Fenton reaction induced in-situ redox and re-complexation of polyphenol-Cr complex and their products

Accurately recognizing chromium species with multi-functionalized nano Au-based sensor array

High-resolution identification of chromium (Cr) species, especially various organic-Cr complexes, in a convenient and economically-feasible manner is the prerequisite for achieving the advanced treatment of chromium wastewater. To this end, a colorimetric nano-Au sensor array was developed by taking advantage of the UV-spectra shift of gold nanoparticles (Au NPs) upon interaction with Cr species; specifically, four molecular modifiers [i.e., iminodiacetic acid (IDA), tripolyphosphate (TPP), cetyltrimethylammonium bromide (CTAB), and 1,5-diphenylcarbazide (DPC)] were intentionally employed for assembling nano-Au array receptors, which showed respective responses toward different Cr species through the formation of coordination, hydrophobic interaction, electrostatic attraction, and redox reaction, respectively; the "fingerprint" differences of the unique optical properties were then integrated for semi-quantitatively recognizing Cr species by pattern recognition techniques. Eleven ubiquitous Cr species [i.e., Cr(III), Cr(VI), and various Cr(III)-organic complexes] served as the model samples, which could be sensitively identified, no matter in individual or mixture mode, by the developed nano-Au sensor array on the basis of the colorimetric responses resulted from diverse nano-Au-aggregation behaviors, with excellent anti-interference ability in the simulated or actual water scenario. Attractively, the nano-Au sensor array can achieve very sensitive detection limit of the quantitative analyses of Cr species in a prompt in-situ manner, which usually requires a two-step process of separation and detection for the conventional analytical methods. Such a convenient strategy of Cr species discrimination conduces to rationally designing specific protocols for the advanced treatment of chromium wastewater.

Unraveling the toxicity response and metabolic compensation mechanism of tannic acid-Cr(III) complex on alga Raphidocelis subcapitata

Due to their assembly properties and variable molecular weights, the potential biological toxicity effects of macromolecular organic ligand heavy metal complexes are more difficult to predict and their mechanisms are more complex. This study unraveled the toxicity response and metabolic compensation mechanism of tannic acid-Cr(III) (TA-Cr(III)) complex on alga Raphidocelis subcapitata using multi-omics approaches. Results showed TA-Cr(III) complex caused oxidative damage and photosystem disruption, destroying the cell morphology and inhibiting algal growth by >80 % at high exposure levels. TA-Cr(III) complex stress down-regulated proteins linked to proliferation, photosynthesis and antioxidation while upregulating carbon fixation, TCA cycle and amino acid metabolism. The increase of fumarate, citrate, isocitrate and semialdehyde succinate was validated by metabolomics analysis, which improved the TCA cycle, amino acid metabolism and carbon fixation. Activation of the above cellular processes somewhat compensated for the inhibition of algal photosynthesis by TA-Cr(III) complex exposure. In conclusion, physiological toxicity coupled with downstream metabolic compensation in response to Cr(III) complex of macromolecular was characterized in Raphidocelis subcapitata, unveiling the adaptive mechanism of algae under the stress of heavy metal complexes with macromolecular organic ligands.

An Investigation into the Stability Source of Collagen Fiber Modified Using Cr(III): An Adsorption Isotherm Study

The enhanced hydrothermal stability of leather, imparted by little Cr(III), has traditionally been ascribed to strong coordinate bonds. However, this explanation falls short when considering that the heat-induced shrinking of collagen fiber is predominantly driven by rupturing weak H-bonds. This study explored the stability source via adsorption thermodynamics using collagen fiber as an adsorbent. Eleven isotherm models were fitted with the equilibrium dataset. Nine of these models aptly described Cr(III) adsorption based on the physical interpretations of model parameters and error functions. The adsorption equilibrium constants from six models could be transformed into dimensionless thermodynamic equilibrium constants. Based on the higher R2 of the van't Hoff equation, thermodynamic parameters (∆G°, ∆H°, ∆S°) from the Fritz-Shluender isotherm model revealed that the adsorption process typifies endothermic and spontaneous chemisorption, emphasizing entropy increase as the primary driver of Cr(III) bonding with collagen. Thus, the release of bound H2O from collagen is identified as the stability source of collagen fiber modified by Cr(III). This research not only clarifies the selection and applicability of the isotherm model in a specific aqueous system but also identifies entropy, rather than enthalpy, as the principal stability source of Cr-leather. These insights facilitate the development of novel methods to obtain stable collagen-based material.

