Fe(VI)-assisted photocatalytic degradating of microcystin-LR using titanium dioxide

Comprehensive strategies for microcystin degradation: A review of the physical, chemical, and biological methods and genetic engineering

Addressing the threat of harmful cyanobacterial blooms (CyanoHABs) and their associated microcystins (MCs) is crucial for global drinking water safety. In this review, we comprehensively analyze and compares the physical, chemical, and biological methods and genetic engineering for MCs degradation in aquatic environments. Physical methods, such as UV treatments and photocatalytic reactions, have a high efficiency in breaking down MCs, with the potential for further enhancement in performance and reduction of hazardous byproducts. Chemical treatments using chlorine dioxide and potassium permanganate can reduce MC levels but require careful dosage management to avoid toxic by-products and protect aquatic ecosystems. Biological methods, including microbial degradation and phytoremediation techniques, show promise for the biodegradation of MCs, offering reduced environmental impact and increased sustainability. Genetic engineering, such as immobilization of microcystinase A (MlrA) in Escherichia coli and its expression in Synechocystis sp., has proven effective in decomposing MCs such as MC-LR. However, challenges related to specific environmental conditions such as temperature variations, pH levels, presence of other contaminants, nutrient availability, oxygen levels, and light exposure, as well as scalability of biological systems, necessitate further exploration. We provide a comprehensive evaluation of MCs degradation techniques, delving into their practicality, assessing the environmental impacts, and scrutinizing their efficiency to offer crucial insights into the multifaceted nature of these methods in various environmental contexts. The integration of various methodologies to enhance degradation efficiency is vital in the field of water safety, underscoring the need for ongoing innovation.

UVA-LED-Assisted Activation of the Ferrate(VI) Process for Enhanced Micropollutant Degradation: Important Role of Ferrate(IV) and Ferrate(V)

This paper investigated ultraviolet A light-emitting diode (UVA-LED) irradiation to activate Fe(VI) for the degradation of micropollutants (e.g., sulfamethoxazole (SMX), enrofloxacin, and trimethoprim). UVA-LED/Fe(VI) could significantly promote the degradation of micropollutants, with rates that were 2.6-7.2-fold faster than for Fe(VI) alone. Comparatively, UVA-LED alone hardly degraded selected micropollutants. The degradation performance was further evaluated in SMX degradation via different wavelengths (365-405 nm), light intensity, and pH. Increased wavelengths led to linearly decreased SMX degradation rates because Fe(VI) has a lower molar absorption coefficient at higher wavelengths. Higher light intensity caused faster SMX degradation, owing to the enhanced level of reactive species by stronger photolysis of Fe(VI). Significantly, SMX degradation was gradually suppressed from pH 7.0 to 9.0 due to the changing speciation of Fe(VI). Scavenging and probing experiments for identifying oxidative species indicated that high-valent iron species (Fe(V)/Fe(IV)) were responsible for the enhanced degradation. A kinetic model involving target compound (TC) degradation by Fe(VI), Fe(V), and Fe(IV) was employed to fit the TC degradation kinetics by UVA-LED/Fe(VI). The fitted results revealed that Fe(IV) and Fe(V) primarily contributed to TC degradation in this system. In addition, transformation products of SMX degradation by Fe(VI) and UVA-LED/Fe(VI) were identified and the possible pathways included hydroxylation, self-coupling, bond cleavage, and oxidation reactions. Removal of SMX in real water also showed remarkable promotion by UVA-LED/Fe(VI). Overall, these findings could shed light on the understanding and application of UVA-LED/Fe(VI) for eliminating micropollutants in water treatments.

Pre-oxidation enhanced cyanobacteria removal in drinking water treatment: A review

Cyanobacterial bloom has many adverse effects on source water quality and drinking water production. The traditional water treatment process can hardly achieve satisfactory removal of algae cells. This review examines the impact of pre-oxidation on the removal of cyanobacteria by solid-liquid separation processes. It was reported that the introduction of chemical oxidants such as chlorine, potassium permanganate, and ozone in algae-laden water pretreatment could improve algae removal by the subsequent solid-liquid separation processes. However, over dosed oxidants can result in more serious water quality risks due to significant algae cell lysis and undesirable intracellular organic matter release. It was suggested that moderate pre-oxidation may enhance the removal of cyanobacteria without damaging algae cells. In this article, effects of moderate pretreatment on the solid-liquid separation processes (sedimentation, dissolved air flotation, and membrane filtration) are reviewed.

