ARTICLE IN PRESS

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Water Research 39 (2005) 1652–1662

www.elsevier.com/locate/watres

Heterogeneous catalytic degradation of cyanide usingcopper-impregnated pumice and hydrogen peroxide

Mehmet Kitisa,�, Emine Karakayaa, Nevzat O. Yigita,Gokhan Civelekoglua, Ata Akcilb

aDepartment of Environmental Engineering (MMF, Cevre Muh.), Suleyman Demirel University, Isparta TR32260, TurkeybMineral Processing Division, Department of Mining Engineering, Suleyman Demirel University, Isparta TR32260, Turkey

Received 18 August 2004; received in revised form 11 January 2005; accepted 31 January 2005

Available online 19 March 2005

Abstract

The main objective of this research was to investigate the oxidative destruction of free cyanide with hydrogen

peroxide and copper-impregnated pumice as a heterogeneous catalyst. Original or copper-impregnated pumices added

alone were not effective adsorbents of negatively charged cyanide ions due to incompatible surface interactions.

Peroxide and original pumices added together were also ineffective in removing cyanide. However, for all of the three

natural pumices tested with various particle size fractions, the use of copper-impregnated pumices and peroxide

together significantly enhanced both the initial rate and extent of cyanide removal. Although copper-impregnated

specific surface area was the major factor affecting the rate and extent of cyanide destruction for a particular pumice

source with similar surface chemistries, the type of surface chemistry (i.e., specific functional groups) within different

pumice sources also appears to be a very important factor. Lower rates and extents of cyanide removals were observed

at pH 11 compared to pH 8 probably because of the negative impacts of alkaline conditions in terms of scavenging

peroxide and forming more negatively charged pumice surfaces. Both the initial rate and ultimate extent of cyanide

removals were generally higher at a temperature of 20 1C compared with those found at 10 1C. The use of copper-

impregnated pumice as a light, cheap, readily available, natural, and porous heterogeneous catalyst either in completely

mixed/suspended or fixed-bed reactor configurations may be an effective treatment technology for cyanide removal

from solution. This new approach may minimize downstream metal removal problems experienced in conventional

cyanide oxidation technologies.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Catalyst; Copper; Cyanide; Hydrogen peroxide; Impregnation; Pumice

1. Introduction

Several industries including mining, metal processing,

electroplating, and organic chemicals production use

cyanide in large quantities. It is used at nearly 90% of

e front matter r 2005 Elsevier Ltd. All rights reserve

atres.2005.01.027

ing author. Tel.: +90246 237 1276;

7 0859.

ess: mkitis@mmf.sdu.edu.tr (M. Kitis).

the gold mines worldwide and is potentially toxic

(Mudder et al., 2001). Cyanide can be removed using

chemical, catalytic, electrolytic, biological, ultrasonic,

and photolytic methods (Young and Jordan, 1995;

Augugliaro et al., 1997; Saterlay et al., 2000; Botz,

2001; Mudder et al., 2001). The copper-catalyzed

chemical processes, including the sulfur dioxide and air

(e.g., the patented INCO Ltd. Process) and hydrogen

peroxide, have been successfully applied worldwide for

d.

ARTICLE IN PRESSM. Kitis et al. / Water Research 39 (2005) 1652–1662 1653

the oxidative destruction of cyanide in leaching process

solutions or slurries (Botz, 2001; Mudder et al., 2001;

Mudder and Botz, 2004), but there are still concerns in

mining applications due to the strict environmental

regulations and costs (Akcil, 2001, 2002, 2003; Akcil and

Mudder, 2003).

Hydrogen peroxide can oxidize free and weakly

complexed metal cyanides (i.e., nickel, copper, cad-

mium, and zinc cyanides), forming one or more less

toxic compounds such as cyanate and, when added in

excess, nitrite and carbonate and, eventually, nitrate

(Mudder et al., 2001; Monteagudo et al., 2004):

CN� þH2O2 ! OCN� þH2O; (1)

OCN� þ 3H2O2 ! NO�2 þ CO2�3 þ 2H2Oþ 2Hþ; (2)

NO�2 þH2O2 ! NO�

3 þH2O: (3)

It is a simple process producing environmentally

acceptable effluents without increasing total dissolved

solids as do other chemical processes. Metals complexed

with cyanide are precipitated as metal hydroxides

depending on the pH of the solution, with an optimum

pH of about 9.0–9.5. However, the oxidation process

may operate well over a wide pH range. Soluble copper

catalysts generally increase cyanide destruction rates

significantly. The copper catalyst can be either copper

present in solution (from gold/silver cyanidation pro-

cesses) or can be added as a reagent copper solution

(Botz, 2001; Mudder et al., 2001; Kitis et al., 2005).

While significantly enhancing the cyanide destruction

rate, addition of soluble copper as a catalyst requires

concentrations as high as 30–50mg/L and may thus

result in downstream problems in terms of metal

removal in the treatment system and complying with

stringent metal discharge standards. Therefore, in this

work, destruction of free cyanide with hydrogen

peroxide was tested using copper impregnated on

particle surfaces. The selected heterogeneous particle

support for this purpose was highly porous pumice

stone. To our knowledge, this is the first such report in

the literature.

