A ligand based innovative composite material for selective lead(II) capturing from wastewater

Journal of Molecular Liquids 294 (2019) 111679

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A ligand based innovative composite material for selective lead(II) capturing from wastewater Md. Rabiul Awual a,b,⁎, Md. Munjur Hasan c,⁎⁎ a b c

Materials Science and Research Center, Japan Atomic Energy Agency (SPring–8), Hyogo 679–5148, Japan Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Department of Applied Chemistry & Chemical Engineering, University of Dhaka, Dhaka 1000, Bangladesh

a r t i c l e

i n f o

Article history: Received 19 June 2019 Received in revised form 9 August 2019 Accepted 2 September 2019 Available online 03 September 2019 Keywords: Composite material Pb(II) ions Capturing Building-block Extreme adsorption Contaminated waters

a b s t r a c t The ligand based composite material is an effective material for the removal of diverse contaminants from wastewater. However, the performance of some ligands functionality is limited in many cases due to the functional complexation ability. Therefore, the high immobilization of ligand anchoring by the building-block approach is a better way for high amount adsorption of pollutants. The present study was carried out for the successful detection and removal of lead (Pb(II)) ion from aqueous solution by a novel functionalized composite material. The composite material was successfully prepared by the building-block immobilization of (3-(3(methoxycarbonyl)benzylidene) hydrazinyl)benzoic acid onto the highly ordered mesoporous silica. The factor affecting Pb(II) detection and adsorption by composite material was carried out in a batch-mode with the parameters of solution pH, color optimization, competing ions, initial concentrations, contact time and reuses. The material was enhanced the color formation by stable complexation during the Pb(II) ion detection and adsorption operations. The limit of detection was 0.51 μg/L. The effective pH for Pb(II) ions capturing was 5.50, and the maximum adsorption capacity of the composite material was found to 214.15 mg/g. The data revealed that the capturing system was in a one-step and ultra-trace Pb(II) was captured without using highly sophisticated instrumentations. The results clarified that composite material was exhibited high selectivity toward the Pb(II) ion even in the presence of the high amount of common metal ions. The adsorbed Pb(II) was eluted with 0.20 M HCl and the material were simultaneously regenerated into an initial form for the next capturing operations after rinsing with water. Therefore, composite material is an effective, low-cost and recyclable material and was a great potential to be a promising technique for Pb(II) remediation for safe-guarding the public health. © 2019 Elsevier B.V. All rights reserved.

1. Introduction The presence and highly toxic pollutants, especially the heavy-metal ions in water bodies have been identified as a global challenge to keep safe the human health [1,2]. The excessive amount of metal ions in water is seriously vulnerable for the human health, and these can be attributed to both natural and anthropogenic sources [3,4]. Moreover, the heavy-metal ions are non-biodegradable pollutants pile up in groundwater and on the soil surface as a waste of some industrial processes such as; mining, painting, and anti-corrosive coating [5–8]. Among all the heavy-metal ions, lead (Pb(II)) has been classified as one of the most toxic heavy metals due to its detrimental effects on the human nervous, blood circulation, kidneys, and reproductive system [9,10].

⁎ Correspondence to: M.R. Awual, Materials Science and Research Center, Japan Atomic Energy Agency (SPring–8), Hyogo 679–5148, Japan. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (M.R. Awual), [email protected] (M.M. Hasan).

https://doi.org/10.1016/j.molliq.2019.111679 0167-7322/© 2019 Elsevier B.V. All rights reserved.

The main anthropogenic sources to release lead into natural waters include used batteries, lead smelting, tetraethyl lead industries, mining, plating and ceramic glass industry. Due to the high toxicity, the maximum contaminant level for Pb(II) ion in drinking water has been set to be 15 μg/L (ppb) according to the United States Environmental Protection Agency (USEPA) [11,12]. Higher amount of Pb(II) ion presence in fresh water may pose health risks and enhance the severe diseases such as encephalopathy and hepatitis [13]. Therefore, it is important to develop sensitive and selective material for the simultaneous detection and removal of Pb(II) ion from waste samples. Several analytical methods such as inductively coupled plasma mass spectrometry, inductively coupled plasma atomic emission spectrometry, and graphite furnace atomic absorption spectrometry have been reported for the sensitive determination of specific metal ion in water, food and environmental samples [14–17]. The determination of trace level Pb(II) ion always desires the use of sensitive and selective techniques with cost-effective and easy to use approaches. The level of Pb (II) ion in natural water samples is very low and then the accurate, reliable and sensitive materials are important to the analysis of Pb(II) ions.

