Oyster shell-doped ground coffee waste biochars for selective removal of phosphate and nitrate ions from aqueous phases via enhanced electrostatic surface complexations: A mechanism study

Efficient removal of phosphate (PI) and nitrate ions (NI) from aqueous phases is crucial for securing drinking water sources. This study firstly explored the surface modification of ground coffee waste biochars (GCWB) via co-thermal decomposition of ground coffee wastes (GCWs) and oyster shells (OSs), focusing on their differences in the physicochemical properties and adsorption mechanisms of PI and NI related to the selectivity and adsorption capacity. OSs-doped GCWB (OS@GCWB) exhibited the higher adsorption capacities of PI (106.5 µg/g) and NI (57.9 µg/g) ions compared to GCWB (PI = 12.2 µg/g; NI = 35.2 µg/g) owing to its enriched calcium hydroxides and improved surface charge properties through co-thermal decomposition of GCWs and OSs. From the assessed kinetic, intra-particle diffusion, and isotherm parameters, physisorption and intra-particle diffusion were primarily responsible for the elimination of PI and NI by GCWB and OS@GCWB. Moreover, the obtained thermodynamic parameters revealed that the adsorptive removal of PI and NI using GCWB and OS@GCWB showed spontaneous and endothermic natures over the tested temperature ranges. The shifts in the dominant adsorptive removal mechanisms of GCWB toward PI and NI from the ligand exchanges to the electrostatic surface complexations supported that its improved surface characteristics through co-thermal decomposition of GCWs and OSs could improve the selectivity and adsorption capacities of PI and NI in the co-presence of Cl-, HCO3-, and SO42- ions. Based on these observations, OS@GCWB derived from co-thermal decomposition of GCWs and OSs may be a promising adsorbent for the elimination of PI and NI.