Removal of Hexavalent Chromium from Water Using Manganese Oxide Sorbents

2018-02-08T00:00:00Z (GMT) by Snover K Punia
The increasing occurrence of hexavalent chromium [Cr (VI)] in water resources worldwide and recent evidence for genotoxic and carcinogenic effects of Cr (VI) by ingestion have heightened concern for this contaminant in drinking water. The current U.S. EPA maximum contaminant level (MCL) is 0.1 mg/L for total chromium. In contaminated water, Cr (VI) levels are typically three to five times the U.S. EPA limit and have been mainly traced to the leaking of industrial waste products such as stainless steel, electroplating, pigments, dyes, and leather tanning. The health concerns have not only led to a global trend towards more stringent regulations, but to the subsequent need for more effective and less expensive technologies for removal of Cr (VI). The conventional methods for Cr (VI) removal, which include Cr (VI) reduction and precipitation, ion exchange, and membrane filtration, have several drawbacks such as high capital and operational cost, production of chemical sludge and sludge disposal problems. Adsorption has been considered as an effective but less expensive and simpler technology for the removal of Cr(VI) from water. Manganese oxide based sorbents have shown promise as low- cost adsorbents. In this study, the feasibility and application of manganese (III) oxide (Mn 2 O 3 ) powder and two manganese-aluminum coated sands (New MCS and X MCS) were evaluated as potential sorbents for the removal of Cr (VI) from water. The processes were investigated by batch technique at room temperature and the adsorption data were analyzed using adsorption equilibrium equations and adsorption kinetic models. The Cr (VI) adsorption capacities and reaction rates were determined. The effect of pH and co- existing ions on adsorption of Cr(VI) was investigated. The reuse and regeneration of sorbent was also evaluated. The Cr (VI) adsorption characteristics were best described using the Freundlich adsorption equation. The Dubinin-Radushkevich adsorption equation parameters showed that the Cr (VI) adsorption was predominately physical and reversible in nature. The mainly physical nature of adsorption also correlated with the rapid kinetics of Cr (VI) adsorption, particularly in case of the MCS adsorbents. Both Mn 2 O 3 and MCS sorbents effectively adsorbed Cr (VI) within the pH range of 2 to 10. The adsorption of Cr (VI) was found to be slightly enhanced in the presence of Ca 2+ and was suppressed in the presence of 0.1 mM PO 4 3- , 0.5 mM HCO 3 - , or 1 mM SO 4 2- concentrations. The reuse potential of the MCS adsorbents was greater than that of Mn 2 O 3 ; New MCS showed potential for further reuse and Cr (VI) removal after regeneration using 0.001 N NaOH. The analysis of the point of zero charge (PZC) of Mn 2 O 3 indicated that Cr (VI) anions were most likely adsorbed as inner-sphere complexes, while the more alkaline PZC of the MCS sorbents supported the formation of weakly bound Cr (VI) in outer-sphere complexes. The adsorption capacity and kinetics were favorable for using Mn 2 O 3 and MCS as sorbents for the removal of Cr (VI) from water. The application of MCS sorbents for Cr (VI) removal can offer the following advantages: lower cost, faster kinetics, wider effective pH range, greater stability under competing ion conditions and sorbent reuse potential. It is recommended that the MCS sorbents be developed further to improve the Cr (VI) adsorption capacity. The surface complex structures can be confirmed from spectroscopic studies in future work, and the sand coating mixture and consistency can be optimized for future preparations of the MCS sorbents.