Remediation of Heavy Metal-Contaminated Soils Using Phosphate-Enriched Sewage Sludge Biochar

Abstract

Heavy metals represent long-lasting contaminants that pose significant risks to both human health and ecosystem integrity. Originating from both natural and anthropogenic activities, they bioaccumulate in organisms through the food web, leading to widespread and long-lasting contamination. Industrialization, agriculture, and urbanization have exacerbated soil and water contamination through activities such as mining, industrial production, and wastewater use. In response to this challenge, biochar produced from waste materials such as sewage sludge has emerged as a promising remediation strategy, offering a cost-effective and sustainable means to immobilize heavy metals and reduce their bioavailability in contaminated environments. Here we explore the potential of phosphate-enriched biochar, derived from sewage sludge, to adsorb and stabilize heavy metals in polluted soils. Sewage sludge was pyrolyzed at various temperatures to produce biochar. A soil incubation experiment was conducted by adding phosphate-amended biochar to contaminated soil and maintaining it for one month. Heavy metals were extracted using a CaCl₂ extraction method and analyzed using atomic absorption spectrophotometry. Results demonstrated that phosphate amendment significantly enhanced the biochar’s capacity to immobilize heavy metals. Amending soils with 2.5 wt% phosphate-enriched sewage sludge biochar led to reductions in bioavailable Cd (by 65–82%), Zn (40–75%), and Pb (52–88%) across varying pyrolysis temperatures. Specifically, phosphate-amended biochar reduced the mobility of Cd and Zn more effectively than unamended biochar, with a significant decrease in their concentrations in soil extracts. For Cu and Pb, the effectiveness varied with pyrolysis temperature and phosphate amendment, highlighting the importance of optimization for specific metal contaminants. Biochar generated from elevated pyrolysis temperatures (500 °C) showed an increase in ash content and pH, which improved their ability to retain heavy metals and limit their mobility. These findings suggest that phosphate-amended biochar reduces heavy metal bioavailability, minimizing their entry into the food chain. This supports a sustainable approach for managing hazardous waste and remediating contaminated soils, safeguarding ecosystem health, and mitigating public health risks.

1. Introduction

Heavy metals are elements with relatively high atomic mass and density, and they exhibit toxicity even at trace concentrations. This group includes various metalloids and metals with densities exceeding 4 g/cm³, often surpassing that of water by several times [1]. Due to their persistent nature, toxicity, and tendency to bioaccumulate in organisms, they are classified as significant environmental pollutants, posing substantial public health risks. Although these metals can occur naturally, human activities have intensified their presence in the environment. Anthropogenic sources such as industrialization, mining, agriculture, and urban development contribute heavily to their release into ecosystems [2]. These processes have led to widespread distribution of heavy metals in terrestrial and aquatic systems, with entry pathways including ingestion through contaminated food, water, and air. Over time, these metals tend to bioaccumulate, leading to potential toxic effects in humans and wildlife [1].

Despite the essential physiological roles of metals like nickel, copper, and zinc, others, such as mercury, chromium, lead, thallium, cadmium, and antimony, pose significant environmental risks and can lead to severe health issues, emphasizing the importance of minimizing exposure to them. The extensive application of industrial and agricultural practices has further increased the presence of heavy metals in soils and water bodies. These elements often exhibit environmental persistence, resistance to degradation, and the ability to bind with organic and inorganic materials. Consequently, they can remain in ecosystems for extended periods and may enter food chains, leading to detrimental impacts on ecological stability and human health [3,4]. In many global regions, particularly in Asia, Africa, South America, and parts of Europe and North America, elevated concentrations of heavy metals have been recorded [5]. For instance, China alone reports that 16.1% of its arable land and 20 million hectares are affected. Inorganic pollutants such as cadmium, lead, arsenic, and mercury account for more than 80% of these contaminants. Europe is home to an estimated 3.5 million potentially contaminated sites [6]. These metals are commonly found in environments influenced by industrial activities, waste disposal, and energy production, including landfills, urban soils, and agricultural lands. Therefore, urgent exploration of technologies aimed at remediating pollution and mitigating the effects of heavy metals is imperative [2].

Given the environmental risks, significant research attention has been directed toward sustainable remediation strategies. Among them, biochar, particularly when derived from organic waste like sewage sludge, has gained attention due to its potential to immobilize or adsorb heavy metals [7,8,9]. Biochar retains contaminants such as Cd²⁺, Pb²⁺, Zn²⁺, and Cu²⁺ mainly due to its high surface area, porosity, functional groups, and cation exchange capacity [10]. These characteristics immobilize heavy metals, preventing their leaching into groundwater, uptake by plants, bioaccumulation in the food chain, and aid in restoring contaminated sites [11]. The biochar surface adsorption and cation exchange capacity determine their retention capacity [10]. Ion exchange, surface complexation, and precipitation reactions all contribute to the binding of metal ions onto biochar surfaces. Functional groups such as hydroxyl, carboxyl, phosphate, and amino groups play vital roles in these processes. Moreover, biochar can increase soil pH, thereby reducing metal mobility and enhancing precipitation of metal hydroxides and carbonates [12].

