Positive Effect of Biochar Application on Soil Properties: Solubility and Speciation of Heavy Metals in Non-Acidic Contaminated Soils near a Steel Metallurgical Plant in Southeastern Europe

Abstract

Neutral and slightly alkaline arable soils from the vicinity of the former and the biggest metallurgical plant in southeastern Europe were analyzed for the status of the water soluble pool of heavy metals in 1–20% w/w biochar (BC)-amended contaminated soils. Heavy metal solubility was monitored over a 6-month period. The metals (Fe, Mn, Cu, Zn, Cd, Pb and Ba) exhibited significant relationships between each other and exceeded the maximum permissible concentrations (MPC) in surface waters for domestic and drinking purposes. In most of the investigated sites and BC treatments, metal concentrations decreased with time due to the transfer to more resistant soil pools. Cation exchange capacity, exchangeable Ca and pH increased after BC application, while electrical conductivity decreased. BC amendment led to the prevalence of humic acids (HAs) over fulvic acids (FAs) and increased the fraction of refractory organic carbon. The share of metal–organic complexes increased for the metals Zn, Cd, Mn and Ba in the BC-amended soils, and the share of free Me²⁺ species decreased. This trend was especially pronounced in the soils with the lowest pH of 6.4–6.9. In addition to improving soil physicochemical and ecochemical properties, biochar application contributed to metal species in solutions that were less mobile and bioavailable.

1. Introduction

In today’s century of advanced industrialization and technogenic pressure on agriculture, restoring natural soil resources of intensively polluted lands occupies an increasing share of research in modern soil science and ecology. The irreversible accumulation of heavy metals in soil is an important global problem and lies at the basis of the so-called metallization of the biosphere [1]. The remediation of heavy-metal-contaminated soils is a broad field involving the use of a variety of organic and inorganic additives with diverse effects on individual contaminants [2,3]. Research studies with biochar (BC) as a soil ameliorant of heavy metals are mainly concerned with in situ experiments investigating the impact of the ameliorant on soil pollution and crops. Most of them have been conducted in countries outside Europe, where the problem is more acute [4]. The results confirm the huge advantages of biochar as a remediator. For example, it has been determined that, over a three-year period, the uptake of cadmium and lead by rice after biochar addition decreased by 67.33% and 69%, respectively [5]. Results demonstrating reduced bioavailability of both metals were also obtained after the addition of biochar in a cucumber/sweet potato/rape rotation in a one-year field trial in a mining area of southern China [6]. In research by Zhang et al. (2016), the effect of biochar produced at different temperatures on the availability of heavy metals (Cd, Cu, Mn, Ni, Pb and Zn) to ryegrass in alkaline soil was analyzed [7]. The authors found that biochar even increased the bioavailability of heavy metals. Another conclusion from the above study was that biochar significantly reduced the uptake of heavy metals by plants, except for the element Mn. Biochar produced at 300 °C and at 600 °C had a remediating effect on Cd-contaminated soil, leading to an increase in Cd (the residual fraction) [8]. Studies by Atanassova et al. (2024) [9] and Medyńska-Juraszek and Ćwieląg-Piasecka (2020) [10] found that biochar reduced the mobility of Cu, Pb, Zn and Cd in acidic soils due to an induced liming effect, but, in alkaline soils, the BC effect on the immobilization of metals was negligibly small. In a comparative study by Gong et al. (2024), with the application of biochar and lime, it was found that biochar decreased the content of Cd and Pb in Chinese cabbage [11]. Burachevskaya et al. (2023) and Burachevskaya et al. (2021), found that the introduction of biochar from different sources to Calcaric Fluvic Arenosol increased its adsorption ability under conditions of contamination for the metals Cu(II), Zn(II) and Pb(II) in the order wood  >  rice husk  >  sunflower husk, and decreased Cu and Zn uptake by spring barley [12,13].

The experiment conducted and published in this study on the use of biochar for the purposes of soil remediation is the first of its kind and scale for the area of a steel plant.

The present study deals with soils from an area with long-term anthropogenic and technogenic influence, including initial deforestation, uranium mining and large-scale industrial and metallurgical activity concentrated for a period of nearly 50 years in the former Kremikovtsi metallurgical plant located near Sofia, Bulgaria [14]. According to the data from our previous study [15], the main soil pollutants in the area are Pb and As and, to a lesser extent, Cd and Zn.

The proximity to the capital, and especially the fact that the surveyed lands are used massively for agricultural purposes, necessitates sustainable actions to combat soil pollution in the area. The present study attempts to provide a solution to this problem in line with the EU Action plan “Towards zero pollution for air, water and soil”. In addition, the accumulation of new data on the interaction of biochar with non-acidic soil adds new knowledge that will contribute to solving problems such as the deepening degradation and pollution of soils worldwide.

The aim of the present study is to determine the effect of biochar application on the water solubility and speciation of heavy metals in neutral and slightly alkaline technogenic soils from the area of the largest steel-producing metallurgical plant in southeastern Europe, which terminated its activity in 2009.

2. Materials and Methods

Soils from three sites in the area of the former Kremikovtsi metallurgical plant near Sofia, Bulgaria were analyzed. Sampling sites were located on a first-order soil catena lying at the foot of a mountain along a fan of proluvial cones of young and old Quaternary age (Figure 1) [16].

