J IRAN CHEM SOC (2012) 9:373–382 DOI 10.1007/s13738-011-0047-2

ORIGINAL PAPER

Potential application of termite mound for adsorption and removal of Pb(II) from aqueous solutions N. Abdus-Salam • A. D. Itiola

Received: 18 February 2011 / Accepted: 26 May 2011 / Published online: 10 January 2012 Ó Iranian Chemical Society 2012

Abstract The morphological and mineralogical composition of a termite mound from Ilorin, Nigeria was investigated with a view to understand its sorption properties. The termite hill soil was subjected to some spectroscopic analyses such as X-ray fluorescence (XRF) and Scanning Electron Microscopy. The XRF results revealed that the adsorbent contains a large fraction of Silicon, Iron and Aluminium minerals. The organic matter (OM) content expressed as percentage carbon was 3.45% while the high value of cation exchange capacity of 14.0 meq/100 g is in agreement with high percentage OM, which signifies high availability of exchangeable ions. The maximum Pb(II) adsorption capacity of the mound was found to be 15.5 mg/g. Batch adsorption experiments were carried out as a function of contact time, ionic strength and pH. Maximum and constant adsorption was observed in the pH range of 2–5.5. The experimental results of Pb(II) adsorption were analyzed using Langmuir, Freundlich, and Temkin isotherms. The Langmuir and Temkin isotherms were found to fit the measured sorption data better than Freundlich. The constants obtained from the Langmuir model are maximum sorption value, Qm = 18.18 and Langmuir energy of adsorption constant, b = 0.085, while the constants of the Freundlich model are the intensity of adsorption constant, n = 0.134, and maximum diffusion constant, Kf = 1.36. The adsorption data for Pb(II) was found to fit well into the pseudo-second order model. Desorption experiment was conducted using different concentrations of leachant and this was repeated three times to determine the life span of the adsorbent. It was observed that 0.2 M HCl had the highest desorption efficiency for reuse. N. Abdus-Salam (&)  A. D. Itiola Department of Chemistry, University of Ilorin, P.M.B.1515, Ilorin, Nigeria e-mail: [email protected]

Keywords Termite mound  Cation exchange capacity (CEC)  Adsorption isotherms  Pseudo-first and second order models  Desorption

Introduction Termites are social ants that exert significant influence on the physical and chemical properties of tropical and sub-tropical soils [1–3]. The feeding habit, the food processing and mound construction operations introduce significant modifications to the soil on which the mound is built [4, 5]. Studies have shown that termite mounds in tropical soil like Nigeria may have lower or higher values for exchangeable Ca, Mg and K, effective cation exchange capacity (ECEC), water holding capacity, and water infiltration rates [6]. African farmers are familiar with the practice of applying termite mounds on their farms as it contains required nutrients for plants growth [7]. In a related research, termite-mound soil was reported to contain as much as 20% of the total nitrogen as inorganic nitrogen, an average organic carbon content of 9.3% and 2.25 times more total P than the adjacent soils [8]. Lead is one of the major industrial pollutants of human concern as air-particulate or in dilute aqueous medium. The metal is associated with industries such as paints, pigments, batteries, ceramic glazes, metal products, petroleum and cosmetic products, cable sheathing and ammunition production [9]. The divalent form [Pb(II)] is the stable ionic species of lead. The metal has the tendency to form insoluble compounds with OH-, CO32-, and S2-. The ratio of suspended form to the soluble form of lead may vary from 4:1 in rural streams to 27:1 in urban streams [10]. Different methods for the removal of heavy metals from aqueous solutions have been proposed [11–14]. These include solvent extraction, ion exchange filtration and

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membrane separation, reverse osmosis, chemical precipitation and coagulation. These methods are known for one shortcoming or the other, ranging from incomplete removal, high energy consumption, reagents cost, disposal of large volume of organic solvents and inefficiency when the metal concentrations are \10 mg/l [9]. Adsorption process is a promising alternative technique that is free from the shortcomings of the earlier techniques. Adsorbents from natural or modified materials and synthetic origin are subject of recent research efforts. These adsorbents include activated carbon from agricultural products [15–17], clay and clay materials [11, 18, 19], and oxides of iron [20]. Modifications of soil physical and chemical properties by termite’s mounds and mechanical properties have been reported, however, only limited studies have involved application of the mounds in the decontamination of metal polluted effluents. The objective of this study was to characterize termite mound soils and determine the sorption characteristics. Langmuir isotherm equation was employed to quantify the adsorption equilibrium. The effects of solution pH, ionic strength and variation of time on Pb(II) adsorption were examined.

