H₃PO₄-modified Lagerstroemia speciosa seed hull biochar for toxic Cr(VI) removal: isotherm, kinetics, and thermodynamic study

The novelty of the present work was to prepare biochar by utilizing waste biomass (Lagerstroemia speciosa) seed hull (LSSH) through pyrolysis process followed by chemical modification with H₃PO₄. This chemically modified biochar was employed as an adsorbent for toxic Cr(VI) removal in batch adsorption process due to its high surface area (246.23 m²g⁻¹) and low pore size (2.36 nm). Thermal stability of LSSH was examined in a thermogravimetric analyzer from 20 to 700 °C in an inert environment (N₂ flow rate = 100 ml/min). Proximate analysis of LSSH revealed lower moisture content (3.7%) and higher volatile matter (74.1%), whereas elemental analysis results revealed significant increase in carbon content after pyrolysis (38.94% to 45.85%). Higher heating (HHV) value of LSSH and LSSB was obtained to be 17.57 (MJ Kg⁻¹) and 18.06 (MJ Kg⁻¹) respectively. The maximum amount of Cr (VI) removal was obtained to be 99.97% at optimized operating condition of pH = 2, adsorbent dosage = 2 g L⁻¹, chromium concentration = 50 mg L⁻¹, and temperature = 303 K. The value of maximum adsorption capacity Q_m based on Langmuir model, enthalpy change (∆H°), entropy change (∆S°), and standard free-energy change (∆G°), was obtained as 41.92 mg g⁻¹, 174.28 kJ mol⁻¹, 0.62 kJ mol⁻¹ K⁻¹, and 15.4 kJ mol⁻¹, respectively at 303 K. Adsorption was noticed to be endothermic followed Langmuir isotherms (R² = 0.9996) with pseudo-second-order kinetics (R² = 0.998). The comparative assessment of adsorption capacity along with other biosorbents suggests that CLSSB can be utilized as an effective adsorbent for toxic Cr(VI) removal as well as other pollutants-containing wastewater.