Chitosan Interactions With Metal Ions and Dyes-dissolved-state vs. Solid-state

7/31/2019 Chitosan Interactions With Metal Ions and Dyes-dissolved-state vs. Solid-state

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Chitosan interactions with metal ions and dyes: dissolved-state vs. solid-stateapplication

E. Guibal*, E. Touraud and J. RoussyEcole des Mines dAle s, Laboratoire Ge nie de lEnvironnement Industriel 6, avenue de Clavie res, F-30319 ALES cedex,France*Author for correspondence: Tel. +33-0-466782734, Fax: +33-0-466782701, E-mail: [email protected]

Received 13 October 2004; accepted 22 November 2004

Keywords: Adsorption, chitosan, coagulationocculation, mercury, reactive black 5, ultraltration

Summary

Chitosan is an amino-polysaccharide with highly efcient properties for the binding of metal ions and anionic dyes.Uptake may occur through chelation on free amino functions (at near-neutral pH) or by electrostatic attraction onprotonated amino groups (in acidic solutions). The polymer is soluble in acidic solutions and its binding propertiescan be used in both solid form (sorption) and liquid form (ultraltration coupled with chelation, coagulation occulation). These properties have been used for the recovery of mercury from dilute solutions at initial pH 5(which reveals the most efcient pH in the range pH 46) and for the recovery of Reactive Black 5 (RB5, anionicdye) at pH 3. While in the case of mercury binding saturation of the biopolymer is only slightly higher whenchitosan is used in the liquid form compared to solid-state adsorption, in the case of the coagulationocculation of RB5 (using the liquid-form of chitosan) the saturation of the polymer (calculated on the basis of molar ratio of dyevs. amino groups of the polymer) is reached at a signicantly greater value than when the polymer is used for thesolid-state binding of the dye. There is a much more efcient use of amino groups when chitosan is used in the

liquid-form due to a better availability of amino groups (less hydrogen bonds between the chains of the polymer)and to a better accessibility to internal sorption sites (lower diffusion control).

Introduction

The increasing demand for new processes for wastewatertreatment, more environmentally friendly and morecompetitive, has led to much research on the use of biopolymers such as alginate and chitin/chitosan for therecovery of metal ions (Chen & Wang 2001; Guibal2004) or organic compounds (Juang et al. 1997; An-nadurai et al. 2002; Chiou & Li 2002; McCarrick et al.2003). These biopolymers are characterized by highsorption capacities, easy degradation routes (at the endof life cycle) and relatively low costs (compared tosophisticated resins, Dubois et al. 1995). These proper-ties make them very promising for replacing conven-tional processes. Chitosan has a unique property amongthese biopolymers: its cationicity in acidic solutions.This property is due to the presence of a large number of amino groups, which are very reactive for (a) metalcations by chelation in near-neutral solutions, and for(b) metal anions that can be bound to protonated aminogroups in acidic solutions (Guibal 2004). The p K a of the

amino groups of chitosan strongly depends on thedegree of acetylation of the polymer and charge neu-tralization in solution (Sorlier et al. 2001). The proton-ation of amino groups also involves the dissolving of the

biopolymer in acidic solutions (with the exception of dilute sulphuric acid solutions). For some applicationsinvolving metal sorption at low pH the polymer can becross-linked to prevent its dissolving, but at the expenseof a decrease of the number of free amino groups. Thisparameter is very limiting in the case of metal cationchelation (Dzul Erosa et al. 2001), but less important inthe case of electrostatic attraction mechanisms (Ruizet al. 2000). Alternatively, it is possible to use chitosanwithout cross-linking treatment but, at the saturation of the polymer, it is necessary to recover the metal-boundmacromolecules by an ultraltration process. This is thebase of polymer-enhanced ultraltration processes(PEUF) (Juang & Tseng 2000; Kuncoro et al. 2004).The protonation of chitosan in acidic solution may bealso interesting for the electrostatic attraction of anionicdyes (McKay et al. 1987; Yoshida & Takemori 1997;Annadurai et al. 2002; Gibbs et al. 2003, 2004; Wonget al. 2004). These chitosan-dye interactions are veryinteresting for the sorption of dyes on solid chitosan(using sulphuric acid for pH control) but they can be

also used for dye binding using chitosan in a dissolved-state. In this case, the interactions of the dyes withchitosan result in the formation of mixed colloids (dye/chitosan) that can settle after a long contact time or can

World Journal of Microbiology & Biotechnology (2005) 21: 913920 Springer 2005DOI 10.1007/s11274-004-6559-5


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be recovered (after maturation) by ltration. Protonatedamino groups neutralize anionic charges of the dye(coagulation effect), before neutralized dyes aggregateand settle (occulating effect). This property results incoagulationocculation properties that have been used

elsewhere for the treatment of many groundwaters andindustrial effluents (from the agricultural and foodindustries) (Huang & Chen 1996; Pan et al. 1999;Huang et al. 2000; Roussy et al. 2004).