Several studies have demonstrated that raw materials and pyrolysis conditions affect biochar’s physicochemical properties, such as ash content, pH [13], surface area, and porosity, all of which influence its remediation effectiveness [14,15,16]. Biochar produced from sewage sludge, a semi-solid by-product of wastewater treatment, offers a cost-effective and environmentally friendly option for waste valorization. However, due to the potential presence of residual contaminants, it is crucial to assess its suitability and effectiveness for soil remediation [8,17,18].

When appropriately processed, sewage sludge-derived biochar may significantly reduce the mobility of heavy metals in soils, contributing to soil detoxification. Furthermore, phosphate amendments, such as phosphate rock (PR) and di-ammonium phosphate (DAP), have also shown potential to immobilize heavy metals by enhancing sorption or precipitation reactions [19]. Recent advancements in biochar research emphasize its multifunctional role in addressing environmental challenges beyond soil fertility enhancement. For instance, the in situ synthesis of lanthanum-coated sludge biochar has shown promising results in advanced phosphorus adsorption, highlighting its potential in nutrient recovery from waste streams [20]. Similarly, predictive models assessing ecological restoration technologies demonstrate the capacity of biochar to remediate heavy metal-contaminated agricultural soils effectively [21]. Moreover, emerging studies also explore the interaction between biochar and degradable microplastics, revealing complex effects on soil organic matter decomposition and stability [22]. Integrating insights from these recent studies helps bridge the gap between traditional soil improvement approaches and contemporary multifunctional applications of biochar in sustainable land management.

Although sewage sludge-derived biochar contains only moderate levels of phosphate (approximately 3–5 wt%) [23], it has demonstrated potential in reducing the mobility and bioavailability of heavy metals for both plants and animals [24]. Previous studies suggest that phosphate and biochar can work in tandem to stabilize heavy metals, but their combined effects remain underexplored. It is also shown that phosphate-enriched biochars enhance immobilization of Cd, Pb, and Zn through precipitation and ligand-specific binding [25,26,27,28]. However, most of these studies focus on wood or manure-derived biochar. Less is known about sewage sludge-based variants and how pyrolysis temperature and phosphate levels interact to affect retention mechanisms. In this study, we focus on evaluating how sewage sludge-derived biochar, especially when integrated with phosphate amendments, can enhance heavy metal stabilization. This approach aims to offer insights into improving remediation strategies for metal-contaminated soils.

2. Materials and Methods

2.1. Biochar Preparation

Using an improved sequential batch-reactor technique, biochar was generated from anaerobically processed raw sludge (RS) from Beijing’s Wulituo wastewater treatment facility, which produces 20,000 m³ daily. In this procedure, an iron-based coagulant was introduced to precipitate phosphorus. The resulting excess sludge was mechanically dewatered and allowed to settle by gravity until it reached approximately 80% moisture content. Subsequently, it was thoroughly dried at 80 °C and finely ground into powder to pass through a 40-mesh sieve. A total of 10.0 g of dried sewage sludge (SS) was mixed with 0.16 M potassium phosphate (K₃PO₄) solution at a 1:2 weight/volume (w/v) ratio. The resulting suspension was agitated at 120 rpm for 2 h, then completely dried at 60 °C to a constant weight. The phosphorus incorporated into the sludge constituted 2.5 weight percent (wt%) on a dry basis. The 2.5 wt% amendment rate was selected based on prior optimization trials and aligns with the minimum effective dose shown to enhance phosphate retention and heavy metal immobilization without causing nutrient leaching [29,30,31,32,33]. Four experimental samples were prepared: two containing only dried SS and two comprising dried SS enriched with 2.5 wt% phosphorus (SSP).

The pyrolysis process was conducted in a muffle furnace under low oxygen levels using 10.0 g of dried sewage sludge enriched with 2.5 wt% phosphate (SSP), and then placed in a 50 mL lidded ceramic crucible. The process was, respectively, performed at temperatures of 300 °C, 400 °C, and 500 °C for 2 h of residence time, with a constant nitrogen flow of 300 mL/min and under the heating rate of 10 °C/min from ambient temperature. The resulting biochar after cooling was ground to pass through a 40-mesh sieve. These biochars derived from SSP were designated as SBP300, SBP400, and SBP500 according to their pyrolysis temperatures. Biochars derived from raw sludge without phosphorus amendment were also prepared under the same conditions and labeled as SB300, SB400, and SB500 for comparison.

The retention rate (RR) refers to the proportion of a specific element retained in the biochar compared to its original amount in the feedstock, while biochar yield denotes the ratio of the mass of the biochar to that of the dried feedstock:

$$\left(RR\right)=Y\times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{b}}}{C_f}}\times 100\%$$

where (RR) is the retention rate, Y is the percentage of biochar yield, C_b is the element containing in biochar (mg/g), and C_f is the element containing in the feedstock, taking into account that C_f for the SSP feedstock and those in SS are not the same due to the addition of potassium phosphate.

