Chromate reduction on humic acid derived from a peat soil--exploration of the activated sites on HAs for chromate removal

Effects of soil pH and organic carbon content on in vitro Cr bioaccessibility in Ultisol, Alfisol, and Inceptisol

Hexavalent chromium (Cr(VI)) is a toxic heavy metal and its mobility and bioaccessibility in soils are influenced by soil properties. In this study, the soil pH and organic carbon contents of Ultisol, Alfisol, and Inceptisol were adjusted before they were polluted with 230 mg kg^-1 Cr(VI). Alkaline digestion, sequential extraction, and an in vitro experiment were conducted to study the valence state, species, and bioaccessibility of Cr in the soils. The results showed that a high soil pH was not favorable for reduction of Cr(VI); therefore the Cr(VI) and exchangeable Cr contents were positively related to soil pH. Soil organic carbon promoted the reduction of Cr(VI). Almost all Cr(VI) was reduced to Cr(III) when the soil organic carbon content reached 10 g kg^-1. Chromium bioaccessibility in simulated gastric and intestinal phase solutions was influenced by Cr(VI) and Cr(III) adsorption/desorption, dissolution/precipitation, and redox reactions. Chromium bioaccessibility differences between the gastric and intestinal phases were associated with the Cr(VI)/Cr(III) ratio. Acidic conditions and a high organic carbon content promoted the conversion of Cr(VI) to Cr(III). When soil pH was increased from 4.01 to 5.85, Cr(VI) in Alfisol without the addition of humic acid increased from 96.38 to 174.78 mg kg^-1, the exchangeable Cr proportion increased from 9.7% to 22.6%, and Cr bioaccessibility increased from 41.29% to 49.14% in the gastric phase and from 41.32% to 48.24% in the intestinal phase. When the organic content increased from 3.95 to 9.28 g kg^-1 in Alfisol, Cr(VI) content decreased from 167.66 to 20.52 mg kg^-1, which led to a decrease in Cr bioaccessibility from 49.15% to 13.8% in the gastric phase and from 45.85% to 7.67% in the intestinal phase. Therefore, acidic conditions and increasing soil organic carbon levels can reduce the health risk posed by Cr in soils.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Structural characterization using 2D NMR spectroscopy and TMAH-GC × GC-MS: Application to humic acids from soils of an integrated agricultural system and an Atlantic native forest

Understanding the chemical make-up of soils and their structure is critical for constraining the role of soil organic matter (SOM) into the global biogeochemical cycles, as well as to understand the capability of SOM to sequester carbon and mitigate greenhouse gas emissions. Here, we use two-dimensional ¹H-¹³C heteronuclear single quantum coherence nuclear magnetic resonance (2D ¹H-¹³C HSQC NMR) spectroscopy to structurally characterize the most refractory component of SOM, the humic acid (HA). The observations from 2D ¹H-¹³C HSQC NMR were coupled with lignin phenol and fatty acid measurements using tetramethylammonium hydroxide (TMAH) thermochemolysis - two-dimensional gas chromatography - mass spectrometry (TMAH-GC × GC-MS). We studied humic acids extracted from an integrated Crop - Livestock - Forest System (CLFS) agricultural area and an undisturbed Atlantic Native Forest (NF) area. We evaluated soils from two different depths: the topsoil (0-20 cm) and subsoil (60-100 cm) layers, and reveal the presence of oxidized ligninaceous phenols as we had previously hypothesized. Collectively, our results indicate that there are significant oxidative processes with increasing soil depth which are more pronounced in the CLFS relative to the NF area. Degradation of stearic acid with increasing depth in the CLFS soils indicated that the CLFS is more microbiologically active than NF. Therefore, CLFS is less biochemically stable than we originally perceived. The enhanced bio-reactivity of CLFS is likely driving the enhanced carbon sequestration in the CLFS soils. This is perhaps due to the diversity of biomass remnants available at the CLFS soil rhizosphere which allows for more different types of biomass to be sequestered as oxidized ligninaceous phenols. Our results employing structural characterization with 2D ¹H-¹³C HSQC NMR and TMAH-GC × GC-MS provide a new layer of knowledge about the practice of integrated agricultural systems and allow us to understand the structure and fate of sequestered carbon in soils from different soil environments.

Aging shapes Cr(VI) speciation in five different soils

To make sound decisions regarding management of heavy metal contamination in soils, it is necessary to understand contaminant transformations over extended periods. In this study, sequential extraction methods were applied to quantify the changes of Cr fractions [available Cr(VI), immobile Cr(VI) and immobile Cr(III)] in five contrasting soils spiked with Cr(VI) over a 240-day incubation. Results showed that available Cr(VI) in soils continually decreased during aging, with a sharp decline occurring in the first 30 days. The best fit of available Cr(VI) data was obtained using an Elovich model for Brunisol and Anthrosol-1, a fractional power model for Anthrosol-2, and a pseudo first-order kinetic model for Luvisol-1 and Luvisol-2. After aging for 240 days, immobile Cr(VI) increased by 4.5-31% and immobile Cr(III) increased by 68-95% of total spiked Cr(VI) in Brunisol, Anthrosol-1 and Anthrosol-2. The two Luvisol soils had relatively high reduction rates with no Cr(VI) immobilized. A multireaction model was developed in MATLAB Simulink toolbox to describe transformation flow rates among soluble Cr(VI), adsorbed Cr(VI), immobilized Cr(VI) and immobilized Cr(III) in soils with aging. We conclude that (i) Cr(VI) reduction and immobilization were occurring concurrently in soils and competing for available Cr(VI) species; (ii) Cr(VI) reduction is favored by low soil pH and high organic carbon, while Cr(VI) immobilization occurs with cations (such as Ca²⁺) and Fe oxides.