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Recently, different organic and inorganic based nanomaterials are synthesized for the naked-eye detection and quantification of diverse metal ion based on the charge-transfer induce and stable mechanism in the sense of low-cost methods [18]. This has led to design the ligand based composite material for efficient detection of Pb(II) ions. Then the highly ligand anchored composite material was designed for sensitive detection of Pb(II) ion in wastewater. However, the problem with the solid material and stability remains unresolved according to the bond stability in the presence of other contaminants. Then the designing of suitable detection method that can detect the specific metal ions from wastewater with fast environmental analysis, easy to handle and costeffective is always desired [19]. There is homogeneous network formation by adding the Schiff base organic ligand. Moreover, the case cavity with composite material specificity is obtained, appropriate for a stable complexation with the Pb(II) ions. Therefore, the development of specific ligand anchored ordered composite material that can separate toxic metal ions from water has great importance. Over the last decade, many techniques have been reported for metal ions removal such as chemical precipitation, membrane filtration, liquid-liquid extraction, ion-exchange and adsorption [20–26]. However, the solid-liquid separation method is the suitable candidate as the promising in the second generation attraction. Several methods have many drawbacks such as high toxicity, requiring a high amount, operating complexity and costly. Among these techniques, adsorption is considered as one of the most economical and efficient methods due to its advantages of cost-effectiveness, simple operation, environmental friendliness, facile handling and availability for various adsorbent materials [27,28]. Therefore, the adsorption technique has been recognized as one of the prospective and major techniques for Pb(II) ions removal from wastewater. Then it is necessary to develop an efficient material to capture the ultra-trace vanadium from aqueous solutions that is not only cost effective, but also non-toxic to the environment and environment friendly. Recently, owing to the large surface area and high activity caused by the size quantization effect than bulk materials for adsorption of Pb(II)ions, nanosized metal oxides, such as composite materials, nanosized iron oxides, magnesium oxides manganese oxides, aluminum oxides, titanium oxides and cerium oxides have reported the remarkably enhanced adsorption capacity and been considered as a promising material for heavy metals removal from aqueous systems [29,30]. However, the separation and recycling of these adsorbents from wastewater are still a serious issue, which greatly limits their applications. In this connection, different ligand immobilized composite material for various metal ion adsorptions from aqueous solution have been investigated [31,32]. The materials have high surface area and an absence of internal diffusion resistance, and they have excellent performance to capture the metal ion at the optimum experimental conditions. The development of new nano-sized composite material with a facile separation and recycling process is of great interest in the current study. Therefore, the present study was planned to develop ligand based functionalized mesoporous composite material for efficient Pb(II) ion detection and removal from wastewater. The objective of this study was to develop and characterize a ligand immobilized composite material and its applicability to remediate multiple contaminants, the potential application being the detection and adsorption of toxic Pb(II) ions. The developed composite material was successfully applied in detection and adsorption of Pb(II) ion in contaminated water solutions. The fabricated composite material has potential aspects for the detection and adsorption of Pb(II) ion because of several interesting characteristics such as high surface area, high porosity, and high functionality at the optimum experimental conditions. The chemical functionality was defined by chemical processes to coat the functional groups for the improvement of sensitive and selective capturing ability. Moreover, the prepared composite material can easily be handled and environmental eco-friendly and potential candidate for large-scale wastewater treatment. The new chromogenic Schiff base organic ligand of (3-(3-(methoxycarbonyl)benzylidene) hydrazinyl)

