Sorption of 1-hydroxy-2-naphthoic acid to goethite, lepidocrocite and ferrihydrite: batch experiments and infrared study

Catalytic activity of H2O2 by goethite and lepidocrocite: Insight from 5-bromosalicylic acid removal mechanism and density functional theory calculation (ID:CHEM114760)

We have compared the elimination of 5-bromosalicylic acid (BSA) in the systems of goethite (α-FeOOH)/H₂O₂ and lepidocrocite (γ-FeOOH)/H₂O₂. The results demonstrated that BSA (10 mg L^-1) could be successfully adsorbed on α- and γ-FeOOH (0.5 g L^-1) and then effectively degraded after the addition of H₂O₂ (14.7 mM). BSA adsorption on both α- and γ-FeOOH followed pseudo-second order adsorption kinetic models, with γ-FeOOH having greater adsorption ability than α-FeOOH. In the α-FeOOH/H₂O₂ system, BSA degradation was well fitted with the pseudo-second order kinetics, whereas the oxidation in γ-FeOOH/H₂O₂ system had a two-stage pseudo-first order kinetics. Electron paramagnetic resonance (EPR) results for these two systems revealed the presence of •OH and •OOH, and further tests with radical captures demonstrated their dominance in degrading BSA. Based on the electronic structure analysis, electrons were more easily transferred from the H₂O₂ molecule to the Fe atoms of α-FeOOH, explaining the density functional theory (DFT) calculation results, which showed that α-FeOOH performed better in catalyzing the decomposition of H₂O₂. However, the free radicals are more likely to desorb from γ-FeOOH, which made the γ-FeOOH/H₂O₂ system more efficient in degrading BSA.

Mineral Nanoparticle Aggregation Alters Contaminant Transport under Flow

Iron oxyhydroxide nanoparticle reactivity has been widely investigated, yet little is still known on how particle aggregation controls the mobility and transport of environmental compounds. Here, we examine how aggregates of goethite (α-FeOOH) nanoparticle deposited on 100-300 μm quartz particles (G_agCS) alter the transport of two emerging contaminants and two naturally occurring inorganic ligands-silicates and phosphates. Bromide tracer experiments showed no water fractionation into mobile and immobile water zones in an individual goethite-coated sand (GCS) column, whereas around 10% of the total water was immobile in a G_agCS column. Reactive compounds were, in contrast, considerably more mobile and affected by diffusion-limited processes. A new simulation approach coupling the mobile-immobile equation with surface complexation reactions to surface reactive sites suggests that ∼90% of the binding sites were likely within the intra-aggregate zones, and that the mass transfer between mobile and immobile fractions was the rate-limited step. The diffusion-controlled processes also affected synergetic and competitive binding, which have otherwise been observed for organic and inorganic compounds at goethite surfaces. These results thereby call for more attention on transport studies, where tracer or conservative tests are often used to describe the reactive transport of environmentally relevant molecules.

Organic compounds alter the preference and rates of heavy metal adsorption on ferrihydrite

The availability of heavy metals in terrestrial environments is largely controlled by their interactions with minerals and organic matter, with iron minerals having a particularly strong role in heavy metal fate. Because soil organic matter contains a variety of compounds that differ in their chemical properties, the underlying impact organic matter-soil mineral associations bestow on heavy metal binding is still unresolved. Here, we systematically examine the binding of Cd, Zn and Ni by a suite of organic-ferrihydrite assemblages, chosen to account for various compound chemistries within soil organic matter. We posited that organic compound functionality would dictate the extent of association with the organic-ferrihydrite assemblages. Increased heavy metal binding to the assemblages was observed and attributed to the introduction of additional binding sites by the organic functional groups with differing metal affinities. The relative increase depended on the metal's Lewis acidity and followed the order Cd > Zn > Ni, whereas the reverse order was obtained for metal binding by pristine ferrihydrite (Ni > Zn > Cd). Citric acid-, aspartic acid- and cysteine-ferrihydrite assemblages also enhanced the metal binding rate. X-ray absorption spectroscopy revealed that the organic coating contributed significantly to Zn binding by the assemblages, despite relatively low organic surface coverage. Our findings provide valuable information on the nature of heavy metal-organic-mineral interactions and metal adsorption processes regulating their bioavailability and transport.

