The removal of uranium(VI) from laboratory and environmental waters using polyurea-cross-linked calcium alginate (X-alginate) aerogels has been investigated by means of batch-type experiments. The experimental data revealed that the material presents extremely high adsorption capacity for uranium(VI) (up to 2023 g kg −1 of aerogel). The adsorption process is endothermic, entropy-driven, and follows the Langmuir isotherm model. The Fourier transform infrared spectroscopy (FTIR) data corroborate the results of the batch experiments and indicate that adsorption occurs via the formation of inner-sphere complexes between the surface functional groups of Xalginate beads and UO 2 2+. The post-adsorption presence of uranium in the adsorbent was confirmed and quantified with energy-dispersive X-ray spectra (EDS) analysis. Compared to other aerogel adsorbents of UO 2 2+ from the literature, X-alginate aerogels show one of the highest sorption capacities per weight and the highest per volume. Uranium could be recovered almost quantitatively (∼100%) in aqueous solutions of Na 2 CO 3 (pH 11) or ethylenediaminetetraacetic acid (EDTA) (pH 10). The aerogel material has been effectively applied for the removal of uranium(VI) from acid mine drainage (AMD), groundwater, and seawater samples. It should be noted that X-alginate aerogel beads are stable in all of the above environments (i.e., no swelling, shrinking, or disintegration was observed). The extraordinary adsorption capacity, even in the presence of competing metal cations, and the stability of X-alginate aerogel beads in environmental waters render them excellent candidates for the specific application.

Two types of preformed alginate wet gels, one with a low (30−35%) and the other with a high (65−75%) content of glucuronic acid, were reacted with an aliphatic triisocyanate that was priorly allowed to diffuse in the pores. This reaction formed urethane groups on the surface of the alginate framework and also formed a polyurea (PUA) network connecting these urethane groups via respective reactions of the triisocyanate with alginate surface −OH groups or with gelation water remaining adsorbed on the inner surfaces of the wet gels. These processes formed a conformal nanothin film of PUA around the alginate network. After drying the wet gels with the supercritical fluid CO 2 , we obtained PUA/polyurethane-crosslinked alginate (X-alginate) aerogels. Although X-alginate aerogels are essentially copolymers, unlike all copolymers mentioned in previous literature reports, the relative topology of the alginate and the cross-linker is defined at the nanoscopic scale rather than at the molecular level. For the systematic study of X-alginate aerogels as a function of synthetic conditions, the experimental protocol was designed according to the central circumscribed rotatory design model using the alginate and the triisocyanate concentration as independent variables. Empirical models were derived for all relevant material properties by fitting experimental data to the two independent variables. The chemical identity of all samples was confirmed with attenuated total reflectance−Fourier transform infrared spectroscopy and solid-state 13 C and 15 N cross-polarization magic angle spinning NMR spectroscopy. The percentage of PUA uptake in X-alginate aerogels (58−98%) was calculated from skeletal density data. Scanning electron microscopy showed that all samples were nanofibrous, indicating that PUA coated conformally the skeletal network of both alginates, and the micromorphology remained the same as in the native (non-cross-linked) samples. X-alginate aerogels are mechanically strong materials, in contrast to their native counterparts, which are extremely weak mechanically. Compared to various organic aerogels from the literature, X-alginate aerogels can be as stiff as many other polymeric aerogels with 2 or 3 times higher densities. In addition, X-alginate aerogels are good candidates for sound insulation applications, as the speed of sound in most samples was estimated to be significantly lower than the speed of sound in dry air.

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Polyurea-crosslinked alginate (X-M-alginate; M: Co, Ni, Cu) aerogels, prepared from the corresponding M-alginate wet-gels and the aromatic triisocyanate Desmodur RE, are precursors for metal- and nitrogen-doped carbon (X-M-C) aerogels.

Polyurea-crosslinked calcium alginate and chitosan aerogel beads: novel fibrous biopolymer-based aerogels.