Optical detection and efficient removal of transition

Sensors and Actuators B 242 (2017) 595–608

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Optical detection and efficient removal of transition metal ions from water using poly(hydroxamic acid) ligand Md Lutfor Rahman a,∗ , Shaheen M. Sarkar b , Mashitah Mohd Yusoff b , Mohd Harun Abdullah a a b

Faculty of Science and Natural Resources, Universiti Malaysia Sabah, 88400 Kota Kinabalu, Sabah, Malaysia Faculty of Industrial Sciences and Technology, University Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia

a r t i c l e

i n f o

Article history: Received 22 February 2016 Received in revised form 1 November 2016 Accepted 2 November 2016 Available online 11 November 2016 Keyword: Optical detection Poly(hydroxamic acid) Transition metals Adsorption Complexation

a b s t r a c t A copolymer, cellulose-graft-poly(methyl acrylate), was synthesized by a free-radical initiating process, and the ester functional groups converted into the hydroxamic acid ligand. The pH of the solution acts as a key factor in achieving optical color signals of metal-complexation. The reflectance spectra of the [M-ligand]n+ complex was found to be at the highest absorbance, ranging from 92 to 99% at pH 6, with absorbance noted to increase as metal ion concentrations were increased. A broad peak at 673 nm for Cu2+ was observed, indicating the presence of the charge transfer (␲–␲ transition) complex. The developed ligand demonstrated superior adsorption capacity for copper (305.3 mg g−1 ), as well as strong adsorption capacity for other metals; the adsorption capacities for Fe3+ , Mn2+ , Co2+ , Cr3+ , Ni2+ , and Zn2+ were 275.6, 258.5, 256.6, 254.3, 198.5, and 190.1 mg g−1 , respectively. The experimental data of the adsorption kinetics of the metal ions fitted well with a pseudo-second-order rate equation. The obtained data demonstrated that the observed metal ion sorption was well fitted with the Langmuir isotherm model (R2 > 0.99), suggesting that the surface of the adsorbent is homogenous and monolayer. The reusability of the ligand was verified using the sorption/desorption process, demonstrating that the developed adsorbent can be reused for 12 cycles without significant loss of its original sensing and removal performance. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Cellulose is a natural polymer; in terms of biomass, it is the most abundant polymer found in nature. Owing to its abundance and particular properties, cellulose and its derivatives can be used in a variety of versatile applications [1]. Cellulose, classified as a linear syndiotactic homopolymer, is composed of d-anhydroglucopyranose units (AGUs) linked by b-(1,4)-glycosidic bonds, with AGUs carrying three hydroxyl groups at the C2, C3, and C6 positions [2]. As the hydroxyl groups of cellulose play a key role in typical reactions, the use of primary and secondary alcohol reactions allows for the preparation of functional materials in heterogeneous systems [3], where the structure of cellulose can be tailored with chelating or metal-binding functionalities through attachments and modifications of the primary or secondary hydroxyl groups. An easy and cost-effective method of preparing such materials

∗ Corresponding author. E-mail address: [email protected] (M.L. Rahman). http://dx.doi.org/10.1016/j.snb.2016.11.007 0925-4005/© 2016 Elsevier B.V. All rights reserved.

involves grafting of selected monomers to the cellulose backbone via direct attachment, followed by subsequent functionalization of the grafted copolymer chains with known chelating moieties; the grafted copolymer is recognized as the side chains covalently attach to the main chain of the cellulose backbone through ionic or free-radical initiating processes [4]. The initiator-created radicals at various sites on the cellulose backbone are highly reactive, and the ceric ion (Ce4+ ) has a profound effect on the initiator due to its high efficiency in grafting [3]. Cellulose possesses good chemical stability and mechanical strength, and can be chemically modified through grafting reactions with desired monomers. Cellulose grafted polymers can be further transformed into known chelating ligands, which readily form metal complexes, an essential step in the chelating mechanism [4,5]. The varied consequences of climate change, such as the melting of the ice caps and rising sea levels, alongside other human activities, have drastically reduced the availability of fresh water for human utilization. However, the global need for increased quantities of pure water is an indisputable fact. To address this key issue, various water treatment technologies have been proposed and

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M.L. Rahman et al. / Sensors and Actuators B 242 (2017) 595–608

applied both experimentally and in-field. While certain progress has been made in this regard, there is still a need for new wastewater treatment technologies that meet the current regulatory limits for discharge of various metal ions into water bodies. Indeed, aboveregulation contamination levels of toxic metal ions are commonly found in aqueous wastes carrying industrial discharges from metal plating, mining, tanneries, chloro-alkali processes, radiator manufacturing, smelting, alloy industries, and storage battery industries. To counteract this, several measures have been taken to prevent or reduce environmental pollution. For example, techniques such as precipitation, adsorption, ion exchange, and reverse osmosis have been applied to remove metal ion toxins from wastewater [6,7]. In some cases, wastewater treatment by precipitation is followed by adsorption onto activated carbons to induce metal-ion removal at the highest level. The increasing costs and environmental considerations associated with the use of commercial adsorbents have led to a significant body of research aimed at developing new adsorbents derived from renewable resources in recent years [8–12]. As a result, many types of adsorbents are available today, such as activated carbon [13], mineral oxides [14], synthetic resins [7], bio-sorbents [15], and chelating resins [16,17]. Although these adsorbents have been shown to be stable under various chemical and physical conditions, they are still hindered by some parameters that can affect biosorption processes, such as pH, metal concentration, and the presence of competing ions [18]. Thus, these materials still need to undergo chemical modifications that yield desired functional groups for the removal of heavy metals from industrial waters [19]. Chemical coagulation, although quite effective in treating industrial effluents, often induces secondary pollution due to the added chemical substances it introduces [6]. Toxic metal precipitation also produces intractable sludge, which is associated with a high cost of disposal. Together with the rapidly declining supply of clean water globally, the drawbacks of the currently available techniques highlight the significant need for more effective techniques for treatment of industrial effluents. In cellulose, the hydroxamic acid group exhibits a ketone (C O) and an amide ( NH OH) group at the same carbon atom, which can produce the fused features of amide, oxime, and hydroxamic acid functionalities. Hydroxamic acid deprotonates to give hydroxamates, which bind to metals ions as bidentate ligands [20]. While both hydroxamic acid and the amidoxime groups have no affinity for common metallic cations (e.g. Na+ , K+ , Ca2+ , and Mg2+ ), they exhibit a strong tendency to form chelating complexes with a wide range of transition metal ions in aqueous solutions [20]. Selective chelation of metal cations of amidoxime groups has been applied for fabrication of functional materials for a variety of applications, such as for preparation of chelating resin. In the present study, pure white cellulose was first extracted from kenaf fibre with the use of a conventional method, then grafted with acrylonitrile. Next, the obtained product was submitted to chemical modification with the use of the radical initiation method [20]. The grafting of methyl acrylate onto kenaf cellulose was conducted under various experimental conditions, and the main characteristics of the grafting parameters determined. A new poly(hydroxamic acid) chelating ligand, which can be applied for the removal of transition metal ions from water, was then synthesized from the kenaf cellulose-graft-poly(methyl acrylate) (PMA). This polymeric ligand was found to have superior recyclability, yielding more stable hydroxamic acid ligands than the amidoxime ligand from kenaf cellulose [21]. To test its feasibility as a metal-removing agent, the obtained poly(hydroxamic acid) was then employed towards the purification of model and real wastewater samples from the electroplating industry containing different amounts of heavy metals.

