Ramichloridium apiculatum-mediated leaching of Mg and Si from serpentine: Process enhancement and kinetic modelling

Abstract

The leaching of Mg²⁺ is a key step in serpentine mineral carbon dioxide sequestration. Microbial-enhanced leaching of elements from serpentine offers significant advantages over traditional physical–chemical methods, including environmental friendliness, sustainability, and safety. However, challenges still remain due to the limited diversity of alkaline heterotrophic microorganisms suitable for serpentine bioleaching, the complexity of optimizing bioleaching conditions, and the unclear kinetic control mechanisms governing ion leaching during microbial action. In this study, the previously screened strain Ramichloridium apiculatum (CMS-1) was employed to activate serpentine. Bioleaching parameters were optimized by single-factor and response surface methodology (RSM) experiments. The regulatory mechanisms of Mg²⁺ and Si⁴⁺ leaching were elucidated by comparative analysis of different kinetic models. The analysis of variance (ANOVA) results confirmed that the leaching models for Mg²⁺ and Si⁴⁺ are statistically significant at the 95 % confidence level, with R² values all exceeding 0.95, the normal probability plots of residuals and the residuals versus run plots further verified the reliability and validity of the regression models. For Mg²⁺ leaching, the order of significance for single factors and their interactions was C (temperature) > B (inoculum size) > AB > AC > A (initial pH) > BC. For Si⁴⁺ leaching, the order was C > AC > A > AB > B > BC; Under the optimized bioleaching conditions (initial pH 6.50, pulp density 0.5 %, inoculum size 1.3 %, temperature 34.9 °C), the leaching efficiencies of Mg²⁺ and Si⁴⁺ reached 22.70 ± 1.70 % and 20.28 ± 1.28 %, respectively, closely matching the predicted values (Mg²⁺ =23.15 %, Si⁴⁺=20.37 %). The leaching processes of both Mg²⁺ and Si⁴⁺ from serpentine were governed by a mixed control model, involving both surface reactions and diffusion through product layer. The activation energy for Mg²⁺ (Ea = 44.26 kJ/mol) was lower than that for Si⁴⁺ (Ea = 48.60 kJ/mol), indicating easier release of Mg²⁺. This preferential leaching resulted in the formation of a porous silicon-rich residue layer on the serpentine surface.