The present work focuses on (a) the study of mercuryrecovery on chitosan by sorption (chitosan in solid-state) and PEUF (chitosan in dissolved-state), and (b)the study of Reactive Black 5 (RB5, or Intracron Black)recovery on chitosan by sorption and by coagulation occulation. The main objective of this work is thus todemonstrate that, despite the high efciency and easyuse of chitosan for sorption processes (in the solid-state), the use of chitosan in the dissolved-state allowsimproving the reactivity of the polymer. This improve-ment in uptake properties will be measured throughequilibrium performance but also kinetic considerations.

Material and methods

Material

Chitosan was used as supplied by Aber Technologies(Plouvien, France). The material was previouslycharacterized by the FT-IR technique and SEC

(coupled with refractometer and laser light scatteringmeasurements) for the determination of the degreeof deacetylation and polymer weight, which were87% and 125,000 g mol

) 1, respectively. Chitosanwas ground and sieved in 4 size fractions:G1 < 125 l m < G2 < 250 l m < G3 < 500 l m < G4< 710 l m. For the experiments using chitosan indissolved-state, the polymer was dissolved in aceticacid solution (1 g of chitosan for 1 g of acetic acid,80% (w/w)).

Reactive Black5 (RB5) wassuppliedby Aldrich(USA).This dye is characterized as a diazo compound bearing 4sulphonyl groups (Figure 1), M r 991.82 g mol

) 1 . Thecommercial salt is supplied as a mixture of the active

material and an inert product: the actual true dye contentis 55%. This must be taken into account for theevaluationof true molar ratio and true sorption capacities (concen-trations). Theexperiments have been performed using thecommercial salt without purication and the concentra-

tions are given in function of the total amount of commercial salt used for the preparation of the solutions.However, for the evaluation of the molar ratio betweenthe dye and amino groups of chitosan, the correction wasmade taking into account the purity of the salt, as well asthe humidity and the degree of acetylation of chitosan.

Mercury (in the form of nitrate salt) was supplied byFluka AG (Switzerland). Other common reagents(acids, bases) were supplied by Fluka (Switzerland)and Carlo Erba (Italy) as analytical grade products.

Sorption Processes

Sorption isotherms were performed in batch systemsusing a standard procedure (Procedure 1) consisting inthe mixing of a xed volume of solution (at givenconcentrations) with different amounts of chitosan forat least 48 h (depending on particle size). In the case of mercury two different procedures have been obtained:(a) the previously described method (Procedure 1); and(b) an alternative procedure consisting in mixing a xedamount of sorbent with a xed volume of solutionprepared at concentrations ranging between 10 and100 mg Hg l

) 1 (Procedure 2). The initial pH was con-trolled with sulphuric acid and NaOH for experiments

on both mercury and RB5. Solutions were ltered at theend of the experiment using a 1.2 l m Whatman ltra-tion membrane. The residual concentration was mea-sured by Inductively Coupled Plasma Atomic EmissionSpectrometry (ICP-AES) (JY 2000, Jobin-Yvon,France) for mercury concentration and by u.v.visiblespectrophotometry for dye concentration at the wave-length of 598 nm (UVVIS spectrophotometer Shima-dzu UV-160 A). A drop of nitric acid (10%) was addedto mercury solution before ICP-AES analysis, while adrop of sodium hydroxide (5 M) was added to dyesamples to maintain a constant pH of the samples beforeanalysis (indeed, a change in the pH of the sample maycause a change in the dyes u.v.visible spectrum andmay affect the determination of solution absorbance anddye concentration). The mass balance equation was usedfor determining solute concentration on the sorbent atequilibrium. The pH of the solution at equilibrium wassystematically measured using a WTW 526 pH-metre(WTW, Germany): the pH measurements were per-formed with a 0.05 pH-unit precision.

Sorption kinetics were performed by mixing 1 l of solution at xed concentrations (either mercury salt ordye) with xed amounts of sorbent. The solution wasagitated for at least 48 h and samples were regularly

collected, ltered and analysed for the determination of residual solute concentration.

Ultraltration experiments were performed in batchsystem using an Amicon 8400 ultraltration unit with

=SO

3Na

SO 3 Na

HO

2HN

N=NNaO 3 SOCH 2 CH 2 S =

=

O

O

N=NNaO 3 SOCH 2 CH 2 S

O

= O

Reactive Black 5

Figure 1. Structure of Reactive Black 5.

914 E. Guibal et al.


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polyethersulphone membranes (Cut-off: 50 and100 kDa). The pressure was xed at 2 bars. Typically,the solution containing the polymer (dissolved in HCl)was mixed with mercury solution at xed pH for 1 h andnally the solution was permeated though the cell.

