Ferrihydrite Transformation Impacted by Coprecipitation of Phytic Acid

Arsenic redistribution associated with Fe(II)-induced jarosite transformation in the presence of polygalacturonic acid

Jarosite exists widely in acid-sulfate soil and acid mine drainage polluted areas and acts as an important host mineral for As(V). As a metastable Fe(III)-oxyhydoxysulfate mineral, its dissolution and transformation have a significant impact on the biogeochemical cycle of As. Under reducing conditions, the trajectory and degree of abiotic Fe(II)-induced jarosite transformation may be greatly influenced by coexisting dissolved organic matter (DOM), and in turn influencing the fate of As. Here, we explored the impact of polygalacturonic acid (PGA) (0-200 mg·L-1) on As(V)-coprecipitated jarosite transformation in the presence of Fe(II) (1 mM) at pH 5.5, and investigated the repartitioning of As between aqueous and solid phase. The results demonstrated that in the system without both PGA and Fe(II), jarosite gradually dissolved, and lepidocrocite was the main transformation product by 30 d; in Fe(II)-only system, lepidocrocite appeared by 1 d and also was the mainly final product; in PGA-only systems, PGA retarded jarosite dissolution and transformation, jarosite might be directly converted into goethite; in Fe(II)-PGA systems, the presence of PGA retarded Fe(II)-induced jarosite dissolution and transformation but did not alter the pathway of mineral transformation, the final product mainly still was lepidocrocite. The retarding effect on jarosite dissolution enhanced with the increase of PGA content. The impact of PGA on Fe(II)-induced jarosite transformation mainly was related to the complexation of carboxyl groups of PGA with Fe(II). The dissolution and transformation of jarosite drove pre-incorporated As transferred into the phosphate-extractable phase, the presence of PGA retarded jarosite dissolution and maintained pre-incorporated As stable in jarosite. The released As promoted by PGA was retarded again and almost no As was released into the solution by the end of reactions in all systems. In systems with Fe(II), no As(III) was detected and As(V) was still the dominant redox species.

Impact of organic phosphates on the structure and composition of short-range ordered iron nanophases

Organic phosphates (OP) are important nutrient components for living cells in natural environments, where they readily interact with ubiquitous iron phases such as hydrous ferric oxide, ferrihydrite (FHY). FHY partakes in many key bio(geo)chemical reactions including iron-mediated carbon storage in soils, or iron-storage in living organisms. However, it is still unknown how OP affects the formation, structure and properties of FHY. Here, we document how β-glycerophosphate (GP), a model OP ligand, affects the structure and properties of GP-FHY nanoparticles synthesized by coprecipitation at variable nominal molar P/Fe ratios (0.01 to 0.5). All GP-FHY precipitates were characterized by a maximum solid P/Fe ratio of 0.22, irrespective of the nominal P/Fe ratio. With increasing nominal P/Fe ratio, the specific surface area of the GP-FHY precipitates decreased sharply from 290 to 3 m2 g-1, accompanied by the collapse of their pore structure. The Fe-P local bonding environment gradually transitioned from a bidentate binuclear geometry at low P/Fe ratios to monodentate mononuclear geometry at high P/Fe ratios. This transition was accompanied by a decrease in coordination number of edge-sharing Fe polyhedra, and the loss of the corner-sharing Fe polyhedra. We show that Fe(iii) polymerization is impeded by GP, and that the GP-FHY structure is highly dependent on the P/Fe ratio. We discuss the role that natural OP-bearing Fe(iii) nanophases have in biogeochemical reactions between Fe-P and C species in aquatic systems.

Ferrihydrite transformation impacted by coprecipitation of lignin: Inhibition or facilitation?

