Defluoridation efficiency of a green composite reactive material derived from lateritic soil and gastropod shell

Sustainable Chemistry and Pharmacy 12 (2019) 100131

Contents lists available at ScienceDirect

Sustainable Chemistry and Pharmacy journal homepage: www.elsevier.com/locate/scp

Defluoridation efficiency of a green composite reactive material derived from lateritic soil and gastropod shell

T

N.A. Oladojaa, , G.A. Belloa, B. Helmreichb, S.V. Obisesana, J.A. Ogunniyia, E.T. Anthonya, T.D. Saliua ⁎

a b

Hydrochemistry Research Laboratory, Department of Chemical Sciences, Adekunle Ajasin University, Akungba Akoko, Nigeria Technical University of Munich, Chair of Urban Water Systems Engineering, Am Coulombwall, 85748 Garching, Germany

ARTICLE INFO

ABSTRACT

Keywords: Composite material Gastropod shell Laterite Fluoride Defluoridation Groundwater

In the bid to exploit the synergy in mixed metal matrix that enhances substrate binding affinity, a composite reactive material (GLT) was prepared using lateritic soil and Gastropod shell as precursors. The defluoridation efficiency of the GLT was studied in laboratory grade fluoride contaminated water and groundwater matrix. Using experimental evidences, insight into the underlying defluoridation mechanisms of GLT was provided. The monolayer adsorption capacity (qm, mg/g) of the GLT (43.7 mg/g) was higher than that of any of the precursors (i.e. lateritic soil (qm = 2.8 mg/g) and Gastropod shell (qm = 19.8 mg/g)), which confirmed the synergistic effect of the constituents of the composite. Premised on the results of kinetic analysis, the Gibbs (∆G) free energy value of formation of insoluble metal fluoride and the results of the evaluation of the effects of water chemistry on the fluoride removal efficiency of the GLT, mechanism of the defluoridation process was found to be a combination of adsorption (via ion exchange or outer sphere complexation) and precipitation reaction. Relative to the performance of the GLT in laboratory grade water, the GLT was also effective in groundwater but the value of the Langmuir monolayer adsorption capacity (qm mg/g) was lower in the groundwater system (29.76 mg/g) than in the laboratory grade water system (43.7 mg/g). The leached constituents of the GLT enhanced the values of the pH, electrical conductivity and the total dissolved solids of the treated water.

1. Introduction Consequent upon the need, in groundwater (GW) contaminated with fluoride (F-), to reduce the F- concentration to values below the permissible WHO standard (˂1.5 mg/L), different adsorbents and reactive materials have been studied for use in adsorption-based water treatment systems and filtration units. In these genre of materials, the Fbinding sites are provided by multivalent metallic species (Zhang et al., 2011; Oladoja et al., 2015a, 2015b, 2016a, 2016b, 2016c, 2017; Sun et al., 2011; Jagtap et al., 2011; Oladoja and Helmreich, 2014, 2016; Xu et al., 2011; Prathna et al., 2017, 2018; Rathore et al., 2016). These metallic species, with high affinities for F-, traverse the spectrum (i.e. S, D and F-block metals) of the metallic genre. The features of metallic species that enhanced their performance(s) in aqua defluoridation include the amphoteric nature, high point zero charge pH (pHPZC)value, ease of surface protonation, and ability to form insoluble metal fluoride salts (Oladoja et al., 2015a, 2016a, 2016b, 2017). Sequel to the high affinity of metallic species for F-, researchers have focused on the synthesis of functional reactive materials for potable ⁎

water defluoridation using metal rich materials. Few of the metal rich materials that have been studied include calcium chloride modified zeolite (Zhang et al., 2011), Gastropod shell (Oladoja et al., 2016a), sand-nano magnesium oxide composite (Oladoja et al., 2015, 2016b, 2017) natural stilbite zeolite modified with Fe3+ (Sun et al., 2011), chitosan based microporous alumina (Jagtap et al., 2011), calcium aluminate–diatomaceous earth composite (Oladoja and Helmreich, 2016), fly ash cenospheres loaded with magnesium oxide (Xu et al., 2011), aluminum-diatomaceous earth composite (Oladoja and Helmreich, 2014; Oladoja et al., 2015b), granular anionic clay composite (Oladoja et al., 2016c), iron oxide nanoparticle (Prathna et al., 2017), chemically treated laterite (Rathore et al., 2016), iron oxide/ alumina-based nano-adsorbents (Prathna et al., 2018). As part of the continuous systematic efforts towards the development of functional materials for aqua defluoridation from metal rich materials, this study evaluated a green composite reactive material, prepared by combining the shell of Gastropod (GS) with laterite (LS), in aqua defluoridation. In the choice of the precursors for the preparation of the composite material, the pervasiveness and the economic

Corresponding author. E-mail address: [email protected] (N.A. Oladoja).

https://doi.org/10.1016/j.scp.2019.100131 Received 28 August 2018; Received in revised form 21 January 2019; Accepted 25 January 2019 2352-5541/ © 2019 Elsevier B.V. All rights reserved.

