C A R B O N 4 8 ( 2 0 1 0 ) 1 0 3 2 1 0 3 7

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Evidence for the presence of cyanide during carbonactivation by KOH

E. Fuente a, R.R. Gil a, R.P. Giron a, M.A. Lillo-Rodenas b, M.A. Montes-Moran a,*,M.J. Martin c, A. Linares-Solano b

a Instituto Nacional del Carbon, CSIC, Apartado 73, E-33080 Oviedo, Spainb Grupo de Materiales Carbonosos y Medioambiente (MCMA), Departamento de Qumica Inorganica, Universidad de Alicante,

Apartado 99, E-03080 Alicante, Spainc Labotarori dEnginyeria Qumica i Ambiental (LEQUIA), Institut de Medi Ambient, Facultat de Ciencies, Universitat de Girona,

17071 Girona, Spain

A R T I C L E I N F O

Article history:

Received 27 July 2009

Accepted 9 November 2009

Available online 12 November 2009

0008-6223/$ - see front matter 2009 Elsevidoi:10.1016/j.carbon.2009.11.022

* Corresponding author: Fax: +34 985 297662.E-mail address: miguel@incar.csic.es (M.A

A B S T R A C T

The observation that carbon activation by KOH gives rise to a significant formation of cya-

nide moieties is confirmed. However, contrary to what has been reported, our results show

that the N2 used as an atmosphere during the activation process has little to do with the

formation of such cyanide moieties. The main source of cyanides is ascribed to structural

nitrogen already present in the precursors. Reducing species (H2 and metallic K) formed

during the KOH carbon activation process might promote the transformation of that struc-

tural nitrogen to cyanides. In order to minimise cyanide formation (and related environ-

mental concerns), materials with low nitrogen content should be selected as precursors

for the preparation of KOH-activated carbons.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Research carried out using alkaline hydroxides as activating

agents for different carbon precursors has been profusely re-

ported. NaOH and KOH activation makes possible activated

carbons with rather unique textural properties, i.e., extremely

high apparent specific surface areas (SBET values as high as

4000 m2 g1 being eventually claimed) and pore volumes.

Thus, chemical activation with hydroxides has become a

recognised method to obtain the sometimes referred to as

super activated carbons [18].

The growing interest on alkaline activation has also

prompted several studies to better understand the mecha-

nism involved. It is now well-established that different chem-

ical pathways occur depending on the nature of the precursor,

raw materials (i.e., lignocellulosic materials) on one side [9],

and carbonaceous precursors (chars, coals, etc.) on the other

er Ltd. All rights reserved. Montes-Moran).

[1,1012]. For this latter type of carbon precursors, the follow-

ing redox mechanism has been proposed from the reaction

products observed during activation performed at tempera-

tures around 750 C [1,1012]:

6MOH 2C! 2M2CO3 2M 3H2 1

M being K or Na. This chemical reaction accounts for the C

oxidation that should develop the porous network in the

remaining carbon material.

In principle, chemical reactions involving other elements

than C (heteroatoms) should be also prone to occur during

the activation, as far as they are present in the precursor. Re-

search contributions on the possible evolution of those other

elements during alkaline activation are scarce, apart from the

interesting recent work of Robau-Sanchez et al. [13,14] where

evidence of cyanide formation during the KOH activation of a

Quercus agrifolia char has been reported. These authors found

.

http://dx.doi.org/10.1016/j.carbon.2009.11.022mailto:miguel@incar.csic.eswww.sciencedirect.comhttp://www.elsevier.com/locate/carbon

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 3 2 1 0 3 7 1033

that considerably more cyanide was formed under nitrogen

than under argon, leading to the conclusion that cyanide

was produced mainly from N2. These unexpected results are

very surprising in view of the kinetic stability of dinitrogen.

Additionally, they are not in agreement with what was re-

ported in Refs. [1,15], where the effect of the type of gas used

during alkaline activations on the textural properties of the

activated materials was analysed. It was concluded that,

independently of the nature of the gas used (He, Ar or N2),

they only work as purging gases, displacing the global chem-

ical activation reaction towards the products.

In a series of recent contributions, using alkaline hydrox-

ide activation of sludge-based precursors [1619], cyanide

moieties were detected in the activated materials [17,19],

which confirms the results of Robau-Sanchez et al. [13,14].

Such confirmed cyanide formation requires further studies

because of its importance for environmental protection.

