Washing Procedure with Several Reagents for Ecological Rehabilitation of Soil Polluted with Heavy Metals

Abstract

Soil contamination by heavy metals poses serious risks to human health and the environment. This study investigates the removal of Pb, Cu, Zn, Cd, and Cr from heavily contaminated slightly acidic to neutral soil (pH 6.5) using organic washing agents (humus, malic acid, and gluconic acid) at concentrations of 1% and 3% and a solid-to-liquid ratio (S/L) of 1:8. The results reveal that metal mobilization depends strongly on the type and concentration of the extraction agent, the target metal, and soil properties. Cd was highly mobilized, reaching more than 90% with 3% gluconic acid, whereas Cu and Pb remained largely immobile (<3%), and Cr (40–78%) and Zn (8–26%) showed intermediate extraction. This study establishes a clear hierarchy of metal mobility (Cd > Cr > Zn > Cu ≈ Pb) and demonstrates that metal speciation, soil chemistry, pH, and S/L ratio critically govern extraction efficiency. These findings provide mechanistic insights into metal–ligand interactions and practical guidance for optimizing soil remediation strategies using organic acids.

1. Introduction

Soil contamination with heavy metals is a global environmental problem, primarily caused by industrial activities, mining operations, improper waste management, and intensive agricultural practices [1,2,3]. Metals such as lead (Pb), cadmium (Cd), chromium (Cr), mercury (Hg), and arsenic (As), have a high tendency to accumulate in soil, where they can persist for long periods, affecting its physicochemical properties and entering the food chain, with toxic effects on ecosystems and human [4,5].

Various methods for remediating soils contaminated with heavy metals have been developed, including stabilization, phytoremediation, chemical immobilization, and soil washing. Among these, soil washing is recognized as an effective, rapid, and relatively inexpensive method capable of significantly reducing heavy metal concentrations by transferring them from the solid phase to the liquid phase [6,7,8,9,10,11,12]. The efficiency of this technology depends on the soil type, the nature and concentration of the metals, pH, contact time, solid/liquid ratio, and, essentially, the choice of washing agent [13].

An optimal washing agent must combine efficiency in mobilizing metals with biodegradability, low toxicity, and minimal environmental impact [1,14,15]. Humic substances, malic acid, and gluconic acid have been identified in the literature as promising, naturally occurring, and environmentally friendly agents [1,14,15,16]. Unlike strong mineral acids (e.g., HCl), which can cause secondary contamination and alter soil properties [13,16], these organic compounds are biodegradable, non-toxic, and sustainable. They also represent distinct categories of biodegradable organics with complementary chemical characteristics and pH-dependent complexation behaviors, allowing the study of different mechanisms of metal mobilization under safe conditions. Compared to synthetic chelating agents such as EDTA or NTA, which may similarly affect soil properties, these agents offer a low-toxicity alternative [16,17,18,19].

Table 1. Summary of heavy metal removal efficiencies with various biodegradable washing agents.

Washing Agent	Reported Removal Efficiency (%)	Optimal pH Range	Observations/Remarks	References
Humic substances	Zn, Pb: 15% Cu, Cd, Cr: 20–40%	6–8	high affinity for Pb2+ and Cu2+; adsorption capacity increases with pH	[7,10,16,20,21,29,35,36,37]
Malic acid	Zn, Pb, Cr: 50–88% Cd: 1.9–60% Cu: 5%	3–7	efficient, biodegradable; pH strongly affects metal mobility; low cost	[1,14,23,24,25,26,27,28,29,33,38,39,40,41]
Gluconic acid	Pb, Cu: 62–80% Zn, Cd, Cr: 34–60%	7–9	non-toxic polyhydroxycarboxylic acid; effective at alkaline pH	[1,29,30,31,32,33]

provides a comparative summary of the reported efficiencies and working conditions for these agents in the literature. Humic substances exhibit high affinity for Pb²⁺ and Cu²⁺, with adsorption increasing with pH [7,20,21,22]. Malic acid effectively forms soluble complexes with metals, with efficiency influenced by solution pH, especially for Cr and Cu [1,14,23,24,25,26,27,28]. Gluconic acid, a non-toxic polyhydroxycarboxylic acid, shows high metal mobilization capacity under alkaline conditions [1,29,30,31,32,33]. Literature also indicates that combinations of organic acids (e.g., citric and malic acid, 4:1) can achieve Pb removal rates up to 88.65% [14], and that soil particle size significantly affects washing efficiency (e.g., Cd: 40–70% with citric acid) [34].

As shown in Table 1, humic substances, malic acid, and gluconic acid exhibit variable efficiencies depending on metal type and pH, which supports their selection for the present study.

Based on these observations, we selected these agents for a systematic comparison of their efficiency in washing soils contaminated with Pb, Cu, Zn, Cd, and Cr. We analyze the influence of solid/liquid ratio (S/L = 1:8), extraction solution concentration (1% and 3%), and agitation time (2–8 h) on metal mobilization, taking into account metal speciation and soil pH. This comprehensive approach provides mechanistic insights into metal–ligand interactions, highlights differences between organic agents, and contributes to the development of sustainable and efficient soil remediation strategies. The scientific contribution of this study lies in the fact that, although the experimental part is mainly based on FAAS spectroscopy, the results obtained have real relevance and applicability for specialists in related fields—such as environmental engineering, agriculture, or decontamination processes—and can be easily integrated into the basic documentation of future research.

