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16 June 2026

20 Pages

Brownfield Remediation with Phosphates: A Nature-Based and Circular Economy Approach—A Case Study from Central Italy

,
and
1
Cerege (Durabilité Environnementale), Aix-Marseille Université, 13545 Aix-en-Provence, France
2
Ministero dell’Istruzione e del Merito (MIUR), 00153 Rome, Italy
3
Department of Earth Sciences, Sapienza University of Rome, 00185 Rome, Italy
*
Author to whom correspondence should be addressed.
Land2026, 15(6), 1063;https://doi.org/10.3390/land15061063
This article belongs to the Special Issue Brownfield Redevelopment: Soil Remediation for Sustainable Cities
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Abstract

Soil contamination by heavy metals (HMs) [or potential toxic elements (PTEs)] poses serious risks to ecosystems and human health. Metals persist in the environment and can reach groundwater and freshwater as part of the food-chain. In soils, anthropogenic inputs dominate over geogenic sources. Metal mobility is strongly controlled by factors such as pH, mineralogy, and erosion processes that transport metal-bearing clay fractions. Wind and water can transport soil, mainly clay particles that can usually bind contaminants such as HMs. Using waste material is a tool suggested from the circular economy, so waste becomes a valuable resource. This study evaluates the immobilization efficiency of several heavy metals (Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn) using phosphate amendments—synthetic hydroxyapatite, phosphatic rock from Florida and Morocco—applied to a brownfield site. Heavy metal immobilization followed a two-step mechanism: first rapid surface complexation and secondly partial dissolution of hydroxyapatite and ion exchange with Ca, leading to the precipitation of metal-substituted hydroxyapatite phases. Synthetic hydroxyapatite generally shows the best efficiency, whereas phosphatic rocks were less effective but still provided a measurable immobilization. From a circular economy perspective, however, phosphatic rocks remain attractive due to their lower cost, availability, and waste-valorization potential.

1. Introduction

Soil contamination by heavy metals (HMs) is a pervasive environmental problem worldwide and cleaning up a brownfield is a hard task because HMs are non-degradable, persistent, and often difficult to remove. Heavy metal inputs arise from anthropogenic activities—mining, smelting, industrial manufacturing, fossil fuel combustion, waste disposal, and certain agricultural practices—as well as natural processes such as rock weathering, volcanic emissions, and wildfire deposition [1]. Once in the soil, the fate of a given metal is governed by its speciation and interactions with soil constituents: adsorption to mineral surfaces and organic matter, incorporation into secondary minerals, complexation with organic ligands, and precipitation as insoluble phases [2]. These processes determine mobility, bioavailability, and potential for plant uptake [3]. Importantly, the mineralogical form of the metal (for example, adsorbed ion, sulphide, carbonate, or phosphate phase) and the local soil chemistry (pH, redox potential, ionic strength, and organic carbon content) strongly influence both short-term behaviour and long-term stability [1]. Remediation technologies are broadly classified as ex situ and in situ, with trade-offs in cost, duration, and site disruption; the optimal choice depends on contaminant speciation, concentration, soil properties, and climate.
Ex situ approaches are costly, energy intensive, or disruptive to soil structure and function [4,5]. In situ remediation is a cost-effective stabilization strategy for the mobile and exchangeable fraction of metals. The use of phosphates among Nature-based Solutions (NbSs), defined as actions to sustainably manage and restore natural and modified ecosystems, is essential. Waste from phosphatic rock mine activity and/or from the fertilizer industry is a way to apply the circular economy principle. Phosphate-based immobilization is a widely studied in situ strategy that reduces metal mobility by promoting the formation of low-solubility metal-phosphate minerals or by enhancing sorption to phosphate-rich phases.
A sustainable remediation treatment should minimize energy and material inputs, enable subsequent land uses (notably agriculture or green infrastructure), and ideally contribute to broader ecosystem services such as carbon sequestration and biodiversity support [6].
Due to the increasing population, increasing the food production is necessary to reach the goals from Agenda 2030. Soil is increasingly becoming a resource of primary importance for agriculture activity and for climate change; soil can sorb carbon dioxide (CO2) from the atmosphere, increasing soil organic carbon (SOC) concentration, a way to improve soil quality and health [7].
This approach can be implemented using commercial phosphate amendments or by valorising phosphate-rich wastes (e.g., certain industrial by-products, bone meal, or P-rich waste from the fertilizer industry), aligning remediation with circular economy principles. The effectiveness of P-induced immobilization is site-specific; it depends on the identity and concentration of target metals, competing ions, soil pH and redox conditions, and the mineralogical form of both the contaminant and the phosphate source. Crucially, the particle size distribution and chemical reactivity of the phosphate amendment control dissolution kinetics and P availability, which in turn govern the rate and extent of metal-phosphate precipitation and long-term stabilization [8].
Despite promising results in many field and laboratory studies, several knowledge gaps limit wider adoption of phosphate-based remediation. These include limited understanding of long-term stability of metal-phosphate phases under fluctuating environmental conditions, the influence of amendment physical properties (such as particle size and surface area) on reaction pathways, potential unintended consequences for nutrient leaching or eutrophication, and the performance of waste-derived phosphate materials compared with purified reagents. Addressing these gaps requires integrated studies that combine mineralogical characterization, geochemical modelling, controlled experiments, and field trials to evaluate both efficacy and ecological safety.
This study aims to (1) characterize how phosphate source properties—particularly particle size distribution and mineralogy—influence dissolution behaviour and P availability in representative soils; (2) quantify the resulting pathways of HMs immobilization, including precipitation versus sorption mechanisms; and (3) assess the persistence of immobilization under variable pH and redox regimes relevant to real-world sites. By linking amendment properties to mechanistic outcomes, the work seeks to inform cost-effective, reuse-oriented remediation strategies that preserve soil function while reducing metal risks.

