Highly Dispersed Blast-Furnace Sludge as a New Micronutrient Fertilizer: Promising Results on Rapeseed

: Due to the growing population of Earth, the problem of providing food comes to the fore. Therefore, the search for new, economically available sources of trace elements for crop production is relevant. One of these potential sources is blast-furnace sludge: highly dispersed metallurgical waste, the industrial processing of which is difficult due to its high zinc content. We studied the effect of blast-furnace sludge on rapeseed plants in laboratory, greenhouse, and field experiments and also assessed the accumulation of sludge components in plant organs. The studied sludge sample consisted of micron and submicron particles containing compounds of iron, silicon, aluminum, zinc, calcium, and sulfur. Used concentrations: laboratory—0.01, 0.1, 1%, 10, and 100 g L − 1 ; greenhouse—0.01, 0.1, 1, 10, and 100 g kg − 1 ; ﬁeld—0.5, 2, and 4 t ha − 1 . During a laboratory experiment, a decrease in the germination of rapeseed seeds exposed to 0.01, 0.1, 10, and 100 g L − 1 waste was observed, but 1 g L − 1 promoted the increase of this indicator by 7% regarding control (0 g L − 1 ). While inhibiting seed germination, the sludge had a beneﬁcial effect on the vegetative performance of plants. Reverse effects were noted in the greenhouse experiment as an increase in seed germination (introduction of 1 g kg − 1 of sludge to the substrate caused maximum stimulation) and a decrease in rapeseed morphometric parameters were observed. However, at a concentration of 10 g kg − 1 , the root mass increased by 43% and the stem mass by 63%. In the same group, the highest content of chlorophylls was noted. The number of pods in all experimental groups of plants was less than in control (0 g kg − 1 ) plants, but at the same time, in the variants of 0.01 and 1 g kg − 1 , the weight of seeds was noticeably increased, by 15.6 and 50%, respectively. Under the conditions of the ﬁeld experiment, the sludge had a positive effect on the indicators of biological and economic productivity. Thus, exposure to 0.5 and 2 t ha − 1 of sludge signiﬁcantly increased the dry matter and leaf area. The highest values of photosynthetic capacity were recorded at a dose of 2 t ha − 1 . The maximum increase in yield was ensured by the introduction of sludge at a concentration of 0.5 and 2 t ha − 1 . The sludge dose of 4 t ha − 1 , which was also used, either had no effect or suppressed the development of the analyzed traits. The study of the accumulation of zinc and iron in the organs of plants showed the absence of a pronounced dose-dependent accumulation of zinc in the organs of rapeseed, while for iron, an increase in the content of the element in the organs of plants associated with an increase in the concentration of sludge in the soil was recorded. Our results demonstrate the promise of further research and development of methods for the agricultural use of highly dispersed sludge from wet gas cleaning of blast furnace production.


Introduction
The growth of Earth's population has been accompanied by the aggravation of the problem of providing food and other crop products [1][2][3].One of the directions for solving this problem is the search for new, economically available sources of trace elements for plants.Technogenic wastes have become a promising sustainable source [4,5].The problem of the formation and processing of industrial waste is one of the most important and insufficiently studied problems of the 21st century [6].A significant proportion of unused waste is metallurgy waste [7].According to available data, smelting 1 million tons of steel at modern metallurgical enterprises produces 800 thousand tons of slag and 30 thousand tons of sludge [8].
In developing countries, due to the lack of economically accessible industrial technologies for recycling fine ash and sludge at most metallurgical enterprises, such waste is disposed of by being placed in dumps [9].Blast-furnace sludge, potentially being an iron-rich metallurgical raw material, cannot be directly used in the blast-furnace sintering process due to the high zinc content (more than 0.5%), which destroys the lining of blast furnaces [10].Uncontrollably flowing from there into groundwater and soil, sludge waste poses an environmental threat being a source of heavy metals, the excess of which negatively affects the state of natural ecosystems and the quality of crop production [11][12][13][14].
At the same time, the presence of iron, zinc, silicon, aluminum, etc., in the composition of the sludge [15][16][17][18] makes it promising to use in crop production as a source of microelements in a highly dispersed form.For example, iron is a part of a number of plant enzymes and is also involved in the synthesis of chlorophyll in respiration and metabolism [19,20].Zinc plays an important role in protein, carbohydrate, and phosphorus metabolism in the biosynthesis of vitamins and growth hormones [21,22].Silicon increases the viability of plants and increases their resistance to exogenous stresses [23,24].However, despite the rich microelement composition, there are currently no studies on the effect of metallurgical sludge on agricultural plants.
Among the most studied metallurgy wastes, in terms of use in crop production, are steel slags.There is a significant number of works showing the promise of using such slags in agriculture to reduce soil acidity and immobilize heavy metals [25][26][27].In addition to CaO, which is responsible for acidity neutralization, the composition of metallurgical slag contains such effective components as FeO, SiO 2 , MnO, and P 2 O 5 [28,29], which increase crop yields [30][31][32].
There are known examples of the use of sewage sludge as an organomineral fertilizer for plant nutrition and increasing soil fertility [33][34][35].Sewage sludge contains about 20-45% dry matter, and its suitability for agricultural use is determined mainly by the high content of organic matter and biogenic elements [36].However, in addition to these advantages, there is evidence that sewage sludge can also contain hazardous biological and chemical compounds (persistent organic pollutants, heavy metals, antibiotics, pathogenic microorganisms, etc.), which prevent their use in agriculture [37][38][39][40][41].
Sludge from metallurgical industries, due to its origin, does not contain such hazardous components as persistent organic pollutants, antibiotics, non-steroidal anti-inflammatory drugs, and pathogenic microorganisms.At the same time, such wastes may contain significant amounts of heavy metals [15,16], and, therefore, the possibility of their use in crop production requires serious, comprehensive studies.
Rapeseed (Brassica napus L.), which is a popular crop for basic and breeding research, was used as a test object in our work.In addition, rapeseed is the second most important oilseed crop in the world [42], and therefore, the search for new agrochemicals to increase its productivity is relevant.
Thus, the purpose of our study was to analyze the effect of blast-furnace sludge on rapeseed plants in laboratory, greenhouse, and field experiments in terms of morphometric and biochemical parameters, as well as taking into account the accumulation of sludge components in plant organs.

