Reduction of hexavalent chromium using zerovalent iron spheres packed in a rotating basket reactor: kinetic and mass transfer study

Abstract

A B S T R A C TKinetics of hexavalent chromium reduction to trivalent chromium onto iron spheres, packed in a rotating basket reactor was investigated under different conditions, such as H 2 SO 4 acid concentration, basket rotational speed, sphere diameter, and solution temperature. The rate of Cr(VI) reduction was found to increase with increasing basket rotational speed, H + ions concentration, and temperature. On the other hand the rate decreases with increasing the diameter of iron spheres. The activation energy was found to be 4.9 kcal/mol which confirm the diffusion controlled nature of the reaction. The thermodynamic parameters for the present study were calculated at 25˚C and was found to be 17.99 kJ mol À1 , 278.98 J mol À1 K À1 , and 103.6 kJ mol À1 for DH, ÀDS, and DG, respectively. Rate of Cr(VI) reduction expressed in terms of the rate of mass transfer was correlated to the controlling parameters by the dimensionless equation Sh ¼ 0:3463 Re 0:55 Sc 0:33The present mass transfer data was found to agree with the surface renewal theory.

Progress in Understanding the Mechanism of CrVI Removal in Fe0-Based Filtration Systems

“…Unfortunately, these reports co-exist in the literature with publications demonstrating that Fe 0 surface is universally covered by oxide layers [61][62][63][64][65][66], and that Fe 0 is additionally passivated with corrosion products during the remediation process [44,52,61,67,68]. Numerous recent studies, aimed to gain insight into the principles governing the removal of Cr VI in Fe 0 /H 2 O systems, have also presumed that this process is exclusively the result of direct electron transfer from Fe 0 to Cr VI [69][70][71][72][73][74][75][76][77][78][79]. Since the surface of commercially Fe 0 materials is permanently covered by an outer layer of low electric conductive air-formed oxides (hematite, maghemite) [64], the electron transport from Fe 0 to Cr VI should be severely inhibited [69][70][71][72][73][74][75][76][77][78][79].…”

“…Numerous recent studies, aimed to gain insight into the principles governing the removal of Cr VI in Fe 0 /H 2 O systems, have also presumed that this process is exclusively the result of direct electron transfer from Fe 0 to Cr VI [69][70][71][72][73][74][75][76][77][78][79]. Since the surface of commercially Fe 0 materials is permanently covered by an outer layer of low electric conductive air-formed oxides (hematite, maghemite) [64], the electron transport from Fe 0 to Cr VI should be severely inhibited [69][70][71][72][73][74][75][76][77][78][79]. Moreover, Fe 0 efficiency should significantly decrease during the time, as its surface is progressively covered with additional secondary mineral coatings that prevents penetration of the Cr VI and stops the electron transfer [11,[80][81][82].…”

Removal of Cr(VI) from Aqueous Solutions Using Amino‐Functionalized Carbon Nanospheres Adsorbents

“…Chromium ions are among the most dangerous heavy metal species. These potentially carcinogenic ions, found in various industrial waters, were placed at the top of the priority list of toxic pollutants by the U.S. EPA (Amin, 2013;Kebir et al, 2015;Liu et al, 2012;Luo et al, 2013;Qian et al, 2012;Sun et al, 2014;Tang et al, 2013;Zhang et al, 2013;Zhou et al, 2013). A maximum acceptable concentration of 0.1 mg/L is proposed for total chromium in drinking water (U.S. .…”