2. Materials and Methods

2.1. Studied Area

Soil samples were taken in the surroundings of the “Rieti basin” (Central Italy) where the factory “Nuova Rayon S.p.A.” (Rieti, Italy) and some plots of “Immobiliare SNIA S.r.l.” (Milan, Italy) are located (Figure 1). This area is defined as a brownfield according to EPA definition: brownfield is a previously developed land—often former industrial or commercial property—that is abandoned or underused and whose redevelopment is complicated by real or perceived contamination from hazardous substances, pollutants, or contaminants.
Figure 1. Map of Italy (a) and a picture of Snia Viscosa (nowadays) (b). The regions of central Italy are represented in red.
This complex was used for the production of rayon during the twenties of XXth century. The production of this material means the use of carbon disulphide (CS2), sulphuric acid, Zn sulphate and Ti dioxide. Nowadays this site is considered highly dangerous and polluted according the Italian law D.L. 17/08/1999 # 334.

2.2. Sampling

Sampling was carried out from soil-derived waste resulting from past industrial activity. These samples from a brownfield do not have all the pedological characteristics of a soil; here the term soil-derived waste is used broadly as in [9,10]. Sampling area was from industrial area “Nuova-Snia” (central Italy) (Figure 2).
Figure 2. Example of sampled material with high concentration of hydrocarbons (dimensions: 23 m × 20 m × 8 m). The numbers in the figure represent the sampling points. (Photograph taken by Author Mignardi, with consent from Coccia).
Samples, labelled with Roman and Arabic numbers, were taken directly from some accumulation of soil-derived waste before the inertization treatment. The choice of these two different labels was from the society (Frankar S.r.l.) in charge of the inertization treatment. Arabic numbers stand for samples with a high concentration of hydrocarbons from past activity, whereas Roman numbers stand for samples with no hydrocarbons (Table 1). The presence of hydrocarbons was estimated at the limit quantity for hydrocarbons of 750 mg/kg, as imposed by Italian Ministerial Decree No. 471/1999 (legislation on contaminated sites management: a site is defined as contaminated if screening values or limit values were exceeded and those values represented the targets for remediation).
Table 1. List of samples from the accumulation of soil-derived waste.

2.3. Amendments

Phosphates used as amendments are: synthetic apatite (HA) commercially synthesized from Alfa Aesar Italia (Segrate, Italy), named calcium phosphate tribasic; Fluroapatite (FLO) is supplied from CT Industries, Inc. (Plant City, FL, USA); and two Moroccan phosphate rocks from the Moroccan plateau named MR and MN (black coloured because of the amount of organic material). Phosphate rocks (FLO) are wastes from fertilizer industry (Florida-USA); this rock comes from the Hawthorne Formation (Lower-Middle Miocene), which outcrops extensively in Florida. It consists primarily of sands, silt, clays, and organic phosphate deposits.
MR and MN are from mining activity (Morocco). Moroccan phosphate rocks were sampled1 in 2004 near the town of Youssofia (Phosphate Plateau). The Phosphate Plateau constitutes a sector of the central Moroccan Meseta; these phosphatic rocks are from the Ganntour Basin. In this area, phosphates are associated with clays, sulphates, and abundant organic material (Figure 3). These rocks are not suitable for the scope of mining activity and are considered waste.
Figure 3. Map of the Phosphate Plateau in Morocco [11]. (A) Model of several narrow gulfs. (B) Large Atlantic Ocean connection with intercon-nected gulfs. (C) Model of a single epicontinental sea with the presence of land masses; PMS: Paleozoic Massif of Skoura. (D) A single epicontinental sea without land masses. For reference citation in the figure see [11].

2.4. Sample Preparation

About 200 g for each sample has been interacted with hydrogen peroxide (H2O2) to oxidize the organic matter. Later, samples were washed with distilled water and dispersed in H2O2. This procedure was repeated twice. Samples were oven-dried (T = 40 °C) and weighed. Afterwards, samples were dispersed again in 500 mL of distilled water and subjected to mechanical agitation for 5 h using a Falc ATM agitator (Treviglio, Italy). Last step is the sieving to separate coarse fraction (>2 mm) from fine fraction (<2 mm), exsiccated in oven (40 °C) and weighted.

