OPTIMIZATION OF ADSORPTION OF PB (II) AND CR (VI) FROM AQUEOUS SOLUTION USING MODIFIED FELDSPAR COMPOSITE: ISOTHERM AND KINETIC STUDIES

OPTIMIZATION OF ADSORPTION OF PB (II) AND CR (VI) FROM AQUEOUS SOLUTION USING MODIFIED FELDSPAR COMPOSITE: ISOTHERM AND KINETIC STUDIES

PhD Student, Department of Pure and Applied Chemistry , Kebbi State University of Science and Technology Aliero Kebbi State, Nigeria
Associate Professor, Department of Pure and Applied Chemistry, Kebbi State University of Science and Technology Aliero Kebbi State, Nigeria
Senior Lecturer, Department of Pure and Applied Chemistry, Kebbi State University of Science and Technology Aliero Kebbi State, Nigeria
Research Fellow, Department of Chemistry, University of Ibadan Oyo State, Nigeria

How to cite this article (APA): Sanusi, KA, Yahaya, Y, Ambrusa, MM, Rabiu, AK, & Hammed, AM (2021). Optimization of adsorption of pb (ii) and cr (vi) from aqueous solution using modified feldspar composite: isotherm and kinetic studies. International Journal of Engineering Science Technologies, 5(4), 18. doi: 10.29121/IJOEST.v5.i4.2021.200

Abstract

Keywords

Characterization, Composite, Feldspar, Calcination, FTIR, SEM, XRD

INTRODUCTION

The presence of heavy metals in water sources has become a significant topic of concern for scientists in various fields associated with quality of life of living things and health of the natural environment as well as concern of the general public (Dang, Doan, Dang, & Lohi, 2009). Heavy metals are one of the most worrisome pollutants in the environment due to their potential toxicity, persistence and bio-accumulation problems. Direct toxicity to man and other forms of life and indirect toxicity through the food chains are the focus of this concern (M., ABIA, & A.I., 2003).

Lead (II) ion is widely used in many industries including painting, petrochemical, smelting, metal electroplating, mining, battery industries and plumbing. It is a cumulative poison that enters the body from water pipes made of lead, lead-based paints and leaded petrol (Sanusi & Umar, 2016). Presence of Pb(II) ions even at trace level in environment cause many fatal diseases including malfunctioning of renal blood and neurological systems, blood, kidney, reproductive system, nervous system and brain damage (Karl, 2004). The permissible level for Pb in drinking water is 0.01 mg/L (Jimoh, Egila, Dauda, & Iyaka, 2011).

Chromium exists in stable form as Cr (III) ion and Cr (VI) ion, all other oxidation states of Cr are not stable in aqueous solutions. Cr (VI) is primarily present in the form of dichromate (Cr2O7 2−) and poses higher toxicity than the other oxidation states (Saeed, Iqbal, & Akhtar, 2005). Chromium-polluted water may be from dyeing industry, wood preservation, electroplating and leather tannery. Ingestion of Cr (VI) can result in ulcer and perforation of the mucous membranes of the nasal cavity, dermatitis, respiratory problems, kidney and liver damage (Khan et al., 2001).

Removal of toxic heavy metals by adsorption using low-cost adsorbents is a latest method of choice in the water treatment (Dang et al., 2009). This is due to the several techno-economic advantages associated with low-cost adsorbents when compared to conventional water treatment techniques. Such techno-economic merits include low-cost, easy to obtain, process and utilize, available in abundance as wastes, good efﬁciency of adsorption and environmental friendly (Olu-Owolabi, Alabi, Unuabonah, Paul, & Diagboya, 2016).

Cocoa (Theobroma cacao) is an important agricultural and economic crop. It is one of the key economic crops cultivated in Nigeria. As the third largest producer of cocoa in Africa and one of the highest cocoa producer in the world, the production capacity of Nigeria was reported to have reached about 385,000 tonnes per annum (Olu-Owolabi, Oputu, Adebowale, Ogunsolu, & Olujimi, 2012); Lateef et al., 2016). Cocoa beans are the primary economic part of the cocoa fruit and are the principal ingredients in the manufacture of chocolate and other derived products such as cocoa liquor, cocoa butter, cocoa cake and cocoa powder. The implication of this is that as the cocoa production industry is expanding, the Theobroma cacao pods (TCP) which are the main by-products of cocoa processing are discarded as wastes hence constituting environmental pollution. Theobroma cacao pod (TCP) is a lignocellulosic waste consisting of cellulose 19.7-26.1%, hemicellulose 8.7-12.8% and lignin 14-28% (Tsai et al., 2020).

Feldspar is a crystalline aluminosilicate minerals and appertains to the tectosilicate group that characterized by a larger density framework of layered silicate structure. It is the greatest and most abundant rock-forming mineral in the earth crust. It constitutes about 60% of igneous rocks. The mineralogical composition of feldspar consist of minerals such as orthoclase (KAlSi3O8), albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). In a considerable ratio of Feldspar ores, K-Feldspar (KAlSi3O8, microcline or orthoclase) and Na-Feldspar (albite, NaAlSi3O8) exist in the similar matrix (Yazdani et al., 2012). It has a formula Si4O8 but up to two of Si (VI) may be replaced by Al (III) with the charge deficiency balanced by large univalent cation. The feldspar surface is composed of negative and positive sites. The positive charges are Na+ ions in albite and K+ ions in orthoclase, and the negative sites are nonpolar siloxane groups or polar silanol groups (Awala & Jamal, 2011). The inorganic cations at the surface of feldspar (e.g. Na+, K+ and Ca2+) could be replaced by organic functional groups in Theobroma cacao pod (TCP), such as hydroxyl, carbonyl, carboxyl and methylene groups, through ion exchange, and hence improve the adsorption capacity of the feldspar surface in the modification process (Unuabonah, Günter, Weber, Lubahn, & Taubert, 2013).

