Rinsing biochar can ruin its ability to clean toxic lead from water

Biochar has a good reputation for cleaning up polluted soil and water, but a new study shows that much of its magic comes from something easy to wash away.

A small, dissolved fraction of biochar – called dissolved organic matter (DOM) – turns out to be doing a surprising amount of the work in locking up toxic lead. When that fraction is removed, biochar’s ability to trap lead collapses.

The study, led by researchers from Northeast Agricultural University, helps clear up a puzzle that has bothered environmental scientists for years.

Why does biochar made at lower temperatures often remove metals better than high-temperature material? The answer, the team says, lies in the chemistry of this dissolved organic matter.

Biochar’s big environmental role

Biochar is made by heating crop residues or other organic waste under low-oxygen conditions. It’s porous, carbon-rich, and cheap, so it has become a favorite for immobilizing heavy metals such as lead in soil and wastewater.

But not all biochars are equal. Lower-temperature biochars usually contain more oxygen-rich functional groups and more soluble organics.

These versions often outperform “cleaner,” harsher pyrolyzed materials, but the reason wasn’t fully nailed down.

This work shows that the dissolved fraction – which some operators actually rinse off – can account for most of the metal-binding capacity. In other words, the stuff some people throw away is actually the stuff doing the heavy lifting.

Washing biochar changed everything

The team tested the role of DOM by comparing ordinary biochar with samples that were water-washed to remove dissolved material.

The difference was dramatic. Unwashed biochar absorbed about 96 milligrams of lead per gram of material. Once the DOM was removed, capacity fell to about 35 milligrams per gram – nearly a two-thirds drop.

That single comparison pinpointed the dissolved organic material as an essential player. It’s not just the solid biochar surface capturing lead. It’s the flexible, reactive molecules that leach out of it.

Chemistry drives the cleanup

Using infrared spectroscopy, X-ray photoelectron spectroscopy, and multidimensional fluorescence, the researchers traced how lead actually binds.

They found that oxygen-containing groups – hydroxyl, carboxyl, carbonyl, and ether – form strong complexes with lead. These are classic metal-binding sites, and they were particularly abundant in the dissolved fraction.

Most of the lead ended up as stable products such as basic lead carbonate, which tells us the main mechanism isn’t just physical adsorption into pores. It’s chemical complexation – lead is being tied up, not just stuck.

“By combining several complementary spectroscopic methods, we could visualize how different molecular sites within dissolved organic matter interact with lead,” said corresponding author Song Cui from Northeast Agricultural University.

“These insights help explain why certain types of biochar perform better in removing heavy metals and how we can design them more effectively.”

Inside biochar’s dissolved organics

The team also showed that the dissolved organic matter in biochar is chemically diverse. Fluorescence and UV analyses revealed several humic-like components, each with different reactivity toward lead.

One fraction, rich in humic and tyrosine-like substances, had the strongest binding affinity. Two-dimensional correlation analysis further picked out carboxyl groups in these humic substances as the fastest and strongest responders to lead.

That’s important because it means we can stop talking about DOM as a single blob and start talking about which fractions actually matter. If carboxyl-rich, humic-like components are the real stars, we can target them in biochar production.

Cooler chars clean better

This work offers a neat explanation for the earlier puzzle. Lower pyrolysis temperatures tend to preserve oxygenated functional groups and keep more soluble organics in the material. That makes the resulting biochar richer in reactive sites for metals.

High-temperature chars, by contrast, are more carbonized and aromatic – great for stability, less great for binding cations like lead.

The takeaway: don’t always strip your biochar “clean.” In some cases, the “dirty” version is the more effective adsorbent.

Smarter design, stronger cleanup

The study doesn’t just solve a mystery – it points a way forward. Knowing that carboxyl- and humic-like structures do most of the binding allows scientists to engineer future biochars richer in those components.

That could mean adjusting feedstocks, tuning pyrolysis temperature, or even post-treating biochar to add or protect reactive groups.

Such tailored materials could make lead immobilization more stable under real-world conditions, where pH, competing ions, and organic contaminants can all interfere.

Proving biochar’s power

The authors are clear that the next step is complexity. Real waters and soils don’t contain only lead. They contain mixtures – cadmium, zinc, copper, even organic pollutants – and the chemistry can change with pH, salinity, and microbial activity.

Future work will need to see whether DOM-enhanced biochars can keep their edge when several metals are competing for the same binding sites or when acidity rises.

But the core insight stands: a small, soluble piece of biochar can massively boost metal capture. Instead of treating that fraction as a nuisance, we can harness it.

Biochar’s power to lock up lead doesn’t just come from its sponge-like structure. It comes from reactive, oxygen-rich dissolved organics that form strong chemical bonds with the metal.

Washing those away can gut performance. Keeping – or even enriching – them could give us a new generation of biochar-based, low-cost tools to protect water and soil from heavy-metal pollution.

The study is published in the journal Biochar.

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