Blast-Furnace Sludge
For the study, a highly dispersed waste from metallurgical production was chosen: blast furnace gas cleaning sludge.A sample of metallurgical sludge was taken from the sludge collector of the wet gas cleaning of the blast furnace shop, filtered on a vacuum filter, and dried at room temperature for 48 h.Then the dry sludge was homogenized using a mechanical mortar Fritsch Pulverisette 2 (Fritsch, Germany).The dry sludge was stored in a sealed plastic bag at room temperature.The moisture content of the sample used in the course of these studies did not exceed 0.8%.
The microstructure and elemental composition of the sample were studied on a Tescan Vega 3 scanning electron microscope (Czech Republic) using an Oxford Instruments X-act (UK) energy-dispersive analysis attachment.In addition, the content of cadmium, chromium, copper, nickel, magnesium, and manganese was analyzed by flame atomic absorption spectrometry using a microemulsion preparation method [43,44].An Analysette 22 NanoTec instrument (Fritsch, Germany) was used to analyze the particle size distribution.The phase composition was determined by X-ray diffraction using a Difrey 401 diffractometer (Russia).From each variant, 30 plants were selected, which were divided into 3 groups of 10 plants.Root, stem, and seed samples from 10 plants were mixed and analyzed.Thus, for each variant, 3 measurements were carried out, and the average value was calculated.
At the end of the experiment, the content of heavy metals (Pb, Ni, Mn, Cu, Cd, Zn) was analyzed in the soils of the experimental plots by flame atomic absorption spectrometry method [45].For analysis, 3 soil samples were taken from each experimental plot.

Laboratory Experiment
To conduct a laboratory study, suspensions of sludge based on distilled water were used.Samples of sludge (100-0.01g) were weighed using a ViBRA HT analytical balance (Shinko Denshi, Japan (accuracy ±0.0001 g)), poured into a pre-prepared 1-L container with water, and stirred with a glass rod for 20 s.After that, samples were dispersed using ultrasound for 5 min, 300 W power, 23.740 kHz frequency (Ultrasonic Cleaner CD-4800, Codyson, China).Thus, dispersions with sludge concentrations of 0.01, 0.1, 1%, 10, and 100 g L −1 were obtained.The resulting suspensions were moistened with paper filters placed in Petri dishes on which seeds were sown (50 seeds in each dish).The control contained distilled water.
The studies were carried out in laboratory conditions without access to light (ambient air temperature in the laboratory was 18-25 • C, relative air humidity 80 ± 5%, atmospheric pressure 84-106 kPa (630-800 mm Hg, temperature for biotesting (20 ± 2 • C)).Seed vigor and germination were determined on days 3 and 7, respectively.The lengths of roots and stems were measured, after which the plants were dried at 90 • C for one hour and weighed.
The experiment was carried out in three analytical repetitions.

Greenhouse Experiment
Greenhouse experiments were carried out according to ISO 11269-2:2012.The substrate for the germination of plants in the greenhouse was artificially prepared.Soil consisting of a mixture of sand (69%), kaolin clay (20%), neutral peat (10%), and calcium carbonate (about 1%) with the addition of metallurgical sludge in 0.01, 0.1, 1, 10, and 100 g kg −1 concentrations.Soil without sludge was used as a control.Before sowing, the resulting substrate was laid out in a container with a volume of 7.2 L (0.8 m length, 0.3 m width, 0.3 m height).Seeds were sown in an amount of 30 pieces per container.The experiment was carried out in three analytical repetitions.
The calculation of the content of pigments in a sample of plant material was carried out according to the Formulas (1)- (4).
where Chla, Chlb, Ccar-concentrations of chlorophylls a and b and carotenoids in the extract, respectively (in mg L −1 ).D-optical density at the corresponding wavelength (nm).

Field Experiment
The field experiment was conducted at the agrobiological station of Michurinsk State University, Russia (Figure 1).
substrate was laid out in a container with a volume of 7.2 L (0.8 m length, 0.3 m width, 0.3 m height).Seeds were sown in an amount of 30 pieces per container.The experiment was carried out in three analytical repetitions.
The calculation of the content of pigments in a sample of plant material was carried out according to the formulas (1)- (4).
where Chla, Chlb, Ccar-concentrations of chlorophylls a and b and carotenoids in the extract, respectively (in mg L −1 ).D-optical density at the corresponding wavelength (nm).