2.5. Granulometric Analysis

Granulometric analysis has been carried out on eight representative samples (I, II, IV, VIII, 2, 6, 20, 26) with an Endecotts EFL mk3, London, UK.
The main particle size fraction is represented by the one over 4 phi, or particles with a diameter smaller than 62 microns, with values between 27 and 59%. The remaining fractions rate between a minimum of 0% and a maximum of approximately 6% (Supplementary Data S1).

2.6. Chemical Composition

Sample fine fraction (~0.3 g) was pulverized in an agate mortar. Fine fraction bound most of the HMs because of a large specific surface area [12,13] and HMs associated with the finer soil fraction are more mobile, subject to long-range transport and consequently also more likely to pollute groundwater and freshwater [14].
Chemical attack on this fraction was performed using a Milestone MLS-1200 MEGA (Labexchange GmbH, Burladingen, Germany) microwave mineralizer in Teflon pressure vessels. Samples were in a solution of HNO3 + HCl + HF (HF-assisted digestion) and heated for approximately 45 min, until dryness began to develop. The residue was then dissolved in a 10% HNO3 deionized water solution. The resulting solutions were poured into calibrated glass flasks to a volume of 100 mL. For each sample, three replicate solutions were prepared and subjected to chemical analysis by ICP-AES Varian Vista RL CCD Simultaneous (Agilent, Santa Clara, CA, USA) to verify the reproducibility of the analytical data. ICP-AES detection limits are 0.01 ppm for Cd, 0.026 ppm for Co, 0.015 ppm for Cr, 0.027 ppm for Cu, 0.0087 ppm for Mn, 0.044 ppm for Ni, 0.25 ppm for Pb, 0.014 ppm for Zn, 0.026 for Ca, 0.73 ppm for K, and 0.005 ppm for Mg. The analytical error was estimated to be on the order of 3%.
Synthetic HA characterization is from Alfa-Aesar Italia (Segrate, Italy); only Ca and P concentration is valued, to reckon differences among the amendments. The three phosphate rocks were characterized too. Calcium and phosphorus values were determined to calculate Ca/P ratio; this ratio ranges from 1.51 (FLO) to a maximum value of 1.91 (MN). The amount of Ca in the amendment is important parameter to infer the metal concentration that could be immobilized (Table 2).
Table 2. Ca and P amount and Ca/P ratio in the four phosphate amendments.
MN phosphate rock showed higher values of Cd, Ni, Zn and Cr than MR amendment. FLO phosphate rock showed Ga, As and Cs values close to the detection limit, whereas U, Rb, Sr, Y, Zr, Nb, Mo, Sb, Ba, La, Ce, Pr are well detected (Supplementary Data S2).
Sample chemical analyses do not apparently show a relationship between hydrocarbons presence/absence and metal content. Nutrients in multi-contaminated soil-derived waste containing hydrocarbons and HMs are cost-effective for degrading total petroleum hydrocarbons (TPHs); it is of paramount importance to determine a suitable and sustainable P amount to avoid eutrophication. A low P concentration can be effective and fertilizer use may be a cost-effective option in a multi-contaminated matrix [15,16,17].
From the results, Mn, Pb and Zn show high values, whereas Cd is generally under the detection limit, Co show values between 2 ppm and 13 ppm, and Cr shows higher values. The range of Cu values is slightly wider (1–44 ppm). Ni contents vary between 10 and 30 ppm (Table 3).
Table 3. Heavy metal chemical composition in soil-derived waste.

2.7. Metal Mobility in the Amendments and in Soil-Derived Waste

Mobility of HM was evaluated by batch leaching tests at pH = 4, 5, 6, and 7 on both amendment (Supplementary Data S3) and soil-derived waste. Tests were performed at room temperature (25 ± 2 °C) interacting 5 g of both amendment and soil-derived waste in 100 mL of ultra-pure water (solid:liquid ratio 1:20 w/v) in Nalgene beakers for about 24 h; stirred with a magnetic stirrer at 500 rpm. Suspensions and blanks were prepared in triplicate. Suspension pH was measured with a Eutech 510 pH meter (Eutech Instruments, Singapore), and adjusted at the target values by adding HNO3 or NaOH at t = 0; pH was not re-adjusted during the test. After 24 h, suspensions were filtered through 0.2 μm Nucleopore polycarbonate membrane filters (Sigma-Aldrich, Saint Louis, MO, USA). Leachates were immediately acidified to 1% HNO after filtration, collected and analyzed by XRD and SEM-EDS; supernatants were analyzed by ICP-AES (Varian Vista RL CCD Simultaneous ICP-AES spectrometer, Agilent, Santa Clara, CA, USA) to determine the concentrations of the eight heavy metals. Procedural blanks, duplicates, calibration standards, and certified reference materials were included for QA/QC.

