Development and application of microcapsules based on rice husk and metallurgical sludge to improve soil fertility

Abstract

The increasing environmental issues and growing interest in utilizing natural resources have led to heightened attention towards renewable energy sources. This has spurred the exploration of sustainable approaches, including ecosystem restoration. The soil’s ability to retain moisture increases with the rise in organic carbon content. As a result of this study, a unique product was developed—microcapsules based on carbonized rice husk and metallurgical slag. Conducted field trials, agrochemical analysis, elemental analysis, and X-ray diffraction analysis allowed for the assessment of the effectiveness of this additive. The research findings confirmed the positive impact of the mineralogical additive on soil properties, opening up prospects for its utilization in agriculture and environmental management.

Introduction

Growing environmental concerns and the use of natural sources have increased interest in renewable sources. This has led to the exploration of sustainable approaches, including ecological restoration^1–5. Biomass is a rich and renewable resource. Current research indicates^6–9 that converting biomass waste into biomass oil, an alternative to fossil fuels, using thermochemical conversion technology is an attractive environmentally friendly method. Among all the thermochemical biomass conversion methods, pyrolysis is a reliable way to convert biomass into liquid bio -oil and solid biochar. Biochars (carbonaceous) are considered sustainable stabilizers because they are produced from pyrolytic biomass wastes (i.e., plants, organic wastes, and animal wastes)^10–12.

Biochar is characterized as a carbon-rich material formed when biomass—such as wood, manure, or leaves—is subjected to heat in a low or oxygen-free environment¹¹. Shackley et al. (2012) offered a more detailed description, defining biochar as a porous carbon-containing solid generated through the thermochemical conversion of organic materials in an oxygen-depleted atmosphere, with properties that enable safe and long-term carbon storage in the environment¹³. Verheyen et al. (2010) described biochar as biomass that has undergone pyrolysis in a low or zero oxygen setting, applied to soil at a specific location, and intended to sustainably sequester carbon while enhancing soil function under current and future management conditions, all without causing short- or long-term harm to the environment, human, or animal health¹⁴. The International Biochar Initiative (IBI) defines biochar as a solid material formed through the thermochemical conversion of biomass in an oxygen-limited environment¹⁵. All these definitions, either directly or indirectly, emphasize the conditions necessary for biochar production and its application to soil.

The resulting biochar by its nature has high porosity, high specific surface area and an abundance of hydrophilic groups. Biochars are widely used for soil reclamation, ecological restoration, waste disposal, engineering lining materials, and water purification^16–19.

Despite the benefits of incorporating biochar into soil, the mechanisms that explain the interaction between biochar and soil properties are not fully understood. Long-term effects of biochar application to different soils should also be monitored²⁰. Both qualitative and quantitative assessments of emissions from conventional pyrolysis of biomass waste should be carried out to assess their impact on health and safety¹⁴.

The water-holding capacity of soil increases with increasing organic carbon content. An increase in the water holding capacity of soil containing biochar has been reported by approximately 18% ²¹. Soil water holding capacity is related to the hydrophobicity and surface area of biochar, as well as improved soil structure after biochar application²². Reduced nutrient leaching due to biochar application has also been reported²³. Biochar typically has a neutral to alkaline pH; however, acidic biochars have also been reported²¹. The pH of biochar depends on various factors, including the type of feedstock and the thermochemical production process¹¹. The alkaline pH of biochar causes liming of acidic soils, thereby possibly increasing plant productivity. The degree of liming of biochar depends on its ability to neutralize acid, which varies depending on the feedstock and pyrolysis temperature. For example, biochar produced from paper mill waste pyrolyzed at 550 °C had a liming rate approximately 30% higher than CaCO₃²⁴. Significant increases in seed germination, plant growth, and crop yield have been reported in soils treated with biochars. Applying biochar along with organic or inorganic fertilizers can even improve crop yields. There was also an increase in microbial population and microbial activity in soils treated with biochar. Significant changes in soil microbial communities and enzyme activities affect biogeochemical processes in soils. The impact of biochar on soil fauna has been largely unstudied, with the exception of a number of studies on the activity of earthworms in the soil. Weyers and Spokas reported that short-term negative effects were followed by long-term null effects on earthworm populations in soils treated with biochar²². The negative effects of biochar on earthworm populations are hypothesized to be due to the increase in soil pH due to biochar, derived from sludge, manure, or crop residues. However, wood-based biochars showed zero positive effect on earthworm populations²². Lee et al. recommended that incorporating wet biochar into the soil can help reduce earthworms by preventing desiccation²⁵.

