Plant-based nanostructured silicon carbide modified with bisphosphonates for metal adsorption

Microporous and Mesoporous Materials 324 (2021) 111294

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials
journal homepage: www.elsevier.com/locate/micromeso

Plant-based nanostructured silicon carbide modified with bisphosphonates
for metal adsorption
ăla
ăinen b,
Ondrej Haluska a, Arezoo Rahmani a, Ayobami Salami a, Petri Turhanen b, Jouko Vepsa
a
a
a, *
Reijo Lappalainen , Vesa-Pekka Lehto , Joakim Riikonen
a
b

Department of Applied Physics, University of Eastern Finland, P.O.Box 1627, Yliopistonranta 1, FI-70211, Kuopio, Finland
School of Pharmacy, University of Eastern Finland, P.O.Box 1627, Yliopistonranta 1, FI-70211, Kuopio, Finland

A R T I C L E I N F O

A B S T R A C T

Keywords:
Barley
Biogenic silica
Biogenic silicon carbide
Bisphosphonates

Metal adsorption

Nanostructured silicon carbide possesses superior properties such as excellent hardness, high chemical stability,
large surface area and good sintering ability at relatively low temperatures compared to bulk silicon carbide.
However, its synthesis with conventional methods is still challenging. In the present study, we produced
nanostructured silicon carbide from barley husks with a simple self-propagating high-temperature synthesis.
Barley husks were chosen as the raw material because they are agricultural residues widely available and contain
large amount of nanostructured silica suitable as a precursor in the synthesis. We studied the effect of two
processes to valorize the barley husks on the extracted silica particles: burning in an industrial scale furnace to
produce heat energy and pyrolysis to extract organic compounds as well as controlled calcination as a reference.
The processing prior to the extraction affected morphology and composition of the nanostructured silica. The
highest purity and surface area of 187 m2/g was obtained for the silica extracted from pristine barley husks
through calcination. On the other hand, pyrolysis allows additional valorisation of the biomass by producing biobased organic chemicals and still the silica particles with relatively high surface area, 105 m2/g, can be extracted.
Nanostructured silicon carbide was produced from the extracted nanostructured silica with magnesiothermic
reduction via self-propagating high-temperature synthesis. Nanostructured silicon carbide produced from silica
particles undergone calcination had the highest surface area of 196 m2/g. Furthermore, it was functionalized
with bisphosphonates to be used as a metal adsorbent and examined in adsorption of manganese from landfill
water with pH 8. The functionalization of the silicon carbide with bisphosphonates increased the adsorption
capacity by 32 % and the material was able to withstand at least 5 adsorption/desorption cycles.

1. Introduction
Silicon carbide (SiC) is a semiconducting ceramic which is used in
many applications such as abrasives, functional ceramics and catalysis
[1,2]. The beneficial characteristics of SiC are due to its high Young’s
modulus and hardness, resistance to oxidation and corrosion, and
excellent mechanical stability [2,3]. Many physical and chemical
properties of SiC depend on its grain size. Nanostructured SiC (nSiC),
consisting of grains below 100 nm in size, possesses certain advantages
over its bulk counterpart. For instance, nSiC can be sintered at a lower
temperature, it can be harder and have larger surface area than con­

ventional SiC powders [2].

Nanostructured SiC has been synthesized with various methods,
including chemical vapor deposition (CVD), sol-gel method, and thermal
and laser pyrolysis of organic molecules. However, these conventional
methods have several disadvantages including the use of toxic reagents,
excessive grain growth of the final product as well as high production
cost [2–4]. Magnesiothermic reduction is an alternative method to
produce nSiC. The overall reaction can be described as follows: SiO2 (s) +
C(s) + 2Mg(s)→SiC(s) + 2MgO(s), but the exact reaction mechanism is
still unclear [2,5,6]. Magnesiothermic reduction can be performed at a
relatively low temperature, ~600 ◦ C [2,3], and it can be conducted as a
self-propagating high-temperature synthesis (SHS). The SHS takes place
as a self-sustained high-temperature combustion reaction propagating

* Corresponding author.
E-mail addresses: (O. Haluska), (A. Rahmani), (A. Salami),
(P. Turhanen), (J. Vepsă
ală
ainen), (R. Lappalainen), (V.-P. Lehto), joakim.riikonen@uef.
fi (J. Riikonen).
/>Received 16 April 2021; Received in revised form 30 June 2021; Accepted 7 July 2021
Available online 9 July 2021
1387-1811/© 2021 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY license ( />

O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

through the precursor mixture. The main advantages of SHS are simple

reactor design, short reaction time, low energy consumption, high purity
and preservation of the nanostructures [4,5,7].
To produce nSiC with magnesiothermic reduction, nanostructured
silica (nSiO2) is required as a precursor. Commercial nSiO2 powders
have been used but they are relatively expensive [4]. Currently, the
emphasis is placed on the production of nanomaterials from renewable
and sustainable sources supporting circular economy approaches
because these sources are environmentally friendly and affordable [2].
For example, there are affordable but less known sources of nSiO2 –
phytoliths which are naturally cumulated in some plants including rice
[8,9], oat [10], bamboo [11] and barley [12]. The other main advan­
tages of the plant-based nSiO2 are relatively high abundance of nSiO2,
good availability and large surface area [10,13,14].
In our previous studies, we demonstrated the synthesis of nSiC from
nSiO2 extracted from barley husks and bamboo through SHS for the first
time [11,12]. Barley husks are a promising source of phytoliths because
approximately 4 % of their dry mass consists of nSiO2 and they are
produced as agricultural residue in large quantities. Currently, barley
husks are often used in fodder or burnt to produce energy. After burning,
nSiO2 is concentrated in the ash. However, potentially more value can be
received from barley husks using pyrolysis to obtain valuable bio-based
organic molecules [15]. Therefore, first part our study is focused on the
comparison of nSiO2 obtained from barley husks after industrial
burning, pyrolysis or laboratory scale leaching and calcination. Pro­
duced nSiO2 batches were converted to high-surface-area nSiC with high
purity to assess the effect of the nSiO2 source on the nSiC material
characteristics.
High surface area of nSiC combined with good chemical stability [2]
makes it a potential material for metal adsorption application. Chemical
stability is important because high surface area makes materials inher­

ently less stable and many nanostructured adsorbents suffer from poor
stability. For example, silica-based mesoporous adsorbents, such as
functionalized SBA-15, are unstable at high pH because of decomposi­
tion of the silicon-oxygen bond by hydroxide ions [16].
The surface of nSiC can be functionalized with organic molecules
such as bisphosphonates (BPs) which act as active adsorption sites [11].
For example, BPs were grafted on thermally carbonized porous silicon
(pSi), and the material demonstrated relatively high chemical stability
especially in acidic and neutral solutions [17]. However, in basic con­
ditions the silicon framework was observed to dissolve.
The BPs are acidic functional groups demonstrating ion-exchange
properties with positively charged ions [18,19]. Industrial waste wa­
ters often contain toxic heavy metals such as manganese. According to
WHO, it can cause irreversible damage to the nervous system [20,21] at
concentrations higher than 100 ppb [22]. Manganese is currently
removed by precipitation, ion-exchange, reverse osmosis, solvent
extraction, flocculation, membrane separation or adsorption [21].
The final part of our study is focused on the testing of
bisphosphonate-functionalized nSiC (BP-nSiC) as a low-cost adsorbent
for extraction of Mnn+ at pH 3 and 8. The high pH during the adsorption
tests was chosen because the landfill water samples may be basic and to
demonstrate the stability of the nSiC material in the basic conditions
that are problematic for typical mesoporous adsorbents such as meso­
porous silica. The functionalized nSiC is an excellent candidate as an
adsorbent at a high pH because of its superior stability compared to
many other adsorbents.

procedure reported earlier [15]. The utilized chemicals were: D(+)-Su­
crose (AnalaR NORMAPUR, VWR Chemical), Mg powder (< 0.1 mm
particles size, purity ≥ 97.0%, Merck), NaOH pellets (purity 99.5%,

