Efficient detection and adsorption of cadmium(II) ions using innovative nano-composite materials

Accepted Manuscript Efficient detection and adsorption of cadmium(II) ions using innovative nanocomposite materials Md. Rabiul Awual, Majeda Khraisheh, Nabeel H. Alharthi, Monis Luqman, Aminul Islam, Mohammad Rezaul Karim, Mohammed M. Rahman, Md. Abdul Khaleque PII: DOI: Reference:

S1385-8947(18)30326-7 https://doi.org/10.1016/j.cej.2018.02.116 CEJ 18595

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

12 January 2018 24 February 2018 26 February 2018

Please cite this article as: d.R. Awual, M. Khraisheh, N.H. Alharthi, M. Luqman, A. Islam, M. Rezaul Karim, M.M. Rahman, d.A. Khaleque, Efficient detection and adsorption of cadmium(II) ions using innovative nano-composite materials, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.02.116

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Efficient detection and adsorption of cadmium(II) ions using innovative nano-composite materials

Md. Rabiul Awual

a, c

*, Majeda Khraisheh b, Nabeel H. Alharthi

c, e

, Monis Luqman c,

Aminul Islam d, Mohammad Rezaul Karim e, Mohammed M. Rahman f, Md. Abdul Khaleque g a

RENESA, Japan, 3–3–22 Sakuraguchi-cho, Nada-ku, Kobe-shi, Hyogo 657–0036, Japan Department of Chemical Engineering, College of Engineering, Qatar University, P.O. Box2713, Doha, Qatar c Mechanical engineering department, College of Engineering, King Saud University, Riyadh 11421, Saudi Arabia d Department of Petroleum and Mining Engineering, Jessore University of Science and Technology, Bangladesh. e Center of Excellence for Research in Engineering Materials, Advanced Manufacturing Institutes, King Saud University, Riyadh 11421, Saudi Arabia f Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia g Department of Environmental Science, School of Environmental Science and Management, Independent University, Bangladesh, Dhaka 1229, Bangladesh b

* Corresponding author. Tel./Fax: +81 787 79 2231. E–mail address: [email protected] and [email protected] (M.R. Awual).

Research highlights:

 Ligand based nano-composite materials was fabricated for optical Cd(II) capturing.  The specific color was developed upon addition of Cd(II) ions at optimum condition.  Nano-composite material was highly selective and sensitive to Cd(II) ions.

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Graphical Abstract

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ABSTRACT In this study, a ligand was anchored with mesoporous silica, named as nano-composite materials, was applied in the detection and adsorption of cadmium (Cd(II)) ions from wastewater samples. The effects of solution pH, color optimization, limit of detection, contact time, initial concentration, ion selectivity and regeneration were systematically performed in the case of detection and adsorption operations. The solution pH was played an important factor both in the case of detection and adsorption, and the optimum pH were selected at 5.50 based on the high absorbance and adsorption ability. Upon addition of Cd(II) ions, the nanocomposite materials was provided an excellent color, which was observed by the naked-eye. The detection limit was calculated to be 0.33 µg/L, which was lower than the permissible limit. Therefore, the Cd(II) ions was detected without using any sophisticated instruments. The equilibrium isotherm has been analyzed using Langmuir isotherm models, and the maximum adsorption capacity was 148.32 mg/g. The results clarified that the nano-composite material had the higher selectivity towards Cd(II) ions even in the presence of high concentration of divers metal ions. The material was reused in several cycles after elution operation with a suitable eluent of 0.25 M HCl. The nano-composite materials exhibited an excellent reusability because of its remarkable mechanical strength and highly efficient elution/regeneration operations ability. In the static treatment process, after seven cycles, Cd(II) ions was adsorbed efficiently and holding over 93% adsorption efficiency. The developed ligand functionalized nano-composite materials is quite simple and rapid with excellent repeatability for Cd(II) ions capturing and has a great potential for potential scale up for field application in real wastewater samples.

Keywords: Cadmium(II) ions; Nano-composite material; Detection and adsorption; High adsorption; Elution/reuse.

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1. Introduction The toxic metal ions contamination is a growing concern worldwide. Among various toxic metals ions, cadmium (Cd(II)) ions is an extremely toxic metal that usually exists in industrial workplaces, including electroplating, paint pigmentations, and nickelcadmium batteries and can cause water, soil, and air pollution [1,2]. The Cd(II) ions can easily accumulate in various organs of the human body such as the kidneys, lungs, and liver, and exposure may cause flu-like symptoms, renal tubular dysfunction and bone degeneration and also serious damage like broken bones, cancer, hypertension, and renal dysfunction [3-6]. Therefore, the maximum permissible level of Cd(II) ions in drinking water by the United States Environmental Protection Agency (ESEPA) standard is only 3.0 µg/L [7,8] and this led to increasing demands to determine even to trace level of Cd(II) ions in wastewater samples. Then it is important to develop a new method that can rapidly detect Cd(II) ions in a sensitive manner and selective even in the presence of different matrices to enable the timely remediation of unexpected accidents to safe-guard the human health. Various analytical methods have been developed, until now, to detect the metal ions in wastewater samples such as ICP-MS, AAS, ICP-OES/AES, FAAS, fluorescence spectrometry, graphite furnace atomic absorption spectrometry (GFAAS), chemical sensing and fluorescence turn on and off sensing techniques [9-17]. Many of these methods are expensive and time-consuming, and need to be carried out by trained personnel due to the complex sample preparation and unable to use onsite operation. Then it is highly desired the use of other kinds of materials with low and operation simplicity. In recent years, developments of suitable ligands are emerging for monitoring of anions, cations or neutral molecules based on charge transfer mechanism. Recently, optical materials based methods have been recognized as the potential technique for detecting metal ions or small molecules [18-22]. These are essential requirements to impart selectivity of chemical signaling upon