Field Experiment
The field experiment was conducted at the agrobiological station of Michurinsk State University, Russia (Figure 1).The period of the experiment was characterized by dry, hot weather with a minimum amount of precipitation and an average air temperature of +25 °C.The soil of the agricultural biostation was alluvial-meadow saturated gleyic medium loamy.Detailed information about the physical and chemical properties of the soil is added in Supplementary Materials.
For research, we used sludge doses of 0.5, 2, and 4 t ha −1 .The site was leveled prior to the commencement of work.The tillage included spring plowing to 20 cm and cultivation.Before laying the experiment, the soil was once again cultivated.The experimental area was divided into rectangles with an area of 5.5 m 2 , on which different doses of sludge were applied.The experiment was set up in triplicate.The period of the experiment was characterized by dry, hot weather with a minimum amount of precipitation and an average air temperature of +25 • C. The soil of the agricultural biostation was alluvial-meadow saturated gleyic medium loamy.Detailed information about the physical and chemical properties of the soil is added in Supplementary Materials.
For research, we used sludge doses of 0.5, 2, and 4 t ha −1 .The site was leveled prior to the commencement of work.The tillage included spring plowing to 20 cm and cultivation.Before laying the experiment, the soil was once again cultivated.The experimental area was divided into rectangles with an area of 5.5 m 2 , on which different doses of sludge were applied.The experiment was set up in triplicate.
The application was carried out before sowing seeds, spilling the soil with an aqueous suspension of sludge.Ten liters of water containing the selected dose (50, 200, 400 g) were spilled per 1 m 2 .The seed application rate was 6 kg ha −1 .
After 90 days, the indicators of biological (dry matter, leaf surface area, photosynthetic capacity) and economic productivity (crop yield, number of pods, number of seeds per pod, weight of 1000 seeds) were evaluated.
After the completion of the experiment, the accumulation of zinc and iron in the organs of plants was analyzed by atomic absorption spectrometry.
Analysis of the distribution of metals in the tissues of experimental plants was carried out using the scanning electron microscope Tescan Vega 3 (Czech Republic).A weighed Agronomy 2022, 12, 2929 5 of 19 portion of plant tissue weighing 250 mg was ground in a chilled mortar in 0.5 mL of distilled water.The resulting homogenate was applied to a substrate and dried at room temperature.

Data Analysis
Descriptive statistics methods included the estimation of the arithmetic mean (M) and standard deviation (S).Statistical data processing was carried out using Microsoft Excel 2010 (descriptive statistics package) using one-way analysis of variance (ANOVA) with the calculation of Fisher's F-test at a 5% significance level.

Sludge Analysis
In the course of electron microscopic analysis, it was found that the material under study consists of particles of various sizes and irregular shapes.The size of most of the aggregates does not exceed 15 µm.It is noted that the surface of large particles is covered with a significant number of inclusions with a size of not more than 1 micron (Figure 2).capacity) and economic productivity (crop yield, number of pods, number of seeds per pod, weight of 1000 seeds) were evaluated.
After the completion of the experiment, the accumulation of zinc and iron in the organs of plants was analyzed by atomic absorption spectrometry.
Analysis of the distribution of metals in the tissues of experimental plants was carried out using the scanning electron microscope Tescan Vega 3 (Czech Republic).A weighed portion of plant tissue weighing 250 mg was ground in a chilled mortar in 0.5 mL of distilled water.The resulting homogenate was applied to a substrate and dried at room temperature.

Data Analysis
Descriptive statistics methods included the estimation of the arithmetic mean (M) and standard deviation (S).Statistical data processing was carried out using Microsoft Excel 2010 (descriptive statistics package) using one-way analysis of variance (ANOVA) with the calculation of Fisher's F-test at a 5% significance level.

Sludge Analysis
In the course of electron microscopic analysis, it was found that the material under study consists of particles of various sizes and irregular shapes.The size of most of the aggregates does not exceed 15 μm.It is noted that the surface of large particles is covered with a significant number of inclusions with a size of not more than 1 micron (Figure 2).A study was also conducted using laser diffraction to build a histogram of particle size distribution (Figure 3).The distribution of the test sample is polydisperse and has a bimodal character.It is shown that the particle size is in the range from 0.2 to 15 μm with distribution peaks corresponding to the size of 0.4 and 1.2 μm.A study was also conducted using laser diffraction to build a histogram of particle size distribution (Figure 3).The distribution of the test sample is polydisperse and has a bimodal character.It is shown that the particle size is in the range from 0.2 to 15 µm with distribution peaks corresponding to the size of 0.4 and 1.2 µm.The elemental composition of the sludge was determined using X-ray microanalysis.The test sample contains iron, oxygen, silicon, aluminum, zinc, calcium, and other elements.The results of determining the elemental composition are shown in Figure 4.The elemental composition of the sludge was determined using X-ray microanalysis.The test sample contains iron, oxygen, silicon, aluminum, zinc, calcium, and other elements.The results of determining the elemental composition are shown in Figure 4.The elemental composition of the sludge was determined using X-ray microanalysis.The test sample contains iron, oxygen, silicon, aluminum, zinc, calcium, and other elements.The results of determining the elemental composition are shown in Figure 4. X-ray diffraction revealed that the sample contained Fe2O3, ZnFe2O4, and C phases (Figure 5).These phases describe almost all reflections present in the diffraction pattern.The conducted studies demonstrate that blast-furnace sludge contains a high content of iron oxides.In addition to iron, the presence of silicon, aluminum, zinc, calcium, and sulfur was recorded in the sample.The granulometric composition of the sludge is represented by particles of various sizes, including submicron ones.Heavy metals, such as lead, were not found in the study of the elemental composition.

Laboratory Research Results
During the laboratory experiment, a decrease in the germination of rapeseed seeds under the influence of blast-furnace sludge was observed.The maximum suppression was noted at 0.1 g L −1 waste in the cultivation medium.The indicator decreased by 30% (Figure 6a).The concentration of 1 g L −1 was an exception.In this variant, the maximum germination of seeds was recorded; the increase was 7% regarding control (0 g L −1 ).The conducted studies demonstrate that blast-furnace sludge contains a high content of iron oxides.In addition to iron, the presence of silicon, aluminum, zinc, calcium, and sulfur was recorded in the sample.The granulometric composition of the sludge is represented by particles of various sizes, including submicron ones.Heavy metals, such as lead, were not found in the study of the elemental composition.

Laboratory Research Results
During the laboratory experiment, a decrease in the germination of rapeseed seeds under the influence of blast-furnace sludge was observed.The maximum suppression was noted at 0.1 g L −1 waste in the cultivation medium.The indicator decreased by 30% (Figure 6a).The concentration of 1 g L −1 was an exception.In this variant, the maximum germination of seeds was recorded; the increase was 7% regarding control (0 g L −1 ).
sented by particles of various sizes, including submicron ones.Heavy metals, such as lead, were not found in the study of the elemental composition.

