Immobilization of cobalt ions using hierarchically porous 4A zeolite-based carbon composites: Ion-exchange and solidification

Journal of Water Process Engineering 33 (2020) 101059

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Immobilization of cobalt ions using hierarchically porous 4A zeolite-based carbon composites: Ion-exchange and solidification

T

Sama M. Al-Jubouria,*, Stuart M. Holmesb a b

Department of Chemical Engineering, College of Engineering, University of Baghdad, Aljadria, Baghdad, P.O.Box: 47024, Iraq Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, UK

A R T I C LE I N FO

A B S T R A C T

Keywords: Hierarchical porosity Zeolite Carbon Composites Ion-exchange Vitrification

The efficacy of the synthesized 4A zeolite and the 4A zeolite-based carbon composites were examined for the removal of cobalt ions from an aqueous solution by ion-exchange. Two types of agricultural waste were chosen as precursors to prepare the porous supports for composites synthesized at hydrothermal conditions. The maximum exchange capacity of zeolite portions present in 4A zeolite/Almond shells carbon (AAS) and 4A zeolite/Walnut shells carbon (AWS) reached to 235.175 and 202.887 mg ion/g zeolite; but it reached to 99.525 mg ion/g zeolite for 4A zeolite (A) at same conditions. Removal of cobalt ions by ion-exchange process enhanced when the temperature and pH of the cobalt solution increased. The equilibrium data are precisely fitted by the Freundlich model with a correlation factor (R2) of 0.9807, 0.9859 and 0.9857 for A, AAS and AWS. The cobalt uptake by zeolite-based carbon composites process is controlled by ion-exchange. The Weber and Morris intra-particle diffusion kinetics and Boyd model equations confirmed enhancement of cobalt removal using the composites. The seepage of cobalt ions from the solidified ion-exchangers encapsulating ions reduced to 0.01 ppm in H2O and to 0.05-0.25 ppm in 0.5 N NaCl solution.

1. Introduction

the environment [2,9]. Cobalt is one of the harmful radioactive elements discharged into the effluents of many industries such as, electronic and semiconductors industries, nuclear medicine, catalysts for petrochemical industry, mining, grinding wheels, manufacturing vitamin B12, pigments, electroplating, paints and varnishes, and metallurgy [10–12]. The physical property of the pure cobalt is shiny, hard metal, odorless, steely-grey. Cobalt is naturally present in the Earth’s shell in form of metals or salts and it is present in the sea water, soil, and dust; it is also present in the human body. According to the EPA, the acceptable level of cobalt in drinking water is 0.002-0.107 ppm [13]. While, it is 0.05−1 ppm for the irrigation water according to Environmental Bureau of Investigation and Canadian Water Quality Guidelines; which means that everybody is subjected to cobalt through water, food and air [10]. The presence of cobalt in the contaminated water causes serious problems due to its toxic nature and harmful effects to the humans health such as, allergy, nausea, asthma, vomiting, loss of smell either completely or partially, genetic changes in living cells, damage to the thyroid, heart failure, damage to the heart, damage to the liver. In addition, it causes neuro toxicological symptoms like, headaches and changes in reflexes, cancer risks resulting of exposure to cobalt radiation. Also, cobalt isotopes send ionizing radiation [10–12,14]. That all

Human and industrial activities produce streams of wastewater associating with manufacturing and processing different materials. These effluents are considered as polluted waters when containing an unallowable concentration (lethal dose) of potential pollutants causing harmful changes to the receiving water bodies [1,2]. Engineers and scientists consider treating industrial wastewaters as a big issue due to increasing the industrial activities, especially wastewater containing heavy metals and radionuclides because they hazardously influence human health and other organisms in the environment [3,4]. About 20 metals are released into the environment are classified as toxic and 50 % of those are discharged with quantities harmful to human health [5]. Numerous industries release effluents containing surplus heavy metals into the aqueous environment such as paper, pulp, cloth and leather, plastics industries, microelectronics and electroplating industries, fertilizers and pesticides productions, mining operations, metal smelters and cleaning, paint, dyes and pigments, tannery, galvanizing and refining industries [6–8]. Untreated or poorly treated industrial effluents contain heavy metals, such as Ni, Cr, Pb, Cd, Hg, Co, As, Cu and Zn which are highly toxic, highly soluble and extremely tend to bioaccumulate in the tissues of living organisms and causes major problems to

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Corresponding author. E-mail address: [email protected] (S.M. Al-Jubouri).

