Optimization of an innovative composited material for effective monitoring and removal of cobalt(II) from wastewater

Journal Pre-proof Optimization of an innovative composited material for effective monitoring and removal of cobalt(II) from wastewater Md Rabiul Awual, Md Munjur Hasan, Aminul Islam, Abdullah M. Asiri, Mohammed M. Rahman PII:

S0167-7322(19)33129-0

DOI:

https://doi.org/10.1016/j.molliq.2019.112035

Reference:

MOLLIQ 112035

To appear in:

Journal of Molecular Liquids

Received Date: 3 June 2019 Revised Date:

23 October 2019

Accepted Date: 29 October 2019

Please cite this article as: M.R. Awual, M.M. Hasan, A. Islam, A.M. Asiri, M.M. Rahman, Optimization of an innovative composited material for effective monitoring and removal of cobalt(II) from wastewater, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.112035. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Optimization of an innovative composited material for effective monitoring and removal of cobalt(II) from wastewater Md. Rabiul Awual

a, b

*, Md. Munjur Hasan c, Aminul Islam c, Abdullah M. Asiri b, Mohammed M. Rahman

a

b

Materials Science and Research Center, Japan Atomic Energy Agency (SPring–8), Hyogo

679–5148, Japan b

Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz

University, Jeddah 21589, Saudi Arabia c

Department of Petroleum and Mining Engineering, Faculty of Engineering and Technology,

Jessore University of Science and Technology, Bangladesh

* Corresponding author at: Materials Science and Research Center, Japan Atomic Energy Agency (SPring–8), Hyogo 679–5148, Japan E–mail address: [email protected] (M.R. Awual).

Research highlights:
Novel composite adsorbent was designed for efficient Co(II) ion detection & removal.
The adsorbent was efficiently removed Co(II) ion with high selectivity & sensitivity.
The competing ion was not affected the Co(II) capturing by the composite adsorbent.

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Graphical Abstract

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ABSTRACT A newly designed composite adsorbent (CpAD), ultra-trace detection ability and a superior adsorption capability to that novel material, was fabricated by functional ligand (3-(((5ethoxybenzenethiol)imino)methyl)–salicylic acid) combining with mesoporous silica. The resultant CpAD was maintained a high surface area with ordered porosity even after successful ligand anchoring. The application of cobalt (Co(II)) detection and adsorption was measured at an optimum experimental protocol with exhibition of significant color visualization. The experiment conditions were optimized based on contact time, solution pH, initial Co(II) concentration and diverse competing metal ions. The CpAD was able to detect the low level Co(II) ion as the detection limit was 0.39 µg/L. The data were clarified that the CpAD was not affected with the existing competing ions and the signal intensity and specific color was observed only toward the Co(II) ion. When used as an adsorbent, the CpAD was demonstrated a very quick adsorption property for the removal of Co(II) ions. The adsorption capability approaches 185.23 mg/g, which is one of the highest capabilities of today’s materials. The data also revealed that the removal capabilities of the CpAD for Co(II) ions depended on the material functionality and initial concentration of Co(II) ions. The elution of Co(II) ions from the saturated composite adsorbent was desorbed successfully with 0.30 M HCl. The regenerated adsorbent that remained maintained the high selectivity to Co(II) ions and exhibited almost the same adsorption capacity as that of the original adsorbent. The highest percentage adsorption of Co(II) exceeds 96% in the presence of competing ion, indicating that the CpAD is one of the very suitable composite adsorbent in environmental pollution management. Keywords: Composite adsorbent; Cobalt(II) ions; Sensitive detection; Optimum experimental protocol; Wastewater samples.

