Photo-reductive decolorization of an azo dye by natural sphalerite: case study of a new type of visible light-sensitized photocatalyst

Natural marmatite photocatalyst for treatment of mineral processing wastewater to help zero wastewater discharge

Mineral processing wastewater (MPW) with large discharge and high toxicity affects environmental safety, and the realizing zero discharge of MPW is of great significance for reducing environmental pollution, saving water resources, and promoting the sustainable development of the mining industry. In this study, we reported natural marmatite (NM) as a low-cost and efficient photocatalyst for the treatment of MPW to help zero wastewater discharge. The photocatalytic activity of NM was evaluated by the removal of total organic carbon (TOC) from MPW under visible-light illumination, and the optimal degradation conditions were discussed. Results showed that superoxide free radicals (·O2-) were the dominant active species responsible for organic pollutants degradation, and 74.25% TOC removal was obtained after 120 min reaction under the optimum treatment conditions. Meanwhile, the wastewater treated by NM photocatalysis can be reused in the flotation system without adverse impact on the product index. Based on these findings, a model of zero wastewater discharge for flotation with the help of photocatalytic treatment was established, it indicated that the water of the whole system can be balanced without affecting the ore dressing index, which showed that visible light-driven photocatalyst has a promising application prospect in the treatment and recycling of industrial wastewater.

Remediation of preservative ethylparaben in water using natural sphalerite: Kinetics and mechanisms

As a typical class of emerging organic contaminants (EOCs), the environmental transformation and abatement of preservative parabens have raised certain environmental concerns. However, the remediation of parabens-contaminated water using natural matrixes (such as, naturally abundant minerals) is not reported extensively in literature. In this study, the transformation kinetics and the mechanism of ethylparaben using natural sphalerite (NS) were investigated. The results show that around 63% of ethylparaben could be absorbed onto NS within 38 hr, whereas the maximum adsorption capacity was 0.45 mg/g under room temperature. High temperature could improve the adsorption performance of ethylparaben using NS. In particular, for the temperature of 313 K, the adsorption turned spontaneous. The well-fitted adsorption kinetics indicated that both the surface adsorption and intra-particle diffusion contribute to the overall adsorption process. The monolayer adsorption on the surface of NS was primarily responsible for the elimination of ethylparaben. The adsorption mechanism showed that hydrophobic partitioning into organic matter could largely govern the adsorption process, rather than the ZnS that was the main component of NS. Furthermore, the ethylparaben adsorbed on the surface of NS was stable, as only less than 2% was desorbed and photochemically degraded under irradiation of simulated sunlight for 5 days. This study revealed that NS might serve as a potential natural remediation agent for some hydrophobic EOCs including parabens, and emphasized the significant role of naturally abundant minerals on the remediation of EOCs-contaminated water bodies.

Mineralogical characteristics and photocatalytic properties of natural sphalerite from China

Different natural sphalerites have a range of photocatalytic properties that can potentially be exploited for environmental remediation purposes. To develop value in the exploitation of sphalerite, samples were collected from 19 ore deposits in China and characterized for their mineralogical and photocatalytic properties. X-ray diffraction (XRD) and electron probe micro analysis (EPMA) measurements indicated that all the natural sphalerites from various localities crystallized in cubic phases with various chemical compositions. The substitution of Fe for Zn ranged from 0.235% to 14.826% by weight, Mn from 0.004% to 4.868%, Cu from 0.009% to 5.529% and Cd from 0.133% to 1.576%. As Fe became more abundant, the color of natural sphalerite darkened, becoming almost black; and higher Fe content was associated with stronger visible light absorption. Photoluminescence spectra showed emission mainly related to S-vacancies and progressively decreasing fluorescence intensity with increasing Fe content. Tests of the photocatalytic degradation of methyl orange indicated that the sample with the highest Cd content but moderate Fe content had the highest photocatalytic activity. Specifically, the degradation of Methyl Orange (30 mg/L) attained 82.11% efficiency under visible light irradiation for 4 hr of natural sphalerite with 4.262% Fe and 1.576% Cd. Overall, the Fe content in sphalerite was found to contribute to the visible light absorption ability and the recombination rate of photo-generated electrons and holes, while substitution by Cd was observed to have a greater effect on the photocatalytic properties. These findings provide a scientific basis for the profitable utilization of base metal resources like sphalerite.

