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Journal of Hazardous Materials 166 (2009) 1362–1366

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

reatment of cyanide effluents by oxidation and adsorption in batchnd column studies

.Y. Yazıcı, H. Deveci ∗, I. Alpineral Processing Division, Department of Mining Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey

r t i c l e i n f o

rticle history:eceived 23 September 2008eceived in revised form 5 November 2008ccepted 9 December 2008

a b s t r a c t

In this study the removal of free cyanide from aqueous solutions by air oxidation and adsorption wasinvestigated. Effects of air and pure oxygen, and catalyst on the rate and extent of the removal of cyanidewere studied. It was found that the oxidative removal of cyanide by air/oxygen was very limited althoughit tended to improve in the presence of pure oxygen and catalyst such as activated carbon (AC) and copper

vailable online 14 December 2008

eywords:yanide removalir oxidationdsorption

sulphate. In the presence of continuous aeration, the non-oxidative removal of cyanide was correlatedwith a decrease in pH effected apparently by the transfer of carbon dioxide from air phase into the medium.The removal of cyanide by adsorption on activated carbon, nut shell (NS) and rice husk (RH) was also exam-ined. Adsorption capacity of activated carbon was shown to be significantly enhanced via impregnationof activated carbons with metals such as copper (AC–Cu) and silver (AC–Ag). In the column tests, the

adso

ctivated carbonnvironment

breakthrough capacity of

. Introduction

Cyanide has been the most preferred solvent in the extractionf gold and silver ores over a century due to its strong complexingapability, ready availability, relatively low cost and its well-knownhemistry [1,2]. Cyanide is also most extensively used in metal fin-shing and production of plastics [3–5]. Wastewaters generatedn these operations often contain cyanide species i.e. free and

etal–cyanides and cyanide related compounds at various lev-ls [5–7]. Regarding the toxicity of cyanide species with the freeyanide being the most toxic, treatment of cyanide containing efflu-nts is prerequisite to lower cyanide content to admissible levels toulfill environmental regulations.

Natural attenuation, chemical and biological oxidation, com-lexing/precipitation and recovery/recycling processes are cur-ently exploited to remediate the effluents containing cyanide8–16]. Natural attenuation is a slow process and closely con-rolled by climate conditions [1,11,17–19]. Oxidative treatment

ethods e.g. ozonation, hydrogen peroxide and SO2/air are effec-ively used for the removal of weak acid dissociable (WAD) cyanideompounds (pK < 30) [11,20,21]. Advanced oxidation processes e.g.

hotochemical oxidation, ultrasonic waves for the treatment ofyanide effluents has been also studied by some researchers [22,23].owever, these are expensive and not effective for the treatment of

trong acid dissociable (SAD) cyanides (pK > 30). Activated carbon

∗ Corresponding author. Tel.: +90 462 377 3681; fax: +90 462 325 7405.E-mail address: hdeveci@ktu.edu.tr (H. Deveci).

304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2008.12.050

rbents was found to be in an increasing order of RH < AC < AC–Cu « AC–Ag.© 2008 Elsevier B.V. All rights reserved.

and various agricultural products can be used as adsorbent for theremoval of cyanide [1,24–27] from effluents. Although its adsorp-tive properties were exploited in most studies, activated carboncan act as an oxidation catalyst in the presence of oxygen [28,29].The aeration can significantly improve the adsorptive properties ofactivated carbon probably linked with the oxidation of chromene-and/or quinone-type functional groups and, thus, the generationof positively charged active sites for further adsorption [27–30].However, the contribution of aeration to the adsorptive removal ofcyanide has not been considered in most studies.

In this study the removal of cyanide from aqueous solutionsby catalytic air oxidation and adsorption was investigated. Effectof solution pH and catalyst such as copper and activated carbonin the presence of air/oxygen on the rate and extent of cyanideremoval were examined. Removal of cyanide using plain and metal-impregnated activated carbons and agricultural by-products i.e. ricehusk and nut shell was also studied through batch and column tests.The importance of aeration for the adsorptive removal of cyanideby these adsorbents was demonstrated.

2. Experimental

2.1. Reagents and adsorbents

Reagent grade sodium cyanide (NaCN), copper chloride(CuCl2·2H2O), copper sulphate (CuSO4·5H2O), silver nitrate(AgNO3), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2,35%, w/w) were used to prepare stock solutions in distilled water.

E.Y. Yazıcı et al. / Journal of Hazardous M

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[1]. Cyanide exists essentially in the form of HCN at pHs below9.3 (pKa) under non-oxidising conditions. The level of free cyanide(CN−) in equilibrium with HCN decreases further with increasingacidity. These theoretical calculations confirm the substantial loss

Fig. 2. Effect of aeration (0.27 l/min) on the removal of cyanide ([CN−]0: 100 mg/l inoxidation reactor (OR)).

ig. 1. Experimental setup for air oxidation ([CN−]0: 100 mg/l and [CuSO4]0:–20 mg/l in OR).

