Removal of Cyanide in Ni(II)–Cyanide,Ni(II)–Cyanide–EDTA, and Electroplating RinseWastewater by Ferrate(VI)

Khemarath Osathaphan & Patarawan Tiyanont &Ria A. Yngard & Virender K. Sharma

Received: 10 August 2010 /Accepted: 8 December 2010 /Published online: 5 January 2011# Springer Science+Business Media B.V. 2010

Abstract Cyanide is found as free cyanide and metal–cyanide complexes in metal finishing rinse wastewaters.Experiments were performed to seek removal of cyanidein Ni(II)–cyanide and Ni(II)–cyanide–ethylenediamine-tetraacetate (EDTA) solutions by the environmentallyfriendly oxidant, ferrate(VI) (FeO4

2−, Fe(VI)) as afunction of pH (8.0–11.0). Incomplete removal ofcyanide in Ni(II)–cyanide solutions (≤60%) was ob-served at the studied pH range. However, cyanideremoval efficiency approached to 100% in Ni(II)–cyanide–EDTA solutions. Formation of Ni(II)–cyanideand Ni(II)–EDTA complexes and relative rates of thereactions of Fe(VI) with various species (water,cyanide, Ni(II)–cyanide, and EDTA) present in solu-tions were responsible for the variation in removalefficiencies in mixtures at various pH. The oxidation ofcyanide by Fe(VI) produced cyanate. Tests using

electroplating rinse wastewaters demonstrated that Fe(VI) was highly effective in removing cyanide.

Keywords Ferrate . Removal . Metal finishing . Rinsewater . Speciation . Rates

1 Introduction

Cyanide is used or produced in several types of industriesthat include gas production, metal plating, pharmaceut-icals, and mining (Young 2001a, b; Zagury et al. 2004;Mudder et al. 2008; Yngard et al. 2008; Acheampong etal. 2010). The speciation of cyanide (HCN, CN−, andmetal-complex species) determines its degree of toxicityand the free cyanide is the most toxic form (Young2001a, b). Metal-complex cyanides are classified asweak-acid dissociables (WADs) and strong-acid disso-ciables (SADs). WADs are relatively unstable com-plexes of cyanide with transition metals such as Cd, Cu,Ni, and Zn that dissociate under neutral or mildly acidicconditions. SADs form strong cyanide complexes withmetals such as Fe, Co, Ag, and Au that are dissociableonly under very acidic conditions. WADs are less toxicthan free cyanide while SADs are relatively non-toxic(Wild et al. 1994; Shifrin et al. 1996).

The gold mining and metal finishing industries areamong the largest consumers of cyanide and releaseconsiderable quantities of cyanide in their effluents (Heet al. 2007; Abou-Elela et al. 2008; Khodadad et al.2008; Acheampong et al. 2010). Effective treatment of

Water Air Soil Pollut (2011) 219:527–534DOI 10.1007/s11270-010-0725-1

K. OsathaphanDepartment of Environmental Engineering,Faculty of Engineering, Chulalongkorn University,Bangkok, Thailand

P. TiyanontEnvironmental Research Institute,Chulalongkorn University,Bangkok, Thailand

R. A. Yngard :V. K. Sharma (*)Chemistry Department, Florida Institute of Technology,150 West University Boulevard,Melbourne, FL 32901, USAe-mail: vsharma@fit.edu

these effluents must take place in order to meetregulatory safe discharge standards. Cyanides ineffluents can be removed by physical, biological,electrochemical, and chemical processes (Young2001b; Osathaphan et al. 2008a, b, c; Acheampong etal. 2010). Chemical methods usually involve oxidationof cyanide by Cl2, KMnO4, H2O2, air, and O3 (Young2001a, b; Acheampong et al. 2010). The disadvantagesof these conventional treatments include partial degra-dation of cyanides and generation of secondary pollu-tants (Young 2001b). For the last few years, the use ofiron in a +6 valence state, ferrate(VI) as FeVIO4

2− (Fe(VI)) to oxidize cyanide in water has been investigated(Sharma et al. 1998, 2002, 2005a, 2008b; Wang et al.2008). The Fe(VI) has multiple properties such as highoxidizing power, selectivity, and formation of a non-toxic by-product Fe(III), which acts as a coagulant(Jiang and Lloyd 2002; Sharma 2002, 2004, 2010a, b;Jiang and Wang 2003; Sharma et al. 2005b, 2008a;Jiang 2007; He et al. 2009; Jiang et al. 2009). Fe(VI) isthus a promising candidate for application in a cleaner(“greener”) technology for removal of cyanides inwastewater effluents. This paper presents the results onthe oxidation of cyanide in the presence of organicligands, which has not been conducted previously.