2.8. Dose of Amendment

To determine the suitable phosphate dose, tests were carried out on 5 g of representative samples (sample IV and 26), for FLO, MN and MR, and 5 different amounts have been chosen: 2; 5; 10; 15 and 20 g/L. Sorption experiments were performed in batch, and solutions were equilibrated in high-density polyethylene beakers (Nalgene, Sigma-Aldrich, Saint Louis, MO, USA) with 100 mL of ultra-pure water. The suspensions and blanks were prepared in triplicate; they were shaken at 500 rpm at 25 ± 2 °C for 24 h, which was considered to be sufficient to attain apparent equilibrium based on our preliminary kinetic experiments. The suspensions were filtered through 0.2 μm Nucleopore polycarbonate membrane filters and analyzed by ICP-AES. Previous results show that the suitable amount of phosphate is about 1.0–1.5 g [1,18,19].
Results from interaction tests between samples and phosphate from Florida (FLO) show a variability that depends on the heavy metal (HM) content amount in soil-derived waste, composition of the amendment, and interaction time (t = 24 h).
From the chemical analyses on the supernatants, Cd is always below the detection limits; Co, Cr, Cu, Ni and Zn are well immobilized with 1.5 g of amendment, whereas Mn shows a different behaviour. Indeed, Mn is better immobilized with a low amount of amendment (0.5 g) and increasing this amount, a release of this metal is observed. Lead shows a strange behaviour, and it is not always well immobilized as in previous studies [9,20].
One of the rocks from Morocco (MR) is not a good amendment as in a previous study [21]. The immobilization is influenced by pH and amendment concentration, resulting in variable outcomes. Good Co immobilization was obtained with 0.2 and 0.5 g of MR; unfortunately, immobilization for Cr and Cu are under 20%, and Pb is not well immobilized. On the contrary, Zn shows good immobilization; Mn shows the same behaviour as using FLO as amendment. High amount of amendment does not increase the immobilization. Ni shows contrasting results according the samples. The second Moroccan rock (MN) shows better results than the previous one but not higher than FLO. Cobalt is well immobilized as Cu, whereas Cr immobilization depends on the amendment concentration but not high values. Mn shows always the same behaviour: increasing amendment concentration, a decrease in the immobilization is observed. On the contrary, Pb shows very good immobilization as in previous studies [9,20,21,22,23]. Increasing the amount of the amendment does not increase Ni immobilization. For MN amendment, Zn shows a very low percentage of immobilization.

2.9. Metal Immobilization Experiments

The immobilization of HMs by phosphate amendments in soil-derived waste was conducted at room temperature (25 ± 2 °C) for t = 24 h. Batch tests were performed in Nalgene beakers among soil-derived wastes and any of the four phosphate amendments, with 5 g of sample and a phosphate dose of 1.0 g and 1.5 g (determined from preliminary tests) in ultra-pure water to a final volume of 100 mL. Suspensions and blanks were prepared in triplicate and pH was measured at t = 0 and t = 24 h [24]. pH is not superimposed to mimic a realistic situation during the remediation process. Suspensions were filtered through 0.2 μm Nucleopore polycarbonate membrane filters and were analyzed by ICP-AES. To improve the quality of the analysis in determining HMs concentration (Cd, Cr, Cu, Mn, Ni, Pb and Zn), they were acidified (10% HNO3). Supernatants were analyzed by XRD and SEM-EDS. Procedural blanks, triplicates, calibration standards, and certified reference materials were included for QA/QC.

2.10. Phosphate Uptake Kinetic

The immobilization reaction from phosphate amendment is very rapid, e.g., Pb is partially immobilized in 1 h; HMs such as Co and Ni need 24 h to be completely immobilized [24,25], so that the suitable time (t) in this study was 24 h for all of the HMs, in agreement with previous studies [23,24].
The removal capacity q (mg ion metal/g phosphate) was calculated as follows:
q = (C0 − Ce) × (V/M)
where C0 and Ce are the initial and equilibrium metal ion concentrations (mg/L), respectively, V is the volume of the solution (L) and M the weight of amendment (g). The percentage sorption of heavy metals from solution was computed using the equation:
% Sorption = (C0 − Ce)/C0 × 100

2.11. pH Adjustment

pH has not been adjusted, first of all to assess the efficiency in a real in situ case where it is not always simple to adjust it [1,26]. Secondly, recent study has demonstrated that pH values between 4 and 7 do not influence the P sorption capacity; on the contrary, to increase Pb immobilization, pH must be at pH = 2–3, modifying the soil characteristics. Lead is well immobilized by phosphate amendments in the human stomach where pH can vary from 1.7 to 7 as stated in [27].