2. Materials and Methods

2.1. Soil Sampling and Analyzing

The soil sample was collected from an area recognized as a polluted industrial area, located in Baia Mare, Romania (47°41′03″ N, 23°37′26″ E), where industrial activity ceased in 2012. The sampling site corresponds to a former non-ferrous metallurgical platform historically affected by emissions and deposition of heavy metals such as Pb, Cd, Cu, and Zn, resulting from ore processing and smelting activities [42,43]. This area was selected because it represents a typical example of long-term heavy metal contamination in post-industrial soils, allowing the evaluation of the washing efficiency of different organic agents under realistic pollution conditions. The sampling was carried out in accordance with the Romanian standard STAS 7184/1-84 [44]. The sample was subsequently processed according to ISO 11464:1998 [45] and analyzed for soil nutrient content, pH and concentration of the main polluting metals (Pb, Cu, Zn, Cd, Cr). The soil samples were prepared by drying to a constant weight at 105 °C using a Binder oven (Binder GmbH, Tuttlingen, Germany) and then manually ground using a porcelain mortar and pestle until a fine, homogeneous powder was obtained. This manual grinding was performed exclusively to obtain a homogeneous particle size distribution and to ensure the reproducibility of subsequent analyses. The obtained material was passed through a 2 mm sieve, and only this fraction was used in all experiments. Within this homogenized fraction, finer particles (clay and silt) tend to bind metals more strongly, affecting extraction efficiency [46,47]. Subsequently, to analyze the possibility of extracting polluting metals by washing with one of the analyzed solutions, the soil samples prepared in this way were immersed in one of the 6 extraction solutions based on humus, malic acid, or gluconic acid, as presented in Section 2.2 and Section 2.3, resulting in one liquid soil extract for each extraction solution. All parameters including nitrogen (N), phosphorus (P), potassium (K), pH, organic matter, and salinity were determined using the same multiparameter analyzer (Biobase BK-YW-6A, Biobase Biodustry Co., Ltd., Jinan, China; repeatability precision ≤ 0.05%, drift < 0.1%). The instrument combines an electrochemical pH sensor with optical colorimetric detection modules for N, P, and K analysis. The photometric detection operates within a wavelength range of 400–700 nm, with a measurement precision of ±0.01 absorbance units. pH measurement was performed using the built-in glass electrode with a range of 0–14 pH and a resolution of 0.01. The reported values of N, P, and K correspond to the mobile (available) fraction of the initial soil sample, obtained through extraction in water. For the analysis, soil samples were first prepared by drying and grinding, followed by dissolution in distilled water or an extracting solution. Calibration of the pH analyzer was performed using standard buffer solutions (Biobase PH-30S, Biobase, Biodustry Co., Ltd., Jinan, China). The Biobase analyzer employs two independent measurement principles: an electrochemical sensor for pH determination and colorimetric modules for N, P, and K. Therefore, only nutrient concentrations were obtained colorimetrically, while pH was determined electrochemically. The detection of N, P, and K was performed through colorimetric (photometric) analysis. Specific reagents were added to the soil extract, triggering a color reaction. The Biobase Soil Nutrient Analyzer operates on a colorimetric principle, in which specific reagents are added to soil extracts to produce characteristic color reactions. The built-in optical sensor then measures light absorption at pre-defined wavelengths—620 nm for nitrogen and 440 nm for phosphorus. By quantifying the intensity of these color changes, the device accurately determines the concentrations of nitrogen, phosphorus, and other soil nutrients. Additionally, spectrophotometric UV–Vis analysis was conducted using a Jenway 7305 UV/Vis Spectrophotometer (Bibby Scientific Ltd., Staffordshire, UK). This single-beam benchtop instrument operates across a wavelength range of 198 to 1000 nm, with a spectral bandwidth of 5 nm, and provides a resolution of 1 nm and wavelength accuracy of ±2 nm. Its photometric capabilities include absorbance measurements from −0.300 to 2.500 A and transmittance from 0 to 199.9% T, with photometric accuracy of ±1% T or ±0.01 A at 1.000 Abs. The instrument was calibrated with a blank solution to establish a baseline for precise absorbance measurements. Each sample was placed in the spectrophotometer, where absorbance values were recorded at the selected wavelengths. A full spectrum scan was performed to identify the maximum absorption peak, which was subsequently used for quantitative analysis. UV–Vis spectroscopy was used to complement flame atomic absorption spectrometry (FAAS) measurements by characterizing the chemical environment and complexation behavior of metals in the soil extract. While FAAS quantified total metal concentrations, UV–Vis provided qualitative evidence of metal–organic interactions (e.g., charge-transfer and ligand-to-metal transitions) that influence metal speciation and mobility in soil.

Fourier Transform Infrared (ATR-FTIR) spectra of soil samples were recorded using a Bruker Tensor 27 spectrophotometer equipped with an ATR accessory (Bruker Optik GmbH, Ettlingen, Germany). Spectra were collected in the range of 4000–500 cm⁻¹ from ground samples at a resolution of 4 cm⁻¹, averaging 32 scans per sample. The concentration of metals in the soil was determined by FAAS using a SHIMADZU AA-6800 spectrometer (Shimadzu, Tokyo, Japan), equipped with an air–acetylene flame. From the homogeneous sample of dry soil, ground and sieved thru a 100 μm mesh sieve, as indicated at the beginning of Section 2.1, 3 g of soil were weighed, to which 21 mL of HCl (35–38% p.a., Merck, Darmstadt, Germany) and 7 mL of HNO₃ (65% p.a., Merck, Darmstadt, Germany) were added, and mineralization was carried out for 3 h. The resulting supernatant was filtered through a 0.45 μm pore size filter (Roth, Karlsruhe, Germany), diluted to 100 mL with distilled water, and then subjected to heavy metal analysis by FAAS. The analysis was performed in the laboratory under constant temperature (T = 26 ± 1 °C), relative humidity (RH = 63 ± 2%), and ventilation conditions ensuring repeatability. The reported results represent the average of three measurements. Following the measurements, the concentrations of metals (Cd, Cr, Cu, Ni, Zn, and Pb) in the soil samples were compared with the permissible values from Order 756/1997 issued by the Ministry of Waters, Forests, and Environmental Protection of Romania, for soils with sensitive use (Figure 1) [48].

