Control of Mg²⁺/Ca²⁺ Activity Ratio on the Formation of Crystalline Carbonate Minerals via an Amorphous Precursor

The formation of amorphous calcium carbonate (ACC) and its transformation to crystalline phases plays a key role in the formation of carbonate minerals on Earth’s surface environments. Nonetheless, the physicochemical parameters controlling the formation of crystalline CaCO₃ via an amorphous precursor are still under debate. In the present study we examine whether crystalline CaCO₃ formation occurs via an ACC precursor in the pH range from 7.8 to 8.8 and at initial Mg/Ca ratios from 1/3 to 1/8. The obtained results document that the transformation of Mg-rich ACC (Mg-ACC) to a crystalline phase is strictly controlled by the prevailing ratio of the Mg²⁺ to Ca²⁺ activity, <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺, of the reactive solution after Mg-ACC was synthesized: Mg-ACC transformed to (i) Mg-calcite at 5 ≤ <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺ ≤ 8 and to (ii) monohydrocalcite at 8 ≤ <i>a</i>_Mg²⁺/<i>a</i>_Ca²⁺ ≤ 12. Our findings suggest that the formation of the crystalline phase induces undersaturation of the reactive solution with respect to the ACC and triggers its dissolution. Thus, the metastability of Mg-ACC in the reactive solution is not determined by its Mg content but is related to the formation kinetics of the less soluble crystalline phase. The experimental results highlight the importance of prevailing physicochemical conditions of the reactive solution on Mg-ACC transformation pathways

Determination of Methanogenic Pathways through Carbon Isotope (δ¹³C) Analysis for the Two-Stage Anaerobic Digestion of High-Solids Substrates

This study used carbon isotope (δ¹³C)-based calculations to quantify the specific methanogenic pathways in a two-stage experimental biogas plant composed of three thermophilic leach bed reactors (51–56 °C) followed by a mesophilic (36.5 °C) anaerobic filter. Despite the continuous dominance of the acetoclastic <i>Methanosaeta</i> in the anaerobic filter, the methane (CH₄) fraction derived from carbon dioxide reduction (CO₂), <i>f</i>_mc, varied significantly over the investigation period of 200 days. At organic loading rates (OLRs) below 6.0 gCOD L^–1d^–1, the average <i>f</i>_mc value was 33%, whereas at higher OLRs, with a maximum level of 17.0 gCOD L^–1d^–1, the <i>f</i>_mc values reached 47%. The experiments allowed for a clear differentiation of the isotope fractionation related to the formation and consumption of acetate in both stages of the plant. Our data indicate constant carbon isotope fractionation for acetate formation at different OLRs within the thermophilic leach bed reactors as well as a negligible contribution of homoacetogenesis. These results present the first quantification of methanogenic pathway (<i>f</i>_mc values) dynamics for a continually operated mesophilic bioreactor and highlight the enormous potential of δ¹³C analysis for a more comprehensive understanding of the anaerobic degradation processes in CH₄-producing biogas plants