Ionic Liquid Extraction Behavior of Cr(VI) Absorbed on Humic Acid-Vermiculite

Cr(VI) can be released into soil as a result of mining, electroplating, and smelting operations. Due to the high toxicity of Cr(VI), its removal is necessary in order to protect ecosystems. Vermiculite is applied in situations where there is a high degree of metal pollution, as it is helpful during the remediation process due to its high cation exchange capacity. The Cr(VI) contained in the vermiculite should be extracted in order to recover it and to reduce the impact on the environment. In this work, adsorption equilibrium data for Cr(VI) in a simulated sorbent for soil remediation (a mixture that included both humic acid (HA) and vermiculite) were a good fit with the Langmuir isotherm model. The simulated sorbent for soil remediation was a favorable sorbent for Cr(VI) when it was in the test soil. An ionic liquid, [C₄mim]Cl (1-butyl-3-methylimidazolium chloride), was studied to determine its efficiency in extracting Cr(VI) from the Cr- contaminated simulated sorbent in soil remediation. At 298 K and within 30 min, approximately 33.48 ± 0.79% of Cr(VI) in the simulated sorbent in soil remediation was extracted into [C₄mim]Cl. Using FTIR spectroscopy, the absorbance intensities of the bands at 1032 and 1010 cm⁻¹, which were attributed to C-O bond stretching in the polysaccharides of HA, were used to detect the changes in HA in the Cr-contaminated simulated sorbent for soil remediation before and after extraction. The results showed that Cr(VI) that has been absorbed on HA can be extracted into [C₄mim]Cl. Using ¹H NMR, it was observed that the 1-methylimizadole of [C₄mim] Cl played an important role in the extraction of Cr(VI), which bonded with HA on vermiculite and was able to be transformed into the [C₄mim]Cl phase.

New insight into the adsorption mechanism of PCP by humic substances with different degrees of humification in the presence of Cr(VI)

Humic substances (HSs) have great retention effects on pentachlorophenol (PCP) migration in subsurface environment, but the adsorption mechanism of PCP by HSs with various aromatic/aliphatic moieties and acidic functional groups in the presence of Cr(VI) is still unclear. In this study, the adsorption mechanism of PCP by undissolved humic acid (HA) and humin (HM) extracted from peat, black soil, lignite and coal was investigated under the presence of Cr(VI). According to the results, HA samples had much lower adsorption capacity for hydrophobic PCP than HM samples due to their higher contents of hydrophilic polar oxygen-containing functional groups. In respect to PCP adsorption mechanism, the molecular unsaturation of HSs associated with humification degree was found to be the determinant instead of polarity. Notably, after reacting with Cr(VI), significant decreasing of PCP adsorption quantities occurred on HSs extracted from lignite and coal with higher degrees of unsaturation (H/C < 0.64), while HSs extracted from peat and black soil with lower degrees of unsaturation (H/C > 0.83) kept almost unchanged, which can be attributed to the much higher reactivity of aromatic domains of HSs for Cr(VI) reduction compared with aliphatic moieties. This indicated that the adsorption mechanism of PCP by HSs with higher and lower degrees of unsaturation might be respectively driven by π-π interaction and hydrophobic interaction. This study highlighted the diverse adsorption mechanisms of PCP on HSs with different degrees of humification, and emphasized the coexisting Cr(VI) only have significant effect on PCP adsorption by HSs with higher humification degrees instead of the lower ones.

Synchronous reduction-fixation of reducible heavy metals from aqueous solutions: Application of novel mesoporous MFT/SBA-15 composite materials

The usual treatment for Cr(VI)-contaminated wastewater primarily included reduction, adsorption, and the subsequent separation of the Cr-laden adsorbent. Among these factors, the adsorbent is the most critical factor in determining Cr removal efficiency. In this study, a novel melamine-formaldehyde-thiourea (MFT) chelating resin/mesoporous silica composite material (MFT/SBA-15) was synthesized via a co-condensation method and used for the reduction and fixation of Cr(VI)-contaminated water. Cr(VI) adsorption onto MFT/SBA-15 obeyed the pseudo-second-order model, and the chemical adsorption was the rate-limiting step in the adsorption process. Also it followed the Langmuir adsorption model, with single molecular layer adsorption characteristics. The organic components within MFT/SBA-15 were the core functional groups for Cr(VI) adsorption, and the formation of a coordination bond (CS→Cr) between the lone electron pairs of the S atom and Cr during the adsorption process led to the synchronous reduction-fixation processes of Cr(VI). These synchronous effects were further demonstrated for other reducible heavy metals, including As(V) and Cu(II), but negligibly observed in chemically stable elements, such as Zn(II), Ni(II), Pb(II), Cd(II), and As(III). The novel mesoporous MFT/SBA-15 materials combine the advantages of the chelating resin and mesoporous silica and have excellent potential for the wastewater treatment of reducible heavy metals through synchronous reduction-fixation.

