The effect of struvite and triple superphosphate on lead immobilization in contaminated soil in the presence of malic acid

Abstract

Lead (Pb) contamination in soils poses a persistent environmental hazard due to its toxicity, mobility, and resistance to degradation. The application of phosphorus (P)-based amendments has recently been considered as an effective strategy for Pb immobilization. However, the impact of low-molecular-weight organic acids (LMWOAs) as a common rhizosphere exudate on this process is not fully understood. This study evaluated the efficiency of various P amendments on Pb immobilization, both individually and in combination with malic acid. The soil was spiked with 400 mg Pb kg⁻¹ and six treatments were applied: The treatments included: S (struvite), T (triple superphosphate), M (malic acid), SM (struvite + malic acid), TM (triple superphosphate + malic acid, and C (control). Soil pH, organic carbon, basal respiration, microbial biomass carbon, extractable Pb with CaCl₂, and available P were measured at 1, 3, 7, 14, 21, and 28 days of incubation. At the end of the incubation, Pb fractionation and X-ray diffraction (XRD) analysis were conducted. The lowest pH (4.75) during incubation was observed in the M treatment, while treatments containing S significantly increased soil pH. The greatest reduction in extractable Pb with CaCl₂ compared to the C was recorded in the S (44.1%), followed by SM (34.1%), T (30.1%), TM (24.5%), and M (16.7%). XRD analysis confirmed the formation of Pb phosphate mineral in treated soils. Struvite demonstrated superior performance over T for Pb immobilization as a recyclable P source, offering more benefits of environmental safety and resource recovery.

Introduction

Lead (Pb) contamination in soil is a pervasive environmental issue, arising either from natural sources, such as parent rocks and minerals that are present in the Earth’s crust, or from man-made sources, which include agriculture, metallurgy, mining, electroplating, energy production, and the improper disposal of sewerage^1,2. The presence of Pb in soils poses severe risks to human and ecosystem health due to its high toxicity, persistence, and mobility³. Therefore, effective strategies for remediating Pb-contaminated soils are essential to mitigate their detrimental effects. Various physical, chemical, and biological methods have been developed in recent decades to remediate Pb-contaminated soil⁴. Chemical immobilization is often preferred over other remediation technologies because it is fast, effective, easy to implement, and inexpensive, particularly in large-scale applications⁵. This technique involves binding Pb from the soil through adsorption, complexation, and precipitation⁶. During this process, various inorganic and organic compounds and clay minerals are added to the soil to facilitate the formation of stable Pb compounds, reducing the Pb mobility, bioavailability, and toxicity^7,8.

Several types of phosphate compounds, such as phosphate rock⁹, phosphoric acid¹⁰, calcium phosphate¹¹, KH₂PO₄¹², and hydroxyapatite⁹, have been extensively used for immobilizing heavy metals (HMs) in soil. Actually, in the presence of sufficient phosphorus (P), Pb can be transformed into an insoluble Pb-phosphate mineral known as pyromorphite. This transformation significantly reduces Pb bioavailability, making it less likely to pose risks to the environment and human health^13,14. P amendments, such as triple superphosphate (TSP,Ca(H₂PO₄)₂·H₂O), are highly soluble and quickly deliver phosphate ions to the soil, facilitating the formation of Pb-phosphate minerals. However, this type of amendment presents a trade-off: phosphate is a valuable resource with a limited global supply¹⁵, and applying it at high rates can increase the risk of eutrophication¹⁶. Over the last decade, the use of recovered P from various municipal and industrial waste streams in the form of struvite (S,MgNH₄PO₄·6H₂O) has gained increasing attention as a P amendment for Pb stabilization¹⁷. S, as a novel P fertilizer with reduced P loss risk, provides both phosphate and magnesium ions, which can increase Pb immobilization by promoting the precipitation of Pb-magnesium-phosphate compounds^18,19. Zhang et al.⁸ reported that struvite and K-struvite can effectively remediate Pb-contaminated soil.