2.12. Solid Examinations: SEM-EDS and XRD Analyses

Samples, phosphates and solid residues extracted from the metal immobilization experiments were analyzed by SEM-EDS (Scanning Electron Microscopy—Energy Dispersive System) ZEISS MD 940 (Zeiss, Oberkochen, Germany). Each sample was mounted on a stub and layered with graphite. SEM analyses for HA show a spherical and plane morphology, whereas natural phosphate rocks (FLO, MR and MN) show only a spherical dress (Supplementary Data S6a–d).
Mineralogical characterization was performed using X-ray diffraction (XRD) analyses on phosphates, soil-derived wastes, and solid residues after the interaction with phosphates. These analyses were carried out using a SEIFERT MZ IV automatic powder diffractometer (Seifert Systems, Hamburg, Germany) with Cu K-α radiation operating at 35 kV and 20 mA. Measurements were performed using a step-scanning technique with a fixed interval of 2 s/0.025° 2θ, and a range from 5 to 60° 2θ. Diffractometric analyses were performed on powders obtained by finely grinding each sample on an agate mortar.

3. Results

3.1. Soil-Derived Waste Chemical Composition

From the chemical analyses on soil-derived wastes, it can be inferred that Cd is generally under the detection limit, while in samples 6, 19, VIII and III it shows values between 0.1 and 0.3 ppm. Co shows values between 2 and 13 ppm; both Cr and Ni show values between 10 and 30 ppm; Cu has values between 1 and 44 ppm; Mn, Pb and Zn show high values between 70 and more than 700 ppm. In addition, a relation among hydrocarbons and the concentration of metals seems not to exist.

3.2. Solid Examinations: XRD and SEM-EDS Analyses

X-ray diffraction analyses show that HA does not have impurities, as certified from Alfa-Aesar. Concerning FLO, MR and MNXRD analyses show the presence of calcite and quartz.
Rocks from Florida contain F-hydroxyapatite, whereas rocks from Morocco contain F-Cl-hydroxyapatite and very rarely dolomite (Supplementary Data S7a–d).
Soil-derived wastes present mainly calcite and quartz; in some cases they are associated with gypsum and sphalerite (Figure 4).
Figure 4. Example of two diffractograms from samples (1,2), showing the same mineralogical composition as far as the other samples.
Results from XRD analysis on leached samples do not show a different mineralogical composition (Figure 5); in few cases, a calcite dissolution phenomenon was observed.
Figure 5. XRD results on two samples, VIII (a) and 26 (b), before and after leaching test, representing the two typologies of samples.
XRD analysis on solid residues—after phosphate interaction—does not show a new mineral phase; therefore, the presence of an amorphous or less crystalline phase is a feasible hypothesis [16,28,29] (Figure 6). Pyromorphite crystallinity is pH dependent, so that at pH values close to neutrality (6–7) the precipitation of amorphous minerals is more presumable [30,31].
Figure 6. XRD results on two samples, 6 (a) and VI (b), representing the two typologies of samples, before and after the reaction with the four amendments.
SEM analyses on soil-derived wastes do not show any difference before and after phosphate interaction, whereas EDS analyses indicate HMs presence on the surface of phosphates in all samples and some encrustations of Fe, Co, and Ni were highlighted on the phosphate surface. The peak intensity suggests HMs are on the phosphate surface in a different amount. The semi-quantitative nature of this analytical methodology does not allow further considerations of the significance of the observations (Figure 7).
Figure 7. Examples of SEM images. (a) sample VI after interaction with phosphate MR. (b) sample 26 after interaction with the MN phosphate. The light areas indicate Fe, Co, Ni encrustations on the apatite (backscattered electrons image). (c) EDS spectrum of Fe, Co and Ni encrustations for sample 26.
EDS analyses mainly show the peak for Ca, Na, K, and Si. Metals were detected in only a few samples and are mostly Fe and Al peaks. Lead peak is associated with S peak, indicating that Pb occurs as galena (Figure 8). Porosity of calcium carbonate particles would indicate rainwater erosion and prolonged acid input to the soil-derived waste (Figure 9).
Figure 8. EDS spectrum from sample 2 where Pb and S peaks are visible.
Figure 9. SEM picture of sample 1. Porosity on CaCO3 particle is visible.

3.3. Sorption Analyses

Mobility tests for amendments show values under detection limits (Supplementary Data S3), whereas metal behaviour in soil-derived waste varied with pH (Supplementary Data S4). At acidic pH (=4), Cd, Cr and Ni are mobile, whereas when increasing pH, mobility decreases. On the other hand, Co mobility does not seem to depend on pH, whereas Cu, Mn, Pb and Zn are not mobile at all. On the contrary, Ni is mobile at pH = 4 and 6. Samples have been analyzed through XRD and it is possible to infer a rarely partial dissolution of calcite.
The ICP-AES results on supernatants, after phosphate interaction, show HA can immobilize HMs quite well; natural amendments show variable results (Supplementary Data S5; Figure 10).
Figure 10. Examples of the immobilization in sample 6.
In particular, Cd immobilization ranges from 35 to 99% and natural amendments display higher results than the synthetic one; the immobilization efficiency order is FLO > MN > MR ≈ HA.
The immobilization percentages of Co generally range between approximately 15 and 99%, except for three samples where FLO immobilized less than 1% of the metal. In a few samples, natural phosphates are more efficient than synthetic apatite. The two Moroccan rocks have substantially similar immobilization capacities. In general, the phosphate efficiency is: HA > FLO > MR > MN.
The immobilization of Cr varies between 8 and 99%, with average values over 40%. HA is less efficient than natural amendments; the immobilization efficiency order is MN > FLO > HA > MR.
The immobilization percentages for Cu range between 3.5 and 99%, with values generally greater than 10%. All phosphates have similar efficiencies in immobilizing the element.
Mn is immobilized by phosphates with values ranging from <1 to 99%; HA is more effective. The immobilization efficiency order is: HA > MR > MN > FLO.
The immobilization percentages for Ni vary between <1 and 99%, with values generally greater than 40%. In most cases, HA, MR, and MN provided excellent results and their efficiency is as follows: HA > MR > MN > FLO.
Pb was immobilized by phosphates in percentages ranging from <1 to 99%, with values typically above 70%. All phosphates generally provided good results.
The immobilization percentages for Zn ranged from <1 to 99%, with values typically above 40%. In general, the efficiency ranking was HA > MR > MN > FLO.