Lignin is a common soil organic matter that is present in soils, but its effect on the transformation of ferrihydrite (Fh) remains unclear. Organic matter is generally assumed to inhibit Fh transformation. However, lignin can reduce Fh to Fe(II), in which Fe(II)-catalyzed Fh transformation occurs. Herein, the effects of lignin on Fh transformation were investigated at 75°C as a function of the lignin/Fh mass ratio (0-0.2), pH (4-8) and aging time (0-96 hr). The results of Fh-lignin samples (mass ratios = 0.1) aged at different pH values showed that for Fh-lignin the time of Fh transformation into secondary crystalline minerals was significantly shortened at pH 6 when compared with pure Fh, and the Fe(II)-accelerated transformation of Fh was strongly dependent on pH. Under pH 6, at low lignin/Fh mass ratios (0.05-0.1), the time of secondary mineral formation decreased with increasing lignin content. For high lignosulfonate-content material (lignin:Fh = 0.2), Fh did not transform into secondary minerals, indicating that lignin content plays a major role in Fh transformation. In addition, lignin affected the pathway of Fh transformation by inhibiting goethite formation and facilitating hematite formation. The effect of coprecipitation of lignin on Fh transformation should be useful in understanding the complex iron and carbon cycles in a soil environment.

Degradation of enoxacin with different dissociated species during the transformation of ferrihydrite-antibiotic coprecipitates

Ferrihydrite acts as a natural reservoir for nutrient elements, organic matter, and coexisting pollutants through adsorption and coprecipitation. However, the degradation of emerging fluoroquinolone antibiotics during the transformation of ferrihydrite coprecipitates, especially those with various dissociated species, remains insufficiently explored. In this study, Enoxacin (ENO), employed as a model antibiotic, was introduced to prepare ferrihydrite-ENO coprecipitates. The influence of coprecipitated ENO on the transformation of the ferrihydrite-ENO coprecipitate was investigated across different pH conditions. The results revealed that ferrihydrite-ENO coprecipitates thermodynamically transformed into more stable goethite and/or hematite under all pH conditions. In neutral and alkaline conditions, ENO promoted the transformation of coprecipitates into goethite while hindering hematite formation. Conversely, under acidic conditions, ENO directly obstructed the transformation of coprecipitates into hematite. Different dissociated species of ENO displayed distinct degradation pathways. The cationic form of ENO exhibited a greater tendency for hydroxylation and defluorination, while the zwitterion form leaned toward piperazine ring oxidation, with limited preference for quinolone ring oxidation. The anionic form of ENO exhibited the fastest degradation rate. It is essential to emphasize that the toxicity of the degradation products was intricately connected to the specific reaction sites and the functional groups they acquired post-oxidation. These findings offer fresh insights into the role of antibiotics in coprecipitation, the transformation of ferrihydrite coprecipitates, and the fate of coexisting antibiotics.

Artificial Humic Acid Mediated Carbon-Iron Coupling to Promote Carbon Sequestration

Fe (hydr)oxides have a substantial impact on the structure and stability of soil organic carbon (SOC) pools and also drive organic carbon turnover processes via reduction-oxidation reactions. Currently, many studies have paid much attention to organic matter-Fe mineral-microbial interactions on SOC turnover, while there is few research on how exogenous carbon addition abiotically regulates the intrinsic mechanisms of Fe-mediated organic carbon conversion. The study investigated the coupling process of artificial humic acid (A-HA) and Fe(hydr)oxide, the mechanism of inner-sphere ligands, and the capacity for carbon sequestration using transmission electron microscopy, thermogravimetric, x-ray photoelectron spectroscopy, and wet-chemical disposal. Furthermore, spherical aberration-corrected scanning transmission electron microscopy-electron energy loss spectroscopy and Mössbauer spectra have been carried out to demonstrate the spatial heterogeneity of A-HA/Fe (hydr)oxides and reveal the relationship between the increase in Fe-phase crystallinity and redox sensitivity and the accumulation of organic carbon. Additionally, the dynamics of soil structures on a microscale, distribution of carbon-iron microdomains, and the cementing-gluing effect can be observed in the constructing nonliving anthropogenic soils, confirming that the formation of stable aggregates is an effective approach to achieving organic carbon indirect protection. We propose that exogenous organic carbon inputs, specifically A-HA, could exert a substantial but hitherto unexplored effect on the geochemistry of iron-carbon turnover and sequestration in anoxic water/solid soils and sediments.

