Article
Utilization of Rice Waste (
Oryza
sativa
L) for The Production of Silica
Gel with Var`iations in Mass Ratio of Raw Materials and Reactant
Solutions
Aisyah Suci Ningsih
, Endang Supraptiah
, Idha Silviyati
, Sri Murda Niati*
, Ibnu Hajar
,
Marghis Nur Dila
Chemical Engineering Department, Politeknik Negeri Sriwijaya, Palembang, Sumatera Selatan -30139,
Indonesia
A
bstract
Rice
is
one
of
the
major
agricultural
commodities
that
generates
large
amounts
of
biomass
waste,
particularly rice husks and rice straw. These by-products contain high silica content and have potential
as raw materials for silica gel production. This study aimed to synthesize silica gel from rice husks and
rice straw using the sol–gel method with different biomass ratios and acid activators. The ratios of rice
husk to rice straw used were 100:0, 75:25, 50:50, 25:75, and 0:100 (w/w). The synthesis process was
carried out using 3 M HCl and 3 M H
₂
SO
₄
solutions as reactants. The obtained silica gel products were
evaluated
based
on
yield,
moisture
content,
water
adsorption
capacity,
and
functional
group
characteristics
using
Fourier
Transform
Infrared
(FTIR)
spectroscopy.
The
results
showed
that
the
optimum silica gel was produced from a biomass ratio of 75 % rice husk and 25 % rice straw using
H
₂
SO
₄
as the reactant. The resulting silica gel exhibited a yield of 82.5 %, moisture content of 14.7 %,
and water adsorption capacity of 0.0092 g/g. FTIR analysis confirmed the presence of silanol (Si–OH),
siloxane (Si–O–Si), and siloxy functional groups, indicating successful silica gel formation. Overall, the
synthesized
silica
gel
demonstrated
physicochemical
properties
consistent
with
standard
silica
gel
characteristics and showed potential for application as an adsorbent material.
Keywords:
Silica gel, rice straw, rice husk
*
Corresponding author
Email addresses:
sri.murda.niati@polsri.ac.id
(Sri Murda Niati)
DOI:
https://doi.org/10.22437/chp.v10i1.49539
Received
November 08
th
2025;
Accepted
March 08
th
2026;
Available online
May 26
th
2026
Copyright © 2026 by Authors, Published by Chempublish Journal. This is an open access article under the CC BY License
(
https://creativecommons.org/licenses/by/4.0
)
138
Graphical Abstract
Introduction
Indonesia
is
one
of
the
countries
with
extensive
agricultural
land
distributed
across major islands such as Java, Sumatra,
and
Sulawesi.
The
total
agricultural
land
area
in
Indonesia
reaches
approximately
10.4
million
hectares,
with
South
Sumatra
recorded
as
one
of
the
provinces
with
the
largest agricultural areas [1]. This condition
supports
Indonesia
as
one
of
the
major
rice-producing
countries
in
the
world.
However,
rice
cultivation
activities
also
generate
large
quantities
of
agricultural
waste, including rice straw, rice husks, bran,
and rice polish.
Agricultural
waste
derived
from
rice
production
has
attracted
considerable
attention
due
to
its
potential
for
value-
added
applications.
Among
these
by-
products, rice husks and rice straw are rich
in
silica
compounds
and
can
be
utilized
as
alternative
raw
materials
for
silica-based
materials.
Rice
husks
contain
silica
at
concentrations ranging from approximately
90–98
%,
while
rice
straw
contains
silica
levels
of
around
82
%
[2,3].
The
high
silica
content
makes
these
materials
promising
renewable
resources
for
silica
extraction
and silica gel synthesis.
Silica
gel
is
an
amorphous
silica
material
with
high
porosity
and
strong
water
adsorption
capacitys
[2].
Due
to
these
characteristics,
silica
gel
has
been
widely
applied as an adsorbent, desiccant, catalyst
support,
anti-caking
agent,
cosmetic
additive,
and
filler
material
in
industrial
products.
One
of
the
commonly
used
methods
for
silica
gel
synthesis
is
the
sol–
gel
method
because
it
is
relatively
simple,
cost-effective,
and
capable
of
producing
silica
with
high
purity
and
controlled
pore
structure [3].
Previous
studies
have
reported
the
utilization
of
rice
husks
or
rice
straw
individually
for
silica
production
[3,4].
However,
studies
evaluating
the
combined
use
of
rice
husks
and
rice
straw
with
different
composition
ratios
for
silica
gel
synthesis
are
still
limited.
Therefore,
this
study
aimed
to
synthesize
silica
gel
from
rice
husks
and
rice
straw using
the
sol–gel
method
with
different
biomass
ratios
and
139
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
acid
activators.
The
synthesized
silica
gel
was characterized based on yield, moisture
content,
water
adsorption
capacity,
and
functional
group
analysis
using
Fourier
Transform Infrared (FTIR) spectroscopy.
Furthermore, abundant rice waste has also
been utilized as animal feed and bioenergy
material.
However,
the
utilization
of
rice
straw
as
feed
is
limited
due
to
its
low
nutritional
value
and
high
fiber
content.
Rice
husks
and
bran
are
considered
more
suitable as concentrated feed because they
contain higher energy content [4].