.2. Air oxidation studies

Air oxidation tests were performed in a closed system consistingf two glass reactors i.e. oxidation and absorption reactors (Fig. 1).he oxidation reactor (OR, 1 l) contained cyanide waste solution100 mg/l CN−, 800 ml) and NaOH solution (400 ml) was used in thebsorption reactor (AR) to entrap HCN gas formed in the oxidationeactor. Air was supplied using a sparger to oxidation reactor at aow rate of 0.27 l/min. Effect of pH control, the use of pure oxygennd the catalytic effect of activated carbon (10–20 g/l) and copperulphate (20 mg/l) on the oxidative removal of cyanide was alsoetermined.

A separate experiment was also designed to confirm the gener-tion of H2O2 by activated carbon in the presence of air. A knownmount of activated carbon (10%, w/vol.) was added into distilledater and air (0.27 l/min) was blown into the solution. The con-

entration of hydrogen peroxide was determined using a filterhotometer at 520 nm.

.3. Adsorption studies

Coconut shell activated carbon (−4 + 1 mm, BET: 546 m2/g)s plain (AC) and impregnated with copper (AC–Cu) and silverAC–Ag), hazel nut shell (−4 + 1 mm) with/without heat treat-

ent (for 15 min at 300 ◦C) and rice husk (−2 + 1 mm) were usedn adsorption tests. Copper and silver impregnation of activatedarbons were conducted in CuCl2 (1 g/l) and AgNO3 (1–10 g/l) solu-ions, respectively over a period of 72 h. Activated carbons with anal metal content of 5.07% Ag and 0.43% Cu by weight were used

n the experiments. Details of the preparation of these adsorbentsor experiments can be found elsewhere [30].

The adsorption tests were carried out in Pyrex beakers (600 ml)ith an initial cyanide concentration of 100 mg/l (300 ml) at an

dsorbent dosage of 1 g/l. Magnetic stirrers were used to agitatehe reactor contents. Column tests were performed in PTFE columnsith an inside diameter (I.D.) of 26 mm (145 mm in length) at an

dsorbent charge of 5 g. Fresh cyanide solution (100 mg/l CN−; pH1) was fed from the top of the column at a flow rate of ≈1.5 ml/min.amples were taken from the effluent solution at certain intervalso construct the breakthrough curve.

Concentration of cyanide in solution was determined by

ilver nitrate titration in the presence of p-dimethylamino-enzylrhodanine (0.02%, w/w in acetone) as indicator [31]. Thoughhe titration method used is suitable for the determination of freeyanide, some species of WAD cyanide e.g. Zn(CN)4

2− and copperyanide complexes (Cu(CN)n

1−n), can also be detected, if they are

aterials 166 (2009) 1362–1366 1363

present in solution; albeit, the recovery of cyanide associated withcopper is not complete [1]. pH was controlled by the addition of 1 MNaOH, if required.

3. Results and discussion

3.1. Removal of cyanide by air oxidation

No significant change in cyanide concentration in oxidationreactor was observed over an initial period of 22 h as shown inFig. 2. However, cyanide level in solution decreased to 76 mg/lbetween 22 and 46 h apparently in coincidence with the corre-sponding decrease in pH (Fig. 3). Following this period, a furthersubstantial reduction in cyanide level (from 76 to 3 mg/l) was alsoconsistent with the further decrease in pH (Fig. 3) and with the accu-mulation of cyanide in absorption reactor over the same period. Thischange in cyanide level depending on pH was strongly related withvolatilisation of cyanide as HCN (g) due to the acidifying effect ofatmospheric carbon dioxide in air:

H2O + CO2(g) ↔ H2CO3 ↔ HCO3− + H+ (1)

H+ + CN− ↔ HCN(g) (2)

Fig. 4 represents the Eh–pH diagram of the aqueous cyanide sys-tem (a) and the pH-dependent fractionation of HCN and CN− (b)

Fig. 3. Evolution of pH in oxidation (OR) and absorption reactors (AR).

1364 E.Y. Yazıcı et al. / Journal of Hazardous Materials 166 (2009) 1362–1366

Fig. 4. (a) Eh–pH diagram for the free cyanide–water system at 25 ◦C ([CN−] = 10−3 M)

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2CN− + H2O2 + 2OH−Act. Carbon−→ 2H2O + CNO− (5)

ig. 5. Effect of copper as catalyst on the oxidative removal of cyanide in the presencef aeration (0.27 l/min) ([CN−]0: 100 mg/l and [CuSO4]0: 20 mg/l in oxidation reactorOR)).

f cyanide in the form of HCN (g) observed with a decrease in pHFigs. 2 and 3).

