Degradation Of Parabens Research Articles

Catalytic performance of FeCo2O4 spinel cobaltite for degradation of ethylparaben in a peroxymonosulfate activation process: Response surface optimization, reaction kinetics and cost estimation

The presence of cosmetics and personal care products in water resources has become an increasing concern for environment due to their toxicity and persistence in aquatic medium. Peroxymonosulfate mediated advanced oxidation methods have a great potential for the efficient removal of complex organic compounds due to the generation of highly reactive radicals with high oxidation ability. Iron cobaltite, FeCo2O4 catalyst was used as a peroxymonosulfate (PMS) activator for the degradation of ethylparaben which was selected as a model personal care product ingredient. Paraben degradation was modelled and the optimum operating parameters were determined by using Box-Behnken design. The ethylparaben degradation efficiency was calculated as 94.6 % under the optimized conditions which were determined as 0.9 g/L FeCo2O4 loading, pH 6.15, and 0.26 g/L PMS dosage. The ethylparaben degradation reaction followed second-order reaction kinetics. The activation energy was evaluated as 40.1 kJ/mol. Quenching radical tests showed that the dominant reactive species were hydroxyl radicals. Toxicity of the treated samples in terms of L. sativum growth inhibition was evaluated as 1.5 %. The total cost of the treatment including the operating and the amortization costs was calculated as 0.91 €/L.

Cobalt-Doped Hydrochar Derived from Microalgae as an Efficient Peroxymonosulfate Activator for Paraben Degradation

Hydrochar, an attractive member of the carbonaceous materials, is derived from biomass and projects great potential in peroxymonosulfate (PMS) activation, but has not been studied much. Herein, by using the large-scale cultured Chlorella vulgaris and field-collected bloom algae, a series of porous hydrochar was synthesized via a facile hydrothermal carbonization reaction, while Co doping significantly increased their specific surface areas, carbonization degree, and surface functional groups. These Co-doped hydrochar (xCo-HC, x: amount of the Co precursor) could efficiently activate the PMS, resulting in nearly 100% removal of five common paraben pollutants within 40 min. A dosage of 0.2Co-HC of 0.15 g/L, a PMS concentration of 0.6 g/L, and an unadjusted pH of 6.4 were verified more appropriately for paraben degradation. The coexistence of Cl−, SO42−, and humic acid inhibited the degradation, while HCO3− showed an enhancing effect. No observable change was found at the presence of NO3−. Quenching results illustrated that the produced •SO4− during the conversion of doped Co3+/Co2+ acted as the dominant active species for paraben degradation, while •O2−, 1O2, and •OH contributed relatively less. The algae-based hydrochar potentially facilitated the electron transfer in the xCo-HC/PMS system. Overall, this study develops a new strategy for resource utilization of the abundant algae.

Analyzing Paraben Degradation in Parenteral Formulations with High-Performance Liquid Chromatography

Analyzing Paraben Degradation in Parenteral Formulations with High-Performance Liquid Chromatography

Fighting poison with poison: Degradation of ethyl paraben by Bi2Ti2O7-TiO2 doped with ethyl paraben

In this paper, a variety of titanium-bismuth-based composite photocatalysts were prepared by molecularly imprinted sol–gel method using ethyl paraben (EP) as the target pollutant and tetra-n-butyl titanate as the precursor for the degradation of EP. The photocatalytic removal of 81.4 % of 10 mg/L EP wastewater was achieved by synthesising Bi-EP-TiO2 for 30 min at 500°C, 5 g Bi and 2 g EP. The main component of the prepared Bi-EP-TiO2 and Bi-TiO2 is Bi2Ti2O7-TiO2. In addition, the forbidden bandwidth of Bi-EP-TiO2 is 0.28 eV larger than that of Bi-TiO2 but 0.30 eV smaller than that of TiO2, which suggests that, compared with TiO2, Bi-EP-TiO2 requires a lower electron-leap energy, which is capable of generating more. This suggests that Bi-EP-TiO2 requires lower electron transition energy than TiO2, and therefore can generate more photogenerated electron holes, thus improving the catalytic efficiency. Hence, Bi2Ti2O7-TiO2 doped with ethyl paraben degrades EP with excellent performance.

