g as adsorption on activated carbons from pet mixtures with a metal salt

7/31/2019 G as Adsorption on Activated Carbons From PET Mixtures With a Metal Salt

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Carbon 41 (2003) 823831

Gas adsorption on activated carbons from PET mixtures with ametal salt

*Kyuya Nakagawa , Shin R. Mukai, Tetsuo Suzuki, Hajime Tamon

Department of Chemical Engineering, Kyoto University, Kyoto 606-8501, Japan

Received 4 October 2002; accepted 16 November 2002

Abstract

Activated carbons were prepared from carbonized PET by steam activation via pretreatment by mixing PET with a metal

salt [Ca(NO ) ?4H O, Ca(OH) , CaCO , ZnO, and AlNH (SO ) ?12H O], and with acid treatment after carbonization. The3 2 2 2 3 4 4 2 2porous properties of the activated carbons were determined by the nitrogen adsorption method. The adsorption isotherms of

CO , C H , nC H and iC H at 298 K on the prepared activated carbons were measured to determine practical2 2 6 4 1 0 4 1 0

applications and to obtain a better understanding of the porous structure of the prepared carbons. Steam-activated carbons via

pretreatment have a larger mesoporosity than carbons with no pretreatment. The metal salt used in the pretreatment for steam

activation has no influence on the microporous structure, but it does influence the mesoporous structure of the prepared

carbons. Activated carbons prepared via pretreatment show a large adsorption capacity for nC H and iC H . These4 1 0 4 10

carbons are suitable as adsorbents for canisters, etc. Application of the potential theory to adsorption data for the prepared

carbons suggests that the pretreatment contributes to the formation of pores larger than 0.50 nm at high burnoff.

2002 Elsevier Science Ltd. All rights reserved.

Keywords:A. Activated carbon; B. Activation; C. Adsorption; D. Porosity

1. Introduction raw materials (such as waste materials) and to optimize the

preparation conditions to obtain activated carbons with the

Activated carbons are widely used in gas purification, desired porous properties. Optimization of the activation

solvent recovery, waste water treatment, etc. It is recog- process has been extensively investigated [26]. As re-

nized that the pore structure is the most important property ported in the literature, pore development by activation

of activated carbons for their application in adsorption reactions is governed by the diffusivity and reactivity of

processes [1]. Activated carbons have a very wide range of the activation agents. Mesopores develop under the con-pore sizes, from the Angstrom scale of micropores to the ditions where micropore development is restricted, and

micrometer scale of macropores. They are used in various vice versa. These conditions are determined by complex

applications depending on their porous properties. For combinations of the activation temperature, activation

example, activated carbons with many micropores are used agents, concentrations of activation agents, and the pore

for gas adsorption, mesopores are necessary for the structures of the activated materials, which influence theadsorption of large molecules, etc. diffusivity and/or reactivity of the activation agents. This

Woods, coconut shell, coal, lignite, peat, etc. are usually knowledge is very important not only to decide on the

chosen as raw materials for activated carbons. Various activation conditions, but also to deduce methods for

carbons differing in porous properties can be obtained by developing many pores in activated carbons. The applica-

changing the raw materials and/ or the preparation con- tion of catalytic activation is one method for developing

ditions, i.e. the carbonization and activation conditions. many pores. This is the way to produce reactive sites on

Many investigations have been performed to explore novel the surface of carbons and to accelerate pore development.

It has been reported that the formation of micropores is

restricted in the presence of catalysis and many mesopores*Corresponding author. Tel.: 181-75-753-5594; fax: 181-75-are obtained [79]. Siemieniewska et al. reported that the753-3346.

E-mail address: [email protected] (K. Nakagawa). application of a graphite intercalation process during the

0008-6223/02/$ see front matter 2002 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0008-6223(02)00404-9
mailto:[email protected]:[email protected]


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824 K. Nakagawa et al. / Carbon 41 (2003) 823831

preparation of carbon adsorbents is applicable for control- after acid treatment contained no ash, which was confirmed

ling the porous properties [10]. by thermogravimetric analysis (Rigaku, TG-8120).