In natural attenuation process, loss of cyanide by volatilisationas proposed to be the most important contributing to ≈90% of

yanide removal [4,32]. This can be related with the ready volatil-sation of HCN (g) due to its high vapor pressure and low boilingoint compared with water [33,34]. Consistent with the literature,urrent findings (Figs. 2 and 3) apparently showed that volatilisa-ion of cyanide as HCN is the most significant mechanism on theate and extent of cyanide removal.

Fig. 5 illustrates the cyanide removal (19%) in the presence oferation and copper at ≥pH 11 over 72 h. More extensive (by 3.4- to.3-fold over 45 h) removal of cyanide was observed in the presencef pure oxygen with/without copper (Fig. 6). The concentration of

ig. 6. Removal of cyanide in the presence of pure oxygen (0.27 l/min) ([CN−]0:00 mg/l and [CuSO4]0: 20 mg/l, pH 11).

and (b) pH-dependent fractionation of HCN and CN− at 25 ◦C ([CN−] = 10−3 M).

dissolved oxygen were also monitored and found to be around 6 and11 mg/l for the tests performed with air (Fig. 5) and pure oxygen(Fig. 6), respectively. These also reveal the relationship betweenhigher cyanide removal and oxygen content in the medium.

3.2. Catalytic effect of activated carbon on cyanide removal

Fig. 7 illustrates that aeration exerted a significant effect on thecatalytic removal of free cyanide in the presence of AC (10 g/l) i.e.44% CN− removal (with aeration) compared with 12% (no aeration).The increase in AC dosage from 10 to 20 g/l led to an improvementin cyanide removal (Fig. 7). Addition of copper sulphate (10 mg/l)and hydrogen peroxide (10 mg/l) was found to produce a negligibleeffect on the cyanide removal (Fig. 7).

Activated carbon is known to act as a catalyst in the presenceof air since it promotes chemical reactions such as oxidation ofcyanide (Eqs. (3)–(5)) [29,35,36]. The generation of H2O2 (0.17 mg/lover 0.5 h at pH 10.5–11) in the presence of air was confirmed ina separate experiment. This suggests that a portion of CN− couldhave been catalytically oxidized by activated carbon (Eqs. (4) and(5)) in the current tests. It is mooted that cyanide is first adsorbedon the surface of activated carbon, and then catalytically oxidized[1,28]:

CN− + 0.5O2 (aq)Act. Carbon−→ CNO− (3)

2H2O + O2 + 2e−Act. Carbon−→ H2O2 + 2OH− (4)

Alicilar et al. [37–39] also studied the air oxidation of cyanidein fixed bed reactors packed with solid catalysts i.e. delrin and/or

Fig. 7. Removal of cyanide from solutions via catalyst assisted air oxidation(0.27 l/min) over a period of 10 h (activated carbon dosage: 10–20 g/l, [CN−]0:100 mg/l, [CuSO4]0: 10 mg/l, [H2O2]0: 10 mg/l, and pH 10.5–11).

E.Y. Yazıcı et al. / Journal of Hazardous Materials 166 (2009) 1362–1366 1365

Table 1Adsorption parameters of Thomas and Yoon-Nelson models for the removal of cyanide by plain (AC), copper- (AC–Cu) and silver-impregnated (AC–Ag) activated carbons andrice husk (RH) ([CN−]0: 100 mg/l and pH 11).

Adsorbent qexp (mg/g) Thomas Yoon-Nelson

q0 (mg/g) kT (×10−4 l/(min mg)) R2 kYN (×10−2 l/min) � (min) R2

A 0.9739 0.65 603.122 0.9739A 0.9748 1.59 212.969 0.9748A 0.9367 2.58 85.596 0.9367R 0.9784 19.98 65.225 0.9784

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C–Ag 16.426 18.094 0.65C–Cu 6.032 6.389 1.59C 3.516 2.568 2.58H 0.401 1.957 19.99

2O5, under co- or counter-current flow conditions. The authors37] reported that both delrin and V2O5 were effective as catalystsince over 95% of cyanide was removed under the conditions of botho- and counter-current flows.

.3. Adsorption of cyanide by activated carbon and agriculturaly-products in batch and column studies

Fig. 8 illustrates that activated carbon was more effective thangricultural by-products for the removal of cyanide and impreg-ation of activated carbon with metals remarkably enhances thextent of cyanide removal in the absence/presence of aeration. Aer-tion appeared to induce a profound effect on the extent of cyanideemoval for all type of adsorbents used in this study i.e. 3.2- to 25.6-old higher than that in the absence of air, in consistent with currentndings (Fig. 8). This enhancement in the extent of cyanide removalould be attributed to the catalytic activity of these adsorbentsnd/or the air oxidation of cyanide [27]. The oxidation of surfaceunctional groups (i.e. chromene- and/or quinone-type functionalroups on carbon material such as activated carbon) in the pres-nce of air could lead to the formation of positively active sites onhe adsorbents and to the increase in their adsorption capacity foregatively charged ions such as cyanide [27,29].

