ORIGINAL PAPER

Thiocyanate separation by imprinted polymeric systems

Ebru Birlik Özkütük & Elif Özalp & Arzu Ersöz &

Erol Açıkkalp & Rıdvan Say

Received: 4 November 2009 /Accepted: 28 February 2010 /Published online: 19 March 2010# Springer-Verlag 2010

Abstract Novel polymers for the specific recognition ofthiocyanate ion (SCN) have been prepared by templatepolymerization of SCN with chitosan-Zn(II), the N-(2-aminoethyl)-3-aminopropyl-trimethoxysilan-Zn(II) com-plex, epichlorohydrin, and tetraethoxysilane. After removalof SCN, the imprinted beads have been used for solid-phaseextraction of SCN from aqueous solutions. Optimisedconditions for SCN separation are reported with respect tosample pH for the quantitative preconcentration anddesorption. Competitive adsorption of fluoride and phos-phate has been investigated and selectivity coefficients aregiven for these ions with respect to SCN.

Keywords Ion imprinted polymer . Thiocyanatedetermination . Chitosan . Solid phase extraction . Sol-gel

Introduction

Thiocyanate (SCN-) is present as a normal constituent inmammalian tissues and body fluids [1]. It is also present inindustrial wastewaters, pesticides residues and organism

metabolites. Since thiocyanate is an end product ofdetoxification of hydrogen cyanide included in cigarettesmoke, its extraction in urine and human serum saliva canprovide a useful probe for distinguishing between smokersand non-smokers [2]. If the content of thiocyanate is a littlehigher in the body than normal, the protein dialysis will beaffected and it may even result in coma. Therefore, thedetermination of thiocyanate at low levels especially infood, biological and water samples is important [3].

Chronically elevated levels of blood thiocyanate inhibitsthe uptake of iodine by the thyroid gland, thereby reducingthe formation of thyroxine [4]. Thiocyanate is mildlyneurotoxic at serum levels of 1 mmol L-1 (60 mg L-1).Ingles and Scott [5] have reported thiocyanate toxicity forfish in the range 90–200 mg L-1. The toxic effects ofthiocyanate include inhibition of halide transport to thethyroid gland, stomach, cornea and gill as well as theinhibition of a variety of enzymes [6].

The destruction of thiocyanate has been studied usingboth biological and chemical methods [7-12]. Chemicalmethods use chlorine, hydrogen peroxide and ozone tooxidize thiocyanate [13]. Ozonation is limited by masstransfer process and thereby leads to the requirement ofexcess amount of ozone. Additionally, ozone must bemanufactured where it is used which adds a high capitalcost to this treatment process [14]. The most commonlyused method for removal of thiocyanate in wastewater isdirect alkaline chlorination or the addition of hypochlorite.This method, although adequate, has disadvantages such aschloride contamination and involvement of reactants thatare hazardous and unsafe to handle [15]. Alternativemethods addressing environmental considerations are need-ed to remove and recover the thiocyanate.

The molecular imprinting method is a useful techniquefor preparing host compounds for molecular recognition.

E. B. Özkütük (*) : E. Özalp : E. AçıkkalpDepartment of Chemistry, Eskişehir Osmangazi University,Eskişehir 26480, Turkeye-mail: ebirlik@ogu.edu.tr

A. Ersöz : R. SayDepartment of Chemistry, Anadolu University,Eskişehir 26470, Turkey

R. SayBİBAM (Plant, Drug and Scientific Researches Center),Anadolu University,Eskişehir 26470, Turkey

Microchim Acta (2010) 169:129–135DOI 10.1007/s00604-010-0319-z

Various kinds of host compounds have been a highrecognition property which is specific for imprintedtemplate molecules [16-24]. In the process of molecularimprinting, appropriate functional monomers are introducedto interact with template molecules, and then the functionalgroups on the monomers are fixed with chemical cross-linker. Finally, after removing the template molecule fromthe imprinted polymers, recognition sites can be used tobind the template or its analogue selectively. These highlycross-linked polymers possess microcavities, which arecomplementary in size and shape to the template molecule.

Ion imprinting polymers (IIPs) are similar to MIPs, buttemplated resins have prepared for polyvalent metal ions[25-29]. There are some papers, recently, for removal ofanions by using molecular imprinting technique [30-32].