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

considerations were paramount factors. Aside these two factors, the thesis that informed the preparation of the composite material hinged on the postulation that mixed metal matrix that is produced from the combination of the precursors can enhance the F- binding capacities via synergistic effects. Functional materials containing mixed metals have been produced and used as advanced materials in environmental management and remediation (Prathna et al., 2018; Tripathy and Kanungo, 2005; Xu and Axe, 2005; Mustafa et al., 2010). In the development of functional materials for environmental management, the prevalence of mixed metal oxides over the single metal oxides in the natural environment system was the impetus. It was hypothesised that the interaction of the metallic constituents in the mixed metal systems influenced the proclivity of such materials for the targeted substrate (Honeyman, 1984; Li et al., 1993; Goh et al., 2008; Zhang et al., 2008). Thus, mixed metal materials are continually being evaluated as functional materials in environmental management and remediation (Chan et al., 2009; Zhang et al., 2005, 2007; Gupta and Ghosh, 2009). In the present study, the constituents of the composite reactive material are Gastropod shell (GS) and lateritic soil (LS). Tropical soils that is rich in iron oxide is called Laterite (Norton and Esposito, 1995). It is a rusty red soil that is widely distributed in the tropics, subtropics and Mediterranean climatic zones. The mineralogical assemblage of this soil is flexible, thus in addition to the rich iron oxide content, they often contain minerals of phyllosilicates, iron (goethite, hematite) and aluminum (gibbsite and bauxite) (Norton and Esposito, 1995). The GS was obtained from the giant African land snail (Achatina achatina). Gastropods have worldwide distribution and the shell has the same chemical composition as other Mollusk shells (contains, mainly, CaCO3 and nominal organic compounds) (Oladoja and Aliu, 2009a; Oladoja et al., 2011). In potable water defluoridation, LS and GS have been used, separately, (Oladoja et al., 2016a, 2019) as green reactive materials but herein a composite is prepared from the two materials. In order to validate the thesis that underlie the aim of this study, different combination ratios of GS and LS were prepared and tested as reactive materials for potable water defluoridation, to determine the best combination ratio. The best combination ratio of the GLT was tested in a systematic defluoridation procedure to obtain the kinetics and the equilibrium isotherm data. The influence of the water chemistry on the defluoridation efficiency of the GLT was assessed and the defluoridation efficiency of the GLT was evaluated in GW spiked with different concentrations of F-.

The best combination ratio of the GS to LT was determined by evaluating the defluoridation efficiency of each composite material viz: A known weight (0.1 g) of the GLT was added to a fixed volume (50 mL) and concentration (32.1 mg/L) of the F- solution in a batch reactor and agitated at 200 rpm for 2 h. Sample was withdrawn, filtered and the Fconcentration in the filtrate was determined using an ion analyzer. The defluoridation efficiency of each composite material was evaluated using the mass balance equation. The characteristic of GLT was determined by using X-ray diffractometer (XRD) and X-ray fluorescence (XRF) to elucidate the mineralogy and chemical composition, respectively; Fourier Transform Infra-red spectrophotometer (FTIR) was used to explicate the surface functional groups while the solid addition method (Oladoja and Aliu, 2009b) was used to determine the pH of the point zero charge (pHPZC). 2.2. Batch defluoridation The kinetic parameters were derived from the data obtained from the determination of the time-concentration profiles of the process of defluoridation. The kinetic experiment was conducted by adding 1.0 g of the GLT into a beaker containing 500 mL of synthetic F- solution of varying initial F- concentrations. The concentration of the F- solution ranged between 1.56 mg/L and 31.00 mg/L. At regular time intervals between zero (0) and 5 h, samples were withdrawn, filtered and the residual F- concentration in the filtrate was quantified appropriately. The equilibrium isotherm experiment was conducted by adding 0.l g of the GLT into a beaker that contains 50 mL of the synthetic F- solution and agitated for a period of 5 h before sample was withdrawn for Fanalysis. The concentration range of the F- solution used was between 1.56 mg/L and 31.38 mg/L. The influence of solution chemistry on the performance of the GLT was assessed by simulating the process variables that are comparable to what obtain in the GW viz: F- solution pH value that ranged between 5.0 and 9.0; interfering anionic species ( NO3−, Cl− PO43-, CO32-and SO42−) and ionic strength that ranged between 0 and 0.17 mol/L. 3. Results and discussion 3.1. Preparation of materials and characterization In order to determine the best combination ratio of the constituents of the GLT for aqua defluoridation, the defluoridation efficiency of each combination ratio was determined. The values of the defluoridation efficiencies (mg/g) for the respective combination ratio (i.e. GLT1:1, GLT1:2 and GLT2:1) are 9.3 mg/L, 6.9 mg/L and 8.9 mg/L. The three combination ratios exhibited appreciable defluoridation efficiencies but the GLT1:1 gave the highest value, thus it was chosen for subsequent studies. The results presented in Table 1 showed that Si, Al and Fe were the major oxides in LT while Ca was the major oxide in GS but the major oxides contained in the GLT are Si, Al, Fe and Ca. This showed that the composite was derived from the precursors. The XRD peak patterns of the GLT, GS and LT are presented in Fig. 1. The XRD peak patterns of the GLT differed from that of the precursors but prominent diagnostic peaks identified in the XRD patterns of the LT were more visible in the GLT peak patterns (Fig. 1). The prominent LT peaks identified in the GLT peak patterns are the quartz (2θ value = 26.7, 40.2, 60.4, 76.04,

2. Materials and methods 2.1. Preparation of material and characterization Sample of the LT was obtained from the University town, ground, sieved and kept in an airtight plastic bag pending usage. The procedure described in our previous expositions [28,29 and 31] was adopted in preparing the GS. The GLT of different combination ratios were prepared by mixing different ratios of the LT and GS (i.e. ratio 1:1, 1:2 and 2:1) and labelled as GLT1:1, GLT1:2 and GLT2:1 (the subscript showing the LT to GS ratio). Sequel to the report (Oladoja et al., 2016a) on the effect of thermal treatment on the defluoridation efficiency of the GS, appropriate mixture of the composite material was calcined at 1000 , allowed to cool in a desiccator and stored in a plastic bag pending usage. Table 1 Results of the XRF analysis of GLT and the precursors. Reactive material

