From Waste to Water Retention: Tailoring Biochar Pyrolysis Temperature for Enhanced Hydraulic Performance in Landfill Covers

Biochar is produced through the thermal decomposition of biomass under specific temperature conditions. Temperature variations in biochar production may lead to differences in the physicochemical properties of the biochar. This study evaluated how pyrolysis temperature affects the physicochemical properties of tea- and straw-derived biochar and the soil’s water retention capacity. Tea- and straw-derived biochar underwent pyrolysis under limited-oxygen conditions at 300 °C and 550 °C. Clay samples incorporated biochar at 5–15% (w/w). A temperature- and humidity-controlled chamber was employed to simulate wet–dry cycles (25 °C; relative humidity 50–90%) to investigate water retention behavior concerning soil suction and water content. Soil amended with 15% (w/w) tea waste biochar (TB) pyrolyzed at 300 °C had the highest volumetric water content (10.601 cm³). Across both biochar (TB and SB), increased biochar content consistently enhanced water retention of yellow clay at each pyrolysis temperature. Specifically, the 15% TB-300 treatment increased soil–water content by 23.7% relative to bare soil (BS), whereas the 5% SB-550 treatment increased it by 10.1%. Under high suction conditions, BAS maintained higher moisture levels than BS, indicating enhanced water retention capabilities in arid environments. In summary, relative to unamended soil, biochar significantly increased soil moisture retention across the tested suction range. Across all relative humidity levels, adsorption and desorption remained stable during testing.