Scholarly article on topic 'Evaluating the potential of a Nigerian soil as an adsorbent for tartrazine dye: Isotherm, kinetic and thermodynamic studies'

Evaluating the potential of a Nigerian soil as an adsorbent for tartrazine dye: Isotherm, kinetic and thermodynamic studies Academic research paper on "Chemical sciences"

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{Tartrazine / "Anionic dye" / Sorption / Soil}

Abstract of research paper on Chemical sciences, author of scientific article — M.O. Dawodu, K.G. Akpomie

Abstract The release of toxic tartrazine dye from industrial effluent into the environment is of public health concern. This study therefore aimed at the removal of tartrazine from solution using Nigerian soil as a low cost potential sorbent. The sorbent was characterized by the Fourier transform infrared spectrophotometer and Scanning electron microscope. Batch sorption methodology was used to investigate the effect of pH, adsorbent dose, dye concentration, contact time and temperature. The sorbent recorded a Brunauer, Emmett and Teller surface area of 9.8m2/g and pH point of zero charge of 5.8. Optimum sorption was achieved at pH 2.0, contact time of 120min, adsorbent dose of 0.05g and tartrazine concentration of 50mg/L. Equilibrium isotherms were analyzed by the Langmuir, Freundlich, Scatchard and Flory-Huggins isotherm models. The pseudo-first-order, pseudo-second-order, Elovich and Bangham models were used for kinetic analysis. Thermodynamics revealed a spontaneous, feasible and endothermic sorption process. The soil was found to be suitable as a low cost sorbent for tartrazine from contaminated solution.

Academic research paper on topic "Evaluating the potential of a Nigerian soil as an adsorbent for tartrazine dye: Isotherm, kinetic and thermodynamic studies"

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ORIGINAL ARTICLE

Evaluating the potential of a Nigerian soil as an adsorbent for tartrazine dye: Isotherm, kinetic and thermodynamic studies

M.O. Dawodu a, K.G. Akpomie b*

a Department of Chemistry and Industrial Chemistry, Bowen University, Iwo, Osun State, Nigeria b Projects Development Institute (PRODA), Federal Ministry of Science and Technology, Enugu, Nigeria

Received 20 May 2015; revised 4 August 2016; accepted 7 August 2016

KEYWORDS

Tartrazine; Anionic dye; Sorption; Soil

Abstract The release of toxic tartrazine dye from industrial effluent into the environment is of public health concern. This study therefore aimed at the removal of tartrazine from solution using Nigerian soil as a low cost potential sorbent. The sorbent was characterized by the Fourier transform infrared spectrophotometer and Scanning electron microscope. Batch sorption methodology was used to investigate the effect of pH, adsorbent dose, dye concentration, contact time and temperature. The sorbent recorded a Brunauer, Emmett and Teller surface area of 9.8 m2/g and pH point of zero charge of 5.8. Optimum sorption was achieved at pH 2.0, contact time of 120 min, adsorbent dose of 0.05 g and tartrazine concentration of 50 mg/L. Equilibrium isotherms were analyzed by the Langmuir, Freundlich, Scatchard and Flory-Huggins isotherm models. The pseudofirst-order, pseudo-second-order, Elovich and Bangham models were used for kinetic analysis. Thermodynamics revealed a spontaneous, feasible and endothermic sorption process. The soil was found to be suitable as a low cost sorbent for tartrazine from contaminated solution. © 2016 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

The increasing use of dyes by the paper, pulp, textile, leather, food and drug industries has led to the release of effluents containing colored substances (dyes) into the environment. Most dyes are resistant to light, temperature and oxidizers, non-biodegradable, bio-accumulate in living organisms and toxic at certain levels [1]. Tartrazine is an important dye used at very

* Corresponding author.

E-mail address: kovoakpmusic@yahoo.com (K.G. Akpomie).

