Manganese(II)-Catalyzed and Clay-Minerals-Mediated Reduction of Chromium(VI) by Citrate

Abstract

Unlike lower valent iron (Fe), the potential role of lower valent manganese (Mn) in the reduction of hexavalent chromium (Cr(VI)) in soil is poorly documented. In this study, we report that citrate along with Mn(II) and clay minerals (montmorillonite and kaolinite) reduce Cr(VI) both in aqueous phase and in the presence of dissolved organic carbon (SDOC) extracted from a forest soil. The reduction was favorable at acidic pH (up to pH 5) and followed the pseudo-first-order kinetic model. The citrate (10 mM) + Mn(II) (182.02 μM) + clay minerals (3% w/v) system in SDOC accounted for complete reduction of Cr(VI) (192.32 μM) in about 72 h at pH 4.9. In this system, citrate was the reductant, Mn(II) was a catalyst, and the clay minerals acted as an accelerator for both the reductant and catalyst. The clay minerals also serve as a sink for Cr(III). This study reveals the underlying mechanism of the Mn(II)-induced reduction of Cr(VI) by organic ligand in the presence of clay minerals under certain environmental conditions.

A green approach towards simultaneous remediations of chromium(VI) and arsenic(III) in aqueous solution

“…In addition, the rate of Cr(VI) reduction also increased in As(III)/Cr(VI) system because As(III) was thought to alleviate Cr(III) re-oxidation to Cr(VI) by scavenging partial Å OH (Eqs. (11) and (12)). In this process, ionic As(V) species (H 2 AsO 4 À ) and Cr(III) species (Cr 3+ and Cr(OH) 2+ ) were simultaneously generated resulting in the rising solution conductivity from 5.0 to 5.46 mS cm À1 as shown in Fig.…”

Reduction of Hexavalent Chromium Using Sorbaria sorbifolia Aqueous Leaf Extract

“…Many remediation strategies have been advanced to deal with Cr(VI) pollution, including adsorption [6], ion exchange [7], chemical precipitation [8], membrane filtration [9] and reduction [10][11][12][13][14][15][16][17], among others. The reduction of Cr(VI) is among the most widely used strategies in view of the redox activity of chromium.…”

“…Reductive remediation approaches can address the toxicity of Cr(VI) by transformation into Cr(III), that can be easily removed after precipitation with alkaline substances [8]. Several synthetic and natural electron-rich compounds are capable of reducing Cr(VI): Fe(II) bearing clay minerals, zero-valent aluminum and Mn(II) have been reported as reducing, and being catalyzing agents in the reduction processes of Cr(VI) [10][11][12]. Natural organic acids including citric, oxalic, ascorbic, tartaric and formic acid have also been examined for the reduction of Cr(VI) from contaminated aqueous and soil matrices [13][14][15][16].…”

“…Sarkar et al report 100% Cr(VI) reduction efficiency at pH 2.25 that becomes negligible at pH > 7.5 [12]. Similarly, Lan et al observed that montmorillonite and illite significantly catalyze the reduction of Cr(VI) by citric acid at pH < 5.0; in contrast, their effectiveness are dramatically diminished at pH 5.0 [18].…”

Acid-treated g-C3N4 with improved photocatalytic performance in the reduction of aqueous Cr(VI) under visible-light

“…Hexavalent chromium (Cr(VI)) is a common pollutant in the effluents from chromate-related industries [1][2][3][4][5][6]. It is highly toxic and highly mobile in water, and can cause great harm to the environment and human health [1][2][3][4][5][6].…”