The release of pollutants into the environment from industrial, residential and commercial sources leads to the degradation of surrounding ecosystems. Soil and aquatic environments in an ecosystem are often contaminated by organic and inorganic pollutants, mainly due to anthropogenic activities. Technologies are being developed to remediate contaminated soil and water. One of the most important technologies is to reduce the bioavailability of pollutants and therefore reduce their accumulation and toxicity to plants and animals. Biochar is emerging as an ameliorant to reduce the bioavailability of pollutants in the environment with the added benefits of soil fertilization and climate change mitigation²⁶.

Recently, environmental restoration has been recognized as a promising area where biochar can be successfully applied. This article discusses in detail the influence of pyrolysis conditions, including residence time, feedstock types, temperature and heat transfer rate, on the properties of biochar and therefore its effectiveness in removing contaminants. Particular attention is paid to mechanistic evidence for the interaction of biochar with soil contaminants. A particular innovation is the improvement of biochar by coating it with a carbon-silicon framework, which has increased the beneficial properties of the carbon material. Thus, this paper demonstrates the successful application of microcapsule carbon material as soil fertilizer.

Materials and methods

Pyrolysis

The carbonization process of enterosorbent samples based on plant fiber was carried out under isothermal conditions. The modification of the samples was carried out in a rotating reactor in an inert environment at a temperature of 300–900 °C; argon supply rate 50 cm³/min, contact time 30–60 min. The installation diagram is shown in Fig. 1.

Fig. 1.

Carbonization installation diagram.

The reactor is made of heat-resistant chromium steel. It is equipped with a heater and a rotation mechanism. The temperature in the reactor was maintained with an accuracy of ±5 °C. Loading up to 500 the catalyst. The volume of the reaction chamber is 3000 cm³. The carbonization time was constant. The gas mixture supply rate is 50 ml per minute. Contact time 60 min. Carbonization was carried out at temperatures of 650–750 °C with an interval of 25 degrees. Propane was used as a carbon source.

The electric furnace is heated using a temperature controller and maintains the required temperature in the rotating reactor. Hydrocarbon is supplied through the gas supply system at a predetermined speed. The hydrocarbon vapors entrained in this process are introduced into the reactor by the carrier gas.

Demineralization of carbonized samples

The process of demineralization of carbonized sorbents from rice husk was carried out in a glass container. The container is equipped with a spiral-shaped electric heater. The heater is connected to a current source. The container is equipped with a reflux condenser on top to avoid leakage of nitric acid vapors.

The sorbent is placed in a container and filled with a mixture of concentrated nitric acid solution (65%);

This mixture is then heated to a boil and boiled for 4 h.

After boiling, the mixture is left overnight for more complete demineralization.

After this, the spent nitric acid is drained by decantation, the sorbent is transferred to another container and washed several times with distilled water by boiling to establish a neutral environment. Granulation of the mixture is carried out in TG-10. With constant rotation of the plate, the mixture is granulated for 5–10 min to obtain a loose granular mass. After 10 min, the wet granulate is fed onto the sieves using a polyethylene scoop;

Drying of wet granules is carried out in a drying chamber at a temperature of 45–50 °C. The relative humidity in the chamber should not exceed 80 °C. The residual moisture content of dry granules should be no more than 30–35%. Next, granules with a size of 40–45 micron were selected from the resulting sample using sieves²⁷.

Sewing sludge into the activated rice husk

Integrating metallurgical sludge into activated rice husk is a method for recycling waste from metallurgy and agro-industry to create valuable materials such as sorbents, catalysts or additives for building composites. This process occurs in several stages: It begins with drying and fine grinding of the metallurgical slurry, which ensures its better mixing with the rice husk. Next comes mixing, in which 2% volume of metallurgical sludge is combined with 98% activated rice husk.