Peer review under responsibility of Faculty of Engineering, Alexandria

University.

low concentrations for drugs especially for the shells of medicinal capsules, syrups, cosmetics and food additives. However it is very toxic when present in high concentrations and is highly soluble in water, which makes it difficult to identify its presence in industrial effluents [2]. High concentrations of tar-trazine in humans can cause behavioral problems, such as asthma, migraines, eczema, thyroid cancer, lupus, hyperactiv-ity and infertility [3]. The removal of such dye from effluent before discharge into water bodies is therefore important. Different methods have been utilized for dye removal which includes ozonation, microbial decomposition, coagulation/ flocculation, photo-catalytic decolorization, adsorption and sono-chemical method [1]. Adsorption has been found to be

http://dx.doi.org/10.1016/j.aej.2016.08.008

1110-0168 © 2016 Faculty of Engineering, Alexandria University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

the most effective among the methods and activated carbon the most effective adsorbent [4]. However, the use of activated carbon is expensive which limits its widespread use, encouraging the use of cheaper alternative adsorbents such as agricultural wastes [5], sawdust [6], fly ash [7] and clay [8]. Very few works have been performed for the removal of dyes especially tartrazine from solution using soil as adsorbent [7]. This study therefore investigates the potential of soil of Nigerian origin for the removal of tartrazine from solution. The soil is present in Osun state in excess amount and can be utilized as an alternative low-cost adsorbent for sorption of tartrazine if found to be effective. The soil was utilized without any treatment or modification in order to keep the process cost low. Adsorption data were evaluated to determine the conditions of maximum sorption along with relevant isotherm, kinetic and thermody-namic equations.

2. Experimental

2.1. Characterization and sorption

The soil sample was obtained from Bowen University, Iwo, Osun state, Nigeria, and utilized without any purification. The sample was sundried for 5 days, then dried in an oven at 105 0C for 6 h, crushed and then passed through a mesh sieve of size 100 im to obtain the prepared adsorbent.

The chemical composition of the adsorbent was determined by the Atomic Absorption Spectrophotometer (AAS) (Buck scientific model 2010VGP) after digestion of the sample with nitric and hydrofluoric acid. The pH point of zero charge (pHpzc) of the soil was determined by the method described in our previous work [9]. The ammonium acetate method was used to determine the Cation Exchange Capacity (CEC) [10]. Pore properties and BET surface area were assessed via N2 adsorption-desorption isotherms with a micromeritics ASAP 2010 model analyzer. The Fourier Transform Infrared (FTIR) spectra of the soil were obtained by the FTIR spectrophotometer (Shimadzu FTIR 8400s), while the surface morphology was assessed with the Scanning Electron Microscope (SEM) (Hitachi S4800 model).

All the reagents utilized in this study were of analytical grade, obtained from sigma-Aldrich and used without any purification. Stock solution of the anionic dye tartrazine (Acid yellow 23) with molecular formula C16H9N4Na3O9 was prepared by dissolving appropriate amount of the dye in distilled water to obtain a concentration of 500 mg/L. Lower concentrations of tartrazine with concentrations ranging from 50 to 250 mg/L were prepared from the stock solution by serial dilution. The pH of the solutions was adjusted to values from 2 to 8 by the minute addition of 0.1 M NaOH or 0.1 M HCl when required.

Batch adsorption experiment was performed by adding 0.05 g of the adsorbent to 40 ml of a given solution in a pre-treated glass bottle at room temperature of 300 K. The influence of pH (2.0-8.0), initial tartrazine concentration (50, 100, 150, 200 and 250 mg/L), adsorbent dose (0.01, 0.02, 0.03, 0.04 and 0.05 g), contact time (10, 20, 30, 40, 50, 60, 90, 120, 150, 180, 300 min) and temperature (300, 313, 323 K) were evaluated. The bottles were placed in a thermo-stated water bath for temperature regulation when the effect of temperature was studied. In order to evaluate the effect of a particular

parameter, that parameter was varied while others were kept constant at the optimum conditions of pH 2.0, contact time 120 min, tartrazine concentration 50 mg/L. At the end of a given contact time of sorption the solutions were centrifuged for 15 min at 5000 rpm. The UV-Visible spectrophotometer operating at 426 nm was then used to determine the equilibrium concentration Ce (mg/L) of tartrazine in the supernatant. The following equations were utilized in the calculation of percentage adsorption of tartrazine and the uptake capacity of the adsorbent for tartrazine respectively:

C — C

% Sorption = x 100

v(C, - Ce)

(1) (2)

where Ci (mg/L) is the initial concentration of tartrazine in solution, qe (mg/g) is the uptake capacity, m (g) is the adsorbent dose and v (L) is the volume of solution used.