Laboratory Research Results
During the laboratory experiment, a decrease in the germination of rapeseed seeds under the influence of blast-furnace sludge was observed.The maximum suppression was noted at 0.1 g L −1 waste in the cultivation medium.The indicator decreased by 30% (Figure 6a).The concentration of 1 g L −1 was an exception.In this variant, the maximum germination of seeds was recorded; the increase was 7% regarding control (0 g L −1 ).While inhibiting seed germination, the sludge had a beneficial effect on vegetative performance.Thus, an increase in the average length of the root by 50-90% and the stem by 15-74% was noted (Figure 6b).The root mass practically did not change, and the stem mass increased by almost 30% at concentrations above 0.1 g L −1 (Figure 6c).It should be said that the maximum increase in plant biomass was observed at maximum concentrations, which is probably due to the macro and microelements contained in the sludge.In general, the laboratory experience demonstrated the absence of a pronounced toxic effect of the sludge since, while the germination rates were suppressed, plant growth was stimulated, and the maximum rates were noted at high waste concentrations.According to the totality of indicators, the best for plants was the concentration of sludge 1 g L −1 .

Greenhouse Research Results
During the cultivation of rapeseed in a greenhouse, an increase in seed germination relative to control (0 g kg −1 ) was observed.The maximum stimulation was noted at a sludge concentration in the substrate of 1 g kg −1 (+ 23.3%) (Figure 7a).While inhibiting seed germination, the sludge had a beneficial effect on vegetative performance.Thus, an increase in the average length of the root by 50-90% and the stem by 15-74% was noted (Figure 6b).The root mass practically did not change, and the stem mass increased by almost 30% at concentrations above 0.1 g L −1 (Figure 6c).It should be said that the maximum increase in plant biomass was observed at maximum concentrations, which is probably due to the macro and microelements contained in the sludge.In general, the laboratory experience demonstrated the absence of a pronounced toxic effect of the sludge since, while the germination rates were suppressed, plant growth was stimulated, and the maximum rates were noted at high waste concentrations.According to the totality of indicators, the best for plants was the concentration of sludge 1 g L −1 .

Greenhouse Research Results
During the cultivation of rapeseed in a greenhouse, an increase in seed germination relative to control (0 g kg −1 ) was observed.The maximum stimulation was noted at a sludge concentration in the substrate of 1 g kg −1 (+ 23.3%) (Figure 7a).
increased by almost 30% at concentrations above 0.1 g L −1 (Figure 6c).It should be said that the maximum increase in plant biomass was observed at maximum concentrations, which is probably due to the macro and microelements contained in the sludge.In general, the laboratory experience demonstrated the absence of a pronounced toxic effect of the sludge since, while the germination rates were suppressed, plant growth was stimulated, and the maximum rates were noted at high waste concentrations.According to the totality of indicators, the best for plants was the concentration of sludge 1 g L −1 .

Greenhouse Research Results
During the cultivation of rapeseed in a greenhouse, an increase in seed germination relative to control (0 g kg −1 ) was observed.The maximum stimulation was noted at a sludge concentration in the substrate of 1 g kg −1 (+ 23.3%) (Figure 7a).The average length of the root and stem decreased when the cuttings were introduced into the cultivation medium (Figure 7b).However, at a concentration of 10 g kg −1 , the mass of the vegetative parts increased significantly, the mass of the root by 43% and the mass of the stem by 63% (Figure 7c).It should be noted that this effect was observed in the group of plants with a minimum value of stem length.The number of pods on plants of all experimental groups was less than on plants of the control group (Figure 7d), while in the 0.01 and 1 g kg −1 variants, the seed weight was noticeably increased by 15.6 and 50%, respectively (Figure 7e).
As can be seen from the presented results, the effects of sludge exposure differ depending on the experimental conditions.In a laboratory study, the germination of rapeseed The average length of the root and stem decreased when the cuttings were introduced into the cultivation medium (Figure 7b).However, at a concentration of 10 g kg −1 , the mass of the vegetative parts increased significantly, the mass of the root by 43% and the mass of the stem by 63% (Figure 7c).It should be noted that this effect was observed in the group of plants with a minimum value of stem length.The number of pods on plants of all experimental groups was less than on plants of the control group (Figure 7d), while in the 0.01 and 1 g kg −1 variants, the seed weight was noticeably increased by 15.6 and 50%, respectively (Figure 7e).
As can be seen from the presented results, the effects of sludge exposure differ depending on the experimental conditions.In a laboratory study, the germination of rapeseed decreased, but the morphometric parameters of plants increased under the influence of waste.In a greenhouse study, germination increased in all experimental cases.The maximum values were noted at medium concentrations of sludge in the substrate (at 1 g kg −1 , the increase was 23%).At the same time, a decrease in the length of stems, roots, and plant biomass was observed.An exception was the 10 g kg −1 variant, as with the minimum length of the stems, their mass increased by 63%.The mass of the root increased in this case by 43%.The development of generative organs was suppressed at all doses of sludge, maximum in the 10 g kg −1 group.
The study of the activity of antioxidant enzymes showed a decrease in catalase activity at medium concentrations (0.1 and 1 g kg −1 ) of sludge in the substrate, as well as in the group treated with 100 g kg −1 (Figure 8a).At the same time, at concentrations of 0.01 and 10 g kg −1 , a significant increase in the catalase activity was recorded compared to the control (0 g kg −1 ).The study of the activity of antioxidant enzymes showed a decrease in catalase activity at medium concentrations (0.1 and 1 g kg −1 ) of sludge in the substrate, as well as in the group treated with 100 g kg −1 (Figure 8a).At the same time, at concentrations of 0.01 and 10 g kg −1 , a significant increase in the catalase activity was recorded compared to the control (0 g kg −1 ).*-significant differences with the control.
In the case of peroxidase, the introduction of metallurgical sludge into the medium led to the inhibition of enzyme activity.The minimum values were noted at 1 g kg −1 concentration (Figure 8b).
Figure 9 shows the results of the current study of pigment content in rapeseed plants.The largest increase in the concentration of chlorophylls a and b in rapeseed compared to control (0 g kg −1 ) plants was noted when the content of sludge in the substrate was 10 g kg −1 .In the same group, a significant increase in biomass was noted (Figures 6c and 7b).At 100 g kg −1 sludge, the pigment concentrations did not differ from the control (0 g kg −1 ) values.In the case of peroxidase, the introduction of metallurgical sludge into the medium led to the inhibition of enzyme activity.The minimum values were noted at 1 g kg −1 concentration (Figure 8b).
Figure 9 shows the results of the current study of pigment content in rapeseed plants.The largest increase in the concentration of chlorophylls a and b in rapeseed compared to control (0 g kg −1 ) plants was noted when the content of sludge in the substrate was 10 g kg −1 .In the same group, a significant increase in biomass was noted (Figures 6c and 7b).At 100 g kg −1 sludge, the pigment concentrations did not differ from the control (0 g kg −1 ) values.
Figure 9 shows the results of the current study of pigment content in rapeseed plants.The largest increase in the concentration of chlorophylls a and b in rapeseed compared to control (0 g kg −1 ) plants was noted when the content of sludge in the substrate was 10 g kg −1 .In the same group, a significant increase in biomass was noted (Figures 6c and 7b).At 100 g kg −1 sludge, the pigment concentrations did not differ from the control (0 g kg −1 ) values.Thus, the study of antioxidant enzyme activity showed the suppression of catalase and peroxidase, except for the increase in catalase activity at concentrations of 0.01 and 10 g kg −1 .The maximum increase in the chlorophylls a and b content in rapeseed compared to the control (0 g kg −1 ) values was noted at 10 g kg −1 of sludge.In the same group, a significant increase in biomass was shown.

