NaOH-assisted H2O2 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 H2O2 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 (RCWBP), H2O2 post-modified (RCWBHP), and NaOH-assisted H2O2 post-modified residual coffee waste biochars (RCWBNHP) were experimentally and theoretically investigated. The highest adsorption capacity of Sr(II) for RCWBNHP (10.91 mg/g) compared to RCWBHP (5.57 mg/g) and RCWBP (5.07 mg/g) was primarily attributed to higher negative surface zeta potential (RCWBNHP = –5.66 → −30.97 mV; RCWBHP = –0.31 → −11.29 mV; RCWBP = 1.90 → −10.40 mV) and decoration of Na on the surfaces of RCWBP via NaOH-assisted H2O2 post-modification. These findings agree entirely with the theoretical observations that the adsorption of Sr(II) onto RCWBP and RCWBHP was controlled by electrostatic interactions involving carbonyls whereas enriched carboxylic acids and decorated Na on the surfaces of RCWBNHP through the replacement of Mg and K by NaOH-assisted H2O2 modification stimulated electrostatic interactions and cation exchanges governing the adsorption of Sr(II). Hence, NaOH-assisted H2O2 post-modification seemed to be practically applicable for improving the adsorption capacity of Sr(II) using RCWB-based carbonaceous adsorbents in real water matrices. [Display omitted] •NaOH-assisted H2O2 post-modification enhanced surface charge properties of RCWBP.•RCWBNHP showed highest Qe values of Sr(II) than RCWBP and RCWBHP.•Adsorption of Sr(II) by RCWBP, RCWBHP, and RCWBNHP was endothermic process.•Carbonyls were responsible for adsorption of Sr(II) by RCWBP and RCWBHP.•Carboxylic acids and Na played a key role in adsorption of Sr(II) by RCWBNHP.