Sorption of Arsenic, Mercury, Selenium onto Nanostructured Adsorbent Media and Stabilization via Surface Reactions
Han, Dong Suk
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The overall goal of this study is to evaluate the ability of novel nanostructured adsorbent media (NTAs, iron sulfides (FeS2 and FeS)) to remove arsenic, selenium and mercury from ash and scrubber pond effluents. The NTAs aim to enhance arsenic removal from solution compared to conventional adsorbents. The iron sulfides are expected to produce stable residuals for ultimate disposal after removing As, Se and Hg from solution, so that removal of these compounds from wastewaters will not result in contamination of soils and groundwaters. Methods for reliably and economically producing these materials were developed. The synthesized NTAs and iron sulfides were characterized by surface analysis techniques such as XRD, FT-IR, SEM-EDS, TEM, XPS, AFM and N2-adsorption. These analyses indicated that Ti(25)-SBA-15 has highly ordered hexagonal mesopores, MT has interparticle mesopores, pyrite (FeS2) forms crystalline, nonporous rectangular nanoparticles (<500 nm), and mackinawite (FeS) forms amorphous, nonporous nanoparticles (<100 nm). Kinetic and equilibrium tests for As(III, V) removal were conducted with NTAs over a range of pH (4, 7, 9.5). The rates of arsenic uptake were very fast and followed a bi-phasic sorption pattern, where sorption was fast for the first 10 minutes, and then slowed and was almost completed within 200 minutes. Distinct sorption maxima for As(III) removal were observed between pH 7 and pH 9.5 for MT and between pH 4 and pH 7 for Ti(25)-SBA-15. The amount of As(V) adsorbed generally decreased as pH increased. In addition, a surface complexation model (SCM) based on the diffuse layer model (DLM) was used to predict arsenic adsorption envelopes by NTAs under various environmental conditions. The SCM for As(III, V) adsorption by NTAs demonstrated the role of mono- and bidentate surface complexes in arsenic adsorption. A batch reactor system was employed in an anaerobic chamber to conduct experiments to characterize both the removal of As, Se, Hg from solution and their subsequent reactions with iron sulfides. Experiment variables for removal experiments included: contaminant valence state (As(V), As(III), Se(VI), Se(IV), Hg(II)); adsorbent/reactant type (FeS, FeS2); adsorbent/reactant concentration; pH (7, 8, 9, 10); and competing ion (SO42-) concentration (0, 1, 10 mM). Experimental variables for reaction experiments were reaction time (up to 30 days) at pH 8 and oxidation states of contaminants. In addition, the stability of iron sulfides (FeS2, FeS) combined with target compounds was investigated by measuring the ability of the target compounds to resist release to the aqueous phase after removal. These experiments showed that iron sulfides were good adsorbent/reactants for target contaminants in spite of the presence of sulfate. This was particularly true at intermediate concentrations of target compounds. The experiments also demonstrated that iron sulfides interacted with target contaminants in such a way to improve their resistance to being released back to solution as pH was changed. Therefore, this study demonstrates the ability of novel nanostructured adsorbent media to remove arsenic, selenium and mercury from ash and scrubber pond effluents and the ability of iron sulfides to produce residuals that are stable when disposed in landfills.