2. Experimental 2.1. Materials Kenaf fibre was obtained from the National Kenaf and Tobacco Board at Kuantan, Pahang, Malaysia, and cut into small pieces of approximately 0.3 cm in length (see Fig. S1a at ESI). Cellulose was extracted from raw fibre using our recently reported method [21]. In its pure form, the cellulose is white in colour, as shown in the electronic Supporting information (see Fig. S1b at ESI). Real wastewater samples were obtained from a local electroplating plant BI-PMB Waste Management Sdn. Bhd, Shah Alam, Kuala Lumpur (Malaysia). A methyl acrylate monomer, purchased from Aldrich, was passed through columns filled with chromatographicgrade activated alumina to remove inhibitors. Other chemicals, such as ceric ammonium nitrate (CAN) (Sigma-Aldrich), methanol (Merck), sulphuric acid (Lab Scan), metal salts, and other analyticalgrade reagents, were used without further purification. 2.2. Graft copolymerization Purified kenaf cellulose (3.0 g) was mixed with 300 mL distilled water. Reactions were carried out in a 1 L three-neck round bottom flask fixed with a stirrer and condenser, and placed in a thermostat water bath. Nitrogen gas was purged into the flask to remove oxygen during the grafting process. The mixture was heated to 55 ◦ C while stirring, and 1.1 mL of diluted sulphuric acid (50%) was added to the mixture. After 5 min, 1.1 g of CAN (10 mL solution) was added, and the reaction mixture stirred under N2 gas. After 20 min, a 10 mL methyl acrylate purified monomer was added into the cellulose suspension and stirred for 4 h under nitrogen flow. At the end of the reaction, the mixture was cooled, and the product precipitated from excess amounts of methanol, then washed several times with a methanolic solution (methanol: water: 4:1, v/v). The resulting product was then oven dried at 50 ◦ C to a constant weight [21]. 2.3. Determination of grafting fractions A Soxhlet purification method consisting of the use of acetone for 12 h was used to remove the homopolymer from the crude grafting product. The purified grafted copolymer was dried at 50 ◦ C to a constant weight (see Fig. S1c at ESI). The percentage of grafting (Gp) was determined using the following Eq. (1): Grafting percentage (gp) =

W2 × 100 W1

where W1 is the weight of the parent polymer (cellulose), and W2 is the weight of the grafted polymer (polyacrylonitrile). 2.4. Synthesis of poly(hydroxamic acid) ligand The hydroxylamine solution was obtained by dissolving approximately 12.0 g of hydroxylamine hydrochloride (NH2 OH·HCl) in 300 mL of methanolic solution (methanol: water; 4: 1). Sodium hydroxide (NaOH) solution (50%, w/v) was added in cold condition until the pH of the solution reached 11. Following, the precipitate form of NaCl was removed through filtration. The ratio of methanol to water was maintained at 4: 1 (v/v). Next, poly(methyl acrylate) grafted kenaf cellulose (5.0 g) was placed into a two-neck round bottom flask fixed with a stirrer and condenser, and placed in a thermostat water bath [21]. The prepared hydroxylamine solution was then added to the flask, and the reaction carried out at 70 ◦ C for 6 h. After completion of reaction, the chelating polymeric ligand was separated from the hydroxylamine solution by filtration, then washed with a methanolic solution (methanol: water/4:1). In order to convert the obtained ligand into an H-form ligand, the


solution was then treated with 200 mL of 0.1 M HCl in methanolic solution for 5 min. Following, the obtained H-form ligand was filtered and washed several times with methanol, then dried at 50 ◦ C to a constant weight (see Fig. S1d at ESI).

One hundred and fifty milligrams of the polymeric ligand was immersed into 10 mL of a buffer solution (adjusted to appropriate pH from 2 to 9; 0.1 M sodium acetate with acetic acid) for typical optical metal ion sensing. Then, a metal ion (Cu2+ , Fe3+ , Co2+ , Cr3+ and Ni2+ ) solution of concentration 5 mg L−1 was added to each pH solution (pH 2–9) with shaking in a temperature-controlled shaker machine (Lab companion, SI-600) at 30 ◦ C for 2 h, at a constant agitation speed of 180 rpm to achieve good colour separation. For colour optimization, 150 mg of the polymeric ligand was also immersed into 10 mL of acetate buffer at pH 6, with metal ion solution (10 mL of each metal ion) concentrations of 5, 10, and 15 mg L−1 added at a constant volume (20 mL) with shaking in a similar manner for 2 h, at a constant agitation speed of 180 rpm to achieve good colour separation. A blank solution was also prepared following the same procedure for comparison of colour formation and detection [22,23]. After equilibration, the solid ligand was separated using filtration, and the ligand dried at 50 ◦ C for 2 h. Optical colour assessment and absorbance were measured by a solid state UV–vis NIR spectrophotometer (UV-2600 Shimadzu). 2.6. Batch adsorption For typical removal of transition metal ions (Cu2+ , Fe3+ , Co2+ , Mn2+ , Cr3+ , Ni2+ and Zn2+ ), 150 mg of the polymeric ligand was also immersed into a metal ion (single metal) solution of 10 mL (0.1 M), with the appropriate pH (3–6) set using an acetate buffer (10 mL). The solution was then left to shake for 2 h at an agitation speed of 180 rpm. After equilibration, the ligand was separated by filtration, and metal ion concentrations were determined by ICP-OES (Perkin Elmer, Optima 8300). The initial and final readings (after adsorption) of metal ion concentrations were calculated according to Eq. (3). qe =

(Co − Ce )V L

(3)

where qe is the equilibrium adsorption amount (mg g−1 ), Co is the initial concentration of metal in the solution (mg L−1 ), Ce is the equilibrium concentration of metal (mg L−1 ) after adsorption, V is the volume of the metal solution (L), L is the mass of polymeric ligand (g). In order to investigate isothermal behavior, batch adsorption experiments (Cu2+ , Fe3+ , Co2+ , Cr3+ and Ni2+ ) were performed using the traditional bottle-point method, with constant temperature at 30 ◦ C, and 180 rpm agitation (Lab companion, SI-600). One hundred and fifty milligrams of poly(hydroxamic acid) ligand were equilibrated with 20 mL of the aqueous metal ion solution (single metal) and an acetate buffer (10 mL) for 2 h. In the isothermal study, the initial concentrations of metal ions ranged from 5 ppm to 1500 ppm. The initial metal solution and final solution (after adsorption) were analysed by ICP-MS (Agilent 7500 series), with analysis of data carried out according to Eq. (3). 2.7. Kinetic study A kinetic study was carried out with 150 mg of ligand immersed into 10 mL of 0.1 M metal ion solution (single metal) and 10 mL of acetate buffer at pH 6, with shaking at a speed of 180 rpm at various time intervals (2, 5, 10, 20, 30, 60 and 120 min). Metal ion concentrations were estimated by ICP-OES (Perkin Elmer, Optima 8300).

Kenaf cellulose Kenaf-g-PMA Poly(hydroxamic acid)

890

T % ( a r b .u n i t s )

2.5. Optical sensing of metal ions

597

1630 1425 1370

2920

1160 1064 3432

827 1740

1400 1682 1652

3180

4000

3500

3000

2500

2000

cm

1500

1000

500

-1

Fig. 1. FTIR spectra of (a) kenaf cellulose, (b) poly(methyl acrylate) grafted kenaf cellulose and (c) poly(hydroxamic acid) chelating ligand.