The present authors proposed a new method to prepare

activated carbons with different pore size distributions 2.3. Steam activation

from PET in a previous report [11]. In that article, we used

two preparation methods. One was a conventional steam- Activated carbons were prepared from chars by steam

activation method. The other was steam activation via activation. 0.4 g of the char was placed in a quartz reactor,

pretreatment, that is, mixing the raw material (PET) with a and heated in an electric furnace from room temperature to3

calcium compound and then acid treatment after carboniza- 1123 K at 20 K/ min. The gas flow rate was 200 cm /min3

tion to wash out the mixed calcium compound. The and the steam concentration was 0.0025 g /cm . The

activated carbons prepared by the latter method have many activation time (i.e., the holding time at 1123 K) was

mesopores and micropores. In contrast, activated carbons changed from 0 to 240 min in order to prepare carbons of

prepared by the conventional method have mainly micro- different burnoff. Four series of carbons were prepared.

pores. These results suggest that the pretreatment method The HT series of carbons were heat treated at 1123 K for

is useful for preparing mesoporous activated carbons. a few minutes without steam flow. The 0A series of

This article reports the preparation of activated carbons carbons were held at 1123 K for 5 min under steam flow.

from carbonized PET by steam activation via pretreatment, The 1A series of carbons were held at 1123 K for

that is, mixing the raw materials (PET) with a metal salt 90120 min under steam flow. The 2A series of carbons

[Ca(NO ) ?4H O, Ca(OH) , CaCO , ZnO, or were held at 1123 K for 180240 min under steam flow.3 2 2 2 3

AlNH (SO ) ?12H O] and acid treatment after carboniza-4 4 2 2tion. The influence of the metal salt on the porous 2.4. Characterization of activated carbons

properties of the prepared activated carbons was investi-

gated. The porous properties of the prepared carbons were Adsorption and desorption isotherms of N on the2

determined by the nitrogen adsorption method. Adsorption activated carbons were measured at 77 K using a volu-

isotherms of CO , C H , nC H and iC H at 298 K on metric adsorption apparatus (BEL Japan, BELSORP28).2 2 6 4 1 0 4 1 0

the prepared activated carbons were also measured to The BET method was used to evaluate the specific surface

confirm the micropore structures and to determine the area of the activated carbons from the adsorption iso-

practical application of the prepared activated carbons as therms. The pore size distribution (range of pore radii

adsorbents. 1.020 nm) was estimated by applying the Dollimore

Heal method [12] to the desorption isotherms, and the

mesopore volume was determined. The micropore volume

2. Experiment was evaluated by the t-plot method [13]. The adsorptionisotherm of N on Spheron 6 was adopted, which was2

2.1. Carbonization measured by BEL Japan as the standard isotherm. Ad-

sorption isotherms of CO , C H , nC H and iC H2 2 6 4 10 4 1 0

Pellets (diameter |2 mm, length |2 mm) of PET were also measured on the activated carbons at 298 K

(polyethylene terephthalate) were used for the raw material using the same apparatus.

of the activated carbons. Ten grams of PET pellets were

mixed with 5 wt% of metal salts, and the samples were

then set in a quartz reactor and heated by an electric 3. Results and discussion

furnace. The reactor was kept under an inert atmosphere3

with a N flow of 80 cm /min. The reactor was heated 3.1. Burnoff and activation time2

from room temperature to 773 K at 0.4 K/min, and kept at

773 K for 1 h. The obtained chars were powdered by a The activation time and the burnoff of the prepared

mortar. Six groups of chars were prepared, i.e. from PET carbons are shown in Table 1. The weight loss of the HTpellets (P1), and from PET pellets mixed with Ca(NO ) ? series carbons varies from 12 to 25%. This means that a

3 2

4H O (P2), Ca(OH) (P3), CaCO (P4), ZnO (P5), and relatively large weight loss occurs before reaching 1123 K2 2 3

AlNH (SO ) ?12H O (P6). (i.e., before starting the activation reaction with steam).4 4 2 2

The weight loss is due to the vaporization of volatile

2.2. Acid treatment matter. One can see that P2P6 group carbons have more

volatile matter and disorganized carbon than group P1. It is3

Two grams of the char was impregnated in 100 cm 1.0 suggested that the different carbon structure is produced by

