| Summary: | 博士 === 國立成功大學 === 環境工程學系碩博士班 === 101 === Methods to economically and efficiently remove arsenic (As) from drinking water supplies are urgently needed in many parts of the world. Iron oxides are known to have a strong affinity for As in water. However, they are commonly present in the forms of fine powder or floc, limiting the applicability in water treatment. In this study, a novel granular adsorbent, iron-oxide-coated diatomite (IOCD), was developed and examined for its adsorption of As from water. The iron-based adsorbent was first studied for its surface properties and adsorption equilibrium and kinetics for As in de-ionized water. To understand the effect of water matrix on the adsorption of As, IOCD was further tested for its adsorption in a groundwater collected from Chiayi and Yanshuei, Taiwan. Finally IOCD was compared with two commercially available iron-based anion exchange resins for their adsorption of As.
Iron-oxide (-Fe2O3, hematite) coated onto diatomite improves sharply (by about 30 times) the adsorption of arsenate (As(V)) from water by the IOCD compared with that by raw diatomite. This improvement was attributed to increases in both surface affinity and surface area of the IOCD. The surface area of IOCD increased to an optimal value with the iron-oxide coating on diatomite. However, as the IOCD surface area (93 m2/g) was only 45% higher than that of the raw diatomite (51 m2/g), the enhanced As(V) adsorption resulted primarily from the enhanced association of negatively charged As(V) ions with the partial positive surface charge of the iron oxide. The As(V) adsorption decreased with increasing solution pH from 3.5 to 9.5, as expected by the partial charge interaction between As(V) and IOCD. The adsorption data at pH 5.5 and 7.5 could be well fitted to the Freundlich equation, and both the pseudo-second-order model and the pore-diffusion model simulated well the adsorption kinetics. A moderately high exothermic heat was observed for the As(V) adsorption, with the calculated molar isosteric heat ranging from -4 to -9 kcal/mol. The observed heats fall between those for physical adsorption and chemisorption and are indicative of the formation of a series of ion-pair complexes of As(V) ions with iron-oxide surface groups. Compared with other iron oxide-based adsorbents reported in the literatures, the adsorption capacity of IOCD is relatively high, and the kinetics is fast.
Removal of As using IOCD was examined for the groundwater near a black foot disease epidemic area, Chiayi, Taiwan. The groundwater was first analyzed for As speciation and water quality that may influence As removal. In the present groundwater, As was present predominately as the penta-valent species and in dissolved form (〈 1 kDa) with a concentration of ~ 30 g/L. Adsorption uptake of As(V) onto the studied IOCD in the groundwater was found to be only 2/3 of that from de-ionized water. Among the 10 anions, metals and organic species examined, only phosphate and natural organic matter (NOM) were found to influence the adsorption of As(V) onto iron oxide coated diatomite (IOCD). To further identify the major factor for the suppression of As(V) uptake on IOCD, adsorption experiments were conducted using coagulated groundwater to remove phosphate and NOM to different degrees. Experimental results confirmed that phosphate was the only important species to suppress the As(V) adsorption to IOCD in the studied groundwater.
IOCD and two hybrid anion exchange resins (A33E and FO36) were compared for their adsorption characteristics for arsenite (As(III)) and arsenate (As(V)) in synthetic As solution and natural groundwater. IOCD was found to outperform the two resins tested for adsorption of As(III). However, for As(V), the two resins had better As uptakes. It is found that in groundwater system, adsorption and oxidation of As(III) were both important processes for As(III). Oxidation of As(III) only occurred in the groundwater with NOM presence. Once adsorbed, no oxidation was observed. Since adsorption of As(III) is fast, no oxidation was found for As(III) on IOCD, while significant oxidation was observed for As(III) with A33E and FO36. The results were also confirmed by X-ray absorption near edge structure (XANES) spectra for As-loaded adsorbents.
This study provides some fundamental information on the adsorption equilibrium and kinetics of As onto three iron oxide based adsorbents. The iron oxide based adsorbent, iron oxide coated diatomite (IOCD), was successfully prepared and tested for the adsorption of As species. IOCD and the two commercially available hybrid anion exchange resins showed different adsorption properties. Depending on As speciation and water quality, they may reach different degrees of As uptakes. In addition, oxidation of As(III) may be observed in the presence of NOM in groundwater. It is suggested that in applying iron based adsorbents for the removal of As, an extensive investigation of groundwater components, particularly anions and NOM that may affect adsorption, should be conducted.
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