2.2. Adsorption Isotherm

The equilibrium adsorption data were evaluated by the Lang-muir, Freundlich, Scatchard and Flory-Huggins isotherm models. The Langmuir isotherm describes a monolayer adsorption on a heterogeneous adsorbent surface and the linear form of the equation is expressed as [11] follows:

qe q-LKL q-

where qL (mg/g) represents the maximum monolayer adsorption capacity and KL (L/mg) corresponds to the Langmuir adsorption constant. A dimensionless constant equilibrium parameter (RL) gives an appropriate description of the Langmuir isotherm and is represented as [11] follows:

[1 + KlC, ]

The Rl value classifies the adsorption process as favorable (0 < Rl < 1), irreversible (RL = 0), linear (RL = 1) and unfavorable (Rl > 1).

The Freundlich isotherm describes a multilayer adsorption on a heterogenous adsorbent surface and the linear form of the equation is given as [12] follows:

log qe = log KF + [1/w] log Ce

where KF (L/g) and n are the Freundlich constants corresponding to the adsorption capacity and intensity, respectively. Values of n between 1 and 10 indicate a favorable adsorption process [13].

The Scatchard isotherm was applied to verify the homogenous or heterogonous nature of the adsorbent in comparison with the data obtained from the Langmuir and Freundlich isotherm. The linear form of the Scatchard isotherm also called independent site oriented model is expressed as [14] follows:

= qsb - qeb

where qS (mg/g) and b (L/mg) represent the Scatchard isotherm sorption parameters. If a straight line is obtained from the Scatchard plot of qe/Ce against qe, then the adsorbent presents only one type of binding site (Homogenous surface), but

if a deviation from linearity is obtained, then the adsorbent has more than one type of binding site (heterogonous surface) [14].

The Flory-Huggins isotherm was used to evaluate the degree of surface coverage characteristics of adsorbate on the adsorbent and is expressed in its linear form by the following equation [15]:

log(h/C,) = log KFH + nFH log(1 - h) (7)

where h = (1 — Ce/Ci) is the degree of surface coverage, KFH (L/g) and nFH represent the Flory-Huggins equilibrium isotherm constant and model exponent, respectively. A linear plot of log (h/G) versus log (1 — h) indicates the application of this isotherm to the sorption process.

2.3. Kinetic modeling

Adsorption kinetic mechanism was evaluated by the application of the Pseudo-first-order, Pseudo-second-order, Elovich and Bangham kinetic rate equations.

The Pseudo-first-order or Lagergren kinetic model is based on the assumption that one dye molecule is sorbed unto one sorption site on the adsorbent and the linear form of the equation is given as [16] follows:

log(qe — q) = log qe — (K,t/2.303) (8)

where qt (mg/g) corresponds to the adsorption capacity at a given time t (min) and K (min—*) is the Pseudo-first-order rate constant.

The pseudo-second order model is based on the assumption that the rate of sorption sites occupation is proportional to the square of the number of unoccupied sites and is expressed in its linear form as [17] follows:

t/qt = 1/K2q2 + t/qe (9)

where K2 (g/mg/min) is the rate constant of pseudo-second order sorption and h = K2qe2 is the initial sorption rate (mg/ g/min).

The Elovich equation was applied in the kinetic analysis and is expressed in its linear form as [16] follows:

qt = [1/b ln(ab) + [1/b] ln t (10)

where b (g/mg) is the Elovich constant representing the surface coverage and activation energy for chemisorptions, and a (mg/g min) is the initial sorption rate constant.

The linear form of the Bangham kinetic model equation was also applied and expressed as [17] follows:

log log[C,/(C, — qtm)] = log(Kom/2.303V) + ab log(t) (11)

where V (ml) is the volume of solution used, and KO (g) and aB (<1) are the Bangham kinetic constants. This model was applied by a linear plot of log log [C!-/(C!- — qtm)] versus log(t) and the constants aB and KO were evaluated from the slope and intercept, respectively.

2.4. Thermodynamic evaluation

Thermodynamic parameters of sorption such as Gibbs free energy change (AG0), Enthalpy change (AH0) and Entropy change (AS0) were determined to assess the spontaneity, feasibility and heat change of the sorption process. The following thermodynamic equations were applied [18]:

AG° = -RTln Kc (12)

ln Kc = -(AH°/RT) + (AS°/R) (13)

where Kc = Ca/Ce is the distribution coefficient, Ca (mg/L) and Ce (mg/L) correspond to the concentration of tartrazine adsorbed and that in solution at equilibrium, respectively. R (8.314 J/mol K) is the ideal gas constant and T(K) is the absolute temperature at which the adsorption was conducted.