4. Discussion

4.1. pH and Heavy Metal Mobility

Soil pH values in soil-derived waste–phosphate system strongly influence HMs mobility (speciation of metals) and phosphate dissolution, showing low solubility at high pH [32].
Generally, at low pH, competition between H+ and dissolved metals for ligands (OH−, CO3 2−, SO42− and phosphates) intensifies, reducing adsorption capacity and increasing metal bioavailability and mobility as stated by [33]. Rising pH favours the formation of less soluble hydroxide and phosphate minerals, which lowers metal solubility [34,35].
Sorption mechanisms are pH dependent; as pH increases, the ionic exchange process and surface complexation are more important [30]; whereas the amount and crystallinity of the products decrease with increasing pH [1,23,36]. Metals can exhibit different mobilities even at similar pH values. Under alkaline conditions, the relative potential mobility of metal decreases in the order Cd > Cr > Zn = Mn = Ni = Pb, which differs from the sequence Zn > Cd > Ni > As > Cu > Pb reported by [24].
Metal immobilization by sorption is a primary process reducing the pollutant mobility. The mobile fraction of Cd is highly variable; it ranges from 39.72% to 0.09%, whereas Cu, Mn, Ni, Pb and Zn typically show a mobile fraction near 0.01%. Finally, Cr values range from 0.01% to 2%.
Soil pH variation can break bonds between metal ions and organic and inorganic compounds [37,38,39]. Imposed pH experiments show HMs mobility is highest at pH = 4; and decreases as pH rises to 5–7 [40]. Cd shows a higher mobility at low pH (~4), possibly because of calcium carbonate deficiency [38,41]; increasing pH, Cd mobility decreases due to enhanced sorption.
Cd is rapidly sorbed onto phosphate (hydroxyapatite) via rapid surface complexation and in a second moment through Ca ion exchange [18]. Lead shows a different behaviour; near-neutral pH promotes precipitation as carbonate and phosphate [38,42]. Zn is a very mobile element [43], but in the presence of phosphate it can be retained by surface sorption through an ion exchange mechanism with Ca [10,44].
pH increases after P application, even though it decreases at the beginning. This increase is due to phosphate dissolution and because of protonation of functional groups, but preventing the acidification that would otherwise remobilize the metals [25]:
≡PO− + H+ → ≡POH0
Sorption encompasses processes from adsorption to precipitation and vice versa [36], so selecting a suitable and sustainable amendment aligns with NbSs.
Contaminants and amendments undergo an ageing effect characterized by progressive diffusion into remote sites via downward migration [44,45,46,47] and may further immobilize metals, partly through phosphate migration [45,46,47,48,49,50]. Phosphate adsorption in soil typically decreases with increasing pH. Actually, at high pH in the presence of calcareous soil, hydroxyapatite may precipitate and later sorb HMs; iron oxides often dominate phosphate retention, accounting for roughly 80–90% of phosphate adsorption [51]. Sorption and desorption dynamics therefore affect the bioavailability and mobility of both metals and amendments. Some constituents, like iron and aluminum, are responsible for sorption of cation and anions, and for phosphate retention [51]. Moreover, organic matter, phyllosilicate and Mn-oxide also contribute to the sorption process of cations and anions [36]. pH alters the number of available sites for HMs sorption [35] and a phenomenon defined as a competition for these sites is observed: in particular, internal competition between ions of the same metal, competition with H+ and competition among each different cation for precipitation and adsorption on these sites [1,30]. This explains why an increase in amendment dose (e.g., up to 10 g/L) optimizes immobilization for elements like Cd, but can occasionally lead to a relative reduction in the immobilization efficiency of competing cations like Ni, Mn, and Cr.
Furthermore, pH influences the sorption mechanism and, particularly, the quantity and crystallinity of the products [30]. At high pH, phosphate solubility decreases from the interaction between the solution and applied amendments. Initial pH values range from 7 to 9 and phosphate application often induces a slight pH increase, whereas reductions are observed only rarely. Phosphate addition does not seem to decrease pH value [20], nor to increase HMs solubility; therefore, under alkaline conditions, hydroxypyromorphite precipitation could be unlikely to be excluded.

