Fluoride Removal from Water using Bio-Char, a Green Waste, Low

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Fluoride Removal from Water using Bio-Char, a Green Waste, Low-Cost Adsorbent: Equilibrium Uptake and Sorption Dynamics Modeling Dinesh Mohan,*,† Rupa Sharma,† Vinod K. Singh,† Philip Steele,‡ and Charles U. Pittman, Jr.§ †

School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, India Forest Products Department, Mississippi State University, Mississippi State, Mississippi 39762, United States § Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States ‡

ABSTRACT: Drinking water containing fluoride >1 mg/L is unsafe for human consumption. Higher intake of fluoride can cause potential health hazards. Low-cost pine wood and pine bark chars, obtained as a byproduct from fast pyrolysis in an auger reactor at 400 and 450 °C, were characterized and used as received for water defluoridation. Sorption studies were performed at different temperatures, pH values, and solid to liquid ratios in the batch mode. Maximum fluoride adsorption occurred at pH 2.0. A kinetic study yielded an optimum equilibrium time of 48 h with an adsorbent dose of 10 g/L. Sorption isotherm studies were conducted over a concentration range of 1100 mg/L. Fluoride adsorption decreased with an increase in temperature. The char performances were evaluated using the Freundlich, Langmuir, RedlichPeterson, Toth, Temkin, Sips, and Radke adsorption models. Based on average percent error, the best isotherm fits follow the orders for pine wood and pine bark: PWLangmuir ≈ PWRedlich‑Peterson > PWToth > PWSips > PWRadke‑Prausnitz ≈ PWFreundlich > PWTemkin and PBToth > PBRadke‑Prausnitz ≈ PBFreundlich > PBRedlich‑Peterson > PBLangmuir > PBSips > PBTemkin. The pine chars successfully treated fluoride-contaminated groundwater at pH 2.0. The chars swelled in water due to their high oxygen content (811%), opening new internal pore volume. Fluoride could also diffuse into portions of the chars’ subsurface solid volume promoting further adsorption. Ion exchange and metal fluoride precipitation are modes of adsorption. Remarkably, these chars (SBET: 13 m2g1) can remove similar amounts or more fluoride than activated carbon (SBET: ∼1000 m2g1).

Haryana.1,3,11,12 Groundwater control or decontamination of fluoride-containing water has been employed.13 The fluoride contamination control in groundwater is difficult since fluoride contamination is influenced by many hydrogeologic and physicochemical parameters. However, artificial recharge techniques, including the aquifer storage recovery (ASR) technique, may be applied to improve water quality by dilution.13 Alternatively, fluoride remediation of water is employed. Defluoridation methods have been critically reviewed.14,15 Adsorption is now evolving as a front line of defense. Selective adsorption utilizing titanium-rich bauxite,16 various forms of quartz,17 aluminum sorbent,18 aluminum-impregnated carbon,19 activated alumina,20 alum-impregnated activated alumina,21 aluminum-type superparamagnetic adsorbents,22 activated carbons (ACs), carbon black,23 red mud,24 zeolite,25 chitin and chitosan,26 magneticchitosan particle,27 lime stone,28 biomass and carbonized biomass,29 carbonaceous materials,30 and bone char31,32 were reported. Sorption properties of biochars have been explored.3340 Recently we have reported the successful use of biochar from biooil production for the removal of arsenic, lead, and cadmium from water.34 In the present study, pine wood and pine bark chars were utilized for fluoride remediation from water. Pine wood and pine bark were fast pyrolyzed in an auger-fed reactor (1 kg/h) at

1. INTRODUCTION Fluoride impact on human health makes its hydrogeochemistry an important topic.1 Fluorides are released naturally through weathering and dissolution of fluorospars (CaF2), fluorapatite (Ca10F2(PO4)6), and cryolite (Na3AlF6) minerals in volcanic emissions and in marine aerosols. Furthermore, emissions occur during the manufacturing of pesticides, disinfectants, wood preservatives, metals, glasses, steel, aluminum processing, TV picture tubes, phosphate fertilizers, glass, brick, plastics, and textile dyeing.2,3 Fluoride intake by humans occurs from drinking water in areas where fluoride concentrations in groundwater and surface water are high. The recommended concentration of fluoride in drinking water is 1.50 mg/L.4 An excess or absence of fluoride in drinking water is harmful to health. Acceptable fluoride concentrations are between 0.5 and 1.5 mg/L.5 Over 200 million people worldwide are affected by fluoride concentrations in excess of 1.5 mg/L in drinking water. Fluorosis caused by high fluoride concentrations has been reported in various countries including India, Argentina, UK, South Africa, the United States, Norway, Mexico, China, and Poland.3,610 A review of fluoride contamination in drinking water covering the global scenario has been published,3 discussing the geochemistry of fluoride in detail. Fluoride contamination in India was also described in the review.3 Statistical modeling of global geogenic fluoride groundwater was reported by Amini and co-workers.9 Fluoride contamination was reported in different parts of India including Rajasthan, Andhra Pradesh, Gujarat, Assam, and r 2011 American Chemical Society

Received: March 26, 2011 Accepted: December 3, 2011 Revised: December 2, 2011 Published: December 03, 2011 900

dx.doi.org/10.1021/ie202189v | Ind. Eng. Chem. Res. 2012, 51, 900–914

Industrial & Engineering Chemistry Research

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temperatures ∼400 and 450 °C. The chars obtained at these two temperatures were mixed and used without grinding or chemical treatment as reported earlier.34 This work investigates whether as-received biochar can be useful as an adsorbent for fluoride removal without further treatment. This would minimize cost. As the importance of biomass pyrolysis to liquid and gas fuels grows, large char amounts should be available. Any use with a value in excess of its fuel value will be welcome. The following studies were conducted in this work: 1. Fluoride removal by char from water at different pHs, temperatures, and solid to liquid ratios was demonstrated. 2. Sorption kinetics and swelling studies to investigate the mechanism were obtained. 3. These chars were applied to real water systems. 4. These chars were compared to commercially available carbons/ adsorbents.