3. Result and discussion

3.1. Characterization of sorbent

The chemical analysis of the soil revealed the following composition: SiO2 (69.6%), Al2O3 (16.4%), Fe2O3 (1.94%), Na2O (1.01%), K2O (2.64%), MgO (1.84%), CaO (1.04%) and loss on ignition of 4.32%. This indicated silica as the major component. A CEC of 31.2 mEq/g and pHpzc of 5.8 were obtained for the adsorbent, which was very close to that of 36 mEq/g and 5.5, respectively obtained by Das and Mondal [13] for calcareous soil. At pH values lower than the pHpzc the surface of the adsorbent is positively charged favoring anionic species adsorption, while at pH values higher than the pHpzc, the adsorbent is negatively charged favoring adsorption of cationic species [16]. This suggests that the sorption of the anionic dye (tartrazine) will be optimum at pH values below 5.8. A BET surface area of 9.8 m2/g was obtained for the adsorbent, which was within the range of 1.8-38.5 reported by Sangiumsak and Punrat-tanasin [19] for various soils. An average pore diameter and total pore volume of 32.41 A and 0.00794 cm3/g, respectively were obtained. This indicates that the soil has pores with good width but poor pore volume suggesting that much of the sorption on this sorbent would mainly be by surface reaction mechanism rather than by pores. It also suggests that the adsorbent might not be very useful in the removal of non-ionic or molecular pollutants from aqueous solutions [20].

The functional groups on the adsorbent were observed by the FTIR spectra as shown in Fig. 1. Absorption bands at 3441-3697 cm-1 indicate the presence of OH— groups of alcohols, while peaks at 2920 and 2852 cm-1 correspond to the C—H stretching vibration [13]. Absorption bands at 1797 and 1631 cm-1 represent the C=O group and peak at 1427 cm-1 corresponds to the C=C of alkenes [21]. Bands of absorption at 1008-1105 cm-1 represent the C—O stretching vibration, while peaks at 694 and 538 cm-1 correspond to the C—Cl and C—Br stretching vibrations respectively [13]. Furthermore, porosity of an adsorbent is desirable for effective sorption of the adsorbate. The SEM morphology reveals useful information about adsorbent porosity. Fig. 2 shows the SEM morphology of the soil at different magnifications of 3500 and 2000. It is observed from the SEM images that the adsorbent is not porous in nature although some level of porosity was revealed at a magnification of 2000 as seen in Fig. 2b, which indicates that the removal of tartrazine would mainly occur via surface phenomenon rather than by pores as stated earlier.

3.2. Effect of some variables on sorption 3.2.1. Effect of pH

The pH of a solution determines the degree of ionization of the adsorbate and the adsorbent surface charge and soil is very

ARTICLE IN PRESS

4 M.O. Dawodu, K.G. Akpomie

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2920.32-. 2852.81 ss 1 K g 1

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Figure 1 FTIR spectra of the soil.

Figure 2 SEM morphology of the soil at magnifications of (a) 3500 and (b) 2000.

important to be determined for any sorption process [16]. Fig. 3a shows the influence of solution pH on the adsorption of tartrazine onto the soil. A decrease in adsorption with increase in solution pH from 2.0 to 8.0 was observed.

Optimum sorption of tartrazine was obtained at pH 2.0, so this pH was chosen and utilized in all subsequent experiments. Due to the fact that tartrazine is an anionic dye, it means it will be attracted to a positively charged adsorbent surface. At pH values lower than the pHpzc (5.8) of the soil, the surface is positively charged favoring the adsorption of anionic species [22]. This must have accounted for the higher sorption recorded at low pH values. Also, at low pH values, fewer OH- ions are available in solution to compete with the anionic dye for the active sites of the adsorbent. As pH increases, the number of OH— ions in solution increases thereby leading to increased competition between the dye and OH— ions for the active sites resulting in less dye removal. Similar results have been reported by other workers in the sorption of tartrazine [1,2,23].

3.2.2. Effect of initial concentration

The initial concentration of adsorbate in solution is also a very important factor affecting the amount of the adsorbate adsorbed by the adsorbent [16]. Therefore it was determined in this study as shown in Fig. 3b. As observed, a decrease in the percentage adsorption of tartrazine by the soil with increase in initial tartrazine concentration was recorded. This is due to the fact that the adsorbent dose is constant and thus has a fixed number of active sites; therefore, at low concentration of dye, more molecules can be adsorbed by the abundant active sites of the soil but the active sites get saturated at higher dye concentration, leading to less removal of tartrazine. A Similar result was reported on the sorption of methylene blue on low cost activated carbon [24]. An initial tartrazine concentration of 50 mg/L was chosen in this study due to optimum removal attainment at this dye concentration.