Field Research Results
As mentioned above, doses of sludge of 0.5, 2, and 4 t ha −1 were used for the field experiment.The maximum dose corresponds to a concentration of 1 g kg −1 , which had a favorable effect on the economically valuable traits of rapeseed (germination, seed weight).
The photosynthetic activity of plants serves as the biological basis for the formation of crops, the main indicators of which should include the area of the assimilation surface, the accumulation of dry matter, and the photosynthetic capacity and the net productivity of photosynthesis.
The analysis of experimental data showed that, on average, the maximum indicators of the amount of dry matter (Figure 10a) and leaf surface area (Figure 10b) were formed in the flowering phase of rapeseed and amounted to 12.6 g per plant and 704.7 cm 2 per plant in the control (0 t ha −1 ).Almost the same indicators were noted in the variant containing 4 t ha −1 .A significant increase in dry matter and leaf area was noted in the variants with the application of sludge at concentrations of 2 t ha −1 (21.1 g per plant and 1189.5 cm 2 per plant) and 0.5 t ha −1 (20.6 g per plant and 1118.4 cm 2 per plant).Thus, the study of antioxidant enzyme activity showed the suppression of catalase and peroxidase, except for the increase in catalase activity at concentrations of 0.01 and 10 g kg −1 .The maximum increase in the chlorophylls a and b content in rapeseed compared to the control (0 g kg −1 ) values was noted at 10 g kg −1 of sludge.In the same group, a significant increase in biomass was shown.