Field tests

The resulting microcapsule sample was carefully prepared for testing. Samples of a certain size and shape were formed, adapted to the test conditions. A laboratory site with controlled conditions was selected, where the soil has characteristics close to natural conditions. The soil is prepared, cleared of large inclusions and provided with sufficient drainage.

The sample was introduced into the soil at a given depth, at the level of the plant root system. The insertion site of the microcapsules was clearly marked for subsequent sampling and analysis.

For three months, the condition of the sample and the surrounding soil was regularly monitored, including visual inspection, photo documentation and measurement of physicochemical soil parameters.

After three months, soil samples are taken from the area around the sample to analyze changes in the composition and properties of the soil. Data obtained from laboratory testing of soil samples is analyzed to assess the effect of the material on soil processes, biodiversity and, where appropriate, plant growth and development. This approach made it possible not only to assess the environmental safety of using the developed materials, but also to study their positive impact on soil processes and agroecosystems as a whole.

Agrochemical soil analysis

Agrochemical soil analysis is a set of activities aimed at determining the chemical composition and fertility of the soil, which includes measuring the content of nutrients, pH, organic matter and other important characteristics. The agrochemical analysis process usually consists of the following steps.

The first step was to determine the sampling site and plan the number of samples. Using special instruments, soil samples were taken at different depths. Soil samples are dried at room temperature to remove moisture. Once dry, the soil is crushed and sifted through a fine mesh sieve (e.g. 2 mm) to remove stones, roots and other large particles. pH levels are measured using a pH meter to determine the acidity or alkalinity of the soil. The content of essential nutrients such as nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and trace elements (iron, manganese, zinc, etc.) is determined using various chemical and spectrometric methods. Soil organic matter content is often assessed by calcination, heating the sample to a high temperature, and measuring the weight loss. Based on the data obtained, a conclusion is made about the fertility of the soil, its deficiencies and excess nutrients²⁸.

Elemental analysis, IR, XRF

Elemental analysis was carried out using a Quanta 200i 3D scanning electron microscope. It provides high-resolution imaging of nanoscale objects down to 2.5 nm, allowing detailed qualitative and quantitative analysis. This method is equipped with a thermionic gun and a station with a focused ion beam, the microscope includes an integrated energy-dispersive microanalysis system for in-depth study of the structure and texture of materials. The combination of thermionic electron optics, a dual anode source and intralens differential pumping provides high accuracy and clarity of images. The microscope’s emission field ion optics guarantees high etching rates thanks to its high-resolution column. The oil-free differential vacuum system supports three vacuum modes—high, low and atmospheric, allowing adaptation to various operating conditions. The microscope’s electron optics operate with an accelerating voltage from 200 V to 30 kV, achieving a resolution of 2.5 nm in high vacuum mode. Ion optics, with accelerating voltages from 500 V to 30 kV, provide a resolution of 10 nm at a voltage of 30 kV. The microscope is equipped with three secondary electron detectors capable of operating in all vacuum modes with the ability to switch between modes depending on pressure. Additionally, there is a backscattered electron detector and a STEM detector for transmission imaging. The EDAX microanalysis system expands the microscope’s functionality, allowing the identification of elements from boron to uranium with an energy resolution of 132 eV. Heating modules maintain sample temperatures up to + 1000 °C, while gas chemistry ensures the deposition of platinum and tungsten through a specialized gas module. This makes it possible to initiate chemical reactions from the gas phase using electron and ion beams, including precise gas dosage and process control²⁹.

Results and discussion

In aerobic composting, the goal is to produce highly humified compost products in a short period. The formation and amount of humus not only reflect the degree of humification of composting products, but also indicate the intensity of biodegradation and stability Table 1.

Table 1.

Results of agrochemical soil analysis.

