Scholarly article on topic 'Efficient abstraction of nickel(II) and manganese(II) ions from solution onto an alkaline-modified montmorillonite'

Efficient abstraction of nickel(II) and manganese(II) ions from solution onto an alkaline-modified montmorillonite Academic research paper on "Chemical sciences"

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Abstract of research paper on Chemical sciences, author of scientific article — Kovo Godfrey Akpomie, Folasegun Anthony Dawodu

Abstract We tested alkaline-activated montmorillonite as a low-cost adsorbent for simultaneous removal of Ni(II) and Mn(II) ions from solution. The experiment was performed by batch adsorption to evaluate the effects of pH, initial metal ion concentration, particle size, adsorbent dose, contact time and temperature on adsorption. Alkaline modification of montmorillonite increased the specific surface area from 23.2 to 30.7m2/g and the cation exchange capacity from 90.78 to 94.32mEq/100g. The adsorption capacity of the montmorillonite for Ni(II) and Mn(II) ions increased with alkaline modification. The adsorbent was characterized by Fourier transform infrared, X-ray diffraction and scanning electron microscopy. When four isotherms, the Langmuir, Freundlich, Temkin and Dubinin–Radushkevich models, were applied to the experimental data, the best fit was obtained with the Freundlich model. The kinetic data were analysed with pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion rate equations; greater conformity was found with the Elovich equation. Thermodynamic studies revealed a spontaneous, endothermic physical adsorption process.

Academic research paper on topic "Efficient abstraction of nickel(II) and manganese(II) ions from solution onto an alkaline-modified montmorillonite"

Accepted Manuscript

Title: Efficient Abstraction of Nickel(II) and Manganese(II) ions From Solution Unto an Alkaline Modified Montmorillonite

Author: Godfrey Kovo Akpomie Folasegun Anthony Dawodu

PII: DOI:

Reference:

S1658-3655(14)00041-7

http://dx.doi.org/doi:10.1016/j.jtusci.2014.05.001 JTUSCI 72

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Received date: Revised date: Accepted date:

9-1-2014

26-3-2014

5-5-2014

Please cite this article as: G.K. Akpomie, F.A. Dawodu, Efficient Abstraction of Nickel(II) and Manganese(II) ions From Solution Unto an Alkaline Modified Montmorillonite., Journal of Taibah University for Science (2014), http://dx.doi.org/10.1016/j.jtusci.2014.05.001

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'Manuscript

Efficient Abstraction of Nickel(II) and Manganese(II) ions From Solution Unto

an Alkaline Modified Montmorillonite.

Godfrey Kovo Akpomieab , Folasegun Anthony Dawodua

aDepartment of Chemistry (Industrial) University of Ibadan, Ibadan, Nigeria bProjects Research and Development Institute (PRODA), Enugu, Nigeria

E-mail: kovoakpmusic@yahoo.com Tel: +2348037617494

Abstract

This paper describes the studies on the alkaline activation of a montmorillonite as a low-cost adsorbent for the simultaneous removal of Ni(II) and Mn(II) ions from solution. The experiment was performed using batch adsorption to evaluate the effect of pH, initial metal ion concentration, particle size, adsorbent dosage, contact time and temperature on adsorption. Alkaline modification of the montmorillonite was found to increase the specific surface area from 23.2 to 30.7m2/g and the cation exchange capacity from 90.78 to 94.32meq/100g. The adsorption capacity of the montmorillonite for Ni(II) and Mn(II) ions was found to increase with alkaline modification. FTIR, XRD and SEM analysis were used to characterize the adsorbent. Four isotherms namely, the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models were applied to the experimental data and the best fit was obtained by the Freundlich model. The Pseudo-first order, pseudo second order, Elovich and intraparticle diffusion rate equations were utilized to analyze the kinetic data, which showed a greater conformity with the Elovich equation. Thermodynamic studies revealed a spontaneous, endothermic and physical adsorption process.

Keywords: Adsorption, Alkaline modification, Nickel, Manganese, Water treatment.

1. Introduction

The problem of pollution of water with toxic substances generated from industrial effluents is of major concern to human health as well as environmental quality. Among different toxic substances, heavy metals are the most dangerous to the environment. It has been reported that the toxicity due to metallic discharge annually into the environment far exceeds the combined total toxicity of all radioactive and organic waste [1]. Heavy metals are commonly found in aqueous wastewaters of many industries such as paints, batteries, ceramic glazes, pigments, dyes, glass, ink, match and firework and metal products [2]. Although many heavy metals are necessary in small amounts for the normal development of the biological cycles, most of them become toxic at high concentrations. Heavy metals are toxic to living organisms because they tend to persist in the environment, are non-biodegradable

and accumulate in living organisms getting concentrated through the food chain [3]. Various metals such as lead, nickel, cadmium, manganese, chromium, copper, mercury, zinc e.t.c are known to be significantly toxic [4]. Manganese in particular is considered a pollutant mainly because of its organoleptic properties and in high concentration causes neurological disorders [5]. Nickel when present in high concentration in humans causes serious health effects such as liver and heart damages, skin irritation, nasal cancer, headache and dermatitis [6]. Therefore the removal of these metals from industrial wastewater is a very important issue.