the DubininRadushkevich equation (eq 3)43,44 log W ¼ log W o Dlog2 ðp0 =pÞ

where W is the micropore volume that has been filled with liquid N2 and W0 is the total micropore volume. D is a characteristic constant of the adsorbent’s micropore structure. Vmi, Vme, and W0 were expressed as liquid volumes. The total pore volume (VT) was calculated using the expression 4. V T ¼ 1=FHg 1=FHe

FHe ¼ M=V s 0

ð4Þ

Char constituents were analyzed by standard chemical methods. 34,4547 Pine wood and pine bark char surface morphologies were examined using a scanning electron micrograph (Zeiss, EVO 40). SEM samples were coated with a thin layer of gold and mounted on a copper stab using double-stick carbon tape. Pine wood and bark biochars were produced by pyrolyzing 26 mm particle size pine wood or pine bark in an auger reactor at 400 and 450 °C as previously reported.34 Pine bark samples were air-dried for ∼12 days to keep 810% moisture content while pine wood samples were used as received (68% moisture).34 The chars obtained at the two temperatures were blended at 1:1 (wt/wt) before use as adsorbents. Chars were used without any chemical treatment or grinding to reduce cost.34 Chars were sieved (>600, 600250, 250177, 177149, and pHzpc the superficial charge is negative, favoring cation adsorption. To determine the pHzpc of biochars, different amounts of biochars (2, 4, 8, 16, 30 mg/L) were added into doubly distilled water adjusted to different initial pH values (210). Blanks with no biochars were run for some randomly selected samples. The flasks were agitated intermittently for 48 h at room temperature.

ultimatea C (%)

83.47(85.78)

68.25(69.86)

H (%)

2.99(3.07)

2.51(2.57)

N (%)

0.27(0.28)

0.34(0.35)

S (%)

0.03(0.03)

0.04(0.04)

Oc (%) ash (%)

8.25(8.48) 2.30(2.36)

10.80(11.06) 15.75(16.12)

elemental analysisd: values (%) Al2O3

13.0

1.47

BaO

0.06

0.46

CaO

11.9

42.0

Fe2O3

6.22

2.04

K2O

3.13

5.24

MgO

1.97

13.2

MnO Na2O

0.33 0.41

6.84 3.50

SiO2

60.0

5.68

SrO

0.04

0.21

TiO2

0.83

0.05

textural data SBET (m2 g1)

e

Vmi (cm3 g1)

e,f

WO (cm3 g1) Vme (cm3 g1)

e

e,f

2.73

1.88

9.1 104

6.6 104

7.381 104 1.64 103

6.575 104 2.94 103

FHg (g cm3)

0.91

0.57

FHe (g cm3)

1.45

1.43

0.41

1.06

e e

VT (cm3 g1)

e,f

a

Values in parentheses are for dry char and values without parentheses are for as-received char. b Calorific values of chars were measured as HHVs. The lower heating values (LHV) were calculated from the HHVs and the total weight percents of hydrogen in the chars using LHV [J/g]= HHV[J/g] 218.13 H% [wt%]. c Oxygen was determined by difference. d Chars were ashed to eliminate all organic matter to allow the determination of the inorganic constituents. The ash was then totally dissolved in HF and analyzed by plasma emission spectroscopy. The different chemical constituents (dry weight % basis) of the ashed chars are reported. e SBET: BET specific surface area; Vmi and W0: micropore volumes; Vme: mesopore volume; VT: total pore volume; FHg: mercury density; FHe: helium densitiy. f N2 adsorption isotherms: Vmi = Vad at p/p0 = 0.1, Vme = Vad at p/p0 = 0.95 Vad at p/p0 = 0.1, VT = 1/FHg 1/FHe.

Kinetic studies were performed to determine the effects of contact time, adsorbent dose, and adsorbate concentration. At desired temperatures, predetermined amounts of char were added to fluoride solution (50 mL). The flasks were closed and 902

dx.doi.org/10.1021/ie202189v |Ind. Eng. Chem. Res. 2012, 51, 900–914


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Figure 1. Effect of adsorbent amount on the equilibrium pH of the water with no fluoride concentration.

Figure 2. Micrographs of pine bark biochar at different magnifications: (a) 431 (b) 1.32K.

When the biochars settled, the equilibrium pH of each solution was measured. The variation in the equilibrium pH at different char amounts is shown in Figure 1. The equilibrium pH increases nonlinearly in the pH range 2.04.0. At higher pH there is no pH increase up to 9.0. Thereafter the equilibrium pH again increases. The pHZPC was approximated as 9.0 since the pH of the water did not change after contact with the samples. Similar results were reported earlier.49 3.2. Morphological Analysis by Scanning Electron Microscope. Surface morphology is an important factor in adsorbent adsorbate interactions. Char SEM images are shown in Figures 2 and 3. The figures clearly demonstrate porous surfaces with a disorganized structural pattern that still contains some of the wood cells’ original morphology. The macropore size distribution has discrete groups of pore sizes rather than a continuum.50,51 These pores are of importance to many liquidsolid adsorption processes. Figure 2a shows the predominant pine bark char morphology. Pine wood char particles are shown in Figure 3a where a semimelt structure appeared to form perhaps due to longer exposure to higher temperatures. These SEM micrographs show that residual wood porosity and wood cell morphology remain in the char (they look like a somewhat distorted picture of a cut through wood itself). The short pyrolysis heating time does not totally destroy the original wood cell morphological structure. Instead, chemical decomposition has occurred with loss of water and

organic fragments which reduces the total mass by at least 80% (assumes a 20% maximum char yield). The remaining solid is more highly carbonized than before pyrolysis but it still has 811% oxygen present and maintains some of the wood or bark initial morphology. Macropores formed on the pine bark char surface are coarser and rougher than those of pine wood char surface. The smooth surface was likely developed by some melting and fusion of the lignin, cellulose, and other macromolecules in pine wood. The vesicles on the smooth surface of pine wood resulted from the release of volatile gas contained in the softened biomass matrix during the pyrolysis.5254 Overall, the low char BET surface areas indicate that, for high amounts of fluoride adsorption (similar to that for activated carbon) to occur, some different mechanisms must be operating. 3.3. Sorption Studies. Experiments were conducted to determine the optimum pH between 2 and 10 for fluoride uptake by both chars (Figure 4). Initial concentrations of 10 and 25 mg/L were used. The fluoride removal efficiency decreased as the pH values increased from 2 to 10. Maximum adsorption occurs at pH 2.0. Above pH 2.0, an increase in pH causes a sharp decrease in fluoride removal efficiency. Higher hydronium ion concentrations at low pH values will protonate basic functions in the char, creating counterion sites to bind fluoride ions. Similar results 903