Enhanced Radical Generation in an Ultraviolet/Chlorine System through the Addition of TiO2

Ultraviolet (UV)/chlorine draws increasing attention for the abatement of recalcitrant organic pollutants. Herein, it was found that TiO₂ would significantly promote the degradation of dimethyl phthalate (DMP) in the UV/chlorine system (from 19 to 84%). Hydroxyl radicals (HO^•) and chlorine radicals (Cl^•) were the dominant reactive species for DMP degradation in the UV/chlorine/TiO₂ system. Chlorine decayed much faster in UV/chlorine/TiO₂ compared with UV/chlorine, which is possibly because photogenerated electrons (e_cb^-) and superoxide radicals (O₂^•-) have high reactivity with chlorine. As a result, the recombination of photogenerated holes (h_vb⁺) and e_cb^- was inhibited and the accumulation of HO^• and Cl^• was facilitated. A kinetic model was established to simulate the reaction process, and it was found that the concentrations of HO^• and Cl^• were several times to dozens of times higher in UV/chlorine/TiO₂ than that in UV/chlorine. The contributions of HO^• and Cl^• to DMP degradation were 70.3 and 29.7% by model simulation, respectively, and were close to the probe experiment result. In the UV/chlorine/TiO₂ system, the degradation of DMP did not follow pseudo-first-order kinetics but the degradation of benzoate fitted well with pseudo-first-order kinetics. This phenomenon was elucidated by the structure of the pollutant and TiO₂ and further tested by calculating the adsorption energy (E_ads)/binding energy (E_b) with density functional theory. Due to faster decay of chlorine, lower amounts of disinfection byproducts formed in UV/chlorine/TiO₂ compared with UV/chlorine. Adding TiO₂ into the UV/chlorine system can promote the degradation of recalcitrant organic pollutants in an aqueous environment.

Improved ciprofloxacin removal by a Fe(VI)-Fe3O4/graphene system under visible light irradiation

In this paper, Fe₃O₄/graphene (Fe₃O₄/GE) nanocomposites were prepared by a co-precipitation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectra (UV-vis DRS). The composites were used in combination with Fe(VI) to construct a Fe(VI)-Fe₃O₄/GE system in order to degrade ciprofloxacin (CIP) in simulated water samples. The photocatalytic properties of Fe(VI)-Fe₃O₄/GE were evaluated under visible light irradiation. The concentration of CIP in solution was detected by high performance liquid chromatography (HPLC). A series of results showed that Fe(VI), as a good electron capture agent, could significantly improve the treatment performance. Major determining factors during CIP degradation were also investigated, in which solution pH of 9, Fe(VI) to Fe₃O₄/GE dosage ratio of 1:25 and GE content in the Fe₃O₄/GE nanocomposites of 10 wt% were found to be the best experimental conditions. The results demonstrated that the Fe(VI)-Fe₃O₄/GE system could offer an alternative process in water treatment in addition to the current Fe(VI)-UV/TiO₂ process.

A comparative study of microcystin-LR degradation by electrogenerated oxidants at BDD and MMO anodes

This study investigated the electrochemical degradation of microcystin-LR (MC-LR) using boron-doped diamond (BDD) anode and mixed metal oxides (MMO, IrO₂Ta₂O₅/Ti) anode in different medium. In-situ electrogenerated oxidants including hydroxyl radical, active chlorine, and persulfate were confirmed in phosphate, chloride, and sulfate medium, respectively. Different from MMO anode, hydroxyl radical was observed to play a significant role in chlorine generation at BDD anode in chloride medium. Besides, BDD anode could activate sulfate electrochemically due to its high oxygen evolution potential, and MC-LR degradation rate increased with the decrease of solution pH. The effects of natural organic matters (NOM) and algal organic matters (AOM) on MC-LR degradation were evaluated and NOM presented stronger inhibition ability than AOM. Furthermore, the intermediates generated in MC-LR degradation in chloride and sulfate medium were identified by LC/MS/MS and possible degradation pathways were proposed based on the experiments results. Benzene ring and conjugated diene bonds of Adda group and double bonds of Mhda group were found to be the reactive sites of MC-LR. Overall, this study broadens the knowledge of electrochemical oxidation in removing microcystins in algae-laden water.

