Effect of natural organic matter on Cr(VI) reduction by reduced nontronite

Fe(II)-bearing clay minerals are abundant in soils and are critical to control the fate of the highly toxic hexavalent chromium (Cr(VI)). Even though the reduction of Cr(VI) by Fe(II)-bearing clay minerals has been extensively studied, the effect of the commonly co-existing natural organic matter (NOM) on this process is poorly under-stood. This study aimed to understand the kinetics of Cr(VI) reduction by a typical Fe(II)-bearing clay mineral (i. e., reduced nontronite (rNAu-2)) in the presence of Suwannee River NOM and to unveil the possible mechanisms. Cr(VI) reduction experiments were conducted with different NOM concentrations in a range of 0 to 100 mg C L-1 under circumneutral pH. The time-course aqueous-phase measurements and solid-phase characterizations were performed with a combination of wet chemical methods, transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analyses. The results demonstrated that NOM inhibited Cr(VI) reduction overall, mainly due to the decrease of Cr(VI) sorption on rNAu-2 by mineral-bound NOM. Besides the predominant solid Cr(III) species, soluble and mobile Cr(III) complexes were formed during reduction. Interestingly, the reduction rate (k) decreased with the increase of NOM concentration from 0 to 20 mg C L-1, but kept increasing when the concentration of NOM increased from 30 to 100 mg C L-1. The contrary pattern is due to the increasing promotion effects that offset the inhibitory effect caused by competitive sorption between Cr(VI) and NOM. The promotion effects may include weakening the mineral passivation by forming stable aqueous NOM-Cr(III) complexes, accelerating the electron transfer rate by dissolving rNAu-2, and donating electrons through NOM-Fe(II/III) complexes and/or NOM-Cr(V) complexes. This study extends our knowledge of Cr(VI) reduction by clay minerals in organic matter-rich soils and sediments and has important implication for understanding the transport and long-term behavior of Cr in the environment.

Automated summary

Fe(II)-bearing clay minerals are abundant in soils and play a critical role in controlling the fate of hexavalent chromium (Cr(VI)), a highly toxic substance. However, the influence of co-existing natural organic matter (NOM) on the reduction of Cr(VI) by Fe(II)-bearing clay minerals is poorly understood. This study investigated the kinetics of Cr(VI) reduction by a typical Fe(II)-bearing clay mineral in the presence of NOM. The results showed that NOM inhibited Cr(VI) reduction mainly by decreasing its sorption on the clay mineral. Additionally, soluble and mobile Cr(III) complexes were formed during the reduction process. The rate of reduction initially decreased with increasing NOM concentration, but then increased at higher concentrations, likely due to promotion effects offsetting the inhibitory effect of competitive sorption between Cr(VI) and NOM.