Several
studies
have
also
reported
the
conversion
of
rice
waste
into
environmentally
beneficial
products.
A
community-based study in Padengo Village
demonstrated
that
rice
straw
could
be
processed
into
compost
using
Effective
Microorganism-4
(EM4)
technology,
which
improved
soil
fertility
while
reducing
agricultural
waste
accumulation
[5].
In
addition,
rice
husks
have
been
utilized
for
biopellet production using tapioca flour as a
binder,
resulting
in
moisture
contents
of
6.6–8.4
%,
ash
contents
of
6–16
%,
and
calorific
values
of
approximately
4.064–
4.135
J/g,
indicating
their
potential
as
alternative
biofuel
materials
[6].
Both
rice
straw
and
rice
husks
therefore
represent
promising renewable biomass resources for
sustainable applications [7].
Silica
(SiO
₂
)
is
an
inorganic
compound
naturally
found
in
materials
such
as
sand,
quartz,
and
silicate
rocks.
Naturally
occurring
silica
generally
possesses
a
crystalline
structure
with
highly
ordered
atomic
arrangements,
whereas
synthetic
silica materials such as silica gel exhibit an
amorphous
structure
without
long-range
crystallinity.
Amorphous silica
is
commonly
synthesized
through
methods
such
as
the
sol–gel
process,
which
enables
the
formation of nano-sized particles with high
surface
area
and
reactivity
[8].
Due
to
its
porous
structure
and
strong
adsorption
properties,
silica
gel
has
been
widely
used
in industrial and laboratory applications as
an
adsorbent,
desiccant,
catalyst
support,
and moisture control material.
Silica
gel
(SiO
₂
·xH
₂
O)
is
characterized
by
solid
glass-like
particles
with
highly
porous
structures.
It
is
commonly
synthesized
through
the
coagulation
of
sodium
silicate
sol derived from sodium silicate precursors.
During
the
dehydration
process,
the
gel
structure
transforms
into
rigid
amorphous
particles with high adsorption capability [9].
Silica
gel
can
absorb
moisture
effectively
without
undergoing
structural
changes,
making
it
highly
suitable
as
a
desiccant
material
[10].
The
adsorption
capability
of
silica
gel
is
strongly
influenced
by
the
presence of silanol (Si–OH) and siloxane (Si–
O–Si) functional groups on its surface [11].
Previous
studies
have
investigated
the
influence of acid catalysts on the synthesis
of silica materials from rice husk ash. Hayati
et al. reported the use of hydrochloric acid
(HCl), sulfuric acid (H
₂
SO
₄
), and phosphoric
acid (H
₃
PO
₄
) at concentrations of 3 M during
the
sol–gel
synthesis
process
to
promote
silica hydrolysis and condensation reactions
[12]. In addition, silica gel synthesized from
rice
husks
using
HCl
solution
showed
promising
physicochemical
properties,
including
a
surface
area
of
130.867
m²/g,
total
pore
volume
of
0.1420
cc/g,
and
average
pore
diameter
of
4.34
nm
[12].
These
findings
indicate
that
rice
husk-
derived silica has strong potential as a raw
material for silica gel production.
Compared
with
previous
studies,
the
novelty of this research lies in the utilization
of
mixed
rice
husk
and
rice straw
biomass
as silica sources for silica gel synthesis. The
incorporation
of rice
straw was considered
because rice straw also contains high silica
140
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
content,
approximately
83
%.
Therefore,
this study aimed to investigate the synthesis
of silica gel from mixtures of rice husks and
rice
straw
using
strong
acid
solutions,
namely HCl and H
₂
SO
₄
, through the sol–gel
method
to
produce
environmentally
friendly
silica
gel
materials
with
potential
industrial applications.
Materials and Methods
Materials
The
materials
used
in
this
study
were
rice
straw
and
rice
husks
collected
from
local
rice
fields
in
Indonesia.
The
chemical
reagents
used
included
sodium
hydroxide
(NaOH, Merck, Germany), hydrochloric acid
(HCl
37
%,
Merck,
Germany),
sulfuric
acid
(H
₂
SO
₄
96 %, Merck, Germany), and distilled
water. All chemicals used were of analytical
grade without further purification.
The equipment used in this study included a
furnace
(Nabertherm,
Germany),
analytical
balance
(Ohaus
Pioneer™,
USA),
hot
plate
magnetic
stirrer
(IKA®
C-MAG
HS
7,
Germany),
laboratory
oven
(Memmert,
Germany), desiccator (Pyrex®, USA), mortar
and pestle, aluminum foil, pH paper (Merck,
Germany),
filter
paper
(Whatman
No.
42,
UK),
stainless
steel
sieve
(100
mesh),
and
standard
laboratory
glassware
including
beakers,
volumetric
flasks,
measuring
cylinders, Erlenmeyer flasks, and glass rods.
Functional
group
characterization
of
the
synthesized
silica
gel
was
performed
using
Fourier-Transform
Infrared
Spectroscopy
(FTIR) (Shimadzu IRPrestige-21, Japan).
Experimental Design
The
independent
variables
in
this
study
were the type of acid reagent solution and
the composition ratio of rice husk and rice
straw
used
as
raw
materials
for
silica
gel
synthesis.
