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Оригинальная статья / Original article УДК 546.284'31 + 543.429.22
DOI: http://dx.doi.org/10.21285/2227-2925-2019-9-2-159-169
Silicon-containing compounds in horsetail (Equisetum Equisetaceae) composition
© Lyudmila A. Zemnukhova***, Olga D. Arefieva***, Anna V. Kovekhova***, Natal'ya V. Polyakova*, Aleksandr E. Panasenko***, Antonina Yu. Kamaeva**
* Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, Vladivostok, Russian Federation ** Far Eastern Federal University, Vladivostok, Russian Federation
Abstract: The article presents the results of a chemical and phase composition study of ash obtained from the aerial parts of three horsetail species growing in different regions of Primorsky Krai, namely forest (Equise-tum sylvaticum L.), wintering (Equisetum hyemale L.) and field (Equisetum arvense L.) horsetail. It is shown that the conditions for processing raw materials affect the content of silicon dioxide (32-98%) in the ash residues. X-ray phase analysis has shown that the amorphous or amorphous-crystalline state of ash residues depends on the conditions of their production. The absorption bands typical for amorphous silicon dioxide are observed in the IR spectra of ash samples, which correspond to bending (467 cm1) and stretching vibrations (802 and 1092 cm1) of Si-O-Si siloxane bonds. The elemental analysis of silicon-containing products indicates an increase of silicon dioxide content in the samples with an increase in acid concentration during the processing of the aerial part of the horsetails, which also depends on the plant species: the field horsetail contains the lowest quantity of SiO2 as compared to the forest and wintering horsetails, but is characterised by a high content of potassium and calcium compounds. The sorption characteristics of ash obtained from the aerial part of horsetails are assessed: iodine adsorption capacity (5-42%) and methylene blue (164-260 mg/g) and methyl orange (40-241 mg/g) organic dye adsorption capacities. The obtained information can be used in the development of sorbents from vegetable raw materials for the purification and after-purification treatment of natural and waste waters from pollutants of various types.
Keywords: horsetail (Equisetum), ash, amorphous silica, elemental composition, sorption properties
Information about the article: Received August 1, 2018; accepted for publication June 7, 2019; available online June 28, 2019.
For citation: Zemnukhova L.A., Arefieva O.D., Kovekhova A.V., Polyakova N.V., Panasenko A.E., Kamaeva A.Yu. Silicon-containing compounds in horsetail (Equisetum Equisetaceae) composition. Izvestiya Vuzov. Prikladnaya Khimiya i Biotekhnologiya [Proceedings of Universities. Applied Chemistry and Biotechnology]. 2018, vol. 9, no. 2, pp. 159-169. (In Russian). DOI: 10.21285/2227-2925-2019-9-2-159-69
Кремнийсодержащие соединения в составе хвощей (Equisetum Equisetaceae)
© Л.А. Земнухова***, О.Д. Арефьева***, А.В. Ковехова***, Н.В. Полякова*, А.Е. Панасенко***, А.Ю. Камаева**
* Институт химии ДВО РАН, г. Владивосток, Российская Федерация
** Дальневосточный федеральный университет, г. Владивосток, Российская Федерация
Резюме: Приведены результаты исследования химического и фазового состава золы, полученной из надземной части трех видов хвощей, произрастающих в разных районах Приморского края: лесного (Equisetum sylvaticum L.), зимующего (Equisetum hyemale L.) и полевого (Equisetum arvense L.). Показано, что условия переработки сырья оказывают влияние на содержание диоксида кремния (32-98%) в зольных остатках. Методом рентгенофазового анализа было установлено, что зольные остатки находятся в аморфном или аморфно-кристаллическом состоянии в зависимости от условий получения. В ИК-спектрах образцов золы имеются типичные для аморфного диоксида кремния полосы поглощения, отвечающие деформационным (467 см1) и валентным колебаниям (802 и 1092 см1) силоксановых связей
Si-O-Si. Элементный анализ кремнийсодержащих продуктов показал, что с увеличением концентрации кислоты в процессе переработки надземной части хвощей увеличивается содержание диоксида кремния в образцах, которое также зависит и от вида растения: хвощ полевой содержит наименьшее количество SiO2 по сравнению с лесным и зимующим, но характеризуется большим содержанием соединений калия и кальция. Дана оценка сорбционных характеристик золы, полученной из надземной части хвощей: определена адсорбционная емкость образцов по отношению к йоду (5-42%) и органическим красителям - метиленовому синему (164-260 мг/г) и метиленовому оранжевому (40-241 мг/г). Полученные сведения могут быть использованы при разработке сорбентов из растительного сырья для очистки и доочистки природных и сточных вод от загрязняющих веществ различной природы.
