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Screening of native plant species from mining sites can lead to identify suitable plants for phytoremediation approaches. In this study, we assayed heavy metals tolerance and accumulation in native and dominant plants growing on abandoned Pb/Zn mining site in eastern Morocco. Soil samples and native plants were collected and analyzed for As, Cd, Cu, Ni, Sb, Pb, and Zn concentrations. Bioconcentration factor (BCF), translocation factor (TF), and biological accumulation coefficient (BAC) were determined for each element. Our results showed that soils present low organic matter content combined with high levels of heavy metals especially Pb and Zn due to past extraction activities. Native and dominant plants sampled in these areas were classified into 14 species and eight families. Principal components analysis separated Artemisia herba-alba with high concentrations of As, Cd, Cu, Ni, and Pb in shoots from other species. Four plant species, namely, Reseda alba, Cistus libanotis, Stipa tenacissima, and Artemisia herba-alba showed strong capacity to tolerate and hyperaccumulate heavy metals, especially Pb, in their tissues. According to BCF, TF, and BAC, these plant species could be used as effective plants for Pb phytoextraction. Stipa tenacissima and Artemisia herba-alba are better suited for phytostabilization of Cd/Cu and Cu/Zn, respectively. Our study shows that several spontaneous and native plants growing on Pb/Zn contaminated sites have a good potential for developing heavy metals phytoremediation strategies.

International audience

International audience

Organo-mineral interactions are recognized as a key factor in stabilizing organic matter (OM) in soils and short-range order mineral phases are increasingly considered as key mineral phases in the control of OM dynamics (Rasmussen et al., 2018). Coprecipitation is the main mechanisms involved (Chen et al., 2014; Mikutta et al., 2014). A recent conceptual model proposes that coprecipitates form continuously upon soil mineral weathering in contact with organic compounds of the soil solution (Basile-Doelsch et al., 2015). For silicate minerals, this process implies that Si may also take part in the structure of coprecipitates. However, only Fe and Al coprecipitates have been considered as coprecipitating cations in the literature. Nanophases were precipitated from a solution containing ionic Fe, Al, Si, Mg and K, obtained from a biotite weathered leachate. TEM and Fe K-edge EXAFS showed that they were structured mainly by small oligomers of Fe, together with Si and Al (Tamrat et al., 2018). By adding an organic ligand (DOPA, initial M:C_1), coprecipitates were structured by a loose and irregular 3D network of small oligmers of Fe, Si and Al forming a highly reactive open-structured mineral skeleton on which OM was bond. A conceptual 3D model of these “nanoCLICs” (Nanosized Coprecipitates of inorganic oLIgomers with organiCs) has been proposed (Tamrat et al., submitted). It differs significantly from the previous models presented for Ferrihydrite and amorphous Al(OH)3 coprecipitates (Kleber et al., 2015). The present work seeks to validate this model using TEM imaging method, whose spatial resolution is adapted to the heterogeneity observed at the nanometer scale. The EELS elemental mapping revealed that the coprecipitates were composed of a mix of Fe Al Si and C down to the nanometer scale. No pure zone of Fe or C were observed, but Fe rich and C rich areas could be detected. The M/C ratio varied from 0.5 to 1 between areas spaced of a few nanometers only. These observations are concordant with OM bonds to small oligomers of Fe, Si and Al, as described in the nanoCLICs model. This work also shows that the high spatial resolution and the diversity of analyzed elements by TEM-EELS make this technique an excellent choice for the characterization of soil organo-mineral associations.

International audience

Zinc (Zn) is the most abundant trace element in organic waste (OW). Therefore, it is relevant to assess the environmental risk of OW agricultural recycling regarding soil contamination by Zn. Zn speciation in OW is a key parameter to evaluate its fate in environment and toxicity to living organisms. We have previously shown that Zn speciation changes radically according to the treatment applied to OW (anaerobic digestion and composting). Nanosized zinc sulphide (nano-ZnS) is the main Zn species in OW after anaerobic digestion, i.e. in digestates. Whereas Zn speciation is dominated by amorphous Zn phosphate and Zn sorbed to ferrihydrite in composts. We also showed that Zn speciation in OW drive Zn availability in an OW-amended soil. As anaerobic digestion and composting are gaining interest in many countries, it is crucial to: assess the fate of Zn following organic waste application on soil, with a particular emphasis on nano-ZnS determine the biogeochemical drivers that control this fate. A mechanistic approach was developed at the microcosm scale. First, we sampled OW on a full-scale plant at four different steps of the treatment. A clayey soil was incubated with the four OW samples having contrasted Zn speciation (0 to 100% nano-ZnS). In a second experiment, nano-ZnS enriched with 68Zn were synthesized to discriminate Zn released by nano-68ZnS dissolution from natural Zn in soil. Two soils with contrasted characteristics were incubated with the nano-68ZnS: the previous clayey soil and a sandy soil. In both experiments, Zn availability patterns was assessed in situ and over one to three months with the technique of the diffusive gradient in thin film (DGT). The results showed that: (i) Zn availability in the amended soil was positively correlated with the percentage of nano-ZnS in OW, (ii) Zn availability is controlled by soil characteristics, as significant differences were detected for the sandy soil and the clayey soil. Therefore, Zn speciation in OW and soil characteristics must be considered when assessing the environmental impact of agricultural recycling regarding Zn contamination.