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2 O

Na2O

M2O5

P2O5

TiO2

LOI

Total

LT GS GLT

74.31 2.2 43.86

11.08 0.32 6.6

5.07 0.21 2.86

0.31 52.85 24.75

0.08 0 0.11

0 0.06 0.03

0.69 0.13 0.47

0 0 0.08

0.05 0.01 0

0.04 0.21 0.02

0.97 0.05 0

6.26 44.43 23.56

98.86 100.46 102.33

2

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 1. XRD pattern of the GLT, GS and LT.

and 68.7) nackrite (46.5) and hematite (50.6) peaks. Some of the peaks synonymous with the calcium carbonate polymorphs that were prominent in the GS were not identified in the GLT diffractogram. Thus, it is posited that the GS carbonate peaks were subsumed by the stronger and prominent quartz peaks of the LT. Although the major LT peaks were prominent in the GLT diffractogram but the intensities of these peaks greatly reduced in the GLT (Fig. 1). It has been posited (Li et al., 1993) that the reduction in the peak intensity in the diffractogram of a material is an indication of surface interaction, coverage or impregnation on such material. Thus, it could be inferred that interfacial interactions occurred between the constituents of the precursors from which the GLT was derived. The FTIR spectra of the GLT, GS and LT (Fig. 2) showed that the spectra pattern of the GLT and LT were similar while the spectra pattern of the GS was totally different. The LT peaks were the prominent peaks identified in the GLT. The peak at 3640 cm- l in the GLT was ascribed to the water molecules incorporated in the lattice structure of a crystalline molecule. The peaks at 1406 cm- l and 873 cm- l were attributed to the CO32- and O-C-O bending vibrations, respectively. The peaks identified at 1086 cm−1 and 462 cm−1 were assigned to the Si-O-Si stretching and

Si-O-Si deformation, respectively. The Si-O and Si-O of the stretching vibration of quartz and silica and stretching vibration of quartz, were observed at 796 cm−1 and 776 cm−1, respectively. The pHPZC of the GLT obtained at a pH value of 11.78, showed that the surface of the GLT is predominantly positively charged over a wide pH window. 3.2. Batch defluoridation The results presented in Fig. 3 highlighted the time-concentration profiles of the process of aqua defluoridation. The magnitude of F- removed from the aqua phase by the GLT was initial F- concentration and contact time dependent. The attainment of the equilibrium time for defluoridation was faster at lower initial F- concentrations but the magnitude of F- removed was more at higher initial F- concentrations. The results of the analysis of the time-concentration profiles with pseudo first order and pseudo second order kinetic equations, whose linear versions are presented in Eqs. (1) and (2) are presented in Table 2.

Fig. 2. FTIR spectra of the GLT, LT and GS. 3

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 3. Time-concentration profiles of the defluoridation process.

3.3. Equilibrium isotherm analysis

Table 2 Kinetic parameters of the defluoridation process in synthetic feed water. Pseudo 1st order Initial Conc. (mg/L)

qe1 (mg/ g)

1.56 4.64 9.40 19.95 31.00

Log [qe

K1 (g/mg min) × 10−3 0.0122 0.0106 0.0129 0.0092 0.012

qt ] = log [qe]

t 1 1 = qe + t qt k qe

The linear versions of the different equilibrium isotherm equations used to analyse the experimental data are presented in Eqs. (3)–(5).

Pseudo 2nd order r2

0.7033 0.9514 0.9531 0.781 0.7482

k1 t 2.303

qe2 (mg/ g) 0.281 1.572 3.034 4.085 7.462

h0 (mg/ g min) 0.01 0.92 5.45 9.56 14.66

K2 (g/ mg min) 0.0709 0.3719 0.5924 0.5730 0.2633

r2

Langmuir:

ce 1 1 = ce + qe qm ka qm

(3) -

Where, qe is the quantity of F adsorbed at equilibrium per gram of GLT (mg/g); Ce is the aqueous phase F- concentration at equilibrium; Ka is a constant; qm (mg/g) is the monolayer adsorption capacity. The Langmuir isotherm parameters were got from the slope and intercept of the plot of Ce/qe versus Ce.

0.988 0.9993 0.9998 0.9996 0.9994

Freundlich:logqe = logkf +

(1)

1 logce n

(4)

-

Where, qe is the quantity of F adsorbed at equilibrium per gram of GLT (mg/g), Ce is the aqueous phase F- concentration at equilibrium, kf is a constant that defined the adsorption capacity and 1 is a constants that is n linked with the adsorption intensity of the adsorbent. The Freundlich constants were obtained from the intercept and slope of the linear plot of log qe versus log Ce

(2)

Where, qe, is the adsorption capacity at the equilibrium time, qt is the adsorption capacity (mg/g) at time, t, k1, is the pseudo first order rate constant and k2, is the pseudo second order rate constant (g (mg/min)). The initial F- sorption rate was obtained as qt/t approaches zero viz:

Temkin:qe = B1 lnkT + B1lnce

h 0 = kq2

(5)