The chemical environment of the rhizosphere and the changes in the soil properties caused by root exudates may significantly affect the dissolution rate and, consequently, the effectiveness of phosphate amendments in immobilizing Pb¹⁴. Naturally occurring compounds such as acetic, citric, oxalic, and malic acids, classified as low-molecular-weight organic acids (LMWOAs), originate in soil through root exudation, microbial processes, and the breakdown of organic matter²⁰. These components can alter the HMs mobility and bioavailability in the soil through the formation of complexes and chelates with HMs²¹, proton or ligand-promoted dissolution, and adsorption/sorption mechanisms²², thereby affecting the uptake of HMs by plants²¹. On the other hand, LMWOAs can influence P availability in the soil via chemical and microbial processes²³. The increase in P availability through chemical processes is primarily due to enhanced dissolution of phosphates caused by the acidification of rhizosphere soils²⁴ or through ligand exchange between the anions of LMWOs and phosphates²⁵. The microbial processes involve supplying carbon (C) sources for microorganisms, which then release phosphorus from P-bearing compounds by secreting phosphatase and LMWOAs anions²⁶. We hypothesized that the efficiency of different phosphorus compounds in lead immobilization varies, and that the presence of malic acid may influence this process through acidification and complexation. Hence, understanding how LMWOAs affect the immobilization of HMs during the application of P compounds can provide valuable insights into the role of LMWOAs in chemical remediation processes. This knowledge can help to introduce more effective strategies for managing HMs-contaminated soils. However, few studies have focused on the influence of LMWOAs on Pb immobilization when different phosphorus sources are applied. Therefore, this research aims to investigate the effects of S and TSP on Pb immobilization, both in the presence and absence of malic acid.

Materials and methods

Soil sampling and preparation

A sample of surface soil (depth: 0–30 cm) was collected from Taleb Abad, Bandar Anzali, Guilan Province, Iran (37°27^′36.0″N, 49°34^′28.1″E). After air-drying, the soil sample was ground and passed through a 2 mm sieve, and then characterized for some physicochemical properties. Soil pH and electrical conductivity (EC) were determined in a 1:2.5 soil/water mixture, organic carbon (OC) content by wet dichromate oxidation²⁷, and calcium carbonate equivalent (CCE) by the back titration method²⁸. Soil particle size distribution was measured by the Bouyoucos hydrometer method²⁹, and water holding capacity (WHC) was determined gravimetrically after drying the soil at 105 °C for 24 h³⁰. The soil available Pb concentration was analyzed using atomic absorption spectrometry (AAS) (Varian 220) following extraction with 0.01 M CaCl₂³¹, and the total concentrations of Pb, Fe, Mn, and Cu were determined through acid digestion procedures³². The total and available P concentration were determined using the spectrophotometric phosphomolybdenum blue method³³. The soil characterization results revealed that the soil was slightly acidic with low organic content and a total Pb concentration of 15.5 mg kg⁻¹, classified as loamy sand (Table 1). Subsequently, the soil was spiked with Pb at a concentration of 400 mg kg⁻¹ using lead nitrate (Pb(NO₃)₂). After thoroughly mixing the soil to achieve equilibrium, it was incubated under laboratory conditions at 25 °C for two weeks, while maintaining a WHC of 70%.

Table 1.

Some properties of the studied soil.

Property	Unit	Initial soil	Spiked soil
pH 1:2.5	–	6.2	5.3
Electrical conductivity (EC1:2.5)	(dS m−1)	0.1
Organic carbon (OC)	(g100g−1)	0.9
Water holding capacity (WHC)	(g100g−1)	30.3
Calcium carbonate equivalent (CCE)	g100g−1	1.7
Textural class (USDA)	–	Loamy sand
Clay	(g100g−1)	3.9
Sand	(g100g−1)	85.9
Silt	(g100g−1)	10.2
Total P	(mg kg−1)	1500.0
Available P	(mg kg−1)	6.3
Total Pb	(mg kg−1)	15.5	410
Available Pb	(mg kg−1)	8.2	47
Total Zn	(mg kg−1)	120.0
Available Zn	(mg kg−1)	2.0