Field Research Results
As mentioned above, doses of sludge of 0.5, 2, and 4 t ha −1 were used for the field experiment.The maximum dose corresponds to a concentration of 1 g kg −1 , which had a favorable effect on the economically valuable traits of rapeseed (germination, seed weight).
The photosynthetic activity of plants serves as the biological basis for the formation of crops, the main indicators of which should include the area of the assimilation surface, the accumulation of dry matter, and the photosynthetic capacity and the net productivity of photosynthesis.
The analysis of experimental data showed that, on average, the maximum indicators of the amount of dry matter (Figure 10a) and leaf surface area (Figure 10b) were formed in the flowering phase of rapeseed and amounted to 12.6 g per plant and 704.7 cm 2 per plant in the control (0 t ha −1 ).Almost the same indicators were noted in the variant containing 4 t ha −1 .A significant increase in dry matter and leaf area was noted in the variants with the application of sludge at concentrations of 2 t ha −1 (21.1 g per plant and 1189.5 cm 2 per plant) and 0.5 t ha −1 (20.6 g per plant and 1118.4 cm 2 per plant).Along with leaf area, the crop's productivity is also determined by the period of the photosynthetic apparatus functioning, which is characterized by such an indicator as the photosynthetic capacity (PC).As a result of the active growth of the leaf surface, the PC value during the budding-flowering period reached its maximum value of 491,537 m 2 per day ha −1 (Figure 11).The highest rate was recorded at a dose of 2 t ha −1 , 537,000 m 2 per day ha −1 .The flowering-maturing period was characterized by a decrease in the PC index compared to the budding-flowering period due to a decrease in the leaf surface area.Along with leaf area, the crop's productivity is also determined by the period of the photosynthetic apparatus functioning, which is characterized by such an indicator as the photosynthetic capacity (PC).As a result of the active growth of the leaf surface, the PC value during the budding-flowering period reached its maximum value of 491,537 m 2 per day ha −1 (Figure 11).The highest rate was recorded at a dose of 2 t ha −1 , 537,000 m 2 per day ha −1 .The flowering-maturing period was characterized by a decrease in the PC index compared to the budding-flowering period due to a decrease in the leaf surface area.However, the highest rates (463,000 m 2 per day ha −1 ) were again obtained for the 2 t ha −1 treated variant.At the same time, in the control (0 t ha −1 ), the value of the PC was 433,000 m 2 per day ha −1 .At 4 t ha −1 , no significant effect of sludge was recorded.Thus, the application of metallurgical sludge at doses of 0.5 and 2 t ha −1 had a positive effect on the indicators of photosynthetic activity of spring rapeseed plants which may be due to the presence of transition metals in the sludge a bioavailable form since metalloproteins are involved in the photosynthetic electron transport [50].
The yield is the main criterion that reflects the efficiency of growing any agricultural crop, including rapeseed.Depending on the concentration of sludge, the crop yield varied from 1.55 t ha −1 to 2.2 t ha −1, while the control (0 t ha −1 ) values were 1.6 t ha −1 .The maximum increase in yield was ensured by the application of sludge at a concentration of 0.5 and 2 t ha −1 (Figure 12a).The yield directly depends on the number of fruits and their weight.Depending on the concentration of the sludge, such indicators as the number of pods, seeds in the pod, as well as their weight changed.The largest number of pods was in the 2 t ha −1 variant and exceeded control by almost 2.5 times (Figure 12b).The maximum number of seeds, 24.8 and 25.7 per pod, was also noted at 0.5 and 2 t ha −1 application rates of sludge against 20.7 seeds in the 0 t ha −1 group (Figure 12c).At 4 t ha −1 , the indicator was lower than the control (0 t ha −1 ) by 5.8 pieces.The weight of 1000 seeds increased at all doses of sludge, maximum at 2 t ha −1, reaching 5.36 g against 4.18 g in the 0 t ha −1 variant (Figure 12d).Thus, the application of metallurgical sludge at doses of 0.5 and 2 t ha −1 had a positive effect on the indicators of photosynthetic activity of spring rapeseed plants which may be due to the presence of transition metals in the sludge in a bioavailable form since metalloproteins are involved in the photosynthetic electron transport [50].
The yield is the main criterion that reflects the efficiency of growing any agricultural crop, including rapeseed.Depending on the concentration of sludge, the crop yield varied from 1.55 t ha −1 to 2.2 t ha −1, while the control (0 t ha −1 ) values were 1.6 t ha −1 .The maximum increase in yield was ensured by the application of sludge at a concentration of 0.5 and 2 t ha −1 (Figure 12a).The yield directly depends on the number of fruits and their weight.Depending on the concentration of the sludge, such indicators as the number of pods, seeds in the pod, as well as their weight changed.The largest number of pods was in the 2 t ha −1 variant and exceeded control by almost 2.5 times (Figure 12b).The maximum number of seeds, 24.8 and 25.7 per pod, was also noted at 0.5 and 2 t ha −1 application rates of sludge against 20.7 seeds in the 0 t ha −1 group (Figure 12c).At 4 t ha −1 , the indicator was lower than the control (0 t ha −1 ) by 5.8 pieces.The weight of 1000 seeds increased at all doses of sludge, maximum at 2 t ha −1, reaching 5.36 g against 4.18 g in the 0 t ha −1 variant (Figure 12d).
the pod, as well as their weight changed.The largest number of pods was in the 2 t ha −1 variant and exceeded control by almost 2.5 times (Figure 12b).The maximum number of seeds, 24.8 and 25.7 per pod, was also noted at 0.5 and 2 t ha −1 application rates of sludge against 20.7 seeds in the 0 t ha −1 group (Figure 12c).At 4 t ha −1 , the indicator was lower than the control (0 t ha −1 ) by 5.8 pieces.The weight of 1000 seeds increased at all doses of sludge, maximum at 2 t ha −1, reaching 5.36 g against 4.18 g in the 0 t ha −1 variant (Figure 12d).Summing up the results of the field experiment, we can note the positive effect of the used sludge from the wet gas cleaning of blast-furnace production in doses of 0.5 and 2 t ha −1 on the indicators of photosynthetic activity, as well as the biological and economic productivity of spring rapeseed plants.The maximum application rate of 4 t ha −1 either did not affect the recorded indicators or, as in the case of yield, slightly suppressed the development of traits.

Accumulation of Sludge Components in Plant Organs
Among the components of the sludge, from the point of view of environmental safety, the greatest threat is zinc, which is a heavy metal [51], and iron, characterized by the maximum content in the analyzed waste.Analysis of the zinc content in the organs of rape plants grown at various doses of blast-furnace sludge showed the absence of a dose-dependent accumulation of the element in the organs of rape (Table 1).Summing up the results of the field experiment, we can note the positive effect of the used sludge from the wet gas cleaning of blast-furnace production in doses of 0.5 and 2 t ha −1 on the indicators of photosynthetic activity, as well as the biological and economic productivity of spring rapeseed plants.The maximum application rate of 4 t ha −1 either did not affect the recorded indicators or, as in the case of yield, slightly suppressed the development of traits.

Accumulation of Sludge Components in Plant Organs
Among the components of the sludge, from the point of view of environmental safety, the greatest threat is zinc, which is a heavy metal [51], and iron, characterized by the maximum content in the analyzed waste.Analysis of the zinc content in the organs of rape plants grown at various doses of blast-furnace sludge showed the absence of a dose-dependent accumulation of the element in the organs of rape (Table 1).In the case of iron, an increase in the metal content in all plant organs was noted (Table 1).The maximum increase in the amount of iron relative to the control was recorded in the organs of plants grown at 4 t ha −1 of sludge.The mapping of iron in rapeseed plant samples is shown in Figure 13.As can be seen from the micrographs, iron is not uniformly distributed in plant parts.Thus, in homogenized samples of stems and, especially, seeds of plants, there are metal accumulation centers (Figure 13b,c).In plant roots, the distribution of Fe is relatively uniform (Figure 13a).
Other authors also showed the accumulation of ultrafine iron in plants [52] and the ability of nano iron to be adsorbed on sandy soil and to improve the availability of iron for plants was shown, which is important when cultivating plants in soils easily leached from fertilizers.