Where, qe is the quantity of F- adsorbed at equilibrium per gram of GLT (mg/g), Ce is the aqueous phase F- concentration at equilibrium, B1 = RT , T (K) is the absolute temperature, R is the universal gas constant b (8.314 J/mol), KT is the equilibrium binding constant (l/mg), and BI is defined the heat of adsorption. The Temkin constants were obtained from the slope and intercept of the linear plot of qe versus lnce. An overview of the equilibrium isotherm parameters (Table 3) showed that the fitting of the Temkin isotherm equation was the best (r2 = 0.8362). The Temkin isotherm assumes that there is rectilinear diminution of the heat of adsorption of the adsorbate molecules with surface coverage. This assumption was ascribed to the adsorbent–adsorbate interactions and the fact that the process of adsorption entails a uniform distribution of the bond energy, up to some maximum value. Premised on the r2 values (Table 3) obtained for the GLT, LT and GS in the equilibrium isotherm studies, it could be inferred that the mode of adsorbent-adsorbate interaction during the defluoridation process, when GLT was used as the reactive material, was similar to the mode of interaction, when the GS was used as the reactive material but different from the mode of interaction when the LT was

where ho (mg/g min) is the rate of initial F- uptake. The values of the pseudo second order rate constants, the initial adsorption rate and the equilibrium adsorption capacity of the GLT increased with increasing initial F- concentrations (Table 2). The fitting of the pseudo second order kinetic equation (r2 value ˃ 0.98) to the time-concentration profiles was better than the pseudo first order. In order to validate the values of the coefficients of determination (r2) obtained for the second order kinetic equation, the theoretical predictions of qt values, at different initial F- concentrations, and the corresponding experimental qt values were plotted (Fig. 4) against time, t (min). The results presented in Fig. 4 showed that for the entire period of the defluoridation process and within the initial F- concentrations studied, the second order equation adequately predicted the time-concentration profiles of the process. Premised on the underlying assumptions of the pseudo second order equation, it could be inferred that the mechanism of aqua defluoridation using the GLT must have occurred via one or the combination of chemisorption or ion exchange reaction. 4

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 4. Comparison of the theoretical and experimental values of qt versus time (min.) at varying initial F- concentration.

used as the reactive material. Aside the use of the values of the correlation coefficient (r2) to determine the best fit, the modelled qe values (i.e. the theoretical values) were also plotted with the experimental data against Ce (Fig. 5) to further ascertain the best fit amongst the equations used. The modelled equilibrium adsorption curves presented in Fig. 5 were derived from the following equations:

Langmuir = qe =

qm ka ce 1+

ka ce

=

0.437ce 1 + 0.01ce

metals oxides are amphoteric, the pHPZC value of the GLT was determined and found to be 11.8. It is noteworthy that surface interaction with cationic species is preferred at pH value above the pHPZC while interaction with anionic species is preferred at pH value below the pHPZC. Juxtaposing the implication of the pHPZC value of the GLT with the trend in the defluoridation efficiency at varying pH values, the defluoridation efficiency is expected to be similar, despite the difference in the initial F- solution pH value. This is because the range of the initial solution pH values of the process of defluoridation was below the pHPZC value of the GLT (pH = 11.8). In the present study, the trend in the defluoridation efficiency of the GLT (Fig. 6a), when the initial solution pH value was varied, agreed with the expected influence of the pHPZC value. The pH value of the reacting mixtures of varying initial solution pH values at the equilibrium time (designated as pHeq) showed that the GLT in the F- solution equalised the solution pH values. In spite of the difference in the initial pH values of the different F- solutions (between 5.01 and 9.02), the pHeq values were in the same range (between 8.4 and 8.5). In our previous studies (Oladoja et al., 2016a), the similar pHeq values obtained when GS was used as an adsorbent for fluoride sequestration from aqua system was attributed to the presence of basic oxide (CaO), as the main constituent. The leaching of the basic oxide into the aqua stream caused the solution pH levelling, to within the same pH range. Consequent upon the closeness in the pHeq values of the reacting mixtures and the defluoridation efficiencies exhibited at different initial solution pH values, it is hereby postulated that the pHeq value and the very high pHPZC were the determinants of the trend in the defluoridation efficiency and not the solution pH values a the inception of the reaction. The similar pHeq values obtained is an indication that the surface chemistry of the GLT and the hydrochemistry of F- were similar in all the reacting mixtures, in spite of the variations in the initial

(6)

1

Freudlich = qe = kf cen = 0.61ce0.9

(7) (8)

Temkin: = qe = Bln (kt ce ) = 2.24 ln(1.192ce) -

The plot in Fig. 5 represent the F removed at equilibrium from the aqua phase by the GLT against the residual F- concentrations at equilibrium. Amongst the isotherm equations studied, Temkin isotherm model gave the best fitting to the process of defluoridation (Fig. 5). Despite the best fitting of the Temkin model, the ability of this isotherm equation to effectively predict the experimental data was greater at lower F- concentration (< 10 mg/L) than at higher F- concentration ( ≥10 mg/L). The possible synergistic influence of the constituents of the GLT on the performance was appraised by comparing the value of the Langmuir monolayer adsorption capacity qm(mg/g) of the GLT with the precursors (i.e. GS and the LT) (Table 3). The highest qm(mg/g) value obtained for the GLT pointed to the synergistic effects of the constituents of the reactive material. 3.3.1. Effects of Initial solution pH value The difference in the initial F- solution pH value had no impact on the performance of the GLT (Fig. 6a). Consequent upon the fact that

Table 3 Comparison of the equilibrium isotherm parameters of the defluoridation process using the GLT and the precursors. Reactive materials

GLT GS LT

Langmuir parameters

Freundlich parameters

Temkin parameters

qm

Ka

r2

1/n

Kf

r2

B1

KT

r2

43.7 19.8 2.8

0.01 0.03 0.47

0.0194 0.037 0.9281

0.9 1.4 0.5

0.61 0.63 0.93

0.8049 0.2718 0.683

2.24 0.347 0.11

1.192 1.109 0.051

0.8362 0.9407 0.8768

5

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 5. Plots of theoretical qe and experimental qe against Ce.