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carboxylic acid, ketone, quinone, ether, lactone, pyrone, catechol, hydroxyketone, and others. Chars contain hydrophilic and hydrophobic sites and provide both acidic and basic sites. The relative contributions of these functional groups and properties depend upon both the feedstock and on the thermal decomposition process variables (time, temperature, heat transfer rates, particle size distribution, reactor configuration, etc.).59 Oxygenated functions exist throughout the solid thickness of the char, unlike some activated carbons where these groups are only on pore surfaces. Thus, these oxygen functions will enable water penetration into the solid char pore walls, unlike the behavior of activated carbon. Oxygenated functional groups contain lone pairs of electrons and will not adsorb fluoride themselves. However, as the pH is lowered, particularly below 2.5, surface group protonation can occur. These protonated functions can sorb hydrated fluoride ions. Additionally, if some water swelling occurs, H3O+ F hydrated ion pairs can diffuse into the solid char walls and F can be exchanged to protonated functions within this subsurface region. This will aid fluoride removal from the water solution being remediated. As pH values are raised, fewer protonated sites are present and fluoride is not efficiently remediated. Another possible mode of fluoride adsorption involves the role of ash that is present in the chars. Metal fluoride bonds are strong. The solubility of CaF2 and TiF3 in water is extremely low (solubility of CaF2: 0.0016 g/100 mL at 20 °C; solubility product, Ksp: 3.9 1011; while TiF3 is insoluble). Thus, Ca2+ and Ti3+salts in the char could form insoluble fluorides. Magnesium, aluminum, barium, manganese, and iron fluorides have only slight water solubilities. All of these metals were found to be present in the biochars (Table 1). Fluoride adsorption via a two-step ligand exchange was claimed by Daifullah et al.60 where the pH of the remediated water solution increased after fluoride sorption. However, if char is placed in water at low pH and if protonation of internal functions of the char occurs, the pH of the fluoride solution would increase by the lowering of hydronium ion concentration due to protonation. The specific mechanisms by which fluoride is sorbed by the biochars are not known. However, the possibilities suggested above fit the existing observations. 3.4. Kinetic Studies and Modeling. The effect of adsorbent amount on the fluoride uptake rate is shown in Figure 5. Uptake increases with more char addition. There is a substantial increase in the adsorption rate when pine wood biochar dosage increases from 5 to 10 g/L, while the additional rate increase on introducing an additional 5 g/L of biochar is not as great. Thus, the amount of pine wood char was held at 10 g/L in all subsequent kinetic studies. On the other hand, the increase in fluoride uptake onto pine bark char was not substantial when char dosage increased from 5 to 10 g/L, but the increase on introducing an additional 5 g/L of char was significant. For the comparative evaluation and convenience in laboratory batch studies, 10 g/L of pine bark char was selected for all the subsequent sorption and kinetic studies. Preliminary investigations indicated the rate of fluoride uptake on biochars was quite rapid. Typically, 5070% of the ultimate adsorption occurs within the 2 h of contact (Figure 5). This fast initial adsorption subsequently gives way to a slow approach to equilibrium. In 48 h, saturation is reached. Kinetic studies were also carried out at different fluoride concentrations viz., 20, 40, and 80 mg/L (Figures 6). An increase in the initial fluoride concentration enhances the sorption rate. The kinetics of fluoride removal has also been studied at 10, 25, and 40 °C (figures omitted for brevity). The extent of fluoride

Figure 3. Micrographs of pine wood-char at different magnifications: (a) 1.77K (b) 4.31K, and (c)10.17K.

have been reported for fluoride19,5557 and anionic surfactant adsorption.58 Furthermore, fluoride removal was also followed by a slight increase in the equilibrium pH. This appears to be an ion exchange mechanism where protonation generates cationic sites to which fluoride becomes the immobilized counterion. The biochar surfaces and subsurfaces contain a rich variety of oxygen-containing functional groups including hydroxy, anhydride, 904



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Figure 4. Effect of pH on fluoride removal using pine wood and pine bark chars at an initial fluoride concentration of 10 and 25 mg/L and at 25 °C.

Figure 6. Pseudo-second-order kinetic plots for fluoride adsorption by pine wood and pine bark at pH 2.0 at different fluoride concentrations (temperature 25 °C and adsorbent concentration of 10 g/L).

To further characterize the adsorption process, first-order and second-order kinetic models were tested to fit the batch experimental data, Determination of a good fitting model could allow design of a treatment process. 3.4.1. Pseudo-First-Order Kinetic Model. A simple adsorption kinetic model, the pseudo-first-order eq 6 suggested by Lagergen61 and further cited by Ho et al.,62 did not apply throughout the range of tested contact time (figures omitted for brevity). t

qt ¼ qe ð1 ek 1 Þ ðnonlinear formÞ Figure 5. Pseudo-second-order kinetic plots for fluoride adsorption by (a) pine wood and (b) pine bark at pH 2.0 and at different adsorbent concentrations (temperature: 25 °C and fluoride concentration of 40 mg/L).