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1. Introduction Cobalt (Co(II)) is an important cofactor in Vitamin B12, which is responsible for proper functioning of the brain and nervous system and for formation of blood [1]. It is well known that Co(II) is one of the most important transition metals for human beings. Excessive amounts of Co(II) may produce goiter and reduce thyroid activity and deficiency of Co(II) may lead to cardiovascular disease [2]. However, excess intake of Co(II) will be hazardous to both humans and animals. The exposition to this element at high levels may induce toxic effects and causes nausea, reproductive problems, hypertension (high blood pressure), pulmonary diseases and hyperglycemia (high blood sugar), bone defects, and may also cause mutations (genetic changes) in living cells [3-6]. The permissible limits of Co(II) in the irrigation water and live-stock watering are 0.05 and 1.0 mg/L, respectively (Environmental Bureau of Investigation, Canadian Water Quality Guidelines). Therefore the detection and removal of trace Co(II) in an environmental samples a key in the fields of environmental analysis, process control and medicine. The determination of trace amounts of Co(II) is of great significance, not only for industry, but for biological study as well. Several methods have been used for Co(II) determination, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES), flame atomic absorption

spectrometry (FAAS),

electrothermal

atomic absorption

spectrometry (ETAAS), stripping voltammetry and spectrophotometry, the latter being favored for rapid analysis in view of its ease of automation, simple and low-cost instrumentation [7-12]. According to advantages, a wide variety of spectrophotometric methods for the determination of Co(II) have been reported. The ICP-MS, FAAS and ETAAS test procedures are time-consuming and costly, requiring sophisticated and expensive instrumentation. Therefore, simple, sensitive and selective methods are demanded for Co(II) ions detection. Compared to these methods, optical methods allow in-situ visual 4

detection with a naked eye is more suitable because these are cost-effective and simple without the need for sophisticated apparatus [13-15]. Recently, several ligands impregnated materials for sensitive diverse metals detection is reported even in the presence of foreign ion [16-18]. The complexation mechanism and optimum condition are the key factors for selective metal ions affinity [19]. In this connection, the specific functional group containing ligand is developed for sensitive, selective and efficient Co(II) detection at optimum experimental protocol. Liquid–liquid extraction is known as solvent extraction (SX), allows separation of a specific compound or many compounds from one liquid and its transfer into another pertinent liquid. This process usually requires vigorous stirring of both liquids in order to allow mass transfer of the solute from one phase to another. Metal ion extraction from aqueous phase requires the use of an organic solution of a special extractant that will allow solubilization of the metal salts in the organic phase. However, organic solvent and long extraction times make this method inefficient for low concentration of metal ions separation [20-22]. Activated carbon is the most common material due to its effectiveness, versatility, and good capacity for the removal of metals, dyes and other organic compounds. However, it suffers from a number of disadvantages; mainly its high cost on largescale uses [23]. One of the most popular techniques used to remove a wide variety of substances is adsorption. This technique allows the use of environmentally friendly materials whose cost of production is usually very low. Adsorption is widely used in preconcentration procedures, which are mandatory for many analytical techniques [24]. Adsorbents have been widely used because they are reusable, easy to handle and characterized by high adsorption rate and high adsorption efficiencies as well as high selectivity to some metal ions [25-27]. However, the low adsorption capacity and selectivity of these adsorbents limit their potential applications in wastewater treatment. It is noted that most of the adsorbent materials have consisted with 5

various functional groups for target ions adsorption. The solid porous nanomaterials are considered to be the most efficient, reliable, simple, and cost-effective adsorbents for the adsorption and recovery of diverse ions at solution acidity [28-33]. Recently, we have used different ligand immobilized silica based nanomaterials for various metal ions detection and removal from wastewater [34-36]. The functionalized nanomaterials have gained special attention due to the intrinsic properties such as high specific surface area, high mechanical stability, high sorption capacity and reusability. This study was aimed to prepare ligand anchored composite adsorbent (CpAD) for ultra-trace level of Co(II) ion detection and removal from wastewater samples. The organic functional ligand of 3-(((5-ethoxybenzenethiol)imino)methyl)–salicylic acid (EBMS) was synthesized and then anchoring onto the mesoporous silica by direct immobilization technique. The EBMS ligand was exhibited the functionality to form significant color upon contacting with the Co(II) ion. Then, the efficiency of this CpAD for Co(II) ion detection and removal from waste solution was evaluated. In the part of this study, surface morphology of the carrier mesoporous silica monolith and fabricated CpAD was characterized systematically. The experimental parameters such as solution pH, contact time, initial Co(II) concentration, elution and regeneration were performed. The adsorption capacity was compared with the other forms material to classify the propose CpAD as potential material. The color assessment, limit of detection and ion selectivity were also investigated from the stand point of onsite use for wastewater treatment. The adsorption data were fitted with the Langmuir isotherm model as expected from the order structure homogeneity to define the maximum enrichment capacities of Co(II) adsorption on the CpAD. The developed adsorbent based on direct organic-inorganic composition is cost-effective and suitable to the large-scale treatment in the real field samples analysis. Therefore, it is expected that proposed material

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and its application would be an interesting that grants experiments with process efficiency and easier operation methods.

