Preparation and application of EDTA-functionalized underutilized Adansonia digitata seed for removal of Cu(II) from aqueous solution

Accepted Manuscript Preparation and application of EDTA-functionalized underutilized Adansonia digitata seed for removal of Cu(II) from aqueous solution Adewale Adewuyi, Fabiano Vargas Pereira PII:

S2468-2039(17)30249-2

DOI:

10.1016/j.serj.2017.12.002

Reference:

SERJ 110

To appear in:

Sustainable Environment Research

Received Date: 8 August 2017 Revised Date:

23 October 2017

Accepted Date: 20 December 2017

Please cite this article as: Adewuyi A, Pereira FV, Preparation and application of EDTA-functionalized underutilized Adansonia digitata seed for removal of Cu(II) from aqueous solution, Sustainable Environment Research (2018), doi: 10.1016/j.serj.2017.12.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Received 8 August 2017 Received in revised form 23 October 2017

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Accepted 20 December 2017

Preparation and application of EDTA-functionalized underutilized Adansonia digitata seed

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for removal of Cu(II) from aqueous solution

a

Department of Chemical Sciences, Redeemer’s University, Ede 230, Nigeria

Department of Chemistry, Federal University of Minas Gerais, Belo Horizonte 6627, Brazil

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b

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Adewale Adewuyia,b,*, Fabiano Vargas Pereirab

*Corresponding author E-mail address: [email protected] 1

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ABSTRACT The application of ethylenediaminetetraacetic acid surface functionalized Adansonia digitata seed (EAD) from Nigeria was evaluated as a possible means of treating Cu contaminated

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water. The seed of A. digitata (AD) and EAD were characterized using Fourier Transform Infrared spectrometer, Thermogravimetric analysis, X-ray Diffraction analysis, Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Particle Size Dispersion (PSD), zeta

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potential and elemental analysis (CHNS/O analyzer). The kinetics studies revealed the adsorption of Cu(II) onto AD and EAD to follow pseudo-second order model which fitted well

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for Temkin and Freundlich isotherm. The adsorption was controlled by both intra-particle diffusion and liquid film diffusion with adsorption capacity of AD (33 mg g-1) higher than that of EAD (27 mg g-1). The PSD of AD and EAD were found to be mono-modal, the ∆Go values increased as the temperature increased in EAD unlike AD where the value was reduced as

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temperature increased. The present work showed A. digitata seed as a potential resource for treating Cu contaminated water. The present work also reveals that it is important and necessary

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to pretreat plant sourced materials before being used for wastewater treatment.

1.

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Keywords: Adansonia digitata; Adsorption; Cu; EDTA; Wastewater

Introduction

Heavy metals have been reported to be non-biodegradable, toxic and capable of

accumulating in living tissues, causing various diseases and disorders [1]. Although they are important in many applications but they become dangerous when they get into the environment. Cu is an example of heavy metal that has found application in architecture, automotive, tubing 2

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and piping, electricity supply, roofing and chemical production. Most times, The Cu contaminated wastewaters are directly or indirectly discharged into the environment causing health issues, especially in developing countries such as Nigeria. Recent studies conducted at

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Onitsha, Anambra State which is the most urbanized city in Southern Nigeria revealed heavy pollution of Cu at both surface and subsurface water with adverse consequence of noticeable increase in disease incidence especially cancer, kidney infection, cardiovascular disorder and

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gastroenteritis among the populace [2]. Similar observation of heavy metal pollution has also been reported by Iweala et al. [3] in foods and drinks from Ota, Ogun State, Nigeria.

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Cu pollution is one of the serious environmental problems among other heavy metals pollution problems in Nigeria. Although Cu is considered to be an essential heavy metal which is required for physical and mental development [4], Cu becomes toxic when its concentration exceeds 5.0 mg kg-1 body weight in humans, especially to the gastrointestinal system [5]. The

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treatment of Cu contaminated water is very important due to its recalcitrance and persistence in the environment. So, this toxic heavy metal should be removed from contaminated water in order to protect human and the environment from its hazardous effect. Several methods have been used

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in the past but most of these methods suffer from one limitation or the other [6]. Among the previously reported methods, adsorption is recognized as an effective and

