Biochar from green waste for phosphate removal with subsequent disposal

•Cotton stalk green waste was carbonized as biochar (CSB).•Prepared CSB was investigated for its phosphate absorbing potential systematically.•Spectral, microscopic and diffraction studies proved the adsorption potential of CSB.•Thermodynamics justified the physical and chemical mode of adsorption.•Active sites depleted CSB were safely discharged in the environment. Biochar prepared from cotton stalk solid waste provides a new material for contaminant removal. In the present study, experiments were conducted to investigate the removal of phosphate from aqueous solution by cotton stalk biochar (CSB). The characterization of CSB & phosphate adsorbed biochar (PCSB) were done to substantiate the adsorption of phosphate on CSB surface. FT-IR studies disclosed the functional groups present in CSB and supported adsorption phenomena. Scanning electron microscope showed the porous nature of CSB and EDS measurements justified the phosphate adsorption process. XRD analysis revealed that the calcium and magnesium ions of CSB were also responsible for adsorption process. Experimental results fitted nicely with the heterogeneous isotherm models viz Freundlich and Temkin isotherm. The calculated Freundlich constant (n) suggested the cooperative adsorption. The heat of adsorption calculated from Temkin isotherm indicated the process to be exothermic in nature. The free energy of adsorption calculated from equilibrium studies justified physical as well as chemical means of adsorption. Hence CSB serves as a good adsorptive material and can provide viable solution for environmental protection. However, discharging active site depleted CSB to environment may pose subsequent problems. To combat the same, the PCSB was tested as nutrient enhancer for plant growth in soil and population multipliers of microbes in microbial fuel cells-a device for power generation. The disposal study concluded the feasibility of safe PCSB removal.