2. Materials and methods 2.1. Materials All materials and chemicals were of analytical grade and used as purchased without further purification. Tetramethylorthosilicate (TMOS) and the triblock copolymers of poly(ethylene

oxide–b–propylene

oxide–b–ethylene

oxide)

designated

as

F108

(EO141PO44EO141) was obtained from Sigma–Aldrich Company Ltd. USA. The standard Co(II), and metal salts for the source of metal ions were purchased from Wako Pure Chemicals, Osaka, Japan. For pH adjustments in optical detection operation, buffer solutions of 3–morpholinopropane sulfonic acid (MOPS), 2–(cyclohexylamino) ethane sulfonic acid (CHES), and N-cyclohexyl-3-aminopropane sulfonic acid (CAPS) were procured from Dojindo Chemicals, Japan, and KCl, HCl, NaOH from Sigma-Aldrich.

2.2. Synthesis and characterization of EBMS ligand The preparation step of 3-(((5-ethoxybenzenethiol)imino)methyl)-salicylic acid (EBMS) is reported [14b], and the structure is depicted in Scheme 1. The EBMS was prepared by the reaction of 2-amino-5-ethoxybenzenethiol (10 mmol) and 3-formylsalicylic acid (10 mmol) in ethanol and small amount of acetic acid. Then the mixture was heated under reflux for 6 h and left to cool at room temperature. The solid formed upon cooling was collected by suction

filtration. The separated product was recrystallized using

dichloromethane/methanol 1/1. Then dry the purpose product at 50 °C for 24 h.

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2.3. Preparation of mesoporous silica and composite adsorbent (CpAD) The preparation procedure involved adding TMOS to triblock copolymers (F108) to obtain a homogenized sol-gel mixture based on specific copolymer/TMOS mass ratio. An acidified aqueous solution was added to the mixture to quickly achieve the desired liquid crystal phase and then to promote hydrolysis of the TMOS around the liquid crystal phase assembly of the triblock copolymer surfactants. Mesoporous silica monoliths were synthesized following the reported methods with slight modification [37]. Mesoporous silica monoliths were synthesized by using direct templating method of lyotropic liquid crystalline phase of F108 as soft template. In typical conditions, the composition mass ratio of F108:TMOS:HCl/H2O was 1.4:2:1 respectively. Homogeneous sol-gel synthesis was achieved by mixing F108/TMOS in a 500 mL beaker and then shaking at 50 °C until homogeneous. The exothermic hydrolysis and condensation of TMOS occurred rapidly by addition of acidified aqueous HCl (at pH = 1.3) solution to this homogeneous solution. Then the methanol produced from the TMOS hydrolysis was removed by using a diaphragm vacuum pump connected to a rotary evaporator at 45 °C. The organic moieties were then removed by calcination at 520 °C for 6 h under normal atmosphere [38]. After that the material was grinded properly and ready to use for immobilization with synthesized EBMS ligand to prepare as new type mesoporous composite adsorbent. The composite adsorbent (CpAD) was assembled by straight immobilization of EBMS (75 mg) in ethanol into 1.0 g grounded silica. The immobilization procedure was performed under vacuum at 30 °C until EBMS ligand saturation was achieved. The ethanol was removed by a vacuum connected to a rotary evaporator at 45 °C, leading to the direct contact of the EBMS ligand into the mesoporous silica materials. The resulting materials

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were rinsed with water until no elution of EBMS was observed. Then the materials was dried at 45 °C for 10 h and grinded into fine powder for optical Co(II) detection and removal experiments. The EBMS immobilization amount was determined by the following equation: Q = (C0– C) V/m

(1)

where Q is the adsorbed amount (mmol/g), V is the solution volume (L), m is the mass of substrates (g), C0 and C are the initial concentration and supernatant concentration of the EBMS, respectively.