The studied soil groups were Fluvisols, Arenic (Site T1), Fluvisols, Loamic (Site T2) and Luvisols, Chromic (Site T3) [17]. Fluvisols were weakly alkaline and Luvisols were weakly acidic. Fluvisols were sown with alfalfa (Site T1) and rapeseed (Site T2), and Luvisols were non-cultivated and of mixed forest vegetation (Site T3).

Soil samples were collected from a depth of 0–20 cm and represented composite soil samples averaged from 5 points close to each other. The main soil properties were determined according to the following methods. Total organic carbon (TOC) was determined by the Tjurin method, i.e., oxidation with K₂Cr₂O₇/H₂SO₄ at 125 °C for 45 min, in the presence of catalyst Ag₂SO₄ and titration with (NH₄)₂Fe(SO₄)₂·6H₂O. The degree of humification and fractionation into humic organic carbon (HOC) and fulvic organic carbon (FOC) was carried out by the method of Kononova, 1966 [18]. Total cation exchange capacity (CEC) was analyzed by saturation with K malate at pH 8.2, and soil pH and Eh were measured in a 1:2.5 soil/water suspension [19]. Soil texture was analyzed by the Kachinsky method [20]. Electrical conductivity was determined at a soil/water ratio of 1:5, ISO 11265:2002 [21]. Total N% and C% were measured on Primacs SNC100-SKALAR (Breda, the Netherlands) through combustion at 1200 °C.

The main physicochemical characteristics of the soils (composite samples of 5 subsamples for each site) for Sites T1, T2 and T3 were the following: pH values for Site T1/Fluvisols, Arenic; Site T2/Fluvisols, Loamic; Site T3/Luvisols, Chromic were, respectively, 7.8, 8.0, 6.9; CEC, cmol/kg, 26.8, 29, 23; clay, %, 3.3, 4.5, 3.8; humus, %, 4.81, 2.16, 3.97 [15].

The total contents of some environmentally significant heavy metals and metalloids for the three sites (T1; T2; T3) were (ppm) Ba, 836, 2210, 1493; Cd, 2.8, 2.8, 2.3; Pb, 249, 273, 208; As, 69, 109, 98; Zn, 143, 384, 259; Cu, 110, 84, 72; Cr, 63, 40, 40; %, Fe, 6.4, 4.9, 4.04; Mn, 0.36, 0.45, 0.26 [15]. The three investigated sites had elevated total contents of Cd, Pb and As compared with the maximum permissible loads according to Decree 3/2008 [22]. The naturally and technogenically accumulated Ba contributed to the irreversible alkalization of the contaminated soils in the area [23] and exhibited higher contents, a couple of times exceeding the reference range of 362 to 580 mg/kg [24]. The results obtained for some contaminants in the soil extracts, e.g., barium, cadmium, copper, iron, manganese and lead, exceeded the limit values for surface waters designated for drinking and domestic purposes, and other guidelines in soil leachates [25,26].

The incubation experiment was conducted in a thermal climatic chamber with a phytotron system KK 350 FIT DS–POL-EKO, Wodzisław Śląski, Poland, 2022 at T 20 °C, air humidity 65%, light level 30 and fan level 3, in order to simulate spring/summer climatic conditions most suitable for crop vegetation (alfalfa was used as a test plant in a pot experiment in project KΠ 06 H66/2) in the ameliorated soils.

The soils were dried and sieved in a 2 mm sieve, and mixed and homogenized with biochar (ground to <2 mm) in five different variants, control (no BC) and BC added at 1, 5, 10 and 20% w/w (two replicates). The biochar used was of mixed wood origin, i.e., birch, sycamore, ash and maple, and pyrolyzed at 400–420 °C (Nikimol, OOD, Asenovgrad, Bulgaria). The pyrolysis process took place in four stages as follows: (1) drying, T was ≤150 °C; (2) initial stage of wood decay T from 150° to 270–275 ° C, the process was endothermic and hemicellulose and individual parts of lignin began to break down and low molecular weight products were formed; (3) self-pyrolysis stage at 270–275 °C, the process was exothermic with intensive decomposition of cellulose and lignin and formation of the bulk of the decomposition products and the actual structure of biochar; (4) tampering (hardening) of biochar at 400–420 °C, holding time was 4.5 h. The main biochar properties and heavy metal contents (total and water soluble) are presented in

Table 1. (a) Chemical and physicochemical characteristics of the biochar. CEC_8.2—cation exchange capacity [9] (SD cmol/kg 0.03); Exch. Ca (SD cmol/kg 0.03); Exch. Mg (SD cmol/kg 0.02); TOC (SD% 0.2). (b) Metal concentrations (mg·kg⁻¹) in biochar (aqua-regia) (means ± 1–12% SD). (c) Soluble (H₂O) forms (mg/L) of biochar (means, SD ± 1–10%).