Several techniques have been utilized for the removal of heavy metals from effluents, such as oxidation, coagulation-flocculation, membrane filtration, sedimentation, solvent extraction, ion exchange, reverse osmosis, evaporation, reduction and adsorption [7]. However, most of these processes have the problem of high cost, low efficiency, inapplicable to a wide range of pollutants and the generation of toxic waste difficult to treat [8]. Among the various techniques mentioned, adsorption is generally preferred for the removal of heavy metal ions in terms of the initial cost, flexibility and simplicity of design, ease of operation, insensitivity to toxic pollutants and the availability of different adsorbents [9]. Also, it does not result in the formation of harmful substances like in most other cases. However, activated carbon has been utilized primarily for the treatment of wastewater containing heavy metals because of its very good adsorption ability [4,10], but it is expensive and this limits its widespread use. As a result, the search for cheaper alternative adsorbent has been the focus of many scientists in recent years. Several low cost adsorbents have been utilized for the treatment of wastewaters containing heavy metals such as biomass and agricultural waste, algae, clay, zeolites, soil and sawdust [11-18]. The uses of natural and modified forms of montmorillonite for the removal of heavy metals from solution have been reported by some researchers [19-22]. Montmorillonite have been found to have a high specific surface area and effective as adsorbents. It mainly adsorbs by two different mechanisms which are cation exchange in the interlayer's resulting from the interactions between ions and negative permanent charge and also through the formation of inners sphere complexes through Si-O- and Al-O- groups at the clay particle edges [19,21]. The modification of different clay minerals using alkaline treatment has been found to improve their adsorption capacity [23,24,25]. As a result, this study is an extension in the same direction in utilizing a low cost, eco-friendly and easily accessible montmorillonite obtained from ugwoba, Enugu state, Nigeria as a cheap alternative adsorbent for the removal of Ni(II) and Mn(II) ions from aqueous solution. Alkaline modification of the clay was performed in order to investigate the effect on the adsorption potential of the montmorillonite. The effect of various experimental conditions such as pH, initial metal ion concentration, adsorbent particle size, adsorbent dose, contact time and temperature were determined. Isotherms, kinetic and thermodynamic models were applied for proper understanding of the mechanism of the adsorption process.

2. Experimental

2.1. Adsorbent Preparation

The montmorillonite was collected from ugwuoba in Oji River local government area, Enugu state, Nigeria. It was then dissolved in excess distilled water in a pretreated plastic container with proper stirring, the mixture was filtered through a 500pm mesh sieve and the filtrate was kept for 24hrs to settle. Excess water was then decanted from the top of the filtrate and the residue obtained was sun dried for several days. The montmorillonite was oven dried at 1050C for 4hrs, then pulverized to obtain the unmodified montmorrillonite clay (UUC).

2.2. Alkaline Modification of Adsorbent

200g of UUC was contacted with 500ml of 1M NaOH solution, stirred for 30mins and left for 24hrs at a room temperature of 270C. The aqueous phase was decanted and the process was repeated using 500ml of 0.5M KOH, the aqueous phase was decanted and the residue washed with excess distilled de-ionized water to a neutral pH. The clay was oven dried at 1000C for 16hrs, then pulverized and passed through various mesh sieves of sizes 100 to 500pm to obtain the alkaline modified montmorillonite clay (KMUC). The adsorbent was stored in a glass bottle for further use.

2.3. Characterization of Adsorbents

The adsorbents were characterized to determine their chemical composition by the use of the Atomic Absorption Spectrophotometer (AAS) (Buck scientific model 210VGP) after digestion of the samples with nitric and hydrofluoric acid. The methylene blue absorption test method was used to determine the specific surface area [26]. The cation exchange capacity was determined by the ammonium acetate method [27]. The pH point of zero charge was obtained as described [28], while the slurry pH was determined by soaking 1g of the adsorbent in 50ml of distilled water and stirred for 24hrs, after which it was filtered and the final pH of the filtrate was determined. The X-ray diffraction analysis of the adsorbents was determined by the use of the X-ray diffractometer (Randicon MD 10 model). An FTIR spectrum of the adsorbent was taken with a Fourier-Transform Infrared Spectrophotometer (Shimadzu FTIR 8400s) model. The pore properties and BET surface area of the adsorbents were determined via nitrogen adsorption-desorption isotherms using a micromeritics ASAP 2010 model analyzer, while scanning Electron Microscope (SEM) (Hitachi S4800 model) was used to assess the morphology of the adsorbents.

