Nanostructure of humic acid adsorption layer in the presence of Cs and Sr ions on the surface of waste material obtained from residue after supercritical extraction of hops

Abstract

Adsorption of ionic molecules at the solid–liquid interface depends on various interactions, particles surface properties as well as the presence of the additional substance in the analyzed systems. The waste material obtained from the supercritical fluid extraction process was used as an adsorbent. The plant-based biosorbent structure was fully characterized. Then the humic acid (constituting the common interfering substance found in the environmental samples) sorption on the hop cones was investigated in the absence and presence of Sr/Cs ions under various pH conditions. Hence, the metal cation valence can influence the interactions between the colloidal system constituents and the adsorption layer structure. Determining the mutual interactions in the complex systems is important due to the potential possibility of using the waste sorbent to remove the radioactive Cs and Sr isotopes from the aquatic environment. Due to the lack of a porous structure of the sorbent and the high surface density of the charge, the metal ions are bound directly on the particles surface. The humic acid (HA) adsorption is greatly pH dependent—the largest removal was achieved under the acidic conditions (Qads = 88.69 mg/g), while the HA sorption process at pH 9 is impeded by a large negative charge deposited at the solid–liquid interface (Qads = 7.06 mg/g). At pH 3, formation of multilayer adsorption structure contributes to the effective removal of organic impurities. The metal ions valence significantly affects the humic acid binding mechanism. Divalent Sr acts like a “bridge” increasing the number of biosorbent active sites (at pH = 3 the HA adsorption increases almost twofold from 88.69 to 156.46 mg/g in the Sr ions), whereas the Cs+ ions leads to the reduction in the HA removal efficiency (Qads. = 46.31 mg/g under the same conditions).