The main objective of this work is to use ion imprintingmethod to synthesize novel selective sorbents for thedetermination at thiocyanate ion, and to investigatedadsorption of thiocyanate under batch conditions such as(pH effect, selectivity, kinetic and isotherm).

Experimantal

Materials

Chitosan, AAPTS (N-(2-Aminoethyl)-3-aminopropyl-trimethoxysilan) and epichlorohydrin were supplied fromAldrich Chemical (USA) (www.sigmaaldrich.com). TEOS(Tetraethyl orthosilicate) were purchased from Acros(Belgium) (www.acros.com). All other chemicals (ZnCl2,NaOH, KSCN, HNO3, formaldhyde) were analyticalreagent grade and purchased from Merck (Darmstadt,Germany) (www.merck-chemicals.com.)

Instrumentation

Metler Toledo Seven Multi pH-ionmeter was used tomesaure pH values and ion concentration. Anions concen-tration was mesaured by unicam UV spectrometer. FTIRspectroscopy was used in the 4,000-400 cm-1 range for thechemistry of functional monomer, pre-organized complex,and imprinted beads in the solid state (FTIR 100 series,Perkin Elmer, USA).

SCN--imprinted polymers of synthesis

Synthesis of MIP1 (chitosan-Zn(II)-SCN)

In the preparation of Zn(II)-chitosan complex, chitosan(2.00 g, corresponding to approximately 12.40 mmolsglucoseamine) was dissolved in the acetic acid aqueoussolution (2%, 100 mL) at ambient temperature, and 1.635 g

of ZnCl2 was added dropwise with vigorous stirring. Then,the mixture solution was slowly dropped into 150 mL of 1 MNaOH. The suspended solution was stirred at 200 rev/min for12 h. Then, the solution was filtered. The solid was separatedand dried. The concentrated solution was throw away.

In the preparation of MIP-1(chitosan-Zn(II)-SCN)-imprinted polymer, chitosan-Zn(II) complexes (2 g) wasdissolved in the acetic acid aqueous solution (5%, 100 mL)and KSCN (1.1657 g) was added slowly to this solutionwith continuous stirring at room temperature. Then, themixture solution was slowly dropped into 150 mL of 1 MNaOH. The suspended solution was stirred at 200 rev/minfor 12 h. After the filtering of this yellow suspendedsolution and the solid was dried in vacou, thiocyanate-complexed chitosan-Zn(II) was crosslinked by 10 mL ofepichlorohydrin in 250 mL of the acetic acid (5%) underrefluxing conditions in an oil bath (ca. 110°C) for 2 h.Then, 250 mL of 0.1 M NaOH solution was added tocomplete the crosslinking reaction. After vacuum filtration,the product of crosslinked polymer was washed with 1 MNaOH and deionized water several times.

The IR spectra of the MIP-1 showed -OH band at3,435 cm-1, I S-C≡N band at the 2,072 cm-1 and aliphaticC-NH2 band at the 1,055 cm-1.

Synthesis of MIP2 (chitosan-Zn(II)-AAPTS-SCN)

Chitosan-Zn(II) complexes (2 g) was dissolved in the aceticacid aqueous solution (5%, 100 mL) and AAPTS (N-(2-Aminoethyl)-3-aminopropyl-trimethoxysilan) (5.54 mL)was added slowly to this solution with continuous stirringat room temperature for 12 h. KSCN (2.332 g) was addedto chitosan-Zn(II)-AAPTS mixture. Then, the mixturesolution was slowly dropped into 150 mL of 1 M NaOH.The suspended solution was stirred at 200 rev/min for 12 h.After filtering this solution and drying in vacou, theobtained mixture was crosslinked by 50 mL of TEOS(Tetraethyl orthosilicate) in 250 mL of the acetic acid (5%)under refluxing conditions in an oil bath (ca. 110°C) for2 h. Then, 250 mL of 0.1 M NaOH solution was added tocomplete the crosslinking reaction. After vacuum filtration,the product of crosslinked polymer was washed withformaldehyde and deionized water several times.

The IR spectra of the MIP-2 showed -OH band at the3,401 cm-1, -NH band at the 1,579 cm-1, C-H band at the1,444 cm-1, Zn-S band at the 994 cm-1and Zn-N band atthe 468 cm-1.