https://doi.org/10.1016/j.jwpe.2019.101059 Received 19 June 2019; Received in revised form 1 November 2019; Accepted 6 November 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

aluminate (55–56%wt Al2O3 from Riedel-deHaën) was used as alumina and sodium oxide source. NaOH pellets (99.9 %wt from Fisher Scientific) was used as sodium oxide source. Deionised water was implemented for different uses in this study. Cobalt nitrate hexahydrate (99 %wt from Lancaster Synthesis) to prepare cobalt solution. A commercial zeolite A supplied by BDH Chemicals Ltd was used to prepare nanoparticles suspension solution. Agricultural waste namely, almond and walnut shells were collected from yards located in the north of Iraq.

elucidate the significant increase in the studies related to eliminate pollution with cobalt. Several methods have been improved over the years to achieve best and cheapest removal of cobalt. The most commonly used techniques to remove heavy metals and radioactive elements from wastewater are chemical precipitation, electroflotation, evaporation, adsorption, ionexchange, membrane filtration, reverse osmosis, electrodialysis, coagulation, flocculation and solvent extraction [15,16]. However, most of these methods are there are generally expensive, generating toxic sludge or secondary wastes and not appropriate for low metal concentrations and small scale industries [14,17,18]. Adsorption and ion-exchange methods have been diagnosed as effective, cheap, and easy-adaption methods for cobalt removal, especially when inexpensive and chemico-physically feasible adsorbents are used to minimize the hazards of harmful contaminates [15]. Many different media have been used for adsorption and ion exchange purposes to remove Co ions and other heavy metals such as, coir pith [14], wood ash [19], black carrot residue [20], lemon peel [11], Trichoderma reesei [21], Spirulina platensis [22], coal dust [23], cashew nut shells [24], sunflower shell [25], Al-Khriet [16], iron-based water treatment residuals [26], chitosan, egg shells, humate potassium and sugar beet factory lime [27]. Also, rice shells, lentil shells, wheat shells, sawdust, peanut hulls, wheat straw, rubber tree leaf, oil palm leaf powders, polyethylenimine modified aerobic granules and modified meranti sawdust were used as low cost adsorbents [28]. However, works of literature have mentioned several works based on developing synthetic materials as effective adsorbents for heavy metals removal such as acetonitrile stannic (IV) selenite composite cation-exchanger [29], nano sodium dodecyl sulfate acrylamide Zr (IV) selenite composite cationexchanger [30], Alizarin red-S-modified amberlite IRA-400 resin [31]. Moreover, other work used some materials to provide required surface area for adsorption and rapid degradation of hazardous metals such as ZrO2/Fe3O4/Chitosan nanocomposite [32]. Zeolites are hydrated aluminosilicates of alkali and alkaline earth metals with a crystalline microporous structure consisting of tetrahedral units linked with each other through a shared oxygen [33]. Utilizing of zeolites to treat wastewater is very effective, especially when they are natural zeolite. This is because they are inexpensive, abundant, easily regenerated and have a good cation exchange capacity, good selectivity for cations, high surface area, a rigid porous structure and good stability in acidic conditions [34]. Synthetic zeolites have been utilized to eliminate cobalt such as, pure zeolite X [35], pure zeolite Y [36] and cancrinite [13]. The ion-exchange performance of zeolite can be enhanced when it is synthesized in form of composites incorporating inexpensive porous supports such as carbon made from natural/agricultural waste materials [37–40]. Presence of macro/mesoporous supports adds advantages of reducing the resistance to mass transfer and enhancing the rate of diffusion of ions and molecules to and from active sites. Also, composites contain only a small portion of zeolites which adds the advantages of the small volume of vitrified capsules in comparison with pure zeolite used for the same purpose [37,38]. For example, composite zeolite Y/carbon was used to remove cobalt ion from an aqueous solution [41]. The current work present a detailed study about removal of cobalt ion using 4A zeolite-based carbon composites at different conditions. Also, the experimental data would be fitted using isotherm models available in the literature to elucidate the trend of the cobalt in the removal process.