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stable complaxation to use selective modifiers which show a strong affinity for the specific ion at optimum condition. Moreover, a number of recent studies on organic-inorganic based functional nanomaterials have been reported to detect and adsorb of various metal ions [2325]. However, it is still a great challenge to design such nanomaterials for highly selective and sensitive detection and removal of Cd(II) ions in various samples. In recent years, many methods applied to adsorb the pollutants such as solvent extraction, ion exchange, electrochemical precipitation and reverse osmosis are mainly based on applications in single systems containing either metal ions or organic compounds [26-31]. Many of these exhibited high costs, possible production of secondary toxic compounds and the generation of sludge leading to high disposal costs [32,33]. Adsorption are one of the most popular and effective methods for the adsorption of diverse pollutants because of the flexibility in design and operation offered by the adsorption process [34]. Ligand functionalized mesoporous silica based nanomaterials received greater attention during last decade due to its diverse applications in various research areas such as column chromatography, immobilization of enzymes or proteins, adsorption and drugs delivery, etc. [35-37]. These possess high surface area, well organized porosity with tunable high pore volumes for high adsorption operation with high chemical, mechanical, and thermal stability [38,39]. It is noted that various types of ionic and non-ionic surfactants have been employed for the preparation of different pore sizes and morphological characteristics exhibited porous silica materials. The ligand based materials have been developed due to some outstanding advantages such as low mass–transfer resistance, convenient to prepare, good accessibility to the target ions, higher affinity and selectivity with naked-eye observation. In this study, we have aimed to develop a novel ligand anchored composite materials for efficient Cd(II) ions capturing from wastewater samples. The ligand plays a significant role as the molecular receptor which transforms their chemical information into analytical

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signals upon binding with Cd(II) ions at specific condition as stated in the experimental section. Here, a novel ligand of 4-tert-Octyl-4-((phenyl)diazenyl)phenol (TPDP) based on nano-composite material was designed for the ultra-sensitive detection and adsorption of Cd(II) ions optically. The association between the mesoporous silica monoliths and the ligand can be based on non-covalent interactions, such as hydrogen bonds, hydrophobic interactions, Van der Waals forces and reversible covalent bonds [40,41]. The performance of the composite material was systematically evaluated by the important experimental parameters such as solution pH, color optimization, limit of detection, initial concentration and reuses with using a suitable eluent. The results indicated that the nano composite material exhibited superior repeatability, sensitivity and low detection limit, adsorption capacity compared with the others forms of materials. Moreover, the process is simple and rapid, reversible, economic and eco-friendly technology with high adsorption capacity. This study highlighted a new concept for the preparation of optical composite materials using a simple combination for organic-inorganic materials for selective capturing of Cd(II) ions from wastewater samples.

2. Materials and methods 2.1. Materials All materials and chemicals were of analytical grade and used as purchased without further purification. The tetramethylorthosilicate (TMOS), the triblock copolymers of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) as Pluronic F108, designated as F108 (EO141PO44EO141), and 4-(hexyloxy)aniline were obtained from Sigma-Aldrich Company Ltd. USA. The buffer solutions buffer solutions of 3–morpholinopropane sulfonic acid (MOPS), 2–(cyclohexylamino) ethane sulfonic acid (CHES), and N-cyclohexyl-3aminopropane sulfonic acid (CAPS) were purchased from Dojindo Chemicals, Japan, and KCl, HCl, NaOH from Wako Pure Chemicals, Osaka, Japan. The CdCl2 as a source of Cd(II) 6

ions and other metal ions reagent were purchased from Wako Pure Chemicals, Osaka, Japan. Ultra-pure water prepared with a Milli-Q Elix Advant 3 and was used throughout in this experimental work.

