Introduction
Soil contamination with heavy metals has become a global environmental issue due to the continuous input of industrial effluents, agricultural runoff, mining wastes, and other anthropogenic sources. These toxic metals—commonly including Pb, Cd, Cu, Zn, and Hg—are non-biodegradable, persist in the soil matrix, and pose a substantial threat to plant health, ecosystem stability, and human well-being. In many cases, these contaminants infiltrate groundwater, bioaccumulate in crops, and enter the food chain, thereby increasing the risk of chronic exposure (Bolan et al., 2014; Chaney et al., 2004).
Urban and industrial soils, in particular, are frequently exposed to metal pollutants originating from vehicular emissions, construction materials, and industrial discharge (Todorovic et al., 2014; Tchounwou et al., 2012). Once introduced into the soil, heavy metals interact with soil components in complex ways, affecting their mobility, bioavailability, and long-term fate.
Heavy metals become bioavailable primarily in their soluble forms. Therefore, research into strategies that reduce the solubility and mobility of these metals is critical. Several soil properties—including pH, organic matter, cation exchange capacity (CEC), and soil texture—govern the extent of metal bioavailability (Fijałkowski et al., 2012). Conventional remediation methods such as excavation and landfill disposal are costly and environmentally intrusive. Hence, more sustainable and ecologically friendly approaches are gaining attention.
Among the innovative solutions is chemical immobilization, which involves amending contaminated soils with materials that can adsorb or bind metal ions, rendering them less mobile. Biochar, produced from incomplete combustion of biomass, has emerged as a highly efficient immobilizing agent due to its high surface area, abundant functional groups, and porous structure (Beesley et al., 2011; Yuan and Xu, 2011). It can adsorb a variety of heavy metals and organic pollutants, thereby enhancing soil quality and reducing metal toxicity.
In contrast, Delonix regia, commonly known as the flame tree, is an ornamental plant whose pods are often treated as waste. Recent studies have hinted at the potential of Delonix regia biomass, especially in modified or activated forms, to serve as an effective adsorbent for aqueous-phase heavy metal removal (Babalola et al., 2019a; Festus et al., 2013). However, its application in remediating heavy metal-contaminated soils remains underexplored.
The objective of this study is to comparatively assess the immobilization efficiency of hardwood biochar and Delonix regia pod powder in remediating tropical soils contaminated with Pb, Cu, and Cd. The study evaluates their performance under varying conditions—soil pH, organic matter content, contaminant concentrations, and different aging periods—to determine their practical applicability in field-scale remediation.
Material and Methods
This study utilized both natural and laboratory-prepared materials to evaluate the potential of Delonix regia pod powder and hardwood biochar as remediation agents for metal-contaminated soils. The following chemical reagents and materials were employed:
- Metal Salts: Analytical-grade copper(II) chloride dihydrate (CuCl₂·2H₂O), lead(II) nitrate [Pb(NO₃)₂], and cadmium nitrate [Cd(NO₃)₂] were used to prepare metal solutions for soil contamination experiments.
- Soil Amendments:
- Biochar: Commercially available hardwood-derived biochar was sourced from Bodfari Environmental, UK. It was selected based on its high surface area and structural stability.
- Delonix regia Pods: Mature pods were collected from the grounds of Ekiti State University, Ado-Ekiti, Nigeria. They were sun-dried, ground into fine powder, and sieved to obtain uniform particle sizes for experimental use.
- Other Reagents:
- Humic Acid Suspension: Prepared and added at 2% w/w to certain soil samples to simulate increased organic matter.
- Sodium Acetate and Ammonium Acetate: Utilized for determining cation exchange capacity (CEC).
- Calgon Solution (sodium hexametaphosphate): Employed in particle size analysis.
- 0.01 M Calcium Chloride (CaCl₂): Used as an extractant to determine the bioavailable metal fractions.
- Methylated Spirit: Utilized in sterilization and sample handling procedures.
Soil Sampling and Preparation
Tropical soil samples were collected from multiple locations in southwestern Nigeria known for their historical exposure to artisanal mining activities. Sampling was conducted along transects at various depths, as outlined below:
- Sampling Depth: 0–20 cm
- Transect Points: 0 m, 2 m, 4 m, 10 m, and 50 m
- Locations: Ijero, Ilesha, Ekiti, and Ife regions
After collection, samples from each transect were homogenized, air-dried at room temperature, and sieved through a 2 mm mesh to remove coarse debris. Processed samples were stored in airtight plastic containers and transported to Lancaster University, UK, for laboratory analysis.
Soil Characterization
The physicochemical properties of the soil samples were analyzed using standard protocols:
- Soil pH: Measured in a 1:2.5 soil-to-water suspension using a calibrated pH meter (PHM 220 model).
- Organic Matter Content: Determined by loss-on-ignition (LOI) method at 550°C following the procedure of Konare et al. (2010).
- Cation Exchange Capacity (CEC): Assessed via sodium saturation and ammonium displacement methods based on Lu (2000).