studied, while the aqua phase Ca2+ concentration ranged between 12.95 mg/L and 17.98 mg/L, the presence of Fe2+ was not detectable. In order to affirm the occurrence of the formation of these insoluble metal salts, the saturation index (SI) value of CaF2, (Ksp = 3.45 × 10−11) which is the only insoluble species that could be produced from the reaction of Ca2+ with F− was determined. The value of the SI was evaluated through the determination of the solution activities of these two ionic species (i.e., Ca2+and F−), at varying initial solution pH values. The ionic activities of the aforementioned ions were determined in solution samples withdrawn at the equilibrium time. The SI value of the insoluble metal fluoride salt was calculated using Eq. (11) (Turner et al., 2005) viz:

solution pH values. On the basis of the pHeq values, the possible underlying mechanism that guided the interaction of the GLT with the aqua phase F- is posited to be similar. GLT is predominantly made up of Al, Si, Fe, Ca and other trace metals, thus the main constituents are essentially metal oxides and silicates. In aqua system, metals easily form metal-hydroxide complexes and the surficial charge is determined by protonation or deprotonation. While protonation, which gives a positive surficial charge (Eq. (9)), occurs below the pHPZC value, deprotonation, which gives a negative surficial charge (7) occurs above the pHPZC value thus:

M

M

OH + H+ =

OH + OH =

M

M

(9)

OH+2

(10)

O + H2 O

SI = log10

Since the value of the pHPZC obtained for the GLT was 11.8, the protonation and deprotonation of the GLT surface is posited to occur thus:

OH+2 + F =

M

OH+2

F

(11′)

At varying initial F- solution pH values, the values of the SI (ranged

The pH dependent aqua speciation of F−, obtained using the Medusa and Hydra Software, showed that the predominant species is F−. In the defluoridation system, since the pHeq value was below the pHPZC value, Eq. (9) predominated and the surface of the GLT became predominantly positively charged (Eq. (9)), thereby, an electrostatic attraction is created between the positively charged GLT surface and the negatively charged F- specie in the aqua system viz:

M

(activity ofCa2+) (activity of F )2 solubility product of CaF2

between 14.1 and 14.3) and ∆G (ranged between −40,773.4 and −41,608.3) obtained were within the same value range for the formation of CaF2. The SI value of FeF2 could not be determined because the ionic activity of the Fe2+ was undetectable. When the SI value of a molecular specie is estimated, a positive value reveals the occurrence of supersaturation of the aqua phase ionic species that resort in the formation of an insoluble species but a negative value indicate a dominant adsorption process (Song et al., 2002). Taken into cognizance the kinetics of precipitation, supersaturation is not an assurance for spontaneous precipitate formation. The value of the Gibbs free energy (∆G) indicates a reaction that is spontaneous when the value is below zero, in equilibrium when the value is zero and non-spontaneous when the value is greater than zero. The value of the ∆G was determined using Eq. (12) (Song et al., 2002):

(11)

The GLT is a metal rich material and the stability is greatly influenced by solution pH value. This fact was corroborated by the observed fractional dissolution of the GLT, which caused the pH levelling in the reacting mixtures. In different studies [2–5, 8, and 12], it has been proven that the fractional dissolution of the metallic fraction of metal rich materials could results in the removal of the aqua phase F- via the formation of the insoluble metal fluoride salts. Consequent upon the need to establish the role of precipitation reaction on aqua phase Fremoval, at different initial solution pH value, the aqua phase concentration of some of the metallic components (i.e. Ca2+ and Fe2+) of the GLT that are capable of forming insoluble metal fluoride salts (CaF2 and FeF2) salts were determined. Within the initial solution pH values

G=

RT IAP ln n Ksp

(12)

Where, R is the ideal gas constant (8.314 J/mol), T is the absolute temperature (K), IAP and Ksp are, respectively, the free ionic activities product and the thermodynamic solubility product of the precipitate phase and, n, is the number of ions in the precipitated compound. 6

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 6. a: Influence of initial solution pH value on the defluoridation efficiency. (b): Influence of initial solution pH value on occurrence of precipitation in the Fremoval process.

The supersaturation propensity of ionic species is defined are defined viz:

SI = log

IAP Ksp

When SI = 0, ∆G = 0, then the ionic species in aqua phase are in a state of equilibrium; when SI < 0, ∆G > 0, then the ionic species in the solution are below the saturation point, then precipitation is not feasible; when SI > 0, ∆G < 0, the ionic species in the solution is supersaturated and the precipitation of the ionic species is spontaneous. In the present study, since the ∆G values obtained (Fig. 6b) for CaF2 formation were all negative, it showed the spontaneity of formation of insoluble CaF2 by the ionic species in solution.

(13)

Therefore

G=

2.303RT SI n

(14) 7

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 7. a: Influence of ionic strength on the defluoridation efficiency. (b): Influence of interfering anionic species on the defluoridation efficiency.