A versatile β-cyclodextrin and polyethyleneimine bi-functionalized magnetic nanoadsorbent for simultaneous capture of methyl orange and Pb(II) from complex wastewater

Ascribing to their significant differences in physicochemical properties, it is extremely challenging to treat complex wastewater containing more than one class of pollutants via one-step treatment. Here, we focused on disposal of complex wastewater bearing organic dye and heavy metal by using adsorptive method. Thus, by combining the advantages of polyethyleneimine (PEI), β-cyclodextrin (β-CD) as well as Fe₃O₄ magnetic nanoparticles, a versatile β-CD and PEI bi-functionalized magnetic nanoadsorbent (Fe₃O₄-PEI/β-CD) with spatially separated sorption sites was successfully constructed for simultaneous capture of methyl orange (MO) and Pb(II) in complex wastewater. In this setting, β-CD cavities and positively charged N-containing groups of PEI were mainly responsible for removal of MO via host-guest inclusion and electrostatic attraction, respectively, and oxygen-bearing groups on the edge of β-CD as well as the free amino moieties in PEI acted as the active sites for Pb(II) uptake. In their individual mono-pollutant system, the adsorption processes can be better described via applying pseudo-second-order kinetic and Langmuir isotherm models. Interestingly, presence of MO in Pb(II)-MO binary system significantly promoted the uptake of Pb(II). But the coexisting Pb(II) had almost no effects on MO uptake. Such results demonstrated that both MO and Pb(II) could be simultaneously and synergistically removed by Fe₃O₄-PEI/β-CD through multiple mechanisms (such as electrostatic attraction, host-guest inclusion, chelating, etc.). Particularly, the excellent regeneration and stability make Fe₃O₄-PEI/β-CD an ideal integrative adsorbent for purification of actual wastewater contaminated by MO and Pb(II). Thus, this study provides some insights into designing a well-performed and easily recyclable adsorbent for simultaneous and synergetic capture of both organic and inorganic contaminants in complex wastewater.

The Fine Characterization and Potential Photocatalytic Effect of Semiconducting Metal Minerals in Danxia Landforms

The Danxia landform is representative of the Cretaceous continental red sediment. The careful identification and potential environmental effects of minerals in Danxia red beds have yet to be clearly reported. In this work, reddish sandstone samples were collected from Lang Mountain Danxia landform in Xinning, Hunan province, China, and their mineral phases, element distribution, microstructure, and the spatial relationship of different minerals were investigated using polarizing optical microscope, environmental scanning electron microscopy, energy-dispersive X-ray analysis, electron probe microanalysis, micro-Raman spectra, micro- X-ray diffraction, X-ray fluorescence spectroscopy, and high-resolution transmission electron microscopy. The results revealed that iron oxide (mainly hematite) and titanium oxide (mainly anatase) were the dominant minerals in Danxia red layers. Microcrystalline hematite was suggested as being the coloring mineral. Anatase, reported here for the first time in Danxia red beds, constituted the content of titanium in the red layer (0.17–0.57%) and was present in a significantly higher amount than the adjacent limestone formation (0.13%). Over 95% of Fe/Ti oxides served as a cementation agent along the framework of coarse-grain minerals (quartz and feldspar). The hematite and anatase were visible-light-responsive semiconductors, with a band gap of 2.01 eV and 3.05 eV, respectively. Photoelectrochemical experiments were performed on synthetic hematite, anatase, and their coupled material. The inactive hematite displayed an enhanced 23-fold photocurrent at 0.8 V (vs. Ag/AgCl) when coupled with anatase. Furthermore, in a photodegradation experiment using methyl orange dye under simulated sunlight, the coupled material showed decolorizing efficiency 2.4 times that of hematite. The anatase, therefore, prominently improved the photocatalytic activities of hematite. It is proposed that these semiconducting minerals in red beds produce oxygen reactive species and have significant environmental effects, which is of great importance.