(a)
pH/ H2O				CEC cmol/kg					Exch. Ca cmol/kg				Exch. Mg cmol/kg				EC mS/cm						TOC K2P2O7 %					(N%)						(C%)
8.2				10.4					7.2				3.2				0.12						19.8					0.027						64.25
(b)
Cd		Co			Cr			Cu			Fe				Mn			Mo			Ni			Pb			Zn			As			Hg			Se
0.28		0.43			1.20			5.81			923.5				212			1.45			2.24			13.16			186.9			0.03			0.015			0.23
(c)
NO3−	SO42−		PO43−			Cl−	DOC			Cd		Cu		Cr		Ni			Pb	Hg		Zn			Fe	Mn			Mg		Ca	Na			K		As
4.0	4.65		3.5			4.25	16.8			0.01		0.02		0.002		nd			0.007	nd		0.012			0.079	0.14			1.4		5.245	0.38			3.75		nd

a–c. We hypothesized that this comparatively low-temperature BC was more suitable for the removal of polar organic and inorganic pollutants [27] because it contained a higher fraction of polar and non-polar functional groups, through which metal cations are electrostatically retained or chemisorbed by specific and non-specific adsorption processes [28].

The sieved soil samples (300 g) of each soil type were placed in 500 mL capacity test vessels. Distilled water was added to achieve 75% of field capacity (FC) [29]. The weight of each test vessel was recorded before it was placed in the chamber, distilled water was added once a week and soils were homogenized by stirring in order to recover the evaporated water to 75% of FC. Every month, for a period of six consecutive months, the change in pH, EC and Eh and the contents of metals (Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Zn, As, Se, Ba, B, etc.) and anions (NO₃⁻, Cl⁻, SO₄²⁻, PO₄³⁻, HCO₃⁻ and DOC) (dissolved organic carbon) in soil water extracts were measured. Water-soluble forms of metals were analyzed at a soil/water ratio 1:5, after shaking for 1 h, centrifuging and filtering (0.45 μm acetate cellulose filter) [30]. Aqueous extracts of metals and metalloids were analyzed by ICP-OES, Agilent 5800. Each element was determined by the two most prominent lines, free from line interference from the matrix elements (Al, Ca, Fe, Mg and Ti). Anions in the soil solution, including DOC, were analyzed with Spectroquant assays, Merck Millipore, Darmstadt, Germany (PHARO 100) and HCO₃⁻ with titration of water extracts with 0.02 N H₂SO₄.

Thus, for each chemical element, results from a total of 90 single samples (3 × 6 × 5) were displayed. Heavy metal speciation in solution and statistics were performed by geochemical software Visual MINTEQ V. 3.1 and IBM SPSS V.26.

3. Results and Discussions

The results shown in Table 1a–c indicate that the biochar used had a measurable CEC (10.4 cmol·kg⁻¹ [9]) due to the presence of negatively charged carboxylic and phenolic groups as a result of its production at a considerably lower temperature (400–420 °C). This biochar could be an active cation exchanger, apart from its ability to retain both polar and non-polar compounds, e.g., by physical adsorption and/or van der Waals interactions. The contents of N and C analyzed by dry combustion were 64.2% C (19.8%, total organic carbon (TOC) by oxidation with K₂Cr₂O₇) and 0.027% N. The total and, especially, the water soluble metal concentrations and anionic species in biochar did not pose a threat to the soil or surface and ground waters (Table 1b,c), according to the national legislation [22,25].

The investigated soils had a relatively low clay content (3.3–4.5%) and CEC (23–29 cmol·kg⁻¹) [15]. Significantly higher was the OC content of the soil from the lowest point of the soil catena (Site T1, 2.79% TOC), where soil formation took place at conditions of shallow groundwater and abundant moisture-loving vegetation before the lands were transformed for agricultural use.

The fractionation of soil organic carbon (SOC) (

Table 2. Content and composition of total organic carbon in soils (controls C and soil + 20% BC) and fractionation of TOC in the experimental soils at the beginning of the experiment and at the 6th month (end of experiment). Markings: ^a % of the soil sample, ^b % of the total carbon.

		Humified OC (%)				Unextracted	E4/E6
Site *	TOC K2Cr2O7 oxidation	Extracted with 0.1 M Na4P2O7 + 0.1 M NaOH				OC	Total HA
	%	TOC	Humic acids	Fulvic acids	Ch/Cf	(%)
Control T1 *	2.79	0.32 a 11.47 b	0.21 7.53	0.11 3.94	1.9	2.47 88.53	4.25
T1 20% BC	4.79	0.32 6.68	0.23 4.80	0.09 1.88	2.56	4.47 93.32	4.11
Control T2	1.25	0.17 13.60	0.17 13.60	0.00	-	1.08 86.40	4.89
T2 20% BC	3.52	0.16 4.54	0.16 4.54	0.00	-	3.36 95.46	3.75
Control T3	2.30	0.33 14.35	0.21 9.13	0.12 5.22	1.75	1.97 85.65	5.17
T3 20% BC	5.41	0.26 4.81	0.18 3.32	0.08 1.48	2.25	5.15 95.19	3.71

* Site T1/Fluvisols, Arenic; Site T2/Fluvisols, Loamic; Site T3/Luvisols, Chromic.