Fisher Scientific), 65 wt % HNO3 (Merck), 28 wt % NH4OH (AnalaR
NORMAPUR, VWR Chemical), 37 wt % HCl (Merck) for purification of
biomass and nanostructured silicon carbide, 30 wt % HCl (suprapure,
Merck) for making Mn solutions, 95–97 wt % H2SO4 (J.T. Baker),
mesitylene (99% extra pure, ACROS Organics), Mn standard solution
(1000 mg/L MnCl in HO, Titrisolđ, Merck) and the landfill water
ătekukko Oy containing Mnn+. Bisphosphonates (tetrakis
sample from Ja
(trimethylsilyl) 1-(trimethylsilyloxy) undec-10-ene-1,1-diyl bisphosph­
onate) were synthesized as reported previously [17].
2.2. Extraction of nanostructured silica
Nanostructured SiO2 was extracted from A, P and H (Fig. 1) in the
following steps. i) Leaching 128 g of the biomass in 1500 ml of 10 wt %
HCl at 100 ◦ C for 2 h. ii) Washing the leached biomass with Milli-Q
water on the filter paper to filter out Cl− and to neutralize pH. The
washing was carried out until no Cl− was detected in the filtrate. Pre­
cipitation of AgCl with AgNO3 was used to test the presence of Cl− in the
filtrate. iii) Drying the leached biomass at 100 ◦ C for 2 h. iv) Calcination
of the dried biomass under air at 550 ◦ C.
2.3. Synthesis of nanostructured silicon carbide
Nanostructured SiC was synthesized from the purified nSiO2 pow­
ders (Fig. 1). Sucrose solution was utilized as a source of carbon with
5.72 g of sucrose was mixed with 4.04 ml of 2.7 vol % H2SO4. Then, 4 ml
of sucrose solution was mixed with 5 g of the nSiO2 powder. Carbon­
ization of the material was performed in two steps. i) Carbonization at
160 ◦ C for 5 h under air. ii) Carbonization at 700 ◦ C for 2 h under N2 to
finalize the carbonization and to remove H2SO4. Magnesium was mixed
with the carbonized nSiO2/C composite in a planetary ball mill (Pul­
verisette 7, Fritsch) at 500 rpm for 5 min using 0.5 mass ratio of nSiO2/
Mg. The magnesiothermic reduction was performed in 15.6 g batches of

the precursor (5 g - nSiO2, 0.6 g - C and 10 g - Mg) in a custom-made steel
reactor at 100 ◦ C under N2. The reduction was initiated with a tungsten
wire heated resistively with a 5 A current. The synthesized nSiC was
washed in two steps. i) With 37 wt % HCl at 70 ◦ C for 1 h. ii) With 1 M
NaOH at room temperature (RT) for 16 h to remove by-products and
produce nSiC with possibly some free/unreacted carbon. After each
washing step, nSiC was washed on the filter paper with Milli-Q water to
remove Cl− and OH− and dried at 65 ◦ C for 2 h [12].
2.4. Bisphosphonates conjugation
The synthesized nSiC powders were functionalized with BPs (Fig. 1)
using 0.5 mass ratio BPs/nSiC. First, 10 ml mesitylene was bubbled for
30 min under N2 to remove dissolved O2. Then, BPs were mixed with
mesitylene and again bubbled for 30 min under N2. Meanwhile, nSiC
was placed into a quartz tube and flushed at RT for 15 min under N2 and
further placed into the pre-heated tube oven at 150 ◦ C for 30 min. Then,
the quartz tube was left to cool down to RT under N2. Finally, BPs were
mixed with nSiC under N2 and placed into an oven to incubate at 120 ◦ C
for 19 h. The BP-nSiC powders were washed with 150 ml methanol on a
filter to remove mesitylene and unreacted BPs and then dried at 65 ◦ C for
2 h. Finally, the BP-nSiC powders were sieved through a 120 μm mesh. A
reference nSiC powder was produced identically except no BPs were
added into mesitylene and used to accurately determine the content of
BPs in BP-nSiC.

2. Material and methods
2.1. Materials

2.5. Material characterization

Barley husks (H) and barley husk fly ash (A) were provided by Altia

Oyj as residues of their processes. The A was obtained after burning the
husks in an industrial scale furnace at approximately 650–700 ◦ C. Py­
rolyzed barley husks (P) were obtained after slow pyrolysis with the

Thermogravimetric analysis of nSiO2, nSiC and BP-nSiC powders was
conducted with TA instruments Q50 using an open platinum pan. First,
2


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

Fig. 1. Graphical scheme of the extraction of nanostructured silica (nSiO2) from barley husk fly ash (A), pyrolyzed barley husks (P) and pristine barley husks (H),
synthesis of nanostructure silicon carbide (nSiC) and surface modification of nSiC with bisphosphonates (BPs). (For interpretation of the references to colour in this
figure legend, the reader is referred to the Web version of this article.)

samples were heated isothermally at 80 ◦ C for 30 min to remove
adsorbed water and then heated up with a ramp rate of 20 ◦ C/min up to
700 ◦ C or 900 ◦ C. The measurements to determine carbon and BPs
contents were performed under synthetic air up to 900 ◦ C and under N2
up to 700 ◦ C, respectively. The content of BPs after adsorption/
desorption cycles was measured with Netzsch TG 209 F1 Libra using and
open alumina crucible. The method used to determine BPs was identical
to the one described above.
N2 sorption measurements were done with Micromeritics Tristar II
3020 at 77 K. Before the measurements, nSiO2 and nSiC powders were
dried at 120 ◦ C and BP-nSiC powders at 65 ◦ C under vacuum for 1 and 2
h, respectively. The specific surface area of powders was calculated
according to Brunauer–Emmett–Teller (BET) theory in the relative

pressure range from 0.05 to 0.3.
X-ray powder diffraction experiments were carried out with BraggBrentano geometry utilizing Bruker D8 Discover diffractometer equip­
ped with a Cu-tube, λ = 1.54 Å. The generator was set to 40 kV and 40
mA and the Kβ-radiation was removed with a 0.02 mm Ni-filter. The
measurements were performed in a 2θ range from 20◦ to 130◦ with a
step size 0.038◦ and step time 1.2 s. The crystalline phases of nSiO2 and
nSiC powders were determined with PDF 2–2015 database. The crys­
tallite size of nSiC powders was determined with TOPAS software using
the integrated breadth method applying whole profile fitting.
The particle size distribution was measured with laser diffraction
using Malvern instruments Mastersizer 2000 with Hydro 2000S (A)
sample dispersion unit. Ethanol was utilized as a dispersant and the
nSiO2, nSiC and BP-nSiC powders were first mixed at 2975 rpm and
sonicated with 50% ultrasound power for 1 min and then the suspension
was mixed at 2975 rpm for 1 min before the measurement.
Zeta potential of H-BP-nSiC particles was measured with Malvern
instruments Zetasizer Nano ZS. 5 mg of particles were mixed with 5 ml
of Milli-Q water. The pH was adjusted between 2 and 9 with HCl and
NH4OH.
Morphology and the elemental composition of the powders were
analysed with scanning electron microscope (SEM), Sigma HD|VP with
energy-dispersive X-ray spectrometer (EDS), Thermo Scientific Noran
System 7. The imaging was conducted at 4 keV accelerating voltage and
recorded with SE2 detector. The elemental analysis was done at 6 keV

for nSiO2 powders and 4 and 15 keV for nSiC/BP-nSiC powders with
Thermo Pathfinder software. Nanostructured SiO2, nSiC and BP-nSiC
powders were placed on a standard aluminium stub without adhesives
to ensure a more reliable quantification of Si and C. The depth of elec­
tron beam penetration at 4, 6 and 15 keV was estimated based on Potts’

equation [23].
Morphology of nSiC powders was also characterized with a trans­
mission electron microscope JEOL, JEM-2100F. Particles were sus­
pended in ethanol and deposited on a copper grid coated with a carbon
film and dried under air before the analysis. The imaging was conducted
at an accelerating voltage of 200 keV.
Characterization of chemical composition of nSiO2, nSiC and BPnSiC powders was also performed with Fourier-transform infrared
spectrometer (FT-IR), Thermo Nicolet iS50. Spectra were measured in
reflex mode with an ATR module. The spectral range was 400–4000
cm− 1 and resolution 4 cm− 1.
The concentration of metals in waters before and after the adsorption
experiments was measured with an inductively coupled plasma mass
spectrometer (ICP-MS), PerkinElmer NexION 350D. Thorium and
yttrium internal standards were used during the measurement. The in­
strument settings are mentioned in Supplementary data Table S1.
All measurements were performed three times (n = 3). The error
limits are presented as standard deviations. The statistical evaluation of
data was done with the Student’s t-test.
2.6. Adsorption studies
2.6.1. Adsorption in batch setup
Adsorption capacity for Mnn+ on BP-nSiC particles at various pH,
adsorption kinetics on BP-nSiC particles and adsorption isotherms on
nSiC, BP-nSiC and nSiO2 particles were studied in batch type setup.
Before all adsorption experiments, the particles were primed by
immersing them in 10 ml of 5 M HCl for 1 h and then washed three times
with 10 ml of Milli-Q water to remove HCl and to neutralize pH. For
adsorption tests of Mnn+ between pH 2 to 8, the concentration 10 mg/L
of Mn solution was used. For adsorption kinetic experiments Mn solu­
tions 1.5 and 6 mg/L at pH 3 and 8, respectively, were used. The
adsorption isotherms were performed in the range of Mn concentration