The
composition
ratios
of
rice
husk to rice straw were 100:0, 75:25, 50:50,
25:75,
and
0:100 (w/w).
The
fixed
variables
were
heating
temperature
and
heating
duration
during
the
synthesis
process.
The
dependent
variables
observed
were
silica
gel
yield,
moisture
content,
and
water
adsorption capacity.
Preparation
of
Rice
Husk
and
Rice
Straw
Ash
Rice
husks
and
rice
straw
were
washed
thoroughly
using
distilled
water
to
remove
adhering
impurities
and
dirt,
followed
by
sun
drying
until
constant
weight
was
achieved.
The
dried
rice
husks
were
then
oven-dried at 110 °C for 2 h, while the rice
straw was dried at 200 °C for 10 min.
Subsequently, the rice husks and rice straw
were immersed separately in 200 mL of 1 M
HCl solution and heated at 75 °C for 1 h to
remove
metallic
impurities
and
reduce
mineral contaminants. The suspension was
filtered using Whatman No. 42 filter paper,
and
the
solid
residue
was
washed
repeatedly with distilled water until neutral
pH was reached.
The cleaned rice husks and rice straw were
then
calcined
in
a
furnace
(Nabertherm,
Germany) at 700 °C for 2 h to produce rice
husk ash and rice straw ash. After cooling to
room
temperature,
the
resulting
ash
was
ground
using
a
mortar
and
pestle
and
sieved through a 100-mesh sieve to obtain
homogeneous ash particles.
Silica Gel Synthesis
Rice
husk
ash
and
rice
straw
ash
were
mixed
according
to
the
predetermined
composition
ratios,
and
40
g
of
the
mixed
ash was dissolved in 200 mL of 2.5 M NaOH
solution. The mixture was heated at 100 °C
for
2
h
on
a
hot
plate
magnetic
stirrer
(IKA®,
Germany)
under
continuous
stirring
and covered with aluminum foil to minimize
evaporation.
141
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
After heating, the mixture was filtered using
Whatman
No.
42
filter
paper
to
separate
insoluble
residues,
and
the
sodium
silicate
filtrate
was
collected.
The
filtrate
was
then
titrated using either 3 M HCl or 3 M H
₂
SO
₄
solution under continuous stirring until pH
7
was
achieved,
resulting
in
hydrogel
formation through the sol–gel process.
The
formed
hydrogel
was
aged
at
room
temperature
for
24
h
to
complete
the
gelation
process.
The
hydrogel
was
subsequently
washed
repeatedly
with
distilled water to remove residual ions and
impurities.
The
washed
hydrogel
was
filtered
and
dried
in
an
oven
(Memmert,
Germany) at 80 °C for 2 h to obtain silica gel
powder.
Product Analysis Method, Moisture Content
Test (SNI 3751-2009)
Heat the porcelain dish at 130 °C in an oven
for
1
hour.
Cool
the
porcelain
dish
in
a
desiccator
for
30
minutes
and
weigh
it.
Weigh
1
g
of
silica
(W)
and
place
it
on
the
porcelain
dish,
then
bake
it
in
an
oven
at
130
°C
for
1
hour.
Cool
the
porcelain
dish
containing
silica
in
a
desiccator
for
30
minutes, then weigh the sample (W1). Once
a constant weight is obtained, calculate the
moisture
content
using
the
formula
1
(SNI
3751-2009) [13].
(1)
where
W
=
Initial
sample
weight
(g);
W1
=
Weight of sample after heating (g).
Product Analysis Method, Absorption Test
1 g of silica and prepare 100 ml of distilled
water.
Store
the
silica
gel
in
a
desiccator
with
the
distilled
water,
then
leave
it
for
1
day until the mass of the silica gel does not
change.
Weigh
the
mass
of
the
silica
gel
after
it
has
been
stored
in
the
desiccator.
Calculate
the
change
in
silica
gel
mass
before and after storage, and determine the
adsorption
capacity
of
the
silica
gel
by
comparing
the
amount
of
water
absorbed
per gram of silica gel (Formula 2) [14].
q
(%)
=
(W
t
– W
0
)/ W
0
100 %
(2)
where
q
is
the
water
adsorption
capacity
(%),
W
0
is
the
initial
dry
weight
of
silica
gel
(g),
and
W
t
is
the
weight
of
silica
gel
after
adsorption equilibrium (g).
Functional Group Characteristics (FTIR)
Infrared
spectroscopy
is
used
to
identify
functional groups present in silica gel. Each
functional
group
in
silica
gel
has
characteristic
absorption
at
specific
wavenumbers,
enabling
qualitative
identification
using
infrared
spectroscopy.
This characterization is expected to be used
for
qualitative
indication
of
the
success
of
silica
gel
synthesis
[15,16].
FTIR
analysis
results
are
presented
in
the
form
of
a
spectrum
graph,
where
the
X-axis
represents the absorption spectrum plotted
against
wavenumber,
and
the
Y-axis
represents the percentage of transmittance
[17].
The
silica
gel
production
process
is
shown
in
Figure
1.
The
silica
gel
testing
process is shown in Figure 2.
Figure
1.
