Process optimization of rice husk-based activated carbon production for water vapor adsorption

Run	Factor
Run	Concentration (% w/v)	Activation time (h)
1	10.00	8.00
2	10.00	12.00
3	15.00	7.17
4	15.00	10.00
5	15.00	10.00
6	15.00	10.00
7	15.00	10.00
8	15.00	10.00
9	15.00	12.83
10	20.00	8.00
11	20.00	12.00
12	7.93	8.00
13	22.07	10.00
	Factor and experimental range
Variable	−α	−1	0	+1	+α
Concentration (% w/v)	7.93	10.00	15.00	20.00	22.07
Activation time (hour)	7.17	8.00	10.00	12.00	12.83

Illustration of activated carbon preparation.

Table 2

Representative samples for each activating agent.

Agent concentration (% w/v)	Activation time (h)
15.00	7.17
	10.00
	12.83
7.93	8.00
10.00
20.00

Table 3

Adsorption models used in this research [27,28].

		Equation
Adsorption kinetics models	Pseudo-first-order	$q_{t} = q_{e} (1 - e^{- k_{PFO} t})$
Adsorption kinetics models	Pseudo-second-order	$q_{t} = \frac{q_{e}^{2} k_{PSO} t}{q_{e} k_{PSO} t + 1}$
Diffusion models	Boyd’s external diffusion	$q_{t} = k_{B} (1 - e^{- Rt})$
Diffusion models	Weber–Morris intraparticle diffusion	$q_{t} = k_{WM} t^{1 / 2} + I$

Table 4

Water vapor adsorption isotherm models in this research.

Isotherm model	Formula	Parameters
Freundlich [31]	$q_{e} = k_{f} {a_{w}}^{1 / n_{f}}$	k _f = Freundlich constant
Freundlich [31]	$q_{e} = k_{f} {a_{w}}^{1 / n_{f}}$	n _f = adsorption intensity
Langmuir [32]	$\frac{1}{q_{e}} = \frac{1}{k_{L} m_{0}} \frac{1}{a_{w}} + \frac{1}{m_{0}}$	k _L = Langmuir constant
Langmuir [32]		m ₀ = monolayer adsorption
BET (Brunauer, Emmett, and Teller) [33]	$q_{e} = \frac{m_{0} c_{B} a_{w}}{(1 - a_{w}) (1 - a_{w} + c_{B} a_{w})}$	c _B = BET constant
FHH (Frenkel-Halsey-Hill) [31]	$q_{e} = m_{0} {[- ln (a_{w})]}^{- 1 / r}$	r = FHH constant

Table 5

CCD factors and response.

Run	Factor		Response
	Concentration (% w/v)	Activation time (h)	Adsorption capacity (g/g)
			KOH	NaOH
1	10.00	8.00	0.1027	0.0751
2	10.00	12.00	0.0892	0.0751
3	15.00	7.17	0.1340	0.1607
4	15.00	10.00	0.1050	0.1328
5	15.00	10.00	0.1541	0.1338
6	15.00	10.00	0.1602	0.0938
7	15.00	10.00	0.1653	0.0947
8	15.00	10.00	0.1283	0.0923
9	15.00	12.83	0.2133	0.1347
10	20.00	8.00	0.0976	0.1946
11	20.00	12.00	0.2693^h	0.0847^l
12	7.93	8.00	0.0794	0.0672
13	22.07	10.00	0.1439	0.1267

h = highest adsorption capacity.

l = lowest adsorption capacity.

Table 6

Analysis of variance of activated carbon water vapor adsorption from RSM.

Source	DF	KOH				NaOH
Source	DF	Adj SS	Adj MS	F-value	P-value	Adj SS	Adj MS	F-value	P-value
Model	5	0.031	0.006	14.350	0.001*	0.012	0.002	4.820	0.032**
Linear	2	0.025	0.012	28.790	0.000*	0.007	0.004	7.380	0.019*
X ₁	1	0.012	0.012	27.640	0.001*	0.004	0.004	8.020	0.025*
X ₂	1	0.010	0.010	22.250	0.002*	0.004	0.004	8.720	0.021*
Square	2	0.006	0.003	6.460	0.026*	0.003	0.001	2.700	0.135**
X ₁ ²	1	0.004	0.004	9.130	0.019*	0.001	0.001	1.330	0.287**
X ₂ ²	1	0.002	0.002	4.610	0.069**	0.002	0.002	4.370	0.075**
Two-way interaction	1	0.010	0.010	22.920	0.002*	0.002	0.002	3.160	0.118**
X _₁ X _₂	1	0.010	0.010	22.920	0.002*	0.002	0.002	3.160	0.118**
Error	7	0.003	0.000			0.004	0.001
Lack-of-fit	3	0.000	0.000	0.240	0.865**	0.002	0.001	1.160	0.427**
Pure error	4	0.003	0.001			0.002	0.000
Total	12	0.034				0.016
R ²		0.911				0.765

*Significant at 0.05.