In the electroplating industry, cyanide is used tokeep metallic ions such as Ni(II) in solution. Organicligands such as ethylenediaminetetraacetate (EDTA)form strong complexes with heavy metal ions and arealso commonly applied in industrial cyanide platingprocesses for the same purpose (Beck 1987). Therefore,rinse wastewaters contain metal ions, cyanide, andEDTA (Cho et al. 2007). The focus of the presentpaper is to seek removal of cyanide in Ni(II)–cyanideand Ni(II)–cyanide–EDTA solutions by Fe(VI) atvarious pH (8.0–11.0). The products of the reactionswere also identified to ensure environmentally friendlytreatment processes using Fe(VI). The efficiency of Fe(VI) to remove cyanide from five different electro-plating rinse wastewater samples, containing varyingconcentrations of metal ions (Cr(III), Cu(II), Fe(III), Ni(II), and Zn(II)), was also determined.

2 Experimental Methods

All chemicals (Sigma, Aldrich, and Antec) werereagent grade or better and were used without furtherpurification. All solutions were prepared with distilled

water that had been passed through an 18 MΩ cmresistivity Milli-Q water purification system. Thepotassium salt of iron(VI) (K2FeO4) was synthesizedusing a wet method and was of high purity (>98%)(Thompson et al. 1951). Fe(VI) solutions wereprepared by addition of solid K2FeO4 to 0.005 MNa2HPO4/0.001 M Na2B4O7

.9H2O at pH 9.0, a bufferin which Fe(VI) is most stable. A molar absorptioncoefficient (ε510nm=1,150 M−1 cm−1) was used tocalculate the [FeO4

2−] (Rush and Bielski 1986). Stocksolutions were prepared by adding solid KCN into0.05 M NaOH while Ni(NO3)2 and tetrasodiumEDTA salts were added into deionized water. Theexperiments were carried out by mixing equalvolumes of Fe(VI) and Ni(II)–cyanide or Ni(II)–cyanide–EDTA solutions, and the concentrations oftotal cyanide in the mixed solutions were determinedafter the disappearance of the purple color of Fe(VI).Reactions were completed within 5 h.

Cyanide concentrations were determined usingDionex ion chromatography (IC): Model ICS 2500.The IC system consisted of an AS7 ion exchange columnconnected in-line to an electrochemical cell with silver asthe working electrode (0.00 V vs. Ag/AgCl reference).The eluent consisted of 0.5 M NaAc, 0.1 M NaOH, and0.5% (v/v) ethylenediamine at a flow rate of 1 ml min−1.Concentrations of cyanate, nitrite, and nitrate weredetermined using the Dionex IC. This IC systemconsisted of an AS16 ion exchange column connectedin-line to a suppressor and a ASRS 4-mm conductivitycell. The eluent was KOH, which varied from 2.8 to 55mM in a gradient mode at a flow rate of 1 ml min−1.Three replicates of cyanide and cyanate analyses wereconducted, and averages of the values are reported. Theanalytical precision of analysis was within 5.5%.

Metals in the rinse wastewater were analyzed byatomic absorption spectrophotometer (GBC ModelAvanta). Samples were filtered using 0.45 μm filtersprior to analysis. Calibration curves were constructedusing standard solutions, prepared using 100 mg L−1

stock solutions (Fluka). Three replicates of sampleswere performed. The precision of the analysis waswithin 1%.