Thus, the aim of the present paper is to delve into the causes

of the cyanide formation. For that purpose, precursors with

different nitrogen contents have been selected to address

the contribution of the two possible N sources to cyanide for-

mation: structural nitrogen, or nitrogen coming from the

flowing gas as suggested by Robau-Sanchez et al. [13,14].

2. Experimental

2.1. KOH activation

Chars were prepared from selected residues attending to

their nitrogen content: a lignocellulosic (Quercus Robur) saw

dust (QUE), two chromium-free solid wastes from the leather

industry (CUA and CUB) and two sewage sludges (SB and

BIOS) collected at two different Spanish wastewater treat-

ment plants (WWTP). SB sludge has been used in previous

works ([1619]). The BIOS sludge was collected directly from

the aeration tank of the activated sludge unit (Girona

WWTP).

Char precursors were obtained from the pyrolysis of the

abovementioned residues for their subsequent activation pro-

cess study. Approx. 20 g of dried residue were put into an alu-

mina crucible and placed in a ceramic tubular furnace (55 mm

i.d.). The residue was heated up to 750 C in 150 ml min1 offlowing nitrogen, using a heating rate of 5 C min1. The max-imum temperature was held for 60 min. Then, the samples

were allowed to cool down to room temperature in the same

atmosphere (N2). Precursors (chars) were finally ground and

sieved to obtain a particle size smaller than 212 lm, before

proceeding to their chemical activation. Nomenclature used

for the chars is the same as that used for their precursors

adding, at the end, -P.

Powdered KOH (Aldrich) was selected as activating agent.

Approx. 15 g of KOH/precursor mixtures prepared using vari-

ous weight ratios were placed into an alumina crucible and

heated up to 750 C, using 5 C min1 heating rate and 1 h ofdwelling in the same horizontal furnace used for pyrolysis.

The activation process was carried out in N2 or Ar at different

flow rates.

The analysis of major elements (C, H, N, S and O) of

solid samples was performed as described elsewhere [16].

The oxygen content determination corresponds to the total

content, i.e., including the metal oxides (ash).

2.2. Cyanide analysis

Six grams of the activated mixtures were split in two parts,

for duplicate analysis. Each part weighing 3 g was washed

with 80 ml of deionised water (MilliQ) in four (20 ml) succes-sive stages, at room temperature. The supernatant liquid was

collected by centrifugation of the solid suspensions. The four

aqueous solutions obtained in every washing plus centrifuga-

tion step were finally mixed. pH of the aqueous solutions

were measured using a TIM870 titration system from

Radiometer.

Total cyanide contents of the two duplicate solutions per

sample (80 ml each) were quantified combining the US EPA

9014 [20] and 9010C methods [21]. Basically, cyanides are re-

leased from the aqueous solutions as hydrocyanic acid

(HCN), by means of a reflux-distillation operation under acidic

conditions, and absorbed in a scrubber containing sodium

hydroxide solution. Interferences are mostly eliminated

by using this distillation procedure [21]. The cyanide concen-

tration in the absorbing solution was then determined [20]

using a UV 2401PC UVvis recording spectrophotometer

(Shimadzu).

3. Results and discussion

The elemental compositions of the five selected residues and

their corresponding chars are collected in Table 1. As men-

tioned in the experimental section, the N content of the dif-

ferent residues spans a relatively wide range of values, from

less than 1% for the lignocellulosic residue QUE to more than

10% for the CUA sample. In terms of their inorganic (ash) con-

tent, the residues could be grouped in two sets, with the QUE

and CU* samples (especially CUA) having very low ash con-

tents and the sludges having relatively high contents (around

30%). After pyrolysis, the initial range of N contents shortens,

especially if daf values are considered. The nitrogen content

in the pyrolysed samples (precursors) is assumed to be linked

to the carbon-rich (char) fraction of the materials, since any

inorganic nitrogen species in the original residues should

not withstand the thermal treatment.

It is confirmed, in agreement with Refs. [13,14,17,19] that

cyanide species were produced in all activation experiments,

regardless of the precursor and conditions employed. They

were analysed indirectly by washing the activated mixtures

using water at room temperature. Preliminary tests (results

not shown) allowed us to conclude that four washing steps

(as described in Section 2) were enough to virtually attain

the complete transfer of the different cyanide moieties,

mainly KCN, to the aqueous solutions. Table 2 shows the cya-

nide concentration of the solutions obtained when washing

activated mixtures prepared using 1/1 KOH/precursor weight

ratios, under nitrogen. The repeatability of the washing pro-

cedure within a given sample was excellent in all cases.

