Physical and Chemical Properties of Silver-Containing Nanosorbent Obtained from Rice Straw Biochar

Abstract

Improving the quality of natural water purification is one of the priority areas in the research conducted by scientific communities in the field of ecology. At the same time, the task is to achieve the optimal efficiency of the technological process at a low cost. The solution, in this case, is the use of materials necessary for cleaning, in particular, sorbents from natural raw materials, including agricultural waste. At present, a sufficient number of research results have been published confirming the effectiveness of the sorbent from biochar from various types of agricultural waste, as well as from rice straw biochar (RSB). This article proposes an innovative method for modifying biochar from rice straw, which allows the use of the material as a sorbent with a disinfecting effect. The method consists of processing biochar in a process activation plant (PAP) using a solution of silver nitrate, which is released in the form of a carbon nanomaterial with attached metallic silver ions on the surface of biochar particles. The biochar impregnated with a solution of silver nitrate was contacted with ferromagnetic particles under electromagnetic influence, followed by thermal treatment of the sample. The resulting silver-containing sorbent was subjected to a physicochemical analysis of its properties; photographs of electron microscopy were also obtained, and a bacteriological analysis of the effectiveness of the sorbent on natural water was carried out. The analysis was carried out on three indicators—total microbial count (TMC), total number of coliform bacteria (TCB), and thermotolerant coliform bacteria (TCB). According to the research results, the sorbent showed its disinfecting properties and confirmed its high efficiency (90.48–100%).

1. Introduction

In the modern world, one of the most pressing issues is the environmental crisis that arose as a result of insufficient responsibility towards use of natural resources in many countries with developed industry. In the absence of a systematic approach to solving environmental problems in some regions of the planet, in others, a complex of negative consequences arises as a result. For example, the populations of many countries do not have access to clean drinking water, despite the presence of water resources in sufficient volumes to provide water to all residents. Wastewater from various types of industries are discharged into water bodies either in the absence or with an insufficient degree of purification. To improve the quality of purified water, as a rule, the technological schema of treatment facilities include a filtration stage using various types of natural and synthetic filtration and sorption loads. As such, anthracite is often used, which has a relatively low cost and a wide range of uses [1,2,3,4]. However, in the global coal mining industry, there is a trend towards a reduction in the number of mines and, accordingly, in the volume of mined minerals [5,6].

The observed trend of environmental degradation and depletion of natural resources led to the need for researchers to develop innovative sorption materials that are consistent with the principles of “green” world ecopolitics [7].

Without a doubt, such materials include biochar synthesized via pyrolysis/carbonization of plant and animal biomass. Potential applications for biochar include, among other things, the elimination of heavy losses in nature and natural waters, as well as the significant processing of by-products/waste. The main parameters that take into account its properties include pyrolysis/carbonization temperature, observation time, heat transfer rate, and type of feedstock. The effectiveness of biochar in the fight against pollution depends on its surface area, pore size distribution, and ion exchange capacity. The physical architecture and molecular composition of biochar can be critical for practical applications in water.

Relatively high pyrolysis temperatures typically produce biochars that efficiently sorb organic contaminants by increasing surface area, microporosity, and hydrophobicity, while low temperature biochar is more suitable for removing inorganic/polar organic contaminants via oxygenated functional groups, electrostatic attraction, and sedimentation [8,9,10,11,12,13].

It is known that biochar materials obtained directly from biomass pyrolysis usually have insufficiently effective surface functionality, a limited number of active groups, low porosity (in the range of 0.016–0.083 cm³·g⁻¹), and minimal surface area (typically <150 m²·g⁻¹) due to which their general applications as functional materials are limited. However, surface features and properties such as porosity can be easily controlled via modification [14]. At the same time, it should be noted that the mechanism of adsorption, functional groups, the pore structure of the adsorbent, and changes in the active sites of the adsorbent have direct impacts on the choice of the modification method with the appropriate selection of chemical reagents [15].

The scope of application of biochar synthesized via pyrolysis/carbonization of plant and animal biomass in the field of water treatment is very extensive. Studies show that biochar is a very effective material in removing pesticides [16] and heavy metals [17,18]; it also has brightening properties [19]. Rice straw can be used as a raw material for the production of biochars, which is confirmed by multiple scientific publications [20,21,22]. The achievement of optimal structural characteristics is carried out by treating rice straw with KOH alkali and performing subsequent carbonization at a temperature of 650 °C [23].

It is important to note that modern “nano” forms of carbon are being actively studied all over the world, which, of course, is justified by their incredible characteristics [24].