2.4. Instruments The N2 adsorption-desorption isotherms were measured using the 3Flex analyzer (Micromeritics, USA) at 77 K for determining the material morphology in terms of surface characterization. The pore size was measured based on the Barrett–Joyner–Halenda (BJH) method. Both silica and CJA were preheated at 110 °C for 3 h. The TEM micrographs were received by using JEOL (JEM-2100F). To detect the color form of Co(II) ion by UV–vis in the solid state was utilized for color optimization and their corresponding signal intensity measurement. The Co(II) ion determination was checked using ICP-AES (PerkinElmer, Germany, 8300).

2.5. The Co(II) detection procedures In the detection of Co(II), the CpAD was combined in a mixture of 2.0 mg/L Co(II) and then adjusted pH of 2.0, 3.50, 5.50, 7.0, 8.0, 9.50 with the specific volume of 10 mL and shaking in water bath. The blank sample was also arranged according to the same method to

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understand the color formation in the comparison of color measurement. The CpAD was filtered and used for color assessment and absorbance intensity measurement. The CpAD was grounded properly to get the smooth in the absorbance spectra by the machine. The detection limit (LD) of Co(II) is determined based on the below Eq. [39]: LD = KSb/m

(2)

Where K is equal to the value of 3, Sb is the standard discrepancy and m the slope of the linear plot, respectively.

2.6. The Co(II) removal procedures In the adsorption case, the CpAD was also added with Co(II) mixing solution where the pH was also conformed using HCl or NaOH. After that the CpAD was isolated by filtration method, and the filtrate solution containing the Co(II) amount was analysed by ICPAES. The removal quantity was evaluated in accordance with the below equations: Mass balance qe = (C0 – Cf) V/M (mg/g) and Co(II) ion adsorption efficiency Re =(C0 – Cf) 100/ C0 (%)

(3) (4)

Where qe removal amount (mg/g), C0 and Cf were the initial and supernatant amounts of Co(II) (mg/L), respectively, V is the total solution (L), m represents the weight of the CpAD (g). To define the equilibrium time, 5.0 mg/L Co(II) amount of fixed volume solution was used in each fraction, and the CpAD amount was fixed at 20 mg in each case. After 10 min interval, the solid CpAD was separated and filtrate was analyzed by the ICP-AES. The

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highest adsorption amount was defined based on the different initial concentrations of Co(II), and each sample was analyzed by ICP-AES. To check the suitable eluent, the CpAD was equilibrated with fixed Co(II), and the CpAD was checked for several eluents such as NaOH, H2SO4, and HCl and the filtrate were relocated to the test tubes, and then Co(II) was checked by the ICP-AES. According to the experimental protocol, the 0.30 M HCl solution was used as appropriate eluent. It was also noted that the CpAD was regenerated in the elution time, and the CpAD could use several cycles for an efficient and economical optical composite adsorbent.

2.7. Effect of competing ion The Co(II) and the competing ions were carried out to understand the selective adsorption ability where diverse counter-ions were used. The solution was contained 15 mg/L of each Zn(II), Ba(II), Al(III), Fe(III), Mg(II), Ca(II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Hg(II), Bi(III) ion and the Co(II) amount was 1.0 mg/L. The sample volume was to be 30 mL and CpAD amount was 20 mg. Then the mixture was stirred for 3 h and then separated by filtration. The solution was analyzed using the ICP-AES. All experimental demonstrations were duplicated to confirm the extracted data in this study. The highest variation for each adsorption operation was 4.0%.

3. Results and discussion 3.1. Characterization of mesoporous silica and composite adsorbent (CpAD)

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The N2 adsorption–desorption isotherms of mesoporous silica monoliths showed IVtype isotherm with an obvious hysteresis loop that was representative of the mesoporous regular framework pore, such as a textural pore as shown in Fig. 1. The mesoporous silica showed apparent appreciable specific surface area (SBET), mesoporous volume, and tunable mesoporous diameters based on data analysis (Fig. 1 inset). Moreover, the framework porosity in the N2 isotherm indicated the porosity contained within the uniform channels with well pore size distribution having pore diameter of 8.90 nm, high surface area (573 m2/g), and high pore volume (0.75 cm3/g). However, a decrease in the surface area and pore volume after functionalization of mesoporous adsorbent provided further evidence that the organic moieties were anchored inside the mesopore (Fig 1 (b)). The TEM images showed a well arrangement pores and continuous arrays along all directions (Fig. 2(A, B)) which indicate the direct interaction between the EBMS and inorganic silica into the rigid condensed pore surfaces with retention of the ordered structures, leading to high flux and Co(II) ion transport during detection and removal operations. In addition, the TEM image exhibited well–organized parallel channels and clarified a hexagonally ordered array in the direction parallel to the pore which clarified the prepared material has a typical hexagonal [40]. The TEM images showed a well-organized parallel channels and clarified as hexagonally ordered array of circles in the direction parallel indicating that mesoporous silica material has a typical hexagonal with ordered mesoporous structure. Therefore, the CpAD chains were partially entrapped in the composite mesopores of the pore mouth as defined from Fig. 2 (C,D). The conjugate adsorbent was fabricated by direct immobilization method. After immobilization of EBMS, a decrease in the surface area (SBET) and average pore diameter of conjugate adsorbent provided further the presence of EBMS ligand to the surface partially blocks the adsorption of nitrogen molecules as judged from Fig. 1(b); however, a significant amount might also anchor on the outer surfaces of the 12

cage [41]. The materials characterization also confirmed that the ability to achieve flexibility in the specific activity of the electron acceptor or donor strength of the chemically responsive EBMS ligand molecule may lead to easy generation and transduction of visual color signal in the naked–eye detection and sorption of ultra–trace Co(II) ions by a stable complexation mechanism with specific selectivity. It was confirmed that the cage character was maintained even after incorporated of the ligand molecules onto mesoporous silica.

3.2. Optical detection of trace Co(II) ions The pH of the solution is an important factor for selective and optical detection of trace Co(II) ions. The present adsorbent of Co(II) detection was significantly affected by the solution pH. The reflectance spectra of the [Co(II)-ligand]n+ complex at λ = 435 nm was carefully monitored over a wide pH ranges as shown in Fig. 3. The data clarified that the amount of CpAD was sufficient to achieve good color separation “signal” between the mesoporous adsorbent :blank” and Co(II) ion–detection :sample,” even at low levels of Co(II) ions. The CpAD was sensitive in terms of its optical color intensity and signal response for Co(II) ions at pH 8.0. In signal responses, the highest absorbance at 435 nm was indicated as 100% in pH evaluation by the CpAD. The results implied that the adsorbent has high functionality and affinity in terms of selectivity and sensitivity to Co(II) ions at pH 8.0. This pH region also suggested that the Co(II) ions can bind strongly to CpAD with high stability during complexation formation in optical detection system. The sensitivity is another parameter to judge the material effectiveness in the detection of the heavy-metal ion [30,35,40]. Then the sensitive Co(II) ion detection by the CpAD was evaluated, and the data are shown in Fig. 4(A). The proposed CpAD design contributed the high complexation affection between the Co(II) and organic ligand, which

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exhibited high sensitive detection from low to high concentration of Co(II) ion. The data also revealed that the fabricated CpAD showed the high physical and textural properties for naked-eye detection of Co(II) ion at this experimental procedure. Increasing the concentration of Co(II) from 0.002 to 2.0 mg/L, when the experimental protocol was fixed, the absorbance spectra was enhanced as judged from Fig. 4(A). The data are also confirming that the increasing the signal intensity corresponds to color formation combining the Co(II) and CpAD is the noticeable observation for the sensitive detection of ultra–trace Co(II) ion without using expensive instrumentations. Therefore, the CpAD is able to detect the Co(II) ion with specific functionality from µg/L to mg/L level with high sensitivity. The low limit of detection of Co(II) ions for the CpAD was estimated from the linear part of the calibration plot of the absorbance spectra of the [Co(II)–EBMS]n+ complex at 435 nm against the Co(II) ion concentration. The calibration plots of the CpAD show a linear correlation at low concentration of Co(II) ions as shown in Fig. 4(B). A linear correlation in the range from 0.002–0.10 mg/L was observed (Fig. 4(B) inset). At higher concentration, the dependence is nonlinear due to saturation effects, indicating that sensitive detection of low concentrations of Co(II) ions. The resultant detection limit value (0.39 µg/L) indicated that the CpAD enabled Co(II) detection of ultra–trace Co(II) ions, even in the presence of the several matrices (calibration curve with dotted lines). Therefore, the fabricated CpAD can effectively separate and preconcentrate the Co(II) ions even at trace levels. Several metal ions are coexisted with Co(II) containing wastewater in real samples. Then the specific ion selectivity study of the CpAD based on signal intensity was measured. The absorbance intensity of each metal ions concentration compared with the blank and Co(II) ions containing samples intensity [14,31,33]. The changes in the absorption spectra of CpAD upon addition of different metal ions were observed and the data are shown in Fig. 5. The absorption spectra of the CpAD was exhibited strong charge transfer complexation upon 14