benzoic acid (Scheme 1) was synthesized and successfully anchored onto the mesoporous silica by the building-block approach. The affecting parameters to the Pb(II) ion capturing such as solution pH, contact time, initial concentration, competing ion and elution/regeneration were measured systematically. In addition, this study highlights a new concept in the designing of composite material for the easy to use with simple high ligand anchoring to avoid the need for complicated process for the Pb(II) ion capturing from wastewater. 2. Materials and methods 2.1. Materials All materials and chemicals were of analytical grade and used as purchased without further purification. Tetramethylorthosilicate (TMOS), the triblock copolymers of poly(ethylene oxide–b–propylene oxide–b– ethylene oxide) designated as F108 (EO141PO44EO141), methyl 3formylbenzoate and 3-hydrazinylbenzoic acid were obtained from Sigma–Aldrich Company Ltd. USA. The standard Pb(II) solution (1000 mg/L) was provided in analytical grade. For pH adjustments in detection systems, the buffer solutions of 3–morpholinopropane sulfonic acid (MOPS), 2–(cyclohexylamino) ethane sulfonic acid (CHES), and N-cyclohexyl-3-aminopropane sulfonic acids (CAPS) were procured from Sigma-Aldrich, USA. Deionized water was used in the all experimental work. 2.2. Synthesis and characterization of MBHB ligand The (3-(3-(methoxycarbonyl)benzylidene) hydrazinyl)benzoic acid (MBHB) is prepared according to the steps depicted in Scheme 1, and the detail procedure is reported elsewhere [10d]. The process was described for reader understanding. The MBHB was synthesized by the reaction of methyl 3-formylbenzoate (1 mol) and 3-hydrazinylbenzoic acid (1 mol) in ethanol and small amount of acetic acid. The resultant mixture was then heated under reflux for 4 h and left to cool at room temperature. The solid formed material upon cooling was collected by suction filtration. The separated product was recrystallized from the system ratio of dichloromethane/methanol 1/1. Then the product was dried at 50 °C for 24 h. 2.3. Synthesis of mesoporous silica and composite material The mesoporous silica monolith was prepared by direct template method with slight change of the reported method [33]. In this composition, each of the reagent ratios was 1.4:2:1 of the F108:TMOS:HCl/H2O. After mixing of F108/TMOS in a flask, then this was heated at 65 °C for complete dissolving. After addition of HCl (at pH = 1.3) to the mixture, the hydrolysis was happened and the methanol was separated by rotary evaporator. The prepared silica monolith containing hydro-carbon was dried in the oven at 45 °C for 12 h. After that, the monolith was calcined at 520 °C under the air mode as the standard protocol. The final porous silica carrier was grounded for the characterization and grounded as possible for suitable carrier mesoporous silica materials in the fabrication of conjugated adsorbent. The composite material was prepared via indirect anchoring immobilization of MBHB onto mesoporous silica monoliths. Firstly, the 0.20 M dilauryl dimethyl ammonium bromide (DDAB) in ethanol solution was immobilized onto 1.0 g of mesoporous silica monoliths [27]. Then MBHB (75 mg) in ethanol solution was contacted with 1.0 g DDAB– mesoporous silica materials. Immobilization was performed under vacuum at 30 °C until MBHB saturation was achieved. Then the ethanol was removed by a vacuum connected to a rotary evaporator at 45 °C and the resulting adsorbent was rinsed with warm water to check the stability and elution of MBHB from DDAB–mesoporous silica. Then the material was dried at 45 °C for 6 h and ground to fine powder for Pb(II) ions detection and removal experiments. In MBHB anchoring onto mesoporous

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Scheme 1. Synthetic route for the fabrication of (3-(3-(methoxycarbonyl)benzylidene)hydrazinyl)benzoic acid (MBHB) ligand.

silica, the final concentration/supernatant concentration of MBHB was counted the total washing amount of solution after MBHB immobilization operation. The MBHB immobilization amount (0.12 mmol/g) was determined by the following equation: Q ¼ ðC 0 –C Þ V=m

ð1Þ

where Q is the uptake (mmol/g), V is the total amount (L), m represents the amount of composite material (g), Ci and Cf was the original and last amount in solution of MBHB, respectively.

2.6. The Pb(II) adsorption procedures In the adsorption case, the composite material was also added with Pb(II) mixing solution where the pH was also conformed using HCl or NaOH. After that the composite material was isolated by filtration method, and the filtrate solution containing the Pb(II) amount was checked by ICP-AES. The removal quantity was evaluated in accordance with the below equations: qe ¼ ðC0 –C f Þ V=M ðmg=gÞ