One-step strategy for efficient Cr(VI) removal via phytate modified zero-valent iron: Accelerated electron transfer and enhanced coordination effect

The toxic Cr(VI) from industrial wastewater pose serious threat to the human beings and eco-systems. To reduce the operation processes and enhance the removal efficiency of Cr(VI), targeted design of functionalized material is critical in practical applications. Herein, we developed a one-step strategy for simultaneous Cr(VI) reduction and total Cr capture by a novel phytate modified zero-valent iron (PA-ZVI). The reaction kinetics of Cr(VI) removal by PA-ZVI (0.2225 min-1) was 53 times higher compared to ZVI (0.0042 min-1). The Fe(0) content on the surface of PA-ZVI increased from 2.2% to 15.6% compared to ZVI. Meanwhile, Cr(VI) was liable to adsorb on the surface of PA-ZVI due to its lower adsorption energy compared with the original ZVI (-2.09 eV vs -0.85 eV). The incorporation of the phytate ligand promoted electron transfer from iron core to Cr(VI), leading to the rapid in-situ reduction of Cr(VI) adsorbed on the surface of PA-ZVI to Cr(III). PA-ZVI exhibited a satisfactory performance for Cr(VI) removal at a broad pH range (3-11) and in the presence of coexisting ions and humic acid. Moreover, the reactor with the addition of PA-ZVI achieved more than 90% Cr(VI) removal within 72 h in continuous flow experiments. The feasibility of PA-ZVI for the removal of Cr(VI) is also validated in authentic wastewater. This work provides novel ZVI materials that can effectively address decontamination challenges from Cr(VI) pollution.

Direct observations of nanoscale brushite dissolution by the concentration-dependent adsorption of phosphate or phytate

With the development of agricultural intensification, phosphorus (P) accumulation in croplands and sediments has resulted in the increasingly widespread interaction between inorganic and organic P species, which has been, previously, underestimated or even ignored. We quantified the nanoscale dissolution kinetics of sparingly soluble brushite (CaHPO4·2H2O, DCPD) over a broad range of phosphate and/or phytate concentrations by using in situ atomic force microscopy (AFM). Compared to water, we found that low concentrations of phosphate (1-1000 µM) or phytate (1-100 µM) inhibited brushite dissolution by slowing single step retraction. However, with increasing phosphate or phytate concentrations to 10 mM, there was a reverse effect of dissolution promotion at brushite-water interfaces. In situ observations of the coupled dissolution-reprecipitation showed that phosphate precipitated more readily than phytate on brushite surfaces, with the formation of amorphous calcium phosphate (ACP). For a fundamental understanding, zeta potential and in situ Raman spectroscopy (RS) revealed that the concentration-dependent dissolution is attributed to the reverse of outer-sphere to inner-sphere adsorption with increasing phosphate or phytate concentrations. In addition, the mineralization of phytate with outer-sphere adsorption by phytase was higher than that with inner-spere adsorption, and the presence of phytate delayed ACP phase transformation to hydroxylapatite (HAP). These in situ observations and analyses may fill the knowledge gaps of interaction between inorganic and organic P species in P-rich terrestrial and aquatic environments, thereby implicating their biogeochemical cycling and the associated availability.

Elevated nano-α-Fe2O3 enhances arsenic metabolism and dissolved organic carbon release of Microcystis aeruginosa under a phytate environment