2.2. Preparation of 4A zeolite 4A Zeolite with a gel composition of 4 Na2O: Al2O3: 2 SiO2: 180 H2O was prepared according to the procedures mentioned in the previous work [42,43]. Preparation of the gel started with dissolving 0.377 g of sodium hydroxide pellets in 20 g of deionized water. Then, 2.785 g of hydrous sodium metasilicate was mixed with only half of the NaOH solution for 15−30 min. 1.19 g of anhydrous sodium aluminate was mixed with the other half of the hydroxide solution for 15−30 min until the solution became clear. Silica solution was then added to the alumina solution and mixed for 20−30 min until the formation of a thick creamy gel. Crystallization of the gel produced was conducted using a Teflon-lined autoclave at 100 °C for 4 h. After that, the autoclave was quenched and the product was taken out, washed with deionized water and dried overnight at 50−80 °C. Then, characterization by XRD, SEM, EDAX and N2 adsorption/desorption was conducted for the samples. 2.3. Preparation of 4A zeolite-based carbon composites A nanoparticles suspension solution was prepared from commercial 4A zeolite using a ball mill, according to procedures mentioned in [39], to create nucleation sites on the support surface via ultrasonication. Almond shells and walnut shells were used as precursors for a porous support. Both almond and walnut are abundant crops in the north of Iraq and their extensive consumption generates a huge amount of shells which is considered as municipal solid waste. The procedures and conditions followed in this study were chosen in accordance with the literature [37,38]. The precursors were properly cleaned, washed with deionized water, dried and imaged by SEM. Then, the shells were weighed and heated at 800 °C for 3 h using a tubular furnace in an atmosphere of nitrogen gas. The product was weighed, manually ground and sieved to obtain the support. Characterizing of samples before and after being used as supports was conducted by both SEM, EDAX and N2adsorption/desorption isotherms. The surface of the carbonous support was modified using the nanoparticle suspension prior to be used in the hydrothermal treatment according to the producers used in a previously published work [39]. The procedures used for the preparation of composites were the same as those used to prepare pure 4 A zeolite, just one step was introduced to the process. This step is adding carbon support to the aluminosilicates gel before crystallization. The weight of support added to a batch of the prepared gel was according to the weight ratio of 2.5 carbon: 1 silica in the silica source. The mixture of carbon and gel was continuously stirred for 15 min and then crystallized inside Teflon-lined autoclaves at 100 °C for 4 h. The resulting composites were recovered, filtered, washed with deionised water and dried at 50−80 °C. Samples of the composites were taken to be characterized by XRD, SEM, EDAX, TGA and N2-adsorption/desorption isotherms. 2.4. Ion-exchange experiments

2. Experimental work Ion-exchange experiments were conducted using a series of 125 ml glass containers (batch system) includes 100 ml of cobalt solution under continuous shacking. Cobalt solution showed a graduated pink color according to the concentrations. Initially, two ion-exchange experiments were conducted to examine the uptake ability of the carbonized

2.1. Chemicals and materials Hydrous sodium metasilicate (Na2SiO3.5H2O from BDH Technical) was used as a silica and sodium oxide source. Anhydrous sodium 2

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Fig. 1. SEM images of (a) almond shells, (b) walnut shells.

and chemical composition of the synthesized zeolite were examined by conducting scanning electron microscopy (SEM) and energy dispersive analysis by X-ray (EDAX) using a model FEI Quanta 200. A sputter coater was used to coat the samples with gold to conduct the SEM imaging. ImageJ software [44] was used for SEM images to obtain the crystal size of 4A zeolite. Thermogravimetric analysis was conducted using TGA, model Q5000 IR by TA-Instruments Company to obtain the weight of zeolite present on the composite materials. The temperature of composite samples was raised from room temperature to 400 °C in the presence of nitrogen gas to discard the moisture, volatile materials and other wastes. Then, the temperature was raised from 400 °C to 600 °C in the presence of air to burn off carbon. The surface area of zeolite and zeolite composites were measured using nitrogen adsorption/desorption isotherms performed using a Micrometrics Accelerated Surface Area and Porosimetry (ASAP) 2010. The samples were degased using nitrogen gas at −200 °C. Manganese concentration in the liquid samples was measured using Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES, model Vista-MPX by Varian).

almond shells and carbonized walnut shells without zeolitization. Where, 0.2 g of carbon support was added to 100 ml of ∼200 ppm cobalt solution under continuous shaking at 250 rpm and temperature ∼15 °C for 24 h. The pH of cobalt solution was measured and it was ∼6. The samples were taken after 0.5, 1, 2, 3, 4 and 24 h to be filtered, diluted and measured by ICP-OES. For the effect of initial ion concentration, a fixed weight of ion-exchange material (0.2 g) was placed in several container filled with 100 ml of cobalt solution (50, 200, 400 and 500 ppm) and shaken for 4 h at room temperature. The equilibrium isotherms were studied by varying the amount of ion-exchange material used as 2, 4, 8, 10 g/l at a fixed initial cobalt concentration of 200 ppm and room temperature. The temperature effect on the ion-exchange process was performed at 15, 35, 45 and 55 ( ± 0.5)⁰C for ion-exchangers dose of 0.2 g and the initial ion concentration of 200 ppm. During the above studies, the pH of cobalt solutions was not modified. But it was changed to study the effect of the solution pH on the performance of ion-exchange media. The pH of solutions was altered using 1 N NaOH solution and 1 N HCl solution. 1 ml of the solution was taken from each container to obtain the initial cobalt concentration before conducting the ion-exchange step. Then, samples were taken periodically to obtain the concentration of cobalt remaining in the solution by ICP-OES. Calibration was conducted for ICP-OES before being utilized, and the samples were diluted before the analysis. The wavelengths used to detect cobalt ions for all samples was 228.615 nm.