2.2. Characterization Methods

A 1.0 g sample was incinerated at 800 °C for 1 h to measure ash content. The pH was determined by combining the sample with deionized water at a 1:20 (w/v) ratio, and the resulting mixture was shaken at 180 rpm for 24 h. Total phosphorus was quantified using ICP-AES following acid digestion with a HClO₄, HF, and HNO₃ mixture (1:2:3). Elemental composition (H, S, C, and N) was analyzed using an Elementar Vario EL, which is an elemental analyzer from Hanau, Germany. Metal concentrations were measured using ICP-AES (SPECTRO Analytical Instruments GmbH, Kleve, Germany) after sample digestion according to the US EPA Method 3050B.

2.3. Soil Incubation Experiment for Heavy Metal Remediation

The samples were taken from Hubei province, China, and were significantly contaminated with heavy metals, including Cu, Pb, Ni, Zn, and Cd. P-amended biochar was added at a dosage of 1 wt% into 100 g of contaminated soil, mixed homogeneously, and incubated for 1 month. The moisture content was maintained at around 40%, with water added every 3 days. Subsamples were collected on the 1st, 2nd, and 4th week, then dried and passed through a 2 mm sieve before measurements. To assess heavy metal remediation, CaCl₂ extraction was performed. In this method, 3.0 g of dry soil was added into 30 mL of 0.01 M CaCl₂ (m/v = 1:10) and shaken for 2 h. The suspension was then prepared for centrifugation, yielding a supernatant and solid residue. The supernatant was filtered, and two drops of 5% HNO₃ were added for preservation before storage at 4 °C until analysis. Given the potential risks heavy metals pose to both animal and plant health, even at low concentrations, highly sensitive measurement methods are essential. Atomic absorption spectrophotometry was employed, which includes adding lanthanum chloride to the sample, followed by atomization and flame spraying. Each metal in the treated sample produced a distinct flame color, whose intensity is quantitatively analyzed using spectrophotometry [34].

3. Results

3.1. Material Characterization

The study of the aspects of biochars derived from sewage sludge involved a detailed analysis of several key parameters and elemental compositions across various samples. The physicochemical properties of the treated biochars (Figure 1) were compiled using previously reported data by [35]. Firstly, the yield of biochar, representing the percentage of the final product obtained after pyrolysis, ranged from 59.0% for the raw sewage sludge pyrolyzed at 300 °C (SB300) to a higher value of 63.7% for biochar produced at 300-degree Celsius and enriched with 2.5% P (SBP300), indicating variations in the efficiency of biochar production under different pyrolysis conditions. The higher yields observed in the SBP samples compared to those of SBs indicate the effectiveness of the addition of K₃PO₄ in stabilizing species during pyrolysis. The ash content of biochar, an important indicator of its mineral content, showed a notable increase when the pyrolysis temperature increased. For instance, biochars made at 500 °C (SBP500, SB500) exhibited a high amount of ash content at 87.3%, 78.1%, respectively, suggesting greater mineral retention at higher pyrolysis temperatures. The pH values of produced biochar samples varied from 7.2 for SS and SBs to 11.1 for SBPs, indicating neutral to alkaline properties across all samples. This variation in pH could influence the biochar’s behavior in soil and the way it interacts with other soil components.

Other characteristics of biochar samples involved analyzing their elemental compositions, which included hydrogen, carbon, nitrogen, potassium, sulfur, calcium, magnesium, iron, aluminum, and phosphorus. Carbon content exhibited a decreasing trend with increasing pyrolysis temperature, while phosphorus content showed an increasing trend with increasing temperature. Additionally, notable changes were significantly identified in the concentrations of other elements such as nitrogen, hydrogen, sulfur, calcium, potassium, magnesium, iron, and aluminum across the samples, suggesting differences in the biochar’s chemical composition based on pyrolysis conditions and feedstock properties. The retention rate of nitrogen, hydrogen, and sulfur declined with increasing temperature for the corresponding group of samples. Sulfur was highly retained, and, on the other hand, nitrogen decreased to a similar extent as carbon. The loss of hydrogen was almost unaffected, whereas nitrogen loss was significant across all SBPs.

3.2. Interaction Mechanisms Between Biochar and Heavy Metals in Soil

This current study assessed the effects of sewage sludge and biochar on the concentrations of heavy metals, including copper (Cu), lead (Pb), cadmium (Cd), zinc (Zn), and nickel (Ni) in soil solution extracts. The results revealed significant variations in metal concentrations after comparing them to the control soil, demonstrating the impact of P-amended sewage sludge-derived biochars on the mobility and availability (both mobility and immobilization) of metals present in soil, highlighting the potential for these amendments to influence metal concentrations in the environment.

3.2.1. Copper (Cu)

For copper (Cu), compared to control soil (17.10–28.68 ppb), the addition of dried sludge (SS) and dried sewage sludge enriched with 2.5 wt% phosphorus (SSP) significantly increased Cu levels in the extracts, with concentrations ranging from 221 to 313.68 ppb and 366.90 to 508.82 ppb, respectively. Conversely, the addition of SBP500 decreased Cu concentrations to 8.05–9.20 ppb, while SBP300 slightly increased concentrations to 25.2–32.45 ppb (Figure 2).

