DOI: 10.3303/CET25117037 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 26 December 2024; Revised: 12 March 2025; Accepted: 16 May 2025 
Please cite this article as: Csaszar K.A., Virag L., Zsinka V., Miskolczi N., 2025, The Effect of Metal Impregnated Biochars in Carbon Dioxide 
Decrement, Chemical Engineering Transactions, 117, 217-222  DOI:10.3303/CET25117037 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 117, 2025 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Fabrizio Bezzo, Flavio Manenti, Gabriele Pannocchia, Almerinda di Benedetto 

Copyright © 2025, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-17-5; ISSN 2283-9216 

The Effect of Metal Impregnated Biochars in Carbon Dioxide 

Decrement 

Kiara Atina Császár, Lilla Virág, Viktória Zsinka*, Norbert Miskolczi 

University of Pannonia, Department of MOL Hydrocarbon and Coal Processing, Egyetem u. 10, H-8200 Veszprém, Hungary 

zsinka.viktoria@mk.uni-pannon.hu 

The rapid increment in CO₂ levels in the atmosphere, mainly of anthropogenic origin, requires innovative 

solutions in the field of carbon dioxide capture and storage (CCS). Due to their improved physicochemical 

properties, metal-impregnated biochars should be promising materials for CO₂ reduction. The efficiency of CO₂ 

storage in case of biochar is significantly enhanced by the impregnation of metallic elements such as alkali, 

alkaline earth and transition metals, which provide additional active sites and change the electron structure of 

the surface. The mechanism of CO₂ capture in metal-impregnated biochars includes both physisorption and 

chemisorption. Metal ions on the biochar surface increase the electrostatic interactions with CO₂, which led to 

improving physisorption. Transition metals such as Fe, Cu, and Ni promote the chemisorption, in which CO₂ 

undergoes more stable binding or reduction effects to form new compounds. For example, impregnation with 

calcium and potassium improves the alkaline nature of biochar, resulting better adsorption through acid-base 

interactions with CO₂. The large surface area and porosity of biochar increase the availability of CO₂, while the 

metallic elements provide catalytic activity that accelerates the CO₂ adsorption and conversion processes. 

Metal-impregnated biochar, which is sustainably derived from biomass, is an energy-efficient and 

environmentally friendly solution for carbon dioxide capturing and storing. In this work, different metals were 

used for surface modification of sewage sludge derived biochar in order to obtain efficient waste-based 

adsorbent for CCS/CCU process. 

Keywords: metal-impregnated biochar, CO₂ capture, carbon reduction, physisorption, chemisorption  

1. Introduction 

Global industrialization and economic development are accelerating the use of fossil fuels and thus the 

excessive emission of greenhouse gases, in particular carbon dioxide (CO2), which poses a serious threat to 

the ecological environment. Increasing atmospheric carbon dioxide levels contribute to rising average 

temperatures, leading to global problems such as sea-level rise, extreme weather events, and ecological and 

economic damage. The UK's weather forecasting service said that carbon dioxide concentrations in 2024 will 

be around 420 ppm. This is dramatically higher than before the industrial revolution, when CO₂ levels were only 

about 280 ppm (Martina Igini, 2024). For this reason, CO2 capture is seen as a potential strategy to reduce the 

amount of CO2 emitted into the atmosphere. Solvent adsorption, membrane separation, cryogenic separation 

and adsorption with solids are commonly used to capture carbon dioxide after combustion (A.L. Yaumi et al., 

2017). Adsorption is considered to be the best technique because of its low energy consumption, the wide 

temperature and pressure range where the technology can be applied, and the sorbent regeneration without 

generating unfavorable by-products. Thus, biochar can be a promising adsorbent for CO2 reduction (M.S. 

Shafeeyan et al., 2010). Despite the fact, that biochar can be used directly for CO2 capture, it generally shows 

limited adsorption performance. Therefore, the surface of biochar has to be modified (e.g. physicochemical 

properties, specific surface area, pore structure and surface functional groups, etc.). Different adsorbents have 

been extensively studied over the past decades. Solid adsorbents, such as amine-supported silica (Lee et al. 

2018), carbonaceous materials (Kamran and Park, 2021), zeolites (Kumar et al. 2020), organo-metallic 

framework materials (Younas et al. 2020). Compared to the above mentioned solid adsorbents, biochar is a 

substance that is stable, highly aromatized and carbon-rich, therefore it is naturally able to adsorb carbon 

217



dioxide. It has excellent potential due to its high porosity and its surface contains a large number of active sites. 