- Particle Size Distribution: Measured using the hydrometer method with Calgon as the dispersing agent. Texture classification followed the Soil Survey of England and Wales triangle chart.
- Water Holding Capacity (WHC): Determined by saturating 20 g of soil in a plant pot, draining for 24 hours, oven drying, and applying Equation (1):
WHC(%)=(Wet Weight−Dry Weight)Dry Weight×100\text{WHC} (\%) = \frac{(\text{Wet Weight} – \text{Dry Weight})}{\text{Dry Weight}} \times 100WHC(%)=Dry Weight(Wet Weight−Dry Weight)×100
- Exchangeable Metals: Extracted using 0.01 M CaCl₂ and analyzed by Inductively Coupled Plasma–Mass Spectrometry (ICP-MS).
Table 1. Physicochemical Properties of Collected Soil Samples
| Location | Longitude | Latitude | pH | Organic Matter (%) | Particle Size Distribution (%) | Texture | CEC (meq/100g) | WHC (%) |
| Sand | Silt | Clay | ||||||
| Ijero | 5.0742 | 7.8139 | 5.20 | 0.018 | 64.72 | 17.50 | 17.78 | Sandy Loam |
| Ilesha | 4.6500 | 7.5333 | 6.72 | 0.033 | 59.62 | 12.50 | 27.88 | Loamy Sand |
| Ekiti | 5.0742 | 7.8139 | 5.40 | 0.026 | 58.20 | 15.00 | 26.80 | Sandy Clay Loam |
| Ife | 4.5667 | 7.4667 | 5.65 | 0.018 | 61.94 | 10.00 | 28.06 | Sandy Clay Loam |
Experimental Design and Treatment Setup
To assess the performance of biochar and Delonix regia pod powder under varying environmental conditions, the following experimental conditions were evaluated:
- Effect of Soil pH: Soil samples were adjusted to pH levels of 3, 4, 5, 6, and 7 using dilute HCl or NaOH solutions.
- Contaminant Concentration:
- Low: 100 mg/kg Pb and Cu; 5 mg/kg Cd
- High: 300 mg/kg Pb and Cu; 15 mg/kg Cd
- Organic Matter Content: Humic acid was added at 2% w/w to simulate organic-rich soil conditions.
- Aging Time: Immobilization efficiency was monitored at different time intervals (0, 1, 5, and 10 days post-treatment).
Soils were treated with 2% (w/w) of either biochar or Delonix regia powder and thoroughly mixed. Following treatment, samples were incubated under controlled conditions and later analyzed for bioavailable metal content using the 0.01 M CaCl₂ extraction method.
Results & Discussion
1. Soil Physicochemical Properties
The characterization of the tropical soils revealed distinct variations in pH, cation exchange capacity (CEC), and organic matter (OM) content, which are critical factors influencing metal retention and mobility.
- The pH values ranged between 5.20 and 6.72, indicating slightly acidic conditions.
- Organic matter content was extremely low (0.018–0.033%), highlighting the degradation of soil quality, possibly due to land degradation and mining activities.
- The CEC varied significantly among samples, with Ilesha soil exhibiting the highest value (25.22 meq/100g), possibly because of its prior use as a cocoa plantation.
These parameters provide insight into the background metal sorption potential of the soils prior to amendment treatment.
2. Effect of Aging on Immobilization Efficiency
The influence of contaminant aging on the immobilization performance of biochar and Delonix regia pod powder was assessed over a period of 10 days.
Key Observations:
- Immobilization efficiency improved with time, indicating progressive stabilization of metals in the amended soils.
- Cadmium immobilization remained relatively constant across time, a trend previously observed by de Barros Amorim et al. (2005).
- Biochar consistently outperformed Delonix regia in immobilizing Pb and Cu across all soils, particularly under low pH conditions.
Table 2. Average Immobilization Efficiency (%) Over Aging Period
| Metal | Amendment | Day 1 | Day 5 | Day 10 |
| Pb | Biochar | 81.5 | 89.2 | 91.7 |
| Pb | D. regia | 48.2 | 58.1 | 63.4 |
| Cu | Biochar | 83.7 | 91.5 | 93.6 |
| Cu | D. regia | 45.6 | 55.7 | 60.2 |
| Cd | Biochar | 89.6 | 90.1 | 91.3 |
| Cd | D. regia | 86.7 | 87.0 | 88.2 |
These results demonstrate that biochar induces longer-term stabilization, while D. regia may require optimization or pretreatment to reach similar efficiencies.
3. Effect of Contaminant Concentration
To evaluate how contaminant load affects immobilization efficiency, soils were spiked with low and high concentrations of Pb, Cu, and Cd.
- Low concentration (within regulatory limits): Both amendments showed high performance, with D. regia reaching up to 83% efficiency for Cu.
- High concentration (3× regulatory limits): Biochar maintained high efficiency (>82%) while D. regia’s performance dropped significantly, especially for Pb.
