This package provides our most detailed standard characterisation of the nitrogen-accessible surface area and pore structure of biochar. A 40-point nitrogen adsorption and desorption isotherm is collected, providing substantially greater data density than our five-point BET package and twice the number of isotherm datapoints used in P364 (Surface Area and Pore-Size Distribution).

 

The increased number of measurements provides a more detailed representation of how nitrogen interacts with the biochar across the relative-pressure range. This allows adsorption and desorption behaviour, hysteresis and changes in pore filling to be examined with greater resolution. It also supplies a denser experimental dataset for the BET, BJH and QSDFT calculations used to evaluate surface area, pore volume and pore-size distribution.

 

The isotherm and pore-size-distribution results are provided as interactive charts through the Celignis Database and are also included in the analytical PDF report.

 

Nitrogen Adsorption and Desorption Isotherm

The isotherm shows the quantity of nitrogen adsorbed by the biochar as relative pressure is increased and the quantity remaining adsorbed as the pressure is subsequently reduced.

 

The adsorption branch provides information on the progressive filling of the biochar’s pores. The desorption branch describes how nitrogen leaves the pore network. Differences between these branches, known as hysteresis, can provide information on mesopore structure, pore connectivity, pore blocking, restricted pore entrances and ink-bottle-type pore geometries.

 

The 40-point isotherm provides improved definition of these features compared with analyses based on fewer datapoints. This can be particularly valuable for biochars containing complex mixtures of micropores and mesopores or showing distinct changes in adsorption behaviour over relatively narrow pressure ranges.

 

BET Surface-Area Analysis

The Brunauer–Emmett–Teller method is used to calculate the apparent nitrogen-accessible specific surface area of the biochar in m²/g.

 

BET surface area is commonly used to compare biochars produced from different feedstocks, pyrolysis temperatures, residence times and activation treatments. A higher surface area generally indicates that more internal and external surface is accessible to nitrogen, although surface area alone does not describe the dimensions, accessibility or connectivity of the pores.

 

The BET analysis also provides:

- the number of isotherm datapoints;

- the BET correlation coefficient;

- the BET C constant;

- the BET slope; and

- the BET intercept.

 

These parameters describe the mathematical fit of the selected BET region and are used to calculate the monolayer nitrogen capacity and specific surface area. The correlation coefficient indicates the linearity of the transformed BET data, while the C constant provides an indication of the relative strength of first-layer nitrogen adsorption.

 

The package also reports the total nitrogen-accessible pore volume and an equivalent average pore diameter. The average pore diameter is calculated from the total pore volume and BET surface area and should be regarded as a whole-sample equivalent value rather than the most frequently occurring pore width.

 

BJH Mesopore Analysis

The Barrett–Joyner–Halenda method is used primarily to evaluate the biochar’s mesopore structure. Mesopores are conventionally defined as pores approximately 2–50 nm wide.

 

The BJH pore-size-distribution chart plots pore volume in cc/g against pore width in nm. It shows how the BJH-assigned mesopore volume is distributed across the analysed pore-width range and identifies the principal BJH pore-width mode, reported as the main cluster of pores.

 

The BJH results include:

- cumulative BJH surface area;

- cumulative BJH pore volume; and

- modal BJH pore width.

 

The additional isotherm datapoints used in this deluxe package can provide greater definition of the mesopore distribution and adsorption–desorption hysteresis than a lower-density isotherm. However, BJH remains a classical capillary-condensation method and is not suitable for accurately characterising micropores below approximately 2 nm. It may also be affected by pore blocking, network effects and the assumptions used to represent pore geometry.

 

QSDFT Micro- and Mesopore Analysis

Quenched Solid Density Functional Theory provides a molecular-level model of nitrogen adsorption within pores of different widths. It is particularly useful for biochar because it can evaluate the narrow micropores that often account for a substantial proportion of the material’s surface area.

 

The QSDFT pore-size-distribution chart plots pore volume in cc/g against pore width in nm. It describes the modelled distribution across the micropore and mesopore ranges and identifies the principal pore-width mode.