3


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

of solutions 0.1–10 mg/L at pH 3 and 1–20 mg/L at pH 8. The pH of
utilized Mn solutions was adjusted with HCl and NH4OH. The batch type
adsorption experiments were conducted by mixing 10 mg of particles
with 10 ml of Mn solution in 15 ml centrifuge tubes. The prepared
suspensions for pH test and adsorption isotherms were mixed in an
orbital shaker at RT at 80 rpm for 24 h and then suspensions were
centrifuged at 5000 rpm for 3 min to separate the particles and the su­
pernatant. In the kinetic experiments the contact time between particles
and solution varied from 2 min to 24 h and at pre-determined time 100
μL of supernatant was taken out. During the experiments the suspensions
were centrifuged at 2000 rpm for 20 s. Furthermore, the supernatant
aliquot was measured with ICP-MS and the adsorbed amount Qe (mg/g)
of Mnn+ was calculated according to the equation
Qe =

(C0 − Ce )⋅V
m

3. Results and discussion
3.1. Extraction of nSiO2
Nanostructured SiO2 was extracted from A, P and H using acid
leaching and calcination. The extracted nSiO2 powders from A and P had
a grey colour and H extracted by calcination was white (Supplementary

data Fig. S1). Colour has been established as good measure of the purity
of nSiO2 [24]. Based on the colour of the extracted nSiO2 powders, the
purity increased in the order A-nSiO2 < P-nSiO2 < H-nSiO2.
The particle morphology and size of nSiO2 powders were studied
with SEM and laser diffraction, respectively. Well-preserved phytolith
structures were observed in all nSiO2 powders (Supplementary data
Fig. S2) and A-nSiO2 and P-nSiO2 also contained more irregular SiO2
structures. The mean particle size of nSiO2 powders was 22.8–42.0 μm
(Table 1, Supplementary data Fig. S3). The particle size increased in the
order H-nSiO2 < P-nSiO2 < A-nSiO2.
The N2 sorption analysis of nSiO2 powders demonstrated II type
isotherm with the H3 hysteresis loop (Supplementary data Fig. S4A).
The highest surface area of 187 m2/g was observed for the H-nSiO2
powder whereas the surface area of the A-nSiO2 powder was relatively
low, only 17 m2/g (Table 1). Obviously, the thermal history of the
phytoliths is critical for preservation of their morphology and porous
structure.
The extracted P-nSiO2 and H-nSiO2 powders were amorphous
showing no diffraction peaks in the XRPD diffractograms (Supplemen­
tary data Fig. S5). Low intensity diffraction peaks associated with the
crystalline phase of SiO2 (cristobalite) were observed with A-nSiO2
powder. Most likely, partial crystallization can be caused by the pres­
ence of metallic impurities during the high temperature process causing
a decrease in melting point of nSiO2 and amorphous-crystallization
transition. Similar effect of the temperature and metallic impurities on
nSiO2 extracted from rice husks was shown by Umeda et al. [25].
The elemental composition of nSiO2 powders were studied with EDS.
The composition of nSiO2 powders showed Si and O as the main ele­
ments with C, Mg, Al, P, K, Ca and S as the main impurities (Table 2,
Supplementary data Fig. S6). The estimated atomic ratio Si/O was 0.81,

0.80 and 0.84 for A-nSiO2, P-nSiO2 and H-nSiO2, respectively. Based on
the results, Si was slightly in excess indicating Si-rich impurities.
Possible compounds causing the skewed Si/O ratio are SiC and SiOC.
The excess Si might also be caused by slight carbothermal reduction
facilitated by impurities. However, the content of impurities detected by
EDS was 4.8, 3.6 and 0.6 wt % for A-nSiO2, P-nSiO2 and H-nSiO2,
respectively.
The mass loss during the TG measurement for nSiO2 powders from
100 to 900 ◦ C was up to 1.4 wt % (Table 1, Supplementary data Fig. S7).
The mass loss was most likely associated with decomposition of carbonbased residues [26], even though 550 ◦ C calcination temperature was
utilized. The A-nSiO2 powder demonstrated the lowest mass loss even
though it contains the highest amount of carbon impurities. It seems that
carbon in this material is contained in thermally stable compounds such
as SiC and SOC.
The chemical composition of nSiO2 powders was also studied with
FT-IR (Supplementary data Fig. S8). The absorption bands associated
with Si–O–Si asymmetric and symmetric vibration modes were observed
around 1080–1040 cm− 1 and 800–790 cm− 1, respectively [25,27]. No
significant absorption peaks related to Si–OH vibration modes were
observed for the nSiO2 powders because of the high calcination
temperature.
The processing routes of H had a significant effect on the physico­
chemical properties of the extracted nSiO2 particles. The purity of the
particles increased in the order of A-nSiO2 < P-nSiO2 < H-nSiO2 based
on the colour and EDS analysis. It seems that removing metallic impu­
rities by acid washing before heating the biomass at high temperatures
increases the purity. This observation is in agreement with previous
studies by Chen et al. and Liou et al. [8,9]. As shown earlier by Umeda

(1)

where, C0, Ce are Mn concentrations before and after the adsorption
experiment, respectively, V is the volume of Mn solution used in the
adsorption experiment, m is the mass of the particles in the suspension.
2.6.2. Adsorption from landfill water
The adsorption efficiency of BP-nSiC particles was examined with a
landfill water sample in the batch setup using the procedure for pH test
and adsorption isotherms mentioned in section 2.6.1. The pH of the
water sample was adjusted from 6.79 to pH 8 before the adsorption
experiment and the water was used immediately. The separation factor
(SF) for chosen metals in the landfill water sample was calculated ac­
cording to the equation
∑Cne,1
SF =

i=2

Ce,i

∑Cn0,1

(2)

C
i=2 0,i

where, C0,i, Ce,i are concentrations before and after the adsorption
experiment of metal i, respectively. The metal of the interest for which
the SF is calculated is marked as i = 1.
2.6.3. Adsorption in a flow-through setup

Flow-through setup was used to study the reusability and stability of
BP-nSiC during 5 adsorption/desorption cycles of Mnn+ at pH 8. The
flow-through setup consisted of a syringe pump (AL-1600, New Era
Pump Systems Inc.), 10 ml syringes and filter holders (13 mm Swinnex
filter holder) with O-rings (Silicone Gaskets). First, high-density poly­
ethylene (HDPE) filters (1 μm pores) were cut (d = 13 mm) and placed
inside column and subsequently 10 mg of BP-nSiC particles were placed
on the filter. To prime the BP-nSiC particles, 5 ml of 5 M HCl was filtered
through the particles with the flow rate of 0.5 ml/min. Then, BP-nSiC
particles were washed with 10 ml of Milli-Q water with the same flow
rate to neutralize the pH around the particles. Adsorption and desorp­
tion experiments were performed by filtering 10 ml of 15 mg/L Mn
solution and 10 ml of 1 M HNO3, respectively, with flow rate of 0.5 ml/
min. After each adsorption/desorption cycle and between the adsorption
and the desorption steps, BP-nSiC particles were washed with 10 ml of
Milli-Q water. The metal concentrations in the filtrate aliquot were
measured with ICP-MS and the adsorbed amount, Qe, of Mnn+ was
calculated according to Eq. (1) and the desorbed amount Qdes (mg/g) of
Mnn+ was calculated according to the equation
Qdes =

Cdes ⋅V
,
m

(3)

where, Cdes is the Mn concentration in the 1 M HNO3 filtrate after the
desorption experiment.