ð3Þ

and PbðIIÞ ions adsorption efficiency Re ¼ ðC0 –C f Þ 100=C0 ð % Þ

ð4Þ

2.4. Analyses The N2 adsorption-desorption isotherms were measured using 3Flex analyzer (Micromeritics, USA) 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-2010) and operated at the accelerating voltage of the electron beam at 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. The metals concentrations were measured by ICP–AES (PerkinElmer, Germany, 8300.). 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 curve was higher than 0.9999. In addition, sample solutions having complicated matrices were not used and no significant interference of matrices was observed. 2.5. The Pb(II) detection procedures In the detection system, the composite material was added in a mixture of 2.0 mg/L Pb(II) and then conformed at pH of 2.0, 3.50, 5.50, 7.0, 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 understand the color formation in the comparison. The composite material was filtered and used for color estimated and absorbance intensity evaluation. The composite material was grounded properly to get the smooth in the absorbance spectra by the machine. The limit detection (LD) of Pb(II) is determined based on the below Eq. [34]: LD ¼ KSb =m

ð2Þ

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

where qe removal amount (mg/g), Ci and C were the initial and supernatant amounts of Pb(II) (mg/L), respectively, V is the total solution (L), m represents the weight of the composite material (g). To define the equilibrium contact time, 5.0 mg/L Pb(II) amount of fixed volume solution was used in each fraction, and the composite material amount was fixed at 20 mg in each case. After 10 min interval, the solid composite material was separated and filtrates were checked by the ICP-AES. The highest adsorption amount was defined based on the different initial concentrations of Pb(II), and each sample was checked by ICP-AES. To check the suitable eluent, the composite material was equilibrated with fixed Pb(II), and the composite material was checked for several eluents such as NaOH, H2SO4, and HCl and the filtrate were relocated to the test tubes, and then Pb(II) was checked by the ICPAES. According to the experimental protocol, the 0.20 M HCl solution was used as appropriate eluent. It was also noted that the composite material was regenerated in the elution time, and the composite material could use several cycles for an efficient and economical optical conjugated adsorbent. 2.7. Effect of competing and co-existing ion The Pb(II) and the competing ion were carried out to understand the selective adsorption ability where the diverse counter-ion was used. The solution was contained 15 mg/L of each Zn(II), Li(I), Al(III), Fe(III), Mg(II), Ca(II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Ba(II), Bi(III) ion and the Pb(II) amount were 1.0 mg/L. The solution volume was to be 30 mL and composite material amount was 15 mg. Then the mixture was stirred for 2 h and separated by filtration. The solution was analyzed using ICP-AES analyzer. All experimental demonstrations were duplicated to confirm the extracted data in this study. The highest variation for each adsorption operation was 2.5%.

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3. Results and discussion 3.1. Characterization of mesoporous silica and composite material The sample prepared show type IV isotherms for nitrogen adsorption at 77 K (Fig. 1(a)), which is typical of mesoporous silica monoliths. The volume adsorbed for isotherms sharply increased at a relative pressure (P/P0) of approximately 0.5, representing capillary condensation of nitrogen within the uniform mesopore structure. In addition, adsorption branches were significantly shifted toward lower relative pressure (P/P0). Desorption occurs from the mesopores by evaporation and usually takes place in lower pressure region than that of capillary condensation resulting in hysteresis. According to Fig. 1(a), the mesoporous silica monoliths have high surface area (SBET), large pore volume and tunable pore sizes. The present synthesis method was clarified to generate the bulk form of mesoporous silica monoliths [35–39]. A decrease in the surface area and pore volume in the functionalization sensor ensemble adsorbent provided further evidence that the organic moieties of MBHB were embedded inside the mesopore (Fig. 1(b)). The TEM images show uniform arrangement pores and continuous arrays along all directions (Fig. 2(A, B)) which indicate the direct interaction between the MBHB and silica into the rigid condensed pore surfaces with retention of the ordered structures, leading to high flux and Pb(II) ion transport during detection and recovery processes. Moreover, the TEM images were recorded perpendicularly to the direction of the pore channels and showed well-organized parallel channels and clarified a hexagonally ordered array of circles in the direction parallel to the pore channels which clarified the prepared material has a typical hexagonal. The composite adsorbent was fabricated by indirect immobilization as the building-block approach [40,41]. First, the polarity of the silica surface was fine–tuned by the dense dispersion of cationic surfactant of DDAB. Here, the DDAB was physisorbed phases on silica surfaces by electrostatic interactions leading to the formation of positively charged silica surfaces as “island”. The large amount of DDAB dispersion on the silica surface will enhance the polarity of the mesoporous silica and imply the highly dense immobilization of the MBHB ligand. Moreover, the positively charged islands led to enhanced robustness in the sequential construction of Silica-DDAB-MBHB composite adsorbent. The fine-tuning of DDAB modified silica surfaces has significant enhancement for immobilization of MBHB in which high loading capacity of MBHB was achieved with open specific activity of the MBHB functional groups. In such immobilization, the MBHB led to high flexibility in binding with Pb(II) ions even at ultra-trace levels. After successful ligand anchoring onto the building mesoporous silica,