addition of Co(II), which was characterized according to the peak occurring at 435 nm. The CpAD was tested with different cations Zn(II), Ba(II), Al(III), Fe(III), Mg(II), Ca(II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Hg(II), Bi(III) and with Co(II) ions. The others metals has no significant signal intensity even their concentration were 15 times higher than the Co(II) ions. Results were highlighted the affinity of CpAD towards the Co(II) ions at pH 8.0, this means the high selectivity at the present experimental conditions. In addition, the designed CpAD remains well suitable materials when the purpose is to detect the ultra-trace level of Co(II) without necessarily know the diverse competing ions in wastewater solutions.

3.3. The Co(II) adsorption A series of batch experiments was carried out to define and evaluate the suitable parameter for complete removal of Co(II) ion. Therefore, pH effect was measured over wide pH ranges from 2.0 to 9.50 in the removal operation to obtain high removal of Co(II) ions from solution and pH was adjusted by adding HCl and NaOH in water as needed. Fig. 6(A) shows the pH effect for Co(II) removal when the feed solution concentration was 2.0 mg/L. The data clarified that the CpAD has high affinity to Co(II) ions at pH 8.0, where the Co(II) ions was completely removed. Then the other experimental parameters for removal operation were carried out at pH 8.0 region. The CpAD has excellent capability to take up Co(II) ions completely even an initial concentration was low level at 2.0 mg/L because of the high surface area and large pore volume of the mesoporous inorganic silica [42-44]. At high pH area, the removal efficiency was high and this may be explained by the stable complexation mechanism between the EMBS ligand of the CpAD and Co(II) ions. Moreover, the composite material has the specific adsorption ability at a specific pH region where the metal ions make stable complexation for selective adsorption operation [34,36,37-39].

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The adsorption kinetic in wastewater treatment is very important because it provides valuable insights into the mechanism and reaction pathways of adsorption reaction [40,45]. Therefore, adequate contact time between Co(II) ions and CpAD for equilibrium adsorption was measured. A series of batch contact time experiments was performed for evaluating the highest Co(II) adsorption by the CpAD. Fig. 6(B) shows the Co(II) adsorption by the CpAD as a function of time. Rate of Co(II) adsorption onto the CpAD was very high in the first 20 min resulting in 80% adsorption. Complete adsorption equilibrium was attained within 50 min, whereas, adsorption of other metal ions by functional resins is very slow kinetics because metal ions penetrate into the polymeric matrix which is a rate-controlling step and the adsorption kinetics also depend on diffusion rate of metal ions [46,47]. The adsorption isotherm investigates the relationship between the metal ions concentrations in the solution and the amount of metal ions adsorbed on the solid phase when both phases are in the equilibrium state. It was also noted that the adsorption isotherm was also investigated by batch approach. To evaluate the adsorption isotherm, 30 mL of sample solutions containing various concentrations of Co(II) ions and the adjusted solution at pH 8.0 and the adsorbent amount was kept constant (10 mg). At equilibrium, the filtrate solution was checked to evaluate the remaining Co(II) ion in each case and the data are shown in Fig. 7. The Co(II) ion’s adsorption amount was increased with increasing the initial Co(II) ion concentrations and reached a plateau to obtain the maximum adsorption capacity. The decrease in the adsorbed amount is due to less active sites being available at the end of the adsorption process and Co(II) ions partially blocking the adsorbent surface of active sites. To understand the nature of the adsorbent based on the adsorption system, it is important to confirm the most appropriate correlation for the equilibrium plot. Therefore, several adsorption isotherms are followed to evaluate the metal ions interaction with the prepared material [48,49]. Among the several isotherms, the Langmuir adsorption isotherm is valid for 16

monolayer adsorption onto the adsorbent surface based on the assumption of surface homogeneity in which all adsorption sites are energetically same. Then the Langmuir equation was applied to this adsorption operation as follows: Ce/qe = 1/(KLqm) + (1/qm)Ce

