In Figure we present the BET measurement results from the Ni-struvite thermal series. Four different stages can be defined in all obtained isotherms (inset in Figure ). Firstly, in stage (I) at low partial pressures (p/p 0 < 0.4) a constant adsorption of N2 can be observed, associated to the formation of a mono-to multilayer on the surface. In the intermediate stage (II) at partial pressures between 0.4 < p/p 0 < 0.55 progressive multilayer condensation and filling of pores smaller size with N2 gas occurs probably. The third (III) and fourth (IV) stages show the absence of significant amounts of macropores as a plateau region occurs at high relative pressures (0.55 < p/p 0 < 0.92) followed by a steep increase due to bulk condensation of N2 at the highest partial pressures (p/p 0 > 0.92). Based on the slope changes in the observed isotherms in the stage (II) and the BJH pore size distribution (Figure ), heated Ni-struvite demonstrated type IV isotherms/hysteresis loops indicative of significant amount of mesopores. The surface area and the cumulative pore volume increased from 3.61(1) m²/g @ t = 10 min heating time to 90.9(5) m²/g @ t = 150 min heating time, while the average desorption BJH pore size decreased from 12.3 nm to 5.2 nm for a longer heating time (Figure ). After extended heating times t = 120-150 min the BET surface area remained nearly the same. As a reference, and for comparison of the BET data, Mg-struvite was heated at T = 90°C for one day and measured (last data point in Figure ). At a BET surface area of 240.5 (9) m²/g, it demonstrated a significantly higher value, accompanied by an average desorption BJH pore size of 4.2 nm compared to thermally treated Ni-struvite (Figure , ESI: Table ). The adsorption/desorption BJH pore size distribution of the Ni t-series clearly showed a shift to lower average pore sizes for longer heating times. Considering higher temperatures T = 120°C, 150°C (ESI: Figure , S5, Table ), the Mg and Ni system showed systematically lower BET surface area (Mg: 195.9(2) m²/g for T =120°C, 138.2(2) m²/g for T =150°C; Ni: m²/g for T = 120°C and 54.5(1) m²/g for T = 150°C).