2.4. Batch Adsorption

All the reagents used in this study were of analytical grade obtained from Sigma-Aldrich and were used without further purification. A binary stock solution of concentration 1000mg/L of Ni(II) and Mn(II) ions was prepared by dissolving appropriate amounts of NiSO4.6H2O and MnSO4.H2O in 1 liter of double distilled water. Working solutions of concentrations 100 to 500mg/L were prepared from the stock solution by serial dilution. By the drop wise addition of 0.1 M HCL or 0.1M NaOH, the pH of each of the solution was adjusted to values from 2.0 - 8.0 using a pH meter.

Batch adsorption was performed by the addition of 0.1g of the adsorbent to 50ml of a given solution of metal ions in a 100ml pretreated plastic bottle at a room temperature of 300K. However, when the effect of temperature was investigated, the bottles were placed in a thermo-stated water bath to help regulate the temperature. The effect of various operating conditions such as pH (2.0 - 8.0), adsorbent dosage (0.1 - 0.5g), particle size (100 - 500^m), contact time (20 - 300mins), initial metal ion concentration (100 - 500mg/L) and temperature (300 to 323K) were investigated. In each experiment, one parameter was varied while the others were kept constant. The contacted solution was filtered at the end of a given contact time and the concentration of metal ions remaining in the filtrate was determined by the AAS. Each experiment was performed twice and the mean value was computed to ensure minimization of errors. The amount of metal ions adsorbed was calculated by the following equations:

qe = V(Co - Ce)/m (1)

% Removal = 100[(Co - Ce)/ Co] (2)

Where qe (mg/g) is the adsorption capacity of the adsorbent for the adsorbate at equilibrium, Co (mg/L) and Ce (mg/L) are the initial and equilibrium concentrations of metal ions in solution respectively, m (g) is the mass of adsorbent and V (L) is the volume of solution used.

3. Results and Discussion

3.1. Adsorbent Characterization

The physicochemical characterizations of the adsorbents are shown in Table 1. Silica and alumina are observed to form the major composition of the montmorillonite. Alkaline modification of UUC led to a slight decrease in silica from 47.32% to 42.98% and an increase in alumina from 25.91% to 26.25% resulting in a decrease in the silica to alumina ratio from 1.83 to 1.64. The decrease in the concentration of silica with alkaline treatment is due to the etching reaction between

silica and a strong base used in the modification. Similar result has been reported [23]. A very important point to be noted is the utilization of both NaOH and KOH in the modification, classically; researchers utilize either one of the bases for modification but not both. The simply reason for modification with KOH was to further increase the adsorption capacity. NaOH treatment helped to remove the surface impurities thereby increasing porosity and surface area for metal adsorption [29], KOH was added for replacement of the Na+ ions by K+ ions which could possibly enhance further the surface porosity and adsorption potential of the adsorbent. In fact at an initial metal concentration of 500mg/l, pH 6.0, contact time of 180min, adsorbent dosage 0.1g and particle size of 100^m, the adsorption potential of the clay increased from 117.45 to 125.95mg/g and 102.76 to 111.95mg/g for Ni(II) and Mn(II) ions respectively after KOH treatment. Further increase in the concentration of KOH or the utilization of NaOH led to a decrease in the adsorption potential. This decrease may be due to excessive alkaline leaching of the clay structure which may affect the porous nature of the adsorbent. KOH treatment has been reported to be effective for removing silica and increasing the pore volume and surface area of adsorbents [30].

Also, the specific surface area (SSA) of an adsorbent is a very important property in determining the adsorption capacity of an adsorbent and a high SSA is desirable for optimum adsorption. In general, it is expected that the higher the SSA of a material the higher its adsorption potential and vice versa, although not in all cases. Alkaline modification was found to increase the SSA of the montmorillonite

determined by the methylene blue absorption method from 23.2 to 30.7m2/g and the BET specific surface area (SBET) from 55.76 to 87.24m2/g, which is desirable. It has been reported that the specific surface area of a particular adsorbent may differ significantly with the method of determination [31,32]. The difference in the specific surface area of the montmorillonite is simply due to differences in the method of determination. The SBET of UUC (55.76m2/g) is close to the value (61 ± 2m2/g) determined by Macht et al [32] for montmorillonites. Interestingly, a low SBET of 26.33m2/g has been reported by Guo et al [33]. A number of factors seemingly contribute to the variation in the SBET values of montmorillonites, which include the type and purity of the montmorillonite, the saturating cation, the out-gassing temperature and in general the preparatory treatment received by the sample. The increase in SSA and SBET is attributed to an increase in pore property caused by the etching reaction between silica and a strong base [34]. An increase in the total pore