3.2.2. Lead (Pb)

Regarding lead (Pb), SS and SSP additions increased Pb levels to 0.14–1.38 ppb and 0.22–4.22 ppb, respectively. However, SBP300 reduced its concentration to 0.13–3.41 ppb, whereas SBP500 increased concentrations to 0.16–2.96 ppb compared to control, 0.14–1.38 ppb (Figure 3).

3.2.3. Cadmium (Cd)

For cadmium (Cd), SS and SSP additions led to significant reductions in Cd concentrations to 7.02–10.57 ppb and 6.10–10.00 ppb, respectively, compared to the initial concentration of 14.48–17.72 ppb in the control soil (Figure 4). Additionally, both SBP300 and SBP500 applications decreased Cd concentrations to 4.38–6.62 ppb and 3.94–4.88 ppb, respectively.

3.2.4. Zinc (Zn)

All zinc (Zn) treatments resulted in decreased concentrations compared to the control soil (548.04–797.07ppb), with reductions ranging from 185.22 to 333.23 ppb for SS, 136.08 to 283.73 ppb for SSP, 94.26 to 161.75 ppb for SBP300, and 113.55 to 228.54 ppb for SBP500 (Figure 5). Notably, SBP300 showed greater effectiveness in Zn immobilization compared to SBP500.

3.2.5. Nickel (Ni)

Regarding nickel (Ni), SS treatment slightly reduced Ni concentrations to 69.60–140.00 ppb, while SSP moderately increased its concentrations to 92.30–174.98 ppb compared to the control soil range of 93.80–149.27 ppb. Furthermore, both SBP300 and SBP500 applications significantly decreased Ni concentrations to 41.68–55.65 ppb and 43.26–44.36 ppb, respectively (Figure 6).

4. Discussion

4.1. Characteristics of Sewage Sludge-Derived Biochar

The physicochemical properties of the treated biochars (Figure 1) were compiled using previously reported data by [35]. The study revealed significant variations in biochar properties based on production conditions. The yield ranged from 59.0% to 63.7%, with phosphorus-amended samples showing higher yields due to improved stabilization during pyrolysis. These values were intermediate compared to previous studies [36,37] but higher than that of [38], suggesting that factors like residence time and feedstock composition significantly influence production efficiency [39]. Notably, the high ash content (>10 wt%) categorizes this biochar as high-ash biomass according to established classification systems [40]. The ash content increased substantially with rising pyrolysis temperatures, indicating greater mineral retention at higher temperatures [41,42]. pH measurements showed a range from neutral to alkaline values, consistent with findings from other studies that have documented biochar’s alkaline nature and its implications for soil chemistry [16,43].

Elemental analysis demonstrated distinct temperature-dependent patterns. While concentrations of Al, Mg, Ca, S, K, Fe, and P increased with pyrolysis temperature, carbon and nitrogen levels decreased, likely due to thermal degradation of organic functional groups [44,45]. The observed changes in molar ratios suggest reduced preservation of original organic matter at higher temperatures, while the increasing N/C ratio indicates enhanced surface polarity [46]. These physicochemical properties collectively determine the biochar’s effectiveness in heavy metal remediation.

4.2. Sewage Sludge-Derived Biochar Interplay with Heavy Metals

Heavy metal immobilization by phosphate-enriched biochar involves distinct mechanisms that vary depending on the metal’s chemical properties, biochar surface chemistry, and soil conditions. For cadmium (Cd) and zinc (Zn), immobilization is primarily governed by the formation of low-solubility phosphate precipitates, such as Cd₃(PO₄)₂ and Zn₃(PO₄)₂ [47]. These reactions are thermodynamically favored under alkaline conditions, often enhanced by the elevated pH of biochar-amended soils. Such precipitation reduces the mobility and bioavailability of these metals significantly [48,49].

In contrast, lead (Pb) demonstrates a different behavior. Although phosphate-induced Pb₃(PO₄)₂ precipitation can occur, Pb exhibits a stronger affinity for surface complexation, particularly with phosphate groups and oxygen-containing functional moieties (e.g., –OH, –COOH, and –PO₄) present on the biochar matrix. This inner-sphere complexation is particularly effective due to Pb’s higher polarizability and coordination capacity, allowing for stable immobilization on the biochar surface [49,50]

For copper (Cu), both precipitation (mainly as Cu(OH)₂) and complexation contribute to immobilization. Cu can form inner-sphere complexes with surface ligands (e.g., carboxyl and phosphate groups), but its behavior is also highly pH-dependent. At near-neutral to slightly alkaline pH, precipitation becomes more dominant, whereas at lower pH, ligand exchange and electrostatic sorption mechanisms prevail [51,52].

Nickel (Ni), however, shows limited immobilization efficiency due to its relatively weak interaction with phosphate ligands. Ni has a lower affinity for phosphate complexation and a higher solubility under a wide range of pH conditions, making it more prone to remain in the exchangeable or bioavailable form. This suggests that phosphate-enriched biochar may be less effective for Ni remediation unless additional stabilization strategies are used [53,54].