Relationship of arsenic and chromium availability with carbon functional groups, aluminum and iron in Little Washita River Experimental Watershed Reservoirs, Oklahoma, USA

Sediment from three reservoirs located in the Little Washita River Experimental Watershed (LWREW) in Oklahoma, USA with contrasting dominant land uses were analyzed for total and extractable concentrations of arsenic (As) and chromium (Cr), and the potential ecologic risk to benthic organisms. Extractable As ranged from 0.24 to 1.21 mg kg^-1, in the order grazing>cropland>forest and 0.13-0.58 mg kg^-1 for extractable Cr, in the order of forest>grazing>cropland. However, only approximately < 1.5% of total As and < 4% of total Cr were extractable. Total As ranged from 16.2 to 141 mg kg^-1 and total Cr ranged from 5.06 to 40.1 mg kg^-1 both in the order of cropland>grazing>forest. The sediment exhibited an alkaline pH (8.0-8.7). As sorption exhibited a positive relationship with Al (r = 0.9995; P = 0.0001), Fe (r = 0.9829; P = 0.0001), and C (r = 0.4090; P = 0.0017) and Cr correlated positively with Al (r = 0.9676 P = 0.0001), Fe (r = 0.9818; P = 0.0001), and C (r = 0.3368; P = 0.0111). In addition, both As and Cr exhibited positive relationships with carbon (C) functional groups in the order of O-alkyl C> methoxyl C> alkyl C> aromatic C> carboxyl C> phenolic C. The sediment concentration analysis results illustrated that As in all reservoirs exceeded their respective Threshold Effect Level (TEL) and/or Probable Effect Level (PEL) indicating that existing concentrations of metals in these sediments were sufficiently high to cause adverse effects. However, Cr concentrations in all reservoirs evaluated was lower compared to the TEL and PEL.

CrVI reductive transformation process by humic acid extracted from bog peat: Effect of variables and multi-response modeling

The present paper reports the efficiency of bog peat-derived humic acid (HA) in the reductive removal of hexavalent chromium (Cr^VI) from aqueous solution as affected by solution pH, the dose of Fe^III and reaction time (numeric variables) and light irradiation (categorical variable). A three-level Box-Behnken design (BBD) applied to design experimental matrix, model the effects and interactions of variables on four determined responses (residual concentration of dissolved Cr^VI, dissolved Cr^III, dissolved Fe^II and total Cr^VI) and optimize the experimental conditions for highest Cr^VI removal efficiency (Cr^VI RE). Reaction mechanisms are also well discussed. Regression models were developed and analyzed by the ANOVA test and models determination coefficient R². Obtained models were significant (F values > 13) and an excellent relationship between experimental and predicted responses (R²: 98.1-99.6%) was observed. The optimum conditions were established corresponding to the residual concentration of dissolved Cr^VI as an index for Cr^VI removal efficiency (RE). In the dark system, the highest Cr^VI RE (98.1%) was obtained under the following conditions: pH = 1, reaction time = 7 d and Fe^III dosage = 0.110 mM. In the light-irradiated system, the optimal Cr^VI RE of 98.3% was observed in pH = 1, reaction time = 5 d and Fe^III dosage = 0.075 mM. Almost all reduced Cr^III remained in the solution even at high pH value. No adsorption or precipitation of Cr^III on the HA surface at pH 5 was confirmed by surface analyses of HA using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).

Evaluating Cr behaviour in two different polluted soils: Mechanisms and implications for soil functionality

This work investigates the mechanisms determining Cr speciation and availability in two different soils polluted with two chromium sources (an industrial sludge, highly polluted with Cr, and Cr(VI) solution) and the influence of these parameters on the recovery of the soil functions related with biological quality and plant growth. The experiment was carried out in greenhouse conditions using 36 pots of 17 kg for the growth of Silene vulgaris for 21 months. Logistic Regression Model using Lasso estimator shows that soil organic matter (SOM) and pH control Cr availability in studied soils. In soils treated with the sludge, X ray Absorption spectroscopy showed that Cr was present as Cr(III), biological quality indicators increased and plants were able to grow. However, in soils polluted with Cr(VI), Cr availability was significantly different in the two soils. In the alkaline and poor in organic matter soil, 12% of Cr(VI) remained in the soil leading to the decrease of soil quality indicators and the total inhibition of plant growth. In the neutral soil, Cr(VI) was totally reduced to Cr(III) by soil organic matter (SOM), quality indicators were not affected and plants grown properly. Infrared Spectroscopy showed that different functional groups reacted with Cr in the two soils. This study highlights the importance to understand the mechanisms underlaying Cr redox and adsorption reactions in Cr polluted soils as they determine the potential recovery of the functions related with biological quality indicators and plant growth. The methodology proposed allows this study in complex soil samples at realistic concentrations and may be useful for risk assessment and for the planning of managing strategies in Cr polluted soils.