volume (TPV) from 0.0688 to 0.0981cm3/g and decrease in the average pore diameter (APD) from 49.35 to 44.98A was obtained after alkaline modification. The cation exchange capacity (CEC) of clay, which is the maximum quantity of total cations the clay can hold at a given pH available for exchange with a given solution, was found to increase from 90.78 to 94.32meq/100g after alkaline modification. The negative charges introduced by OH group in the presence of fewer H+ ions under alkaline condition led to the slight increase in CEC recorded [35]. The high CEC of KMUC indicates that the removal of Ni(II) and Mn(II) ions from solution may also involve an ion exchange mechanism. The slurry pH and the pHpzc of the adsorbents

usually give useful information about the surface oxygen complexes and the electronic surface charge of an adsorbent. This surface charge arises from the interaction between adsorbent surface and the aqueous solution. The complexes on adsorbent surfaces are generally classified as acidic, basic or neutral. The slurry pH indicated the adsorbents (UUC and KMUC) to be slightly acidic (pH 4.2 and 5.8) respectively. There exist a relationship between the pHpzc and the adsorption capacity of an adsorbent: when the solution pH is lower than the pHpzc, the surface of the adsorbent is positively charged thereby favoring the adsorption of anionic species, while for pH values greater than the pHpzc, the surface is negatively charged thereby favoring the adsorption of cationic species [36]. Alkaline modification resulted to an increase in pHpzc from 3.7 to 5.2, this increase does not imply a lower potential or affinity of KMUC to adsorb metal ions from solution compared to UUC. It only implies that optimum adsorption of metal ions on KMUC is most likely to be achieved at pH values higher than 5.2, when the surface of the adsorbent is negatively charged.

3.2. FT-IR Analysis

The FTIR spectra of UUC and KMUC were determined in order to determine the surface functional groups responsible for binding of Ni(II) and Mn(II) ions and the

effect of alkaline modification as illustrated in Fig.1. The spectrum of UUC showed

absorptions at 3697.66cm-1 and 3620.51cm-1 which corresponds to the inner surface -OH stretching vibration of kaolinte, this might indicate the presence of kaolinite in the clay. The presence of the outer surface -OH stretching was indicated by the bands at 3441.12cm-1 and 3410.26cm-1. Absorption band at 1627.97cm-1 represents the OH bending vibration and can also be assigned to the -COO- symmetric stretching vibration [37]. The presence of the outer OH stretching and the symmetric -COO- stretching vibrations indicate the presence of smectite [38]. The Si-O bending

vibration was observed by the bands at 1114.89 - 1006.88cm-1 while the stretching vibrations were observed at 796.63cm-' and 694.4cm-'. Absorption bands at 912.36cm-1 corresponds to the Al-O bending vibration while bands at 536.23, 470.65 and 430.14cm-1 represent the Al-O-Si skeletal vibrations [39,40]. The alkaline modification of the adsorbent was indicated by the shift in absorption bands observed on KMUC from 3441.12 and 3410.26cm-1 to 3427.62 and 3389.04cm-1, respectively for the outer -OH stretching. Shifts in the -OH bending from 1627.97 to 1629.9cm-1, the Si-O bending from 114.89, 1097.53 and 1006.88cm-1 to 1112.96, 1101.39 and 1008.8cm-1 respectively and the Al-O-Si vibration from 470.65 to

472.58cm-1. The FTIR spectra revealed the aluminol and silanol groups as major sites for binding of the positively charged ions unto KMUC.

3.3 XRD and SEM Analysis

The XRD spectrum of UUC and KMUC are shown in Fig.2. The d-spacing values presented by UUC reflect the presence of montmorillonite as the major constituent and also the presence of quartz, kaolinite and gibbsite. The sharp increase in the intensity of the montmorillonite peak observed on KMUC after alkaline modification suggest that the leached silica must have been from the silica phase and not the montmorillonite phase. The XRD also gives useful information about the change(s) in the crystalline and amorphous nature of an adsorbent. The presence of the sharp and weak peaks suggests the amorphous nature of the adsorbent [41]. The amorphous nature of KMUC indicates that Ni(II) and Mn(II) ions in solution can easily penetrate into its surface for efficient adsorption [42].

The SEM analysis of UUC and KMUC was taken in order to examine their surface morphology and is presented in Fig.3. The porous nature of the adsorbents was revealed by the SEM images and an increase in surface porosity as observed on KMUC with alkaline modification was obtained. The porous nature of KMUC when compared to UUC signifies an enhance sorption potential of the adsorbent for metal ions and suggest that physical adsorption mechanism would play a major role in the overall sorption process [43].