Indeed, large amounts of reports are available on the use of biosorbent materials, clay/aluminosilicate and biopolymer/clay composites for metal ion removal from aqueous solution. In contrast, there are virtually limited research reports on feldspar-biobased composites (Unuabonah et al., 2013); (Yazdani, Bahrami, & Arami, 2014). Based on synergistic combinations of FS and TCP, the modified feldspar composite material have potential of a good adsorbent for varieties of pollutants in wastewater besides other industrial applications.

The present work is aimed at investigation of the adsorption of Pb (II) and Cr (VI) onto the modified composite (MFC) adsorbent using the experimental design and optimization of the adsorption parameter with application of Surface response methodology (RSM). The effects of initial Pb(II) and Cr(VI) concentration, adsorbent dose, solution pH, and contact time were optimized, and the isotherm and kinetics of Pb (II) and Cr (VI) ions adsorption onto the novel adsorbent were also studied.

MATERIAL AND METHODS

THEOBROMA CACAO PODS (TCP)

The Theobroma cacao (Cocoa) pods were collected from cocoa plantations in Ifetedo town, Osun State. The TCP samples were washed thoroughly with tap water and then distilled water to remove dust and dirt. The samples were sun-dried and then oven-dried at 700C for 72h to remove moisture. The dried TCP was ground in a mortar into powder. The powdered sample was sieved in a 0.230 mm sieve and stored in a pre-cleaned dry air-tight plastic container (Olu-Owolabi et al., 2010; (Sanusi et al., 2016).

FELDSPAR SAMPLE (FS

The feldspar sample was obtained from the Federal Institute of Industrial Research Oshodi (FIIRO), Lagos, Nigeria. The sample was pre-treated by suspension in deionized water for 24 h and the suspension was carefully decanted to obtain the feldspar (Unuabonah et al., 2013). The organic matter in the sample was removed by adding 30% hydrogen peroxide solution with stirring until effervescence ceased. The mixture was decanted and the feldspar washed with deionized water to remove all traces of hydrogen peroxide and then oven dried at 105 0C (Adebowale, Unuabonah, & Olu-Owolabi, 2006). The sample sieved through a 230 mm mesh size sieve and stored in a pre-cleaned air-tight plastic container (Olu-Owolabi et al., 2016).

Equal weight of feldspar and Theobroma cacao pods (50 g each) were weighed into a 500 mL beaker and 300 mL of 0.1M NaOH was added. The content was thoroughly stirred and heated in an oven at 150 oC until dryness (Olu-Owolabi et al., 2016). The samples of the dried mixtures were weighed into crucibles and calcined at 300 oC for 6 h. The resulting dark powdery material was washed with deionized water to remove residual NaOH from the surface of the composite and subsequently oven-dried at 105oC to remove all moisture. The dried sample was stored in an airtight container (Unuabonah et al., 2013). It was subsequently referred to as Modified Feldspar Composite (MFC).

SELECTION OF OPTIMUM THEOBROMA CACAO POD/FELDSPAR PERCENTAGE COMBINATION IN THE COMPOSITE PREPARATION

In order to select the optimum percentage combination of feldspar (FS) and Theobroma cacao pod (TCP) in the novel composite (MFC), five different combos were prepared which comprise of 10% TCP and 90% FS (10FCP−90FS), 20% TCP and 80% FS (20TCP−80FS), 30% TCP and 70% FS (30TCP−70FS), 40% TCP and 60% FS (40TCP−60FS) and 50% each of TCP and FS (50FCP−50FS). The procedure was followed so as to optimize the preparation of the composite (MFC) adsorbents (Ogbu, Akpomie, Osunkunle, & Eze, 2019).

The cation exchange capacity (CEC) of TCP, FS and MFC samples were determined using the ammonium acetate method as reported by (Unuabonah et al., 2013). Na+ and K+ were determined using AES (Agilent PG 10T), while Ca2+ and Mg2+ were determined by atomic absorption spectrophotometer (SHIMAZU 360H Model). The summation of the exchangeable cations (Ca2+, Mg2+, Na+, and K+) gave the cation exchange capacity (CEC). (Unuabonah et al., 2013).

The functional groups present in the adsorbents (TCP, FS and MFC) were obtained using Shimadzu 8400S FTIR spectrometer operating on Platinum-Attenuated Total Reflectance (ATR) method. The samples were mounted on a diamond crystal disc and recorded with 20 scans in the frequency range of 4000 - 400 cm−1 with a resolution of 2 cm 1 (Adebowale et al., 2006), (Singh & Bhateria, 2020).

The surface morphologies of the adsorbents were analyzed using scanning electron microscopy (SEM), Philips XL30 model. After gold coating (Philip J1050 sputter coater), the samples were placed in the SEM sample chamber where the micrographs of the TCP, FS and MFC were scanned and recorded at 32 scan and 15keV (Hossain, 2013); (Dawood, 2018).

