Environmental Remediation with Functional Aerogels and Xerogels

Biopolymer-Polysiloxane Double Network Aerogels

A new series of transparent aerogels of biopolymer-polysiloxane double networks is reported. Biopolymer aerogels have attracted much attention from green and sustainable aspects but suffered from strong hydrophilicity and difficulty to make homogeneous structures in nanoscale; these drawbacks are overcome by compositing with a polysiloxane network. Alginate-polymethylsilsesquioxane aerogel has high optical transparency, water repellency, comparable superinsulation property and improved bending flexibility compared to pure polymethylsilsesquioxane aerogel. The nanoscale homogeneity is realized by separating the crosslinking steps for two networks in a sequential protocol: condensation of siloxane bonds and metal-crosslinking of biopolymer. The crosslinking order, biopolymer-first or siloxane-first, and universality/limitation of biopolymer-crosslinker pairs are discussed to construct fundamental chemistry of double network systems for their further application potentials.

Iodine Capture with Mechanically Robust Heat-Treated Ag-Al-Si-O Xerogel Sorbents

Various radionuclides are released as gases during reprocessing of used nuclear fuel or during nuclear accidents including iodine-129 (129I) and iodine-131 (131I). These isotopes are of particular concern to the environment and human health as they are environmentally mobile and can cause thyroid cancer. In this work, silver-loaded heat-treated aluminosilicate xerogels (Ag-HTX) were evaluated as sorbents for iodine [I2(g)] capture. After synthesis of the base NaAlSiO4 xerogel, a heat-treatment step was performed to help increase the mechanical integrity of the NaAlSiO4 gels (Na-HTX) prior to Ag-exchanging to create Ag-HTX xerogels. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, gravimetric iodine loading, nanoindentation, and dynamic mechanical analysis. The structural and chemical analyses of Ag-HTX showed uniform distribution of Ag throughout the gel network after Ag-exchange. After I2(g) capture, the AgI crystallites were observed in the sorbent, verifying chemisorption as the primary iodine capture mechanism. Iodine loading of this xerogel was 0.43 g g-1 at 150 °C over 1 day and 0.52 g g-1 at 22 °C over 33 days. The specific surface area of Ag-HTX was 202 m2 g-1 and decreased to 87 m2 g-1 after iodine loading. The hardness of the Na-HTX was >145 times higher than that of the heat-treated aerogel of the same starting composition. The heat-treatment process increased Young's modulus (compressive) value to 40.8 MPa from 7.0 MPa of as-made xerogel, demonstrating the need for this added step in the sample preparation process. These results show that Ag-HTX is a promising sorbent for I2(g) capture with good iodine loading capacity and mechanical stability.