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economic method for the treatment of heavy metal contaminated water [6]. The operational process is flexible in design and reusable over a period of time. Interestingly, focus has been on the development of cheap adsorbents to replace costly ones in wastewater treatment. Use of biomass like plant wastes has the potential of serving this purpose. The use of plant wastes for purification of polluted water has several advantages such as; good adsorption capacity, simple to use, relatively cheap, readily available, non-toxic, easily reused and requires little operational 3

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steps. Underutilized plant seed falls into this category of plant waste that can be used as low-cost adsorbents to replace costly conventional methods or adsorbents for wastewater treatment. Adansonia digitata seed is an example of underutilized plant seed in Nigeria which

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belongs to the Bombacaceae plant family. The seed is considered as waste because it has no specific use presently in Nigeria. Currently, it is discarded as waste and most times create disposal problem. Finding application for the seed of A. digitata in Nigeria will be worth

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investigating. So, in continuation of our previous work [7], this present study investigated the use of A. digitata seed as a low-cost and readily available adsorbent for the removal of Cu(II) ions aqueous

system.

The

seed

was

pretreated

and

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from

surface

modified

with

ethylenediaminetetraacetic acid (EDTA). EDTA was used as the modifying agent due to its ability to exchange its hydrogen atom for metal cation when it ionizes in solution, e.g., it is able to complex Ca2+, Mg2+ and some heavy metal ions from solution [8-10]. Since Cu(II) also

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belongs to the +2 state cations, the present study modified the surface of AD with EDTA (EAD) with the aim of producing a low-cost adsorbent for removing Cu(II) ions in aqueous solution. The Cu adsorption capacity of EAD was compared with that of AD. The effects of pH,

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temperature, contact time, initial concentration and adsorbent dosage on the removal of Cu(II)

2. 2.1.

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ions from the aqueous solution by the EAD and AD were investigated.

Material and methods Materials

AD used was obtained from the Botanical garden at the University of Ibadan, Ibadan,

Oyo state, Nigeria. AD was ground in an industrial mill, air dried and stored in an airtight container. Stock solutions of 1000 mg L-1 was prepared by dissolving accurately weighed 4

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amounts of (Cu(SO4)· 5H2O) in 1000 mL deionized water. Experimental solutions were prepared by diluting the stock solution with deionized water. Sodium bisulfite, sodium chlorite, EDTA, glacial acetic acid, NaOH and all other chemicals used in this study were purchased from Sigma-

2.2.

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Adrich (Brazil). Pretreatment of A. digitata seed

The seed of AD was extracted with n-hexane in a soxhlet extractor in order to remove fat

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soluble materials. Lignin and hemicellulose were further removed from the seed as previously described by Adewuyi and Pereira [11]. Briefly, the defatted seed powder was treated with 0.7%

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sodium chlorite solution at 60 °C for 2 h under continuous stirring. This was filtered, washed severally with deionized water and placed in 2% sodium bisulfite solution for 1 h; further filtered using Whatman no 4 filter paper, washed with deionized water and dried. The dried mass was then treated with alkali (17.5% NaOH) for 2 h to remove hemicelluloses; this was later filtered,

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washed severally with deionized water until it was completely free of alkali and oven dried at 50 °C. 2.3.

Preparation of EDTA-functionalized A. digitata adsorbent

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The pretreated seed material was finally functionalized with EDTA via simple surface reaction. To achieve this, the pretreated seed material was weighed (30 g) into a two-necked

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round bottom flask containing a 100 mL solution of EDTA (10 mg L-1) while the temperature was maintained at 60 °C for 10 h. The final product was filtered using Whatman no 4 filter paper, washed severally with deionized water and oven dried at 50 °C to obtain the EAD of about 98% yield. 2.4.

Characterization of AD and EAD

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The functional groups on the surface of AD and EAD were determined using Fourier Transform Infrared spectrometer (FTIR) (Perkin Elmer, Spectrum RXI 83303). X-ray diffraction pattern was obtained using X-Ray diffractometer (XRD) (7000, Shimadzu) with filtered Cu Kα

1

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radiation operated at 40 kV and 40 mA. The XRD pattern was recorded from 10 to 80 °C of 2θ swith a scanning speed of 2° at 2θ min-1. Zeta potential was determined using a zeta potential

analyzer (DT1200, Dispersion Technology) while thermal stability and fraction of volatile