) indicated that BC amendment led to the prevalence of humic acids (HA) over fulvic acids (FA) in the humified organic fraction, and increased the fraction of unextracted (non-humified OC) acids. A similar effect on OC from the BC application was observed in another paper (submitted to Bulg. J. Agric. Sci., under revision) and also in acidic Cu-contaminated Technosols in our previous study [9]. We suspected that this was attributed to the release of soluble organic compounds that contributed to the humification of SOC.

Similarly, the E4/E6 ratio of optical densities of humic acids in the alkaline extracts related to the degree of condensation of aromatic carbon and the molecular weight of humic substances decreased as the share of FA decreased [31].

3.1. Acid/Alkaline and Cation Exchange Properties of Soils

The following trends were noticed with the addition of BC at regular periods each month (called “stage”):

• The initial pH values in the successive measurement stages gradually decreased for all three investigated soils and for the five investigated variants.
• As the % of added BC increased, soil pH increased, as observed by Gull et al. (2015) and Atanassova et al. (2024) [9,32]. In the weakly alkaline Fluvisols from Sites T1 and T2, this trend was gradual, with the maximum values reached (when adding 20% BC) between the range of 7.5–8 and 7.7–8.2, respectively (Figure 2a). In the weakly acidic to neutral Chromic Luvisols from Site T3, this trend was jump-like in the last maximally BC-enriched variant (20%). The measured maximum values were within pH 6.85–7.05. Here, again, this could be attributed to the adsorption capacity of biochar towards the soluble compounds of various salts and complexes in the solution through ion exchange reactions, complexation and/or precipitation.
• The cation exchange capacity (CEC) increased significantly at 20% of added BC for the three sites (Figure 2c) due to the CEC of biochar [9] and, respectively, had a higher efficiency for reducing the phytoavailability of heavy metals, similar to the results of Domingues et al. (2020) [33] and Zhang et al. (2024) [34].

3.2. Electrical Conductivity

Two trends were observed in the change of electrical conductivity (EC) of the studied soils with the addition of increasing amounts of biochar (Figure 2b).

With the passage of time (per variant) in the chronologically consecutive stages of the study, the initial control EC values after the third month gradually increased due to the dissolution of specific mineral phases and desorption of salts in the soil solution with time. In a global meta-analysis by Sun et al. (2022), it was concluded that, in most of the studies, EC increased due to the higher content of ash in biochar, its solubilization and the release of ions in the solution [35].

In our study, as the % BC increased, the measured electrical conductivity decreased, similar to the results of Kane et al. (2021) and Tan et al. (2021) [36,37], who found that biochar enhanced the sorption of nutrients, reduced nutrient leaching and alleviated salt stress due to the desorption of Na⁺ ions. In addition, Kane et al. (2021) concluded that decreases in oxygen content and changes in particle size of biochar were associated with increases in electrical conductivity [36]. The biochar tested in our study had measurable CEC (10.4 cmol/kg), which could lead to a decrease in EC. In the studies by Sigh et al. (2017) [38], it was demonstrated that biochar produced at higher pyrolysis temperatures had higher EC values, attributed to the increased concentration of ash due to the loss of volatile material during the pyrolysis process. Lawrinenko and Laird (2015) [39], examining a cellulose-based biochar, found a significant anion exchange capacity (AEC) at pH 8 due to the contribution of pH independent O-containing oxonium heterocycles and pyridinic N.

In summary, there was a notable increase in pH, CEC/decrease in EC with BC addition, and an increase in EC/decrease in pH with time per variant. Similar effects were noted in acid technogenic Cu-contaminated soils around a Cu-smelter in Bulgaria [9]. These effects could be attributed to the alkalinity effect of biochar, which led to heavy metal and other cation and anion sorption [39] in soil, making them less mobile and bioavailable to plants.

3.3. Relationships between Metals in Aqueous Extracts

Correlation coefficients were derived based on the results obtained for the metal content in the water extracts from all 90 samples (3 × 5 × 6) for each element (

Table 3. Correlation matrix between the measured elements. Brave’s linear correlation coefficient (R) with statistical significance in intervals (−1 ÷ −0.3) and (0.3 ÷ 1), n = 90, calculated by the formula |r| ≥ 3 * σr, where σr = (1 − r²) ÷ √n and “n” is the number of pairs in the correlation [40].

El.	AL	B	Ba	Ca	Cd	Co	Cr	Cu	Fe	Mn	Mo	Ni	Pb	Zn
Al	1.00	−0.07	−0.08	−0.25	0.51	0.62	−0.16	0.04	0.89	0.56	−0.28	0.22	0.60	0.40
B		1.00	0.08	0.05	0.25	0.23	0.27	0.12	−0.10	0.14	0.09	0.23	−0.01	0.09
Ba			1.00	0.06	−0.20	−0.30	−0.04	−0.41	−0.06	0.05	−0.24	−0.33	−0.26	−0.25
Ca				1.00	−0.13	−0.27	0.27	0.21	−0.09	0.09	0.42	0.12	0.13	0.07
Cd					1.00	0.71	−0.29	0.25	0.54	0.42	−0.12	0.23	0.51	0.51
Co						1.00	−0.30	0.12	0.51	0.40	−0.14	0.26	0.40	0.31
Cr							1.00	0.34	−0.07	0.04	0.47	0.47	0.03	0.00
Cu								1.00	0.18	0.18	0.23	0.74	0.36	0.45
Fe									1.00	0.68	−0.24	0.29	0.76	0.52
Mn										1.00	−0.24	0.25	0.60	0.54
Mo											1.00	0.30	−0.04	−0.13
Ni												1.00	0.31	0.37
Pb													1.00	0.72
Zn														1.00

).

