Copper sorption onto a lignocellulosic substrate from wheat bran impregnated with a lipophilic tetraazamacrocycle

Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 126–132

Copper sorption onto a lignocellulosic substrate from wheat bran impregnated with a lipophilic tetraazamacrocycle St´ephanie Sayen a,∗ , Franc¸oise Chuburu a , Emmanuel Guillon a , Michel Aplincourt a , Henri Handel b , Michel Le Baccon b , V´eronique Patinec b a b

GRECI, Universit´e de Reims Champagne Ardenne, B.P.1039, 51687 Reims Cedex 2, France UMR CNRS 6521 Universit´e de Bretagne Occidentale, B.P.809, 29285 Brest Cedex, France

Received 20 January 2006; received in revised form 12 April 2006; accepted 13 April 2006 Available online 25 April 2006

Abstract The topic of this paper is the impregnation with tetraazamacrocyclic ligands of a lignocellulosic substrate extracted from wheat bran. A preliminary N-functionalization by an alkyl pendant arm makes the cyclic tetraamine ligands lipophilic, so as to ensure their immobilization on the substrate. The macrocyclic ligands thus fixed at the surface of the lignocellulosic substrate interact with copper ions in solution to form complexes, as proven by EPR spectra. The end result is an increased retention capacity of the lignocellulosic substrate for pH values lower than 4. Indeed, while the unmodified lignocellulosic substrate does not retain Cu(II) at pH 3, the impregnated substrate exhibits a retention capacity of 4.13 mg g−1 . As a result, this technique of impregnation seems to be a good solution to extend the application field of lignocellulosic substrates, and consider their use in the treatment of acidic effluents. © 2006 Elsevier B.V. All rights reserved. Keywords: Copper(II); Lignocellulosic substrate; Lipophilic tetraazamacrocycle; Impregnation; Adsorption

1. Introduction As the desire for a better environment has become more and more widespread in our communities, the attention of the health authorities has been drawn to this subject, and environmental regulations have become more and more severe. The water is no exception, and its contamination by industrial waste filled with heavy metals is less and less tolerated. As a result, the subject of decontaminating these waste waters before their release into the environment has become important. The methods used to reduce the heavy metal content of these waste waters are rather expensive. They consist in either a chemical precipitation, a solvent extraction, an exchange of ions or an adsorption on activated carbon [1,2]. In the last few years, numerous studies have focused on developing alternative and low-cost sorbents for the removal of heavy metals [1,3]. For instance, studies have investigated the use of biosorbents such as M. rouxii biomass [4], marine green

∗

Corresponding author. Tel.: +33 3 26 91 31 42; fax: 33 3 26 91 32 43 E-mail address: [email protected] (S. Sayen).

0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.04.020

alga Ulva reticulata [5], dried activated sludge [6], Aspergillus niger [7], plant route tissues [8] as well as forest and agricultural by-products including grape stalk wastes [9], wheat shell [10], sawdust of pinus sylvestris [11], apple wastes [12] or peat [13]. For the majority of these substrates, the sorption capacity strongly decreases with the pH. Lignocellulosic substrates (LS) from wheat bran have revealed themselves to be efficient sorbents for metal ions such as Cu(II), Ni(II), Zn(II), Pb(II), or Cd(II) [14–18]. The residues issued from the treatment of wheat bran are known to be able to strongly adsorb copper present in aqueous solution, with a maximum capacity of about 0.3 mmol g−1 (19 mg g−1 ) reached at pH 5 [17]. However, this retention capacity dramatically decreases below this pH value. The chemical modification of substrates represents a good solution to improve their sorbing capacity. The grafting of acrylic acid on starch and the grafting of copolymers on cellulose have been reported to help in the retention of Cu(II) [19,20], Cd(II) and Pb(II) [20]. The graft leads to an increase in the metal ion binding capacity of the material. The nature of the compound incorporated at the LS surface must be judiciously chosen to ensure a good uptake ability. Polyazamacrocyclic ligands such