Ключевые слова: хвощ (Equisetum), зола, аморфный кремнезем, элементный состав, сорбционные свойства
Информация о статье: Дата поступления 1 августа 2018 г.; дата принятия к печати 7 июня 2019 г.; дата онлайн-размещения 28 июня 2019 г.
Для цитирования: Земнухова Л.А., Арефьева О.Д., Ковехова А.В., Полякова Н.В., Панасенко А.Е., Ка-маева А.Ю. Кремнийсодержащие соединения в составе хвощей (Equisetum Equisetaceae) // Известия вузов. Прикладная химия и биотехнология. 2019. Т. 9, N 2. С. 159-169. DOI: 10.21285/2227-2925-20199-2-159-169
INTRODUCTION
Horsetails belonging to the Equisetum L. genus, which grow on all continents except Antarctica, have an ancient history. According to various estimates, the number of their species is between 15 and 60. The size of plants of this genus varies from a few centimetres to several metres [1]. 13 horsetail species are represented in the flora of the Russian Federation [2]. As studies have shown [3], the plants of the horsetail genus contain more than 35 macro- and microelements. A comparison of the elemental content indicates a similar chemical composition among the different horsetail types. In addition to silicon, there is a general tendency towards the accumulation of such elements as calcium, sodium, iron and zinc. However, amongst the various species there is a significant difference in the quantitative content of these elements. In the aerial part of horsetails, there is a large amount of silica (up to 25% of the dry weight), which structure is considered in a number of works [4-8]. Depending on species and other factors, the silicon content in horsetails can differ by a factor of up to three, which probably affects their pharmacological activity [1, 9]. Silica deposits are observed in all organs of the field horsetail, including the rhizome, stem, leaves and spores. Numerous plant structures, including cell walls and stomata, are silicified. Silicon is associated with hemicellulose and callose in horsetails in the same way as in other silicophile plants [10]. It was established that the callose polysaccha-ride can serve as the basis for the silicon deposition [11]. The mechanical role of silica in strengthening and hardening of tissues is described in [5, 12, 13]. The existence of a unique connection between Si(OH)4 and callose is indicated as providing protection against fungal infection [11, 14]. Horsetails have fodder and tech-
nical value [15, 16], as well as being of great interest for the production of pharmacological preparations [3, 17, 18]. Horsetail (E. arvense L.) is used in mainstream medicine. However, although raw materials containing a large amount of silicon are formed following the extraction of biologically-active substances, the composition and properties of this residue have been little studied.
The aim of the work is to study the silicon-containing products of aerial part of three types of horsetails (forest, field and wintering).
EXPERIMENTAL PART
The aerial parts of forest horsetail (E. sylvati-cum L.; Primorsky Krai, Kiparisovo), wintering horsetail (E. hyemale L.; Primorsky Krai, Roschino, Parti-sansk, Ussuriisk) and field horsetail (E. arvense L.; Primorsky Krai, Pshenitsyno, commercial drug by OOO "Zdorovye") were used as a (raw) material for the study.
The ash was obtained by the oxidising firing of the raw material (Scheme 1) or raw materials after processing (Schemes 2a-3b) (Table 1). The hydrolysis of the raw material was carried out in a ratio S:L = 1:13 when heated up to 90 °C for 1 h and stirred. The residue after hydrolysis was filtered, dried and calcined in a muffle furnace at 650 °C to constant weight.
The numbers of horsetail ash samples (No. 1-15) are given in Table 2. The ash samples of rice fruit shells (No. 16, 17) obtained according to Schemes 1 and 3a (see Table 1) were used for comparison.