Overview
process;
(a)
Drying
of
rice husks and straw; (b) Furnace output of
rice husks and straw; (c) Furnace feedstock;
(d)
Ground
rice
husk
and
straw
ash;
(e)
Suspension
filtration
process;
(f)
Heating
142
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
the
mixture
of
ash
and
HCl
solution;
(g)
Preparation
of
sodium
silicate
solution;
(h)
Sol-gel
process
with
H
₂
O
and
CuSO
₄
;
(i)
Filtered and washed hydrogel.
Figure
2.
Photograph
of
samples.
(a)
Silica
gel sample; (b) Silica gel absorption test; (c)
Silica gel moisture content test; (d) Silica gel
usage test
Result and Discussion
Effect
of
Reagent
Type
and
Biomass
Composition on Silica Gel Characteristics
The synthesis of silica gel from mixtures of
rice husk (RH) and rice straw (RS) produced
materials
with
varying
yields,
moisture
contents,
water
adsorption
capacities,
and
silica functional group characteristics (Table
1).
Since
each
treatment
was
performed
without
replication,
the
data
were
interpreted
descriptively
to
identify
trends
associated
with
biomass
composition
and
acid reagent type.
The results indicate that both RH proportion
and
acid
reagent
influenced
the
physicochemical
properties
of
the
synthesized
silica
gel.
Generally,
increasing
the RH content improved silica recovery and
adsorption
performance,
reflecting
the
higher silica content of rice husk relative to
rice
straw.
For
HCl-treated
samples,
the
highest
yield
was
obtained
from
the
100:0
RH:RS
ratio
(52.0%),
while
the
lowest
yield
was
observed
in
the
0:100
treatment
(5.75%). Water adsorption capacity showed
a similar pattern, decreasing from 0.0084 g
g
⁻
¹ at the 75:25 ratio to 0.0052 g g
⁻
¹ in the
rice-straw-only
sample.
These
results
suggest
that
increasing
the
proportion
of
rice
straw
reduced
silica
recovery
and
adsorption
capacity
due
to
its
lower
silica
content
and
greater
abundance
of
non-
siliceous constituents.
Table 1.
Results of Rice Husk and Rice Straw Silica Gel Synthesis
Sample
Code
Type of
solution
Reagent
Mass
Ratio
SP : JP (%)
Yield (%)
Moisture Content
(%)
Water Adsorption
(g/g)
Result
SNI
Standard
Result
SNI
Standard
A1
HCl
100:0
52
14.1
≤15
0.0080
≥0.0075
A2
HCl
75:25
37
14.6
≤15
0.0084
≥0.0075
A3
HCl
50:50
21.5
10.8
≤15
0.0068
≥0.0075
A4
HCl
25:75
13
7
≤15
0.0056
≥0.0075
A5
HCl
0:100
5.75
2.9
≤15
0.0052
≥0.0075
B1
H
2
SO
4
100:0
78.25
14.6
≤15
0.0076
≥0.0075
B2
H
2
SO
4
75:25
82.5
14.7
≤15
0.0092
≥0.0075
B3
H
2
SO
4
50:50
61.25
11.8
≤15
0.0080
≥0.0075
B4
H
2
SO
4
25:75
29.25
11.6
≤15
0.0064
≥0.0075
B5
H
2
SO
4
0:100
13.5
7.5
≤15
0.0063
≥0.0075
Description: SP : JP Mass Ratio (%)
→
SP = Rice Husk, JP = Rice Straw
Compared
with
HCl,
H
₂
SO
₄
consistently
produced
silica
gel
with
superior
performance across all biomass ratios. The
optimum
formulation
was
achieved
at
an
RH ratio of 75:25 using 3 M H
₂
SO
₄
, yielding
82.5%
silica
gel
with
the
highest
water
143
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
adsorption
capacity
(0.0092
g
g
⁻
¹)
while
maintaining an acceptable moisture content
(14.7%).
Notably,
this
adsorption
capacity
exceeded
the
minimum
requirement
specified in SNI 06-2477-1991 (0.0075 g g
⁻
¹),
indicating good adsorbent quality.
The
superior
performance
of
the
H
₂
SO
₄
-
treated samples indicates that sulfuric acid
was more effective than hydrochloric acid in
promoting
silica
gel
formation
during
the
sol–gel
process.
Across
comparable
biomass
compositions,
H
₂
SO
₄
treatment
generally
resulted
in
higher
yields
and
water
adsorption
capacities,
suggesting
enhanced
silica
precipitation
and
a
more
developed
gel
structure.
The
maximum
yield (82.5%) was achieved at a rice husk-to-
rice
straw
ratio
of
75:25,
whereas
the
highest
yield
obtained
using
HCl
was
37%.
Thus,
H
₂
SO
₄
increased
silica
recovery
by
approximately
123%,
demonstrating
the
substantial
effect
of
acid
reagent
selection
on
synthesis
efficiency.
These
findings
highlight the importance of both precursor
composition
and
acid
catalyst
type
in
optimizing
silica
gel
production
from
agricultural residues.
Table 2.
