Biochar‑Amended Soil Remediation for PAHs and PCBs

Harnessing Biochar’s Power to Trap Persistent Organic Pollutants

Introduction

Soil contamination with persistent organic pollutants (POPs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) remains a significant environmental and human health concern globally. These compounds resist natural breakdown, accumulate through the food chain, and pose serious long‑term cancer and ecosystem risks.

Recently, a 2025 MDPI study has demonstrated a promising approach: amending sludge–soil systems with biochar to significantly inhibit plant uptake of PAHs by approximately 27–34 per cent, while simultaneously enhancing key soil health indicators and plant biomass. Leveraging biochar as an amendment brings together pollutant sequestration, soil conditioning, carbon retention, and improved plant growth into a single, multifunctional solution.

In this article, we explore:

• What biochar is, and how it functions
• Key research that underpins the claimed reductions in plant uptake
• Mechanisms of action for biochar with respect to PAHs and PCBs
• Broader scientific evidence and meta‑analyses
• Benefits and limitations of the approach
• Practical considerations for field application
• Future prospects for integrated remediation strategies

What Is Biochar?

Biochar is a highly porous, carbon‑rich material produced by pyrolysis—the thermochemical decomposition of biomass under oxygen‑limited conditions. Common feedstocks include crop residues, wood chips, sewage sludge, or other agricultural wastes. The process yields biochar (solid), syngas and bio‑oil (liquid and gases) (PubMed, Wikipedia, SpringerLink).

It is characterised by:

• High surface area and pore volume
• Aromatic carbon structures that are chemically stable
• A range of surface functional groups (e.g. carboxyl, phenolic) depending on temperature and feedstock

Properly produced biochar is beneficial in soils for carbon sequestration and improved fertility. However, if poorly produced, it can contain contaminants like PAHs, metals or volatile organic compounds (VOCs)—hence processing standards such as those from the International Biochar Initiative (IBI) and European Biochar Certificate (EBC) are vital (MDPI, Wikipedia).

The 2025 MDPI Study: PAH Uptake and Soil Health Benefits

The focal MDPI study (2025) investigated sludge–soil mixtures amended with biochar and showed:

• 27–34 per cent reduction in PAH uptake by plants (ryegrass) compared to unamended sludge–soil systems
• Enhanced biomass and chlorophyll content, signifying improved soil fertility and plant growth
• Better soil buffering capacity and nutrient availability, attributed to biochar’s high porosity and nutrient retention
• Recommendations for using biochar as a simple additive during sludge composting or remediation to reduce pollutant mobility and improve ecological outcomes (PubMed, PMC).

This result is highly relevant: agricultural or industrial sites often involve sludge containing PAHs. Incorporating biochar creates a remediation strategy that does double duty: it sequesters contaminants and builds productive soil.

Mechanisms: How Biochar Reduces PAH and PCB Mobility

Biochar’s effectiveness stems from several interlocking mechanisms:

1. Adsorption and Immobilisation

Biochar’s large surface area and aromatic structure enable strong hydrophobic/π–π interactions with PAHs, and multiple bonding types (hydrogen bonds, dipole, coordination) with PCBs. It also physically entraps pollutants in its micropores, which reduces bioavailability (SpringerLink).

2. Pore‑Filling and Microbial Habitat

Biochar’s porous structure not only retains contaminants but also offers a habitat for microorganisms, including PAH‑ and PCB‑degrading bacteria. Populations such as Sphingomonas, Haliangium, and others colonise biochar surfaces and enhance biodegradation (PubMed).

3. Rhizosphere Enhancement and Phytostimulation

In soil–plant systems, biochar supports root growth and microbial symbiosis. Roots grow into biochar pores, releasing exudates that feed pollutant‑degrading microbes. Additionally, plant phenolic compounds may bind to biochar and activate microbial PCB degradation pathways (PMC).

4. Enhanced Microbial Biodegradation

Biochar can stimulate enzymatic activity (e.g. dehydrogenase, urease) and microbial abundance within the soil microbial community—including genes encoding PAH‑ring hydroxylating dioxygenases—resulting in higher biodegradation rates, especially with high‑temperature biochar (PubMed).