N HCl solution at room temperature for 24 h and stirred carbonization under influence of the mixture.

using a magnetic stirrer. After acid treatment, the samples One can see that the burnoff of 0A series carbons

were washed with distilled water until the pH of the filtrate varies from 13 to 29%. Burnoff then increases linearly to

was |5.0, and then dried in an oven at 383 K. Carbons the holding time for all groups of 1A and 2A series


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K. Nakagawa et al. / Carbon 41 (2003) 823831 825

Table 1

Preparation conditions and porous properties of the prepared activated carbons

a b c

Group Mixture Series Activation Burnoff S V VBET m ic m es

2 3 3

time (min) (wt%) (cm /g) (cm /g) (cm /g)

P1 None HT 0 12.5 394 0.21 0.04

0A 5 12.7 456 0.24 0.06

1A 120 68.7 1450 0.72 0.132A 240 77.8 1740 0.93 0.15

P2 Ca(NO ) ?4H O HT 0 13.2 396 0.20 0.043 2 2

0A 5 18.0 521 0.25 0.08

1A 150 63.7 1460 0.66 0.27

2A 180 85.7 2190 0.81 0.90

P3 Ca(OH) HT 0 25.0 419 0.21 0.072

0A 5 29.4 495 0.23 0.08

1A 150 60.6 1200 0.55 0.19

2A 180 81.0 1960 0.86 0.50

P4 CaCO HT 0 14.4 421 0.22 0.093

0A 5 20.7 446 0.23 0.05

1A 150 57.1 1170 0.59 0.142A 240 87.5 2180 0.93 0.78

P5 ZnO HT 0 13.9 416 0.20 0.09

0A 5 20.5 459 0.25 0.05

1A 1150 61.6 1240 0.66 0.16

2A 240 85.9 2240 1.10 0.87

P6 AlNH (SO ) ?12H O HT 0 15.6 370 0.17 0.034 4 2 2

0A 5 23.3 493 0.27 0.04

1A 150 63.5 1220 0.66 0.11

2A 240 85.1 2080 0.82 0.61

a

S , BET surface area.BET

b

V , micropore volume.mic

c

V , mesopore volume.mes

3

carbons, and the overall reaction rates of P2P6 group for the BET surface area, from 0.2 to 1.1 cm / g for the3

carbons are a little higher than group P1 carbons. It is micropore volume, and from 0.04 to 0.9 cm / g for the

suggested that the activation reactions of all groups of mesopore volume. The porous properties of P1 group

carbons are typical steam-activation reactions, and their carbons are fairly similar to those reported in the literature

activation processes are similar. [14]. These activated carbons have a BET surface area2

.1000 m /g and mainly contain micropores. It is

3.2. Porous properties of the prepared activated carbons noteworthy that the mesopore volume of 2A series

carbons of groups P2P6 are quite large. Although the

3.2.1. N adsorption isotherms porous properties of HT series carbons of all groups are2

Fig. 1 shows examples of N adsorption isotherms at 77 quite similar, the properties of the carbons become differ-2

K on the prepared activated carbons. The amount of N ent with an increase in burnoff. Micropore volume and2

adsorbed on each group of carbons increases with burnoff, mesopore volume are plotted as a function of burnoff inas expected. All groups of HT and 0A series carbons Fig. 2. It is confirmed that all groups of carbons have

have type I adsorption isotherms according to the BDDT similar micropore volumes (Fig. 2a). On the contrary, it

classification. The P1 group of carbons exhibited type I can be seen that the development of mesopores of P2P6

isotherms with increasing burnoff (Fig. 1a). On the other group carbons is much larger than that of group P1 with a

hand, the shape of the isotherms changed for the P2 group burnoff of more than 60% (Fig. 1b).

of carbons as burnoff increased (Fig. 1b). These isotherms

become type IV, and suggest the formation of mesopores. 3.2.3. Pore size distributions