Study of the spectroscopic properties and deprotonation of 6-bromo-2-naphthoic acid

The present work studies the UV-Vis spectroscopic properties of 6-bromo-2-naphthoic acid (BNA) dissolved in methanol, tetrahydrofuran (THF) and ethyl acetate. It has been found that there are significant differences between the emission spectra for the samples with different concentrations of BNA in methanol. Also, the analogical changes have been observed in the case of the samples of BNA in THF and ethyl acetate which have been studied immediately after preparation and 24 h after preparation. It could be explained by deprotonation processes that take place in BNA. Moreover, it has been found that the anion formation process in BNA is more intensive in the case of methanol solvent in comparison with the cases of THF and ethyl acetate. This fact could be explained by the different polarity of these solvents. As methanol is characterized by higher polarity, the deprotonation is more intensive in the case of this solvent. To verify our assumption the quantum-chemical calculations have been also performed. It has been found that the enthalpy of dissociation (the bond-dissociation energy) is equal to 183.2, 200.2 and 205.4 kcal/mol for BNA dissolved in methanol, THF and ethyl acetate, respectively. It explains why the deprotonation process in methanol is more intensive than in THF and ethyl acetate. Thereby, the study shows that the obtained theoretical calculations are in good agreement with the experimental results.

Goethite-modified biochar restricts the mobility and transfer of cadmium in soil-rice system

Cadmium (Cd) contamination of paddy soils has raised serious concerns for food safety and security. Remediation and management of Cd contaminated soil with biochar (BC) and modified biochar is a cost-effective method and has gained due attention in recent years. Goethite-modified biochar (GB) can combine the beneficial effects of BC and iron (Fe) for remediation of Cd contaminated soil. We probed the impact of different BC and GB amendments on Cd mobility and transfer in the soil-rice system. Both BC and GB effectively reduced Cd mobility and availability in the rhizosphere and improved the key growth attributes of rice. Although BC supply to rice plants enhanced their performance in contaminated soil but application of 1.5% GB to the soil resulted in prominent improvements in physiological and biochemical attributes of rice plants grown in Cd contaminated soil. Sequential extraction results depicted that BC and GB differentially enhanced the conversion of exchangeable Cd fractions to non-exchangeable Cd fractions thus restricted the Cd mobility and transfer in soil. Furthermore, supplementing the soil with 1.5% GB incremented the formation of iron plaque (Fe plaque) and boosted the Cd sequestration by Fe plaque. Increase in shoot and root biomass of rice plants after GB treatments positively correlates with incremented chlorophyll contents and gas exchange attributes. Additionally, the oxidative stress damage in rice plants was comparatively reduced under GB application. These findings demonstrate that amending the soil with 1.5% GB can be a potential remediation method to minimize Cd accumulation in paddy rice and thereby can protect human beings from Cd exposure.

The impacts of Cu(II) complexation on gatifloxacin adsorption onto goethite and hematite

Gatifloxacin (GAT) is a new generation fluoroquinolone antibiotic and its adsorption onto iron minerals influenced by coexisting trace elements [e.g., Cu(II)] has not been well investigated. To evaluate the adsorption behavior of GAT and Cu(II) onto goethite and hematite, the complexation constants of GAT with Cu(II) were determined using potentiometric titration, and the effects of Cu(II) concentration and solution pH on GAT adsorption were investigated using batch experiments. It was observed that GAT adsorption was negatively correlated with molar concentration ratio of Cu(II) to GAT. In our experimental pH range (i.e., 3.0-10.8), the calculated main species involved in GAT adsorption were Cu(GAT^± )²⁺ and Cu(GAT^± )₂ ²⁺ under acidic to neutral conditions, and formation of Cu(GAT^- )₂ (s) facilitated the removal of GAT from solution under alkaline condition. The adsorption data were well fitted by the Freundlich model and showed high nonlinearity. In adsorption onto goethite, the primary interactions shifted from electrostatic repulsion to formation of goethite-Cu(II)-GAT ternary surface complexes with increase of GAT concentration. For hematite, electrostatic repulsion was the main inhibiting mechanism and became stronger with increase of Cu(II) concentration. Our findings suggest that it is necessary to consider the complexation between GAT and coexisting metal cations in evaluating its transport in soils rich in different iron minerals.

