Speciation of chromium in chromium yeast

Aerobic Fe transformation induced decrease in the adsorption and enhancement in the reduction of Cr(VI) by humic acid-ferric iron coprecipitates

Humic substance (HS)-ferric iron (Fe(III)) coprecipitates are widespread organo-mineral associations in soils and aquifers and have the capacity to immobilize and detoxify Cr(VI). These coprecipitates undergo transformation owing to their thermodynamic instability; however, the effects of this transformation on their environmental behaviors remain unclear, particularly in aerobic environments. In this study, the aerobic transformation of humic acid (HA)-Fe(III) coprecipitates, a representative of HS-Fe(III) coprecipitates, was simulated. The environmental effect was then evaluated after conducting an adsorption-reduction batch experiment toward Cr(VI). The aerobic transformation characteristics, as well as the adsorption/reduction capacity of HA-Fe(III) coprecipitates, were found to depend strongly on their structures. In ferrihydrite (Fh)-like coprecipitates, amorphous Fh is readily transformed into crystalline hematite and goethite at aerobic environments, leading to a much lower specific surface area and adsorption capacity. However, this increasing degree of crystallization enhanced the inductive reduction ability towards Cr(VI) owing to the more significant shift of electron pairs in the FeOC bond toward the HA direction. In HS-like coprecipitates, Fe(III) always serves as a cation bridge connecting HA molecules, but can be reduced to Fe(II) by the associated HA after aerobic transformation. The produced Fe(II), therefore, drove the reduction of the adsorbed Cr(VI). These findings emphasize the pivotal role of aerobic transformation in enhancing the reduction capacity for Cr(VI), which opens a new avenue for the development of in-situ remediation agents for Cr(VI)-contaminated sites.

Chromium speciation by the combination of high-performance liquid chromatography and inductively coupled plasma-optical emission spectrometry

A hyphenated instrumental method high-performance liquid chromatography-inductively coupled plasma-optical emission spectrometry (HPLC-ICP-OES) was used for the separation and determination of Cr(III) and Cr(VI). After the chromatographic separation of chromium species by anion exchange column, their spectrophotometric detection was carried out by ICP-OES system. Important instrumental and chromatographic parameters were investigated via univariate optimization approach to obtain high signal to noise ratio and good resolution for chromium species. Under the optimum HPLC-ICP-OES conditions, limit of detection (LOD) values for Cr(III) and Cr(VI) were found to be 0.27 and 0.05 mg/kg, respectively. In addition, accuracy and applicability of developed method were checked by recovery experiments performed with the spiked soil, grass, and water samples. High percent recovery results (88-104%) were achieved by utilizing matrix matching calibration strategy for the selected samples. The results showed that this method was accurate and applicable to soil, grass, and water samples.

Synergistic/antagonistic effects and mechanisms of Cr(VI) adsorption and reduction by Fe(III)-HA coprecipitates

Widespread Fe(III)-humic acid (HA) coprecipitates (FHCs) have substantial impacts on the adsorption and reduction of Cr(VI) in soils and sediments, but whether this process is equal to the sum of their individual components remains unknown. In this study, ferrihydrite (Fh)- and HA-like FHCs (C/Fe<3 and C/Fe>3, respectively) were synthesized by controlling the initial C/Fe ratios (0.5-18) to explore the potential synergistic/antagonistic effects during the adsorption and reduction of Cr(VI). According to the results, antagonistic effects on Cr(VI) adsorption (5%-80%) were observed on Fh- and HA-like FHCs, where the antagonistic intensity increased with increasing HA proportions, respectively caused by the more serious occupation of adsorption sites and the stronger electrostatic repulsion to Cr(VI). Notably, significant synergistic reduction effects (5%-650%) occurred on Fh-like FHCs were found to be achieved by the activation of low-molecular HA (0.1-0.3 kDa) with primary/secondary hydroxylic groups, which might be induced by the inductive effect of Fh on complexed HA molecules according to density-functional theory (DFT) calculation. While slight antagonistic reduction effects (2%-45%) by HA-like FHCs were attributed to the decreasing accessibility of Cr(VI) to reductive phenolic groups, which might be blocked within FHC particles or complexed with Fe(III) ions through cation bridges.

Speciation Analysis of Chromium in Chromium-Enriched Yeast by Ion Chromatography-Inductively Coupled Plasma Mass Spectrometry

Chromium-enriched yeast (CrY) is a popular Cr dietary supplement, but suitable speciation analysis of highly toxic Cr(VI) in CrY is not available. Ion chromatography-inductively coupled plasma mass spectrometry method was firstly developed and validated for the quantification of Cr(III) and Cr(VI). Ultrasound-assisted weakly alkaline EDTA solution combined with boiling was used to extract two Cr species in CrY. Two species were separated on two successive anion-exchange columns using a mobile phase of 0.6 mmol/L EDTA and 76 mmol/L NH₄NO₃ solution. The method was sensitive, accurate (92.4-100.9%), and precise (0.8-3.1%). Species of Cr(VI) were not found in CrY.