Materials and methods Collection of samples and pretreatment The termite mound soil was collected within the compound of Ilorin Grammar School, Ilorin, Kwara State. A sample of about 2 kg was collected and air-dried in the laboratory at room temperature for 5 days. The sample was then placed in an oven set at 60 °C for 10 h before crushed in a mortar, grounded and thoroughly mixed by repeated (more than 10 times) conning and quartering. The grounded mound was then sieved into different fractions: U B 0.09 B 0.25 B 0.4 B 0.5 mm and the fraction with diameter U B 0.25 mm was used for subsequent experiment. This fraction was pretreated to remove non-clay material such as carbonate and quartz minerals in order to concentrate the active minerals and improve the sorption property of the adsorbent. The pretreatment involved washing a pre-heated slurry of termite mound at 90 °C with 0.1 M HNO3 to leach the non-clay materials. The mixture was stirred for 2 h and then thoroughly washed with distilled water until the pH of the washings remains constant and finally air-dried [21]. Soil pH was determined potentiometrically in 0.01 M CaCl2 at a soil-solution ratio of 1:2.5 [22, 23]. The organic carbon content of the mound sample was determined by the dichromate oxidation, a Walkley–Black method [24]. The ammonium acetate method was used to determine the cation exchange capacity (CEC). The CEC was obtained by

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summation of the exchangeable acidity and exchangeable cations, which are mostly Ca, Mg, Na, K (collectively termed as bases). The K and Na were determined by flame photometry method while Ca and Mg were determined by titrimetric method [24]. The point of zero charge (pzc) was determined by both mass titration and potentiometric methods [25]. This involve measurement of pH of the mound (0.5–2.0 g) suspension in 25 ml of different ionic strength (10-3 to 10-1 M). The pH values at the start of experiment and after 24 h were recorded. The pH of the mixture was then adjusted to pH 5–6 with 0.01 M HNO3, equilibrated for 24 h and pH measured. The pH of these mixtures was then re-adjusted to pH 10–11 with 0.01 M KOH, equilibrated for 24 h and the pH measured. The acid and base adjustments were made in order to determine the influence of pH on the pzc [25]. The termite mounds sample was analyzed to determine its constituents and qualities using X-ray flourescence (XRF) model Axios of Analytical type with a 2.4kWatt Rh X-ray tube. The scanning analysis was accomplished on a Leo 1430 VP Scanning Electron Microscope. Contact experiment A 0.5 g portion of the various soil samples collected was weighed in a 100 mL conical flask and 25 cm3 of working standard solutions (500, 350, 250, 150, 100 and 50 ppm) of Pb(II) prepared by serial dilution of the standard solution, was added to the soil sample in the beaker. Each solution was agitated on a flat orbital mechanical shaker for 7 h and then filtered. The filtrate was analyzed using Atomic Absorption Spectroscopy (AAS) to determine the quantity of Pb(II) remaining in the solution. The quantity sorbed was then evaluated using the following equation [26]. qe ¼

Ci  Cf V M

ð1Þ

where qe is the amount sorbed at equilibrium (mg/g), Ci is the initial concentration of lead solution (mg/L), Cf is the final concentration of lead solution (mg/L), V is the volume of lead nitrate solution used (mL), M is the mass of soil used (g). The quantity sorbed was plotted against the initial concentration to determine the equilibrium concentration which is the concentration with the highest sorption capacity [the adsorption capacity of the mound soil for Pb(II)]. This equilibrium concentration was used subsequently for the sorption kinetics. Effect of reaction time The effect of reaction time was investigated using 25 mL of 350 mg/L of Pb(II) solution and 0.5 g of the mound