Pumice is a light, highly porous (pore volumes up to

85%) volcanic stone. Pumice particles, resembling a

sponge, consist of a network of irregular or oval-shape

internal voids/pores or vesicles, some of which are

interconnected and open to the external surface, while

others are isolated inside the particle (Gunduz et al.,

1998; Wesley, 2001). Due to its microporous structure it

has a relatively high specific surface area. Pumice stone

can exhibit acidic or basic character. Since the majority

of internal pores, especially micropores, are not con-

nected, pumice has a low permeability, providing very

high isolation for heat and sound (Gunduz et al., 1998).

It may also float in water for a long time depending on

its source and density (generally about 0.5–1 kg/L). Its

hardness is about 5–6 on the Moh scale. Pumice stone

has a high silica content (generally 60–75% SiO2), which

makes it abrasive. It has been used as an abrasive,

supplementary and polishing material in textile and

chemical industries. It is used in construction sector as a

lightweight aggregate in precast masonry units, poured

concrete, insulation and acoustic tiles and plaster.

Pumice has also been placed underground in agricultural

fields in draught areas since it may hold water for

extended period of time, thus decreasing water loss and

providing moisture for plant roots (Gungor and

Tombul, 1997; Gunduz et al., 1998; Farizoglu et al.,

2003).

The high porosity of volcanic pumice rock and its

richness in silica, alumina, and natural zeolites makes

this material a promising candidate as a catalyst in

reactions that require active centers or as a support for

the metallic function required in isomerization or

hydrogenation reactions (Brito et al., 2004). Therefore,

pumice has been used as a support for the preparation of

metal catalysts (i.e., nickel, palladium, copper–palla-

dium, silver, platinum, and rhodium), which are

employed for hydrogenation of various compounds

such as CO, alkadienes and alkynes, phenylacetylene,

but-1-ene, unsaturated hydrocarbons, nitrate, nitrite,

and for hydroisomerization of n-pentane (Deganello et

al., 1994, 2000; Fagherazzi et al., 1994; Venezia et al.,

1995, 1998; Balerna et al., 1999; Guczi et al., 1999; Brito

et al., 2004). Pumice is an ideal support that presents low

microporosity and minimizes the interference between

the support and metal particles (Venezia et al., 1992;

Brito et al., 2004).

Pumice stone has also been tested and used in various

environmental applications mainly as an adsorbent, as

filtration media and for biolfilm support. It was used

successfully for slow filtration to remove pathogens

from irrigation waters at low cost in horticulture

(Wohanka et al., 1999). It also exhibited a high potential

for use as a filter bed material for turbidity removal

under rapid filtration conditions (Farizoglu et al., 2003).

The slow filtration of mine processing slurries through a

mixture of pumice and common clay demonstrated

excellent retention of Cu, Fe, Mo and As in batch

filtrations. The retention of oil and grease by pure

pumice was also excellent (Kelm et al., 2003). It was

suggested that sepiolite and pumice stone would be

excellent solid supports for use in biological fluidized

bed processes, and would consume less energy than sand

supports (Balaguer et al., 1997). Similarly, a fixed film

column reactor was operated with Paracoccus denitrifi-

cans (heterotrophic microorganisms), which were im-

mobilized on pumice as filling material (Karagozoglu

et al., 2002). Pumice has also been used as adsorbent for

grease and fats (Geitgey, 1994), Sr, Cs, U and Th ions

(Bassari et al., 1996), phosphorus (Njau et al., 2003),

and Cu, Zn and Ni (adsorption on to formaldehyde

ARTICLE IN PRESSM. Kitis et al. / Water Research 39 (2005) 1652–16621654

cross-linked Saccharomyces cerevisiae immobilized on

pumice) (Lale et al., 2001). Organopumice, prepared by

modification of the surface with a cationic surface-active

compound, was able to sorb hydrophobic pesticides

from water as effectively as organoclay materials (Akbal

et al., 2000). Pumice stone was a promising porous

support for the immobilization of TiO2 used for the

photocatalytic degradation of organic pollutants in

aqueous solutions including 3-nitrobenzenesulfonic acid,

Acid Orange-7 (a dye) and wastewaters collected after

biological treatment (Rachel et al., 2002; Rao et al.,

2003).

The main objective of this work was to investigate the

destruction of free cyanide with hydrogen peroxide and

copper-impregnated pumice as the catalyst. The use of

pumice for such purpose has not been investigated in the

available literature. The impacts of pumice type and size,

peroxide and pumice doses, pH, and temperature on the

extent and rate of cyanide destruction were studied. For

comparison, cyanide adsorption by original and copper-

coated pumices in the absence of peroxide was also

investigated.

2. Materials and methods

2.1. Experimental procedures

All experiments were conducted in distilled and

deionized water (DDW) employing the variable-dose

completely mixed batch reactor (CMBR) bottle-point

method. Reagent grade sodium cyanide was used to

prepare synthetic cyanide solutions in DDW with target

initial concentrations of about 100mg/L as CNWAD(weak acid dissociable cyanide). CNWAD was selected to

quantify the cyanide concentrations since it represents

the total concentration of free cyanide (CN� and HCN)

and weak and moderately strong metal-cyanide com-

plexes of Ag, Cd, Cu, Hg, Ni, and Zn. CNWAD is the

appropriate selection for most situations since it includes

the toxicologically important forms of cyanide (Botz,

2001).