3.4. Effect of initial metal ion concentration

The amount of metal ions adsorbed by an adsorbent is a function of the initial concentration of the adsorbate (metal ion), which makes it an important factor to be considered for effective adsorption. The effect of initial metal ion concentration on the adsorption of Ni(II) and Mn(II) ions unto UUC and KMUC is shown in Fig.4. An increase in adsorption capacity of both adsorbents for metal ions with increase in metal concentration was recorded. This is due to increasing concentration gradient which acts as an increasing driving force to overcome the resistances to mass transfer of metal ions between the aqueous phase and the adsorbent [17]. Higher concentration in solution implies higher metal ion to be fixed on the surface of the adsorbent [4]. Also, KMUC recorded a higher adsorption capacity for both metal ions than UUC which indicates the effectiveness of alkaline modification. At a concentration of 500mg/L, 125.95 and 111.95mg/g were adsorbed by KMUC for Ni(II) and Mn(II) ions, respectively, while UUC recorded an adsorption of 110.45 and 89.4mg/g for Ni(II) and Mn(II) respectively. The higher adsorption of KMUC may be due to the higher surface area and improved cation exchange capacity induced by alkaline modification compared to UUC. Also, both UUC and KMUC showed a higher adsorption for Ni(II) than Mn(II) ions. This may be due to the smaller ionic radii of Ni(II) (0.72Â), which makes sorption easier compared to the larger Mn(II) ions of atomic radii, 0.80Â.

3.5. Effect of pH

The initial pH of a solution is an important controlling parameter in adsorption processes and metal ion removal usually increases with increase in pH. The pH of a solution affects the ionization degree of the adsorbate and the surface property of the adsorbent [17]. The effect of initial pH of solution on the percentage removal of Ni(II) and Mn(II) ions from solution unto KMUC is shown in Fig.5. An increase in adsorption of both metal ions with increase in pH was obtained. Significant adsorption of the metal ions was achieved at pH values of 6 to 8 greater than the pHpzc of 5.2 of KMUC, when the surface of the adsorbent becomes negative. However, an optimum pH of 6.0 was selected and utilized for all experiments, in order to avoid the formation of metal hydroxides precipitates associated with higher pH values. At lower pH values, higher concentrations of H+ ions are present in solution which creates a competition between the protons and metal ions for the active sites of the adsorbent. The low adsorption thus recorded at low pH values is due to the saturation of the active sites of KMUC with hydrogen ions. Also, the increase in Ni(II) and Mn(II) ions removal with increase in pH can be explained on the basis of the decrease in competition between protons and metal ions for the active sites and by the decrease in positive surface charge, which results in a lower electrostatic repulsion between the surface and metal ions before ion exchange [5].

3.6. Effect of Adsorbent Dosage

Adsorbent dosage is an important parameter useful in the determination of the capacity of an adsorbent for a given initial adsorbate concentration. The effect of KMUC dosage on the adsorption of Ni(II) and Mn(II) ions from solution is shown in Fig.6. An increase in the percentage removal of both metal ions with increase in adsorbent dosage was obtained. This increase is mainly attributed to an increase in the number of available active sites with increase in adsorbent dose [14]. However, the equilibrium adsorption capacity per unit mass of KMUC decreased considerably with increase in sorbent dose for both metal ions. In fact, increasing the adsorbent dose from 0.1 to 0.5g, the equilibrium adsorption capacity decreased from 34.85 to 8.51mg/g and 32.35 to 8.05mg/g for Ni(II) and Mn(II), respectively. This may be due to a decrease in the total adsorption surface area available for metal ions resulting from the overlapping or aggregation of adsorption sites [37]. Another reason may be as a result of higher adsorbent dose providing more active adsorption sites which results in the adsorption sites remaining unsaturated during adsorption [44].

3.7. Effect of Particle Size

The experimental result indicating the effect of adsorbent particle size on the percentage removal of Ni(II) and Mn(II) from solution unto KMUC is shown in Fig.7. A slight decrease in the percentage removal of both metal ions with increase in the particle size of KMUC was observed. In fact, increasing the adsorbent particle size from 100 to 500pm the percentage removal decreased from 69.7 to 55.8% and from 64.7 to 50.6% for Ni(II and Mn(II) respectively. This decrease in percentage removal is attributed to a decrease in the surface area of the adsorbent available for metal ions with increase in particle size. The breaking of larger particles to smaller ones tend to open up tiny cracks and channels on the particle surface of the adsorbent, resulting in more accessibility to better diffusion, due to smaller particle sizes [45]. Similar results have been reported [4,46].

3.8. Equilibrium Isotherm modeling

Equilibrium adsorption isotherms are used to relate the adsorbate concentration in solution and the amount on the adsorbent at equilibrium [47]. These parameters of equilibrium isotherms often provide fundamental information on sorption mechanism, surface properties and affinity of the adsorbents, which helps to determine the applicability of the sorption process as a unit operation. Therefore, it is important to establish the most suitable correlation of equilibrium curves in order to optimize the conditions for designing adsorption systems. In this regard, the most frequently used isotherm, which are the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherm models were used to analyze the experimental data. The isotherm parameters obtained are shown in Table 2.