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sample each in an 100 mL reactor flask. A total of ten reactors were set up for time duration ranging between 5 and 420 min. The mixtures in reactors was then agitated on a flat orbital shaker and allowed to equilibrate at different time intervals. The flaks were removed at designed intervals and content filtered. The Pb(II) in the filtrates were analyzed using AAS and the amount sorbed were calculated from Eq. 1. Effect of ionic strength The influence of ionic strength on the adsorption of Pb(II) was investigated using three different concentrations (0.1, 0.01, 0.001 M) of potassium nitrate (KNO3) solution. The different concentration was added to the solution of 350 mg/L of Pb(II) solution and the mound sample in different flasks. The mixtures were equilibrated for 7 h and then filtered; this filtrate was analyzed for Pb(II) using AAS and the quantity adsorbed was calculated from Eq. 1 above. Effect of pH 25 mL of 350 mg/L of Pb(II) solution was added to 0.5 g of soil sample in a 100 mL conical flask and the initial pH was noted. The pH of the resulting solution was then varied between pH 2 and 7.5 using 0.1 M HNO3 and/or KOH solution. The mixture was equilibrated for 7 h and then filtered. The filtrate was analyzed for Pb(II) using AAS and the quantity adsorbed calculated. Fitness of Pb(II) adsorption data into Langmuir, Freundlich, and Temkin equations

from the graphs were used to calculate the Langmuir and Freundlich constants. In the case of Temkin, the quantity sorbed Qe was plotted against lnCe and the constants were determined from the slope and intercept.

Adsorption kinetics The adsorption kinetic models are important in the process of removal of toxic heavy metals from the environment because it provides information on pollution flux. Both pseudo- first and second order models were tested with the data obtained to establish the model that best fits the data. Pseudo-first order model [30, 31] The equation for this reaction is dqt ¼ kðqe  qt Þ dt

ð5Þ

where qe is the quantity of solute adsorbed at equilibrium per unit mass of adsorbent (mg/g), qt is the amount of solute adsorbed at any given time t, (mg/g) and k is the rate constant of first order sorption (1/min). By using the boundary conditions t = 0 to t = t and qt = 0 to qt = qt and simplifying, Eq. 5 becomes log(qe  qt Þ ¼ logqe 

k t 2:303

ð6Þ

The plot of log (qe - qt) versus t gives the slope and intercept from which k and qe were evaluated. Pseudo-second order model

The equilibrium concentrations data obtained in the contact experiment were subjected to the Langmuir, Freundlich, and Temkin adsorption isotherms to find the equation that best fits the data. Data were fed into Eqs. 2–4 separately and constants were calculated.

The pseudo-second order model [30, 31] was used for the sorption data and the equation for this is dqs  kz ðqe  ql Þ2 dt

ð7Þ

Ce 1 Ce ¼ þ Qe bQm Qm

ð2Þ

On integration for boundary conditions when t = 0 to t = t and qt = 0 to qt = qt then Eq. (7) can further simplification results in the following equation

1 Freundlich : LogQe  LogKf þ LogCe n

ð3Þ

Temkin : Qe ¼ BlnA þ BlnCe

ð4Þ

t 1 1 ¼ þ t qt k2 q2e qe

Langmuir :

where Qe is the quantity sorbed at equilibrium (mg/g), Ce is the equilibrium concentration of adsorbate (mg/L), A and B are Temkin isotherm constants related to adsorption efficiency (dm3/mmol) and energy of adsorption, respectively. The Langmuir adsorption isotherm constants were determined by plotting Ce/Qe against Ce [27–29]. The Freundlich adsorption isotherm was determined by plotting log Qe against log Ce. The slopes and intercepts obtained