3.3.2. Ionic strength The variation in the ionic strength of the F- solution negatively impacted the defluoridation efficiency of the GLT (Fig. 7a) The defluoridation efficiency reduced from a value of 9.7 mg/g, when the ionic strength was not influenced, to 6.98 mg/g at ionic strength value of 0.0085 M and gradually reduced to 2.01 mg/g at ionic strength value of 0.17 M. Premised on the principles of the pseudo second order kinetic equation, the fitting of experimental data to this equation is an indication that the mode of adsorbent-adsorbate interaction must have occurred via chemisorption. Amongst the possible mode of chemisorption are the normal covalent bond, coordinate bond and ion exchange. In view of the nature of the reactive material and the influence of ionic strength on the performance of the adsorbent, it is posited that the essential mode of adsorbate-adsorbent interaction also occurred via either ion exchange or complexation. Metallic species interact with anionic species (ligand) via coordinate bond to form complexes. Since F- is a monodentate ligand, it is capable of complex formation with the metallic species in the GLT matrix. During complexation reaction, the central metal atom or ion tend to satisfy both primary and secondary valences. The ligand that satisfies only the primary valency of the metal ion is found outside the

coordination sphere (i.e. the outer or ionization sphere) and these species are ionisable and thus can be precipitated. In the case of adsorption reaction, if an adsorbate is bound to the central metal ion via the outer-sphere mode of coordination, the interaction is greatly influenced by ionic strength. In the present context, since the defluoridation efficiency reduced when there is increase in the solution ionic strength, it is hereby posited that the mode of F- abstraction from the aqua phase by the GLT must have occurred via the outer- sphere complexation mode thus:

[M(H2 O)6]n + + nF

[M(H2 O)6]Fn

(12′)

N.B: M is any metal with high charge density that is required for complex formation Since an outer sphere complexation reaction is proposed as the underlying mechanism of F- uptake from the aqua phase, ion exchange of the outer-sphere complexed F- on the GLT surface for the aqua phase Cl- must have occurred in a high ionic strength system viz:

[M(H2 O)6]Fn + nCl 8

[M(H2 O)6]Cln + Fn

(13′)

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

University and dosed with different concentrations of F- (range = 2.20 mg/L and 28.87 mg/L). The pH, conductivity, total dissolved solids (TDS) and the F- concentration were determined in the raw and treated GW. Despite the multicomponent nature of GW, the defluoridation efficiency of the GLT was relatively high. The defluoridation efficiency of the GLT improved with increasing initial F- concentration (Table 5). The results of the equilibrium isotherm fittings showed that the Freundlich isotherm equation described the process best (r2 = 0.9856). For the synthetic water, Temkin equation was the best fit (Table 4). The difference in the isotherm equations that described the process of defluoridation in the two aqua systems was ascribed to the possibility that the multicomponent nature of the GW system influenced the mode of adsorbent-adsorbate interaction during the process of defluoridation. The difference in the magnitude of the value of the isotherm parameter, qm (mg/g), which was greater in synthetic water (43.7) than in the GW (29.76) was attributed to the multicomponent nature of the GW, which contained some interfering anionic species that impeded the F- uptake by the GLT. The diminution of the defluoridation efficiency of GLT by interfering anionic species have been reported herein in section 3.4.2. The determination of the impacts of the treatment procedure on selected physicochemical parameters of GW revealed that the values of the pH, electrical conductivity and the TDS of the treated water increased with increasing initial F- concentration (Table 5). The enhanced pH value in the treated GW was ascribed to the fractional dissolution (especially, Ca2+) of the GLT in the aqua stream, which enhanced the alkalinity of the aqua system. The fractional leaching of the GLT constituents also caused the elevation of the TDS and electrical conductivity (Table 5).

Table 4 Equilibrium isotherm parameters of the defluoridation process in real GW. Langmir

Freundlich

Temkin

qm = 29.76 Ka = 0.0085 r2 = 0.0067

1/n = 1.02 Kf = 6.07 r2 = 0.9856

B1 = 1.71 KT = 2.36 r2 = 0.9223

Table 5 Physicochemical characteristics of raw and treated GW. Parameters

Raw GW

Treated Water (at different initial F-concentrations, mg/ L) 2.2

pH Elec. Cond. (mS/m) TDS Fluoride

5.09

10.05

22.22

28.87

7.01 14.9

7.8 16.4

8.5 17.8

8.7 19.3

9.05 20.5

9.10 24.9

95 0.18

102 1.54

110 3.12

125 6.54

130 13.52

136 20.24

3.4. Anionic interference The presence of interfering anions negatively impacted the performance of the GLT to varying degrees. Albeit, all the interfering anions reduced the efficiency of the GLT to varying capacities but carbonate had greater influence than other interfering anionic specie on the defluoridation efficiency (Fig. 7b). In our previous treatise, we reported the effects of the presence of interfering anionic specie on the performance of the parent materials (i.e. LT and GS) that constituted the GLT. The presence of carbonate had greater influence on the GS than in the LT. The negative impact of carbonate on the GS was ascribed to the CaO rich nature of the adsorbent, which made it to show higher affinity for carbonate than for F-. In the GLT, since CaO is one of the active metallic specie, the preference of carbonate to F- is adduced for the reduction in the defluoridation efficiency of GLT (Fig. 7b).

3.6. Perspective on the defluoridation mechanism The comparison of the surficial architecture, diffractogram and FTIR spectra of the pristine and F- laden GLT are presented in Figs. 8–10. The uptake of F- on the GLT did not cause any major alteration in the surficial architecture of the two samples (Fig. 8). The only change observed was the transformation of the loosely held particles of the GLT to more compact particles. Significant enhancement of the intensities of the peaks, attributed to the presence of quartz, nackrite, and hematite, in the virgin GLT was observed in the F-laden GLT (Fig. 9). The enhanced peak intensities were ascribed to diffraction from the plane of fluorite

3.5. Defluoridation efficiency of GLT in GW matrix In the bid to evaluate the defluoridation efficiency of the GLT in GW contaminated with F-, GW sample was collected from a borehole in our

Fig. 8. Surfical architectre of the of the raw (A) composite material and the F-laden (B). 9

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

Fig. 9. XRD pattern of the raw composite material and the F-laden.

Fig. 10. FTIR spectra of the raw composite material and the F-laden.