Synthesis of MIP3 (AAPTS-zn(II)-SCN)

In the preparation of AAPTS-Zn(II) complex, ZnCl2(0.8178 g) was dissolved in the methanol (20 mL) andthen, AAPTS (5.54 mL) was added slowly to this solution

130 E. B. Özkütük et al.

with continuous stirring at room temperature for 3-4 days.The product was crystalized.

In the preparation of (AAPTS-Zn(II)-SCN)-imprintedpolymer, AAPTS-Zn(II) complex was dissolved in themethanol (20 mL) and KSCN (0.5830 g) was added slowlyto this solution with continuous stirring at room tempera-ture. Then, the mixture solution was crosslinked by 25 mLof TEOS in 250 mL of the acetic acid (1%) under refluxingconditions in an oil bath (ca. 110°C) for 7-8 h. Aftervacuum filtration, the product of crosslinked polymer waswashed with 1 M of formaldhyde and deionized waterseveral times.

The IR spectra of the MIP-3 showed N-H band at the3,232 cm-1, aliphatic C-H band at the 2,935 cm-1, C≡Nband at the 2,085 cm-1, Zn-N band at the 471 cm-1.

Adsorption studies

Effects of the pH of the medium and the initial concen-trations of SCN- on the adsorption rate and capacity wereinvestigated in batch adsorption-equilibrium experiments.The effect of pH on the adsorption rate of the SCN-imprinted polymer was investigated in the pH range 2.0–12.0 at 25°C. The suspensions were brought to the desiredpH by adding NaOH and HNO3. The pH was maintained ina range of ±0,1 U until equilibrium was attained. In allexperiments, polymer concentration was kept constant at25 mg (25 mL)-1. Anions were treated with the SCN--imprinted particles at room temperature, in the flask stirredmagnetically at 600 rpm.

The concentration of the ions in the aqueous phases afterdesired treatment periods were measured by using UVspectrometer [33]. The effect of the initial ion concentrationon the adsorption was investigated at a suitable pH asdescribed above except that the concentration of thiocya-nate ions in the adsorption medium was varied between 25and 500 mg L-1

Competitive adsorption of SCN-/F-, SCN-/PO43- from

their mixture was also investigated in a batch sysytem. Asolution (25 mL) containing 25 mg L-1 from each anionswas treated with the thiocyanate imprinted microparticles ata pH of 3.0 at room temperature, in the flask stirredmagnetically at 600 rpm. After the adsorption equilibrium,the concentration of anions in the remaining solution wasmeasured by UV spectrometer [33].

Quantification of adsorbed thiocyanate in the thiocyanateimprinted microparticles

In all experiments, polymer concentration was kept con-stant at 25 mg (25 mL)-1. The known concentration of thethiocyanate ions in the aqueous phases was treated with thethiocyanate imprinted microparticles at a pH of 3.0 at room

temperature at adsorption time, in the flask stirred magnet-ically at 600 rpm. Mixture was filtrated. Solution was usedto measure thiocyanate concentration. A series of thiocya-nate standard solution was prepared for calibration curve.Standard solution and thiocyanate concentration in theremaining solution was measured by UV spectrometer at460 nm [33].The instrument response was periodicallychecked with known thiocyanate ion solution standarts. Foreach sample, the mean of 10 UV measurements wasrecorded. The amount of adsorbed ions was calculatedusing the following equation:

Q ¼ Co � Cð Þ:V½ �=M ð1ÞQ is the amount of ions adsorbed onto the unit amount of

the polymer (mg g-1); Co and C are the concentrations ofthe ions in the initial solution and in the aqueous phase afteradsorption, respectively (mg L-1); V is the volume of theaqueous phase (mL); and M is the amount of polymer (g).

Desorption and reuse

Adsorbed thiocyanate anions were desorbed by the treat-ment with 1 M NaOH solution. Adsorption conditions wereas follows: initial concentration of the anion: 25 ppm;amount of the imprinted polymers: 25 mg; volume of theadsorption medium: 25 mL; pH 3; temperature: 25°C; andadsorption time: 30 and 180 min. Then, these imprintedpolymers were placed in this desorption medium and stirredat a stirring rate of 600 rpm. The imprinted polymers werewashed a few times with 1 M NaOH for MIP-1 and 1 Mformaldehyde for MIP-2, MIP-3 new solution and water.Desorption time was 3 h.