3.2.3. Effect of adsorbent dose

The effect of adsorbent dose on the percentage adsorption of tartrazine onto the soil is shown in Fig 3c. An increase in adsorption of dye with increase in adsorbent dose from 0.01 to 0.05 g was obtained. This is due to the fact that since the

295 300 305 310 315 320 325 Temperature (K)

Figure 3 Percentage sorption of tartrazine unto the soil as a function of (a) pH, (b) Initial tartrazine concentration, (c) Sorbent dosage, (d) Contact time and (e) Temperature.

tartrazine concentration in solution is fixed at 50 mg/L, increasing the adsorbent dose results in an increase in the number of active sites which results in more removal of tartrazine molecules on the additional active sites. It has also been attributed to an increase in surface area of the adsorbent and decrease in electrostatic potential near the solid surface which favors adsorbate-adsorbent interactions [25]. An adsorbent dose of 0.05 g was chosen as optimum sorption of tartrazine by the soil was recorded.

3.2.4. Effect of contact time

A good adsorbent must not only have a high adsorption potential but also a fast rate of removal. In this regard the effect of contact time on the sorption of tartrazine by the soil

was determined as shown in Fig. 3d. The adsorption was rapid initially and then gradually diminished to attain an equilibrium beyond which there was no change in the rate of sorption. Equilibrium was achieved around 120 min and so this time was utilized in this study in order to ensure optimum adsorption and equilibrium attainment. The fast adsorption at the initial stage is due to the availability of abundant vacant active sites on the adsorbent. The sorption rapidly occurs and is normally controlled by diffusion process from the bulk to the surface of the adsorbent. In the later stages, the sorption is likely an attachment controlled process due to less available active sites and eventually the saturation of the active sites at equilibrium [13]. Similar findings have been reported by other investigators [1,2,23,26,27].

3.2.5. Effect of temperature

The influence of temperature on the adsorption of tartrazine from solution onto the soil is presented in Fig. 3e. As observed, an increase in adsorption with increase in solution temperature from 300 to 323 K was obtained. This suggests that high temperature favors the process, indicating an endothermic adsorption process. The increase has been attributed to pore size enlargement of adsorbents with increase in temperature. It is also due to an increase in the kinetic energy of dye molecules with temperature, prompting their diffusion to the surface of the adsorbent [18]. Similar results have been reported by other researchers in the adsorption of dyes [28,29].

3.3. Isotherm, kinetics and thermodynamics

Equilibrium isotherms provide the basic physico-chemical data for evaluating the applicability of the sorption process as a unit operation. It also provides information on the affinity between the adsorbate and adsorbent. The equilibrium isotherm parameters obtained are presented in Table 1. The value of the linear regression (R2) was used to evaluate the best fitted model. The closer R2 is to one, the best fit the isotherm model. From Table 1, it is seen that the Langmuir model (R2 = 0.982) did not represent the sorption process properly as the Freundlich model (R2 = 0.999). However, the Langmuir RL values were in the range of 0.091-0.333 which indicates a favorable adsorption of tartrazine unto the soil. The Freundlich model showed a very good fit to the adsorption data indicating that the process involves a multilayer adsorption unto a heterogenous soil surface and not a monolayer homogenous sorption of the Langmuir isotherm. Also, the value of n (2.29) showed a favorable adsorption process of tar-trazine on the adsorbent, which corroborates the RL values of the Langmuir isotherm. Furthermore, to verify whether the adsorbent is made up of more than one type of active site (heterogeneous surface) as revealed by the Freundlich model, the Scatchard plot analysis was applied. The R2 of 0.871 of the Scatchard analysis showed a deviation from linearity

Table 2 Comparision of maximum adsorption capacity of different adsorbents for tartrazine.

