New Insights into Novel Catalysts for Treatment of Pollutants in Wastewater

Water scarcity has become a worldwide problem. Wastewater treatment and reuse has become an effective way to expand water resources. Among many treatment methods, the wastewater treatment method using catalysts as media is unique. Obviously, catalysts are the core of these treatment methods, and their properties directly determine the treatment effect and cost. In recent years, with the development of science and technology, a variety of new catalysts has emerged in an endless stream, and the related fields have become the focus of current scientific research. To this end, we are organizing a Special Issue: New Insights into Novel Catalysts for Treatment of Pollutants in Wastewater, focusing on the preparation, modification, and application of novel catalysts for catalytic wastewater treatment. Below, we will introduce each of the 13 works published in this Special Issue.

To achieve sustainable, low-carbon development, it is imperative to explore water treatment technologies in a carbon-neutral model. Because of the advantages of high efficiency, low consumption, and a lack of secondary pollution, electrocatalytic oxidation technology has attracted increasing attention for tackling the challenges of organic wastewater treatment. The performance of an electrocatalytic oxidation system depends mainly on the properties of the electrode materials. Compared with the instability of graphite electrodes, the high expenditure of noble metal electrodes and boron doped diamond electrodes, and the hidden dangers of titanium-based metal oxide electrodes, a titanium sub-oxide material has been characterized as an ideal choice of anode material due to its unique crystal and electronic structure, including high conductivity, decent catalytic activity, intense physical and chemical stability, corrosion resistance, low cost, long service life, etc. Guo’s paper [1] systematically reviews the electrode preparation technology of the Magnéli phase titanium sub-oxide and its research progress in the advanced electrochemical oxidation treatment of organic wastewater in recent years, with technical difficulties highlighted. Future research directions are further proposed in terms of process optimization, material modification, and application expansion. It is worth noting that Magnéli phase titanium sub-oxides have played very important roles in organic degradation. There is no doubt that titanium sub-oxides will become indispensable materials in the future.

Lead dioxide electrodes are also typical electrocatalytic anode materials. In Kang’s paper [2], active granule (WC/Co₃O₄)-doping Ti/Sb-SnO₂/PbO₂ electrodes were successfully synthesized by composite electrodeposition. The as-prepared electrodes were systematically characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical performance, zeta potential, and accelerated lifetime. It was found that the doping of active granules (WC/Co₃O₄) can reduce the average grain size and increase the number of active sites on the electrode surface. Moreover, it can improve the proportion of surface oxygen vacancies and non-stoichiometric PbO₂, resulting in outstanding conductivity, which can improve the electron transfer and catalytic activity of the electrode. Electrochemical measurements implied that Ti/Sb-SnO₂/Co₃O₄-PbO₂ and Ti/Sb-SnO₂/WC-Co₃O₄-PbO₂ electrodes have superior oxygen evolution reactions (OERs) relative to those of Ti/Sb-SnO₂/PbO₂ and Ti/Sb-SnO₂/WC-PbO₂ electrodes. A Ti/Sb-SnO₂/Co₃O₄-PbO₂ electrode is considered as the optimal modified electrode due to its long lifetime (684 h) and the remarkable stability of plating solutions. The treatment of copper wastewater suggests that composite electrodes exhibit low cell voltage and excellent extraction efficiency. Furthermore, pilot simulation tests verified that a composite electrode consumes less energy than other electrodes. Therefore, it is inferred that composite electrodes may be promising for the treatment of wastewater containing high concentrations of copper ions.

The composition of the electrode is also an important issue. Magnetic activated carbon particles (Fe₃O₄/active carbon composites) as auxiliary electrodes (AEs) were fixed onto the surface of Ti/Sb-SnO₂ foil by a NdFeB magnet to form a new magnetically assembled electrode (MAE) in Shao’s paper [3]. Characterizations including cyclic voltammetry, Tafel analysis, and electrochemical impedance spectroscopy were carried out. The electrochemical oxidation performances of the new MAE towards different simulated wastewaters (azo dye acid red G, phenol, and lignosulfonate) were also studied. Series of the electrochemical properties of MAE were found to be varied with the loading amounts of AEs. The electrochemical area, as well as the number of active sites, increased significantly with the loading of AEs, and the charge transfer was also facilitated by these AEs. Target pollutants’ removal of all simulated wastewaters were found to be enhanced when loading appropriate amounts of AEs. The accumulation of intermediate products was also determined by the loading amount of AEs. This new MAE may provide a more cost-effective and flexible method of electrochemical oxidation wastewater treatment (EOWT).