 

The QSDFT results include:

- cumulative QSDFT surface area;

- cumulative QSDFT pore volume;

- modal QSDFT pore width;

- lower confidence limit; and

- QSDFT fitting error.

 

The lower confidence limit is the smallest pore width that can be determined reliably from the measured isotherm and selected QSDFT model. Where the modal pore width coincides with this limit, the dominant pore feature occurs at the lower reliable resolution of the analysis and may extend into still narrower pores.

 

The QSDFT fitting error describes the agreement between the measured nitrogen isotherm and the theoretical isotherm reconstructed from the calculated pore-size distribution. A lower value indicates a closer mathematical fit, although the resulting pore distribution remains dependent on the selected carbon model and assumed pore geometry.

 

The greater number of pressure measurements in P366 gives the QSDFT fitting process a more detailed experimental isotherm to reproduce. This can improve the definition of pore-size-distribution features, particularly where the biochar contains several overlapping pore populations. It does not, however, remove the inherent model assumptions or the lower resolution limit of nitrogen adsorption.

 

Why BET, BJH and QSDFT Results Differ

BET, BJH and QSDFT analyse different aspects of the nitrogen adsorption data and should not be expected to return identical surface areas or pore dimensions.

 

- BET provides the overall apparent nitrogen-accessible specific surface area and associated regression parameters.

- BJH principally characterises the mesopore portion of the biochar pore network.

- QSDFT provides a model-based assessment of both micropores and mesopores and is generally more appropriate for evaluating the narrow carbon pores commonly found in biochar.

 

For example, a biochar may have a high BET surface area, a relatively modest BJH pore volume and a strong QSDFT peak in the micropore region. This indicates that much of the accessible surface is associated with narrow micropores rather than larger mesopores.

 

Advantages of the 40-point Deluxe Package

Increasing the isotherm from 20 to 40 datapoints does not simply produce more values of the same type. It provides a more densely defined adsorption and desorption profile that can offer:

- improved visual definition of the isotherm and hysteresis loop;

- better resolution of transitions in pore filling and emptying;

- more detailed BJH and QSDFT pore-size-distribution plots;

- improved identification of overlapping or secondary pore-size features;

- a more robust basis for comparing similar biochars; and

- greater sensitivity to changes caused by feedstock, pyrolysis or activation conditions.

 

The benefit of the additional datapoints depends on their placement across the relative-pressure range and on the suitability of the selected calculation models. Nevertheless, the 40-point analysis is preferred where small differences between samples are important or where the biochar is expected to have a complex or multimodal pore structure.

 

Applications to Biochar

Detailed surface-area and pore-size analysis can support the assessment of biochar for applications including:

- contaminant adsorption and environmental remediation;

- nutrient retention and soil amendment;

- water treatment and filtration;

- catalyst and catalyst-support applications;

- gas adsorption and storage;

- activated-carbon-type applications;

- microbial habitat and pore accessibility;

- comparison of activation treatments; and

- evaluation of the effects of feedstock and production conditions.

 

P366 is recommended for research, product development, detailed process optimisation and high-resolution comparison of biochars. Where only a basic BET surface-area value is required, P360 may be sufficient. Where detailed pore-size information is required but the additional resolution of a 40-point isotherm is not essential, P364 provides a lower-data-density alternative.

Constituents Determined

For every constituent determined via wet-chemical analysis each sample is analysed in duplicate and we provide the compositional data for each replicate along with the average value and the standard deviation between the duplicates. Our analytical protocols typically provide results with a high level of precision, as detailed here.

Examples of the data reports generated can be viewed on the Celignis Database. Please log on to the guest account using email "test@celignis.com" and password "celignis".

Click here to place an order for determining Surface Area and Pore-Size Distribution Deluxe.

Sample Weight Requirements

Under nornal conditions there is a minimum requirement of 1g of sample for this analysis package with a recommended weight of 5+g of sample.

However, it may be possible for us to undertake the anaysis with lower quantities than the minimum specified above. Please email us at info@celignis.com to find out.

Equipment Used for Surface Area and Pore-Size Distribution Deluxe