Breakthrough curves produced in the column tests for the mostffective adsorbents (AC, AC–Cu and AC–Ag) are shown in Fig. 9.he data for RH was also included for comparison with AC-baseddsorbents. Consistent with the earlier findings (Fig. 8) metal-mpregnated activated carbons had greater capacities than plainctivated carbon (and RH) with AC–Ag having the highest break-hrough capacity (≈500 ml, 50-bed volume). Adhoum and Monser

26] also reported that the silver impregnation improved by 3.7-foldhe breakthrough capacity of activated carbon for CN− adsorption.

Thomas (Eq. (6)) and Yoon-Nelson (Eq. (7)) models are widelysed to describe breakthrough curves for the removal of inorganicnd organic constituents [40,41]. In the current study, these mod-

ig. 8. Removal of cyanide by plain (AC), copper- (AC–Cu) and silver-impregnatedAC–Ag) activated carbons, rice husk (RH), nut shell (NS) and activated nut shellANS) over 10 h in the absence/presence of air (adsorbent dosage: 1 g/l, [CN−]0:00 mg/l, and pH 10.5–11).

Fig. 9. Breakthrough curves for cyanide adsorption by plain (AC), copper- (AC–Cu)and silver-impregnated (AC–Ag) activated carbons and rice husk (RH) ([CN−]0:100 mg/l and pH 11).

els were used to describe breakthrough curves for the adsorptionof cyanide by plain (AC), copper- (AC–Cu) and silver-impregnated(AC–Ag) activated carbons and rice husk (RH).

Ce

Co= 1

1 + exp[kT(q0 × m − Co × V)/�](6)

Ce

Co= 1

1 + exp[kYN(� − (V/�))](7)

where Ce (mg/l) and Co (mg/l) are the cyanide concentration ofeffluent and initial solution, respectively; V (l) and � (l/min) are theeffluent volume and volumetric flow rate, respectively; q0 (mg/g)and m (g) are the maximum capacity and mass of adsorbent, respec-tively; kT (l/(min mg)) and kYN (l/min) are the rate constant forThomas and Yoon-Nelson model, respectively; � (min) is the timerequired for 50% cyanide breakthrough [40].

The parameters of Thomas (kT and q0) and Yoon-Nelson (kYN and�) models were determined using non-linear regression (Table 1).Experimental capacities (qexp) of the adsorbents were also pre-sented to compare with the calculated capacities (q0) using Thomasmodel. Both models were seemed to be consistent with the data asindicated by high correlation coefficients (Table 1). It is also per-tinent to note that these experimental capacities in the columnstudies are lower than those previously determined values in equi-librium studies (e.g. 16.43 mg/g in column studies c.f. 29.6 mg/g inthe equilibrium studies of AC–Ag [27]). This can be attributed tothe increased contact time and probably to the positive effect ofaeration of the cyanide solution in the latter. In this regard, the per-formance of these adsorbents in the columns could be improved viathe aeration of cyanide solutions prior to the feeding to the column.

4. Conclusions

This study has shown that non-oxidative removal of cyanideinduced by the decrease in pH is the leading mechanism in thepresence of aeration since the oxidation of cyanide by air is ratherlimited. Oxidative removal of cyanide can be improved by supply-ing pure oxygen into the solution in the absence/presence of copper.

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urthermore, activated carbon was found to catalyse the oxidativeemoval of cyanide in the presence of air. The catalytic effect ofhe activated carbon appeared to be derived from the generationf the strong oxidant, H2O2, in the presence of air resulting in thenhanced oxidation of cyanide.

Adsorption studies have also shown that activated carbon andgricultural by-products have relatively low capacity for cyanideemoval. However, adsorption capacity of activated carbons washown to be significantly enhanced via impregnation with met-ls, silver (AC–Ag), in particular. Column studies have indicatedhat AC–Ag has the highest breakthrough capacity (≈500 ml, 50-ed volume) compared with plain activated carbon and agriculturaly-products. Breakthrough curves for plain and metal-impregnatedctivated carbons and rice husk can be described by both Thomasnd Yoon-Nelson models.

cknowledgements

The authors would like to express their sincere thanks andppreciation to the Research Foundation of Karadeniz Technicalniversity for the financial support (Project No. 2002.112.8.3), torol Yılmaz (University of Quebec in Abitibi-Temiscamingue) andr. Celal Duran (KTU) for their technical support and to the New-ont Mining Co. (Ovacik Gold Mine, Turkey) for kindly providing

he activated carbon samples.

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