2.2. Preparation and characterization of TPDP ligand The preparation of 4-tert-Octyl-4-((phenyl)diazenyl)phenol (TPDP) is reported elsewhere [10d]. The process and preparation steps were described for reader understanding. The major steps for the preparation of TPDP are shown in Scheme 1. Around 0.1 mol of 4(hexyloxy)aniline was dissolved in 100 mL of HCl; the solution was then cooled to 0°C, and 8 g of NaNO2 in water was slowly added to this mixture. The reaction progress was controlled by a starch iodide paper. Finally, the solution of diazonium salt was slowly poured into a well-cooled solution of 8 g 4-tert-Octylphenol in 100 mL of 4.0 M HCl. The resultant solution was also cooled at 0°C for 30 min and subsequently a solution of 60 g of sodium acetate in 180 mL of water was added. The coupling reaction was carried out at 0°C for 2 h. The precipitate was washed with water. The purity of the TPDP was analyzed by CHN elemental analyses as follows: C, 76.02%; H, 9.24%; N, 6.81%, as consistent with the C28H24N2O2S2 molecular formula, which requires C, 76.10%; H, 9.27%; N, 6.83%. The product was characterized by 1H NMR spectroscopies. 1H NMR (400 MHz, CDCl3): δ 5.53 (H, OH), 0.94 (9H, methyl, t-butyl), 0.91 (6H, 2 methyl), 1.46 (2H, methylene), 7.62 (H, m, OH-Ph), 7.92 (H, m, OH -Ph), 7.07 (H, O, OH-Ph), 7.89 (2H, o, benzylideminin), 7.19 (2H, p, benzylideminin), 4.06 (2H, CH2, 1-hexyloxy), 1,76 (2H, CH2, 2-hexyloxy), 1.43 (2H, CH2, 3-hexyloxy), 1.29 (2H, CH2, 4-hexyloxy), 1.31 (2H, CH2, 5-hexyloxy), 0.88 (3H, methyl, hexyloxy).

2.3. Preparation of mesoporous silica and nano composite materials

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Mesoporous silica monoliths were prepared following the reported methods with slight modification [35]. Mesoporous silica monoliths were synthesized by using direct templating method of lyotropic liquid crystalline phase where F108 was used as the soft template. In typical conditions, the composition mass ratio of F108:TMOS:HCl/H2O was 1.3:2:1 respectively. Homogeneous sol-gel synthesis was achieved by mixing of F108/TMOS. The exothermic hydrolysis was occurred rapidly by addition of acidified aqueous solution of HCl (at pH = 1.3) to this solution. The methanol produced from the TMOS hydrolysis was removed by using a diaphragm vacuum pump. After that the materials were dried at 45°C for 24 h. The organic moieties were also removed by calcination at 530°C for 6 h under normal atmosphere. Then the material was ground properly for the fabrication of nano-composite materials. Here, the optical adsorbent was prepared by direct immobilization of TPDP (50 mg) in ethanol solution with 1.0 g mesoporous silica materials. The immobilization procedure was performed under vacuum at room temperature until TPDP 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 TPDP into the mesoporous silica. The resulting composite materials were washed with warm water until no elution of TPDP color was observed. Then materials was dried at 45°C for 6 h and grinded into fine powder for Cd(III) ions detection and removal experiments. The TPDP immobilization amount (0.062 mmol/g) was calculated from the following equation:

Q = (Ci - Cf) V/m

(1)

where Q is the adsorbed amount (mmol/g), V is the solution volume (L), m is the mass of substrates (g), Ci and Cf are the initial concentration (M) and supernatant concentration (M) of the TPDP, respectively.

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2.4. The Cd(II) ions detection, removal and regeneration studies In detection operation, the composite material was immersed in a mixture of specific Cd(II) ions concentrations (2.0 mg/L) and adjusted at appropriate pH of 2.0, 3.50 (0.2 M of KCl with HCl), 5.50 (0.2 M CH3COOHCH3COONa with HCl), 7.01 (0.2 M MOPS with NaOH) and 9.50 (0.2 M 2–(cyclohexylamino) ethane sulfonic acid (CHES) with NaOH) in the specific amount of the composite material (10 mg) at constant volume (10 mL) with shaking in a temperature–controlled water bath with a mechanical shaker at 25°C for 15 min at a constant agitation speed of 110 rpm to achieve good color separation. The material was separated using filtration system by Whatman filter paper (25 mm; Shibata filter holder). The color assessment was carried out by solid-state UV–Vis-NIR spectrophotometer. The limit of detection (LD) for Cd(II) ions was evaluated from the linear part of the calibration plot according to the following equation [19]: LD = KSb/m

(2)

where, K value is 3, Sb is the standard deviation for the blank and m the slope of the calibration graph in the linear range, respectively. In adsorption operation the composite material was also immersed in Cd(II) ions concentrations and adjusted at specific pH values by adding of HCl or NaOH in 20 mL solutions. The solid materials were separated by filtration system and Cd(II) concentrations in before and after sorption operations were analyzed by ICP–AES. The adsorption efficiency was determined according to the following equations:

Mass balance qe = (C0 – Cf) V/M (mg/g)

(3)

(C0 – Cf) and metal ion uptake efficiency Re =

x 100 (%) C0

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(4)

where V is the volume of the aqueous solution (L), and M is the weight of the composite material (g), C0 and Cf are the initial and final concentrations of Cd(II) ions in solutions, respectively. In regeneration study, first 40 mL of 4.0 mM Cd(II) ion solution was adsorbed on the 20 mg composite material and then regeneration operation was carried out using 0.25 M HCl solution. The adsorbed Cd(II) ions onto material was washed with deionized water in several times and transferred into 50 mL beaker. The washing solution was also checked by ICPAES. The concentration of Cd(II) ions released from the composite material into aqueous solution was analyzed by ICP–AES. Then the composite material was reused several cycles to investigate the reusability for the potential application in real operation.

