Adsorption of trace levels of arsenic and selenium from aqueous solutions by conditioned layered double hydroxides.

摘要

Arsenic (As) and Selenium (Se) are found in water in the form of oxyanions. Relatively high concentrations of As and Se have been reported both in power plant discharges, as well as, in fresh water supplies. The International Agency for Research on Cancer currently classifies As as a group 1 chemical, that is considered to be carcinogenic to humans.;The focus of the present work is a systematic study of the adsorption of As and Se by conditioned layered double hydroxide (LDH) adsorbents. Conditioning the adsorbent significantly reduced the Mg, Al dissolution observed with uncalcined and calcined LDH. The adsorption rates and isotherms have been investigated in batch experiments using particles of four different particle size ranges. As(V) adsorption is shown to follow a Sips-type adsorption isotherm. The As(V) adsorption rate on conditioned LDH increases with decreasing adsorbent particle size; the adsorption capacity, on the other hand, is independent of the particle size. A homogeneous surface diffusion model (HSDM) and a bi-disperse pore model (BPM) - - the latter viewing the LDH particles as assemblages of microparticles and taking into account bulk diffusion in the intraparticle pore space, and surface diffusion within the microparticles themselves - - were used to fit the experimental kinetic data. The HSDM estimated diffusivity values dependent on the particle size, whereas the BPM predicted an intracrystalline diffusivity, which is fairly invariant with particle size.;The removal of As(V) on conditioned LDH adsorbents was also investigated in flow columns, where the impact of important solution and operational parameters such as influent As concentration, pH, sorbent particle size and flow rate were studied. An early breakthrough and saturation was observed at higher flow rates and at higher influent concentrations, whereas a decrease in the sorbent particle size and a decrease in influent pH resulted in an increase in the bed volumes treated at breakthrough. Both the HSDM and BPM were shown capable of predicting the column behavior.;Analysis of power plant effluents from various power plants in the greater Los Angeles area shows that, in addition to As and Se, these streams also consist of ions like fluorides, nitrates, chlorides, carbonates, sulphates and phosphates. In this study we have also tried to understand the effect of each of these individual ions on the As and Se removal capacity of LDH. For that, we carried out binary isotherm experiments with varying concentrations of As and of the individual competing ions in order to better understand the competition for the available adsorption sites. We fitted the data using the extended Sips isotherm equation. It was observed that the As adsorption capacity decreases in the presence of various individual competing ions in the order of PO42- > SO42- > CO 32- > Cl- > NO3- ≈ F-. For the Se removal from aqueous solutions in the presence of individual competing ions, calcined LDH proved more effective than conditioned LDH.;We also studied the regeneration of LDH and it was shown that even after regenerating the LDH 15 times, the adsorption capacity only decreased by 10%, making it a very economical adsorbent. The TCLP test carried out on As- and Se-saturated LDH, showed that it can be categorized as a non hazardous waste.