Assessing of lead(III) capturing from contaminated wastewater using ligand doped conjugate adsorbent

Chemical Engineering Journal 289 (2016) 65–73

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Assessing of lead(III) capturing from contaminated wastewater using ligand doped conjugate adsorbent Md. Rabiul Awual ⇑ Actinide Chemistry Group, Quantum Beam Science Centre, Japan Atomic Energy Agency (SPring–8), Hyogo 679–5148, Japan

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Ligand doped conjugate adsorbent

prepared for efficient Pb(II) uptake from water.
The Pb(II) can be detected in ultratrace level with high sensitivity and selectivity.
Conjugate adsorbent exhibited the high adsorption capacity and extreme reusability.

a r t i c l e

i n f o

Article history: Received 6 November 2015 Received in revised form 20 December 2015 Accepted 22 December 2015 Available online 28 December 2015 Keywords: Conjugate adsorbent Lead(II) ions Detection and adsorption Groundwater Reusable

a b s t r a c t A ligand doped conjugate adsorbent was prepared by indirect ligand immobilization onto the mesoporous silica for lead (Pb(II)) ions detection and adsorption from aqueous media. The solution acidity played a key factor for sensitive Pb(II) ions detection. The data clarified that the adsorbent was able to detect low level Pb(II) ions with one-step detection procedure without using any sophisticated instruments. The detection platform exhibited the high sensitivity and selectivity toward Pb(II) ions and determined limit of detection was 1.2 lg/L. The effects of different parameters such as solution acidity, interaction time, initial concentration, ion selectivity and regeneration were investigated on the Pb(II) adsorption by the conjugate adsorbent. The data clarified that the Pb(II) ions adsorption process was found to be relatively fast. The adsorption isotherms of Pb(II) ions from aqueous solutions onto the adsorbent were measured, and the results showed that the Langmuir isotherm was found to be the best represent the measured adsorption data. The maximum adsorption capacity was determined to be 188.67 mg/g. The Pb(II) adsorption efficiency was not affected in the presence of co-existing ions. Moreover, this technique achieved residual Pb(II) concentration less than 10 lg/L, which is acceptable by water quality regulations. In addition, the conjugate adsorbent was regenerated by 0.10 M HCl treatment the Pb(II) ion adsorption efficiency was retained after nine adsorption-elution-regeneration cycles. The results suggested that conjugate adsorbent had the potential to become a promising technique for in situ Pb(II) contaminated groundwater remediation with naked-eye monitoring. Therefore, the ligand doped conjugate adsorbent is efficient and cost-effective potential materials for sensitive and selective Pb(II) detection and adsorption from aqueous solution. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction ⇑ Tel.: +81 791 58 2642; fax: +81 791 58 0311. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.cej.2015.12.078 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

Lead (Pb(II)) ions have received continuing concern due to its increased discharge and adverse effects on the environment and