Fig. 1. N2 adsorption-desorption of (a) mesoporous silica, (b) ligand functionalized composite material with different surface areas, pore sizes and pore volumes.

the significant pore with the ordered homogeneous porous morphology of the composite material was exhibited as the TEM images are shown in Fig. 2(C, D). 3.2. Pb(II) ion detection parameters In Pb(II) detection by composite material, the solution pH is playing an important role based on the stable complexation ability with the signal enhancement. The signalling of Pb(II) ability is mainly dependent upon the binding stability on the composite material surfaces and the solution acidity. In this study, different buffer solution in each pH region were investigated for Pb(II) detection system. The color formation and maximum signal responses were observed at pH 5.50 (Fig. 3). The reflectance spectra at λ = 362 was carefully recorded for Pb(II) detection. The data also confirmed that the dose was sufficient to achieve good color separation between the conjugate adsorbent “blank” and Pb(II)– MBHB “sample,” at low level of Pb(II) concentrations. Therefore, to get a high sensitive response, pH 5.50 was chosen as optimum experimental condition in the optical Pb(II) ion detection and all other effects on the detection operation were investigated at pH 5.50. It is also noted that ligand immobilized adsorbent has specific ion selectivity for optical detection of diverse ions [42]. The proposed composite material was highly sensitive in terms of its optical color formation and signal intensity responses to the Pb(II) ions even in the low-level concentrations. The present composite material showed high physical features and textural properties in optical detection of Pb(II) ions. Increasing the Pb(II) ion concentration from 0 to 2.0 mg/L, increases the signal intensity corresponding to the equilibrium color formation complexes between the composite material surface of MBHB ligand function and Pb(II) ions as depicted in Fig. 4(A). The signal intensity exhibited a bathochromic shift from λmax at 450 nm to lowenergy sidebands at 362 nm with addition of wide range of Pb(II) ions concentration indicating the charge transfer (intense π-π transition) complex formation with the high stability at this optimum conditions. Fig. 4(A) also clarified that the increasing of signal intensity corresponds to the optimum color formation between the composite material and Pb (II) ions, which enhances the sensitive detection of ultra–trace Pb(II) ion without using the highly sophisticated apparatus. This finding shows that proposed composite material has highly sensitive and effective in detection of Pb(II) ions at low to high concentration level. The calibration curve during the Pb(II) ion optical detection by the composite material was investigated based on the linear part of the calibration plot. Fig. 4(B) shows the calibration plot of the composite material during optical detection of Pb(II) ions. Several quantification measurements were evaluated using a wide range of concentrations of standard Pb(II) ion solutions under specific detection conditions. The standard deviation of the Pb(II) ion analysis using composite material was 1.5% as evidenced by the fitting plot of the calibration graphs (Fig. 4(B), inset). The linear correlation at low Pb(II) ion concentration ranges indicated that the Pb(II) ion can be detected with the highest sensitivity and selectivity over a wide-range of concentration levels. The detection limit (LD) of Pb(II) ions using the composite material was 0.51 μg/L according to Eq. (2), which indicates that the composite material can simultaneously detect of Pb(II) ions at the ultra-trace concentration level [26–28]. The specific ion selectivity of Pb(II) ion detection system is crucial because of the coexisting diverse metal ions. The color formation and signal response of the composite material for Pb(II) over a series of metal ions at pH 5.50 was evaluated. In the experimental condition, the diverse metal ions amount was 15-fold (15 ppm) compared to the Pb(II) ions concentration. Fig. 5 shows the signal intensity of the blank sample and after the addition of competing cations such as Zn(II), Li (I), Al(III), Fe(III), Mg(II), Ca(II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Ba(II) and Bi(III) ions. The Pb(II) ion-selectivity studies by the conjugate material clarified that no measurable spectral interferences from other metals ion even 15–fold concentration as judged from Fig. 5. The ion