While our study provides valuable insights into the short-term immobilization of heavy metals by phosphate-amended biochar, it does not address the long-term leaching potential or aging behavior of the immobilized metals. Over time, environmental processes such as oxidation, microbial activity, and changes in pH can alter biochar surface chemistry and metal-binding forms, potentially affecting retention efficiency. Previous studies have shown that aging may lead to both enhanced stabilization and remobilization, depending on the specific metal and environmental context [55,56]. Therefore, long-term field studies or extended incubations are recommended to evaluate the durability and environmental safety of biochar-based remediation strategies.

Moreover, this study focused on total metal concentrations to assess the immobilization capacity of phosphate-amended biochars. We then acknowledge that this approach does not distinguish between metal fractions with varying bioavailability. Speciation analysis, such as sequential extraction into exchangeable, reducible, oxidizable, and residual forms, would provide a more comprehensive understanding of metal stability and environmental risk. Future studies should incorporate such analyses to better evaluate the long-term effectiveness of biochar-based remediation strategies.

4.2.1. Dual Mechanisms of Copper Retention: Surface Complexation and Hydroxide Precipitation

The study found complex behavior in copper mobility depending on treatment type. While raw sewage sludge amendments increased Cu concentrations, the SBP500 biochar effectively reduced mobility, while SBP300 slightly increased Cu concentrations through several mechanisms. The higher pH of this biochar enhanced Cu(II) complexation with surface functional groups such as carbonyl and phenolic hydroxyl groups. It was reported that Cu is stabilized through increasing soil pH to enhance Cu(II) complexation with biochar surface functional groups such as C=O and phenolic-OH [57,58]. It has also been reported by [59] that biochar pH value increases with the rise in pyrolysis temperature due to the biochar’s increased ash content. Additionally, the abundant hydroxides and carbonates in the high-temperature biochar ash promoted the formation of insoluble Cu(OH)₂ and CuCO₃ compounds [60,61]. However, the potential for increased dissolved organic carbon to enhance Cu solubility, as reported in other studies [14,62], underscores the need for careful amendment selection. It may also be added that the divergent behavior of Cu²⁺ arises from its intermediate binding affinity. In treatments with high pH and ash content, Cu may co-precipitate as hydroxide or phosphate phases. However, in lower-temperature biochar, where surface ligands dominate, Cu retention is governed by surface complexation and ion exchange. This dual mode of retention explains the observed variability across treatments [63].

4.2.2. Phosphate-Facilitated Lead Immobilization Through Complexation and Precipitation

Addition of SS and SSP increased Pb levels in the extracts, while SBP300 showed effective Pb immobilization compared to SBP500. Lead immobilization showed temperature-dependent effectiveness, with SBP300 demonstrating superior performance compared to higher-temperature biochars. This phenomenon appears related to several factors: the increased presence of exchangeable cations like Ca²⁺, Mg²⁺, Na⁺, and K⁺ [64], π-cation electron donor–acceptor interactions with biochar’s graphene surfaces [65], and precipitation reactions with carbonate and hydroxide groups [66]. The biochar’s phosphate content played a particularly important role, facilitating the formation of insoluble lead phosphate compounds that are highly stable in soil environments. These findings align with previous work showing that lower pyrolysis temperatures may preserve more effective phosphate forms for lead immobilization [66]. In particular, the sewage sludge-derived biochar contains plentiful phosphates [67], which are capable of forming insoluble compounds, such as β-Pb₉(PO₄)₆ and Pb₅(PO₄)₃Cl, Pb₅(PO₄)₃OH, with Pb(II), to decrease the mobility of Pb [68]. The chemical modifications of coconut fiber-derived biochar (CFBs) with nitric acid and ammonia and produced at 300 °C were also reported to increase the ability of the biochar to immobilize Pb from the aqueous solution, while all modification treatments of biochar pyrolyzed at 500 and 700 °C reduced the ability of the biochar to remove Pb due to the changes in the biochar basic properties, particularly the functional groups [69].

4.2.3. Cadmium Immobilization via Apatite Formation and pH-Induced Precipitation

The SS and SSP significantly reduced Cd concentrations in the soil solution. This immobilization could be due to the conversion of phosphorus fractions to apatite, which affects the precipitation of phosphorus-containing minerals, thereby being replaced by Cd [62,70]. Furthermore, the immobilization of Cd by SBP500 was observed to be more effective than that of SBP300, and this could be explained by an increase in soil pH [71] and induction of the conversion of Ca phosphates into hydroxyapatite (Ca₁₀(PO4)₆OH₂) and apatite, which in turn are substituted by Cd in soil [62]. The same trend of results was also reported by [72], who found that the biochar pyrolysis temperature decreases Cd bioavailability in the sequence of 700 °C > 500 °C > 300 °C. Furthermore, the biochar alkaline materials such as CO₃²⁻, OH⁻, and PO₄³⁻ generally have a significant binding and adsorption capacity to Cd in soils, which transform the free Cd(II) into CdCO₃, Cd(OH)_2, and Cd₃(PO4)₂ precipitates [60]. The effects of cation exchange between soil calcite (CaCO₃) and Cd(II) produce high adsorption affinity, which is the main factor to decrease Cd bioavailability in soil, and the large specific surface area and large functional groups in biochar are also important in immobilizing Cd [73]. For example, the corn straw-derived biochar reduced the CaCl₂-extractable Cd concentration levels up to 91% in the agricultural soil as reported by [71]. Ref. [74] applied 1~2% of rice straw-derived biochar to the mining area soil and reported that the Cd content in pore water of rhizosphere soil was significantly reduced. Therefore, the increase in soil pH value by biochar is extremely effective in immobilizing Cd in soil. It is important to mention that the alkaline materials in biochar can also enhance the deprotonation of biochar acid functional groups and further stimulate the adsorption capacity of Cd [75].