Glomalin-related soil protein enhances the sorption of polycyclic aromatic hydrocarbons on cation-modified montmorillonite

This study investigated the sorption of phenanthrene (as a representative PAH) by cation-modified montmorillonites (Ca-MMT and Fe-MMT) under the influence of Glomalin-related soil protein (GRSP) fractions (EE-GRSP and T-GRSP). Batch sorption studies were carried out as a function of GRSP concentrations (0-500 mg/L), results suggested that the sorption capacities of Ca-MMT and Fe-MMT for phenanthrene were greatly enhanced. The phenanthrene sorption isotherms were in good agreement with the Linear and Freundlich models (R² = 0.886-0.999). The K_d values increased from 4.14 to 60.76 L/kg for Ca-MMT and from 15.57 to 153.80 L/kg for Fe-MMT with the GRSP concentrations adding from 0 to 500 mg/L, respectively. Furthermore, the sorption of phenanthrene was higher on Fe-MMT than that on Ca-MMT. It is believed that GRSP developed a higher sorption level on Fe-MMT, resulting in higher phenanthrene sorption. Microscopic and Spectroscopic analyses confirmed that the effects of GRSP on phenanthrene sorption were attributed to the changes in the surface structure and the hydrophobic property of montmorillonites. In the sorption process, GRSP may sorb onto montmorillonites through cation-π interaction when a bridge linkage was formed, and phenanthrene bound with GRSP mainly via π-π electron donor-accepter interaction. The findings could provide an in-depth understanding of the ecological functions of GRSP and provide new insights into the pathways of PAH transport and fate in the contaminated fields.

Sorption specificity and desorption hysteresis of gibberellic acid on ferrihydrite compared to goethite, hematite, montmorillonite, and kaolinite

The pesticide gibberellic acid (GA₃) is a potential endocrine disruptor and environmental toxin; therefore, research into its environmental fate is warranted. Batch studies were conducted to investigate the sorption and desorption characteristics of GA₃ on aquifer media. The results demonstrated special sorption characteristic of GA₃ on ferrihydrite compared to goethite, hematite, montmorillonite, and kaolinite, where the sorption kinetics of GA₃ on ferrihydrite was fitted well with the pseudo-second-order, Elovich, and intra-particle diffusion models. The sorption kinetics of GA₃ on ferrihydrite indicated an initial high sorption rate followed by a slow reaction process. The initial high GA₃ sorption rate may be related to electrostatic sorption and surface complexation reactions on the outer surfaces and at the macropore entrances of ferrihydrite. While the slow step was controlled by GA₃ diffusion into mesopore of ferrihydrite. Analysis of the desorption hysteresis indicated a high hysteresis index (HI) ranging from 0.68 to 17.32, and a low desorption percentage ranging from 18 to 48%. After sufficient desorption, the calculated maximum residual GA₃ quantity due to surface complexation reactions with the ferrihydrite coordinated unsaturated sites was 9.05 ± 0.12 mg g^-1. The calculated maximum quantity of GA₃ trapped within the mesopore was 16.23 ± 0.91 mg g^-1. Graphical Abstract Schematic overview of GA₃ sorption and desorption on five minerals in groundwater.

Origin of Magnetism in Hydrothermally Aged 2-Line Ferrihydrite Suspensions

As an iron oxyhydroxide, nanosized ferrihydrite (Fh) is important in Earth science, biology, and industrial applications. However, its basic structure and origin of its magnetism have long been debated. We integrate synchrotron-based techniques to explore the chemical structures of 2-line ferrihydrite and to determine the origin of its magnetism during hydrothermal aging in air. Our results demonstrate that both the magnetism and X-ray magnetic circular dichroism (XMCD) signal of 2-line ferrihydrite are enhanced with aging time, and that XMCD spectral patterns resemble that of maghemite (γ-Fe₂O₃) rather than magnetite (Fe₃O₄). Fe L-edge and K-edge X-ray absorption spectroscopy (XAS) further indicate formation of both maghemite and hematite (α-Fe₂O₃) with increasing concentrations with longer hydrothermal aging time. Thus, magnetic enhancement with longer hydrothermal aging time is attributed to increasing maghemite concentration instead of a magnetically ordered ferrihydrite as previously reported. Moreover, L-edge and K-edge XAS spectra with different probing depths yield different ratios of these Fe oxides, which suggest the formation of a core (ferrihydrite-rich)-shell (with a mixture of both allotropes; α-Fe₂O₃ and γ-Fe₂O₃) structure during hydrothermal aging. Our results provide insights into the chemical evolution of 2-line ferrihydrite that reveal unambiguously the origin of its magnetism.

