Biochar: Lab Characterization Options

The purpose of this page is to guide consulting soil scientists to readily available analytical procedures suitable to support client biochar projects. At this point it is an informal work in progress which I update with abandon as new information comes to me.  Last updated June 6, 2010. Philip Small.

1. Must-Have References

Biochar for Environmental Management: Science and Technology has chapters on biochar characterization, classification and test methods. This book, highly recommended for NSCSS members, can be purchased from Earthscan and through Amazon.com. Another must-have resource for consulting soil scientists is All Biochars are Not Created Equal, and How to Tell Them Apart by Hugh McLaughlin, Paul Anderson, Frank Shields, and Tom Reed (October, 2009). A white paper, posted for discussion purposes, it proposes revising proximate analysis to better address biochar. It explores a simple measure of adsorption within reach of most of our members. An excellent complement to Biochar for Environmental Management and to All Biochars.... is Jim Amonette's Introduction to Biochar with an Emphasis on its Properties and Potential for Climate Change Mitigation. Especially valuable is Jim's treatment of titrateable acidity and basicity in biochar.

2. The Caveats

David Laird, Soil Scientist and USDA-ARS' lead biochar researcher, kindly writes us that:

A USDA-ARS pyrolysis researcher wisely comments to us, that, considering the complexities involved, it would be preferable for the biochar-interested community-at-large to wait on the research community to complete their analysis. Preferences aside, the reality is that charcoal intended as a soil amendment is being produced and marketed. Consulting soil scientists are involved with biochar today specifically because our professional judgement is needed in lieu of the guidelines that USDA-ARS, and others, have yet to develop.

3. Sample Preparation

Biochar varies considerably, and proper sample preparation is key to achieving reproducible results. Take samples from various portions of the biochar product stream. These are ground, mixed, and then reduced in volume to a single gallon sample to be sent to the lab. Confidence in the results is affected by sample size, best practice is to log the number of samples used to make the composite. Coefficients of variability (CV) have not been published for charcoal, least not that I could determine. If we assume a CV of 35, then a composite of 10 samples will be within 15% of the true value with 99% confidence. A composite of 3 samples has a comparable 32% margin of error. Bottom line: more samples is better.

4. Basic Tests

Tests available to the home practitioner. They won't have the accuracy of laboratory analysis, but certainly good for initial assessment.

Crush Test

The portion which cannot be crumbled easily in the hands is incompletely pyrolyzed.

Soap Test

Crumble and rub charcoal into your hand. Rinse with cold water. What is removed by rinsing is the charcoal. What remains can be washed off with soap. These remains are tar compounds, not-charcoal. (via Hugh McLaughlin)

Loss on Ignition

In simple charcoal, this provides a workable approximation of carbon versus ash content. Generally, ash content increases with duration and temperature of pyrolysis. Short term effects increase with ash content. Long term effects increase with carbon content.

pH

Generally pH increases with pyrolysis temperature. pH above 8.2 indicates pyrolysis temperatures above 500⁰C (via Frank Shields). The pH of biochar can change over time, especially after the biochar has equilibrated with atmospheric carbon dioxide, which converts many of the alkaline hydroxides into corresponding carbonates and shifts the pH lower (via Hugh McLaughlin).

Acid fizz test

Using dilute muriatic acid, confirms the presence of carbonates. Carbonates are replaced by oxides at pyrolysis temperatures above 500⁰C. They can also be removed by post-process washing. Dilute muriatic acid can be purchased from pool supply or hardware store.

Adsorption Capacity

The higher, the better. All Biochars....(p33) presents an adaptation of the Gravimetric Adsorption Capacity Scan such that this approach can be "accessible to the home practitioner." I haven't tried it yet, but am excited at the prospect.

5. Tests in Support of Client Projects

The following are ranked roughly on anticipated importance from a consulting soil scientist's client project perspective:

5.1 Fuel: Proximate /Ultimate analysis

Review All Biochars.... to understand how these apply to biochar. I recommend sticking to ASTM standards (or the very similar British/Australian standard) for now. This analysis determines

• Total Carbon (full loss-on-ignition minus ash and moisture)
• Volatile Matter (based on partial loss on ignition)
• Fixed Carbon (difference between TC and VM)
• Ash
• Moisture