Octogen (HMX) is widely used as a high explosive and constituent in plastic explosives, nuclear devices, and rocket fuel. The direct discharge of wastewater generated during HMX production threatens the environment. In Qian’s study [4], they used the electrochemical oxidation (EO) method with a PbO₂-based anode to treat HMX wastewater and investigated its degradation performance, mechanism, and toxicity evolution under different conditions. The results showed that HMX treated by EO was able to achieve a removal efficiency of 81.2% within 180 min at a current density of 70 mA/cm², Na₂SO₄ concentration of 0.25 mol/L, interelectrode distance of 1.0 cm, and pH of 5.0. The degradation followed pseudo-first-order kinetics (R² > 0.93). Degradation pathways of HMX in the EO system have been proposed, including cathode reduction and indirect oxidation by •OH radicals. The molecular toxicity level (expressed as the transcriptional effect level index) of HMX wastewater first increased to 1.81 and then decreased to a non-toxic level during the degradation process. Protein and oxidative stress were the dominant stress categories, possibly because of the intermediates that evolved during HMX degradation. This study provides new insights into the electrochemical degradation mechanisms and molecular-level toxicity evolution during HMX degradation. It also serves as initial evidence for the potential of the EO-enabled method as an alternative for explosive wastewater treatment with high removal performance, low cost, and low environmental impact.

Zhao [5] reported and demonstrated a photoelectrochemical filtration system that was shown to enable the effective decontamination of micropollutants from water. The key to this system was a photoelectric-active nanohybrid filter consisting of a carbon nanotube (CNT) and MIL–101(Fe). Various advanced characterization techniques were employed to obtain detailed information on the microstructure, morphology, and defect states of the nanohybrid filter. The results suggest that both radical and nonradical pathways collectively contributed to the degradation of antibiotic tetracycline, a model refractory micropollutant. The underlying working mechanism was proposed based on solid experimental evidence. This study provides new insights into the effective removal of micropollutants from water by integrating state-of-the-art advanced oxidation and microfiltration techniques.

Noble metal nanoparticle-loaded catalytic membrane reactors (CMRs) have emerged as a promising method for water decontamination. In Yan’s study [6], a convenient and green strategy was proposed to prepare gold nanoparticle (Au NPs)-loaded CMRs. First, a redox-active substrate membrane (CNT-MoS₂), composed of carbon nanotubes (CNTs) and molybdenum disulfide (MoS₂), was prepared by an impregnation method. Water-diluted Au(III) precursor (HAuCl₄) was then spontaneously adsorbed on the CNT-MoS₂ membrane through filtration and reduced into Au(0) nanoparticles in situ, which involved a “adsorption–reduction” process between Au(III) and MoS₂. The constructed CNT-MoS₂@Au membrane demonstrated excellent catalytic activity and stability; a complete 4-nitrophenol transformation could be obtained within a hydraulic residence time of <3.0 s. In addition, thanks to the electroactivity of CNT networks, the as-designed CMR could also be applied to the electrocatalytic reduction of bromate (>90%) at an applied voltage of −1 V. More importantly, by changing the precursors, one could further obtain the other noble metal-based CMR (e.g., CNT-MoS₂@Pd) with superior (electro)catalytic activity. This study provided new insights into the rational design of high-performance CMRs for various environmental applications.

The water pollution caused by industry emissions makes effluent treatment a serious matter that needs to be settled. Heterogeneous Fenton oxidation has been recognized as an effective means to degrade pollutants in water. Attapulgite can be used as a catalyst carrier because of its distinctive spatial crystal structure and surface ion exchange. In Zhou’s study [7], iron ions were transported on attapulgite particles to generate an iron-supporting attapulgite particle catalyst. BET, EDS, SEM, and XRD characterized the catalysts. The particle was used as a heterogeneous catalyst to degrade rhodamine B (RhB) dye in wastewater. The effects of H₂O₂ concentration, initial pH value, catalyst dosage, and temperature on the degradation of the dyes were studied. The results showed that the decolorization efficiency was consistently maintained after consecutive use of a granular catalyst five times, and the removal rate was more than 98%. The degradation and mineralization effect of cationic dyes by granular catalyst was better than that of anionic dyes. Hydroxyl radicals play a dominant role in RhB catalytic degradation. The dynamic change and mechanism of granular catalysts in the catalytic degradation of RhB were analyzed. In this study, the application range of attapulgite was widened. The prepared granular catalyst was inexpensive, stable, and efficient, and could be used to treat refractory organic wastewater.