Little information is available on the effects of nano-α-Fe2O3 on arsenic (As) metabolism of algae and potential associated carbon (C) storage in As-contaminated water with dissolved organic phosphorus (DOP) as a phosphorus (P) source. In this study, Microcystis aeruginosa (M. aeruginosa) was used to investigate impacts of nano-α-Fe2O3 on cell growth and As metabolism of algae under a phytate (PA) environment as well as potential associated C storage. Results showed that nano-α-Fe2O3 had a subtle influence on algal cell growth in a PA environment. Herein, algal cell density (OD680) and chlorophyll a (Chla) were inhibited at elevated nano-α-Fe2O3 levels, which simultaneously limited the decrease of Yield. As suggested, the complexation of PA with nano-α-Fe2O3 could alleviate the negative influence on algal cell growth. Furthermore, the elevated nano-α-Fe2O3 increased As methylation in the PA environment due to higher monomethylarsenic (MMA) and dimethylarsenic (DMA) concentrations in the test media. Additionally, microcystins (MCs) in the media changed consistently with UV254, both of which were relatively lower at 10.0 mg·L-1 nano-α-Fe2O3. Enhanced As(V) methylation of algal cells was found to simultaneously reduce the release risk of As(III) and MC while increasing dissolved organic carbon (DOC) content in media, suggesting unfavorable C storage. Three-dimensional fluorescence analysis revealed that the main DOC constituent was the tryptophan-like component in aromatic proteins. Correlation analysis showed that decreases in pH and the zeta potential and an increase in Chla may lead to metabolic As improvements in M. aeruginosa. The obtained findings highlight the need for greater focus on the potential risks of DOP combined with nano-α-Fe2O3 on algal blooms as well as the biogeochemical cycling processes of As and C storage in As-contaminated water with DOP as the P source.

Influence of montmorillonite incorporation on ferrihydrite transformation and Cr(VI) behaviors during ferrihydrite-Cr(VI) coprecipitates aging

Hexavalent chromium (Cr(VI)) is a pollutant with high migration ability, and the destiny of Cr(VI) is highly correlated with ferrihydrite (Fh). Montmorillonite (Mt) is a clay mineral abundantly presents in nature. Although Cr(VI) adsorption on montmorillonite or ferrihydrite has been studied, Cr(VI) behaviors during the Fh-Cr-Mt coprecipitates transformation still remain unknown. In this study, calcium montmorillonite (Ca-Mt) or sodium montmorillonite (Na-Mt) was coprecipitated with ferrihydrite and Cr(VI). Effect of Ca-Mt (or Na-Mt) incorporation on coprecipitates transformation and Cr(VI) behaviors during aging were investigated. The results showed that Ca-Mt or Na-Mt incorporation inhibited the transformation of ferrihydrite in Fh-Cr-Ca-Mt or Fh-Cr-Na-Mt at the initial pH of 5.0, 7.0 and 9.0. During aging, two kinds of Mt were supposed to interact with Fh to form the FeOSi and FeOAl bonds, and thus the formation of hematite and goethite were limited. By testing the Cr(VI) distribution in each phase of coprecipitates during transformation, delay on Cr(VI) migration and redistribution could be found in systems added with montmorillonite, and Cr(VI) was retained in coprecipitates to a greater extent compared with the systems without montmorillonite addition. The results of this study contribute to increasing our knowledge about the role of clay minerals on the coprecipitates transformation when they coexist at different pH values. It is also significant for the heavy metals polluted sites repairing.

Anti-Oil-Adhesion Property of Superhydrophilic/Underwater Superoleophobic Phytic Acid-FeIII Complex Coatings

Crude oil adhesion issues are widespread in the petroleum industry, leading to inefficient production and high maintenance costs. Developing efficient antifouling materials and investigating the microscopic adhesion mechanism are of substantial significance. In the present work, a superhydrophilic/underwater superoleophobic PAFC coating with excellent antifouling properties was constructed by the coordination-driven self-assembly of phytic acid (PA) and FeCl₃ (FC). The atomic force microscope (AFM) droplet probe technique was employed to elucidate the underlying mechanism of the anti-oil-adhesion property of the PAFC coating. Results showed that the PAFC modification achieved the optimum effect at a molar ratio of 1:3 between PA and Fe^III. Applying a (3-aminopropyl)triethoxysilane (APTES) interlayer can effectively improve the performance of the PAFC coating on silica substrates. AFM droplet probe experiments indicated that the adhesion force between submerged micrometer-sized oil droplets and PAFC-modified substrates was significantly weaker than that with the untreated substrate. Meanwhile, the adhesion forces between oil droplets and surfaces were inversely proportional to the contact angle of the oil in water and were enhanced by higher salinity, lower collision velocity, and stronger loading force. The oil injection and wall sticking tests also confirmed the effectiveness of the PAFC modification in resisting the adhesion of crude oil.