Elemental analysis was performed by energy dispersive X-ray fluorescence spectroscopy using an EDX 800 HS spectrometer (Shimadzu, Japan) with a Rh anode tube and an exposure time of 100 s. The concentration of elements was calculated using a calibration curve.
Table 1
Schemes for obtaining the ash from the aerial parts of horsetails
Таблица 1
Схемы получения золы из надземной части хвощей
Scheme number Conditions
1 2a 2b 2c 3a 3b Calcination at 650 °C The hydrolysis of raw materials with water, then according to Scheme 1 Processing raw materials with ethyl alcohol, then according to Scheme 1 Processing raw materials with ethyl alcohol, then the hydrolysis of the residue of raw materials with 0.1 M HCl solution, then according to Scheme 1 The hydrolysis of raw materials with 0.1 M HCl solution, then according to Scheme 1 The hydrolysis of raw materials with 1.0 M HCl solution, then according to Scheme 1
Table 2
Characteristics of ash obtained from the horsetail aerial parts
Таблица 2
Характеристика золы, полученной из надземной части хвощей
Ash characteristics
Raw Ash production scheme XRF data
materials, collection place Sample number Ash yield, %, Colour Bulk density, g/m3 State Crystalline phase composition Amorphous phase interplanar distance, Â
Field horsetail, commercial drug 1 2a 3a 1 2 3 16.8 grey 9.5 light grey 6.5, light beige 99 74 56 amorph.-cryst. amorph.-cryst. amorph. sylvite (KCl), anhydrite (caSO4) sylvite (KCl), larnite (P-Ca2SiO4) 4.08 4.17 4.03
3b 4 5.5 white 62 amorph. - 4.08
Forest horsetail, Kiparisovo settlement 1 3a 5 6 18.8 light grey 12.6 white 60 amorph.-cryst. amorph. cristobalite (SO2), wollastonite (CaSiOa), sylvite (KCl) 3.86 4.05
Wintering horsetail, Roshchino settlement 1 3a 7 8 14.4 light grey 10.3 white 66 amorph.-cryst. amorph. cristobalite (SiO2), quartz (SiO2) 3.85 2.85 4.05
Wintering horsetail, Ussuriisk town 2b 2c 9 10 22.4 beige 12.7 white 41 amorph.-cryst. amorph. kieserite (Mg (SO4)H2O), syngenite (K2Ca (SO4V2H2O) 4.10 4.03
Wintering horsetail, Partisansk town 1 3a 11 12 15.0 grey 10.2 white 48 amorph.-cryst. amorph. cristobalite (SO2), brucite (Mg (OH)2) 4.15 3.03 4.05
Field horsetail, Pshenitsyno settlement 1 3a 3b 13 14 15 14.9 grey 6.0 light beige 4.6 white — amorph.-cryst. amorph.-cryst. amorph.-cryst. calcium carbonate (CaCOa) calcium silicate (CaSiOa) calcium carbonate (CaCOa) calcium oxide (CaO) quartz (SiO2) —
Rice husks 1 3a 16 17 15.0 light grey 12.0 white 37 25 amorph. amorph. — 4.07 4.07
The content of silicon dioxide in the obtained samples was also determined by the gravimetric method. About 2.0 g of the preparation was placed in a platinum crucible, calcined to constant weight at 800-900 °C and weighed. The calcined residue was moistened with 2 cm3 of water, followed by the
3 3
addition of 0.25 cm3 of sulphuric acid and 10 cm3 of hydrofluoric acid and heating on a hotplate until the volatilisation of the liquid and the sulphuric acid vapours was complete. The preparations were further calcined for another 5 min at 600-700 °C and weighed. At the same time, a non-volatile residue in the reagents used was determined under the same conditions.
The mass fraction of silicon oxide (IV) X in percent was calculated by the formula:
v _ m - m1 - (m2 - m3)) 100 x = ,
m
where m is the mass of the preparation, g; m1 is the mass loss on ignition, g; m2 is the mass of residue after the treatment of the calcined preparation with hydrofluoric acid, g; m3 is the mass of the residue of the used reagents after the treatment with hydrofluoric acid.