3. Results and discussion 3.1. Characterization The SEM images of almond shells and walnut shells before carbonization are shown in Fig. 1. Almond shells shows almost a uniform porous surface, whereas walnut shells shows apparently different pore sizes. The porous system of the two precursors reflected on the porosity of carbon produced as shown by the SEM images presented in Fig. 2. Approximately, almond shells produced a favorable porous carbon with more uniform pores after removing moisture and high volatiles materials during the carbonization process. But, walnut shells produced carbon with less apparent voids. The weight of carbon resulted from these two precursors was 26.3 % and 27.6 % of the initial precursors weight (almond shells and walnut shells respectively). The results of EDAX analysis of almond and walnut shells before and after carbonization are shown in Tables 1 and 2, respectively. These results indicate that carbon is the major element in the support chemical composition. Also, Al and Si consist very small percentage within the composition of the carbon support. The small surface area of the carbon supports are 13.287 and 10.636 m2/g indicating the macro/ mesoporosity of the carbon supports. The XRD pattern of the synthesized 4A zeolite was identical to the pattern of its counterpart commercial 4A zeolite as shown in Fig. 3. Also, the produced zeolite formed as high crystalline cubic shaped crystals with well-defined edges and has Si/Al ratio of ∼1. The XRD patterns of 4A zeolite/almond shells carbon composite (AAS) and 4A zeolite/walnut shells carbon composite (AWS) are shown in Fig. 3. The composites showed similar XRD patterns to this of the synthesized 4A

2.5. Immobilization of cobalt ion The immobilization of cobalt ion, which has a boiling point of 2870⁰C, carried by the spent materials was conducted by vitrification. Certain weights of the spent materials were heated in a muffle furnace at 1200⁰C for 2 h. Then, the resulted solidified product was weighed and removed from the boats. After that, 0.04 g of the solidified materials were placed in tubes containing 20 ml of deionized water and 0.5 N NaCl and left in contact for 9 months to examine leaching of cobalt ions from the solidified samples. Also, 0.04 g of the non-vitrified A, AAS and AWS were placed in another group of tubes containing 20 ml of deionized water and 0.5 N NaCl. 0.5 ml of liquid was taken from the tubes at intervals of 1 day, 1 week, 1 month and 9 months to be analyzed. 2.6. Analytical techniques A Miniflex Rigaku X-ray analytical instrument was used to obtain XRD patterns of the zeolite samples, with CuKα radiation source (λ =1.5418 Å), voltage =30 kV, current =30 mA, scan speed = 3 °min-1, step size = 0.03, 2θ = 5-50° and total time ∼ 20 min. The morphology 3

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Fig. 2. SEM images of (a) carbonized almond shells, (b) carbonized walnut shells. Table 1 The elemental analysis of the almond shells and walnut shells before carbonization. Precursor materials

Almond shells Walnut shells

The elemental analysis wt.% Al

Si

Na

O

C

K

Ca

Fe

0.34 0.22

0.71 0.30

0.28 0.22

28.88 28.13

68.24 62.05

0.53 2.70

0.37 6.21

0.12 0.17

and the peaks kept the same intensity as this of pure 4A zeolite. Since, presence of carbon support did not affect crystallization of the required phase and no other phases or impurities generated during the synthesis. Fig. 4 shows SEM images of the 4A zeolite-based carbon composites. The images show a homogeneous distribution and coverage of 4A zeolite crystals over the supports surface. Since, modification of the support surface via ultrasonication in the presence of nanoparticles facilitated the growth of zeolite crystals on the nucleation centres created on the support structure [39]. Thus, a homogeneous growth and coverage, and favourable attachment of crystals were obtained over the carbon surface. Table 3 present the elemental analysis and structural properties of A, AAS and AWS. The average crystal size of A was 1916, and it was 1050 and 1452 nm for AAS and AWS, respectively. The composites grew with relatively smaller crystal size due to the effect of nucleation sites created on the support surface by the impact of ultrasonication in the presence of nanoparticles [39]. The Si/Al ratio of AAS and AWS was 1 as obtained by EDAX results. The percentage of zeolite in AAS and AWS was obtained using TGA results and it was 40.81 % and 44.69 %, respectively. The BET surface area of A, AAS and AWS was 246.47, 183.36 and 157.65 m2/g respectively. The small zeolite content in the composites gave rise to the reduction in the microporous surface area of the composites.