According to the analyzed results, associations of elements with statistically significant positive correlations between each other could be distinguished, e.g., Al, Fe, Mn, Co, Cd, Pb, Zn, Cu, Ni, Cr, Mo and Ni.

According to the derived correlations, barium (Ba) was an antagonist to the rest of the metals, and the correlation coefficients showed statistically insignificant and moderate negative correlation with Cd, Co, Cu, Ni, Pb and Zn (Table 3).

3.4. Changes in Metal Solubility

In the interpretation of the data, we focused on the heavy metals in Figure 3, i.e., Fe, Mn, Cu, Zn, Cd, Pb and Ba, because they exhibited significant relationships between each other and, in addition, were present in excess of the maximum permissible concentrations (MPC) according to Decree No. 12/2002 for recommended and mandatory levels of metals in surface waters. In addition, the total concentrations of the heavy metals Pb, Cd and Zn and the metalloid arsenic (As) exceeded MPL for soils according to Decree No 3 (2008) [15].

3.4.1. General Trends in Total Water Soluble Metal Concentrations

• Manganese. There was a decrease in concentration with Stage or the so called “time effect” and an increase in solubility with BC treatment.
• Iron. After the second stage, there was a sharp decrease in concentration and no visible biochar effect at the three sites studied. In general, Fe decreased with “time” (only at Site 3 was there mobilization with BC addition).
• Copper. There was no consistent trend in the variation of the Cu concentration. In general, Cu increased its concentration with time and there was no significant effect of BC treatment, except at the first stage of Site T1 soil.
• Zinc. There was a mobilization effect from BC application at Site T1 and Site T3 soils and no visible “time” effect.
• Cadmium. There was neither significant “time” effect nor “BC” effect, but a visible effect at 20% BC applied at Site T2 soil.
• Lead. There was a “time” effect at Site 1 soil and no significant “BC” effect.
• Barium. There was neither “time” nor “BC” effects at Sites T1 and T2 soils, only at Site T3, Ba increased with “time”; however, there was a fixation effect from BC addition.

It should be concluded that, in most of the investigated sites and treatments, metal concentrations in soil solutions decreased with time due to transfer of more labile fractions into more resistant metal pools [41]. As found in studies of Sun et al. (2022) [35], Farhangi-Abriz et al. (2021) and Atanassova et al. (2024) [9,42], acidic mineral soils responded more efficiently to biochar application than neutral and alkaline soils, mainly because of the liming effect and increase in the cation exchange and adsorption capacities.

3.4.2. Metal Species Distribution

In our previous studies with acidic Cu-contaminated Technosols, BC application led to a decrease in total Cu concentration by ~70%, while free Cu²⁺ species decreased to ~3.7% at the fourth month of incubation for some soils [9]. Biochar favored a rise in metal–organic complexes in soil solution with incubation time and BC addition.

The speciation modeling using Visual-Minteq v. 3.1 revealed some interesting trends in metal species distribution. The concentrations of major ions obtained from direct measurements were used to model metal speciation. The Stockholm humic model (SHM) was used for modeling DOC properties. As observed by Wu et. al. (2000) and Ren et al. (2015) [43,44], the free metal species Meⁿ⁺ were most mobile in surface and ground waters and also in the major species absorbed by plants and soil biota; therefore, the mobility and bioavailability of most heavy metals were better depicted by the free ion species than the total concentration (sum of free ion species and complexes).

The contributions of various species of metals differed significantly among the control variants and the BC-amended soils (

Table 4. (a) Species distribution in Site T1 (Fluvisol, Arenic) at the end of month 6 determined by geochemical modeling. The data used for modeling were the means of three replicates. (b) (Continued) species distribution in Site T2/Fluvisols, Loamic at the end of month 6 determined by geochemical modeling. (c) Species distribution in Site T3/Luvisols, Chromic at the end of month 6 determined by geochemical modeling.