Effects of tannic acid on the transport behavior of trivalent chromium in soils and its mechanism

Trivalent chromium [Cr(III)] and tannins serve as necessary substances in leather processing and coexist in tannery site, which lead to the chromium contamination in site soil when disposed improperly. However, coexisting tannins are very likely to complex with Cr(III) and affect its properties, ultimately changing the mobility of chromium in soil. In this study, tannic acid (TA) was selected to investigate the complexation with Cr(III) and the influence on the solubility and sorption of Cr(III) in soils. Then, the transport behavior and mechanism of Cr(III)-TA complexes in soil was clarified. Dialysis results showed that the increase of TA concentration and solution pH promoted the formation of complexed Cr(III). The results of UV-Vis absorption spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations indicated that the adjacent ionized phenolic hydroxyls in TA functioned as the binding sites with Cr(III) to form the Cr-O bonds and the degree of complexation increased with pH. The Cr(III)-TA complexes had higher solubility than free Cr(III) at pH ≥ 6.0. Batch sorption experiments demonstrated that the sorption capacity of Cr(III)-TA to soils with different pH was always lower than that of free Cr(III). These reasons led to the stronger mobility of Cr(III)-TA in soil columns than Cr(III). Our research reveals that the enhanced mobility of Cr(III) in soils coexisting with TA.

Complexation behaviour and removal of organic-Cr(III) complexes from the environment: A review

Chromium (Cr) is mainly found in the form of organic-Cr(III) complexes in the natural environment and industrial waste. The widespread existence of composite contaminants composed of organic matter (OM) and Cr pose a serious ecological threat, and its potential interaction and removal need to be further summarised. Organic ligands, such as carbohydrates, nitrogen compounds, phenolic compounds, humus substances (HS), and low molecular weight organic acids (LMWOAs), play an important role in governing the speciation, mobility, and absorption and desorption of Cr in the environment. Moreover, growing evidence indicates that oxygen-containing functional groups (e.g., carboxyl, hydroxyl, and phosphate) are closely related to the complexation of Cr(III). Advanced oxidation processes (AOPs) are efficient and widely applicable technologies. However, the re-complexation of oxidation intermediates with Cr(III) and the formation and accumulation of much more toxic Cr(VI) species hinder the possible utilisation of AOPs. In this paper, the sources and harmful effects of organic-Cr(III) complexes are reported in detail. The complexation behaviour and structure of the organic-Cr(III) complexes are also described. Subsequently, the application of AOPs in the decomplexation and degradation of organic-Cr(III) complexes is summarised. This review can be helpful for developing technologies that are more efficient for organic-Cr(III) complex removal and establishing the scientific background for reducing Cr discharge Cr into the environment.

Electro-peroxone enables efficient Cr removal and recovery from Cr(III) complexes and inhibits intermediate Cr(VI) generation in wastewater: Performance and mechanism

Available oxidation processes for removing Cr(III) complexes from water/wastewater usually encounter the formation of highly toxic Cr(VI) and the generation of Cr enriched waste sludge, posing challenges on the subsequent disposal. Herein, we achieve efficient removal of Cr(III)-organic complexes and simultaneous recovery of Cr from wastewater with enhanced curtailment of intermediate Cr(VI), by using an electrochemically driven peroxone (i.e., electro-peroxone) process with activated carbon fiber (ACF) electrodes. For Cr(III)-EDTA, electro-peroxone could remove ∼90% total Cr from 11.50 mg/L to 1.20 mg/L and ∼80% total organic carbon, with a strong curtailment of Cr(VI) to less than 0.2 mg/L. Additionally, the process could obtain a complete recovery of the removable Cr, of which 78.3% are enriched at ACF cathode as amorphous Cr(OH)₃ deposits and the remaining 21.7% are adsorbed at the anode, thus avoiding the generation of Cr laden sludge. Mechanism studies show the electro-generated H₂O₂ reacts with O₃ to generate abundant HO· for decomplexation, which sequentially oxidizes Cr(III) to Cr(VI), and degrades the released EDTA via stepwise decarboxylated process, as confirmed by HPLC analysis. Multiple pathways including electro-reduction, H₂O₂ reduction and electro-adsorption synergistically curtail and immobilize the formed intermediate Cr(VI). ACF characterizations and continuous 5-cycle experiments substantiate the excellent reusability of the ACF electrodes. Moreover, this process exhibits satisfactory effectiveness to Cr(III) complexed with other ligands (e.g., citrate and oxalate), and complexed Cr(III) in the real electroplating wastewater. We believe this study would provide an efficient and eco-friendly alternative for Cr(III) complexes removal from wastewater.

Reductive immobilization of Cr(VI) in contaminated water by tannic acid

The rapid reductive immobilization of Cr(VI) from the aqueous solution was achieved by reduction to Cr(III) using tannic acid (TA), and subsequent pH-triggering precipitation of the organo-Cr(III) complexes formed in the redox reaction. The effects of TA concentration, temperature, and solution pH on the reduction of Cr(VI) were examined by batch experiments, and the rapid redox reduction followed a second-order kinetics with respect to Cr(VI) concentration in the pH range of 2.0-3.0. UV-visible spectra, FTIR, and XPS confirmed the complete detoxification of Cr(VI) concomitant with carboxylation of partial phenolic hydroxyls in TA. Synchronously, the reduced Cr(III) coordinated with carboxyl groups in oxidized TA (OTA) to form complexes, which exhibited remarkable pH-dependent size distribution characteristics as illustrated by SEM images and sequential filtration/ultrafiltration. The resulted Cr(III) complexes could aggregate into colloids with larger size and precipitate out at pH range of 6.0-8.0 via cross-linking, thereby leading to 93% Cr and 89% TOC immobilization. An eco-friendly and cost-effective method for Cr(VI) elimination and immobilization is provided because polyphenols are natural polymers derived from plants.