X-ray diffraction patterns were recorded using a D8 Advance diffractometer (Bruker, Germany) with Cu Ka-radiation. Phase identification was performed using the EVA software and PDF-2 database on powder diffraction.
IR absorption spectra of samples suspended in petroleum jelly were recorded in the range of 400-4000 cm-1 using a FTIR Prestige-21 Fourier transform spectrophotometer (Shimadzu, Japan).
The pH determination of the aqueous extract of ash samples was carried out as follows. 5 g of the ash sample was placed in a flask, 50 cm of distilled water was added and boiled for 3 minutes in a flask with reflux condenser. Further the flask content was filtered, the filtrate was cooled and the pH was determined using a FiveEasyPlus 20 pH metre (Mettler Toledo, Switzerland).
To determine the mass fraction of water-soluble substances (X?) in the ashes, 5 g of the sample was placed in a conical flask, 250 cm3 of water was added and boiled in a reflux condenser for 2 h, then the flask content was filtered. The filtrate was evaporated, and the resulting residue was dried at 110 °C to constant weight. The mass fraction of water-soluble substances X? was calculated by the formula:
v _ m1•100
X1 = -,
1m
where mi is the mass of dry residue, g; m is the sample weight, g.
Ash bulk density pb was determined by the following method. The test sample was poured into a pre-weighted cylinder 10 cm high until a cone was formed, which was removed with a ruler up to the
brim (without compaction), after which the cylin-derwith the sample was weighed.
The bulk density of the sample was calculated by the formula:
p _ (m1 - m)
pb- Г7 ,
V
where pb is the bulk density of the sample, kg/m ; m? is the mass of the measuring cylinder with the sample, kg; m is the mass of the measuring cylinder, kg; V is the volume of the measuring cylinder, m3.
To determine the methylene blue (MB) and methyl orange (MO) adsorption capacity, 0,1 g of the ash sample was placed in a conical flask, 25 cm3 of dye solution (1500 mg/dm3) was added and shaken with a stirrer during 20 min. After shaking, the suspension was centrifuged. The dilution factor for the determination of the methylene blue (MB) adsorption capacity is 10, and methyl orange (MO) adsorption capacity is 100. The optical density of the solutions was measured using an UNIC0-1201 spectrophotometer (United Products & Instruments Inc., USA) with a light filter with A = 400 nm wavelength. Sorbent's indicator adsorption capacity X in milligrams per 1 g of the product was calculated by the formula:
(C1 - C2) • K • 0.025
m ,
where C? is the mass concentration of the initial solution, mg/dm3; C2 is the mass concentration of the solution after contacting with the sample, mg/dm3; K is the dilution factor of the solution; 0.025 is the volume of the model solution, dm3; m is the sample mass, g.
To determine the iodine adsorption capacity of ashes, 1.0 g of ash sample was placed in a conical flask with the volume of 250 cm3, 100 cm3 of iodine solution in potassium iodide with a molar iodine concentration of 0.1 mol/dm3 was added, closed with a stopper and stirred for 15 min at an intensity of at least 100-125 vibrations per minute. Further a 10 cm3 of solution was taken by a pipette, placed in a conical flask with the volume of 50 cm3 and titrated with a solution of sodium thiosulphate (0.1 mol/l). At the end of the titration, 1 cm3 of starch solution was added and titrated until the blue colour had disappeared. At the same time, the initial iodine content in the solution was determined. For this, 10 cm of iodine solution in potassium iodide was taken and titrated with sodium thiosulphate solution, with a starch solution added at the end of the titration. The iodine adsorption activity of sorbents X in percent was calculated by the formula:
_ (V1 - V2) • 0.0127 • 100 • 100 X 10 • m ,
where V? is the volume of sodium thiosulphate so-
lution used for the titration of 10 cm of iodine solution in potassium iodide, ml; V2 is the volume of sodium thiosulphate solution used for the titration of 10 cm3 of iodine solution in potassium iodide, after treatment, ml; 0.0127 is the mass of iodine, corresponding to 1 cm of sodium thiosulphate solution with a concentration of exactly 0.1 mol/l, g; 100 is the volume of iodine solution in potassium iodide taken for clarification, ml; m is the weight of the sample, g.