(a)
T1 + 20% BC pH 7.5	% of Total Conc.	Species Name	Control T1 pH 7.2	% of Total Conc.	Species Name
Zn+2	9.331	Zn+2	Zn+2	12.991	Zn+2
	1.032	/FA-Zn+2G (aq)		1.067	/FA-Zn+2G (aq)
	0.254	ZnOH+		0.179	ZnOH+
	0.097	Zn(OH)2 (aq)		0.034	Zn(OH)2 (aq)
	0.014	ZnCl+		0.013	ZnCl+
	0.244	ZnSO4 (aq)		0.247	ZnSO4 (aq)
	0.122	ZnHPO4 (aq)		0.013	ZnNO3+
	0.183	ZnCO3 (aq)		0.102	ZnHPO4 (aq)
	0.071	ZnHCO3+		0.153	ZnCO3 (aq)
	40.983	/FAZn+ (aq)		0.119	ZnHCO3+
	47.66	/FA2Zn (aq)		45.45	/FAZn+ (aq)
Pb+2	0.054	Pb+2		39.634	/FA2Zn (aq)
	0.037	PbOH+	Pb+2	0.101	Pb+2
	0.062	PbCO3 (aq)		0.035	PbOH+
	0.01	PbHCO3+		0.07	PbCO3 (aq)
	1.462	/FAPb+ (aq)		0.023	PbHCO3+
	98.363	/FA2Pb (aq)		2.144	/FAPb+ (aq)
Mn+2	73.786	Mn+2		97.613	/FA2Pb (aq)
	0.051	MnOH+	Mn+2	79.014	Mn+2
	0.038	MnCl+		0.027	MnOH+
	1.57	MnSO4 (aq)		0.026	MnCl+
	0.04	MnNO3+		1.219	MnSO4 (aq)
	1.239	MnHPO4 (aq)		0.052	MnNO3+
	0.357	MnHCO3+		0.796	MnHPO4 (aq)
	1.261	MnCO3 (aq)		0.456	MnHCO3+
	8.16	/FA-Mn+2G (aq)		0.809	MnCO3 (aq)
	13.499	/FAMn+ (aq)		6.492	/FA-Mn+2G (aq)
Fe+3	99.99	/FA2FeOH (aq)		11.108	/FAMn+ (aq)
Cu+2	74.761	/FA2Cu (aq)	Fe+3	99.985	/FA2FeOH (aq)
	0.24	/FACu+ (aq)		0.013	/FA2Fe+ (aq)
	24.979	/FA2CuOH− (aq)	Cu+2	0.011	CuCO3 (aq)
Cd+2	15.952	Cd+2		85.098	/FA2Cu (aq)
	0.035	CdOH+		0.353	/FACu+ (aq)
	0.782	CdCl+		14.524	/FA2CuOH (aq)
	0.447	CdSO4 (aq)	Cd+2	21.428	Cd+2
	0.017	CdNO3+		0.023	CdOH+
	0.511	CdHPO4 (aq)		0.686	CdCl+
	0.123	CdHCO3+		0.436	CdSO4 (aq)
	0.127	CdCO3 (aq)		0.028	CdNO3+
	1.764	/FA-Cd+2G (aq)		0.412	CdHPO4 (aq)
	39.401	/FACd+ (aq)		0.196	CdHCO3+
	40.838	/FA2Cd (aq)		0.103	CdCO3 (aq)
Ba+2	86.526	Ba+2		1.761	/FA-Cd+2G (aq)
	0.041	BaCl+		42.159	/FACd+ (aq)
	1.396	BaSO4 (aq)		32.766	/FA2Cd (aq)
	0.15	BaNO3+	Ba+2	89.484	Ba+2
	0.09	BaHPO4 (aq)		0.028	BaCl+
	0.015	BaCO3 (aq)		1.047	BaSO4 (aq)
	0.2	BaHCO3+		0.184	BaNO3+
	9.569	/FA-Ba+2G (aq)		0.056	BaHPO4 (aq)
	1.993	/FABa+ (aq)		0.247	BaHCO3+
	0.02	/FA2Ba (aq)		7.353	/FA-Ba+2G (aq)
				1.584	/FABa+ (aq)
(b)
T2 + 20% BC pH 7.7	% of Total Conc.	Species Name	Control T2 pH 7.5	% of Total Conc.	Species Name
Zn+2	7.574	Zn+2	Zn+2	14.086	Zn+2
	1.037	/FA-Zn+2G (aq)		1.055	/FA-Zn+2G (aq)
	0.338	ZnOH+		0.394	ZnOH+
	0.206	Zn(OH)2 (aq)		0.151	Zn(OH)2 (aq)
	0.124	ZnSO4 (aq)		0.01	ZnCl+
	0.087	ZnHPO4 (aq)		0.256	ZnSO4 (aq)
	0.381	ZnCO3 (aq)		0.152	ZnHPO4 (aq)
	0.093	ZnHCO3+		0.305	ZnCO3 (aq)
	35.326	/FAZn+ (aq)		0.118	ZnHCO3+
	54.823	/FA2Zn (aq)		40.158	/FAZn+ (aq)
Pb+2	0.038	Pb+2		43.306	/FA2Zn (aq)
	0.043	PbOH+	Pb+2	0.087	Pb+2
	0.113	PbCO3 (aq)		0.061	PbOH+
	0.012	PbHCO3+		0.112	PbCO3 (aq)
	1.08	/FAPb+ (aq)		0.018	PbHCO3+
	98.706	/FA2Pb (aq)		1.55	/FAPb+ (aq)
Mn+2	71.171	Mn+2		98.159	/FA2Pb (aq)
	0.08	MnOH+	Mn+2	80.083	Mn+2
	0.016	MnCl+		0.056	MnOH+
	0.946	MnSO4 (aq)		0.02	MnCl+
	0.039	MnNO3+		1.185	MnSO4 (aq)
	1.052	MnHPO4 (aq)		0.026	MnNO3+
	0.551	MnHCO3+		1.112	MnHPO4 (aq)
	3.118	MnCO3 (aq)		0.424	MnHCO3+
	9.744	/FA-Mn+2G (aq)		1.513	MnCO3 (aq)
	13.284	/FAMn+ (aq)		5.996	/FA-Mn+2G (aq)
Fe+3	99.992	/FA2FeOH (aq)		9.585	/FAMn+ (aq)
Cu+2	0.016	CuCO3 (aq)	Fe+3	99.988	/FA2FeOH (aq)
	68.349	/FA2Cu (aq)	Cu+2	0.018	CuCO3 (aq)
	0.167	/FACu+ (aq)		75.323	/FA2Cu (aq)
	31.459	/FA2CuOH- (aq)		0.262	/FACu+ (aq)
Cd+2	13.347	Cd+2		24.38	/FA2CuOH- (aq)
	0.047	CdOH+	Cd+2	23.209	Cd+2
	0.292	CdCl+		0.052	CdOH+
	0.234	CdSO4 (aq)		0.547	CdCl+
	0.014	CdNO3+		0.453	CdSO4 (aq)
	0.376	CdHPO4 (aq)		0.015	CdNO3+
	0.164	CdHCO3+		0.614	CdHPO4 (aq)
	0.273	CdCO3 (aq)		0.195	CdHCO3+
	1.827	/FA-Cd+2G (aq)		0.205	CdCO3 (aq)
	35.006	/FACd+ (aq)		1.738	/FA-Cd+2G (aq)
	48.418	/FA2Cd (aq)		37.209	/FACd+ (aq)
Ba+2	84.904	Ba+2		35.762	/FA2Cd (aq)
	0.018	BaCl+	Ba+2	90.402	Ba+2
	0.856	BaSO4 (aq)		0.021	BaCl+
	0.146	BaNO3+		1.015	BaSO4 (aq)
	0.077	BaHPO4 (aq)		0.094	BaNO3+
	0.038	BaCO3 (aq)		0.077	BaHPO4 (aq)
	0.314	BaHCO3+		0.017	BaCO3 (aq)
	11.624	/FA-Ba+2G (aq)		0.229	BaHCO3+
	1.995	/FABa+ (aq)		6.769	/FA-Ba+2G (aq)
	0.027	/FA2Ba (aq)		1.362	/FABa+ (aq)
				0.013	/FA2Ba (aq)
(c)
T3 + 20% pH 6.85	% of Total Conc.	Species Name	Control T3 pH 6.4	% of Total Conc.	Species Name
Zn+2	7.685	Zn+2	Zn+2	21.908	Zn+2
	5.733	/FA-Zn+2G (aq)		1.662	/FA-Zn+2G (aq)
	0.049	ZnOH+		0.048	ZnOH+
	0.143	ZnSO4 (aq)		0.025	ZnCl+
	0.026	ZnCO3 (aq)		0.304	ZnSO4 (aq)
	0.045	ZnHCO3+		0.032	ZnNO3+
	79.581	/FAZn+ (aq)		0.014	ZnHPO4 (aq)
	6.721	/FA2Zn (aq)		61.609	/FAZn+ (aq)
	0.143	ZnSO4 (aq)		14.397	/FA2Zn (aq)
	0.026	ZnCO3 (aq)	Pb+2	0.5	Pb+2
	0.045	ZnHCO3+		0.038	/FA-Pb+2G (aq)
	79.581	/FAZn+ (aq)		0.028	PbOH+
	6.721	/FA2Zn (aq)		0.016	PbSO4 (aq)
Pb+2	0.195	Pb+2		8.686	/FAPb+ (aq)
	0.146	/FA-Pb+2G (aq)		90.721	/FA2Pb (aq)
	0.031	PbOH+	Mn+2	83.156	Mn+2
	0.039	PbCO3 (aq)		0.033	MnCl+
	0.029	PbHCO3+		0.938	MnSO4 (aq)
	14.154	/FAPb+ (aq)		0.077	MnNO3+
	85.395	/FA2Pb (aq)		0.066	MnHPO4 (aq)
Mn+2	43.685	Mn+2		6.307	/FA-Mn+2G (aq)
	0.659	MnSO4 (aq)		9.419	/FAMn+ (aq)
	0.031	MnNO3+	Fe+3	99.913	/FA2FeOH (aq)
	0.16	MnHCO3+		0.085	/FA2Fe+ (aq)
	0.129	MnCO3 (aq)	Cu+2	0.045	Cu+2
	32.588	/FA-Mn+2G (aq)		96.118	/FA2Cu (aq)
	22.731	/FAMn+ (aq)		1.37	/FACu+ (aq)
Fe+3	99.947	/FA2FeOH (aq)		2.459	/FA2CuOH− (aq)
	0.048	/FA2Fe+ (aq)	Cd+2	32.867	Cd+2
Cu+2	0.037	Cu+2		1.232	CdCl+
	0.013	CuCO3 (aq)		0.489	CdSO4 (aq)
	0.028	/FA-Cu+2G (aq)		0.06	CdNO3+
	91.562	/FA2Cu (aq)		0.05	CdHPO4 (aq)
	4.135	/FACu+ (aq)		2.493	/FA-Cd+2G (aq)
	4.216	/FA2CuOH− (aq)		51.976	/FACd+ (aq)
Cd+2	12.409	Cd+2		10.825	/FA2Cd (aq)
	0.268	CdCl+	Ba+2	90.743	Ba+2
	0.247	CdSO4 (aq)		0.033	BaCl+
	0.018	CdNO3+		0.777	BaSO4 (aq)
	0.072	CdHCO3+		0.265	BaNO3+
	0.017	CdCO3 (aq)		6.883	/FA-Ba+2G (aq)
	9.257	/FA-Cd+2G (aq)		1.294	/FABa+ (aq)
	72.265	/FACd+ (aq)
	5.439	/FA2Cd (aq)
Ba+2	54.726	Ba+2
	0.012	BaCl+
	0.626	BaSO4 (aq)
	0.124	BaNO3+
	0.096	BaHCO3+
	40.824	/FA-Ba+2G (aq)
	3.585	/FABa+ (aq)