The residual metal concentrations were determined by deducting final concentrations (after adsorption) from initial metal ion concentrations, as calculated by Eq. (4): qt =

(Co − Ct )V L

(4)

where qt is the adsorption amount at time t (mg g−1 ), Co is the initial concentration of metal solution (mg L−1 ), and Ct (mg L−1 ) is the metal concentration at time t. 2.8. Wastewater purification Experiments were performed using two different wastewater models, namely ww1 and ww2, containing different amounts of transition metals, as shown in Table 1. In addition, experiments were also conducted using real wastewater samples collected from untreated wastewater containing discharges from a local electroplating plant. This sample, namely ww3, also contained different amounts of transition metals. The transition metals content of the untreated industrial wastewater samples was analyzed by ICP-OES (Table 1). Removal of metal ions from model wastewater samples (ww1; ww2) and real wastewater samples (ww3) was achieved by adding 10 mL of metal ion solution (wastewater, ww1–ww3) and 5 mL of sodium acetate buffer at pH 6 to 150 mg of dried ligand. The obtained solution was then submitted to shaking for 4 h at an agitation speed of 180 rpm. Initial concentrations of metals (Cu, Fe, Cr, Ni, Zn, Co and Mn; analysed by ICP-OES) and final solutions (after adsorption) were analysed by ICP-MS (Agilent 7500 series); results are presented in Table 1. 3. Results 3.1. FT-IR analysis The synthesized chelating ligand, kenaf cellulose, and the cellulose-grafted copolymer were characterized by infrared (IR) spectra using an FT-IR Spectrometer (Perkin–Elmer). The IR spectrum of kenaf cellulose showed adsorption bands at 3432 and 2920 cm−1 , which corresponded to the O H and C H stretching, respectively (Fig. 1a). The band at 1630 cm−1 was determined to correspond to the bending mode of the absorbed water [24]. A smaller band at 1425 cm−1 was observed for the symmetric bending of CH2 . The observed absorbances at 1370 and 1160 cm−1 originated from the O H bending and C O stretching, respectively. The C O C pyranose ring skeletal vibration produced a strong band

598


Table 1 Metal content in model wastewater (ww1 and ww2) and real electroplating wastewater (ww3) analyzed by ICP-OES and ICP-MS.a

Metals

Cu Fe Cr Ni Zn Co Mn a

Model ww1

Model ww2

Real ww3

Before treatment (mg L−1 )

After treatment (mg L−1 )

Removal, %



Removal, %



Removal, %

436.2 100.7 488.2 317.3 388.1 387.1 310.5

0.5 0.56 1.43 1.89 1.32 1.19 1.32

99.88 99.44 99.70 99.40 99.65 99.69 99.57

126.1 290 140.3 121.8 110.6 113 105.3

0.16 0.98 0.49 0.82 0.91 0.86 0.78

99.87 99.66 99.65 99.32 99.17 99.23 99.25

63.8 135.1 91.6 87.2 46.9 47.5 45.8

0.05 0.71 0.78 0.42 0.51 0.56 0.49

99.92 99.47 99.14 99.51 98.91 98.82 98.91

Image of 6 wastewater samples, before and after treatment.

at 1064 cm−1 . A small sharp peak at 890 cm−1 corresponded to the glycosidic C1 H deformation, with ring vibration contribution and OH bending, which is characteristic of ␣-glycosidic linkages between glucose units in cellulose [24]. The IR spectrum of purified poly(methyl acrylate) grafted cellulose (kenaf-g-PMA) showed new absorption bands at 1740 cm−1 and 827 cm−1 due to the C O and CH2 stretching, respectively, of methyl acrylate (Fig. 1b) and other bands retained from kenaf cellulose (3432, 2920, 1630, 1425, 1370, 1160, 1065 and 890 cm−1 ). The hydroxamic acid functional group from the poly(hydroxamic acid) ligand showed new absorption bands at 1682 and 1652 cm−1 corresponding to the C O stretching and N H bending modes, respectively (Fig. 1c). In addition, new broad bands were observed at 3180 cm−1 , for N H stretching, and at 1400 cm−1 , for OH bending (Fig. 1c). The C O band for 1740 cm−1 was found to have disappeared, while new absorption bands for the hydroxamic acid group were noted to appear at 3180, 1682, 1652, and 1401 cm−1 , confirming the successful synthesis of a hydroxamic acid ligand from poly(methyl acrylate) grafted kenaf cellulose.

3.2. FE-SEM, HR-TEM and BET analysis FE-SEM measurement was performed with JEOL (JSM-7800F). The SEM micrograph of kenaf cellulose showed a morphology resembling a wooden stick, with no impurities observed (Fig. 2a). The SEM micrograph of the poly(methyl acrylate) grafted kenaf cellulose showed that distinguishable grafting occurred on the surface of this stick-like cellulosic structure, with a rough surface surrounding the stick resulting from the PMA grafting (Fig. 2b). The poly(hydroxamic acid) ligand showed cracking at the surface (Fig. 2c), exhibiting a slightly different morphology compared to the grafting copolymer. However, an enlarged view of the poly(hydroxamic acid) ligand showed that spherical beads were present, as shown in Fig. 2d. After adsorption of copper(II) metal ions, the poly(hydroxamic acid) ligand was characterized by compact spherical shapes (Fig. 2e) of differing sizes. HR-TEM was measured with a Hitachi instrument (HT-7700), and a nanoscale micrograph (Fig. 2f) showed scattered Cu2+ complexes (average

5 nm size), strongly evidencing that adsorption by the polymeric ligands took place. The textural properties of the synthesized poly(hydroxamic) acid ligand were analysed using the isothermal nitrogen adsorption–desorption method. Pure poly(hydroxamic) acid ligand exhibits a typical isotherm of type III with a H3 hysteresis loop, as can be seen in Fig. 2g. A gradual increase in adsorption at relative pressures of 0.5–0.9, and a steep increase in adsorption at relative pressures of 0.9–0.99 were attributed to the capillary nitrogen condensation in the polymer ligand (Fig. 2g). The BET surface area, pore volume, and average pore diameter were 110.5 m2 g−1 , 0.51 cm3 g−1 , and 10.5 nm, respectively. Although the surface area of this polymer ligand is smaller in comparison to reported materials [22,23], it should nonetheless be considered as a promising sorbent in view of the high density of functional groups such as the ligand-metal complexation system.

3.3. Reaction mechanism The mechanism of the grafting reaction of acrylic monomers with starch or cellulose materials through use of the free-radical initiation method has been extensively reported in the literature [25,26]. Recent studies have proposed an alternative mechanism of cellulose units containing primary OH groups; metal ions form a free radical on the oxygen atom, which then reacts with vinyl or acrylic monomers for copolymerization reactions [27]. In this study, kenaf cellulose is grafted with methyl acrylate using a freeradical chain reaction with a ceric ion as the initiator (Scheme 1). The ceric (IV) ion forms a complex with the OH groups of the glucose units in the kenaf cellulose, and the hydrogen atom is oxidized by reduction of the Ce4+ ion to a Ce3+ ion. The cellulose free radicals then induce the initiation of grafting by addition of a double bond in the acrylonitrile monomer, resulting in the formation of a radical which can be then submitted to a subsequent propagation reaction. The termination reaction of the growing polymer chain consisting of cellulose-monomer molecules results in the combination of grafting, as shown in Scheme 1, although termination by disproportionation is also possible [28].


599

Fig. 2. FE-SEM micrographs of (a) kenaf cellulose, (b) PMA grafted cellulose, (c) poly(hydroxamic acid) ligand (d) enlarged view of poly(hydroxamic acid) ligand (e) poly(hydroxamic acid) ligand after adsorption of Cu2+ , (f) HR-TEM micrographs of poly(hydroxamic acid) ligand after adsorption of Cu2+ , (g) Nitrogen adsorption–desorption isotherms of poly(hydroxamic acid) ligand.

3.4. Poly(hydroxamic acid) ligand A free-radical polymerization process was used to synthesize the kenaf cellulose-graft-poly(methyl acrylate) copolymer from a reaction between kenaf cellulose and a methyl acrylate monomer. The optimum reaction conditions of graft copolymerization for cellulose (AGU), mineral acid (H2 SO4 ), ceric ammonium nitrate (CAN), and methyl acrylate (MA) were found to be at

0.030, 0.038, 0.005, and 0.621 mol L−1 , respectively. Subsequently, the grafted copolymer with an acrylate group was reacted with hydroxylamine for conversion of the polymeric chelating ligand known for having poly(hydroxamic acid) functional groups, using a Beckmann- or Lossen-type rearrangement (Scheme 2). Hence, the hydroxamic acid functional group participates to enhance the significant metal ion binding properties of this ligand.