Activation of persulfate by Fe(III) species: Implications for 4-tert-butylphenol degradation

In this study, the activation of persulfate induced by Fe(III) species, including 5 kinds of iron oxhydroxides (IOs) and dissolved Fe³⁺ under dark condition were investigated. Ferrihydrite (FH) and akaganeite (AK) showed the highest activity in 4-tert-butylphenol (4tBP) removal. The 4tBP degradation rate constant decreased as the solution pH increased from pH 3.2 to 7.8 in FH/S₂O₈^2- system. However, the pH value had no significant effect on the 4tBP degradation in AK/S₂O₈^2- system. The degradation of 4tBP in Fe³⁺/S₂O₈^2- system was also performed to investigate the role of ferric species in persulfate activation. The pH dependency of 4tBP degradation rate was closely related to the speciation of Fe^III, whereas the Fe(H₂O)₆³⁺ was found to be the most active soluble iron complex form in the activation of S₂O₈^2-. 4tBP degradation was mainly due to the SO₄^- in IOs/S₂O₈^2- system, while SO₄^- and HO₂ both had great contribution on 4tBP degradation in Fe³⁺/S₂O₈^2- system. Further investigations showed clearly that 4tBP degradation efficiency was decreased significantly due to the trapping of SO₄^- by chloride. This finding may have promising implications in developing a new technology for the treatment of contaminated waters and soils, especially where Fe³⁺ species are naturally occurring.

An overview of the role of goethite surfaces in the environment

Goethite, one of the most thermodynamically stable iron oxides, has been extensively researched especially the structure (including surface structure), the adsorption capacity to anions, organic/organic acid (especially for the soil organic carbon) and cations in the natural environment and its potential application in environmental protection. For example, the adsorption of heavy metals by goethite can decrease the concentration of heavy metals in aqueous solution and immobilize; the adsorption to soil organic carbon can decrease the release of carbon and fix carbon. In this present overview, the possible physicochemical properties of the goethite surface contributing to the strong affinity of goethite to nutrients and contaminants in natural environment are reported. Moreover, these chemicals adsorbed by goethite were also summarized and the suggested adsorption mechanism for these adsorbates was elucidated, which will help us understand the role of goethite in natural environment and provide some information about goethite as an absorbent. In addition, the feasibility of goethite used as catalyst carrier and the precursor of NZVI was proposed for removal of environmental pollution.

New insight into the effect of the formation environment of ferrihydrite on its structure and properties

The adsorption and photocatalysis activities of ferrihydrites formed under different environments are porosity-, surficial- and local structure-dependent.

Bio-inspired fabrication of hierarchical FeOOH nanostructure array films at the air-water interface, their hydrophobicity and application for water treatment

Hierarchical FeOOH nanostructure array films constructed by different nanosized building blocks can be synthesized at the air-water interface via a bio-inspired gas-liquid diffusion method. In this approach, poly(acrylic acid) (PAA) as a crystal growth modifier plays a crucial role in mediating the morphology and polymorph of FeOOH crystals. With the increase of PAA concentration, the shape of the building blocks assembling into FeOOH films can be tailored from nanosheets, to rice spikes, then to branched fibers, and finally to nanowires. What is more, a low concentration of PAA will induce the formation of α-FeOOH, while a high one could stabilize FeOOH in the form of the γ-FeOOH phase. After being modified with a thin layer of polydimethylsiloxane (PDMS), the as-prepared FeOOH films exhibited strong hydrophobicity with water contact angles (CA) from 134° to 148° or even superhydrophobicity with a CA of 164° in the sample constructed by nanosheets. When the FeOOH nanostructures were dispersed in water by ultrasound, they displayed quite promising adsorption performance of heavy metal ions for water treatment, where the highest adsorption capacity can reach 77.2 mg·g⁻¹ in the sample constructed by nanowires. This bio-inspired approach may open up the possibilities for the fabrication of other functional nanostructure thin films with unique properties.

