulphate in gypsumcalcinated at temperature of 350oC, but theamount is found to be within therecommended standards and iron level wasfound to be below the detection limit.

Surface area was determined using BET(Brunauer–Emmett–Teller) method andpore diameter was determined using(Barrett-Joyner-Halenda) BJH method.Magnesite prepared at 6000C had a smallersurface area of 83.1 m2/g compared tobauxite with 103 m2/g and gypsum 115.5m2/g. Bauxite prepared at 4000C had thelargest pore diameter of 32.4 Å comparedto magnesite 31.8 Å and gypsum with 32.3Å. The highest removal efficiency forbauxite, gypsum and magnesite wereobserved at temperatures of 4000C, 3500Cand 6000C, respectively. Similarly,Singano (2000), reported best fluorideremoval capacity of magnesite to be at6000C. For the case of gypsum,performance was observed to be best atlower temperature as reported by Thole et



al. (2011), hence 3500C calcinationtemperature for gypsum was best in thisstudy.

The composite used comprised adsorbentswith best performance calcination

temperatures that is bauxite calcinated at4000C, gypsum at 3500C and magnesitecalcinated at 6000C. The ratios used were1:2:3, 2:3:1 and 3:2:1 bauxite: gypsum:magnesite, respectively. The results areshown in Table 4.

Table 3: Treated water analysis results

Figure 4: Effect of adsorbent calcination temperature on fluoride removal

Table 4: Treated water test results of the composite ratios

Parameter CompositeRatio 1:2:3

CompositeRatio 2:3:1

CompositeRatio 3:2:1

WHOStandards

NationalStandard (TBS)

Removal Efficiency(%)

90.9 87.6 84.4

Fluoride (mg/L) 1.52 2.07 2.6 1.5 1.5-4.0Sulphate (mg/L) 175 165 168 500 200-600Iron (mg/L) BDL BDL BDL 0.3 0.3-1.0

All values were within the WHO and TBSallowable drinking water quality standards.

Figure 5 illustrates the residual fluoridewith the respective ratios.

Material Initial Fluoride(mg/L)

Final pH Fluoride(mg/L)

Iron(mg/L)

Sulphate(mg/L)

Bauxite 16.7 8.04 8.5 1.55 24Gypsum 16.7 8.06 14.4 BDL 190

Magnesite 16.7 8.62 14.7 0.75 18WHO (2006) Standards 6.5-8.5 1.5 0.3 500TBS (2009) Standards 6.5-9.2 1.5 – 4.0 0.3-1.0 200-600



Figure 5: The effect of composite ratio on residual fluoride

Adsorption Isotherms

Composite ratio 1:2:3 bauxite, gypsum andmagnesite, respectively, as shown inFigures 6 and 7 demonstrate sorptionisotherms obtained from the experimentaldata fitted. Most of the data set adheredmore strongly to Langmuir sorption modelwhich gave the value of r = 0.00575 whichlies in the range 0 < r < 1 which indicates afavourable adsorption. For Freundlichmodel, the value of 1/n was 0.0521 in

which the value of n was greater than 10hence out of range.

The results obtained from fitting theexperimental data into the Langmuir andFreundlich equations have beensummarized in Table 5, whereas when rvalue lies in the range 0 < r < indicates afavourable adsorption and n valueindicates good adsorption characteristicswhen is in range 2 to 10.

y = 0.056x + 0.585R² = 0.806

00.10.20.30.40.50.60.70.80.9

0 0.5 1 1.5 2 2.5 3

1/qe

1/Ce

Figure 6: Langmuir isotherm for composite ratio 1:2:3



Figure 7: Freundlich isotherm for composite ratio 1:2:3

Table 5: Fitted parameters for Langmuir and Freundlich isotherms of the composites

Langmuir Isotherm Freundlich IsothermR2 R 1/KLQm 1/Qm 1/n R2

1:2:3 0.8065 0.0058 0.0566 0.5859 0.0521 0.85542:3:1 0.9697 0.1895 0.1528 0.5965 0.1308 0.99133:2:1 0.9525 0.1022 0.3096 0.5886 0.1772 0.9724

CONCLUSIONS

Drinking water defluoridation with thecomposite materials of bauxite, gypsumand magnesite is possible and provides thebest results, where the dosage of theadsorbent is inversely proportional to theremoval capacity with constant initialfluoride concentration. The adsorptionprocess was observed in the first 2 minutescontact time of the reaction.

Bauxite gave the highest fluoride removalefficiency compared to gypsum andmagnesite at all calcination temperaturestested, though it altered the water qualityparameters such as pH, hence with thecomposite of the three adsorbents thewater quality parameters were balanced.

Calcination temperatures of 3500C forgypsum, 4000C for bauxite and 6000C formagnesite were found to be the best in theremoval of fluoride. The findings illustratethat the adsorbent material composite of

the best calcination temperatures gave thehighest removal efficiency at all the ratiostested; also, all gave the lowest sulphateand iron levels as impurities.

The best composite ratio 1:2:3 bauxite,gypsum and magnesite respectively hadthe highest removal efficiency with lowsulphate and iron levels which were foundto be within the recommended WHOstandards. Whereas adsorption isothermsof Langmuir model had the best fit, theadsorption characteristics were good andthe adsorption was favourable. Hence bestfor the removal of excess fluoride indrinking water.

Therefore, using the naturally occurringmaterials for the case of bauxite, gypsum,magnesite and its composite of bestcalcination temperature of each adsorbent,can be used as a treatment technology ofdrinking water in highly contaminatedfluoride regions when upscaled to largemunicipal treatment plants.



AKNOWLEDGEMENT

Our sincere gratitude and thanks to theAlmighty God for his guidance, protection,strength and help throughout the researchwork. The authors are grateful toDAFWAT project for the support in thelaboratory research activities that has ledto this output. Thank you and may GodAlmighty bless you.

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