Recycling food waste to clean water: the use of a biodigester's residual liquid inoculum (RLI) to decolourise textile azo dyes

A residual liquid inoculum (RLI) was used to decolourise solutions of Acid Yellow 25 (AY25) and Direct Violet 51 (DV51) azo dyes. The RLI was obtained through anaerobic digestion of food waste from a university restaurant. The concentration of bacteria in the RLI was 8.45 × 10⁷ CFU mL^-1. Dye solutions (50 μg mL^-1) were inoculated with the RLI (20% v/v) and incubated at room temperature. The decolourisation studies took place at microaerophilic and in-batch conditions and at pH = 2.50. Initially, the dyes were taken up from solution by biosorption; maximum colour removal was achieved after 3 hours of incubation, with 88.66% for AY25 and 77.65% of DV51. At prolonged incubation times (3-96 hours) decolourisation was mainly attributed to biodegradation of the azo solutions, with breakage of the azo bond, as detected by UV-VIS spectroscopy and Fourier transform infrared (FT-IR) analysis. Analysis of UV-VIS absorption rates of dyes showed, however, that AY25 was more readily biodegradable whereas DV51 was more recalcitrant to the action of the RLI.

Degradation pathway and mechanism of Reactive Brilliant Red X-3B in electro-assisted microbial system under anaerobic condition

The degradations of Reactive Brilliant Red X-3B (RBRX-3B) in an electric-assisted microbial system (EAMS), a microbial system (MS) and an electrochemical system (ECS) were compared. The degradation efficiency of RBRX-3B in EAMS (99.8%) was 10.8% higher than the sum in MS (61.9%) and ECS (27.1%) at 24h at the optimal voltage of 0.4V, indicating that there was a synergistic effect between the electrode reaction and the biodegradation. The RBRX-3B degradation in EAMS followed first-order kinetic model. The activation energy of RBRX-3B degradation in EAMS was calculated to be 60.53kJmol^-1 by the Arrhenius equation, showing that the degradation rate of RBRX-3B mainly depended on bio-chemical reaction. RBRX-3B was degraded to both low-strength toxic compounds and nontoxic compounds in EAMS and those intermediates were easier to be further degraded. The pathway of RBRX-3B degradation in EAMS was different from that in MS.

Degradation of Orange II by Fenton reaction using ilmenite as catalyst

This work deals with the degradation of the azo-dye Orange II (OII) by a heterogeneous process with dark Fenton. Natural and purified ilmenites from Colombian soil were used as catalysts. The catalysts have different physicochemical properties and are basically composed of TiO₂ and Fe₂O₃. Ilmenites (FeTiO₃), raw materials highly available at low cost, were studied by means of conventional metallography (polished grain mounts), as well as BET, XRD, and XRF in order to determine their possible source area and the factors that influence their use as a catalyst for OII degradation. The pH, the ilmenite amount, the initial CH₂O₂, and the temperature of the reaction system were studied. Complete degradation of dye was observed within 7 h, while 90 % of OII was removed in 7 h using Cumaribo Ilmenite. Graphical Abstract ᅟ.

Mild fabrication of silica-silver nanocomposites as active platforms for environmental remediation

A new, simple and convenient approach to prepare silver nanoparticles supported on SiO₂–PEI composites for the decoloration of organic dyes.

Catalytic bubble-free hydrogenation reduction of azo dye by porous membranes loaded with palladium nanoparticles

Catalytic bubble-free hydrogenation reduction of azo dye by porous membranes loaded with palladium (Pd) nanoparticles was studied for the first time. The effects of Pd loading, dye concentration and reuse repetitions of membranes were investigated. In reduction, the dye concentration decreased whereas the pH rose gradually. An optimal Pd loading was found. The catalytic membranes were able to be reused more than 3 times.

