Enhanced adsorption of Congo red dye onto polyethyleneimine-impregnated biochar derived from pine needles

Biochar derived from waste pine needles was chemically modified using polyethyleneimine (PEI) to increase its adsorptive potential for withdrawal of anionic dye Congo red from aqueous solution. PEI impregnation on biochar was confirmed from scanning electron microscopy and energy-dispersive X-ray analysis, Fourier transform infrared spectroscopy, and X-ray diffraction analysis. The surface area of biochar decreased after PEI treatment, but the amine groups increased on biochar surface. PEI-treated biochar displayed considerable increase in adsorption at acidic conditions. Adsorption isotherm was best explained by Langmuir model (R² > 99) and the adsorption kinetics agrees well with pseudo-second-order model. The maximum adsorption capacity of PEI-treated biochar was observed to be 294.11 mg g^-1 and 30.76 mg g^-1 for pristine biochar displaying a 9.5-fold increase. The positive value of standard enthalpy of adsorption (∆H° = 14.96 KJmole^-1) indicated the endothermic nature of adsorption, and positive value of entropy (∆S° = 74.43 Jmole^-1 K^-1) revealed the affinity of biochar towards dye molecules. Negative value of Gibb's free energy ∆G° (- 7.2 KJmole^-1) revealed that the process was spontaneous. Electrostatic interaction appeared to be the key mechanism governing the adsorption process. Thus, PEI-impregnated biochar represents novel low-cost sorbent that can effectively remove anionic dyes which are poorly removed by pristine biochar.