Phosphate amendment preparation

The triple superphosphate (TSP) was purchased from a local fertilizer company. Struvite was prepared in the laboratory by utilizing analytical-grade ammonium chloride (NH₄Cl), magnesium chloride hexahydrate (MgCl₂·6H₂O), and phosphoric acid (H₃PO₄) at a stoichiometric molar ratio of 1:1:1, in a 1-L beaker. The solution was continuously stirred with a magnetic stirrer at 100 rpm for 60 min. To create favorable conditions for the struvite crystallization (pH 8.5–10;³⁴), the pH was adjusted to 9.0 by the dropwise addition of 0.5 mol L⁻¹ NaOH. All reactions were carried out at room temperature (25 °C), and continuous pH measurement was taken throughout the 60-min reaction period. The sample was allowed to settle overnight, after which the resulting precipitate was filtered and dried in an oven at 35 °C⁸. Finally, the struvite powder was analyzed using X-ray diffraction (XRD) and FT-IR techniques.

The total and water-soluble P in phosphate amendments were also measured colorimetrically with a spectrometer (Varian Gary 100 Scan) by the ascorbic acid method³³. The total P in TSP and struvite was 21.5% and 11.88%, and the water-soluble P was 18.3% and 1.25%, respectively.

Incubation experiment

During the incubation experiment, 500 g of Pb-spiked soil was allocated to each treatment, and all treatments were conducted in triplicate. For the soil remediation, five treatments were applied: struvite (S), TSP (T), malic acid (M), S + M (SM), and T + M (TM). Contaminated soil without any treatment was also considered as a control (C). The rate of T treatment was calculated using a 4:1 molar ratio of P to Pb + Zn¹⁸. This ratio is slightly higher than the 3:1 ratio proposed by Basta et al.³⁵ and Melamed et al.³⁶ to ensure the formation of pyromorphite. The S was added to the soil in an amount equivalent to the P introduced through T. The M was applied to the soil at a rate of 10 mmol kg⁻¹³⁷ based on the organic acids contents in the rhizosphere, which can generally be as high as 1–10 mmol kg⁻¹, and sometimes even higher^38,39.

The treated soils were incubated under 70% WHC at 25 °C for 30 days, and the soil samples were collected at 1, 3, 7, 14, 21, and 28 days of incubation. The soil pH, OC, available Pb, and P were measured by applying the procedures mentioned above. Soil basal respiration (BR) and microbial biomass carbon (MBC) were also determined based on the method proposed by Anderson and Domsch⁴⁰. At the end of incubation, the Pb fractions in the soil samples were analyzed by Tessier’s method⁴¹. Furthermore, XRD analysis was performed using a Smart Lab SE (Japan) to investigate the mineral phase present in the C, S, and T treatments. The expert High Score Plus software was utilized to analyze the X-ray diffraction patterns.

Statistical analysis

The experiment was conducted in a factorial arranged in a completely randomized design with three replications. Data analysis and mean comparisons were performed using the Tukey HSD method (p ≤ 0.05) with the SAS (SAS⁴²) software package. The graphs illustrate the means ± standard error, represented as error bars.

Results and discussion

Struvite and soil characterization

Based on the XRD analysis, the spectra of the prepared struvite match their standard powder diffraction files (96-900-7675 PDFs) (Fig. 1a). Differences in the intensity or position in some of the characteristic peaks may be due to differences in the degree of compression of the samples when they were prepared for spectrum acquisition. In such a case, the compression of the samples to different degrees causes certain surfaces of the crystals to become aligned and affects the diffraction intensity⁴³.

Fig. 1.

The XRD (a) and FTIR (b) spectra of the prepared struvite.

The FTIR spectrum of the struvite is also presented in Fig. 1b. The identified peaks observed within the range 2200 to 3400 cm⁻¹ correspond to the stretching vibrations of O–H and N–H groups. A strong band observed at 1681 cm⁻¹ was attributed to O–H bending vibrations. A distinct peak at 1436 cm⁻¹ was linked to N–H bending vibrations within the ammonium group. Additionally, the band observed at approximately 1006 cm⁻¹ corresponds to P–O stretching vibrations in phosphate units, while the feature at 760 cm⁻¹ is indicative of O–H bending associated with magnesium coordination⁴⁴. Meanwhile, the intense bands at around 569 cm⁻¹ and 463 cm⁻¹ were associated with metal–oxygen bonds and the bending vibrations of Mg–O bonds in the struvite⁴⁵. The FTIR results verified the presence of water of hydration, phosphate (PO₄³⁻), ammonium (NH₄⁺), and metal–oxygen bonds within the struvite structure.

