NaOH-assisted HO Post-modification As a Novel Approach to Enhance Adsorption Capacity of Residual Coffee Waste Biochars Toward Radioactive Strontium: Experimental and Theoretical Studies

In this study, NaOH-assisted HO post-modification was proposed as a novel strategy to enhance the adsorption of radioactive strontium (Sr) onto residual coffee waste biochars (RCWBs). To validate its viability, the adsorption capacities and mechanisms of Sr(II) using pristine (RCWB), HO post-modified (RCWB), and NaOH-assisted HO post-modified residual coffee waste biochars (RCWB) were experimentally and theoretically investigated. The highest adsorption capacity of Sr(II) for RCWB (10.91 mg/g) compared to RCWB (5.57 mg/g) and RCWB (5.07 mg/g) was primarily attributed to higher negative surface zeta potential (RCWB = -5.66 → -30.97 mV; RCWB = -0.31 → -11.29 mV; RCWB = 1.90 → -10.40 mV) and decoration of Na on the surfaces of RCWB via NaOH-assisted HO post-modification. These findings agree entirely with the theoretical observations that the adsorption of Sr(II) onto RCWB and RCWB was controlled by electrostatic interactions involving carbonyls whereas enriched carboxylic acids and decorated Na on the surfaces of RCWB through the replacement of Mg and K by NaOH-assisted HO modification stimulated electrostatic interactions and cation exchanges governing the adsorption of Sr(II). Hence, NaOH-assisted HO post-modification seemed to be practically applicable for improving the adsorption capacity of Sr(II) using RCWB-based carbonaceous adsorbents in real water matrices.