The molar ratio Hg/polymer was varied in order toevaluate polymer saturation. Permeate samples wereregularly collected and analysed for metal determination(ICP/AES) and polymer content (TOC metre, Shima-dzu, Japan).

Experiments on dye coagulation and occulation wereperformed using a jar-test equipment. The dye solution,which pH was controlled using HCl, was mixed afterpolymer addition under strong agitation (ca. 200 rev/min) for 3 min and then under slow agitation (ca.40 rev/min) for 20 min. After stopping the agitation,samples were regularly collected and analysed for dyecontent in the supernatant. Two different experimentalprocedures have been used depending on the experi-ments. The amount of dye on the polymer was calcu-lated by a mass balance equation assuming that theamount of dye that was removed from the solution wasbound to the total amount of polymer added in thesolution. The dye/polymer molar ratio was varied inorder to determine the optimum relationship betweendye functional groups and amino groups of chitosan.The optimization took into account both the removalefciency and the molar ratio dye/amino groups.

Results and discussion

Chitosan interactions with mercury ions

Adsorption processThe sorption process on chitosan is strongly controlledby the pH of the solution due to protonation of aminogroups on the biopolymer. This may have a positiveeffect on the uptake of anionic solutes or a negativeimpact for example in the case of cationic molecules. Inacidic solutions, the protons compete with target cationsand signicantly reduce their sorption. Figure 2 showsthe sorption isotherms obtained at pH 46 using 2

different experimental procedures for initial mercuryconcentrations below 50 mg l

) 1. The pH was notcontrolled during the experiment; it was only monitoredat the end of the experiment. The variation of the pHduring sorption experiments is an experimental param-

eter frequently underestimated in the literature dedi-cated to biosorption properties of chitosan; for thisreason special attention has been paid to pH variation inthe present work. The change in the pH in the course of metal sorption was directly inuenced by experimentalconditions; i.e. initial pH, amount of sorbent and initialmetal concentration (Figure 3). When the initial pH of the solution approaches the p K a of chitosan, whichdepends on the degree of deacetylation and the extent of amino groups dissociation (Sorlier et al. 2001), thechange in the pH decreased (Figure 3a). This weak pHvariation may be explained by a buffering effect of chitosan on the solution: the protonation/deprotonationof amino groups tends to decrease and the pH is lessaffected. At low initial pH (i.e. pH 4) the change in thepH increased with initial metal concentration, this trendwas reversed at higher initial pH (Figure 3b). Noexplanation was found to this increased effect of initialmetal concentration on pH variation at pH 4. At pH 5and 6, increasing the amount of metal resulted in anincrease of the amount of metal adsorbed and thereforea reduced number of amino groups available for theuptake of protons (additionally to the buffering effect of chitosan when approaching the p K a of amino groups of chitosan).

When initial pH was 4 the pH variation reached 0.8pH unit (as a mean value), while when the initial pHs

0

100

200

300

400

500

0 10 20 30 40Ceq (mg Hg/L)

q ( m g

H g / g )

pH 4

pH 4

pH 5

pH 5

pH 6

pH 6

Figure 2. Inuence of pH on mercury sorption on chitosan akes(closed symbols: Procedure 1; open symbols: Procedure 2).

4

5

6

7

0 25 50 75 100Initial Hg concentration (mg/L)

F i n a l p H

pHi 4

pHi 5

pHi 6

Vsol: 150 mL

(a)mchitosan : 25 mg

3.5

4.5

5.5

6.5

7.5

0 0.02 0.04 0.06 0.08

Chitosan amount (mg)

F i n a l p H

pHi: 4; Co: 10 mg/LpHi: 4; Co: 25 mg/LpHi: 4; Co: 50 mg/LpHi: 5; Co: 10 mg/LpHi: 5; Co: 25 mg/LpHi: 5; Co: 50 mg/LpHi: 6; Co: 10 mg/LpHi: 6; Co: 25 mg/LpHi: 6; Co: 50 mg/L

Vsol : 150mL

(b)

Figure 3. Inuence of mercury concentration (a) (Procedure 1) andchitosan amounts (b) (Procedure 2) on equilibrium pH (pH i: initial pH,and C 0: initial metal concentration).

Chitosan interactions with mercury and dyes 915


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were 5 and 6 the pH variation tended to 0.5 and 0.3 pHunit, respectively. Figure 2 shows that the pH hardlychanged the prole of the sorption isotherm in the rstpart of the curve: this weak effect is due to thisbuffering effect of the polymer on the nal pH; the

polymer smoothed the differences in the initial pHs.Figure 4 shows a more complete isotherm obtained atpH 5 with different particle sizes (G1 < 125 l m

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Chitosan Interactions With Metal Ions and Dyes-dissolved-state vs. Solid-state