600


OH H

H H H

O

OH

Ce4+

H

O

HO OH

HH H

+

Cell

.

CellH CH2

CH

n H2C

CellH CH2

CH C

O

CH

n

O

CH3

O

O

H3C

CellH CH2

CH

n

CH2

CHH

O

n

CH2

C CH3

O

O

H3C

CH

O

C

C O

CellH CH2

CH

CH

O

O C

C

.

O

O

H3C

CH3

CHH C

O

O

CH3 termination

CH2

C

O

H3C

CH3

.

propagation

O

O

H+

.

CHH C

O

+

OH

Cell

initaition

Ce3+

O OH

C

OH

HO

OH

H2C

.

CH2

CH

CH2

n

HCell

C O

O

O

CH3

CH3

Scheme 1. Graft copolymerization of methyl acrylate onto kenaf cellulose (Cell • i s a glucose unit of cellulose).

H3C

CH3

C

C CellH CH2

CH

n

CH2

C

CH2

CH

n

HCell

C O

O

CH2

CH

CH

O

O

O

O

O

CH3

O

CH3 NH2OH

HO

OH C

C CellH CH2

CH

n

CH2

C HN OH

CH

CH

NH

NH O

O

CH2

CH

CH2

n

HCell

C O HN

Fig. 3. Effect of solution pH for Cu2+ ion sensing by polymeric ligand (150 mg) at different pH conditions, with 6 mg L−1 of Cu2+ ion at 30 ◦ C in 20 mL volume for 2 h.

O

OH

3.5. Optical detection of metal ions

Scheme 2. Kenaf cellulose-g-PMA converted into poly(hydroxamic acid) ligand.

Following the methodology developed in our previous works [28], the amidoximation reaction was maintained with a methanolto-water ratio of 4:1 (vv), with optimum amidoximation achieved at pH 11 and 70 ◦ C for 4 h. The H-form ligand was obtained after treatment with a 0.1 M HCl solution. The physical and chemical properties of the poly(hydroxamic acid) ligand are summarized as follows: percentage of grafting (Gp = 135), swelling capacity (14%), average exchange rate (t1/2 = 10 min), and highest adsorption capacity (304.9 mg g−1 ).

3.5.1. Effect of solution pH The pH of the solution was found to be a significant factor when using the ligand to sense transition metal ions. The effect of solution pH is a key factor for selective optical detection of metal ions [29,30]. The reflectance spectra of the [Cu-ligand]n+ complex was observed over a wide pH range, spanning from 2 to 9. The amount of Cu2+ ion adsorbed by the polymeric ligand was adequate for attainment of good color separation (signal) between the ligand (blank) and Cu2+ ion-sensing samples, as shown in Fig. 3. The ligand was robust at pH 6–7 for optical color intensity and signal response for Cu2+ ions, in which the highest absorbance was 99.5% at pH 6. The obtained results suggest that the ligand has a high functionality and affinity toward the Cu2+ ion at pH 6, which was attained with


601

Table 2 Colour optimization with increasing concentration of Copper, Cobalt, Chromium, Nickel & Iron at pH 6.

Fig. 4. Colour optimization with increasing concentrations of Cu(II) ions at pH 6 with reflectance spectra.

3.6. Color optimization In this study, the adsorbent exhibited high physical and textural properties for visual inspection of transition metal ions. The reflectance spectra was noted to increase as the Cu2+ ion concentration was increased from 0 to 15 ppm, with all other experimental conditions held constant, as shown in Fig. 4. In addition, a broad peak at approximately 673 nm was created when the Cu2+ ion was adsorbed by the ligand, whereas the blank polymeric ligand did not show a peak at 673 nm. The reflectance spectra of the polymeric ligand exhibited a new peak at approximately 673 nm with addition of the Cu2+ ion, indicating the presence of the charge transfer (␲–␲ transition) complex. In addition, other transition metal ions also developed new peaks at 685, 675, 680, and 688 nm for Ni2+ , Fe3+ , Cr3+ , and Co2+ , respectively (see Fig. S3 at ESI). Color optimization of selected metal ions is shown in Table 2. It is evident that the increase in absorbance corresponds to equilibrium color formation between ligand and metal ions; thus, the increased absorbance is sensitive for determination of metals in ultra–trace concentrations [23]. 3.7. Adsorption of metal ions by ligand 3.7.1. Effect of pH on the removal of metal ions To study the effect of pH on adsorption behavior, the polymer chelating ligand was evaluated over a pH range spanning from 3 to 6, with pH adjustments performed through addition of a sodium acetate buffer solution, and the adsorption behavior of the polymeric ligand determined by the binding of selected transition metal ions. The obtained analytical results demonstrated that the adsorption capacities of the selected metal ions were found to increase as pH was increased from 3 to 6. The ligand was shown to have high affinity towards the Cu2+ ion in the neutral pH region; on the other hand, other common transition metals such as iron, cobalt, manganese, and chromium showed

Sorptoon capacity (mg/g)

the use of a sodium acetate buffer solution (where a 0.1 M sodium acetate solution was used to adjust pH by addition of acetic acid). For other transition metal ions, the [M-ligand]n+ showed similar behavior, with the highest absorbance ranging from 97–99% at pH 6 (see Fig. S2 at ESI). Therefore, pH 6 was chosen as the optimum experimental condition in the optical recognition system for a high sensitive response towards metal ions. These findings justify that the selective recognition of target metal ions by a ligand-supported adsorbent at a specific pH region is an important factor in the capture of selective metal ions [29,30].

300 250 200

Cu Zn Ni Co Mn Cr Fe

150 100 50 3

4

5

6

7

pH Fig. 5. Metal ion adsorption capacity by the ligand as a function of pH. Reaction conditions were 150 mg of dried ligand, 10 mL of 0.1 M sodium acetate buffer solution at pH 3–6, and 10 mL of 0.1 M metal ion solution shaken for 2 h.

higher affinities at pH 6. The binding capacities of Cu2+ , Fe3+ , Mn2+ , Co2+ , Cr3+ , Ni2+ , and Zn2+ were found to be 305.3, 275.6, 258.5, 256.6, 254.3, 198.5, and 190.1 mg g−1 , respectively, at pH 6 (Fig. 5). The uptake of metal ions by the ligand was found to be pH-dependent. The adsorption capacity of the synthesized chelating ligand towards the metal ions can be represented in the order Cu2+ > Fe3+ > Mn2+ > Co2+ > Cr3+ > Zn2+ > Ni2+ . The ligand functional group of the polymeric adsorbent was observed to be very active in complexation with metal ions. However, the kenaf cellulose modified ligand was found to be pH sensitive for target ion detection and removal [15,21,28]. After batch adsorption with metal ions, a highly coloured polymer chelating ligand was observed to appear as a result of the formation of complexes of hydroxamic acid groups with the metal ions. The hydroximate anions, as bidentate ligands, trapped the metal ions and formed five-membered ring complexes [5]. The

602


CellH CH2 HN O

CH

n

CH

CH2

C

C O HN

O

Adsorbate

Pseudo-first-order

O

Cu

Cu O

O CH

Cu Fe Co Cr Ni Mn Zn

O

NH

C CH2

Table 3 Adsorption kinetic parameters of the pseudo-first-order model for transition metals on the poly(hydroxamic) acid ligand.