Fig. 3. XRD patterns of synthesised 4A zeolite (A), 4A zeolite/almond shells (AAS) and 4A zeolite/walnut shells (AWS).

qt =

(Co − Ct ) ×V W

(1)

Where qt is the amount of cobalt ions removed per the amount of ionexchanger (mg/g) at time is t, Co is the initial cobalt concentration (mg/ l), Ct is the remaining cobalt ion concentration (mg/l), W is the weight of ion-exchanger (g) and V is the volume of cobalt ion solution (l). Moreover, some results are presented in the form of qTGA (mg ions/g zeolite) plotted against t. Where, qTGA represents the amount of metal ions removed by the actual weight of zeolite present in a composite based on TGA results using Eq. (2), therefore qTGA was calculated for both AAS and AWS only.

qTGA =

3.2. Ion-exchange

qt A fraction of zeolite in W

(2)

Fig. 5 shows the results of a comparative study which was conducted using carbonized Almond shells and Walnut shells without an activation. Since, there is no detectable change in the cobalt concentration along 24 h of contact with cobalt solution. Fig. 6 shows that a distinguished cobalt ion removal from an

The results of ion-exchange experiments are presented in the form of ion-exchange capacity (q mg ions/g ion-exchange material) calculated using Eq. (1) against parameters.

Table 2 The elemental analysis and structural properties of the carbonous supports prepared from almond shells and walnut shells. Support

Almond shells Walnut shells

The elemental analysis wt.% Al

Si

Na

O

C

K, Ca, Fe

0.12 0.06

0.05 0.03

0.11 0.15

5.05 6.15

94 92.78

0.68 0.84

4

Ash Content%

BET surface area (m2/g)

VTotal (cm3/g)

3.7 3.8

13.287 10.636

0.076 0.068

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Fig. 4. SEM images of (a) synthesised 4A zeolite, (b) 4A zeolite/almond shells, (c) 4A zeolite/walnut shells.

aqueous solution was obtained using the synthesized 4A zeolite. This is because 4A zeolite possesses low Si/Al ratio which provides a high concentration of easily exchangeable cations. The ion-exchange occurring between cobalt cations and Na cations present in the structure of zeolite 4A can be represented by the following equation: 2Na+(z) + Co2+(s) → 2Na+(s) + Co2+(z) Where s refers for solution and z refers to zeolite. When a comparison based on the total weight of ion-exchange material used in the experiment, the ion-exchange capacity, which was calculated using Eq. (1), of pure 4A zeolite is higher than this of both AAS and AWS. This is because the percentage of zeolite present in AAS and AWS is 40.81 % and 44.69 % as given by TGA results presented in Table 3. And this small percent decreases the number of ion-exchange sites provided per weight of media. However, both AAS and AWS showed higher ion-exchange capacity (235.175 and 202.887 mg ion/g zeolite) than pure 4A zeolite (99.525 mg ion/g zeolite) when the capacity was calculated with respect to the actual weight of zeolite using Eq. (2). Spreading the zeolite crystals over a macro/mesoporous carbon support reduces the film resistance and improves the cations diffusivity into the zeolite micropores and hence increases ion-exchange efficacy. Providing a hierarchical porosity within the composite prevents the accumulation of zeolites crystals and restrict creation of died zones which are difficult to be accessed by ions. Fig. 7 shows SEM image and EDAX result of A after ion-exchange with cobalt. Where, the zeolite crystals kept there morphology and the Si/Al reminded 1after being utilized for cobalt removal. Also, a peak belongs to cobalt appeared in the EDAX plot. Figs. 8 and 9 show SEM images and EDAX results of AAS and AWS after ion-exchange with cobalt. 4A zeolite crystals remained to attach on the supports surface even after 24 h of shaking. EDAX results for these materials shows additional peak relevant to cobalt ion and the Si/ Al ratio remained 1. Also, the peak of cobalt appeared in the EDAX plot of both AAS and AWS.