Effect of ethylenediamine-N,N'-disuccinic acid on Fenton and photo-Fenton processes using goethite as an iron source: optimization of parameters for bisphenol A degradation

The main disadvantage of using iron mineral in Fenton-like reactions is that the decomposition rate of organic contaminants is slower than in classic Fenton reaction using ferrous ions at acidic pH. In order to overcome these drawbacks of the Fenton process, chelating agents have been used in the investigation of Fenton heterogeneous reaction with some Fe-bearing minerals. In this work, the effect of new iron complexing agent, ethylenediamine-N,N'-disuccinic acid (EDDS), on heterogeneous Fenton and photo-Fenton system using goethite as an iron source was tested at circumneutral pH. Batch experiments including adsorption of EDDS and bisphenol A (BPA) on goethite, H(2)O(2) decomposition, dissolved iron measurement, and BPA degradation were conducted. The effects of pH, H(2)O(2) concentration, EDDS concentration, and goethite dose were studied, and the production of hydroxyl radical ((•)OH) was detected. The addition of EDDS inhibited the heterogeneous Fenton degradation of BPA but also the formation of (•)OH. The presence of EDDS decreases the reactivity of goethite toward H(2)O(2) because EDDS adsorbs strongly onto the goethite surface and alters catalytic sites. However, the addition of EDDS can improve the heterogeneous photo-Fenton degradation of BPA through the propagation into homogeneous reaction and formation of photochemically efficient Fe-EDDS complex. The overall effect of EDDS is dependent on the H(2)O(2) and EDDS concentrations and pH value. The high performance observed at pH 6.2 could be explained by the ability of O (2) (•-) to generate Fe(II) species from Fe(III) reduction. Low concentrations of H(2)O(2) (0.1 mM) and EDDS (0.1 mM) were required as optimal conditions for complete BPA removal. These findings regarding the capability of EDDS/goethite system to promote heterogeneous photo-Fenton oxidation have important practical implications for water treatment technologies.

Transport of two naphthoic acids and salicylic acid in soil: experimental study and empirical modeling

In contrast to the parent compounds, the mechanisms responsible for the transport of natural metabolites of polycyclic aromatic hydrocarbons (PAH) in contaminated soils have been scarcely investigated. In this study, the sorption of three aromatic acids (1-naphthoic acid (NA), 1-hydroxy-2-naphthoic acid (HNA) and salicylic acid (SA)) was examined on soil, in a batch equilibrium single-system, with varying pH and acid concentrations. Continuous flow experiments were also carried out under steady-state water flow. The adsorption behavior of naphthoic and benzoic acids was affected by ligand functionality and molecular structure. All modeling options (equilibrium, chemical nonequilibrium, i.e. chemical kinetics, physical nonequilibrium, i.e. surface sites in the immobile water fraction, and both chemical and physical nonequilibrium) were tested in order to describe the breakthrough behavior of organic compounds in homogeneously packed soil columns. Tracer experiments showed a small fractionation of flow into mobile and immobile compartments, and the related hydrodynamic parameters were used for the modeling of reactive transport. In all cases, the isotherm parameters obtained from column tests differed from those derived from the batch experiments. The best accurate modeling was obtained considering nonequilibrium for the three organic compounds. Both chemical and physical nonequilibrium led to appropriate modeling for HNA and NA, while chemical nonequilibrium was the sole option for SA. SA sorption occurs mainly in mobile water and results from the concomitancy of instantaneous and kinetically limited sites. For all organic compounds, retention is contact condition dependent and differs between batch and column experiments. Such results show that preponderant mechanisms are solute dependent and kinetically limited, which has important implications for the fate and transport of carboxylated aromatic compounds in contaminated soils.

Surface complexation of 2,5-dihydroxybenzoic acid (gentisic acid) at the nanosized hematite-water interface: an ATR-FTIR study and modeling approach

In this study, characteristic interactions of 2,5-dihydroxybenzoic acid (or gentisic acid, GA) with the surface of 15-nm-sized hematite (α-Fe2O3) were studied by combining batch macroscopic experiments, in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic investigations, and surface complexation modeling. A correlation between the pH, the amount of adsorbed GA, and the amount of Fe(III) released from the hematite surface was observed, whereas the dissolution of hematite nanoparticles became significant only at low pH and high ligand loading. From the ATR-FTIR results, two aqueous complex structures have been identified depending on pH. At the hematite-water interface, the occurrence of one deprotonated inner-sphere "bidentate" complex and one outer-sphere complex was suggested through all of the investigated pH range. At high surface coverage, variations of vibrational band intensities were observed, suggesting the occurrence of nonspecific molecular interactions. The macroscopic results (i.e., GA batch sorption and the ligand-promoted dissolution of hematite) obtained under a wide range of experimental conditions corroborated the ATR-FTIR microscopic findings. GA adsorption was described by a surface complexation model fitted to pH-adsorption curves with 1 mM sorbate concentration in the pH range of 3-9. Two surface complexes (one outer-sphere species (≡FeOH2)2···H2L((1+,1-)) and one inner-sphere species (≡Fe)2H2L) were proposed using the three-plane model. The inner-sphere complexes were predominant at low pH values, and the relative concentrations of the outer-sphere species increased with the pH increase. The formation of inner-sphere complexes at acidic pH values can promote the dissolution of nanosized hematite. At high solute loading, GA oxidation into carboxybenzoquinone compounds by ferric species was suspected, suggesting the occurrence of a redox reaction analogous to that of hydroquinone compounds.