CH2

O C

CH2

NH

CH CH2

HCell n

Experimental

qe (mg g−1 )

K1 (min−1 )

R2

qm (mg g−1 )

292.9 279.8 287.5 266.0 197.7 274.7 191.3

0.0276 0.0262 0.0261 0.0258 0.0246 0.0264 0.0247

0.997 0.993 0.988 0.988 0.990 0.994 0.995

305.3 275.6 256.6 254.3 198.5 258.5 190.1

Fig. 6. Polymeric ligand with metal ions.

3.0

0.5

Cu Fe Cr Co Ni Mn Zn

log (qe - qt)

2.0 1.5 1.0

0.4 0.3

t/qt

2.5

Cu Fe Cr Ni Co Mn Zn

0.2 0.1

0.5

0.0

0.0

0 0

10

20

30

40

50

60

70

80

10

20

30

90

40

50

60

70

80

90

Time (min)

Time (min) Fig. 7. Kinetic plots of pseudo-first-order model for transition metals on the poly(hydroxamic) acid ligand. Other conditions: 0.1500 g of dried ligand, 10 mL of 0.1 M sodium acetate buffer, 10 mL of 0.1 M metal ion solution.

metal ions, in turn, were bound to both oxygen atoms; the chelate complex is shown in Fig. 6.

Fig. 8. Kinetic plots of pseudo-second-order model for transition metals on the poly(hydroxamic) acid ligand. Other conditions: 0.1500 g of dried ligand, 10 mL of 0.1 M sodium acetate buffer, 10 mL of 0.1 M metal ion solution. Table 4 Adsorption kinetic parameters of the pseudo-second-order model for transition metals on the poly(hydroxamic) acid ligand. Adsorbate

3.8. Adsorption kinetic As shown in the literature, in complexation reaction mechanisms, the kinetic of sorption for the uptake of target ions is slower than the kinetics of ion-exchange and hydrogen-bonding reaction mechanisms [22,23]. Thus, in order to determine the required contact time between metal ions (Cu2+ , Fe3+ , Co2+ , Mn2+ , Cr3+ , Ni2+ , and Zn2+ ) and the polymeric ligand for selected metal ion removal, a series of batch contact time experiments were performed. The time-dependent metal ion sorption by the ligand was determined using the filtrate solution analyzed by ICP-OES. Several metal ions were used to study the rate of adsorption in a buffer solution at pH 6. The efficiency of adsorption can be explained by the kinetic model, allowing for an appropriate understanding of the mechanism of adsorption. A pseudo-first-order kinetic equation is widely used for the adsorption of solute from solution, as shown in Eq. (5): k1 × t log(qe − qt ) = logqe − 2.303

(5)

where qt and qe are the adsorption capacity at time t and at equilibrium (mg g−1 ), respectively, and k1 is the rate constant of the pseudo-first-order adsorption process (min−1 ). The values of qe and k1 can be determined from the intercept and slope of plots of log (qe –qt ) versus t (Fig. 7); corresponding values are shown in Table 3. Although R2 values are acceptable for all adsorbates, the experimental values of adsorption capacity (qm exp.) show a significant difference compared to the calculated values (qe cal.) from the

Cu Fe Co Cr Ni Mn Zn

Pseudo-second-order

Experimental

qe (mg g−1 )

K2 (g mg−1 min−1 ) × 10−3

R2

qm (mg g−1 )

315.4 284.8 282.3 239.2 209.6 255.1 243.9

0.0121 0.0170 0.0230 0.0244 0.0292 0.0296 0.0296

0.999 0.985 0.975 0.986 0.981 0.977 0.975

305.3 275.6 256.6 254.3 198.5 258.5 190.1

first-order plot (Table 3). These results suggest a poor fit between the pseudo-first-order model and the experimental data. The pseudo-second-order model represents the adsorption rate relationship, with differences in adsorption capacities at equilibrium and at different contact times. The pseudo-second-order kinetic model is given in Eq. (6): 1 t t = + qt qe k2q2e

(6)

where k2 is the rate constant of the pseudo-second-order sorption (g mg−1 min−1 ), qe is the amount of metals adsorbed (mg g−1 ) at equilibrium, and qt is the amount of adsorption (mg g−1 ) at time t. The values of k2 and qe can be calculated from a plot of t/qt versus t (Fig. 8); corresponding values are presented in Table 4. While a significant difference was found between the parameters depicted in Tables 3 and 4, Table 4 shows that the correlation coefficients for the pseudo-second-order adsorption are high, and that the calculated qe values agree well with the experimental values. These results

M.L. Rahman et al. / Sensors and Actuators B 242 (2017) 595–608 Table 5 Langmuir and Freundlich isotherm parameters obtained by nonlinear fitting for the poly(hydroxamic) acid ligand.

Cu Fe Co Cr Ni

Langmuir

Cu - Langmuir Fe - Langmuir Cr - Langmuir Ni - Langmuir Co - Langmuir

350 300

Freundlich KL (L g−1 )

R2

n

KF (L mg−1 )

R2

312.1 274.6 263.5 230.1 212.5

0.171 0.152 0.150 0.129 0.117

0.971 0.986 0.976 0.962 0.964

0.172 0.154 0.153 0.129 0.125

73.75 71.04 57.96 56.42 44.80

0.979 0.987 0.960 0.954 0.977

-1

qm (mg g−1 )

qe (mg L )

Adsorbate

603

250 200 150

Cu - Freundlich Fe - Freundlich Cr - Freundlich Ni - Freundlich Co - Freundlich

100 50

suggest that the second-order mechanism is predominant, indicating that the chemical process is an adsorption mechanism occurring due to valence forces resulting from the sharing or exchange of electrons between the transition metal ions and the poly(hydroxamic) acid ligand [31,32].

Sorption isotherm studies were carried out to understand the nature of metal uptake at various metal ion concentrations. Transition metal ions (Cu2+ , Fe3+ , Co2+ , Cr3+ , Ni2+ ) were adsorbed by the cellulose-based polymeric ligand. Initial concentrations of metals were increased from 5 to 1500 mL L−1 as ligand dose, pH, and agitation period were held constant. The obtained results indicated that adsorption capacity increased in relation to increases in initial concentrations of metal ions, and that adsorption values also gradually increased, but only up to a certain limit. 3.9.1. Nonlinear forms of the isotherm models The Langmuir isotherm theory represents a saturated monolayer of solute molecules on the adsorbent surface with no migration of adsorbate molecules, and a constant energy of adsorption [32]. The nonlinear Langmuir isotherm model is expressed as Eq. (7): qe =

qm KL Ce 1 + KL Ce

where equilibrium values of qe and Ce are defined as Eq. (7), KF is the Freundlich constant (L mg−1 ) indicating the adsorption capacity, and 1/n is related to the heterogeneity factor, and indicates the adsorption capacity. Here, n gives the degree of non-linearity, in which if n = 1, then the adsorption is linear, but if n < 1, the adsorption is nonlinear [33]. All model parameters were evaluated by both nonlinear regression and the linear least-squares method using Origin 8.0 software. Fig. 9 shows Langmuir and Freundlich adsorption isotherms of the polymer adsorbents by nonlinear analysis, while the values of the corresponding isotherm parameters are shown in Table 5. The maximum adsorption capacity values determined by the Langmuir model were 312.1, 274.6, 263.5, 230.1, and 212.5 mg g−1 for Cu, Fe, Co, Cr, and Ni, respectively. All adsorption values obtained by the Langmuir model were close to the experimental adsorbed amounts, indicating that the Langmuir modeling for the adsorption system is in good agreement with the experimental results.

400

600

800

1000

1200

1400

-1

Fig. 9. Nonlinear fitting of Langmuir isotherms (solid lines) and Freundlich curves (dot lines); experimental conditions: initial metal ion concentration range 5–1500 mg L−1 , sample dose 150 mg/20 mL, solution pH 6, temperature 32 ◦ C, contact time 2 h.