S. Sayen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 126–132

as cyclen (1,4,7,10-tetraazacyclododecane) or cyclam (1,4,8,11tetraazacyclotetradecane) are known to exhibit strong chelating properties towards transition metal ions such as Cu(II), giving rise to very stable complexes [21,22]. Different tetraazamacrocycles bound to silica gels have proven to be well-suited for the extraction of uranyl ions from aqueous solution [23] or for the removal of lead from drinking water [24]. Other studies have reported on the immobilization of these cyclic polyamines on an organic polymer and the chelating properties of the resulting materials towards Cu(II), Ni(II) and Co(II) [21,25,26]. In particular, a polymer-supported tetraazamacrocycle was applied to the extraction of copper from sea water [26]. However, these ligands have never been used to modify the surface of low-cost solids. The focus of the present study is the impregnation of a LS with the latter ligands in order to enhance its retention capacities towards Cu(II) for pH values lower than 4. It may consequently extend the application of the lignocellulosic residue to a broader pH range and make its use possible in the treatment of acidic effluents. 2. Experimental 2.1. Chemicals and reagents KNO3 , HNO3 and KOH were of analytical grade and used without further purification. Copper(II) stock solution was prepared from Cu(NO3 )2 purchased from Fluka. Cyclen (1,4,7,10-tetraazacyclododecane) was used as received from Strem Chemicals. The LS solid was provided by Agro Industrie Recherche et D´eveloppements (ARD) and was used after acido-basic treatments: (i) an acidic hydrolysis (H2 SO4 10%) was first necessary to remove the hemi-celluloses, polysaccharides, lipids and proteins, (ii) an alkali treatment (pH 8) was then carried out with KOH to dissolve lignins of low molecular weight, (iii) the solid was suspended in a nitric acid solution (pH 3) in order to protonate the surface sites. Successive Soxhlet extractions were finally realized with dichloromethane and ethanol to eliminate soluble molecules in these solvents. After water washing and drying, the resulting residue was ground at a granulometry lower than 100 ␮m using a standard sieve. These different treatments were necessary to obtain a quasiinsoluble substrate, both in solvents where the impregnations will be carried out and in acidic or basic aqueous solutions (for further applications). 2.2. Impregnation of LS Two modified LS were prepared: one impregnated with cyclen and the other impregnated with a lipophilic cyclam-based macrocycle containing a C14 -alkyl pendant arm (Scheme 1). The latter, denoted C14 -cyclam, was prepared according to a previously published procedure [27]. Impregnation was conducted by dispersing 100 mg of LS in 40 mL of ethanol under stirring during 24 h (which corresponds

127

Scheme 1.

to the solid/solution equilibrium time [28]). Then, 10 mL of an ethanolic solution of cyclen or C14 -cyclam was added at a concentration of 2.66 mmol L−1 (which corresponds to an initial concentration of 0.53 mmol L−1 ). After 48 h of mixing, the suspension was filtered and the solid was dried at 40 ◦ C for several hours. Finally, it was suspended in 40 mL of distilled water for 24 h under a constant shaking, filtered and dried again. The macrocyclic resulting materials (cyclen-LS or C14 -cyclamLS) were used for the retention of Cu(II) in aqueous solutions. The behavior of these two solids was compared to the one of the unmodified LS substrate, which had undergone exactly the same treatment: dispersion in ethanol (50 mL) and stirring during 72 h. The impregnation filtrates were kept and analyzed using UV–vis spectroscopy (Perkin Elmer Lambda 6) to quantify the amount of ligand impregnated onto the LS. Tetraazamacrocyles are known to form very stable complexes with Cu(II), thus the rate of impregnated macrocycles onto the LS was deduced from the quantification by UV–vis spectroscopy of the remaining copper complex in solution (in these experiments, the metal ion was added in excess to ensure the whole complexation of the macrocycle). For the titration of cyclen and C14 -cyclam, the absorbance was measured at 603 nm and 522 nm, respectively (d–d transition band of the [Cu(cyclen)]2+ and [Cu(C14 cyclam)]2+ copper complexes). The cyclen UV–vis titration required a signal deconvolution in two Gaussian bands, the first one corresponding to the [Cu(cyclen)]2+ complex, the second one to the free copper ion in excess (Fig. 1). The impregnated amounts on the LS were determined by difference. The washing water was also analyzed in order to study the leaching of the ligands in the aqueous medium. X-ray photoelectron spectroscopy (XPS) data were collected using the facilities of the “Laboratoire de Chimie Physique et Microbiologie pour l’Environnement (LCPME)”, located in Villers-l`es-Nancy, France. Measurements were performed on a Kratos (Axis-Ultra) hemi-spherical spectrometer using an Mg K␣ (1235.6 eV) X-ray source run at 90 W. The X-ray gun was operated at 15 kV and 6.0 mA. The pressure in the sample analysis chamber was about 10−9 mbar. Before measurements, the test chamber was first evacuated for 30 min to remove the residual moisture from the sample. All spectra were calibrated (to correct for the static charging effects) by using the “adventitious” C1s peak at 284.6 eV, which provided a consistent set of data with all compound peaks at their expected values. The overlapping peaks were resolved by the peak synthesis method, applying Shirley-type background subtraction. Spectra were measured