Kinetic experiments in CMBRs were performed with

constant initial CNWAD concentrations but variable

hydrogen peroxide and/or pumice doses, pumice type

(i.e., original pumices from different sources or pumices

impregnated with copper), pH and temperature. For the

kinetic experiments, reaction times selected to quantify

cyanide removal were 0.5, 1, 2, 4, 8, and 24 h. Peroxide

and pumice doses ranged from 0 to 300mg/L and from 0

to 3000mg/L, respectively. Temperature and pH values

investigated were 10 and 20 1C, and 8 and 11,

respectively. Solution pH values were adjusted using

reagent grade NaOH and/or HCl solutions with

different molar concentrations. In an effort to minimize

the loss of cyanide during solution preparation espe-

cially for lower pH experiments (i.e., o11), the solutionpreparation (in a 2-L pyrex glass volumetric flask) and

pH adjustment were performed very quickly by adding

and mixing pre-determined amounts of cyanide and

acid/base. Solutions were prepared in a narrow-neck

volumetric flask filled almost completely and sealed with

parafilm to minimize the air–water interface. CMBRs

(100-mL pyrex glass bottles sealed with PTFE screw-

caps) used in all experiments were filled headspace free.

Control experiments (without peroxide and pumice

dosing) were also periodically conducted to check for

the possible losses of cyanide (i.e., via photodegradation,

hydrolysis, volatilization, natural degradation) in

CMBRs during mixing. After up to 24 h of mixing, the

loss of cyanide was negligible (72mg/L) with initialCNWAD concentrations of about 100mg/L.

For each experimental matrix, after dosing hydrogen

peroxide and/or pumice, the solutions containing

cyanide in headspace-free CMBRs were kept well-mixed

(100 rpm) in a temperature-controlled orbital shaker and

incubator (Gallenkamp). The bottles were covered with

aluminum foil to prevent photodegradation of cyanide

or peroxide during mixing. After the pre-selected

reaction time, the bottles were opened in a vacuum

hood and quickly dosed with sodium sulfite (Na2SO3) to

quench the residual peroxide (if dosed) and stop the

oxidation reactions. Sulfite was added in slight excess

(i.e., 1.2 times) of the stoichiometric requirements to

ensure complete quenching. Bottles were then closed and

mixed about 3min at 100 rpm in the shaker for

quenching reaction. Then, bottles were opened, and

about 5–10mL of sample was filtered (0.45 mm celluloseacetate sterile syringe filters) quickly to remove pumice

particles before CNWAD analysis. An aliquot of

25–30mL was filtered if the sample was to be analyzed

for copper in addition to CNWAD.

2.2. Materials

Pumice samples were obtained from the Pumice

Research and Application Center at Suleyman Demirel

University, Isparta, Turkey. Pumices from three differ-

ent sources (Isparta, Kayseri and Nevsehir) with varying

physicochemical characteristics were used. The follow-

ing codes for these pumices are used throughout this

article: Isp: Isparta; Kay: Kayseri; Nev: Nevsehir. The

raw samples were ground by a primary crusher (hammer

type), and then sieved to four different particle size

fractions (o63, 63–125, 125–250, and 250–1000 mm) foreach pumice type tested.

Pumice samples were impregnated with copper using

reagent grade Cu(NO3)2 � 3H2O (Riedel-de Haen), em-

ploying the method reported by Lai and Chen (2001)

with minor modifications. They were first pretreated in

acid solution (HCl) at pH 1, for 24 h at room

temperature, rinsed with DDW several times, and then

ARTICLE IN PRESS

Table 1

Physicochemical characteristics of the original pumice samples

Parameter Chemical characteristics (% mass)a

Nevsehir Kayseri Isparta

SiO2 74.1 68.5 59.0

Al2O3 13.5 14.9 16.6

Fe2O3 1.4 3.1 4.8

CaO 1.2 2.9 4.6

MgO 0.4 0.95 1.8

Na2O 3.7 4.1 5.2

K2O 4.1 2.8 5.4

SO3 NDb ND 0.4

TiO2 0.07 0.2 0.6

Ash content 1.7 2.6 1.6

Particle size

(mm)Internal porosity (%)c

250–500 58.5 69.2 46.3

500–1000 62.5 73.3 55.3

1000–2000 66.5 76.9 61.5

Specific gravity

(without pores)

(g/cm3)d

2.33 2.21 2.47

aObtained from XRD analysis.bBelow detection.cIncludes all interconnected open and closed pores.dBulk densities (with pores) for all above crushed pumices are

generally 0.5–1.0 g/cm3.

