Treatment Of Organic Pollutants Research Articles

Performance improvement of polyethersulfone membranes with Ti3AlCN MAX phase in the treatment of organic and inorganic pollutants

In this work, the hydrophobic polyethersulfone (PES) membrane was modified by incorporating Ti3AlCN MAX phase. Synthesis of Ti3AlCN MAX phase was performed using the reactive sintering method. The scanning electron microscopy (SEM) images showed a 3D compressed layered morphology for the synthesized MAX phase. The Ti3AlCN MAX phase was added to the casting solution, and the mixed-matrix membranes were fabricated by the non-solvent induced phase inversion method. The performance and antifouling features of bare and modified membranes were explored by pure water flux, flux recovery ratio (FRR), and fouling resistance parameters. Through the modification of membranes by introducing the Ti3AlCN MAX phase, the enhancement of these features was observed, in which the membrane containing 1 wt% of MAX phase showed 17.7 L/m2.h.bar of permeability and 98.6% for FRR. Also, the separation efficiency of all membranes was evaluated by rejecting organic and inorganic pollutants. The Ti3AlCN MAX membranes could reject 96%, 95%, and 88% of reactive blue 50, Rose Bengal, and azithromycin antibiotics, respectively, as well as 98%, 80%, 86%, and 36% of Pb2+, As5+, Na2SO4, and NaCl, respectively. Finally, the outcomes indicated the Ti3AlCN MAX phase was an excellent and efficient novel additive for modifying the PES membrane.

Photocatalytic degradation of sulfamethazine using g-C3N4/TiO2 heterojunction photocatalyst: Performance, mechanism insight and degradation pathway

In the present research, Z-scheme g-C3N4/TiO2 heterojunction composite photocatalysts with varied g-C3N4 content were successfully prepared using the sol-gel technique combined with calcination methods. The morphology, crystallinity, chemical composition, and optical properties of the synthesized catalysts were characterized employing a series of techniques. Sulfamethazine (SMZ) was selected as the model pollutant to evaluate the photocatalytic activity of the prepared catalysts employing a 500 W xenon lamp as the light source to simulate sunlight. The results demonstrated that within 180 min of irradiation, 94.8 % of SMZ was removed by using the optimal photocatalyst (0.4CN/TiO2 composite sample), affording an apparent first-order rate coefficient of 0.0152 min−1 which was 3.8 or 2.0 times as much as that involving single g-C3N4 or TiO2 catalyst, respectively. The effects of catalyst dosage, initial pH, different water matrices, different substrates, and inorganic ions on the performance of 0.4CN/TiO2 photocatalyst were explored. Furthermore, the 0.4CN/TiO2 composite also displayed satisfactory cyclic stability. The results of radical quenching experiments revealed that the key active species in SMZ degradation were hydroxyl radicals (•OH), superoxide radicals (O2•−), and holes (h+). The efficient separation of photo-generated electrons and holes originating in the formation of a Z-scheme g-C3N4/TiO2 heterojunction was evidenced by photoluminescence and electrochemical impedance spectroscopy determination and should be responsible for the enhanced catalytic activity of the 0.4CN/TiO2 sample. In addition, the plausible pathways for the aqueous degradation of SMZ were suggested according to the identified degradation intermediates employing the HPLC/MS technique. In summary, the current research provides a novel insight into the development of Z-scheme heterojunctions in the realm of organic pollutant treatment.

Catalytic Janus Nanoparticle-Based Recyclable Emulsifiers for Collaborative Treatment of Water-Soluble and Water-Insoluble Organic Pollutants

Catalytic Janus Nanoparticle-Based Recyclable Emulsifiers for Collaborative Treatment of Water-Soluble and Water-Insoluble Organic Pollutants

Congo red dye reduction mediated by the electron (e−) transfer route of BH4 − ions using synthesized NiCo2O4/rGO hybrid nanosheets