3 Results and Discussion

Initially, the solubility of Ni(II) in a Ni(II)–cyanidesolution as a function of pH was determined by

528 Water Air Soil Pollut (2011) 219:527–534

keeping the concentration of Ni(II) and cyanide atfixed concentrations of 500 μM. These experimentalconditions were used to study the removal of cyanidein Ni(II)–cyanide solutions by Fe(VI). In the mixedsolution of Ni(II)–cyanide, the concentration ofsoluble Ni(II) was determined experimentally afterfiltering the solution. The values of the solubility ofNi(II) at different pH are given in Table 1. Thesolubility of Ni(II) decreased sharply from pH 8.0 to9.0. The decrease slowed down in the pH range from9.0 to 10.0 and no change in the solubility withfurther increase in pH to 11.0 was observed. The pHdependence of the solubility suggests the precipitationof Ni(II) as a solid Ni(OH)2 (Ksp=1.0×10

−15 at 25°C),which would increase with an increase in pH. In thesolution phase, the affinity of cyanide for Ni(II) insolution results in the formation of different Ni(II)–cyanide species according to the following equilibriumreactions (Beck 1987; Hefter and May 1991).

Ni2þ þ CN� ¼ NiCNþ logb1 NiCNþð Þ ¼ 7:0 ð1Þ

Ni2þ þ 2CN� ¼ Ni CNð Þ2o logb2 Ni CNð Þ20� �

¼ 14:0 ð2Þ

Ni2þ þ 3CN� ¼ Ni CNð Þ3� logb3 Ni CNð Þ3�� � ¼ 22:0 ð3Þ

Ni2þ þ 4CN� ¼ Ni CNð Þ42� logb4 Ni CNð Þ42�� �

¼ 31:1 ð4Þ

The overall stability constants of Ni(CN)+, Ni(CN)2,and Ni(CN)3

− are much less than that of Ni(CN)42−

and the stepwise formation of the latter occurs very

rapidly. Ni(CN)42− is thus the only major species

present in the studied pH range (Hefter and May1991), which would determine the speciation of Ni(II)and cyanide at different pH (Table 1). The maximumamount of Ni(II) that could complex with 500 μMcyanide to result in the Ni(CN)4

2− species is 125 μM;hence, most of the soluble Ni(II) at pH 8.0 would be inthe free form (Table 1). Similarly, 45 μM free Ni(II) ofthe 170 μM total soluble Ni(II) would be present at pH9.0. The solubility of Ni(II) at pH 10.0 and 11.0 is 100μM, and therefore, Ni(II) is the limiting species in theformation of Ni(CN)4

2− in the Ni(II)–cyanide solution.This would result in 100 μM free cyanide in thesolutions having free Ni(II) (Table 1).

The influence Ni(II) ions on the removal ofcyanide by Fe(VI) at different pH was investigatedin Ni(II)–cyanide solutions (open symbols, Fig. 1).Figure 1 shows the amount of cyanide remaining afterreacting with a specific amount of Fe(VI). In all cases,with increasing amounts of Fe(VI), the removalefficiency for cyanide increased in Ni(II)–cyanidesolutions; however, removal of cyanide was incom-plete at all pH values studied. The maximum removalefficiency of 60% was found at pH 8.0 in the Ni(II)–cyanide solution, followed by ~50% at pH 9.0 and10.0. The least amount of cyanide removed (30%)was at pH 11.0. The effect of EDTA on the removalefficiency was also studied (filled symbols, Fig. 1).Interestingly, addition of EDTA to the Ni(II)–cyanidesolution increased the removal efficiency of cyanideat all pH values studied, except at pH 8.0 (Fig. 1).Cyanide was almost completely removed by additionof Fe(VI) into Ni(II)–cyanide–EDTA at pH 9.0 and10.0. Removal of cyanide was ~80% at pH 11.0 (Fig.1). The results of cyanide removal with and without

Table 1 Experimentally determined Solubility and speciation of Ni(II) in Ni(II)–cyanide solutions as a function of pH (Experimentalconditions: [Ni(II)] = [Cyanide] = 500 μM)

pH [Ni(II)]solublea Speciationb

[Ni(II)]Free [Ni(II)]Complex [Cyanide]Free [Cyanide]Complex

μM μM μM μM μM

8.0 450 325 125 0.0 500

9.0 170 45 125 0.0 500

10.0 100 0.0 100 100 400

11.0 100 0.0 100 100 400

a Experimentally determinedb Considering Ni(CN)4

2− as only major species present in Ni(II)–cyanide solution

Water Air Soil Pollut (2011) 219:527–534 529

EDTA in the Ni(II)–cyanide solutions can beexplained by considering reactions of Fe(VI) withindividual species present in the mixed solutions.