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Fig. 2. The TEM images of meso-channel mesoporous silica monolith (A, B) and MBHB anchoring composite material with ordered mesoporosity (C, D).

selectivity study was also evaluated by adding of several anions. However, specific adverse effect was not found by measuring the signal responses. This is probably the high bonding affinity to the composite material of MBHB ligand and Pb(II) ion over the common metal ions. 3.3. Pb(IV) adsorption parameters The pH is an important factor affecting the forms and solubility of ions in water and the activity of functional groups [43]. In the Pb(II)

Fig. 3. Effect of solution acidity for Pb(II) detection by the ligand functionalized composite material when equilibrated individually at different pH conditions with 2.0 mg/L of Pb(II). The standard deviation was N2.5% for the analytical data of duplicate analyses.

chemistry, when pH is kept below 3.30, Pb(II) mainly exists in the form of Pb(II) ions. Only a small fraction of Pb(II) can hydrolyze into Pb(OH)+ when the pH is raised to 4.2–7.0, but still mainly in Pb(II) form [44]. After minimising according to the solubility product constant of Pb(OH)2 and the initial Pb(II) concentration, Pb(II) would start to generate Pb(OH)2 precipitate at pH 7.0 in this study. Then the adsorption experiments under alkaline condition are not feasible by the proposed composite material. However, the influence of pH from 2.0 to 9.50 on the Pb(II) adsorption was defined to comply the study of the detection operation. The pH seldom had influenced on Pb(II) adsorption of composite as shown in Fig. 6(A). The data clarified that the adsorption process of composite material was strongly pH dependent. With the pH increased from 2.0 to 5.50, the Pb(II) adsorption capacity of composite material was increased sharply. After that, the Pb(II) adsorption was decreased with increasing the pH. The protonated site of the composite material surface would reject Pb(II) ion according to the theory of same electric charge mutual repulsion, leading to the low Pb(II) adsorption efficiencies at high pH region. With the pH increased from 2.0 to 3.50, the H+ concentration decreased quickly and the deprotonation reaction enhanced, which made the repulsive interaction between functional surface and Pb(II) ion weakened and the Pb(II) adsorption increased sharply. When the pH continually increased from 3.50 to 5.50, most of the adsorptive sites on the composite material was occupied and the Pb(II) adsorption capacity changed a little. Hence, the maximum Pb (II) adsorption capacity happened at the pH close to 5.50. Therefore, the highest adsorption was obtained at pH 5.50 and this pH was chosen for the adsorption of Pb(II) ion to determine the others parameters of the adsorption experiment in this study. The material surface is a critical factor that affects the adsorption rate parameters [45]. In addition, the adsorption kinetics is an essential aspect for the operation to define the adsorption efficiency of a process.

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Fig. 4. Increasing the signal intensity in color optimization with increasing Pb(II) concentrations (A) and calibration profiles with signal intensity in different Pb(II) concentration (B). The inlets in graph (B) optical responses for Pb(II) ions with a linear fit in the linear concentration ranges. The A and A0 is the absorption signal responses of the adsorbent after and before addition of Pb(II).

It is indicated in terms of the rate of metal ions adsorption that controls the residue time of the material in the solid-solution interface [40,42]. Therefore, a series of batch contact time experiments was performed to define and evaluate the maximum adsorption of Pb(II) by the composite material at optimum conditions. Therefore, the Pb(II) adsorption kinetics was investigated to understand the adsorption behavior of Pb (II) by the synthesized composite material. The adsorption kinetics plots of Pb(II) on composite material is shown in Fig. 6(B). The data revealed that Pb(II) ions adsorption was very fast in the beginning and then become slowly after the contact time increases further to 70 min. The process almost reaches equilibrium within 50 min, which is much shorter than that of other composite material [32–34]. The Pb(II)