Mesopore size distributions calculated by the Dolli-

3.2.2. Effect of burnoff on porous properties moreHeal method are shown in Fig. 3. It is clear that the

As shown in Table 1, the porous properties of the distributions of group P1 differ greatly from those of2

prepared activated carbons vary from 370 to 2200 m / g groups P2P6. In group P1, mesopores are formed in the


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Fig. 1. Adsorption and desorption isotherms for N at 77 K. (a) Group P1, (b) group P2. Closed symbols, adsorption; open symbols,2

desorption.

region narrower than pore radii of 2 nm and the region Great differences are not observed in pore size dis-

wider than pore radii of 9 nm as burnoff increases (Fig. tributions of HT, 0A and 1A series carbons of groups

3a). In groups P2P6, the development of mesopores is P2P6. On the other hand, differences in the development

remarkable in the region 29 nm, and mesopores are also of pore radii of 29 nm are observed in 2A series

formed in the region narrower than pore radii of 2 nm and carbons. The shapes of the pore size distributions of groups

the region wider than pore radii of 9 nm (Fig. 3bf). P2, P4 and P5 are similar (Fig. 3b, d and e), that is, the

Fig. 2. Pore volumes of the prepared activated carbons as a function of burnoff. (a) Burnoff vs. micropore volume; (b) burnoff vs. mesopore

volume.


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Fig. 3. Pore size distributions. (a) Group P1, (b) group P2, (c) group P3, (d) group P4, (e) group P5, (f) group P6.


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peaks in the region 29 nm are sharper and larger than amounts of C H adsorbed on HT and 0A series2 6

those of groups P3 and P6 (Fig. 3c and f). Hence, the carbons of all groups are similar. The amount of C H2 6

mixture of metal salts influences the porous structure of adsorbed on all groups of carbons increases with burnoff,

carbons when burnoff is high. and a large increase of the amount adsorbed is observed

between 0A and 1A series carbons. However, the

3.3. Gas adsorption characteristics amount adsorbed on 2A carbon greatly decreases in

group P1 (Fig. 5a), as observed in the CO adsorption2

3.3.1. Adsorption isotherms isotherms. The amounts adsorbed on 1A and 2A

It was demonstrated in the previous section that acti- carbons of group P2 are almost the same, and much larger

vated carbons prepared via pretreatment have unique than the commercial activated carbon (Fig. 5b). Similar

mesopore size distributions. It is also expected that these results were also observed on P3P6 group carbons.

activated carbons will have different adsorption charac- Adsorption isotherms for nC H are shown in Fig. 6.4 10

teristics from commercial activated carbons. The adsorp- The amounts of nC H adsorbed on all groups of carbons4 10

tion isotherms of gases are useful for confirming the increase with burnoff. This result is different from CO or2

applications of the prepared activated carbons as adsor- C H adsorption. The amounts adsorbed on 1A series2 6

bents, and for a better understanding of the pore structure carbons are much larger than those on 0A series carbons.

of the prepared carbons. The adsorption isotherms of CO , Fig. 6a shows that the amounts of nC H adsorbed on2 4 10

C H , nC H and iC H at 298 K on the prepared 1A and 2A series carbons are almost the same as those2 6 4 10 4 1 0

activated carbons were measured. The adsorption iso- on the commercial carbon. On the other hand, Fig. 6b

therms of CO , C H and nC H on a commercial shows that the adsorption capacity of nC H of 1A and2 2 6 4 10 4 1 0activated carbon [15] were used as the reference isotherms. 2A series carbons are much larger than that of the

Adsorption isotherms for CO are shown in Fig. 4. The commercial carbon. These results are also observed for2

amount of CO adsorbed on all groups of carbons did not P3P6 group carbons.2

change much as burnoff increased and the amount of CO Adsorption isotherms for iC H are shown in Fig. 7.2 4 10

adsorbed on 2A series carbons was smaller than on Figs. 6 and 7 show that the amounts of iC H adsorbed4 10

carbons of the other series. This trend is also found for are much smaller than those of nC H on HT series4 10

P3P6 group carbons, and is the most remarkable for carbons of groups P1 and P2. Hence, HT series carbons

group P1 carbons. One can see that the amount of CO would show a molecular sieving effect for the adsorption2

adsorbed on the 2A series carbon of group P1 is much of nC H and iC H . 0A series carbons adsorb iC H4 10 4 10 4 1 0

smaller than that of commercial activated carbon (Fig. 4a). more than HT series carbons because of steam activation.