The concentrations of anions in the aqueous phase werefollowed as mentioned before. Desorption ratio wascalculated from the following expression.

Desorption ratio ¼ Amount of ions desorbed to the elution mediumð Þ=Amount of ions adsorbed onto the sorbentð ÞX100

ð2ÞAdsorption-desorption cycle was repeated seven times

using the same sorbent for the detection of reusability of theSCN--imprinted polymers.

Result and discussion

Adsorption capacity of SCN--imprinted polymer

Adsorption rate

Figure 1 shows adsorption rates of thiocyanate ions ontothe SCN--imprinted polymers from aqueous solutions

Thiocyanate separation by ımprinted polymeric systems 131

containing 25 mg L-1 of thiocyanate ions at a constant pHof 3.0. Note that the ordinate values on this were calculatedby using the expression as given in Eq. 1. As seen here,from the figure high adsorption rates are observed at thebeginning, and then plateau values (i.e., adsorption equi-librium) are gradually reached within 30 min for MIP-1 andMIP-2 and 300 min.for MIP-3.

Several experimental data on the adsorption of variousions by molecular imprinted polymer have shown a widerange of adsorption rates. For example, Guo et al. usedmolecularly imprinted chitosan beads for the separation ofhemoglobin and found the equilibrium time as 10 h [34].

Adsorption capacity

Figure 2 shows effects of initial concentration of thiocyanateions onto the adsorption capacity of the SCN--imprintedpolymer at pH 3.0. The amount of thiocyanate ions adsorbedper unit mass of the polymer (i.e., adsorption capacity)

increased with the initial concentration of thiocyanate ions.The maximum adsorption (corresponding a 500 ppm thio-cyanate ion initial concentration), which represents saturationof active points (which are available for thiocyanate ions) onthe polymers, was 305 for MIP-1, 315 for MIP-2 and 307 forMIP-3 mg thiocyanate (g polymer)-1.

During the batch experiments, adsorption isotherms wereused to evaluate adsorption properties. For the systemsconsidered, the Langmuir model was found to be applicablein interpreting thiocyanate adsorption by imprinted polymer.Table 1 shows the Langmuir adsorption ishotherm constantsfor the SCN--imprinted polymers. Langmuir adsorptionmodel assumes that the molecules are adsorbed at a fixednumber of well-defined sites, each of which can only holdone molecule. These sites are also assumed to beenergetically equivalent, and distant to each other so thatthere are no interactions between molecules adsorbed toadjacent sites [35]. The corresponding transformations ofthe equilibrium data for thiocyanate ions gave rise to alinear plot, indicating that the Langmuir model could beapplied in these systems and described by the equation:

Q ¼ QmaxbCe= 1þ bCeð Þ ð3ÞWhere Q is the concentration of bound thiocyanate ions

on the adsorbent (μmol g-1), Ce is the equilibriumthiocyanate ions concentration in solution (μmol L-1), b isthe Langmuir constant (g μmol-1) and Qmax is themaximum adsortion capacity (μmol g-1). This equationcan be linearized.

The maximum adsorption capacity (Qmax) data for theadsorption of thiocyanate ions was obtained from theexperimental data. The correlation coefficient (R2) was0.94; 0.97; 0.97 for MIP-1, MIP-2 and MIP-3, respectively.The Langmuir adsorption model can be applied in this affinityadsorbent systems. It should be also noted that maximumadsorption capacity (Qmax) was found to be 290 mg g-1.

In order to examine the controlling mechanism ofadsorption process, pseudo-first and second-order kineticmodels were used to test experimental data [36]. Acomparison of the experimental adsorption capacity andthe theoretical values are presented in Table 2. Thetheoretical Q value estimated from pseudo-first-order

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

Time (min.)

