Minnesota Stormwater Manual test page 5

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Biochar

Overview and description

Image source: Illinois Sustainable Technology Center

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 250-800^oC in a limited oxygen environment. Gasification involves temperatures greater than 700^oC 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), as well as yard, food and forestry wastes, and animal manures. Clean feedstocks with 10 to 20 percent moisture and high lignin content must be used. 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 the following.

• enhancing soil structure
• increasing water retention and aggregation
• decreasing acidity
• reducing nitrous oxide emissions
• improving porosity
• regulating nitrogen leaching
• improving electrical conductivity
• improving microbial properties

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.

Variants

The properties of biochar depend on the feedstock and the conditions under which the biochar is produced.

• biochar yield and contents of N, hydrogen and oxygen decrease as pyrolysis temperature increased from 400˚C to 700˚C
• contents of ash, pH and carbon increase with greater pyrolysis temperature
• The ability of biochars to adsorb NH4+-N is a funtion of cation exchange capacity. For example, adsorption of corn-straw > peanut > > wheat straw and the adsorption amount decreased with higher pyrolysis temperature [1]

Properties and specifications for biochar

This section is divided into chemical-physical properties, hydraulic properties, retention-leaching properties, and other properties.

Chemical-physical properties of biochar

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.

• Biochar has a large surface area.
• Cation exchange capacity (CEC) decreases as pyrolysis temperature increases. This is due to the loss of volatile organic content and associated functional groups as temperature increases. As CEC decreases, the ability of biochar to retain negatively charged chemicals, such as phosphate, decreases.
• Non-wood vegetative feedstocks have a greater CEC than wood feedstocks. This is due to a greater percentage of aliphatic compounds and associated functional groups. Non-wood feedstocks primarily consist of grasses.
• Sludges and manure-based biochars have high nutrient content and are thus not satisfactory for managing stormwater

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: Gai et. al, 2014 Krishna et al., 2014 Yaoa et al., 2011 Zhaoa et al., 2013 Rimena et al., 2017 Jindo et al., 2014

Biochar standards

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.

Effects of biochar on physical and chemical properties of soil and bioretention media

In this section we provide information on effects of biochar on pollutant attenuation and the physical properties of soil and bioretention media.

Effect of biochar on retention and fate of phosphorus

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.

• Mohanty et al. (2018) observed that biochar does not absorb phosphate efficiently. Phosphorus retention can be enhanced by impregnating biochar with cations such as magnesium and zinc.
• Reddy et al. (2014) found that biochar reduced influent phosphate concentrations by 47% in column experiments. Influent concentrations were 0.57 and 0.82 mg/L for unwashed and washed biochar, respectively. These concentrations are on the high end of concentrations found in urban stormwater.
• Yaoa et al. (2011) observed retention in biochar-(sugar beet source)amended soils that were fertilized. Adsorption was dominated by magnesium oxides and maximum adsorption occurred at pH values less than 4.
• Zhaoa et al. (2013) studied different feedstocks and observed high phosphorus concentrations in animal-based feedstocks and wastewater sludge (0.065 - 0.44%) compared to other feedstocks (0.007 - 0.07%)
• Iqbal et al. (2015) examined leaching of phosphorus from compost (80% yard and 20% food waste) and co-composted biochar (100% fir-forest slash). They found biochar amendments did not significantly reduce the leaching of phosphorus compared to the compost only treatment. Phosphorus leached from biochar, but because phosphorus concentrations in biochar are low, this leaching contributed little total phosphorus. Leached phosphorus was primarily in the form of orthphosphate.
• Han et al. (2018) found that addition of biochar to soil led to increased desorption of phosphorus during winter freeze-thaw cycles.
• Soinne et al. (2014) observed no effect of biochar on phosphorus retention in a sandy and two clay soils.

Effect of biochar on retention and fate of other pollutants

• Nitrogen. Biochar effects on nitrogen retention depend on the properties of the biochar and stormwater runoff. Biochars produced at relatively low temperatures (less than 600^oC) provide some retention of organic nitrogen and ammonium nitrogen in stormwater runoff. Mechanisms for nitrogen retention include adsorption of ammounium and nitrogen immobilization. Leaching of nitrogen may decrease due to increased water holding capacity (Iqbal et al., 2015; Gai et al., 2014; Zheng et al., 2013; Ding et al., 2010).
• Metals. Biochar enhance retention of metals in stormwater runoff. (Reddy et al., 2014; Domingues et al., 2017; Iqbal et al., 2015)
• Organics. Biochar significantly retains polynuclear aromatic hydrocrabons in stormwater runoff (Reddy et al., 2014; Domingues et al., 2017; Ulrich et al., 2017; Iqbal et al., 2015)
• Bacteria and viruses. Biochar effects on bacteria and virus retention are a function of the particle size of the biochar. Fine-grained biochars enhance removal of bacteria in stormwater runoff through straining of microorganisms (Reddy et al., 2014; Sasidharan et al., 2016; Yang et al., 2019).
• Dissolved organic carbon. Biochar shows limited retention of dissolved carbon in stormwater runoff (Iqbal et al., 2015).
• Greenhouse gas emissions. Biochar effectively sequesters carbon and reduces loss of greenhouse gases when incorporated into soil or media, particularly soil with high organic matter content (Zhaoa et al., 2013; Mohanty et al., 2018; 37. Agyarko-Mintah et al., 2017).

Effect of biochar on soil physical properties

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.

• Porosity and surface area. Biochar significantly increases the porosity of most soils.
• Water holding capcity. Biochar significantly increases the water holding capacity of soil.
• Hydraulic conductivity. Biochar increases the hydraulic conductivity of fine- and medium-grained soils and may decrease the hydraulic conductivity of coarse-grained soils.
• Structure. Biochar enhances aggregation in soils, thus enhancing soil structure and potentially increasing soil macroporosity.

Distributors

Caution: The Minnesota Pollution Control Agency does not endorse specific distributors of biochar or biochar products

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.

References

• Regeneration International
• WHAT’S BIOCHAR? HOW TO STABILIZE CARBON IN YOUR SOIL
• Biochar feedstocks
• Effects of Feedstock and Pyrolysis Temperature on Biochar Adsorption of Ammonium and Nitrate
• THE PROPERTIES OF FRESH & AGED BIOCHAR
• characterization and engineering
• Heterogeneity of biochar properties as a function of feedstock sources and production temperatures