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human health since it can be accumulated in living tissues throughout the food chains as the non-biodegradable pollutants [1–3]. Pb(II) is widely used in many industrial purposes, including storage battery manufacturing, painting pigment, fuels, photographic materials, coatings, the automotive industries and hundreds of other products in many countries [4–6]. Also Pb(II) can be accumulated in bones, brain, muscles and kidneys, which lead to damage of nervous system, renal function, nervous disorders and weakness of muscles [7,8]. The maximum permissible limit of Pb(II) in drinking water is 10 lg/L and 15 lg/L recommended by World Health Organization (WHO) and United States Environmental Protection Agency (USEPA), respectively [9,10]. Therefore, it is highly desired to develop new approaches efficient Pb(II) detection and removal from wastewater with simplicity, sensitivity and selectivity. Various analytical methods have been using for Pb(II) ions detection such as chromatography, spectroscopy, and electrochemistry methods [11–13]. These are exhibited prominent accuracy with satisfactory sensitivity. However, these are also expensive sophisticated instruments, needing well skilled operators, complicated sample matrix adjustment [14,15]. In addition, these are unable to use in-site monitoring with real-time measurement. In this connection, optical sensor-ensemble materials have been reported for diverse metal ions monitoring based on the solution acidity at optimum conditions. We have also reported that ligand doped nanomaterials are able to detect and remove the target metal ions with optimum color formation by naked-eye observation [16,17]. Based on this concept, we have designed a novel ligand doped conjugate adsorbent for sensitive and selective Pb (II) ions detection with high selectivity and selectivity. Several methods have been also employed to remove the Pb(II) ions from wastewater samples such as co-precipitation, coagulation, solvent extraction, floatation, electrochemical treatment, solid-phase adsorption extraction, including ion exchange, evaporation, membrane filtration and reverse osmosis [17–24]. Among these methods, adsorption technology is popular due to its ecofriendly, effective and cost-effective simplicity [25,26]. The intrinsic characteristic of the adsorbent exhibited the key point for successful adsorption of the metal ions. Therefore, there are still some challenges that restrict the adsorption approach considering limited adsorption capacity and low adsorption rate by the ordinary absorbents. Then the novel adsorbent materials for the removal of heavymetal ions with high adsorption capacity and fast adsorption rate are of great importance. In recent years, the nanotechnology has attracted a lot of attention to the scientific community [27,28]. The various nanomaterials like multi-walled carbon nanotubes, ligand immobilized composites are utilized to remove the organic contaminants and heavy metals ions from contaminated water [29–31]. The unique characteristics of nanomaterials including large specific surface area with high pore sizes and stable interconnected frameworks with an active pore surface for easy modification have increased their potential uses for clean-up of toxic heavy-metal ions [21,32]. We have developed several types of nanomaterial including a large surface area, large pore volume, controllable and narrowly distributed pore sizes and an ordered pore arrangement in the meso-structure [33,34]. The mesoporous silica is exhibited the abundant hydroxyl group to make hydrogen bonding with heteroatoms bond for stable immobilization of organic compounds. Then the introduction of additional functional groups by ligand immobilization is convenient for easy orientation of metal ions capturing from aqueous media [35–38]. In this study, we have aimed to design a conjugate adsorbent that can simultaneously detect and remove of toxic Pb(II) ions using a two-step process. First, the mesoporous silica was synthesized. Second, the surface of mesoporous silica was modified with ammonium (4-chloro-2-mercaptophenyl)carbamodithioate (ACMPC) to