2.2. Preparation of Extraction Solutions

Three extraction solutions were used in the experiment: (1) humic acid, obtained by alkaline extraction from German leonardite available commercially under the name Powhumus WSG-85, supplied by Humintech GmbH (Grevenbroich, Germany); (2) malic acid, supplied by Penta Chemical (Wuchterlova, Czech Republic); (3) gluconic acid, sup-plied by Alfa Aesar GmbH (Karlsruhe, Germany).

The quantity of 10 g and 30 g of each substance, respectively, were weighed using an electronic balance (Kern, Balingen, Germany), to which 100 mL of distilled water was added. This resulted in two extraction solutions with concentrations of 1% and 3%, respectively. These solutions were used as extraction agents for the remediation of polluted soils. The concentrations of 1% and 3% were selected based on preliminary tests and literature data indicating that this range provides effective metal desorption without causing excessive dissolution of soil constituents or instability of the organic acids [30,40,49]. These concentrations are commonly used for evaluating the comparative extraction efficiency of organic complexing agents in soil remediation studies.

2.3. Metal Extraction

To extract metals (Pb, Cu, Zn, Cd, Cr) from polluted soil, a series of experiments were conducted using the extraction solutions presented in Section 2.2. Experiments on the possibility of heavy metal extraction were conducted at the laboratory scale, under constant temperature (T = 27 ± 1 °C), actual air humidity (RH = 65 ± 2%), and ventilation conditions, without forced ventilation of the ambient air, ensuring repeatability conditions, with the reported results representing the average of three measurements.

In the experiment, the following solutions were used: 1% humic solution, 3% humic solution, 1% malic acid solution, 3% malic acid solution, 1% gluconic acid solution, and 3% gluconic acid solution. In 250 mL Erlenmeyer flasks, 5 g of soil were weighed and 40 mL of the extraction solution was added, depending on the washing agent contained (humus, malic acid, gluconic acid) and its concentration (1% and 3%), resulting in a solid/liquid ratio (S/L) of 1:8. The solid-to-liquid ratio of 1:8 was selected based on preliminary experiments and previous work in which a 1:4 ratio was used [49]. Increasing the liquid volume was intended to improve contact between soil particles and the washing solution, enhance metal desorption, and align with ratios commonly reported in the literature [30,40]. The samples prepared in this way were stirred in a continuous orbital rotation-oscillation stirrer VDRL 711 CT (ASAL S.R.L., Cernusco sul Naviglio, Milan, Italy) at 200 oscillations/min for 2 h, 4 h, 6 h, or 8 h. Subsequently, the soil was separated from the extraction solution by filtration, and the leachate was analyzed by atomic absorption spectrometry to determine the metal concentrations (Pb, Cu, Zn, Cd, Cr). These metal concentrations, determined in the leachate, represent the extraction capacity of the solution used, under the specific conditions of concentration and contact time for each situation, from the total metal content available in the soil sample subjected to treatment. To ensure repeatability and reproducibility, the results were recorded as the average of three consecutive measurements.

Based on the recorded experimental results, the extraction yield of heavy metals was determined at the laboratory scale as a function of the washing solution concentration and agitation time. The extraction yield was calculated for the maximum agitation and contact time of 8 h as the mass percentage of metal extracted relative to the total metal content in the soil, previously determined by FAAS [49], using the following formula [50]:

$$\mathrm{Extraction}\mathrm{efficiency}(\%)={\displaystyle \frac{C_{\mathrm{extracted}}}{C_{\mathrm{initial}}}}\times 100$$

(1)

where

C_extracted is the metal concentration measured in the extraction solution (mg/L);

C_initial is the initial metal concentration of soil (mg/L).

3. Results

3.1. Soil Characterization

The soil analysis revealed a pH of 6.5 (

Table 2. Physicochemical characteristics of the soil sample.

pH [-]	Ammonium Nitrogen N [ppm]	Potassium K [ppm]	Phosphorus P [ppm]	Humus [%]	Organic Carbon [%]	Electrical Conductivity [μs/cm]	Salinity [PSU]
6.5 ± 0.1	14.87 ± 0.8	14.45 ± 0.7	23.06 ± 1.0	2.5 ± 0.2	1.7 ± 0.1	1100 ± 50	2.1 ± 0.1

Note: Values refer to the mobile (available) fraction of the initial soil sample.

), indicating a slightly acidic condition that is favorable for nutrient availability and suitable for most crops. N (14.87 ppm) and P (23.06 ppm) were found at moderate levels, sufficient to support healthy growth, though long-term cultivation may require additional nitrogen inputs. K content (14.45 ppm), however, was relatively low, suggesting the need for supplementation to meet crop demands. The soil contained 2.5% humus and 1.7% organic carbon, reflecting a moderate organic matter status that contributes to nutrient cycling and soil structure. The electrical conductivity of the soil extract was measured at 1100 μS/cm, corresponding to a salinity of approximately 2.1 PSU, which classifies the soil as a moderate saline soil. At this level, many crops may experience reduced growth and yield due to osmotic stress, restricted water uptake, and potential nutrient imbalances. Overall, while the soil shows moderate fertility, the elevated salinity poses a significant constraint for crop production [51,52]. All analyses were performed in triplicate, and the results presented in Table 2 are expressed as mean ± standard deviation (SD).