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components was monitored using Thermogravimetric analysis (TGA). Surface morphology was also studied using Scanning Electron Microscopy (SEM, JEOL JSM-6360LV, Japan) coupled

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with Energy Dispersive Spectroscopy (EDS, Thermo Noran, 6714A-ISUS-SN, USA). Equilibrium study

Sorption of Cu(II) ions on AD (0.5 g) or EAD (0.5 g) was studied at various time intervals using 250 mL varying concentration of Cu(II) solution (25–100 mg L-1) in 500 mL

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beaker at 298 K and 200 rpm for 130 min. Periodic agitations at 298 K and 200 rpm were repeated in order to establish the equilibrium time. Equilibrium concentration of Cu was determined by withdrawing clear samples at different interval and analyzing using Atomic

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Absorption Spectrometer (Varian AA240FS).

Effect of adsorbent dose, pH and temperature on sorption of Cu(II) ions

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Effect of adsorbent weight was determined by varying the weight of AD or EAD

separately from 0.1–1.0 g. The effect of pH was evaluated using sample weight of 0.5 g (AD or EAD) while adjusting the test solution over a pH range of 1.5–6.0 using 0.1 M HCl and 0.1 M NaOH as appropriate. For the effect of temperature, sample weight of 0.5 g (AD or EAD) was also used with varying concentration of Cu(II) ions (25 to 100 mg L-1) at different temperatures (298 to 333 K) in 250 mL of 100 mg L-1 solution of Cu(II) while stirring at 200 rpm in a 500 mL 6

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beaker for 130 min. Clear supernatants were withdrawn at different time interval and analyzed using Atomic Absorption Spectrometer (Varian AA240FS) for Cu concentrations.

Results and discussion

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3.

AD was defatted via extraction with n-hexane in a soxhlet extractor in order to remove fat and other nonpolar compounds present. Hemicellulose and lignin were also removed in order

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to avoid their presence in the treated water since hemicellulose, lignin and other water soluble compounds present in AD can further contaminate the treated water. The FTIR results of AD and

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EAD are presented in Fig. 1. Part characterization of AD had been previously reported [11]. Both AD (Fig 1a) and EAD (Fig. 1b) revealed a peaks at 2950 and 2830 cm-1 which were attributed to signals from the methyl (-CH3) and methylene (-CH2) functional groups, respectively. The peak at 3350 cm-1 in AD and EAD was considered as being due to the presence of –OH functional

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groups; the –OH peak was broader in EAD than AD suggesting the dominancy of the –OH group of the carboxyl functional group; mainly from EDTA. The C=O stretching of the carbonyl functional group appeared at 1725 cm-1 in both AD and EAD while the peak at 1419 and 1296

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cm-1 in EAD were attributed to the –OH bending and C-O stretching of the carboxyl functional group, respectively. The peak at 1350 cm-1 in EAD was also accounted for as being due to C-N

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stretching indicating the presence of EDTA on EAD. The x-ray diffraction patterns of AD and EAD are also presented in Fig. 1 with 2θ peaks

which are typical of semicrystalline cellulosic material with an amorphous broad hump. The crystallinity index (Ic) was determined using the height of 200 peak (I002, 2θ = 22.05°) and the minimum intensity between the 200 and 110 peaks (IAM, 2θ = 18.15°) which can be expressed as:

7

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 - 

× 100

(1)

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I % =

where I002 represents both crystalline and amorphous material while IAM represents amorphous material. The crystallinity of AD was 29.8% but after the modification with EDTA, it increased

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to 36.2% in EAD. The increase in crystallinity may be due to the introduction of the EDTA group to the surface of the material reinforcing its crystalline nature.

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The TG and zeta potential plots are shown in Fig. 2. The TG plot presents the characteristic decomposition pattern of the contents of AD and EAD. Mass loss at temperature range 50–150 °C in AD and EAD may be due to removal of internally bound water molecules. Both AD and EAD also exhibited mass loss around temperature range of 190–250 °C which was considered to be due to decomposition of hemicelluloses. This also confirmed that EAD still

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contains some amount of hemicellulose. Mass loss in the range 250–350 °C was attributed to the decomposition of cellulose as previously reported [12] which also indicated that AD and EAD are cellulosic materials. Mass loss found above 350 °C was accounted for as being due to

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decomposition of lignin.