obtain conjugate adsorbent using the indirect doping technique. The ACMPC ligand was used as molecular receptor and transformed the signal when binding with Pb(II) ion at the specific pH region. The usage of specific functional organic ligand emphasis the metal ions at certain condition and save a lot of experimental efforts when studying the sensitivity, selectivity, adsorption and reusability. The Pb(II) ion was captured based on the pH dependent matter. The conjugate adsorbent exhibited the high surface area, large pore volumes even after ACMPC doped onto the mesoporous silica. Several experimental parameters were systematically evaluated including solution acidity, color optimization, limit of detection, initial Pb(II) concentration, contact time, ion selectivity and reuses. 2. Materials and methods 2.1. Materials All materials and chemicals were of analytical grade and used as purchased without further purification. Silica source of tetramethylorthosilicate (TMOS), F108 (EO141PO44EO141), 2-Amino-4chlorobenzenethiol, carbon disulphide and ammonium hydroxide solution were purchased from Sigma–Aldrich Company Ltd. USA. The standard Pb(II) and other metal ions solutions were prepared from their corresponding AAS grade (1000 lg/mL) solutions and purchased from Wako Pure Chemicals, Osaka, Japan. The buffer solutions of 3-morpholinopropane sulfonic acid (MOPS), 2-(cyclohexylamino) ethane sulfonic acid (CHES), N-cyclohexyl-3aminopropane sulfonic acid (CAPS) and sodium acetate were procured from Dojindo Chemicals, Japan, and KCl, HCl and NaOH from Wako Pure Chemicals, Osaka, Japan. Ultra-pure water prepared with a Millipore Elix Advant 3 was used throughout in this work. 2.2. Analyses The SEM analysis was performed on Hitachi S-4300 operated at 16 keV. TEM images were obtained by using a JEOL (JEM-2100F) and operated at the accelerating voltage of the electron beam 200 kV. The absorbance spectrum was measured by UV–Vis–NIR spectrophotometer (Shimadzu-3700). The metals concentrations were measured by ICP–AES (SII NanoTechnology Inc.). The N2 adsorption–desorption isotherms were carried out by the 3Flex analyzer (Micromeritics, USA) at 77 K. The specific surface area (SBET) was calculated using multi-point adsorption data from the linear segment of the N2 adsorption isotherms using Brunauer–E mmett–Teller (BET) theory. The NMR spectra was obtained on a Varian NMR System 400 MHz Spectrometer. 2.3. Synthesis and characterization of ACMPC ligand The preparation steps and structure of (4-chloro-2-mercaptophe nyl)carbamodithioate (ACMPC) is reported elsewhere [39]. However, the main steps are describing here for reader understanding as shown in Scheme 1. In the syntheses steps, In a 100 mL roundbottomed flask, fitted with a mechanical stirrer and surrounded by an ice-salt cooling bath, were placed 5.4 g (0.071 mol) of carbon disulfide and 9.0 mL (0.13 mol) of concentrated aqueous ammonia. The stirrer was started, and 9.58 g (0.06 mol) of 2-amino-4chlorobenzenethiol were run into the mixture from a separators funnel at such a rate that the addition was completed in about twenty minutes. The stirring was continued for thirty minutes after that the 2-amino-4-chlorobenzenethiol was added, and then the reaction mixture was allowed to stand for another thirty minutes. During this time, a heavy precipitate of ammonium (4-chloro-2-mer captophenyl)carbamodithioate (ACMPC) was separated and stopped the stirring. The product was characterized by 1H NMR

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S HN

NH2

C

S

NH4

Ammonia soln. SH

+ S

C

S

SH ice bath / stirring

Cl

Cl

Scheme 1. Preparation steps of organic ligand of ammonium (4-chloro-2-mercaptophenyl)carbamodithioate (ACMPC).

spectroscopy. The 1H NMR (400 MHz, CDCl3): d 17.30 (s, H, Sec. amine), 7.43 (d, H, C(3) aromatic), 7.21 (q, H, C(5) aromatic), 7.13 (q, H, C(4) aromatic), 6.84 (d, H, C(6) aromatic), 4.00 (s, H, C-SH aromatic) and 1.5 (s, 4H, NH4). 2.4. Mesoporous silica and conjugate materials The mesoporous silica preparation method is reported elsewhere [40]. In preparation of inorganic mesoporous silica, the F108 was used as a soft template in the direct templating method of lyotropic liquid crystalline phase. Typically, the composition mass ratio of F108:TMOS:HCl/H2O was 1.2:2:1, respectively. After mixing of F108/TMOS in a 200 mL beaker at 60 °C, the exothermic hydrolysis and condensation of TMOS occurred by addition of HCl acid (at pH = 1.3). 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 550 °C for 6 h under the normal atmosphere. After that the material was ready to use for buildingblock step for highly immobilization of ACMPC ligand. The conjugate adsorbent was prepared by the building-block approach [41]. Here, 0.20 M dilauryl dimethyl ammonium bromide (DDAB) was dissolved ethanol solution and 1.0 g of mesoporous silica were immobilized. Then ACMPC (65 mg) was dissolved in DMF solution and 1.0 g of DDAB-mesoporous silica materials was added into the solution. The mixture was stirred for 8 h at 55 °C until ACMPC ligand saturation onto the DDAB-mesoporous silica. The DMF was removed by a vacuum connected to a rotary evaporator at 80 °C. The resulting materials were washed several times with water to check the leaching of ACMPC. Then the material was dried at 45 °C for 6 h and ground to fine powder for Pb(II) detection and removal operations. In the ACMPC immobilization onto mesoporous silica, the total washing amount of ACMPC solution was counted, and ACMPC immobilized amount was determined. The ACMPC immobilization amount (0.11 mmol/g) was determined by the following equation:

Q ¼ ðC 0  CÞV=m

ð1Þ

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 ACMPC, respectively.

at 25 °C for 35 min at a constant agitation speed of 110 rpm to achieve optimum color separation. The blank solution was also prepared, following the same procedure for comparison of color formation before and after addition of Pb(II) ions. After optimum color formation, the solid adsorbent was separated by Whatman filter paper (25 mm; Shibata filter holder) and used for color assessment and absorbance measurements by solid-state UV–Vis–NIR spectrophotometer. It is also noted that the conjugate adsorbent was ground into fine powder to achieve homogeneity in the absorbance spectra. The detection limit (LD) was determined from the linear part of the calibration plot according to the following equation [42]:

LD ¼ KSb =m

ð2Þ

where, the value of K is 3, Sb is the standard deviation for the blank and m the slope of the calibration graph in the linear range, respectively. The single and multi-component adsorption experiments were performed at room temperature (25 °C) and the Pb(II) ion adsorption was calculated from the following equations:

Mass balance qe ¼ ðC 0  C f ÞV=Mðmg=gÞ and metal removal efficiency R ¼

ðC 0  C f Þ  100ð%Þ C0

ð3Þ ð4Þ

where V is the volume of the aqueous solution (L), and M is the weight of the conjugate adsorbent (g), C0 and Cf are the initial and final concentrations of Pb(II) in solutions, respectively. In order to define the eluting agent, Pb(II) ion was adsorbed by the adsorbent and then washed with water several times and transferred into a 50 mL beaker. The washing solution was also checked by the ICP-AES. Then 0.10 M HCl was used as a suitable eluent to desorb the Pb(II) ions from the adsorbent, and the Pb (II) ions concentration were checked by ICP–AES. After the regeneration operation, the adsorbent was reused for several cycles to investigate the reusability as a cost-effective adsorbent. All experiments in this study were duplicated at least to assure the consistency and reproducibility of the results. 3. Results and discussion 3.1. Characterization of mesoporous silica and conjugate adsorbent

2.5. Optical detection and removal of Pb(II) ions In optical detection, the conjugate adsorbent was immersed in a mixture of specific Pb(II) ion at appropriate pH of 2.01, 3.50 (0.2 M of KCl with 2.0 M HCl), 5.20 (0.2 M CH3COONa with 1.0 M HCl), 7.01 (3-morpholinopropane sulfonic acid (MOPS) with NaOH), 9.50 (0.2 M 2-(cyclohexylamino) ethane sulfonic acid (CHES) with NaOH) and 11.01 (0.2 M N-cyclohexyl-3-aminopropane sulfonic acid (CAPS) with NaOH) with adding of known amount of conjugate adsorbent (10 mg) at constant volume (10 mL) with shaking in a temperature-controlled water bath with a mechanical shaker

The porosity of the mesoporous silica was clarified by N2 adsorption–desorption measurement as shown in Fig. 1(a). The N2 adsorption–desorption isotherms displayed the H2 type hysteresis loop with typical IV isotherms confirming the highly ordered mesoporous silica materials [43–45]. The specific surface area and pore volume of the mesoporous silica were indicated in Fig. 1 (inset). After making the building-block by insertion of DDAB, the surface of mesoporous silica was decreased. The lower pore volume of DDAB-silica is probably due to cationic substance substitution in the mesoporous framework, arising from a synthesis

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Fig. 1. N2 adsorption–desorption isotherms of mesoporous silica (a) and ligand immobilized conjugate adsorbent (b) with different surface area, pore volume and pore sizes.