**Not Significant at 0.05.

Comparison between the predicted data (model) from RSM and experimental data for (a) KOH and (b) NaOH.

3D surface plot of adsorption capacity of rice husk-based activated carbon prepared using (a) KOH and (b) NaOH.

Table 7

Optimum conditions for preparing activated carbon from rice husk using activating agents of KOH and NaOH.

Activating agent	Concentration (% w/v)	Activation time (h)	Adsorption capacity (g/g)	Composite desirability
KOH	22.07	12.83	0.32	1
NaOH	22.07	7.17	0.23	1

SEM and EDX analysis of RHC (a and b), ACK (c and d), and ACNa (e and f).

Table 8

Chemical composition of RHC and activated carbon.

Compound	Composition (%)			Reduction (%)		Increase (%)
Compound	RHC	ACK	ACNa	ACK	ACNa	ACK	ACNa
SiO₂	87.00	92.70	87.70			6.55	0.80
P₂O₅	2.20	2.70	3.80			22.73	72.73
K₂O	4.52	0.67	0.88	85.18	80.53
CaO	4.03	1.60	3.16	60.30	21.59
TiO₂	0.07	0.26	0.34			271.43	385.71
MnO	0.53	0.16	0.32	69.81	39.62
Fe₂O₃	1.04	1.51	2.27			45.19	118.27
NiO	0.02	0.08	0.10			300.00	400.00
CuO	0.08	0.16	0.29			100.00	262.50
ZnO	0.03		0.19	100.00			533.33
BaO	0.10		0.20	100.00			100.00
Yb₂O₃	0.06		0.20	100.00			233.33
Re₂O₇	0.20		0.60	100.00			200.00
Eu₂O₃		0.20		—	—	—	—

(a) N₂ adsorption–desorption and (b) pore size distribution of samples.

Table 9

Surface area, pore radius, and pore volume of samples.

Sample	Surface area (m²/g)	Average pore radius (nm)	Pore volume (cm³/g)
RHC	25.88	3.14	0.04
ACK	138.86	2.93	0.20
ACNa	45.80	3.20	0.07

Water vapor adsorption of activated carbon prepared using (a) KOH concentration of 15% w/v, (b) NaOH concentration of 15% w/v, (c) KOH activation for 8 h, and (d) NaOH activation for 8 h.

Water vapor adsorption process on the surface of activated carbon.

Water vapor adsorption of rice husk and activated carbon at a water activity of (a) 0.7 and (b) 0.85.

Table 10

Comparison of the adsorption capacity by several desiccants.

Desiccant	Adsorption capacity (g water/g desiccant)	Reference
Synthesized zeolite 13X	0.47	[53]
Synthesized zeolite Na-A	0.18	[53]
Silica gel	0.29–0.32	[53,54]
Sulfuric acid-treated activated carbon	0.36	[55]
Nitric acid-treated activated carbon	0.27	[55]
Acorn nutshell-based activated carbon	0.29	[56]
Rice husk-based activated carbon	0.27	This research

Water vapor adsorption of (a) RHC and (b) ACNa at different temperatures.

(a) Water vapor adsorption of a single ACK at same condition and (b) water vapor desorption at different temperatures.

Table 11

Parameters of nonlinear pseudo-first-order model of rice husk and activated carbons.