According to the International Biochar Initiative, biomass derived biochar, can be carbon neutral (Lee et al. 

2018).  

The economic feasibility of biochar production depends mostly on the cost of the feedstock, therefore biomass 

waste can be an advanced solution. Due to its relatively low cost, biochar can be produced using biomass and 

other waste such as crop residues, wood waste, animal manure, food waste, municipal solid waste and sewage 

sludge. In addition, the use of biochar from waste promotes sustainable waste management. According to Xue 

et al. (2022), carbon, hydrogen, oxygen and nitrogen are the primary components of biochar, with carbon content 

for more than 40% of all elements. Biochar produced from different raw materials generally exhibit different 

physicochemical properties (e.g. pore structure, specific surface area, surface oxygen-containing chemical 

groups and elemental composition, etc.) (Nguyen and Lee 2016; Mulabagal et al. 2015; Goldfarb et al. 2017; 

Lee et al. 2017) The carbon dioxide capacity can be further enhanced by various chemical treatments of biochar, 

such as surface oxidation or impregnation with metals (e.g. Ca, Mg, Zn, Ni, Cu). These impregnations facilitate 

reactions in the presence of alkali and alkaline earth metals, which enhance the adsorption process, for example 

by forming carbonate that binds carbon dioxide to the biochar surface (X. Xu et al., 2016).  

In this work, sewage sludge biochars were modified with different metals and the carbon dioxide capture was 

also investigated. 

2. Materials and methods 

2.1 Preparation and characterization of biochar  

For biochar production animal sewage sludge was used. Raw material was dried and pyrolyzed in a batch steel 

reactor (inside volume ~0.5 dm3) under a nitrogen atmosphere at 400°C. The sewage sludge derived biochar 

was divided into different fractions by sieving: 3.15-2.50 mm (hereafter CM1) and 0.80-0.40 mm (hereafter CM2) 

(Figure 1). 

 

Figure 1: Production and application of metal impregnated biochar 

The biochar fractions with different particle sizes were impregnated by wet impregnation. Five different metal 

salts were used: calcium-chloride (Sigma-Aldrich, 97-103%), magnesium-chloride-hexahydrate (VWR 

Chemicals, 99-101%), nickel-nitrate hexahydrate (thermo-scientific, 98%), zinc-nitrate hexahydrate (Molar 

Chemicals Kft.) and copper-sulphate pentahydrate (Molar Chemicals Kft.). The impregnation was carried out in 

a spherical flask with a recirculating cooler, which was placed in a temperature-controlled water bath to adjust 

the temperature of the impregnation. For the impregnation 60 g of the biochar was placed in the flask and 240 

ml of 2 M metal-salt solution was added in a 1:4 ratio. The wet impregnation process was carried out at 80°C 

and 2°C for 2 hours using a magnetic stirrer (Faithful) with constant stirring. The samples were then filtered and 

dried in an oven (POL-EKO Aparatura) at 80°C to constant weight. Finally, after the impregnation, the materials 

were treated in in a tubular reactor (Carbolite Gero). Samples were inertized in a nitrogen atmosphere for 20 

minutes at room temperature before heat treatment. After the inertization, the metal impregnated biochars were 

heated at 400 and 600°C for 1 hour, with nitrogen flowrate of 2 l/h. 

2.2 Adsorption 

The CO2 absorption properties of the biochar-based adsorbents were investigated at 25°C using 15 vol.% CO2-

N2 gas mixture. For the adsorption, 14 g of impregnated biochar was loaded into a U-tube, which was placed in 

thermostat (Julabo) to ensure a constant temperature (30°C). The gas flow rate was set to 4.1 l/. The CO2 

concentration was measured by a Draeger X-am 7000 multi-gas detector. The acquired CO2 breakthrough 

curves were used to determine the breakthrough time (tbr) of the CO2, which was used to determine the 

adsorption capacity (q) in mg CO2/g sorbent unit. 