M. Kitis et al. / Water Research 39 (2005) 1652–1662 1655

dried at 103 1C for 36 h. The dried pumice sample (about

200 g) was placed in a 1-L pyrex glass beaker, and a

stock solution of 0.5M Cu(II) was added until all

pumice was wetted. NaOH (3N) was added dropwise

during mixing until a pH of about 9.5 was reached. After

pH adjustment, mixing was continued for another

30min. The mixture was dried at 5071 1C for about100 h; it was mixed during the first 50 h. Then, the dried

mixture was rinsed with DDW several times until the

rinsed water was free of color and turbidity (i.e., o0.1NTU). Finally, the pumice samples were dried at 80 1C

for 24 h, and 50 1C for 72 h until all moisture was

removed and constant weight was achieved.

2.3. Analytical methods

In the CNWAD analysis, free and complexed cyanide

reacts with a picric acid reagent to produce an orange

color that can be measured colorimetrically at a

wavelength of 520 nm. A UV-visible spectrophotometer

(UV-1601, Shimadzu) was used. Calibration standards

were prepared from reagent grade sodium cyanide

employing the same CNWAD analysis methodology for

samples. Calibration curves were prepared for each test

(R2 values ranged between 0.98 and 0.99 for all tests). In

all experiments, measured concentrations in triplicate

exhibited coefficient of variations around 2–8%. A 30%

hydrogen peroxide aqueous solution (Merck) was used

as the stock solution for all experiments. All chemicals

used were reagent grade. DDW was used for stock

solution preparations and dilutions.

Copper concentrations were measured using atomic

absorption spectrometry based on Standard Methods. A

scanning electron microscope (SEM) (Philips XL 30S

FEG) and SEM-EDX (energy dispersive X-ray spectro-

meter) (Philips CM 10) were used for observation of

pumice morphology and identification of atomic ele-

ments and relative concentrations of elements in pumice

samples. Operational conditions in SEM-EDX analysis

were as follows: 15.0 kV, magnification: 350–5000. X-ray

diffraction analysis (XRD) was performed using a

Philips 1710 diffractometer (40 kV, 45mA, angle resolu-

tion: 0.03 2y; analysis time: �45min). Specific surfacearea measurements were conducted using a Micromeri-

tics Flowsorb II-2300 analyzer employing conventional

multipoint BET technique (N2 gas adsorption).

Pumice surface chemistry was characterized in terms

of point of zero charge (pHPZC, the pH at which the

total net surface charge is zero), and acid and base

neutralization capacities. pHPZC value of a pumice

sample was determined through a pH equilibration

method (Karanfil, 1995). Total surface acidic groups

(NaOH uptake) and total surface basic groups (HCl

uptake) were measured employing the Boehm method

(alkalimetric titration) with minor modifications (Sum-

mers, 1986; Karanfil, 1995).

3. Results and discussions

3.1. Material characterization

Physicochemical characteristics of the original (i.e.,

untreated) pumice samples are shown in Table 1. SiO2contents ranged between 59% and 74%, values typical

for natural pumices. While the lowest SiO2 content was

in Isparta pumice, the contents of all of the other

compounds (oxides of Al, Fe, Ca, Mg, Na, K, S, and Ti)

were highest in this source. In addition, Isparta pumice

also exhibited the lowest internal porosity for all size

fractions (Table 1). It appears that there may be a

correlation between silica content and internal porosity

in natural pumices. As the particle size of fractions

obtained from sieving increased (from 250–500 to

1000–2000mm), the percent internal porosity also

increased, ranging about 11–32% for the three pumice

sources. On the other hand, specific surface areas

decreased with increasing particle sizes (Table 2),

suggesting that larger size pumice fractions mainly

consist of large pores (i.e., macropores) with lower

surface area, consistent with the literature (Gunduz et

al., 1998). For the Isparta pumice, surface areas found

ARTICLE IN PRESSTable2

Elementalcompositionandsurfaceareasoforiginalandcopper-coatedpumicesamples

Pumice

Specificsurfacearea(m2/g)

pHPZC

Totalsurfaceacidicgroupsa

Totalsurfacebasicgroupsb

Elements(%)c

(meq/g)

(meq/m2)

(meq/g)

(meq/m2)

Cu

Si

OAl

Na

KCa

Isp(o63mm)

14.19

9.0

1.33

0.09

0.80

0.06

1.6

34.2

48.4

8.6

1.4

4.7

1.1

Isp(63–125mm)

9.55

9.2

0.40

0.04

0.60

0.06

—d

——

——

——

Isp(125–250mm)

8.34

8.8

0.35

0.04

0.73

0.09

——

——

——

—

Isp(250–1000mm)

2.09

9.2

0.80

0.38

0.95

0.45

1.2

29.1

49.0

8.4

3.4

6.1

4.5

Isp(o63mm)CIe

4.51

7.0

1.08

0.24

0.68

0.15

14.1

29.7

39.9

9.5

2.2

4.6

NDf

Isp(250–1000mm)CI

3.04

7.7

1.03

0.34

0.80

0.26

26.7

18.4

39.2

7.7

4.6

3.4

ND

Kay(250–1000mm)

7.25

6.9

0.35

0.05

0.83

0.11

——

——

——

—

Kay(250–1000mm)CI

3.30

7.0

0.28

0.08

1.38

0.42

——

——

——

—

Nev(250–1000mm)

4.61

7.1

0.28

0.06

0.78

0.17

——

——

——

—

Nev(250–1000mm)CI

9.74

7.3

0.15

0.02

0.35

0.04

——

——

——

—

Codesforpumicesources:Isp:Isparta;Kay:Kayseri;Nev:Nevsehir.

aNaOHuptake.

bHCluptake.

cByweight.ObtainedfromSEM-EDXanalysis.Averageoftriplicateanalysis.

dNotmeasured.

eCI:Copperimpregnated.

f Belowdetection.