nature and structural disorder. With further the introduction of ACMPC organic functional groups, the surface area and pore volume gradually decreased as Fig. 1(b). This is evident that the pore entrances were blocked by the organic groups. However, the hysteresis loop width was slightly decreased with the embedding of organic moieties indicating a slight decrease in nanoscale pore size of the adsorbent. Therefore, the N2 isothermal data indicated that ACMPC might be incorporated into inner pores of the DDAB-silica monoliths. The TEM images of mesoporous silica provide an evidence for the presence of the hexagonal mesostructured in the frame work, which is representative of mesoporous silica materials as judged from Fig. 2(A and B). Also the pore structure throughout the entire particle was regular for homogenous capturing of organic molecules [46]. The appreciable pores were also evident after successful organic ligand immobilization as shown in Fig. 2(C and D). In the conjugate adsorbent, the hydrogen bond might be formed between abundant hydroxyl groups of pore surface silicates and heteroatoms of ligand molecules [42,46]. 3.2. Pb(II) ions detection 3.2.1. Effect of pH The solution pH plays a unique role in metal-conjugate adsorbent complex formation and its signal enhancement due to the existing form of metal ions and conjugate adsorbent is pH dependent. In the detection of Pb(II) ions, the effect of pH was evaluated in the range of 2.0–11.0 and the absorbance intensities are listed in Fig. 3. As can be seen, the optimum pH for optimum color formation with highest absorbance intensity of Pb(II) ions is 7.0. At the low pH region, the significant color was not found due to the absence of charge-transfer active sites on the conjugate adsorbent with the positively charged Pb(II) species. A further increase over neutral pH area, the signal intensity also decreased, and this is probably due to the formation of Pb(II) hydroxide. Therefore, a pH value of 7.0 was chosen for further experiments to evaluate the other parameters of the detection methods. 3.2.2. Sensitivity, color optimization and limit of detection To explore the sensitivity and the color optimization of the conjugate adsorbent, the absorbance intensity was measured toward with different concentrations of Pb(II) ions. The absorbance inten-

sity was increased with the Pb(II) ions concentration increased from 0 to 4 mg/L at 400 nm as shown in Fig. 4(A). The data also clarified that signal intensity enhancement corresponds to the color optimization between conjugate adsorbent and Pb(II) ions confirmed the sensitive detection of low level Pb(II) ions in water samples without using sophisticated instruments. Therefore, the proposed conjugate adsorbent could detect the Pb(II) ions at any concentration level (from lg/L to mg/L) with extreme sensitivity and naked-eye observation. Fig. 4(B) indicates the relative absorbance intensity against with Pb(II) ions concentration. The inset in Fig. 4(B) shows the linear responses at the low-level of Pb(II) concentrations. The linear correlation at the low-level Pb(II) ions clarified that Pb(II) can be detected with the highest sensitivity even in the trace level of Pb (II) ions in water. The linear regression equation was Y = 0.03632 + 0.22492 X with a correlation coefficient of 0.9991. Based on the equation of limit of detection, the lower limit of detection for Pb (II) ions is 1.2 lg/L. The maximum permissible limit by USEPA of Pb(II) ions in drinking water is 15 lg/L [47,48]. Therefore, the proposed conjugate adsorbent is fully applicable for Pb(II) ion detection in the contaminated water at pH 7.0. 3.2.3. Selectivity The ion selectivity of the proposed conjugate adsorbent was determined with diverse metal ions such as Na+, K+, Ag+, Ni2+, Cd2+, Ca2+, Mg2+, Zn2+, Mn2+, Hg2+, Al3+, Sb3+ and Cr3+ was tested at 2 ppm of Pb(II) ions while others were at 10 ppm. A significant and distinct color formation and high signal intensity were observed in the case Pb(II) ions compared with other metal ions as shown in Fig. 5. It is also noted that slight signal intensity was found in Hg2+ ions compared to the blank sample as judged from Fig. 5. However, the Hg2+ concentration was 5 times higher than the Pb(II) ions. It can be estimated that Hg2+ ions can be detected based on the pH dependent by the conjugate adsorbent. This result suggests that the present detection procedure is selective toward Pb(II) ions and exhibited the potentiality to be applied for real sample analysis. 3.3. Pb(II) ions adsorption 3.3.1. Solution acidity The solution acidity plays an important role in adsorption process by the ligand immobilized nano-adsorbents and significantly affected the adsorption capacity and selectivity [49]. It is also noted that solution pH would affect both solution chemistry and surface binding sites of the adsorbents and therefore, solution pH can easily modify the surface charge of the adsorbent surface. Therefore, the solution acidity is an important parameter in the adsorption process [32,50,51]. Fig. 6(A) shows the effect of pH on Pb(II) ions adsorption efficiency by the conjugate adsorbent. The maximum adsorption efficiency was observed at pH 7.0. The basic pH region is neglected due to the deposition of Pb(OH)2, which is insoluble. Therefore, Pb (II) ions adsorption over pH 7.0 is unsuitable to understand the adsorption mechanism by the conjugate adsorbent. The same pH effect is also reported by other scientists [52]. 3.3.2. Effect of contact time The interaction time between metal ions and adsorbent surface is important to exhibit the high efficiency of the adsorbing materials [13,25,53–55]. Then we have defined the adsorption efficiency based on the contact time effect. Fig. 6(B) shows the effect of contact time of Pb(II) ions adsorption by the proposed conjugate adsorbent. With increasing the contact time, the Pb(II) adsorption efficiency was increased and the equilibrium adsorption was reached in 30 min. The data also clarified that the adsorption efficiency was very fast at the beginning and then the rate of