ð6Þ

Here, k1 (min1 ) is the first order adsorption rate constant, q e is the amount adsorbed at equilibrium, and q t is the fluoride amount adsorbed at time t. The k1 values together with regression coefficients are provided in Table 2. The qe values as calculated from these plots

adsorption on both pine wood and pine bark chars decreased upon raising the temperature, indicating the process is exothermic. 905


4.06

2.06

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3.48 3.74

4.06

1.78

4.01

3.45

2.11

3.27

Figure 7. Langmuir adsorption isotherm of fluoride by pine wood char at different temperatures (pH 2.0, adsorbent concentration of 10 g/L). Solid lines represent the data fitted by the Langmuir isotherm model.

0.7986

did not match well with the experimental values at different concentrations. 3.4.2. Pseudo-Second-Order Kinetic Model. For reactions involving pseudo-second-order kinetics, rate is directly proportional to the number of active sites on the adsorbent surface. The pseudo-second-order rate expression (eq 7) is t 1 t ¼ þ 2 qt k2 qe qe

0.211 0.7890 0.8145

0.716

qe ¼ K F Ce 1=n

80

0.552

ð7Þ

Here k 2 (g mg1 min1 ) is the rate constant of pseudosecond-order adsorption, qe is the amount adsorbed at equilibrium, and qt is the amount of fluoride adsorbed at time t. The k2 and q e values were determined from the plots at different concentrations (Figure 6, Table 2). The correlation coefficients (R2) are clearly superior. The qe values are presented in Table 2 at different concentrations. The excellent agreement between the theoretical and the experimental qe values suggests that this sorption process occurs by a pseudo-second-order model. 3.5. Sorption Studies and Modeling. Models capable of predicting the fate and transport of groundwater contaminants require a reliable prediction of the adsorption data. Approaches for comparing adsorption data include the Freundlich,45 Langmuir,45 RedlichPeterson,63 Toth,64 Temkin,45 Radke and Prausnitz,65 and Sip (LangmuirFreundlich)66 adsorption isotherm models (eqs 815, respectively). They were compared here at 25, 35, and 45 °C to determine the best fits to adsorption results. The best fitting isotherms for fluoride adsorption at the optimum pH (2.0) on pine wood and pine bark chars are shown at three different temperatures (Figures 7, 8, and 9). The isotherms are positive, regular, and concave to the concentration axis. For both the chars, fluoride uptake decreases with an increase in temperature, indicating the process is exothermic. The sorption equilibrium isotherms, tested here by fitting the pine wood and pine bark adsorbents, are provided below. The isotherm parameters for fitting all seven models are summarized in Table 3. The Freundlich isotherm is45

0.9482

0.331

4.08

1.93

1.69 1.95

2.06 0.7782 0.419

3.68

0.8592 1.121

0.5522

0.9713 0.511

0.905 0.8760

0.8618

0.4850

1.144 0.9160

2.075

20

0.710

0.525

pine wood pine bark pine wood concn. (mg/L)

40

pine bark

2.11

1.76

pine wood pine bark pine bark pine wood pine bark pine wood

model (mg/g) order kinetic model (mg/g) constant, k1 (h ‑1) R2 constant, k1 (h ‑1)

R2

constant, k2 (g mg‑1 h‑1)

R2

constant, k2 (g mg‑1 h ‑1)

R2

(mg/g)

qe calculated using second-order kinetic qe calculated using firstqe experimental second-order rate second-order rate first-order rate first-order rate

Table 2. First-Order and Pseudo-Second-Order Rate Constants and Comparative Evaluation of qe as Calculated Experimentally and by Using First- and Second-Order Rate Equations at Different Fluoride Concentrations


ð8Þ

where qe is the amount of solute adsorbed per unit weight of adsorbent (mg/g), Ce is the equilibrium solute concentration 906



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Figure 9. Toth adsorption isotherms of fluoride by pine bark char at different temperatures (pH 2.0, adsorbent concentration of 10 g/L). Solid lines represent the data fitted by the Toth isotherm model.

Figure 8. RedlichPeterson adsorption isotherms of fluoride by pine wood char at different temperatures (pH 2.0, adsorbent concentration of 10 g/L). Solid lines represent the data fitted by the RedlichPeterson isotherm model.

where KT, BT, and βT are the Toth constants. The nonlinear Toth adsorption isotherm fits for pine wood char are given in Figure 9. The Temkin isotherm is69,70

in the solution (mg/L), constant KF indicates the adsorbent’s relative adsorption capacity (mg/g), and 1/n is the constant representing adsorption intensity. Langmuir isotherm is45 Q 0 bCe qe ¼ 1 þ bCe

qe ¼

ð9Þ

1 1 þ bC0

qe ¼

K RP Ce ð1 þ aRP Ce βRP Þ

qe ¼

ð10Þ

K T Ce ð1 þ BT Ce βT Þ1=βT

ð14Þ

K LF Ce nLF 1 þ ðaLF Ce ÞnLF

ð15Þ

where KLF, aLF, and nLF are the Sips constants. All the model parameters were evaluated using nonlinear regression employing Sigma Plot 8.0 software. The average percent error between the experimental and calculated values for all seven models was calculated using eq 16. n

APE ¼

∑ ðqe, experimental qe, calculatedÞ=qe, experimental i¼1 N

100 ð16Þ

ð11Þ

Here, qe,calculated (mg/g) and qe,experimental (mg/g) represent the calculated and measured amounts of fluoride adsorbed by biochars at equilibrium. The average percent errors calculated for each of the seven isotherms are given in Table 3. From these average error values, the best fits to the isotherms follow the orders for pine wood and pine bark shown as follows: Pine woodLangmuir ≈ Pine woodRedlich‑Peterson > Pine woodToth > Pine woodSips > Pine woodRadke and Prausnitz ≈ Pine woodFreundlich > Pine woodTemkin; and Pine barkToth > Pine barkRadke and Prausnitz ≈

where KRP, aRP, and βRP are RedlichPeterson constants. The exponent, β, lies between 0 and 1. The nonlinear RedlichPeterson adsorption isotherms fits for pine wood char are given in Figure 8. The Toth isotherm is68 qe ¼

abCe β a þ bCe β 1

Here a, b, and β are adjustable parameters. The Langmuir Freundlich or Sip equation is71