M. Kitis et al. / Water Research 39 (2005) 1652–16621656

were 14.2 and 2.1m2/g for size fractions o63 and250–1000mm, respectively. Similarly, it was found that,in addition to specific surface areas, bulk densities also

decrease with increasing particle sizes in many natural

pumices (Gunduz et al., 1998). However, when the same

size fractions of different pumice sources were com-

pared, there is a positive relation with internal porosity

and surface area, i.e. Kayseri pumice has the largest

porosity and surface area. This finding indicates that

pore structures and pore size distributions vary sig-

nificantly depending on the pumice source affecting the

available surface areas for adsorption reactions.

The impact of copper impregnation on pumice surface

areas was not consistent. While the surface area

decreased significantly with copper impregnation for

the smallest size fraction (o63 mm) of Isparta pumice, itincreased slightly for the largest size fraction

(250–1000mm) (Table 2). This suggests that coppercoating reduced available adsorption sites for nitrogen

(i.e., micropores) during BET analysis; however, such

impact was minimal for the largest size fraction with

larger and more pores. Furthermore, opposite trends

were found for Kayseri and Nevsehir pumices, suggest-

ing that the impact of copper coating on surface area

may depend upon the pumice source, pore structure,

surface chemical characteristics and how the coating

changes the surface chemistry and pore structure.

The original Isparta pumice has a higher pHPZC value

than the other pumice sources, consistent with its low

silica content and high concentration of total surface

basic groups, both on a mass- (meq/g) and surface area-

normalized (meq/m2) basis (Table 2). Kayseri and

Nevsehir pumices had pHPZC values in the neutral

range, indicating that these pumices will have minimal

surface charge in waters with neutral pHs. While copper

impregnation increased the total surface acidic groups

and thus decreased the pHPZC of Isparta pumice

significantly (250–1000mm) making its surface moreacidic, it showed minimal impact on the surface

chemistries of Kayseri and Nevsehir pumices with

neutral pHPZC values. On the other hand, copper

impregnation impacted the surface areas of these two

pumices. Thus, these results overall suggest the sig-

nificant variations between the characteristics of natural

pumices, especially their surface chemistries, which may

affect their reactivity if used as a support material,

adsorbent, or filtration media.

SEM-EDX elemental analysis showed that for both

particle sizes (o63 and 250–1000 mm) of Isparta pumicecopper impregnation significantly increased the copper

contents (by percent weight) of the pumices, suggesting

an effective impregnation process (Table 2). On the

other hand, percent Si, O, K, Ca, and Al contents were

reduced after impregnation due to the increased

contribution of Cu to the total mass of elements.

Copper impregnation was more effective for the larger

ARTICLE IN PRESS

Fig. 1. SEM images of original (A) and copper-impregnated

(B) Isparta pumice (size fraction: o63mm).

0

20

40

60

80

100

0 5 10 15 20 25

CN

WA

D (

mg/

l)

PIsp, Original PIsp, C-I

PKay, Original PKay, C-I

Time (h)

Fig. 2. Removal of cyanide by original or copper-impregnated

pumices by adsorption. Size fraction: 250–1000mm; pumicedose: 1000mg/L; no peroxide dose; pH: 8; T: 20 1C. C-I: copper

impregnated. Error bars indicate the 95% confidence intervals.

M. Kitis et al. / Water Research 39 (2005) 1652–1662 1657

size fraction (250–1000 mm) in terms of increasing thepercent copper content. This may be due to the presence

of more and larger pores in this size fraction, which may

provide more access to copper. Fig. 1 shows the SEM

images of original and copper-impregnated Isparta

pumice.

3.2. Cyanide removal

The removal of cyanide by adsorption (i.e., no

peroxide addition) by original or copper-impregnated

pumices were generally less than 20% after 24 h of

reaction time in CMBRs (Fig. 2). Although only Isparta

and Kayseri pumices with size fraction of 250–1000 mmare shown in this figure, other pumices with different

size fractions exhibited similar trends. These relatively

low removals through adsorption found from kinetic

experiments indicate that natural pumices, either origi-

nal or copper impregnated, are not effective adsorbents

of negatively charged cyanide ions, probably due to

incompatible surface interactions. Based on the pHPZC

values, the surfaces of all pumices (except original

Isparta pumice) should be dominantly negatively

charged at the experimental pH of 8, decreasing the

sorption of cyanide ions. This is consistent with the

work of Dictor et al. (1997), in which pumice stone

tested as a support in fixed-bed reactors did not adsorb

either total or free cyanide. However, it successfully

allowed fixation of bacteria for biodegradation of

cyanide. For the pumice doses tested (ranging between

30 and 1000mg/L), adsorption capacities obtained from

bottle-point equilibrium tests (24 h of mixing) were also

very low due to these low cyanide removals (data not

shown). It was generally found that removal did not

increase after 4–8 h of adsorption time, suggesting the

rapid kinetics to achieve adsorption equilibrium com-

pared with many adsorbents. In terms of the impact of

copper impregnation on adsorption, while copper

impregnation slightly improved both the rate and extent

of cyanide adsorption in all size fractions of Kayseri

pumice, it slightly decreased the adsorption rates in all

size fractions of Isparta pumices, probably due to

increased surface acidity (thus negative surface charge)