The Langmuir isotherm model is used to describe a monolayer adsorption onto a surface of an adsorbent with finite number of identical adsorption sites without any interaction between the sites. The Langmuir model can be expressed as [48]:

Where qL(mg/g) is the Langmuir monolayer adsorption capacity of adsorbent, KL (L/mg) is the Langmuir adsorption constant which reflects the affinity between the adsorbent and adsorbate. Where qL and KL were determined from the slope and intercept of the plots of Ce/qe versus Ce. An essential feature of the Langmuir isotherm can be expressed in terms of a dimensionless separation factor (RL), defined as:

The adsorption is said to be favorable if (0<RL<1) and is unfavorable if (RL>1). From

Table 2, the correlation coefficient (r2) obtained for Ni(II) and Mn(II) are low ( less

Ce/qe = 1/qLKL + Ce/qL

Rl = 1/[1 + KLCO]

than 0.95). Therefore the adsorption does not follow the Langmuir isotherm closely. However, a favorable adsorption process was indicated by the RL values, which lies within the range of 0.25 - 0.65 and 0.33 - 0.71, at all concentrations studied, for Ni(II) and Mn(II), respectively.

The Freundlich isotherm [49] is based on the assumption that sorption takes place on a homogenous adsorbent surface, where the sorption energy distribution decreases exponentially and can be expressed as:

logqe = logKF + 1/nlogCe (5)

Where KF (mg/g)(mg/L)1/n and n are the freundlich constants related to the adsorption capacity and intensity respectively. Therefore a plot of logqe versus logCe gives a straight line of slope 1/n and intercepts logKF. As can be seen in Table 2, r2 values of both Ni(II) and Mn(II) ions are high ( greater than 0.95) indicating a good fit of the Freundlich model to the adsorption process. This suggests that the surface of KMUC is heterogeneous in nature. Also, the values of n obtained for both metal ions lies between 1 and 10 indicating a favorable adsorption [50].

The Temkin isotherm is based on the assumption that the free energy of adsorption is a function of the surface coverage and is expressed as [51]:

qe = BlnA + BlnCe (6)

where B = RT/b (mg/g) is the Temkin isotherm constant related to the heat of adsorption and A (L/g) is the equilibrium binding constant corresponding to the maximum binding energy, R is the gas constant and T (K) is the absolute temperature. The constants A and B were determined from the plot of qe versus lnCe. The r2 values obtained (Table 2) for both metal ions showed that the Temkin model did not fit the data properly.

The Dubinin-Radushkevich (D-R) isotherm does not assume a homogenous surface or a constant adsorption potential as the Langmuir isotherm and is expressed as [52]:

lnqe = lnqm - /e2 (7)

Where qm (mg/g) is the D-R theoretical saturation capacity, / (mol2/J2) is a coefficient related to the mean free energy of adsorption and e is the Polanyi expressed as:

e = RTln(1 + 1/Ce) (8)

The constants qm and / were calculated from the intercept and slope of lnqe versus

e2. The D-R isotherm did not provide a good fit for both metal ions indicated by the r2 values obtained. Therefore the adsorption of Ni(II) and Mn(II) ions unto KMUC showed greater conformity to the Freundlich adsorption model.

The difference in the adsorbed amount deduced from the four models is simply as a result of the different equations presented by the models for analyzing the experimental data. Similar results have been reported by many researchers [4,5,8,13,14,20,29].

3.9. Effect of Contact Time

The effect of contact time on the adsorption of Ni(II) and Mn(II) ions unto KMUC is shown in Fig.8. An initial increase in percentage removal with increase in contact time was observed and the adsorption became fairly stable with time. Equilibrium removal was achieved around 120mins for Ni(II) and 160mins for Mn(II), after which further increase in contact time did not result in any significant adsorption. However, maximum adsorption was achieved at 180mins for both metal ions. A contact time of 180mins was utilized in this study for all experiments performed, in order to ensure maximum removal. The fast removal rate for Ni(II) compared to Mn(II) may be due to the smaller ionic radii of Ni(II) (0.72A) compared to Mn(II) (0.80A), which makes for an easier and more rapid diffusion to the surface of KMUC. The fast adsorption at the initial stages may be due to the availability of abundant active sites on the surface of KMUC which becomes saturated with time. At the initial stage, the sorption is mainly controlled by the diffusion process from the bulk to the surface of KMUC. In the later stages, the sorption is likely an attachment controlled process due to the presence of less available active sites [17].

3.10. Kinetic Modeling

The controlling mechanism of the adsorption process was investigated by the use of the Pseudo-first order, Pseudo-second order, Elovich and the Intraparticle diffusion rate equations. The kinetic parameters are given in Table 3.

The Pseudo-first order or Lagergern equation is based on the assumption that the rate of adsorption site occupation is proportional to the number of unoccupied sites [53]. The linear form of the Lagergern equation is given as:

log(qe - qt) = logqe -(K¡t/2.303) (9)

Where K (min ) is the pseudo-first order rate constant, qt and qe are the amounts of metal ion adsorbed at time t and equilibrium respectively. The constant K¡ and qecal were obtained from the slope and intercept of the plot of log(qe - qt) versus t. The r2 value (Table 3) obtained for both metal ions showed a poor fit of this model to the experimental data.