2.5. Analyses The N2 adsorption-desorption isotherms were measured using the BELSORP MINI– II analyzer (JP. BEL Co., Ltd.) at 77 K.. The pore size distribution was measured from the BJH adsorption. Mesoporous inorganic silica was pre-treated at 100°C for 3 h under vacuum until the pressure was equilibrated to 10–5 Torr before the N2 isothermal analysis. The specific surface area (SBET) was measured by using multi–point adsorption data from linear segment of the N2 adsorption isotherms using Brunauer–Emmett–Teller (BET) theory. Transmission electron microscopy (TEM) was obtained by using a JEOL (JEM–2100F) and operated at the accelerating voltage of the electron beam 200 kV. The TEM samples were prepared by dispersing the powder particles in ethanol solution using an ultrasonic bath and then dropped on copper grids. The absorbance spectrum was measured by UV–Vis–NIR spectrophotometer (Shimadzu, 3700). The metals ion concentrations were measured by ICP– AES. The ICP–AES instrument was calibrated using five standard solutions containing 0, 0.5, 1.0, 1.5 and 2.0 mg/L (for each element), and the correlation coefficient of the calibration

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curve was higher than 0.9999. In addition, sample solutions having complicated matrices were not used and no significant interference of matrices was observed. All experiments in this study were duplicated at least to assure the consistency and reproducibility of the results.

3. Results and discussion 3.1. Mesoporous silica and nano composite materials The N2 isotherm isotherms are typical IV isotherms of mesoporous silica and shows hysteresis loop as shown in Fig. 1 (a). However, the mesoporous silica materials showed a well pore size distribution with pore diameter, high surface area, and high pore volume for the successful immobilization of the organic ligand for high adsorption of metal ions adsorption. The N2 isotherms also indicated that the porosity contained within the uniform channels through the framework to comply with the presence of both framework porosity and textural porosity according to the regular pore size distribution in cage monoliths. The present synthesis method was clarified to generate the bulk form of mesoporous silica monoliths [25]. A decrease in the surface area, diameter and pore volume in the functionalization of composite material provided further evidence that the TPDP moieties were embedded inside the mesoporous silica (Fig 1(b)). Fig. 2 (A, B) shows the SEM image of direct templating method synthesized of mesoporous silica monoliths with uniform particles. This synthesis method was confirmed that the bulk form of mesoporous silica monoliths exhibited the specific shape and sizes of as prepared of the monolith. The TEM photographs were well-organized parallel channels (Fig. and depicted as hexagonally ordered array through the all direction parallel indicating that mesoporous silica material has a typical hexagonal with ordered mesoporosity [35]. The TEM images also confirmed the uniform arrangement with continuous arrays in all directions

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to ensure the direct interaction between the TPDP ligand molecule and silica into the rigid condensed pore surfaces leading to high flux for maximum Cd(II) ions adsorption at optimum condition. It was estimated that the mesoporous silica material has great advantages for fabrication of ligand anchoring for optical metal ions capturing as reported.

3.2. Cd(III) detection operations 3.2.1. Effect of pH The solution pH is the most important factor and critical parameter for the detection of metal ions on ligand anchored composite materials. Moreover, the acidity of sample solution also plays a unique role in metal-chelate formation ion the complexation mechanism. The signaling and color formations of Cd(II) ions are mainly dependent on the stability of [Cd-TPDP]n+ binding events on the nano-composite materials surfaces. The absorbance spectra of [Cd(II)–TPDP]n+ complex at λ = 515 nm was carefully recorded. The effect of pH is shown in Fig. 3 and the highest absorbance intensity reached the maximum when pH was 5.50. This pH was selected through the detection experiment procedures for this study. In is noted that the specific pH area is an important in case of ligand functionalized composite materials for selective detection and recovery of the suitable metal ions [18]. It is postulated that Cd(II) ions was formed the stable complexation with the functional groups of the nanocomposite materials.

