Arsenate adsorption from aqueous solution using iron-loaded Azadirachta indica roots: batch and fixed-bed column study

Batch and fixed-bed column studies were carried out to investigate the potential of an economic and eco-friendly adsorbent (iron impregnated Azadirachta indica roots (AIR)), for the remediation of arsenate. Maximum adsorption of 96.811% (qe exp = 10.586 μg/g) was obtained at an initial arsenate concentration of 100 μg/L, pH of 6.0, adsorbent dose of 1 g/L and contact time of 30 min. The presence of co-existing anions like PO43–, SiO32– decreases the adsorption capacity whereas cation does not have any effect on the same. Kinetics, isotherms, and thermodynamic studies were also conducted for understating the adsorption process in a better way. The adsorption data fitted best in the Langmuir model (R2 = 0.9918 and qmax = 29.82 μg/g), which reveals the removal takes place as monolayer adsorption with homogenous energy levels. Adsorption kinetics study reveals that the pseudo- second-order kinetics (R2 = 0.9966, qe = 10.2832) controls the adsorption process. Thermodynamics studies show exothermic and spontaneous adsorption of arsenate on Fe-AIR. In fixed-bed column operations, effects of various operating parameters like bed height, the influent concentration of As(V), flow rate, column diameter were investigated to evaluate the removal efficiency. At optimum experimental conditions (Ci = 1,000 μg/L, bed height = 9 cm, flow rate = 3 mL/min, column diameter = 3 cm and pH 7.0), maximum uptake (qe = 93.3 μg/g) with maximum breakthrough (450 min) was achieved. Breakthrough curves were used to analyze the effect of operating parameters on adsorption. Thomas model was used to determine the saturated concentration and the Yoon–Nelson model was applied to determine the required time for 50% adsorbate breakthrough. The present system developed for arsenate remediation is rapid, economic, and reproducible and can be applied to any resource of arsenic-contaminated water. The Fe-AIR fixed column can be regenerated and reused many times. The adsorption capacity decreases to 31.5% after 5 cycles.