Evidence from Meta‑Analyses and Broader Research

PAHs

A global meta‑analysis of 2,236 observations across 56 studies found biochar reduced soil PAH concentrations by an average of ~25%, with reductions varying by PAH ring number. Longer ring PAHs (5–6 rings) showed up to 40 per cent reductions when biochar application and feedstock were optimised, especially at high pyrolysis temperatures and selected feedstocks like wood‑derived biochar (PubMed).

Other systematic reviews show biochar enhances soil health and pollutant immobilisation while promoting biodegradation of organic contaminants, including pesticides and dyes, through adsorption and microbial stimulation (frontiersin.org, PMC).

PCBs

According to a 2021 review, biochar in PCB‑contaminated soils:

• Reduced PCB uptake by plant roots by up to 89% at 11 % biochar application by mass
• Also reduced PCB concentrations in stems, and in organisms such as earthworms by 52–88%
• Supported better plant growth and soil biomass in contaminated sites
• Promoted microbial attachment and dechlorinating communities on its surfaces, enhancing PCB degradation (PubMed, PMC).

These results confirm that biochar can both immobilise PCBs chemically and biologically promote their breakdown through microbial community cooperation.

Benefits of Biochar‑Amended Remediation

1. Dual Functionality: Sequesters pollutants while boosting soil fertility, plant growth and structure.
2. Low Environmental Impact: No harsh chemicals or heating, unlike thermal or chemical treatments.
3. Adaptable and Scalable: Feedstocks range from agricultural wastes to sludge; suitable across scales including field trials and industrial sites.
4. Carbon Sequestration Co‑Benefit: Biochar locks stable carbon into the soil long‑term, helping climate mitigation efforts (Wikipedia).
5. Supports Biodegradation: Biochar’s microhabitats foster beneficial microbial communities that actively degrade PAHs and PCBs.
6. Reduced Bioaccumulation: Lower plant uptake reduces risk to crops and food chains, protecting ecosystem and human health.

Limitations and Risks to Consider

Endogenous Contaminants

Biochar can itself contain PAHs, heavy metals, PCDD/Fs or PCBs depending on feedstock and pyrolysis process. This risk is minimised when high pyrolysis temperatures (>500 °C), inert carrier gases and stringent protocols are used. Standards defined by IBI or EBC help ensure safe biochar (PubMed).

Soil pH and Biota Disruption

Biochar is often alkaline, which can alter soil pH and potentially impact sensitive soil organisms like earthworms or microbial communities. Application rates must be calibrated and soil monitoring conducted to prevent unintended disruption (frontiersin.org, frontiersin.org).

Pesticide Efficacy Interference

Due to its high adsorption, biochar may sequester pesticides, reducing their efficacy in agricultural contexts. This dual role necessitates careful management if biochar is used alongside crop protection regimes (frontiersin.org).

Variable Field Performance

Lab and greenhouse experiments often show consistent benefits, but field performance can vary due to soil texture, climate, organic loading, pollutant type and hydrology. Field validation remains critical (MDPI, SpringerLink, frontiersin.org).

Practical Guidelines for Implementation

Selecting Biochar

• Use high‑temperature pyrolysis (>500 °C) for lower PAH impurity and better aromatic structure for high‑ring PAHs (SpringerLink, SpringerLink).
• Choose feedstocks low in contaminants, e.g. clean plant biomass rather than sewage sludge, unless sludge‑specific controls and testing are implemented.
• Confirm compliance with standards such as IBI or EBC for PAH, metals and VOC content.

Application Rates

• Studies show better uptake suppression at around 5 per cent (by mass) biochar amendment, yet crop yield improves also at lower rates like 2 per cent depending on biomass and feedstock type (PubMed, PMC, SpringerLink).
• The 2025 study noted 27–34 per cent PAH uptake reductions at tested amendment levels (typical sludge–soil mix applications) (PMC).

Integration with Sludge or Compost Remediation

• Blend biochar into composted or dewatered sludge before land application. The amendment improves compost chemistry, reduces PAH mobility, and yields cleaner, more fertile biosolids.
• When planting is involved (e.g. ryegrass), biochar both supports plant growth and restricts pollutant transfer.