RESULTS AND DISCUSSION
Below are the results of the study of ash samples obtained from the aerial parts of horsetails.
Ash yield, product colour, bulk density. Ash content in horsetails depended on the type of plant and the scheme for processing of raw materials (see Table 2). Thus, the ash yield obtained according to Scheme 1 ranged from 14.4 (Sample 7) to 18.8% (Sample 5), and according to scheme 3a -from 6.0 (Sample 14) to 12.6% (Sample 5). The amount of ash did not depend on the plant collection place, as shown by the example of wintering horsetail. The ash colour changed from grey to white, depending on the conditions of production. The bulk density of the ash obtained according to
Scheme 1 also depended on the type of horsetail and varied within 99.0 (Sample 1) - 48.0 kg/m3 (Sample 11).
Phase composition. X-ray phase analysis of ash showed that, unlike the reference sample (No. 16), all samples obtained according to Scheme 1 (No. 1, 5, 7, 11, 13) were in an amorphous-crystalline state (Table 2; Fig. 1, X-ray diffraction Patterns 1, 5 and 7). The presence of an amorphous phase in the samples is indicated by a diffuse peak on the X-ray diffraction patterns in the region 20 = ~ 15-35°. In the amorphous phases, the interplanar distances have different values (Table 2), indicating their varying structure. The composition of the crystalline phase varies depending on the plant type and its collection place (Samples 7 and 11). All ash samples obtained according to Schemes 2b, 3a and 3b (No. 3, 4, 6, 8, 10, 12, 17) are in an amorphous state, with the exception of Samples 14 and 15 (see Table 2). A typical X-Ray diffraction pattern for Sample 3 is shown (Fig. 1, Pattern 3), on which a blurred - but narrower as compared with Patterns 1, 5 and 7 - peak is observed; this is characteristic of the amorphous structure of the substance [19]. The interplanar distance of this reflex for these samples is 4.03-4.08 A, indicating their similar structure.
wollastonite (волластонит) cristobalite (кристобалит) quartz (кварц) sylvite (сильвин) anhydrite (ангидрид)
—I— 20
—I— 50
—I— 60
—I— 70
10
30
40
2©, град
80
Fig. 1. X-Ray diffraction patterns of silicon-containing samples obtained from the horsetail aerial parts according to different Schemes (the pattern number corresponds to the sample number in Table 2)
Рис. 1. Рентгенограммы кремнийсодержащих образцов, полученных из надземной части хвощей по разным схемам (номер линии соответствует номеру образца по табл. 2)
IR absorption spectra. Fig. 2 shows the IR absorption spectra of ash samples obtained according to Scheme 1 (Samples 1, 5, 7) and Scheme 3a (Sample 3). IR spectrum of Sample 3 is typical for the amorphous silicon dioxide [19]: there are absorption bands corresponding to bending (467 cm-1) and stretching (802 and 1092 cm-1) vibrations of Si-O-Si siloxane bonds. Weak absorption bands at 3181-3684 (stretching) and 1639 (bending) cm-1 indicate the presence of a small number of O - H bonds. According to the IR spectra, siloxane bonds are also present
in Samples 1, 5, 7. The band in the region of 615 cm-1 indicates the presence of a crystalline silica phase, which is consistent with the ash phase composition (see Table 2). A more complex spectrum of Sample 1 and the appearance of additional band at 1047 cm-1 do not exclude the presence of Si - OM bonds (M - Mg, Al) due to the possible presence of silicate compounds in the sample [20]. The primary substance comprising horsetail ash is silicon dioxide, as is the case with ash from rice tissues (husks and straw) [19].