Changes in soil pH, OC, BR, and MBC

The mean value of pH, OC, BR, and MBC influenced by the different treatments and incubation times is presented in Fig. 2a–d. The lowest pH value (4.47) was observed on the first day of incubation in the M treatment, which increased with time and then reached a constant value, which could be related to the separation of H⁺ ions from the carboxyl group of organic acids (Fig. 2a). A similar trend was observed by Rukshana et al.⁴⁶, who reported that the addition of organic acid (acetic, malic, citric, benzoic, and ferulic acid) to soil instantly decreased pH due to H⁺ dissociation. However, the magnitude of pH decrease depended on the dissociation degree of the acids, the initial soil pH, soil buffering capacity, soil structure, clay composition, organic matter content, and microbial activity³⁷. Subsequently, the pH gradually returned to its initial level during incubation as the mineralization of organic anions consumed H⁺ ions.

Fig. 2.

The effect of different treatments and incubation times on soil pH (a) and soil organic carbon (b), soil basal respiration (c), and microbial biomass carbon (d). C: control, T: TSP, S: Struvite, M: Malic acid, TM: TSP + Malic acid, and SM: Struvite + Malic acid. The error bars represent the standard error (n = 3).

Moreover, applying organic compounds can increase cumulative respiration in soils, affecting the soil pH.

The pH level in the S treatment increased significantly compared to the C treatment on the first day of incubation. The rise in pH was attributed to the struvite’s high alkalinity, suggesting that adding struvite is beneficial and can effectively treat acidic soils⁴⁷. The struvite-induced pH increase was also reported by Ramut et al.⁴⁸. In the S treatment, the soil pH remained relatively constant until day 14 of incubation, after which it began to decline. This decline may be linked to the acidic nature of the soil, which markedly accelerated the struvite dissolution⁴⁷. Struvite solubility is strongly influenced by pH, being higher under acidic conditions due to proton-mediated dissolution⁴⁹, whereas it decreases as pH rises⁵⁰.

The application of SM treatment slightly reduced the soil pH during the incubation times compared to the S treatment. This implies that, in acidic soils, the presence of organic acids did not significantly affect struvite solubility⁴⁷. Also, the application of TSP caused a slight increase in pH compared to the C treatment in all incubation times. This increase may be attributed to the addition of H₂PO₄⁻ to soils with a pH below 7.2⁵¹. In such cases, orthophosphate can react with FeOOH or AlOOH to form complexes such as Fe–O–PO₄, which, by releasing hydroxide ions (OH⁻) make the soil environment more alkaline (Fe│AlOOH + HPO₄^–2 → FeOHPO₄^- + OH^-). Also, the application of TM reduced the soil pH during the incubation times compared to the T treatment.

The mean value of soil OC influenced by the different treatments and incubation times is presented in Fig. 2b. In all treatments, soil OC exhibited a decreasing trend over the incubation period, indicating the decomposition of organic matter by microorganisms. The M treatment showed the highest amount of OC during the first three days of incubation; however, it then declined sharply and reached its lowest level by the end of the incubation period compared to the other treatments. After adding LMWOAs to the soils, the rate of decomposition initially increases due to the availability of a rapidly metabolizable carbon pool. However, this rate eventually decreases over time as the added carbon pool becomes exhausted. A slower decomposition rate after two weeks of incubation is thought to represent the turnover of secondary metabolites resulting from microbial metabolism of the added LMWOAs^52,53. Furthermore, the subsequent decline of soil OC during incubation may be attributed to the positive priming effect (PE), which is influenced by microbial types (r and k strategists) and co-metabolism. However, the extent of PE depends on the soil nutrient status, quality and quantity of soil C, nature and quantity of LMWOAs, and soil microbial composition⁵². Guenet et al.⁵⁴ found that applying easily degradable carbon to the soil enhances microbial activity by providing an energy source for the microbial population, which increases the decomposition of soil organic carbon (positive PE).

