Doctoral dissertation
Andreina Laera

Dissertation: DGT technique improves precision in bio-accessible trace element analysis

Estimating bio-accessible trace elements in digestate is key to inform farmers about the immediately effective fractions in fertilizers. In her doctoral dissertation, Andreina Laera examined the diffusive gradients in thin films (DGT) technique and found it promising in observing the most bio-accessible trace element species.

In an effort to minimize organic waste generation and polluting emissions into the environment, the anaerobic digestion process is considered one of the best techniques currently available. Formally, the effort is framed in the EU climate and energy objectives of the Paris agreement.

The anaerobic digestion of organic waste generates two valuable products: biogas rich in methane and digestate rich in organic matter and nutrients for the soil. The utilization of digestate as soil fertilizer is supported by the European fertilizer regulation, which will come into force by the end of 2019. Despite the agronomic usefulness of digestate related to its organic matter content, however, the high concentration of metals may preclude the utilization of digestate on soils.

Andreina Laera's doctoral dissertation focuses on the utilization of an organic matter sequential extraction procedure and the DGT technique in identifying the fraction of trace elements potentially accessible for microorganism or plant uptake.

“For regulation purposes, the total element concentration is a standard criterion but it is actually less than perfect in indicating the fraction of trace elements available. Non-accessible trace elements, such as sulfide precipitates or trace elements chelated to organic matter ligands, contribute to the overall pool of element concentration in digestate,” Laera says.

Results from the organic matter sequential extraction procedure demonstrated that a small proportion of trace elements was associated with the dissolved organic matter fraction in digestate, suggesting that those elements are very likely bio-accessible for uptake. A high proportion of trace elements was associated with the mineral and residual fractions, however, meaning that most of the elements are not immediately bio-accessible after anaerobic digestion. Some elements were also not efficiently extracted in the procedure, which suggests that the method may not be ideal for the purpose.

Following these findings, Laera applied the DGT technique to a new digestate sample and was the first to demonstrate that such a technique can be used with matrices rich in organic matter content to estimate labile elements concentration, in other words the most bio-accessible species of trace elements.

“This technique proved more sensitive than the conventional dissolved element measurement due to its accumulation ability. I used it to estimate the labile elements in digestate exposed to different aeration regimes to simulate digestate storage in open tanks and handling before spreading on land,” Laera explains.

“The technique needs further investigation, however, as we need to determine the influence of organic matter on the diffusion rate of trace elements in DGT devices. Once we establish that, we can estimate the concentration of labile trace elements in digestate with great precision,” Laera notes.

The doctoral dissertation of MSc Andreina Laera in the field of environmental biotechnology titled Fate of trace elements during and after anaerobic digestion: a sequential extraction method and DGT technique to assess bio-accessible trace elements in digestate will be publicly examined in the Faculty of Engineering and Natural Sciences at Tampere University in Finland at 9:15 am on Wednesday, 22 May 2019 (address: Hervanta campus, Korkeakoulunkatu 8, Festia, room: FA032 (Pieni sali 1)). The opponents will be Professor Gabriel Billon from Université de Lille (France) and Associate Professor Joyanto Routh from Linköping University (Sweden). The Custos will be Professor Gilles Guibaud from Université de Limoges, PEIRENE (France).

Andreina Laera's doctoral research has been conducted in the framework of the Advanced Biological Waste to Energy Technologies (ABWET) Innovative Training Network (ITN) European Joint Degree Programme. ABWET has four partner organizations: University of Cassino and Southern Lazio (Italy, coordinator), Université Paris-Est (France), IHE Delft (the Netherlands) and Tampere University (Finland). The Examination Committee of Andreina Laera's doctoral dissertation includes one representative from each partner organization.

Andreina Laera comes from Noci (Bari) in Italy and worked as a researcher in the Laboratoire Géomatériaux et Environnement at the University of Paris-Est, in the Equipe Développement d’indicateurs ou prévision de la qualité des eaux at the University of Limoges and in the Department of Civil and Mechanical Engineering at the University of Cassino and Southern Lazio.

The dissertation is available in the Faculty of Engineering and Natural Sciences (Konetalo building K2222A) at Tampere University and by request from suvi.ikonen@tuni.fi

Watch the defence online here.