| Summary: | 博士 === 國立高雄第一科技大學 === 工程科技研究所 === 96 === Arsenic is a known human carcinogen, and its contamination in source drinking water is presently of a worldwide concern. The USEPA (2001) announced to lower the maximum contaminant level of arsenic in drinking water from 50 to 10 μg L-1 to respond to the increasing awareness of arsenic toxicity. This study aims at the adsorption kinetics and competition behavior of As(V) and As(III) by reclaimed iron-oxide coated quartz sands (RIOCS). The RIOCS was a reclaimed solid waste, which was collected from the process of a Feo/CO2-nitrate system. Batch experiments were performed to examine the adsorption isotherm and removal performance of arsenic systems by using the RIOCS. In addition, a 27-3 fractional factorial design (FFD) was employed for screening main competitive factors in this adsorption process.
The results showed that the pHzpc (zero point of charge) of RIOCS was about 7.0 ± 0.4, favoring the adsorption of As(V) onto the RIOCS surface. As the adsorbent dosage and initial arsenic concentration were fixed, both the As(V) and As(III) removals decrease with increasing initial solution pH. It was found that the total removal efficiencies of arsenic species were in the order of As(V) > As(III) + As(V) > As(III) under the conditions of the same initial pH and adsorbent dosage. This demonstrated that little chemical interaction or competition occurred between As(V) and As(III) in the removal of arsenic by RIOCS. The adsorption isotherms of As(V) and As(III) fit the Langmuir model satisfactorily for the four different initial pH conditions as well as for the studied range of initial arsenic concentrations. Based on the Langmuir model, the calculated saturated adsorption capacities of arsenate were 22.1, 21.5, 21.0, and 17.5 mg g-1 at pH values of 5, 6, 7, and 8, respectively. As for the arsenite species, the saturated adsorption capacities were 12.6, 11.8, 11.5, and 10.5 mg g -1. It implied that lower pH favors the adsorption for both arsenic species. Our results concluded that the RIOCS can be considered as adsorbent for arsenic removal.
Moreover, this study investigated the competitive effects of selected ions and natural organic matter on As(V) removal using RIOCS in the single- and multi-ion systems. The results of kinetic adsorption showed that As(V) of 500 μg L-1 was removed with a maximum efficiency of 61% at adsorption time 12 h in the presence of 24 g RIOCS/L. The interference of ionic strength was found to be insignificant, with the difference of As(V) removal less than 10% under the initial pH range of 5-8. The inhibitive competition effects of the anions on As(V) removal in the single ion system were in the following sequence: HPO42-> SiO32-> CO32-> HA> SO42-> Cl-, whereas the cation Ca2+ was observed to promote the As(V) removal. The optimum initial pH for As(V) removal in single-ion tests was 5. In addition, based on the FED estimates of major effects and interactions, HPO42-, SiO32-, Ca2+ and HA were important factors on As(V) removal in the multi-ion system. The promoters for the As(V) removal were Ca2+ and, to a lesser extent, SO42-. The competitive effects of these ions on As(V) removal were in the order large to small of HPO42-, SiO32-, HA, HCO3-, and Cl-. In the single ion system, the efficiencies of As(V) removal range from 75 to 96%, much higher than those in the multi-ion system (44%). This implies that there were some complex anion interactions in the multi-ion system. Moreover, the results of this study showed that 150 mg L-1 of Ca2+ was not high enough to overcome the competitive effects of 5 mg L-1 of HPO42- and 20 mg L-1 of SiO32-. To promote the removal of As(V) by RIOCS, it is proposed to lower the pH in the single-ion system, while in the multi-ion system, the increase of the Ca2+ concentration, or decreases of HPO42-, SiO32- and HA concentrations is favored.
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