Sample	Parameter
Sample	k	q _e	R ²	SSE
KOH 15% w/v 7.17 h	0.0202	0.1321	0.9994	1.124 × 10⁻⁶
KOH 15% w/v 10.0 h	0.0007	0.1174	0.9184	9.947 × 10⁻⁵
KOH 15% w/v 12.83 h	0.0001	0.3069	0.8059	0.0056
KOH 7.93% w/v 8.00 h	0.0148	0.0765	0.9877	6.686 × 10⁻⁶
KOH 10.00% w/v 8.00 h	0.0138	0.1027	0.9981	2.010 × 10⁻⁶
KOH 20.00% w/v 8.00 h	0.0008	0.0954	0.9985	1.488 × 10⁻⁶
NaOH 15% w/v 7.17 h	0.0143	0.1592	0.9992	2.195 × 10⁻⁶
NaOH 15% w/v 10.0 h	0.0008	0.1223	0.9499	6.990 × 10⁻⁵
NaOH 15% w/v 12.83 h	0.0688	0.1328	0.9984	3.708 × 10⁻⁶
NaOH 7.93% w/v 8.00 h	0.0192	0.0643	0.9853	5.100 × 10⁻⁶
NaOH 10.00% w/v 8.00 h	0..0112	0.1142	0.9791	2.234 × 10⁻⁵
NaOH 20.00% w/v 8.00 h	0.0052	0.1928	0.9548	1.846 × 10⁻⁴
Relative humidity of 85%: RHC	0.0247	0.0911	0.9880	1.346 × 10⁻⁵
Relative humidity of 85%: ACK	0.0075	0.1843	0.9164	4.235 × 10⁻⁴
Relative humidity of 85%: ACNa	0.0096	0.1137	0.9515	8.525 × 10⁻⁵
Relative humidity of 70%: RHC	0.0080	0.0578	0.9310	2.306 × 10⁻⁵
Relative humidity of 70%: ACK	0.0103	0.0864	0.9612	3.018 × 10⁻⁵
Relative humidity of 70%: ACNa	0.0124	0.0775	0.9521	3.670 × 10⁻⁵
Average			0.9597	3.672 × 10 ⁻⁴

Table 12

Parameters of nonlinear pseudo-second-order model of rice husk and activated carbons.

Sample	Parameter
Sample	k	q _e	R ²	SSE
KOH 15% w/v 7.17 h	0.2884	0.1331	0.9998	3.208 × 10⁻⁷
KOH 15% w/v 10.0 h	0.0097	0.1256	0.9569	5.180 × 10⁻⁴
KOH 15% w/v 12.83 h	0.0001	0.4349	0.6630	0.0062
KOH 7.93% w/v 8.00 h	0.3448	0.0799	0.9968	1.741 × 10⁻⁶
KOH 10.00% w/v 8.00 h	0.2616	0.1069	0.9807	1.999 × 10⁻⁵
KOH 20.00% w/v 8.00 h	0.1171	0.0953	0.9986	1.719 × 10⁻⁶
NaOH 15% w/v 7.17 h	0.1383	0.1608	0.9999	1.154 × 10⁻⁷
NaOH 15% w/v 10.0 h	0.0097	0.1309	0.8741	3.782 × 10⁻⁴
NaOH 15% w/v 12.83 h	1.4591	0.1341	0.9990	2.322 × 10⁻⁶
NaOH 7.93% w/v 8.00 h	0.5912	0.0667	0.9976	8.268 × 10⁻⁷
NaOH 10.00% w/v 8.00 h	0.1934	0.1163	0.9978	2.215 × 10⁻⁶
NaOH 20.00% w/v 8.00 h	0.0417	0.1976	0.9731	1.072 × 10⁻³
Relative humidity of 85%: RHC	0.6570	0.0915	0.9994	7.453 × 10⁻⁷
Relative humidity of 85%: ACK	0.0345	0.1868	0.9957	3.080 × 10⁻⁵
Relative humidity of 85%: ACNa	0.0983	0.1151	0.9982	3.300 × 10⁻⁶
Relative humidity of 70%: RHC	0.5095	0.0580	0.9858	7.410 × 10⁻⁶
Relative humidity of 70%: ACK	0.5774	0.0865	0.9903	6.347 × 10⁻⁶
Relative humidity of 70%: ACNa	0.1385	0.0780	0.9809	1.410 × 10⁻⁵
Average			0.9660	4.583 × 10 ⁻⁴

Table 13

Parameters of Boyd’s external diffusion model of rice husk and activated carbons.