The treatment of toxic organic pollutants is extremely important for the conservation of clean air, soil, and water. In this study, (reduced graphene oxide) NiCo2O4/ rGO hybrid nanocomposite was prepared by a facile hydrothermal technique and employed for organic dye adsorption from wastewater. The synthesized NiCo2O4/rGO hybrid nanocomposite was studied using FTIR, XRD, SEM, TEM, BET, Raman spectroscopy, and UV–visible. The physical characterizations prove the deposition of NiCo2O4 particles on the rGO surface. The transmission electron microscope image demonstrated that the NiCo2O4 particles with an average size of ∼46 nm was dispersed on the rGO surface. The obtained nanoparticles show a higher specific surface area of 56.4 m2 g−1. Adsorption dynamics as investigated by time and concentration variation show that the adsorption data follows pseudosecond order kinetics and the Langmuir isotherm model, with a maximum adsorption capacity of 106.2 mg g−1, indicating homogeneous physiochemical adsorption of CR dye on the adsorbent surface. Besides, the catalytic effectiveness of synthesized nanocomposite towards Congo red (CR) dye reduction mediated by the electron (e−) transfer route of BH4 − ions was explained in detail. The electrostatic interaction used between the NiCo2O4/rGO hybrid composite and Congo red increased the adsorption ion effectiveness of the dye sample.

High-Efficient photocatalytic degradation of Levofloxacin via a novel ternary Fe2O3/CeO2/ZnO Heterostructure: Synthesis Optimization, Characterization, toxicity assessment and mechanism insight

Considering the serious problem of lack of freshwater worldwide and the effectiveness of the photocatalysis process in water purification, we prepared a novel Ternary Fe2O3/CeO2/ZnO photocatalyst toward degradation of Levofloxacin (LEF) antibiotic. In this study, the CeO2 was synthesized through the hydrothermal route and the nanocomposite was prepared using a simple-impregnation method. Then the synthesized materials with various weight percentages 5 %FeCe (10, 20, 30 and 40 %) on the ZnO were completely characterized in the aspects of structural, morphological and optical properties with the aid of XRD, XPS, BET, TEM, SEM, EIS, ESR, Band-gap, Photocurrent, Mott Schottky, etc. The XPS analysis depicts the successful construction of required bonds among FeCe/ZnO materials. The enhancement of LEF decomposition was investigated utilizing RSM-CCD method, considering factors such as the amount of catalyst used, irradiation time, pH levels, and the initial concentration of LEF. The trapping experiments of free radicals highlight the significant contributions of the •OH and •O2– radicals. A notable degradation rate of 98.9 % was achieved under optimal operational conditions, which included catalyst dosage of 0.69 g/L, irradiation period of 83.64 min, initial LEF concentration of 37.9 mg/L, and pH of 6.18. Furthermore, the findings of EIS, photocurrent, and Mott-Schottky analyses allowed for proposing a suitable charge transfer mechanism and demonstrated effective separation of photo-generated electron-hole pairs. Additionally, LC-MS analysis was utilized to elucidate the intermediate products generated during the photodegradation of LEF, leading to the proposal of plausible degradation pathways. The exceptional efficiency of the 30FeCe/ZnO photocatalyst highlights its promising potential for the efficient treatment of persistent organic pollutants on a large scale in wastewater treatment plants.

Application of Bi0.5Na0.5TiO3 −based catalysts for piezocatalytic, photocatalytic and piezo-photocatalytic degradation of organic pollutants in wastewater remediation

In order to attain visible light driven piezo-photocatalyst for treatment of organic pollutants, Ag doped Bi0.5Na0.5TiO3 particles (Bi0.5Na0.5-x AgxTiO3 particles with x = 0, 0.01, 0.03, and 0.05) have been synthesised via sol–gel route. X-ray diffraction (XRD) study confirmed the monoclinic phase structure with Cc space group for all compositions. The Rietveld refinement using FullPROF program was performed to ensure the phase purity of the samples. The structure is reconfirmed by matching vibrational modes in Raman spectra as well. Morphological and elemental analysis were done using scanning electron microscopy (SEM) equipped energy dispersive x-ray spectra (EDX) and found the particle size ranging from 300-400 nm. UV–vis absorbance spectra were employed to find the optical bandgap for all synthesised particles, found in the range of 3.65–2.63 eV. The best piezo-photocatalytic activity found for sample x = 0.01 tested for rhodamine B dye, which is ∼ 82 % in 80 min under the exposure of UV light and mechanical disturbances, whereas degradation was ∼ 52.4 % and ∼ 22.7 % for photo- and piezocatalysis, respectively. Kinetics study and rate constant values were found using first order rate kinetics equation. Trapping experiment and stability test for the catalysts were done systematically, finding that hydroxyl and superoxide radicals are major active species and ∼ 9 % decrement in degradation capacity over three cycles were found due to loss of catalyst amount. The improvement in degradation is attributed to least recombination rate, greater oxygen vacancies, high degradation yield, high absorption capacity and optimum bandgap values over the other compositions. Therefore, 1 % Ag doped BNT sample can be novel efficient piezo-photocatalyst for treatment of organic dyes.