Henceforth, for each activation experiment, only average con-

centration values obtained from the two measurements will

be reported. For two of the samples, namely CUA-P and

QUE-P

CUA-P

CUB-P

SB-P

BIOS-P

0 20 40 60 80 100 120[CN- ] (mg g-1)

Ar N2

Fig. 1 Comparison between the amount of cyanides

obtained during the activation of the different precursors

under N2 and Ar.

Table 1 Elemental analysis (dry basis) of the residues and corresponding chars.

Sample Ash content Elemental analysis (wt.%)

(wt.%) C H N S O

QUE

150/1

150/3

500/3

0 10 20 30 40[CN- ] (mg g-1)

Ar N2

Fig. 3 Cyanide formation in the course of the activation of

SB-P using different KOH/precursor ratios and flow rates,

under N2 and Ar.

C A R B O N 4 8 ( 2 0 1 0 ) 1 0 3 2 1 0 3 7 1035

Other important issue that should be addressed when

dealing with alkaline hydroxide activation of carbons is the

effect of both nitrogen flow rate and the KOH/precursor ratio.

Two of the precursors under study, namely SB-P and CUB-P,

were selected to carry out additional experiments using dif-

ferent activation conditions. Labelling of the new samples

obeys the following rationale: A/B, A being the flow rate (in

ml min1) and B being the KOH/precursor weight ratio. Re-

sults are shown in Figs. 2 and 3.

Figs. 2 and 3 confirm that the precursor with higher nitro-

gen content leads to higher cyanide amounts and that both

char precursors behave similarly. Although differences are

not very pronounced, it seems that an increase in the carrier

gas flow rate or in the relative proportion of the activating

agent produces, interestingly, solutions with a higher concen-

tration of cyanides. This observation has not been reported

before, as far as Robau-Sanchez et al. failed to find any rela-

tionship between the activation conditions and the amount

of cyanides detected [13].

A nitrogen mass balance study was carried out to better

understand the effect of the activating conditions on the for-

mation of cyanides. Elemental analyses of the water-washed

samples were carried out to determine the nitrogen remain-

ing in the activated solids after cyanide removal. Results are

shown in Table 3, keeping a constant basis for the calcula-

tions (100 g of precursor). The increase of the CN concentra-

tion in the aqueous solutions obtained when activation is

carried out at higher flow rates or KOH/precursor ratios (Figs.

2 and 3, and NL values in Table 3) are attained at the expense

of the nitrogen remaining in the washed solids, so to keep the

amount (Ns,f + NL) relatively constant for a given precursor,

regardless of the activation conditions.

Having more CN transferred from the activated solids to

the aqueous solution when the activation is carried out under

higher flow rates or activating agent/precursor ratios could be

due not only to the formation of cyanide species during the

process. It might be also a consequence of an eventual change

in the solubility conditions of the cyanide salts. The similarity

between the pH values of the solutions obtained from a given

precursor activated in Ar and in N2 under different conditions

150/1

500/1

150/3

500/3

0 20 40 60 80 100[CN- ] (mg g-1)

Ar N2

Fig. 2 Cyanide formation in the course of the activation of

CUB-P using different KOH/precursor ratios and flow rates,

under N2 and Ar.

(Table 3) rules out this possibility. Thus, the increment of the

cyanide concentration in the aqueous solutions measured at

higher flow rates or KOH/precursor ratios is most likely linked

to the completion of a chemical reaction involving the forma-

tion of cyanides during the activation. This increase in the

cyanide amount detected in these experiments is consistent

and must be related to a higher activation degree [13].

All the above results indicate that cyanides, ascribed to

structural nitrogen already present in the precursors, are

formed during the activation reaction, as XRD analysis of

the solids obtained after activation (i.e., before being washed)

confirms [19]. These cyanides are produced as a consequence

of the reducing atmosphere formed during the KOH carbon

activation process. Thus, when heating the carbon precur-

sor/KOH mixture in an inert atmosphere, usually in the tem-

perature range of 750850 C, carbon oxidation (carbonconversion to potassium carbonate) takes place, together

with the formation of reducing products (metallic K and H2),

responsible of cyanide formation.

The cyanide concentrations reported by Robau-Sanchez

et al., when activating a lignocellulosic char in N2 (i.e., approx.

300 mg g1) [13], are two orders of magnitude higher than

those obtained for the QUE-P sample (Table 2, Fig. 1). Compo-

sitional differences between the two lignocellulosic chars un-

der consideration (see Table 2 in [13] and Table 1 above)

cannot justify such a significant variation in the amount of

cyanides detected. A possible explanation is that they em-

ployed a stainless steel reactor (in the present work an alu-

mina crucible) where the char/KOH mixtures were placed in

direct contact with the reactor wall. Fe has been traditionally

used as catalyst in the production of alkaline cyanides from

gaseous N2 [22,23]. The nitrogen mass balances of Table 3 ex-

cludes N2 fixation, within experimental error (DN = 0 1).