Figure 1. Immobilization efficiency under low and high contaminant levels
Table 3. Average Immobilization Efficiency (%) in Low vs. High Contaminated Soils
| Metal | Amendment | Low Conc. | High Conc. |
| Pb | Biochar | 84.6 | 82.1 |
| Pb | D. regia | 67.2 | 49.3 |
| Cu | Biochar | 86.5 | 85.8 |
| Cu | D. regia | 83.0 | 61.2 |
| Cd | Biochar | 90.4 | 89.1 |
| Cd | D. regia | 86.3 | 82.0 |
Final Remarks: Biochar exhibited resilience across contamination levels, while Delonix regia was more effective under low-to-moderate contamination.
4. Effect of Soil pH on Metal Immobilization
Soil pH plays a pivotal role in the sorption and desorption of heavy metals. Treatments were applied across a pH gradient (3 to 7) to evaluate performance.
- Biochar: Not only retained high immobilization efficiency (>87%) across all pH levels but also slightly elevated the soil pH, especially in highly acidic soils.
- Delonix regia: Showed a direct correlation between increasing pH and enhanced metal sorption—indicative of its reduced efficacy in acidic conditions.
Table 4. Immobilization Efficiency (%) at Different Soil pH
| pH | Amendment | Pb (%) | Cu (%) | Cd (%) |
| 3 | Biochar | 87.4 | 87.8 | 89.1 |
| 3 | D. regia | 42.4 | 50.0 | 86.6 |
| 5 | Biochar | 95.1 | 95.9 | 95.9 |
| 5 | D. regia | 58.7 | 56.5 | 88.4 |
| 7 | Biochar | 98.3 | 92.6 | 98.9 |
| 7 | D. regia | 68.2 | 74.5 | 90.0 |
These findings reinforce that biochar is suitable for a broader pH range, while D. regia requires near-neutral to alkaline pH for optimal performance.
5. Effect of Organic Matter on Immobilization
Organic matter significantly alters metal availability via complexation or competition. Soils were amended with humic acid to assess this effect.
- In biochar-treated soils, organic matter addition reduced metal sorption, likely due to decreased pH and competition for binding sites.
- In contrast, Delonix regia’s efficiency improved in OM-rich soils—likely due to synergistic interactions between pod powder and humic substances.
Figures 2 & 3. illustrate the difference in response across Ekiti and Ilesha soils.
Table 5. Average Immobilization Efficiency (%) With and Without Organic Matter
| Soil | Amendment | Pb (No OM) | Pb (+OM) | Cu (No OM) | Cu (+OM) | Cd (No OM) | Cd (+OM) |
| Ekiti | Biochar | 91.2 | 81.4 | 93.5 | 84.1 | 91.8 | 92.5 |
| Ekiti | D. regia | 45.0 | 70.2 | 50.0 | 70.1 | 87.6 | 87.9 |
| Ilesha | Biochar | 94.6 | 88.5 | 95.1 | 89.3 | 95.0 | 92.1 |
| Ilesha | D. regia | 50.2 | 55.0 | 52.3 | 62.2 | 88.4 | 89.0 |
These findings underscore that biochar is more effective in mineral soils, while Delonix regia is more compatible with organic-rich agricultural soils.
General Insights
- Biochar demonstrated consistent performance across all test conditions and is particularly effective in degraded, acidic, and high-contaminant soils.
- Delonix regia exhibited moderate but situational performance—optimal in soils with moderate to high organic matter and near-neutral pH.
- The overall immobilization efficiency of both amendments improved with aging, suggesting time-dependent stabilization mechanisms.
Conclusion
This study demonstrates the comparative potential of Delonix regia pod powder and hardwood biochar as soil amendments for remediating heavy metal-contaminated tropical soils. The immobilization efficiency of these amendments was tested under various environmental parameters including aging period, contaminant concentration, soil pH, and organic matter content. The following conclusions can be drawn:
- Biochar consistently exhibited superior immobilization efficiency across all tested metals (Pb, Cu, and Cd), and its performance was largely unaffected by changes in soil pH or contaminant concentration. It also proved to be more effective in low organic matter soils typical of weathered or disturbed environments.
- Delonix regia pod powder, although less efficient overall, showed improved performance in soils rich in organic matter and with near-neutral to slightly alkaline pH. Its affordability, abundance as agricultural waste, and moderate efficiency under specific conditions make it a viable amendment for remediating agricultural soils with low-to-moderate metal pollution.
- Soil pH was a critical factor influencing metal immobilization, particularly for Delonix regia, whose efficiency improved significantly with increasing pH. In contrast, biochar’s alkalinity-buffering capacity allowed for stable immobilization across the pH range.
- Aging influenced amendment performance, with longer contact times enhancing immobilization, especially for lead and copper. This highlights the importance of considering equilibrium timeframes when applying amendments in field conditions.
- Organic matter influenced sorption behavior differently: it reduced the immobilization potential of biochar but enhanced the performance of Delonix regia, likely due to increased complexation and sorption site availability.
Conflict of Intrest
The authors declare no conflict of interest pertaining to the publication of this research.
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