No.	Place of selection	Defined indicators
		General humus (%)	Movable
			Nitrogen (mg/kg)	Phosphorus (mg/kg)	Potassium (mg/kg)
1	1—soil with modified biochar	12.60	89.6	172	1200
2	2—control sample	6.91	89.6	134	1140

Sample 2 shows a humus content of 6.91%, which is significantly lower than sample 1, indicating lower organic matter content. This may mean adding organic amendments to improve soil health. Nutrient levels across the board are slightly lower compared to sample 1: nitrogen is 89.6 mg/kg, phosphorus is 134 mg/kg, and potassium is 1140 mg/kg. This accession may benefit from targeted fertilization to correct these slightly lower nutrient levels. Further, after adding the microcapsule, the soil quality was improved. Thus, sample 1 has a humus content of 12.60%, which indicates a relatively high level of organic matter, which has a beneficial effect on soil fertility. Levels of available nitrogen (89.6 mg/kg), phosphorus (172 mg/kg) and potassium (1200 mg/kg) indicate that the soil is well supplied with these key nutrients, potentially requiring less fertilizer for healthy plant growth^30,31.

The results of agrochemical analysis showed that sample 1 had a richer organic matter content and slightly higher levels of key nutrients than sample 2, which provides evidence of the beneficial effects of the microcapsule on the soil, that it may be more fertile and potentially more suitable for intensive agricultural production. use without significant changes. Sample 2, although remaining relatively fertile, may require addition of organic matter and minor adjustments to nutrient management practices to optimize crop production.

RS contains polysaccharides and a mineral part. During the carbonization process, changes in the crystallinity of substances can be expected, i.e. their structures.

At carbonization temperatures above 750 °C, amorphous silicon dioxide transforms into cristobalite. The carbohydrate part of the raw material undergoes more complex transformations³⁰. Thus, the carbonization of cellulose occurs in four stages³².

IR analysis data of the initial composition of rice husk showed that, along with the OH groups of cellulose and hemicelluloses, aliphatic and phenolic hydroxyl groups of lignin may be present, and combined carboxylated derivatives (3400.66 cm⁻¹) are likely to appear in the structure of carboxylated biomass, shown in Fig. 2.

Fig. 2.

IR spectrum of carbonized rice husk.

The composition of rice husk according to IR spectroscopy data represents a set of functional groups in the region of 1400–1707.08 cm⁻¹ corresponding to aromatic components. There are also absorption regions at 807.09 cm ⁻¹, 824.94 and 762.26 cm ⁻¹ of the corresponding C–C, C = C, C–O bonds, carbonyl and carboxyl groups.

The main structural fragments of biomass are high-molecular substances: namely, cellulose macromolecules are linear, hemicelluloses are branched, lignin are network polymers.

In this case, the melting temperature of the main high-molecular fragments of biomass is higher than the temperature of their decomposition, and they are in a solid state of aggregation. Whereas, carboxylation of the main structural fragments of biomass contributes to the appearance of thermoplastic properties in the reaction products, due to a decrease in the binding of hydroxyl groups and a decrease in the melting temperature relative to the temperature at which decomposition begins.

According to SEM data and elemental analysis, it has been established (Fig. 3) that the equilibrium compositions of the reaction system corresponding to the average elemental composition of rice husk (RH) in the temperature range 300–900 °C contain 83.63% carbon and 15% silicon dioxide in a condensed state %.

Fig. 3.

Results of SEM and elemental analysis of rice husk.

This indicates the possibility of modifying the structure of rice husk in this temperature range and obtaining composite materials by simple carbonization. Samples carbonized at different temperatures (300…500 °C) visually differ from samples carbonized at higher temperatures, having a dark brown rather than black color.

It follows from this that the carbonization process up to a temperature of 500 °C is not complete. It has been established that the specific surface area of carbonized RS increases with increasing process temperature and reaches a maximum of 150 m²/g at a temperature of 700 °C, and then decreases. RS contains polysaccharides and a mineral part. During the carbonization process, changes in the crystallinity of substances can be expected, i.e. their structures.

At carbonization temperatures above 750 °C, amorphous silicon dioxide transforms into cristobalite. The carbohydrate part of the raw material undergoes more complex transformations.