It is important to note that the soil investigated in this study corresponds to a single textural class (fraction < 2 mm), with a slightly acidic pH (6.5) and moderate humus content. These characteristics define a specific physicochemical environment that strongly influences the mobility and binding of metals. Therefore, the extraction efficiencies reported here should be interpreted as representative of soils with similar composition and texture, rather than universally applicable to all soil types. Soils with higher clay content, alkaline pH, or different organic matter levels could exhibit markedly different metal–ligand interactions and extraction behaviors. This limitation is acknowledged and discussed to delineate the scope of the present findings.

Figure 2 presents the FTIR spectrum of the soil extract, with the UV–VIS spectrum shown in the inset for comparison. The FTIR spectrum displays characteristic absorption bands associated with both organic matter and mineral components of the soil. The broad vibration mode around 3400 cm⁻¹ corresponds to O–H stretching from hydroxyl and carboxylic groups, while weak signals near 2920 cm⁻¹ are attributed to aliphatic C–H stretching. The band at approximately 1630 cm⁻¹ is assigned to C=O stretching vibrations of carboxylate groups, indicating the presence of oxygenated organic functional groups capable of metal binding [53]. The absorption near 1380 cm⁻¹, attributed to C–H bending vibrations of aliphatic structures. The intense absorption observed in the 1000–500 m⁻¹ region indicates overlapping contributions from M–O bond vibrations, consistent with the formation of metal–organic complexes, as well as characteristic Si–O stretching (quartz and silicate minerals), Al–O vibrations (clay minerals), and CO₃²⁻ asymmetric stretching (calcite) [54]. Together, these bands confirm the coexistence of organic matter, mineral components, and metal–organic interactions within the soil extract matrix.

The UV–VIS spectrum (inset) exhibits strong absorbance below 300 nm, characteristic of dissolved organic matter and metal–ligand charge-transfer transitions, particularly involving Cu–organic species [55]. Absorbance decreases sharply above 300 nm, indicating that the main chromophores are active only in the UV region and that colored inorganic phases such as Fe oxides are scarce. It is important to note that the UV–VIS data were used only for qualitative interpretation of electronic transitions and complexation behavior. Together, the FTIR and UV–VIS results confirm that the heavy metals in the soil extract are predominantly associated with organic ligands, forming stable complexes within the dissolved organic matrix.

Spectrophotometric analysis revealed severe contamination, with Pb and Cu exceeding the intervention thresholds for sensitive soils by almost 28 times for Pb and Cu content, and by approximately 3.7 times and 2.3 times for Zn and Cd content, respectively (Figure 3). These values have the immediate consequence of the need to identify and apply rapid intervention treatments for soil decontamination. The Cr concentration does not exceed the corresponding intervention threshold, but given its high toxicity, it was considered useful to analyze the possibilities of extraction from the soil even in this situation.

3.2. Metal Extraction from Soil

The influence of the extraction medium and its concentration over time on the extraction of Pb, Cu, Zn, and Cd is shown in Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8. All FAAS measurements were performed in triplicate, and the results are expressed as mean ± standard deviation (n = 3). Error bars indicate the standard deviation, reflecting data variability and measurement reproducibility.

Before analyzing the effect of the different extraction solutions, it is important to note that the extraction of metals using water alone (i.e., without any leaching agent) was evaluated in our previous experiments to assess the influence of the soil’s natural pH (6.5) on metal mobility. The results indicated that Pb and Cu exhibited extremely low mobilization (<1%), Zn and Cr showed limited mobilization (~1.5% and <3%, respectively), while Cd was slightly more mobile (~11%), although still considerably lower than the yields obtained with organic acids [49]. These findings confirm that at slightly acidic to neutral pH, the majority of metals remain largely fixed in the soil matrix, and that natural pH alone is insufficient to promote significant metal leaching, particularly for Pb and Cu. This observation aligns with literature reports, which indicate that metal solubility generally increases under acidic conditions, whereas slightly acidic to neutral soils favor stabilization within the solid matrix [46,47]. In contrast, when organic leaching agents were introduced, metal mobilization increased significantly, confirming that complexation and desorption mechanisms induced by these agents effectively overcome the retention capacity of the soil matrix.

3.2.1. Individual Metal Extraction

Pb extraction as a function of the solution used and stirring time (Figure 4) highlights that the malic acid extraction solution yields the highest concentrations of extracted Pb. The humus solution shows the lowest Pb concentrations across all time intervals, indicating reduced solubilization. There are no significant differences between the 1% and 3% concentrations, suggesting that a higher humus concentration does not improve Pb extraction.

In the case of malic acid, it is observed that Pb is extracted from the very first hours of the experiment, both in the variant using a 1% concentration solution and a 3% concentration solution, but at a concentration of 3%, after 6 and 8 h of agitation, the Pb concentration decreases slightly. Also, 1% malic acid has a lesser effect than the 3% version, but it still solubilizes more Pb than humus. For 3% malic acid, the largest amount of Pb is extracted in the first 4 h, reaching a peak at 4 h a Pb concentration of ~26 mg/L is found in the extraction solution.