Fig. 5. Equilibrium data for the uptake of cobalt ion by carbonized almond shells and carbonized walnut shells. 200 ppm cobalt ion, 2 g carbon/l solution, pH 6 and room temperature.

3.2.1. Equilibrium isotherms study The linear regression of Freundlich isotherm model and DubininRadushkevitch isotherm model (D–R) were chosen to fit the experimental data. Numerous papers have been published on applying those models to the equilibrium data of metals removal. The linearized form of Freunlich isotherm model represented by Eq. (3) includes the heterogeneity of a surface and the exponential distribution and the

Fig. 6. Equilibrium data for the uptake of cobalt ion by A, AAS and AWS. 200 ppm cobalt ion, 2 g ion-exchange media/l solution, pH 6, room temperature and contact time 24 h.

Table 3 The elemental analysis and structural properties of A, AAS and AWS. Material

A AAS AWS

The elemental analysis wt.% Al

Si

Na

O

C

K, Ca, Fe

20.82 12.36 10.30

23.73 12.85 10.80

13.01 6.55 5.30

42.45 41.30 36.81

– 26.20 36.19

– 0.74 0.58

Zeolite percentage wt.%

Crystal size (nm)

BET surface area (m2/g)

VTotal (cm3/g)

VMesopores (cm3/g)

100.00 40.81 44.69

1916 1050 1452

246.47 183.36 157.65

0.235 0.116 0.128

– 0.063 0.086

5

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Fig. 9. SEM image and EDAX analysis of AWS after ion-exchange with cobalt ion solution.

energies of active sites [45].

Fig. 7. SEM image and EDAX analysis of A after ion-exchange with cobalt ion solution.

lnqe = lnKf + 1/ nlnCe

(3)

Where qe is the amount of cobalt ions removed per the amount of ionexchanger at equilibrium (mg/g), Ce is an equilibrium cobalt ions concentration (mg/l), Kf (mg/g) and 1/n are Freundlich constants indicating the ion-exchanger relative capacity and the intensity of the process, respectively. Freundlich constants (listed in Table 4) can be easily obtained from plotting lnqe against lnCe [46]. The linearized form of D–R isotherm model represented by Eq. (4) predicts the process mechanism and assumes occurring of adsorption process on a homogeneous and heterogeneous surfaces.

lnqe = lnqm + Kε 2

(4)

ε = RTln (1 + 1/ Ce )

(5)

Where qm is the maximum capacity of ion-exchange (mg/g), K is a constant related to the energy of ion-exchange (mol2/kJ2), Ɛ is the Polanyi potential (kJ/mol), R is the gas law constant (kJ/kmol.K), T is the absolute temperature (K), qe is the equilibrium capacity of ion-exchange (mg/g) and Ce is the equilibrium cobalt concentration (mg/l) [47,48]. Estimating the mechanism nature of the process can be conducted by calculating the energy of transferring mole of ion from solution to the adsorbent surface. Eq. (6) can be used to obtain the uptake energy E (kJ/mol).

E = (2K )−1/2

(6)

When E is in the range of 20–40 kJ/mol indicates chemisorption. While, E is in the range of 8–16 kJ/mol indicates adsorption is governed by ion-exchange. However, adsorption is affected by physical forces when E < 8 kJ/mol [49]. D–R model constants (listed in Table 4) can be obtained by plotting lnqe versus Ɛ2. The correlation factor (R2), is a statistical amount shows how the experimental data fit the regression line, was depended to compare between Freundlich and D–R models. The results presented in Table 4 shows that the uptake of cobalt ions occurred at the external

Fig. 8. SEM image and EDAX analysis of AAS after ion-exchange with cobalt ion solution.

6

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Table 4 The isotherm parameters for the cobalt ions removal. Zeolitic materials

A AAS AWS

Freundlich model

Dubinin-Radushkevitch model 2

Kf (mg/g)

1/n

R

24.94 23.07 20.88

0.357 0.282 0.237

0.9807 0.9859 0.9857

RMSE

qm (mg/g)

K (mol2/kJ2)

E (kJ/mol)

R2

RMSE

4.821 1.431 1.208

64.17 51.22 60

2E-7 5E-7 3E-7

1.58 1 1.29

0.833 0.9232 0.9149

12.29 9.113 5.616

Fig. 10. Equilibrium data for the uptake of cobalt ion by A, AAS and AWS conducted at different cobalt initial concentration. 2 g ion-exchange media/l solution, pH 6 and room temperature.