The lowest amount of soil OC during the first day of incubation was found in the T treatment, showing a decreasing trend up to day 14, after which it remained constant. Sánchez-Rodríguez et al.⁵⁵ reported that P fertilization increased short-term carbon turnover and may lead to reduced carbon storage in soil. Xia et al.⁵⁶ also found that P addition significantly increased the cumulative mineralization of soil OC. Two processes are proposed to explain these findings: (i) an increase in microbial activity due to the alleviation of microbial P limitation, and (ii) the release of previously inaccessible organic carbon as a result of P addition, transforming the organic carbon into dissolved organic carbon, which is more accessible to microbes and contributes to the mineralization of soil OC. Previous studies indicated that adding nutrients to nutrient-poor soils can stimulate microbial activity, as measured by CO₂ production^57,58. Additionally, both the TM and SM treatments showed less reduction in OC than the single treatments. Overall, the combination of malic acid with P sources contributed to greater stability of soil OC, which can be effective in improving the quality of HMs-contaminated soils.

The mean values of soil BR and MBC influenced by the different treatments and incubation times are presented in Fig. 2c, d. In all treatments except T, the highest amount of BR was observed on the first day of incubation, with a decreasing trend during incubation. Treatments containing malic acid showed a higher amount of BR than other treatments. Organic acids can directly reduce the toxicity of HMs by forming chelates, which prevent these metals from reacting with bacterial active groups. On the other hand, organic acids can serve as carbon sources for microbes and easily cross the thinner cell walls and membranes of bacteria, enabling them to use these acids directly for energy³⁷. By applying the T treatment, microbial activity was enhanced, and soil OC mineralization accelerated during the first week of incubation, aligning with the findings of Xia et al.⁵⁶.

The MBC in all treatments was higher compared to the C at all incubation times, showing a general upward trend, except for T. Treatments containing struvite demonstrated the highest amount of MBC during the incubation period. At the end of the incubation, the MBC amount in the S and SM treatments was 1.8 and 1.7 times greater, respectively, than that of the C. Koizumi et al.⁵⁹ also reported that adding P sources caused a higher cumulative CO₂ emission and total MBC in comparison to the control at 20 days of incubation. The increase in MBC following the application of S and SM treatments may be related to the rise in pH. An increase in pH stimulates enzyme activity and also influences the microbial community⁴⁸. Also, an increase in MBC coupled with a lower BR in the struvite treatments suggests a higher carbon use efficiency. This means that a larger portion of the carbon was utilized for microbial growth, resulting in less carbon being lost as CO₂. As a result, this contributes to greater carbon stability in the soil⁶⁰.

In the M and TM treatments with lower pH values than other treatments, lower MBC content was observed during the first week of incubation. Tian et al.⁶¹ also observed positively significant effects of soil pH and organic matter content on MBC. The decrease in microbial activity due to lower soil pH likely indicates a change in the composition of fungal and bacterial communities, as well as the associated sensitivity to pH and the carbon or nutrient utilization efficiency of various microflora⁶².

Changes in soil Pb and P

The mean values of soil Pb and P influenced by the different treatments and incubation times are presented in Fig. 3a, b. The highest concentration of Pb (37.42 mg kg⁻¹) was observed at the end of the incubation period in the C treatment, which can be attributed to the soil’s low pH, enhancing the solubility and mobility of Pb. However, the C treatment also demonstrated the ability to immobilize Pb throughout the incubation period. Since the soil can serve as a bio-retention filter medium, effectively retaining HMs on its own, as demonstrated by Davis et al.⁶³.

Fig. 3.

The effect of different treatments and incubation times on lead (Pb) (a) and phosphorus (P) (b). C: control, T: TSP, S: Struvite, M: Malic acid, TM: TSP + Malic acid, and SM: Struvite + Malic acid. The error bars represent the standard error of the mean (n = 3).

