Tuned synthesis of carbon spheres from resorcinol-formaldehyde resins for effective Cd(II) removal

In this study, the tunable synthesis of resorcinol-formaldehyde (RF) resins from resorcinol and formaldehyde, with ammonia as a catalyst, was investigated. RF resins with uniform sphere sizes were successfully prepared by optimizing the concentrations of resorcinol and ammonia. The resin spheres were subsequently carbonized into amorphous carbon spheres with slightly smaller diameters. The selection of reaction parameters enabled the adjustment of RF carbon sphere (RFCS) diameters within the range of 179 to 1546 nm. The prepared RFCSs exhibited type I adsorption isotherms, and the pore size distribution was dominated by micropores (1.9-1.6 nm), with a Brunauer-Emmett-Teller surface area of 479-650 m2/g. X-ray diffraction and Raman spectroscopy confirmed the presence of amorphous carbon with a high defect density in RFCSs. Fourier transform infrared spectroscopy indicated incomplete carbonization of RF resins, resulting in the retention of a substantial number of functional groups on their surfaces. Adsorption tests showed that RFCSs synthesized with 0.6 mL ammonia and 1.2 g resorcinol at the carbonization temperature of 800 °C exhibited the highest adsorption capacity for Cd(II) (813 mg/g), demonstrating their potential as effective adsorbents for heavy metal removal from wastewater. In conclusion, RFCSs are promising as an excellent matrix material for further modification and performance enhancement across diverse applications due to their regular spherical shape, adjustable pore size, hydrophobic surface, outstanding performance and low-cost preparation.