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Fig. 1. UV–vis spectrum of a cyclen solution at a concentration of 2.66 mmol L−1 in the presence of an excess of Cu(II) (about 1.5 times excess over cyclen) dotted line: deconvolution of the signal in two gaussians; dashed line: result of the deconvolution; solid line: experimental signal (the dashed and the solid lines are perfectly superposable).

by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) (Varian Liberty—Series II). The amount of adsorbed copper was deducted by difference with respect to the starting concentrations. The equilibrium time was predetermined by kinetic studies. The sorption experiments as a function of pH were carried out at an initial copper(II) concentration of 2 × 10−4 mol L−1 . Experiments as a function of Cu(II) concentration were conducted at a fixed pH value equal to 5.1 with an introduced metal concentration ranging from 5 × 10−5 mol L−1 to 1.4 × 10−3 mol L−1 . In comparison, experiments were carried out in the same conditions with the unmodified LS. Blank experiments without solid were also performed to check that no Cu(II) was retained during the experiments other than by adsorption on solid particles. The solid state electron paramagnetic resonance (EPR) spectra were recorded at 150 K with a Br¨uker ESP 300 spectrometer operating at X-band frequency of 9.44 GHz with a 100 kHz modulation frequency. The solids were prepared by adsorption of a 2 × 10−4 mol L−1 copper(II) solution at pH 4.1. 3. Results and discussion

on well-powdered samples with a thickness of about 1.0 mm, placed in copper sample holders. Potentiometric measurements were carried out in a thermostated cell at 20 ◦ C under an argon atmosphere using a combined glass electrode and a Dosimat 665 automatic burette (Metrohm) and a 654 pHmeter (Metrohm). Titrations were carried out in aqueous medium at an ionic strength of 0.1 mol L−1 (with using KNO3 ) after a solid/solution equilibrium time of 24 h [28]. The pH values were collected every 3 min. Titrations by 0.1 mol L−1 KOH and HNO3 allowed us to obtain the surface saturation curves necessary for the determination of the acidic and basic sites concentration at the surface of LS and C14 -cyclam-LS. 2.3. Adsorption experiments Adsorption experiments were carried out in batch conditions at 20 ◦ C, by suspending a given amount of impregnated LS in an aqueous solution of 0.1 mol L−1 KNO3 (typically 2 g L−1 ), during 24 h to ensure the hydration of the solid particles [28]. The adjustment of the pH was realized by drop wise addition of 0.1 mol L−1 HNO3 or KOH. The final volume was adjusted at 10 mL by introducing Cu(II) at a selected concentration. After sorption equilibrium under constant stirring, the pH of each suspension was measured and the quantitative analysis of the remaining Cu(II) in solution was performed after filtration