Heavy Metal Concentration in Soils
The analysis of the content of heavy metals in the soil after harvesting the crop grown using highly dispersed blast-furnace sludge did not show an excess of the norms according to Russian regulatory documents [53].At the same time, with an increase in the application rate of sludge, the zinc content in the soil increased (Table 2).As can be seen from the micrographs, iron is not uniformly distributed in plant parts.Thus, in homogenized samples of stems and, especially, seeds of plants, there are metal accumulation centers (Figure 13b,c).In plant roots, the distribution of Fe is relatively uniform (Figure 13a).
Other authors also showed the accumulation of ultrafine iron in plants [52] and the ability of nano iron to be adsorbed on sandy soil and to improve the availability of iron for plants was shown, which is important when cultivating plants in soils easily leached from fertilizers.

Heavy Metal Concentration in Soils
The analysis of the content of heavy metals in the soil after harvesting the crop grown using highly dispersed blast-furnace sludge did not show an excess of the norms according to Russian regulatory documents [53].At the same time, with an increase in the application rate of sludge, the zinc content in the soil increased (Table 2).*-significant differences with the control.

Discussion
Thus, we found that the studied sludge sample had a predominantly negative effect on the germination of rape seeds in the laboratory experiment but positive in the greenhouse and in the field.This may be due to the peculiarities of the action of sludge in various environments-in the aquatic environment and soil.Some authors point out that the results of research in aqueous suspension of the biological impact of ultrafine materials on plants hardly correlate with the results in soil [54].
We have determined the optimal range of sludge concentrations under controlled conditions-1-10 g/kg of substrate.At the same time, stimulation of seed germination and intensive growth of vegetative organs were observed.Apparently, the content of micronutrients in the medium in these cases is optimal for plants.
In field conditions, the application of 0.5-2 t/ha of sludge into the soil turned out to be effective.At these application rates, an increase in yield and indicators of photosynthetic activity was noted, which may be due to the content of a significant amount of iron in the sludge.Iron is one of the structural elements of organic components that play an essential role in the photosynthesis and nitrogen assimilation of plants [55].The photosynthetic electron transport chain is known to require a large amount of Fe cofactors [56].Fe homeostasis is an important determinant of photosynthetic efficiency in algae and higher plants [57].
At the same time, the application of 4 t/ha led to the termination of the stimulating effect and inhibition of plants.Probably, in the latter case, there was an excess of the normal content of micronutrients in the soil for plants, which was partially confirmed by its elemental analysis.
Although the information on the effects of blast-furnace sludge on crops is limited, there is evidence of positive effects on crops from other iron-containing wastes.Thus, under the conditions of a greenhouse experiment, it was shown that iron-containing waste from steel production facilitates the chlorosis of sorghum grown in iron-deficient limestone soil [5,58,59].Other authors have studied the possibility of using iron-containing converter sludge as a (Fe) fertilizer [60].The results showed an increase in the dry matter of the shoots and the uptake of Fe, Cu, and P by the plants.Iron is an essential micronutrient for almost all living organisms as it plays a critical role in metabolic processes such as DNA synthesis, respiration, and photosynthesis.In addition, many metabolic pathways are activated by iron, which is part of the prosthetic group of many enzymes [61].The availability of iron in soils provides the distribution of plant species in natural ecosystems and limits the yield and nutritional value of agricultural crops [62].
It has been established that metallurgical sludge has a multidirectional effect on the growth of leguminous crops at the early stages of ontogenesis [63].In soil culture, the stimulating effect of 1% sludge dispersion on pea seedlings was shown, while 10% inhibited plant growth.When an aqueous solution of sludge was added to the cultivation medium, the germination energy of sunflower seeds increased by more than 30% [64].In addition, at a sludge concentration of 10%, the root length increased by almost five times and the stem length by 2.5 times compared to the control.Kuznetzov et al. [65] found a dose-dependent increase in the length and mass of stems and roots of wheat, barley, and corn was shown.Among the investigated sludge concentrations of 0.001, 0.01, 0.1, 1, and 10%, the maximum positive effect was noted in the 10% variant.When rapeseed seeds were germinated in a cultivation medium containing 0.001-10% blast-furnace sludge, significant stimulation of stem and root length, as well as biomass growth, was observed [66].Biochemical analysis showed the greatest increase of chlorophylls and carotenoids concentration in plants treated with 0.1 and 1% of sludge.In a laboratory experiment, the inhibitory effect of metallurgical production sludge on the germination of flax seeds was shown.However, an increase in the production of seedling biomass was observed [9].When replacing the aquatic environment with a sandy one, the suppression of biological productivity under the action of sludge was recorded when the activity of photosynthetic system II was stimulated.In addition to data on positive effects, there are results showing a negative effect of sludge on plants.Thus, when studying the effect of aqueous suspensions of highly dispersed sludge wastes of metallurgy on the growth and development of cultural tomatoes (Lycopersicon esculentum Mill.), a negative effect of 1 and 10% sludge suspensions on the growth and development of seedlings at the early stages of ontogenesis was shown.Treatment of the root system of 40-day-old tomato seedlings also negatively affected the development of vegetative and generative organs [67].
There are examples of the use of metal-containing wastes, in particular, the so-called "red mud" in biotechnology and crop production [68,69].Red mud (bauxite residue) is a solid alkaline residue formed in the Bayer process [70].Red mud consists of a mixture of solid oxides and metal oxides.Iron oxides are responsible for the red color, which can be up to 60% of the mass.In addition to iron, other dominant components include silica, residual non-leached aluminum compounds, and titanium oxide [71].The sludge has a very alkaline reaction, and its pH varies from 10 to 13 [72].The study showed [73] that the greenhouse cultivation of Paraserianthes falcataria in a mixture consisting of 750 g of red mud, 750 g of Ultisol soil, and 117.18 g of compost improved the chemical properties of the cultivation medium, as well as plant growth.It has been reported [74] that red mud applied at 5% by weight had a positive effect on degraded acidic sandy soil by significantly increasing the pH of the soil, thereby improving its structure and water-holding capacity.At the same time, various ecotoxicological studies have revealed negative effects of red mud in soil in more than 5% volumes on various test organisms [75], with some evidence of effects at the molecular level, for example, genotoxic effects on plants [76].The example of metal-containing galvanic sludge also showed a toxic effect on living organisms, such as a decrease in the germination rate of Lactuca sativa seeds and the appearance of chromosome aberrations [77].The authors suggest that the negative effects are associated with heavy metals that are part of the galvanic sludge.
The positive effects noted in our study can be associated not only with iron but also with zinc and silicon present in the composition.Zinc is a catalytic and structural protein cofactor in hundreds of enzymes and performs key structural functions in protein domains that interact with other molecules [78].Silicon is not classified as an essential plant nutrient, but numerous studies show its beneficial effects on various species [79][80][81][82].The prevailing research findings suggest that silicon does not promote plant growth, function, or metabolic activity in itself but rather prevents or alleviates stress-induced strain, and this is ultimately reflected in improved plant growth and physiological processes [79].
In addition, it is important to note that the composition of the used sludge includes the compounds in the ultra-dispersed state, which suggests the effect of nanoparticles on rapeseed.It was found that low concentrations of Fe nanoparticles promote plant growth [83].Light and electron microscopy analysis showed that the nanoparticles promoted plant growth by altering leaf organization and increasing chloroplast number and granule stacking, as well as regulating the development of vascular bundles.In another study, Fe 2 O 3 nanoparticles promoted peanut growth by regulating the content of phytohormones and the activity of antioxidant enzymes [52].Foliar spraying of Moringa oleifera with Fe 3 O 4 nanoparticles promoted growth, an increase in photosynthetic pigments and indoleacetic acid content, and a decrease in hydrogen peroxide and lipid peroxidation levels in plant leaves [84].Different concentrations of Fe 3 O 4 significantly increased the percentage of crude protein, fiber, and ash, as well as the content of some nutrients in moringa leaves.
Summarizing the available data, we can talk about the favorable effect of ultradispersed iron particles on plants [85][86][87] and the prospects for further research and development of methods for the agricultural use of highly dispersed sludge from wet gas cleaning of blast-furnace production.