(RL > 1 unfavorable; RL = 1 linear; 0 < R < 1 favorable, and RL = 0 irreversible). RL was determined at different temperatures over a broad concentration range. The RL values at different temperatures were between 0 and 1 indicating that fluoride adsorption on pine wood and pine bark chars is favorable (data omitted for brevity). Nonlinear Langmuir adsorption isotherms are given for pine wood char in Figure 7. The RedlichPeterson equation is63 qe ¼

ð13Þ

The Temkin constant, bTe, is related to heat of sorption; aTe is the Temkin isotherm constant, R is the gas constant, and T is the absolute temperature. The Radke and Prausnitz equation is65

where qe is the solute amount adsorbed per unit weight of adsorbent (mg/g), Ce is the solute equilibrium concentration in solution (mg/L), Q0 is the monolayer adsorption capacity (mg/g,) and constant b is related to the net enthalpy, H, of adsorption (bαe ΔH/RT). More precisely, b is the reciprocal of concentration at which half saturation of the adsorbent is attained. A Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor, RL, originally defined by Hall et al.67 This RL factor is defined by RL ¼

RT lnðaTe Ce Þ bTe

ð12Þ 907



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Table 3. Freundlich, Langmuir, RedlichPeterson, Toth, Sips, Radke and Prausnitz, and Temkin Isotherm Parameters for Fluoride Removal on Pine Wood and Pine Bark Chars at Different Temperatures pine wood char isotherm parameters

25 °C

35 °C

pine bark char 45 °C

25 °C

35 °C

45 °C

Freundlich (eq 8) KF (mg/g)

2.28

2.01

1.26

1.18

0.58

1.17

1/n R2

0.318 0.8033

0.301 0.8027

0.338 0.8812

0.5126 0.9440

0.6283 0.9907

0.4281 0.7681

%APE

4.97

7.52

4.16

3.12

1.21

3.99

Q0(mg/g)

7.66

6.34

4.46

9.77

10.53

8.40

b

0.355

0.457

0.556

0.075

0.032

0.06

R2

0.9560

0.9159

0.7992

0.9209

0.9962

0.7890

%APE

1.71

6.48

5.10

2.84

1.12

4.94

Q0(mg/g) in presence of interfering ions

2.45

6.43

Qmix /Q0

0.32

0.66

Langmuir (eq 9)

RedlichPeterson (eq 11) KRP (L/g)

1.90

2.51

4112.1

15.66

0.42

0.25

aRP (L/mg)βRP

0.11

0.28

3275.8

12.39

0.10

4.29 106 3.14

βRP

1.24

1.09

0.66

0.50

0.78

R2

0.9735

0.9217

0.8812

0.9439

0.9991

0.8448

%APE

2.42

6.69

4.15

3.02

0.75

4.81

Toth Isotherm (eq 12) KT (L/g)

6.86

5.79

112.1

85.59

16.22

6.04

BT(mg/L)βT βT

0.23 0.399

0.30 0.021

97.37 9.371

0.09 4.40

0.27 1.50

0.04 0.02

R2

0.9791

0.9325

0.8759

0.9417

0.9970

0.8449

%APE

2.83

6.48

4.24

2.52

0.74

4.85

Sips (eq 15) KLF (L/g)

2.13

2.94

0.78

1.18

0.40

0.0004

aLF (L/mg)aLF

0.305

0.498

0.379

0.0035

0.0325

6.08 105 3.78

nLF

1.52

1.60

0.15

0.506

0.903

R2

0.9774

0.9308

0.8888

0.9440

0.9968

0.8324

%APE

3.42

7.08

5.74

3.15

0.72

5.73

a

2.23 107

2.23 107

2.23 107

2.10 106

1.31 106

4.18 107

b

2.28

2.02

1.25

1.18

0.58

1.19

β

0.32

0.30

0.33

0.5127

0.6283

0.4282

R2

0.8033

0.8027

0.8812

0.9440

0.9907

0.7681

%APE

4.96

7.53

4.16

3.12

1.21

3.99

bTe

1.77

1.54

1.05

1.86

1.47

1.29

aTe R2

1.00 0.2384

1.00 0.2632

1.00 0.2673

1.00 0.7210

0.99 0.9317

1.00 0.2162

%APE

24.26

14.16

13.48

11.66

5.49

13.59

Radke and Prausnitz (eq 14)

Temkin (eq 13)

Pine barkFreundlich > Pine barkRedlich‑Peterson > Pine barkLangmuir > Pine barkSips > Pine barkTemkin. Langmuir and Redlich Peterson equations best fitted the data for pine wood (Figures 7 and 8), while the Temkin approach gave the worst fit to the data. The Toth equation gave the best fit for pine bark (Figure 9), while the Temkin equation produced the poorest fit. The Langmuir model is used to estimate

maximum uptake values, where these could not be obtained experimentally. 3.6. Influence of Coexisting Ions on Fluoride Adsorption. The influence of coexisting ions commonly present in water viz., sulfate, chloride, and nitrate, on fluoride adsorption was studied using both chars. Tests were conducted in the presence of interfering ions in the broad range of concentrations (1100 mg/L). 908



