Different classes of artificial pollutants, collectively called emerging pollutants, are detected in various water bodies, including lakes, rivers, and seas. Multiple studies have shown the devastating effects these emerging pollutants can have on human and aquatic life. The main reason for these emerging pollutants in the aquatic environment is their incomplete removal in the existing wastewater treatment plants (WWTPs). Several additional treatments that could potentially supplement existing WWTPs to eliminate these pollutants include a range of physicochemical and biological methods. The use of enzymes, specifically, oxidoreductases, are increasingly being studied for their ability to degrade different classes of organic compounds. These enzymes have been immobilized on different supports to promote their adoption as a cost-effective and recyclable remediation approach. Unfortunately, some of these techniques have shown a negative effect on the enzyme, including denaturation and loss of catalytic activity. This review focuses on the major challenges facing researchers working on the immobilization of peroxidases and the recent progress that has been made in this area. It focuses on four major areas: (1) stability of enzymes upon immobilization, enzyme engineering, and evolution; (2) recyclability and reusability, including immobilization on membranes and solid supports; (3) cost associated with enzyme-based remediation; and (4) scaling-up and bioreactors.

As a result of their unique structural and multifunctional characteristics, organic–inorganic hybrid nanoflowers (hNFs), a newly developed class of flower-like, well-structured and well-oriented materials has gained significant attention. The structural attributes along with the surface-engineered functional entities of hNFs, e.g., their size, shape, surface orientation, structural integrity, stability under reactive environments, enzyme stabilizing capability, and organic–inorganic ratio, all significantly contribute to and determine their applications. Although hNFs are still in their infancy and in the early stage of robust development, the recent hike in biotechnology at large and nanotechnology in particular is making hNFs a versatile platform for constructing enzyme-loaded/immobilized structures for different applications. For instance, detection- and sensing-based applications, environmental- and sustainability-based applications, and biocatalytic and biotransformation applications are of supreme interest. Considering the above points, herein we reviewed current advances in multifunctional hNFs, with particular emphasis on (1) critical factors, (2) different metal/non-metal-based synthesizing processes (i.e., (i) copper-based hNFs, (ii) calcium-based hNFs, (iii) manganese-based hNFs, (iv) zinc-based hNFs, (v) cobalt-based hNFs, (vi) iron-based hNFs, (vii) multi-metal-based hNFs, and (viii) non-metal-based hNFs), and (3) their applications. Moreover, the interfacial mechanism involved in hNF development is also discussed considering the following three critical points: (1) the combination of metal ions and organic matter, (2) petal formation, and (3) the generation of hNFs. In summary, the literature given herein could be used to engineer hNFs for multipurpose applications in the biosensing, biocatalysis, and other environmental sectors.

In the present study, soybean peroxidase (SBP) was covalently immobilized onto two functionalized photocatalytic supports (TiO₂ and ZnO) to create novel hybrid biocatalysts (TiO₂-SBP and ZnO-SBP). Immobilization caused a slight shift in the pH optima of SBP activity (pH 5.0 to 4.0), whereas the free and TiO₂-immobilized SBP showed similar thermal stability profiles. The newly developed hybrid biocatalysts were used for the degradation of 21 emerging pollutants in the presence and absence of 1-hydroxy benzotriazole (HOBT) as a redox mediator. Notably, all the tested pollutants were not equally degraded by the SBP treatment and some of the tested pollutants were either partially degraded or appeared to be recalcitrant to enzymatic degradation. The presence of HOBT enhanced the degradation of the pollutants, while it also inhibited the degradation of some contaminants. Interestingly, TiO₂ and ZnO-immobilized SBP displayed better degradation efficiency of a few emerging pollutants than the free enzyme. Furthermore, a combined enzyme-chemical oxidation remediation strategy was employed to degrade two recalcitrant pollutants, which suggest a novel application of these novel hybrid peroxidase-photocatalysts. Lastly, the reusability profile indicated that the TiO₂-SBP hybrid biocatalyst retained up to 95% degradation efficiency of a model pollutant (2-mercaptobenzothiazole) after four consecutive degradation cycles.

Organic pollutants, especially those found in water bodies, pose a direct threat to various aquatic organisms as well as humans. A variety of different remediation approaches, including chemical and biological methods, have been developed for the degradation of various organic pollutants. However, comparative mechanistic studies of pollutant degradation by these different systems are almost non-existent. In this study, the degradation of a model thiazole pollutant, thioflavin T (ThT), was carried out in the presence of either an advanced oxidation process (ultraviolet (UV) + H₂O₂) or a chloroperoxidase enzyme system (CPO + H₂O₂). The degradation was followed both spectrophotometrically and using liquid chromatography-mass spectroscopy (LC-MS), and the products formed were identified using tandem liquid chromatography-mass spectrometry-mass spectrometry (LC-MS-MS). The results show that the two remediation approaches produced different sets of intermediates, with only one common species (a demethylated form of ThT). This suggests that different degradation schemes were operating in the two systems. Interestingly, one of the major intermediates produced by the CPO + H₂O₂ system was a chlorinated form of thioflavin. Phytotoxicity studies showed that the CPO + H₂O₂-treated ThT solution was significantly (p <0.05) less toxic than the UV + H₂O₂-treated ThT solution. This is the first time that a comparative mechanistic study showing in detail the intermediates generated in chemical and biological remediation methods has been presented. Furthermore, the results show that different remediation systems have very different degradation schemes and result in products having different toxicities.