Molecular mechanisms of Chromium(III) sorption by organo-ferrihydrite coprecipitates induced by crop straws

Agricultural organo-ferrihydrite (Fh) coprecipitates (OFCs), resulting from the coprecipitation of Fe(III) and dissolved organic carbon (DOC) from returned straws, significantly affect the bioavailability of heavy metals in farmland. However, the molecular sorption mechanisms of Cr(III) by the OFCs remain unclear. Here, we explored the sorption behaviors of Cr(III) by the OFCs formed with wheat or maize straws derived-DOC (wheat-DOC or maize-DOC) under various environmental conditions, and further underlying molecular sorption mechanisms using Cr K-edge X-ray absorption near edge structure (XANES) spectroscopy. Results showed that high C loadings reduced the specific surface areas (SSAs) and Cr(III) sorption capacities of the OFCs, implying the blockage of binding sites by C loading. Additionally, although the wheat-DOC induced OFC had a smaller SSA than the maize-DOC induced OFC, their Cr(III) sorption were comparable, which was likely to be compensated by the more carboxyl in the wheat-DOC. Moreover, at a higher ionic strength, the increased or slightly decreased Cr(III) sorption indicated that the inner-sphere sorption was dominant regardless of high or low C loadings, which was also supported by the extremely low Cr(III) extraction percentage. The Cr K-edge XANES spectroscopy suggested that Cr(III) could be immobilized by both the Fh and organic fractions, with the Fh fractions playing a significant role. These findings contribute to a molecular-level mechanistic understanding of Cr(III) sorption by the OFC, which will aid in the prevention and control of Cr-contaminated agricultural soils.

Phytic acid and graphene oxide functionalized sponge with special-wettability and electronegativity for oil-in-water emulsion separation in single-step

Developing an emulsion separation material with one-step in-situ purifying capability and improved security in applications, especially for subsequent scale-up, is valuable but remains a challenge. Herein, the amphiphilic sponge (PA@RGO@MS) was prepared via impregnation and in-situ growth of the negatively charged hydrophilic phytic acid (PA) and the hydrophobic reduced graphene oxide (RGO) on the surface of the melamine sponge (MS) and applied in emulsion purification. The mechanics, wettability, absorption performance of the PA@RGO@MS were analyzed to identify its potential for stable demulsification. Results show that the PA@RGO@MS could purify emulsions (turbidity removal rate = 99.7%; TOC removal rate = 94.14%) in-situ in one step by simple shock absorption, profited from the hydrophilic and demulsification capability of PA, oil absorption of RGO, and wide reaction and storage space of MS. Targeting the emulsion with distinct properties (density, viscosity, and concentration) of the oil phase, the PA@RGO@MS could efficiently enable the purification. Meanwhile, the powerful flame-retardant granted from PA ensures the safe shipment and storage of sponges. The favorable cyclability (turbidity removal rate > 98.5% and TOC removal rate > 89.5% after 10 cycles) and diversified operating modes enhance the practical value of the PA@RGO@MS.

Enhancing Phytate Availability in Soils and Phytate-P Acquisition by Plants: A Review

