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

A’yuni, Dewi Qurrota; Djaeni, Moh; Mufti, Nandang; Subagio, Agus

doi:10.2478/msp-2025-0044

Figures & Tables

Illustration of activated carbon preparation.

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.

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

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

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.

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.

Residual plots of different adsorption and diffusion models.

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

Residual plots of several adsorption isotherm models.

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

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

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

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

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

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 isotherm models in this research_

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

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

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

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

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		—	—	—	—

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

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

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

CCD on activation of RHC using KOH and NaOH_

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

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

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

Figures & Tables

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

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

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

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

Water vapor sorption isotherm models of rice husk and activated carbon_

Comparison of the adsorption capacity by several desiccants_

Water vapor adsorption isotherm models in this research_

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

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

Chemical composition of RHC and activated carbon_

Representative samples for each activating agent_

CCD factors and response_

Analysis of variance of activated carbon water vapor adsorption from RSM_

CCD on activation of RHC using KOH and NaOH_

Surface area, pore radius, and pore volume of samples_

Paradigm

My account