ð8Þ

where k2 is the rate constant of second order of sorption (g/mg min). The plot of t/q versus t was made to evaluate k2 and qe. Sorption–desorption experiment The normal sorption procedure was carried out using 1.0 g of mound sample in 25 cm-3 of 500 ppm Pb(II) solution. The mixture was equilibrated for 7 h and then filtered and

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the filtrate was analyzed for Pb(II) while the residue was used for desorption experiment. A 25 cm-3 of different concentrations of HCl (0.1, 0.2, 0.5 and 1.0 M) were added to the residues in different conical flasks and each mixture was equilibrated for 7 h and then filtered. The filtrate was analyzed for the amount of Pb(II) released back into the solution. This sorption and desorption processes were repeated on the same sample for three times. The quantity sorbed or desorbed was calculated using Eq. 1 above. A graph of the quantity desorbed was plotted against the steps of the desorption process for the different ionic strengths.

Results and discussion The physical properties of the termite hill soil are summarised in the Table 1. The termite hill soil obtained from Ilorin Grammar School in Ilorin was brown with fine texture. The colour is within the variable colours of mound hills (brownish to blackish) depending on the soil morphology of the environment. The pH of the supernatant fluid of a mixture of the termite hill soil and water was slightly lower than neutral (pH 6.7).The termite hill soil is relatively rich in organic matter as expressed by percentage organic carbon (3.45%). Lower percentage organic matters were reported for soil fractions such as utisol (1.86%) [18], montmorillonite (2.1%) [32], and soil mound 0.8–1.32 [33]. The CEC depends greatly on the soil organic matter. The more the soil’s organic matter content, the higher will be its CEC. The high CEC value of 14.0 meq/100 g is in agreement with high percentage of OM, signifying high availability of exchangeable ions. This translates to enhance adsorptive capability of the soil. Spectroscopic studies The XRF elemental analyses of virgin (A) and after sorption of Pb(II) onto the mound are reported in Table 2. The result showed that SiO2, Al2O3 and Fe2O3 were present as major components as indicated from their high intensities. TiO2, CaO, K2O, H2O and MgO were present as minor components while P2O5, Cr2O3 and MnO were present as trace components. This implies that the adsorbent contains a Table 1 Physico-chemical properties of termite hill soils Properties

Quantity/value

Colour

Brown

Texture

Fine

pH

6.7

Weight organic carbon (%)

3.45

CEC (meq/100 g)

14.0

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large fraction of silicon, iron and aluminium minerals. Sample B data also indicate an increase in the %weight of aluminium oxides after sorption of Pb(II) and a decrease in %weight of silicate after sorption of Pb(II) and consequently a decrease in the total weight which may be due to the presence of some minerals which were not present in the list of the major elements used in the analysis. This trend is attributable to the loss on ignition (LOI) which was found to increase from 6.38 to 13.25 after sorption of Pb(II). The result of SEM elemental and weight per cent of their oxides for virgin and after sorption of Pb(II) onto the mound are reported in Table 3. Although, there were variations in the percent weights of elements obtained from the two analyses (XRF and SEM), Si, Al and Fe remained major elements while K, and Ti are minor elements. Figures 1 and 2 are the results of scanning electron micrographs of virgin samples. Round crystals arranged well next to each other and forming large round aggregates of crystals were observed. A similar image was observed in an SEM photograph of lead hydroxide [34]. The presence of many open pores is visible from Figs. 1 and 2. Figures 3 and 4 show the filled pores and signify adsorption of Pb(II). The point of zero charge (pzc) of the termite hill soil by potentiometric method was found to be 7.8 for the different ionic strengths which is similar to what was obtained for natural goethite (pzc = 7.8) [35]. Comparative values obtained for the same sample by mass titration were between 6.1 and 6.5 range for the different ionic strengths (0.1, 0.01, 0.001 M), but after acid adjustment, the pzc increased proportionately to the range 6.4–6.9. After alkaline adjustment, the pzc was observed at pH 8.2. The variable pzc obtained from mass titration is often affected by the nature of contaminants such as basic and acidic elements in the sample [34]. The potentiometric method may therefore be preferred because it is free from the interference of acidic/basic contaminants. Therefore, the pzc of this termite hill is 7.8. At pH below this pzc value, the acidic water donates more protons than hydroxide groups and so the termite hill soil will have a positive surface charge characteristic and will therefore electrostatically repel cations and attract anions to its surface. Conversely, above pzc, the surface charge characteristic will be negative [35]. The presence of ionic species or complexing agents in the reaction medium may change this adsorption pattern by conferring a net negative surface charge at pH below pzc value. This explains the adsorption of metal or hydrated metal ions onto surfaces at pH lower than its pzc value. Sorption capacity of termite mound Figure 5 represents a two steps process. An initial linear rise in the uptake of Pb(II) which is followed by a less steep

