Comparative analysis of biochar carbon stability methods and implications for carbon credits.

Biochar is an emerging negative emission technology. Its ability to sequester carbon and subsequent carbon credit valuation hinge on the stability of its carbon structure. The widely used indicators of carbon stability H:Corg and O:Corg provide conservative results as these are based on limited incubation experiments and associated modeling results. The results from these accepted methods and other derived methods have not been compared as indicators of carbon stability in a variety of biochar samples. Furthermore, the influence of contrasting feedstock and production techniques on biochar carbon stability is not well explored. Therefore, to address these challenges, a comprehensive stability analysis of 21 different biochar samples with contrasting feedstocks and pyrolysis techniques was conducted using a combination of instrumental methods and derived indicators of carbon stability. Methods such as biochar carbon half-life, thermo-stable fraction, oxidation resistance (R50), and carbon sequestration potential (CS) were used. Based on the initial carbon content of the biochar, simple pyrolysis techniques have similar potential for carbon credits as biochar produced from advanced pyrolysis techniques. Results indicate that the carbon stability of a biochar product is primarily a factor of feedstock type. We found that biochar carbon stability is not related to volatile matter or fixed carbon content for biochar produced using a simple pyrolysis technique and mixed feedstock. Biochars with H:Corg < 0.4 were deemed to have lower carbon stability when compared using different methods. No correlation was observed between the carbon stability of biochar using H:Corg and other methods, however, correlations were observed between half-life, O:Corg, fixed carbon, number of aromatic peaks in FTIR spectrum, R50, and CS. Therefore, it is recommended that data from additional incubation and modeling studies need to be considered to increase the confidence in carbon stability results having major implications to carbon credits.

Evaluating fundamental biochar properties in relation to water holding capacity.

Biochar products that hold and release water within a stable carbonised porous structure provide many opportunities for climate mitigation and a range of applications such as for soil amendments. Biochar that are produced from various organic feedstocks by pyrolysis can provide multiple co-benefits to soil including improving soil health and productivity, pH buffering, contaminant control, nutrient storage, and release, however, there are also risks for biochar application in soils. This study evaluated fundamental biochar properties that influence Water Holding Capacity (WHC) of biochar products and provides recommendations for testing and optimising biochar products prior to soil applications. A total of 21 biochar samples (locally sourced, commercially available, and standard biochars) were characterised for particle properties, salinity, pH and ash content, porosity, and surface area (with N2 as adsorbate), surface SEM imaging, and several water testing methods. Biochar products with mixed particle size, irregular shapes, and hydrophilic properties were able to rapidly store relatively large volumes of water (up to 400% wt.). In contrast, relatively less water (as low as 78% wt.) was taken up by small-sized biochar products with smooth surfaces, along with hydrophobic biochars that were identified by the water drop penetration test (rather than contact angle test). Water was stored mostly in interpore spaces (between biochar particles) although intra-pore spaces (meso-pore and micropore scale) were also significant for some biochars. The type of organic feedstock did not appear to directly affect water holding, although further work is needed to evaluate mesopore scale processes and pyrolytic conditions that could influence the biochemical and hydrological behaviour of biochar. Biochars with high salinity, and carbon structures that are not alkaline pose potential risks when used as soil amendments.

Optimising water holding capacity and hydrophobicity of biochar for soil amendment - A review.

Biochar is a product of the thermal treatment of biomass, and it can be used for enhancing soil health and productivity, soil carbon sequestration, absorbance of pollutants from water and soil, and promoting environmental sustainability. Extensive research has been done on applications of biochar to enhance the Water Holding Capacity (WHC) of biochar amended soil. However, a comprehensive road map of biochar optimised for enhanced WHC, and reduced hydrophobicity is not yet published. This review is the first to provide not only quantitative information on the impacts of biochar properties in WHC and hydrophobicity, but also a road map to optimise biochar for enhanced WHC when applied as a soil amendment. The review shows that straw or grass-derived biochar (at 500-600 °C) increases the WHC of soil if applied at 1 to 3 % in the soil. It is clear from the review that soil of varying texture requires different particle sizes of biochar to enhance the WHC and reduce hydrophobicity. Furthermore, the review concludes that ageing biochar for at least a year with enhanced oxidation is recommended for improving the WHC and reducing hydrophobicity compared to using biochar immediately after production. Additionally, while producing biochar a residence time of 1 to 2 h is recommended to reduce the biochar's hydrophobicity. Finally, a road map for optimising biochar is presented as a schematic that can be a resource for making decisions during biochar production for soil amendment.

Direct stable isotope porewater equilibration and identification of groundwater processes in heterogeneous sedimentary rock.

The off-axis integrated cavity output spectrometry (ICOS) method to analyse porewater isotopic composition has been successfully applied over the last decade in groundwater studies. This paper applies the off-axis ICOS method to analyse the porewater isotopic composition, attempts to use the isotopic shift in groundwater values along with simple geochemical mixing model to define the groundwater processes in the Sydney Basin, Australia. Complementary data included geophysical, hydrogeological, geochemical, and mineralogical investigations. Porewater from core samples were analysed for δ(18)O and δ(2)H from various sedimentary units in the Basin and compared to endpoint water members. Stable δ(18)O and δ(2)H values of porewaters in the Basin (-9.5 to 2.8‰ for δ(18)O and -41.9 to 7.9‰ for δ(2)H) covered a relatively narrow range in values. The variability in water isotopes reflects the variability of the input signal, which is the synoptic variability in isotopic composition of rainfall, and to a minor extent the subsequent evaporation. The porosity, bulk density and mineralogy data demonstrate the heterogeneity that adds the complexity to variations in the isotope profile with depth. The source of chloride in the sedimentary sequence was related to rock-water and cement/matrix-water interaction rather than to evaporation. The heterogeneous character of the sedimentary rock strata was supported by a change in pore pressures between units, density and variability in rock geochemical analyses obtained by using X-ray fluorescence (XRF) and X-ray power diffraction analyses. This research identified distinct hydrogeological zones in the Basin that were not previously defined by classic hydrogeological investigations. Isotopic signature of porewaters along the detailed vertical profile in combination with mineralogical, geochemical, geophysical and hydrogeological methods can provide useful information on groundwater movement in deep sedimentary environments. The findings of the study are valuable in management of sensitive ecosystems and potable resources above mining areas.

Coupling centrifuge modeling and laser ablation inductively coupled plasma mass spectrometry to determine contaminant retardation in clays.

Quantifying the retardation (Rd) of reactive solutes as they migrate through low-permeability clay-rich media is difficult, thus motivating this study to assess the viability of combining centrifuge modeling and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) techniques. An influent solution containing Cl-, trace metals, and lanthanide species flowed at 1.0 mL x h(-1) through an undisturbed clay-rich core sample (33 mm diameter x 50 mm long) mounted in a UFA Beckman centrifuge operating at 3000 rpm (N factor = 876 g). During the 87 day experiment the hydraulic conductivity of the core was 3.4 x 10(-10) m x s(-1). Effluent breakthrough data indicate the Rd of Tl to be 10; incomplete breakthrough (non-steady-state) data for 145Nd and 171Yb suggest Rd values of >>75 and >>85, respectively. At the completion of the transport experiment, longitudinal sections of the core solid were analyzed for 145Nd and 171Yb using a Cetac laser ablation system coupled with an ICP-MS. The longitudinal core sections yielded Rd values of >10000 for 145Nd and 171Yb. This study demonstrates coupling these techniques can provide Rd values for a wide range of reactive solutes with relatively rapid testing of small-scale, low hydraulic conductivity core samples.