Assessment of UV-activated persulfate process for the degradation of parabens and its degradation product p-hydroxybenzoic acid in environmental matrices

In the present study the simultaneous degradation of methyl paraben (MeP) and propyl paraben (PrP), as the most often used parabens, as well as its major transformation product p-hydroxybenzoic acid (p-HBA) by UV-C/PS in different aqueous matrices was investigated. The effects of various parameters such as the initial oxidant concentration, solution pH, humic acid content and inorganic anions on the efficiency of the process were evaluated. The degradation pathways were also evaluated through the assessment of the role of the reactive radicals as well as the identification of the transformation products. Moreover, the toxicity evolution during the process was estimated using two different microalgae species, i.e. Scenedesmus rubescens and Dunaliella tertiolecta. UV-C/PS was effective to degrade all the studied contaminants in all the aqueous matrices that is correlated with the formation of the reactive radicals HO· and SO4·−. However, the organic and inorganic content that co-exist in lake and drinking water as well as wastewater significantly affected the rate of the removal. The degradation was found to proceed through hydroxylation and ring opening reactions. The process was also found to proceed without the formation of compounds of concern. Moreover, UV-C/PS process had a positive effect on transferring toxic organics and inorganics to harmless compounds and was able to significantly reduce the toxicity of the wastewater.

High surface area g-C3N4 nanosheets as superior solar-light photocatalyst for the degradation of parabens

The rational design and development of highly-active photocatalytic materials for the degradation of dangerous chemical compounds, such as parabens, is one of the main research pillars in the field of photocatalysis. Graphitic carbon nitride (g-C3N4) is a 2D non-metal material and is considered one of the most promising photocatalysts, because of its peculiar physicochemical properties. In this work, porous g-C3N4 nanosheets (CNNs) were successfully prepared via thermal exfoliation of bulk g-C3N4 (CNB). A thorough physicochemical characterization analysis before and after the exfoliation process was performed, revealing the improved textural characteristics (surface area of 212 m2/g), chemical stability, and optical properties (wide band gap of 2.91 eV) of CNNs compared to the CNB. Then, both CNB and CNNs were comparatively assessed as photocatalysts for the degradation of methyl-, ethyl- and propylparaben (MP, EP, and PP), as well as of their mixture. CNNs with high surface area display superior photocatalytic performance under solar irradiation, offering > 95% degradation efficiency to all parabens, in contrast to the much inferior performance of CNB (< 30%). Several experimental parameters, involving catalyst concentration, initial concentration of parabens, and irradiation type were thoroughly investigated for the degradation of MP over CNNs. Moreover, various scavengers were employed to discriminate the role of different reactive species, revealing that superoxide anion radicals (·O2–) play a pivotal role in the degradation process, in contrast to hydroxyl radicals (·OH). The present results pave the way towards the facile synthesis of high surface area CNNs with improved textural and electronic characteristics, which can be applied in various environmental applications.

Phosphorus‐Doped Porous Carbon Coated Graphite Felt as An Efficient and Reusable Cathode for Electro‐Fenton Degradation of Parabens

AbstractPharmaceuticals and personal care products (PPCPs) are currently drawing significant attention due to their environmental impact. Electro‐Fenton, one of the advanced oxidation processes (AOPs), is an efficient and comprehensive technology to degrade all kinds of organic pollutants, but the degradation efficiency relies on cathode material. In this work, phosphorus‐doped porous carbon material coated graphite felt (P‐GF) was prepared by adopting phytic acid as a phosphorus source through high temperature solid‐state method. The microstructure and the elements composition of this fabricated material were investigated by SEM, BET and XPS. With phosphorus doping and plenty of carbon porous structures, the hydrophilicity and infiltrating property of the P‐GF had been greatly improved. The prepared P‐GF material enhanced the electrochemical reduction of O2 and could significantly enhance the yield of H2O2. It was used as a cathode to degrade pollutants under a constant potential, and methylparaben (MePa) was chosen as a research model for optimizing the experimental conditions and analyzing the possible degradation pathway. The P‐GF cathode achieved an excellent ability of degrading nearly 100 % of methylparaben within 3 h. The simultaneous removal ability of the P‐GF to four parabens was impressed. Additionally, the P‐GF material exhibited outstanding reusability.