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Table 2 XRF analysis of Ilorin virgin termite hill soil (A) and after sorption of Pb (B) Wt%

Al2O3

CaO

Cr2O3

Fe2O3

K2O

MgO

Na2O

MnO

P2O5

SiO2

TiO2

LOI

H2O

Total

A

11.36

0.10

0.01

6.38

0.72

0.10

bd

0.08

0.06

73.46

0.97

6.38

0.89

100.52

B

17.86

0.62

0.01

8.35

0.97

0.14

0.01

0.04

0.16

43.41

1.08

13.25

1.64

87.52

A: element analysis before sorption of Lead B: element analysis after sorption of Lead Bd below detection Table 3 Scanning electron microscopy elemental and % compound results Analysis

Al (Al2O3)

Si (SiO2)

K (K2O)

Ti (TiO2)

Fe (FeO)

O

Total

Sample A (wt%)

18.02 (34.048)

20.38 (43.59)

0.39 (0.468)

0.75 (1.254)

16.04 (20.64)

44.42

100.00

Sample B (wt%)

16.93 (31.988)

25.04 (53.576)

0.56 (0.678)

1.29 (2.155)

9.02 (11.603)

47.15

100.00

curve. As the initial concentration increases the amount adsorbed increases proportionally until a plateau was reached, where there was no corresponding rise in the amount adsorbed. The favourable sites with lower adsorption energies have been filled making the unfavourable sites more difficult to fill. This is an indication of surface saturation or a monolayer adsorption. The second rise is a continuation of monolayer adsorption given a twostep Langmuir curve Influence of contact time on the adsorption of Pb(II) Figure 6 which shows the effect of time on the adsorption of Pb(II), can be classified into three portions according to the behaviour of the curve. An initial decrease in the quantity of Pb(II) (5–20 min) adsorbed due to inability of the system to establish equilibrium. This is followed by sharp rise in adsorption where majority of the metal ion was adsorbed. This is a fast uptake (between 30 and 90 min) in which about 65% of Pb(II) was sorbed. The last portion (90–600 min) represents a slow kinetics. It is characterized by an increase in the quantity of Pb(II) adsorbed but at a relatively slow rate compared to the second portion of the curve. This occurs when the available (favourable) binding sites on the adsorbent is low. The energy of adsorption becomes relatively higher than before. Similar trends were reported for adsorption of Pb(II) on a natural goethite [35]. Effect of ionic strength on adsorption of Pb(II) A steady decrease in the amount of Pb(II) adsorbed was observed as the ionic strength increases (0–0.5 M) until it gets to a point where a further increase in ionic strength has no effect on the quantity of Pb(II) adsorbed. Higher ionic strength of the reacting medium has no influence on the adsorption of Pb(II) (Fig. 7). The decrease in the quantity of

Fig. 1 SEM photograph of termite hill soil using K91.50 magnification

Fig. 2 SEM photograph of termite hill soil after sorption using K91.50 magnification

Pb(II) sorbed between 0 and 0.2 M ionic strength is due to modification of surface charge characteristics. As the ionic strength increases, the net negative charge on the termite hill surface decreases, thereby decreasing the attraction between the metal and the surface.

