Degradation of organic contaminants in the CoFe2O4/peroxymonosulfate process: The overlooked role of Co(II)-PMS complex

Recent advances in catalyst design, performance, and challenges of metal-heteroatom-co-doped biochar as peroxymonosulfate activator for environmental remediation

The escalation of global water pollution due to emerging pollutants has gained significant attention. To address this issue, catalytic peroxymonosulfate (PMS) activation technology has emerged as a promising treatment approach for effectively decontaminating a wide range of pollutants. Recently, modified biochar has become an increasingly attractive as PMS activator. Metal-heteroatom-co-doped biochar (MH-BC) has emerged as a promising catalyst that can provide enhanced performance over heteroatom-doped and metal-doped biochar due to the synergism between metal and heteroatom in promoting PMS activation. Therefore, this review aims to discuss the fabrication pathways (i.e., internal vs external doping and pre-vs post-modification) and key parameters (i.e., source of precursors, synthesis methods, and synthesis conditions) affecting the performance of MH-BC as PMS activator. Subsequently, an overview of all the possible PMS activation pathways by MH-BC is provided. Subsequently, Also, the detection, identification, and quantification of several reactive species (such as, •OH, SO4•-, O2•-, 1O2, and high valent oxo species) generated in the catalytic PMS system by MH-BC are also evaluated. Lastly, the underlying challenges associated with poor stability, the lack of understanding regarding the interaction between metal and heteroatom during PMS activation and quantification of radicals in multi-ROS system are also deliberated.

Investigating the evolution of reactive species in the CuO-mediated peroxymonosulfate activation process

The utilization of copper oxide (CuO) as a catalyst in the peroxymonosulfate (PMS) activation process holds great promise for effectively degrading aqueous organic pollutants, while the relevant mechanism remains inadequately understood. In this study, we delve into the evolution pathways of reactive species in the CuO/PMS system through a comprehensive series of experimental analyses. Our findings indicate that various reactive species are generated in the CuO/PMS system with the specific sequence, where the decomposition of surface Cu(II)-OOSO3- leads to the formation of surface Cu(III) species, which are responsible for the subsequent generation of HO•. The reactivity of these reactive species and the sequence of their generation explain the distinct oxidation behaviors of pollutants with different values of ionization potential (IP). In addition, singlet oxygen (1O2) may be produced during the PMS activation process, while its involvement in the oxidation of substrates is deemed negligible. This investigation presents a novel perspective, enhancing our comprehension of the mechanism underlying transition metal-mediated PMS activation processes. ENVIRONMENTAL IMPLICATION: The removal of refractory organic contaminations in water constitutes a fundamental concern within the realm of environmental pollution management. Peroxymonosulfate activation induced by transition metal oxides has garnered significant recognition as a promising technological approach for the degradation of aqueous organic contaminants, while the underlying mechanism remains enigmatic. In this study, we systematically investigate the evolution pathways of reactive species in the CuO/peroxymonosulfate system to reveal the mystery of the reaction mechanism between CuO and peroxymonosulfate. The outcomes of our study contribute to enhancing the practical applicability of transition metal-triggered PMS activation processes.

CoFe2O4 as a source of Co(II) ions for imidacloprid insecticide oxidation using peroxymonosulfate: Influence of process parameters and surface changes

Nanometric cobalt magnetic ferrite (CoFe2O4) synthesized by distinct methods was used for in situ chemical activation of peroxymonosulfate (PMS) under neutral conditions to oxidize imidacloprid (IMD) insecticide. The effect of CoFe2O4 load (0.125-1.0 g L-1) and PMS concentration (250-1000 μM) was investigated as well as the influence of phosphate buffer and Co(II) ions. PMS activation by Co(II) ions, including those leached from CoFe2O4 (>50 μg L-1), exhibited a strong influence on IMD oxidation and, apparently, without substantial contributions from the solid phase. Within the prepared solid materials (i.e., using sol-gel and co-precipitation methods), high oxidation rates (ca. 0.5 min-1) of IMD were attained in ultrapure water. Phosphate buffer had no significant influence on the IMD oxidation rate and level, however, its use and solution pH have shown to be important parameters, since higher PMS consumption was observed in the presence of buffered solutions at pH 7. IMD byproducts resulting from hydroxylation reactions and rupture of the imidazolidine ring were detected by mass spectrometry. At optimum conditions (0.125 g L-1 of CoFe2O4 and 500 μM of PMS), the CoFe2O4 nanoparticles exhibited an increase in the charge transfer resistance and an enhancement in the surface hydroxylation after PMS activation, which led to radical (HO● and SO4●-) and nonradical (1O2) species. The latter specie led to high levels of IMD oxidation, even in a complex water matrix, such as simulated municipal wastewater at the expense of one-order decrease in the IMD oxidation rate.