Moreover, literature studies show that dispersing metal oxide in a carbonaceous matrix causes the creation or enhancement of surface properties, as well as catalytic or, in some cases, magnetic activity. The addition of metal particles to biochars increases their weight, facilitating their separation by enabling the settling process and, thus, facilitating the extraction of materials from the aqueous medium after the purification process (Figure 1) [25].

In the presented study, metallic silver is impregnated onto the surface of the sorbent via the electromagnetic method in a setting process activation without the use of additional reagents, which greatly simplifies and reduces the cost of manufacturing a silver-containing sorption material.

2. Materials and Methods

2.1. Feedstock

To obtain a silver-containing sorbent from biochar, rice straw was used as a starting material.

Rice straw and husks contained a combination of cellulose (Figure 2), hemicelluloses (Figure 3), and lignin (Figure 4), along with appreciable amounts of silica and other minor components, as summarized in

Table 1. Major components of rice straw and husks (%) [26].

	Cellulose	Hemicellulose	Lignin	Ash
Straw	32.0–38.6	19.7–35.7	13.5–22.3	10–17
Husk	28.6–43.3	22.0–29.7	19.2–24.4	17–20

[26].

Due to the stability of lignocellulosic materials, separation of the individual cell wall polymers was difficult, although there were various physical, chemical, and/or biological treatments available that could release the constituent sugars and phenols. Thermal decomposition products, such as chars and ashes, also showed useful properties, although these were strongly influenced by the pyrolysis conditions and subsequent additional treatments. However, high value utilization of rice-derived biomass is still poorly developed [27,28], and field burning and soil accumulation of rice straw remain common practices, despite their contributions to environmental problems as a result of air pollution and ecosystem deterioration.

Physical activation consisted of two stages—carbonization and actual activation—as a result of mechanical impact on the processed materials. In addition, as a result of the interaction between the field and manifestations of bodies, the independent mechanical working action of bodies, with a wall and a number of materials and physicochemical effects, such as magnetostriction, cavitation, ultrasound and electrophysical phenomena, their role, in turn, had a significant impact on materials. In the working zone, in a unit of its volume, a huge energy was concentrated, which directly affected the substance. In places of collision between ferromagnetic bodies, pressure can rise up to thousands of MPa. Thus, in the vortex layer of ferromagnetic bodies, any processed materials were strongly mixed, subjected to intense deformation, and activated to a large extent [29,30].

The nature of the movement of magnetic particles was determined by many factors, such as the speed of rotation and the intensity of the applied field, the viscosity of the medium, the size, shape, mass and magnetic properties of the particles, etc. Each particle performed 2 types of motion: translational motion in the direction of rotation of the field at a speed that can reach the speed of its rotation, and rotational motion around its smallest axis at a speed of up to 10 revolutions per second. Finely dispersed particles, being magnetized, were attracted to each other and forme chains. Coarse particles, as a rule, moved separately from each other.

2.2. Biochar Production/Activation

In the course of obtaining a modified sorption material from rice straw, sodium hydroxide (NaOH) and silver nitrate (AgNO₃) were used as activating agents.

2.2.1. Physico-Chemical Activations of Activation of the Initial Source from Rice Straw

Processing (activation) of the source material (rice straw) to obtain its specific sorption properties was possible in various ways. Activation could be carried out via physical or chemical methods. With chemical activation, two stages—carbonization and activation—were carried out simultaneously. The raw material was impregnated with a specific chemical, such as phosphoric acid, sulfuric acid, potassium (sodium) hydroxide, or zinc chloride, for several hours [31]. The impregnated product was carbonized at moderate or high temperatures (500–800 °C) in a muffle furnace. To remove the residual activating agent, the impregnated product was washed with distilled water until the pH value of the washed water (pH) became equal to the pH value of distilled water.

In this study, the chemical activation of rice straw was carried out in the following order: the waste was washed to remove dust, dried at a temperature of 105 °C to a mass, cut into lengths of 1–1.5 cm, and soaked in 1.0 M NaOH solution for 24 h, while the ratio of the hardness of the substance/liquid was set at 1.0 g/2.5 mL. After soaking in chemical solutions of rice straw wash water, tap water was distilled until a pH value of 7–8 was established, after drying under special conditions.

2NaOH + SiO₂⟶ Na₂SiO₃ + H₂O

(1)

Thermal activation of rice straw in a muffle furnace was carried out in the following sequence: rice straw (after chemical activation) was heated to a temperature of 600 °C in the furnace, then maintained at this temperature for 30 min.