M.R. Awual / Chemical Engineering Journal 289 (2016) 65–73

69

Fig. 2. TEM micrographs mesoporous silica (A and B) and ligand embedded conjugate adsorbent (C and D) existed with ordered porous structure.

1.4

Signal intensity (a.u.)

1.2 1.0 0.8 0.6 0.4 0.2 0.0 2

4

6

8

10

12

Solution pH Fig. 3. Effect of pH for optical detection of Pb(II) ions where the initial Pb(II) ion concentration was fixed at 2.0 mg/L in each pH area.

adsorption was reduced. This is obvious that all adsorbent sites are vacant in the beginning and their capabilities were high to adsorb the metal ions. After that the Pb(II) ion adsorption was slow due to the occupied adsorbent surface. The results demonstrated that conjugate adsorbent has high affinity and short response time toward the low-level of Pb(II) ions, which can be used for rapid removal of Pb(II) ions from water solution. Similar trends are also reported by the other researchers [22,51]. Therefore, based on the results of this study, equilibration time of 3 h was selected for to understand the maximum adsorption capacity when the initial Pb(II) ion concentration was high.

3.3.3. Adsorption isotherms The developed conjugate adsorbent exhibited the high surface area, large porous structure and then we have expected the high Pb(II) ions adsorption. The adsorption capacity increased with an increasing adsorbate concentration as shown in Fig. 7. Fig. 7 also clarified that the adsorption increased rapidly at the beginning, then slowed down until it formed a plateau following reaching the equilibrium with the saturation or possible pore blocking effect [56,57]. At higher initial Pb(II) ions concentration, a higher flux allowed the solute to penetrate deeper into the inner pore surface of the conjugate adsorbent until it was saturated or clogged [57]. The relationship between the amount adsorbed on the adsorbent and the concentration of dissolved metal ions in the solution at equilibrium gives the adsorption isotherm. The isotherms models are implied the equilibrium relations between the metals ions concentrations in the solid phase and that in the liquid phase. Also the metal ions transferred from the aqueous media to a solid phase when the adsorption process reaches an equilibrium position. The Langmuir isotherm is often used to describe the solute adsorption from liquid solutions, and this model assumes a monolayer adsorption onto a homogeneous surface of the adsorbent sites with the uniform energies of adsorption for each sorption site to be equivalent [58,59]. The linear form of the Langmuir isotherm is as follows:

C e =qe ¼ 1=ðK L qm Þ þ ð1=qm ÞC e ðlinear formÞ

ð5Þ

where qe is the amount of heavy metal ions adsorbed (mg/g) by the conjugate adsorbent at equilibrium time, Ce is the concentration of Pb(II) ions (mg/L) at equilibrium time in solution, qm is the maximum adsorption capacity (mg/g) that forms a complete monolayer on the surface, and KL is the Langmuir adsorption constant (L/mg) related to the energy of adsorption and the affinity of binding sites for ions, respectively. The qm and KL were calculated from the slope and intercept of the linear plot of Ce/qe versus Ce. The Langmuir model was fitted well with the experimental data according to

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(A)

(B)

Fig. 4. The color optimization at different concentrations of Pb(II) ions corresponds to the absorbance spectral intensity (A) and calibration profile with spectral absorbance measured at 400 nm with Pb(II) ion concentrations adjacent to spectral intensity. The inlets in graphs (B) show the low–limit of detection with a linear fit in the linear concentration range. The dotted line represents the calibration plot of the Pb(II) ions in the presence of active interfering species under the optimum detection conditions.

0.65

13 2 1 1 1 0 1 9

Io

An ion s 8

ns

7

6

5

4

3

2

A-A0 (a.u.)

1.30

0.00

Ca tio ns 1

Fig. 5. Ion selectivity profile of the conjugate adsorbent at pH 7.0 with adding diverse metal ions corresponds to the color and signal intensity. The interfering cations (10.0 mg/L) listed in order (1–11): (1) Hg2+, (2) K+, Na+, (3) Ag+, Cr3+, (4) Ni2+, (5) Cd2+, (6) Ca2+, (7) Mg2+, (8) Zn2+, (9) Mn2+, (10) Sb3+, (11) Al3+, (12) Blank and (14) Pb2+. The interfering (150 mg/L) anions listed in order (6 to 11): (6) nitrate, (7) bicarbonate, (8) chloride, (9) carbonate, (10) sulfate and (11) phosphate.

Fig. 7. Effect of initial Pb(II) ion concentration by the conjugate adsorbent and the linear form of the Langmuir plot (initial Pb(II) ion concentration range 2.10–70.26 mg/L; solution pH 7.0; dose amount 10 mg; solution volume 30 mL and contact time 3 h).

100

100 80

Efficiency (%)

Efficiency (%)

(A)

60 40 20

80

(B) 60

40

20

0 0

2

4

6

8

pH

10

12

0

20

40

60

Contact time (min.)

Fig. 6. Adsorption efficiency of Pb(II) ion with changing the initial pH solution (A) and equilibrium contact time to understand the equilibrium Pb(II) adsorption with varying different contact time where the initial Pb(II) ion concentration was fixed at 4.0 mg/L (B).

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the high correlation coefficient (R2 = 0.98) as shown in Fig. 7 (inset). Also the maximum adsorption capacity of the conjugate adsorbent was 188.67 mg/g for Pb(II) ions by the proposed conjugate adsorbent. Moreover, the Langmuir constant KL has a positive value (2.13), indicating a favorable adsorption system. The Langmuir fitting also suggested homogenous adsorption sites on the conjugate adsorbent surface where it restricted to monolayer adsorption due to a strong interaction between the adsorbent and the Pb(II) ions [60]. The maximum adsorption capacities of various adsorbents for Pb(II) ions are summarized in Table 1 with reported articles [18,22a,24,26,37,51,57,58,61]. This comparison showed that the present conjugate adsorbent exhibited promising adsorption capacities than some of the commercially available and low cost adsorbents. 3.3.4. Effect of foreign ions Several types of co-existing ions such as Na+, K+, Ag+, Ni2+, Cd2+, Ca2+, Mg2+, Zn2+, Mn2+, Hg2+, Al3+, Sb3+ and Cr3+ are existed in the groundwater. These ions effects were investigated to evaluate the Pb(II) ions selectivity by the conjugate adsorbent. The data clarified that these common ions were not obviously affected the Pb(II) ions adsorption as depicted in Fig. 8(A). However, the Hg2+ ion was little interfered as already in the preceding section. Due to the difference of stability constant, complex formation, electronegativity, massto-charge ratio and size of hydrated ion of these metal ions [61], the selective adsorption on the conjugate adsorbent might happen in a competitive adsorption. Therefore, the Pb(II) ion was preferably adsorbed by the conjugate adsorbent. In addition, the major