Oxidation of microcystin-LR by ferrate(VI): kinetics, degradation pathways, and toxicity assessments

The presence of the potent cyanotoxin, microcystin-LR (MC-LR), in drinking water sources poses a serious risk to public health. The kinetics of the reactivity of ferrate(VI) (Fe(VI)O4(2-), Fe(VI)) with MC-LR and model compounds (sorbic acid, sorbic alcohol, and glycine anhydride) are reported over a range of solution pH. The degradation of MC-LR followed second-order kinetics with the bimolecular rate constant (kMCLR+Fe(VI)) decreasing from 1.3 ± 0.1 × 10(2) M(-1) s(-1) at pH 7.5 to 8.1 ± 0.08 M(-1) s(-1) at pH 10.0. The specific rate constants for the individual ferrate species were determined and compared with a number of common chemical oxidants employed for water treatment. Detailed product studies using liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) indicated the oxidized products (OPs) were primarily the result of hydroxylation of the aromatic ring, double bond of the methyldehydroalanine (Mdha) amino acid residue, and diene functionality. Products studies also indicate fragmentation of the cyclic MC-LR structure occurs under the reaction conditions. The analysis of protein phosphatase (PP1) activity suggested that the degradation byproducts of MC-LR did not possess significant biological toxicity. Fe(VI) was effective for the degradation MC-LR in water containing carbonate ions and fulvic acid (FA) and in lake water samples, but higher Fe(VI) dosages would be needed to completely remove MC-LR in lake water compared to deionized water.

Progress of TiO2 Photocatalytic Oxidation Technology in Algae Removal

In this paper, the development in the degradation of microcystic toxins and algae removal by TiO2photocatalysts is reviewed, analyzing the prospect and pinpointing the key factor of the technology. TiO2, which is insoluble, nontoxic, inexpensive and catalyst-efficient, shows good chemical inertness. For the degradation of microcystic toxins and algae removal, TiO2 photocatalysts is superior to the traditional processing technology and method, contributing a lot to environmental protection.

Removal of microorganisms and their chemical metabolites from water using semiconductor photocatalysis

Semiconductor photocatalysis has been applied to the remediation of an extensive range of chemical pollutants in water over the past 30 years. The application of this versatile technology for removal of micro-organisms and cyanotoxins has recently become an area that has also been the subject of extensive research particularly over the past decade. This paper considers recent research in the application of semiconductor photocatalysis for the treatment of water contaminated with pathogenic micro-organisms and cyanotoxins. The basic processes involved in photocatalysis are described and examples of recent research into the use of photocatalysis for the removal of a range of microorganisms are detailed. The paper concludes with a review of the key research on the application of this process for the removal of chemical metabolites generated from cyanobacteria.

Photocatalytic degradation efficiency and mechanism of microcystin-RR by mesoporous Bi₂WO₆ under near ultraviolet light

Microcystin-RR (MC-RR) is one of the most common cyanotoxin microcystins in fresh water and is of great concern due to its potential hepatotoxicity. In the present study, Bi(2)WO(6) was synthesized with a hydrothermal method by varying the pH of the reaction solution in the range of 1-11. The surface area of the catalysts decreased, but the crystallinity and crystal size increased with the pH. The adsorption and degradation capacities of the catalysts decreased with increasing the preparation solution pH. The Bi(2)WO(6) prepared at pH 1 (Bi(2)WO(6)-pH1) displayed the highest adsorption and degradation capacity to MC-RR even though it consisted of randomly aggregated particles. Nearly 100% of MC-RR at 10 mg L(-1) was removed after 30 min of irradiation of near-ultraviolet light (300-400 nm) in a solution with Bi(2)WO(6) concentration of 0.2 g L(-1). The photodegradation efficiency of Bi(2)WO(6)-pH1 was greater in acid medium than in basic solutions. Several intermediate products were observed and identified by liquid chromatography/mass spectrometry/mass spectrometry, and a unique photodegradation pathway was proposed. It was assumed that a photo-Kolbe process happened at the site carboxyl acid group of the d-Glu residue by the photogenerated holes, producing a hydroperoxyl product at m/z 513.8. This intermediate could be further decomposed to an alcohol product at m/z 505.8 and a ketone product at m/z 504.8. The aromatic ring and diene bond of the Adda chain could also be attacked by the holes and form phenol and diol products.