3.2.2. Color optimization and limit of detection To clarify the sensitive Cd(II) ions detection using the nano-composite materials, the color optimization was investigated and the Cd(II) ions in different concentration was tested. The absorbance intensities were proportionally increased with increasing the Cd(II) ions concentration as evident in Fig. 4 (A). Increasing the absorbance corresponds to equilibrium

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color formation between Cd(II) ions and nano-composite material clarified the sensitive detection in ultra–trace concentration without need of sophisticated instruments. The rapid and sensitive Cd(II) ions detection at ultra-trace concentrations in the color formation by using such composite material indicated the high performance and reliability of this detection system. The spectral change was the results of Cd(II) ions addition and stable complex formation for sensitive detection of Cd(II) ions efficiently. The limit of detection, which is the lowest Cd(II) ions concentration required to produce a signal greater than three times the standard deviation (3s) as determined by the Eq. (2). The limit of detection of Cd(II) ions by the nano-composite materials was determined from the linear part of the calibration plot as the data were plotted in Fig. 4 (B). The Fig. 4(B) shows the calibration plot of the nano-composite material during determination of Cd(II) ions in low concentration levels. From the calibration curve, the Cd(II) ions limit of detection was determined, and the lower detection limit was found 0.33 µg/L. In all detection systems, the lower detection value is the first evidence of a higher degree of sensitivity. The present method was generating the analytical signal as a response to binding with Cd(II) ions even in low concentration level was technologically advantageous because of reliability, easy-to-use, low cost, high selectivity and sensitivity [41].

3.2.3. Specific selectivity towards Cd(II) ions detection The ion selectivity of the optical detection ability for various metal ions such as Na(I), K(I), Ca(II), Mg(II), Ba(II), Zn(II), Al(II), Fe(III), Co(II), Mn(II), Pb(II), and Cd(II) ions was carried out to know the specificity of the nano-composite materials at the optimum conditions the Cd(II) ions concentration was 1.0 mg/L and others metal ions concentrations were 10 times greater than the Cd(II) ions. Fig. 5 shows the absorbance intensity in the signal of composite materials by the addition of Cd(II) ions; however, other metal ions exhibit a

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negligible effect on the absorbance intensity and color formation. A little intensity was observed in the presence of Pb(II) ions but it was neglected from the stand point of color formation. This result demonstrated that effective color formation only occurred when Cd(II) ions was present in the sample, which allows the non-interfering detection of Cd(II) ions by the proposed materials.

3.3. The Cd(II) ions adsorption 3.3.1. Effect of pH The solution pH significantly affected the speciation of heavy-metals and the surface-active sites of the functional materials. Then the acidity of the adsorption medium is an important factor of heavy metal adsorption in terms of sensitivity and selectivity. The effect of pH value on the Cd(II) ion adsorption by nano-composite material was thoroughly carried out within a range of pH 2.0–11.0. This is noted that the speciation form of Cd(II) ions as a function of pH is exited as Cd(II) soluble cationic species up to pH 8.5. After pH 8.50, the Cd(II) ions is started to form insoluble Cd(OH)2 species and over pH 10.0, the Cd(OH)2 is the only dominant species. In addition, the insignificant amount of Cd(OH)+ species is also formed between pH 8.0 and 10.0 [42]. Fig. 6 (A) shows the effect of solution pH on Cd(II) ion adsorption onto the nano-composite materials. At the low pH region, the Cd(II) ions adsorption was low as depicted in Fig. 6 (A). When increasing pH, the protonation reaction becomes weak and the Cd(II) ions interaction with the functional groups on the surface of the nano-composite adsorbent becomes dominant, and then the Cd(II) ions adsorption was increased. The adsorption was increased with increasing the pH up to 5.50. In the strong acidic area, the Cd(II) ions was adsorbed in a small amount due to the high concentration of hydrogen ions which compete with the diffusion of metal ions resulting in the hydrogen ions to be adsorbed on the surface of the nano-composite material instead of the

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specific metal ions [6]. The maximum adsorption was observed by the proposed materials was at pH 5.50. This pH gave the advantages because the solutions acidity below 6.0 have reflected no precipitation occurs during the adsorption operation.

3.3.2. Effect of contact time Contact time between Cd(II) ions in solution and the nano-composite materials is an important factor for the Cd(II) ions adsorption from wastewater samples. A series of batch experiments was carried to determine the optimum time for the maximum Cd(II) ions adsorption, and the filtrate solution was checked by ICP–AES after each fraction. It is noted that the initial Cd(II) ions concentration was 10.0 mg/L in 20 mL volume, and the nanocomposite material amount was 10 mg. Fig. 6 (B) illustrates the suitable contact time by the material when other experimental conditions were identical. The Cd(II) ions adsorption as a function time curve was single, smooth and continuous indicating monolayer adsorption of Cd(II) ions on the nano-composite material surfaces [20]. Increasing the contact time increases the Cd(II) adsorption efficiency and the maximum Cu(II) was adsorbed from solution by 30 min. The rapid adsorption is quantitatively predominant due to the availability of active functional sites and then gradually occupancy of the pore surface active functional sites to attain the maximum adsorption ability. Increasing contact time up to 60 min, the adsorption efficiency almost the same as 30 min contact time. Usually, the complexation mechanisms are slow kinetics than the ion–exchange and hydrogen bonding reaction mechanism [43-45]. Then the contact of 3 h was assumed to be suitable for subsequent adsorption for the determination of maximum adsorption capacity.