This is because the adsorption potential becomes weak as It can be seen that the amounts of iC H adsorbed on4 10

the pore size increases by activation. One can see that the 1A series carbons are much larger than those on 0Aamount of CO adsorbed on all series of carbons is larger series carbons. The differences in the amounts adsorbed on2

than that of the commercial activated carbon, except for 1A and 2A series carbons of group P2 are much larger

the 2A series carbon of group P1. than those of group P1. This is the same trend as for the

Adsorption isotherms for C H are shown in Fig. 5. The nC H adsorption isotherms, and also observed for P3P62 6 4 10

Fig. 4. Adsorption isotherms for CO at 298 K. (a) Group P1, (b) group P2.2


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Fig. 5. Adsorption isotherms for C H at 298 K. (a) Group P1, (b) group P2.2 6

group carbons. Hence, it is suggested that these activated unit weight adsorbent, R is the gas law constant, T is the

carbons can be applied as adsorbents for canisters, etc. adsorption temperature, and V9 is the molal volume of

saturated liquid at the vapor pressure equal to the ad-

sorption pressure. If the adsorption potential theory can be3.3.2. Application of adsorption potential theory to gasapplied to adsorption equilibrium, plots of NV9 versusadsorption isotherms(T/V9) ln(f /f) for all the adsorbates would fall on one

sThe modification of the PolanyiDubinin adsorptioncurve. When the potential theory is applied to the ad-theory proposed by Lewis et al. [16] is given bysorption isotherms of several adsorbates of different mo-

[(RT/V9) ln(f /f)] 5 [(RT/V9) ln(f /f)] (1) lecular sizes (CO , 0.33 nm; C H , 0.40 nm; nC H , 0.43s I s I I 2 2 6 4 10

nm; iC H , 0.50 nm), the formation of pores in the4 10

when activated carbons during activation can be postulated.

Fig. 8 shows plots of the adsorption capacities versus9 9N V 5N V (2)I I I I II the adsorption potential. The amount of CO and C H2 2 6

where f is the fugacity of the gas at the adsorption pressure adsorbed on HT series carbons can be correlated with the

and temperature, f is the fugacity of the saturated liquid at potential, as shown in Fig. 8a. Hence, it was found thats

the adsorption temperature, N is the moles adsorbed per HT series carbons of all groups have the same micropore

Fig. 6. Adsorption isotherms for nC H at 298 K. (a) Group P1, (b) group P2.4 10


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Fig. 7. Adsorption isotherms for nC H at 298 K. (a) Group P1, (b) group P2.4 10

Fig. 8. Correlation of adsorption isotherms. (a) HT series, (b) 1A series, (c) 2A series.


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structure in the region ,0.40 nm. On the other hand, the References

adsorption data for nC H do not fit the correlation curve4 10

and the data for iC H greatly deviate from the curve. [1] Wigmans T. Industrial aspects of production and use of4 10

activated carbons. Carbon 1989;27:1322.Since the amount of iC H adsorbed is extremely small4 10

[2] Pastor-Villegas J, Duran-Valle CJ. Pore structure of activatedcompared with nC H , it can be seen that the carbons4 10

carbons prepared by carbon dioxide and steam activation atshow the molecular sieving effect and that all their poresdifferent temperatures from extracted rockrose. Carbonare smaller than 0.50 nm.2002;40:397402.As steam activation proceeds, the micropore structure of

[3] Molina-Sabio M, Gonzalez MT, Rodrguez-Reinoso F, Sep-the carbon prepared via pretreatment for steam activation

ulveda-Escribano A. Effect of steam and carbon dioxide(groups P2P6) becomes different from that of the carbons activation on the micropore size distribution of activatedprepared by conventional steam activation (group P1). Fig. carbon. Carbon 1996;34:5059.8b suggests that the adsorption data for CO , C H , and [4] Gergova K, Eser S. Effect of activation method on the pore2 2 6nC H on 1A series carbons of groups P1P6 can be structure of activated carbons from apricot stones. Carbon

4 10

1996;34:87988.correlated by the potential theory. However, data for the [5] Gonzalez MT, Molina-Sabio M, Rodrguez-Reinoso F.carbons of group P1 deviate in the low potential region.