A 3% gluconic acid solution has a significant effect on Pb solubilization after 6 h, with a Pb concentration of ~11 mg/L in the extraction solution, indicating a slower but effective extraction process. At 1% concentration, gluconic acid has a weaker effect than 3% during the first 2–4 h, but it still solubilizes more Pb than the humus solution.

In the case of 1% gluconic acid, the extraction increases linearly with the duration of sample stirring. However, for 3% gluconic acid, no extraction is observed in the first 4 h, followed by an upward increase at 6 h a Pb concentration of ~11.05 mg/L is found in the extraction solution, after which it decreases at 8 h (~9.99 mg/L).

Regarding the extraction of Pb with the three solutions analyzed, humus does not favor its solubilization, maintaining low concentrations throughout the experiment. 3% malic acid proves to be the most efficient extraction agent, reaching a maximum peak at 4 h. On the other hand, gluconic acid acts more gradually but consistently, with significant extraction after 6 h.

Figure 5 shows the evolution of the Zn extraction process depending on the specific extraction solution (active substance, concentration, exposure time). In the case of humus treatment, the 1% extraction solution shows a low initial concentration of extracted Zn, around 4 mg/L at 2 h, which increases to 8 mg/L at 4 h, then decreases at 6 h and remains relatively constant at 8 h. The 3% variant has reduced solubilization in the first few hours, but after 6 and 8 h, the concentration increases, reaching almost 6 mg/L. 3% malic acid proves to be the most effective in solubilizing Zn throughout the experiment, maintaining high values of approximately 16–17 mg/L. 1% malic acid has a similar effect, but slightly less than a 3% malic acid solution. These results suggest that malic acid has a strong impact on Zn solubilization, acting quickly and stably.

Regarding gluconic acid, the 3% solution has a weak effect in the first few hours, but after 6 and 8 h, Zn solubilization increases significantly, reaching values of approximately 14–15 mg/L. The 1% variant shows an intermediate effect, with a significant increase at 6 and 8 h. It can be observed that gluconic acid acts more slowly than malic acid, but in the long term, their effects become comparable.

In conclusion, 3% malic acid is the most efficient Zn solubilizing agent, ensuring rapid and consistent extraction. Gluconic acid has a progressive effect, with a significant increase in solubilization after 6–8 h.

Cu solubilization (Figure 6) varies less depending on the characteristics of the extraction solutions used, indicating lower mobility of this metal in the presence of the substances used.

When using humus as an extraction solution at a concentration of 1% treatment, the extracted Cu concentration remained almost constant throughout the experiment, around 3.4 mg/L. This stability suggests that the 1% humic solution does not favor Cu solubilization, but rather fixes it, preventing its mobilization into the solution. On the other hand, the humus-based extraction solution with a concentration of 3% initially shows a higher Cu retention, with very low values at 2 and 4 h, but after 6 and 8 h, the metal concentration in the extraction solution increases significantly, reaching approximately 3.3 mg/L at the end of the experiment. This trend indicates that humus with a concentration of 3% has a stronger adsorption effect in the first few hours, but over time, the Cu becomes more available for transfer into solution.

Regarding malic acid, a 1% solution has a moderate effect on Cu solubilization, with values close to those recorded for 1% humus. A slight increase at 6 and 8 h suggests progressive mobilization. On the other hand, 3% malic acid is the most effective in solubilizing Cu, reaching a maximum extracted metal concentration of approximately 4.1 mg/L at 8 h. These results show that 3% malic acid can significantly increase Cu mobility in solution, having a clear impact on its solubilization. It is the most effective in solubilizing Cu, and its effect increases with mixing time.

1% gluconic acid has a moderate but progressive effect, with a constant increase in Cu concentration over time. The 3% variant has a similar effect to 1% malic acid, showing a gradual increase and reaching a solubilization level comparable to the other treatments, with the concentration of extracted metal (Cu) in the solution being approximately 3.6 mg/L at 8 h. These data suggest that gluconic acid acts more slowly, but over time, it can have a significant impact on Cu mobility. In the first 4 h, no extraction is observed when using the extraction solution with gluconic acid, but this is followed by an upward increase at 6–8 h (~3.5 mg/L). Therefore, it can be estimated that gluconic acid gradually increases Cu solubility, with a more pronounced effect observed after 6 h. After 8 h of stirring, all three solutions used, regardless of their concentration, achieve maximum Cu extraction around a value of 3.5 mg/L. Therefore, it can be concluded that the extraction behavior of Cu is less influenced by the type of extraction solution used when compared to Pb and Zn. Unlike these metals, whose concentrations vary significantly between solutions, Cu exhibits smaller fluctuations, indicating a greater affinity for the solid phase and higher resistance to solubilization. 3% malic acid remains the most effective in mobilizing Cu, reaching the highest concentration in solution at 8 h, while 3% gluconic acid has a similar but less pronounced effect. Humus, on the other hand, plays a stabilizing role, maintaining constant concentrations in the 1% variant and initially retaining Cu in the 3% concentration, before allowing for a slight solubilization at the end of the experiment.

