Molecular Sorption Mechanisms of Cr(III) to Organo-Ferrihydrite Coprecipitates Using Synchrotron-Based EXAFS and STXM Techniques

Abstract

Ubiquitous organo-ferrihydrite coprecipitates (OFC) significantly affect the mobility and availability of Cr in soil through sorption, but the underlying sorption mechanisms remain unclear at the molecular level. Due to the potential formation of OFC in agricultural soils with returned crop straws, we synthesized OFC with rice/rape straw-derived carbon (C) sources and different loadings. The molecular sorption mechanisms of Cr(III) to the synthesized OFC under different conditions were investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Cr(III) sorption by OFC decreased with increasing C loading and decreasing pH, regardless of C sources. Moreover, inhibition of Cr(III) sorption to OFC with high C loading occurred when ionic strength (IS) increased, suggesting the presence of outer-sphere complexed Cr(III). EXAFS analysis revealed that more Cr(III) were bound to ferrihydrite of the OFC at a relatively high pH, and organically bound Cr(III) enhanced when increasing C loading and decreasing IS. STXM analysis strongly suggested that C loading reduced Cr(III) sorption through blocking the binding sites on the ferrihydrite, which overwhelmed Cr(III) retention by the direct binding of Cr(III) to carboxyl of the particulate organic matter (OM) and OM coated on the Fh fractions of the OFC. These findings facilitated the comprehensive understanding of the sorption mechanisms of Cr(III) to OFC at the molecular level, which will assist the prediction of Cr(III) mobility in soils, particularly for Cr(III)-contaminated agricultural soils with the application of crop straws.

Coprecipitation of Fe/Cr Hydroxides with Organics: Roles of Organic Properties in Composition and Stability of the Coprecipitates

“…After 24 h of coprecipitation, the Fe/Cr-OM coprecipitates were collected, using centrifuging filters (Millipore, 100 kDa) from solution, and were quickly washed multiple times with deionized (DI) water to remove excess salts and organics, following procedures from previous studies. , During rinsing with DI of pH 6, the remaining Fe/Cr ions may hydrolyze and precipitate out which could lower solution pH quickly. After quick rinsing, the pH of the slurry slightly increased the pH to ∼3.8 ± 0.1, only slightly higher than the pH condition (3.0 ± 0.1) of coprecipitation experiments.…”

Extraction and purification of glomalin-related soil protein (GRSP) to determine the associated trace metal(loid)s

“…1 d, arrow d1) after GRSP extraction, indicating the coupling of carbon and iron is disrupted during the extraction of GRSP. The local Fe:C molar ratio affects this organo–ferrihydrite co-precipitates process [ 6 , 7 ]. Therefore, when the local Fe and C+N coupling distribution was lost, some heavy metal(loid)s immobilized via organo-ferrihydrite co-precipitates may be released into the solution.…”

Scanning Transmission X-Ray Microscopy At The Canadian Light Source: Progress And Selected Applications In Geosciences

“…To better understand and model the OMAs, natural soils are often too complex to be used as model systems for accurate and systematic investigations. Recently, Fe hydroxides have gained increased interest as model adsorbents or co-precipitates for deliberately designed sorption studies that involve important metal species such as Ca or Cr, together with the dissolved organic carbon (DOC) of soils or pure organic acids such as citric acid, [102][103][104][105][106][107][108][109] in order to investigate the interaction/association mechanisms at the molecular level. Among these studies, a novel Fe-Ca-organic carbon ternary system was proposed and confirmed using STXM and other laboratory-based techniques.…”

“…[104][105][106] Recently, we have been investigating organo-Fe hydroxide co-precipitates (OFC) through sorption and co-precipitation batch experiments involving Cr, which were further characterized using STXM. 108,109 The OFC is significantly important in the environment, affecting the immobilization of heavy metals and carbon sequestration. Fig.…”