Facile one-step solvothermal synthesis of flower-like BiOBr microspheres for the efficient degradation of organic dyes under visible light irradiation

In this study, BiOBr microspheres were successfully synthesized through a facile solvothermal route in the presence of glycerol. The photocatalysts were characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The morphology of BiOBr-6, which was synthesized using glycerol, comprised flower-like structures with diameters of ∼2 μm, which were composed of two-dimensional flakes with the sizes of tens of nanometers. The photocatalytic activity of the optimized BiOBr-6 was evaluated through the photocatalytic degradation of rhodamine B, methylene blue, methyl orange, and chrome blue-black R under visible light irradiation. Within 60 min, the degradation rates of Rh B、MB and MO reached 99.7 %, 99.6 % and 98.1 %, and within 90 min，the degradation rates of C-R reached 98.0 %. We concluded that the highly visible-light-driven catalytic performance of the flower-like BiOBr sample was associated with its high surface area, porous structure, and low bandgap energy, as well as its unique microsphere morphology. The resulting BiOBr microspheres may serve as promising adsorbents and photocatalysts for the treatment of organic pollutants.

Carbon Nanofiber Membranes Loaded with MXene@g-C3N4: Preparation and Photocatalytic Property

In this study, a Ti3C2 MXene@g-C3N4 composite powder (TM-CN) was prepared by the ultrasonic self-assembly method and then loaded onto a carbon nanofiber membrane by the self-assembly properties of MXene for the treatment of organic pollutants in wastewater. The characterization of the TM-CN and the C-TM-CN was conducted via X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectrometer (FTIR) to ascertain the successful modification. The organic dye degradation experiments demonstrated that introducing an appropriate amount of Ti3C2 MXene resulted in the complete degradation of RhB within 60 min, three times the photocatalytic efficiency of a pure g-C3N4. The C-TM-CN exhibited the stable and outstanding photocatalytic degradation of the RhB solution over a wide range of pH values, indicating the characteristics of the photodegradation of organic pollutants in a wide range of aqueous environments. Furthermore, the results of the cyclic degradation experiments demonstrated that the C-TM-CN composite film maintained a degradation efficiency of over 85% after five cycles, thereby confirming a notable improvement in its cyclic stability. Consequently, the C-TM-CN composite film exhibits excellent photocatalytic performance and is readily recyclable, making it an auspicious eco-friendly material in water environment remediation.

Preparation of a New and Effective Heterogeneous Catalyst for Treatment of Organic Pollutant Using Fenton Process

Preparation of a New and Effective Heterogeneous Catalyst for Treatment of Organic Pollutant Using Fenton Process

The effective polymeric macromolecule as peroxymonosulfate activator as an alternative to metal oxides in the treatment of organic dye pollutants

Catalyst efficiency is affected by inhomogeneous catalyst atom placement on the substrate, leading to nano-level aggregation and reduced performance. Research on single-atom catalysts aims to place them at the atomic level to maximize efficiency, but faces challenges and high costs. This study presents a cost-effective approach using polymeric phthalocyanine Co(II) complex (poly-CoPc) prepared by a simple method. Poly-CoPc shows superior catalytic activity by acting as a catalyst without requiring additional processing. Co(II) atoms in the polymer matrix are covalently coordinated into single atoms and inhibit nanoparticle growth. The poly-CoPc/PMS system outperforms Co3O4 and unsubstituted cobalt phthalocyanine, achieving 97.06% Rhodamine B degradation in 20 min. Furthermore, the poly-CoPc proves to be the most effective, with a degradation efficiency of 69.94% for tap water and 95.76% for seawater, producing predominantly 1O2. These results position poly-CoPc as a promising PMS activator for environmental treatment that surpasses metal oxide derivatives in terms of activity and properties.

