Biochar Control of Water Regime and Adsorption Rate in Soils

The effect of adding 10% biochar (B) or sludgechar (SL) on the water regime and adsorption properties of soils was tested on composites prepared by mixing two standard soils of loamy and clay type with B or SL in a 90:10 weight ratio. Water-holding capacity was assessed as initial (2 h) and equilibrium (24 h). Water retention time was estimated by evaporation from saturated samples at 20 °C to a constant weight. The composites exhibited a 60–90% increase in water absorption compared to the individual soils, retaining water up to 3–6 days longer than the individual soils. The adsorption properties were tested for cation (Pb2+) and anion (Sb(OH)6−) adsorption and for Pb2+ and Sb(OH)6− co-adsorption from model solutions under laboratory conditions. All samples showed higher selectivity for Pb2+, with the adsorption efficiency from 40% to 99%. Sb(OH)6− adsorption achieved a maximum efficiency of only 10%. Pb2+ and Sb(OH)6− co-adsorptions were efficient for Sb(OH)6− adsorption, reaching efficiency levels above 95%. At prolonged reaction times, the adsorption efficiency elevated by more than 20%. Only 10% wt. addition of biochar or sludgechar enhanced not only the water regime of soils but also their adsorption capacity for ionic contaminants.