4


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

Table 1
Physical and structural properties of nanostructured silica (nSiO2), nanostructured silicon carbide (nSiC) and bisphosphonate-modified nSiC (BP-nSiC) extracted from
barley husk fly ash (A), pyrolyzed barley husks (P) and barley husks (H).
Sample

Yielda (%)

D(0.5)b
(μm)

SBETc(m2/g)

Phase amountd (%)

Crystallites
3C–SiCe (nm)

Crystallites
2H–SiCf (nm)

Mass lossg (wt %)

A-nSiO2

P-nSiO2
H-nSiO2
A-nSiC

–
–
–
54

42 ± 2
27.8 ± 0.5
22.8 ± 0.4
5.0 ± 0.4

16.6 ± 0.6
105 ± 2
187 ± 9
64.8 ± 0.2

–
–
–
14.2 ± 0.1

–
–
–
8.3 ± 0.1

0.68 ± 0.03

1.08 ± 0.06
1.4 ± 0.1
13.46 ± 0.06

P-nSiC

61

5.0 ± 0.8

181.3 ± 0.2

5.43 ± 0.02

13.24 ± 0.02

0.71 ± 0.04

H-nSiC

70

5.11 ± 0.05

196 ± 1

5.32 ± 0.02

14.3 ± 0.1

1.24 ± 0.04

A-BP-nSiC
P-BP-nSiC
H-BP-nSiC

–
–
–

5.2 ± 0.2
5.07 ± 0.02
5.3 ± 0.3

47.5 ± 0.4
145.2 ± 0.8
141 ± 5

–
–
–
53.2 ± 0.5e
46.8 ± 0.5f
71.47 ± 0.07e
28.53 ± 0.07f
65.67 ±0.03e
34.33 ± 0.03f
–
–
–

–
–
–

–
–
–

0.92 ± 0.05
2.42 ± 0.08
2.6 ± 0.1

a
b
c
d
e
f
g

Calculated yield of nSiC as a mass of nSiC without free carbon divided by the theoretical maximum mass of synthetic nSiC (based on the mass of the utilized nSiO2).
Mean value D(0.5) of particles measured with laser diffraction.
Surface area of the powders calculated based on BET theory.
Fraction of 3C–SiC and 2H–SiC calculated based on the XRPD diffractograms.
Calculated for cubic phase 3C–SiC from the XRPD diffractograms.
Calculated for hexagonal phase 2H–SiC from the XRPD diffractograms.
Mass loss determined with TG.

Table 2

Elemental composition (wt %) of nanostructured silica (nSiO2), nanostructured silicon carbide (nSiC) and bisphosphonate-modified nSiC (BP-nSiC) extracted from
barley husk fly ash (A), pyrolyzed barley husks (P) and barley husks (H). Limit detection was 0.1 wt %.
Sample Elements

A-nSiO2

P-nSiO2

H-nSiO2

A-nSiC

P-nSiC

H-nSiC

A-BP-nSiC

P-BP-nSiC

H-BP-nSiC

Si
C
P
O
Mg
Al
Cl
Ca

K
S
Free Ca
BPsb

55.9 ± 0.4
<0.1
0.40 ± 0.08
39.3 ± 0.1
0.40 ± 0.05
0.10 ± 0.06
<0.1
<0.1
3.9 ± 0.2
–
–
–

56 ± 1
<0.1
0.47 ± 0.06
40.0 ± 0.6
0.9 ± 0.1
0.2 ± 0.1
–
<0.1
1.8 ± 0.2
0.2 ± 0.3
–
–

60 ± 2
<0.1
0.27 ± 0.03
40 ± 2
–
<0.1
–
–
–
–
–
–

47 ± 3
48 ± 2
–
3.0 ± 0.6
1.1 ± 0.4
0.2 ± 0.1
0.2 ± 0.1
0.2 ± 0.2
<0.1
<0.1
28 ± 4
–

55.0 ± 0.7
40.7 ± 0.6
–

3±1
0.79 ± 0.02
0.125 ± 0.004
0.12 ± 0.02
–
–
<0.1
17.6 ± 2
–

54 ± 3
42 ± 3
–
3.1 ± 0.4
0.76 ± 0.06
0.20 ± 0.09
<0.1
–
–
–
20 ± 5
–

45.5 ± 0.6
47.5 ± 0.4
0.26 ± 0.07
4.7 ± 0.7
1.14 ± 0.02
0.1 ± 0.04
0.31 ± 0.02

0.21 ± 0.03
0.11 ± 0.02
<0.1
–
1.4 ± 0.4

53 ± 1
41 ± 2
0.60 ± 0.03
4.5 ± 0.5
0.81 ± 0.05
0.12 ± 0.02
0.12 ± 0.01
<0.1
–
0.123 ± 0.009
–
3.2 ± 0.2

54.5 ± 0.9
40.3 ± 0.4
0.5 ± 0.1
3.7 ± 0.5
0.71 ± 0.02
0.19 ± 0.06
<0.1
–
–
–
–

2.6 ± 0.5

a
b

Amount of free carbon calculated based on EDS results at accelerating voltage 15 keV.
Amount of bisphosphonates calculated based on EDS results of phosphorus at accelerating voltage 15 keV.

et al., metallic impurities, mainly Na+ and K+, form ternary oxides with
SiO2, such as Na6Si8O19 and Na2Si2O5, decrease the melting point of SiO2
from 1713 ◦ C to 789 ◦ C [25]. Subsequently, partial melting or enhanced
diffusion of the Si and O atoms induced by the ternary oxides may cause
carbon to become enclosed inside the SiO2 particles, further decreasing
the purity of nSiO2. This is seen as reduced purity in A-nSiO2 as well as
P-nSiO2 which were heated during burning or pyrolysis before removing
the inorganic impurities by acid washing. A-nSiO2 seemed to have the
lowest purity, probably because of the higher temperature experienced
during the processing.
The heat treatments of husks also clearly affected the morphology,
porous structure, and crystallinity of the extracted nSiO2. A-SiO2 that
had experienced the highest temperature before the acid wash had the
lowest surface area, while H–SiO2 without heat treatment before the
acid wash had the largest surface area. The decrease in surface area is
likely caused by metallic impurities, which caused lowering of the
melting temperature. Because of their thermal instability, the small
nanostructures were liable to degradation during the heat treatments
prior to the acid wash. The structural changes caused by high temper­
ature processing were further evidenced by the higher content of crys­
talline cristobalite in A-SiO2 compared with the other two samples.

3.2. Synthesis of nSiC
Nanostructured SiC powders were produced from nSiO2 powders
when the reaction was locally initiated with the tungsten wire at an
ambient temperature 100 ◦ C.
3.2.1. Microstructure of nSiC
The particle size and morphology of nSiC powders were character­
ized with SEM, TEM and laser diffraction. Based on SEM, nSiC powders
were composed of large secondary polycrystalline particles of approx.
5–10 μm in diameter as well as of finer particles of approx. 20–50 nm
(Fig. 2A and B). Individual primary particles were not easily seen even at
high magnification most likely because of the small particle size and
tight packing of the particles. There was no difference in the morphology
of particles between nSiC powders. The grain size of approx. 20–100 nm
for all nSiC powders was also confirmed with TEM (Fig. 2C). The mean
secondary particle size of nSiC powders measured with laser diffraction
was around 5 μm (Table 1, Supplementary data Fig. S9). There was also
a peak indicating weak larger agglomerates around 1000 μm in size in AnSiC and P-nSiC. The weak agglomerates were formed in the suspension
and constantly decreasing during the measurement as the secondary
particles broke to smaller ones.
The XRPD studies revealed that the synthesized A-nSiC, P-nSiC and
H-nSiC materials were composed of two SiC polytypes (Fig. 2D), i.e.
5


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

Fig. 2. A, B) Scanning electron microscopy images of nanostructured silicon carbide H-nSiC, C) the transmission electron microscopy image of H-nSiC and D) X-ray
diffractogram of nanostructured silicon carbide A-nSiC, P-nSiC and H-nSiC. Nanostructured SiC powders were composed of two crystalline SiC polytypes marked as