3.3.3. Effect of initial concentration and adsorption capacity The concentration dependence adsorptions of Cd(II) ions by the nano-composite materials were performed within a wide range of initial Cd(II) ions concentrations. The initial 15

Cd(II) ions concentration was increased from 2.0 to 72.2 mg/L at equilibrium pH 5.50. The equilibrium adsorptions against its initial concentrations are depicted in Fig. 7. An increase in initial concentration of Cd(II) ions, the amount of adsorptions were significantly increased from aqueous solutions. A significant increase in the amount of Cd(II) ions adsorbed while increasing the initial concentrations suggested the potential applicability of nano-composite materials in the remediation of Cd(II) ions contaminated wastewater even at higher concentration levels [36]. At low initial concentrations, the availability of the adsorption sites is relatively high, and the Cd(II) ions can be adsorbed easily with the high amount. Contrary at high initial concentration, a decrease in the adsorption efficiency because of the limitation of the total available adsorption sites as expected. In order to understand the maximum adsorption capacity of the nano-composite materials, the equilibrium data were plotted according to the Langmuir adsorption isotherms based on the contact time evaluation. According to the Langmuir isotherms model, the adsorption sites are homogeneous with monolayer coverage. However, the experimental data always completely not fit the Langmuir isotherm because some of the functional groups. Based on the Langmuir assumption, the adsorption seems to follow the monolayer adsorption process since only Cd(II) ions can bond with the functional groups on the surface with homogeneity. Therefore, the Langmuir adsorption isotherm has been successfully applied as follows:

Ce/qe = 1/(KLqm) + (1/qm)Ce

(linear form)

(5)

where Ce is the solution concentration at equilibrium (mg/L), qe the amount adsorbed at equilibrium (mg/g), KL the Langmuir constant (L/mg) which can be considered as a measure of the adsorption energy and qm is the maximum adsorption capacity (mg/g) corresponding to complete monolayer coverage. The qm and KL are the Langmuir constants which are related to

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the adsorption capacity and energy of adsorption, respectively, and can be calculated from the intercept and slope of the linear plot, with Ce/qe versus Ce as shown in Fig. 7 (inset). The Cd(II) ions adsorption was homogeneous surface and adsorption was limited to the formation of monolayer or the number of adsorbed species did not exceed the total surface active sites based on the Langmuir hypotheses. The maximum adsorption capacity was 148.32 mg/g, and this adsorption capacity was also compared with other forms of adsorbent [6,36,46-50] as shown in Table 1. The correlation coefficients value was R2=0.987 and therefore, the Langmuir adsorption equation can be considered an accurate model of Cd(IV) ions adsorption by the proposed material.

3.3.4. Effect of competing ions The coexisting concomitant ionic species always compete with Cd(II) ions and hinder the adsorption process by the adsorbent materials. Because of the difference of stability constant, complexation ability, mass-to-charge ratio and size of hydrated metal, the selective adsorption on the nano-composite materials might happen in a competitive adsorption for specific selectivity. Therefore, the potential interfering effects of divers commonly competing ionic species were investigated the adsorption efficiency by the proposed nano-composite materials. Fig. 8 (A) shows the selective adsorption of heavy-metal ions (Na(I), K(I), Ca(II), Mg(II), Ba(II), Zn(II), Al(II), Fe(III), Co(II), Mn(II), Pb(II), and Cd(II)) onto the nano-composite materials. For Cd(II) ions, more than 94% adsorption efficiency was observed; however, the Pb(II) ions also slight adsorbed. Such high selectivity of the nano-composite materials toward the Cd(II) ions over other bivalent ions was observed due to the strong complexation ability between TPDP and Cd(II) ion in the adsorption operation.

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3.4. Elution and reuses Desorption and reuses of the nano-composite materials is a crucial parameter for practical implication of the proposed material for its potential application in real wastewater treatment. In addition, the reusability could be the emphasis the cost-effectiveness of the process. The acidic solutions are commonly used for elution from the solid materials surface. In this study, 0.25 M HCl was found suitable to elute Cd(II) ions from nano-composite solid materials. The materials were exhibited high mesostructure stability with uniform surface area with large pore volume after the elution/regeneration process which implies the several reuses of the adsorbent. After elution of Cd(II), the nano-composite materials was simultaneously regenerated into the initial form without significant deterioration in its original structure. In the first adsorption cycle, the adsorption capacity was 150.1 mg/g, whereas the adsorption capacity was decreased to 139.12 mg/g after seven cycles as judged from Fig. 8 (B). The results revealed that nano-composite materials was highly practical materials for the detection and removal of Cd(II) ions from wastewater samples.