Fig. 4 shows a clear dependence of the amount of CN de-

tected in the liquid phase on Ns,0 (see Table 3), for activations

carried out under the same experimental conditions (i.e.,

100 g of precursor, 1:1 KOH:precursor ratio) and 150 ml min1 of Ar or N2. The trend depicted in Fig. 4 can be fairly ac-

cepted to be valid for precursors whose inorganic fraction

does not promote N2 fixation (a significant presence of transi-

tion metals, for example). Nevertheless, in that case, impor-

Table 3 Nitrogen mass balances of the activation process.

Precursor Conditionsa NS,0b NS,f

c NLd DNe pH

QUE-P N2/150/1 0.8 0.6 0.18 0.0 13.08Ar/150/1 0.7 0.10 0.0 12.99

CUA-P N2/150/1 8.9 2.0 6.91 0.0 12.83Ar/150/1 2.4 6.52 0.0 12.82

CUB-P N2/150/1 6.0 2.1 3.91 0.0 12.78Ar/150/1 1.9 3.93 0.3 12.78N2/150/3 1.0 4.40 0.6 12.49Ar/150/3 0.8 4.65 0.6 12.45N2/500/1 1.5 4.13 0.4 12.75Ar/500/1 1.2 4.57 0.3 12.76N2/500/3 0.5 4.85 0.6 12.59Ar/500/3 0.4 5.24 0.4 12.60

SB-P N2/150/1 2.6 1.0 1.51 0.1 13.09Ar/150/1 1.0 1.52 0.1 13.07N2/150/3 0.8 1.57 0.2 13.02Ar/150/3 0.8 1.67 0.1 12.95N2/500/3 0.5 1.99 0.4 13.07Ar/500/3 0.5 1.85 0.2 13.07

BIOS-P N2/150/1 3.1 1.6 0.96 0.5 12.79Ar/150/1 1.4 1.26 0.4 12.81

a Activation atmosphere/flow rate/KOH/precursor ratio.b Gram of N in 100 g of the precursor.c Gram of N in the washed, activated samples obtained from 100 g of the precursor.d Gram of N in the washing solutions (from total CN content) of activated samples obtained from 100 g of the precursor.e N balance: NS,0 (NS,f + NL).

0 2 4 6 8 100

2

4

6

8

10

12

14

CN

- (g)

NS,0 (g)

N2 Ar (R2 = 0.94)

Fig. 4 Dependence of the amount of cyanides detected on

NS,0 (Table 3) during the activation of the different

precursors under the same experimental conditions (100 g

of precursor, 1:1 KOH:precursor ratio) and 150 ml min1 of

Ar or N2.

1036 C A R B O N 4 8 ( 2 0 1 0 ) 1 0 3 2 1 0 3 7

tant differences in the amount of cyanides formed under

nitrogen or argon should be expected.

4. Conclusions

Soluble cyanide is formed during the alkaline activation of

nitrogen-containing chars as a consequence of structural

nitrogen present in the precursor. The amount of cyanides ob-

tained when the activation is carried out under nitrogen or ar-

gon atmospheres is essentially the same, as far as iron or

other cyanide formation promoters are absent from the reac-

tion chamber. Increasing the flow rate or KOH/precursor ratio

tends to raise the amount of cyanides formed. A correlation

between the cyanides detected in solution and the nitrogen

content of the precursors could be established. Cyanide for-

mation during chemical activation with hydroxides can be

minimised by reducing the N content of the precursor. Finally,

attention should be paid to the KOH activation process of

some carbon precursors because high cyanide concentrations

(e.g., as high as 130 mg of CN per gram of precursor) can be

eventually reached for chars with relatively high N contents

(e.g., 9 wt.%, dry basis). This remarks that materials with

low nitrogen content should be selected, from an environ-

mental point of view, to be used as precursors for the prepa-

ration of activated carbons by KOH activation.

Acknowledgements

This work was funded by MEC and MICINN (CTM2005-07524-

C02-00, CTM2008-06869-C02-00/PPQ and PRT2007-0421

projects).

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Evidence for the presence of cyanide during carbon activation by KOHIntroductionExperimentalKOH activationCyanide analysisResults and discussionConclusionsAcknowledgementsReferences