Characterization of Fe-Mn concentric nodules from Luvisol irrigated by mine water in a semi-arid agricultural area Vojtěch Ettler a, ⁎, Martin Chren a , Martin Mihaljevič a , Petr Drahota a , Bohdan Kříbek b , František Veselovský b , Ondra Sracek c , Aleš Vaněk d , Vít Penížek d , Michael Komárek e , Ben Mapani f , Fred Kamona f a Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic b Czech Geological Survey, Geologická 6, 152 00 Prague 5, Czech Republic c Department of Geology, Faculty of Science, Palacký University in Olomouc, 17. listopadu 12, 771 46 Olomouc, Czech Republic d Department of Soil Science and Soil Protection, Faculty of Agrobiology, Food and Natural Resources, Czech University of Life Sciences Prague, Kamýcká 129, 165 21 Prague 6, Czech Republic e Department of Environmental Geosciences, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kamýcká 129, 165 21 Prague 6, Czech Republic f Department of Geology, Faculty of Science, University of Namibia, Private Bag 13301, Windhoek, Namibia abstract article info Article history: Received 6 December 2016 Received in revised form 13 March 2017 Accepted 25 March 2017 Available online xxxx We studied Fe-Mn concentric nodules from Cutanic Luvisol in the northern part of Namibia, where agricultural fields are irrigated with the drainage water from the Kombat Cu-Pb-(Zn) mine (pH 7, metal concentrations in μgl -1 : Fe 7, Mn 10, Zn 7, Cu 18). Concentric nodules (0.5–2 cm in size) were mostly found towards the bottom of the sampled soil profile (Btcg2 horizon, depth 100–120 cm). Comparisons with the bulk chemical composition of the soil matrix showed that Fe-Mn nodules were enriched in metals, metalloids and other trace elements (with a few exceptions, enrichment factors varied in the range 1.3–6.4). The concentrations of the elements of interest in the Fe-Mn nodules were as follows (mg kg -1 ): As 23.1, Ba 3840, Cd 6.83, Cu 450, Pb 597, Zn 137. The X-ray diffraction analysis indicated that the nodules were composed of quartz, goethite, hematite, illite/mica, pyrochroite, lithiophorite and birnessite. The scanning electron microscopy (SEM) observations confirmed that the internal structure of the nodules with concentric rings reflected seasonal changes in the redox conditions. Spot analyses and X-ray elemental maps performed using energy/wavelength dispersion spectrometry (EDS/ WDS) showed that the concentrations of metalloids were rather low except for the slightly elevated Ba concen- trations, which were observed only within the Mn oxide-rich zones. Selective extractions were used to under- stand the partitioning of trace elements within the individual phases. Whereas Mn oxides sequestered the majority of the Cd (up to 98%), Ba, Pb and rare earth elements (REEs) (up to 78%), other metals such as Cu and Zn exhibited much lower values (47–65%) and, together with oxyanionic species (As, Cr, V), were also significant- ly bound to Fe oxides. The pH-static leaching test conducted in the pH range 2–12 indicated that the majority of the trace elements were mostly leached under acidic conditions with the exception of As, which was highly sol- ubilized at pH 12 (up to 17%). Whereas Ba, Cd, Cu and Zn were significantly released under acidic conditions (up to 12%), the leaching of Pb was almost negligible over the entire pH range. Our results show that Fe-Mn nodules act as significant traps for trace metals in Luvisols irrigated by mine water. However, an abrupt decrease in the pH and changes in the redox conditions may cause the dissolution of Fe-Mn nodules and subsequent release of contaminants into the soil system. © 2017 Elsevier B.V. All rights reserved. Keywords: Luvisol Fe-Mn concentric nodules Trace elements Irrigation Mine water Contamination 1. Introduction Seasonal changes in the redox potential (Eh) and pH of soils, primar- ily due to waterlogging, may lead to the formation of Fe-Mn nodules, corresponding to hard redoximorphic features with spherical shapes and dark colors containing grains of soil material cemented with Fe and Mn (oxyhydr)oxides (D'Amore et al., 2004; Gasparatos et al., 2005; Gasparatos, 2013 and references therein; Jien et al., 2010; Sipos et al., 2016). Concentric nodules have distinct internal structure gener- ally with concentric rings (layers) reflecting variation in the redoximorphic conditions during their formation (Hickey et al., 2008; Gasparatos, 2013; Stoops, 2003). Nodules are generally enriched in trace elements compared to the host soils (Cornu et al., 2005; Gasparatos et al., 2005; Gasparatos, 2013; Latrille et al., 2001; Laveuf et al., 2012; Liu et al., 2002; Neaman et al., 2004a, 2008; Palumbo et al., 2001; Szymański and Skiba, 2013; Tan et al., 2005, 2006; Timofeeva, 2008; Timofeeva and Golov, 2010; Timofeeva et al., 2014) and their mineralogical composition is often very complex and standard phase characterization techniques (such as X-ray diffraction, XRD) Geoderma 299 (2017) 32–42 ⁎ Corresponding author. E-mail address: ettler@natur.cuni.cz (V. Ettler). http://dx.doi.org/10.1016/j.geoderma.2017.03.022 0016-7061/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma