Introduction
Heavy metal contamination of soil is a global issue arising from industrial activities, mining, urban runoff, and agricultural misuse of fertilizers and wastewater (Zhang et al., 2013; Tchounwou et al., 2012). Lead (Pb), cadmium (Cd), and copper (Cu) are particularly hazardous due to their toxicity and non-biodegradable nature (Bolan et al., 2014). They persist in soils, leach into groundwater, and enter the food chain (Chaney et al., 2004; El Khalil et al., 2008), posing health and ecological risks.
Recent interest in sustainable remediation has driven the exploration of carbon-rich materials like biochar and plant-derived waste as soil amendments. Among these, Delonix regia (commonly known as flame tree or Royal Poinciana) pods—an agricultural byproduct—have shown adsorption potential in aqueous media (Babalola et al., 2019a; Festus et al., 2013), but their role in soil remediation is underexplored.
2. Mechanisms of Soil Remediation
Remediation technologies range from physical and chemical to biological approaches. Conventional techniques such as excavation, vitrification, and soil washing are effective but disruptive and costly (Beesley et al., 2011). In contrast, in situ chemical immobilization using amendments such as lime, compost, and biochar offers a more sustainable alternative (Kumpiene et al., 2008; Bolan & Duraisamy, 2003).
Biochar, produced through pyrolysis of biomass under limited oxygen, exhibits high porosity, surface area, and cation exchange capacity (Chen & Chen, 2009). These properties facilitate heavy metal immobilization via adsorption, complexation, and precipitation (Ippolito et al., 2012; Inyang et al., 2016). Similarly, Delonix regia pod powder, rich in lignocellulosic content, can chelate metal ions, though its interaction with soil systems has been less studied.
3. Materials for Soil Remediation
3.1 Biochar
Biochar’s effectiveness is attributed to its alkaline nature, porous structure, and surface functional groups (Chan & Xu, 2009). It has been widely studied for remediating Pb, Cu, Zn, and Cd in soils (Yuan & Xu, 2011; Karami et al., 2011). Moreover, its capacity to raise soil pH enhances heavy metal precipitation and reduces solubility.
3.2 Delonix regia Pod Powder
Derived from a widely distributed tree, Delonix regia pods are readily available and inexpensive. Prior studies have shown their efficacy in removing Pb and Cr from aqueous solutions (Babalola et al., 2019b; Louis et al., 2018), but applications in soil remain limited. Its performance is hypothesized to depend on organic matter interaction and surface sorption characteristics.
4. Results and Discussion (Based on Experimental Study)
4.1 Soil Characteristics
Tropical soils studied were acidic (pH 5.2–6.7) and low in organic matter (<0.04%). Cation exchange capacities (CEC) varied significantly, influencing metal retention potential. High sand content resulted in low water-holding capacities, a factor impacting amendment performance.
4.2 Effect of Aging
Aging (i.e., the time after contaminant application before treatment) influenced immobilization efficiency. Biochar consistently outperformed Delonix regia in Pb and Cu immobilization (>90% vs. ~60%). For Cd, both amendments were effective (>90%), confirming Cd’s low pH sensitivity. Results align with findings by de Barros Amorim et al. (2005) and Lanno et al. (2004).
4.3 Contaminant Concentration
At both low and high contaminant levels, biochar maintained high immobilization efficiencies (75–95%). Delonix regia performed better at lower concentrations, suggesting saturation of sorption sites at higher metal loads. This concentration dependency is crucial for field-scale application planning.
4.4 Soil pH Effects
Biochar enhanced soil pH, thereby improving heavy metal immobilization. Its efficacy remained high (87–98%) across pH 3–7. In contrast, Delonix regia showed pH-dependent performance, with immobilization efficiency rising from ~50% at pH 3 to >70% at pH 6. This reflects H⁺ ion competition at low pH, hindering metal adsorption (Rieuwerts et al., 2006).
4.5 Organic Matter Influence
Organic matter reduced biochar’s performance—likely due to humic acid-metal complex formation limiting biochar-metal interaction. Conversely, Delonix regia’s performance improved in organic-rich soils, with Pb and Cu immobilization rising by ~20%. These findings support its potential in agricultural soils with higher organic content.
5. Comparative Insights and Field Implications
| Parameter | Biochar | Delonix regia Pod Powder |
|---|---|---|
| pH Tolerance | Wide (3–7) | Moderate (5–7) |
| Organic Matter Compatibility | Better in low OM soils | Better in high OM soils |
| Contaminant Range | High and low levels | Effective at low concentrations |
| Immobilization Efficiency | Pb, Cu, Cd (>85%) | Cd (>85%), Cu & Pb (moderate) |
| Aging Impact | Consistent improvement | Moderate improvement |
These observations suggest that biochar is a versatile amendment suitable for diverse contamination scenarios, while Delonix regia is better suited to nutrient-rich agricultural soils with lower contamination loads.
6. Challenges and Future Directions
- Field Validation: Most studies remain laboratory-based; large-scale field trials are needed.
- Long-term Stability: The fate of immobilized metals under fluctuating environmental conditions is not well understood.
- Synergistic Use: Combined use of biochar with Delonix regia or compost may offer complementary benefits.
- Policy Integration: Local availability and cost-effectiveness of amendments must be considered in environmental remediation policy frameworks.
7. Conclusion
This review affirms the promise of biochar and Delonix regia pod powder as sustainable amendments for heavy metal remediation. While biochar is broadly applicable, Delonix regia offers a localized, low-cost solution for managing contamination in organic-rich, moderately impacted soils. Integrating these materials into soil management strategies requires further interdisciplinary research to validate long-term performance and ecological impacts.
References
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