3C–SiC (*) and 2H–SiC (#). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

cubic phase (Moissanite-3C) and hexagonal phase (Moissanite-2H). The
average crystallite size was 5.3–14.2 nm for 3C–SiC and 8.3–14.3 nm for
2H–SiC (Table 1). The crystallite size of nSiC powders decreased with
decreasing size of nSiO2 precursor particles. The same trend was found
by Yermekova et al. who showed that smaller SiO2 particles utilized in
magnesiothermic reduction lead to smaller SiC crystallites [4]. The
relative amount of SiC–3C phase was 53–71 % (Table 1). As shown by
Jepps the crystalline phase 2H–SiC should be formed in the temperature
range 1300–1600 ◦ C as a polytypic transformation 3C → 2H [28]. Su
et al. and Zhao et al. produced only 3C–SiC when magnesiothermic
reduction was performed at 600 ◦ C [2,3]. Most likely, the temperature
inside the powder during the SHS synthesis used in our study reaches
high values and is not homogenous throughout the whole powder bed
unlike in the more conventional slowly progressing synthesis methods.
Also, trace metal impurities in the biomass could facilitate the formation
of 2H polytype [28].
A diffraction peak of low intensity around 26.5◦ (2θ) was also
observed in diffractograms of P-nSiC and H-nSiC (Fig. 2D). This 2θ value
would correspond to diffraction peak with the highest theoretical in­
tensity of crystalline phase of carbon (Graphite-2H). However, no
diffraction peak for carbon was observed for A-nSiC. Instead, a more
intense diffraction halo possibly associated with amorphous carbon was
seen. We can assume that any free carbon in the nSiC powders is mainly
amorphous because of the low intensity of the Grapihe-2H peak and
because no other significant diffraction peaks were observed.
The N2 sorption analysis of nSiC powders demonstrated type II
isotherm with the H3 hysteresis loop (Supplementary data Fig. S4B)
indicating mesoporous character as the original nSiO2 powders. The

measured surface area was 64.8 m2/g, 181.3 m2/g and 196 m2/g for AnSiC, P-nSiC and H-nSiC, respectively (Table 1). The lower the purity of
the utilized nSiO2 powder the lower surface area of synthesized nSiC
powder was achieved.
The nanostructure of nSiC powders was influenced by the nano­
structure and purity of the precursor nSiO2. No notable differences

between nSiC powders in the surface morphology or particle size were
observed. However, the crystallite size of the nSiC powders decreased
with the increasing purity of nSiO2 precursor powders. The highest
surface area of 196 m2/g was measured for the H-nSiC prepared with the
highest purity nSiO2 precursor with the highest surface area, 187 m2/g.
The surface area of P-nSiC, 181 m2/g, was also high despite the lower
surface area of its nSiO2 precursor, 105 m2/g. Even the A-nSiC had a
relatively high surface area of 65 m2/g, even though the surface area of
A-nSiO2 was only 17 m2/g. Therefore, to get the highest surface area
nSiC it is necessary to leach the husks before calcination/pyrolysis in
order to maintain high surface area of the nSiO2 precursor.
3.2.2. Chemical properties of nSiC
The TG analysis was performed in synthetic air for the nSiC powders
(Fig. 3A) to determine the amount of free carbon. Amorphous carbon is
oxidized into CO2 and seen as mass loss between 400 and 650 ◦ C [29].
Amount of free carbon was 13.5 wt % in A-nSiC, 0.7 wt % in P-nSiC and
1.2 wt % in H-nSiC. The mass increase in P-nSiC between 300 and 520 ◦ C
and H-nSiC between 300 and 540 ◦ C might be caused by creation of
metastable COx compounds on the surface or oxidation of SiC surface
which is partially superimposed with oxidation of free carbon. Oxidation
of nSiC powders was observed as the mass increased above 650 ◦ C.
The elemental composition of nSiC powders was measured with EDS
at accelerating voltage of 4 keV (Supplementary data Table S2, Fig. S10)
corresponding approx. 0.2 μm depth of electron beam penetration. It

showed two main elements C (28.5–36 wt %) and Si (60–69 wt %), with
trace contamination of O, Mg and Al. The atomic ratio C/Si was 1.4, 1.2
and 1.0 for A-nSiC, P-nSiC and H-nSiC, respectively. The increased
amount of C in A-nSiC and P-nSiC was mainly caused by unreacted
carbon from the carbonized sucrose. Assuming that all the measured
oxygen was present as SiO2, the synthesized A-nSiC, P-nSiC and H-nSiC
would contain approx. 4.5 wt %, 4.0 wt % and 3.3 wt % of SiO2,
respectively. However, most likely O was also present in the form of
MgO and SiOC. Therefore, the above values for SiO2 can be regarded as
6


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

takes place only close to the SiO2–C interface. In the SiO2/C precursor, a
large fraction of carbon is presumably in the pores of the nSiO2. How­
ever, if larger carbon particles exist in the precursor, the conversion to
SiC occurs only at the external surface of these particles and the inside
remains as carbon. This conclusion is also supported by clearly higher
amount of free carbon calculated based on EDS results (Table 2)
compared to those obtained with TG assuming that the SiC shell pre­
vents the oxidation of the carbon core (Fig. 3A). The low porosity of
A-nSiO2 and P-nSiO2 also decreases the reaction interface of SiO2–C and
promote formation of individual carbon particles in the precursor. This
conclusion is supported by increasing yield of nSiC powders up to 70 %
(Table 1), with increasing surface area and high free carbon content
determined by TG in the A-nSiC powder as discussed above.
The chemical composition of nSiC powders was also studied with FTIR (Supplementary data Fig. S12). One intensive absorption peak was

observed around 790 cm− 1 with a small shoulder around 910 cm− 1. The
signals were associated with transverse and longitudinal phonon modes
of Si–C, respectively [4,30]. No significant absorption peaks related to
Si–O–Si vibration modes were observed for the nSiC powders.
3.3. Synthesis of BP-nSiC
BP-nSiC powders were prepared from all nSiC powders and labelled
as A-BP-nSiC, P-BP-nSiC and H-BP-nSiC. The functionalization of nSiC
powders did not change the morphology or the microstructure of the
particles based on SEM and laser diffraction analysis (Table 1). The
mean particle size was around 5 μm for all three BP-nSiC samples.
The N2 sorption analysis of BP-nSiC powders demonstrated similar
hysteresis loop as before the conjugation of BPs (Supplementary data
Fig. S4B). The surface area was 47.5 m2/g, 145 m2/g and 141 m2/g for
A-BP-nSiC, P-BP-nSiC and H-BP-nSiC, respectively. The surface areas
were modestly reduced by the BP grafting suggesting that the grafted
BPs may have blocked part of the accessible surface area from N2
adsorption.
The elemental analysis of BP-nSiC powders conducted at 4 keV
(Supplementary data Table S2, Figs. S13) and 15 keV (Table 2, Sup­
plementary data Fig. S14) showed different content of BPs on particles.
The content of BPs calculated based on phosphorus content measured at
4 keV for A-BP-nSiC, P-BP-nSiC and H-BP-nSiC was 4.8, 6.9, and 8 wt %,
respectively. The BP content calculated based on phosphorus measure­
ment at 15 keV (Table 2) was 1.4, 3.2, and 2.6 wt % in A-BP-nSiC, P-BPnSiC and H-BP-nSiC, respectively. Based on these differences and the
knowledge of penetration depths of the electron beams at 4 and 15 keV
we can assume that BPs were preferentially grafted closer to the external
surfaces of the particles rather than deeper in the structure. This
observation differs from electrochemically etched pSi, which has a
narrow pore size distribution and pores go through pSi as shown by
Thapa et al. [18]. There are two possibilities that could explain the re­

sults. Either the surface is preferentially functionalized compared to the
inner porous structure, or the surface of the particles is more porous and
contains more surface area than the inside of the particles. The latter
seems to be the case at least for the particles having the carbon rich core
surrounded by SiC structure as discussed earlier.
The TG curves of BP-nSiC powders (Fig. 3B) measured under N2
consisted of two parts. i) Thermal decomposition of BPs from 100 to
650 ◦ C. ii) Thermal oxidation of the surfaces of nSiC above 650 ◦ C
probably caused by residual O2 in the measurement system. The amount
of BPs was 0.92 wt % in A-BP-nSiC, 2.42 wt % in P-BP-nSiC and 2.6 wt %
in H-BP-nSiC. The content of BPs measured with TG was comparable to
EDS results at 15 keV. The coverage of the surface with BPs was calcu­
lated from the content of BPs (TG) and nSiC surface area. The coverage
was similar between the samples as the A-BP-nSiC, P-BP-nSiC and H-BPnSiC contained 0.26, 0.24 and 0.25 molecules per nm2, respectively.
This observation supports the explanation of the higher BP content close
to the external surfaces being caused by the BPs being evenly grafted on
the surfaces but the material being more porous close to the external