Adsorbent qe (mg/g) Reference

Activated carbon 18,581 [30]

Coconut husk carbon 3366 [30]

Chitosan 350 [23]

Nigerian soil 83.33 This study

Chitin 30 [23]

Sawdust 4.71 [1]

Romanian Soil 1.82 [26]

Hen Feather 0.000064 [27]

De-oiled soya 0.0000212 [2]

Bottom ash 0.0000101 [2]

indicating a heterogeneous adsorbent surface and this supports the good fit presented by the Freundlich model [14]. The R2 of 0.991 presented by the Flory-Huggins model was high but lower than that of the Freundlich, therefore the later was found to give the best fit and description for the sorption of tartrazine onto the soil. The maximum adsorption capacity of tartrazine obtained in this study was compared with that of other adsorbents in literature as shown in Table 2. The Nigerian soil was found to give a higher adsorption of tar-trazine than most adsorbents in the literature indicating the potential of this soil as an effective low cost adsorbent.

The kinetic mechanism of sorption was analyzed by the Pseudo-first-order, Pseudo-second-order, Elovich and Bang-ham rate equations and the model constants which are presented in Table 3. It is observed from R2 that the Pseudofirst-order and Pseudo-second-order models gave close and good fit to the sorption process. The fit of the Pseudo-first order model implies that sorption occurs due to a driving force generated by a concentration difference of the external coefficient mass transfer, corresponding to physisorption [31]. On the other hand, the fit of the Pseudo-second order model implies the involvement of internal diffusion mechanism and considers that adsorption is of a chemical nature [32]. The

Table 1 Equilibrium Isotherm parameters for

the sorption of tartrazine unto the soil.

Isotherm model Value

Langmuir

qL (mg/g) 83.33

Kl (L/mg) 0.04

R2 0.982

Freundlich

Kf (L/g) 6.53

N 2.29

R2 0.999

Scatchard

qs (mg/g) 117.7

b (L/mg) 0.03

R2 0.871

Flory-Huggins

KFH 0.001

nFH 0.76

R2 0.991

Table 3 Kinetic model parameters for

the sorption of tartrazine unto the soil.

Kinetic model Value

Pseudo-first-order

qe (mg/g) 56.63

K (min-1) 0.018

R2 0.977

Pseudo-second-order

h (mg/g min) 1.432

K2 (g/mg min) 3.06 x 10-4

qe (mg/g) 68.42

R2 0.978

Elovich

a (mg/g min) 1.13

ß (g/min) 0.072

R2 0.991

Bangham

aB 0.263

Ko (g) 2.84

R2 0.799

Table 4 Thermodynamic parameters for the sorption of tartrazine on soil.

Temp. (K) AG° (kJ/mol) AH° (kJ/mol) AS° (J/mol K)

300 -5.425 60.101 217.7

313 -6.396

323 -8.409

close and good fit presented by both models might indicate the occurrence of several processes occurring simultaneously in the sorption. However, the Elovich equation was found to present the best fit to the sorption experiment as revealed by its highest R2 value of 0.991. Several researchers have found the Elovich model to be more suitable in the description of kinetic mechanism of sorption [33,34]. The good fit of the Elovich model to the sorption confirms clearly that physisorption is not the rate controlling mechanism but its involvement cannot be ruled out due to the good fit obtained by the Pseudo-first-order model. The R2(0.799) presented by the Bangham model showed a deviation from linearity indicating that the diffusion of tar-trazine into the pores of the adsorbent was not the rate controlling mechanism [17]. This signifies that film diffusion and pore diffusion were both important to different extents in the adsorption process [17]. This supports our suggestion that as a result of the poor pore volume and porosity of the soil as revealed by the BET and SEM analysis, much of the sorption will mainly be by surface phenomenon rather than by pores as stated earlier.

Furthermore, from thermodynamic analysis as presented in Table 4, a Positive AH0 value indicates an endothermic sorption which is supported by the increase in sorption with temperature increase. Also, positive AS0 value of 217.7 J/mol K indicates an increase in randomness at the solid solution interface [35]. The sorption was found to be spontaneous and feasible as negative values of AG0 were obtained at all temperatures. Finally, AH0 value from 2.1-20.9 kJ/mol to 80-200 kJ/mol indicates physisorption and chemisorptions, respectively [18]. The AH0 value of 60.101 kJ/mol was higher than 20 but less than 80 kJ/mol indicating the participation of both physical and chemical sorption mechanisms [36]. This supports our kinetic deduction which suggests the participation of several mechanisms in the overall sorption of tartrazine on the soil. Such physicochemical adsorption has been reported in other studies [36,37].

4. Conclusion

The Nigerian soil was found to have a higher adsorption of tartrazine than most sorbents in the literature and can therefore be utilized as an alternative low-cost adsorbent for removal of the hazardous dye tartrazine from aqueous solution.

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