This page provides information on biochar. While providing extensive information on biochar, there is a section focused specifically on stormwater applications for biochar.
Biochar is a charcoal-like substance that’s made by burning organic material from biomass. The two most common proceesses for producing biochar are pyrolysis and gasification. During pyrolysis, the organic material is heated to 250-800oC in a limited oxygen environment. Gasification involves temperatures greater than 700oC in the presence of oxygen.
Biomass waste materials appropriate for biochar production include crop residues (both field residues and processing residues such as nut shells, fruit pits, bagasse, etc); yard, food and forestry wastes; and animal manures. Clean feedstocks with 10 to 20 percent moisture and high lignin content are recommended. Examples are field residues and woody biomass. Using contaminated feedstocks, including feedstocks from railway embankments or contaminated land, can introduce toxins into the soil, drastically increase soil pH and/or inhibit plants from absorbing minerals. The most common contaminants are heavy metals—including cadmium, copper, chromium, lead, zinc, mercury, nickel and arsenic, and polycyclic aromatic hydrocarbons (PAHs).
Biochar is black, highly porous, lightweight, fine-grained and has a large surface area. Approximately 70 percent of its composition is carbon. The remaining percentage consists of nitrogen, hydrogen and oxygen among other elements. Biochar’s chemical composition varies depending on the feedstocks used to make it and methods used to heat it.
Biochar benefits for soil may include but are not limited to
Biochar is also found to be beneficial for composting, since it reduces greenhouse gas emissions and prevents the loss of nutrients in the compost material. It also promotes microbial activity, which in turn accelerates the composting process. Plus, it helps reduce the compost’s ammonia losses, bulk density and odor (Spears, 2018; Hoffman-Krull, 2019).
Although the basic structure of all biochars is similar, the physical-chemical properties of biochar varies with the source material and with the temperature used in production.
Since a wide variety of organic material can be used to produce biochar, it is not feasible to discuss each material separately. We provide the following general conclusions and summarize results from several studies.
Literature review
Changes in the properties of biochar result from loss of volatile organic matter as temperature increases. This leads to a gradual loss in the number of functional groups on the biochar and increased aromaticity as temperature increases.
In general, the following conclusions are applicable for biochar used in stormwater applications.
The following information comes from a literature review of the effects of production temperature on biochar (Zhaoa et al., 2013; Zhao et al., 2017; Mohanty et al., 2018; Jindo et al., 2014; Wang et al., 2018)
For detailed discussion of structural, chemical, and physical changes in biochar as a function of temperature, see the following articles.
This section is divided into chemical-physical properties, hydraulic properties, retention-leaching properties, and other properties.
The properties of biochar vary depending on the feedstock and conditions, primarily the pyrolysis temperature, under which the biochar is produced. Consequently there is considerable variability in the chemical and physical properties of different biochars. The table below summarizes data from our literature review. Some conclusions from the literature are summarized below.
Chemical and physical properties of biochar.
Link to this table
Property | Range found in literature1 | Median value from literature |
---|---|---|
Total phosphorus (%) | 0.0061 - 1.086 | 0.0618 |
Total nitrogen (%) | 1.2 - 2.4 | 0.88 |
Total potassium (%) | 0.0079 - 1.367 | 0.181 |
Total carbon (%) | 24.2 - 90.9 | 66 |
Total hydrogen (%) | 0.67 - 4.3 | 2.8 |
Total oxygen (%) | 2.69 - 28.7 | 16.3 |
pH | 6.43 - 10.4 | 9.66 |
Cation exchange capacity (cmol/kg) | 0.1 - 562 | 43.1 |
Surface area (m2/g | 2.78 - 203 | 30.6 |
Electrical conductivity (μs/cm) | 100 - 2221 | 231.5 |
Pore volume (cm3/g) | 0.006 - 0.51 | 0.036 |
Total calcium (%) | 0.0954 - 3.182 | 0.590 |
Total magnesium (%) | 0.0297 - 0.2801 | 0.0587 |
Total copper (%) | 0.0001 - 0.0078 | 0.00025 |
Total zinc (%) | 0.0002 - 0.0152 | 0.00135 |
Total aluminum (%) | 0.001 - 0.1929 | 0.0290 |
Total iron (%) | 0.0009 - 0.2209 | 0.0333 |
Total manganese (%) | 0.0001 - 0.1025 | 0.00145 |
Total sulfur (%) | 0.01 - 0.44 | 0.05 |
Primary references for this data:
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Potential contaminants associated with biochar are a function of the source material and production temperature. Of greatest concern are metals and polycyclic aromatic hydrocarbons (PAHs). Oleszcuk et al. (2013) examined metal and PAH concentrations in four biochars (elephant grass, coconut shell, wicker, and wheat straw). Metal concentrations (mg/kg) in the biochars are summarized below. Tier 1 Soil Reference Values (SRVs) are included in parentheses.