The amount of Pb in the T, M, and TM treatments showed an increasing trend during the first three days of incubation, followed by a decline up to the end of incubation time. In contrast, the S and SM treatments exhibited the highest amount of Pb on the first day of incubation, after which a decreasing trend was observed. The high amount of Pb at the beginning of the incubation compared to the T and TM treatments may be attributed to cation exchange reactions involving the introduced cations, specifically ammonium (NH₄⁺) and magnesium (Mg²⁺) from struvite. After which, Pb immobilization is likely facilitated through the interaction of free Pb²⁺ ions with phosphate ions released from struvite⁸. The highest reduction in Pb amount was observed in the S treatment (44.1%) on the last day of incubation compared to the C, followed by SM (34.1%), T (30.1%), TM (24.5%), and M (16.7%) treatments. Gu et al.¹⁸ also reported that Pb immobilization efficiency differed by amendment type; struvite and PR (phosphate rock) induced similar Pb immobilization (~ 18%), while TSP was less efficient (10%). Moragaspitiya et al.⁴⁷ reported that the application of struvite to contaminated soils markedly decreased (99%) the water-soluble fractions of copper (Cu) and zinc (Zn). They proposed that Cu immobilization occurs through a combination of amorphous Cu phosphate precipitation and adsorption onto the surfaces of struvite particles. The application of S treatment increased the soil pH (Fig. 2a), enhancing the retention of HMs on the soil surfaces through processes such as adsorption, inner-sphere surface complexation, and/or precipitation and multinuclear type reactions^64,65. A negative and significant correlation was observed between pH and Pb (p ≤ 0.001, r = − 0.54) in this study.

A positive effect of M treatment compared to C was also observed in reducing Pb. This decrease may be attributed to the reaction between Pb and soil native P in the presence of malic acid⁶⁶. The presence of LMWOAs promotes the release of P through soil acidification and the formation of metal cation complexes with organic anions. These complexes compete with P for adsorption sites on soil colloids, resulting in the release of both precipitated and adsorbed phosphorus⁶⁷. Ultimately, this process causes Pb to precipitate as pyromorphite, which has a low solubility constant. The application of TM and SM increased the Pb amount compared to applying S and T individually. This enhancement may be linked to a slight reduction in pH caused by the M. Liu et al.⁶⁸ suggested that organic acids, particularly malic acid, citric acid, and oxalic acid, can not only acidify the soil but also form soluble complexes and chelates with metal ions. Chen et al.⁶⁹ also confirmed that LMWOAs can increase the mobility of HMs, which may be due to the formation of soluble complexes and ligand exchange reactions on soil adsorption surfaces. LMWOAs also contribute to the release of HMs by affecting the dissolution of iron oxides and soil organic matter⁷⁰. However, the mobility of HMs largely depends on the type and concentration of LMWOAs, soil characteristics, and other environmental factors⁷¹. Moragaspitiya et al.⁴⁷ reported no significant difference in the immobilization of HMs between struvite treatments with and without organic acids.

The highest amount of available P in the soil was observed on the first day of incubation across all treatments, showing a decreasing trend until the end of incubation, except in the C and M treatments. Nevertheless, the lower P amounts were recorded in the T and TM treatments compared to the struvite-containing treatments. A closer look at the data in Fig. 3a indicates that the highest Pb immobilization was observed after 28 days; however, a high immobilization rate in S and SM treatments was achieved after 3 days of incubation. Moreover, in this study, a significant negative correlation (p ≤ 0.001, r = − 0.55) has been observed between the P and Pb. These findings indicate that while the Pb immobilization kinetics are rapid⁸, the slower dissolution rate of S causes a delayed release of phosphate compared to TSP, resulting in Pb being effectively immobilized throughout the incubation period. The dominant reaction mechanism for Pb immobilization has been reported to be through P sources dissolution and subsequent precipitation of sparingly soluble pyromorphite-like minerals [PY, Pb₁₀(PO₄)₆(OH, Cl, F)₂] across a wide range of pH (3–9). However, other mechanisms, such as cation substitution, adsorption, and precipitation of different Pb minerals, may also be involved⁷².