). It can be outlined that the species of Cu, Zn and Pb complexed with soluble organic ligands (represented as fulvic acid complexed with FA-Me⁺ and FA₂Me) prevailed in the solutions of both the control and BC-amended soils but increased in the BC-amended soils for the metals Zn, Mn, Cd and Ba; at the same time, the share of free Meⁿ⁺ ions decreased. In the slightly alkaline soils of Sites T1 and T2, Stage 6 (soil pH increased by ~0.3 units to pH 7.5–7.7), the fraction of Cd²⁺, Pb²⁺ and Zn²⁺ decreased on average 1.3~2 times in the 20% BC variants compared with the fraction of these metals in the control variants (Table 4a,b). The metal Mn showed the slightest decrease in the free Mn²⁺ species content and the lowest increase in the organic complexation due to the lowest stability constant of Mn complexes with FA ligands and the lowest hydrolysis constant compared with the other studied metals [45]. At the two sites T1 and T2, the free Cu²⁺ ions were lacking and only organic complexes were present, with FA₂CuOH⁻ increasing at the 20% BC variants. FA₂Cu complexes also increased at the 20% BC addition and at the higher pH. Uchimiya et al. (2010) observed the mobilization of copper in alkaline soils, which they assigned to a rise and solubilization of organic fractions of biochar and, respectively, of dissolved organic matter [46].