Steam activation of olive stone chars, development ofHence, it can be seen that all carbons prepared viaporosity. Carbon 1994;32:140713.pretreatment (groups P2P6) have the same micropore

[6] Rodrguez-Reinoso F, Molina-Sabio M, Gonzalez MT. Thestructure. For the adsorption of CO , C H , nC H , and

2 2 6 4 1 0 use of steam and CO as activating agents in the preparation2iC H on 2A series carbons, correlation of the ad-

4 10 of activated carbons. Carbon 1995;33:1523.

sorption data is shown in Fig. 8c. The deviation of the data [7] Yoshizawa N, Yamada Y, Furuta T, Shiraishi M, Kojima S,for group P1 from the correlation curve is larger than that Tamai H et al. Coal-based activated carbons prepared withfor the 1A carbons shown in Fig. 8b. Hence, it can be organometallics and their mesoporous structure. Energyseen that steam activation pretreatment contributes to the Fuels 1997;11:32730.

[8] Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A.formation of pores larger than 0.50 nm.Formation of mesopores in phenolic resin-derived carbon

fiber by catalytic activation using cobalt. Carbon

1995;33:108590.4. Conclusion [9] Cazorla-Amoros D, Ribes-Perez D, Roman-Martnez MC,

Linares-Solano A. Selective porosity development by cal-Activated carbons were prepared by steam activation

cium-catalyzed carbon gasification. Carbon 1996;34:86978.from carbonized PET via a mixture of several metal salts [10] Siemieniewska T, Albiniak A, Broniek E, Kaczmarczyk J,and acid treatment. The effects of different mixtures on the Jankowska A, McEnaney B et al. Porosity development in

porous properties of the prepared activated carbons are steam activated chars from mixtures of coal tar pitch withdiscussed. The adsorption isotherms for CO , C H , graphiteFeCl intercalation compounds. Fuel 1998;77:50932 2 6

17.nC H and iC H on the prepared activated carbons4 1 0 4 1 0

[11] Tamon H, Nakagawa K, Suzuki T, Nagano S. Improvementwere measured to confirm practical applications and for aof mesoporosity of activated carbons from PET by novelbetter understanding of the porous structure of the preparedpre-treatment for steam activation. Carbon 1999;37:16435.carbons, and the following conclusions were obtained.

[12] Dollimore D, Heal GR. An improved method for the

calculation of pore-size distribution from adsorption data. J1. Activated carbons prepared from PET have a large Appl Chem 1964;14:10914.

porosity. Steam-activated carbons via a mixture of [13] Lippens BC, Boer JH. Pore system in catalysts, v. themetal salts and acid treatment as the pretreatment for t-method. J Catal 1965;4:31923.

activation have a larger mesoporosity than carbons with [14] Laszlo K, Bota A, Nagy LG, Cabasso I. Porous carbon frompolymer waste materials. Colloids Surfaces Ano pretreatment.1999;151:31120.2. The kind of metal salt used in the pretreatment for

[15] Valenzuela DP, Myers AL, editors, Adsorption equilibriumsteam activation does not influence the microporousdata handbook, Englewood Cliffs, NJ: Prentice-Hall, 1989,structure, but does influence the mesoporous structurep. 33, 51, 80.

of the prepared carbon.[16] Lewis WK, Gilliland ER, Chertow B, Cadogan WP. Ad-

3. Since the activated carbons prepared via pretreatmentsorption equilibria. Pure gas isotherms. Ind Eng Chem

show a large adsorption capacity for nC H and4 10 1950;42:132632.

iC H compared with commercial carbon, the prepared4 10

carbons are suitable as adsorbents for canisters, etc.

4. Application of the potential theory to adsorption data

for the prepared carbons suggests that the pretreatment

contributes to the formation of pores larger than 0.50

nm at high burnoff.

g as adsorption on activated carbons from pet mixtures with a metal salt

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