Fig. 3. TG curves of A) thermal oxidation of nanostructured silicon carbide AnSiC, P-nSiC and H-nSiC in synthetic air (20 vol % O2) and B) thermal
decomposition of bisphosphonates (BPs) in N2 of A-BP-nSiC, P-BP-nSiC and HBP-nSiC. Thermal oxidation of P-nSiC and H-nSiC was divided into three
dominant steps. I) Formation of meta-stable COx compound or oxidation of nSiC
surface partially superimposed with decomposition of free carbon as CO2. II)
Decomposition of unreacted/free carbon as CO2. III) Oxidation of nSiC surface
above 650 ◦ C. Thermal oxidation of A-nSiC was composed of step II and III, the
step I was not observed. Thermal decomposition of BPs took place in the range
100–650 ◦ C. Slight oxidation of BP-nSiC powders with oxygen impurities was
observed above 650 ◦ C. (For interpretation of the references to colour in this
figure legend, the reader is referred to the Web version of this article.)

the upmost limits. The amount of free carbon was also calculated based

on EDS data (Supplementary data Table S2) and compared to TG anal­
ysis. The results were comparable, except for the P-nSiC powder. The
origin of the discrepancy for P-nSiC is unknown.
The elemental composition of nSiC powders were also examined at
accelerating voltage 15 keV (Table 2, Supplementary data Fig. S11)
which gives information from approx. 2 μm deep in SiC. The main
detected contaminants were O, Mg and Al, which were observed with 4
keV and in addition also K, Ca, S and Cl were observed. The atomic ratio
C/Si was approx. 2.4, 1.7 and 1.8 for A-nSiC, P-nSiC and H-nSiC,
respectively. Deeper inside the material the carbon content seems to be
higher compared to the surface. As shown earlier by Zhao et al. the
contact area between SiO2–C is an important parameter during the
conversion of SiO2 into SiC [3]. Especially, because the reaction time of
magnesiothermic reduction proceeding via SHS is short, the reduction
7


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

surfaces than deeper inside the particles.
The surface functionalization of nSiC powders was also studied with
FT-IR (Fig. 4). The strong absorption peak at 790 cm− 1 with small
shoulder around 910 cm− 1 was associated with Si–C vibrations. There
were absorption bands at 2920 cm− 1 and 2850 cm− 1 associated with
CHx stretching vibrations, in agreement with work reported by Riikonen
et al. [17]. However, there were no visible CHx bending vibrations in the
range 1500–1300 cm− 1, which can be explained with the relatively low
concentration of BPs. The absorption peak around 1620 cm− 1 was most

– C species, as reported also by Guo
likely related to the vibrations of C–
et al. [31,32]. The low intensity absorption band at 1080 cm− 1 was
probably associated with the P–O vibration [32]. The absorption band at
1250 cm− 1 might be associated with either C–O/C–C–O in
– O [17,33]. The low in­
phosphorus-containing group [31,32], or P–
tensity of absorption bands associated with BPs was affected by rela­
tively low concentration of BPs grafted on the particles.

model (Supplementary data Fig. S17) fitted data very well with the
coefficient of determination R2 = 0.99 at both pH. The data up to 1 h was
used to calculate the rate constant k2, because there was no significant
adsorption after 1 h. We can assume that, at least at the later stages of
adsorption, the adsorption of Mnn+ on H-BP-nSiC at pH 3 and 8 follows
pseudo-second-order kinetic model with the availability of active
adsorption sites as a limiting factor [34]. Accordingly, diffusion inside
the porous structure of the particles does not seem to restrict the
adsorption. It should be noted that, the adsorption of Mnn+ on
H-BP-nSiC was very fast and because of practical limitations we were not
able to determine the kinetic model at the early stages of the adsorption.
The adsorption isotherms for Mnn+ on BP-nSiC samples were
measured in batch setup at pH 3 and 8. The H-nSiC powder was chosen
as a reference. H-nSiC adsorbed negligible amount of Mnn+ at pH 3
because of the lack of specific binding sites (Fig. 5A). The experimental
data of A-BP-nSiC, P-BP-nSiC and H-BP-nSiC powders measured at pH 3
were fitted with Freundlich and Sips models (Fig. 5B–D) and parameters
derived from the fittings are listed in Supplementary data Table S4.
Freundlich model did not fit experimental data well with the coefficient
of determination R2 between 0.59 and 0.91. However, the Sips model

fitted the data better with R2 of 0.86–97. The Sips model is usually used
to predict the heterogeneity of the surface and overcome the limitations
of the Langmuir and Freundlich models [35–37]. The calculated
adsorption capacity Qmax was between 0.3 and 1.4 mg/g for BP-nSiC
powders (Supplementary data Table S4). Based on ns values close to 1,
we can assume that the adsorption on BP-nSiC takes place mainly on
homogenous adsorption sites [35–37]. Therefore, the adsorption was
most likely caused by a specific interaction between Mnn+ and the
phosphonate groups, as proposed by Thapa et al. in the case of
adsorption of Sc [18] and U [19] on BP-functionalized pSi.
The adsorption behaviour for Mnn+ on H-nSiC and BP-nSiC powders
was also examined at pH 8 (Fig. 5). The measured dataset of H-nSiC was
fitted with Freundlich and Sips models and the calculated parameters
are listed in Supplementary data Table S4. Both models gave R2 of 0.86.
Even though, the parameter ns was 1, which would correspond to the
adsorption on homogeneous surface [35–37], the low value of R2
demonstrates that the adsorption did not strictly follow Langmuir or
Freundlich model.
The adsorption isotherms of BP-nSiC powders (Fig. 5B–D) were also
measured at pH 8. Freundlich and Sips models gave again identical R2
between 90 and 96. The simple adsorption models did not fit the data
well and because of the complex shape of the isotherms we might as­
sume the presence of heterogeneous active sites on the surface of BPnSiC powders. Therefore, the exact adsorption mechanism of Mnn+ on
nSiC and BP-nSiC powders remains unclear.
The higher adsorption capacity of BP-nSiC powders at pH 8
compared to pH 3 might be associated with more negative charge on BPnSiC powders as was demonstrated with H-BP-nSiC (Supplementary
data Fig. S18), and therefore higher affinity to Mnn+. A similar trend was
shown earlier by Singh et al. [38] showing an increased adsorption af­
finity to ammonium polycarbonate on unmodified nSiC increased above
isoelectric point at pH 4.9. The adsorption capacity of H-nSiC increased

after grafting with BPs approx. by 32 % based on the experimental data.
The increase of the capacity due to the BP grafting was 1.4 and 1.9 mg/g
at pH 3 and 8, respectively. The results emphasize the significance of the
surface modification with suitable metal chelating functional groups to
enhance metal adsorption properties of the mesoporous materials.
The reusability and stability of BP-nSiC powders were demonstrated
on H-BP-nSiC at pH 8 by performing five consecutive adsorption/
desorption cycles in a flow-through setup (Supplementary data
Fig. S19). The average adsorption of Mnn+ during the cycling experi­
ment was 58 ± 6 % of the adsorption capacity (based on the experi­
mental data) and 2 ± 4 % of Mnn+ remained in the particles after the
desorption step. The differences in adsorbed or desorbed amounts be­
tween the cycles were not statistically significant (p > 0.17 for adsorp­
tion and p > 0.11 for desorption). The stability of BPs on the surface was

3.4. Adsorption studies
The H-BP-nSiC powder was chosen for studies of adsorption capacity
for Mnn+ in various pH because of the highest BP content and the high
surface area. The adsorption of Mnn+ was measured in batch type setup
from pH 2 to 8 (Supplementary data Fig. S15). The adsorbed amount of
Mnn+ increased from 0.6 to 4.9 mg/g between pH 2 and 8. There was a
significant increase in the adsorbed amounts between pH 2 and pH 3
(from 0.6 to 1.5 mg/g) as well as between pH 7 and pH 8 (from 2.2 to 4.9
mg/g). Between pH 4 and 7, there was only modest increase in adsorbed
amount (from 1.8 to 2.2 mg/g).
Adsorption kinetics for Mnn+ on the H-BP-nSiC powder were studied
at pH 3 and 8 (Supplementary data Figs. S16A and B) in batch setup. The
majority of the adsorption took place before the first time point at 2 min
at both pH. The adsorption of Mnn+ proceeded fast up to 15 min. Sub­
sequently, the uptake of Mnn+ slowed down significantly. After 1 h, no

further adsorption took place. Linearization of the experimental data
was done according to pseudo-first-order and pseudo-second-order ki­
netic models. The derived parameters of fitting were listed in Supple­
mentary data Table S3. Pseudo-first-order kinetic model did not fit
experimental data well with the coefficient of determination R2 = 0.80
and 0.30 for pH 3 and 8, respectively. Pseudo-second-order kinetic

Fig. 4. FT-IR spectra of bisphosphonate-modified nanostructured silicon car­
bide A-BP-nSiC, P-BP-nSiC and H-BP-nSiC. (For interpretation of the references
to colour in this figure legend, the reader is referred to the Web version of
this article.)
8


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

Fig. 5. Adsorption isotherms of Mnn+ with
A) nanostructured silica H-nSiO2 and nano­
structured silicon carbide H-nSiC, B)
bisphosphonate-modified silicon carbide ABP-nSiC, C) bisphosphonate-modified silicon
carbide P-BP-nSiC and D) bisphosphonatemodified silicon carbide H-BP-nSiC at pH 3
and 8. Solid/open symbols describe adsorp­
tion isotherms at pH 3 and 8, respectively.
(For interpretation of the references to
colour in this figure legend, the reader is
referred to the Web version of this article.)

investigated with TG analysis before and after adsorption/desorption

cycles. The content of BPs was 2.6 ± 0.1 wt % and 2.4 ± 0.2 wt % before
and after five cycles, respectively, and the difference was not statistically
significant (p > 0.14). The chemical stability of the adsorbent resulted in
the stable performance of the adsorbent in the adsorption/desorption
cycles.
Finally, the adsorption efficiency of H-BP-nSiC particles towards
Mnn+ in a landfill water sample was tested in a batch type setup. The
initial concentrations C0 of metals are shown in Supplementary data
Table S5. The highest initial concentration, 0.6 mg/L, was measured for
Mnn+. After the adsorption test Qe of Mnn+ was 0.046 mg/L, approx. 93
% of Mnn+ was adsorbed. Separation factor was calculated for Al, Cr,

Mn, Fe, Co, Ni, Zn, Pb, (Fig. 6). The highest separation factor 1.14 ± 0.01
was obtained for Mnn+, showing a slight selectivity of the H-BP-nSiC
towards Mnn+ compared to other metals. The lack of significant speci­
ficity towards adsorption of any specific metals was expected as the
majority of the adsorption is expected to be unspecific on the H-BP-nSiC
surface at pH 8.
Mn adsorption isotherms were investigated with nSiO2 powder in
batch setup at pH 3 and 8 to compare the performance with the nSiC
powders. H-nSiO2 powder was chosen because it had the highest purity
and the surface area of all the nSiO2 powders. The adsorption of Mnn+ on
H-nSiO2 powder (Fig. 5A) was negligible at pH 3 similar to H-nSiC
powder. The low capacity was most likely caused by the lack of the
specific binding sites on the nSiO2 surface and low adsorption affinity
between the surface and Mnn+ ions.
Adsorption of Mnn+ on H-nSiO2was observed at pH 8. The experi­
mental data were fitted with Freundlich and Sips adsorption models and
the calculated parameters are listed in Supplementary data Table S4.
Freundlich and Sips models fitted data with the coefficient of determi­

nation R2 0.87 and 0.90, respectively. The ns value was approx. 1 but the
fit of the Sips model was not optimal which may indicate some hetero­
geneity in the adsorption sites on the surface. Based on the experimental
data, the adsorption capacity Qmax of the H–SiO2 powder was less than
half of the capacity of the H-nSiC powder at pH 8.
The adsorption tests showed the benefits of BP-nSiC powders
compared to extracted nSiO2 powders. The adsorption capacity of
H–SiO2 was half compared to synthesized H-nSiC at pH 8. BP modifi­
cation also increased adsorption capacity of H-nSiC powder by 32 %.
Except for adsorption capacity of the nSiC and BP-nSiC powders at pH 8,
they demonstrated good chemical stability in a basic solution. It high­
lights the advantage over the silicon/silica-based adsorbents [17,39]
which do not possess the long term stability in basic solutions. Analo­
gous bisphosphonate-modified nanoporous silicon adsorbents have also
shown promising results in selective adsorption of Sc [18] and U [19].

Fig. 6. Separation factor (SF) of chosen metals in landfill water sample on HBP-nSiC at pH 8.
9


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294

4. Conclusions

Declaration of competing interest

Nanostructured silica was effectively extracted from barley husk fly
ash, slow pyrolyzed barley husks and pristine barley husks. The highest

purity silica was obtained from the pristine barley husks and it also
possessed the highest surface area of 187 m2/g. Furthermore, nano­
structured silicon carbide was synthesized from the three kinds of
extracted nanostructured silica powders with magnesiothermic reduc­
tion via self-propagating high-temperature synthesis at ambient tem­
perature of 100 ◦ C. The synthesized nanostructured silicon carbide had
high surface area up to 196 m2/g and were composed of cubic 3C and
hexagonal 2H polytypes with average crystallite sizes of 5 and 14 nm,
respectively.
The results showed that the method used to extract the nano­
structured silica precursors had a significant effect on the structure and
composition of the synthesized nanostructured silicon carbide. To
obtain the highest surface area of nanostructured silicon carbide, it is
advisable to leach the inorganic impurities before high temperature
removal of the organic components. However, relatively high surface
area material (181 m2/g) can also be synthesized from pyrolyzed barley
husks. Furthermore, optimization of slow pyrolysis process parameters
may further improve surface area. The advantage of pyrolysis is that it
allows additional valorisation of the biomass by extracting organic
components. It also reduces the volume of the biomass making the
subsequent processes such as leaching and calcination easier to perform.
Nanostructured silicon carbide powders were successfully function­
alized with bisphosphonates for metal adsorption. Material prepared
from the pristine barley husk showed the best performance for manga­
nese adsorption because of the highest surface area and content of
bisphosphonates. The functionalization of the particles with
bisphosphonates improved the adsorption capacity by 32 %. The ma­
terial also showed no obvious degradation at pH 8, withstanding at least
five adsorption/desorption cycles.
We have previously demonstrated the applicability of bisphospho­

nate functionalized thermally carbonized porous silicon in selective
extraction of scandium and uranium. However, this material is very
expensive to produce and its stability is not optimal at high pH. The
functionalized nSiC has similar chemical properties as the functionalized
thermally carbonized porous silicon but is more affordable and has a
chemically stable SiC framework. Therefore, the developed material has
potential as an adsorbent in several applications at a wide pH range.
The developed synthesis method for producing nanostructured sili­
con carbide allows taking advantage of the low-value agricultural resi­
dues, barley husks, and turning them into a high-performance
nanomaterial without utilizing expensive or toxic chemicals. Besides
adsorption, the produced biogenic nanostructured silicon carbide holds
potential for various applications that require high surface area and
chemical or thermal stability.