Resource utilization of rice straw to prepare biochar as peroxymonosulfate activator for naphthalene removal: Performances, mechanisms, environmental impact and applicability in groundwater

Herein, biochar was prepared using rice straw, and it served as the peroxymonosulfate (PMS) activator to degrade naphthalene (NAP). The results showed that pyrolysis temperature has played an important role in regulating biochar structure and properties. The biochar prepared at 900°C (BC900) had the best activation capacity and could remove NAP in a wide range of initial pH (5-11). In the system of BC900/PMS, multi-reactive species were produced, in which 1O2 and electron transfer mainly contributed to NAP degradation. In addition, the interference of complex groundwater components on the NAP removal rate must get attention. Cl- had a significant promotional effect but risked the formation of chlorinated disinfection by-products. HCO3-, CO32-, and humic acid (HA) had an inhibitory effect; surfactants had compatibility problems with the BC900/PMS system, which could lead to unproductive consumption of PMS. Significantly, the BC900/PMS system showed satisfactory remediation performance in spiked natural groundwater and soil, and it could solve the problem of persistent groundwater contamination caused by NAP desorption from the soil. Besides, the degradation pathway of NAP was proposed, and the BC900/PMS system could degrade NAP into low or nontoxic products. These suggest that the BC900/PMS system has promising applications in in-situ groundwater remediation.

Efficient activation of peroxymonosulfate via Cu2+/Cu+ cycle enhanced by hydroxylamine for the degradation of Rhodamine B

The application of Cu²⁺/peroxymonosulfate (PMS) process for the elimination of refractory pollutants in industrial wastewater is limited by the slow transformation from Cu²⁺ to Cu⁺. In this research, hydroxylamine (HA) was employed to improve the degradation capacity of the Cu²⁺/PMS process. Rhodamine B (RhB) was selected as the target compound to indicate the performance of HA/Cu²⁺/PMS process. Compared with the Cu²⁺/PMS process, the reduction of Cu²⁺ to Cu⁺ was effectively promoted by HA in the HA/Cu²⁺/PMS process, which increased the decomposition rate of PMS by 29.2%, correspondingly, promoted the removal rate of RhB by 77.6%. The degradation of RhB followed pseudo-second-order kinetics in the proposed process. The active species analysis subsequently indicated hydroxyl radicals (·OH) and sulfate radicals (SO₄·^-) played important roles for degrading RhB with ·OH as the dominant active radical. The effects including initial pH, RhB concentration, PMS concentration, and Cu²⁺ concentration on the degradation of RhB were further investigated and discussed in detail. Additionally, the HA/Cu²⁺/PMS process exhibited effective RhB removal in simulated wastewater. From the perspective of waste utilization (Cu²⁺) and the remediation of organic contamination, the work would provide a valuable and promising process.

Robust polystyrene resin-supported nano-CoFe2O4 mediated peroxymonosulfate activation for efficient oxidation of 1-hydroxyethane 1,1-diphosphonic acid

Nanosized spinel cobalt ferrite (CoFe₂O₄) shows high performance in peroxymonosulfate (PMS) activation for decontamination in water, but is yet challenged by the easily leached Co(II) with high toxicity. Herein, macroporous polystyrene resin is used as the support to improve the stability of CoFe₂O₄ nanoparticles during PMS activation. CoFe₂O₄@S201 exerted high catalytic activity toward PMS activation for oxidation of 1-hydroxyethane 1,1-diphosphonic acid (HEDP), with the apparent rate normalized by Co content 38.2 times higher than that of the unsupported CoFe₂O₄. Meanwhile, one order of magnitude lower Co leaching (< 2.1 μg L^-1) was detected during the catalytic oxidation. The Co(II)-PMS complex was the primary oxidant responsible for the oxidation of HEDP. The catalytic durability and stability of CoFe₂O₄@S201 for degradation of HEDP in actual wastewater were systematically evaluated in both batch and continuous-flow mode. It is found that the organic resin, which is often considered to be intolerant to oxidation, is rather stable during the non-radical process. The total cobalt leaching of the fresh CoFe₂O₄@S201 cannot be ignored in the 100-h continuous-flow run. In contrast, much lower cobalt leaching and slightly higher oxidation efficiency were observed for the regenerated CoFe₂O₄@S201, which might be due to the removal of unreactive and unstable Co sites on the surface in the first trial. The findings shed light on the potential of organic supports for improving the stability and activity of nanosized CoFe₂O₄ and other nano-catalysts toward practical application.