The XRD spectra for TCP, FS and MFC were obtained using Agilent X-ray diffractometer with Ni filtered CuKα radiation at (λ = 1.5406 Å, 40 mA and 40 kV), a diffracted-beam graphite monochromator, and a detector (Salih, 2017). The X-ray diffractograms of TCP, FS and MFC were recorded by step scanning from 1.5° to 50° (2θ) at 0.02° s−1 (Unuabonah et al., 2013); (Salih, 2017). XRF analysis technique was used in this study to determine the chemical composition of the TCP, FC and MFC adsorbents using Bruker AXS S4 Pioneer model X-ray fluorescence (XRF) Spectrometer with 60 kV energy of the X-ray tube (Unuabonah et al., 2013); Yazdani et al., 2012; (Salih, 2017).

Brunauer, Emmet and Teller (BET) results for specific surface area and porosity of adsorbents were obtained from adsorption/desorption isotherm of N2 at 77 0K (Hossain, 2013); (Unuabonah et al., 2013); (Dawood, 2018).

All experiments were carried out in triplicate for all the adsorbents prepared in the study.

Batch adsorption studies were carried out in 100mL plastic bottles by contacting 0.5 g of adsorbent with 20 mL of Pb (II) and Cr (VI) solutions of 100mg/L concentration. The adsorbent/metal ions mixtures were placed on a rotary shaker and shaken at 120 rpm at a room temperature (3000K) for a period of 150min to attain equilibrium (Chukwuemeka-Okorie, Ekemezie, Akpomie, & Olikagu, 2018). The effect of adsorption parameters such as adsorbent dose (0.5-3.0 g), pH (1±0.2 to 8±0.2) contact time (15-300 min) and adsorbate concentration (50-300 mg/L) were investigated for Pb (II) and Cr (VI) onto FS and MFC adsorbents. On completion of experimental time, the sample mixtures were filtered using whatman 0.45 micron filter paper and the filtrates were be analyzed for the concentration of the metal ions left in solutions using atomic absorption spectrophotometric (AAS) method (Unuabonah et al., 2013); (Anusa, Ravichandran, & Sivakumar, 2017). All adsorption experiments were carried out in triplicate for all the four metals studied.

A standard response surface methodology (RSM) design, central composite design (CCD), was utilized to determine the optimum set of adsorption process variables, and this includes three steps of experimental design, response surface modeling, and process optimization. Generally, in a 5 level study. The CCD consists of 2 n factorial runs, 2n axial runs (indeed these 2n points fixed axially at a distance, $\alpha$ , from the center to produce the quadratic terms), and n c center runs (2𝑛+2𝑛+𝑛𝑐 ), where 𝑛 is the number of adsorption parameters.

Number of experiments (𝑁) = (2𝑛+2𝑛+𝑛𝑐) = 16+ (2×4) +8 = 32 32

DESIGN OF EXPERIMENT USING RESPONSE SURFACE METHODOLOGY

Design-Expert 12 software (“MiniTab” version 15) was applied in the design of experiment. In the design process, four factors (adsorption parameters) were considered for optimization, viz; initial metal concentrations (50-150 mg/L), pH (2-8), adsorbent dosage (0.5-2.5g) and contact time (30-210 min) and adsorption efficiencies of Pb (II) and Cr (VI) ions onto MFC (response).

DETERMINATION OF EFFECT OF INTERACTION OF ADSORPTION PARAMETERS (FACTORS)

A quadratic model in form of expression below was developed for the adsorption of the metal as it was influenced by the adsorption parameters considered during the experiments. The response model may be represented as

Where 𝑌 is the response, and the objective is to optimize the response (𝑌). 𝛽0 is the constant coefficient, is 𝛽𝑖 the linear coefficients, 𝛽𝑖𝑖 is the quadratic coefficients, 𝛽𝑖𝑗 is the interaction coefficients, and 𝑋𝑖and 𝑋𝑗 are the coded values of the independent process variables. Coefficient of determination (R2) was used to determine the quality of the model as it compared the data from predicted model with data got from experimental runs (Yazdani et al., 2013; (Adetokun, Uba, & Garba, 2019).

DATA TREATMENT

The amount of metal ions adsorbed per gram of FS and MFC was calculated using equation 1.

Where; and are the initial and final metal ion concentrations in solutions; , V and M are the amount of metal ions adsorbed (mg/g), volume of the solution (mL) used for experiment and mass (g) of sample, respectively.

The percentage removal of metal ions by FS and MFC was calculated using the equation 2.

Where; R is the removal efficiency of the metal ions adsorbent studied; CO is the initial metal ions concentration in solution (mg/L); ${C}_{e}$ is the metal ions concentration removed or adsorbed by adsorbent at equilibrium (mg/L).

The relationship between the amount of Pb (II) and Cr (VI) ions adsorbed onto the adsorbents (FS and MFC) and their equilibrium concentrations in aqueous solution were evaluated using of Langmuir and Freundlich models.

The Langmuir isotherm is represented by the equation 3.

or its linear form in equation 4.

When $\frac{{C}_{e}}{{q}_{e}}$ is plotted against Ce, the slope is equal to $\frac{1}{{Q}_{o}}$ and the intercept is equal to

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter (RL) which is defined by:

Where b is the Langmuir constant and C0 the highest metal ion concentration (mg/L). The value of RL indicates the type of the isotherm to be either unfavourable (RL > 1), linear (RL = 1), favourable (0 < RL < 1) or irreversible (RL = 0).