Table 1 suggests that Fe(VI) can possibly reactwith cyanide and Ni(CN)4

2− species present in Ni(II)–cyanide solutions (Eqs. 5 and 6).

Fe VIð Þ þ cyanide ! Fe IIIð Þ þ product sð Þ ð5Þ

Fe VIð Þ þ Ni CNð Þ42� ! Fe IIIð Þ þ product sð Þ ð6ÞFe(VI) can also self-decompose due to its reactionwith water (Eq. 7).

2FeO42� þ 5H2O ! 2Fe3þ þ 3=2O2 þ 10OH� ð7Þ

In the presence of EDTA in Ni(II)–cyanidesolutions, an additional reaction (Eq. 8) needs to beconsidered.

Fe VIð Þ þ EDTA ! Fe IIIð Þ þ product sð Þ ð8Þ

The variation in removal efficiency with variable pHis related to the competing reactions taking place in thereaction mixtures (Eqs. 5–8). The rates of reactions 5,6, and 8 follow second-order kinetics, first-order withrespect to concentrations of Fe(VI) and the otherreactant. The values of second-order rate constants(k, M−1 s−1), determined independently, are given inTable 2 (Sharma et al. 1998; Noorhasan and Sharma2008; Yngard et al. 2008; Sharma 2010a). Reaction (7)goes through mixed first- and second-order kinetics(Carr 2008). In the present work, decay of Fe(VI) bywater (reaction 7) largely follows first-order kinetics(k’, s−1). The reactivity of Fe(VI) with the studiedreactants varies significantly with pH (Table 2). Thevalues of the second-order rate constants (k, M−1 s−1)for the reactions of Fe(VI) with cyanide, Ni(CN)4

2−,and EDTA were converted to first-order rate constants(k’, s−1) in order to compare rates of various reactionsin the mixed solutions (Table 2).

The removal of cyanide in Ni(II)–cyanide solutionsat different pH will involve reactions of Fe(VI) with

pH 8.0

[Fe(VI)], µM

0 100 200 300 400 500 600

[Fe(VI)], µM

0 100 200 300 400 500 600

[Fe(VI)], µM

0 100 200 300 400 500 600

[Fe(VI)], µM

0 100 200 300 400 500 600

[Cya

nide

] rem

aine

d, µ

M

0

100

200

300

400

500

600pH 9.0

0

100

200

300

400

500

600

pH 10.0

[Cya

nide

] rem

aine

d, µ

M

0

100

200

300

400

500

600pH 11.0

0

100

200

300

400

500

600

Fig. 1 Oxidation of cyanidein Ni(II)–cyanide solutions atdifferent pH (open symbolsNi(II)–cyanide solution;filled symbols Ni(II)–cya-nide–EDTA solution)

530 Water Air Soil Pollut (2011) 219:527–534

Ni(CN)42− and water. The incomplete removal of

cyanide suggests that reaction (7) is competing withreaction (5). However, the rate constants of thereaction of Fe(VI) with Ni(CN)4

2−, except at pH11.0, are at least one-order of magnitude faster thanthe self-decomposition of Fe(VI) (k6’/k7’=10–103,Table 2). The use of inaccurate values of k7’ underour experimental conditions may be the cause of thediscrepancy in the expected decay of cyanide usingrate constants and experimental results. The values ofk7’ reported in Table 2 are for only the bufferedsolutions while the buffered Ni(II)–cyanide solutionsin the present study also contain Ni2+ ions (see Table1). It has been demonstrated that Ni2+ ions in solutionenhance the decomposition of Fe(VI) (Veprek-Siskaand Ettel 1967; Licht et al. 1999). The Ni2+ ions inthe solution can be inherently present (for example atpH 9.0 and 10.0; Table 1) and would also beproduced in the decomposition of Ni(CN)4

2− by Fe(VI). Hence, the values of k7’ are expected to be fasterunder our experimental conditions than those given inTable 2 and reaction (7) would be expected to competesuccessfully with reaction (5) to result in incompleteoxidation of cyanide in the Ni(II)–cyanide solutions(Fig. 1).