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Fig. 3 SEM photograph of termite hill soil using K91.00 magnification

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Fig. 6 Effect of contact time on the adsorption of Pb(II)

Fig. 7 Quantity of Pb(II) sorbed versus ionic strength

Fig. 4 SEM photograph of termite hill soil after sorption using K91.00 magnification

Fig. 8 Quantity of Pb(II) sorbed versus pH

Fig. 5 Determination of equilibrium concentration

Effect of pH on the adsorption of Pb(II) The graphical illustration of the pH on the adsorption characteristics of mound is shown in Fig. 8. The sorption experiment was carried out at pH ranging from 2.0 to 7.5.

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A steady increase was observed in the quantity of Pb(II) adsorbed as the pH decreased from 7.5 until after a pH of 5.5. The quantity of Pb(II) adsorbed was highest and practically constant between the pH of 2.0–5.5 but decreases from 6.0 to 7.5. Although, the natural surface charge characteristics of soil materials is positive at pHs below the pzc but practically, adsorption of metal ions is found to be favourable at pH less than the pzc values because at this value, the adsorbent is in a monomeric anion form and consequently has affinity for cations [36].

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The presence of anionic species such as nitrate ions from the Pb(II) salt solution used confers a net negative charge on the mound in a monomeric anionic form. This is attributed to a higher adsorption of Pb(II) between pH 2 and 5. Above the pzc value, the surface is positively charged and will repel adsorption of cations. The only process that can increase the quantity of metal adsorbed after pzc is by precipitation of metals. Many metals precipitate out in an alkaline medium. Adsorption isotherms Fig. 10 Freundlich adsorption isotherm for the sorption Pb(II)

The adsorption data obtained from the adsorption of Pb(II) experiment were tested for fitness of data against three common adsorption equations, Langmuir, Freundlich and Temkin adsorption isotherms, respectively. Figure 9 is a Langmuir adsorption isotherm for the sorption of Pb(II) by termite hill soil. The adsorption isotherm data as analyzed from Fig. 9 are 18.8, 0.085 and 0.993 for Qm, b and the regression coefficient R2, respectively. The observed b value (b  1) shows that the mound sample prefers to bind acidic ions and that speciation predominates on sorbent characteristic when ion exchange is the predominant mechanism that takes place in the adsorption of Pb(II) [28]. The observed R2 = 0.993 shows that the Langmuir isotherm fitted fairly well the adsorption data for Pb(II). The favourability of this adsorption process was subjected to the equation of separation factor RL [37] given as: RL ¼

1 1 þ bCi

ð9Þ

where b is the Langmuir equilibrium constant (K), Ci is the initial concentration. For a favourable adsorption, 0 \ RL \ 1, while for an unfavourable adsorption, RL [ 1 and when RL = 0, adsorption is linear and irreversible. The RL value obtained from this adsorption process is 0.033 which indicates that the adsorption process is favourable. The Freundlich adsorption constants obtained from Fig. 10 are n = 0.134, Kf = 1.36 and the regression coefficient R2 = 0.972. The relatively small slope, n  1,

indicates that sorption intensity is favourable over the entire range of concentrations studied [36, 38]. Likewise, the high value of the intercept Kf, is indicative of a high sorption capacity of termite hill soil. The regression coefficient R2 = 0.972 shows that the Freundlich adsorption isotherm equation fits the experimental data too. When the R2 values are compared, the data fits into Eqs. 2 and 3, while Langmuir predicts a physical and mono-layer adsorption process, the possibility of two-step (Fig. 5) and chemo-sorption may not be ruled out because of the fitness into Eq. 3. The adsorption data was subjected to Temkin equation and Fig. 11 is a representation of the isotherm obtained. The Temkin adsorption constant A, obtained from Fig. 11 is -0.122 and the regression coefficient R2 is 0.985. In comparison, the regression coefficients, R2, for the isotherms are: Langmuir (0.993), Freundlich (0.972) and Temkin (0.985). It shows that the Langmuir and Temkin isotherms fitted the adsorption data for lead ions better than the Freundlich isotherm. Table 4 is a summary of isothermal constants and regression coefficients. Comparing these data with similar data for goethite, where the Langmuir constants (b and Qm) obtained for Pb(II) on goethite are 4.99 and 2.40, respectively [39], it can be concluded that Pb(II) is better sorbed on to termite hill soil than natural goethite. Adsorption kinetics

