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The bioavailability of pesticides in soils is affected by the addition of crop‐residue‐derived char, which alters the sorption and desorption characteristics of the soils. Sorption, desorption, and biodegradation experiments were performed using atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine] as the target compound in two soils (Hartsells and Grady) with and without a wheat (Triticum aestivum L.) char, and the char alone. The sorption isotherms for both soils were highly linear, whereas the isotherms for 1% char‐amended soils showed an increased sorption capacity and isotherm nonlinearity. The amount of atrazine sorbed by the char alone was 800 to 3800 times greater than sorbed by the soils. Successive desorption experiments showed that char‐amended soils contained higher nondesorbable fractions of atrazine than soils without char amendment. Pseudomonas sp. strain ADP capable of rapidly mineralizing atrazine was used to evaluate atrazine bioavailability. The presence of char reduced 14CO2 production by 11 and 20% in the char‐amended soils and char, respectively. The two‐step successive desorption experiments appeared to be able to predict the bioavailabilty of atrazine in char‐amended soils. This study shows that the presence of char in soils significantly influences the sorption and desorption processes and, ultimately, the bioavailability of atrazine.

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The reactive transport of metal and radionuclide contaminants in the subsurface often significantly influences their long‐term fate and effect in the environment. Typically, predictions of contaminant migration at a site involve the measurement of a distribution coefficient (KD), which is used to describe the interactions between the contaminant and the subsurface. The typical implicit assumption is that the adsorption isotherm (e.g., KD) is independent of the solid/solution ratio. Many geochemical factors, however, play a significant role in the reactive transport of contaminants in groundwater. The adsorption and transport of U(VI), for example, is strongly influenced by the presence of Fe oxyhydroxides and the carbonate system. However, these solutes or adsorbates and adsorbent interact with one another in a complex and highly nonlinear manner. Modeling of U(VI) adsorption has shown that under certain conditions, the solid/solution ratio can theoretically have a significant impact on the U(VI) adsorption isotherm. In particular, combining strongly interacting solutes [U(VI) and carbonate] and adsorbents [Fe(III) oxyhydroxides] that have monocomponent solute or adsorbate adsorption isotherms that are independent of the solid/solution ratio results in a multicomponent system where adsorption isotherms become dependent on the solid/solution ratio. The solid/solution ratio can therefore be critical when extrapolating the results of batch experiments, generally conducted at low solid/solution ratios, to column experiments and then to the field. These results have implications for modeling, scaling, and predicting the reactive transport of U(VI) and other strongly interacting solutes (e.g., metals and dissolved organic C) in subsurface environments.

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