In the presence of EDTA, stoichiometries of 1:1(Fe(VI)/cyanide) were obtained at pH 9.0 and 10.0while ratios >1 were obtained for the consumption ofFe(VI) per mole of cyanide at pH 8.0 and 11.0 (Fig.1). Such stoichiometric consumptions as a function ofpH are similar to the results obtained for the oxidationof uncomplexed cyanide by Fe(VI) (Sharma et al.1998). This indicates that the addition of EDTA,eliminated the additional Fe(VI) amount needed to

oxidize cyanide in Ni(II)–cyanide mixed solution. Onepossibility would be that Ni(II) ions form strongercomplexes with EDTA (logK(NiEDTA2−=17.1) thanwith cyanide (logK(Ni(CN)4

2−=9.1); causing the re-lease of cyanide from the Ni(II)–cyanide complex.The exchange reaction may also be kinetically driven(Eq. 9).

Ni CNð Þ42� þ EDTA4� ! Ni IIð Þ � EDTA½ �2� þ 4CN�

ð9ÞAnother possibility is that there may be mixed Ni

(II)–EDTA–cyanide complexes formed in the solutions.Such complexes have been reported in the literature(Margerum et al. 1961; Kolski and Margerum 1969;Kumar and Nigam 1980). The mixed complexes mayhave similar reactivity with Fe(VI). This would explainthe observed results of adding EDTA into Ni(II)–cyanide solutions (Fig. 1).

The reaction of Fe(VI) with cyanide released fromNi(CN)4

2− by EDTA is also supported by therelatively sluggish reactivity of Ni(CN)4

2− and EDTAwith Fe(VI) (Table 2). Comparatively, Fe(VI) reactsrapidly with cyanide. This can be seen in the ratios ofk5’/k6’ and k5’/k8’, which varied from 11.0 to 779(Table 2). The incomplete removal of cyanide in Ni(II)–cyanide–EDTA mixed solutions at pH 8.0 and11.0 may be due to competing reactions of Fe(VI)with water in the presence of Ni2+ ions. The ratios ofk5’/k7’ are so high at pH 9.0 and 10.0, that reaction (7)was not able to compete with reaction (5) (Table 2).However, the results at pH 8.0 and 11.0 in Fig. 1clearly indicate the possibility of simultaneous reactionsof Fe(VI) with water and cyanide.

Table 2 Rate constants for reactions of Fe(VI) with water, cyanide, Ni(CN)42−, and EDTA at different pH

pH Water Cyanideb Ni(CN)42−c EDTAd k6’/k7’ k5’/k7’ k5’/k6’ k5’/k8’

k7’, s−1 k5, M

−1 s−1 (k5’, s−1) k6, M

−1 s−1 (k6’, s−1) k8, M

−1 s–1 (k8’, s−1)

8.0 3.8×10−3a 4.9×102 (9.5×10−1) 1.7×102 (8.6×10−2) 1.2×101 (6.0×10−3) 22.6 250 11.0 158

9.0 9.7×10−5a 3.0×102 (6.7×10−1) 2.0×101 (1.0×10−2) 1.7×100 (8.6×10−4) 103 6907 67.0 779

10.0 9.7×10−5 7.3×101 (1.7×10−1) 2.0×100 (1.0×10−3) 6.0×10−1 (3.0×10−4) 10.3 1753 170 566

11.0 1.0×10−4 8.6×100 (2.0×10−2) 2.0×10−1 (1.0×10−4) 1.7×10−1 (8.5×10−5) 1.0 200 200 235

First-order rate constants were calculated using [Fe(VI)] = 500 μMaCarr 2008b Sharma et al. 1998c Yngard et al. 2008d Noorhasan and Sharma 2008

Water Air Soil Pollut (2011) 219:527–534 531

The products of the reaction between Fe(VI) andcyanide in Ni(II)–cyanide–EDTA solutions at differentpH were determined. Fe(III) was the final reducedproduct of Fe(VI). The products of cyanide oxidationtested were cyanate, nitrite, and nitrate ions. Cyanatewas the only final product detected while the otherpossible products, nitrite and nitrate of cyanide destruc-tion were not detected. Figure 2 shows the relationshipbetween cyanide degradation and cyanate formation.The ratio of cyanide removed to cyanate formed was0.99±0.03 and is independent of pH (Fig. 2). A similarratio of cyanide oxidation by Fe(VI) in an oxygenatedenvironment was previously determined (Sharma et al.1998). A recent study of the correlation of rates withredox potential of cyanides demonstrated that theoxidation of cyanide occurs through 1−e− transfersteps (Sharma 2010b). The sequential 1−e− reductionof FeV to FeIV to FeIII by CN− in an alkaline mediumwas spectroscopically observed (Sharma et al. 2001). Afree radical mechanism explains the requirement of onemole of Fe(VI) for oxidation of each cyanide ligand inNi(CN)4

2− (Yngard et al. 2008). The stoichiometry ofthe reaction between Fe(VI) and Ni(CN)4

2− can bewritten as Eq. 10.