after copper impregnation. However, the extents of

cyanide removal by adsorption after 24 h were compar-

able. Consistent with the findings from material

characterization studies, the impact of copper impreg-

nation on cyanide adsorption may vary depending on

the pumice source and its physicochemical character-

istics.

Fig. 3A shows the removal of cyanide by original

pumices (i.e., untreated) and/or hydrogen peroxide

through adsorption and/or oxidation. Similar to results

presented in Fig. 2, adsorption with pumice doses of 30

ARTICLE IN PRESS

0

20

40

60

80

100

0 5 10 15 20 25

CN

WA

D (

mg/

l)

P = 30; HP = 0

P = 1000; HP = 0

P = 0; HP = 150

P = 30; HP = 150P = 1000; HP = 150

0

20

40

60

80

100

0 5 10 15 20 25

CN

WA

D (

mg/

l) P = 30; HP = 0P = 1000; HP = 0P = 0; HP = 150P = 30; HP = 150P = 200; HP = 150P = 500; HP = 150P = 1000; HP = 150P = 3000; HP = 150

Time (h)

Time (h)

(A)

(B)

Fig. 3. Removal of cyanide by original (A) and copper-

impregnated pumices (B) by adsorption and/or catalytic

hydrogen peroxide oxidation. Size fraction: o63mm (Isparta);pH: 8; T: 20 1C. HP: hydrogen peroxide. All doses are in mg/L.

Error bars indicate the 95% confidence intervals.

0

20

40

60

80

100

0 5 10 15 20 25

pH=8,P=1000,HP=0

pH=11, P=1000, HP=0

pH=8,P=0,HP=150

pH=11, P=0, HP=150

pH=8,P=200, HP=150

pH=11, P=200, HP=150

pH=8,P=1000,HP=150

pH=11, P=1000, HP=150

CN

WA

D (

mg/

l)

Time (h)

Fig. 4. Impact of solution pH on the removal of cyanide by

copper-impregnated pumice and/or peroxide. Size fraction:

o63mm (Isparta); T: 20 1C. P: pumice. HP: hydrogen peroxide.

All doses are in mg/L. Error bars indicate the 95% confidence

intervals.

M. Kitis et al. / Water Research 39 (2005) 1652–16621658

or 1000mg/L removed up to 15% cyanide. Peroxide

(150mg/L) alone removed about 43% free cyanide

(from 97 to 55mg/L CNWAD) through oxidation,

consistent with the results in the literature obtained

from experiments for tailings slurry or low-solid

containing solution samples from gold mining waste-

waters (Knorre and Griffiths, 1984; Botz, 2001; Mudder

et al., 2001; Kitis et al., 2005). Peroxide and original

pumice added together were also ineffective in removing

cyanide. The removals achieved in this case were even

lower than that of peroxide alone, suggesting that

original pumice surfaces scavenged peroxide, thus

decreasing the initial rate and extent of cyanide removal.

Impregnation of pumices with copper did not improve

the removals through adsorption alone compared to

those of original pumices (Figs. 3A,B). However, for all

of the three natural pumices tested with various size

fractions, the use of copper-impregnated pumices and

peroxide together significantly enhanced the cyanide

removals. Both the initial rate and extent of cyanide

removal increased with increasing copper-impregnated

pumice doses at a constant peroxide dose (Fig. 3B).

Cyanide removals of more than 90% were achieved with

pumice and peroxide doses of X500 and 150mg/L,

respectively, providing CNWAD concentrations of less

than 5mg/L after reaction times from 4 to 8 h. Kinetic

experiments showed that at pumice doses of X500mg/

L, 2–4 h of oxidation time was adequate to achieve the

ultimate extent of cyanide removal. These results overall

indicate that the impregnation of copper into natural

pumice particles used as heterogeneous supports sig-

nificantly catalyzes the degradation of cyanide and also

enhances the ultimate removals through peroxide

oxidation. As employed in industrial treatment applica-

tions (i.e., for gold mining wastewaters), soluble copper

catalyst, either added as a reagent copper solution or as

copper naturally present in the ore, also significantly

increase both the rate and extent of cyanide destruction

(Botz, 2001; Mudder et al., 2001; Kitis et al., 2005).

However, the addition of soluble copper introduces

another contaminant and creates new problems in terms

of downstream metals removal. Thus, the use of light,

cheap, readily available, natural, and porous supports

(i.e., pumice) impregnated with copper either in com-

pletely mixed/suspended or fixed-bed reactor configura-

tions may help minimize this problem. In addition to the

batch experiments conducted in this work, experiments

for continuous flow suspended and fixed-bed reactor

configurations are in progress to further test the

effectiveness of this approach.