Organic fragments newly released from heat-treated peat soils create synergies with dissolved organic carbon to enhance Cr(VI) removal

Surface fires occur naturally or anthropogenically and can raise the temperature at the soil surface up to 600 °C. The heat derived from the surface fire can be subsequently transferred into CO₂-enriched subsoils. As a result, the chemical compositions of soil organic matter (SOM) may be altered in fire-impacted anaerobic environments, indirectly influencing the redox transformations of pollutants, such as Cr(VI). In this study, a peat soil was heated up to 600 °C with limited air flow to simulate the effects of heat on the SOM during surface fire events. Then, Cr(VI) removal, including reduction and sorption, by the heat-treated peat soils was determined in relation to changes in the soil organic components. The results showed that the amount of O-containing functional groups, -CH₂/-CH₃ units of aliphatic groups, and dissolved organic carbon (DOC) in the SOM gradually decreased with an increase in the heating temperature. The removal of 0.1932 mM Cr(VI) did not exhibit a consistent decline along with the changes in these soil components. The heating temperatures of 200 and 250 °C were the thresholds that led to the decomposition of temperature-sensitive soil organic components such as lignin and other labile SOM. Such newly released organic fragments synergized lignin-like substances and carboxyl groups, resulting in up to 99% removal of the initially added Cr(VI). As the heating temperatures were increased from 300 to 600 °C, Cr(VI) reduction decreased from 66% to 20%. The black carbon-like materials and/or aromatic-containing moieties were the major components responsible for Cr(VI) reduction in 600°C-treated peat soils.

Redox reactions between chromium(VI) and hydroquinone: Alternative pathways for polymerization of organic molecules

Chromium (VI) reduction by organic compounds is one of the major pathways to alleviate the toxicity and mobility of Cr(VI) in the environment. However, oxidative products of organic molecules receive less scientific concerns. In this study, hydroquinone (H₂Q) was used as a representative organic compound to determine the redox reactions with Cr(VI) and the concomitant oxidative products. Spectroscopic analyses showed that Cr(III) hydroxides dominated the precipitates produced during redox reactions of Cr(VI) and H₂Q. For the separated filtrates, the acidification induced the oxidative polymerization of organic molecules, accompanied with the complexation with Cr(III). The aromatic domains dominated the chemical structures of the black and fluffy organic polymers, which was different to the natural humic acids due to the shortage of aliphatic chains. Results of linear combination fitting (LCF) for Cr K-edge X-ray absorption near edge structure (XANES) spectra demonstrated that up to 90.4% of Cr inventory in precipitates derived after the acidification of filtrates was Cr(III) complexed with humic-like polymers, suggesting that Cr(III) possibly acted as a linkage among organic molecules during the polymerization processes of H₂Q. This study demonstrated that Cr(VI) may lead to the polymerization of organic molecules in an acidic solution, and thus, it could raise scientific awareness that the oxidative decomposition of organic molecules may not be the only pathway while interacting with the strong oxidant of Cr(VI).

Iron mineral-humic acid complex enhanced Cr(VI) reduction by Shewanella oneidensis MR-1

Microorganisms, iron minerals, and humic acid are widely common in the soil and water environment and closely interact within environmental processes. In this study, the Cr(VI) removal by Shewanella oneidensis MR-1 (S. oneidensis) was examined in the presence of goethite and humic acid (HA) to mimic the real environment situation. Scanning electron microscopy (SEM) combined with energy disperse spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) technologies were used to probe the Cr(VI) reduction mechanism. Our results showed that S. oneidensis alone could reduce 65% of 1.0 mM Cr(VI) after 8 h of the reduction process. Meanwhile, Cr(VI) reduction rate was declined to 56% in the presence of goethite or humic acid. Contrary, the Cr(VI) reduction rate was mightily increased to 79% by the ternary complex of S. oneidensis-goethite-HA where reduced humic acid (HA_red) acted as electron shuttles and diminished the bacterial adhesion to the goethite surface thereby enhanced electron transfer and increased the extent of Cr(VI) reduction by 1.3 fold. XPS analysis indicated that Cr(VI) was reduced to Cr(III), and the final yields were Cr(OH)₃ and Cr₂O₃ precipitated on the surface of bacterial cells. S. oneidensis could also reduce Fe(III) in goethite to Fe(II), which in turn reduced Cr(VI). These results suggested that iron mineral-humic acid complex could enhance the microbial reduction of Cr(VI) and revealed the promotion role of HA in the Cr(VI) bioreduction process. This study affords inclusive insights on the Cr(VI) reduction kinetics and mechanisms in the most complicated systems.

Evaluation of Commercial Humic Substances and Other Organic Amendments for the Immobilization of Copper Through 13C CPMAS NMR, FT-IR, and DSC Analyses

The application of organic amendments to contaminated agricultural lands can immobilize metals and improve soil conditions. The chemical structures and long-term stability of commercial humic substances and other composted organic amendments (sheep and horse manure, vermicompost, pine bark, and pruning waste) were analyzed using 13C CPMAS NMR, FT-IR, and DSC to evaluate their use in soil remediation. The interactions of humic substances and manure with Cu (0 and 5000 mg kg−1) at different pH (2.5 and 5.0) were studied through a batch adsorption experiment observing the changes in their molecular structure using spectroscopic techniques. Humic substances exhibited high aromaticity and phenolic and carboxylic group content, with great affinity for Cu complexation. Humic substances and pruning waste were the most stable due to their high recalcitrant organic matter contents, whereas manure was the least stable, given the labile nature of its organic matter content. There were considerable changes in the carboxylic and phenolic groups of humic substances with pH, and also with Cu, albeit in a lesser extent, especially at pH 5.0, suggesting the great sorption capacity of humic substances and the key role of pH and these functional groups in metal complexation. Manure did not exhibit such changes. Commercial humic substances could be useful amendments for the remediation of contaminated agricultural soils due to their high sorption capacities and long-term stability.