Conclusions
In the course of the study, a multidirectional effect of blast-furnace sludge, depending on the concentrations and the experimental conditions on rapeseed plants, was established.During the laboratory experiment, a decrease in the rape seeds germination exposed to 0.01, 0.1, 10, and 100 g L −1 waste was shown.At the same time, exposure to 1 g L −1 of sludge led to the increase of this indicator by 7%, while the beneficial effect on vegetative parameters was not revealed.On the contrary, in greenhouse conditions, seed germination was upregulated with a decrease in the morphometric parameters of plants.However, at a concentration of 10% g kg −1 , the mass of roots and stems increased significantly.The number of pods on plants of all experimental groups was less than on control plants, while the weight of seeds exposed to 0.01 and 1% g kg −1 was noticeably increased.
Under the conditions of the field experiment, the sludge had a positive effect on the indicators of biological and economic productivity at application rates of 0.5 and 2 t ha −1 .An increase in the dose of sludge to 4 t ha −1 either had no effect or suppressed the development of the analyzed traits.
The study of the accumulation of zinc and iron in the organs of plants showed the absence of a pronounced dose-dependent accumulation of zinc in the organs of rapeseed, while, for iron, an increase in the content of the element in the organs of plants associated with an increase in the concentration of sludge in the soil was recorded.
The study shows good potential for the use of highly dispersed blast-furnace sludge as a source of trace elements in crop production.However, it is important to take into account concentration and species-specific effects, as well as crop growing conditions.
The prospect for agricultural producers is to obtain new inexpensive and effective micronutrient fertilizers based on highly dispersed blast-furnace sludge.At the same time, the metallurgical industry will be able to reduce its negative impact on the environment through the commercialization of innovative products based on industrial waste.

Figure 7 .
Figure 7. Influence of sludge on rapeseed in a greenhouse experiment: (a) Germination rates; (b) morphometric indicators; (c) biomass growth; (d,e) development of generative organs.n = 90.*-significant differences with the control.

Figure 7 .
Figure 7. Influence of sludge on rapeseed in a greenhouse experiment: (a) Germination rates; (b) morphometric indicators; (c) biomass growth; (d,e) development of generative organs.n = 90.*-significant differences with the control.

Agronomy 2022 ,
12, 2929 10 of 20 of the stems, their mass increased by 63%.The mass of the root increased in this case by 43%.The development of generative organs was suppressed at all doses of sludge, maximum in the 10 g kg −1 group.

Figure 8 .
Figure 8.The activity of enzymes of the antioxidant system: (a) Catalase; (b) Peroxidase.*-significant differences with the control.

Figure 8 .
Figure 8.The activity of enzymes of the antioxidant system: (a) Catalase; (b) Peroxidase.*-significant differences with the control.

Figure 9 .
Figure 9.The content of pigments in rapeseed plants.*-significant differences with the control.Figure 9.The content of pigments in rapeseed plants.*-significant differences with the control.

Figure 9 .
Figure 9.The content of pigments in rapeseed plants.*-significant differences with the control.Figure 9.The content of pigments in rapeseed plants.*-significant differences with the control.

Figure 10 .
Figure 10.Indicators of photosynthetic activity of spring rapeseed plants: (a) Amount of dry matter; (b) Leaf area.n = 90.*-significant differences with the control.

Figure 10 .
Figure 10.Indicators of photosynthetic activity of spring rapeseed plants: (a) Amount of dry matter; (b) Leaf area.n = 90.*-significant differences with the control.

Table 1 .
The content of metals in the tissues of spring rapeseed, mg kg −1 .
*-significant differences with the control.
*-significant differences with the control.