Pyrolysis of 
animal sewage 
sludge (biochar 

production)

Sepration of 
biochar (3.15-
2.50 mm and 

0.80-0.40 mm)

Impregnation of 
biochar

Post treatment 
of biochar (at 

400 and 600°C)

Adsorption 
(15% CO2)

218



3. Results and discussion 

3.1 Adsorption isotherms 

The CO2 adsorption breakthrough curves of the biochar-based sorbents obtained are shown in Figure 2. Based 

on the results, it can be concluded that for the reference samples, the breakthrough occurs slightly earlier (25 

seconds) for the 0.80-0.40 mm particle size fraction than for the 3.15-2.50 mm particle size fraction. Accordingly, 

the reference samples CM1 and CM2 have a capacity of 2.9 mg CO2/g sorbent and 3.5 mg CO2/g sorbent 

respectively. This is probably because the CM1 sample may have a larger particle size but higher porosity than 

the CM2 sample, so their specific surface areas may be nearly the same. 

  

Figure 2: CO2 breakthrough curves of adsorbents: a. impregnated at 25°C and b. impregnated at 80°C 

As function of particle size, the results show that the CO2 breakthrough of the impregnated sorbents occurred 

at least at the same time as for the reference sample (2,1 min<tbr<4,0 min), so the capacity of sorbents increased 

or remained almost the same as a result of impregnation. The change in the capacity was independent of the 

temperature to be used during the impregnation process. For most of the samples, the 0.80-0.40 mm particle 

size had higher capacity, than that of 3.15-2.50 mm. The capacity increased by an average of 1.9 mg CO2/g 

sorbent compared to the reference, an overall increase of 89% on average. The highest capacity was found for 

CM2_80_Cu, which was 4.1 mg CO2/g sorbent. However, the results show that the capacity for some samples 

impregnated at 25°C was worse than that of samples impregnated at 80°C. As it is known, the adsorption is 

favored by the low temperature, therefore at room temperature more metals are likely to have been impregnated 

onto the surface of the biochar, blocking the pores of the sorbent and reducing its capacity. 

  

Figure 3: CO2 breakthrough curves of adsorbents: a. impregnated at 25°C and heat treated at 400°C and b. 

impregnated at 80°C and heat treated at 400°C 

Regarding the heat treatment, the results show that the heat treatment after impregnation had a positive effect 

on the CO2 capture of the impregnated sorbents (Figure 3 and Figure 4). Similarly to the reference samples, 

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.
%

)

Time (min)

CM1
CM2
CM1_25_Ni
CM2_25_Ni
CM1_25_Cu
CM2_25_Cu
CM1_25_Mg
CM2_25_Mg
CM1_25_Ca
CM2_25_Ca
CM1_25_Zn
CM2_25_Zn

a

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.
%

)

Time (min)

CM1
CM2
CM1_80_Ni
CM2_80_Ni
CM1_80_Cu
CM2_80_Cu
CM1_80_Mg
CM2_80_Mg
CM1_80_Ca
CM2_80_Ca
CM1_80_Zn
CM2_80_Zn

b

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.

%
)

Time (min)

CM1_400
CM2_400
CM1_25_Ni_400
CM2_25_Ni_400
CM1_25_Cu_400
CM2_25_Cu_400
CM1_25_Mg_400
CM2_25_Mg_400
CM1_25_Ca_400
CM2_25_Ca_400
CM1_25_Zn_400
CM2_25_Zn_400

a

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.

%
)

Time (min)

CM1_400
CM2_400
CM1_80_Ni_400
CM2_80_Ni_400
CM1_80_Cu_400
CM1_80_Cu_400
CM1_80_Mg_400
CM2_80_Mg_400
CM1_80_Ca_400
CM2_80_Ca_400
CM1_80_Zn_400
CM2_80_Zn_400

b

219



due to the heat treatment, most of the impregnated CM1 sorbents showed a longer breakthrough time, resulting 

sorbents with higher specific capacities than the impregnated CM2 sorbents. This observation is true for samples 

with both particle size ranges treated at 400°C and 600°C. The increase in the breakthrough times at 400°C 

and 600°C was at least 6.5% and 17.5% respectively. 

The results show that in case of 400°C heat treatment, the capacity of the CM2 sample did not change 

significantly (0.7 mg CO2/g sorbent increase in capacity), whereas the capacity of the reference sample 

increased by 2.5 times using CM1 biochar. At higher temperatures (600°C), the heat treatment can increase the 

capacity of the reference samples (CM1 and CM2) by at least 1.5 times. However, the capacity of the CM1_600 

particle size fraction decreased compared to the samples heated at 400°C. The capacity of the CM1_600 sample 

(3.6 mg CO2/g sorbent) was 25% lower than that of the CM1_400 sample (4.9 mg CO2/g sorbent). However, 

the capacity of the other particle fraction (CM2) increased by 34% after heat treatment at 600°C. 