Concentrations in biochar are well below Tier 1 SRVs.
In the study by Oleszcuk et al. (2013), total PAHs ranged from 1124.2 ng/g to 28339.1 ng/g. The dominant group of PAHs were 3-ring compounds which comprised 64.6% to 82.6% of total PAHs content. The primary compounds included, in order of abundance, phenanthrene, fluorene, anthracene, and pyrene. No 6-ring PAHs were observed. Concentrations of PAHs and other organic contaminants, such as dioxins, decreases with increasing pyrolysis temperature (Lyu et al., 2016).
In general, biochars mixed with soil do not inhibit germination or root growth. Biochar may enhance soil fertility by providing nutrients or more commonly by slowing the release of nutrients from materials such as compost. was observed. Toxic effects have been observed for some invertebrates, indicating that in sensitive environments, biochar testing is advisable (Oleszcuk et al., 2013; Getz et al., 2018; Flesch et al., 2019; Wang et al., 2017) .
The Internation Biochar Initiative (IBI) developed Standardized Product Definition and Product Testing Guidelines for Biochar That Is Used in Soil, also referred to The Biochar Standards. These standards provide guidelines and is not a formal set of industry specifications. The goal of The Biochar Standards is to "universally and consistently define what biochar is, and to confirm that a product intended for sale or use as biochar possesses the necessary characteristics for safe use. The IBI Biochar Standards also provide common reporting requirements for biochar that will aid researchers in their ongoing efforts to link specific functions of biochar to its beneficial soil and crop impacts." The IBI also provides a certification program. Information on the standards and certification are found on International Biochar Institute's website or at the IBI's Standardized Product Definition and Product Testing Guidelines for Biochar That Is Used in Soil.
In this section we provide information on effects of biochar on pollutant attenuation and the physical properties of soil and bioretention media.
Biochar is not likely to provide significant phosphorus retention in bioretention practices unless impregnated with cations (e.g. magnesium) during production at relatively low temperatures (e.g. less than 600oC.) |
Biochar may have several properties for managing stormwater, such as increased water and pollutant retention, improving soil physical properties, and attenuating bacteria and pathogens. Biochar has been examined as a potential amendment to engineered media in bioretention or other stormwater control practices. With respect to phosphorus, information from the literature is mixed. Below are summaries from several studies.
Because of a large surface area and internal porosity, biochar can affect soil physical properties (Mohanty et al., 2018; Herrera Environmental Consultants, 2015; Iqbal et al., 2015; Imhoff, 2019; Jien and Wang, 2013). These effects are most pronounced in soils with low organic matter concentration.
A list of biochar distributors is provided on the United States Biochar Initiative website (USBI). Note the USBI neither provides endorsements nor accepts liability for any particular product or technology listed below.
Because biochar is produced from biomass, including wastes, it is sustainable from an availability or supply standpoint. Sustainable biochar production, however, is less certain based on current economic constraints. Biochar has several potential markets and exploiting these markets is necessary for biochar production to be sustainable. Examples of specific markets include stormwater media, soil health and fertility, and carbon sequestration [Biogreen http://www.biogreen-energy.com/biochar-production/] (accessed December 10, 2019). Sustainable biochar production must also meet certain environmental and economic criteria, includign the following.
For more information, see the International Biochar Initiative discussion on sustainable biochar production. For a discussion of biochar sustainability, see sustainability and Certification (Vereijen et al., 2015).