Remediation performance and mechanism of hexavalent chromium in alkaline soil using multi-layer loaded nano-zero-valent iron

Remediation of soil chromium (Cr) pollution is becoming more and more urgent. In this study, a multi-loaded nano-zero-valent iron (nZVI) material (CNH) was prepared by carboxymethyl cellulose (CMC) and humic acid (HA) as dispersant and support agent, respectively, and the remediation effect of CNH, HA and CN (CNH without HA) for Cr contaminated soil was investigated within 90 d cycle. After 7 d treatment of CNH, the HOAc-extractable Cr decreased significantly. After the 90 d remediation, the HOAc-extractable Cr decreased most in the treatment of 3% CNH, about 74.48% lower than control. All treatments eventually caused different decline of soil pH, with a range of 0.12-0.54, in which the CNH treatment group had the least depression. HA loading significantly weakened the toxicity of nZVI, resulting in the higher soil microbial quantity and enzyme activities compared with CN. Additionally, the improvement of soil microecology by CNH and HA was positively correlated with the ratio of application, while CN was negatively correlated (except FDA enzyme activity) with these indexes. These results emphasized the potential of the synthesized CNH as a promising material to remediate Cr contaminated soil. Furthermore, details of possible mechanistic insight into the Cr remediation were carefully discussed.

Removal and simultaneous reduction of Cr(VI) by organo-Fe(III) composites produced during coprecipitation and coagulation processes

Composites formed during the coprecipitation and/or coagulation of ubiquitous dissolved organic matter (DOM) and Fe in natural and waste water systems might be potential scavengers for Cr(VI) in terms of sorption and reduction. Our objective here was to determine sorption and simultaneous reduction of Cr(VI) on organo-Fe(III) composites (OFC) in relation coprecipitated pH and C/(C + Fe) ratios. Results showed the greatest Cr sorption of 51.8 mg g^-1 on the OFC sample that was precipitated at pH 3 and contained the C/(C + Fe) molar ratio of 0.71. Wherein the Cr(VI) removal subsequent to the coprecipitation was dominated by the sorption on Fe hydroxides. Although amounts of total sorbed Cr decreased with increasing C/(C + Fe) molar ratio, the reverse trend on Cr(VI) reducibility compensated the Cr(VI) removal capability of OFC samples. With C/(C + Fe) molar ratios ≥ 0.89, the increasing amounts of coprecipitated organic matter that homogeneously distributed with Fe domains on OFC surfaces could trigger a significantly pronounced Cr reduction. Collectively, our results suggested an alternative method for Cr(VI) remediation by manipulating C/Fe ratios in suspensions. After the sorption of most Cr(VI) on Fe hydroxides, increasing C/Fe ratio in systems could further improve the Cr(VI) removal efficiency by the reduction of remaining Cr(VI) to Cr(III).

Carbon-coated montmorillonite nanocomposite for the removal of chromium(VI) from aqueous solutions

A carbon-coated montmorillonite nanocomposite (CMt), obtained by hydrothermal carbonization of montmorillonite suspension in glucose, was used to remove Cr(VI) from aqueous solutions. The distribution and speciation of Cr immobilized by CMt were assessed by transmission electron microscopy with energy-dispersive X-ray analysis, X-ray absorption near edge structure (XANES), and scanning transmission soft X-ray microscopy (STXM). The variation in the functional groups and molecular structures of CMt was also investigated. The capacity of CMt for adsorbing Cr(VI) was markedly superior to that of the parent montmorillonite, showing maximum uptake of 100 and 12.4 mg g^-1 at pH 2 and 8, respectively. The Cr K-edge XANES and STXM analyses indicated that Cr(VI) was reduced to Cr(III) under both acidic and alkaline conditions, while a Cr(OH)₃ precipitate and Cr(III)-acetate complex were the predominant species present on the CMt surface. The Fourier transform infrared spectroscopy and C K-edge XANES further suggested that the phenolic groups in CMt could serve as electron donors, facilitating Cr(VI) reduction. The combined results indicate that electrostatic attraction, Cr(VI) reduction, complexation, and precipitation are involved in the removal of Cr(VI) by CMt.

Reduction and immobilization of hexavalent chromium in chromite ore processing residue using amorphous FeS2

In this work, a series of long-term treatment trials were conducted to evaluate the remediation performance of amorphous iron pyrite (FeS₂₍am₎) toward hexavalent chromium (Cr(VI)) in chromite ore processing residue (COPR). The effectiveness of FeS₂₍am₎ was assessed using alkaline digestion, the synthetic precipitation leaching procedure (SPLP) and the physiologically based extraction test (PBET). Reaction mechanisms were explored by monitoring the changes in the solid pH, redox potential (Eh), the chemical states of relevant elements as well as the crystal forms present in COPR. The results showed that, using a proper dosage, the total content of Cr and Cr(VI) in the leachate from treated COPR met the Chinese standard regulatory limits for the extraction toxicity of hazardous wastes (GB 5085.3-2007). In addition, the in vitro bioaccessibility of Cr(VI) in COPR was also significantly reduced. Moreover, the remediation effect was maintained for the subsequent six months. This long-term effect was attributed to the presence of reductive sulfur (S) species and Fe(II) that remained even after 180 days of treatment, which were identified using high-resolution X-ray photoelectron spectroscopy (HR-XPS) and X-ray diffraction (XRD). To maximize the effect of FeS₂₍am₎, 5% lime was employed as an additive to adjust the pH and thus contributed to Cr(VI) reduction and immobilization. To remediate COPR within 30 days, the use of 5% lime and a specific FeS₂₍am₎ dosage (FeS₂₍am₎:Cr(VI) = 1.25:1) is recommended based on the results. For rapid remediation (< 1 day), the use of 5% lime and a 2.5:1 FeS₂₍am₎:Cr(VI) ratio is recommended. In comparison with widely studied reductants, FeS₂₍am₎ showed excellent efficiency for the remediation of COPR over both short- and long-term treatment trials, demonstrating it is a very promising alternative treatment method.

