[Adsorption of Iopamidol by NaHCO₃-activated Buckwheat Biochar]

NaHCO₃-activated buckwheat biochar was studied, and an optimal biochar of 0.25N-BC [m(NaHCO₃):m(buckwheat bark)=0.25:1]was selected. SEM, BET, XRD, Raman, FTIR, and XPS methods were applied to analyze the effects of NaHCO₃ on the physicochemical properties of buckwheat biochar. The adsorption properties and mechanism of NaHCO₃-activated buckwheat biochar for iopamidol(IPM), a nonionic iodol X-ray contrast agent, were also investigated. The results showed that compared with buckwheat skin biochar(BC), NaHCO₃-activated biochar had higher structural defects(surface area and pore volume increased, respectively, from 480.40 m²·g^-1 and 0.29 cm³·g^-1 to 572.83 m²·g^-1 and 0.40 cm³·g^-1, with I_D/I_G being 1.22 times that of BC), the carbon and oxygen functional groups on the BC surface changed significantly, and the polarity increased [(N+O)/C from 0.15 to 0.24]. The maximum adsorption capacity of 0.25N-BC for IPM was 74.94 mg·g^-1, which was 9.51 times that of BC(7.88 mg·g^-1). The pseudo-second-order adsorption kinetics and Langmuir and Freundlich isotherm models could well fit the adsorption of 0.25N-BC for IPM. The adsorption processes were mainly chemical, monolayer, and heterogeneous multilayer adsorption. Pore filling, hydrogen bonding, π-π, and n-π interactions were the main mechanisms of 0.25N-BC adsorption for IPM. Comparing the activated buckwheat biochar by different bases [KOH, Na₂CO₃, NaHCO₃, KHCO₃, and Ca(HCO₃)₂], 0.25N-BC exhibited high adsorption capability and short equilibrium time and could effectively remove the IPM residue in the actual water(secondary sedimentation tank effluent and lake). The removal rate of IPM remained at 74.91% after three adsorption-desorption cycles. The results showed that NaHCO₃-activated buckwheat biochar was a green, effective, and sustainable adsorbent for the removal of iodine-containing organic matter.