6

Cu Fe Cr Ni Co

4

2

0 0

(8)

200

Ce (mg L )

(7)

Where qe is the equilibrium adsorption capacity of metal ions on the adsorbent (mg g−1 ), qm is the maximum capacity of the adsorbent (mg g−1 ), KL is the Langmuir adsorption constant (L mg−1 ), and Ce represents the equilibrium concentration of metal ions in the solution (mg L−1 ). The Freundlich model describes the adsorption of a reversible heterogeneous surface, which can be applied to multilayer adsorption [31]. The nonlinear empirical equation of the Freundlich isotherm model is described as Eq. (8): qe = KF Ce1/n

0

Ce/qe (g/L)

3.9. Sorption isotherms

0

250

500

750

1000

1250

1500

Ce (mg/L) Fig. 10. Linear fitting of Langmuir adsorption isotherms of metal ions; experimental conditions: initial metal ion concentration range 5–1500 mg L−1 , sample dose 150 mg/20 mL, solution pH 6, temperature 32 ◦ C, contact time 2 h.

3.9.2. Linear forms of isotherm models The Langmuir and Freundlich isotherm models are the most common isotherms models used to describe and investigate the equilibrium data of adsorption from aqueous solutions [31]. The Langmuir isotherm model is derived to model the assumptions of monolayer adsorption, a certain number of identical active sites, active sites distributed on the surface of the adsorbent, and no interaction between adsorbents and adsorbates [31]. The linear form of the Langmuir isotherm equation is described by the following Eq. (9): Ce 1 Ce = + qe qm KL qm

(9)

Where equilibrium values of qe and Ce are defined as Eq. (7), while qm and KL represent the maximum adsorption capacity of adsorbents (mg g−1 ) and the Langmuir adsorption constant (L mg−1 ). Data was utilized according to the Langmuir sorption isotherms model (linear form) by plotting Ce /qe against Ce, as shown in Fig. 10. The values of qm and KL are calculated from the slope and intercept of the linear plot of Ce/qe against Ce. The data suggests that the metal ion sorption onto ligand is well fitted with the Langmuir isotherm model, as indicated by the R2 values (>0.99). The calculated data for the maximum sorption capacity (qm ) and the sorption

604


Table 6 Langmuir and Freundlich isotherm parameters obtained by linear fitting for the poly(amidoxime) ligand. Adsorbate

Cu Fe Co Cr Ni

Langmuir

Freundlich

qm (mg g−1 )

KL (L g−1 )

R2

n

KF (L mg−1 )

R2

303.9 272.4 265.2 231.4 209.6

0.011 0.010 0.008 0.011 0.009

0.994 0.991 0.990 0.994 0.993

2.739 2.724 2.570 2.610 2.579

7.452 8.776 12.57 13.70 16.50

0.938 0.967 0.989 0.980 0.984

3.00 2.75

log qe (mg/g)

2.50 Fig. 12. Wide scan of X-ray photoelectron spectra of poly(hydroxamic) acid ligand with copper sorption (a) and poly(hydroxamic) acid ligand (b).

2.25 2.00

Cu Fe Cr Ni Co

1.75 1.50 1.25 1.00 0.5

1.0

1.5

2.0

2.5

3.0

3.5

log Ce (mg/L) Fig. 11. Freundlich isotherms for the adsorption of metal ions; experimental conditions: initial metal ion concentration range 5–1500 mg L−1 , sample dose 150 mg/20 mL, solution pH 6, temperature 32 ◦ C, contact time 2 h.

coefficient KL are presented in Table 6. The value of qm obtained from the linear plot of the Langmuir isotherm model correlates well with the measured qm , where copper adsorption is shown to be higher than adsorption of nickel, iron, chromium, and cobalt, which suggests that monolayer adsorption by the polymeric ligand has occurred. Owing to their mathematical simplicity, linear isotherms models are also widely adopted to determine isotherm parameters, or the best fitted model for adsorption systems. The Freundlich isotherm model, known as a multilayer model of adsorption and adsorption on heterogeneous surfaces [31,32], is exemplified by the following Eq. (10): logqe = logKF +

logCe n

(10)

where KF and n are the Freundlich constants, namely the adsorption capacity and the adsorption strength, respectively. KF and n can also be calculated from the intercept and the slope of the linear plot of log qe versus log Ce . The obtained experimental data was utilized to test the Freundlich isotherm model by plotting the log of qe versus Ce , as shown in Fig. 11. The calculated KF and n values are presented in Table 6. The Freundlich equation shows a non-significant correlation (R2 < 0.95) with the experimental data, which suggest that no multilayer adsorption occurred by the selected transition metals. A comparison between the results obtained by the Freundlich adsorption model and experimental data is presented in Table 6. 3.9.3. Comparison of maximum adsorption capacities (qm ) The maximum adsorption capacities (qm ) of the adsorbents, which were calculated using both the linear and nonlinear Langmuir models according to the reported method [33], were compared in this study. The obtained results clearly show that the theoretical values of qm obtained from the nonlinear Langmuir model are close to experimental values. The values of qm obtained

from the linear Langmuir model are also close to experimental values for Cu, Fe, and Ni, although the obtained value for Cr differs significantly from experimental data. Differences (Da ) between qm values derived from the nonlinear Langmuir model and the experimental data are presented in Table 7. Similarly, differences (Db ) between qm values derived from the linear Langmuir model and the experimental data are also presented in Table 7. The obtained results indicate that the adsorption system of the transition metal ions is coordinated with the amidoxime-functionalized polymeric ligand for metal removal, although the results derived from the linear fitting of the isotherm models can cause discrepancies (Table 7). 3.9.4. Wastewater purification Table 1 shows the concentrations of transition metals in two different wastewater models before and after treatment by poly(hydroxamic acid) at fixed reaction parameters. The wastewater purification power of the poly(hydroxamic acid) process can reach percentages as high as 99.92% for both the higher concentration wastewater model (ww1) and the low concentration wastewater model (ww2). Real wastewater samples obtained from a local electroplating plant (ww3) differed in raw composition from wastewater models, with relatively higher concentrations of chromium, nickel, copper, and iron ions. As a proof of concept, the polymer ligand was applied for transition metal removal of these real samples using the batch method. Results from the ICP analysis demonstrated that the metal ion removal was 99.70% on average (Table 1), indicating that the created ligand is a suitable candidate for metal removal by batch method from industrial wastewaters, especially electroplating wastewater. 3.9.5. Adsorption mechanism of metal ions sorption by polymer ligand Efficient adsorption of copper cations by the poly(hydroxamic acid) ligand is achieved by virtue of the strong chelating ability of the hydroxamate groups, which act as bidentate ligands. X-ray photoelectron spectra (Scanning X-ray Microprobe PHI Quantera II) were obtained to interpret the sorption mechanisms of Cu2+ on the polymeric ligand. The wide scan XPS spectra of the polymer ligand after adsorption of copper were compared to spectra taken prior to sorption, as shown in Fig. 12. Peaks at binding energies (BEs) of 284.0, 399.9, and 530.8 eV, corresponding to the C 1s, N 1s, and O 1s spectra, respectively, were observed for both adsorbed and unadsorbed samples (Fig. 12a and b). The adsorption of Cu2+ is evident by the appearance of two new peaks with BEs of 933.1 eV and 953.0 eV for the signals of Cu2p3/2 and Cu2p1/2, as shown in Fig. 12b.