The Elimination of Levofloxacin from High-Salinity Wastewater via the Electrochlorination Process

The electrochlorination (E-Cl) process has attracted much attention as it is a highly efficient method for treating organic compounds in hypersaline wastewater. In this study, the E-Cl process was utilized for the removal of antibiotics. The optimal experimental conditions were determined to be a NaCl concentration of 100 mM, a current density of 1.5 mA/cm2, a pH of 7.0, and a plate spacing of 1 cm, with a levofloxacin (LEV) degradation efficiency reaching as high as 99% using this setup. The effects of the presence of other ions and humic acid on the E-Cl process were investigated, and it was found that the degradation of LEV was not significantly affected by the presence of coexisting substances. In addition, free chlorine was identified as the primary active species for the degradation of LEV by means of a quenching experiment. It was demonstrated by 3D EEM and TOC that LEV was not completely mineralized and that intermediate products may be present. In order to reveal the degradation pathways of LEV, its degradation products were also analyzed via LC-MS, and some possible pathways of LEV degradation in this system were proposed. The successful degradation of LEV demonstrated that the E-Cl process is an efficient and promising technique for the treatment of organic pollutants in high-salinity wastewater.

Efficient Catalytic Elimination of Toxic Volatile Organic Compounds via Advanced Oxidation Process Wet Scrubbing with Bifunctional Cobalt Sulfide/Activated Carbon Catalysts.

Advanced oxidation process (AOP) wet scrubber is a powerful and clean technology for organic pollutant treatment but still presents great challenges in removing the highly toxic and hydrophobic volatile organic compounds (VOCs). Herein, we elaborately designed a bifunctional cobalt sulfide (CoS2)/activated carbon (AC) catalyst to activate peroxymonosulfate (PMS) for efficient toxic VOC removal in an AOP wet scrubber. By combining the excellent VOC adsorption capacity of AC with the highly efficient PMS activation activity of CoS2, CoS2/AC can rapidly capture VOCs from the gas phase to proceed with the SO4•- and HO• radical-induced oxidation reaction. More than 90% of aromatic VOCs were removed over a wide pH range (3-11) with low Co ion leaching (0.19 mg/L). The electron-rich sulfur vacancies and low-valence Co species were the main active sites for PMS activation. SO4•- was mainly responsible for the initial oxidation of VOCs, while HO• and O2 acted in the subsequent ring-opening and mineralization processes of intermediates. No gaseous intermediates from VOC oxidation were detected, and the highly toxic liquid intermediates like benzene were also greatly decreased, thus effectively reducing the health toxicity associated with byproduct emissions. This work provided a comprehensive understanding of the deep oxidation of VOCs via AOP wet scrubber, significantly accelerating its application in environmental remediation.

Construction of Fe-based amorphous active surface microenvironment and its synergistic Fenton-like treatment of organic pollutants

Fe-based amorphous materials with the superiorities of high efficiency and low Fe sludge secondary pollution can be seen as a potential alternative to Fenton catalysts. However, catalyst deactivation on the catalyst surface hinders the sustainability of high degradation activity. In this study, the degradation capabilities of FeBCCr amorphous alloys in methylene blue (MB) solution by Fenton-like method were explored. The optimum (Fe81B10C9)99Cr1 ribbon with a more uniform Fe3B-like amorphous structure, which exhibits a thin and easily-peeled oxide layer, provides unique dynamic self-renewing to obtain excellent efficiency and recyclability. The degradation rate with 0.05 g/L (Fe81B10C9)99Cr1 ribbon reached 0.248 min−1 and the recyclability reached a charming record of over 90 times with a dosage of 0.1 g ribbon. The activation energy of (Fe81B10C9)99Cr1 ribbon (12.97 kJ mol−1) was found to be superior to that of most metallic catalysts. More importantly, the boron-rich film in the interfacial region can both retard further oxidation of iron and provides a channel for electron transmission, which can contribute to the persistence of a highly active microenvironment with more Fe(Ⅱ) sites and highly utilization of H2O2. The limit segmentation method has verified the good performance in both heterogeneous and homogeneous reactions. This study introduces a strategy for designing high-performance non-noble metallic catalysts through cluster structure regulation, which provides a novel Fenton-like amorphous material with long recyclability of high degradation activity for water remediation.