(linear form)

(5)

where qe is the amount (mg) of adsorbed Co(II) ion per unit of mass of the CpAD (g), whereas Ce is the concentration of Co(II) ion remained in the solution (mg/L) at the equilibrium. The qm is theoretical maximum adsorption capacity defined by the proposed CpAD and KL is the Langmuir constant (L/mg). On the plot of Ce/qe against Ce, qm and KL were decided from the slope and intercept. The data were well fitted to the Langmuir isothermal model as shown in Fig. 7 (inset) and the coefficient in the linear regression was 0.995; the highest amount adsorption capacity was 185.23 mg/g, and the adsorption coefficient KL was 1.77 L/mg. The CpAD is exhibited the surface homogeneity and able to accommodate the high amount of Co(II) ion as the surface functional was active. In addition, the high surface area is also one of the reasons for this high adsorption ability. A comparison has also been defined between the present CpAD and the conventional or previously reported [27,50-55] materials for Co(II) adsorption and these are summarized in Table 1. The comparison data clarified that the fabricated CpAD has significant capacity compared to other forms of diverse materials. In natural water, the metal ions are consisted with several monovalent and divalent or trivalent metal ions [56]. These diverse ions are usually competed due to the affinity as well similar ionic radii of metal ion [52]. Then the effect of competing ion was evaluated for the adsorption efficiency by the CpAD at pH 8.0. In order to assess the selectivity and potential application of the proposed CpAD to environmental samples, 1.0 mg/L of Co(II) and other 17

foreign ion such as the Zn(II), Ba(II), Al(III), Fe(III), Mg(II), Ca(II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Hg(II), Bi(III) was used where each foreign ion concentration was 20 mg/L. The concentrations of foreign ions were chosen to be close or higher to their contents in natural waters and many ions are also in higher concentration level. However, the same concentration of each foreign ion was chosen to obey the experimental procedure for understanding the CpAD affinity. The effect of competing ion on the Co(II) adsorption on the CpAD is shown in Fig. 8(A). The data clarified that the presence of these competing ions did not affect the Co(II) adsorption by the fabricated CpAD. Therefore, the CpAD has exhibited high selectivity toward the Co(II) ion and is a potential candidate for Co(II) containing natural water treatment. The possible complexation mechanism between the functional group of CpAD and the Co(II) ion is illustrated in Scheme 2.

3.4. Elution-regeneration-reuses An elution, regeneration and reuses of the solid adsorbent are crucial parameters for potential implication in real wastewater treatment technologies [30,35,38]. The reusability of the CpAD is also recognizing the cost effectiveness to draw the high attention from the potential users. Ligand anchoring materials are highly reusability and remaining high almost the original functionality for the metal ion capturing [14]. In this desorption operation the 0.30 M HCl was selected as the suitable amount of eluent. After desorption, the CpAD was simultaneously regenerated into the original position and uses as like parents CpAD for Co(II) ion detection and removal after washing with water. Fig. 8(B) shows the efficiency after eight sequential cycles of Co(II) on the CpAD. The removal efficiency was not significantly encountered even after eight cycles. Therefore, the results clearly clarified the cost-effective CpAD for efficient Co(II) ion detection and removal from wastewater samples.

