Dissolved Organic Matter Composition and Mineral Characteristics both Control Adsorption Processes

Adsorption of dissolved organic matter (DOM) onto minerals plays a critical role in the global carbon cycle, influencing carbon stability and sequestration across terrestrial and aquatic ecosystems. Currently, it remains unclear if mineral properties or DOM composition are more relevant for predicting adsorption. We tested this by quantifying the maximum adsorption capacity (Qmax) of five mineral materials (podzol Bs horizon, agricultural topsoil, glacial stream sediment, kaolinite-dominated clay, synthetic goethite) for five DOM sources (humic lake, peat, leaf litter, algae, and pyrogenic organic matter). Adsorption characteristics were determined using a modified Langmuir model. In addition, aliquots of three mineral samples were treated with sodium hypochlorite to remove pre-existing organic matter, enabling assessment of adsorption capacity onto bare mineral surfaces. Qmax values spanned 31–28,630 mg C kg^-1, exceeding previously reported ranges and showing that both DOM composition and mineral properties variably control adsorption capacity. Even strongly adsorbing minerals such as goethite and clay showed large variation across DOM sources, being highest for peat and lowest for algae. Likewise, DOM from different sources differed in their adsorption affinities for the different mineral surfaces. Treatment with sodium hypochlorite increased DOM adsorption, depending on material type and mineral characteristics, such as hydrous aluminum and iron phases. In summary, carbon adsorption onto minerals depends on the characteristics of both the minerals and the organic matter. This suggests that soil models that do not consider the characteristics of organic matter are limited in accurately describing adsorption and predicting carbon sequestration potentials in soils and aquatic ecosystems.