Abstract
Synthesis of crystalline hydrous ferric oxide (CHFO), a modified iron-based adsorbent, and its arsenic sorption behaviour have been reported. Here, the effects of pH with variation of arsenic concentrations, contact time, pre-drying of CHFO, competition of some other anions and regeneration of arsenicsaturated CHFO are conducted by batch method. The sorption of As(V) is highly dependent on the concentration and pH of the experimental system, while that for As(III) is pH insensitive. As(III) is found to require less contact time to attain equilibrium than that of arsenic(V). Pre-drying of CHFO in the temperature range of 200 to 300°C is found to be effective in removing both As(III) and As(V). Adsorption kinetics follow the first-order Lagergren model. The equilibrium data conform to the Langmuir isotherm. Evaluated Langmuir constants and equilibrium parameter (RL) indicate that CHFO is a better As(III) adsorbent under experimental conditions. Sulphate, phosphate and bicarbonate compete poorly with As(III) sorption. A field test using CHFO-packed fixed-bed column is reported. Effluent water bed volumes of 14,000, 11,000 and 9000 BV (arsenic ≤ 0.01 mg L-1) were obtained in the first, second and third cycle of operation from a groundwater sample (arsenic content: 320.400 μg L-1). Regeneration of the exhausted column was achieved with up to 80 to 85% efficiency using 3 BV of 5 M NaOH solution recycled through the column five times. Arsenic was recovered as As2S3 from the regenerates, to avoid recycling of arsenic into the environment.
