Concurrent removal of elemental mercury and SO2 from flue gas using a thiol-impregnated CaCO3-based adsorbent: a full factorial design study.

Mercury (Hg) emitted from coal-based thermal power plants (CTPPs) can accumulate and bio-magnify in the food chain, thereby posing a risk to humans and wildlife. The central idea of this study was to develop an adsorbent which can concurrently remove elemental mercury (Hg0) and SO2 emitted from coal-based thermal power plants (CTPPs) in a single unit operation. Specifically, a composite adsorbent of CaCO3 impregnated with 2-mercaptobenimidazole (2-MBI) (referred to as modified calcium carbonate (MCC)) was developed. While 2-MBI having sulfur functional group could selectively adsorb Hg0, CaCO3 could remove SO2. Performance of the adsorbent was evaluated in terms of (i) removal (%) of Hg0 and SO2, (ii) adsorption mechanism, (iii) adsorption kinetics, and (iv) leaching potential of mercury from spent adsorbent. The adsorption studies were performed using a 22 full factorial design of experiments with 15 ppbV of Hg0 and 600 ppmV of SO2. Two factors, (i) reaction temperature (80 and 120 °C; temperature range in flue gas) and (ii) mass of 2-MBI (10 and 15 wt%), were investigated for the removal of Hg0 and SO2 (as %). The maximum Hg0 and SO2 removal was 86 and 93%, respectively. The results of XPS characterization showed that chemisorption is the predominant mechanism of Hg0 and SO2 adsorption on MCC. The Hg0 adsorption on MCC followed Elovich kinetic model which is also indicative of chemisorption on heterogeneous surface. The toxicity characteristic leaching procedure (TCLP) and synthetic precipitation leaching procedure (SPLP) leached mercury from the spent adsorbent were within the acceptable levels defined in these tests. The engineering significance of this study is that the 2-MBI-modified CaCO3-based adsorbent has potential for concurrent removal of Hg0 and SO2 in a single unit operation. With only minor process modifications, the newly developed adsorbent can replace CaCO3 in the flue-gas desulfurization (FGD) system.

Quenched phonon drag in silicon nanowires reveals significant effect in the bulk at room temperature.

Existing theory and data cannot quantify the contribution of phonon drag to the Seebeck coefficient (S) in semiconductors at room temperature. We show that this is possible through comparative measurements between nanowires and the bulk. Phonon boundary scattering completely quenches phonon drag in silicon nanowires enabling quantification of its contribution to S in bulk silicon in the range 25-500 K. The contribution is surprisingly large (∼34%) at 300 K even at doping of ∼3 × 10(19) cm(-3). Our results contradict the notion that phonon drag is negligible in degenerate semiconductors at temperatures relevant for thermoelectric energy conversion. A revised theory of electron-phonon momentum exchange that accounts for a phonon mean free path spectrum agrees well with the data.

Porosity control in metal-assisted chemical etching of degenerately doped silicon nanowires.

We report the fabrication of degenerately doped silicon (Si) nanowires of different aspect ratios using a simple, low-cost and effective technique that involves metal-assisted chemical etching (MacEtch) combined with soft lithography or thermal dewetting metal patterning. We demonstrate sub-micron diameter Si nanowire arrays with aspect ratios as high as 180:1, and present the challenges in producing solid nanowires using MacEtch as the doping level increases in both p- and n-type Si. We report a systematic reduction in the porosity of these nanowires by adjusting the etching solution composition and temperature. We found that the porosity decreases from top to bottom along the axial direction and increases with etching time. With a MacEtch solution that has a high [HF]:[H(2)O(2)] ratio and low temperature, it is possible to form completely solid nanowires with aspect ratios of less than approximately 10:1. However, further etching to produce longer wires renders the top portion of the nanowires porous.

Formation of high aspect ratio GaAs nanostructures with metal-assisted chemical etching.

Periodic high aspect ratio GaAs nanopillars with widths in the range of 500-1000 nm are produced by metal-assisted chemical etching (MacEtch) using n-type (100) GaAs substrates and Au catalyst films patterned with soft lithography. Depending on the etchant concentration and etching temperature, GaAs nanowires with either vertical or undulating sidewalls are formed with an etch rate of 1-2 µm/min. The realization of high aspect ratio III-V nanostructure arrays by wet etching can potentially transform the fabrication of a variety of optoelectronic device structures including distributed Bragg reflector (DBR) and distributed feedback (DFB) semiconductor lasers, where the surface grating is currently fabricated by dry etching.