4.2. Solid Examinations: XRD and SEM-EDS Analyses

XRD patterns do not indicate the precipitation of crystalline precipitates, which supports the earlier conclusion that precipitation occurred as amorphous products [1]. Normally, XRD cannot detect phases at a concentration lower than 1% of the sample matrix, so poorly crystalline or low-concentration products may remain undetected.
SEM observations revealed no discernible morphological changes between the solid residues and the starting amendments. EDS analyses, however, detected heavy metals associated with the phosphate amendments, corroborating previous findings [51,52]. This provides direct physical evidence that the heavy metals are migrating from the aqueous solution and fixing onto the solid-state phosphate matrix. Therefore, immobilization is an amendment-driven surface-bound process (surface complexation and surface precipitation) rather than separate, independent precipitation events occurring randomly in the bulk soil solution.
Our experimental monitoring of solution chemistry provided direct evidence of the surface complexation and ion exchange mechanism via proton (H+) and cation dynamics. The experimental data shows a two-stage pH behaviour: an initial rapid modification followed by a low increase in solution pH (less than 1 unit) accompanying metal uptake. This directly supports the protonation/deprotonation mechanisms tracking the surface complexation reaction:
≡POH + Me → ≡POMe + H+
The subsequent stabilization confirms that surface functional groups on the amendments are actively exchanging ions with the aqueous heavy metals, suggesting that heavy metals are present on the amendment surfaces but are either poorly crystalline, amorphous surface precipitates, or are highly dispersed monomolecular layers formed through rapid surface complexation and Ca2+ heavy metal isomorphic substitution, or below XRD detection limits.

4.3. Heavy Metals Immobilization Results

The ICP-AES analysis indicates that phosphate amendments promote the transformation of metals from soluble forms to stable forms; therefore, the minimization of potential migration of contaminants through in situ stabilization is effective.
In particular, the greatest HMs immobilization was observed at the amendment dose of 10 g/L; in a few cases the increasing of the phosphate dose shows a reduction in HMs immobilization such as for Ni, Mn and Cr; occasionally Pb and Co show the same behaviour. These findings agree with the results reported in the literature [53,54,55], whereas Cd is consistently well immobilized.
Metal uptake typically produced a modest increase in pH solution (less than 1 unit change) [53,55]. Concerning Cd immobilization, it ranges from 35 to 99%, with natural amendments showing a superior performance; the immobilization order is FLO > MN > MR≈HA. Co is generally well immobilized (15–99%) by HA, FLO, MR and MN amendments. Cr immobilization ranges from 8 to 99%, with values usually above 40%; the efficiency order is MN > FLO > HA > MR. Cu shows immobilization values over 10%, ranging from 3.5 to 99%. The four amendments show a good efficiency overall, whereas in some samples FLO and MR are less efficient. Pb is consistently well sorbed over 70%, across all of the amendments. Mn (1–99%) results show values generally over 20% and it is well immobilized by HA; in a few cases natural amendments are better; the general order is HA > MR > MN > FLO. In particular, Mn shows a drop in the immobilization efficiency; this phenomenon is driven by multi-metal competition, surface site blocking/saturation, and amendment-induced pH/solubility constraints. Mn sits lower in relative potential mobility/alkaline precipitation affinity compared to Cd and Cr; the excessive presence of dissolved phosphate ligands preferentially targets other components of the multi-element matrix, leaving a fraction of Mn weakly bound or untargeted.
Ni (1–99%) shows values over 40% following the efficiency order HA > MR > MN > FLO. Zn shows immobilization values over 40%; the efficiency order is HA > MR > MN > FLO.