Progress of photocatalytic oxidation-adsorption synergistic removal of organic arsenic in water.

Photocatalytic oxidation-adsorption synergistic treatment of organic arsenic pollutants is a promising wastewater treatment technology, which not only degrades organic arsenic pollutants by photocatalytic degradation but also removes the generated inorganic arsenic by adsorption. This paper compares the results of photocatalytic oxidation-adsorption co-treatment of organic arsenic pollutants such as monomethylarsonic acid, dimethylarsinic acid, phenylarsonic acid, p-arsanilic acid, and 3-nitro-4-hydroxyphenylarsonic acid on titanium dioxide, goethite, zinc oxide, and copper oxide. It examines the influence of the morphology of organic arsenic molecules, pH, coexisting ions, and the role of natural organic matter. The photocatalytic oxidation-adsorption co-treatment mechanism is investigated, comparing the hydroxyl radical oxidation mechanism, the hydroxyl radical and superoxide anion radical cooxidation mechanism, and the hydroxyl radical and hole cooxidation mechanism. Finally, the future prospects of metal oxide photocatalytic materials and the development of robust and efficient technologies for removing organic arsenic are envisioned.

Transformation of metoprolol in UV/PDS process: Role and mechanisms of degradation and polymerization

Advanced oxidation processes for the treatment of organic pollutants in wastewater suffer from difficulties in mineralization, potential risks of dissolved residues, and high oxidant consumption. In this study, radical-initiated polymerization is dominated in an UV/peroxydisulfate (PDS) process to eliminate organic pollutant of pharmaceutical metoprolol (MTP). Compared with an ideal degradation-based UV/PDS process, the present process can save four fifths of PDS consumption at the same dissolved organic carbon removal of 47.3%. Simultaneously, organic carbon can be recovered from aqueous solution by separating solid polymers at a ratio of 50% of the initial chemical oxygen demand. The chemical structure of products was analyzed to infer the transformation pathways of MTP. Unlike previous studies on simple organic pollutants that the polymerization can occur independently, the polymerization of MTP is dependent on the partial degradation of MTP, and the main monomer in polymerization is a dominant degradation product (4-(2-methoxyethyl)-phenol, denoted as DP151). The separated solid polymers are formed by repeated oxidation and coupling of DP151 or its derivatives through a series of intermediate oligomers. This proof-of-concept study demonstrates the advantage of polymerization-dominated mechanism on dealing with large organic molecules with complex structures, as well as the potential of UV/PDS process for simultaneous organic pollution reduction and organic carbon recovery from aqueous solution.

A novel bifunctional particle electrode with abundant Fe/Fe3C and Fe-N-C sites to enhance the performance of electro-Fenton in degrading organic pollutant

Electro-Fenton (EF) technology emerges as a promising approach to address the treatment of persistent organic pollutants. Nevertheless, its practical implementation often comes with certain drawbacks, like the low H2O2 activation efficiency, poor pH adaptability, and difficult recycling of the catalysts. To this end, we introduced highly active Fe-N-C sites onto the surface of nitrogen-doped graphene, improved the chemical environment of its central Fe atoms with Fe/Fe3C nanoparticles, effectively regulated the multi-electron oxygen reduction process of the catalyst in the electrochemical reaction, and realized the efficient production and activation of H2O2 in the electro-Fenton process. The synthesized Fe/Fe3C@FeNG-1000 achieved efficient removal of refractory organic pollutants in neutral conditions. The rate constant of degradation for tetracycline hydrochloride (TC) in the electro-Fenton reaction system at pH 7 was recorded as 0.1011 min−1. Additionally, a removal rate of 67.4% for total organic carbon (TOC) was achieved in just 120 min. The treatment of actual wastewater also showed excellent performance, with an average specific energy consumption of 6.16 kWh kg−1 COD−1. Fe/Fe3C@FeNG-1000 can still achieve more than 90% degradation efficiency after 5 consecutive cycles of the experiment, showing excellent stability, and the catalyst can be conveniently retrieved via magnetic methods following the reaction. The results show that the Fe/Fe3C@FeNG-1000-EF system has a wide range of pH applicability, long-term stability and good recyclability, has a good application value in practical wastewater treatment.