4.2.4. Zinc Sequestration Through Adsorption and Ion Exchange on Biochar Surfaces

All treatments reduced the concentrations of Zn compared to those of the control soil. SBP300 was observed to be very effective for Zn immobilization compared to SBP500. This observation may be attributed to adsorption, complexation, precipitation, reduction in bioavailability, and stabilization properties of biochar [11]. The high surface area and functional groups (carboxyl, hydroxyl, phenolic) of biochar allow it to adsorb Zn, reducing the concentration of free Zn ions in the soil and making them less available. Biochar also acts as a sponge, capturing Zn ions and preventing their leaching into groundwater, thus reducing Zn mobility and environmental toxicity. The process of ion exchange involves Zn ions replacing other cations on the biochar surface and precipitation, forming Zn-containing compounds within the biochar matrix. The increase in soil pH and high cation exchange capacity (CEC) is a function of organic matter content in soil; thus, an increase in CEC enhances the immobilization of heavy metals including Zn. This is consistent with the results from the studies of [76,77]. Furthermore, the study conducted by [78] using sugar cane straw-derived biochar reported that biochar reduced the availability of Zn in mine contaminated soils. In a similar way, ref. [79] combined compost and rice husk-derived biochar produced at 500 °C and studied the speciation, distribution, and availability of heavy metals, including Zn in the wetland soil of Dongting Lake, China. Their results demonstrated that all treatments decreased the availability of Zn, potentially due to the change in soil pH values, as most of the heavy metals become more bioavailable under acidic conditions (pH < 7) [80]. Moreover, ref. [81] used dead pigs and tobacco stalks to produce biochar; when applied, it increased soil pH, thereby decreasing the CaCl₂-extractable Zn concentration levels. Over time, these interactions influence soil properties, nutrient availability, and microbial activity, making biochar an effective tool for immobilizing Zn in contaminated soils.

4.2.5. Limited Nickel Retention: Weak Phosphate Affinity and Surface Complexation Constraints

SS treatment had a minor effect on reducing Ni concentrations, whereas SBP300 and SBP500 applications significantly lowered Ni levels compared to the control soil. This reduction may be attributed to the fact that biochar interacts with nickel in several ways, effectively immobilizing and reducing its bioavailability in soil. Biochar’s high surface area and porous structure provide numerous adsorption sites for Ni ions, with functional groups such as hydroxyl (-OH), carboxyl (-COOH), and carbonyl (-C=O) binding Ni through electrostatic attraction and covalent bonding (e.g., Biochar-OH+Ni²⁺ → Biochar-O-Ni) [16]. Additionally, biochar can form stable complexes with Ni ions via coordination with oxygen-containing groups, further enhancing Ni immobilization (e.g., Biochar-COOH + Ni²⁺ →Biochar-COO-Ni). It increases soil pH, promoting the precipitation of Ni as hydroxides or other insoluble compounds (e.g., Ni²⁺+2OH⁻ →Ni(OH)₂) [12]. By binding Ni, biochar reduces its bioavailability to plants and microorganisms, minimizing toxic uptake [11]. It stabilizes Ni in the soil, preventing leaching into groundwater and reducing environmental contamination risks [13]. Our observations are consistent with the results of [82] who reported that the concentrations of Ni reduced with pyrolysis temperature when they employed biochar derived from co-pyrolysis of calcium sulfate and sewage sludge. Furthermore, it has been reported that the soil pH was increased by wine lees-derived biochar, which decreased soil exchangeable Ni as compared to the control [83].

4.3. Mechanistic Insights: pH-Mediated and Ligand-Specific Pathways of Metal Immobilization

To better interpret the observed differences in metal immobilization efficiency, it is essential to consider the underlying mechanistic pathways. Higher pyrolysis temperatures (>600 °C) increase ash content and pH but reduce surface oxygen-containing functional groups. While this enhances precipitation-based immobilization (particularly for Cd²⁺ and Pb²⁺), it may reduce ligand-based sorption for metals like Cu²⁺ and Ni²⁺ that rely on carboxyl and phenolic groups preserved at moderate temperatures. Two dominant groups of immobilization mechanisms were proposed: (i) pH-mediated precipitation and charge interactions, and (ii) ligand-specific complexation and ion exchange.

