Abstract
Hydrothermal carbonisation (HTC) is an efficient route to convert lignocellulosic residues into carbon-rich materials for environmental applications. This work evaluates the influence of phosphoric acid (H3PO4) concentration (2.5 and 5 M) on the yield, surface properties, and methylene blue (MB) adsorption capacity of activated hydrochars (AHCs) produced from apple tree pruning. HTC was conducted at 200 °C for 24 h followed by activation at 450 °C. The AHC obtained using 2.5 M H3PO4 developed a markedly higher specific surface area (1,343 m²/g) and micropore volume (0.591 cm³/g) compared with the 5 M sample (468 m²/g and 0.346 cm³/g, respectively), while also exhibiting a narrower average pore width (1.76 vs. 2.95 nm). These structural differences translated into significantly enhanced MB adsorption performance, with the 2.5 M material achieving the highest adsorption ratio and removal efficiency under the tested conditions. Ultimate analysis further confirmed stronger deoxygenation and carbon enrichment at 2.5 M (C = 76.7 wt%, O/C = 0.26) relative to 5 M (C = 53.4 wt%, O/C = 0.78). Overall, the results demonstrate that moderate H3PO4 loading optimises microporosity and functionalisation, yielding superior adsorbents for wastewater treatment.
