Characterization of Fe-Mn concentric nodules from Luvisol irrigated by mine water in a semi-arid agricultural area

Abstract

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 (pH7, metal concentrations in μgl−1: Fe 7, Mn 10, Zn 7, Cu 18). Concentric nodules (0.5–2cm in size) were mostly found towards the bottom of the sampled soil profile (Btcg2 horizon, depth 100–120cm). 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 (mgkg−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 concentrations, which were observed only within the Mn oxide-rich zones. Selective extractions were used to understand 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 significantly 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 solubilized at pH12 (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.