4.3.1. pH-Mediated Reactions: Precipitation and Charge Neutralization

One of the primary mechanisms through which P-modified sewage sludge biochars immobilize heavy metals is by altering soil pH. The biochars produced at higher pyrolysis temperatures (>500 °C) are alkaline and rich in ash, notably calcium and magnesium oxides, which contributed to a significant pH increase in the soil matrix [16,58,84]. This pH rise directly influences metal solubility by shifting equilibrium reactions toward the formation of less soluble hydroxide and phosphate minerals [78].

Cadmium (Cd²⁺) and zinc (Zn²⁺), in particular, were effectively immobilized through precipitation as Cd(OH)₂ and Zn(OH)₂, and more stably as Cd₃(PO₄)₂ and Zn₃(PO₄)₂ when sufficient phosphate was available [85]. These processes were favored under alkaline conditions, reducing the free ion concentrations in the soil solution and leading to substantial declines in bioavailable metal fractions. Lead (Pb²⁺) also precipitated efficiently, likely forming hydroxide, carbonate, and phosphate complexes, particularly in biochars with elevated pH and phosphorus content. For copper (Cu²⁺), hydroxide precipitation also contributed to reduced mobility, especially in treatments where pH exceeded 7.5 [86].

Moreover, the rise in pH induced by biochar application enhanced the soil’s cation exchange capacity (CEC), indirectly supporting heavy metal retention through electrostatic attraction and charge neutralization mechanisms. These pH-mediated pathways formed the foundational layer of immobilization effectiveness, particularly in treatments with higher pyrolysis temperature biochars, where pH elevation was most pronounced [87,88].

4.3.2. Ligand-Specific Interactions: Complexation and Ion Exchange

In addition to pH-driven mechanisms, phosphate-modified biochars facilitated metal immobilization through direct interactions with functional groups on their surface. These include oxygen-containing groups such as carboxyl (–COOH), hydroxyl (–OH), and especially phosphate (–PO₄³⁻) functionalities, which act as high-affinity ligands for heavy metal ions [89]. Such interactions enable both surface complexation and inner-sphere coordination, forming stable, often irreversible, metal–ligand bonds [90].

Among all metals studied, lead (Pb²⁺) exhibited the strongest complexation with phosphate moieties, forming low-solubility compounds like Pb₃(PO₄)₂ that remained tightly bound to the biochar matrix. These reactions were particularly efficient in biochars pyrolyzed at moderate temperatures (450–600 °C), where surface functional groups are preserved and remain reactive [91]. For copper (Cu²⁺), both carboxylic and phenolic sites contributed to immobilization via inner-sphere surface complexation. Additionally, Cu²⁺ likely underwent ion exchange with resident cations (e.g., Ca²⁺, K⁺) present in the biochar structure, enhancing sorption capacity [25,92].

Nickel (Ni²⁺) showed the least effective immobilization, which may be attributed to its relatively weak affinity for both phosphate and carbon-based ligands under the tested pH range. This behavior aligns with earlier findings that Ni²⁺ tends to remain in exchangeable forms and requires more specific surface modifications or co-precipitation strategies to achieve stable immobilization [93].

These ligand-mediated interactions complement the pH-induced mechanisms, and their relative contribution appears metal-specific. The success of such interactions depends on the biochar’s surface chemistry, degree of phosphate modification, and preservation of active functional groups during pyrolysis. The synergy between surface complexation and precipitation likely explains the high immobilization efficiency observed for Pb and Cu compared to Ni and Zn [94].

4.4. Environmental Implications and Remediation Potential

Heavy metals can have significant detrimental effects on soil, plants, and organisms when present in critical concentrations [95]. They accumulate and contaminate soil through various sources such as industrial activities, mining, and improper disposal of waste. They can alter soil pH, decrease soil fertility, and inhibit microbial activity, leading to reduced soil productivity and ecosystem health [96]. On top of this, plants can absorb them from contaminated soil through their roots. Once inside the plant, they can disrupt essential physiological processes, impair growth and development, and result in visible symptoms such as stunted growth, chlorosis, necrosis, and reduced yield. Additionally, plants can translocate heavy metals to edible parts, posing risks to human health if consumed [97,98]. Moreover, these metal contaminations in soil can affect the entire ecosystem by harming soil-dwelling organisms such as earthworms, microorganisms, and beneficial insects, disrupting soil food webs and nutrient cycling processes [99]. This, in turn, can affect the health and productivity of higher trophic levels, including animals and humans [100]. They can also leach from contaminated soil into groundwater or surface water bodies, leading to water pollution and severe consequences for aquatic organisms, disrupting aquatic ecosystems, and affecting human health if contaminated water is used for drinking or irrigation [101,102]. Furthermore, heavy metals can bioaccumulate in organisms, particularly in tissues, at concentrations higher than those found in the surrounding environment [103]. Through biomagnification, they can become increasingly concentrated as they move up the food chain, posing risks to predators at higher trophic levels [104]. Heavy metal exposure has been linked to various health problems, including neurological disorders, developmental abnormalities, kidney and liver damage, respiratory issues, and certain types of cancer [105].