These parameters are all fairly straightforward, all important aspects to understand in regards to a char product. Volatile matter will identify the proportion of the charcoal product that is less recalcitrant: torrefied biomass, as well as the more volatile organic compounds Note a: I pick up the remaining ultimate parameters in the compost analysis below, with the exception of H, and O. Whichever way doesn't matter to me, but it would matter when it comes time to define a standard. Note b: There is much P/U data available for charcoal due to its history as a fuel commodity. If I understand this correctly, VM and FC inform us as to the degree of carbonification and perhaps to recalcitrance, an important characteristic for carbon sequestration. It partially informs us as to the potential for remaining wood tar, condensates, and bio-available C (BAC) thus good to know VM from the perspective of ranking for potential toxicity, as well as potential for stimulating soil life. See 5b below for another BAC related test. Note c: The fixed carbon that remains after the proximate VM test will generally ignite between 400-500^o C but (obvious) is more resistant than what is lost during the VM test. Under ASTM the VM test involves placing a covered platinum crucible in a furnace heated to 950^o C for 7 minutes. Loss-on-ignition testing on what remains after the VM test consumes the Fixed Carbon component, leaving ash. In the soil science world, loss-on-ignition (for organic matter in soil, compost, and waste-water solids) is 2 hours at 550^o C, thus does not distinguish a volatile component. Note d: The absolute magnitude of ash measurements in biochars should be taken with the proverbial “grain of salt”, especially the acid soluble fractions. Higher ash levels generally mean that higher levels of non-organic “something” are present in the char. What those ash constituents are, and whether they could impact local soil conditions, needs to be understood before utilization as a biochar. (via Hugh McLaughlin).

5.2 Chemical Characteristics

5.2.1 Adapt standards used for compost chemistry analysis using TMECC standard tests

Test Methods for the Examination of Composting and Compost (TMECC) port well to the task at hand with one exception: Calcium Carbonate Equivalent (CCE). To gauge potential effects on soil pH, CCE is important and alternatives to TMECC's derivation of CCE should be considered per note 2a, below.

• Calcium carbonate equivalent (CCE, see TMECC 04.08a. Use ASTM C602 if pH above 8.2 per note 2a, below)
• pH
• Conductivity (ECe)
• Organic Matter (OM)
• Total organic C (TOC)
• Total Nitrogen (TN)
• Carbon:Nitrogen (C/N) ratio
• Ammonical-Nitrogen (NH₄-N)
• Nitrate-Nitrogen (NO₃-N)
• Total Phosphorus (P)
• Total Potassium (K)
• Total Sulfur (S)
• Total Calcium (Ca)
• Total Magnesium (Mg)
• Total Sodium (Na)
• Total Boron (B)
• Total Zinc (Zn)
• Total Copper (Cu)
• Total Iron (Fe)
• Total Manganese (Mn)
• Cation Exchange Capacity (CEC. as TMECC 04.09)

Note 2a: On CCE. My current understanding from Frank Shields, Control Labs, is that deriving CCE consistent with TMECC 04.08a is based on the evolution of CO₂, thus will not give an accurate measurement of neutralizing potential if the charcoal is made at very high temperature. Temperatures above 500^o C cause oxides to form and carbonates to be lost. Calcium carbonate buffers the pH to 8.2, Ca/Mg/K/Na-oxides jump the charcoal pH towards 12-13, so, we should be able to screen for this: if pH is above 8.2 you will need to determine titrateable basicity using a standard approach such as the ASTM published standard for classifying ag liming materials in terms of calcium carbonate equivalent: ASTM C602. Other applicable standards are AOAC 955.01 (Available in AOAC's Official Methods of of Analysis) and standards for determining Neutralization Potential published in Field and Laboratory Methods Applicable to Overburdens and Minesoils (EPA-600/2-78-054). NP is discussed here. Driving home the point that projecting biochar effect on soil pH is challenging is the observation by David Laird that biochar researchers

Note 2b: With TMECC, nutrients and trace elements (P, K, S, ...) are all totals, not exchangeable/available/extractable as is done for soil agronomy.

5.2.2. Adsorption Capacity

This section is a work in progress, as I look into the commercial availability of the Gravitric Adsorption Capacity Scan (GACS) presented on pages 15-16 in All Biochars are Not Created Equal, and How to Tell Them Apart. All Biochars....(p33) also presents an adaptation "accessible to the home practitioner."

5.2.2.1 Phosphorus Adsorption Capacity

Biochar has a high capacity for adsorption of phosphorus. When that capacity becomes a design parameter for biochar application, it should be characterized according to Graetz, D.A., and V.D. Nair. 2000. Phosphorus sorption isotherm determination. In G.M. Pierzynski (ed.) Methods of phosphorus analysis for soils, sediments, residuals, and waters. Southern Coop. Ser. Bull. 396. Kansas State Univ., Manhattan The following advice related to phosphorus sorption isotherm determination in filter materials applies to biochar. Especially note the recommendation that the "P concentrations range close to that of wastewater or slightly higher."