adsorption was initially rapid and then decelerated prior to reaching a plateau. The maximum adsorption was obtained at 70 min. However, stirring time of 3 h was used in all further adsorption experiments to ensure Pb(II) equilibrium adsorption. The high surface area with the large amount of MBHB content and great availability of them in the pores of the adsorbent, which are required for electrostatic interaction with the Pb(II), significantly improved the adsorption capacity and the process proceeded rapidly. On the contrary, the complexation mechanisms are slow kinetics than the ion–exchange and hydrogen bonding mechanism for cation and anion adsorption by the ion-exchange resins and fibrous materials [46–48]. It is important to evaluate the adsorption isotherm not only for understanding the interaction between the adsorbate and the material, but also for designing the adsorption systems. However, the more concentrated the solution is better for the adsorption rate. The adsorption isotherm is relating to the solution concentration in the bulk and the adsorbed amount on the interface. In the present study, the adsorption data are studied by the most commonly used theoretical models of Langmuir. The Langmuir model is based on the assumption that binding sites are homogeneously distributed over the adsorbent surface. These binding sites have the same affinity for the adsorption of a single molecular layer [49]. The energy of adsorption is constant and there is no migration of adsorbate particles on the surface plane [50]. As the composite material exhibited the homogeneous porosity according to the morphological measurement by the TEM and N2 adsorptiondesorption isotherms, the Langmuir adsorption model was chosen in this study for the determination of Pb(II) ion maximum adsorption capacity The Langmuir equation has been successfully applied to this adsorption operation as follows: C e =qe ¼ 1=ðK L qm Þ þ ð1=qm ÞC e ðlinear formÞ

Fig. 5. Ion selectivity measurement of (1.0 mg/L) Pb(II) ion by the composite material after adding various foreign ion in higher concentrations. The interfering cations (15.0 mg/L) listed in order (1 to 15): (1) Zn(II), (2) Li(I), (3) Al(III), (4) Fe(III), (5)Mg(II), (6) Ca(II), (7) Ni(II), (8) Ag(I), (9) K(I), (10) Na(I), (11) Mn(II), (12) Ba(II), (13) Bi(III), (14) Blank and (15) 1.0 mg/L Pb(II) ion. The interfering (350 mg/L) anions listed in order (7 to 13): (7) chloride, (8) nitrate, (9) bicarbonate, (10) carbonate (11) sulfate, (12) phosphate and (15) citrate.

ð5Þ

where, qm is the maximum adsorption capacity, KL is the Langmuir coefficient (L/mg), qe is the amount of adsorbed Pb(II) by the composite material at equilibrium (mg/g) and Ce is the equilibrium Pb(II) concentration in filtrate solution (mg/L). The adsorption isotherms (qe versus Ce) showed that the sorption capacity, which is the mass (mg) of total Pb(II) adsorbed per unit mass of composite material, increased with increasing equilibrium Pb(II) concentration and eventually attained a constant value (Fig. 7). The qm and KL are the Langmuir constants which are related to the adsorption capacity and energy of adsorption, respectively, and can be calculated from the intercept and

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Fig. 6. The Pb(II) adsorption in various solution pH (A) and effect of contact time for the measurement of equilibrium adsorption when Pb(II) concentration was 5.0 mg/in 20 mL.

slope of the linear plot, with Ce/qe versus Ce as shown in Fig. 7 (inset). The correlation coefficients value (R2 = 0.984) confirm that the Langmuir adsorption equation can be considered an accurate model of the adsorption behavior for Pb(II) by the composite material and also indicating that the adsorption process was monolayer. The maximum adsorption capacity corresponding to complete monolayer coverage showed a capacity of 214.15 mg/g. The adsorption coefficient KL, which is related to the apparent energy of adsorption, was calculated to be 1.71 L/mg. The adsorption capacity compared to literature listed [51–57] values in Table 1, where the composite material is found to be comparable and competitive with other forms of materials. However, the direct comparisons of adsorption properties of composite material with other adsorbents are difficult due to the different applied experimental conditions and affecting parameters. The adsorption capacities of different adsorbents under the optimum conditions were calculated using the reported Langmuir equation. The higher maximum adsorption capacity obtained by the composite material was due to the spherical nanosized cavities with large surface area would allow an increased equilibrium adsorption capacity for Pb (II) ion at this optimum experimental conditions. A variety of ionic compounds coexists in natural waters and competes with each other for available adsorption sites [17,19]. Therefore

the Pb(II) adsorption capacity of the composite material was investigated in the presence of diverse metal ions. In this connection, the impact of interfering metal ion of Zn(II), Li(I), Al(III), Fe(III), Mg(II), Ca (II), Ni(II), Ag(I), K(I), Na(I), Mn(II), Ba(II) and Bi(III) on Pb(II) adsorption by the composite material was measured and the results are depicted in Fig. 8(A). Usually, the introductions of competing metal ion into working solutions have brought some negative effects on the target metal ion adsorption as defined by the several reports [15,26,31]. As regards the impact of diverse metal ion, the existence of the indicated metal ion showed no significant affecting and had not caused a distinct reduction on Pb(II) adsorption capacity of the proposed composite material. The soft donor atoms are always favorable for stable complexation mechanism, and this may be main assumption for high selectivity by the composite material. The possible bonding mechanism is shown in Scheme 2.