Carbon nitride exfoliation for photocatalysis and photocatalytic ozonation over emerging contaminants abatement

The production of graphitic carbon nitride was studied with the addition of a following exfoliation step using ultrasounds, at different times. The treatment produced catalysts with much higher surface areas, from 2.18 m2 g−1 with no treatment to 28.34 m2 g−1 after 36 h of exfoliation. During exfoliation, melem was also produced, and may further enhance the photoluminescence and photocatalytic properties. The photocatalytic reaction also indicated the improved performance of exfoliated g-C3N4 towards the degradation of a mixture of parabens, enhancing their removal from 20% to up to 65% in 2 h under UVA radiation. The combination of ozone and g-C3N4 photocatalysis was able to completely remove the parabens in under 8 min and with a 20% lower consumption of ozone compared to photolytic ozonation. The main reactive species were identified, and the by-products formed during parabens degradation were detected. The efficiency of the combined system was also attested by using a mixture of parabens and other common drugs, maintaining its better performance.

Acid-induced tubular g-C3N4 for the selective generation of singlet oxygen by energy transfer: Implications for the photocatalytic degradation of parabens in real water environments

Parabens are widely present in aquatic environments and pose potential health risk. Although great progress has been made in the field of the photocatalytic degradation of parabens, the powerful Coulomb interactions between electrons and holes are the major limitations to photocatalytic performance. Hence, acid-induced tubular g-C3N4 (AcTCN) was prepared and applied for the removal of parabens from a real water environment. AcTCN not only increased the specific surface area and light absorption capacity, but also selectively generated 1O2 via an energy transfer-mediated oxygen activation pathway. The 1O2 yield of AcTCN was 11.8 times higher than that of g-C3N4. AcTCN exhibited remarkable removal efficiencies for parabens depending on the length of the alkyl group. Furthermore, the rate constants (k values) of parabens in ultrapure water were higher than those in tap and river water because of the presence of organic and inorganic species in real water environments. Two possible pathways for the photocatalytic degradation of parabens are proposed based on the identification of intermediates and theoretical calculations. In summary, this study offers theoretical support for the efficient enhancement of the photocatalytic performance of g-C3N4 for the removal of parabens in real water environments.

G-C3N4 for Photocatalytic Degradation of Parabens: Precursors Influence, the Radiation Source and Simultaneous Ozonation Evaluation

Graphitic carbon nitride (g-C3N4) is a promising catalyst for contaminants of emerging concern removal applications, especially as a visible-light-driven material. In this study, g-C3N4 catalysts were effectively synthesized through a simple thermal polymerization method, using melamine, urea, and thiourea as precursors to elucidate the influence of these compounds on the final product’s photocatalytic performance. The degradation of a mixture of three parabens was investigated under different types of radiation: two artificial, ultraviolet-A (UVA) and visible LED, and natural sunlight. The urea-based catalyst (UCN) presented better results under all radiation sources, followed by thiourea, and finally, melamine. Among the artificial light sources, the degradation of parabens under UVA was considerably higher than visible—up to 51% and 21%, respectively—using UCN; however, the broader spectrum of natural sunlight was able to achieve the highest removals, up to 92%, using UCN. Comparing artificial radiation sources, UVA lamps presented 45% lower energy consumption and associated costs. Photocatalytic ozonation was tested using UCN and MCN, with UCN once more possessing superior performance and a synergetic effect between photocatalysis and ozonation, with complete removal under 12 min. The use of g-C3N4 was then successfully tested in initial screening and found to be an efficient alternative in more low-cost and feasible solar photocatalysis water treatment.

Cloning and rational modification of a cold-adapted esterase for phthalate esters and parabens degradation

Phthalate esters (PAEs) and parabens are environmental pollutants that can be toxic to human health. Herein, a cold-adapted esterase from the Mao-tofu metagenome named Est1260 was screened for its PAE-hydrolyzing potential in cold temperatures. The results showed that purified Est1260 could degrade a variety of PAEs and parabens at temperatures as low as 0 °C. After careful analysis of the structural information and molecular docking, site-saturation mutation was conducted at the identified hotspots. Protein expression of variant A1B6 doubled, and its thermal stability significantly improved (24 times) without sacrificing activity at low temperatures. In addition, Est1260 and its variants were activated by NaCl and demonstrated resistance to high concentrations of saline (up to 5 M), making it a potential biocatalyst for bioremediation of PAE and paraben-polluted environments.

