Contents lists available at ScienceDirect Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem SR micro-XRF to study Pb diffusion using a one-dimensional geometric model in leaves of Brassica napus for phytoremediation M. Rubio a,b,c,* , M.F. Mera a , S. Cazón a , M.E. Rubio b,c , C.A. Pérez d a CEPROCOR, Córdoba, Argentina b FAMAF. UNC, Córdoba, Argentina c IFEG-CONICET, Córdoba, Argentina d LNLS, Campinas, SP, Brazil ABSTRACT The purpose of phytoremediation is the removal of contaminants by plants that grow in soils with high levels of specific contamination. The technological im- plementation of lead phytoremediation depends on the understanding of tolerance and growth phenomena of the plant in contaminated soil, uptake and translocation of this element and its accumulation in aerial foliage of the plant. This work provides results about diffusive transfer of lead across the xylem towards leaf blade (or lamina) of Brassica napus using a 1-dimension model that assumes Fick's Second Law. It uses mapping data of Pb concentrations measured by synchrotron radiation micro x-ray fluorescence in order to calculate the diffusion coefficient in soft tissue of leaves. The diffusion equation is solved for a flat leaf geometric model and the solution is fitted to experimental values of Pb concentrations of equivalent leaves, harvested from the same plant in progressive times of its growing cycle. The results of the diffusion coefficient were calculated in a zone (∼600 μm) outside the leaf xylem, where the Pb concentration profile determines a well-defined gradient. These results are original and provide a contribution to understand the dynamics of Pb retention in leaves. 1. Introduction Some global organizations related to environmental effects on public health determine that lead is the heavy metal with major public health concern (WHO, 2016; IHME, 2015). It can produce serious consequences in organisms and in humans even in low concentrations. Human activities such as mining, smelting, municipal sewage sludge and shooting ranges are main sources of environmental lead con- tamination. Phytoextraction is one of the technological processes pro- posed for the remediation of soils contaminated by Pb and other toxic metals (EPA, 1999). It uses plants that develop the full growth cycle tolerating the toxicity of contaminated soils where they are grown. Phytoremediation is a feasible remediation tool if plants can take large amounts of lead in their roots, translocate this metal to the shoots and produce a large amount of biomass, preferably in its aerial foliar part (Cunningham et al., 1995). These plants should have the ability to grow in poor fertility soils and low rainfall too. Current research continues looking for plants with these characteristics. Not all of them achieve in themselves to satisfy the –so called in the literature–“three paradigms” of an ideal phytoextractive plant. However, given the potential hazard and widespread contamination of Pb, there is a high interest to seeking ecologically sustainable and cheaper Pb cleaning methods. Current phytoextraction knowledgement is mainly based on studies of Pb uptake processes from the soil of the rhizosphere to the root. However, less knowledge exists about the mechanisms of transfer of Pb to the rest of the plant and the bibliography related to the fixation in leaves is almost scarce. It is important to develop new works that cal- culate parameters involved in the translocation and accumulation of Pb in the aerial foliage, such as diffusion and saturation. Simulation models are also important, and identify parameters that contribute to the required knowledge. However, the literature related to this problem discusses almost exclusively the uptake of minerals and metals, and it does mention less about translocation (Brennan and Shelley, 1999). The purpose of this work is to contribute to the translocation mechanism that occurs in leaves, when Pb crosses the main vascular system of the plant. Lead divalent cations flow through the xylem from soil-to-shoots at higher speed integrating the transpiration stream. Johnson and Singhal (2007), proposed a dozen physicochemical processes and their respective equations to model the mechanisms that control plant uptake of metal from the soil and translocation from the roots to the shoots and how these mechanisms interact to control accumulation. In this work, the passive diffusion equation (2nd Fick's Law) is used to determine the diffusion coefficient of Pb in blade tissue of B. napus leaves. This approach describes the system of the leaf at the simplest level in order to estimate, based on SR induced micro-XRF measure- ments of concentrations, the diffusivity of Pb during its accumulation in the leaves. Diffusion coefficients results of this work are non-existent in the literature and may be important in order to shorten the plant crop https://doi.org/10.1016/j.radphyschem.2019.04.041 Received 16 December 2018; Received in revised form 15 April 2019; Accepted 17 April 2019 * Corresponding author. CEPROCOR, Córdoba, Argentina. E-mail address: mrubiocba@yahoo.com (M. Rubio). Radiation Physics and Chemistry xxx (xxxx) xxx–xxx Available online 22 April 2019 0969-806X/ © 2019 Published by Elsevier Ltd. Please cite this article as: M. Rubio, et al., Radiation Physics and Chemistry, https://doi.org/10.1016/j.radphyschem.2019.04.041