With the increasing detection of emerging pollutants (EPs), such as pharmaceuticals, personal care products, and pesticides, in water bodies, there is a growing research priority to develop efficient methods for their degradation. In the present study, the degradation of 21 EPs from different classes was investigated using horseradish peroxidase (HRP) covalently immobilized onto two photocatalytic supports, TiO ₂ and ZnO. The hybrid biocatalysts (TiO ₂ -HRP and ZnO-HRP) were used to degrade EPs with and without the presence of a redox mediator, 1-hydroxybenzotriazole (HOBT). The results revealed that not all EPs were equally degraded by HRP enzymes, with some pollutants showing complete, partial, or no degradation. For example, full degradation was achieved for MBT (100%), meloxicam (99.5%) and caffeic acid (98.6%). In contrast, prometryn and MCPA showed only partial degradation, reaching 19.2% and 20%, respectively, and no degradation was observed for fluometuron and atenolol. The addition of the model redox mediator, HOBT, improved the degradation of some pollutants, for example, lincomycin-HCl degradation increased to 36.9%, while trimethoprim degradation improved to 21.5%. Additionally, several pollutants, including roxithromycin, cimetidine, and caffeine, exhibited significantly higher degradation rates when treated with the immobilized HRP enzyme (TiO ₂ -HRP or ZnO-HRP) compared to free HRP. For instance, roxithromycin degradation increased from 0.5% to 58.2% with ZnO-HRP, and for cimetidine, TiO₂-HRP improved degradation from 0.3% to 60.1% without HOBT and from 9.0% to 66.7% with HOBT. Molecular docking with the catalytic heme cofactor retained further suggested that degradation trends were not explained by docking score alone, but were influenced by ligand access to the distal heme pocket, molecular planarity, and contact topology near catalytic residues. Finally, an integrated HRP enzyme and chemical oxidation remediation strategy was applied to degrade two pollutants, trimethoprim and DEET. For trimethoprim, TiO ₂ -HRP combined with H ₂ O ₂ and UV achieved approximately 30% degradation, while for DEET, both TiO ₂ -HRP + H ₂ O ₂ + UV and ZnO-HRP + H ₂ O ₂ + UV resulted in around 40% degradation, thereby highlighting a novel application for these hybrid peroxidase-photocatalysts.

This greenhouse study examined whether the drought-mitigating effects of elevated ${\mathrm{CO}}_2$ (${\mathrm{eCO}}_2$), reported under favorable temperatures, persist at high temperatures in the subtropical desert climate of the United Arab Emirates (UAE). Tabernaemontana divaricata (L.) R. Br. ex Roem. & Schult. (Apocynaceae), a tropical ${\mathrm{C}}_3$ plant, was grown at 35.5/26 °C (day/night) under three ${\mathrm{CO}}_2$ levels (ambient, 700, and 1000 ppm) and three water regimes (well-watered, mild, and moderate drought). Significant ${\mathrm{CO}}_2$× drought interactions were observed for all variables except stomatal density. Physiological responses to ${\mathrm{eCO}}_2$ relative to respective ambient ${\mathrm{CO}}_2$ (${\mathrm{aCO}}_2$) varied across variables and water regimes. Contrary to the expected decline, ${\mathrm{eCO}}_2$ increased stomatal conductance at 700 and 1000 ppm under mild (+77.8% and +243.4%) and moderate drought (+65.5% and +305.7%), respectively. At 1000 ppm, stomatal area and density showed the expected inverse relationship under well-watered (area +34.7%, density −29.1%) and moderate drought conditions (area +5.4%, density −19.4%). Under both drought levels, 700 and 1000 ppm increased abscisic acid (1.4-fold to 7.7-fold), glycine betaine (2.4-fold to 96.2-fold), and leaf water potential (+9.0% to +42.6%). Proline and chlorophyll b increased only at 700 ppm under mild (+28.8% and +15.3%) and moderate drought (+29.1% and +10.7%), whereas 1000 ppm reduced proline (−7.9% and −15.1%) and, under moderate drought, chlorophyll b (−6.6%). Chlorophyll a increased at 700 and 1000 ppm under mild drought (+35.4% and +16.3%) but decreased at 1000 ppm under moderate drought (−21.8%). Effective quantum yield of photosystem II decreased at both ${\mathrm{CO}}_2$ levels (−14.4% to −19.2%), except at 700 ppm under moderate drought. Relative to well-watered ${\mathrm{aCO}}_2$ plants, 1000 ppm countered mild and moderate drought impacts on biomass and increased biomass under mild drought (+8.3%); however, 700 ppm only countered mild drought impacts. Yet these growth benefits were not directly explained by the measured physiological variables. Overall, similar to ${\mathrm{eCO}}_2$ responses reported under favorable temperatures, 1000 ppm mitigated drought effects on the growth of T. divaricata under high growing temperatures.