4000
3200
-1-
2400
1800
1400
1000
600
Wavenumber, cm" Воллновое число, см"1
Fig. 2. IR absorption spectra of ash samples from the aerial parts of horsetails: (the spectrum number corresponds to the sample number in Table 2; * - petroleum jelly bands)
Рис. 2. ИК-спектры поглощения образцов золы из надземной части хвощей: (номер линии соответствует номеру образца по табл. 2; * - полосы вазелина)
Chemical composition. Table 4 shows the elemental composition of horsetail ash. The content of the main elements is given in oxides, and trace elements in the form of elements. The analysis of samples obtained according to Scheme 1 shows that field horsetail (Samples 1 and 13) contains the smallest SiO2 amount in comparison with the forest (Sample 5) and wintering (Samples 7 and 11) horsetails, whose silica concentration is two times greater. It should be noted that rice straw and husks accumulate more of this compound in comparison with horsetails (Sample 16 [19]). At the same time, the horsetail is characterised by
the significantly higher content of calcium, potassium and magnesium compounds, and the absence of titanium and barium. Processing the horsetails with a 0.1 M acid solution before firing led to the production of ash (Table 2) containing amorphous silica (81.1-91.2%), and did not depend on the type of horsetail (Samples 3, 6, 8, 12, 14). Increasing the acid concentration for treating the raw materials can increase the content of silica in the ash (Sample 4). However, it should be noted that ash with a higher SiO2 content is formed from raw rice materials under the same conditions (Sample 17) [19].
Elemental composition of silicon-containing samples obtained from the aerial part of horsetails according to different Schemes, wt.%
Table 3
Элементный состав кремнийсодержащих образцов, полученных из надземной части хвощей по разным схемам, % масс.
Таблица 3
Concentration, wt.%
Oxide
№ SÍO2 CaO K2O MgO AI2O3 Fe2O3
1 32.92 26.58 20.27 7.56 1.78 0.120
2 50.03 27.33 10.89 4.86 0. 68 0.230
3 87.84 8.06 0.82 0.41 0.88 0.460
4 98.33 0.23 0.11 n/d 0.94 0.120
5 68.43 9.05 14.41 3.82 1.43 0.330
6 91.24 1.11 1.67 0.58 0.57 0.280
7 67.78 14.06 9.57 4.29 2.76 0.790
8 87.57 3.80 1.69 0.95 0.57 0.260
9 57.40 17.85 13.01 4.03 2.12 0.600
10 88.10 3.90 1.70 0.96 0.58 0.450
11 67.27 13.44 12.08 5.55 1.14 0.360
12 89.62 2.28 1.95 0.38 0.38 0.170
13 32.45 8.27 24.42 2.63 0.26 0.040
14 81.10 5.49 1.82 0.41 0.42 0.100
15 89.22 0.09 0.66 - 0.79 0.090
16 91.70 0.86 0.19 0.45 0.06 0.060
17 99.50 0.15 0.16 0.07 0.02 0.021
Element
№ P Stotal Cl Sr Rb Br
1 0.64 3.98 4.930 0.079 0.058 0.028
2 0.95 2.05 2.570 0.084 0.030 0.012
3 0.78 0.40 n/d 0.012 0.003 n/d
4 0.19 0.01 n/d n/d n/d n/d
5 0.51 0.46 2.010 0.098 0.023 0.018
6 - 0.21 - 0.013 - -
7 1.15 0.32 n/d 0.102 0.018 n/d
8 - 0.02 - 0.011 0.002 -
9 1.69 3.16 n/d 0.095 0.014 0.002
10 0.88 0.24 - 0.080 0.010 n/d
11 1.25 0.40 n/d 0.077 0.008 0.003
12 - 0.02 - 0.014 0.002 -
13 0.47 - 0,160 0.040 0.040 0.003
14 0.30 - - 0.020 - -
15 0.10 - - - - -
16 - 0.87 - 0.001 0.003 -
17 - n/d - <0.001 <0.001 -
№ Mn Zn Cu Ti Cr Mo Zr Ni Ba
1 0.025 0.02 0.017 n/d n/d n/d n/d n/d n/d
2 0.035 0.036 0.021 0.029 0.019 0.009 0.005 0.005 n/d
3 0.008 0.015 0.018 0.021 0.010 n/d 0.004 0.002 n/d
4 n/d n/d 0.012 0.014 0.024 n/d n/d 0.003 n/d
5 0.166 0.022 0.012 0.083 n/d n/d n/d n/d 0.250
6 0.041 0.017 0.018 0.078 - 0.001 0.002 - 0.005
7 0.125 0.042 0.014 0.065 n/d n/d n/d 0.005 0.330
8 0.024 0.020 0.003 0.050 n/d n/d 0.001 0.001 0.002
9 0.096 0.032 0.017 0.017 n/d n/d n/d n/d 0.111
10 0.028 0.029 0.011 0.050 n/d n/d n/d n/d 0.007
11 0.052 0.050 0.013 0.026 n/d n/d n/d n/d 0.260
12 0.009 0.020 0.004 0.054 n/d n/d 0.0001 0.001 0.003
13 0.005 0.010 0.004 - - - - - -
14 0.004 0.010 0.006 - - - - - -
15 - - 0.004 - - - - - -
16 0.029 0.002 0.001 0.080 0.003 - - 0.001 -
17 0.018 0.004 <0.001 0.009 n/d <0.001 <0.001 n/d n/d
n/d - not detected
Due to the wide range of applications of silicon-containing products, depending on the structure of the substance and the purity of the product, as well as its physicochemical characteristics [21], we have investigated some properties of the obtained ash samples.