Sample	Parameter
Sample	k	R	R ²	SSE
KOH 15% w/v 7.17 h	0.1321	0.0202	0.9994	1.1239 × 10⁻⁶
KOH 15% w/v 10.0 h	0.1209	0.0010	0.9120	0.0001
KOH 15% w/v 12.83 h	0.3071	4.138 × 10⁻⁵	0.8860	0.0013
KOH 7.93% w/v 8.00 h	0.0764	0.0149	0.9877	6.682 × 10⁻⁶
KOH 10.00% w/v 8.00 h	0.1027	0.0139	0.9981	2.006 × 10⁻⁶
KOH 20.00% w/v 8.00 h	0.0954	0.0008	0.9985	1.488 × 10⁻⁶
NaOH 15% w/v 7.17 h	0.1607	0.0024	0.8826	0.0006
NaOH 15% w/v 10.0 h	0.1369	0.0002	0.7638	0.0007
NaOH 15% w/v 12.83 h	0.1328	0.0689	0.9984	3.708 × 10⁻⁶
NaOH 7.93% w/v 8.00 h	0.0764	0.0149	0.9877	6.682 × 10⁻⁶
NaOH 10.00% w/v 8.00 h	0.1140	0.0115	0.9789	2.219 × 10⁻⁵
NaOH 20.00% w/v 8.00 h	0.1928	0.0052	0.9546	0.0002
Relative humidity of 85%: RHC	0.0883	0.0244	0.9880	6.290 × 10⁻⁶
Relative humidity of 85%: ACK	0.1759	0.0049	0.9687	8.557 × 10⁻⁵
Relative humidity of 85%: ACNa	0.1094	0.0072	0.9729	2.491 × 10⁻⁵
Relative humidity of 70%: RHC	0.0546	0.0153	0.9636	8.438 × 10⁻⁶
Relative humidity of 70%: ACK	0.0846	0.0186	0.9925	4.651 × 10⁻⁶
Relative humidity of 70%: ACNa	0.0742	0.0100	0.9561	2.171 × 10⁻⁵
Average			0.9550	1.720 × 10 ⁻⁴

Residual plots of different adsorption and diffusion models.

Table 14

Parameters of Weber–Morris intraparticle diffusion model of rice husk and activated carbons.

Sample		Parameter
	k ₁	k ₂	k ₃	R ²	SSE
	I ₁	I ₂	I ₃	R ²	SSE
KOH 15% w/v 7.17 h	0.0028	0.00004	—	0.8229	0.0004
KOH 15% w/v 7.17 h	0.0315	0.12880	—	0.8229	0.0004
KOH 15% w/v 10.0 h	0.0019	0.00040	—	0.9702	3.837 × 10⁻⁵
KOH 15% w/v 10.0 h	0.0046	0.07060		0.9702	3.837 × 10⁻⁵
KOH 15% w/v 12.83 h	0.0045	0.00010	0.00200	0.9937	3.618 × 10⁻⁵
KOH 15% w/v 12.83 h	0.0048	0.06910	−0.11050	0.9937	3.618 × 10⁻⁵
KOH 7.93% w/v 8.00 h	0.0059	0.00045	0.00003	0.9982	1.036 × 10⁻⁶
KOH 7.93% w/v 8.00 h	0.0004	0.06184	0.07681	0.9982	1.036 × 10⁻⁶
KOH 10.00% w/v 8.00 h	0.0066	0.00001	—	0.9868	1.366 × 10⁻⁵
KOH 10.00% w/v 8.00 h	0.0042	0.10140	—	0.9868	1.366 × 10⁻⁵
KOH 20.00% w/v 8.00 h	0.0011	0.00010	—	0.9661	3.777 × 10⁻⁵
KOH 20.00% w/v 8.00 h	0.0055	0.08410	—	0.9661	3.777 × 10⁻⁵
NaOH 15% w/v 7.17 h	0.0035	0.00005	—	0.9104	0.0002
NaOH 15% w/v 7.17 h	0.0286	0.15477	—	0.9104	0.0002
NaOH 15% w/v 10.0 h	0.0020	0.00040	—	0.9302	0.0002
NaOH 15% w/v 10.0 h	0.0037	0.07930	—	0.9302	0.0002
NaOH 15% w/v 12.83 h	0.0187	0.00010	—	0.9796	4.716 × 10⁻⁵
NaOH 15% w/v 12.83 h	0.0039	0.12340	—	0.9796
NaOH 7.93% w/v 8.00 h	0.0046	0.00025	0.00003	0.9745	8.860 × 10⁻⁶
NaOH 7.93% w/v 8.00 h	0.0033	0.05626	0.06475	0.9745	8.860 × 10⁻⁶
NaOH 10.00% w/v 8.00 h	0.0064	0.00008	—	0.9706	3.129 × 10⁻⁵
NaOH 10.00% w/v 8.00 h	0.0073	0.10640	—	0.9706	3.129 × 10⁻⁵
NaOH 20.00% w/v 8.00 h	0.0101	0.00186	0.00001	0.9703	0.0001
NaOH 20.00% w/v 8.00 h	0.0013	0.08947	0.19304	0.9703	0.0001
Relative humidity of 85%: RHC	0.0065	0.00022	0.00004	0.9545	2.386 × 10⁻⁵
Relative humidity of 85%: RHC	0.0062	0.08147	0.08768	0.9545	2.386 × 10⁻⁵
Relative humidity of 85%: ACK	0.0078	0.00130	0.00010	0.9829	5.402 × 10⁻⁵
Relative humidity of 85%: ACK	0.0031	0.11190	0.17451	0.9829	5.402 × 10⁻⁵
Relative humidity of 85%: ACNa	0.0053	0.00084	0.00008	0.9888	9.679 × 10⁻⁶
Relative humidity of 85%: ACNa	0.0052	0.07729	0.10614	0.9888	9.679 × 10⁻⁶
Relative humidity of 70%: RHC	0.0029	0.00006	—	0.9608	9.018 × 10⁻⁶
Relative humidity of 70%: RHC	0.0048	0.05031	—	0.9608	9.018 × 10⁻⁶
Relative humidity of 70%: ACK	0.0042	0.00002	—	0.9473	3.261 × 10⁻⁵
Relative humidity of 70%: ACK	0.0091	0.08393	—	0.9473	3.261 × 10⁻⁵
Relative humidity of 70%: ACNa	0.0040	0.00093	0.00002	0.9777	1.292 × 10⁻⁵
Relative humidity of 70%: ACNa	0.0032	0.04694	0.07457	0.9777	1.292 × 10⁻⁵
Average				0.9603	6.980 × 10 ⁻⁵