Molecular structure-reactivity correlations of humic acid and humin fractions from a typical black soil for hexavalent chromium reduction

Different soil humus fractions are structurally distinct from each other molecularly, however, the relationship between their microscopic molecular structures and the macroscopic reduction of Cr(VI) is still unknown, especially for the humin fraction. In this study, different humus fractions (HA, humic acid; HMi, humin linked to iron oxides; HMc, humin linked to clay; and HMr, humin residue) were sequentially extracted from a typical black soil and well characterized. It was found that HA, HMi and HMc were the same type of humus with similar molecular structures, while HMr was structurally different from the other fractions with a high cellulose content. The removal rate of Cr(VI) in solution decreased with progressive humus fractionation, namely, HA > HMi > HMc > HMr. Based on the two-dimensional correlation spectroscopic analysis (2DCOS) of the FTIR data, the changing functional groups of all humus fractions during reacting with Cr(VI) followed a similar order: carboxyl > phenol > hydroxyl > methyl > methylene. According to the correlation analysis, Cr(VI) reduction rates by different humus fractions were mainly determined by the content of phenol (R² = 0.99) instead of carboxyl (R² = 0.28). Except for HMr, the Cr(VI) reduction rates of different humus fractions were also positively correlated with surface and bulk polarity (R² = 0.98 and 0.99) but not with aromaticity or aliphaticity (R² = 0.21).

Abiotic reduction of Cr(VI) by humic acids derived from peat and lignite: kinetics and removal mechanism

Hexavalent chromium contamination of groundwater is a worldwide problem caused by anthropogenic and natural processes. We report the rate of Cr(VI) removal by two humic acids (extracted from Miocene age lignite and younger peat soil) in aqueous suspensions across a pH range likely to be encountered in terrestrial environments. Cr(VI) was reduced to Cr(III) in a first-order reaction with respect Cr(VI) concentration, but exhibited a partial order (~ 0.5) with respect to [H+]. This reaction was more rapid with the peat humic acid, where Cr(VI) reduction was observed at all pH values investigated (3.7 ≤ pH ≤ 10.5). ¹³C NMR and pyrolysis GC-MS spectroscopy indicate that the reaction results in loss of substituted phenolic moieties and hydroxyl groups from the humic acids. X-ray absorption spectroscopy indicated that at all pH values the resulting Cr(III) was associated with the partially degraded humic acid in an inner-sphere adsorption complex. The reaction mechanism is likely to be controlled by ester formation between Cr(VI) and phenolic/hydroxyl moieties, as this initial step is rapid in acidic systems but far less favourable in alkaline conditions. Our findings highlight the potential of humic acid to reduce and remove Cr(VI) from solution in a range of environmental conditions.

Dissolved Mineral Ash Generated by Vegetation Fire Is Photoactive under the Solar Spectrum

Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.

The role of different functional groups in a novel adsorption-complexation-reduction multi-step kinetic model for hexavalent chromium retention by undissolved humic acid

Undissolved humic acid (HA) has a great retention effect on the migration of hexavalent chromium [Cr(VI)] in soil, and HA functional groups play a predominant role in this process. However, the coupled mode between Cr(VI) retention and HA functional groups reaction is still unclear. In this study, it was found that a fair amount of Cr on HA existed in the forms of ion exchangeable and binding Cr(VI) during the reaction resulting from the ion exchange adsorption and complexation of Cr(VI). According to the results of two-dimensional correlation spectroscopic analysis (2DCOS), HA functional groups participated in the reaction with Cr(VI) in the order of carboxyl ≈ chelated carboxyl > phenol > polysaccharide > methyl, and all the functional groups were more likely to be located at aromatic domains. Based on the results of XPS spectra, rather than to be oxidized by Cr(VI), carboxyl more tended to be complexed by chromium, which is regarded as the precondition for Cr(VI) reduction. Phenol, polysaccharide and methyl with distinct reaction activities successively acted as major electron donors for Cr(VI) reduction in different reaction stages. Consequently, it was determined that the retention of Cr(VI) by undissolved HA followed an adsorption-complexation-reduction mechanism, and based on this, a multi-step kinetic model with multiple types of complexation/reduction sites was developed to simulate the retention processes resulting in a much better fitting effect (R² > 0.99) compared with traditional first-order and second-order kinetic models (R² < 0.95). This demonstrated that the multi-step kinetic model is of great potential in accurately simulating the migration and transformation of Cr(VI) in soil environment.