Mass fraction of water-soluble ash. The content of water-soluble ash components (Table 4) varies from 3.2% (Sample 4) to 26% (Sample 1), which correlates with the results of chemical analysis (see
Table 3). The greatest quantity of water-soluble components is found in the field horsetail; the smallest - in the forest horsetail.
The pH of the ash aqueous extract is presented in Table 4. The aqueous extracts of samples obtained according to the Scheme 1, have a pH of 11.2 (Sample 1 (Table 3), containing a large amount of calcium and magnesium) to 7.1 (Sample 5). Sample 4 is characterised by the lowest pH value, which main substance is amorphous silica (Table 3).
Table 4
Characteristics of ash obtained from the horsetail aerial parts (Samples 1-9) and rice husks (No. 16)
Таблица 4
Характеристика золы, полученной из надземной части хвощей (образцы № 1-9) и шелухи риса (№ 16)
Indicator The sample number according to the Table 2
1 2 3 4 5 7 9 11 13 14 15 16
pH of the aqueous extract 11.2 10.7 8.9 6.7 7.1 9.2 8.5 9.7 - - - 10.3
The mass fraction
of water- soluble 25.8 6.4 4.4 3.2 7.5 7.6 18.7 9.3 - - - 7.3
ash, %
MO adsorption capacity, mg/g 240.9 238.1 186.3 43.3 39.7 105.8 223.1 123.4 157.0 216.0 - 87.9
MB adsorption capacity, mg/g 176.8 173.5 243.5 259.7 190.7 181.5 164.3 172.2 139.0 164.0 179.0 150.3
Iodine adsorption capacity, % 41.9 35.6 13.3 9.5 5.1 7.6 7.6 14.0 40.1 17.8 10.2 17.6
MB, MO and iodine adsorption capacity. The adsorption capacity of the studied ash samples was determined using the conventional substances with different molecular weights and ionogenicity, such as iodine and organic dyes (MB, MO), which have a different chemical nature. The obtained values of the adsorption capacity of ash samples are given in Table. 4.
The adsorption capacity of all the studied horsetail ash samples ranged from 164 (Samples 9 and 14) to 260 mg/g (Sample 4), exceeding the activity of Sample 13, obtained from rice husks (150 mg/g). The magnitude of the MO adsorption capacity of ash has a wide scatter from 40 to 240 mg/g. The dependence of this indicator on the plant species is observed when comparing the samples obtained according to Scheme 1. The field horsetail (Sample 1) has the greatest, and the forest horsetail (Sample 5) ash has the lowest adsorption capacity. The wintering horsetail ash (Samples 7 and 11) absorbs the MO two times worse than the field horsetail ash. Such a difference in MB and MO adsorption capacity can be explained by a higher basicity of MB, in the molecule of which there are three nitrogen atoms and a conjugation due to nitrogen and sulphur atoms
between two benzene rings. Evidently, each of the molecules of these substances (MB and MO) has its own active centres, which participate in the interaction with the surface of the sorbent (horsetail ash), obtained from different plant species.
The iodine adsorption capacity of the studied samples ranged from 5 to 42%. Sample 1 is characterised by the highest and Sample 5 by the lowest (Table 4) adsorption capacity, which indicates the dependence of iodine adsorption on the plant species.