Sorption of two naphthoic acids to goethite surface under flow through conditions

While the transport of low molecular weight organic acids was widely investigated, little is known about the mobility of the carboxylated aromatic compounds containing double rings in natural porous media. This study combines macroscopic (batch and column), microscopic (vibration spectroscopy), and surface complexation modeling to evaluate the mobility of two PAH degradation products: naphthoic acid (1-naphthoic acid (NA) and 1-hydroxy-2-naphthoic acid (HNA)), in porous media consisting of goethite-coated sand. The loss of ligands from aqueous solution was attributed to (1) a hydrogen-bonded surface complex present over the entire 3-10 pH range as well as protonated (2) surface and (3) bulk precipitates below pH 5. Mobility in column experiments was strongly affected by ligand functionality. Adsorption breakthrough predictions that make use of surface complexation parameters accurately predicted NA mobility. Those for HNA however predicted much less adsorption reactions than in the batch sorption experiments. Additional breakthrough experiments and test calculations confirmed that these differences were not related to sorption kinetics. HNA adsorption breakthrough data could only be predicted by lowering intrinsic complexation constant of the formation of hydrogen-bonded species, thereby suggesting modifications of the diffuse layer properties under flow conditions. These findings have strong implications in the assessment and prediction of contaminant transport and environmental remediation.

Adsorption and oxidative transformation of phenolic acids By Fe(III)-montmorillonite

Phenolic acids participate in various soil processes and are of great concern due to their allelopathic activity. The interactions of phenolic acids (ferulic, p-coumaric, syringic, and vanillic) with montmorillonite enriched with Fe(III) was investigated. Adsorption of the phenolic acids on Fe(III)-montmorillonite was accompanied by their oxidative transformation and formation of Fe(II). Oxidative transformation of phenolic acids was affected by their molecular structure. The order of maximal transformation at the initial acid concentration of 20 mg/L on the surface of Fe(III)-montmorillonite was ferulic (94%), syringic (60%), p-coumaric (35%), and vanillic (25%). Benzoic acid which was used as a reference aromatic compound exhibited only 5% transformation. Removal of the phenolic acids from solution increased with decreasing pH. LC-MS analysis demonstrated the presence of dimers, trimers, and tetramers of ferulic acid on the surface of Fe(III)-montmorillonite. Oxidation and transformation of ferulic acid were more intense on the surface of Fe(III)-montmorillonite as compared to Fe(III) in solution due to stronger complexation on the clay surface. The results of the current study demonstrate the importance of Fe(III)-clay surfaces for the abiotic formation of humic materials and for the transformation of aromatic (phenolic) pollutants.

The microstructure of ferrihydrite and its catalytic reactivity

Ferrihydrites were prepared by different mixing procedures of ferric and sodium hydroxide solutions. The microstructure of ferrihydrites was characterized by XRD, TG-DTA, BET surface and EXAFS. The catalytic activity of ferrihydrites toward degradation of Mordant Yellow 10 (MY10) and decomposition of hydrogen peroxide was evaluated. The results show that ferrihydrites, prepared by different procedures, display various physico-chemical properties, which can be well explained by the standard multiphase structural model of ferrihydrite.

Sorption and transport of salicylate in a porous heterogeneous medium of silica quartz and goethite

Among transport studies of solutes in porous media, few works have combined microscopic speciation with macroscopic-scale investigations to describe the impact of antecedent sorbed silica on the transport of organic ligands in porous heterogeneous media. In this study, the sorption of salicylate (SA) to goethite-coated sand (GCS) was investigated under static and dynamic conditions by combining batch experiments and column tests with infrared spectroscopy. On the basis of infrared spectra, the salicylate adsorption was described by one type of iron site and a mononuclear bidentate surface complex. The intrinsic complexation constant deduced from batch modeling was successfully applied to estimate the sorbed amount under flow through conditions at various water velocities (0.038-0.768 cm/min). The shape of the breakthrough curve of SA was characterized by two fronts in both SA concentration and pH. This behavior could be likely explained by the mobilization of initially adsorbed silica from goethite surface upon SA sorption. The SA breakthrough can be interpreted as retention of SA on available surface sites up to their saturation and then on additional reactive sites, becoming free due to silicate desorption. This present work demonstrated the importance of sorbed silicate on Fe-oxides in the prediction of reactive transport of organic species on natural surfaces.

