Washed hydrochar from spent coffee grounds: A second generation of coffee residues. Evaluation as organic amendment

Spent coffee grounds (SCG) hydrochar is a second-generation solid waste obtained by hydrothermal carbonization (HTC). Two washed hydrochars from SCG (175 and 185 °C; 12 MPa of N₂) were tested as organic amendments of an agricultural soil (Cambic Calcisol), at doses of 1 and 2.5%, in an in vitro assay using Lactuca sativa as a crop plant. The washed hydrochars differ from the SCG in organic carbon (OC) (56 vs 47%), C/N ratio (29 vs 24), polyphenols (186 vs 77 mg GAE/g), pH (4.1 vs 5.8), assimilable P (186 vs 1274 ppm) and K (32 vs 2475 ppm). The particles of washed hydrochars have a lower size and a more porous structure than SCG particles. Higher HTC temperatures generate greater differences with SCG. The effects on the soil of washed hydrochars are similar to SCG, regarding OC, total N, C/N ratio, available K and P contents. The influence of SCG and washed hydrochars on lettuces is also similar: both give rise to plant growth inhibition and increase in the contents of Ca, Mg, Cu, Fe and Mn. Nevertheless, washed hydrochars seem to be more effective in the mobilization of elements in the soil than SCG, which could be attributed to their higher content of polyphenols. For example: Ca (139 mg/100 g with SCG and 160 mg/100 g with hydrochar) and Fe (0.742 mg/100 g with SCG and 1.45 mg/100 g with hydrochar). Therefore, it can be concluded that SCG hydrochars could be used as organic amendments with similar limitations and advantages to SCG.