Application of hydrophobic selective stimuli‐responsive polymer membranes in organic pollutant treatment in aqueous phase

Application of hydrophobic selective stimuli‐responsive polymer membranes in organic pollutant treatment in aqueous phase

Biochar imparted constructed wetlands (CWs) for enhanced biodegradation of organic and inorganic pollutants along with its limitation.

The remediation of polluted soil and water stands as a paramount task in safeguarding environmental sustainability and ensuring a dependable water source. Biochar, celebrated for its capacity to enhance soil quality, stimulate plant growth, and adsorb a wide spectrum of contaminants, including organic and inorganic pollutants, within constructed wetlands, emerges as a promising solution. This review article is dedicated to examining the effects of biochar amendments on the efficiency of wastewater purification within constructed wetlands. This comprehensive review entails an extensive investigation of biochar's feedstock selection, production processes, characterization methods, and its application within constructed wetlands. It also encompasses an exploration of the design criteria necessary for the integration of biochar into constructed wetland systems. Moreover, a comprehensive analysis of recent research findings pertains to the role of biochar-based wetlands in the removal of both organic and inorganic pollutants. The principal objectives of this review are to provide novel and thorough perspectives on the conceptualization and implementation of biochar-based constructed wetlands for the treatment of organic and inorganic pollutants. Additionally, it seeks to identify potential directions for future research and application while addressing prevailing gaps in knowledge and limitations. Furthermore, the study delves into the potential limitations and risks associated with employing biochar in environmental remediation. Nevertheless, it is crucial to highlight that there is a significant paucity of data regarding the influence of biochar on the efficiency of wastewater treatment in constructed wetlands, with particular regard to its impact on the removal of both organic and inorganic pollutants.

Rational design of a hydrophilic nanoarray-structured electro-Fenton membrane for antibiotics removal and fouling mitigation: An intensified catalysis process in an oxygen vacancy‐mediated cathodic microreactor

Electro-Fenton membranes (EFMs) can synchronously realize organic micropollutants destruction and fouling mitigation in a single filtration process with the assistance of hydroxyl radicals (•OH). Herein, a nanoarray-structured EFM (NS-EFM) was designed by assembling Fenton reactive CoFe-LDH nanowires using a low-temperature hydrothermal method. Combined with a defect-engineering strategy, the oxygen vacancies (OVac) in the CoFe-LDH nanoarrays were tailored by manipulating the stoichiometry of cations to optimize the Fenton reactivity of NS-EFMs. The optimized NS-EFM demonstrated exceptional sulfamethoxazole (SMX) removal (99.4%) and fast degradation kinetics (0.0846 min−1), but lower energy consumption (0.22 kWh m−3 per log removal of SMX). In-depth mechanism analysis revealed that the intrinsic electronic properties of OVac endowed NS-EFM with enhanced reactivity and charge transferability at metallic active sites of CoFe-LDH, thereby intensifying •OH generation. Besides, the nanoarray-structured NS-EFM built a confined microreactor space, leading to expedited •OH microflow to SMX. Meanwhile, the hydrophilic nature of CoFe-LDH nanoarrays synergistically contributed to the high flux recovery (95.0%) and minimal irreversible membrane fouling (5.0%), effectively alleviating membrane fouling within pores and on surfaces. This study offers insights into the potential of defect engineering as a foundational strategy in the design of EFMs, significantly advancing the treatment of organic pollutants and control of membrane fouling.

Construction of S scheme ZnO/g-C3N4 heterojunction for the removal of pyridine from coal chemical wastewater

In order to achieve the removal of nitrogen-containing heterocyclic compound pyridine in coal chemical wastewater. The S scheme ZnO/g-C3N4 heterojunction was constructed by evaporation solvent-high temperature thermal polymerization. The construction of the S scheme heterojunction effectively improved the photocatalytic activity. The S scheme heterojunction well retained the h+ with high oxidation activity of ZnO-VB and the e− with high reduction activity of g–C3N4–CB, achieving efficient separation of e−-h+. The complex band structure and potential difference lead to the bending of the band and the formation of the internal electric field, which effectively realizes the charge separation. ZnO/g-C3N4 heterojunction shows good activity and stability, and has great potential in the field of organic pollutant wastewater treatment. The construction of S scheme heterojunction also provides a new way to improve the activity of photocatalyst.