CONCLUSION
Silicon-containing horsetail ash samples, characterised by chemical, X-ray diffraction and IR spectroscopic analysis, are promising raw materials for use in various industries, including as pure forms of amorphous silica. The product yield (6.5-18.8%), the content of the SiO2 primary substance (32-98%) and the pH of the aqueous suspension depend on the taxonomic affiliation and plant processing conditions. The sorption characteristics of ash obtained from the aerial part of horsetails are assessed for the first time: iodine (5-42%), methylene blue (164-260 mg/g) and methyl orange (40-241 mg/g) organic dye adsorption capacities of the samples are determined.
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Contribution
Lyudmila A. Zemnukhova, Olga D. Arefieva, Anna V. Kovekhova, Natal'ya V. Polyakova, Aleksandr E. Panasenko, Antonina Yu. Kamaeva carried out the experimental work, on the basis of the results summarized the material and wrote the manuscript. Lyudmila A. Zemnukhova, Olga D. Arefieva, Anna V. Kovekhova, Natal'ya V. Polyakova, Aleksandr E. Panasenko, Antonina Yu. Kamaeva have equal author's rights and bear equal responsibility for plagiarism.
Conflict of interests
The authors declare no conflict of interests regarding the publication of this article.
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Критерии авторства
Земнухова Л.А., Арефьева О.Д., Ковехова А.В., Полякова Н.В., Панасенко А.Е., Камаева А.Ю. выполнили экспериментальную работу, на основании полученных результатов провели обобщение и написали рукопись. Земнухова Л.А., Арефьева О.Д., Ковехова А.В., Полякова Н.В., Панасенко А.Е., Камаева А.Ю. имеют на статью равные авторские права и несут равную ответственность за плагиат.
Конфликт интересов
Авторы заявляют об отсутствии конфликта интересов.
AUTHORS' INDEX
Lyudmila A. Zemnukhova, IS)
Dr. Sci. (Chemistry), Professor, Head of Laboratory,
Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, Far Eastern Federal University, e-mail: laz@ich.dvo.ru
Olga D. Arefieva,
Ph.D. (Pedagogics), Associate Professor, Far-Eastern Federal University, Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, e-mail: arefeva.od@dvfu.ru
Anna V. Kovekhova,
Ph.D. (Chemistry), Associate Professor, Far-Eastern Federal University, Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, e-mail: kovekhova.av@dvfu.ru
Natal'ya V. Polyakova,
Ph.D. (Chemistry), Senior Researcher, Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, e-mail: polyakova@ich.dvo.ru
Aleksandr E. Panasenko,
Ph.D. (Chemistry), Senior Researcher, Institute of Chemistry, Far-Eastern Branch, Russian Academy of Sciences, Far Eastern Federal University, e-mail: panasenko@ich.dvo.ru
Antonina Yu. Kamaeva,
Student,
Far Eastern Federal University, e-mail: zelenova.aiu@students.dvfu.ru
СВЕДЕНИЯ ОБ АВТОРАХ
Земнухова Людмила Алексеевна, СЕН
д.х.н., профессор, заведующая лабораторией, Институт химии ДВО РАН, Дальневосточный федеральный университет, e-mail: laz@ich.dvo.ru
Арефьева Ольга Дмитриевна,
к.пед.н., доцент, доцент, Дальневосточный федеральный университет, Институт химии ДВО РАН, e-mail: arefeva.od@dvfu.ru
Ковехова Анна Васильевна,
к.х.н., доцент,
Дальневосточный федеральный университет, Институт химии ДВО РАН, e-mail: kovekhova.av@dvfu.ru
Полякова Наталья Владимировна,
к.х.н., старший научный сотрудник, Институт химии ДВО РАН e-mail: polyakova@ich.dvo.ru
Панасенко Александр Евгеньевич,
к.х.н., старший научный сотрудник, Институт химии ДВО РАН, Дальневосточный федеральный университет, e-mail: anasenko@ich.dvo.ru
Камаева Антонина Юрьевна,
студентка,
Дальневосточный федеральный университет, e-mail: zeIenova.aiu@students.dvfu.ru
