Chemical Reduction of Hexavalent Chromium and Its Immobilisation Under Batch Conditions Using a Slurry Reactor

Abstract

Chemical reduction of the hexavalent chromium, Cr(VI), present in contaminated soil and groundwater was carried out in a slurry reactor under dynamic conditions (120 rpm and 25°C) using different reductants [ferrous sulphate (Fe(II))], sodium bisulphite, sucrose, ascorbic acid and zerovalent iron (ZVI)] in order to evaluate the influence of the reductant on the redox process. Chemical analysis of the contaminated soil revealed a Cr(VI) concentration of 528±31 mg kg −1 . Batch studies under dynamic conditions (slurry reactor) using different [Cr(VI)]/ [reductant] molar ratios revealed that only Fe(II) and ZVI species can promote both reduction of Cr(VI) and immobilisation of Cr(III) (formation of an insoluble hydroxide compound). It was verified that 1.0 g of ZVI is capable of converting 104±5 mg of Cr (VI) in Cr(III). A kinetic redox study was carried out using ZVI in different conditions. In all cases, it was verified that Cr(VI) reduction follows a pseudo-firstorder kinetic behaviour. The dependence of the pseudo-first-order kinetic rate constant, k obs , on [ZVI] indicates that the redox process taking place in the slurry reactor is rather complex. A phenomenological kinetic equation for the redox process taking place in the slurry reactor was presented in order to describe the behaviour of k obs under nonideal conditions.

Hexavalent Chromium Reduction with Zero-Valent Iron (ZVI) in Aquatic Systems

“…Studying the reduction of Cr(VI) as a function of ZVI powder dosage over the ZVI concentration range of 2-14 g/L, Qian et al (2008) reported that at higher ZVI concentrations, higher rates of Cr(VI) reduction were observed. Franco et al (2009b) studied the kinetics of Cr(VI) reduction at Cr(VI)/ ZVI molar ratios ranging from 1:15 to 1:60, revealing a direct dependence of the kinetic rate constant with the molar ratio. found that the rate of ZVI dissolution increased as the amount of ZVI was increased from 2 to 64 g/L; however, this study also noted that the increase in ZVI dissolution rates was not linearly proportional to the increase in the amount of ZVI reacted ).…”

“…In a subsequent study, Franco et al (2009b) studied the overall removal rate of Cr(VI) using a batch slurry reactor, revealing that Cr(VI) reduction obeys a pseudo-first-order kinetic model. However, taking into account the heterogeneous nature of the redox process carried out under constant and intense agitation, and the collisions comprising the suspended soil microparticles (SSM), as well as the collisions of SSM with the ZVI, the authors estimated that the obtained k obs cannot be represented by a simple constant, and therefore, they proposed the following relation in order to represent k obs (Franco et al 2009b):…”

“…Subsequently, the heterogeneous kinetic rate constant (k h ) of the Cr(VI) reduction at the surface of ZVI, under perfect mixing conditions, in contact with SSM was described as follows (Franco et al 2009b):…”

Chemical Reduction of Hexavalent Chromium Present in Contaminated Soil using a Packed‐bed Column Reactor

“…Since the redox reaction between Fe(II) and Cr(VI) is indeed a rapid one [14,15], the treatment time is mainly governed by the spatial reductant distribution inside the soil microstructure, which in turns is controlled by G. Obviously, the use of high G values can be limited by the hydraulic conductivity of the contaminated soil. These changes can be attributed to the following physico-chemical processes [3 -5]: (i) chemical reduction of Cr(VI) leading to the formation of Cr(III); (ii) ion hydrolysis of Cr(III) and Fe(II) forming an insoluble mixed hydroxide, and (iii) an excess of the reductant in the reaction medium after the treatment.…”

“…We recently reported studies concerning reduction and immobilization of Cr(VI) using suspensions containing stabilized and nonstabilized nanoparticles of zerovalent iron [14,15]. Reported in the present paper is the redox treatment of soil samples containing Cr(VI) under flow conditions using a packed-bed column reactor and different reductant solutions.…”