Uptake of technetium and iodine (I^-, IO₃^- and organo-I) by pecan biochar

Remediation of land and groundwater that are contaminated with high-risk driving anionic-radionuclides, such as iodine-129 (I-129) and technetium-99 (Tc-99), remains an intractable problem. The objective of this study was to evaluate biochar materials as a low-cost and effective sorbent for Tc-99 and three iodine species, iodide/iodate/organo-iodine (org-I). Sorption studies were conducted with biochar derived from pecan shells (Carya illinoinensis), that underwent two pyrolysis temperatures (500 °C and 700 °C) and two types of acid-activation (H₃PO₄ and HCl). Acid-activated biochar had much higher sorption capacities (in terms of distribution coefficient, K_d, the concentration ratio of solid:liquid) for Tc-99 and different iodine species, than non-acid-activated biochar. The H₃PO₄-activated biochar (500 °C) was the most effective sorbent with Tc-99 K_d of 49,390 ± 14,268 mL/g, iodide K_d of 2433 ± 312 mL/g, iodate K_d of 410 ± 168 mL/g, and org-I K_d of 857 ± 181 mL/g. The HCl-activated biochar (700 °C) was also effective at sequestering Tc-99 (K_d of 7864 ± 5,585 mL/g) and iodide (K_d of 2481 ± 237 mL/g) but not for iodate/org-I. Solid-state ¹³C-nuclear magnetic resonance (NMR) analysis suggested the strong sorption capacity related to the formation of abundant alkene, aromatic and heteroaromatic functional groups, which was also supported by the Fourier-transform infrared spectroscopy (FTIR) analysis. Additionally, the FTIR suggested the possible electrophilic substitution of C-H by iodine to form C-I bond. Surface area measurements and SEM images indicated the H₃PO₄-activated biochar (500 °C) had especially high surfaces areas. Functionalized biochar may provide a cost effective and efficient sorbent for sequestering I-129 and Tc-99 from the biosphere, thereby reducing human risk.