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Biogeochemical processes and arsenic enrichment around rice roots in paddy soil: results from micro-focused X-ray spectroscopyJ . F r o m m e r a , A . V o e g e l i n b , J . D i t t m a r a , M . A . M a r c u s c & R . K r e t z s c h m a r a SummaryThe spatial distribution and speciation of iron (Fe), manganese (Mn) and arsenic (As) around rice roots grown in an As-affected paddy field in Bangladesh were investigated on soil sampled after rice harvest. Synchrotron micro-X-ray fluorescence spectrometry on soil thin sections revealed that roots influence soil Fe, Mn and As distribution up to 1 mm away from the root-soil interface. Around thick roots (diameter around 500 μm), Mn was concentrated in discrete enrichments close to the root surface without associated As, whereas concentric Fe accumulations formed farther away and were closely correlated with As accumulations. Near thin roots (diameter < 100 μm), in contrast, a pronounced enrichment of Fe and As next to the root surface and a lack of Mn enrichments was observed. X-ray absorption fine structure spectroscopy suggested that (i) accumulated Fe was mainly contained in a two-line ferrihydrite-like phase, (ii) associated As was mostly As(V) and (iii) Mn enrichments consisted of Mn(III/IV) oxyhydroxides. The distinct enrichment patterns can be related to the extent of O 2 release from primary and lateral rice roots and the thermodynamics and kinetics of Fe, Mn and As redox transformations. Our results suggest that in addition to Fe(III) plaque at the root surface, element accumulation and speciation in the surrounding rhizosphere soil must be taken into account when addressing the transfer of nutrients or contaminants into rice roots.

An innovative full-scale implementation of a permeable reactive barrier, consisting of a double-row of cylinders filled with zerovalent iron shavings, for chromate remediation was monitored over four years. Solid samples were analyzed to elucidate (i) the relevant corrosion mechanisms and products, (ii) the pathways of chromate reduction and immobilization, and (iii) the long-term performance of the barrier situated in a hydrological and geochemical complex groundwater regime. Sampling and analysis of groundwater and reactive material revealed an oxidative iron corrosion zone evolving in the inflow and a zone of anaerobic iron corrosion in the center and outflow of the barrier. Chromate reduction was mainly confined to the inflow region. The formation and thickness of corrosion rinds depended on sampling time, position, and depth, as well as on the size, shape, and graphite content. In the inflow, the corrosion rinds mostly consisted of goethite and ferrihydrite. X-ray absorption fine structure spectroscopy revealed two distinct Cr III species, most likely resulting from homogeneous and heterogeneous redox reaction pathways, respectively. The longevity and long-term effectiveness of the PRB appears to be primarily limited by reduced corrosion rates of the ZVI-shavings because of the thick layers of Fe-hydroxides.