Molecular Mechanisms of Chromium(III) Immobilization by Organo–Ferrihydrite Co-precipitates: The Significant Roles of Ferrihydrite and Carboxyl

Abstract

The interaction mechanisms of heavy metals with organo−Fe hydroxides co-precipitates (OFC) remain unclear due to the structural complexity of the OFC. In this study, batch experiments were conducted to investigate the immobilization mechanisms of Cr(III) by the OFC, which was prepared by co-precipitating Fe 3+ with rice/rape straw-derived dissolved organic carbon, through sorption and coprecipitation using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy and scanning transmission X-ray microscopy (STXM). At an Fe/C molar ratio ≥ 0.3, both the sorption and co-precipitation immobilized the majority of Cr(III), but the coprecipitation desorbed less Cr(III) than the sorption regardless of DOC loadings and sources. In contrast, Cr(III) immobilization was significantly reduced at an Fe/C molar ratio of 0.1 for both reactions. Linear combination fitting of Cr K-edge XANES spectra revealed the predominance of ferrihydrite-bound Cr(III), but enhanced organic Cr(III) occurred with increased organic carbon (OC) loading for both the sorption and co-precipitation. STXM coupled with multi-edge XANES analysis confirmed the primary association of Cr(III) with ferrihydrite and directly probed carboxyl as the binding site for Cr(III) retention on the OC constituents of the OFC. These results provided new molecular-level insights into the Cr(III) retention mechanisms on the OFC, particularly for the interactions of Cr(III) and OC constituents of the OFC, which could benefit the management of Cr-contaminated soils with straw returning.

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

“…Anionic functional groups in OM, − primarily carboxylates (COO – ), have been reported to have strong affinity for positively charged iron oxide surfaces under acidic pH conditions. − A higher adsorption affinity of poly­(acrylic acid) (PAA) to iron oxide than Suwannee River natural OM (SRNOM) was previously reported, due to a higher carboxyl density and linear molecular structure of PAA . The adsorption affinity was also reported to vary for monodentate and polydentate ligands. , Beyond OM adsorption, the formation of insoluble Fe­(III)-organic complex was also reported to result in additional OM sequestration in the coprecipitates. ,,,, For Fe 3+ -OM complexation, carboxyl groups (−COOH) in OM were reported to complex with Fe 3+ ions and thus suppress Fe hydrolysis and precipitation. ,− Cr 3+ -OM complexation was highly pH dependent, which controlled Cr speciation (e.g., monomeric or polymeric Cr­(III)-OM complexes) in the environment . Regarding Fe/Cr-OM ternary system, only a few studies have investigated Fe/Cr-OM coprecipitation. , Yang et al studied Fe/Cr coprecipitation with dissolved organics derived from rice and rape straws .…”

“…The adsorption affinity was also reported to vary for monodentate and polydentate ligands. , Beyond OM adsorption, the formation of insoluble Fe­(III)-organic complex was also reported to result in additional OM sequestration in the coprecipitates. ,,,, For Fe 3+ -OM complexation, carboxyl groups (−COOH) in OM were reported to complex with Fe 3+ ions and thus suppress Fe hydrolysis and precipitation. ,− Cr 3+ -OM complexation was highly pH dependent, which controlled Cr speciation (e.g., monomeric or polymeric Cr­(III)-OM complexes) in the environment . Regarding Fe/Cr-OM ternary system, only a few studies have investigated Fe/Cr-OM coprecipitation. , Yang et al studied Fe/Cr coprecipitation with dissolved organics derived from rice and rape straws . The dried coprecipitates were characterized by X-ray absorption spectroscopy and scanning transmission X-ray microscopy (STXM) measurements, which revealed the predominance of ferrihydrite-bound Cr­(III) as well as OM-complexed Cr­(III) with carboxyl group as the binding sites .…”

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.…”