605

Table 7 Comparison between the maximum adsorption capacities by the Langmuir nonlinear and linear isotherm models. Adsorbate

Exp. measured −1

qm (mg g Cu Fe Co Cr Ni

305.3 275.6 256.6 254.3 198.5

)

Nonlinear Langmuir −1

qm (mg g

)

312.1 274.6 263.5 230.1 212.5

Linear Langmuir −1

qm (mg g

)

303.9 272.4 265.2 231.4 209.6

Da

Db −1

(mg g

−6.8 1.0 −6.9 24.2 −14.0

)

(mg g−1 ) 1.4 3.2 −8.6 22.9 −11.1

Da means the differences between the maximum adsorption capacities of nonlinear model and experimental data. Db means the differences between the maximum adsorption capacities of linear model and experimental data.

A detailed investigation of sorption mechanisms was undertaken to further understand the interaction between metal ions and the polymer chelating ligand. For this purpose, the N 1s and O 1s XPS spectra of the poly(hydroxamic) acid ligand before and after adsorption of Cu2+ were submitted to analysis. The core-level N 1s XPS spectra for both samples are shown in Fig. 13a and b. The N 1s peak of the polymer ligand exhibits two peaks at BEs of 398.9 and 400.1 eV, belonging to the nitrogen atoms in the N H and N OH species, respectively (Fig. 13a). After the adsorption of Cu2+ , both peaks for the N H and N O species are slightly shifted to the 399.6 and 400.7 eV alongside the peaks of N 1s (Fig. 13b). The O 1s XPS spectra of the polymer ligand also exhibits two peaks at BEs of 530.5 and 532.0 eV, which can be attributed to the oxygen atoms in the HN OH and C O species, respectively (Fig. 14c). After adsorption of Cu2+ , a new peak at BE of 533.0 eV appeared alongside the peaks of O 1s, 530.4 and 531.9 eV for the HN OH and C O species, respectively (Fig. 13d). These results also evidence that the oxygen atoms in the HN OH and C O species contribute to the formation of coordinate bonds between ligand and metal ions. These results demonstrate that the hydroxamate group of the polymer ligand is the active group that forms coordinate bonds with Cu2+ after adsorption. The lone pair of electrons in the oxygen atoms of the hydroxamate group is donated to form a coordination bond between Cu2+ and the oxygen atoms [34]. It is well documented that when a sorbate is adsorbed on a sorbent through chemical interactions, the chemical state of atoms involved in the sorption process could be changed, resulting in different X-ray photoelectron spectra [35]. After sorption, the O 1s core-level spectrum showed an additional peak at 533.0 eV (Fig. 14b), indicating that O atoms are electron donors during the copper sorption. Since a lone pair of electrons is present in the nitrogen atom, this pair of electrons can be donated to form a coordination bond between a copper species and an oxygen atom. As a consequence, the electron cloud density of the oxygen atom is reduced, resulting in higher BE peaks [35]. Therefore, it can be concluded that the sorption of Cu2+ on the polymer ligand is associated with oxygen atoms belonging to the hydroxamate groups.

3.9.6. Elution and reusability studies In order to evaluate the reusability of the ligand, elution experiments were carried out after removal/sorption tests to evaluate elution and regeneration, as these are important factors for practical applications from the point of view of cost-effectiveness [22,23]. The release of metal ions from the ligand can be accomplished with the use of acidic conditions, as sorption of metals was shown to be very low at pH 3. For such cases, a below pH zero solution can be used for complete extraction of metal ions from the ligand. In this study, 2 M of HCl solution was used to extract the adsorbed Cu2+ ions from the adsorbent. The adsorbent was regenerated into its initial form after rinsing with water several times and by adding a buffer of pH 6 after every elution experiment for reusability of the ligand. The reusability of the ligand was examined by the sorption/elution process for seven cycles. The sorption

process was performed by stirring 150 mg of ligand with 10 mL of 0.1 M Cu2+ solution at pH 6 for 2 h. A desorption study was performed by adding 20 mL of 2 M HCl solution. Sorption/removal and extraction efficiencies were noted to decrease by only 9% after 12 cycles, as estimated from Fig. 14. This polymeric ligand can be reused for 12 cycles without any significant loss of its removal performance. Therefore, the ligand shows high structural stability, which promotes its application towards Cu2+ ion removal from environmental wastewater effluents.

4. Discussion Ions or molecules possessing a pair of non-bonding electrons that bind metals can be defined as ligands. A range of functional or ligand groups in the modified cellulose adsorbent material contain elements in groups V (nitrogen) and VI (oxygen) of the periodic Table. In this report, a polymeric chelating ligand containing the hydroxamic acid ligand was synthesized and introduced in the metal ion adsorption study. This ligand exhibited high affinity to copper (305.3 mg g−1 ) at pH 6, while other metals such as iron, chromium, manganese, and cobalt also displayed excellent uptake at pH 6. In comparison with other cellulose-based adsorbents, our prepared ligand from kenaf cellulose showed higher adsorption capacity. In other work, Liu et al. [36] synthesized a spheroidal cellulose adsorbent through a grafting reaction using acrylonitrile, converting carboxyl groups on its surface. This modified cellulose adsorbent, having carboxyl groups, was used for the removal of Cu ions from aqueous solutions using a bidentate complexation. In related work, Low et al. [37] modified wood pulp by esterification reaction to form carboxyl groups from the citric acid and hydroxyl groups present in the wood surface for subsequent adsorption of Cu and Pb ions from aqueous solutions. A lone pair of electrons exists on the nitrogen of the amino group, which can be used to form covalent bonds with metals. For example, hydroxamic acid groups have a bidentate ligand that loses a proton and a basic lone pair of electrons on the nitrogen that coordinate with metal ions. O’Connell et al. [38] synthesized imidazole by using a binding agent on a glycidyl methacrylate grafted cellulose adsorbent. Imidazole has a five-membered ring containing two nitrogen atoms. An important aspect of unsaturated nitrogen donors such as imidazole is the possibility of p-backbonding between nitrogen donors and metal ions [39]. It has been clearly shown that significant adsorption capabilities can be achieved with modified cellulose materials [39]. Significant variations in adsorption levels for each cationic species can be observed depending on the cellulose modification method used and the nature of the chelating or metal-binding ligands. The detailed mechanism of each adsorption process is difficult to recognize, but a number of fundamental interactions are conceivable, such as ion exchange, complexation, coordination/chelation, electrostatic interactions, acid–base interactions and hydrogen bonding, hydrophobic interactions, physiadsorption, and possibly precipitation [40]. However, the chemical and

606


100

% Removal % Extraction

Efficiency (%)

95 90 85 80 75 70

1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th11th 12th

Number of Cycles Fig. 14. Reusability studies of the polymeric ligand in several cycles of sorption/removal − extraction experiments.

Fig. 13. N 1s core-level XPS spectra of poly(hydroxamic) acid ligand (a) and poly(hydroxamic) acid ligand with copper (b). O 1s core-level spectra of poly(hydroxamic) acid ligand (c) and poly(hydroxamic) acid ligand with copper (d).

