Bromate and chromate removal from water by double layered hydroxides

Villanueva Rangel, Cecilia Patricia (2015) Bromate and chromate removal from water by double layered hydroxides. Maestría thesis, Universidad Autónoma de Nuevo León.

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Resumen

To remove bromate and chromate from water, common methods include ion exchange, coagulation-flocculation processes, reductionprecipitation, etc., but these strategies can present some important disadvantages like incomplete removal, high cost, continuous use of reagents, and high energy costs. One of the most promising removal methods is the adsorption processes because of its high efficiency and low cost. In this study, double-layered hydroxides (DLH) of aluminum and magnesium were synthesized at different Mg/Al molar ratios with Cl- and CO3 -2 as the exchanging anions in order to remove bromate and chromate from water by adsorption processes. The synthesized materials were calcined at 400, 500, and 600°C to evaluate the effect of this treatment on the adsorption of the target pollutants. Contribution and Conclusions: Among all the tested adsorbents, DLH with a Mg/Al molar ratio of 3:1, with carbonate as the exchange anion, and calcined at 500 °C (identified as DLH31500) presented a maximum adsorption capacity for chromate and bromate of 248.9 and 134.1 mg/g at pH 6, respectively. In contrast, adsorption capacity onto granular activated carbon achieved 35.76 and 69.1 mg/g for chromate and bromate, respectively, which is seven times and two fold lower than DLH31500. When the activated carbon was modified with ammonia, bromate adsorption capacity decreased 12% in comparison with the raw activated carbon. Adsorption kinetics of chromate on DHL31500 was affected by the stirring type (mechanical, magnetic, intermittent, and oscillatory mixing), but the intermittent mode allowed the maximum chromate uptake as well as bubbling argon gas to the initial solution in order to displace the air in a closed vessel and to avoid carbon dioxide absorption along the experiment. The adsorption equilibrium was reached at 2880 min and the chromate adsorption capacity was 169.38 mg/g. In the same system, bromate adsorption equilibrium was reached at 5760 min, and the adsorption capacity was 110.38 mg/g. For continuous adsorption tests, a packed bed column with DLH31500 was built and chromate (20 mg/L) and bromate (5 mg/L) solutions were fed to the column. Chromate and bromate adsorption capacities of 104.66 and 45.77 mg/g were achieved, which successfully treated 7.85 and 13.73L, respectively. Finally, it was verified by X-Ray diffraction that DLH is mainly formed of layers, but after thermal treatment this structure is disassembled. Nevertheless, DLH layers reassemble after chromate adsorption, forming again the characteristic layers of a DLH (i.e. a memory effect). In conclusion, DLH is a promising adsorbent for the removal of bromate and chromate pollutants from aqueous solutions.

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