3.4. Elution and reuses studies The regeneration capability and versatility of material in order to be applied in high-efficiency adsorption systems are necessary to measure the potentiality in the practical field operation [40,45]. Then the elution operation was carried out to evaluate the performing sequential adsorption cycles as the data are depicted in Fig. 8(B). Here, the eluent was 0.20 M HCl for the effective desorption of Pb(II) ion from the composite material. The adsorption capacity of Pb(II) ion was slightly encountered a reduction from 99% to 90% after seven consecutive cycles. This result demonstrated that composite material is a cost-effective material with an outstanding potential to be recovered and used in sequential water treatment cycles preserving high stability. This reusability study also proved that the present composite material can be used many cycles without significant deterioration in its original adsorption performance for the Pb (II) containing wastewater samples. Table 1 Comparison of maximum adsorption capacities of Pb(II) using different forms of materials.

Fig. 7. Langmuir adsorption isotherms for Pb(II) ion and the linear form (inlet) of the Langmuir plot (initial Pb(II) concentration from 2.02 to 80.0 mg/L; solution pH 5.50; composite material amount 10 mg; solution volume 30 mL; contact time for 3 h).

Used materials

Maximum sorption capacity (mg/g)

Ref.

Amorphous and crystalline ZrP Inert Organic Matter (IOM) Magnetic CNTs-diatomite Copper oxide nanoflowers Pecan nutshell ZVI-GAM P-MCS Conjugate material

155.40 175.60 60.00 188.7 260.0 78.13 151.06 214.15

[51] [52] [53] [54] [55] [56] [57] This study

8

M.R. Awual, M.M. Hasan / Journal of Molecular Liquids 294 (2019) 111679

Fig. 8. (A) Competing ions effect for Pb(II) removal at optimum conditions while Pb(II) concentration was 1.0 mg/L and competing ion concentration was 15.0 mg/L in each and (B) reuses study in a cycle of adsorption-elution-regeneration by the composite material.

4. Conclusions

Acknowledgment

In this study, the Pb(II) ion capturing was investigated by the (3(3-(methoxycarbonyl)benzylidene)hydrazinyl)benzoic acid ligand based composite material. The main advantages of the method were easy to use with simplicity, the ability to rapidly attain phase equilibration and operation without using highly sophisticated instruments. The experimental results indicated that the composite material effectively detected and removed the Pb(II) ion in aqueous solution at pH 5.50. The material was permitted the rapid detection and removal of low concentrations of Pb(II) via a colorimetric signal visible to the naked-eye. The solution pH was highly influenced, and the 5.50 pH was effective for efficient Pb(II) capturing. The detection limit was 0.51 μg/L. The presence of coexisting ions did not adversely affect during Pb(II) ion detection and removal system, and composite material was efficiently captured the Pb(II) ion from multimixture solutions. The detection and adsorption mechanism was preferred to charge-transfer, electrostatic interactions and stable complexation. The experimental adsorption isotherm data were well fitted with the Langmuir, and the maximum adsorption capacity was found to be 214.15 mg/g. Overall, the fact that the detection and adsorption of Pb(II) ion were not significantly affected by ionic strength of diverse metal ions. Moreover, the composite material was almost remained high performance even seven regeneration cycles, suggest that the composite material as a promising economic and environmentally friendly Pb(II) ion treatment material for complex aqueous systems at large-scale at the optimum experimental protocol.

This research was partially supported by the Grant-in-Aid for Research Activity Start-up (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 author also wishes to thanks to the anonymous reviewers and editor for their helpful suggestions and enlightening comments.

Scheme 2. The possible bonding mechanism of Pb(II) ion with MBHB at pH 5.50 during detection and adsorption operation to exhibit the high sensitivity and selectivity.

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