Biochar as The Eco-Friendly Solution or a Hidden Pollutant Source

Biochar has long been known as a soil amendment that improves land fertility, retains water, and reduces environmental stress on crops. Many farmers and environmental activists view biochar as the “black gold” of sustainable agriculture.But did you know that not all biochar is safe?

In fact, if not produced properly, biochar can contain harmful substances such as heavy metals and polycyclic aromatic hydrocarbons (PAHs) — two types of contaminants that pose serious risks to the environment and human health.

What Are PAHs and Heavy Metals?

• PAHs (Polycyclic Aromatic Hydrocarbons) are chemical compounds formed during the incomplete combustion of organic materials. They are known to be carcinogenic, mutagenic, and toxic.

• Heavy metals like lead (Pb), cadmium (Cd), chromium (Cr), and copper (Cu) can disrupt plant growth, pollute groundwater, and endanger living organisms if they enter the food chain.

Biochar Sources Vary — So Does Its Content

In a study, researchers processed various organic materials (called feedstock) into biochar, including:

• Sewage sludge

• Olive oil processing waste

• Poultry feathers

• Soft offal

• Chicken manure

• Date palm waste

Each material underwent pyrolysis (thermal decomposition in low oxygen) at 300°C, 500°C, and 700°C, resulting in 18 types of biochar with very different characteristics.

What Did the Study Find?

1. Higher Temperature = Higher PAHs

   The higher the pyrolysis temperature — especially up to 700°C — the greater the PAH content, particularly in biochar made from chicken manure.Notable PAHs detected included:

   1. Benzo[b]fluoranthene

   2. Benzo[k]fluoranthene

   3. Benzo(a)pyrene

2. Heavy Metals Also Increase with Temperature

   When pyrolysis temperature was increased from 300°C to 700°C, a significant rise in heavy metal concentration was observed, including:

   1. Chromium (Cr) in biochar from sewage sludge and olive waste

   2. Manganese (Mn) in biochar from chicken manure

   3. Iron (Fe), Nickel (Ni), Copper (Cu), and Zinc (Zn) in sewage sludge biochar

3. Not All Metals Increase

   Interestingly, cadmium (Cd) levels decreased in sewage sludge biochar at 700°C compared to lower temperatures.

Key Factors That Affect Biochar Safety

From the study, two major factors clearly determine the potential toxicity of biochar:

1. Feedstock Type

   Each type of waste carries a unique chemical profile.For example, municipal sewage sludge tends to contain more heavy metals than agricultural waste.

2. Pyrolysis Temperature

   Higher temperatures create more physically stable biochar, but also increase the risk of PAH formation and the concentration of certain heavy metals.

The Hidden Threat of PAHs

PAHs are invisible but deadly. Though biochar may look like ordinary soil, if it contains PAHs, it can:

• Disrupt soil microbes

• Enter plants and accumulate in food crops

• Pollute air and water when broken down or disturbed

That’s why it's essential to test biochar for PAHs and heavy metals before using it extensively in agriculture.

The Solution: Safe Biochar Production

To ensure biochar is truly safe and beneficial, several best practices must be followed:

• Choose low-contaminant feedstock

• Use appropriate pyrolysis temperatures — higher is not always better

• Conduct lab testing to measure PAHs and heavy metals before application

Biochar remains a promising solution for agriculture and the environment — but only when produced and used wisely.

Like two sides of a coin, biochar can be a soil savior or a silent source of pollution. Its benefits can only be realized if we understand how production methods determine its final quality.

Discover more about SAWA’s Innovative Biochar production and its impact on sustainable farming.

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