Recycling of digestates is a strategic aspect for anaerobic digestion (AD) plants suitability. Many studies have shown the fertilizing properties of digestate on croplands (Nkoa, 2014). However, application of digestate on lands is not riskless, particularly regarding metal contamination. Indeed, organic wastes used as input in AD can be highly concentrated in zinc partly due to the addition of high amount of zinc in pig and cattle feeding (Alburquerque et al., 2012). Repeated land application of digestate causes long-term accumulation of zinc in soil. To prevent risk of contamination, some countries implemented regulation on digestate application. Unfortunately, those regulations only focus on total metal concentration while zinc speciation is the crucial parameter to consider when evaluating its bioavailability and potential toxicity (Harmsen, 2007). It has been shown that AD changes zinc speciation: zinc is mostly found as sulfides in digestates. ZnS formed during AD transform very quickly under oxic conditions (total transformation within 2 months (Lombi et al., 2012)) compared with what is observed with natural ZnS (i.e. sphalerite) (

Recycling organic residual by-products (ORB) in agriculture is a common practice. Indeed, it provides agronomic benefits as providing nutrients for crops. However, it may also induce environmental issues. Among these environmental issues are the high potentially toxic heavy metal concentrations that can be observed in ORB. Agricultural recycling of ORB is also a complex undertaking in which many choices must be made. For example, what type of ORB to use? Which quantity? Is a treatment of ORB necessary? What treatment (composting, anaerobic digestion…)? Etc… These choices should be based on a comprehensive assessment of potential benefits (methane potential, nutrient availability, carbon storage) and negative effects due to the presence of contaminants. So far, those choices are made mainly according to agronomical reasons and the negatives effects are only taken into account according to the regulation. While it is now well accepted that the fate of heavy metals in the environment is affected by their speciation, the regulation on heavy metals for agricultural recycling of ORB is solely based on total concentration in the ORB and in the soil. This is mainly due to the lack of understanding regarding the heavy metal speciation in ORB, the impact of treatments on heavy metal speciation, and the scarcity of knowledge on the fate of heavy metal following ORB soil application. These questions are the key scientific challenges that the study aims to meet. The methodological approach used in this study permitted to assess Zn speciation in key points of the agricultural recycling system: in ORB that have been treated or not, and within the soils that had received ORB applications. We used a combination of state-of-the-art physical techniques: X-ray powder diffraction (XRD), scanning electron microscopy with X-ray microanalysis (SEM-EDS), and X-ray absorption spectroscopy (XAS). We performed short-term lab experiment and long-term field experiment. The main results of this study are new insights on Zn speciation of ORB (e.g. the key role of Zn sulphides nanoparticles), on how treatment (particularly anaerobic digestion) alters Zn speciation in ORB, and on how Zn speciation in organic residual by-product drives the fate of Zn in cultivated soil.

Incidental NPs are produced and dispersed in the environment in much larger amounts than manufactured NPs and with much less consideration of their environmental impact. This is potentially the case for ZnS NPs that form in a variety of anaerobic environments such as municipal and agricultural organic wastes. As an example, ZnS NPs with sizes ranging from 2.5 to 7.5 nm in diameter were observed in final sewage sludge products potentially accounting for about 80% of the Zn present in the sludge.1 In most countries, these organic wastes are spread on cultivated soils as fertilizer, which may represent an important source of nanomaterial into the environment. Zinc can be phytotoxic for crops at doses commonly found in organic wastes.2 Although ZnS is poorly soluble from a thermodynamic point of view (Ksp=10-24.7), ZnS NPs are known to transform to oxidized products in oxic environments quicker than bulk ZnS. As an example, the oxidation rate of bulk sphalerite was shown to be relatively slow when incubated in soils under oxic conditions: 0.6% per year in a sub-tropical soil and 1.2 % per year in a temperate soil.3 In contrast, we have recently shown that ZnS NPs formed in pig slurry were quickly oxidized after spreading on crop fields.4 The parameters controlling the oxidation of ZnS NPs are currently unknown. Available data is insufficient to elucidate whether the reported differences in oxidation rate are related to medium properties or to size effects. Given the likely dominance of ZnS NPs in municipal and agricultural wastes and the widespread use of these wastes in agricultural practices, a more fundamental understanding on the factors controlling the formation, chemical reactivity, mobility, bioavailability and toxicity of ZnS NPs under different environmental conditions is needed. Here, we will present some results regarding the characterization of ZnS NPs in different organic wastes (e.g. pig slurry, sewage sludge) as well as preliminary results on the sizedependent properties of these NPs