4. Conclusions In this study, novel nano-composite materials was fabricated for selective and sensitive detection and adsorption of Cd(II) ions based on the Cd(II) ions concentrationdependent formation of absorbance spectroscopy. Upon addition of Cd(II) ions, the absorbance intensity was increased with increasing concentrations of Cd(II) ions under the optimum conditions. The experimental result showed that Cd(II) ion was detected quickly and accurately with excellent discrimination against competing heavy-metal ions. Therefore, the nano-composite materials assay was a capable measuring amount of Cd(II) ions in the wastewater sample. Then the present proposed materials open up a new possibility of an

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assay, reliable and rapid method for of Cd(II) ions detection in drinking water with ultra-trace level. The adsorption isotherm was well fitted by the Langmuir isotherms equation, and the maximum adsorption capacity was 148.32 mg/g. The presence of competing ions did not adversely affect during Cd(II) ions adsorption systems, and material was efficiently adsorbed the Cd(II) ion from multi-mixture solutions. The adsorbed Cd(II) onto nano-composite materials was eluted with 0.25 M HCl acid and simultaneously regenerated into the initial form for next Cd(II) ion operations after rinsing with water without deterioration in it significant initial performances. The proposed material revealed the advantages of nanoscale pore geometry and shape, particle morphology of the support carriers, ligand functionality for efficient Cd(II) ions capturing, in the design of nanomaterials that can efficiently and selectively Cd(II) ions detection and adsorption functionality. Therefore, the nano-composite materials have the potential to be used as effective materials for sensitive and selective Cd(II) ions detection and adsorption from aquatic environmental samples.

Acknowledgment The authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP# 87. The authors also wish to thank the anonymous reviewers and editor for their helpful suggestions and enlightening comments.

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Motokawa, S. Suzuki, Y. Okamoto, T. Yaita, Evaluation of lanthanide sorption and their coordination mechanism by EXAFS measurement using novel hybrid adsorbent, Chem. Eng. J. 225 (2013) 558–566; (d) M.R. Awual, T. Yaita, T. Taguchi, H. Shiwaku, S. Suzuki, Y. Okamoto, Selective cesium removal from radioactive liquid waste by crown ether immobilized new class conjugate adsorbent, J. Hazard. Mater. 278 (2014) 227–235. [24] (a) M.R. Awual, M. Miyazaki, T. Taguchi, H. Shiwaku, T. Yaita, Encapsulation of cesium from contaminated water with highly selective facial organic-inorganic mesoporous hybrid adsorbent, Chem. Eng. J. 291 (2016) 128–137; (b) M.R. Awual, Ring size dependent crown ether based mesoporous adsorbent for high cesium adsorption from wastewater, Chem. Eng. J. 303 (2016) 539–546; (c) M.R. Awual, T. Yaita, Y. Miyazaki, D. Matsumura, H. Shiwaku, T. Taguchi, A reliable hybrid adsorbent for efficient radioactive cesium accumulation from contaminated wastewater, Sci. Rep. 6, 19937; doi: 10.1038/srep19937 (2016). [25] (a) M. R. Awual, N.H. Alharthi, Y. Okamoto, M.R. Karim, M.E. Halim, M.M. Hasan, M.M. Rahman, M.M. Islam, M.A. Khaleque, M.C. Sheikh, Ligand field effect for Dysprosium(III) and Lutetium(III) adsorption and EXAFS coordination with novel composite nanomaterials, Chem. Eng. J. 320 (2017) 427–435; (b) M.R. Awual, M.M. Hasan, H. Znad, Organic–inorganic based nano-conjugate adsorbent for selective palladium(II) detection, separation and recovery, Chem. Eng. J. 259 (2015) 611–61; (c) M.R. Awual, M.M. Hasan, Fine-tuning mesoporous adsorbent for simultaneous ultra-trace palladium(II) detection, separation and recovery, J. Ind. Eng. Chem. 21 (2015) 507–515; (d) S.A. El-Safty, M.R. Awual, M.A. Shenashen, A. Shahat, Simultaneous optical detection and extraction of cobalt(II) from lithium ion batteries using nanocollector monoliths, Sensor. Actuat. B: Chem. 176 (2013) 1015-1025.

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Znad, M.R. Awual, A ligand anchored conjugate adsorbent for effective mercury(II) detection and removal from aqueous media, Chem. Eng. J. 334 (2018) 432–443. [41] A. Shahat, H.M.A. Hassan, M.F. El-Shahat, O. El Shahawy, M.R. Awual, Visual nickel(II) ions treatment in petroleum samples using a mesoporous composite adsorbent, Chem. Eng. J. 334 (2018) 957–967; (b) A. Shahat, H.M.A. Hassan, H.M.E. Azzazy, E.A. El-Sharkawy, H.M. Abdou, M.R. Awual, Novel hierarchical composite adsorbent for selective lead(II) ions capturing from wastewater samples, Chem. Eng. J. 332 (2018) 377–386; (c) A. Shahat, H.M.A. Hassan, H.M.E. Azzazy, M. Hosni, M.R. Awual, Novel nano-conjugate materials for effective arsenic(V) and phosphate capturing in aqueous media, Chem. Eng. J. 331 (2018) 54–63; (d) M.R. Awual, N.H. Alharthi, M.M. Hasan, M.R. Karim, A. Islam, H. Znad, M.A. Hossain, M.E. Halim, M.M. Rahman, M.A. Khaleque, Inorganic-organic based novel nano-conjugate material for effective cobalt(II) ions capturing from wastewater, Chem. Eng. J. 324 (2017) 130–139. [42] S.M. Lee, Lalhmunsiama, D. Tiwari, Sericite in the remediation of Cd(II)- and Mn(II)contaminated waters: batch and column studies, Environ. Sci. Pollut. Res. Int. 21 (2014) 3686–3696. [43] (a) M.R. Awual, A. Jyo, T. Ihara, N. Seko, M. Tamada, K.T. Lim, Enhanced trace phosphate removal from water by zirconium(IV) loaded fibrous adsorbent, Water Res. 45 (2011) 4592–4600; (b) M.R. Awual, M.A. Hossain, M.A. Shenashen, T. Yaita, S. Suzuki, A. Jyo, Evaluating of arsenic(V) removal from water by weak-base anion exchange adsorbents, Environ. Sci. Pollut. Res. 20 (2013) 421–430; (c) M.R. Awual, A. Jyo, M. Tamada, A. Katakai, Zirconium(IV) loaded bifunctional fiber containing both phosphonate and sulfonate as arsenate adsorbent, J. Ion Exchange 18 (2007) 422–427; (d) M.R. Awual, A. Jyo, S.A. El-Safty, M. Tamada, N. Seko, A weak-base

