Uranium Isotope (U-232) Removal from Waters by Biochar Fibers: An Adsorption Study in the Sub-Picomolar Concentration Range

Abstract

The adsorption of the U-232 radionuclide by biochar fibers in the sub-picomolar concentration range has been investigated in laboratory aqueous solutions and seawater samples. The adsorption efficiency (Kd values and % relative removal) of untreated and oxidized biochar samples towards U-232 has been investigated as a function of pH, adsorbent mass, ionic strength and temperature by means of batch-type experiments. According to the experimental data, the solution pH determines to a large degree the adsorption efficiency, and adsorbent mass and surface oxidation lead to significantly higher Kd values. The ionic strength and temperature effect indicate that the adsorption is based on the formation of inner-sphere complexes, and is an endothermic and entropy-driven process (ΔH° and ΔS° > 0), respectively. Regarding the sorption kinetics, the diffusion of U-232 from the solution to the biochar surface seems to be the rate-determining step. The application of biochar-based adsorbents to treat radioactively (U-232) contaminated waters reveals that these materials are very effective adsorbents, even in the sub-picomolar concentration range.

Removal of 241Am from Aqueous Solutions by Adsorption on Sponge Gourd Biochar

“…The solution pH strongly affects the sorption efficiency ( K d values), since both the Am 3+ species distribution in solution and the protonation/dissociation of the surface-active moieties of the biochar materials depend on the proton concentration. The latter applies to the π system–proton interaction and carboxylic dissociation for unmodified and oxidized biochar, respectively [ 20 , 21 , 22 , 23 , 24 ]. A schematic illustration of the interaction of Am 3+ cations with the aromatic (π-system) and the carboxylic groups on the biochar surface is given in Figure 2 .…”

“…Regarding the biochar surface, at pH = 2, the carboxylic moieties were extensively protonated, because the mean value of the acid dissociation constant, pK a , was below 4. At pH 4, a significant number of the carboxylic moieties was deprotonated and only at pH = 7 and 9 did the deprotonated carboxylic groups dominate, and the biochar surface was negatively charged [ 20 , 21 , 22 , 23 ].…”

“…The effect of pH on the Am 3+ adsorption by biochar fibers has been investigated and the corresponding log 10 K d values are summarized in Figure 3 . The oxidized biochar fibers (BC_ox) showed a much higher adsorption affinity for Am 3+ than their non-oxidized counterpart (BC), which may be attributed to the presence of a significantly higher concentration of oxygen-containing moieties (e.g., carboxylic groups) on the BC-ox surface [ 20 , 21 , 22 , 23 , 24 ]. Oxygen-containing moieties (e.g., phenolic, carboxylic groups) are hard Lewis bases and therefore exhibit a higher affinity for hard Lewis acids such as the Am 3+ cationic species.…”

“…Hence, the highest K d values for BC_ox were observed in the acidic pH region log 10 K d (BC_ox) = 4.1 ± 0.1 at pH = 2 and log 10 K d (BC_ox) = 4.2 ± 0.2 at pH = 4, and the lowest values (log 10 K d (ΒC) = 3.4 ± 0.3 at pH = 7 and log 10 K d (ΒC_ox) = 3.5 ± 0.2 at pH = 9. The significantly higher affinity of the oxidized biochar for Am 3+ in the acidic pH region can be attributed to the predominance of the Am 3+ cation in the respective pH region and the partially deprotonated, negatively charged surface carboxylic moieties, which strongly attract and complex the Am 3+ cations, resulting in the formation of inner-sphere complexes [ 20 , 21 , 22 , 23 , 24 ]. These observations agree well with Li et al (2001) and Zhang et al (2014), who reported comparable K d values at acidic pH [ 33 , 40 ].…”

“…A wide spectrum of physical, chemical, and biological processes has been developed and applied for the removal of radionuclides from the nuclear process and contaminated wastewaters including membrane and osmosis technologies, sorption and ion exchange, chemical precipitation, electroremediation, and bioremediation [ 8 ]. Adsorption technologies are of particular interest since effective adsorbents such as metal organic frameworks [ 9 ], aerogels [ 10 , 11 , 12 ], silica-based materials [ 13 ], polymers [ 14 , 15 ], biomasses [ 16 , 17 , 18 , 19 ], and carbon-based composites [ 20 , 21 , 22 , 23 , 24 , 25 ] can be prepared easily and at low-cost. One such material is biochar, prepared by the pyrolysis of residual dry biomass and agricultural waste.…”