Effect of combined Bacillus subtilis on the sorption of phenanthrene and 1,2,3-trichlorobenzene onto mineral surfaces

In the natural environment, minerals are often associated with coexisting microorganisms. These interactions have profound impacts on the fate of a wide variety of contaminants. However, little information is available on the sorption of hydrophobic organic compounds (HOC), such as polycyclic aromatic hydrocarbons and chlorinated benzenes, onto the composites of minerals with bacteria, and knowledge of the influence of combined bacteria on HOC sorption to minerals is limited. In our study, sorption isotherms of phenanthrene (Phen) and 1,2,3-trichlorobenzene (TCB) onto Bacillus subtilis, minerals (kaolinite, montmorillonite, and goethite), and mineral-B. subtilis composites were studied to determine the role of B. subtilis in sorption. For pure mineral systems, the order of Phen and TCB sorption affinity was montmorillonite > kaolinite > goethite. For mineral-B. subtilis composites, the trend was montmorillonite > goethite > kaolinite, consistent with that of their ability to combine with bacteria. The coating of B. subtilis with minerals enhanced the sorption due to the strong sorption of Phen and TCB onto B. subtilis cells and the increase of total organic carbon of minerals. With increasing B. subtilis concentration, sorption of Phen and TCB on pure B. subtilis cells decreased, but sorption on kaolinite surface increased. Sodium azide can greatly reduce sorption capacity but increases sorption linearity for B. subtilis and mineral-B. subtilis composites. Compared with TCB, Phen had higher sorption affinity due to its high hydrophobicity. Our results may be useful for understanding the role of bacteria in regulating the distribution and transport of HOCs in the environment.

Fenton-like oxidation of Rhodamine B in the presence of two types of iron (II, III) oxide

The catalytic efficiency of iron (II, III) oxide to promote Fenton-like reaction was examined by employing Rhodamine B (RhB) as a model compound at neutral pH. Two types of iron (II, III) oxides were used as heterogeneous catalysts and characterized by XRD, Mössbauer spectroscopy, BET surface area, particle size and chemical analyses. The adsorption to the catalyst changed significantly with the pH value and the sorption isotherm was fitted using the Langmuir model for both solids. Both sorption and FTIR results indicated that surface complexation reaction may take place in the system. The variation of oxidation efficiency against H(2)O(2) dosage and amount of exposed surface area per unit volume was evaluated and correlated with the adsorption behavior in the absence of oxidant. The occurrence of optimum amount of H(2)O(2) or of exposed surface area for the effective degradation of RhB could be explained by the scavenging effect of hydroxyl radical by H(2)O(2) or by iron oxide surface. Sorption and decolourization rate of RhB as well as H(2)O(2) decomposition rate were found to be dependent on the surface characteristics of iron oxide. The kinetic oxidation experiments showed that structural Fe(II) content strongly affects the reactivity towards H(2)O(2) decomposition and therefore RhB decolourization. The site density and sorption ability of RhB on surface may also influence the oxidation performance in iron oxide/H(2)O(2) system. The iron (II, III) oxide catalysts exhibited low iron leaching, good structural stability and no loss of performance in second reaction cycle. The sorption on the surface of iron oxide with catalytic oxidation using hydrogen peroxide would be an effective oxidation process for the contaminants.

Interactions of hydrophobic fractions of dissolved organic matter with Fe(3+) - and Cu(2+)-montmorillonite

Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu(2+)- and Fe(3+)-montmorillonite. Adsorption of both samples was higher on Fe(3+)-montmorillonite than on Cu(2+)-montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe(3+)-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe(3+)-montmorillonite surfaces. Adsorption of HoA on Cu(2+)-montmorillonite and HoN on Fe(3+)- and Cu(2+)-montmorillonites was governed mainly by van der Waals interactions. Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe(3+)-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe(3+)-montmorillonite similar to goethite.