Ferrate(VI) enhanced photocatalytic oxidation of pollutants in aqueous TiO2 suspensions

BACKGROUND, AIM AND SCOPE

Photocatalytic oxidation using UV irradiation of TiO(2) has been studied extensively and has many potential industrial applications, including the degradation of recalcitrant contaminants in water and wastewater treatment. A limiting factor in the oxidation process is the recombination of conduction band electrons (e(-)(cb)) with electron holes (h(vb)(+)) on the irradiated TiO(2) surface; thus, in aqueous conditions, the presence of an effective electron scavenger will be beneficial to the efficiency of the oxidation process. Ferrate (FeO(4)(2-)) has received much recent attention as a water treatment chemical since it behaves simultaneously as an oxidant and coagulant. The combination of ferrate [Fe(VI)] with UV/TiO(2) photocatalysis offers an oxidation synergism arising from the Fe(VI) scavenging of e(-)(cb) and the corresponding beneficial formation of Fe(V) from the Fe(VI) reduction. This paper reviews recent studies concerning the photocatalytic oxidation of problematic pollutants with and without ferrate.

MATERIALS AND METHODS

The paper reviews the published results of laboratory experiments designed to follow the photocatalytic degradation of selected contaminants of environmental significance and the influence of the experimental conditions (e.g. pH, reactant concentrations and dissolved oxygen). The specific compounds are as follows: ammonia, cyanate, formic acid, bisphenol-A, dibutyl- and dimethyl-phthalate and microcystin-LR. The principal focus in these studies has been on the rates of reaction rather than on reaction pathways and products.

RESULTS

The presence of UV/TiO(2) accelerates the chemical reduction of ferrate, and the reduction rate decreases with pH owing to deprotonation of ferrate ion. For all the selected contaminant substances, the photocatalytic oxidation rate was greater in the presence of ferrate, and this was believed to be synergistic rather than additive. The presence of dissolved oxygen in solution reduced the degradation rate of dimethyl phthalate in the ferrate/photocatalysis system. In the study of microcystin-LR, it was evident that an optimal ferrate concentration exists, whereby higher Fe(VI) concentrations above the optimum leads to a reduction in microcystin-LR degradation. In addition, the rate of microcystin-LR degradation was found to be strongly dependent on pH and was greatest at pH 6.

DISCUSSION

The initial rate of photocatalytic reduction under different conditions was analysed using a Langmuirian form. Decrease in rates in the presence of dissolved oxygen may be due to competition between oxygen and ferrate as electron scavengers and to non-productive radical species interactions. The reaction between ferrate(VI) and microcystins-LR in the pH range of 6.0-10.0 is most likely controlled by the protonated Fe(VI) species, HFeO(4)(-).

CONCLUSIONS

The photocatalytic oxidation of selected, recalcitrant contaminants was found to be significantly greater in the presence of ferrate, arising from the role of ferrate in inhibiting the h(vb)(+)-e(-)(cb) pair recombination on TiO(2) surfaces and the corresponding generation of highly oxidative Fe(V) species. The performance of the ferrate/photocatalysis system is strongly influenced by the reaction conditions, particularly the pH and dissolved oxygen concentration, arising from the complex nature of the interactions between the catalyst and the solution. Overall, the treatment performance of the Fe(VI)-TiO(2)-UV system is generally superior to alternative chemical oxidation methods.

RECOMMENDATIONS AND PERSPECTIVES

The formation of intermediate Fe(V) species in the photocatalytic reduction of ferrate(VI) requires confirmation, and a method involving electron paramagnetic resonance spectroscopy could be applied for this. The reactivity of Fe(V) with the selected contaminants is required in order to better understand the role of ferrate in the Fe(VI)-TiO(2)-UV oxidation system. To increase the practical utility of the system, it is recommended that future studies involving the photocatalytic oxidation of pollutants in the presence of ferrate(VI) should focus on developing modified TiO(2) surfaces that are photocatalytic under visible light conditions.

On-line production of ferrate with an electrochemical method and its potential application for wastewater treatment--a review

A number of studies on the oxidation of various organic/inorganic contaminants by ferrate(VI) were reported in the 1980s and 1990s. The exploration of the use of ferrate(VI) for water and wastewater treatment has been well addressed recently. However, challenges have existed for the implementation of ferrate(VI) technology in practice due to the instability of a ferrate solution or high production cost of solid ferrate products. The research has been carried out aiming at the generation and application of ferrate(VI) in situ. This paper thus reviews ferrate chemistry and its overall performance as a water treatment chemical, discusses the factors affecting the ferrate yield efficiency using the electrochemical method, and finally, summarises the work on the production and use of ferrate in situ which is currently under study.