Results of Analysis of Silica Gel Function Groups in Rice Husk and Rice Straw
Types of Vibration
Wave Number (cm
-1
)
Sample A2
Sample B2
Span -OH from Si-OH
3391.13
3386.11
Bending -OH from Si-OH
1640.34
1638.9
Asymmetry range Si
-
O from Si
-
O
-
Si
1060.8
1042.16
Symmetry range Si-O from Si-OH
953.93
878.62
Asymmetry range Si
-
O
-
Si
793.99
788.97
Curve Si-O-Si
454.73
432.5
A = HCl Solution Variation, B = H2SO4 Solution Variation, Material Mass Ratio 75%:25%
Moisture
content
analysis
showed
that
all
synthesized
silica
gel
samples
contained
less
than
15%
moisture,
indicating
acceptable
water
content
for
silica
gel
materials.
Low
residual
moisture
is
advantageous because excessive water may
adversely
affect
adsorption
performance
and storage stability.
The
adsorption
capacity
of
silica
gel
is
strongly
influenced
by
its
pore
structure
and
the
density
of
surface
silanol
(Si–OH)
groups,
which
contribute
to
water
adsorption
through
hydrogen-bond
interactions.
The
higher
adsorption
capacities
observed
in
several
treatments
therefore suggest the
formation of a more
developed
porous
network
with
a
greater
number
of
accessible
active
sites.
Notably,
the
silica
gel
synthesized
using
H
₂
SO
₄
at
a
rice
husk-to-rice
straw
ratio
of
75:25
achieved
the
highest
yield
and
adsorption
capacity,
indicating
that
this
formulation
provided the most favorable conditions for
silica gel formation.
Overall,
the
results
highlight
the
feasibility
of
utilizing
rice
husks
and
rice
straw
as
renewable
silica
sources
for
value-added
silica
gel
production.
The
observed
differences
among
treatments
further
confirm
that
both
feedstock
composition
and
acid
reagent
type
are
key
factors
governing
silica
recovery
and
adsorption
properties.
These
findings
support
the
potential
application
of
agricultural
waste-
derived
silica
gel
as
an
environmentally
sustainable adsorbent material.
144
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
Tabel 3.
Silica Gel Usage Test Results
Day Observation
Sample Mass A2 (g)
Sample Mass B2 (g)
1
1.0
1.0
2
1.13
1.26
3
1.15
1.33
4
1.19
1.38
5
1.22
1.41
6
1.28
1.48
7
1.37
1.52
Description: A = Variation in HCl Solution, B = Variation in H2SO4 Solution, Material Mass Ratio 75 % : 25 %.
Yield Results
Yield
is
a
comparison
of
the
weight
of
the
precipitate
obtained
with
the
amount
of
silicate ash used. The yield value produced
will
be
related
to
the
amount
of
bioactive
content
contained
therein
[18].
The
relationship
between
the
composition
of
rice
husk
and
rice
straw
mixtures
and
the
silica gel yield can be seen in Figure 3.
Figure 3.
Graph showing the effect of the composition of rice husk and rice straw mixtures
0
10
20
30
40
50
60
70
80
90
1 (100:0)
2 (75:25)
3 (50:50)
4 (25:75)
5 (0:100)
Yie
ld
(%)
Composition of Rice Husk and Rice Straw Mixture (%)
A (HCl solution)
B (H
2
SO
4
solution)
As shown in Figure 3, the highest silica gel
yield
was
obtained
from
sample
B2
(75%
rice
husk:25%
rice
straw)
synthesized
using H
₂
SO
₄
, reaching 82.5%, whereas the
lowest
yield
was
recorded
for
sample
A5
(0% rice husk:100% rice straw) synthesized
using HCl, with a yield of only 5.75%. These
results
indicate
that
both
biomass
composition
and
acid
type
strongly
influenced
silica
recovery
during
the
sol–
gel process.
An
increase
in
rice
husk
proportion
generally
resulted
in
higher
silica
gel
yields. This trend can be attributed to the
inherently
higher
silica
content
of
rice
husks compared with rice straw, enabling
more
efficient
sodium
silicate
formation
and
subsequent
silica
precipitation.
In
contrast, increasing the proportion of rice
145
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
straw reduced silica recovery, likely due to
its
lower
silica
concentration
and
higher
content
of
non-siliceous
organic
components.
The
use
of
H
₂
SO
₄
consistently
produced
higher yields than HCl across comparable
biomass
ratios.
Notably,
the
optimum
formulation
(B2)
exhibited
a
yield
approximately 123% higher than the best-
performing
HCl-treated
sample
(A2,
37%).
The
superior
performance
of
H
₂
SO
₄
may
be
associated
with
its
stronger
acidic
character
and
diprotic
nature,
which
promotes
more
efficient
protonation
of
silicate
species
and
facilitates
silica
condensation
during
gel
formation.
Consequently,
silica
precipitation
from
sodium
silicate
solution
becomes
more
complete,
resulting
in
greater
silica
recovery
and
improved
gel
formation.
These
findings
are
consistent
with
previous
studies
reporting
that
sulfuric
acid
enhances
silica
formation
efficiency
during sol–gel synthesis from rice-derived
silica sources [12].
Moisture Content Test
T
Moisture content analysis was performed
to
evaluate
the
hygroscopic
properties
of
the
synthesized
silica
gel.
This
parameter
reflects
the
amount
of
water
retained
within
the
silica
structure
and
is
closely
related to the abundance of surface silanol
groups
and
the
porous
characteristics
of
the
material.
