Metal Immobilization on Wood‐Derived Biochars: Distribution and Reactivity of Carbonate Phases

Metals can be immobilized on biochars by precipitation with carbonate. The distribution of metal–carbonate phases at the surface of biochars and the conditions of their formation, however, are unknown. Electron microscopy and X‐photon spectroscopy were used to characterize carbonate phases in various morphological groups of particles of a wood‐derived biochar, both before and after a metal‐sorption experiment. Our results showed that the distribution of metals at the surface of biochar particles depended on the corresponding wood tissues and the presence of carbonate phases. Metals were particularly concentrated (i) within calcium carbonate crystals in bark‐derived particles, which originated from calcium oxalate crystals formed prior to pyrolysis, and (ii) as new phases formed by the reprecipitation of carbonate on specific tissues of biochar. The formation of biochar carbonate phases and their redistribution by dissolution–precipitation mechanisms may primarily control the localization of metals on biochar particles and the durability of metals immobilization. Core Ideas Carbonate phases in wood‐derived biochars can immobilize metals in solution. Some carbonate phases result from oxalate crystals transformation during pyrolysis. Metals are immobilized within preexisting calcium crystals or in new precipitates. The new precipitates are localized on specific features inherited from wood tissues.