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Stability of ZnS formed during anaerobic digestion

Le Bars M., Levard C., Legros S., Ambrosi J.P., Rose J., Borschneck D., Doelsch E.. 2017. In : Antonio Serrano (ed.), Fernando G. Fermoso (ed.). 1st International Congress on Metals in Anaerobic Biotechnologies (IMAB17) book of abstracts. Seville : Events 4u, 2 p.. International Congress on Metals in Anaerobic Biotechnologies IMAB17. 1, 2017-10-04/2017-10-06, Séville (Espagne).

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) (<2% of sphalerite transformed in one year (Robson et al., 2014)) or synthetic ZnS with a crystallite size of 20-40 nm (7 to 76% of ZnS transformed in 2 years depending on the soil type (Voegelin et al., 2011)). This difference remains unexplained and is the main focus of our study in order to assess the fate of ZnS after spreading on crops. We assumed that ZnS formed in organic waste have nanometric sizes due to the presence of high amount of organic matter that inhibit ZnS growth. First, we synthetized nano-ZnS of different sizes to assess the influence of size and crystallinity on ZnS stability in controlled systems and in soil samples. We obtained new accurate data on ZnS-NPs transformation kinetics. These nano-ZnS have been characterized by: - Transmission Electronic Microscopy (TEM) to determine particle morphology (figure 1); - Wide Angle X-ray Scattering with Pair Distribution Function Analysis to obtain crystallite sizes and lattice constrains; - X-ray Absorption Spectroscopy (XAS) at Zn k-edge for local structure. Those well characterized nano-ZnS were also used as references for XAS Zn k-edge in situ measurements to assess the properties (size and crystallinity) of ZnS presents in digestates sampled in various industrial plants. This study evidences the formation of nano-ZnS during anaerobic digestion, the latter being spread in large quantities on cultivated lands during digestate recycling. We gained a new understanding on how AD conditions control the precipitation of ZnS-NPs and on ZnS-NPs fate in soil after digestate application on cultivated lands.

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