Rice straw biochar obtained as a result of activation with sodium hydroxide at a temperature of 600 °C can be considered as a matrix base for imparting specific properties to it. Thus, biochar was modified via electromagnetic treatment in a process activation apparatus into a silver-containing sorption material, which was followed by a study of its physicochemical properties, the results of which are presented below.

2.2.2. Electromagnetic Method for Activating the Surface of Rice Straw Biochar Particles

In the vortex layer of the process activation plant, the energy of the rotating electromagnetic field caused an intense mechanical effect of the working bodies (ferromagnetic particles) on the processed materials. In addition, as a result of the interaction between the field and the working bodies, the mechanical action of the working bodies among themselves, both with the wall and with the materials, caused a number of physicochemical effects arise, such as magnetostriction, cavitation, ultrasound, and electrophysical phenomena, which, in turn, had a significant impact on materials. In the working zone, in a unit of its volume, a huge amount of energy was concentrated, which directly affected the substance. In places of collision with ferromagnetic bodies, pressure could rise up to thousands of MPa. Thus, in the process activation plant of ferromagnetic bodies, any processed materials were strongly mixed, subjected to intense deformation, and largely activated.

A process activation plant with ferromagnetic particles implemented only a certain degree of filling the working area of the chamber, which was characterized by a critical coefficient Kcr (the ratio of the total volume of ferromagnetic bodies to the volume of the working area of the chamber) [32]. The process of mechanical activation of materials in PAP, as in other mixers, was usually described in several stages [33,34]. At the first stage, EMT caused a violation of the crystal lattice of the material, as well as changes in interplanar distances and orientation angles of the structure. In the second stage, i.e., the formation of a new surface of the system, the appearance and development of cracks in the material took place. In this case, the increase in the total energy of the system in line with an increase in the phase interface by 1 cm² was determined via the formula:

$$\Delta H_s=\sigma -T\frac{\delta \sigma }{\delta T}=\sigma +q,$$

(2)

where σ—the specific surface energy; T—temperature; and q—the latent heat of formation of 1 cm³ of a new surface.

In the third stage, the process of fine grinding took place, in which the concentration of energy in the surface layer significantly changed the thermodynamic and chemical properties of the substance.

To describe the kinetics of processes occurring in heterogeneous systems, empirical models were used that established the functional dependence of the degree of conversion (α) on time (t). The Avraami–Erofeev equation, which describes decomposition reactions well, has the form [35]:

$$\propto -1-\exp \left(-k{t}^n\right),$$

(3)

where k—the rate constant; and n—the kinetic parameter of the process.

For the case of a topochemical process involving solid-phase particles occurring in the kinetic region of reaction, the reaction rate was proportional to the surface of unreacted particles and described via the following equation [35]:

$$1-{\left(1-\propto \right)}^n=\frac{kt}{r_0},$$

(4)

where n = 1/3, ½, and 1 for the cases of three-dimensional, two-dimensional, and one-dimensional reacting particles, respectively; and r₀ = the radius of reacting particles. Gray–Weddington’s “shrinking sphere” equation, i.e.:

$${\left(1-\propto \right)}^{1/3}=kt,$$

(5)

also effectively describes the kinetics of processes occurring in the kinetic region. In practice, to describe the recovery processes occurring in the kinetic region, the McKevan equation is used [36]:

$$r_i{d}_0\left[1-{\left(1-\propto \right)}^{1/3}\right]=kt,$$

(6)

where r_i—the average particle size, m; and d₀—the oxygen content in the oxide, measured as fractions of a unit.

Equation (7) is an improved version of Equation (6), since it takes into account the particle size and the oxygen content in the oxide. The rate constant has the dimension m/s and characterizes the speed of movement of the metal–oxide interface. The advantages of this model include simplicity, ease of use, and good agreement with experimental data on the reduction in powder oxides. If the rate of the process determines the mass transfer, and during the process a gradual growth of the product layer occurs, the equation describing this process, which was first obtained by Jander, has the form [36]:

$${\left[1-{\left(1-\propto \right)}^{1/3}\right]}^2=2kD\frac{t}{r_0^2},$$

(7)

where D—the diffusion coefficient.