In Yu’s work [8], the degradation performance of Fe²⁺/PAA/H₂O₂ on three typical pollutants (reactive black 5, ANL, and PVA) in textile wastewater was investigated in comparison with Fe²⁺/H₂O₂. Therein, Fe²⁺/PAA/H₂O₂ showed a high removal of RB5 (99%), mainly owing to the contribution of peroxyl radicals and/or Fe(IV). Fe²⁺/H₂O₂ showed a relatively high removal of PVA (28%), mainly resulting from ·OH. Fe²⁺/PAA/H₂O₂ and Fe²⁺/H₂O₂ showed comparative removals of ANL. Additionally, Fe²⁺/PAA/H₂O₂ was more sensitive to pH than Fe²⁺/H₂O₂. The coexisting anions (20–2000 mg/L) showed inhibition on their removals and followed an order of HCO₃⁻ > SO₄²⁻ > Cl⁻. Humic acid (5 and 10 mg C/L) showed notable inhibition on their removal following an order of reactive black 5 (RB5) > ANL > PVA. In a practical wastewater effluent, PVA removal was dramatically inhibited by 88%. Test results regarding bioluminescent bacteria suggested that the toxicity of Fe²⁺/PAA/H₂O₂-treated systems was lower than that of Fe²⁺/H₂O₂. RB5 degradation had three possible pathways with the proposed mechanisms of hydroxylation, dehydrogenation, and demethylation. The results may favor the performance evaluation of Fe²⁺/PAA/H₂O₂ in the advanced treatment of textile wastewater.

In recent years, with the large-scale use of antibiotics, the pollution of antibiotics in the environment has become increasingly serious and has attracted widespread attention. In Qian’s study [9], a novel CDs/g-C₃N₄/BiPO₄ (CDBPC) composite was successfully synthesized by a hydrothermal method for the removal of the antibiotic tetracycline hydrochloride (TC) in water. The experimental results showed that the synthesized photocatalyst was crystalline rods and cotton balls, accompanied by overlapping layered nanosheet structures, and the specific surface area was as high as 518.50 m²/g. This photocatalyst contains g-C₃N₄ and bismuth phosphate (BiPO₄) phases, as well as abundant surface functional groups such as C=N, C-O, and P-O. When the optimal conditions were pH 4, CDBPC dosage of 1 g/L, and TC concentration of 10 mg/L, the degradation rate of TC reached 75.50%. Active species capture experiments showed that the main active species in this photocatalytic system were holes (h+), hydroxyl radicals, and superoxide anion radicals. The reaction mechanism for the removal of TC by CDBPC was also proposed. The removal of TC was mainly achieved by the synergy between the adsorption of CDBPC and the oxidation of both holes and hydroxyl radicals. In this system, TC was adsorbed on the surface of CDBPC; then, the adsorbed TC was degraded into small molecular products by an attack with holes and hydroxyl radicals and, finally, mineralized into carbon dioxide and water. This study indicated that this novel photocatalyst, CDBPC, has significant potential for antibiotic removal, which provides a new strategy for antibiotic treatment of wastewater.

In Gan’s study [10], nitrogen-doped biochar (N-PPB) and nitrogen-doped activated biochar (AN-PPB) were prepared and used for removing bisphenol A (BPA) in water through activating peroxymonosulfate. It was found from the results that N-PPB exhibited superior catalytic performance over pristine biochar, since nitrogen was able to bring about abundant active sites to the biochar structure. The non-radical singlet oxygen (¹O₂) was determined to be the dominant active species responsible for BPA degradation. Having a non-radical pathway in the N-PPB/PMS system, the BPA degradation was barely influenced by many external environmental factors, including solution pH value, temperature, and foreign organic and inorganic matters. Furthermore, AN-PPB had richer porosity than N-PPB, which showed even faster BPA removal efficiency than N-PPB through adsorptive/catalytic synergy. The finding of this study introduced a novel way of designing hieratical structured biochar catalysts for effective organic pollutant removal in water.

