Heterogeneity in composition and reactivity of dissolved organic matter and metal partitioning in soil

Soil contamination with heavy metals is concerning due to its adverse effects on soil ecosystem services and human health. Metal uptake by plants and effects on soil organisms depend not only on the total metal content but also on their partitioning between the soil solid phase and soil solution, and their solution speciation. Understanding the solid-solution partitioning behavior and speciation of heavy metals is essential for risk assessment and soil management. Soil organic matter (SOM), particularly dissolved organic matter (DOM), plays a crucial role in regulating the bioavailability and transport of heavy metals. DOM can form soluble metal-organic complexes, facilitating the transport of metals to groundwater and surface waters. As the most mobile and reactive component of SOM, DOM influences key biogeochemical processes such as soil carbon sequestration, nutrient cycling, and the degradation and transport of organic pollutants. This thesis investigated the heterogeneity in composition and reactivity of DOM and its effects on metal partitioning in soil using surface complexation modeling (e.g., NICA-Donnan, NOM-CD), laboratory experiments, and advanced techniques (e.g., FTIR, FT-ICR-MS). The first objective was to investigate the physical-chemical processes controlling DOM release in agricultural topsoils under varying pH and Ca concentration. Results showed that DOM release was controlled by mineral-bound organic matter desorption at acidic to near-neutral pH, releasing mainly hydrophilic acids and fulvic acids (FA). At near-neutral to alkaline pH, the dissolution of OM aggregates became the dominant process. The second objective was to study the effects of pH and Ca on soil DOM composition and metal binding reactivity. The highest metal binding capacity of DOM occurred around neutral pH at a given Ca concentration, corresponding to the dominance of FA with a middle weight-average molecular size (Mw). FTIR and FT-ICR-MS analysis indicated that these DOM have a higher ratio of carboxylic to aromatic functional groups and are richer in carboxylic-rich aromatic molecules. The metal binding capacity of DOM is not a simple function of Mw and aromaticity (SUVA254nm), as often suggested. The third objective was to evaluate the effects of composite soil sampling on soluble metal concentrations. The soluble Cu concentrations measured in composite samples were consistently lower than the averages of the two corresponding discrete samples, due to the nonlinear dependency of the soluble Cu concentration on the reactive Cu concentration, OM content and pH. This suggested that composite soil sampling strategy can underestimate the risks of heavy metal contaminated soils. The fourth objective was to apply MSM approach to study solid-solution partitioning and speciation of heavy metals in calcareous clay soils contaminated by mining activities. For these soils, the geochemical reactivity of heavy metals evaluated by 0.005 M DTPA extraction and the reactivity of SOM are required to enable adequate MSM predictions of heavy metal solubility. This thesis contributes to understanding of DOM heterogeneity and its role in metal partitioning and speciation. The findings support strategies to mitigate DOM-induced environmental impacts and inform soil management practices, such as liming and composite sampling, for improved assessment of heavy metal leaching and bioavailability.