On the basis of the Yander model, the Ginstling–Brownstein, Carter, andanti-Yander models (counter-diffusion being taken into account) were developed. Thus, one of the prerequisites for effective grinding in liquid and mixed media was the appearance of acoustic phenomena. The frequency of sound waves ranged from from tens of Hz to tens of MHz. In the liquid and mixed phases, acoustic waves were the source of cavitation. Studies [36] show that the share of acoustic oscillations was no more than 2% of the total energy, yet the resulting cavitation phenomena had a significant impact on the course of many physicochemical processes. In addition, the influence of an alternating magnetic field in the working area of the ferromagnetic needle apparatus led to the appearance of induction currents. The liquid component of processed raw materials was often water combined with dissolved salts; thus, electrolysis processes occur in such systems.

The process activation plant (PAP) allowed us to change the structure and properties of processed materials with small equipment sizes and low specific energy costs (Figure 5).

The principle of operation of the PAP was described by the author in the article studying the properties of concrete combined with biochar from rice straw [36].

A sample of RSB weighing 35 g was stirred in 150 mL of distilled water with silver nitrate AgNO₃ weighing 10 g, after which it was placed in the PAP (Figure 6 and Figure 7) in a replaceable insert 4 (Figure 8a) with ferromagnetic particles (m = 200 g), which have a magnetostrictive effect under the influence of an electromagnetic field. Next, contact between biochar impregnated with a solution of silver nitrate and ferromagnetic particles was carried out for 2 min under electromagnetic influence, after which the treated sample was dried in an oven for 4 h at t = 105 °C, followed by calcination in a muffle furnace for 3 h at 450 °C.

3. Results and Discussion

X-ray studies of the samples were carried out using a two-beam scanning electron/ion microscope ZEISS CrossBeam 340 equipped with an Oxford Instruments X-Max 80 X-ray microanalyzer (Carl Zeiss Industrial Metrology LLC (Maple Grove, MN, USA)) (Figure 9, Figure 10, Figure 11, Figure 12, Figure 13 and Figure 14,

Table 2. Chemical composition of rice straw biochar samples with treatment.

№	Chemical Element, %	EP	S
1	C	8.9	13.7
2	O	54.9	58.3
3	Si	8.1	16.4
4	Ag	16.7	1.1
5	K	2.5	3.9
6	Ca	1.3	0.9
7	Mg	4.4	2.7
8	Na	1.4	0.9
9	Cl	0.7	0.3
10	Fe	0.3	0.1
11	Ni	0.3	0.1
12	Mn	0.3	0.5
13	P	0.2	0.7
14	Al	-	0.3
15	S	-	0.1

). The determination of the structural characteristics of the sorption material was carried out on a D8 Advance powder diffractometer manufactured by Bruker AXS GmbH (Karlsruhe, Germany) (

Table 3. Physico-chemical characteristics of treated rice straw biochar samples.

№	Characteristic	EP	S
1	Ash content	35.8	35.5
2	Moisture contents, %	-	-
3	Specific surface, m2/g	7.45	7.37
4	Relative pore volume diameter up to 900 Å, cm3/g	0.034	0.036
5	Average mesopore diameter via desorption, Å	196	238
6	Micropore volume, cm3/g	0.0026	0.0019
7	Average micropore diameter, Å	4.08	3.68

).

Thermal analysis of rice straw biochar samples impregnated with silver nitrate via the electromagnetic method and via the soaking method during their heating was carried out via the method of synchronous thermal analysis on a STA 449 F1 Jupiter device (Selb, Germany). Thermal analysis was carried out in air. The temperature program involved heating at a rate of 5 °C/min up to 950 °C.

When heated to 500 °C, the calcined sample (Figure 15) practically lost no weight. The loss of 0.7% by the time of heating to 500 °C may indicate a small amount of moisture gained during the preparation of the sample for analysis. Since a very small amount of the sample (less than 20 mg) is needed for thermal analysis, even a small amount of moisture from the air during sample preparation can have some effect on the result. In the range of 500–688 °C, the sample loses were about 3.3% of the mass, which were accompanied by a signal of the endothermic effect on the DSC curve. The nature of the process is probably similar to the transformations occurring at 620–685 °C with sample No. 5, i.e., the release of chemically bound moisture, as well as a certain amount of organic volatile substances.

Upon further heating to 950 °C, the sample continues to lose mass at a lower rate. The residual mass at 950 °C is 95.76%. In this case, the TG signal also does not reach a stationary level, which indicates the future course of the sample decomposition process.