Carbon quantum dots (CQDs) mediated Z-scheme g–C3N4–CQDs/BiVO4 heterojunction with enhanced visible light photocatalytic degradation of Paraben

The construction of a novel Z-scheme system which possesses superior charge separation and high redox ability is highly desirable for efficient photocatalytic degradation of organic pollutants. Herein, a carbon quantum dots (CQDs) modified g-C3N4 (GCN) and BiVO4 (BVO) composite (GCN-CQDs/BVO) was fabricated via an initial loading of CQDs on GCN, and a subsequent combination with BVO during its hydrothermal synthesis. Physical characterization (e.g. TEM, XRD, XPS) verified the intimate heterojunction structure of the composite, while CQDs improved its light absorption. The band structures of GCN and BVO were evaluated, displaying the feasibility for Z-scheme formation. In comparison with GCN, BVO, and GCN/BVO, GCN-CQDs/BVO generated the highest photocurrent and lowest charge transfer resistance, inferring the prominently improved charge separation. Under visible light irradiation, GCN-CQDs/BVO exhibited the significantly enhanced activity in degrading the typical Paraben pollutant——benzyl paraben (BzP), achieving the removal of 85.7% in 150 min. The effects of various parameters were explored, demonstrating that neutral pH was optimal, while coexisting ions (CO32−, SO42−, NO3−, K+, Ca2+, Mg2+) and humic acid impacted the degradation negatively. Meanwhile, trapping experiments and electron paramagnetic resonance (EPR) technique revealed that superoxide radicals (•O2−) and hydroxyl radical (•OH) were primarily responsible for BzP degradation by GCN-CQDs/BVO. In particular, with the assistance of CQDs, the generation of •O2− and •OH was notably augmented. Based on these results, a Z-scheme photocatalytic mechanism was proposed for GCN-CQDs/BVO, where CQDs acted as electron mediators to combine the holes from GCN and electrons from BVO, resulting in significantly improved charge separation and maximized redox ability. Moreover, the toxicity of BzP was remarkably reduced during the photocatalytic process, emphasizing its great potential in abating the risk of Paraben pollutants.

Fabrication of layered In2S3/WS2 heterostructure for enhanced and efficient photocatalytic CO2 reduction and various paraben degradation in water

Because of the excessive use of fossil fuels, CO2 emissions into the environment are increasing. An efficient method of converting CO2 to useful carbonaceous products in the presence of light is one way to address the issues associated with energy and environmental remediation. In2S3/WS2 heterostructure has been fabricated using the efficient hydrothermal method. The results of structural, morphological, optical, and photo/electrochemical characterization confirm the formation of a hierarchical, layered heterostructure of type-II. Enhanced photocatalytic activity is observed in InS/WS heterostructure compared to pristine In2S3 and WS2. InS/WS heterostructure exhibit higher photocatalytic activity than pure In2S3 and WS2. For 12 h, photocatalytic CO2 reduction produces 213.4 and 188.6 μmol of CO and CH4, respectively. Furthermore, the photocatalytic ability of the synthesized materials to degrade different parabens (Methyl: MPB, Ethyl: EPB, and Benzyl: BPB) under visible radiation was evaluated. Under optimized conditions, the InS/WS heterostructure degraded 88.6, 90.4, and 95.8% of EPB, BPB, and MPB, respectively, in 90 min. The mechanism of photocatalysis was discussed in detail. MCF-7 cell viability was assessed and found to exhibit low mortality in InS/WS treated MPB aqueous solution. InS/WS heterostructure could improve the fabrication of more sulphide-based layered materials to combat environmental pollution.

In vitro reciprocal interactions between yeasts from human cutaneous mycobiota and parabens used in cosmetics.