Plotting between experimental data and modeled data of water vapor sorption isotherm of (a) RHC, (b) ACK, and (c) ACNa.

Table 15

Water vapor sorption isotherm models of rice husk and activated carbon.

Isotherm model	Sample	Parameters		R ²	SSE
Freundlich	RHC	k _F = 0.1397	n _F = 0.3821	0.9861	0.0002
	ACK	k _F = 0.3311	n _F = 0.2606	0.9898	0.0005
	ACNa	k _F = 0.2040	n _F = 0.2888	0.9510	0.0008
Langmuir	RHC	m ₀ = 0.0186	k _L = 0.9680	0.9164	0.0005
	ACK	m ₀ = 0.0412	k _L = 0.9448	0.9807	0.0007
	ACNa	m ₀ = 0.0372	k _L = 0.8874	0.9226	0.0006
BET	RHC	m ₀ = 0.0127	c _B = 1.1326	0.9124	0.0009
	ACK	m ₀ = 0.0305	c _B = 0.9708	0.9700	0.0013
	ACNa	m ₀ = 0.0208	c _B = 1.0275	0.9079	0.0016
FHH	RHC	m ₀ = 0.0124	r = 1.0129	0.8935	0.0007
	ACK	m ₀ = 0.0330	r = 1.0921	0.9691	0.0007
	ACNa	m ₀ = 0.0216	r = 1.0702	0.8848	0.0009

Residual plots of several adsorption isotherm models.

References

Authors

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Articles in this issue

DOI: https://doi.org/10.2478/msp-2025-0044 | Journal eISSN: 2083-134X (formerly 2083-124X) | Journal ISSN: 2083-1331

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Language: English

Page range: 198 - 220

Submitted on: Mar 10, 2025

Accepted on: Dec 21, 2025

Published on: Dec 31, 2025

Published by: Wroclaw University of Science and Technology

In partnership with: Paradigm Publishing Services

Keywords:

Activated carbon,

Adsorption,

Dehumidification,

Response surface methodology

Related subjects:

Materials sciences,

Materials sciences, other,

Nanomaterials,

Functional and smart materials,

Materials characterization and properties

© 2025 Dewi Qurrota A’yuni, Moh Djaeni, Nandang Mufti, Agus Subagio, published by Wroclaw University of Science and Technology
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

Volume 43 (2025): Issue 4 (December 2025)

Process optimization of rice husk-based activated carbon production for water vapor adsorption

Figures & Tables

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Table 13

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Table 14

Figure 12

Table 15

Figure 13

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