Abstract

This study investigated the removal of arsenic from groundwater by granular activated carbon (GAC) supported nano scale zero-valent iron (nZVI). GAC supported nZVI (nZVI/GAC) composite was synthesized by hydrolyzing a Fe(III) salt on GAC, reduced by NaBH4 and dried under vacuum. Synthesized nZVI/GAC was characterized using scanning electron microscopy (SEM) along with EDS, BET surface area analysis, X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. The experimental results were produced through the batch and Rapid Small Scale Column Test (RSSCT). The adsorption depends on pH, initial concentration, and reaction time. Arsenite adsorption capacity varies from 800 to 1400 μg/g over the pH 2-11. Arsenate adsorption was higher (3000-3700 μg/g) over the acidic pH range 2-6.5. Among competitive ions, phosphate and silicate affected the most while sulfate, nitrate, chloride, fluoride, manganese, magnesium and calcium had insignificant impact. The experimental data were evaluated with Langmuir and Freundlich isotherms. The adsorption capacity for arsenate, calculated from Langmuir and Freundlich isotherms, were 5000 and 6000 µg/g, respectively at pH 4.5. The reaction kinetics followed the pseudo-second order model. The initial sorption rate (h), determined from pseudo-second order kinetic model, was 666 µg/g.min. The dynamic behaviour of the RSSCT was predicted by the HSDM model using the software FAST 2.0. From the RSSCT results, it was found that the number of bed volumes treated depends on the empty bed contact time (EBCT) as well as the initial arsenate concentration.
The regeneration of spent nZVI/GAC using 0.1M NaOH was effective as it desorbed 87% of adsorbed arsenic. The solid waste can be safely disposed of in a sanitary landfill without any treatment as the concentration of leached arsenate determined by TCLP was much lower than the regulatory limit. The arsenic removal mechanism was due to the combination of electrostatic and the complex formation, either monodentate or bidentate, between As(V) and nZVI corrosion products. The results indicated that nZVI/GAC is a promising adsorbent for arsenic removal.