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The zeta potential of AD (Fig. 2c) increased as pH increased but the value dropped at pH above 11. The zeta potential value also increased in EAD (Fig. 2d) as pH increased but decreased drastically at pH above 9. This drastic drop in zeta potential value may be associated with the presence of more dissociated carboxylic acid groups from the EDTA at the surface of EAD, similar observation had been previously reported by Ostolska and Wisniewska [13]. This is also an indication that the EDTA from the modification process most has dominated the surface of

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EAD. The Particle Size Dispersion (PSD) was found to be monomodal for both AD and EAD with a mean distribution size of 163 µm for AD and 0.04 µm for EAD. This reduction in size distribution may be due to the removal of hemicellulose, lignin and other extraneous materials in

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AD. Moreover, functionalization with EDTA may have also had impact on the structural arrangement of EAD. The CNH analysis revealed the presence of C, H and N. The amount of carbon increased from 39.0% in AD to 44.3% in EAD while hydrogen remained approximately

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the same, or from 6.3% in AD to 6.4% in EAD. Nitrogen was found to be 1.5% in EAD. The SEM result is presented in Fig. 3. The surface of AD appears heterogeneous which may be due

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to the presence of different functional groups on this surface as previously reported [11]. The surface changed after modification with EDTA in EAD. The surface of EAD looks homogeneous with spiral arrangement that seems flaky. The surfaces of AD and EAD changed completely after adsorption of Cu(II) ions as shown in Figs. 3c and 3d; respectively. The surfaces of AD and

3.1.

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EAD turned floppy and jelly after the adsorption process. Kinetic study

   

(2)

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 =

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The amounts of Cu(II) ions adsorbed by AD and EAD were calculated using Eq. 2:

where qe is the amount of Cu(II) ions adsorbed in mg g-1,  and  are initial and final equilibrium concentrations (mg L-1) of Cu(II) ions in solution respectively, while V and M are volumes (L) of Cu(II) ions solution and weight (g) of AD or EAD used. As shown in Figs. 4a and 4b, the adsorption capacity of AD and EAD increased with time until a plateau was attained which was described as the maximum adsorption capacity of the materials. The adsorption 9

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capacities of AD and EAD were also found to increase as the concentration of Cu(II) increased from 25 to 100 mg L-1. This observation may be due to the availability of more Cu(II) ions in solution as the concentration increased from 25 to 100 mg L-1. Moreover, the adsorption rate

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depends on the amount of Cu(II) ions which migrate from the bulk liquid phase to the active adsorption sites on the surfaces of AD and EAD. After several experimental trials the adsorption capacity of AD towards Cu(II) ions was found to be 32 mg g-1 while that of EAD was 27 mg g-1.

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The difference in the adsorption capacity of AD and EAD may be due to the different nature of the surface of these materials. The surface of AD may have contained some naturally present

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functional groups while the surface of EAD contains mainly the functionalized EDTA group. These naturally occurring functional groups in AD may have played a role in the adsorption of Cu(II) ions since some of these groups may contain heteroatoms with nonbonding electrons that may interact with Cu(II) ions in solution. Although the adsorption capacity of AD was higher

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than that of EAD, it was observed that some coloured compounds leached from AD into the treated water whereas no such thing happened when EAD was used. This gave an impression that AD needs to be pretreated to remove water soluble compounds present in it before it can be

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efficiently used to treat contaminated water. Moreover, use of untreated plant materials has been reported to increase chemical oxygen demand (COD), biological oxygen demand (BOD) and

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total organic carbon (TOC) due to release of soluble organic compounds [14, 15]. This increase in COD, BOD and TOC is of disadvantage to optimum use of such plant materials in wastewater treatment. Therefore, pretreatment or modification of plant sourced material as adsorbent in wastewater treatment is important and necessary. 3.2.

Effect of adsorbent dose on sorption of Cu(II) ions

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The effect of AD and EAD doses on the adsorption of Cu(II) ions are presented in Figs. 4c and 4d. In both AD and EAD, the adsorption capacity decreased with increasing dose from 0.1 to 1.0 g which may be due to a decrease in gross surface area made available for adsorption

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by the adsorbents (AD and EAD) and also an increase in diffusion path length from the aggregation of adsorbent particles which became significant as the weight of AD and EAD increased from 0.1 to 1.0 g [16]. The percentage of Cu(II) ions adsorbed by AD and EAD

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increased as the dose increased from 0.1 to 1.0 g. This may be due to an increase in available surface negative charge and decrease in the electrostatic potential near the solid surface that

3.3.

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favors sorbent-solute interaction as the dose of AD and EAD increased. Effect of pH on adsorption of Cu(II) ions

Effect of pH is an important parameter during adsorption study which plays an important role in the interaction between adsorbate and adsorbent. The surface of bio-sourced material has

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functional groups which may be pH dependent in their function when in contact with solution. These functional groups are important in the formation of electrical charges on the surface of these materials and as well play a role in how they interact with adsorbate. So, when the pH

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value of solution is greater than the pKa of these functional groups at the surface of bio-sourced adsorbent, most of the functional groups on the surface dissociates and exchange their H+ with

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adsorbate ions in solution but when the pH value is lower than their pKa value, they pick up adsorbate ions by means of complexation reaction [11,17]. During this study, the pH range of 1.7–6.2 was maintained in order to prevent the Cu(II) ions from precipitating from solution. In AD, the adsorption capacity increased as the pH value of solution increased but reduced on approaching pH 5.5 (Fig. 4e). In the case of EAD, the adsorption capacity increased as the pH value increased all through the pH values examined as shown in Fig. 4f. 11

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Since change in solution pH is responsible for the protonation of metal binding sites, metal speciation and degree of ionization in solution, then the adsorption of Cu(II) ions may be related to the active sites on AD and EAD. Both AD and EAD are cellulosic material and are

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capable of having a negatively charged surface when in contact with water [18]; moreover, the surface of EAD is functionalized with EDTA which when in contact with water can become negatively charged. Dissolved Cu(II) ions are positively charged and are capable of being

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attracted towards the surfaces of AD and EAD. As a result of the net negative charge on AD and EAD, adsorption takes place at pH above the isoelectric point. At this point, the surface of AD

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and EAD are in the ionic state which favours reaction with Cu(II). So, as pH reduces the net charge on AD and EAD tends towards being positive thereby inhibiting the approach of positively charged Cu(II) ions. Whereas, as the pH increases the surface tends towards being negatively charged thereby promoting the attraction of positively charged Cu(II) ions. The EDS

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(Fig. 5) reveals the presence of Cu on the surfaces of AD (Fig 5c) and EAD (Fig. 5d) after the treatment with the Cu contaminated water. This confirms that Cu is adsorbed by AD and EAD. The presence of gold (Au) peak at the surfaces before and after adsorption is due to the coating

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of the surface with Au during sample preparation in order to increase electrical conductivity and thus improve the quality of the micrograph obtained. Adsorption kinetic models

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3.4.

The adsorption of Cu(II) on AD and EAD was evaluated using pseudo-first-order,

pseudo-second-order, intra-particle diffusion and liquid –film diffusion models. Pseudo-first-order model was given as:

 

=   −   (3) 12

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where k1 (min-1) is the rate constant for pseudo first order adsorption. On integrating at boundary

log %q' -q( ) = log q' -

*+

,../.

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condition (qe-qt) = 0 at t = 0, Eq. becomes:

t (4)

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The plot of log (qe-qt) against t gave a linear relationship (Lagergren plot) from which k1 and qe

kinetic was expressed as:

 

= ,  −  ,

(5)

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were determined from the slope and intercept of the plot, respectively. The pseudo-second-order

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For the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, the integrated form of Eq. (5) becomes:



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=  + , 2

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(6)

On rearranging Eq. (6), it becomes:

 =



+  5 3 4

 4

(7)

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The integrated rate law for pseudo second order reaction in Eq. (7) has a linear form which can be expressed as:

 

−



= , 2



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(8)





and intercept of

6 



. Relatively high value of the co-

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gave a linear relationship with a slop of

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where k2 (g mg-1 min-1) is the rate constant for pseudo-second-order model. A plot of against t

efficient of determination (R2) and low standard error of estimate (SE) was used as criteria for the best fit which were calculated as follows [19]:

∑   ∑9 

∑ 

∑ 9 

=,

/.?

>

(10)

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:; = <

(9)

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7, =

where q and q’ are the measured and calculated amounts of Cu(II) adsorbed on the AD or EAD

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respectively, at time t and N is the number of measurements [19]. The results obtained are shown in Table 1. The initial sorption rate of Cu(II) on AD (16 mg g-1 min-1) was found higher than that of EAD (2 mg g-1 min-1) which suggest that Cu(II) was preferentially adsorbed on AD than EAD; this further corroborated the higher adsorption capacity exhibited by AD over EAD. The values obtained for the model sorption capacity (qe) and the R2) of pseudo-first-order and pseudosecond-order kinetics fitted better for the pseudo-second-order kinetic model. So, the adsorption 14

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of Cu(II) on both AD and EAD can be described using pseudo-second-order kinetic model. The R2 values of the pseudo-second-order model were closer to unity and the qe values also

by AD and EAD may be via chemisorption [20].

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correlated better with the experimental sorption values which indicates that the sorption of Cu(II)

The adsorption was further evaluated using intra-particle diffusion model which was

 = @ 2 /.? + 

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described as:

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(11)

where kid is the intra-particle diffusion rate constant (mg g-1 min1/2) and C (mg g-1) is a constant which reflects the thickness of the boundary layer. The larger the value of C is, the greater the boundary layer effect. The value of C for the adsorption of Cu(II) on AD was 24 mg g-1 while it

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was 11 mg g-1 for EAD suggesting greater boundary effect for AD than for EAD. This indicated a greater contribution of the surface sorption in the rate controlling step. Although the plot of qt versus t1/2 gave a straight line but it did not pass through the origin which suggests that the intra-

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particle diffusion was not the only rate controlling step. So, intra-particle diffusion could not have being the only rate controlling step for the sorption of Cu(II) ions on AD and EAD.

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However, liquid film diffusion model was also used to further evaluate whether the movement of Cu(II) molecules from the liquid phase up to the solid phase boundary played a role in the adsorption process according to the equation below:

AB1 − C  = −D 2

(12)

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F = qe/qt is the fractional attainment of equilibrium while kfd is the adsorption rate constant. A linear graph was obtained for the plot of –ln (1-F) against t. As shown in Table 1, the values obtained for kfd and r2 indicated that diffusion through the liquid surrounding AD and EAD

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played a role in the adsorption process. This has shown that both intra-particle diffusion and liquid film diffusion were involved in the adsorption of Cu(II) ions on AD and EAD. By this, it is evident that the sorption of Cu(II) on AD and EAD followed series of steps with the slowest

3.5.

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step being considered as the overall rate determining step [21]. Isotherm

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Data obtained for the equilibrium sorption of Cu(II) on AD and EAD were fitted to Temkin, Langmuir and Freundlich adsorption isotherm models. The data did not fit well for Langmuir isotherm, so, only Temkin and Freundlich isotherm models were used to describe the sorption process. For the Temkin isotherm model, this has a factor that accounts for the

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adsorbent-adsorbate interaction. On ignoring the low and large value of concentrations, Temkin model assumes that heat of adsorption of all molecules in the layer would decrease linearly rather than logarithmic [22] which is characterized by a uniform distribution of the bonding

FG

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energies up to some maximum binding energy. This is described as:

(13)

 = KABJ + KAB

(14)

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 = E I ABJ  H

where A (L g-1) is the Temkin isotherm equilibrium binding constant, corresponding to the maximum binding energy, and constant B (J mol-1) = RT/b, is the constant related to heat of sorption, b is the Temkin isotherm constant. R is the gas constant (8.314 J mol-1 K-1), and T is the 16

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absolute temperature (K). A straight line was obtained on plotting qe against ln Ce from which B and A were determined from the slope and intercept of the straight line plotted. The result obtained is shown in Table 2. Both Temkin isotherm equilibrium binding constant and constant

preference Cu(II) had for AD over EAD.

