Deciphering the distinct roles of endogenous and exogenous alkali/alkaline earth metals in modulating biochar surface charge and electron donating capacity: A mechanistic study

Abstract

Alkali and alkaline earth metals (AAEMs) are ubiquitous in biomass-derived biochars, yet whether their origin (endogenous vs. exogenous) leads to distinct electrochemical functions remains unclear. Here, this study systematically decouples these roles by comparing pristine biochars from four feedstocks, their acid‑demineralized counterparts, and the demineralized biochar subsequently reloaded with K, Na, Mg, or Ca. Using multi-technique characterization (XPS, FTIR, Raman, XRD, SEM‑EDS, zeta potential measurements) and mediated electrochemical analysis, this study confirms that endogenous AAEMs act as structural dopants, catalyzing the formation of oxygen‑containing groups (C-O and carboxyl) and defect sites (high I_D/I_G), resulting in an intrinsically negative surface charge and a high electron donating capacity (EDC up to 1.25 mmol e^-·g^-1 in CMBC). Acid demineralization removes AAEMs, drastically lowering EDC by 85–90% and shifting the surface charge from permanent to pH‑dependent. Reloading AAEMs creates cation-specific effects, and monovalent K⁺ and Na⁺ mainly form soluble salts and enhance EDC modestly through electrostatic screening and cation‑π interactions, while divalent Mg²⁺ and Ca²⁺, especially Ca²⁺, re‑establish oxygen‑rich surfaces (O/C up to 0.48) and induce new defects, achieving the highest EDC recovery among all loaded biochars (0.29 mmol e^-·g^-1). These findings demonstrate that endogenous AAEMs build an effective electron reservoir, while exogenous AAEMs act as tunable surface modifiers, providing a mechanistic framework for designing biochars with predictable electrochemical properties.