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Microbial electrochemical snorkel (MES) is a short-circuited microbial fuel cell applicable to water treatment that does not produce energy but requires lower cost for its implementation. Few reports have already described its water treatment capabilities but no deeper electrochemical analysis were yet performed. We tested various materials (iron, stainless steel and porous graphite) and configurations of snorkel in order to better understand the rules that will control in a wetland the mixed potential of this self-powered system. We designed a model snorkel that was studied in laboratory and on the field. We confirmed the development of MES by identifying anodic and cathodic parts, by measuring the current between them and by analyzing microbial ecology in laboratory and field experiments. An important application is denitrification of surface water. Here we discuss the influence of nitrate on its electrochemical response and denitrification performances. Introducing nitrate caused the increase of the mixed potential of MES and of current at a potential value relatively more positive than for nitrate-reducing biocathodes described in the literature. The major criteria for promoting application of MES in artificial wetland dedicated to mitigation of non-point source nitrate pollution from agricultural water are considered., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

To limit the nitrate contamination of ground and surface water, stimulation of denitrification by electrochemical approach is an innovative way to be explored. Two nitrate reducing bio-cathodes were developed under constant polarization (-0.5 V vs SCE) using sediments and water from a constructed wetland (Rampillon, Seine-et-Marne, France). The bio-cathodes responded to nitrate addition on chronoamperometry through an increase of the reductive current. The denitrification efficiency of the pilots increased by 47% compared to the negative controls without electrodes after polarization. 16S rRNA gene sequencing of the biofilms and sediments evidenced the significant and discriminating presence of the Azoarcus and Pontibacter genera in the biofilms from biocathodes active for nitrate reduction. Our study shows the possibility to promote the development of efficient Azoarcus-dominated biocathodes from freshwater sediment to enhance nitrate removal from surface waters., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Highlights: [•] An artificial wetland and a forest buffer were set-up in a tile-drained watershed. [•] Typically, the first flows after pesticide spraying had the highest concentrations. [•] An open close strategy is proposed for pesticide flow interception. [•] Load reductions were 45–96% in the wetland and −32% to 100% in the forest. [•] Infiltration, sorption and degradation accounted for these results. [Copyright &y& Elsevier]

Abstract: Wetlands are widely used to improve water quality, in particular for removal of nutrients such as nitrates. Watercress farms are artificial wetlands that could potentially reduce transfers of nitrate to rivers; however, little is known about denitrification occurring in these particular wetlands. Watercress farming is a declining activity, which could have effects on nitrate removal in these wetlands. Therefore, we investigated the potential for denitrification in sediments from cultivated and abandoned watercress farms using flow-through reactors in the laboratory. Nitrate reduction rates ranged from 1.03 to 7.51μmol NO3 − cm−3 day−1 (mean±SD: 3.2±2.1μmol NO3 − cm−3 day−1). In general, sediments downstream of watercress beds exhibited higher nitrate reduction rates and contained more organic carbon than upstream sediments. The highest potential nitrate reduction rates were reached by sediments from cultivated watercress beds. As carbon was hypothesized to be a key factor in controlling nitrate reduction rates, the effects of different carbon sources on these rates were investigated by adding carbon to the sediment. Carbon was added either as labile organic acids (acetate or a mixture of lactate, fumarate, and succinate) or in the form of site-specific macrophytes (watercress [Nasturtium officinale] and reed [Phragmites australis]). Nitrate reduction rates increased significantly upon supply of organic acids or macrophytes. The addition of watercress resulted in the highest potential nitrate reduction rates. This study shows that denitrification in watercress farms is carbon-limited and suggests that the abandonment of watercress farms, with declines in macrophyte production and shifting macrophyte dominance, is likely to decrease the ability of these wetlands to remove nitrate by denitrification. Nitrate removal in watercress farms could be improved by supplying easily available organic C or by management of the vegetation, in particular in the upstream part of the beds. [Copyright &y& Elsevier]

• Metal removal is assessed in a wetland implemented in drained agricultural catchment. • Time-integrated tools show efficient mitigation in dissolved and particulate fractions. • Bioaccumulation based-tools reveal metal bioavailability reduced at the outlet. • Spatial-temporal monitoring supports the central role of sediments in metal trapping. • The wetland has a beneficial effect for local invertebrates in terms of exposure. Constructed wetlands are designed to mitigate nutrient and pesticide fluxes from agricultural catchments. Nevertheless, information on their efficiency in removing non-degradable contaminants such as metals is still scarce. This study aimed to explore the metallic signature and fate of metals within the Rampillon wetland (France) receiving water from a drained 355-ha catchment under intensive agriculture. Original monitoring coupling classic, time-integrated and bioaccumulation-based tools was achieved to characterise spatiotemporal dynamics of various metals (As, Cd, Cr, Co, Cu, Mn, Ni, Pb, Sb, Se and Zn). To assess metal inflows and mitigation, samples of dissolved and particulate metals were collected bimonthly at the inlet and outlet of the wetland over 3 months. Simultaneously, time-integrated (sediment traps and passive samplers) and bioaccumulation-based (caged gammarids and biofilms) tools were deployed to monitor temporal changes in metal speciation and bioavailability. To gain insight into the spatial distribution of metals between abiotic and biotic matrices, sediments and indigenous invertebrates with contrasted ecologies were sampled in different cells of the wetland. The results showed time-integrated tools were more suitable than bimonthly samples to quantify metal mitigations because of temporal fluctuations and low contamination levels. Significant mitigations were thus observed in trapped sediments for all metals (ranged 11–23%, except Mn) as well as in the DGT-labile fraction for Cd, Cr, Co, Mn and Ni (ranged 13–51%). Bioaccumulation levels in biofilms also revealed a decrease in metal bioavailability at the outlet. Furthermore, the spatial survey supported the central role of sediments in metal trapping and the beneficial effect of this wetland for local biodiversity in terms of exposure. To conclude, this study provides valuable information on the ecodynamics and bioavailability of metals required for sustainable management of such artificial ecosystems and furthermore, of agricultural areas. [ABSTRACT FROM AUTHOR]