Authors declare no competing interest.
Acknowledgements
The authors would like to thank Mr. Jukka Laakkonen for his
contribution in designing the reduction reactor. Funding by The Foun­
dation for Research of Natural Resources in Finland (1794/16, 1801/17
and 2018003), The Academy of Finland (292601, 314552), Business
Finland (NanOhra) and SEM and EDS facility of Sib Labs of the Uni­
versity of Eastern Finland is acknowledged.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.micromeso.2021.111294.
References
[1] S. Castelletto, B.C. Johnson, C. Zachreson, D. Beke, I. Balogh, T. Ohshima,
I. Aharonovich, A. Gali, ACS Nano 8 (2014) 7938–7947, />nn502719y.
[2] J. Su, B. Gao, Z. Chen, J. Fu, W. An, X. Peng, X. Zhang, L. Wang, K. Huo, P.K. Chu,

ACS Sustain. Chem. Eng. 4 (2016) 6600–6607, />acssuschemeng.6b01483.
[3] B. Zhao, H. Zhang, H. Tao, Z. Tan, Z. Jiao, M. Wu, Mater. Lett. 65 (2011)
1552–1555, />[4] Z. Yermekova, Z. Mansurov, A. Mukasyan, Ceram. Int. 36 (2010) 2297–2305,
/>[5] Rosario Gerhardt, Properties and Applications of Silicon Carbide, IntechOpen,
2011, pp. 389–410.
[6] J. Ahn, H.S. Kim, J. Pyo, J. Lee, W.C. Yoo, Chem. Mater. 28 (2016) 1526–1536,
/>[7] A.G. Merzhanov, J. Mater. Chem. 14 (2004) 1779–1786, />B401358C.
[8] H. Chen, W. Wang, J.C. Martin, A.J. Oliphant, P.A. Doerr, J.F. Xu, K.M. DeBorn,
C. Chen, L. Sun, ACS Sustain. Chem. Eng. 1 (2013) 254–259, />10.1021/sc300115r.
[9] T. Liou, S. Wu, Ind. Eng. Chem. Res. 49 (2010) 8379–8387, />10.1021/ie100050t.
[10] L.A. Zemnukhova, A.G. Egorov, G.A. Fedorishcheva, N.N. Barinov, T.
A. Sokol’nitskaya, A.I. Botsul, Inorg. Mater. 42 (2006) 24–29, />10.1134/S0020168506010067.
[11] J. Riikonen, J. Rantanen, R. Thapa, N.T. Le, S. Rigolet, P. Fioux, P. Turhanen, N.
K. Bodiford, J.R. Kalluri, T. Ikonen, T. Nissinen, B. Lebeau, J. Vepsă
ală
ainen, J.
L. Coffer, V. Lehto, J. Am. Ceram. Soc. 104 (2020) 766775, />10.1111/jace.17519.
[12] A. Lă
ahde, O. Haluska, S. Alatalo, O. Sippula, A. Meˇsˇceriakovas, R. Lappalainen,
T. Nissinen, J. Riikonen, V. Lehto, Nano Express 1 (2020), 010014, />10.1088/2632-959X/ab82e5.
[13] F. Fraysse, O.S. Pokrovsky, J. Schott, J. Meunier, Chem. Geol. 258 (2009) 197–206,
/>[14] S. Neethirajan, R. Gordon, L. Wang, Trends Biotechnol. 27 (2009) 461–467,
/>[15] A. Salami, K. Raninen, J. Heikkinen, L. Tomppo, T. Vilppo, M. Selenius,
O. Raatikainen, R. Lappalainen, J. Vepsă
ală
ainen, Ind. Crop. Prod. 155 (2020)
112760, />[16] M. Mureseanu, A. Reiss, I. Stefanescu, E. David, V. Parvulescu, G. Renard, V. Hulea,
Chemosphere 73 (2008) 1499–1504, />chemosphere.2008.07.039.
[17] J. Riikonen, T. Nissinen, A. Alanne, R. Thapa, P. Fioux, M. Bonne, S. Rigolet,
F. Morlet-Savary, F. Aussenac, C. Marichal, J. Lalevee, J. Vepsă

ală
ainen, B. Lebeau,
V. Lehto, Inorg. Chem. Front. 7 (2020) 631641, />C9QI01140D.
ăttă
[18] R. Thapa, T. Nissinen, P. Turhanen, J. Mă
aa
a, J. Vepsă
ală
ainen, V. Lehto,
J. Riikonen, Microporous Mesoporous Mater. 296 (2020) 109980, />10.1016/j.micromeso.2019.109980.
[19] R. Thapa, A. Rahmani, P. Turhanen, A. Taskinen, T. Nissinen, R. Neitola,
J. Vepsă
ală
ainen, V. Lehto, J. Riikonen, Sep. Purif. 272 (2021) 118913, https://doi.
org/10.1016/j.seppur.2021.118913.
[20] D. Savova, N. Petrov, M.F. Yardim, E. Ekinci, T. Budinova, M. Razvigorova,
V. Minkova, Carbon 41 (2003) 1897–1903, />(03)00179-9.
[21] M.S.M. Zahar, F.M. Kusin, S.N. Muhammad, Procedia Environ. Sci. 30 (2015)
145–150, />[22] World Health Organization, in: Guidelines for Drinking-Water Quality, Fourth ed.,
Incorporating the 1st Addendum, fourth ed., World Health Organization, Geneva,
2017.

CRediT authorship contribution statement
Ondˇrej Haluska: Conceptualization, Methodology, Validation,
Formal analysis, Investigation, Writing – original draft, Visualization.
Arezoo Rahmani: Methodology, Investigation, Writing – original draft.
Ayobami Salami: Methodology, Writing – review & editing. Petri
Turhanen: Methodology, Writing – review & editing. Jouko
ă la
ăinen: Methodology, Writing review & editing. Reijo Lappa­

Vepsa
lainen: Methodology, Writing – review & editing, Funding acquisition.
Vesa-Pekka Lehto: Conceptualization, Resources, Writing – review &
editing, Supervision, Project administration, Funding acquisition. Joa­
kim Riikonen: Conceptualization, Methodology, Investigation, Re­
sources, Writing – review & editing, Supervision, Project administration,
Funding acquisition.

10


O. Haluska et al.

Microporous and Mesoporous Materials 324 (2021) 111294
[31] Y. Guo, D.A. Rockstraw, Microporous Mesoporous Mater. 100 (2007) 12–19,
/>[32] Y. Guo, D.A. Rockstraw, Bioresour. Technol. 98 (2007) 1513–1521, https://doi.
org/10.1016/j.biortech.2006.06.027.
[33] W. Zhang, S. Ning, S. Zhang, S. Wang, J. Zhou, X. Wang, Y. Wei, Microporous
Mesoporous Mater. 288 (2019) 109602, />micromeso.2019.109602.
[34] J. Wang, X. Guo, J. Hazard Mater. 390 (2020) 122156, />jhazmat.2020.122156.
[35] J. Wang, X. Guo, Chemosphere 258 (2020) 127279, />chemosphere.2020.127279.
[36] K.Y. Foo, B.H. Hameed, Chem. Eng. J. 156 (2010) 2–10, />cej.2009.09.013.
[37] M.A. Al-Ghouti, D.A. Da’ana, J. Hazard Mater. 393 (2020) 122383, https://doi.
org/10.1016/j.jhazmat.2020.122383.
[38] B.P. Singh, J. Jena, L. Besra, S. Bhattacharjee, J. Nano Res. 9 (2007) 797–806,
/>[39] L. Sheng, Y. Zhang, F. Tang, S. Liu, Microporous Mesoporous Mater. 257 (2018)
9–18, />
[23] P.J. Orr, S.L. Kearns, D.E.G. Briggs, Palaeogeogr. Palaeoclimatol. Palaeoecol. 277
(2009) 1–8, />[24] B.D. Mattos, G.R. Gomes, M. Matos, L. Pereira Ramos, W.L.E. Magalh˜
aes, Waste

Biomass Valorization 9 (2018) 1993–2002, />[25] J. Umeda, K. Kondoh, Ind. Crop. Prod. 32 (2010) 539–544.
/10.4028/www.scientific.net/AMR.795.701.
[26] I.J. Fernandes, D. Calheiro, A.G. Kieling, C.A.M. Moraes, T.L.A.C. Rocha, F.
A. Brehm, R.C.E. Modolo, Fuel 165 (2016) 351–359, />fuel.2015.10.086.
[27] S. Kamari, F. Ghorbani, Biomass Convers. Bioref. (2020), />s13399-020-00637-w.
[28] N.W. Jepps, T.F. Page, Prog. Cryst. Growth Char. Mater. 7 (1983) 259–307,
/>[29] K. Chen, Z. Bao, A. Du, X. Zhu, G. Wu, J. Shen, B. Zhou, Microporous Mesoporous
Mater. 149 (2012) 16–24, />[30] H. Mutschke, A.C. Andersen, D. Clement, T. Henning, G. Peiter, Astron. Astrophys.
345 (1999) 187–202. />
11