Reaction pathways of dimethyl phthalate degradation in TiO2-UV-O2 and TiO2-UV-Fe(VI) systems

The photocatalytic degradation of dimethyl phthalate (DMP) in aqueous TiO2 suspension under UV illumination has been investigated using oxygen (O2) and ferrate (Fe(VI)) as electron acceptors. The experiments demonstrated that Fe(VI) was a more effective electron acceptor than O2 for scavenging the conduction band electrons from the surface of the catalyst. Some major intermediate products from DMP degradation were identified by HPLC and GC/MS analyses. The analytical results identified dimethyl 3-hydroxyphthalate and dimethyl 2-hydroxyphthalate as the two main intermediate products from the DMP degradation in the TiO2-UV-O2 system, while in contrast phthalic acid was found to be the main intermediate product in the TiO2-UV-Fe(VI) system. These findings indicate that DMP degradation in the TiO2-UV-O2 and TiO2-UV-Fe(VI) systems followed different reaction pathways. An electron spin resonance analysis confirmed that hydroxyl radicals existed in the TiO2-UV-O2 reaction system and an unknown radical species (most likely an iron-oxo species) is suspected to exist in the TiO2-UV-Fe(VI) reaction system. Two pathway schemes of DMP degradation in the TiO2-UV-O2 and TiO2-UV-Fe(VI) reaction systems are proposed. It is believed that the radicals formed in the TiO2-UV-O2 reaction system preferably attack the aromatic ring of the DMP, while in contrast the radicals formed in the TiO2-UV-Fe(VI) reaction systems attack the alkyl chain of DMP.

Aqueous oxidation of dimethyl phthalate in a Fe(VI)-TiO(2)-UV reaction system

The application of a combined ferrate-photocatalysis process for the aqueous degradation of dimethyl phthalate (DMP) has been studied. The behaviour of the Fe(VI)-TiO(2)-UV process at pH 9 in the presence and absence of dissolved oxygen (DO) has been compared and significant differences have been found. In comparative tests under N(2) and O(2) bubbling, the chemical reduction rate of ferrate as conduction band electron acceptor was similar, but the resulting degradation of DMP was substantially lower in the presence of oxygen. It is speculated that the presence of oxygen leads to the formation of Fe-O-(organic) complex species that adsorb to, and deactivate, the surface of the photocatalyst. The presence of surface-adsorbed complex species was indicated by FTIR spectroscopy and a reduced TiO(2) adsorption capacity for DMP. In the presence of typical environmental levels of DO ( approximately 9mgL(-1)), the Fe(VI)-TiO(2)-UV process achieved a modest degree of DMP degradation (40% in 120min).

Mesoporous nitrogen-doped TiO2 for the photocatalytic destruction of the cyanobacterial toxin microcystin-LR under visible light irradiation

The presence of the harmful cyanobacterial toxins in water resources worldwide drives the development of an innovative and practical water treatment technology with great urgency. This study deals with two important aspects: the fabrication of mesoporous nitrogen-doped TiO2 (N-TiO2) photocatalysts and their environmental application for the destruction of microcystin-LR (MC-LR) under visible light. In a nanotechnological sol-gel synthesis method, a nitrogen-containing surfactant (dodecylammonium chloride) was introduced as a pore templating material for tailor-designing the structural properties of TiO2 and as a nitrogen dopant for its visible light response. The resulting N-TiO2 exhibited significantly enhanced structural properties including 2-8 nm mesoporous structure (porosity 44%) and high surface area of 150 m2/g. Red shift in light absorbance up to 468 nm, 0.9 eV lower binding energy of electrons in Ti 2p state, and reduced interplanar distance of crystal lattices proved nitrogen doping in the TiO2 lattice. Due to its narrow band gap at 2.65 eV, N-TiO2 efficiently degraded MC-LR under visible spectrum above 420 nm. Acidic condition (pH 3.5) was more favorable for the adsorption and photocatalytic degradation of MC-LR on N-TiO2 due to electrostatic attraction forces between negatively charged MC-LR and +6.5 mV charged N-TiO2. Even under UV light, MC-LR was decomposed 3-4 times faster using N-TiO2 than control TiO2. The degradation pathways and reaction intermediates of MC-LR were not directly related to the energy source for TiO2 activation (UV and visible) and nature of TiO2 (neat and nitrogen-doped). This study implies a strong possibility for the in situ photocatalytic remediation of contaminated water with cyanobacterial toxins and other toxic compounds using solar light, a sustainable source of energy.