Enhancement of aluminum doping on removal of norfloxacin by iron-loaded biochar: efficacy and mechanism

Abstract

The capacity of aluminum/iron-loaded biochar (AFBC) and iron-loaded biochar (FBC) for removal of norfloxacin (NOR) was investigated by batch experiments, while enhancement efficacy and mechanism was explored by kinetic, isothermal, and thermodynamic analyses, as well as by characterization of the AFBCs and FBCs and solution properties. As the initial pH was raised from 2 to 12, the NOR removal capability of AFBC varied within a narrow range (123.6–124.4 mg g⁻¹), which was 2.06–2.53 times that of FBC. The better description of the pseudo-second-order kinetic model, liquid film diffusion model, and Langmuir model indicated that NOR removal was dominated by surface monolayer chemisorption, and its rate-controlling step was mainly liquid film diffusion. Liquid film diffusion rates of AFBC and FBC were 7.01 × 10⁻² and 1.75 × 10⁻² min⁻¹, whereas their maximum q-values were 205.8/72.77 (298 K), 222.9/84.42 (308 K), and 235.3/95.25 mg g⁻¹ (318 K), respectively. NOR removal was spontaneous and endothermic, while more negative Gibbs energy change and lower enthalpy change of AFBC suggested that it was more beneficial for NOR removal. Adsorption and degradation of AFBC and FBC contributed 53.07 %/46.93 % and 61.76 %/38.24 % to NOR removal (298 K). The amounts of adsorption sites and free radicals (¹O₂, ·OH and ·SO₄) derived from AFBC were 2.43 and 3.47 times those of FBC, respectively. AFBC and FBC had three similar pathways for degrading NOR, while AFBC significantly enhanced the contributions of ·OH and ·SO₄. Furthermore, AFBC and FBC could be effectively desorbed and regenerated by ultrapure water and NaOH. In conclusion, the enhancement of NOR removal by aluminum doping on iron-loaded biochar was attributed to the elevation of affinity and the provision of more adsorption sites and free radicals.