The pseudo-second order model assumes that the rate of occupation of adsorption sites is proportional to the square of the number of unoccupied sites [54]. The linear form of the pseudo-second order equation is given as:

t/qt = 1/K2qe2 + t/qe (10)

Where K2 (g/mgmin) is the pseudo-second order constant. The initial sorption rate h, was calculated from the equation:

If the pseudo-second order equation is applicable, the plot of t/qt versus t yields a straight line. K2 and qe were calculated from the slope and intercept of the plot. This model provided a good fit to the experimental data for both metal ions as seen from the r2 values (Table 3). However, the calculated qe values showed great discrepancy from the experimental ones. Therefore, the adsorption process does not follow the pseudo-second order model closely.

The kinetic data was analyzed further by the use of the Elovich equation, which is expressed in its linear form as [8]:

qt = 1//ln(a/) + 1//lnt (12)

Where a (mg/gmin) is the initial sorption rate constant, /3 (g/mg) is a constant corresponding to the surface coverage and activation energy for chemisorptions. This model equation was applied by a linear plot of qt versus lnt, which showed the conformity of the data to the Elovich model for both metal ions (Table 3), as good r2 values of 0.979 and 0.980 were obtained for Ni(II) and Mn(II) ions respectively.

The diffusion mechanism was further analyzed using the intraparticle diffusion rate equation expressed as [55]:

Where Kd (mg/gmin1/2) is the intraparticle diffusion rate constant and C is the intercept. A linear plot of qt versus t1/2 was used to obtain the constants Kd and C. Intraparticle diffusion is the sole rate controlling step if the plot is linear and passes through the origin (C = 0). From Table 3, the r values obtained for both metal ions showed the existence of intraparticle diffusion mechanism, although it was not the sole rate controlling step (C + 0). The occurrence of the intercept C, showed the existence of some boundary layer effect, which indicates the occurrence of some surface phenomenon such as mass transfer or liquid film diffusion mechanism in the sorption process.

3.11. Thermodynamic Studies

The influence of solution temperature on the adsorption of Ni(II) and Mn(II) ions from solution by KMUC is shown in Fig.9. A slight increase in adsorption of both metal ions with temperature increases from 300 to 323K was recorded. The percentage adsorbed increased from 69.7 - 77.9% and 64.7 to 74.2% for Ni(II) and Mn(II) ions, respectively. This increase may be due to a greater kinetic energy acquired by the metal ions to overcome the energy barrier and the creation of more active sites on the adsorbent due to dissociation of some of the surface components on KMUC [37].

h = Kqe2

qt = Kdt1/2 + C

The thermodynamic parameters such as the standard free energy change (AG0), enthalpy change (AH0) and entropy change (AS0) were calculated to evaluate the feasibility of the adsorption process, from the following equations [22]:

Kc = Ca/Ce (14)

AG0 = -RTlnKc (15)

lnKc = - (AH0/RT) + (AS0/R) (16)

Where Kc is the thermodynamic equilibrium constant, Ca (mg/l) is the concentration of metal ions adsorbed by the adsorbent at equilibrium, Ce (mg/l) is the concentration of metal ions remaining in solution at equilibrium, R is the ideal gas constant (8.314J/molK) and T (K) is the absolute temperature. The values of AH0 and AS0 were estimated from the slope and intercept of the plot of lnKc versus 1/T. The thermodynamic parameters obtained are presented in Table 4.

Negative values of AG0 were obtained at all temperatures for both metal ions indicating a spontaneous adsorption process. Also, positive values of AH0 obtained revealed an endothermic adsorption process, which is supported by the increased adsorption with increase in temperature recorded. The positive values of AS0 reveal an increase in randomness at the solid solution interface [56] and the low AS0 values suggest that no remarkable change in entropy occurs. The magnitude of AH0 provides information about the type of adsorption. The heat evolved during physical adsorption lies within the range 2.1 - 20.9kJ/mol, while that of chemisorptions is in the range of 80 - 200kJ/mol [57]. From Table 4, the magnitude of AH0 obtained indicates a physical adsorption process between the metal ions and the adsorbent.

4. Conclusion

The result of the present study showed that alkaline modification of the montmorillonite enhanced its adsorption capacity for Ni(II) and Mn(II) ions. The FTIR spectra of KMUC revealed the existence of certain functional groups responsible for binding of metal ions from solution. SEM morphology indicated an increase in the porous nature of the alkaline modified adsorbent (KMUC) when compared to the unmodified form (UUC). This porous nature enhances metal ion adsorption on KMUC. The heterogeneous nature of KMUC was revealed by the good fit of the data to the Freundlich model while thermodynamic studies revealed an endothermic, spontaneous and physical adsorption process. These results indicate the potential of KMUC as a low-cost adsorbent for the removal of Ni(II) and Mn(II) ions from solution.