Table 1 Comparison of Pb(II) adsorption capacities with different forms of adsorbents. Used materials

Adsorption capacity (mg/g)

Ref.

NH2-MCM-41 Nano-adsorbent Magnetic composite Cotton stalk activated carbon Nano-adsorbent Nanostructured cedar leaf ash Fumarate ferroxane MWCNT-COOH Polysiloxane-graphene oxide Conjugate adsorbent

57.74 169.34 60.80 119.00 400 7.23 243.90 50.00 256.00 188.67

[18] [22a] [24] [26] [37] [51] [57] [58] [61] This study

ions in the real samples have no obvious influence on the Pb(II) adsorption under the selected optimum conditions. 3.3.5. Elution and recycle The feasibility of using materials to remove contaminants from wastewater depends on the regeneration ability during the several adsorption-elution-regeneration cycles. However, some of the articles have been paid attention along with other affecting parameters of the adsorption process. In the present study, the conjugate adsorbent was regenerated nine times to find adsorption efficiency of Pb(II) ions as shown in Fig. 8(B), where 0.10 M HCl was used suitable eluent. The experimental data also clarified that the adsorption capacity slightly decreased after nine consecutive cycles. However, the conjugate adsorbent can be used several cycles for Pb(II) adsorption. Therefore, the present adsorbent is exhibited the practical potentiality to remove Pb(II) ions from real wastewater solution. 4. Conclusions In this study, functional organic ligand was successfully immobilized onto the mesoporous to prepare conjugate adsorbent for sensitive and selective Pb(II) ions detection and removal from contaminated groundwater solutions. In the presence of Pb(II) ions, the significant color changed evidence, and the change can be easily detected by the naked-eye observation. The prepared adsorbent was demonstrated good sensitivity for Pb(II) ions even in the low concentration level. In addition, the color optimization and absorbance intensity values were linear over the concentration range 0.002–0.10 mg/L with a limit of detection of 1.20 lg/L. The results of this study also indicated that the conjugate adsorbent exhibited good adsorption ability from aqueous solutions. The solution pH played an important factor, and the maximum adsorption efficiency was attained at pH 7.0. Also the Pb(II) ions adsorption efficiency was not adversely affected to considerable extent by the presence of diverse competing ions. The adsorption equilibrium data was well fitted by the Langmuir isotherm model and the maximum adsorption capacity by the conjugate adsorbent for Pb(II) ions was 188.67 mg/g. Therefore, the proposed adsorbent could capture the Pb(II) ions from groundwater samples in terms of high sensitivity, selectivity, efficiency, stability, and reactivity.

(A)

(B)

100

100

Initial

Sorption

Eluted amount

80

Efficiency (%)

Efficiency (%)

80 60

40

20

60 40 20 0

0 Na

K

Ag Pb Ca Mg Ni Mn Cd Zn Sb Cr Hg Al

Ions

1st 2nd 3rd 4th 5th 6th 7th 8th 9th

No. of cycles

Fig. 8. The Pb(II) ion adsorption in the presence of diverse competing metal ions by the conjugate adsorbent at pH 7.0 (A) and regeneration study with successive elution operation in nine cycle with eluent of 0.10 M HCl (B).

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