Fig. 4 shows the impact of solution pH on the removal

of cyanide by copper-impregnated pumice and peroxide.

For all cases (i.e., pumice or peroxide alone, or added

together), both the initial rate and ultimate extent of

cyanide removals were higher at pH 8 as compared to

pH 11. This result is expected for an adsorption

mechanism alone, since the surfaces of pumices become

more negatively charged from pH 8 to 11, thus

interfering with the adsorption of negatively charged

free cyanide ions. Similarly, oxidation of cyanide with

peroxide alone (in the absence of any catalyst) is more

effective at lower pH values (i.e., 8–9), consistent with

our previous findings (Kitis et al., 2005). This is

probably due to the enhanced decomposition of

peroxide in highly alkaline conditions. When peroxide

ARTICLE IN PRESS

0

20

40

60

80

100

0 5 10 15 20 25

CN

WA

D (

mg/

l)

<63

125-250

250-1000

Time (h)

Fig. 6. Impact of pumice particle size fraction on the removal

of cyanide by copper-impregnated pumice and peroxide. Type:

Isparta pumice; pH: 8; T: 20 1C; pumice dose: 1000mg/L;

peroxide dose: 150mg/L. Size fractions are in micrometer.

Error bars indicate the 95% confidence intervals.

0

20

40

60

80

100

0 5 10 15 20 25

PIsp PNev PKay

CN

WA

D (

mg/

l)

Time (h)

Fig. 7. Impact of pumice type on the removal of cyanide by

copper-impregnated pumice and peroxide. Size fraction:

250–1000 mm; pH: 8; T: 20 1C; pumice dose: 1000mg/L;

M. Kitis et al. / Water Research 39 (2005) 1652–1662 1659

and pumice were added together, lower rates and extents

of cyanide removals were observed at pH of 11 because

of the negative impacts of alkaline conditions in terms of

scavenging peroxide and forming more negatively

charged pumice surfaces. However, it should be noted

that most cyanide treatment technologies employ pH

values of about 10.5–11 to minimize the formation of

HCN gas.

Fig. 5 shows the impact of temperature on the

removal of cyanide by copper-impregnated pumice and

peroxide. Consistent with many oxidation reaction

kinetics, both the initial rate and ultimate extent of

cyanide removals were generally higher at a temperature

of 20 1C compared with those found at 10 1C. The

positive impact of elevated temperature, however,

diminished with increasing pumice doses at a constant

peroxide dose. For example, at a pumice dose of

3000mg/L, temperature had essentially no impact. It

appears that the presence of excess amounts of catalyst

overcome the negative impacts of lower temperatures.

The impact of copper-impregnated pumice particle

size fractions on cyanide removals is shown in Fig. 6 for

Isparta pumice. Similar trends were also observed for

the other pumice types, for various pumice and peroxide

doses, and for pH of 11. Although the ultimate extents

of cyanide removals (i.e., after 8–24 h of oxidation time)

were comparable, the initial destruction rates increased

with decreasing size fractions when all the other

variables were constant. The initial destruction rate

(0–2 h) calculated from the smallest size fraction

(o63mm) was about 1.35 times that of the largest sizefraction (250–1000 mm). Since the surface chemistries ofthese different particle size Isparta pumice fractions

(copper impregnated) are relatively similar, this finding

may be explained by the specific surface areas alone,

which were found to increase with decreasing particle

sizes. Increased available surface areas coated by copper

should accelerate the kinetics of the heterogeneous

0

20

40

60

80

100

0 5 10 15 20 25

CN

WA

D (

mg/

l)

T=10, P=0, HP=150

T=20, P=0, HP=150

T=10, P=200, HP=150

T=20, P=200, HP=150

T=10, P=1000, HP=150

T=20, P=1000, HP=150

T=10, P=3000, HP=150

T=20, P=3000, HP=150

Time (h)

Fig. 5. Impact of temperature on the removal of cyanide by

copper-impregnated pumice and/or peroxide. Size fraction:

250–1000mm (Isparta); pH: 8. P: pumice. HP: hydrogen

peroxide. All doses are in mg/L. Error bars indicate the 95%

confidence intervals.

peroxide dose: 150mg/L. Error bars indicate the 95%

confidence intervals.

catalytic surface oxidation of cyanide. Furthermore,

the mass transport of cyanide via diffusion in internal

pores is expected to be faster in smaller particle size

fractions, a trend consistent with many adsorbents such

as activated carbon. However, when used in fixed-bed-

or suspended reactor systems in treatment processes, the

lower size fractions of these pumices may be less feasible

due to more hydraulic headloss or more difficulty in

solids separation (e.g., via sedimentation).