The Freundlich isotherm is represented by equation 6.

or its linear form in equation 7.

The values of ${K}_{f}$ and n may be calculated by plotting log ( ${q}_{e}$ ) versus log ( ${C}_{e}$ ). The slope is equal to $\frac{1}{n}$ and the intercept is equal to log ( ${K}_{f}$ ).

Where is the maximal adsorption capacity per unit weight of adsorbent, b is a solute–surface interaction energy-related parameter, while and are the amount of metal ions adsorbed (mg/g) and metal ions concentrations in solutions after the equilibrium; where and n are the Freundlich model capacity factor and the isotherm linearity parameter respectively (Saeed et al., 2005); Das and Mondal, 2011)).

Four adsorption kinetics models were employed in describing the adsorption experiment data in order to examine the mechanism of the adsorption process. These are the Lagergren pseudo-first-order (PFO), pseudo second-order (PSO), intra–particle diffusion and Elovich models.

The equations are expressed below:

Where qe and qt are adsorption quantity (mg/g) at equilibrium and at time t, respectively; k1 and k2 are the rate constants (min−1) of the PFO and PSO, respectively; ) is the rate parameter of the intra-particle diffusion control stage; α is the initial adsorption rate (mg/g/min) and is related to the extent of surface coverage and activation energy for chemisorption (g/mg).

The qe and rate constants were calculated from the slope and intercept of the plots of log (qe−qt) vs. t; and t/q vs. t for PFO and PSO respectively. From equation 10, a plot of and interceptadsorption mechanism is in line with the intra-particle diffusion process. If the intra-particle diffusion process is the predominant rate-limiting step in the adsorption, the plot would pass through the origin (Olu-Owolabi et al., 2016) (Sanusi et al., 2016).

RESULT AND DISCUSSION

TEST FOR THE PERFORMANCE OF MODIFIED FELDSPAR COMPOSITE (MFC) FOR METAL ADSORPTION

The adsorbents were utilized for the adsorption of the metal ions from the aqueous solution at metal concentration 100mg/L as shown in Figure 13 . It was observed that an increase in percentage adsorption and adsorption capacity of the metal ions with increase in the amount of TCP in the composite were obtained. However, 50TCP−50FS recorded the highest percentages for Pb (97.8%) and Cr (92.1%) respectively. In general, the increase in metal adsorption with TCP concentration is possibly due to increase in the number of active binding sites on the MFC and improved heterogeneous nature of the adsorbent. The 50TCP−50FS composite adsorbent was chosen and utilized in this study for adsorption due to its relatively higher adsorption potential recorded than the other composite mixtures.

CATION EXCHANGE CAPACITY (CEC)

From the results in Table 2 , MFC had the higher value for CEC (30.66 ± 0.21 meq/100g) compared to FS (6.42 ± 0.45 meq/100g). The results indicated that MFC, a composite resulting from the combination of FS and TCP has improved ion-exchange capacity from the surface modification process.

FOURIER TRANSFORM INFRARED (FTIR SPECTROSCOPY ANALYSIS

The infrared spectra of TCP, FS, and MFC were shown in Figure 16; Figure 15; Figure 14. FTIR spectra of TCP showed several absorption peaks between the scanning frequency ranges of 4000-500 cm-1. The peak exhibit a broad and strong −OH band at 3479 cm−1 indicating the presence of hydroxyl, phenolic or carboxylic −OH stretching vibration as typical of cellulose and lignin (i.e. free hydroxyl groups in the polysaccharide structure of its wall). The strong band at 3408 cm−1 is a −N−H stretching indicative of vibration associated with N- substituted amide. The two sharp absorption bands at 2923 and 2854 cm–1 showed the presence of C–H stretch likely from aliphatic (methyl and methylene) groups. The sharp band appearing at 1747cm-1 is characteristic of typical C=O stretch bond from either a free acid or esterified carboxyl group. However, a distinct absorption band at 1651 cm–1 is assigned to a C=O stretching vibration of carbonyl group. The strong band appearing at 1380 cm-1 suggests a carboxylate ester. Moreover, the prominent band at 1057 cm-1 is associated with the C−O bond or −C−C− stretching such as glycosidic linkage which are characteristic of polysaccharides. Similar observations have been reported by Pehlivan et al. (2007), Olu-Owolabi et al. (2012b), and (Unuabonah et al., 2013).

The FT-IR spectra of feldspar (FS) exhibit a strong band at at 3670 cm-1 indicative of the presence of hydroxyl group (–OH stretching vibration) lying between the octahedral sheets in the feldspar mineral surface. This peak at 3670 cm-1 is characteristic of –OH group residing at the surface of the layer that form weak hydrogen bonds with oxygen of the Si–O–Si bonds on the lower surface of the next layer. The absorption peaks noticed at 3447 and 1637 cm–1 are H−O−H stretching and bending vibrations from absorbed water. The strong peak at 1366 cm-1 is suggestive of Si-O stretching vibration band. Bands at 1067 and 952 cm−1 correspond to the Si−O and Al−OH in plane bending vibrations, respectively. The peaks at 993 and 767 cm-1 were attributed to SiO2 and Al−OH deformation. The two medium but weak peaks at 615 and 559 cm-1 indicate the presence of Si-O stretching as well as –OH bending vibration respectively. FT-IR spectra analysis confirmed that the aluminosilicate material composed of feldspar (Awala et al., 2011), 2011; (Unuabonah et al., 2013) and (Yazdani et al., 2014).