Parabens are substances with antifungal and antibacterial properties, suspected to be endocrine disruptors and widely used as preservatives in cosmetics. In this case, exposure to these compounds is mainly dermal and interactions may occur with skin components including cutaneous mycobiota. In this work, we have explored the in vitro reciprocal interactions between three parabens (methylparaben, ethylparaben, and propylparaben) and yeasts from the human cutaneous mycobiota (Candida parapsilosis, Cryptococcus uniguttulatus, and Rhodotorula mucilaginosa) by studying the effect of these parabens on fungal growth and the fungal ability to metabolize the tested compounds. Our results showed that, at the tested concentrations, the growth of three strains of C. parapsilosis was not influenced by the presence of parabens. Whereas, using the same parabens concentrations, growth of C. uniguttulatus and R. mucilaginosa was completely inhibited by ethylparaben since the first day of contact, whereas these same fungi were not sensitive to the two other parabens, even after seven days of incubation. The presence of a lamellar wall in these basidiomycete fungi as well as the physico-chemical properties of ethylparaben could explain this selective inhibition. Additionally, C. parapsilosis and R. mucilaginosa degraded 90% to 100% of propylparaben after seven days of incubation but had no effect on the other tested parabens. Thus, their enzymes seem to only degrade long chain parabens. In the same conditions, C. uniguttulatus did not degrade any paraben. This inability may be due to the absence of fungal enzymes able to degrade parabens or to the possible inaccessibility of intracellular enzymes due to the polysaccharide capsule. Our work has shown that parabens can act differently from one fungus to another within the cutaneous mycobiota. These preliminary results have evidenced that in vitro parabens, contained in cosmetic products, could be involved in the occurrence of a state of dysbiosis. The tested yeasts from the cutaneous mycobiota can also be involved in the degradation of parabens and thereby reduce, according to the produced metabolites and their activities, the risk of endocrine disruption they can induce.

Optimization of thermal exfoliation of graphitic carbon nitride for methylparaben photocatalytic degradation under simulated solar radiation

A full factorial 32 design was used to optimize the exfoliation treatment of g-C3N4 to enhance its photocatalytic performance for methyl paraben degradation.

Mild Fenton Processes for the Removal of Preservatives: Interfering Effect of Methylisothiazolinone (MIT) on Paraben Degradation

The degradation of various preservatives used in the cosmetics industry, including five parabens and their most employed substitute, methylisothiazolinone (MIT), was investigated. A mild photo-Fenton process was applied using low iron concentrations (5 mg/L) at a pH of five, instead of the traditional acidic value of three. At these conditions, the paraben degradation was very low after one hour of reaction and it was necessary to present humic-like substances (HLS) acting as iron chelators to improve the process. Values obtained when MIT was treated were very low, also in the presence of HLS, indicating that their complexing effect was not acting properly. When MIT was added to the mixture of parabens an inhibitory effect was found in the presence of HLS. A possible complex between iron and MIT was suggested and the studies of hydrogen peroxide consumption and Job’s plot technique confirmed this hypothesis. Evidence of the formation of this inactive complex, so far never reported, will be essential in future work when dealing with this compound using Fenton processes. Furthermore, this fact points out the importance of using mixtures of model contaminants instead of a single one or a group of the same family, since their ability to form active or inactive complexes with iron can strongly change the behavior of the whole system.

Architecture of bimetallic-MOF/silicate derived Co/NC@mSiO2 as peroxymonosulfate activator for highly efficient ciprofloxacin degradation

Bimetallic-ZIF@silicate derived Co/NC@mSiO2 hollow sphere was developed as an excellent peroxymonosulfate (PMS) activator for ciprofloxacin (CFL) antibiotic and parabens degradation. First, bimetallic Zn-Co/ZIF frameworks were prepared by a solvothermal method and then coated with a silica layer. Subsequently, Zn-Co/ZIF@mSiO2 was carbonized under nitrogen gas to obtain the Co, N co-doped porous carbon material. The evaporation of zinc metal in Zn-Co/ZIF@mSiO2 by inert gas flow played an important role in the formation of porous carbon material. In the presence of a small quantity of PMS, this hollow sphere material acts as a highly efficient catalyst for the degradation of CFL with a rate constant of 0.07 min−1. The findings obtained from the quenching radical test and electron paramagnetic resonance (EPR) reveal that Co site within the hollow sphere nanoreactor catalyzed PMS to the generation of the HO, SO4−, and 1O2 for CFL degradation. Notably, the presence of Co NPs in the hollow sphere material enables the facile separation of the catalyst from the reaction mixtures using an external magnet. Furthermore, the as-prepared catalysts exhibited antibacterial activity in Staphylococcus aureus (S. aureus). The current study provides new horizons to develop ZIF-derived hybrid hollow materials in eliminating organic pollutants.