No	Element	Concentration, %	CPS
1	Manganese	25.88	260.6
2	Iron	23.89	296.4
3	Silicon	46.35	2.1
4	Potassium	2.15	3.0
5	Calcium	0.932	3.7
6	Chromium	0.0612	0.60
7	Scandium	0.180	1.3
8	Zinc	0.562	3.8

Based on X-ray phase analysis Fig. 4, a high concentration of manganese was found − 25.88% at an intensity of 260.6 cps. This element is important for soil health in small doses, but excess can be harmful to plants. A high iron content is also observed − 23.89% with a maximum intensity of 296.4 cps. Iron plays a key role in plant nutrition, but it may be less available in soils with high pH levels.

Fig. 4.

X-ray phase analysis.

The highest concentration of silicon is recorded − 46.35% at low intensity − 2.1 cps. Although silicon is not an essential plant nutrient, it helps strengthen cell walls and increases resistance to pests and diseases. Potassium is present in an amount of 2.15% at an intensity of 3.0 cps. This element is extremely important for plant health, as it affects their water balance, activates enzymes and enhances photosynthesis. Calcium was detected at a concentration of 0.932% at an intensity of 3.7 cps. Calcium is necessary for the formation of cell wall structure and the absorption of nutrients. Very low chromium concentration − 0.0612%, intensity is 0.60 cps. At high concentrations, chromium can become toxic to plants. A very low concentration of scandium was also detected − 0.180% at an intensity of 1.3 cps. The role of scandium in soil and plant health requires further research. Zinc is present in an amount of 0.562% at an intensity of 3.8 cps. This trace element is necessary for the functioning of enzymes in plants.

In terms of soil benefits, the presence of essential elements such as iron, potassium and calcium can be beneficial for plant growth and soil health. However, high concentrations of manganese and the presence of chromium, even in low concentrations, can potentially be harmful depending on the specific soil chemistry and the form of these elements. Excessive amounts of manganese can interfere with the absorption of other nutrients, and chromium can be toxic.

It is necessary to carefully control the introduction of materials with such elemental composition into the soil. The benefits of elements such as potassium and calcium can improve crop yields and soil structure, while the potential negative effects of elements such as manganese and chromium need to be mitigated. This may include balancing soil pH, ensuring the material is properly diluted, or using it in combination with other additives that bind or neutralize potentially harmful elements.

It is important to note that cps values are relative intensity values and give an indication of how the concentrations were measured, rather than being an absolute measure of quantity. The actual impact on the soil will also depend on the existing soil composition, pH, organic matter content and the specific needs of the plants being grown.

Conclusion

As a result of this work, a unique product was obtained - a microcapsule based on carbonized rice husks and metallurgical sludge.

The presented biochar component, obtained from carbonized rice husk, possesses sorption properties. When finely dispersed particles of 40 microns are intensively mixed with wolframite sludge of 3 mm size, the formed fine particles combine with soil nutrient particles, connecting with humus and clay particles in the soil to form stable capsules with microparticles (microcapsules). The biochar microcapsule absorbs nutrients and water from the soil due to its sorption properties and then releases active substances and nutrients into the soil.

Based on the research and analysis conducted, it can be concluded that the development and application of microcapsules based on carbonized rice husks and metallurgical sludge represents an innovative and environmentally sustainable approach to improving the agrochemical properties of soil. This approach not only helps to increase soil fertility by enriching it with minerals and trace elements, but also offers a solution for the disposal of industrial waste, thereby reducing environmental damage. The introduction of such technologies can have a significant impact on the agricultural industry, increasing yields and improving product quality, as well as contributing to sustainable development and environmental protection.

Author contributions

Alibek Mutushev—Idea and wrote the Material and methods part. Asiya Nuraly—Idea, made a research and wrote the Results and discussion part. Dauren Mukhanov—wrote the introduction part and arranged the article. Ayla Kaya—reviewed the manuscript.

Funding

This research has been funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP19676747).

Data availability

The datasets used and/or analysed during the current study available from the corresponding author Dauren Mukhanov on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.