Fig. 7 shows the impact of pumice source on the

removal of cyanide by copper-impregnated pumices and

peroxide, when all other variables including size fraction

were held constant. Based on the above discussions

regarding the positive impacts of specific surface areas

on kinetics, one may expect that copper-impregnated

ARTICLE IN PRESSM. Kitis et al. / Water Research 39 (2005) 1652–16621660

Nevsehir pumice with the largest surface area (Table 2)

would exhibit the fastest kinetics. However, the opposite

trend was observed, in which this pumice type had both

the lowest rate and extent of cyanide removal, while the

other two pumice sources were comparable. Further-

more, total amounts of surface functional groups and

pHPZC values of these copper-impregnated pumices

(250–1000 mm) are not significantly different. Thus,these results indicate that although copper-coated

specific surface area is the major factor affecting the

rate and extent of cyanide destruction for a particular

pumice source with similar surface chemistries, the type

of surface chemistry (i.e., specific functional groups)

within different pumice sources is also an important

factor. The characterization of such specific surface

groups is not possible with the lumped surface

characterization parameters employed in this work such

as pHPZC, total surface acidic and basic groups. Efforts

are in progress to further characterize the pumice

surfaces in detail.

In terms of the stability of the copper-impregnated

pumices during mixing in batch experiments, it was

generally found that less than about 4–7mg/L of copper

was released to the solution after extended periods of

mixing (i.e., 48–72 h) with the largest pumice (3000mg/

L) and peroxide (150mg/L) doses. With improved

impregnation methods, copper release may be mini-

mized. Such efforts are currently in progress. It should

be noted that the concentration of copper released into

solution was much lower than concentrations typically

added to solutions or slurries in conventional cyanide

oxidation technologies employing hydrogen peroxide or

SO2/air.

4. Conclusions

The tested natural pumice samples and their particle

size fractions varied in physicochemical characteristics

including surface area, elemental composition, porosity,

and surface chemistry, which may affect their reactivity

if used as a support material, adsorbent, or filtration

media. Copper impregnation significantly increased the

copper contents of the pumices, suggesting an effective

impregnation process. However, the impact of the

copper coating on pumice surface area depended on

the pumice source, pore structures and surface chemical

characteristics. Kinetic and bottle-point equilibrium

adsorption experiments indicated that natural pumices,

either original or copper impregnated, are not effective

adsorbents of negatively charged cyanide ions at pH

values 8 and 11, probably due to incompatible surface

interactions. The surfaces of majority of the pumices

tested were dominantly negatively charged above pHs of

about 7–8. However, the adsorption kinetics to achieve

equilibrium was rapid (4–8 h) compared with many

adsorbents.

Peroxide and original pumice added together were

also ineffective in removing cyanide. However, for all of

the three natural pumices tested with various size

fractions, the use of copper-impregnated pumices and

peroxide together significantly enhanced both the initial

rate and extent of cyanide removal, which increased with

increasing pumice doses at a constant peroxide dose.

These findings indicate that the impregnation of copper

into natural pumice particles used as heterogeneous

supports significantly catalyzes the degradation of

cyanide and also enhance the ultimate removals through

peroxide oxidation.

Specific surface areas of the pumice samples were

found to decrease with increasing particle size. Increased

available surface areas coated by copper accelerated the

kinetics of the heterogeneous catalytic surface oxidation

of cyanide. However, although copper-coated specific

surface area was the major factor affecting the rate and

extent of cyanide destruction for a particular pumice

source with similar surface chemistries, the type of

surface chemistry (i.e., specific functional groups) within

different pumice sources also appears to be an important

factor.

Lower rates and extents of cyanide removals were

observed at pH of 11 compared to pH of 8 probably

because of the negative impacts of alkaline conditions in

terms of scavenging peroxide and forming more

negatively charged pumice surfaces, thus hindering the

interaction of cyanide with surfaces for its surface

catalyzed oxidation. Consistent with many oxidation

reaction kinetics, both the initial rate and ultimate extent

of cyanide removals were generally higher at a

temperature of 20 1C compared with those found at

10 1C. The released copper amounts from pumices to the

solutions during mixing were much lower than those

typically added to solutions or slurries in conventional

cyanide oxidation technologies.

Soluble copper catalyst is used in industrial treatment

applications to catalyze the degradation of cyanide by

hydrogen peroxide or SO2/air. However, the addition of

soluble copper introduces another contaminant and

creates new problems in terms of downstream metals

removal. Thus, as tested in this work, the use of light,

cheap, readily available, natural, and porous hetero-

geneous supports (i.e., pumice) impregnated with copper

either in completely mixed/suspended or fixed-bed

reactor configurations may help minimize this problem

for cyanide removal in solutions.

Acknowledgments

The authors thank Professor Lutfullah Gunduz in

Pumice Research and Application Center of Suleyman

ARTICLE IN PRESSM. Kitis et al. / Water Research 39 (2005) 1652–1662 1661

Demirel University (Turkey) for kindly providing the

pumice samples, Assistant Professors Erol Seker and

Sait Sofuoglu in the Chemical Eng. Dept. of Izmir

Institute of Technology (Turkey) for conducting the

SEM and SEM-EDX analysis, and Graduate Research

Assistant Savas Ozun in the Mining Eng. Dept. of

Middle East Technical University (Turkey) for conduct-

ing the BET surface area analysis. We also thank

Graduate Research Assistant Hasan Ciftci in the Mining

Eng. Dept. of Suleyman Demirel University for his

technical assistance during cyanide analysis.

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