  

Figure 4: CO2 breakthrough curves of adsorbents: a. impregnated at 25°C and heat treated at 600°C and b. 

impregnated at 80°C and heat treated at 600°C 

3.2 CO2 capacity of the sorbents 

The specific capacity for the different adsorbents is shown in Figure 5 and 6. For both the CM1_80_400 and 

CM1_80_600 impregnated samples, it can be concluded that the sorbent capacity increased or remained almost 

unchanged after heat treatment. But the opposite trend was observed for CM2_80_400 and CM2_80_600, 

where the capacity was reduced by the heat treatment. On average, the capacity decrease was 45%. A similar 

conclusion was found for samples impregnated at 25°C.  

 

Figure 5: Specific CO2 capacity of the adsorbents3.15-2.50 mm 

It was found that the capacity of the adsorbents increased or remained almost unchanged after heat treatment 

for both CM1_25_400 and CM1_25_600 samples. The average increase of the capacity was 12%. The best 

performance was achieved with Ni, Cu and Zn impregnated sorbents, and the highest capacity was obtained 

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.

%
)

Time (min)

CM1_600
CM2_600
CM1_25_Ni_600
CM2_25_Ni_600
CM1_25_Cu_600
CM2_25_Cu_600
CM1_25_Mg_600
CM2_25_Mg_600
CM1_25_Ca_600
CM2_25_Ca_600
CM1_25_Zn_600
CM2_25_Zn_600

a

0

2

4

6

8

10

12

14

16

0 5 10 15 20

C
O

2
c
o

n
c
e
n

tr
a
ti

o
n

 (
v

o
l.

%
)

Time (min)

CM1_600
CM2_600
CM1_80_Ni_600
CM2_80_Ni_600
CM1_80_Cu_600
CM2_80_Cu_600
CM1_80_Mg_600
CM2_80_Mg_600
CM1_80_Ca_600
CM2_80_Ca_600
CM1_80_Zn_600
CM2_80_Zn_600

b

0

1

2

3

4

5

6

7

CM1_Ni CM1_Cu CM1_Mg CM1_Ca CM1_Zn

q
 (

m
g

 C
O

2
/ 

g
 a

d
s

o
rb

e
n

t)

25°C impregnation 25°C imp., 400°C heat treatment

25°C imp., 600°C heat treatment 80°C impregnation

80°C imp., 400°C heat treatment 80°C imp., 600°C heat treatment

a

220



with CM1_Ni, CM1_Cu and CM1_Zn adsorbents. The average capacity was higher than 4.0 mg CO2/g sorbent. 

Overall, it can be concluded that the thermal treatment after impregnation has a positive effect on the CO2 

adsorption properties of sorbents. To the fact that, after impregnation, the metals are present on the sorbent 

surface in the form of chlorite, nitrate and sulphate salts, which thermally decompose and form oxides at the 

heat treatment temperature. As confirmed by earlier measurements, metal oxides are more efficient for CO2 

absorption. 

  

Figure 6: Specific CO2 capacity of the adsorbents 0.80-0.40 mm particle size fraction 

3.3 Elemental composition of char 

The elemental composition of the used adsorbents was analysed by a Shimadzu EDX-8100 Energy Dispersive 

X-ray Fluorescence spectroscopy. The results showed that both the impregnation temperature and the particle 

size had an effect on the amount of metals impregnated on the char surface. The results show that in the case 

of biochar impregnated at 80°C, less metal was impregnated on CM1 particles than on CM2 particles. 

Considering the amount of metal components impregnated on the sorbent surface, the more metal impregnated 

on the sorbent surface, the higher the sorbent capacity. For sorbents impregnated at 80°C the results show that 

although the nickel fraction was the smallest in the sorbent after impregnation, it was the second largest capacity 

measured. In the presence of nickel, the sorbent capacity increased by 2-3 times. Regarding the effect of metals, 

zinc and magnesium had the least effect on the capacity. 

4. Conclusion 

The CO2 adsorption capacity of metal-impregnated and heat-treated biochar-based sorbents was investigated. 

The effects of impregnation temperature and post-treatment temperature on biochar with different particle sizes 

were investigated. Fractions of 3.15-2.50 mm and 0.80-0.40 mm biochar were used for the investigation. The 

investigated biochar-based sorbents were prepared by a wet impregnation method using 5 different metal 

compounds (Ni, Cu, Zn, Mg and Ca) at 2 different temperatures (25°C and 80°C). The sorbents prepared by 

impregnation were heat treated at 400°C and 600°C. It was found that adsorbents with 3.15-2.50 mm particle 

size have better CO2 adsorption capacity, than adsorbents with 0.80-0.40 mm particle size. The difference in 

capacity was 14%. As a function of particle size, the results show that the capacity of sorbents increased or 

remained almost the same as a result of impregnation. The highest capacity increase was achieved when 

biochars were impregnated with Ni and Cu salts (average capacity of 3.4 mg CO2/g adsorbent). Furthermore, it 

can be concluded that the heat treatment after impregnation has a positive effect on the CO2 adsorption. It was 

found that the capacity of the adsorbents increased or remained almost unchanged after heat treatment, both 

at 400° and 600°C. The capacity increase is mostly favored by the 600°C heat treatment. The average increase 

of capacity after the 600°C heat treatment was 12%. 

Acknowledgments 

Project no. RRF-2.3.1-21-2022-00009, titled National Laboratory for Renewable Energy has been implemented 

with the support provided by the Recovery and Resilience Facility of the European Union within the framework 

of Programme Széchenyi Plan Plus.  

 

0

1

2

3

4

5

6

7

CM2_Ni CM2_Cu CM2_Mg CM2_Ca CM2_Zn

q
 (

m
g

 C
O

2
/ 

g
 a

d
s

o
rb

e
n

t)

25°C impregnation 25°C imp., 400°C heat treatment

25°C imp., 600°C heat treatment 80°C impregnation

80°C imp., 400°C heat treatment 80°C imp., 600°C heat treatment

b

221



References 

Dissanayake P.D., You S., Igalavithana A.D., Xia Y., Bhatnagar A., Gupta S., Kua H.W., Kim S., Kwon J.H., 

Tsang D.C.W., Ok Y.S., 2020, Biochar-based adsorbents for carbon dioxide capture: A critical review, 

Renewable and Sustainable Energy Reviews, 119, 109582  

Guo S., Li Y., Wang Y., Wang L., Sun Y., Liu L., 2022, Recent advances in biochar-based adsorbents for CO2 

capture, Carbon Capture Science & Technology, 4, 100059 

Miskolczi N., Gao N., Quan C., Laszlo A.T., 2025, CO2 reduction by chars obtained by pyrolysis of real wastes: 

Low temperature adsorption and high temperature CO2 capture, Carbon Capture Science & Technology, 

14, 100332  

Miskolczi N., Csutoras B., 2024, Thermo-catalytic pyrolysis of sewage sludge and techno-economic analysis: 

The effect of synthetic zeolites and natural sourced catalysts, Bioresource Technology, 400, 130676 

Robert Monroe, 2024, Daily Measurements, Keeling Curve History  

Martina Igini, 2024, Atmospheric CO2 Jump in 2024 off Track With Trajectory Needed to Meet 1.5 Goal, Met 

Office Says  

Rebecca Lindsey, 2024, Climate Change: Atmospheric Carbon Dioxide  

Xue Q., Xie S., Zhang T., 2022, Biochar production and modification for environmental improvement, Biochar 

in Agriculture for Achieving Sustainable Development Goals, 181-191 

Yaumi A.L., Abu Bakar M.Z., Hameed B.H., 2017, Recent advances in functionalized composite solid materials 

for carbon dioxide capture, Energy, 124, 461-480 

Shafeeyan M.S., Wan Daud W.M.A., Houshmand A., Shamiri A., 2010, A review on surface modification of 

activated carbon dioxide adsorption, Journal of Analytical and Applied Pyrolysis, 89, 143-151  

Xu X., Kan Y., Zhao L., Cao X., Chemical transformation of CO2 during its capture by waste biomass derived 

biochars, Environmental Pollution, 213, 533-540  

Kamran U., Park S-J., 2021, Chemically modified carbonaceous adsorbents for enhanced CO2 capture: A 

review, Journal of Cleaner Production, 290, 125776  

222


	332csaszar.pdf
	The Effect of Metal Impregnated Biochars in Carbon Dioxide Decrement