Phytate (myo-inositol hexakisphosphate salts) can constitute a large fraction of the organic P in soils. As a more recalcitrant form of soil organic P, up to 51 million metric tons of phytate accumulate in soils annually, corresponding to ∼65% of the P fertilizer application. However, the availability of phytate is limited due to its strong binding to soils via its highly-phosphorylated inositol structure, with sorption capacity being ∼4 times that of orthophosphate in soils. Phosphorus (P) is one of the most limiting macronutrients for agricultural productivity. Given that phosphate rock is a finite resource, coupled with the increasing difficulty in its extraction and geopolitical fragility in supply, it is anticipated that both economic and environmental costs of P fertilizer will greatly increase. Therefore, optimizing the use of soil phytate-P can potentially enhance the economic and environmental sustainability of agriculture production. To increase phytate-P availability in the rhizosphere, plants and microbes have developed strategies to improve phytate solubility and mineralization by secreting mobilizing agents including organic acids and hydrolyzing enzymes including various phytases. Though we have some understanding of phytate availability and phytase activity in soils, the limiting steps for phytate-P acquisition by plants proposed two decades ago remain elusive. Besides, the relative contribution of plant- and microbe-derived phytases, including those from mycorrhizas, in improving phytate-P utilization is poorly understood. Hence, it is important to understand the processes that influence phytate-P acquisition by plants, thereby developing effective molecular biotechnologies to enhance the dynamics of phytate in soil. However, from a practical view, phytate-P acquisition by plants competes with soil P fixation, so the ability of plants to access stable phytate must be evaluated from both a plant and soil perspective. Here, we summarize information on phytate availability in soils and phytate-P acquisition by plants. In addition, agronomic approaches and biotechnological strategies to improve soil phytate-P utilization by plants are discussed, and questions that need further investigation are raised. The information helps to better improve phytate-P utilization by plants, thereby reducing P resource inputs and pollution risks to the wider environment.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Carbonate-Enhanced Transformation of Ferrihydrite to Hematite

An elevated activity of (bi)carbonate in soils and sediments (pCO₂, ∼2%) above current atmospheric CO₂ (∼0.04%) could influence the iron cycling in mineral-water interfacial chemistry. However, the impact of (bi)carbonate on mineral transformation is unclear. Here, a model short range-ordered iron oxyhydroxide, two-line ferrihydrite, was used to evaluate the impact of (bi)carbonate on mineral transformation at near-neutral pH using experimental geochemistry, X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Results showed that (bi)carbonate promoted the transformation of ferrihydrite to hematite and retarded the goethite formation. As pCO₂ increased from 408 to 20,000 ppmv at 40 °C, the transformation efficiency of ferrihydrite increased from 53 to 95%, and the formation of hematite increased from 13 to 76%. During the formation of hematite, a terminal ligand on a Fe(III)O₆ octahedral monomer such as a hydroxyl or water was displaced to form Fe(III)O₆ octahedral dimers and/or polymers. Because the Fe-O bond of ≡(Fe-O)₂-CO is much weaker than that of ≡Fe-O-H, the -O₂CO group can be more easily replaced by two terminal -OH groups; the dehydration/rearrangement between Fe(III)O₆ octahedral monomers was enhanced under high pCO₂. Results suggest that high carbonate activity is an important geochemical parameter controlling the occurrence of hematite in oxic environments and, in turn, iron cycling in the critical zone.

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite

This work aimed to determine the effect of various amounts of P admixtures in synthetic ferrihydrite on its thermal stability, transformation processes, and the properties of the products, at a broad range of temperatures up to 1000 °C. A detailed study was conducted using a series of synthetic ferrihydrites Fe5HO8·4H2O doped with phosphates at P/Fe molar ratios of 0.2, 0.5, and 1.0. Ferrihydrite was synthesized by a reaction of Fe2(SO4)3 with 1 M KOH at room temperature in the presence of K2HPO4 at pH 8.2. The products of the synthesis and the products of heating were characterized at various stages of transformation by using differential thermal analysis accompanied with X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. Coprecipitation of P with ferrihydrite results in the formation of P-doped 2-line ferrihydrite. A high P content reduces crystallinity. Phosphate significantly inhibits the thermal transformation processes. The temperature of thermal transformation increases from below 550 to 710-750 °C. Formation of intermediate maghemite and Fe-phosphates, is observed. The product of heating up to 1000 °C contains hematite associated with rodolicoite FePO4 and grattarolaite Fe3PO7. Higher P content greatly increases the thermal stability and transformation temperature of rodolicoite as well.