We have conducted the first study of Tl isotope fractionation during sorption of aqueous Tl(I) onto the manganese oxide hexagonal birnessite. The experiments had different initial Tl concentrations, amounts of birnessite, experimental durations, and temperatures, but all of them exhibit heavy Tl isotope compositions for the sorbed Tl compared with the solution, which is consistent with the direction of isotope fractionation observed between seawater and natural ferromanganese sediments. However, the magnitude of fractionation in all experiments (α ≈ 1.0002–1.0015, where α=205Tl/203Tlsolid/205Tl/203Tlliq is smaller than observed between seawater and natural sediments (α ≈ 1.0019–1.0021; Rehkämper et al., 2002, Earth. Planet. Sci. Lett.197, 65–81). The experimental results display a strong correlation between the concentration of Tl in the resulting Tl-sorbed birnessite and the magnitude of fractionation. This correlation is best explained by sorption of Tl to two sites on birnessite, one with large isotope fractionation and one with little or no isotope fractionation. Previous work ( Peacock and Moon, 2012, Geochim. Cosmochim. Acta 84, 297–313) indicates that Tl in natural ferromanganese sediments is oxidized to Tl(III) and adsorbed over Mn vacancy sites in the phyllomanganate sheets of birnessite, and we hypothesize that this site is strongly fractionated from Tl in solution due to the change in oxidation state from aqueous Tl(I). In most experiments, which have orders of magnitude more Tl associated with the solid than in nature, these vacancy sites are probably fully saturated, so various amounts of additional Tl are likely sorbed to either edge sites on the birnessite or triclinic birnessite formed through oxidative ripening of the hexagonal starting material, with unknown oxidation state and little or no isotopic fractionation. Thus each experiment displays isotopic fractionation governed by the proportions of Tl in the fractionated and slightly fractionated sites, and those proportions are controlled by how much total Tl is sorbed per unit of birnessite. In the experiments with the lowest initial Tl concentrations in solution (∼0.15–0.4 μg/g) and the lowest concentrations of Tl in the resulting Tl-sorbed birnessite (⩽17 μg Tl/mg birnessite), we observed the largest isotopic fractionations, and fractionation is inversely proportional to the initial aqueous Tl concentration. Again, this correlation can be explained by the simultaneous occupation of two different sorption sites; vacancy sites that carry isotopically fractionated Tl and a second site carrying slightly fractionated Tl. The fractionation factors observed in nature exceed those in the experiments likely because the Tl concentrations in seawater and in ferromanganese sediments are three to four orders of magnitude lower than in our experiments, and therefore the second slightly fractionated sorption site is not significantly utilized. Temperature (6–40 °C) and experimental duration (3 min–72 h) appear to have little or no effects on isotope behaviour in this system

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:C_org and O:C_org 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:C_org < 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:C_org and other methods, however, correlations were observed between half-life, O:C_org, fixed carbon, number of aromatic peaks in FTIR spectrum, R_50, 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.