physical composition of the modified cellulose, the nature of the metal, and solution conditions such as pH, metal concentration, and the solubility product principle can affect the adsorption interaction. In another study, cellulose was grafted with the vinyl monomer glycidyl methacrylate using a chemical initiating process, and the cellulose grafted copolymer was further functionalized with thiosemicarbazide for adsorption of Cd2+ and Hg2+ from aqueous solutions [41,42]. The authors further extended the cellulose grafted copolymer with ␤-CD (cyclic oligosaccharides) and quaternary ammonium groups to build a cellulose-g-GMA␤-CDN+ adsorbent, where the maximum adsorption capacity of Cr6+ reached 61.05 mg g−1 . Adsorption-desorption tests of cellulose derivatives exhibited good reproducibility for the adsorbent, showing that the adsorbent could be reused [42]. The mercaptobenzothiazole adsorbent is prepared from cellulose for the adsorption of Hg2+ , where a high adsorption capacity of 204.08 mg g−1 is obtained with the use of the column method [43]. Microwave-induced emulsion copolymerization was conducted for acrylic monomer ethylacrylate and guar gum. The copolymer sample was used to uptake the cadmium ion, and authors claimed that the adsorbent exhibited high reusability and could be successfully recycled [44]. A new adsorbent was synthesized from graft copolymerization of glycidylmethacrylate onto zirconium oxide densified cellulose in the presence of N, N’-methylenebisacrylamide as a cross-linker, having tannin-modified poly(glycidylmethacrylate)grafted zirconium oxide-densified cellulose [45]. The optimum pH for maximum adsorption was found to be 5.5 with 99.2% removal at an initial concentration of 10 mg L−1 . The maximum adsorption capacity was found to be 96.7 mg g−1 for complete removal of Th4+ from simulated seawater. The authors further reported the development of a glycidyl methacrylate grafted cellulose densified quaternary ammonium adsorbent for removal of Cr6+ ion [46]. Dragon et al. [47] developed a potato starch grafted poly amidoxime-chitosan composite for the decontamination of Cu2+ ion from water, with a reported maximum adsorption of 238.14 mg g−1 . The nitrogen and oxygen atoms of the amidoxime group, having a lone pair of electrons each, bind the metal ion through complex formation, enabling Cu2+ ion removal. Another amidoxime-containing adsorbent was reported in the literature, with adsorption capacities of 146.05, 133.86, and 236.2 mg g−1 for Cu2+ , Ni2+ , and Pb2+ ions, respectively [48]. Saravanan et al. [49] modified cellulose with azomethine, forming pendent methyl formylimino groups; the resulting ligand showed adsorption capacities of 157.3 and 153.5 mg g−1 for Cu2+ and Pb2+ ions,


607

Table 8 Adsorption of some metal ions reported in the literature [36–51]. Adsorbent Materials Cellulose Wood pulp Cellulose Chitosan Cellulose Cellulose Guar Gum Cellulose Cellulose Potato starch/Chitosan Resin Cellulose Cellulose Cellulose Cellulose

Modifier

Metal ions

Sodium hydroxide(carboxyl) [36] Citric acid [37] Glycidylmethacrylate [38] Tripolyphosphate [40] ␤-CD and quaternary ammonium [44] Mercaptobenzothiozole [43] Ethyl acrylate [44] Glycidylmethacrylate [45] Glycidyl methacrylate densified quaternary ammonium [46] Poly(amidoxime) ligand [47] Poly(amidoxime) [48] Azomethine [49] Hyper branched aliphatic polyester [50] succinylated mercer-ized [51] triethylenetetramine [51] Poly (hydroxamic acid) ligand(this study)

correspondingly. The adsorption capacity of hyper branched aliphatic polyester grafted cellulose [50] for the Cu2+ ion was reported as only 9.8 mg g−1 , whereas in cellulose modified with succinylated mercerized and triethylene tetramine, adsorption maximums were reported as 56.8 and 69.4 mg g−1 for Cu2+ ; 68.0 and 87.0 mg g−1 for Cd2+ , and 147.1 and 192.3 mg g−1 for Pb2+ ions [51]. This biopolymer cellulose has been utilized along with different modifiers/chelating agents to study the adsorption of other metal ions such as copper, lead, nickel, etc. A comparison of adsorption capacities [36–51] is shown in Table 8; as can be seen, the kenaf-based hydroxamic acid ligand has high adsorption efficiency for several metal ions. 5. Conclusion Successful synthesis was performed to obtain a poly(hydroxamic acid) chelating ligand from poly(methyl acrylate) grafted kenaf cellulose. The chelation behaviour of the ligand towards selected transition metal ions was found to be excellent. The adsorption capacities of the ligand for Cu, Fe, Zn, Ni, and Cr ions were observed to be pH-dependent. Owing to its fast rate of equilibrium, the column technique was determined to be the most efficient method for heavy metal extraction. The low-cost production of the kenaf cellulose-based poly(hydroxamic acid) ligand supports that the method should be considered as an excellent candidate for wastewater treatment processes. In addition, the removal of metal ions was highly efficient, with up to 99% removal of metal ions from water media at low concentrations of metal ions. Further, the purification power of the method towards both synthetic wastewater samples and real wastewater samples obtained from an electroplating plant was shown to be 99.70% on average. A desorption study was also carried out in the present work to determine the reusability of the polymeric ligand. According to the batch adsorptions conducted for the isothermal study, transition metal ion sorption onto ligand was excellently fitted with the Langmuir isotherm model, which suggests that the cellulose-based adsorbent surface is homogenous and has monolayer properties. The results of the sorption/desorption process for 12 cycles demonstrates that the new adsorbent can be reused for many cycles without any significant loss of its original removal performance. Acknowledgement This research was supported by the Ministry of Science, Technology, and Innovation, Malaysia (RDU 130505).

2+

Cu Cu2+ Cu2+ Cu2+ Cr6+ Hg2+ Cd2+ Th4++ Cr6+ Cu2+ Cu2+ /Ni2+ /Pb2+ Cu2+ /Pb2+ Cu2+ Cu2+ /Cd2+ /Pb2+ Cu2+ /Cd2+ /Pb2+ Cu2+ /Fe3+ /Mn3+ /Co2+ /Cr3+

Adsorption capacity (mg g−1 ) 70.5 24.0 68.5 200.00 61.05 204.08 270.27 96.7 123.60 238.14/133.15 146.05/133.86/236.2 157.3/153.5 9.8 56.8/68.0/147.1 69.4/87.0/192.3 305.3/275.5/258.5 /256.6/254.3

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2016.11.007.

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Biographies

Md. Lutfor Rahman received his BSc (Hons) and MSc in Chemistry from University of Rajshahi, Bangladesh. Later he obtained PhD in Polymer Chemistry from Universiti Putra Malaysia in 2000 and he appointed a postdoctoral researcher at UPM. Soon after postdoctoral work (2002), he joined as a lecturer at Universiti Malaysia Sabah (UMS). In 2010, he moved to Universiti Malaysia Pahang as an Associate Professor. In 1016, he again moved to previous Universiti (UMS) as a Professor. Meanwhile he was worked as a short term DAAD fellowship at Martin Luther University, Halle, Germany in 2003. He has wide ranging research interest including the synthesis of novel adsorbents/chelating ligand from biopolymers using metal ions coordination system and also targeting biocatalyst for carbon-carbon bonding reactions. Dr. Md. Shaheen Sarkar received his BSc (Hons) and MSc in Chemistry (2005) from University of Rajshahi, Bangladesh. He joined as a research assistant (2005–2007) at Inha University, South Korea then he moved to Nagasaki University, Japan and obtained PhD in 2010. During his PhD work he developed a new common key intermediate compound which facilitated to synthesis of fostriecin and phoslactomycin family of antibiotics. Later Dr. Shaheen joined as a postdoctoral fellow at RIKEN, Wako, Japan (2010–2013). Currently he is working as an assistant professor of Universiti Malaysia Pahang, Malaysia. His research interests are focusing on Synthetic and catalytic chemistry. Mashitah M Yusoff completed BS and PhD degrees in Chemistry at Wichita State University and MA in Chemistry at Western Michigan University. She is presently Professor at the Faculty of Industrial Sciences and Technology and Deputy Vice Chancellor (Research & Innovation) of Universiti Malaysia Pahang. Her specializations are physical organic and natural products chemistry.

Mohd Harun Abdullah received his BSc in Environmental Science from Universiti Pertanian Malaysia, MSc from Universiti Kebangsaan Malaysia, and PhD from Universiti Teknologi Malaysia. Currently, he is a Professor at the Faculty of Science and Natural Resources at Universiti Malaysia Sabah and Vice Chancellor of the same university (Universiti Malaysia Sabah). He has wide ranging research interest including the environment, hydrology, water quality and analysis, ground water pollution and remediation.