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The stock solution was made by dissolving NaF, NaNO3, Na2SO4, and NaCl in 1:1 ratio and dilutions were made in the concentration rage of 1100 mg/L. The Langmuir adsorption isotherms in the absence and presence of interfering ions at 25 °C are given in Figure 10. The effect of ionic interactions on sorption may also

be represented by the ratio of the sorption capacity for fluoride in presence of the other ions, Qmix, to the sorption capacity for the fluoride when it is present alone in the solution, Q0. When Qmix/ Q0 > 1, the sorption is promoted by the presence of other ions. When Qmix/Q0 = 1, there is no observable net interaction. When Qmix/Q0 < 1, sorption is suppressed by the presence of other ions. The Qmix/Q0 values are less than 1 (Table 3), confirming that the fluoride adsorption is suppressed by the presence of other ions. 3.7. Application of These Biochars to Treat Groundwater Samples. A groundwater sample was collected from Mathura district of Uttar Pradesh, India. Its physicochemical analysis is given in Table 4. Pine bark and pine wood chars were used to remove fluoride from this sample under optimum conditions obtained by batch sorption studies. The pH of the water (50 mL) was adjusted using H2SO4 to 2.0 before applying biochars. A predetermined amount (15 g/L) of biochar was added to each sample. After 48 h of equilibration, the samples were filtered and fluoride concentration was measured. The final fluoride concentrations were reduced significantly from the original value. The fluoride removal was higher with pine wood char versus pine bark char. This agreed with the batch experiments conducted with synthetic samples. Furthermore, solids and interfering ions present in surface water caused a little reduction in the chars’ sorption capacities. Thus, these chars can be applied to remediate fluoride from contaminated surface or ground waters, although their application at pH 2.0 will not always be practical. Wastewater from hydrofluoric acid processes, metal plating, soldering, glass manufacturing, phosphate fertilizer, semiconductor, and other electronic industries with low pH and high fluoride72 concentrations are treatment candidates. 3.8. Comparative Evaluation: Biochars Compared to Other Adsorbents. The monolayer adsorption capacities of both the chars calculated using Langmuir adsorption isotherm model are given in Table 3. The biochar adsorption capacities are compared to those of other adsorbents in Table 5. The BET surface areas of dry pine wood and pine bark chars are very small (∼23 m2 g1) versus commercial activated carbons (∼1000 m2 g1). These biochars adsorb far more fluoride per unit surface area (mg/m2) than activated carbons (Table 5). This remarkably higher fluoride removal per unit surface area by the chars is a result of biochar swelling in water. We demonstrated these chars imbibe water, opening up pores or adsorption sites that are not measured by N2 BET when the chars are dry. This provides more internal surface for adsorption. Diffusion into portions of the solid char also occurs.

Figure 10. Adsorption isotherms of fluoride by (a) pine wood char and (b) pine bark char in absence and presence of interfering ions at pH 2.0, temperature 25 °C, and adsorbent concentration of 10 g/L. The sample with interfering ions was an artificially made sample prepared by dissolving NaF, NaNO3, Na2SO4, and NaCl in 1:1 ratio. Solid lines represent the data fitted by the Langmuir isotherm model.

Table 4. Fluoride Remediation from Contaminated Ground Water Samplea Using Pine Wood and Pine Bark Chars (Adsorbent Dose 15 g/L; Equilibrium Time 48 h; pH 2.0; Temperature 25 °C; Particle Size ; Volume of the Water Sample Taken 50 mL) parameters pH ORP (mV)

2.05

values after treatment with pine wood char 2.08

+291.8

+290.1

values after treatment with pine bark char 2.23 +281.5

conductivity (μS/cm)

4770

4550

4030

TDS (mg/L)

2338

2235

1975

F (mg/L)

a

values (initial pH 8.33 which is adjusted to pH 2)

6.995

0.973

1.754

Na+ (mg/L) K+ (mg/L)

349.0 7.6

348.0 17.6

330.0 24.2

Ca2+ (mg/L)

23.7

24.0

31.6

Groundwater sample was collected from Mathura district of Uttar Pradesh, India. 909



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Table 5. Comparative Evaluation of Langmuir Adsorption Capacities of Bio-Chars Compared to Activated Carbons and Other Adsorbents for Fluoride Removal temperature

concentration

type of water

pH

(°C)

range

pine wood biochar

aqueous solution

2.0

25 35

1100 mg/L

7.66 6.34

pine bark biochar

aqueous solution

2.0

1100 mg/L

9.77

adsorbent

45 25

activated carbon

aqueous solution

aluminum-impregnated activated carbon

aqueous solution

activated carbon derived aqueous solution

7.0

2.0

-

(m2/g)

2.80 2.32

1.88

5.20

refs this study

1.63 this study

5.60

8.40

4.46

2.41

250

0.010

20

27

1.07

1044

0.001025

74

27

0.49

950

0.000516

15.90

123

0.129

60 76

25.0

2.514.0 mg/L

surface area

2.73

10.53

45 aqueous solution

(mg/g)

4.46

35 activated alumina

adsorption capacity surface area adsorption capacity/

20.0 mg/L

from rice straw activated alumina

aqueous solution 7.0 ( 0.2 30.0 ( 2.0

2.030.0 mg/L

1.08

204.13

0.005

2.85

170.39

0.017

02.4 mmol/L

6.29

20.7

0.304

0.1606

7.908

77

0.154

8.247

78

manganese-oxide-coated alumina (MOCA) activated red mud

aqueous solution

5.5

original red mud algal Spirogyra IO1

aqueous solution

2.0

algal biosorbent

aqueous solution

7.0

room temp.