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FIGURE CAPTION

Fig.1: Fourier Transform Infrared (FTIR) Spectra of UUC and KMUC.

Fig.2: XRD spectra of UUC and KMUC

Fig.3: SEM images of UUC and KMUC

Fig.4: Effect of initial metal ion concentration on the adsorption of metal ions unto the adsorbents (pH 6.0, adsorbent dose 0.1g, temp 300K, particle size 100pm, time 180min)

Fig.5: The effect of initial pH of solution on the percentage removal of metal ions unto KMUC. (Co 100mg/L, adsorbent dose 0.1g, temp 300K, particle size 100pm, time 180min).

Fig.6: The effect of adsorbent dose, (A) on the percentage removal of metal ions, (B) on the adsorption capacity of KMUC for metal ions. (Co 100mg/L, pH 6.0, temp 300K, particle size 100pm, time 180min).

Fig.7: The effect of adsorbent particle size on the percentage removal of metal ions unto KMUC. (Co 100mg/L, pH 6.0, adsorbent dose 0.1g, temp 300K, time 180min).

Fig.8: The effect of contact time on the percentage removal of metal ions unto KMUC. ( Co 100mg/L, pH 6.0, adsorbent dose 0.1g, temp 300K, particle size 100pm).Fig.

Fig.9: The effect of temperature on the percentage removal of metal ions from solution unto KMUC. (Co 100mg/L, pH 6.0, adsorbent dose 0.1g, contact time 180min, particle size 100pm)

RUN0413/ UI / KOVO / Free KMUC 1/cm

Figure 2

FileiUUC.smd

Figure 6

90 80 70 60 50

■a <

So 40 ro

30 20 10 0

■ N ¡(11) • Mn(ll)

0 0.1 0.2 0.3 0.4 Adsorbent Dosage (g)

>• 25

ro 21)

■ N ¡(11) • Mn(ll)

0.1 0.2 0.3 0.4 Adsorbent Dosage (g)

295 300 305 310 315 Temperature (K)

Table 1: Physicochemical Characterization of the Adsorbents.

Parameter UUC KMUC

SiO2 (%) 47.32 42.98

AbOs(%) 25.91 26.25

Fe2O3 (%) 2.14 2.41

CaO (%) 3.39 3.52

K2O (%) 1.07 2.57

Na2O (%) 2.86 4.66

MgO (%) 3.14 3.29

TiO2 (%) 0.12 0.20

MnO (%) 0.43 0.46

LOI (%) 13.56 13.60

Si/Al Ratio 1.83 1.64

SSA (m2/g) 23.2 30.7

Sbet (m2/g) 55.76 87.24

TPV (cm3/g) 0.0688 0.0981

APD (Ä) 49.35 44.98

CEC (meq/100g) 90.78 94.32

pHpzc 3.7 5.2

Slurry pH 4.2 5.8

Table 2: Equilibrium Isotherm Parameters

Isotherm models Ni(II) Mn(II)

Langmuir Model

qL (mg/g) 200 197

Kl (L/mg) 0.006 0.004

r2 0.853 0.723

Freundlich Model

Kf (mg/g) (mg/L)1/n 4.67 3.89

n 1.72 1.75

r2 0.981 0.965

Temkin Model

B (mg/g) 39.57 34.57

A (L/g) 0.067 0.056

r2 0.819 0.803

Dubinin-Radushkevich Model

qm (mg/g) 93.59 79.28

ß (mol2/kJ2) 0.2 x 10-6 0.1 x 10

r2 0.778 0.683

Table 3: Kinetic Parameters for the adsorption of Ni(II) and Mn(II) unto KMUC.

Kinetic Models Ni(II) Mn(II)

qeexp (mg/g) 34.85 32.35

Pseudo-First order

qecai (mg/g) 138.4 131.5

Ki (min-1) 0.044 0.037

r2 0.872 0.773

Pseudo-second order

h (mg/gmin) 0.71 0.49

K2 (g/mgmin) 2.56 x 10-4 1.58 x 10-4

qecai (mg/g) 52.63 55.56

r2 0.971 0.965

Elovich Equation

a (mg/gmin) 1.44 1.08

ß (g/min) 0.081 0.084

r2 0.973 0.980

Intraparticle Diffusion

Kd (mg/gmin1/2) 2.86 2.83

I 0.22 3.2

r2 0.918 0.960

Table 4: Adsorption Thermodynamic Parameters

Metal ion Temp (K) Kc AGU AHU ASU

(kJ/mol) (kJ/mol) (J/molK)

Ni(II) 300 2.30 -2.08 15.52 58.53

308 2.56 -2.41

313 2.97 -2.83

318 3.24 -3.11

323 3.52 -3.38

Mn(II) 300 1.83 -1.51 15.87 57.78

308 2.12 -1.92

313 2.23 -2.08

318 2.62 -2.55

323 2.88 -2.84