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This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical K d approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2-0.3 h), a slow desorption rate (half-time of 3-9 days) and a very low sorption capacity (0.02-0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the K d model to predict 137 Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Laurent Garcia-Sanchez reports financial support was provided by Electricité de France., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Radionuclides 129 I (t 1/2  = 15.7 × 10 6 years) and 131 I (t 1/2  = 8.02 days) are both introduced into the environment as a result of nuclear human activities. Environmental transfer pathways and fluxes between and within ecosystems are essential information for risk assessment. In forest ecosystems, humus degradation over time could result in re-mobilization and then downward migration and/or volatilization of intercepted 129 I. In order to estimate the scale of these processes, humus (mull and moder forms) sampled under deciduous and coniferous forests were spiked with 125 I - (t 1/2  = 59.4 days), as a surrogate for 129 I, in order to study the evolution of its water-soluble and organic fractions as well as the volatilization rate during humus degradation at laboratory scale. To our knowledge, this is the first time that interactions between iodine and contrasting forms of forest humus have been investigated. The evolution of native stable iodine ( 127 I) pools in unspiked humus was also studied. The nature of the humus' organic matter appears to be a factor that impacts on the proportions of water-soluble and organic fractions of iodine and on their evolution. Iodine-125 was mainly organically bound (fraction for mulls and moders: ∼54-59 and 41-49%, respectively) and no clear evolution was observed within the 4-month incubation period. A large decrease in 125 I water-solubility occurred, being more marked for mull (from ∼14-32 to 3-7%) than for moder (from ∼21-37 to 7-19%) humus. By contrast, a significant fraction was not extractible (∼38-43%) and varied in inverse proportion to the water-soluble fraction, suggesting a stabilization of iodine in humus after wet deposit. The nature of the humus organic matter also impacted on 125 I volatilization. Although of the same order of magnitude, the total volatilization of 125 I was higher for moders (∼0.039-0.323%) than for mulls (∼0.015-0.023%) within the 4-month incubation period. Volatilization rates for mulls were correlated with the water-soluble fraction, implying that volatilization of 125 I could occur from the humus solution. Our results showed that humus is thus a zone of iodine accumulation by association with organic matter and that potential losses by lixiviation are significantly more important compared to volatilization., (Copyright © 2022 Elsevier Ltd. All rights reserved.)