A recyclable mineral catalyst for visible-light-driven photocatalytic inactivation of bacteria: natural magnetic sphalerite

Motivated by recent studies that well-documented mineral photocatalyst for bacterial inactivation, a novel natural magnetic sphalerite (NMS) in lead-zinc deposit was first discovered and evaluated for its visible-light-driven (VLD) photocatalytic bactericidal properties. Superior to the reference natural sphalerite (NS), vibrating sampling magnetometeric (VSM) analysis revealed the ferromagnetic property of NMS, indicating its potential for easy separation after use. Under the irradiation of fluorescence tubes, NMS could inactivate 7 log10 Gram-negative Escherichia coli K-12 without any regrowth and metal ions leached out from NMS show no toxicity to cells. The cell destruction process starting from cell wall to intracellular components was verified by TEM. Some products from damaged cells such as aldehydes, ketones and carboxylic acids were identified by FTIR with a decrease of cell wall functional groups. The relative amounts of potassium ion leakage from damaged cells gradually increased from initial 0 to approximately constant concentration of 1000 ppb with increasing reaction time. Superoxide radical (•O2(-)) rather than hydroxyl radical (•OH) was proposed to be the primary reactive oxidative species (ROSs) responsible for E. coli inactivation by use of probes and electron spin resonance (ESR). H2O2 determined by fluorescence method is greatly involved in bacterial inactivation in both nonpartition and partition system. Multiple cycle runs revealed excellent stability of recycled NMS without any significant loss of activity. This study provides a promising natural magnetic photocatalyst for large-scale bacterial inactivation, as NMS is abundant, easily recycled and possessed an excellent VLD bacterial inactivation ability.

Flocculation of Quaternary Chitosan on Acid Dyeing Waste Water

In order to increase the positively charge, solubility in water and grafted degree of chitosan, quaternary chitosan was synthesized with chitosan and glycidyl-trimethyl-ammonium chloride by the heterogeneous reaction. FT-IR was used to characterize the structure of the synthesized. The quaternary chitosan was then used to treat simulated acid dyeing wastewater, the effect of quaternary chitosan concentration, agitation mode and pH values on flocculation were discussed. The results indicated that the H+ in nucleophilic center-NH2 was displaced by-CH2CH(OH)CH2N+(CH3)3 and the 2-hydroxypropyltrimethyl ammonium chloride chitosan (quaternary chitosan) was synthesized. When applied to treatment of acid dye waste water, the decolorization rate was 95.5%, and the COD removal was 86.4%.

Photocatalysts for Solar-Induced Water Disinfection: New Developments and Opportunities

Recent years have seen a surge of interest in the application of solar energy for water disinfection by using nanostructured photocatalysts elaborately designed and fabricated. Photocatalysis has its unique advantage for utilizing sunlight to drive the disinfection process. The highly reactive oxygen species (ROS) serve as the main oxidants and are capable of inactivating microorganisms, including viruses, bacteria, spores and protozoa. This chapter presents an overview of current research activities that center on the preparation, characterization and application of highly efficient photocatalysts for water disinfection under both UV and visible light irradiation. It is organized into two major parts. One is the development of TiO2-based photocatalysts including surface noble metal modified, ion doped, dye-sensitized, and composite TiO2. The other part is the introduction of new types of photocatalysts and advanced technologies that have recently fascinated the scientific community. Particular attention is given to the pioneering fields such as graphene-based photocatalysts, plasmonic-metal nanostructures and naturally occurring photocatalysts. Finally, we conclude with a discussion of what major advancements are needed to move the field of photocatalytic water disinfection forward.

Heterogeneous photocatalytic degradation of methyl orange in schwertmannite/oxalate suspension under UV irradiation

INTRODUCTION

Schwertmannite was synthesized through an oxidation of FeSO(4) by Acidithiobacillus ferrooxidans LX5 cell suspension at an initial pH 2.5 and 28°C for 3 days and characterized using X-ray diffraction spectroscopy and scanning electron microscope. The schwertmannite photocatalytic degradation of methyl orange (MO) by oxalate was investigated at different initial pH values, concentrations of schwertmannite, oxalate, and MO.

RESULTS

The results demonstrated that photodegradation of MO in the presence of schwertmannite or oxalate alone was very weak. However, the removal of MO was significantly enhanced when schwertmannite and oxalate coexisted in the reaction system. Low pH (4 or less) was beneficial to the degradation of MO. The optimal doses of schwertmannite and oxalate were 0.2 g L(-1) and 2 mM, respectively. Hydroxyl radicals (·OH) and Fe(II), the intermediate products, were also examined during the reaction to explore their correlation with the degradation of MO.