(CaF2) (Augustyn et al., 1978; Aldaco et al., 2006) in the F- laden GLT. This observation is an indication of the simultaneous precipitate formation of fluorite and fluoride adsorption on the GLT surface. Enhancement of peak intensities of F- laden calcium rich materials has been reported (Oladoja and Helmreich, 2016; Nath and Dutta, 2012). The FTIR spectra (Fig. 10) of the raw and fluoride laden GLT also confirmed the possible modes of surficial interactions that occurred between GLT and the aqua phase F-. A total disappearance of the peak at 3640 cm-1 in the GLT, that was ascribed to the -OH peak of water, was observed in the F- laden GLT. Other prominent peaks identified in the virgin GLT appeared in the F- laden GLT but the diminution of the peak intensities were observed. The disappearance of the -OH peaks in the F-laden GLT was ascribed to the postulated role of the surficial -OH group in F- uptake (Oladoja and Helmreich, 2014). The XRF analysis showed that the GLT is a metal rich material, thus, surface hydrolysis of the metallic species and the process of protonation

and deprotonation played important role in the defluoridation process. The results of the kinetic fitting also indicated that the process of aqua defluoridation occurred via chemical sorption involving valency forces through sharing or exchange of electrons. Albeit, the kinetic analysis gave an insight into the possible mode of adsorbate-adsorbent interaction but the trend in the defluoridation efficiency of the GLT obtained when the influence of solution chemistry was determined provided further insight into the possible underlying mode of GLT-F- interactions. Considering the influence of pH on the performance, vis-a-vis the determination of the value of the thermodynamic driving force of a chemical reaction (i.e. ∆G), it could be posited that in addition to the surface adsorption, precipitation reaction also partook in the defluoridation process. Considering the operational pH value (i.e. pHeq), which was below the pHPZC, the formation of a positively charged surface, which promoted electrostatic mode of attraction was confirmed. The negative ∆G value, which showed the 10

Sustainable Chemistry and Pharmacy 12 (2019) 100131

N.A. Oladoja et al.

formation of CaF2 during the defluoridation process supported the contribution of precipitation to the F- removal mechanism. The enhanced intensification of the peaks in the spectra of the fluoride laden GLT (Fig. 9) also attested to the simultaneous precipitation formation and surface adsorption on the GLT. Using the trend in the effects of ionic strength on the performance of the GLT, it could be posited that adsorption, via ion exchange reaction, was also part of the mechanism of defluoridation. In an aqua system, the hydrolysis of the metallic species in the GLT matrix culminated in the formation of coordinate bond with the water molecule. The ligand derived from the water molecule satisfied only the secondary valency of the central metal ion, thus they are found within the coordination sphere of the complex. In order to satisfy the primary valency of the metal ion, the aqua phase F- is taken up to form an outer-sphere complexation with the complex ion. Consequent upon the fact that the Fposition is outside the coordinating sphere, they are liable to being exchange with Cl- in the aqua stream. This fact accounted for the observed reduction in the performance of the GLT as the ionic strength of the aqua matrix increases. The analysis of the results of the time-concentration profiles and the appraisal of the influence of solution chemistry on the performance of the GLT revealed that the defluoridation mechanism was multifaceted. Thus, the underlying mechanism is posited to be a combination of electrostatic attraction, precipitation and ion exchange (via outersphere complex formation).