3.1. Impregnation of LS The mixing of LS with 0.53 mmol L−1 of tetraazamacrocycle led to close impregnated amount onto the LS (Table 1). Indeed, 79.2% and 71.7% of cyclen and C14 -cyclam were retained on LS, respectively. Despite the lowest amount of C14 -cyclam impregnated, this ligand was not leached by water during the LS washing after impregnation, whereas a significant quantity of cyclen was lost in solution (about 7%). The hydrophilic nature of cyclen and the lipophilic nature of C14 -cyclam indicate that hydrophobic interactions would be responsible for the impregnation of this latter. As a result, the systematic release of cyclen molecules can be expected when suspending cyclen-LS in aqueous solution, which will be unsuited for further sorption experiments. These impregnated amounts indirectly obtained by UV–vis spectrophotometry were directly confirmed by XPS spectroscopy onto the solid. The modification of the LS surface by polyamines was clearly evidenced by the increase of the nitrogen percentage at the solid surface from 2.32% for LS to 3.53% for C14 -cyclam-LS (Table 2). In addition, a correlative enrichment of the carbon content was noted after impregnation of C14 -cyclam (presence of the alkyl chain). The N/O and C/O ratios also confirmed these results. In Table 2, the surface composition of the solids (determined by XPS) is compared to their global composition obtained by elementary analysis. The differences observed between the two analyses, especially after

Table 1 Amount of polyamines (introduced concentration = 0.53 mmol L−1 ) impregnated on LS (2 g L−1 ) Ligand

Amount remaining in solution after impregnation (mmol L−1 )

Amount leached after a water washing (mmol L−1 )

Amount retained on LS (%)

Amount retained on LS (mmol g−1 )

Cyclen C14 -cyclam

0.08 0.15

0.03 0

79.2 71.7

0.21 0.19

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Table 2 XPS analysis: surface composition (%) of LS, cyclen-LS and C14 -cyclam-LS O (%) analysis)*

LS (elementary Cyclen-LS C14 -cyclam-LS (elementary analysis)* *

C (%) (42.14)*

27.11 26.64 24.75 (39.56)*

N (%) (49.65)*

70.57 69.37 71.72 (51.17)*

(1.76)*

2.32 3.99 3.53 (2.30)*

N/O

C/O (0.042)*

0.085 0.150 0.143 (0.058)*

2.603 (1.178)* 2.604 2.898 (1.293)*

Comparison with the composition of bulk solids obtained by elementary analysis.

studies as corresponding to carboxylic and phenolic moieties at the LS surface [15]. Fig. 3 clearly showed that the impregnation of C14 -cyclam onto LS led to the appearance of basic sites, which corresponds to amine moieties (only present after impregnation). The efficiency of the impregnation is also confirmed by the point of zero charge (pHpzc ) values of the solid before (pHpzc 4.1) and after impregnation (pHpzc 5.2). This increase of the pHpzc after impregnation of C14 -cyclam is indicative of the presence of basic sites on the solid. On the other hand, we can observe a slight decrease of the amount of acidic sites on the LS surface after modification by the ligands from 0.56 mmol g−1 onto LS to 0.44 mmol g−1 on C14 -cyclam-LS. This decrease is probably due to the polyamines impregnated at the LS surface, which makes a part of these sites inaccessible. 3.2. Sorption of Cu(II)

Fig. 2. Saturation curves of LS and C14 -cyclam-LS (2 g L−1 ) with OH− ions at an ionic strength of 0.1 mol L−1 (KNO3 ).