4HFeO4� þ Ni CNð Þ42� þ 6H2O ! 4Fe OHð Þ3 þ Ni2þ

þ 4NCO� þ 4OH� þ O2

ð10Þ

Removal of cyanide from the rinse water samplesof a metal plating facility by Fe(VI) was examined.The composition of rinse water samples is given inTable 3. Of the five tested samples, only one sample

(no. 4) had detectable concentration of EDTA (13.5mg L−1). The rinse water samples were diluted tobring the total cyanide concentration to about 100 μMand the pH was adjusted to 9.4. Increasing amounts ofFe(VI) were added to the diluted rinse water andresidual cyanide was determined after completion ofthe reactions. As shown in Fig. 3, complete removalof cyanide was achieved with the addition of Fe(VI).However, a higher amount of Fe(VI) for rinse waterthan that for deionized water was needed for completeremoval of cyanide. Additional components present inrinse water increased the demand for Fe(VI). Asexpected the small amount of EDTA present in rinsewater sample no. 4 did not increase the demand of Fe(VI) to remove cyanide. The results are consistent withthe oxidation of cyanide in the presence of metals(Costarramone et al. 2004; Yngard et al. 2007; Yngardet al. 2008).

[Cyanide]Decrease, µM0 100 200 300 400 500

[Cya

nate

] For

mat

ion,

µM

0

100

200

300

400

500

pH 8.0pH 9.0pH 10.0pH 11.0

Fig. 2 A plot of cyanide decrease versus cyanate formation inthe oxidation of cyanide in Ni(II)–cyanide–EDTA by Fe(VI) atdifferent pH

Table 3 Concentration of constituents in electroplating rinsewater

Sample no. mg L−1

Cyanide Cr Cu Fe Ni Zn EDTA

1 541 26.6 28.7 0.78 30.1 1.32 <1.0

2 503 8.85 12.4 2.5 17.6 8.17 <1.0

3 447 30.2 3.8 0.74 4.5 0.53 <1.0

4 653 0.70 12.7 0.14 2.3 3.08 13.5

5 674 38.0 19.3 2.37 29.0 0.14 <1.0

[Ferrate(VI)], µM0 100 200 300

Res

idua

l Cya

nide

, µM

0

20

40

60

80

100

Sample #1Sample #2Sample #3Sample #4Sample #5

Water

Fig. 3 Removal of cyanide by Fe(VI) in rinse wastewatersamples obtained from a metal plating facility

532 Water Air Soil Pollut (2011) 219:527–534

4 Conclusions

The solubility of Ni(II) in the Ni(II)–cyanide solutionswas limited by the pH; an increase in the pH wasaccompanied by a decrease in solubility. Incompleteremoval of cyanide in Ni(II)–cyanide solutions atdifferent pH values was found to be due to simultaneousreactions of Fe(VI) with water and Ni(CN)4

2−. Additionof EDTA to the Ni(II)–cyanide solutions resulted incomplete removal of cyanide at pH 9.0 and 10.0.Removal patterns of cyanide in Ni(II)–cyanide–EDTAat different pH were successfully explained by consid-ering rates of the reactions of Fe(VI) with water,cyanide, Ni(CN)4

2−, and EDTA. The molar stoichiom-etry of Fe(VI) consumption to cyanide decrease was 1:1and cyanide was transformed to relatively non-toxic,cyanate. Rinse wastewater of a metal plating facilitysubjected to Fe(VI) also showed complete removal ofcyanide.

Acknowledgment This researchwas supported by the GraduateSchool, Chulalongkorn University and the National ExcellenceCenter for Environmental and Hazardous Waste Management,Chulalongkorn University, Bangkok, Thailand. We wish to thankDr. Mary Sohn for useful comments on the paper. Authors wish tothank anonymous reviewers for useful comments to improve thepaper.

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