In the case of humus treatment, the concentration of solubilized Cd (Figure 7) varies depending on the concentration of the extraction solutions and time. In the case of the 1% humus extraction solution, the initial values (~0.18 mg/L at 2 h) increase to ~0.22 mg/L at 4 h, after which they decrease at 6 h. The action of the 3% humus extraction solution causes solubilization to increase gradually throughout the measurement period, reaching almost 0.20 mg/L at 8 h. In the case of malic acid, the extracted Cd concentrations were slightly higher compared to those obtained in the presence of humus. The 1% malic acid extraction solution showed more efficient Cd solubilization than the 3% concentration, with values increasing over stirring time. In the case of gluconic acid, the 3% extraction solution variant results in high solubilization from the first few hours (~0.28 mg/L), remaining constant throughout the experiment, with an increase observed at 8 h (0.42 mg/L). The 1% gluconic acid extraction solution variant shows a constant effect in the first few hours, but registers a notable increase at 8 h, reaching approximately 0.35 mg/L. The results obtained indicate that gluconic acid (3%) represents a favorable situation because it shows a gradual increase in Cd concentration, reaching maximum values after 8 h. Humus and malic acid, regardless of concentration, exhibit a low solubilization capacity, with no significant improvements observed throughout the experiment.

In the case of Cr (Figure 8), in humus treatments, the 1% extraction solution concentration variant maintains relatively constant values throughout the period (3.4 mg/L at 2 h and ~3.2 mg/L at 8 h), while the 3% concentration variant shows a slight increase, reaching ~3.4 mg/L at the end of the test. Malic acid shows the lowest levels of solubilized Cr, with values close to 3.2 mg/L at 6 h for the 3% concentration solution, and the 1% variant follows a similar profile, but with slightly lower values. In contrast, gluconic acid (3%) determines a high extraction from the first hours (~2.5–3 mg/L), and the 1% gluconic acid variant records a notable increase at 4 h (~3 mg/L). General trends show that humus, especially in the 1% concentration variant, provides the most stable and efficient solubilization of Cr, with gluconic acid showing higher values, therefore being more efficient than malic acid.

Although no statistical analysis was included, the observed trends provide valuable qualitative insights that will be further explored in future studies.

A comparative analysis of metal extraction process yields (Figure 9) highlights marked differences in metal behavior depending on the type and concentration of leaching agents. Cd showed the highest yields for all treatments, exceeding 50% even at lower concentrations of the washing agents, and reaching a maximum of over 90% with 3% gluconic acid. This indicates high mobility and weak binding within the sample matrix. Cr ranks second in extraction efficiency, with values ranging from ~40% to 78%. Humus, especially at 1% and 3%, promotes the mobilization of Cr, suggesting an effective interaction between the organic matter and this metal. Zn shows moderate yields (8–26%), with the best results obtained using 3% malic acid and 3% gluconic acid washing, suggesting a higher sensitivity to organic acids compared to humus. This shows that Zn is more easily mobilized than Pb and Cu, especially by organic acids. Cu and Pb have the lowest yields (<3%), regardless of the leaching agents used. This indicates low mobility, likely due to the formation of stable compounds within the solid matrix or a high affinity for the residual mineral fraction. Although Pb and Cu extraction yields were below 3%, such low values indicate a strong association of these metals with the soil matrix, reflecting their low mobility. While these levels are typically considered negligible in terms of remediation efficiency, they remain relevant for understanding metal speciation and comparing the relative performance of different washing agents. This behavior highlights the resistance of Pb and Cu to mobilization under the tested conditions and underscores the need for tailored remediation strategies targeting these metals [56,57,58,59].

Overall, the general order of metal mobility, according to the yields obtained, is: Cd > Cr > Zn > Cu ≈ Pb. Cd and Cr exhibited the highest mobilization, particularly in the presence of 3% gluconic acid, indicating weak binding within the soil matrix. Zn showed moderate extraction, with malic acid (3%) achieving the highest yields, while Pb and Cu remained the least mobile, reflecting their strong association with soil components. Humus generally stabilized metals, resulting in limited solubilization across all concentrations.

The differences in extraction efficiency among metals can be largely attributed to their speciation within the soil matrix. Lead and copper are predominantly associated with stable mineral or organic complexes, such as carbonates and humic–metal bonds, which limit their solubilization even in the presence of organic acids [46,59,60,61]. In contrast, cadmium and zinc are more often present in exchangeable or carbonate-bound forms, which are more readily mobilized under slightly acidic conditions. The FTIR and UV–VIS spectra support the presence of metal–organic complexes in the soil extract, confirming that metals such as Pb and Cu exhibit strong binding to functional groups (C=O, COO–), resulting in their low mobility. These observations highlight the critical role of metal speciation in determining the extraction performance and the need for tailored leaching strategies depending on the predominant metal fraction.

To facilitate a clearer comparison of the metals’ mobility under different conditions, the key trends are summarized in

Table 3. Comparison of the obtained results with the values reported by other authors.