Graetz and Nair (2000) presented a set of guidelines for P sorption isotherm determination in soils based on the investigation of Nair et al. (1984). Based on these guidelines, and the analysis of batch experiment procedures in the selected studies, recommendations on main batch experiment parameters shall include:

• Particle/aggregate diameter of the same order of magnitude (the materials could be classified according to their particle size and hydrological properties).
• Similar material-to-solution ratio, as close as possible to that use[d] in soil science (1:20), and, preferably, similar amount of the material. Graetz and Nair (2000) proposed an amount of 0.5 to 1 g for soils. For filter materials a range between 1 and 3 g may be appropriate. In case of very fine materials, 0.5 or even 0.25 g could be used.
• P concentrations range close to that of wastewater or slightly higher (max. 100–200 mg P L–1).
• Contact times of at least 24 h (preferably longer). Preliminary kinetic studies are recommended.
• Agitation of about 100 rpm, so that the solution moves but not the material.
• Temperature between 21 and 25°C (room temperature)

5.3. Physical Characteristics

• bulk and particle density. Needed to gauge application depth.
• particle size distribution
• crushability
• pore volume
• pore size distribution either as i) directly measured by microscopy - must be very expensive or ii) inferred using an absorbate gas at several pressures - or iii) ranked (my concept) based on measured surface area as determined with by CO2 gas adsorption at 150oC for 10 h. (the oil mallee charcoal study reported this parameter)
• water holding capacity
• permeability

Note on pore size distribution: No commercial USA lab found for pore size distribution options yet. Goal is to gauge pore size as it relates to AM fungi microhabitat. "AM fungi easily extend their extraradical hyphae into charcoal buried in the soil and sporulate in the particles (Ogawa, 1987). Postma et al. (1990) show evidence that rhizobia in pores <50 _m are protected from predation by protozoan predators." (Blackwell, 2007).

5.4. Additional Metals

A chemistry characteristic, but generally ranks well below Physical Characteristics in importance. Elevated metal content of the biochar feedstock would elevate the value of analysis for metals. When metals are a concern, land application of biomass derived solids, including the products of pyrolysis, can result in detrimental accumulation of those listed below as well as what could be covered in the compost package: copper (Cu), and zinc (Zn).

• Total Arsenic (As)
• Total Cadmium (Cd)
• Total Chromium (Cr)
• Total Lead (Pb)
• Total Mercury (Hg)
• Total Molybdenum (Mo)
• Total Selenium (Se)

5.5. Biological testing

a) Bioassay. See Woods End examples. Among those, I am most interested in results from Woods End II: Growth series testing, which clues us in to a maximum loading rate. b) Bioavailable carbon (BAC). Add nutrients and microbes, making C the most limiting constituent. Incubate, and measure evolved CO₂ per unit mass per unit time.

5.6.Volatile organic compounds

This is an area where my terminology is relatively weak. VOC's has a very specific meaning in the regulatory world, and my use here may not be that narrow. My view is that some charcoal does need to be tested for the most volatile organic compounds from a land application perspective. Charcoal produced by Hydrothermal Carbonisation (HTC) retains wood gas condensates, has a high level of proximate volatile matter and HTC-biochar has been observed to have a negative effect on plant growth. One suspects that the damaging agents are the more volatile compounds produced by pyrolysis. Torrefied wood has most of the moisture and most volatile organic compounds (VOC's) driven out. The biochar (or more accurately, torrefied wood) produced by torrefaction has not been demonstrated to have a plant limiting effect despite high proximate volatile matter. I am open to the idea that VOC data could be useful in evaluating risk of exposure in a confined charcoal processing workplace setting.

6. Soil testing

Not charcoal but pretty important. I would say soil testing ranks in importance equal to physical and chemical characteristics of biochar.

6.1. Soil pH and Lime Requirement

1. Test soil pH, and if pH is below the ideal range for the plant community, then
2. Test soil for lime requirement (LR)

Note: With the exception of basing lime requirement on exchangeable aluminum, LR is a relatively inexpensive, readily available test. It is important to match the approach to the soil type and the ideal pH range specific to the plant community. Accordingly, it is best to use a lab serving the specific region of the application site, but barring that, any properly equipped lab would serve. The major lime requirement testing options are nicely introduced in a 1999 Clemson internet studies handout. Since then (1999) alternatives to the Shoemaker, McLean, and Pratt (SMP) method have been developed. I covered some of this in a post in June, 2008.

6.2. Charcoal content of soil

As needed to both to support carbon sequestration bookkeeping and to assess the potential that adding more charcoal will have a significant effect, as some soils have a surprisingly high native charcoal content as a result of indigenous burning. The black color of the upland Mollisol and Chernozem soil types may be largely attributable to charcoal. Quantifying charcoal content in soils is problematic (Hammes, 2007). Forest ecologist Valerie Kurth has worked up a fairly reliable method (Kurth, 2006). It is not perfect, but it is pretty simple and requires little specialized equipment.

7. Acknowledgements

Frank Shields, at Control Laboratories (CA) and Mark Flock, at Brookside Laboratories (OH), were particularly helpful in helping me sort this through at the time the page was created. Control Laboratories provides biochar characterization services, as noted here, and the core analysis package provided by Frank is certainly a solid starting point. Jim Amonette, at Pacific Northwest National Laboratory (WA), helped me connect titrateable basicity of biochar, one of its more important soil affects, to what I know about managing agricultural soil acidity using liming materials.