Selenate removal via continuous fixed-bed column with nanoscale zero-valent iron supported on bentonite-zeolite pellets

Abstract

Excessive levels of selenium in water are caused by discharging from fossil fuels combustion, coal-fired power plant activity, and mining operations. Selenium contamination poses considerable hazards to human health. Selenate (Se6+) is the most oxidized and common form of selenium in contaminated water. Zeolite-supported nanoscale zero-valent iron (Z-NZVI) coated onto bentonite-zeolite pellets (BZ-NZVI) was prepared for the removal of Se6+. The BZ pellets were coated with Z-NZVI powder at a ratio of 1:5. The BZ-NZVI was characterized using X-ray diffraction, point of zero charge analysis, Brunauer-Emmett-Teller method, field emission scanning electron microscopy, and X-ray fluorescence spectrometry techniques. The BZ-NZVI samples were tested in the batch reactor for kinetics and adsorption isotherm studies and subsequently in a continuous fixed-bed column. The kinetic models and adsorption isotherms were best described by the pseudo-first-order kinetics and Langmuir adsorption isotherm, respectively. Both boundary layer diffusion and intraparticle diffusion appeared to affect the intraparticle diffusion process. The efficiency of Se6+ removal in fixed-bed column of BZ-NZVI fitted well with Yoon-Nelson model. The adsorption rate decreased according to the Se6+ breakthrough proportion. The maximum Se6+ adsorption breakthrough capacity was 0.034 mg/g under the test run of 0.5 mg/L-Se6+ solution, pH 7, the flow rate of 5 mL/min, and the temperature of 303 K. Breakthrough times increased with decreasing flow rate and Se6+ initial concentration but decreased as the mass of BZ-NZVI increased. Se6+ contaminated in wastewater was removed by reduction and adsorption mechanisms over BZ-NZVI, which is a promising new area of research.