impregnation of C14 -cyclam onto LS, clearly indicate the location of N and C atoms at the surface of the solid whereas O atoms are distributed deeper. Potentiometric titrations of the solids compared to the background electrolyte one (0.1 mol L−1 KNO3 in absence of solid) allowed us to plot the saturation curves (concentration of introduced OH− or H+ as a function of OH− or H+ concentration remaining in solution) of the LS surface before and after impregnation of polyamines. Concerning the cyclen-LS, the concentrations of acidic and basic sites were not determined because of the cyclen molecules released in aqueous solution during the potentiometric experiments. Figs. 2 and 3 represent the LS and C14 -cyclam-LS saturation curves by OH− ions (titration by KOH 0.1 mol L−1 ) and H+ (titration by HNO3 0.1 mol L−1 ), respectively. The difference between curves relative to LS and to the background electrolyte reflects the consumption of OH− or H+ by the LS surface sites, which corresponds to the total concentration of acidic or basic sites on the LS surface (Table 3). The acidic sites were characterized by FTIR spectroscopy in previous

The sorption equilibrium time was previously determined by a kinetic study. Suspensions were filtered after different solid/solution contact times. At pH 5.1 and for a concentration of copper(II) of 2 × 10−4 mol L−1 , the whole amount of copper was adsorbed within 3 h for LS and C14 -cyclam-LS, which indicates a rapid equilibrium time for sorption of copper(II) ions onto solid particles. For further sorption experiments the contact

Table 3 Concentration of acidic and basic sites at the surface of LS and C14 -cyclam-LS

mmol g−1

[Acidic sites] [Basic sites] mmol g−1

LS

C14 -cyclam-LS

0.56 0

0.44 0.14

Fig. 3. Saturation curves of LS and C14 -cyclam-LS (2 g L−1 ) with H+ ions at an ionic strength of 0.1 mol L−1 (KNO3 ).

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Fig. 4. Copper(II) adsorption on LS, cyclen-LS and C14 -cyclam-LS (2 g L−1 ) as a function of pH at 20 ◦ C in 0.1 mol L−1 KNO3 ([Cu2+ ] = 2 × 10−4 mol L−1 ). Dotted line: precipitation curve of Cu(II) obtained in the same conditions (without solid in the solution).

time between Cu(II) and the solid was fixed at 4 h to ensure that sorption equilibrium was reached. For cyclen-LS, the leaching in solution of the majority of the cyclen ligands was observed during sorption experiments (apparition of a transition band at 603 nm characteristic of the [Cu(cyclen)]2+ complex on UV–vis spectra of the batch supernatants). As a result, compared to the unmodified solid, the retention of copper on cyclen-LS is significantly lower between pH 4 and 6 (from 100% to about 15% of introduced metal amount). A competition likely takes place between the LS solid particles and the cyclen molecules released in solution, which is against the sorption process. The influence of the pH on the C14 -cyclam-LS copper(II) retention is reported in Fig. 4 for an initial metal concentration of 2 × 10−4 mol L−1 . It is compared with the fixation on the unmodified LS and with the copper(II) precipitation curve obtained in the same conditions without LS solid in the solution. The isotherms relative to LS and C14 -cyclam-LS are shifted to the acidic pH compared to the copper precipitation curve. This observation highlights the sorption of the metal at the solid surface, at least until pH 7. Beyond pH 10, one can notice a retention decrease which was imparted to LS dissolution. In each case, the adsorption is strongly pH-dependent, most particularly for unmodified LS since adsorption is very weak below pH 4. The different behaviors of LS and C14 -cyclam-LS at acidic pH are of particular interest. In Table 4, we reported the percentage of copper adsorbed at pH 3 on the three different solids (LS,

Fig. 5. EPR spectra recorded at 150 K for the Cu(II)-LS and Cu(II)-C14 -cyclamLS systems at pH 4.1.