5.025.0 mg/L

3.12 1.27

30.0

5.025.0 mg/L

1.27

5.035.0 mg/L

24

(Spirogyra sp. IO2) alum sludge

aqueous solution

6.0

32.0

alumina supported

aqueous solution

3.0

25.0

biomass carbon (300 °C) aqueous solution

5.8

25.0

5.39 45.32

119.4

0.045

79

165

0.275

80

carbon nanotube 10.0 mg/L

0.52

30.0 ( 2.0

5.0 mg/L

1.54 0.13

15.0

1.020.0 mg/L

6.05

104

0.058

25.0

11.90

104

0.114

5.0

25.0

7.74

104

0.074

7.0

25.0

5.44

104

0.0523

10.0

25.0

2.31

104

0.022

11.0

25.0

4.04

104

0.0388

12.0 7.0

25.0 35.0

1.33 5.07

104 104

0.0127 0.0487

biomass carbon (600 °C) bleaching powder aqueous solution

6.7

bone char

7.0 3.0

charcoal

aqueous solution

drinking water

6.9 ( 0.1 28.0 ( 2.0

2.050.0 mg/L

7.92

AF650/C

13.64

FeAlCe

81

3.77

AF500/C AF900/C

29

31

82

5.67 drinking water

7.0

25.0

0.001 M

2.22

83

25.0

1.0100.0 mg/L

5.97

25.0 ( 1.0 30.0 ( 2.0

5.0150.0 mg/L

8.921

5.9 mg/L

2.68

2.5 1056.34102 mg/L

4.54

0.054

84.07

fluorspar

2.5 1056.34102 mg/L

1.79

0.048

37.30

calcite

2.5 1056.34102 mg/L

1.16

0.057

20.35

quartz

2.5 1056.34102 mg/L

0.39

0.06

6.50

activated quartz hydrous-manganese-

2.5 1056.34102 mg/L 10.070.0 mg/L

0.19 7.09

nanoadsorbent granular ferric hydroxide (GFH)

aqueous solution 6.07.0

granular red mud (GRM) aqueous solution hydrated cement aqueous solution

4.7 6.7

hydroxyapatite

6.0

aqueous solution

aqueous solution 5.2 ( 0.05

30.0

300 10.2

0.20 0.875

84 85 86

315.2

0.022

17

87

oxide-coated alumina (HMOCA) 910



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Table 5. Continued adsorbent

type of water

pH

temperature

concentration

(°C)

range

adsorption capacity surface area adsorption capacity/ (mg/g)

kaolinites

aqueous solution 3.011.0

30.0

2.010.0 mg/L

0.61

charcoal

aqueous solution 6.9 ( 0.1

25.0

210.0 mg/L

3.77

Ca500/C

17.09

Ca650/C

19.05

Ca900/C

(m2/g) 32.43

surface area 0.019

refs 88 75

15.22

laterite

drinking water

magnesia-amended

drinking water

7.5

30.0

10.050.0 mg/L

0.85

6.57.0 30.0 ( 1.0

5.0150.0 mg/L

8.08

193.5

0.042

89

6.57.0 30.0 ( 1.0

5.0150.0 mg/L

4.04

242.07

0.017

1.025.0 mg/L

0.1701

203

0.00083

0.1591

203

0.00078

4.31

629

0.007

90

activated alumina granules (MAAA) activated alumina (AA) manganese dioxide-

aqueous solution

coated activated

4.0

room temp.

7.0

91

alumina (MCAA) waste carbon slurry

aqueous solution

7.58

25.0

10.0 mg/L

92

sorption experiments. The samples were then filtered on a preweighed filter and all the surface water was removed using suction filtration. The weight of the char was recorded as a function of time to follow weight loss due to internal water evaporation (Figure 11). Both wood and bark chars lose weight due to internal water evaporation with time. This internal water is present in both pores and in swollen solid regions of the char where it had diffused during immersion. Clearly, porosity exists in the char, when submerged, that was not available to gas adsorption when the char was dry. The chars’ low density (it does not sink) and slow water weight loss by evaporation with time both show that the char is porous. Proof was obtained that more water was imbibed in the chars than is possible by filling the BET measured surface area. The weight loss due to internal water removal from pine wood char was about 0.370.38 g/g of char and from pine bark char was ∼0.260.57 g/g of char. Thus, this water occupied about 0.370.38 cc/g and 0.260.57 cc/g of these two chars. Some of this was accommodated by diffusion into pore walls and by expansion of internal pore structure. This swelling contrasts with the behavior of almost fully carbonized carbon black. The char has ∼812% by weight oxygen throughout its chemical structure, so it is much more hygroscopic than carbon black. During fast pyrolytic decomposition, gases and steam rapidly generated inside the wood or bark particles are “exploded” outward. As these escape from the pyrolyzing/decomposing wood or bark particles, more porosity is generated, adding to the wood’s porous morphology which survives fast pyrolysis. These internal pore networks can partially collapse or close on cooling. Semimelted regions also decrease the pore areas that might have formed. Therefore, gas adsorption will measure a surface area that is different from the total char contact region that is occurring in water-swollen samples. It is the water-swollen char that is adsorbing the fluoride. Micro- or ultramicropores may exist (Table 1) in the char solids through which gases exited to the larger pores. These cannot be seen at the SEM resolution (Figures 2 and 3). Some are likely closed and only open when water is imbibed. Water either opens such pores or, even more likely, diffuses into these walls (carrying fluoride ions). Thus, much of the solid (seen on micrographs) may be able to adsorb fluoride within these wall structures.

Figure 11. Swelling behavior measured by loss of imbibed water weight by evaporation: (a) pine wood and (b) pine bark at 25 °C in water at pH 7.2 and pH 2.0 after 48 h of immersion.

The dry weight of as-received pine wood and pine bark char samples was recorded. Then char samples were immersed into water for the length of time that corresponds to our longer (48 h) 911


Industrial & Engineering Chemistry Research This cannot happen with many activated carbons because their wall structures are more carbonized and consist of hydrophobic rigid carbon below the pore wall surfaces. Thus, water cannot diffuse into that solid region of activated carbon. In the chars, fluoride adsorbs both on the surfaces of pores and also inside these cell walls as water diffuses into and swells portions of these walls.