The X-ray diffraction patterns obtained from C, T, and S treatments at the end of incubation confirmed the formation of the Pb₅(PO₄)₃F, Pb₅(PO₄)₃Cl, and PbHPO₄ compounds (Fig. 4). This result revealed the effectiveness of the treatments for Pb immobilization through the formation of more stable Pb-containing compounds.

Fig. 4.

The X-ray diffraction analysis of C, T, and S treatment at the end of incubation. C: control, T: TSP, S: Struvite.

Fractionation of Pb

The Tessier sequential extraction method was used to investigate the variation of chemical fractions of Pb at the end of incubation (Fig. 5). As shown by the results, the Pb fractions in the soil were dramatically influenced by the different treatments. Across all treatments, the oxide-bound Pb in the soil increased, while the Pb soluble-exchangeable fraction decreased compared to the C. The highest Pb soluble-exchangeable fraction was observed in C (25.4%), followed by M (23.3%), TM (15.2%), T (15.1%), SM (8.7%), and S (6.9%) treatments. The M treatment exhibited the highest organic-bound Pb fraction, which may be attributed to a reduction in soil pH that enhances Pb desorption from clay minerals and facilitates the dissolution of Pb compounds²¹, leading to the formation of Pb-organic complexes. The findings indicate that applying P sources, particularly S, effectively reduces the Pb soluble-exchangeable fraction, thereby significantly alleviating the environmental risk of Pb.

Fig. 5.

The effect of different treatments on Pb fractions in the soil at the end of incubation. C: control, T: TSP, S: Struvite, M: Malic acid, TM: TSP + Malic acid, and SM: Struvite + Malic acid. F1: Soluble-exchangeable, F2: Carbonate, F3; oxides, F4: Organic, and F5: Residual.

Conclusions

This study demonstrated that phosphorus-based amendments, particularly struvite, effectively immobilize Pb in contaminated soils through the formation of stable Pb–phosphate minerals such as pyromorphite. Although the individual application of malic acid increased Pb mobility through acidification and complexation, its combined application with P sources enhanced Pb immobilization and contributed to improved soil organic carbon stability. Both triple superphosphate and struvite, with or without malic acid, increased oxide-bound Pb fraction while reducing soluble-exchangeable forms, indicating a general decrease in Pb bioavailability. Despite these promising findings, several limitations should be considered. The study was conducted using artificially Pb-spiked soil, which may not fully represent the complexity, aging processes, and heterogeneity of naturally contaminated soils. In addition, the relatively short incubation period (28 days) limits insight into long-term stability, transformation kinetics, and potential remobilization of Pb under fluctuating environmental conditions. Environmental factors further constrain direct field extrapolation, as soil pH buffering capacity, mineralogy, moisture regimes, redox conditions, and competing ions can strongly influence phosphate dissolution, Pb speciation, and immobilization efficiency. In particular, struvite performance may vary across soil types, especially in calcareous or highly buffered soils where phosphate availability and precipitation pathways differ. Furthermore, while this study highlights the dual role of LMWOAs, their behavior under field conditions is more complex than in controlled experiments. In natural rhizosphere environments, LMWOAs are continuously produced, rapidly degraded, and influenced by plant–microbe interactions. Their net effect on Pb mobility, therefore, depends on their concentration, persistence, and prevailing soil biochemical conditions, and may shift between promoting immobilization via enhanced P availability and promoting mobilization through complexation and acidification. Overall, struvite represents a promising, sustainable, and recyclable P source for Pb immobilization; however, its long-term effectiveness requires further validation under field conditions, across diverse soil types, and within dynamic rhizosphere systems. Future research should focus on the long-term stability of immobilized Pb and the coupled interactions among LMWOAs, soil properties, and Pb behavior in realistic environmental systems.

Author contributions

All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Mahnesa Gholamrezaei. The first draft of the manuscript was written by Maryam Khalili Rad, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper. Should any raw data files be needed in another format, they are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Ethics approval

All authors have read, understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors.