Arsenic is found in water in the form of oxyanions. Relatively high concentrations of As have been reported both in power plant discharges, as well as, in fresh water supplies. Inorganic As may be present in the 3+ (As(III), arsenite) and 5+ (As(V), arsenate) oxidation states¹. Protracted contact with As- containing water is thought to cause arsenicosis, a form of poisoning in humans². The International Agency for Research on Cancer (IARC) currently classifies As as a group 1 chemical, which is considered to be carcinogenic to humans.

The focus of the present work was to study the removal of As(V) by conditioned, calcined layered double hydroxide (LDH) adsorbents in continuous-flow, packed-bed columns in order to study the effect of important operating parameters, such as the influent As concentration, the pH, the adsorbent particle size, and the flow rate. As expected, an earlier bed saturation and breakthrough was observed at higher flow rates and influent concentrations. On the other hand, a decrease in the adsorbent particle size and the influent pH resulted in an increase in the number of bed volumes at breakthrough. A column model which accounts for external, liquid-film mass transport and for diffusion and adsorption in the adsorbent particles is utilized. Two different adsorbent particle models are employed, which were shown previously capable of predicting As(V) uptake by LDH adsorbents. A conventional homogeneous surface diffusion model, and a bidisperse pore model; the latter model views the LDH particles as assemblages of microparticles and takes into account bulk diffusion in the intraparticle pore space, and surface diffusion within the microparticles themselves. Both models are found capable of predicting the flow-column experimental results.

Batch experiments with simulated power plant effluents were also carried out. The effect of the presence of anions, namely NO₃^2-, SO₄^2-, PO₄^2-, CO₃^2-, Cl^- and F^-, on arsenic adsorption was studied. We collected data on various power plant effluents in the greater Los Angeles area, in order to identify the concentrations of these ions present in the power plant effluents. LDH was found to be effective adsorbent to remove arsenic in the presence of various ions in the solution.

References:

(1) Ferguson, J.F.; Gavis, J.; A Review of the Arsenic Cycle in Natural Waters. Water Res. 1972, 6, 1259.

(2) Lepkowski, W.; Arsenic Crisis in Bangladesh. Chem. Eng. News 1998, 76 (46), 27.

(3) Yang, L.; Shahrivari, Z.; Liu P.K.T.; Sahimi, M.; Tsotsis, T.T. Removal of Trace Levels of Arsenic and Selenium from Aqueous Solutions by Calcined and Uncalcined Layered Double Hydroxides (LDH), Ind. Eng. Chem. Eng. 2005, 44(17), 6804-6815.

(4) Yang, L; Dadwhal, M; Shahrivari, Z; Ostwal, M; Liu, P.K.T; Sahimi, M; Tsotsis, T.T. Adsorption of Arsenic on Layered Double Hydroxides: Effect of the Particle Size. Industrial & Engineering Chemistry Research , 2006, 45(13), 4742-4751.

Keywords: Layered double hydroxides, Homogeneous surface diffusion model, Bidisperse pore model, Arsenic, Adsorption, Packed bed columns, Multi solute.