4.0. CONCLUSIONS The chars produced by pine wood and pine bark pyrolyzed at 400 and 450 °C in an auger-fed reactor during bio-oil production were characterized and used successfully, without activation, for the fluoride remediation from low pH water, including water with other competing ions added and groundwater. The Langmuir monolayer adsorption capacity was higher for pine bark (Q0= 9.77 at 25 °C) char versus pine wood char. The fluoride sorption capacities of biochars are higher than or comparable to the activated carbons and other materials reported in literature for fluoride decontamination (Table 5). It is remarkable that pine char (surface area of 23 m2 g1) can remove amounts of fluoride similar to activated carbon (with surface area of ∼1000 m2 g1). This occurs because adsorption can occur within the solid char walls. Water swelling and water diffusion in the solid takes place as a result of the high oxygen content present in chars. However, the maximum fluoride adsorption occurs at pH 2.0 which is not a viable option for practical large-scale water treatment systems. Nevertheless, low pH wastewater from hydrofluoric acid processes, metal plating, soldering, glass manufacturing, phosphate fertilizer, semiconductor, and other electronic industries containing high fluoride levels could be treated.72 Fluoride adsorption decreased with an increase in temperature, indicating an exothermic process. The rate of fluoride adsorption followed a pseudo-second-order kinetic model, based on the assumption that the rate-limiting step is a chemical sorption between the adsorbate and adsorbent. The very small (∼15 m2 g1) surface areas of the biochars could someday be greatly increased by steam activation and/or specific chemical treatments to improve their removal efficacy. This could enhance the adsorption rate and might further increase adsorption capacity. The current value of bio-oil-derived biochars is only their heating value when burned for process heat. Bio-oil production may become a distributive effort at many locations near the biomass source. Bio-oil would then be transported to biorefineries. If this occurs, biochars would be widely distributed and available to many water treatment facilities without extensive transportation. Will bio-oil production develop widely as part of the renewable fuels push? We do not know. Therefore, it is too early to discuss or guess at economics. Furthermore, economic factors will differ country to county and region to region. The real value, in our opinion, of this work is to provide a beginning that others can build upon as the biofuels panorama develops (if it develops) worldwide. Obviously, fluoride ion removal is not the only possibility for water treatment. We and others have begun to look more widely at biochars for adsorption.34,73 The game has just begun. Byproduct chars from bio-oil production could be used as readily available and inexpensive adsorbents for water treatment at a value above their pure fuel value. Further char studies, using both untreated samples and samples after activation, seem warranted.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]; phone: 0091-11-26704616; fax: 0091-11-26704616.

’ ACKNOWLEDGMENT Financial support of this work at Jawaharlal Nehru University was provided by University Grant Commission (PAC/SES/ DM/UGC/0210113-491)) New Delhi, Department of Science and Technology (DST-PURSE) New Delhi, and Capacity Buildup funds provided by Jawaharlal Nehru University, New Delhi, India. Financial support of this work at Mississippi State University was provided by the U.S. Department of Energy, through the U.S. DOE SERC-9III grant DE-FG36-06GO86025. ’ REFERENCES (1) Rao, N. S.; Devadas, D. J. Fluoride Incidence in Groundwater in an Area of Peninsular India. Environ. Geol. 2003, 45, 243. (2) Saxena, V. K.; Ahmed, S. Inferring the Chemical Parameters for the Dissolution of Fluoride in Groundwater. Environ. Geol. 2003, 43, 731. (3) Ayoob, S.; Gupta, A. K. Fluoride in Drinking Water: A Review on the Status and Stress Effects. Crit. Rev. Environ. Sci. Technol. 2006, 36 (6), 433. (4) WHO. Guidelines for Drinking Water Quality, 3rd ed.; World Health Organization: Geneva, 2004. (5) Hichour, M.; Persin, F. O.; Sandeaux, J.; Gavach, C. Fluoride Removal from Waters by Donnan Dialysis. Sep. Purif. Technol. 2000, 18, 1. (6) Kruse, E.; Ainchil, J. Fluoride Variations in Groundwater of an Area in Buenos Aires Province, Argentina. Environ. Geol. 2003, 44, 86. (7) Neal, C.; Neal, M.; Davies, H.; Smith, J. Fluoride in UK Rivers. Sci. Total Environ. 2003, 314, 209. (8) Camargo, J. A. Fluoride Toxicity to Aquatic Organisms: A Review. Chemosphere 2003, 50 (3), 251. (9) Amini, M.; Mueller, K.; Abbaspour, K. C.; Rosenberg, T.; Afyuni, M.; Møller, K. N.; Sarr, M.; Johnson, C. A. Statistical Modeling of Global Geogenic Fluoride Contamination in Groundwaters. Environ. Sci. Technol. 2008, 42 (10), 3662. (10) Czarnowski, W.; Niowska, K. W.; Krechniak, J. Fluoride in Drinking Water and Human Urine in Northern and Central Poland. Sci. Total Environ. 1996, 191 (12), 177. (11) Saxena, V. K.; Ahmed, S. Dissolution of Fluoride in Groundwater: A Water-Rock Interaction. Environ. Geol. 2001, 40, 1084. (12) Sujatha, D. Fluoride Levels in the Groundwater of the SouthEastern Part of Ranga Reddy District, Andhra Pradesh, India. Environ. Geol. 2003, 44, 587. (13) Jain, C. K. Fluoride Contamination in Ground Water. In Water Encyclopedia: Ground Water; Lehr, J. H., Keeley, J., Eds.; John Wiley & Sons, Inc., 2005; pp 130136. (14) Ayoob, S.; Gupta, A. K.; Bhat, V. T. A Conceptual Overview on Sustainable Technologies for the Defluoridation of Drinking Water. Crit. Rev. Environ. Sci. Technol. 2008, 38, 401. (15) Bhatnagar, A.; Kumar, E.; Sillanp€a€a, M. Fluoride Removal from Water by Adsorption;A Review. Chem. Eng. J. 2011, 171 (3), 811. (16) Das, N.; Pattanaik, P.; Das, R. Defluoridation of Drinking Water Using Activated Titanium Rich Bauxite. J. Colloid Interface Sci. 2005, 292, 1. (17) Fan, X.; Parker, D. J.; Smith, M. D. Adsorption Kinetics of Fluoride on Low Cost Materials. Water Res. 2003, 37, 4929. (18) Yang, C.-L.; Dluhy, R. Electrochemical Generation of Aluminum Sorbent for Fluoride Adsorption. J. Hazard. Mater. 2002, B94, 239. 912



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Fluoride Removal from Water using Bio-Char, a Green Waste, Low

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