CONCLUSION

A possible mechanism for the photocatalytic decomposition of MO in the study was proposed. The formation of Fe(III)-oxalate complexes on the surface of schwertmannite was a precursor of H(2)O(2) and Fe(II) production, further leading to the yield of ·OH responsible for the decomposition of MO.

Photocatalytic inactivation of Escherichia coli by natural sphalerite suspension: effect of spectrum, wavelength and intensity of visible light

The photocatalytic disinfection of Escherichia coli K-12 is investigated by the natural sphalerite (NS) under different spectra, wavelengths and intensities of visible light (VL) emitted by light-emitting-diode lamp (LED). The spectrum effect of VL on disinfection efficiency is studied by using white LED, fluorescent tube (FT) and xenon lamp (XE), which indicates that the "discreted peak spectrum" of FT is more effective to inactivate bacteria than "continuous spectrum" of LED and XE. Besides, the photocatalytic disinfection of bacteria is compared under different single spectrum (blue, green, yellow and red color) LEDs. The results show that the most effective wavelength ranges of VL for photocatalytic disinfection with the NS are 440-490 and 570-620 nm. Furthermore, a positive relationship is obtained between the disinfection efficiency and the VL intensity. The experiment shows that NS can completely inactivate 10(7)cfu mL(-1)E. coli K-12 within 8h irradiation by white LED with the intensity of 200 mW cm(-2) at pH 8. Moreover, the destruction process of the cell wall and the cell membrane are directly observed by TEM. Finally, no bacterial colony can be detected within a 96 h regrowth test of inactivated bacteria, which reveals that the VL-photocatalytic disinfection leads to an irreversible damage to the bacterial cells.

Naturally occurring sphalerite as a novel cost-effective photocatalyst for bacterial disinfection under visible light

The photocatalytic disinfection capability of the natural semiconducting mineral sphalerite is studied here for the first time. Natural sphalerite can completely inactivate 1.5 × 10(7) cfu/mL E. coli K-12 within 6 h under visible light irradiation. The photocatalytic disinfection mechanism of natural sphalerite is investigated using multiple scavengers. The critical role that electrons play in bactericidal actions is experimentally demonstrated. The involvement of H(2)O(2) in photocatalytic disinfection is also confirmed using a partition system combined with different scavengers. Moreover, the photocatalytic destruction of bacterial cells is observed through transmission electron microscopic analysis. A catalase activity study reveals that antioxidative enzyme activity is high in the initial stage of photocatalytic disinfection but decreases with time due to damage to enzymatic functioning. Natural sphalerite is abundant and easy to obtain and possesses excellent visible-light photocatalytic activity. These superior properties make it a promising solar-driven photocatalyst for large-scale cost-effective wastewater treatment.

Study on adsorption and desorption properties of the starch grafted p-tert-butyl-calix[n]arene for butyl Rhodamine B solution

The adsorbents of starch grafted p-tert-butyl-calix[4,6,8]arene-SGCn (SGC4, SGC6, SGC8) are prepared. The products are characterized by FTIR, elemental analysis, thermal gravimetric analysis and scanning electron microscope. Static adsorption behavior is studied by using SGC8 as adsorbent, butyl Rhodamine B (BRB) solution as simulation dye wastewater. The adsorption of BRB onto SGC8 fits the second order kinetic model and the apparent adsorption rate constant is 0.002 g mg(-1)min(-1) at 25 °C. The equilibrium adsorption data are interpreted using Langmuir and Freundlich models. The adsorption of BRB onto SGC8 is better represented by the Langmuir equation. The thermodynamic parameters for the adsorption reaction are calculated through van't Hoff analysis. The adsorbent may be easily regenerated by using ethanol solution as desorption agent to extract dye from SGC8. The rate of desorption of BRB is dependent on the concentration of ethanol and the temperature. SGC8 exhibits excellent adsorption and desorption properties toward dye molecule. The new-style adsorbent of SGC8 is regarded as a potential adsorbent to deal with dye or organic wastewater.