Speciation of chromium compounds from humic acid-zeolite Y to an ionic liquid during extraction

By synchrotron X-ray absorption spectroscopy, chemical structures of hexavalent chromium (Cr(VI))/trivalent chromium (Cr(III)) adsorbed on humic acid (HA)-zeolite Y and extracted in an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate ([C₄mim][BF₄])) have been studied. By combining the competitive adsorption results and reduction of Cr(VI)-HA with the carboxyl groups of HA, Cr(III)-HA (58%) was shown to be the major compound in HA-zeolite Y using synchrotron X-ray absorption near-edge structure (XANES) spectroscopy. In an ionic liquid phase, the reduction of Cr(VI)-HA to Cr(III)-HA and the desorption of Cr(III) from HA were caused by [C₄mim][BF₄]. The ⁹F nuclear magnetic resonance (NMR) spectra show that the perturbation of the [C₄mim][BF₄] anion was affected by the extractable chromium species. The formation of a Cr(III) ion affected the increase in the bond distance for the 1st shell CrO of the chromium species in [C₄mim][BF₄] using extended X-ray absorption fine structure (EXAFS) spectroscopy. The changes in the non-extractable chromium species remaining in HA-zeolite Y were also caused by [C₄mim][BF₄] during extraction. The desorption of the absorbed Cr(III) on HA and zeolite Y was observed to form Cr(III) ions. As the percentage of Cr(III) ions remaining in HA-zeolite Y increased, a slightly greater bond distance for CrO was found at 2.01 Å. The enhanced reduction of Cr(VI)-HA and desorption of Cr(III) adsorbed on the HA and zeolite Y to form Cr(III) ions were affected by [C₄mim][BF₄]. Increased mobility of Cr(III) in the simulated soil can promote the migration of Cr(III) ions into the H₂O during soil washing for remediation.

A novel method of three-dimensional hetero-spectral correlation analysis for the fingerprint identification of humic acid functional groups for hexavalent chromium retention

Two-dimensional hetero-spectral correlation analysis has been widely used for the interpretation of spectral changes of humic substances involved in various environmental processes. However, when three different types of spectroscopies are utilised, only a pairwise correlation can be achieved. In order to overcome this problem, a novel method of three-dimensional hetero-spectral correlation analysis with scaling technique was developed in this study, which can further establish a direct correlation between three different types of spectroscopies, including FTIR, 13C CP/MAS NMR, and XPS. The proposed method was applied to the fingerprint identification of undissolved humic acid functional groups for Cr(vi) retention, which is one of the most important points for understanding the migration and transformation of Cr(vi) in a subsurface environment. The results indicated that mainly free and dissociated carboxylic groups, phenolic groups, and polysaccharide participated in the reaction with Cr(vi), and these functional groups were mainly located at aromatic domains. Besides, the variations of functional groups were related to the reduction of Cr(vi), and the reduced Cr(iii) mainly bound to aromatic domains. The successful application of the proposed method demonstrated that it can serve as a promising tool for further investigations concerning more complicated environmental processes and even other scientific fields by supplying more detailed, reliable and visualised spectral information.

Mechanism study of humic acid functional groups for Cr(VI) retention: Two-dimensional FTIR and 13C CP/MAS NMR correlation spectroscopic analysis

Undissolved humic acid (HA) is known to substantially effect the migration and transformation of hexavalent chromium [Cr(VI)] in soils. The mechanisms of Cr(VI) retention in soils by undissolved HA have been reported; however, past studies are inconclusive about the types of HA functional groups that are involved in Cr(VI) retention and the retention mechanisms. Utilizing a two-dimensional correlation spectroscopy (2DCOS) analysis for FTIR and ¹³C CP/MAS NMR, this study investigated the variations of HA function groups and molecular structures after reactions with aqueous Cr(VI) under different pH conditions. Based on the changing sequence of functional groups interpreted from the 2DCOS results, a four-step mechanism for Cr(VI) retention was determined as follows: (1) electrostatic adsorption of Cr(VI) to HA surface, (2) complexation of adsorbed Cr(VI) by carboxyl and ester, (3) reduction of complexed Cr(VI) to Cr(III) by phenol and polysaccharide, and (4) complexation of reduced Cr(III) by carboxylic groups. These functional groups that are involved in Cr(VI) retention were determined to occur in aromatic domains.

Kinetics and mechanisms of amorphous FeS2 induced Cr(VI) reduction

A novel reductant, amorphous FeS₂ (FeS₂₍am₎) was synthesized to remove hexavalent chromium (Cr(VI)). The stoichiometry, kinetics and mechanisms of FeS₂₍am₎ induced Cr(VI) reduction were studied under anaerobic conditions with a pH range of 5.0-13.0. Results showed that FeS₂₍am₎ was efficient in removing aqueous Cr(VI) with a stoichiometry of Cr(VI): FeS₂₍am₎ ∼1: 0.9. Moreover, the reduction kinetics decreased with the growth of pH within the pH range from 5.0 to 10.0, but increased within pH 10.0-13.0. The dissolution kinetics of FeS₂₍am₎ was found to be mainly responsible for the varied Cr(VI) reduction kinetics, and the mechanisms involved in the reduction process were further confirmed by X-ray photoelectron spectroscopy analysis. In addition, O₂ was found to compete with Cr(VI) in the reduction process and subsequently impede the Cr(VI) reduction rate.