Metal	Washing Agent	Observations	Optimal Time	Maximum Yield (%)	Yield (%) Other Authors
Pb	Humus	Lowest extraction; reduced solubilization	n/a	0.1%	19% [29]
	Malic acid	Rapid extraction; 3% max ~26 mg/L at 4 h	4 h	1.4%	50–89% [14,29]
	Gluconic acid	Slower extraction; 3% significant after 6 h (~11 mg/L)	6–8 h	0.3–1.2%	30% [29] 66–80% [33]
Cu	Humus	1% constant ~3.4 mg/L; 3% slightly higher after 6–8 h	8 h	2%	20–40% [9,10,16] 15% [35] 5–28% [36]
	Malic acid	3% most effective ~4.1 mg/L; Progressive mobilization	8 h	2.45%	3–5% [49]
	Gluconic acid	Gradual increase; 6–8 h ~3.5–3.6 mg/L	8 h	2%	62–80% [33]
Zn	Humus	1%: 4 → 8 mg/L; 3%: slower, max ~6 mg/L	8 h	8.3–11.4%	13% [35]
	Malic acid	Most efficient; stable ~16–17 mg/L	8 h	24–26%	60% [33]
	Gluconic acid	Increase after 6–8 h; 3% max ~14–15 mg/L	8 h	20–24%	45% [33]
Cd	Humus	Low mobility; ≤0.22 mg/L	8 h	54–60%	25% [10] 20–70% [36] 35–75% [62]
	Malic acid	Moderate extraction; 1% > 3%	8 h	51–77%	60% [33] 1.9–60% [40,41]
	Gluconic acid	Most favorable; 3% max 0.42 mg/L	8 h	63–91%	34% [29] 48–63% [33]
Cr	Humus	Stable ~3.2–3.4 mg/L	8 h	75–78%	23–31% [37]
	Malic acid	Lowest values (~3.2 mg/L)	6 h	40–48%	70% [63]
	Gluconic acid	High initial extraction (~2.5–3 mg/L)	8 h	61–63%	39–60% [33]

. This table allows for an immediate visual comparison of the extraction efficiency of humic substances, malic acid, and gluconic acid, and highlights the approximate time of maximum extraction. As shown in Table 3, malic acid (especially at 3%) achieves the fastest and highest extraction for Pb, Zn, and Cu, whereas gluconic acid provides slower but steady mobilization, particularly for Cd and Cr. Humus generally stabilizes metals, showing limited solubilization throughout the experiment.

The results summarized in Table 3 provide a useful reference for comparing our findings with previous studies. In this study, the application of malic acid resulted in a Pb extraction yield of 1.4%, significantly lower than the value reported by Han et al. (2021), who achieved 19% in an experiment on the use of organic acids for mobilizing heavy metals from contaminated soil [29]. These differences can be attributed to variations in soil physicochemical properties (pH, cation exchange capacity, organic matter content, mineralogical type), Pb speciation, and extraction parameters (solid–liquid ratio, malic acid concentration, contact time). Additional factors, such as soil particle size, the presence of competing metal ions, or slight variations in temperature and agitation, may also have contributed to the lower extraction yield. In contrast, the soils investigated by Han et al. (2021), might have contained more mobile fractions of Pb, susceptible to complexation and solubilization by organic acids [29]. This contrast highlights the need to adapt washing methods based on the specific characteristics of the soil and the speciation of the targeted metal to maximize the process’s efficiency.

When using gluconic acid, the percentage of Pb extracted ranged between 0.3% and 1.2%, considerably lower than the 30% [29] and 80% [33] reported in the literature. Conversely, Cd extraction (63–91%) exceeded previous reports (34%) [26] and aligned with Fischer and Bipp (2002) [33].

The results of this study confirm observations from the specialized literature regarding the influence of pH on the efficiency of humic substances in the process of washing soils contaminated with heavy metals. The soil analyzed in this study had a slightly acidic pH of 6.5, which is within the range favorable for the mobilization of heavy metals by humic substances, malic acid, and gluconic acid. This pH directly influenced the experimental design, guiding the selection of extraction solution concentrations, solid-to-liquid ratios, and agitation times to maximize metal removal efficiency. Cd removal with humus (54–60%) exceeded values reported at pH 3 (20–40%) [36] and was comparable to pH 5 results (36–69%), closer to the soil pH here (6.4). These findings support literature showing that soil washing efficiency depends on pH [9,16,64,65] and agitation time [15,66], with other studies reporting similar efficiencies of 25% [10] and 35–75% [62]. Therefore, the slightly acidic pH of the soil analyzed here was a key factor in shaping the experimental conditions and interpreting the extraction yields obtained.

Regarding Cu, humic substances yielded only ~2%, lower than literature values (5–40%) [10,16,36], likely due to specific soil mineralogy, Cu-organic interactions, and experimental conditions.

3.2.2. Transport Phenomena and Extraction Mechanisms

To better understand the observed differences in extraction efficiency among metals, the underlying transport phenomena and mechanisms governing metal mobilization in the presence of leaching agents are discussed below. The improved metal removal observed in the presence of organic leaching agents can be explained by coupled physicochemical transport phenomena occurring at the soil–solution interface. Initially, metal ions adsorbed onto mineral and organic components undergo desorption and diffusion through the liquid film surrounding soil particles. The introduction of organic acids (humic, malic, and gluconic) enhances these processes by (i) decreasing pH, which promotes partial dissolution of metal-bearing minerals and oxides, and (ii) forming soluble metal–ligand complexes that shift the equilibrium toward the aqueous phase [46,47,67]. Humic acid, containing multiple carboxyl and phenolic groups, exhibits strong chelating capacity, explaining the higher removal of Pb and Cu, while the smaller organic acids facilitate faster diffusion and improved mobilization of Cd and Zn [68,69]. Therefore, the combined effects of acid dissolution, ion exchange, complexation, and diffusion control the overall transport of metals from the solid matrix to the liquid phase, justifying the improved extraction efficiency observed in the presence of leaching agents.

3.2.3. Chemical Species and pH Influence

The efficiency of metal removal using organic acids and humic substances is strongly influenced by the chemical speciation of metals in the soil matrix, the functional groups present in the leaching agents, and the soil pH. Lead and copper are predominantly bound to stable mineral and organic complexes, including carbonates, oxides, and humic–metal chelates, which limits their solubilization [46,47]. Cadmium and zinc, in contrast, are mainly present in exchangeable or carbonate-bound forms, which are more labile and readily mobilized under slightly acidic conditions (pH 6.5) [67,69]. Humic acids contain carboxyl and phenolic groups capable of forming stable chelates with Pb and Cu, slowing their mobilization but stabilizing them in solution once extracted. Malic and gluconic acids, smaller organic acids, promote faster complexation and diffusion of Cd and Zn into solution [68,70]. Thus, the observed differences in extraction efficiency reflect a combination of metal speciation, ligand complexation, and the protonation state dictated by the soil pH.