Remediating heavy metal contamination demands a multi-faceted approach. Techniques like phytoremediation utilize plants to absorb metals from soil, while bioremediation employs microorganisms to break down contaminants [106,107,108]. Chemical methods are also being used to adjust soil chemistry and physical approaches involving the removal or separation of metals [109,110,111]. Additionally, barrier systems preventing further spread and in situ or ex situ methods to determine whether treatment occurs on-site or elsewhere are in place [109,112]. Often, as no method is efficient alone, a combination of two or more of these techniques is recommended for comprehensive immobilization and cleanup, underscoring the complexity of managing heavy metal pollution and the importance of tailored strategies to ensure effective remediation [113,114]. It is worth adding that biochar has also emerged as a promising solution for remediating heavy metals, exhibiting the ability to do so with limited or no adverse side effects while offering other economic benefits [115,116,117]. The interaction mechanisms between biochar and heavy metals in soil are influenced by various factors, including the characteristics of the biochar itself, pH, and pyrolysis conditions [118]. Biochar, with its diverse surface functional groups, alkaline metal ions, mineral substances, organic matter, electrons, and pore structures, serves as an effective binding site for heavy metals, making them unavailable [119]. Through mechanisms like complexation, π bond action, electrostatic attraction, ion exchange, precipitation, and reduction, biochar can absorb or combine with heavy metals in soil, converting them from inorganic to organic states and altering their mobility and bioavailability. This interaction is crucial for soil remediation, as it enhances soil benefits by mitigating heavy metal toxicity and promoting plant growth [116].

Sewage sludge-derived biochars may contain trace levels of polyaromatic hydrocarbons (PAHs), antibiotic residues, or pathogens. Although pyrolysis can reduce these risks, comprehensive risk assessments are necessary [120,121,122]. Sewage sludge-derived biochars may elevate soil pH to levels that negatively affect crop productivity or microbial communities. Moreover, concerns remain about the potential release of emerging contaminants, such as PAHs or heavy metals, and residual pathogens. These aspects must be addressed through stringent feedstock control and post-treatment testing.

5. Conclusions and Recommendations

This study underscores the sustainable way of sewage sludge management as a waste and the role it can play in remediating heavy metal-contaminated soils. However, further research is still needed to optimize their management, valorization, and application of derived biochar for effective metal immobilization and soil remediation. We provided valuable insights into the varied impacts of sewage sludges and derived biochar on the mobility and availability of metals, highlighting their potential to influence metal concentrations in the environment. Biochar interacts with heavy metals through mechanisms like complexation, ion exchange, precipitation, and reduction, effectively reducing their mobility and bioavailability, providing a sustainable and cost-effective solution for remediating heavy metal contamination in soils. Biochar’s diverse surface functional groups, alkaline metal ions, and pore structures serve as effective binding sites for heavy metals, converting them from inorganic to organic states and mitigating their toxic effects on soil and ecosystems.

Specifically, in this study, we employed the innovative technique of co-pyrolysis of sewage sludge with K₃PO₄ to produce nutrient-enriched biochar (SBP300 and SBP500), the physicochemical properties of which were observed to be influenced by pyrolysis temperatures. The addition of sewage sludge-derived biochar to soil led to varied effects on the concentrations of copper, lead, cadmium, zinc, and nickel in soil solution extracts compared to control soil. It was found that different types of sewage sludge-derived biochar and pyrolysis temperature can either increase or decrease the concentrations of these metals in soil solution extracts. The yield ranged from 59.0% to 63.7%, with phosphorus-amended samples showing higher yields. The highest immobilization efficiency was recorded for Pb (88% reduction), followed by Cd (82%), with performance varying by biochar pyrolysis temperature. For each metal studied, different mechanisms appear to be at play. For example, the increase in Cu concentrations with the addition of dried sewage sludge and sewage sludge amended with phosphorus, the effective immobilization of Pb by biochar treatments, reduction in Zn and Ni concentrations with biochar treatments, and the significant reductions in Cd concentrations with sewage sludge and biochar addition suggest a complex interplay between soil pH, electron donor–acceptor interaction, biochar surface functional groups, and metal complexation.

This highlights the economic and green viability of using amended sewage sludge biochar for soil remediation, showing an effective and environmentally friendly alternative to traditional techniques for mitigating heavy metal contamination in soils. However, it is crucial to recognize the influence of different factors such as feedstock type and composition, pyrolysis conditions, residence time, type of soil, cations present in both soil and biochar, heavy metal species, and the performance characteristics of biochar. Moreover, while phosphate-enriched sludge biochar presents a promising remediation strategy, practical application requires careful control over feedstock quality, pyrolysis conditions, and post-treatment stabilization. Variability in feedstock composition and treatment temperatures can lead to inconsistent performance or secondary contamination risks.

Integrating phosphate biochar with phytoremediation or soil washing could enhance remediation performance. However, such combined approaches must consider cost, scalability, and compatibility. For instance, combining with hyperaccumulator plants may increase remediation effectiveness but require prolonged implementation time and regulatory oversight. Therefore, through collective efforts and ongoing research, we can utilize the full potential of biochar as a valuable tool in the remediation of contaminated soils, thereby preserving both environmental and human health integrity.