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related to heat of sorption were found higher in AD than EAD which further corroborated the

Freundlich isotherm model is known to describe the adsorption process for heterogeneous

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surface and multilayer sorption. The model can be expressed as:

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+

 = LD M

(15)

where Kf (mg g-1) is the Freundlich isotherm constant, n is the adsorption intensity, and qe (mg g1

) is the amount of Cu(II) adsorbed at equilibrium. As presented in Table 2, Kf was found to be

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2.0 mg g-1 for AD and 1.6 mg g-1 for EAD. The strength of the adsorption process is a function of 1/n. It has been reported that when 1/n = < 1 it indicates a normal adsorption, if the value equals 1 then the partition between the liquid and solid phases are independent of the

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concentration but when 1/n = > 1 it indicates cooperative adsorption [23]. In this present study,

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1/n was found to be 1 for AD and 0.93 for EAD which suggests a normal adsorption in EAD and in AD with partition between the liquid and solid phases independent of the concentration of Cu(II). This may be due to the fact that the surface of EAD is mainly dominated by the EDTA group which played active role in the adsorption process while the surface of AD has different types of functional groups which may have taken part in the adsorption process. The removal of lignin and hemicellulose from AD resulted in the formation of a cellulocious material which contains mainly hydroxyl functional groups. These hydroxyl functional groups were later 17

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interacted with EDTA to complete the surface modification. So, the surface of EAD may have being dominated by the carboxyl functional groups from EDTA which must have contributed immensely to the adsorption process. Moreover, the zeta potential value also pointed to this fact

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with the drastic reduction in the value as pH increases above 11 suggesting that most of the carboxyl functional groups now exist in the ionic state. This also explains the reason for the observed increase in adsorption capacity of EAD as pH increased because the surface had

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increase in the dominating carboxyl functional group existing in their ionic state thus they were

3.6.

Thermodynamics of adsorption

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able to pick the positively charged Cu(II) ions.

Table 3 shows the thermodynamic parameters for the adsorption of Cu(II) on AD and EAD. Data obtained were analyzed to determine some thermodynamic parameters such as entropy change (∆So), Gibb’s free energy change (∆Go) and enthalpy change (∆Ho). The

 

∆P  = −7QABN

(16) (17)

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N =

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adsorption equilibrium constant (bo) was estimated from the expression:

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∆P  = ∆R  − Q∆:  (18)

The plot of ln bo against reciprocal of temperature (1/T) gave a straight line from which ∆Ho and ∆So were calculated from the slope and intercept. Values obtained for qe increased for AD as the temperature increased whereas in the case of EAD, the values of qe reduced as the temperature increased. The values of ∆Go increased as the temperature increased in EAD unlike in the case of AD where the value reduced as temperature increased. The positive nature of ∆Go is an 18

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indication that energy is required for the adsorption of Cu(II) on AD and EAD. However, the higher value obtained for EAD suggests that higher amount of energy is required for the sorption of Cu(II) on EAD than for AD. The values obtained for ∆Ho and ∆So are presented in Table 4.

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These values were positive for AD but negative for EAD. The negative value of ∆So in EAD (0.11 KJ mol-1 K-1) indicates a stable configuration of Cu(II) ions on the surface of EAD while the positive value obtained for AD (0.05 kJ mol-1 K-1) is an indication of some structural changes in

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AD due to the influence of the different functional groups at its surface.

The value of ∆Ho was higher for AD (16 kJ mol-1) than EAD (-32 kJ mol-1) which

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suggests a fairly strong bonding between AD and Cu(II). The positive nature of the ∆Ho value obtained for AD indicates that the adsorption process in AD was endothermic in nature while the negative nature of the value obtained for EAD suggests the process to be exothermic in EAD. The values of ∆So, ∆Ho, ∆Go and qe obtained for AD and EAD were compared with values

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found in literature for other adsorbents from plant seeds as presented in Table 5. The adsorption capacity of AD and EAD were found higher than other plant seed sourced adsorbents reported. Most previously reported process were through chemisorption just like for AD and EAD except

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the process reported by Meena et al. [24] which was physisorption. The present work found the isotherm for the sorption of Cu(II) on AD and EAD to be via Freundlich but most previous work

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reported both Langmuir and Freundlich except for Garba et al. [25] and Rao and Khatoon [26] who reported the isotherm to obey Langmuir model.

4.

Conclusions

The present work evaluated the functionalization of the surface of underutilized A. digidata seeds from Nigeria with EDTA and its use as a means of treating Cu contaminated 19

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water. The adsorption capacity of AD was higher than that of EAD. The adsorption process was via chemisorption which obeyed second-order-kinetic model and fitted well for Freundlich and Temkin isotherm. The adsorption is controlled by both intra-particle diffusion and liquid film

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diffusion with adsorption capacity which compared better than those of other plant materials found in literature. This work also showed that it is important and necessary to treat plant

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materials before being used in wastewater treatment.

