For thousands of years, innovative farmers have added a charcoal-like substance—biochar—to the soil to improve crop productivity. Between 450 BCE and 950 CE, native farming communities of the Amazon basin successfully converted low-fertility, tropical soils to agricultural soils through the amendment of biochar, compost, manure, and other waste. Unknown to these farmers, the physical and chemical properties of the biochar in these terra preta soils (“black soil” in Portuguese) were helping to retain soil nutrients that otherwise would have been leached away during the tropical precipitation. In recent decades, interest in biochar has experienced a resurgence as insightful scientists have looked to those same properties that helped biochar mitigate nutrient leaching for utility in environmental remediation endeavors.

What is biochar? Analysis of the name likely draws up images of a burned forest, perhaps a biological experiment involving fire, or a new product riding today’s wave of environmental interest. In summary, when any carbon biomass (e.g., wood waste, plants, or even animal waste) is exposed to high heat in an oxygen-free environment, this biomass converts to a solid, charcoal-like product known as biochar. In this form, the carbon is resistant to decomposition and is chemically stable. Macroscopically, biochar appears as dusty, black charcoal pieces with grains of generally 0.1 centimeters or smaller. But it’s the microscopic properties of biochar that make it unique. Consider a cross-section of a plant under a microscope with all the plant’s xylem, stoma, cells, and other tissues shown in detail. Then, imagine removing all liquid components and leaving only this skeletal framework behind. This solid, carbon-based matrix makes up biochar and instills it with tremendous surface area. This surface area and its generally negative surface charge are the basis for biochar’s environmental applications.

The high surface area and negative surface charge of biochar enable it to adsorb various substances and chemicals. Within the terra preta soils, nitrogen and other nutrients from manure and other wastes would adhere to the biochar’s surface rather than be leached away during rainfall. This maintained nutrient availability to the crops grown by the Amazonian farmers. Similarly, pollutants (particularly those with a positive charge) cling to biochar and can include organic compounds and heavy metals. Pollutants adhering to biochar are immobilized and prevented from entering the water supply—much as a charcoal filter will screen particles from drinking water. Adding biochar has been shown to help stabilize heavy metals in soil and thus may make it a valuable tool in mine reclamation and the treatment of industrial spills.

In addition to stabilizing metals and chemicals, research suggests that the simultaneous adsorption of organic compounds and microorganisms to biochar facilitates metabolic interaction and expedites the degradation of these pollutants. Bioremediation, the use of microbes to clean up contaminated soil and groundwater, may be enhanced with biochar. One method of bioremediation—bioaugmentation—involves the introduction of a concentrated culture of microbes to the contaminated site. The physical application of these microbes requires mixing them with a carrier medium and amending the area of treatment with this mixture. A slurry containing biochar not only acts as an effective carrier but also provides a protective habitat for the microbes within the pores and rough surface of biochar. As aforementioned, biochar’s affinity for adsorbing organic pollutants helps to bring both the organic pollutants and the microbes into association, thus accelerating the breakdown of these toxins. One study found enhanced bioremediation of polycyclic aromatic hydrocarbons when using biochar as a carrier. Other studies have found similar success when using biochar in bioremediation.

Biochar may also be used to control ongoing pollution from agriculture and large-scale industrial processes. Wetlands and biofilters (engineered filtration systems composed of soil, sand, gravel, and plants) are often constructed to treat agriculture and industrial wastewater naturally. Biochar has demonstrated promise in augmenting contaminant removal of wetlands and biofilters when included among the substrates. Simply put, biochar's unique but consistent properties make it transferrable to various remediation methods.

Thousands of years ago, the Amazonian farmers understood little of biochar, except that adding it to soils improved the success of their harvests. Today, we know the properties of biochar and learn how it can be used in other applications. As our technology advances and our environmental standards improve, our environmental needs will evolve. Meeting these needs will require viewing old technology with a fresh perspective. There is no single solution to our environmental dilemmas. However, the investigation of novel ideas, such as biochar for remediation, can help us along the way.

Dan Smith is an Environmental Specialist for Cornerstone Environmental, Health and Safety out of our Zionsville office. His seven years in environmental consulting include experience in environmental compliance, site assessments, monitoring, remediation, and health and safety. When not at work, he can be found hiking, biking, or engaging in other outdoor activities.