Carbon nitride photoactivation evaluation and degradation of a mixture of parabens by ozone assistance

Graphitic carbon nitride was investigated as an efficient and cheap catalyst for the degradation of a mixture of parabens through photo-aided processes. The ozone and g-C3N4 system was explored to obtain lacking necessary information regarding this combination for the proposed contaminants. The influence of key parameters was investigated to better understand the behavior of system and optimize it. Ultraviolet-A radiation promoted a more efficient activation of g-C3N4 compared to visible light. The photocatalytic ozonation process achieved higher parabens degradation rates and lower ozone consumption than the single technologies, reducing its overall cost. Basic and neutral conditions (pH = 7–11) promoted a better interaction between catalysts and contaminants, and radicals' formation. A catalyst concentration of 500 mg L−1 and parabens concentration of 1 mg L−1 resulted in >95 % removal of the three parabens within 15 min and with a lower transferred ozone dose required. Toxicity towards Allivibrio fischeri bacteria demonstrated a significant decrease in its luminescence inhibition after photocatalytic ozonation treatment, resulting in a non-toxic solution.

Enhanced photocatalytic treatment using plasmonic Ag@Ag3PO4/Ag@AgCl nanophotocatalyst for simultaneous degradation of multiple parabens and UV-filters in various aquatic environments under visible light irradiation.

In this study, simultaneous photocatalytic degradation of different parabens (methyl-, ethyl-, propyl-, and butyl paraben) and UV filters (benzophenone-3, 4-methylbenzylidene camphor, 2-ethylhexyl 4-(dimethylamino) benzoate, ethylhexyl methoxycinnamate and octocrylene) in water matrices was performed under visible light irradiation using novel double plasmonic Ag@Ag3PO4/Ag@AgCl nanophotocatalyst, synthesized by an easy and fast photochemical conversion and photo-reduction. It was found that the nanophotocatalyst with appropriate mole ratio of Ag@Ag3PO4/Ag@AgCl (1:3) showed superior photocatalytic activity than individual plasmonic nanoparticles. This is because there are two simultaneous surface plasmon resonances (SPR) generated by the metallic Ag nanoparticles, in addition to the hetero-junction structure formed at the interface between Ag@Ag3PO4 and Ag@AgCl. The structures of the synthesized photocatalysts were characterized, and the principal reactive oxygen species in the photocatalytic process were identified via a trapping experiment, confirming superoxide radicals (∙O2-) as the key reactive species of the photocatalytic system. The process of photodegradation of the target pollutants was monitored using an optimized method that incorporated solid-phase extraction in combination with gas chromatography-mass spectrometry. The simultaneous photodegradation process was modeled and optimized using central composite design. The kinetic study revealed that the degradation process over Ag@Ag3PO4 (30%)/Ag@AgCl (70%) under visible light followed a pseudo-first-order kinetic model. The simultaneous degradation of target compounds was further investigated in sewage treatment plant effluent as well as tap water. It was found that the matrix constituents can reduce the photodegradation efficiency, especially in the case of highly contaminated samples.

Doping strategy-tuned non-radical pathway on manganese oxide for catalytic degradation of parabens

Non-radical oxidation processes with singlet oxygen (1O2) and mediated electron transfer have emerged as efficient methods for the selective degradation of aqueous pollutants in peroxymonosulfate (PMS)-based catalytic systems. On this basis, in this study, transition- and lanthanide-metal co-doped manganese oxide octahedral molecular sieve (Cu-Nd-OMS-2) was synthesized, characterized, and tested in the degradation of aqueous parabens. Methylparaben (30 mg/L) was degraded at 100% and mineralized at 92% within 40 min in the presence of Cu-Nd-OMS-2 and PMS (200 mg/L) in pH scale ranging from 3 to 8. Compared with pristine and mono-doped catalysts, the Cu-Nd-OMS-2, produced by the doping strategy, exhibited a superior catalytic activity for the oxidative degradation of several parabens through turning the mechanism with radicals into non-radical pathway. Electron paramagnetic resonance, quenching experiments and mechanism study indicated that 1O2 and mediated electron transfer contributed to the removal of parabens. Besides, the redox cycle of Mn(III)/Mn(IV) might be responsible for PMS activation, and abundant surface hydroxyl groups as well as oxygen vacancies on the catalyst further promoted the adsorption of pollutants and the generation of 1O2. Overall, the present study provides a novel and facile doping strategy on manganese oxide catalysts for fine-tuning radical/non-radical processes towards PMS-based water remediation.