Simultaneous removal of Cr and organic matters via coupling Cr-Fenton-like reaction with Cr flocculation: The key role of Cr flocs on coupling effect

Cr contamination is frequently combined with organic pollution. Cr flocculation using flocculants results in difficulty in Cr recovery and increase of salinity. Moreover, the fates of coexisted organic pollutants are rarely attended. In this study, the Fenton-like reaction based on Cr redox reaction (Cr-Fenton-like reaction) coupled with Cr flocculation was established using H₂O₂ as additives, which gave the opportunity to realize simultaneous removal of Cr and organic matters sustainably. In the coupling system, Cr-Fenton-like reaction generated more OH^- for Cr flocculation via the H₂O₂ decomposition, meanwhile, Cr flocculation provided heterogeneous catalytic regions for Cr-Fenton-like reaction. The formation of Cr flocs was the key to the coupling effect. They created partially alkaline regions, therefore Cr-Fenton-like reaction (reaction condition: pH > 5) and Cr flocculation (reaction condition: pH > 8) occurred in these heterogeneous regions, although the pH of the solutions was below 5. Besides, the Cr flocs in the coupling system tended to adsorb Cr(III) which also contributed to the coupling effect. Although Cr-Fenton-like reaction generated Cr(VI) inevitably, the dissolved Cr(VI) didn't accumulate due to the persistent acidic condition and the heterogeneous catalysis of Cr flocs in the coupling system. When the coupling effect was applied to a stimulated tannery wastewater with Cr and organic co-pollution, the simultaneous removal rates for Cr and total organic carbon were 81.2% and 41.34% respectively after an 8-h running. This study gives deep insights into the coupling effect and provides a sustainable and eco-friendly strategy for the remediation of wastewater with Cr and organic co-pollution.

Migration of leather tannins and chromium in soils under the effect of simulated rain

Chromium (Cr) and tannin are two major pollutants in leather industry. However, little is known about the co-migration of leather tannins and Cr in soils. In this study, column experiments were conducted to estimate Cr leaching behavior from topsoil and the environmental risk of the leachate at various tannin dosages and different ways (tannin either directly adding to the Cr-contaminated soil or adding stepwise through simulated rain) into the soil. The total Cr concentration in leachate was positively related with tannin content in soil, while Cr (Ⅵ) concentration was negatively correlated. The maximum cumulative leaching efficiency of total Cr from soil after six leaching events was 44.65% with 3 mg/g tannin adding into soil directly, and the maximum cumulative leaching efficiency of Cr (Ⅵ) was 38.75% with simulated rain leaching Cr-contaminated soil. With 3 mg/g tannin adding into soil, tannin concentration in the top layer (0-7 cm) lost by 32.67% after leaching, the amount of decomposed tannin was 0.25 mg/g, excluding the amount of tannin in leachate (3.63 mg/L) and the original amount in the soil (0.34 mg/g), indicating a slow degradation under natural condition. Both of the total Cr and Cr (Ⅵ) concentration in each layer of the soil columns decreased under tannin treatments compared with control. Compared with tannin adding stepwise into simulated rain, adding tannin into soil significantly (p < 0.05) affected the migration of Cr. Tannin increased the residual fraction while decreased the exchangeable fraction of Cr in the soils. Overall, this research can provide reference information for environmental risk assessment of contaminants in tanning sites.

Regeneration and reuse of magnetic particles for contaminant degradation in water

Fenton reaction is an oxidation process of interest in wastewater treatment because of its ability to degrade organic compounds. Iron-based magnetic particles can be a very useful catalyst when using heterogeneous Fenton process. The major problem of this heterogeneous process is the saturation of the Fe ³⁺ on the surface, which limits the process. In this study, the possibility of using magnetite particles as a substrate is presented, increasing its degradation efficiency by Fenton reaction through a regeneration process that achieves the electronic reduction of its surface using reducing agents. The results indicate that the regeneration process is quite effective, increasing the efficiency of the degradation of Methylene Blue up to 99%. The concentration of magnetite is the most influential factor in the efficiency of the reaction, while the regeneration time and the concentration of reducing agent do not significantly affect the results considering the range used. The presence of mechanical stirring may adversely affect the reaction in the long term. Increasing the oxidant agent concentration reduces the initial speed of the reaction but not the long-term efficiency. The use of hydrazine in this process allows the successive reuse of these particles maintaining a high percentage of elimination of methylene blue, above 70% even after 10 uses, compared to an elimination below 20% for particles not regenerated after the second use and for particles regenerated with ascorbic acid after the eighth use.