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Trace metals in anaerobic biomass digestate
Date Issued |
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2019 |
One of the most abundant sources for alternative energy is biomass. Energy can be produced either by direct biomass combustion or alternatively using anaerobic biogas production technologies. Digestate – a by-product of anaerobic digestion can be used in agriculture or forestry as an organic fertiliser. However, due to the elevated concentrations of trace metals on-land application of the digestate can be restricted. Furthermore, the fate, chemical speciation as well as transport of trace metals in environments - often agricultural lands – is far from being understood. Metal species are formed exclusively in the extremely reducing, organic and metal rich conditions in anaerobic bioreactors. After anaerobic digestion trace metals are released into the environment via different routes of entry either suspended/dissolved in effluents or in a form of a solid biomass product. Once outside the reducing conditions of anaerobic fermenters, they will face oxidizing conditions that will influence their transformation to other thermodynamically stable species. This work aims to understand trace metal chemical speciation, redox-stability, sorption, mobility as well as bioavailability both in anaerobic technologies and in the soil environment. The total metal concentration, the conditions during digestion such as pH and redox potential, and the kinetics of reduction, precipitation, complexation, and adsorption are expected to play a key role influencing the chemical speciation of microelements in the digestate. To realize this aim, trace metal fractionation experiment was performed. Trace metal concentration and speciation was dependent upon the type of biomass used in the biogas reactor. The increasing pH decreased solubility of metals in the matrix. There were found significant differences in trace metal speciation between anaerobic environment and oxidising conditions.