For the MFC adsorbent, as it could be seen (Figure 16 ), changes in the absorption bands of the feldspar after TCP modification were observed in the spectrum which is an indication of the successful modification of the adsorbent. The absorption band of the TCP/feldspar (MFC) appearing at 3432 and 3696 cm−1 has been enhanced, which resulted from vibration bands of TCP (O–H and N–H stretching) overlap with the bands of feldspar (–OH stretching of H2O). The bands at 2928, 2849 cm−1 and 1380 cm−1 related to the intercalated TCP (–CH2 stretching and C–H bending on methyl and methylene groups) are seen in the spectra of the bio-based (MFC) composite (Unuabonah et al., 2013). The Si–O bending vibration observed at 1034cm–1 showed a significant shift when compared to that of the raw feldspar. This suggests that the peak position is perhaps one of the active sites for interaction of the feldspar with the TCP biomass (Dawodu and Akpomie, 2014). The band at 1626 cm-1 attributed to the deformation vibration of the protonated amine group (−N–H) of TCP was also observed in the MFC. The intensity of the band at 1626 cm−1 of the MFC spectra has increased with a pronounced band shift from the original constituent materials, indicating the first NH–CO group stretching vibration of TCP overlap with –OH bending vibration of H2O in the interlayer of feldspar. More so, corresponding to the vibration bands of silicates, and other adsorption bands are relatively similar to those of the feldspar sample with only slight shift indicating the MFC adsorption active sites responsible for binding of metal ions (Ogbu et al., 2019). Overall, FTIR spectra confirmed that the MFC is indeed a composite consisting of feldspar (FS) and Theobroma cacao pod (TCP).

X-RAY FLUORESCENCE (XRF ANALYSIS)

Showed the results obtained from X-ray fluorescence (XRF) analysis of the adsorbents. XRF detected only a slight change in the Si/Al ratio before and after modification of the feldspar. MFC has a higher silica to alumina ratio (5.52) than the FS (3.95) which might be associated with the leaching of silicon and aluminum by base treatment during the surface modification process (Unuabonah et al., 2013). From XRF results, it was observed that the percentage composition of K is higher than Na and Ca in the feldspar sample, suggesting an orthoclase (K- feldspar) (Awala et al., 2011). XRF also finds a number of other elements (Fe, Ca and Mg) that are typical of the raw feldspar (Unuabonah et al., 2013) and Yazdani et al., 201). XRF did, however, detected the elements only at low levels in TCP except for K and Ca which are essential minerals.

X-RAY DIFFRACTION (XRD ANALYSIS)

X-ray diffraction (XRD) analysis of the FS sample showed that it is a feldspar containing orthoclase, microcline and quartz Figure 17 . This suggests that the feldspar has two variety (phases) of K-feldspar minerals; the orthoclase (O) and microcline (M) (Unuabonah et al., 2013). The reflections at 20.08° (2θ) (3.83 Å, 2̅01) and 25.6° (2θ) (3.11Å, 002) could be assigned to that of orthoclase and microcline minerals respectively. The reflections at at 20.08° (2θ) (3.83 Å, 2̅01), 25.6° (2θ) (3.11 Å, 002) and 27.80° (2θ) (2.89 Å, 040), 30.44° (2θ) (2.67 Å, 13̅1) indicated that the crystals of the feldspar sample used in this study were partly monoclinic and partly triclinic (Unuabonah et al., 2013). The reflection at 26.60° (2θ) is from quartz (3.00 Å, 131), which is also present in the raw material. The XRD pattern of feldspar showed typical diffraction peaks at 2θ = 13.25 (6.6 Å) and 14.40∘ (6.14 Å), whereas, after incorporating with TCP, these peaks shift to lower angles and even disappear. The disappearance of the feldspar reflection at 2θ =14.40° and the appearance of a new reflection at 11.25° 2θ in the XRD pattern of MFC composite showed that intercalation has occurred (Yazdani et al., 2014). These results suggested that the order along the crystallographic c-axis of feldspar is slightly disturbed by the reaction between the TCP and the FS. The rest of the crystal structure is not affected by the MFC preparation process, indicating that the crystallography of the individual aluminosilicate layers is not affected. Similar observation was reported (Ofomaja, Unuabonah, & Oladoja, 2007), (Unuabonah et al., 2013). The XRD patterns of the TCP with two broad and overlapping reflections essentially indicate a low order structure. Correspondingly, the XRD patterns of the MFC have increased intensity than those of the FS indicating that the incorporation of TCP via calcination cause little orderliness in the resulting MFC material, presumed by disturbing the interlayer geometry between individual feldspar mineral layers. This likely confirmed the disordered intercalation in MFC composite as reported (Sanusi et al., 2016).

SEM ANALYSIS

Figure 19; Figure 18 showed scanning electron microscopy (SEM) of FS and MFC respectively. The SEM is used to examine the surface morphology and the porous nature of the material responsible for adsorption of metal ions. The SEM images supported the XRD results, as there was no significant morphological disparity between the FS and MFC. Both adsorbents are porous, consisted of considerable number of heterogeneous pores, irregular surface and particle aggregation of various shapes and sizes with relatively flat facets, which is typical of feldspar mineral (Awala et al., 2011) and (Ogbu et al., 2019). The presence of pores helps in the diffusion of metal ions into the adsorbents during the adsorption of metal ions from solution (Dawodu and Akpomie, 2014). Also, the SEM images of FS and MFC showed some white clumps or particles on the surface, these were probably due to the presence of non-clay minerals like potassium, magnesium, sodium and calcium (Ogbu et al., 2019). From the image of MFC (Figure 19 ), it was observed that the bio-based composite smaller particle sizes compared to the raw feldspar.

BET SURFACE AREA AND PARTICLE SIZE ANALYSIS

The results of cation exchange capacity (CEC) (Table 2 ) and the surface area (SA) (Table 4 ) supported the FTIR, XRF, XRD, and SEM images. The results reflected the similarities and differences between the FS and MFC. The MFC adsorbent showed an increased surface area (53.60 ± 0.3 m2/g) compared to FS (21.56 ± 0.1 m2/g), suggesting that the new material has the smallest particle sizes. This is supported by the results of particle size analysis of the adsorbents in which FS and TCP particles sizes decreased from a mean size of 162.8 μm (FS) to 105.4 μm for the MFC adsorbent. Finally, the CEC increased from 6.42 ± 0.45 meq/100 g (FS) to 30.66 ± 0.21 meq/100 g (MFC) upon functionalization. The narrow pore diameter (8.571± 0.11 nm) for MFC has indicated a micropore dimension as improvement from feldspar mesopores (30.65 ± 0.03nm). These mesopores were responsible for transporting metal ions into the micropores of MFC, consequently improving the adsorption capacity of the MFC (Ogbu et al., 2019).

Overall, FTIR, XRF, XRD and SEM confirmed that the composite (MFC) was essentially identical to the feldspar (FS) starting material; a tectosilicate with only slight modification. These observations were reported in a similar studies by (Awala et al., 2011); (Unuabonah et al., 2013) and (Yazdani et al., 2014), (Sanusi et al., 2016).

EFFECT OF OPERATING PARAMETERS ON METAL ADSORPTION

EFFECT PH OF THE SOLUTIONMETAL ADSORPTION

Comparing adsorption of metals at optimum pH values on all the adsorbents, the adsorption trend of Pb (II) < Cr (VI) may be attributed to differences in behaviour of metal ions in the aqueous solution such as the smaller hydrated radius of Pb (II) (0.401 nm) compared to Cr (VI) (0.512 nm) and the higher electronegativity of Pb (II) (1.69) than Cr (VI) (5.08) (Ogbu et al., 2019). These factors accounts for the higher adsorption of Pb (II) than other metals on adsorbents which has been reported in similar research works (Akpomie and Dawodu, 2014; (Chukwuemeka-Okorie et al., 2018). It is noteworthy that the optimum adsorption at pH 3 for Cr (VI) and pH 6 for Pb (II) was carried out in the batch adsorption experiment to avoid the precipitation of metal ions associated with higher pH values.

EFFECT OF INITIAL METAL CONCENTRATION METAL ADSORPTION

The results of the influence of initial metal concentration on the percentage adsorption of Pb (II) and Cd (II) on the adsorbents is shown in Figure 23; Figure 22 .

Decrease in percentage removal of the metal ions on the adsorbents with increase in initial metal ion concentration was observed. In fact, with increase in initial metal concentration after 100 mg/L, the percentage removal of Pb (II) decreased from 68.76 to 50.66% and 91.86 to 68.16% for FS and MFC respectively. Similarly, the percentage adsorption of Cr (VI) also showed a decrease from 63.86 to 32.00% (FS) and 91.48 to 60.49% (MFC) respectively. The decrease in percentage adsorption with increase in metal ion concentration is due to the fact that at lower concentrations, more of the metal ions would be removed by the abundant active sites on the surface of the adsorbents. However at higher metal ion concentrations, less metal ions were adsorbed due to the saturation of the available active adsorption sites on the adsorbent surface preventing further metal ion binding hence the adsorption percentages decrease (Ozdes, Duran, & Senturk, 2011). This implies that if the concentration is significantly increased further, a corresponding decrease in percentage removal would be expected due to complete saturation of the active sites. Similar result has been reported (Das and Mondal, 2011).

The effect of adsorbent dosage on removal of Pb (II) and Cd (II), As(III) and Cr(VI) is shown in Figure 25; Figure 24 respectively. An increase in the percentage adsorption of all the metal ions with increase in adsorbent dose was observed for all the adsorbents. However, the adsorbent dosage of 0.5 g recorded the highest adsorption because of its high adsorption capacity.

With adsorbent dosage from 0.50 to 3.0 g, an increase in percentage adsorption of Pb (II) from 61.32 to 67.36 and 93.58 to 98.93% was obtained for FS and MFC respectively. For Cr (VI) ions on the respective adsorbents, an increase in removal percentage from 58.12 to 62.32% (FS) and 86.66 to 92.90% (MFC) was obtained. The increase in metal adsorption with dosage increase may be attributed to an increase in the number of active sites available on the adsorbent surface (Akpomie and Dawodu, 2014). Similar results have been reported (Olu-Owolabi et al., 2012); (Unuabonah et al., 2013); and (Sanusi et al., 2016).

EFFECT OF CONTACT TIME METAL ADSORPTION

The results of the effect of contact time on the adsorption of Pb (II) and Cr (VI) ions onto the adsorbents were presented in Figure 29; Figure 28 . An initial rapid adsorption was observed which increased until equilibrium was reached, after then the metal uptake rate became slow steadily.

It was observed that Pb (II) and Cr (VI) ions uptake was rapid within (15-30 min), afterward it continued at a slower rate and finally reached equilibrium in 60 minutes. After equilibrium, there was little or no adsorption (marginal uptake) till 300 minutes. Further increase in contact time beyond the equilibrium time had no significant effect on the amount of Pb (II) and Cr (VI) ions adsorbed. The equilibrium percentage adsorption of Pb (II) ions was 80.92% (FS) and 99.72% (MFC) and that of Cr (VI) was 65.99% (FS) and 96.24% (MFC) at contact time of 60 min respectively. The presence of more vacant adsorption active sites at the surface of FS and MFC initially accounted for the initial rapid metal uptake, afterward the metal adsorption progressed slowly and finally attained equilibrium at 60minutes when the active adsorption sites were saturated. Similar observation on the effect of contact time on the two metal uptake was reported (Sanusi et al., 2016). The faster rate of adsorption of Pb (II) as compared to Cr (VI) is due to the smaller ionic radii of Pb (II) (0.097 nm) than Cr (VI) (0.12 nm) which makes its diffusion faster to the surface of adsorbents than the larger-sized Cr (VI) ion (Unuabonah et al., 2013).

From the batch adsorption study, an optimum contact time of 60min for (Pb and Cr) was used in this study to ensure that accurate equilibrium adsorption of the metal ions on all the adsorbents was attained.

MODEL DEVELOPMENT FOR OPTIMIZATION OF PB (II AND CR (VI ADSORPTION

In this experimental study, the central composite design (CCD) method was applied to design experiments as presented in Table 5 , propose empirical models, investigate the interaction effects of four factors (adsorption parameters) and determine the optimum values for the studied operating variables (pH, initial metal concentration, adsorbent dose and contact time) to achieve the maximum response (adsorption efficiency of Pb (II) and Cr (VI) in aqueous solution (Yazdani et al., 2013; (Singh et al., 2020).

The models (in actual form) for the respective metal ion uptake as presented in equations 2 and 3 were employed to analyze the effect of the independent variables on adsorption amount (𝑌. The results revealed that pH, adsorbent dosage, contact time and metal concentration presented the most significant effects on the metal adsorption. A positive sign coefficient term indicating the variables affecting metal uptake positively (synergetic effect) while negative sign coefficient terms are suggestive of uptake been antagonistic effect (decrease) on response values predicted by the quadratic model equations (Yazdani et al., 2013).

The results of the statistical analysis and significance test for the model applied were obtained from analysis of variance (ANOVA) as displayed in Table 7; Table 6 . A higher 𝐹 value indicates a greater significance of most of the variables in the response indicated in model regression equations. Most of the probability (p values) of the models are less than 0.05 which indicated that they are statistically significant (s) while those greater than 0.05 are insignificant (n) indicated as superscript on the values shown in Table 6, 7. Those significant values were further confirmed by the high R2 of the models (Adetokun et al., 2019). The optimum variables predicted by the model and that of experimental results for maximum uptake of the four metals were presented in Table 9; Table 8 . These were further validated by comparing experimental results to the predicted results. Higher coefficients of determination i.e. adjusted R2 (0.9919-0.9986), is in good agreement with the predicted R2 (0.9966-0.9996) as shown in table 8, 9 signifying less variation between the predicted and actual values (Adetokun et al., 2019). The results confirmed the adequacy of the selected quadratic model (Singh et al., 2020).

RESPONSE SURFACE PLOTTING

The optimum values of experimental (actual) and the maximum values predicted responses are presented in Table 9; Table 8 . The maximum removal percentages for Pb (II) and Cr (VI) were predicted to be 99.95% and 92.48%, respectively. Verification experiments were conducted under the optimum conditions, and the removal percentages were 99.98% and 96.24% for Pb (II) and Cr (VI) respectively at adsorbent dose of 1g, initial metal concentration (100mg/L) and temperature (300K). These experimental results were similar to the predicted values and indicated the suitability, accuracy and adequacy of the models used. The metal uptake capacities for the metals obtained under the optimum experimental conditions were 14.148 mg/g and 3.504 mg/g for Pb (II) and Cr (VI) respectively while the predicted values for Pb (II) and Cr (VI) were 14.021 mg/g and 3.428 mg/g respectively.

PROCESS OPTIMIZATION

The main objective of the present work was to reach the most effective values of process variables for the optimization of Pb (II) and Cr (VI) adsorption onto the MFC composite from aqueous solution. This aim was fulfilled using the quadratic model within the studied experimental range of the selected parameters. The optimization of the selected adsorption parameters at optimum values of pH (2 and 6), temperature 300K, initial metal concentration 100mg/L, and adsorbent dose 0.5 g revealed the maximum metal uptake capacities (predicted values) of 14.021 mg/g and 3.128 mg/g for Pb (II) and Cr (VI) respectively. Correspondingly, the experimental values of the metal adsorption under the optimum condition of the process parameters were obtained as (14.148 mg/g and 3.504 mg/g) for the selected metal ions (Table 8). It should be noted that the experimental and predicted results of metal adsorption efficiency onto MFC at optimum variables values was determined as (99.98%, 99.95%) for Pb (II) and 96.24%, 92.48%) for Cr (VI) respectively as shown in (Table 9 ).