Environmental materials for remediation of soils contaminated with lead and cadmium using maize (Zea mays L.) growth as a bioindicator

Heavy metal pollution is a severe environmental problem. Remediation of contaminated soils can be accomplished using environmental materials that are low cost and environmentally friendly. We evaluated the individual and combination effects of humic acid (HA), super absorbent polymer (SAP), zeolite (ZE), and fly ash composites (FC) on immobilization of lead (Pb) and cadmium (Cd) in contaminated soils. We also investigated long-term practical approaches for remediation of heavy metal pollution in soil. The biochemical and morphological properties of maize (Zea mays L.) were selected as biomarkers to assess the effects of environmental materials on heavy metal immobilization. The results showed that addition of test materials to soil effectively reduced heavy metal accumulation in maize foliage, improving chlorophyll levels, plant growth, and antioxidant enzyme activity. The test materials reduced heavy metal injury to maize throughout the growth period. A synergistic effect from combinations of different materials on immobilization of Pb and Cd was determined based on the reduction of morphological and biochemical injuries to maize. The combination of zeolite and humic acid was especially effective. Treatment with a combination of HA + SAP + ZE + FC was superior for remediation of soils contaminated with high levels of Pb and Cd.

Interactions of the products of oxidative polymerization of hydroquinone as catalyzed by birnessite with Fe (hydr)oxides – an implication of the reactive pathway for humic substance formation

In this study, we aimed to determine the products of oxidative polymerization of hydroquinone with and without catalysis by birnessite, and investigate the adsorption kinetics of the HQs on Fe (hydr)oxides upon their formation.

Comparison of Cr(VI) removal by activated sludge and dissolved organic matter (DOM): importance of UV light

Removal of toxic Cr(VI) by activated sludge and DOM derived from activated sludge was investigated in this study. A rapid increase in TOC concentration from 50.93 to 127.40 mg L(-1) is observed during the Cr(VI) removal process by activated sludge in the pH range of 2-9. Removal efficiencies of Cr(VI) by either activated sludge or DOM greatly decreased with the increasing initial pH. Kinetics of Cr(VI) removal by activated sludge indicate that both biosorption and bioreduction are involved in the Cr(VI) removal. Cr(VI) removal by DOM is slow in dark, but it is greatly enhanced when UV light is applied. The first-order constant increases from 0.0033 (in dark) to 0.079 min(-1) (UV illumination) at pH 2.0 and 1068 mg L(-1) DOM. The enhancement of Cr(VI) reduction is due to the generation of the reactive intermediates such as O2(●-) and DOM* as DOM absorbed light energy, which plays important roles in the reduction of Cr(VI).

Comparison of the spectroscopic speciation and chemical fractionation of chromium in contaminated paddy soils

Sequential extraction has been widely used to classify metal species in soils and sediments; however, the lack of selectivity in extraction reagents may lead to the misinterpretation of metal speciation. In this study, we used X-ray absorption near edge structure (XANES) spectroscopy to classify Cr species based on its molecular form. These results complement the conventional Cr fractionation derived from the Tessier extraction method. The linear combination fitting (LCF) for the Cr-XANES spectra indicated that the Cr species in the soils could generally be described as Cr(III) sorbed on ferrihydrite (Cr-FH), Cr(III) complexed with humic acid (Cr-HA), and precipitated Cr. While the sum of the adsorbed Cr(III) and Cr(III)/Fe coprecipitates showed a nearly 1:1 relationship with reducible Cr, the total of Cr precipitates and organic Cr also followed the same trend with oxidizable Cr. This result indicated that there might be a underestimation in the reducible fraction as pure Cr(III) precipitates associated with surfaces of Fe minerals would not be extracted in the reducible process. Instead, such pure Cr(III) precipitates were dissolved during the oxidizable process, resulting in a overestimation in the fractionation of organic-related Cr.

Evaluation of bacterial biosensors to determine chromate bioavailability and to assess ecotoxicity of soils

Chromate can be considered a potent environmental contaminant and consequently, an understanding of chromate availability and toxicity to soil biology is essential for effective ecological assessment of metal impact in soils. This study shows the response of two bacterial bioreporters, pCHRGFP1 Escherichiacoli and pCHRGFP2 Ochrobactrumtritici, to increasing concentrations of chromate in two different soils. The bioreporters, carrying the regulatory gene chrB transcriptionally fused to the gfp reporter system, exhibited different features. In both, the fluorescence signal and the chromate concentration could be linearly correlated but E. coli biosensor functioned within the range of 0.5-2 μM and O. tritici biosensor within 2-10 μM chromate. The bioreporters were validated through comparative measurements using the chemical chromate methods of diphenylcarbazide and ionic chromatography. The bacterial sensors were used for the estimation of bioavailable fraction of chromate in a natural soil and OECD artificial soil, both spiked with chromate in increasing concentrations of 0-120 mg Cr(VI) kg(-1) of soil. OECD soil showed a faster chromate decrease comparing to the natural soil. The toxicity of soils amended with chromate was also evaluated by ecotoxicological tests through collembolan reproduction tests using Folsomia candida as test organism. Significant correlations were found between collembolans reproduction and chromate concentration in soil (lower at high chromate concentrations) measured by biosensors. Data obtained showed that the biosensors tested are sensitive to chromate presence in soil and may constitute a rapid and efficient method to measure chromate availability in soils.