Ads

orpt

ion

Cap

acit

y m

g/g

MIP-1

MIP-2

MIP-3

Fig. 1 Adsorption rates of thiocyanate ions on the SCN-imprintedpolymer; pH:3.0, T:25°C

0

50

100

150

200

250

300

350

0 200 400 600

Concentration mg/L

Ads

orpt

ion

capa

city

mg/

g

MIP-1

MIP-2

MIP-3

Fig. 2 Adsorption capacity of thiocyanate ions on the SCN-imprintedpolymer; pH:3.0, T:25°C

Table 1 Langmuir adsorption ishotherm constants for the SCN--imprinted polymers

Polymer Langmuir adsorption ıshotherm constants

Qeq b R2

MIP-1 290 mg g-1 63,5 0,94

MIP-2 290 mg g-1 30 0,97

MIP-3 290 mg g-1 46 0,97

132 E. B. Özkütük et al.

kinetic model is more close to the experimental value andthe correlation coefficient. So, the results suggested that thepseudo-first order adsorption mechanism is predominant forthis SCN--imprinted adsorbent systems and that overall rateof the SCN- adsorption process appeared to be controlledby chemical reaction.

Effects of pH

The effect of pH on the thiocyanate ions adsorption usingSCN--imprinted polymers is shown in Fig. 3. As can beseen in Fig. 3, the SCN--imprinted polymers exhibited ahigh affinity in acidic conditions (pH=3.0) for thiocyanateions.

Selectivity studies

Competitive adsorption of SCN-/F-, SCN-/PO43-, from

their couple mixture was also investigated in a batchsystem. The thiocyanate adsorption capacity of the SCN--imprinted particles was higher than that other ions. Thecompatitive adsorption capacity of the SCN--imprintedparticles for thiocyanate ions was also higher than non-imprinted particles. When they exist in the same medium,a competition will start for the same attachment sites. Itcan be concluded that the thiocyanate imprinted particlesshow the following ion affinity order under competitiveconditions:

SCN� > PO43� > F�for MIP� 1;

SCN� > F� > PO43�for MIP� 2

SCN� > F� > PO43�for MIP� 3

It should be noted that the imprinted microparticlesshowed excellent selectivity for the target molecule(thiocyanate ions) due to molecular geometry.

Distribution and selectivity coefficienet of thiocyanatewith respect to F-, PO4

3-and SCN- was calculated asexplained below.

Kd ¼ Ci � Cf=Cf½ �� volume of the solution;mL=mass of microparticles; gð Þ

ð4Þwhere Kd, Ci and Cf represent the distribution coefficient,initial and final solution concentrations, respectively. The

selectivity coefficient for the binding of a anion in thepresence of competitor species can be obtained fromequilibrium data according to the Eq. 5;

k ¼ Kd ðtemplate ionÞ=Kd ðinterferent ionÞ ð5Þwhere k is the selectivity coefficient. A relative selectivitycoefficient k′ Eq. 6 can be defined as

k0 ¼ kimprinted=kcontrol 37½ � ð6Þk′ that results from the comparision of the k values of theimprinted microbeads to non-imprinted microbeads, allowsan estimation of the effect of imprinting on selectivity. Acomparison of the selectivity coefficient of SCN--imprintedpolymers with the selectivity coefficient of non-imprintedpolymers showed that the effect of imprinting on F- ions forMIP-1, MIP-2 and MIP-3 was 1.97; 6.05 and 45.0 timesgreater than non-imprinted matrix, respectively (Table 3).This means that thiocyanate ions can be determined even inthe presence of SCN- and F- interferences.

Table 3) showed that the selectivity coefficient of MIP-1,MIP-2 and MIP-3 on the effect of imprinting on PO4

3- ionswas 1.32; 17.90 and 1,068 times gearter than non-imprintedpolymer, respectively.

Polymer Experimental Q (mg g-1) Pseudo- first order Pseudo- second-order

K1 Qe R2 K2 Qe R2

MIP-1 305 0,0092 277 0,99 2,95.10-5 400 0,91

MIP-2 315 0,368 344 0,97 4,69.10-5 344 0,95

MIP-3 307 0,0322 326 0,99 1,96.10-5 322 0,96

Table 2 Kinetic constants forthe SCN--imprinted polymer

0

5

10

15

20

25

0 2 4 6 8 10 12 14

pH

Ads

orpt

ion

capa

city

mg/

g

MIP1

MIP2

MIP3

Fig. 3 Effects of pH on thiocyanate adsorption; thiocyanate initialconcentration, 25 ppm; T:25°C

Thiocyanate separation by ımprinted polymeric systems 133

Desorption and repeated use

The regeneration of the adsorbent is likely to be a keyfactor in improving process economics. Desorption of theadsorbed thiocyanate ions from the imprinted particles wasalso studied in a batch experimental set up. The adsorption-desorption cycle of SCN--imprinted polymers is shown inFig. 4.