The purpose of Wang’s work [11] was to optimize the catalytic performance of biochar (BC), improve the removal effect of BC composites on organic pollutants in wastewater, and promote the recycling and sustainable utilization of water resources. Firstly, the various characteristics and preparation principles of new BC are discussed. Secondly, the types of organic pollutants in wastewater and their removal principles are discussed. Finally, based on the principle of removing organic pollutants, a BC/zero valent iron (BC/ZVI) composite is designed, in which BC is mainly used for catalysis. The effect of BC/ZVI on the removal tetracycline (TC) was comprehensively evaluated. The research results revealed that the TC removal effect of pure BC is not ideal, and that of ZVI is general. The BC/ZVI composite prepared by combining the two had a better removal effect on TC, with a removal amount of about 275 mg/g. Different TC concentrations, ethylene diamine tetraacetic acid (EDTA), pH environment, tert-butanol, and calcium ions were shown to affect the TC removal effect of BC composites. The overall effect was the improvement of the TC removal amount of BC composites. This reveals that BC has a very suitable catalytic effect on ZVI, and the performance of BC composite material integrating the BC catalyst and ZVI was effectively improved; it can play a very suitable role in wastewater treatment. This exploration provides a technical reference for the effective removal of organic pollutants in wastewater and contributes to the development of water resource recycling.

In Wang’s study [12], single-spherical nanoscale zero valent iron (nZVI) particles with large specific surface areas were successfully synthesized by a simple and rapid chemical reduction method. The XRD spectra and SEM–EDS images showed that the synthesized nZVI had excellent crystal structure, but oxidation products, such as γ-Fe₂O₃ and Fe₃O₄, were formed on the surface of the particles. The effects of different factors on the removal of Cr(VI) by nZVI were studied, and the optimum experimental conditions were found. Kinetic and thermodynamic equations at different temperatures showed that the removal of Cr(VI) by nZVI was a single-layer chemical adsorption, conforming to pseudo-second-order kinetics. Applying the intraparticle diffusion model, the adsorption process was composed of three stages, namely rapid diffusion, chemical reduction, and internal saturation. Analysis of the mechanism demonstrated that the removal of Cr(VI) by nZVI involved adsorption, reduction, precipitation, and coprecipitation. Meanwhile, Cr(VI) was reduced to Cr(III) by nZVI, while FeCr₂O₄, CrxFe₁-_xOOH, and CrxFe_1−x(OH)₃ were formed as end products. In addition, the study found that ascorbic acid, starch, and Cu-modified nZVI were able to promote the removal efficiency of Cr(VI) in varying degrees due to the enhanced mobility of the particles. These results can provide new insights into the removal mechanisms of Cr(VI) by nZVI.

In Xiong’s study [13], a Co-Mn/CeO₂ composite was prepared through a facile sol-gel method and used as an efficient catalyst for the ozonation of norfloxacin (NOR). The Co-Mn/CeO₂ composite was characterized via XRD, SEM, BET, and XPS analysis. The catalytic ozonation of NOR by Co-Mn/CeO₂ under different conditions was systematically investigated, including the effect of the initial solution’s pH, Co-Mn/CeO₂ composite dose, O₃ dose, and NOR concentration on degradation kinetics. Only about 3.33% of the total organic carbon (TOC) and 72.17% of NOR could be removed within 150 min by single ozonation under the conditions of 60 mg/L of NOR and 200 mL/min of O₃ at pH = 7 and room temperature, whereas in the presence of 0.60 g/L of the Co-Mn/CeO2 composite under the same conditions, 87.24% NOR removal was obtained through the catalytic ozonation process. The results show that catalytic ozonation with the Co-Mn/CeO₂ composite can effectively enhance the degradation and mineralization of NOR compared to a single ozonation system alone. The catalytic performance of CeO₂ was significantly improved by modification with Mn and Co. Co-Mn/CeO₂ represents a promising way to prepare efficient catalysts for the catalytic ozonation of organic polluted water. The removal efficiency of NOR in five cycles indicates that Co-Mn/CeO₂ is stable and recyclable for catalytic ozonation in water treatment.

In the future, there will be additional new catalysts and new catalytic methods proposed by scholars, and our Special Issue will continue to focus on the latest progress in this field.