Fig. 9 Langmuir isotherm curve

The data obtained from the influence of time on the adsorption of Pb(II) onto the mound sample were subjected to the pseudo-first order and pseudo-second order kinetics (Lagergren) equations for a test of fitness of data and the plots of which gave Figs. 12 and 13, respectively. A negative and zero slopes were observed for pseudo-first order model; therefore, pseudo-first order model was not sufficient to explain the adsorption kinetics of Pb(II) on termite hill soil. Figure 13 is the plot of t/qt versus t, the

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Therefore, the kinetics is best described by the pseudosecond order kinetics. Desorption experiment

Fig. 11 Temkin adsorption isotherm for the sorption of Pb(II)

Fig. 12 Pseudo-first order plot for the sorption of lead on termite hill soil

Desorption data obtained is graphically represented in Fig. 14. From the graph, it was revealed that the quantity desorbed for the first process was practically the same for the various acid concentrations. There is significant difference in the quantity desorbed in the second process among the different acid concentrations. The quantity of Pb(II) desorbed from the second step was highest for 0.2 M HCl concentration and has the greatest efficiency for reuse. The adsorption efficiency of termite hill soil dropped by 52.8% after the first desorption and by 86% after the second desorption, the quantity desorbed between the processes is consistent. The adsorption process is therefore reversible. Desorption data for 1.0 M is the most irreversible, the adsorption efficiency dropped by 88% after the first desorption process and by 12% after the second. The quantity desorbed between each process is inconsistent. The desorption process using distilled water followed a different pattern. The quantity desorbed during the first process was very low but the efficiency increased after each desorption process. With similar solutions for the desorption process, it can be seen that the quantity desorbed decreased after subsequent desorption for all the acidic strengths except for distilled water which increased after subsequent desorption. The 0.5 M HCl solution has the highest desorption efficiency. The quantity desorbed dropped from 62 to 13% after subsequent desorption, followed by 0.1 M HCl which dropped from 61 to 34% and then 0.2 M HCl with 60% which also dropped to 28%. We can deduce from this that the desorption efficiency of both 0.1 and 0.2 M are the most consistent and reversible which can also be observed on the graph (33).

Conclusion Fig. 13 Pseudo-second order plot for the sorption of lead on termite hill soil. t Time (min), qt quantity adsorbed at time t (mg/g)

pseudo-second order model, yields a very good plot with a regression coefficient R2 = 0.998, pseudo-second order rate constant k2 = 107.8 g/mg and qe = 13.6 mg/g.

The mineralogical details of a Nigerian termite hill soil were reported with the aid XRF and SEM. We have also determined the surface charge characteristics and CEC of this termite hill soil in order to understand its sorptive characteristics. Both the mineralogical compositions and

Table 4 Isothermal constants and correlation coefficients b

Qm

Kf

n

B

A

R2

Langmuir

0.085

18.18

–

–

–

0.993

Freundlich

–

–

1.36

0.134

–

–

0.972

Temkin

–

–

–

–

1.567

-0.22

0.985

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Fig. 14 Quantity desorbed (qe) versus steps for 0.1, 0.2, 0.5, 1.0 M HCl and distilled water. qe Quantity desorbed, steps desorption processes

surface characteristics suggest that the termite hill soil material is a good adsorbent in the class of oxides of metals or manganate–silicate. From the adsorption kinetics, the sorptive property of the mound was found to be dependent on contact time, pH, and ionic strength. The equilibrium adsorption data showed satisfactory correlation with the Langmuir adsorption data and was also found to fit the pseudo-second order kinetics. Termite mound (soil) has also been found to have a high efficiency for desorption and also for reuse even at low concentrations of leachant.

8.

9.

10.

11.

12.

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