For iron Fe(III), there was no sufficient effect of BC addition on the presence of organic species. For Ba²⁺, there was a slight increase in organic complexation and a decrease in free metal species at the alkaline sites, but the effect of BC was more pronounced at the 20%-added BC (Table 4). However, for Cd, Pb and Zn species, there was a 2-time decrease in the free species as a result of BC application, and those species were present at a very low share of the total dissolved metals.

Smebye et al. (2016) concluded that size exclusion in biochar’s micropores could result in a predominant sorption of smaller DOM molecules than larger aromatic ones, which could consequently increase the DOM leached from soil and increase the size and aromaticity of DOM [47].

In the slightly acidic to neutral soil T3 (pH 6.4–6.85), most of the Me²⁺ metals, i.e., Cd, Mn, Ba and Pb, decreased in the BC-amended soils ~2 to 3 times compared with the control soils, with a concomitant increase in organic complexes (Table 4c and Figure 4).

For Cu and Fe, there was no sufficient change in the concentrations, because these metals formed strong complexes with organic ligands at pH ≥ 7 [48,49,50].

In the study by Lee et al. (2011), it was found that, for less hydrated cations, fulvic acid could increase the sorption of divalent cations (Ba, Sr, Pb, Hg) on the muscovite basal surfaces in the order Ba ≈ Sr < Pb < Hg. According to Papadaki et al. (2023), both HA and FA could be effectively utilized as sorbent materials for metal removal from water samples [51,52].

4. Conclusions

The analysis of heavy metal water soluble pool in 1–20% BC (w/w)-amended contaminated non-acidic soils from the vicinity of a former steel metallurgical plant in southeastern Europe, and heavy metals species in soil solution, revealed that associations of elements with statistically significant positive correlations can be distinguished. The metals, Fe, Mn, Cu, Zn, Cd, Pb and Ba, exhibited significant relationships between each other and exceeded the maximum permissible concentrations (MPC) of metals in surface waters for domestic and drinking purposes. In most of the investigated sites and BC treatments, metal concentrations in soil solutions decreased with time due to the transfer to more resistant metal pools. The share of metal–organic complexes calculated by Visual-Minteq Ver. 3.1 increased for the metals Zn, Cd, Mn and Ba in the BC amended soils, and the share of free Me²⁺ species decreased. This trend was especially pronounced in the soil with the lowest pH of 6.4–6.9. Cation exchange capacity, exchangeable Ca and pH increased with BC application, while electrical conductivity decreased. Biochar amendment led to an increase in humic acids (HA) and the fraction of refractory organic carbon.

In addition to improving soil physicochemical and ecochemical characteristics, biochar application in non-acidic heavy-metal-contaminated soils contributed to the generation of metal species that were less mobile and bioavailable, especially for elements Zn, Cd and Ba. The optimal effect of BC treatment on soil characteristics was achieved at 20% of BC applied.