When heated to 450 °C, the calcined sample of biochar from rice straw impregnated with silver nitrate via soaking (S) (Figure 16) practically loses no weight: a loss of 1% may indicate a small amount of moisture gained while preparing the sample for analysis. Since a very small amount of sample (less than 20 mg) is needed for thermal analysis, even a small amount of moisture absorbed from the air during sample preparation can have some effect on the result. In the range of 450–710 °C, the sample loses about 4.3% of the mass, which is accompanied by a signal of the endothermic effect on the DSC curve. The nature of the process is probably similar to the transformations occurring at 620–685 °C with sample No. 5, i.e., the release of chemically bound moisture, as well as a certain amount of organic volatile substances.

Upon further heating to 950 °C, the sample continues to lose mass at a lower rate. The residual mass at 950 °C is 94.36%. In this case, the TG signal also does not reach a stationary level, which indicates the future course of the sample decomposition process.

The nitrogen adsorption–desorption isotherm (Figure 17) belongs to the fourth type of isotherms, according to the international BDDT classification, and is typical for mesoporous materials. On the adsorption branch, a slow increase in the values of specific sorption Vads are observed, along with an increase in the relative index p/p0, and in the region of nitrogen pressures close to the saturation pressure, adsorption increases sharply.

The mesopore size distribution curve for the sample has pronounced peaks corresponding to the presence of groups of pores of the same size (Figure 18).

The distribution curve for the sample has blurry maxima, corresponding to the presence of groups of pores of the same size in the material (Figure 19).

The resulting silver-containing nano-sorbent from rice straw biochar with electromagnetic treatment (Figure 20) was tested on natural water in order to confirm its disinfecting properties toward total microbial count (TMK), common coliform bacteria (CCB), and thermotolerant coliform bacteria (TCB). Microbiological analysis of well water was carried out by the Non-Governmental Institution Testing Laboratory “NIKA and K” (NU “IL “NIKA and K”) (Accreditation No. RA.RU.21AB55 dated 13 May 2015).

Under laboratory conditions, well water was subjected to sorption treatment with silver-containing rice straw biochar. The treatment was carried out in the carbonation mode on a flocculator for 2 min at a speed of 45 rpm, before being repeated for 30 min at a speed of 20 rpm. After carbonization, the treated water was coagulated with SKIF at a dose of 1.0 mg/L in the mixing mode for 5 min at a speed of 10 rpm, before being settled for 40 min (Figure 21).

According to the results of microbiological analysis of the studied samples, it was found that the treatment of rice straw with silver-containing biochar showed high efficiency—according to TMC, efficiency was 90.5%, while according to OKB, efficiency was 100%—which meets the requirements for drinking water (

Table 4. Lab test results.

№	Index	Permissible Levels	Source Water Quality Indicators	Water Quality Indicators after Carbonization Silver-Containing Biochar Sorbent	Efficiency, %
1	2	3	4	5	6
1	TMK, CFU/mL	Not more than 100 CFU in 1.0 mL	210 CFU in 1.0 mL	20 CFU in 1.0 mL	90.48
2	CCB	Not found in 100.0 mL	20 CFU in 1.0 mL	Not found in 100.0 mL	100.0
3	TCB	-	Not found in 100.0 mL	Not found in 100.0 mL	-

).

4. Conclusions

As the results of studies of the physicochemical properties of the samples show, biocharcoal rice straw with electromagnetic treatment is a stable carrier for the population with catalytic properties. As such a substance, silver nitrate (AgNO₃) was used, being impregnated on examples in various cases. A distinctive feature in the preparation of samples through treatment with silver nitrate was the rejection of the use of a reducing agent. Silver was impregnated onto the surface of one of the samples via the electromagnetic method, and the other sample was impregnated by holding the biochar matrix in a solution of silver nitrate. After processing via both methods, the surface morphology of the samples changed. A dense uniform coating is observed on the particles. In the samples, the volumes of micropores are almost the same—0.0026 cm³/g and 0.0019 cm³/g, respectively—as are the volume of mesopores—0.034 cm³/g for the first sample, and 0.036 cm³/g for the sample with soaking. There is low carbon content in the composition of both samples—8.9% and 13.7%—while the oxygen content on the surface of the material are, on average, 54.9% and 58.3%, respectively. Moreover, comparing the concentrations of metallic silver on the surface of the samples, it was found that the sample with electromagnetic treatment has 93.41% (16.7%) more silver ions than the sample with soaking (1.1%).

Thus, the possibility of using biochar both as a sorbent of metals from salts and as a modified carrier of chemically active substances was established. Moreover, the resulting material can be used both independently for disinfection, including with treated water, and as part of composite materials. In addition, the silver-containing material allows the purification of water by removing radioactive elements. Given that the electromagnetic method was used to activate the biochar sample, additional studies of the magnetic properties of the sample are required.