Fig. 11. Equilibrium data for the uptake of cobalt ion by A, AAS and AWS conducted at different temperatures. 200 ppm cobalt solution, 2 g ion-exchange media/l solution and pH 6.

heterogeneous surface of the media (because 1/n < 1) and controlled by physical forces (because E is in the range of 8–16 kJ/mol) [50].

Where, ki is the intra-particle diffusion rate constant (mg/g. min0.5) and C is a constant related to the liquid film thickness (mg/g). If C (which is the intercept) has a smaller value, it indicates that the liquid boundary layer has less contribution in the rate controlling step. β is a Boyed equation constant which can be used to calculate the effective diffusivity (Di m2/s) according to Eq. (9).

3.2.2. Effect of initial ion concentration The amount of Co2+ ions removed increased with increasing the initial cobalt concentration for a specified weight of ion-exchange material as shown in Fig. 10. This trend can be because of increasing the number of ions vying for the same number of sites existing in a fixed amount of ion-exchanger. The ion-exchange process is motivated by increasing the concentration gradient, mass transfer driving force, resulting from increasing the initial cobalt concentration.

β=

(7)

q βt = − 0.4977 − ln ⎛⎜1 − t ⎞⎟ q e⎠ ⎝

(8)

(9)

Where r is the media particles radius (m). The linear plots of the intra-particle diffusion model presented in Fig. 12 shows that the intercept (C values) which refers to the effect of boundary layer for the composites and enhancing the ion diffusivity. However, the boundary layer still contributes to the rate limiting step in the process as confirmed by the results of Boyd model presented in Fig. 13. The linear plots of Boyed models did not pass through origin revealing that the film diffusion controls the ion-exchange process. Also, the intra-particle diffusion, which is represented by the slope in Fig. 12, enhanced for composites, since the slope seems higher for them. This outcome can be attributed to the outstanding distribution of zeolite crystals covering the carbon support surface. And this leads to enhance the diffusion of ions and reduce the resistance arisen from the boundary film. Approximately, comparable findings obtained by AL-Othman and Naushad [53] when used activated carbon prepared from peanut shell to remove for the removal of Cr (VI) from aqueous solutions.

3.2.3. Effect of cobalt solution temperature Temperature is significantly studied because it associates with some thermodynamic parameters. The results describing the effect of solution temperature on the removal of cobalt ion from aqueous solution are presented in Fig. 11. A solution with higher temperature (50⁰C) showed significant removal of cobalt ion from the solution. This is because the ion-exchange process of cobalt ion is an endothermic process as proved in [37]. Similarly, literature work showed that removal of Pb(II) ions by MWCNTs/ThO2 nano-composite [51] and removal of Th(IV) and U(VI) ions by a novel magnetic metal-organic framework composite consisting iron oxide magnetic nanoparticles and AMCA-MIL-53(Al) [52] are endothermic process. Understanding the ion-exchange mechanism and the rate determining step in the cobalt removal process was achieved using the Weber and Morris intra-particle diffusion kinetics equations shown in Eq. (7) [49] and the linearized form of Boyd model shown in Eq. (8) [7].

qt = ki t 0.5 + C

π 2Di r2

3.2.4. Effect of pH of cobalt solution The results representing the effect of solution pH on the removal of cobalt ions are shown in Fig. 14. It was difficult to increase the pH of cobalt solution beyond 6.8 due to precipitation of cobalt hydroxide in the solution. Increasing the pH of solution enhanced the removal of cobalt ions. The ion-exchange process using zeolitic materials is strongly influenced by the pH of solutions because it highly affects zeolite structure and the concentration of active sites. Moreover, the pH 7

Journal of Water Process Engineering 33 (2020) 101059

S.M. Al-Jubouri and S.M. Holmes

Fig. 12. Mechanism plots of Weber-Morris intra-particle diffusion model for the cobalt ions removal.

Fig. 13. Kinetic plots of Boyd model for the cobalt ions removal.

