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Freshwater ecosystems are critical resources for drinking water. In recent decades, dissolved organic matter (DOM) inputs into aquatic systems have increased significantly, particularly in central and northern Europe, due to climatic and anthropogenic drivers. The associated increase in dissolved organic carbon (DOC) concentration can change lake ecosystem services and adversely affect drinking water treatment processes. In this study, we examined spatial and temporal patterns of DOM treatability with granular activated carbon (GAC) and biological reactivity based on 14-day bacterial respiration incubations at 11 sites across Mälaren during six-time points between July 2019 and February 2021. Mälaren is the third largest lake in Sweden and provides drinking water for over 2 million people including the capital city Stockholm. In our spatio-temporal analysis, we assessed the influence of phytoplankton abundance, water chemistry, runoff, and climate on DOM composition, GAC removal efficiency, and biological reactivity. Variations in DOM composition were characterized using optical measurements and Orbitrap mass spectrometry. Multivariate statistical analyses indicated that DOM produced during warmer months was easier to remove by GAC. Removal efficiency of GAC varied from 41 to 87 %, and the best predictor of treatability using mass spectrometry was double bond equivalents (DBE), while the best optical predictors were specific UV absorbance (SUVA), and freshness index. The oxygen consumption rate (k) from the bacterial respiration incubations ranged from 0.04 to 0.71 d⁻¹ and higher in warmer months and at deeper basins and was associated with more aliphatic and fresh DOM. The three deepest lake basins with the longest water residence time (WRT) were temporally the most stable in terms of DOM composition and had the highest DOC removal efficiency and k rates. DOM composition in these three lake basins was optically clearer than in basins located closer to terrestrial inputs and had a signature suggesting it was derived from in-lake processes including phytoplankton production and bacterial processing of terrestrial DOM. This means that with increasing WRT, DOM derived from terrestrial sources shifts to more aquatically produced DOM and becomes easier to remove with GAC. These findings indicate WRT can be highly relevant in shaping DOM composition and thereby likely to affect its ease of treatability for drinking water purposes.

Adsorption of dissolved organic matter (DOM) onto mineral surfaces plays a critical role in the global carbon cycle, influencing carbon stability and sequestration across terrestrial and aquatic ecosystems. We quantified the maximum adsorption capacity (Qmax) of DOM to mineral particles to investigate variations in the adsorption of five different DOM sources, including a humic lake, peat extract, leaf litter, algae extract, and pyrogenic DOM to five distinct mineral types of podzol, agricultural soil, glacial sediment, commercial clay, and pure goethite. Adsorption characteristics were determined by fitting a modified non-linear Langmuir model to data obtained in batch adsorption experiments. We also treated samples of podzol, agricultural soil, and glacial sediment with sodium hypochlorite to remove pre-existing organic matter, allowing for a closer examination of adsorption mechanisms. The resulting range of Qmax values (31 to 28,630 mg kg^-1) spanned the full range of values previously reported in the literature. We found that both DOM composition and mineral properties play significant and variable roles in determining adsorption capacity. DOM sources can exhibit either high or low Qmax depending on the mineral types, while minerals can be more or less susceptible to adsorption based on the specific DOM source. The sodium hypochlorite treatment without pre-existing soil OM showed higher DOM adsorption. However, this depends on the type of soil and other adsorptive characteristics of the minerals, such as amorphous compounds of aluminum and iron. This study contributes to a comparative understanding of DOM-mineral interactions, which has implications for predicting carbon sequestration in soils and aquatic ecosystems.

Dissolved organic matter (DOM) composition in terrestrial and aquatic ecosystems is controlled by various physical and biogeochemical processes, including adsorption to mineral particles. Advancing our knowledge of DOM-mineral interactions is crucial for assessing long-term carbon storage in ecosystems. Considerable efforts have been made to characterize how DOM-mineral interactions influence DOM composition, yet the impact of adsorption extent on shaping DOM composition remains poorly understood. In this study, we investigate how DOM composition changes across a gradient of adsorption capacity by studying adsorption interactions between five widely different DOM sources originating from terrestrial and aquatic ecosystems and five minerals that have largely distinct characteristics. We also used a multi-analytical approach for DOM characterization by combining fluorescence analysis, mass spectrometry (FT-ICR MS) and ¹H NMR, allowing us to gain a more comprehensive overview of changes in DOM chemical composition due to adsorption. By including ¹H NMR analysis, we captured changes to carbohydrate-like compounds, which are potentially energy-rich for microbial uptake and respiration. We found that strong adsorption interactions preferentially removed more oxygen-rich DOM compounds with higher aromatic and polyphenolic fractions, while weak adsorption was less selective and removed a broader range of DOM fractions. Selective removal for strong DOM-mineral interactions led to the enrichment of more aliphatic, protein-like structures and carbohydrates in the remaining DOM pool. The findings of this study have important ecological implications in terms of energy availability for microorganisms and predicting carbon sequestration in terrestrial ecosystems and inland waters, where agricultural, mining, and forestry activities increase mineral particle presence and enhance DOM-mineral interactions.

Organo-mineral interactions exert important controls on DOM concentration and composition by selectively removing specific fractions of the DOM pool and changing the composition of the remaining fractions in the solution phase. The extent to which organo-mineral interactions affect the biodegradability of the remaining pool is important for a better understanding of DOM fate in aquatic and terrestrial ecosystems. In this study, we aim to investigate the effects of adsorption on DOM biodegradation by including a wide range of organo-mineral interactions and studying the microbial degradation of DOM remaining in solution after adsorption. To this end, we conducted 14-day bio-incubation experiments on five DOM sources with and without adsorption to five minerals. The wide range of adsorption capacities across this matrix of 25 DOM-mineral interactions provided an opportunity to establish important controls on DOM biodegradation after adsorption. The rate of biodegradation was obtained by fitting an exponential decay model to the high-resolution time series of oxygen concentration measurements over 14 days. Additionally, changes in biodegradable dissolved organic carbon (%BDOC14) were measured with and without adsorption. Our findings show that adsorption has varying effects on DOM biodegradation but largely depends on the original DOM composition and its interaction with minerals. We found that adsorption enhanced the biodegradation rate and %BDOC14 of certain DOM sources, such as peat extract, humic lake and leaf litter DOM by selectively removing less bioavailable fractions, including polyphenolic aromatics. Conversely, algae-derived DOM had high rates of biodegradation and %BDOC14 with minor changes in biodegradability after adsorption due to weak adsorption interactions with minerals. For pyrogenic DOM, biodegradation rates decreased specifically after exposure to a podzol due to the desorption of less biodegradable fractions into the solution. Our findings indicate that adsorption can enhance DOM mineralization under certain conditions by removing less biodegradable fractions. These findings have important implications for better prediction of carbon fluxes from terrestrial and aquatic ecosystems to the atmosphere.