Reductive Sequestration of Cr(VI) and Immobilization of C during the Microbially Mediated Transformation of Ferrihydrite-Cr(VI)-Fulvic Acid Coprecipitates

Abstract

Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.

Adsorption-Reduction Behavior of Cr(VI) by Ferrihydrite-Fulvic Acid Complexes with Different C/Fe Mole Ratios

“…The Fh-FA complex preparation was adapted from Wang et al [32] and Chen [37] et al with minor modifications. Initial C/Fe mole ratios were set to 0.5, 1.0, and 1.5 (Fh-FA0.5, Fh-FA1.0, and Fh-FA1.5); these C/Fe molar ratios were set to represent the typical conditions in natural environments [38–40]. Differences in C/Fe mole ratios were achieved by adjusting the mass of fulvic acid (FA).…”

Behavior and Fate of Chromium and Carbon during Fe(II)-Induced Transformation of Ferrihydrite Organominerals

“…One of the most important types of OC for Fe mineral–OC associations are carboxyl-rich OC. Carboxyl functional groups are prevalent in natural OC and are highly reactive toward different metal ions, mineral surfaces, and other adsorbents like microplastics. , Our recent NEXAFS spectroscopy and surface complexation modeling show that as the number of carboxyl functional groups present in simple OC increases, the number of carboxylate-Fe bonds formed between carboxyl functional groups and the Fe particles increases, and thus the binding strength and stability of the OC associated with Fh also increase. , At present, studies about OC stabilization during Fh transformation mainly involve the high-weight OC like fulvic acids, polygalacturonic acid, soil organic matter, and dissolved organic matter from litter sample. ,,− When OC gradually changes from low-weight to high-weight OC with increasing carboxyl-richness and binding strength, the mechanisms of OC stabilization are still ambiguous. Therefore, it is important to investigate how OC with different carboxyl-richness influences the transformation pathway of Fh and then influences the OC stabilization in a reductive environment.…”

“…44,52,53 Prior study reports that during reductive transformation of Fh, the formation of nanopores in products facilitates the sequestration of OC, but high Cr(III) loading could impede the transformation of Fh and formation of nanopores, unfavoring OC stabilization. 50 The mechanisms behind OC stabilization in the presence of Cr(III) are still ambiguous for different pH conditions and for OC with different carboxyl-richnesses and binding strength.…”

“…The stabilization of OC may also be influenced by other adsorbates on Fh. Anions like phosphate and arsenate can compete with OC for adsorption sites, increasing the desorption of OC, while polycations like Ca­(II) and Mg­(II) may increase the OC sequestration by cation-bridging effects, forming iron (oxyhydr)­oxide–cation–OC ternary complexes. ,, Prior study reports that during reductive transformation of Fh, the formation of nanopores in products facilitates the sequestration of OC, but high Cr­(III) loading could impede the transformation of Fh and formation of nanopores, unfavoring OC stabilization . The mechanisms behind OC stabilization in the presence of Cr­(III) are still ambiguous for different pH conditions and for OC with different carboxyl-richnesses and binding strength.…”

Enhanced Oxidation of Organic Compounds by the Ferrihydrite–Ferrate System: The Role of Intramolecular Electron Transfer and Intermediate Iron Species

“…Iron oxides, such as ferrihydrite, goethite, and hematite, are ubiquitous in aquatic and most terrestrial environments and participate in various chemical processes. − Ferrihydrite (Fh), characterized by its extensive surface area and unique structure, is particularly reactive and plays a crucial role in regulating the (im)­mobilization contaminants. , On one hand, Fh can contribute to controlling the fate of toxic metals and metalloids by processes such as adsorption/desorption, coprecipitation, and crystallization. − On the other hand, Fh is often utilized as a heterogeneous catalyst to activate H 2 O 2 or persulfate, leading to the degradation of organic contaminants by generating · OH or SO 4 · – . − However, the oxidation of organic contaminants via the combination of Fe­(VI) and Fh remains unexplored.…”