In
this
study,
moisture
content
was
determined
from
the
weight
loss of silica gel after heating at 130 °C for
1 h, representing the removal of physically
adsorbed
water.
The
results
of
the
moisture content analysis are presented in
Figure
4.
Overall,
all
synthesized
silica
gel
samples
exhibited
moisture
contents
below the maximum limit specified by the
silica
gel
quality
standard
(≤15%),
indicating
acceptable
physicochemical
quality
for
adsorbent
applications.
Figure 4.
Graph showing the effect of the composition of rice husk and rice straw mixtures on
moisture content
146
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
Based on the graph in Figure 4, it can
be
seen
that
the
highest
moisture
content
was found in sample B2, which was 14.7 %
with
a
composition
of
100
%
rice
husks
using
H
2
SO
4
solution,
and
the
lowest
moisture content was found in sample A5,
which was 2.9 % with a composition of 100
%
rice
straw
using
HCl
solution.
Factors
that
can
affect
the
moisture
content
percentage include raw materials and acid
solutions.
In
addition,
moisture
content
can
also
be
influenced
by
the
amount
of
water vapor in the air and the duration of
the
cooling
process
[19].
Differences
in
material
percentages
can
affect
the
moisture
content
value
obtained.
The
higher
the
rice
husk
content,
the
higher
the
moisture
content
value
[20].
This
is
because
rice
husks
have
a
high
silica
content,
while
straw
has
a
lower
silica
content.
The
higher
the
silanol
(Si-OH)
content in silica, the
greater the ability of
silica
gel
to
bind
water
molecules
due
to
the increased hydrogen bonds.
Additionally,
strong
acid
solutions
cause
an
increase
in
moisture
content.
This
is
because
sulfuric
acid
has
a
higher
acidity
than hydrochloric acid. The increase in the
acidity
of
the
synthesized
silica
gel
is
estimated to originate from the increase in
the
number
of
silanol
groups
caused
by
the
increased
H
+
ions
used
in
the
production
of
silica
gel
[22].
Sulfuric
acid
has 2H
+
, while HCl only has H
+
ions.
The
higher
the
acidity
value,
the
greater
the
number
of
silanol
(Si-OH)
groups
present in the silica gel, thereby increasing
its ability to donate protons [9]. Based on
the
results
of
the
percentage
of
air
content,
all
treatments
on
the
samples
have met the requirements of SNI silica gel
[20], which states that the maximum water
content
value
of
silica
gel
is
15%,
so
that
the water content in the silica gel samples
meets
the
quality
requirements
with
the
optimal water content being in sample B2.
Water
Adsorption
Capacity
of
Synthesized
Silica Gel
One
application
of
silica
gel
adsorption
capacity
that
was
tested
to
determine
the
quality of the adsorbent was its absorption
capacity
for
H
2
O,
since
adsorbents
are
generally
used
as
absorbent
materials
(adsorption).
Adsorption
in
silica
gel
is
an
absorption
process
that
occurs
when
a
fluid/liquid
binds
to
a
solid
and
eventually
forms a thin layer or film on its surface [9].
Water adsorption capacity is defined as the
maximum
ability
of
silica
gel
to
adsorb
water from saturated vapor. The results of
water
absorption
on
silica
gel
from
rice
husks and straw can be seen in Figure 5.
Based
on
Figure
5,
the
highest
water
adsorption capacity was observed in sample
B2, which was synthesized using a mixture
of 75 % rice husk and 25 % rice straw with
H
₂
SO
₄
solution,
reaching
0.0092
g/g.
In
contrast,
the
lowest
adsorption
capacity
was found in sample A5, consisting of 100 %
rice
straw
synthesized
using
HCl
solution,
with an adsorption value of 0.0052 g/g.
The water adsorption capability of silica gel
is
influenced
by
several
factors,
including
silica
content,
acid
reagent
type,
pore
structure, and the chemical composition of
the
synthesized
silica
material
[23].
Higher
silica content generally contributes to better
adsorption
performance
because
silica-rich
materials
possess
larger
surface
areas
and
more active adsorption sites [9]. Rice husks
contain higher silica content compared with
rice
straw;
therefore,
silica
gel
synthesized
with higher rice husk composition tended to
exhibit superior adsorption properties.
The
type
of
acid
reagent
also
significantly
affected the adsorption performance of the
synthesized
silica
gel.
Silica
gel
prepared
147
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
using
H
₂
SO
₄
solution
showed
higher
water
adsorption
capacity
than
samples
synthesized
using
HCl.
This
phenomenon
may
be
attributed
to
the
stronger
acidity
and
higher
proton
contribution
of
H
₂
SO
₄
,
which
contains
two
dissociable
H
⁺
ions,
whereas
HCl
only
contributes
one
H
⁺
ion.
Higher
acidity
during
the
sol–gel
process
can
enhance
silica
hydrolysis
and
condensation
reactions,
promoting
the
formation
of
larger
numbers
of
silanol
(Si–
OH) groups on the silica surface
[9]. These
silanol
groups
play
an
important
role
in
water adsorption because they can interact
strongly
with
water
molecules
through
hydrogen bonding.
The
presence
of
abundant
silanol
groups
increases the hydrophilic nature of silica gel,
thereby
improving
its
adsorption
capacity.