Comparing the potential of the bio-based composite adsorbent (MFC) for the uptake rate of the selected metal ions with pure feldspar (FS), it is evident from Figure 32 that the adsorption amount of metal ions onto FS lower than the adsorption amount of metal ions onto the MFC. This result has indicated that the metal ions have lower affinity to feldspar surface, while its modification with Theobroma cacao pods resulted the enhancement of metal ions adsorption onto the composite material (MFC). Similar observations were reported in the previous research works on optimization of adsorption process parameters (Yazdani et al., 2013; (Adetokun et al., 2019).

The equilibrium adsorption isotherms are one of the most important data that provide information on surface properties of adsorbents, adsorption mechanism and sorbents affinity (Das and Mondal, 2011). The relationship between the amount of Pb (II) and Cr (VI) ions adsorbed onto the adsorbents (FS and MFC) and their equilibrium concentrations in aqueous solution were evaluated using of Langmuir and Freundlich models.

The equilibrium data obtained from the adsorption of Pb(II) and Cr(VI) ions onto FS and MFC were fitted the Langmuir and Freundlich isotherm models (Fig. 10(a) and (b)). All of the isotherm constants and correlation coefficients were calculated from the linear equations of the models and provided in Table 8. According to the Langmuir isotherm model, the maximum monolayer adsorption capacity of FS and MFS for Pb (II) was found to be higher than for Cr (VI) ions indicating that Pb (II) ions were well adsorbed onto adsorbents surfaces than Cd (II) ions. Between 50 and 300 mgL-1 of initial metal ions concentration, the RL values ranged from 0.76 to 0.11 for Cr (VI) ions and from 0.22 to 0.01 for Pb(II) ions then approached zero with increase in the Co values of both metal ions, indicated that the Pb (II) and Cr (VI) adsorption onto FS and MFS is favorable under studied conditions. Furthermore n values, obtained from Freundlich isotherm model were higher than unity which supported the favorability of adsorption process. By comparing the correlation coefficient values obtained from Langmuir and Freundlich isotherm models, it could be concluded that the Langmuir isotherm model is more suitable for Pb (II) ions while the experimental data obtained for Cr (II) ions adsorption were fitted well to Freundlich isotherm model. This may be due to both homogeneous and heterogeneous distribution of adsorption active sites on the surface of the FS and MFC. Similar observation was reported (Adebowale et al., 2006); (Ozdes et al., 2011); (Unuabonah et al., 2013)

Kinetic modeling evaluates adsorption mechanism, which include mass transfer, chemical reaction and diffusion control (Ogbu et al., 2018). Four adsorption kinetics models were employed to investigate the mechanism and rate of the adsorption process. These are the Lagergren pseudo-first-order (PFO), pseudo second-order (PSO), intra–particle diffusion and Elovich models. The kinetic rate constants obtained from the application of the kinetic model equation for the adsorption of Pb (II) and Cr (VI) ions unto the adsorbents are shown in Table 9 .

The metal ions uptake was rapid for the first 30 min because at the beginning of the adsorption the active adsorption sites on the adsorbent surface were more available thus metal ions could interact easily with these sites. Afterward it continued at a slower rate and finally reached to the equilibrium by virtue of the saturation of adsorption sites on adsorbent surface within 60 min.

The qe and rate constants were calculated from the slope and intercept of the plots of log (qe−qt) vs. t; and t/q vs. t for PFO and PSO respectively.

From equation 10, a plot of and interceptadsorption mechanism is in line with the intra-particle diffusion process. The intra-particle diffusion process, though not the only rate-limiting steis the predominant in the metal adsorption onto FS and MFC, as the plot did not pass through the origin (Olu-Owolabi et al., 2016) , (Sanusi et al., 2016) .Furthermore, the occurrence of the intercept C, showed the involvement of surface active sites onad(Yazdaniet al2013A significant adsorption of both Pb (II) and Cr (VI) ions was achieved during movement of adsorbates from solution to adsorbents surface indicating the conformity of metal uptake process to the IP (Chukwuemeka-Okorie et al., 2018).

By evaluating the intra-particle mass transfer curve, it was observed that the Pb (II) and Cr (VI) adsorption process was found to be followed by two distinct phases. The first phase is attributed to the diffusion of metal ions through the solution to the external surface of FS and MFS while the second phase indicated the intra-particle diffusion (IPD) of Pb (II) and Cr (VI) ions into the pores of the adsorbents. The intra-particle rate constants for the first phase (kid, 1) and second phase (kid, 2) and C parameters were obtained from the plot of qt versus t1/2 and the results were given in Table 11 . By comparing the rate constants, the lower values of (kid 2) than (kid 1) indicating that the rate limiting step for Pb (II) and Cr (VI) adsorption process is intra-particle diffusion. As observed, all the R2 values obtained for IPD for both metal ions were greater than 0.9. This implies that film diffusion (surface phenomenon) also played a dominant role in the adsorption process. As a result, Pb (II) and Cr (VI) adsorption onto FS and MFC is considered as a complex process and both intra-particle diffusion and surface adsorption contributed to the rate limiting step (Ozdes et al., 2011).