cyclen-LS and C14 -cyclam-LS) for an introduced concentration of 2 × 10−4 mol L−1 . When retention is negligible at this pH value on the starting material, more than 65% of the introduced copper is sorbed on the C14 -cyclam-LS (that is 4.13 mg g−1 ). By increasing the introduced Cu(II) concentration, a maximum adsorption capacity was determined to be equal to about 0.3 mmol of copper sorbed per gram of solid at pH 5.1. After impregnation of the residue by the lipophilic C14 -cyclam molecules, no decrease of this value has been encountered. Consequently, the modification of the LS surface allowed us to enlarge the pH range of use of the solid without modifying its initial retention capacity towards Cu(II). The impregnation of such molecules onto LS is a good solution for extending the application field of the solid. In order to estimate the nature of the complexation sites at the surface of the modified material, EPR experiments were carried out and allowed us to compare the complexation of Cu(II) onto the LS and C14 -cyclam-LS surfaces. On Fig. 5, we represented the spectra recorded at 150 K and relative to the two solids after batch experiments at pH 4.1 ([Cu2+ ]initial = 2 × 10−4 mol L−1 ). On the two spectra, a narrow signal appears at g = 2.0025, which is typical of an organic free radical intrinsic to LS [29–31]. In addition, the EPR spectra exhibit a typical anisotropic copper signal with a hyperfine structure of four lines in the paral-

Table 4 Amount of copper(II) adsorbed on LS, cyclen-LS and C14 -cyclam-LS (% and mg per gram of solid) at pH 3 for a Cu(II) initial concentration of 2 × 10−4 mol L−1 (I = 0.1 mol L−1 ) Solid

Percentage of Cu(II) sorbed at pH 3

Amount of Cu(II) sorbed at pH 3 (mg g−1 )

Amount of Cu(II) sorbed at pH 3 (mmol g−1 )

LS Cyclen-LS C14 -cyclam-LS

<5 <5 65.2

≈0 ≈0 4.13

≈0 ≈0 0.065

S. Sayen et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 289 (2006) 126–132 Table 5 EPR parameters of the Cu(II) surface complexes prepared at pH 4.1, recorded at 150 K

≡C14 -cyclam-LS–Cu ≡LS–Cu [Cu(cyclam)]2+* *

g//

A// (10−4 cm−1 )

2.182 2.310 2.186

197 172 205

References [32,33].

lel region, resulting from the interaction between the electron spin and the nuclear spin of the metal. These spectra are axial (g// > g⊥ > 2.0), with spin Hamiltonian parameters reported in Table 5. The positions of the four lines relative to copper in the parallel region and the g// values are different for LS (g// = 2.310) and C14 -cyclam-LS (g// = 2.182) which indicates that Cu(II) is held in a different environment on their surfaces. The experimental values of the A// and g// parameters, which are given in Table 5, are compared with those of the [Cu(cyclam)]2+ complex [32,33]. Concerning Cu(II) adsorbed at the LS surface, these values are characteristic of an inner sphere surface complex between Cu(II) and the surface sites of LS that is a CuO4 chromophore [34]. In the case of the Cu(II)-C14 -cyclam-LS system, the spectroscopic parameters are very similar to those of the [Cu(cyclam)]2+ complex. They are both consistent with a CuN4 chromophore [32,33] included in an axially symmetric d9 copper complex with a dx2−y2 ground state [35] and the formation of an inner sphere surface complex. Therefore, after impregnation of the C14 -cyclam molecules, Cu(II) is essentially adsorbed at the solid surface by complexation with the macrocyclic ligands. 4. Conclusion In this study, a lipophilic tetraazamacrocyclic ligand was successfully impregnated onto a lignocellulosic substrate extracted from wheat bran. The presence of an alkyl pendant arm preliminary grafted onto the macrocyclic cavity permitted to ensure its stay on the substrate without leaching in the aqueous medium. Equilibrium batch experiments realized with this new material have revealed that the retention capacity of the initial residue towards Cu(II) was retained for a pH range comprised between 4 and 10 with a relatively fast kinetic even after impregnation. The most interesting fact is the clearly improved sorption ability of the impregnated residue at more acidic pH, thus allowing the extension of its application field to the whole pH range. Indeed, when the unmodified LS does not significantly retain Cu(II) below pH 4, C14 -cyclam-LS is able to adsorb more than 65% of the initially introduced metal. The EPR spectroscopy evidenced that the complexation of Cu(II) is ensured by the macrocycles located at the LS surface. This surface modification is proven to be a good mean of enlarging the application field of the solid which is consequently well-suited for water depollution even in conditions involving acidic effluents.

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