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.
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 ¨inen b, Ondˇrej 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 The functionalization of the silicon carbide with bisphosphonates increased the adsorption capacity by 32 % and the material was able to withstand at least adsorption/desorption cycles 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: ondrej.haluska@uef.fi (O Haluska), arezoo.rahmani@uef.fi (A Rahmani), ayobami.salami@uef.fi (A Salami), petri.turhanen@uef.fi (P Turhanen), jouko.vepsalainen@uef.fi (J Vepsă ală ainen), reijo.lappalainen@uef.fi (R Lappalainen), vesa-pekka.lehto@uef.fi (V.-P Lehto), joakim.riikonen@uef fi (J Riikonen) https://doi.org/10.1016/j.micromeso.2021.111294 Received 16 April 2021; Received in revised form 30 June 2021; Accepted July 2021 Available online July 2021 1387-1811/© 2021 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 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 % 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 and 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 MnCl₂ 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 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 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, ml of sucrose solution was mixed with g of the nSiO2 powder Carbon ization of the material was performed in two steps i) Carbonization at 160 ◦ C for h under air ii) Carbonization at 700 ◦ C for 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 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 A current The synthesized nSiC was washed in two steps i) With 37 wt % HCl at 70 ◦ C for h ii) With 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 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 under N2 to remove dissolved O2 Then, BPs were mixed with mesitylene and again bubbled for 30 under N2 Meanwhile, nSiC was placed into a quartz tube and flushed at RT for 15 under N2 and further placed into the pre-heated tube oven at 150 ◦ C for 30 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 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 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, O Haluska et al Microporous and Mesoporous Materials 324 (2021) 111294 Fig 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 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 and 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 and then the suspension was mixed at 2975 rpm for before the measurement Zeta potential of H-BP-nSiC particles was measured with Malvern instruments Zetasizer Nano ZS mg of particles were mixed with ml of Milli-Q water The pH was adjusted between and 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 The imaging was conducted at keV accelerating voltage and recorded with SE2 detector The elemental analysis was done at keV for nSiO2 powders and 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, 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− and resolution cm− 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 M HCl for 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 to 8, the concentration 10 mg/L of Mn solution was used For adsorption kinetic experiments Mn solu tions 1.5 and mg/L at pH and 8, respectively, were used The adsorption isotherms were performed in the range of Mn concentration O Haluska et al Microporous and Mesoporous Materials 324 (2021) 111294 of solutions 0.1–10 mg/L at pH and 1–20 mg/L at pH 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 to separate the particles and the su pernatant In the kinetic experiments the contact time between particles and solution varied from 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 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− 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 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 = 2.6.3 Adsorption in a flow-through setup Flow-through setup was used to study the reusability and stability of BP-nSiC during adsorption/desorption cycles of Mnn+ at pH 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, ml of 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 M HNO3, respectively, with flow rate of 0.5 ml/ 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 M HNO3 filtrate after the desorption experiment O Haluska et al Microporous and Mesoporous Materials 324 (2021) 111294 Table 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 ± 27.8 ± 0.5 22.8 ± 0.4 5.0 ± 0.4 16.6 ± 0.6 105 ± 187 ± 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 ± 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 ± – – – 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 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