In
addition,
better
pore
formation
during
gelation
may
contribute
to
increased
surface area and adsorption efficiency. The
results
indicate
that
the
combination
of
high
rice
husk
composition
and
H
₂
SO
₄
reagent
produced
silica
gel
with
more
favorable
physicochemical
characteristics
for adsorption applications.
According
to
the
Indonesian
National
Standard
(SNI
No.
06-2477:1991),
silica
gel
should
have
a
minimum
water
adsorption
capacity
of
0.0075
g/g
[21].
Based
on
the
obtained
results,
several
synthesized
silica
gel samples met or exceeded this standard,
particularly
sample
B2
with
an
adsorption
capacity
of
0.0092
g/g.
This
value
was
also
higher
than
that
of
commercial
silica
gel,
which
exhibited
an
adsorption
capacity
of
approximately 0.0085 g/g.
Conversely,
samples
synthesized
using
HCl
with
higher
proportions
of
rice
straw
(A3–
A5)
showed
relatively
lower
adsorption
capacities,
in
some
cases
below
the
commercial
silica
gel
standard.
The
lower
adsorption performance may be associated
with
reduced
silica
purity
and
less
optimal
pore
formation
due
to
the
higher
organic
and cellulose content of rice straw [22].
Overall,
the
results
demonstrate
that
silica
gel synthesized using H
₂
SO
₄
and a rice husk
to
rice
straw
ratio
of
75:25
exhibited
the
optimum
water
adsorption
performance.
These
findings
indicate
that
agricultural
biomass waste can be effectively utilized as
an environmentally friendly raw material for
silica
gel
production
with
competitive
adsorption properties.
Identification
of
Functional
Groups
of
Synthesized Silica Gel
Functional
group
identification
of
the
synthesized silica gel was carried out using
Fourier
Transform
Infrared
(FTIR)
spectroscopy.
FTIR
analysis
is
commonly
used
to
identify
characteristic
functional
groups
in
silica
materials
based
on
their
absorption bands at specific wavenumbers.
The active functional groups present on the
surface of silica gel mainly consist of silanol
(Si–OH) and siloxane (Si–O–Si) groups, which
play
important
roles
in
adsorption
processes
and
surface
interactions.
Therefore,
FTIR
characterization
was
performed
to
confirm
the
successful
formation
of
silica
gel
synthesized
from
mixtures
of
rice
husks
and
rice
straw.
The
FTIR spectra of silica gel synthesized from a
mixture
of
75
%
rice
husk
and
25
%
rice
straw
using
HCl
solution
(sample
A2)
and
H
₂
SO
₄
solution (sample B2) are presented in
Figures 6 and 7, respectively.
148
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
Figure 6.
IR Spectrum Overlay of Silica Gel Synthesis Results for Sample A2 and B2
Based
on
the
FTIR
spectrum
of
sample
A2
(Figure
6),
a
broad
absorption
band
was
observed at 3391.13 cm
⁻
¹, corresponding to
the
stretching
vibration
of
hydroxyl
(–OH)
groups associated with silanol (Si–OH). The
presence
of
hydroxyl
groups
was
further
confirmed
by
the
absorption
peak
at
1640.34 cm
⁻
¹, which represents the bending
vibration
of
adsorbed
water
or
Si–OH
groups. The absorption band at 793.99 cm
⁻
¹
indicates the symmetric stretching vibration
of
the
siloxane
(Si–O–Si)
group,
while
the
absorption
at
454.73
cm
⁻
¹
corresponds
to
the
bending
vibration
of
the
Si–O–Si
bond.
In
addition,
the
absorption
peak
at
953.93
cm
⁻
¹
was
assigned
to
the
symmetric
stretching
vibration
of
Si–O
from
silanol
groups,
whereas
the
strong
absorption
band
at
1060.80
cm
⁻
¹
indicates
the
asymmetric
stretching
vibration
of
Si–O–Si
bonds.
Similarly,
the
FTIR
spectrum
of
sample
B2
(Figure 7) showed a broad absorption band
at
3386.11
cm
⁻
¹,
indicating
the
stretching
vibration
of
hydroxyl
groups
from
Si–OH.
The bending vibration of the hydroxyl group
was
observed
at
1638.90
cm
⁻
¹.
The
absorption
band
at
788.97
cm
⁻
¹
corresponds
to
the
stretching
vibration
of
siloxane
(Si–O–Si),
while
the
absorption
at
432.50
cm
⁻
¹
represents
the
bending
vibration of the Si–O–Si group. Furthermore,
the
absorption
peak
at
878.62
cm
⁻
¹
indicates the symmetric stretching vibration
of
Si–O
from
silanol
groups,
and
the
absorption
band
at
1042.16
cm
⁻
¹
corresponds
to
the
asymmetric
stretching
vibration of Si–O–Si bonds.
In
general,
the
FTIR
spectra
of
all
synthesized
silica
gel
samples
exhibited
characteristic
absorption
bands
of
silica
materials.
The
identified
functional
groups
included
silanol
(Si–OH),
siloxane
(Si–O–Si),
149
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
and
siloxy
(Si–O)
groups,
which
are
typical
functional
groups
of
amorphous
silica
gel.