4.4. Uptake Mechanism

The variation in performance among the tested phosphate amendments stems from structural and kinetic factors: two-step kinetic uptake, and crystallinity and surface state.
The uptake mechanism is well known, it may occur as: ion exchange, surface complexation, dissolution of phosphate minerals and precipitation of new metal-phosphate, and coprecipitation via substitution of Ca in phosphate minerals by other metals during recrystallization. The relative contribution of each mechanism is not fully determined, particularly in the case of a multi-metal matrix such as soil-derived waste, where all of the mechanisms could occur with different importance as stated in [55]. However, the sorption process is divided in two steps—a first rapid uptake (surface complexation) on binding site of phosphates, and a second step with a slower uptake to reach the equilibrium with the precipitation of a metal-containing phosphate according to the chemical reaction [10,56,57]:
≡POH + Me → ≡POMe + H+
Generally, the sorption equilibrium is reached in 24 h for many metals as already stated. Time is therefore a critical parameter in case of the remediation treatment. The sorption mechanism is also influenced by external physico-chemical factors such as pH, temperature, pollutant concentration, and solubility of the amendment; whereas the internal factors are cohesive energy, lattice energy, and Gibbs free energy [26]. In particular, sorption increases with increasing pH as stated by [40]; pH control is either applicable or not applicable, or very difficult to realize.
A further important and intrinsic parameter is the Ca/P molar ratio of the phosphate amendments. It is a material property governing both the dissolution–precipitation kinetics and the surface ion exchange capacity, effectively serving as a primary driver that balances the two-step uptake mechanism. A lower Ca/P ratio enhances dissolution and subsequent metal-phosphate precipitation, while higher Ca/P structural frameworks like HA favour long-term surface complexation and isomorphic substitution (Ca2+ exchange).
Synthetic hydroxyapatite (HA) possesses a stoichiometric Ca/P ratio near 1.67, yielding a highly stable lattice that favours highly selective, long-term surface complexation and steady ion exchange for specific divalent cations (Mn2+, Ni2+, Zn2+). Conversely, phosphate rocks feature a non-stoichiometric Ca/P ratio (1.51–1.91) due to carbonate (CO32) or organic matter substitutions. This ratio and lower lattice energy increase their chemical reactivity and dissolution rates, facilitating a rapid release of phosphate ions that accelerates the immobilization of highly mobile species through immediate precipitation pathways [18,21,29].
Metal composition strongly affects metal retention onto solid surfaces. HMs sorption is governed by the internal competition, a competition with H+ and among HMs for the adsorption sites and for precipitation [30]. These competitive interactions can substantially alter immobilization efficiencies in multi-metal systems.
Indeed, the HMs immobilization mechanism was by the above-mentioned two-step process, in a multi-metal-contaminated matrix enriched in hydrocarbons. In matrices contaminated with hydrocarbons, the presence of hydrocarbons does not appear to interfere significantly with the immobilization process.
This study simultaneously evaluated the immobilization of Cd, Co, Cr, Cu, Pb, Ni, Mn, and Zn using a cost-effective phosphate amendment. The results demonstrate that phosphate amendments are an effective and economical strategy for stabilizing multiple heavy metal contaminants in multi-element-contaminated soil-derived waste.

5. Conclusions

Soil is a non-renewable resource: forming a single centimetre can take on the order of a thousand years, which makes its protection and restoration an urgent environmental and societal priority.
This work investigated the sorption removal of bivalent metal ions (Ni2+, Mn2+, Co2+, Pb2+, Cu2+, Cd2+, Zn2+) from industrial soils-derived waste by phosphate amendments of different chemical compositions. The phosphate materials sequester heavy metals, forming a metal-phosphate reducing HMs mobility and bioavailability in soil and water. Parameters which affect the sorption process are pH, amendment properties, cations and anions in soil. Competition for sorption sites among HMs and H+ is also important, regulating HMs mobility, sorption on these sites and precipitation of new minerals. To evaluate equilibrium conditions and the speed with which equilibrium is achieved is of primary importance to conduct kinetic experiments to investigate full stabilization and mineral transformation that may require extended contact times and favourable geochemical conditions to ensure long-term permanence.
Using phosphate-rich wastes aligns remediation practice with circular economy goals by converting by-products into valuable amendments, reducing landfill disposal and the demand for virgin phosphate resources. Integrating NbSs into regulatory frameworks and land use planning can accelerate brownfield regeneration, enabling safe reuse for agriculture, biomass production, or urban redevelopment while delivering co-benefits such as habitat restoration, carbon and nitrogen sequestration, and overall ecosystem resilience, while reducing reliance on virgin materials.
Phosphate-based in situ stabilization can be an effective method to reduce the mobility and bioavailability of certain metal contaminants in brownfield soils when applied with appropriate design and controls. When selection of phosphate amendments is guided by rigorous site assessment, tailored amendment dosing and delivery, and sustained post-application monitoring, this approach can support safe reuse of contaminated land and contribute to circular economy goals when low-impact or recycled phosphate sources are used. However, the technology’s scalability, cost-effectiveness, and long-term permanence is site dependent; potential risks such as phosphate leaching, changes in metal speciation, and regulatory acceptance must be addressed through field trials and long-term performance data before claiming broad benefits for food security or biomass production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/land15061063/s1.

Author Contributions

A.C. (Alessandro Coccia): collected and analyzed soil samples (Master thesis) supervised by S.M. This manuscript is the continuation of Corami’s thesis. A.C. (Alessia Corami): conceptualization, designed the structure, wrote, edited and revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors are indeed thankful to V. Ferrini (retired) for his supervision along the project and the writing. The authors are thankful to T. Coppola (ICP-AES lab), S. Stellino and S. Fiori (XRD lab), G. Gaglianone (Sedimentology Lab), Sottili and C. Aurisicchio (SEM-EDS lab), for their laboratory assistance. Thanks are also extended to “Francar S.r.l.”, particularly to Avezzù, L. Carnielutti and Simeoni. Thanks also to ARPA Rieti and Millesimi for helping during the sampling.

Conflicts of Interest

The authors declare no conflicts of interest.

Note

1
Sampling was carried out from some of the authors for Corami’s Ph.D.

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