Valorisation of Macroalgae Biomass through Hydrothermal Carbonization for Soil Improvement

In recent years excessive macroalgal growth was observed in the Orbetello lagoon (Tuscany, Italy) due to eutrophication. At present, macroalgae are harvested and landfilled as a waste product. This work aims to investigate the valorisation of the macroalgae through hydrothermal carbonization (HTC) process. The influences of temperature (200 – 240 °C) and residence time (2 – 4 h) on yields and products properties products were investigated. Considering the hydrochar yield, the temperature was the most influencing parameter and the highest value (66.4 wt.% dry basis) was achieved at 200 °C and 4 h. The reaction conditions slightly influenced the organic carbon content of the samples, ranging between 38.8 and 41.4 wt.%, while the highest ash content (28.1 wt.%), pH (7.0) and electrical conductivity (6.2 mS cm-1) were measured at harsher reaction conditions. This study investigated the possible application of macroalgae hydrochar (HC) as soil improver and green peat substitute. Both pH and carbon content of the HC met the limits imposed by the Italian legislation for neutral peat and the concentration of pollutants comply with the limit for utilization as green soil amendment. Moreover, the HC were rich in macronutrients (Ca, K, Mg) and micronutrients (Fe, Mn) and their concentration ranged between 6.5 and 7.5 wt.% suggesting the potential application as a nutrients source for plant growth.