3.2.4. Cost–Benefit Analysis

To better understand the practical relevance of the extraction procedure, a simple cost–benefit analysis was performed. In this context, cost refers to the amount of extraction solution used (i.e., reagent and operational costs), while benefit corresponds to the metal extraction yield achieved under the given conditions. Although using a 1:8 solid-to-liquid ratio doubles the cost compared to a 1:4 ratio, the corresponding increase in metal extraction efficiency demonstrates that the additional expense is largely justified, highlighting the economic and technical advantages of the higher S/L ratio.

If the metal extraction yield is evaluated using the presented procedure with an S/L ratio of 1:8 and compared to the metal extraction yield using a similar procedure with an S/L ratio of 1:4, as presented in [49], considering the compatibility of these studies because the polluted soil was taken from the same source and under the same conditions, and the extraction methodology was identical, including the sources of humus, malic acid, gluconic acid, the equipment and working conditions, and the 8 h stirring duration, a cost–benefit analysis can be performed that favors the S/L ratio of 1:8. Thus, by comparing the costs of the extraction solutions, (cost for using S/L ratio 1:8)/(cost for using S/L ratio 1:4) = 2, in other words, using the 1:8 S/L ratio doubles the costs. However, the extraction yield has a much more favorable evolution. As shown in Figure 10, the ratio of (extraction yield using S/L = 1:8)/(extraction yield using S/L = 1:4) in most cases exceeds a value of 2 (cost ratio), which indicates that the technology that is twice as expensive yields benefits far greater than the costs. There are also exceptions, such as the extraction of Cu with 1%, 3% malic acid solutions or 3% gluconic acid, respectively, and the extraction of Cd with a 3% malic acid solution. Analyzing the results of this simple calculation, it is observed that:

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for Pb extraction, the yield increases over 100 times using humus-based extraction solutions, over 30 times when using 1% malic acid and 1% gluconic acid solutions, and approximately 2.8 and 9 times when using h3% malic acid and 3% gluconic acid solutions, respectively.

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for Cu extraction, the yield increases 90–100 times using humus-based extraction solutions, and 4 times using a 1% gluconic acid-based extraction solution, but with unsatisfactory results for the other extraction solutions.

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good and very good values are also recorded in the case of Zn extraction, with humus-based extraction solutions and 1% malic acid again standing out, achieving yields 7–8 times better than the other extraction solutions.

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the extraction yield of Cd and Cr maintains an improving trend more than the increase in costs (>2), within the range of 3.9–15. The yield obtained with a 1:8 ratio was 9 times higher compared to a 1:4 ratio, the only exception being in the case of Cd extraction with a 3% malic acid solution (1.95 times).

All of this leads to the conclusion that, although doubling the amount of extraction solution, which doubles the cost, is beneficial in most cases, applied research is still needed to identify the optimal variant, including the active substance in the extraction solution, its concentration, the stirring duration, and the solid/liquid ratio, depending on the type of pollutant metal and the type of extraction solution.

4. Conclusions

The analyzed soil has a slightly acidic to neutral reaction (pH = 6.5), with moderate levels of ammonia nitrogen and phosphorus, while potassium is relatively low. Spectrophotometric analysis revealed severe contamination, with Pb and Cu exceeding the intervention thresholds by approximately 28 times, highlighting the need for remedial measures.

The experimental results show that the mobilization of heavy metals depends on the chemical speciation of each metal, the type and concentration of the extraction solution, and the soil’s physicochemical characteristics. Pb and Cu are predominantly bound to stable mineral and organic complexes, which limit their mobility, or carbonate-bound forms, facilitating their movement. In contrast, Cd and Zn are mainly present in labile, exchangeable, or carbonate-bound forms, facilitating their removal. Malic acid rapidly mobilizes Pb, Zn, and Cu due to fast diffusion and complexation, whereas gluconic acid promotes slower but steady mobilization, particularly for Cd and Cr. Humic substances generally stabilize metals, limiting extraction, which reflects their strong chelating interactions.

Slightly acidic conditions (pH = 6.5) enhance metal solubility and favor complexation with organic acids, influencing both the kinetics and intensity of the extraction process. Transport phenomena such as desorption, diffusion, and complexation at the soil–solution interface governs how metals are transferred from the solid phase into solution, with metals bound to fine particles or soil organic matter requiring ligand-mediated mobilization. Cd mobilization was significant (over 90% with 3% gluconic acid), while Cu and Pb remained largely unmobilized (<3%). Cr (40–78%) and Zn (8–26%) showed intermediate mobilization, highlighting the need to adapt remediation strategies to the specific metal and characteristics of the contaminated soil. These observations highlight the importance of selecting the correct extractant and its concentration for optimizing heavy metal mobility in soil.

Forward-looking perspective. Future research should focus on pilot-scale and field-scale testing of these extraction agents under real environmental conditions. Optimization of solution concentration, solid-to-liquid ratio, contact time, and consideration of metal speciation will be essential to maximize heavy metal removal efficiency. These steps are necessary to develop practical, cost-effective soil remediation strategies that bridge laboratory findings with real-world implementation.