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31

Table 1 Comparison of nano-composite materials with other forms of materials reported in the literature for Cd(II) ions adsorption

Used adsorbents

Adsorption capacity (mg/g)

Ref.

236.04

[6]

Mesoporous silica (AMS)

3.62

[36]

Nanoscale zero-valent iron (NZVI)

48.63

[46]

PS-GO gel

136.98

[47]

Amino functionalized silica

194.40

[48]

234.80

[49]

41.60

[50]

148.32

This study

PCBs

3D sulfonated reduced graphene oxide MnO2/o-MWCNTs) Nano-composite materials

32

Scheme 1. Synthetic route for the preparation organic ligand of 4-tert-Octyl-4-((phenyl)diazenyl)phenol (TPDP) ligand.

33

Fig. 1. N2 adsorption/desorption isotherms of (a) ordered mesoporous silica and (b) ligand anchored nano-composite materials with specific surface area, pore volume and pore sizes (inset).

34

Fig. 2. SEM images of highly ordered mesoporous silica (A, B) and TEM image of mesoporous silica with ordered structure and pore sizes (C, D).

35

Fig. 3. Effect of solution acidity for Cd(II) ions detection by the nano-composite materials at different pH areas when Cd(II) ions concentration was 2.0 mg/L in solution volume was 10 mL.

36

(B)

(A)

Fig. 4. (A) Color optimization with increasing the Cd(II) ions concentration (λ = 515 nm) in detection operation; (B) Calibration curve with spectral absorbance measured at 515 nm with Cd(II) concentrations and limit of detection limit for Cd(II) ions with a linear fit line (inset).

37

0.2 1

13 12 11 10 9

Io n

A -A 0 ( a . u . )

0.4 2

0.0 0 An io 8

s

7

6

5

4

Ca 3

2

tio n

ns

s

1

Fig. 5. Specific ion selectivity for Cd(II) ions detection of the nano-composite materials at pH 5.50. The interfering cations (20.0 mg/L) listed in order (1 to 14): (1) Na+, (2) K+, (3) Ca(II), (4) Mg(II), (5) Ba(II), (6) Zn(II), (7) Al(II), (8) Fe(III), (9) Pb(II), (10) Co(II), (11) Mn(II), (12) Blank and (13) 1.0 mg/L Cd(II). The interfering anions (500 mg/L) listed in order (5 to 11): (5) chloride, (6) nitrate, (7) sulfate, (8) phosphate, (9) sulfite, (10) carbonate and (11) bicarbonate. The A and A0 are the signal responses of the solid nano-composite materials after and before addition of Cd(II) ions.

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A d s o r p tio n e ffic ie n c y (% )

A d s o r p t io n e f f ic ie n c y ( % )

100

(A)

80

60

40

20

100

(B)

80

60

40

20

0

0 0

2

4

6

8

10

12

0

10

20

30

40

50

60

T im e in m in .

S o lu t io n p H

Fig. 6. (A) Effect of pH for Cd(II) ions adsorption and (b) adsorption kinetics by investigating the contact time for complete Cd(II) adsorption from solution by the nano-composite materials.

39

70

Fig. 7. Effect of initial Cd(II) ions concentration and the Langmuir adsorption isotherms with linear form (initial Cd(II) ion concentration range 2.02 - 72.204 mg/L; solution pH 5.50; dose amount 10 mg; solution volume 30 mL and contact time 3 h).

40

(B)

(A) 100

100

75

Adsorption efficiency (%)

Adsorption efficiency (%)

80

60

40

20

0

Na

K

Ca Mg Ba Zn Cd Pb Co Mn Fe

Al

Metal ions

50

25

0 1st

2nd

3rd

4th

5th

6th

7th

No. of cycles

Fig. 8. (A) Effect of competing for selective Cd(II) ions adsorption where Cd(II) ions concentration was 1.0 mg/L and other metal ions concentration was 10.0 mg/L by the nano-composite materials and (B) Elution-regeneration-reuses cycles with using eluent of 0.25 M HCl.

41