and selenite on the binary oxide systems. Water Res. 43, 4412–4420. Goh, K.H., Lim, T.T., Dong, Z., 2008. Application of layered double hydroxides for removal of oxyanions: a review. Water Res. 42, 1343–1368. Gupta, K., Ghosh, U.C., 2009. J. Hazard. Mater. 161, 884–892. Honeyman, B.D., 1984. Cation and anion adsorption at the oxide/solution interface in systems containing binary mixtures of adsorbents. Stanford University, Stanford, CA. Jagtap, S., Yenkie, M.K.N., Labhsetwar, N., Rayalu, S., 2011. Defluoridation of drinking water using chitosan based microporous alumina. Microporous Mesoporous Mater. 142, 454–463. Li, L., Liu, X., Ge, Y., Xu, R., 1993. Structural studies of pillared saponite. J. Phys. Chem. 97, 10389–10393. Mustafa, S., Waseem, M., Naeem, A., Shah, K.H., Ahmad, T., Hussain, S.Y., 2010. Chem. Eng. J. 157, 18. Nath, S.K., Dutta, R.K., 2012. Acid-enhanced limestone defluoridation in column reactor using oxalic acid. Process Saf. Environ. Prot. 90, 65–75. Norton, B.T., Esposito, J.J., 1995. The New Encyclopedia of Britannica, 15th ed. Encyclopedia Britannica Inc, Chicago, pp. 179. Oladoja, N.A., Aliu, Y.D., 2009a. Snail shell as coagulant aid in the alum precipitation of malachite green from aqua system. J. Hazard. Mater. 164, 1494–1502. Oladoja, N.A., Aliu, Y.D., 2009b. Snail shell as coagulant aid in the alum precipitation of malachite green from aqua system. J. Hazard. Mater. 164, 1496–1502. Oladoja, N.A., Helmreich, B., 2014. Batch defluoridation appraisal of aluminium oxide infused diatomaceous earth. Chem. Eng. J. 258, 51–61. Oladoja, N.A., Helmreich, B., 2016. Calcium aluminate–diatomaceous earth composite as a reactive filter material for aqua defluoridation. J. Water Process Eng. 9, 58–66. Oladoja, N.A., Aliu, Y.D., Ofomaja, A.E., 2011. Evaluation of Snail shell as coagulant aid in the alum precipitation of aniline blue from aqua system. Environ. Technol. 32 (6), 639–652. Oladoja, N.A., Drewes, J.E., Helmreich, B., 2015b. Assessment of fixed bed of aluminum infused diatomaceous earth as appropriate technology for groundwater defluoridation. Sep. Purif. Technol. 153, 108–117. Oladoja, N.A., Helmreich, B., Bello, H.A., 2016a. Towards the development of a reactive filter from green resource for groundwater defluoridation. Chem. Eng. J. 301, 166–177. Oladoja, N.A., Chen, S., Drewes, J.E., Helmreich, B., 2015a. Characterization of granular matrix supported nano magnesium oxide as an adsorbent for defluoridation of groundwater. Chem. Eng. J. 281, 632–643. Oladoja, N.A., Hu, S., Drewes, J.E., Helmreich, B., 2016b. Insight into the defluoridation efficiency of nano magnesium oxide in groundwater system contaminated with hexavalent chromium and fluoride. Sep. Purif. Technol. 162, 195–202. Oladoja, N.A., Liu, Y., Drewes, J.E., Helmreich, B., 2016c. Preparation and characterization of a reactive filter for groundwater defluoridation. Chem. Eng. J. 283, 1154–1167. Oladoja, N.A., Seifert, M.L., Drewes, J.E., Helmreich, B., 2017. Influence of organic load on the defluoridation efficiency of nano-magnesium oxide in groundwater. Sep. Purif. Technol. 174, 116–125. Oladoja, N.A., Bello, G.A., Obisesan, S.V., Helmreich, B., Ogunniyi, J.A., Daramola, O.A., Bello, H.A., Anthony, E.T., Saliu, T.D., 2019. Insight into the defluoridation efficiency of Lateritic soil. Environ. Prog. Sustain. Energy. https://doi.org/10.1002/ep. Prathna, T.C., Sharma, S.K., Kennedy, M., 2017. Development of iron oxide nanoparticle adsorbents for arsenic and fluoride removal. Desal. Wat. Treat. 67, 187–195. Prathna, T.C., Sitompul, D.N., Sharma, S.K., Kennedy, M., 2018. Synthesis, characterization and performance of iron oxide/alumina-based nanoadsorbents for simultaneous arsenic and fluoride removal. Desalin. Water Treat. 104, 121–134. Rathore, V.K., Dohare, D.K., Mondal, P., 2016. Competitive adsorption between arsenic and fluoride from binary mixture on chemically treated laterite. J. Environ. Chem. Eng. 4, 2417–2430. Song, Y., Hahn, H.H., Hoffmann, E., 2002. Effects of solution conditions on the precipitation of phosphate for recovery: a thermodynamic evaluation. Chemosphere 755 (48), 1029–1034. Sun, Y., Fang, Q., Dong, J., Cheng, X., Xu, J., 2011. Removal of fluoride from drinking water by natural stilbite zeolite modified with Fe(III). Desalination 277, 121–127. Tripathy, S.S., Kanungo, S.B., 2005. J. Colloid Interface Sci. 284, 30. Turner, B.D., Binning, P.J., Stipp, S.L.S., 2005. Fluoride removal by calcite: evidence for fluoride precipitation and surface adsorption. Environ. Sci. Technol. 39, 9561–9568. Xu, X., Li, Q., Cui, H., Pang, J., Sun, L., An, H., Zhai, J., 2011. Adsorption of fluoride from aqueous solution on magnesia-loaded fly ash cenospheres. Desalination 272, 233–239. Xu, Y., Axe, L., 2005. J. Colloid Interface Sci. 282, 11. Zhang, G.S., Qu, J.H., Liu, H.J., Liu, R.P., Wu, R.C., 2007. Water Res. 41, 1921–1928. Zhang, N., Lin, L.S., Gang, D., 2008. Adsorptive selenite removal from water using iron coated GAC adsorbents. Water Res. 42, 3809–3816. Zhang, Y., Yang, M., Dou, X.M., He, H., Wang, D.S., 2005. Environ. Sci. Technol. 39, 7246–7253. Zhang, Z., Tan, Y., Zhong, M., 2011. Defluoridation of wastewater by calcium chloride modified natural zeolite. Desalination 276, 246–252.

4. Conclusion

• The multi-metallic nature of the GLT enhanced the defluoridation efficiency • The pH value and the high pH values were the determinants of eq

• • • •

PZC

the trend in the defluoridation efficiency of the GLT and not the pH value at the inception of the reaction. The influence of ionic strength on the performance of GLT explicated the role of ion exchange, via outer sphere complex formation. The SI values that is greater than zero and the ∆G values that is less than zero indicated the formation of insoluble metallic fluoride during the defluoridation process Relative to the value of qm (mg/g) in synthetic feed water, the multicomponent nature of the GW reduced the value in GW system The fractional dissolution of the GLT in aqua system enhanced the value of the TDS, pH, and electrical conductivity of GW

Acknowledgement The Authors are grateful to the Alexander von Humboldt Foundation for the award of Equipment Grant to Oladoja N.A. to undertake this research work. The technical support offered by Mr. A. S. Adeyemo, the Technologist in the Department of Chemical Sciences, Adekunle Ajasin University is appreciated. References Aldaco, R., Garea, A., Irabien, A., 2006. Fluoride recovery in a fluidized bed: crystallization of calcium fluoride on silica sand. Ind. Eng. Chem. Res. 45, 796–802. Augustyn, W., Dzieqielewska, M., Kossuth, A., Librant, Z., 1978. Studies of the reaction of crystalline calcium carbonate with aqueous solutions of NH4F, KF and NaF. J. Fluor. Chem. 12, 281–292. Chan, Y.T., Kuan, W.H., Chen, T.Y., Wang, M.K., 2009. Adsorption mechanism of selenate

11