In general, biochar reduces the rate of heavy metal uptake by making heavy metals less available in soil. It does this in four ways:
1. Biochar's relatively high surface area provides a high number of adsorption sites. Adsorption is the process in which a thin film of molecules adheres to a solid substance.
2. Due to biochar's alkaline nature, it's presence will increase soil pH. Heavy metals are less available at a higher pH.
3. A high cation exchange capacity allows biochar to attract and hold onto positively charged heavy metal ions.
4. Organic functional groups in biochar can bind with heavy metals. For example, carboxyl groups will bind with positively charged heavy metal ions and phenolic groups will bind very well is copper.
All these forces work together to help reduce the number of heavy metal molecules available for plant uptake. The effectivness of biochar can vary depending on the type of biochar, what feedstock was used, the heavy metals present in the soil, and the properties of the soil itself. Importantly, even in the best case scenario, biochar will not remove all heavy metals from the soil.