sample reduced to 44 % of the sample weight before vitrification. However, in the case of composites the sample weight reduced to 25 %. During the vitrification, carbon supports burned to CO2 leaving a melt of silica-alumina-metal because 4A zeolite composes about 45 % of the weight of composites. After nine months, the concentration of cobalt ions leached to H2O was of 0.01 ppm and to 0.5 N NaCl solution ranged of 0.05-0.25 ppm. Therefore, leaching of cobalt ions from the spent materials to the solutions was nearly eliminated when samples vitrified even after nine months in contact with 0.5 N NaCl solution. 3.4. Comparison based on cost and performance This section presents a general comparison made among hierarchical composites prepared from 4A zeolite in this study and composites made using X zeolite and clinoptilolite presented in the previously published works [37,38] based on cost and performance. In term of cost, the hierarchical composites prepared from 4A zeolite in the presented work is better than the composites made using X zeolite and clinoptilolite. This is because the cost required to prepare 4A zeolite composites is lower than the cost required to prepare X zeolite and clinoptilolite composites. This low cost is due to that preparation of 4A zeolite composites does not require overnight aging step under continuous aging like X zeolite. Also, crystallization of 4A zeolite composites does not require long crystallization time like X zeolite composites (8 h and more) and clinoptilolite composites (4 days and more). And in term of performance, 4A zeolite and its composites show higher performance of ion-exchange than both X zeolite and clinoptilolite and their composites because 4A zeolite owns lower Si/Al ratio providing higher concentration of easy exchangeable cations.

Fig. 14. Equilibrium data for the uptake of cobalt ion by A, AAS and AWS conducted at different solution pH. 200 ppm cobalt solution, 2 g ion-exchange media/l solution and room temperature.

of solutions affects the concentration of hydrogen ions competing with metal ions for actives sites during the exchange process. In the same context, Naushada and Alothmana [54] found that the removal of Pb2+ ions using the Amberlite IR-120 resin increased with increasing the pH. However, Awual et al., [55] found that acidic and alkaline media inhibit removal of Hg2+ ions by conjugate nano-materials. 3.3. Solidification

4. Conclusions

The samples of the used zeolitic materials bearing cobalt ions before and after vitrification are shown in Fig. 15 and the concentration of Co2+ ions leached to solutions are shown in Table 5. The samples encapsulating cobalt ions showed blue glassy solid due to formation a melt of Al and Co at elevated temperature. The weight of 4A zeolite

This study presents implementation of hierarchically porous 4Azeolite/carbon composites for removal of cobalt ions from aqueous solution. Inclusion of carbon supports prepared from almond shells and 8

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S.M. Al-Jubouri and S.M. Holmes

Fig. 15. Photographic images of the ion-exchange materials before and after ion-exchange with cobalt ions. Table 5 Concentration of Co2+ ions leached to solutions before and after Vitrification process. Sample Code

A AAS AWS

Without Vitrification

WithVitrification

Conc. of Co2+ ions leached to H2O after 9 months

Conc. of Co2+ ions leached to 0.5 N NaCl solution after 9 months

Conc. of Co2+ ions leached to H2O after 9 months

Conc. of Co2+ ions leached to 0.5 N NaCl solution after 9 months

7.40 2.72 1.78

8.85 4.13 3.29

0.01 0.01 0.01

0.05 0.25 0.11

help with samples measurement.

walnut shells within the composites structure did not alter the required crystallization time to obtain fully-crystallized zeolite crystals. Modifying of the carbon surface by nanoparticles achieved a permanent attachment of crystals on the carbon surface even though after using the composites for the ion-exchange process to remove cobalt ions. According to a comparison made based on the actual weight of zeolite used for ion-exchange with cobalt ions it was found that the ion-exchange capacity of zeolite significantly increased when zeolite-based carbon composites was used. Ion-exchange controls the removal process and it was found to be dependent on initial ion concentration, temperature and pH. Also, cobalt ion-exchange by 4A zeolite composites was found to be an endothermic process as ion-exchange capacity increased with increasing temperatures. The experimental data of the ionexchange were successfully fitted by the linear regression of Freundlich isotherm model and Dubinin-Radushkevitch isotherm model. The ionexchange process was found to be controlled by intra-particle diffusion and less by liquid boundary film as confirmed by Weber-Morris intraparticle diffusion and Boyd models. For successful immobilizing of cobalt ions, vitrification was applied to the composites-bearing cobalt ions.

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Declaration of Competing Interest This paper presents a reproducible work dealing with preparation of hierarchically porous 4A zeolite-based carbon composites using agricultural waste and implementing them for best removal of harmful ions from wastewater. Also, it presents a detailed study about the encapsulation of cobalt ions via vitrification process. In conclusion, the effect of boundary film reduced and the intra-particle diffusion of cobalt ion enhanced due to creating hierarchically porosity. Hereby this statement, we are the work team declare there is no conflict of interest to this work. Acknowledgments The authors would like to thank the Higher Committee for Education Development in Iraq for financially supporting Sama M. AlJubouri to conduct this work on the University of Manchester labs. Also, the authors would like to specifically thank Dr. Patrick Hill for 9

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