When desorption agent is used as NaOH for MIP1,formaldhyde for MIP2 and MIP3, the ions are released fromthe thiocyanate templates into desorption medium. In order toshow the resuability of the SCN--imprinted particles,adsorption-desorption cycle was repeated 5 times by usingthe same imprinted particles. The results showed that theSCN--imprinted affinity microparticles can be used repeat-edly without loosing significantly their adsorption capacities.

Conclusions

In the present work, three different SCN--imprinted polymers(MIP-1, MIP2 and MIP3)are synthesized by molecularimprinting thecniques in aqoueos solution of epiclorohydrinand TEOS, in the presence of AAPTS, SCN-, chitosan-Zn(II), AAPTS-Zn(II) complex. The effect of initial concentra-tion of SCN-1 ion, adsorption time and imprinting efficiencyon the adsorption selectivity for MIP-1, MIP-2 and MIP-3were investigated. The amount of thiocyanate ions adsorbedper unit mass of the polymer (i.e., adsorption capacity)increased with the initial concentration of thiocyanate ions.The maximum adsorption capacities of MIP-1, MIP-2 andMIP-3 were 305 mg g-1, 315 mg.g-1 and 307 mg g-1 forSCN-1 ion, respectively. It is too difficult to compare thereported adsorption rates, because there are several param-eters that determine the adsorption rate. The adsorption wasrelatively fast and the time required to reach equilibrium

Tab

le3

The

effect

ofim

printin

gon

selectivity

Polym

erSCN- (mgL

-1)

F-(m

gL-1)

KD(SCN- )

KD(F

- )k

k1Polym

erSCN-(m

gL-1)

PO43-(m

gL-1)

KD(SCN- )

KD(PO43- )

kk1

a)SCN-/F-

b)SCN-/PO43-

NON-M

IP-1

2525

4526

1239

370,03

71,97

NON-M

IP-1

2525

1784

910

816

1,65

1,32

MIP-1

2525

4,5

1644

90,07

3MIP-1

2525

1143

752

822,17

NON-M

IP-2

2525

4022

2523

8316

86,05

NON-M

IP-2

2525

3613

1869

5351

,96

17,90

MIP-2

2525

1922

076

1882

1021

MIP-2

2525

6249

000

6717

930,26

NON-M

IP-3

2525

5742

9080

900,00

645

,0NON-M

IP-3

2525

5601

6990

10,08

0110

68

MIP-3

2525

1614

760

455

0,27

MIP-3

2525

2440

128

585

0

5

10

15

20

25

0 1 2 3 4 5 6

Adsorption-desorption cycle

Ads

orpt

ion

capa

city

mg/

g

MIP-1

MIP-2

MIP-3

Fig. 4 Adsorption-desorption cycle of SCN-imprinted polymer

134 E. B. Özkütük et al.

conditions was about 30 min for MIP-2, MIP-3 and seems tobe very satisfactory. This adsorption equilibrium is mostprobably due to high complexation and geometric shapeaffinity (or memory) between SCN- ion and SCN- ioncavities. Competitive adsorption of SCN-/F-, SCN-/PO4

3-

from their couple mixture was also investigated in a batchsystem. The thiocyanate adsorption capacity of the SCN--imprinted particles was higher than that other ions. Thecompetitive adsorption capacity of the SCN--imprintedparticles for thiocyanate ions was also higher than non-imprinted particles. MIP-3 was exhibited in most of the workto increase ion loading capacities and selectivity, ascompared to MIP-1 and MIP-2 (MIP-3>MIP-2>MIP-1).Sol-gel in the MIP-2 and MIP-3 was attributed to theflexibility of the cavity and which specific binding sitescontained functional groups in a predetermined orientation.Considering all results; it can be results, that this MIP-3 ismost suitable for a selective separation of SCN- from diluteaqueous solutions.

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