Compositional and structural characterization of bioenergy digestate and its aerobic derivatives compost and vermicompost

Introduction: In addition to produce biogas, the anaerobic digestion process releases a significant quantity of solid digestate (DG), a C‐rich byproduct suitable both as a soil improver and as a biosorbent of organic and inorganic pollutants. This study considered a mixed biowaste DG and two aerobic DG‐derivatives such as compost (CP) and vermicompost (VC). Materials and Methods: A multianalytical approach was adopted to comprehensively characterize DG, CP and VC. Elemental composition was determined by total reflection X‐ray fluorescence (TXRF) spectroscopy; micromorphological features, surface elements and surface area were evaluated by scanning electron microscopy coupled with energy dispersive X‐ray (SEM‐EDX) technique and Brunauer–Emmett–Teller (BET) analysis; functional properties were investigated by attenuated total reflectance‐Fourier transform infrared (FTIR‐ATR) spectroscopy and Raman spectroscopy. Results: TXRF analysis showed that DG had the highest K content (~8.9 g kg−1), while VC had the maximum P, S and Ca contents, and appreciable levels of essential plant micronutrients, such as Mn, Fe, Cu and Zn. SEM‐EDX technique evidenced heterogeneous surfaces and low porosity of all materials. BET analysis showed relevant specific surface areas of DG, CP and, especially, VC (~1.6 m2 g−1). FTIR‐ATR and Raman spectroscopy allowed to identify various aliphatic and aromatic functional groups of the materials and provided information on their major molecular classes, such as lignocellulosic moieties and inorganic components. The latter techniques also contributed to understand the chemical transformations of the DG after the aerobic treatments. Conclusion: The overall results obtained confirmed the valuable potential of these materials in improving the physical and chemical fertility of the soil and in retaining pollutants, with co