The
presence
of
these
functional
groups
confirms
that
silica
gel
was
successfully
synthesized
from
rice
husk
and
rice
straw
biomass
through
the
sol–gel
process using
both HCl and H
₂
SO
₄
solutions.
The
abundance
of
silanol
and
siloxane
groups
also
indicates
the
potential
adsorption
capability
of
the
synthesized
silica gel, since
these
functional groups act
as
active
adsorption
sites
for
water
molecules
and
other
polar
compounds.
Therefore,
the
FTIR
results
support
the
adsorption analysis results, where silica gel
synthesized using H
₂
SO
₄
and a rice husk to
rice
straw
ratio
of
75:25
demonstrated
superior adsorption performance.
Usability Testing of Synthesized Silica Gel
The usability test was conducted to evaluate
the
effectiveness
of
the
synthesized
silica
gel
as
a
moisture-absorbing
material.
This
test was performed by observing changes in
mass and physical appearance of the silica
gel
samples
before
and
after
storage
in
a
wardrobe environment for seven days. The
results of the usability test are presented in
Figure 8.
Figure 8.
Graph of Silica Gel Usage Test for 7 Days
Based
on
Figure
8,
both
silica
gel
samples
initially had a mass of 1.0 g on the first day
of
testing.
After
seven
days,
the
silica
gel
synthesized
using
HCl
solution
showed
an
increase
in
mass
to
1.37
g,
while
the
silica
gel
synthesized
using
H
₂
SO
₄
solution
increased
to
1.52
g.
The
increase
in
mass
observed in both samples indicates that the
synthesized silica gel successfully absorbed
moisture
from
the
surrounding
environment inside the wardrobe.
The
higher
mass
increase
observed
in
the
silica gel synthesized using H
₂
SO
₄
suggests
that this sample possessed better moisture
adsorption
capability
compared
with
the
silica
gel
synthesized
using
HCl
[24].
This
result
is
consistent
with
the
previous
adsorption
analysis,
which
demonstrated
that
silica
gel
prepared
using
H
₂
SO
₄
exhibited superior adsorption performance
due
to
the
presence
of
larger
numbers
of
active
silanol
groups
and
potentially better
pore development.
In
addition
to
mass
changes,
visual
observations
also
showed
changes
in
the
physical
appearance
of
the
silica
gel
samples after the usability test. Initially, the
silica
gel
appeared
as
dry
white
granules.
However, after seven days of exposure, the
silica
gel
became
slightly
clumped
and
150
AS Ningsih et al.,
Chempublish Journal, 10(1) 2026, 138-153
exhibited
a
pale
coloration.
The
clumping
phenomenon
occurred
due
to
moisture
absorption
by
the
silica
gel,
causing
the
particles
to
agglomerate.
Meanwhile,
the
slight
color
change
may
have
been
influenced
by
absorbed
moisture
and
environmental impurities during storage.
The
observed
increases
in
mass
and
changes
in
physical
appearance
indicate
that
the
synthesized
silica
gel
effectively
functioned
as
a
moisture-absorbing
material.
These
findings
demonstrate
the
potential
application
of
silica
gel
synthesized
from
rice
husk
and
rice
straw
biomass
as
an
environmentally
friendly
desiccant material for humidity control and
moisture adsorption applications
Conclusions
Based
on
the
results
of
the
research
that
has
been
done,
it
can
be
concluded
that
silica gel with a mixture composition of 75 %
rice husk and 25 % rice straw has the most
optimal
silica
gel
synthesis
results,
where
rice
husk
has
a
higher
silica
content
than
rice straw. The use of H
2
SO
4
solution in the
manufacture
of
silica
gel
has
the
most
optimal
yield,
water
content,
and
water
adsorption
results
compared
to
the
manufacture of silica gel with HCl solution,
with values of 82.5 %, 14.7 %, and 9.2 ml/g,
respectively.
Based
on
the
results
of
the
characteristics
using
FTIR,
the
synthesized
silica
gel
has
the
characteristics
of
the
general
silica
gel
functional
groups,
which
are silanol functional groups (Si-OH), siloxy
functional
groups
(Si-O),
and
siloxane
functional
groups
(Si-O-Si)
with
the
results
of
silica
gel
using
H
2
SO
4
solution
having
better absorption.
Acknowledgement
We
wish
to
extend
my
heartfelt
thanks
to
everyone who supported the completion of
this
research.
Particular
appreciation
is
given
to
Politeknik
Negeri
Sriwijaya,
the
Department
of
Chemical
Engineering,
and
the
Unit
Operations
and
Unit
Process
Laboratories
for
providing
the
facilities,
opportunities,
and
assistance
during
the
conduct of this study.
Author Contributions
Marghis Nurlaila
contributed to validation,
visualization,
and
data
interpretation.
Idha
Silviyati
contributed
to
formal
analysis,
investigation,
and
data
collection.
Endang
Supraptiah
contributed
to
software,
data
curation,
and
writing
of
the
original
draft.
Aisyah
Suci
Ningsih
contributed
to
conceptualization,
methodology,
and
project
administration.
Sri
Murda
Niati
contributed
to
supervision,
resources,
and
writing—review
and
editing.
All
authors
have read and approved the final version of
the manuscript.
Conflict of Interest
The authors declare no conflict of interest.
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