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4. Conclusions The ligand anchoring optical composite adsorbent (CpAD) was fabricated using direct immobilization of organic ligand onto the silica monoliths for Co(II) ion detection and adsorption from wastewater samples. The porous silica and CpAD were characterized systematically to understand the material morphology for efficient Co(II) ion detection and removal. The CpAD was exhibited the high ordered porosity and kept the open ligand functionality for solid-liquid separation in the case of Co(II) ion. After optimization of pH solution, a full factorial experimental design was measured to define the sensitivity, detection limit, competing ion effect, contact time, initial Co(II) concentration and desorption for reuses evaluation. The influence of each affecting factor and of their interactions was clarified at optimum experimental conditions. The limit of detection was low as 0.39 µg/L, which was enhanced the applicability of the Co(II) containing water. The kinetic performance was also high, and the data were highly fitted to the Langmuir adsorption isotherms. The maximum adsorption capacity was 185.23 mg/g, which was remarkable compared to the other forms of different materials. The existing diverse competing ion was not interfered with Co(II) ion both in the case of detection and removal operation. The CpAD was exhibited the regeneration ability and reuses with eluent of 0.30 M HCl and able to use several cycles without deterioration in its initial performances. Overall, these results illustrated the potentiality of the CpAD, and the design proposed functional composite adsorbent here as a new tool to tackle the problem of water pollution by Co(II) in a cheap, easy and ecological way, especially in the developing countries.

Acknowledgments 19

This research was partially supported by the Grant-in-Aid for Research Activity Startup (24860070) from the Japan Society for the Promotion of Science. The part of the experimental works was carried out at Japan Atomic Energy Agency. The authors also wish to thank to the anonymous reviewers and editor for their helpful suggestions and enlightening comments.

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28

Table 1 Comparison of adsorption capacities to the other forms of diverse adsorbents reported in the literature for Co(II) ion

Used materials

Adsorption capacity (mg/g)

Ref.

Composite adsorbent

189.37

[27]

Silica SBA-15

181.67

[50]

SICCM

144.90

[51]

Swine bone char

108.70

[52]

B12C4

137.01

[53]

PET-TSC fibers

78.08

[54]

6.16

[55]

185.23

This study

Functionalized hybrids Composite material

29

Fig. 1. The N2 adsorption-desorption isotherms of mesoporous silica monolith (A) ligand functionalized composite adsorbent (B) under experimental conditions.

30

Fig. 2. The TEM images of the carrier silica monolith (A, B) and organic ligand functionalized composite adsorbent (C, D) with ordered and homogeneous porosity.

31

Fig. 3. Determination of optimum condition based on the solution pH, where the Co(II) ion concentrations and composite adsorbent amount were fixed. The concentration of the Co(II) ion was 2.0 mg/L and the solution volume was 10 mL.

32

Fig. 4. Effect Co(II) ions concentration in the increment of color formation using the signal intensity optimization of the composite adsorbent at pH 8.0 (A) and determination of detection limit with the connection of different signal intensity upon addition of different amount of Co(II) ions.

33

Fig. 5. (A) Ion selectivity of the composite adsorbent using diverse at pH 8.0. The interfering cations (15.0 mg/L) listed in order (1 to 15): (1) Zn(II), (2) Ba(II), (3) Al(III), (4) Fe(III), (5) Mn(II), (6) Ca(II), (7) Ni(II), (8) Ag(I), (9) K(I), (10) Na(I), (11) Mn(II), (12) Hg(II), (13) Bi(II), (14) Blank and (15) 1.0 mg/L Co(II). The interfering anions (300 mg/L) anions listed in order (7 to 13): (7) chloride, (8) nitrate, (9) bicarbonate, (10) carbonate (11) sulfate, (12) phosphate and (13) citrate. The RSD value was ~3.0%. 34

Fig. 6. Effect of pH in terms of Co(II) adsorption when all others parameters were fixed (A) and evaluation of contact time effect for an equilibrium adsorption when the initial Co(II) concentration was fixed at each fraction.

35

Fig. 7. Determination of maximum adsorption capacity by the Langmuir adsorption isotherms with linear form (initial Co(II) concentration range from 2.0 to 72.22 mg/L; solution pH 8.0; dose amount 10 mg; solution volume 30 mL and contact time 3 h).

36

Fig. 8. (A) The Co(II) adsorption in the presence of competing ion where each ion concentration was 20.0 mg/L and Co(II) concentration was 1.0 mg/L and (B) Elution operation with 0.30 M HCl for successful regeneration. The RSD values were ∼3.00%.

37

Scheme 1. Preparation of 3-(((5-ethoxybenzenethiol)imino)methyl)-salicylic acid (EBMS) ligand for mesoporous composite adsorbent.

38

Scheme 1. The possible complexation mechanism between the Co(II) ions and the functional group of CpAD during the selective detection and removal operations.

39

All authors are confirming that there is no conflict of interest of this work.