Monitoring and Assessment

• Monitor soil pH, organic matter, nutrient levels, microbial enzyme activity and biomass to assess both soil health and remediation efficacy.
• Test plant tissue for residual PAH or PCB uptake to quantify environmental risk reductions.

Ecosystem and Food Safety Considerations

• If food crops or grazing species are involved, plant uptake measurements are essential to verify food safety.
• Trace pollutant transfer into earthworms or soil fauna where relevant: studies show biochar can reduce tissue PCB levels by >50 per cent (PMC).

Case Example: Ryegrass in Sludge–Soil System

In the 2025 MDPI sludge–soil study:

• Ryegrass was grown in sludge-soil mixtures with biochar amendments.
• Biomass increased by approx. 23–49 per cent, chlorophyll content by 8–10 per cent, indicating stronger plant health.
• PAH uptake decreased by 27–34 per cent compared to non-amended controls.
• Soil buffering capacity and nutrient retention also improved, highlighting multiple beneficial outcomes (PMC).

This illustrates how biochar can convert a contaminated matrix into a more productive, less risky system via a single, manageable intervention.

Broader Policy and Remediation Implications

• Municipal sludge application: Cities can blend biochar into treated sludge before land distribution to reduce pollutant transfer risk and improve agricultural utility.
• Brownfield or post-industrial sites: Biochar amendments can mitigate residual PAHs or PCBs, stabilising soils for ecological restoration or redevelopment.
• Agricultural lands: In areas affected by PAH contamination (e.g. near traffic corridors or agrochemical drift), biochar helps reduce crop uptake while improving fertility.
• Carbon‑negative climate strategies: Biochar is increasingly valued not only for pollutant control, but also for sequestration potential—some recent analyses highlight that carbon permanence in biochar may be significantly underestimated in policy models (Reddit).

Future Research Directions

1. Field‑scale validation: More long‑term trials in varied soil types, climates, and pollutant mixtures to verify laboratory findings.
2. Optimised biochar design: Engineering biochars with tailored functionality—e.g. doped with nutrients, activated surfaces, or co‑materials—for enhanced PAH or PCB binding and microbial colonisation.
3. Integrated remediation systems: Combining biochar with microbial inoculants or plants (phytostimulation) to accelerate biodegradation and site recovery.
4. Ecotoxicology and safety studies: Assess unintended impacts on soil biota or pollutant binding affecting pesticide efficacy.
5. Policy alignment: Incorporating verified biochar application protocols into safer sludge management, brownfield swaps, and agricultural best practices.

Conclusion

Biochar‑amended soil remediation presents a multi‑benefit, sustainable strategy for dealing with stubborn organic pollutants such as PAHs and PCBs. The 2025 MDPI study showing 27–34 per cent reductions in plant uptake of PAHs—alongside improved soil fertility and plant biomass—highlights biochar’s potential to convert contaminated matrices into healthier, productive landscapes.

Supported by meta-analyses and existing literature on PCB reduction in plant and soil biota systems, biochar offers:

• Sequestration of pollutants, reducing spread and food chain transfer
• Enhanced microbial degradation, via microbial colonisation and enzyme stimulation
• Soil conditioning, boosting fertility, nutrient retention and biomass
• Climate mitigation benefits, by storing stable carbon over long timescales.

However, benefits rely on careful feedstock selection, pyrolysis control, compliance with safety standards, balanced application rates, and thorough monitoring. Field validation remains essential to scale these findings for real‑world practice.

When thoughtfully applied, biochar can transform remediation—from a costly chemical intervention to an ecosystem-arming tool that detoxifies, restores and revitalises soil in one go. As environmental pressures mount, embracing biochar-amended strategies helps us seize a greener, cleaner future for soil, food, water—and communities.

References

• Biochar description and properties (frontiersin.org).
• 2025 MDPI sludge–soil biochar study (PAH uptake, soil/plant benefits) (PMC).
• Global meta‑analysis of biochar effects on PAHs (PubMed, SpringerLink).
• PCB remediation review and plant/earthworm uptake reduction (PubMed, PMC).
• Potential risks (endogenous pollutants, pH effects, pesticide adsorption) (SpringerLink).
• Limits in field efficacy (MDPI).
• Climate and carbon mitigation potential of biochar (Reddit).

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