Acknowledgements

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The authors are grateful to International Foundation for Science for awarding a water research grant (No W/5401-1). Authors are also grateful to the Department of Chemistry, Federal University of Minas Gerais, Brazil.

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Table 1.

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Kinetic model parameters for the sorption of Cu(II) on AD and EAD Model Parameter AD EAD Pseudo-First-order qe (mg g-1) 6.5 13.8 -1 0.014 0.007 k1 (min ) 0.928 0.914 R2 Pseudo-second-order qe (mg g-1) 33 27 k2(g mg-1 min-1) 0.015 2.857E-03 R2 0.999 0.979 2.1 h (mg g-1 min) 16.1 -1 1/2 Intra-particle diffusion Kid (mg g min ) 0.91 1.34 24.2 10.6 C (mg g-1) r2 0.700 0.918 0.034 0.025 Liquid film diffusion kfd 0.902 0.706 r2 -1 Experiment qe (mg g ) 33 27

Table 2. Cu(II) sorption parameters for Temkin and Freundlich models

2.33 27.0 91.7 0.998

2.27 16.9 146.3 0.957

2.00 1.00 1.000

1.56 0.93 0.975

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Temkin A (L g-1) B b R2 Freundlich Kf 1/n r2

Parameter AD EAD

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Isotherm

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298

313

323

333

32.8

37.8

39.0

39.4

1.05

0.73

0.67

0.66

298

313

323

333

26.6

23.3

17.70

11.15

1.57

1.98

4.66

6.08

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AD T (K) qe (mg g-1) ∆G (kJ mol-1 K-1) EAD T (K) qe (mg g-1) ∆G (kJ mol-1 K-1)

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Table 3. ∆G and qe obtained at various temperatures

Table 4. Thermodynamic parameters obtained from plot of ln bo vs 1/T for sorption of Cu(II) AD 16.1 0.05

EAD -31.5 -0.11

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Parameters ∆H (kJ mol-1) ∆S (kJ mol-1K-1)

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- = Not reported

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Table 5. Comparison of the adsorption of Cu(II) on AD and EAD with other plant sourced adsorbents in literature Material qe (mg g-1) Adsorption ∆Goads ∆Hoads ∆Soads Mechanism -1 -1 isotherm (kJ mol ) (kJ mol ) (J mol-1 K-1) NFAD 9.4 Langmuir & 27.5 231.4 13.7 Chemisorption Freundlish Acacia Arabica 5.6 Langmuir & 0.07 29.8 0.10 Physisorption Freundlich Papaya seeds 17.3 Langmuir Caryota urens seeds 9.4 Langmuir 54.6 0.2 Grape seed 1.4 Langmuir & -85.3 Chemisorption Freundlich Sugar cane saw dust 3.9 Freundlich & Langmuir Adenanthera 10.9 Chemisorption pavonina seed Rape seed 15.4 Langmuir & -15.6 7.5 0.08 Chemisorption Freundlish Cocoa pod 12.2 -1.6 Chemisorption AD 32.80 Freundlich & 1.05 16.1 0.05 Chemisorption Temkin EAD 26.65 Freundlich & 1.57 -31.5 -0.11 Chemisorption Temkin

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Reference [11] [24] [25] [26] [27] [28] [29] [30] [31] This study This study

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Fig. 1. (a) FTIR of AD [8], (b) FTIR of EAD, (c) XRD of AD [8] and (d) XRD of EAD.

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Fig. 2. (a) TG of AD [8], (b) TG of EAD, (c) Zeta potential plot of AD and (d) Zeta potential plot of EAD.

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Fig. 3. SEM micrograph of (a) AD surface before adsorption [8], (b) EAD surface before adsorption, (c) AD surface after adsorption and (d) EAD surface after adsorption.

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Fig. 4. (a) qt of AD against time, (b) qt of EAD against time, (c) effect of AD dose on qe and % adsorbed, (d) effect of EAD dose on qe and % adsorbed, (e) effect of pH on qe of AD and (f) effect of pH on qe of EAD.

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Fig. 5. ESD of (a) surface of AD before adsorption, (b) surface of EAD before adsorption, (c) surface of AD after adsorption and (d) surface of EAD after adsorption. (Note: C = carbon, O = oxygen, Au = gold and Cu = copper)

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