Oxidative removal of phenol by HRP-immobilized beads and its environmental toxicology assessment

Removal and detoxification of pentahalogenated phenols using a photocatalytically induced enzymatic process

Poly-halogenated phenols generated from a range of industrial processes can find their way into rivers and ground water. Here we report on a potential treatment for reducing the toxicity of these aqueous pollutants using two highly toxic penta-halogenated phenols (pentachlorophenol (PCP) and pentabromophenol (PBP)) as surrogates. Solutions were passed through a glass column packed with a silica support fused with titanium dioxide (TiO2) and horseradish peroxidase (HRP) immobilized on its TiO2/glass surface (HRP-Tglass). TiO2 photocatalysis was activated through irradiation with UVB (320 nm) which in turn activated the HRP. Two operational flow rates (0.5 and 1.25 mL min-1; hydraulic retention times (HRTs) of 20 and 8 min, respectively), tested the effect of retention time on the extent of degradation and reduction in toxicity of the treated effluent. Microtox® was used to measure the toxicity of the substrate and its by-products at both flow rates. At the highest flow rate, dehalogenation was limited (removal of 37 % chlorine and 22 % bromine) and the toxicity of the reaction products increased. At the lowest flow rate, the longer exposure time resulted in approximately 97 % and 96 % transformation of PCP and PBP, respectively, a greater degree of dehalogenation (removal of 65 % chlorine and 70 % bromine) and a substantial decrease in toxicity of the treated solutions. The higher toxicity of effluent from the higher flow rate was attributed to the initial degradation products being more toxic than the substrates. With a longer HRT, these were then further broken down to less toxic products. Additional toxicity tests (Hydra hexactinella (Hydra) and Chinese Hamster Ovary (CHO) cell toxicity were conducted on the effluent from the lowest flow rate. Both were less sensitive than the Microtox test, with Hydra proving more sensitive than CHO. The novelty of this work is the toxicity risk assessment of the products resulting from the use of a spatially separated immobilized enzyme and photooxidation system. The system was robust and showed no decrease in treatment efficacy over 10 h.

Hierarchical porous MoS2 particles: excellent multi-enzyme-like activities, mechanism and its sensitive phenol sensing based on inhibition of sulfite oxidase mimics

It is important to exploit highly efficient methods for detecting pollutants selectively and sensitively. Artificial enzymes are promising to replace natural enzymes with diverse functions for sustainable developments and various applications. However, it remains the challenge to develop novel mimic enzymes or multi-enzyme mimics for pollutant detection. Herein we report hierarchical porous MoS₂ particles prepared by a simple hydrothermal method, which demonstrated excellent sulfite oxidase (SuO_x)-, nicotinamide adenine dinucleotide (NADH) oxidase- and superoxide dismutase-mimicking activities. In addition, the catalytic conditions for SuO_x-like and NADH oxidase-like activities of MoS₂ were optimized. The catalytic mechanism of the NADH oxidase mimics is that O₂ involves in the oxidation of NADH, to generate O₂^.- intermediate and finally turn to H₂O₂, while SuO_x mimics comes from that MoS₂ particles can effectively catalyze sulfite to reduce [Fe(CN)₆]^3-. Based on the excellent SuO_x-like activity of MoS₂ particles, while phenol can inhibit the oxidation of sulfite, a phenol colorimetric sensor was explored with the dynamic range of 2-1000 μM and the limit of detection of 0.72 μM, applicable to detect phenol in effluents. Therefore, MoS₂ particles with the SuO_x-like, NADH oxidase-like and SOD-like activities has broad application prospects in environmental monitoring and bio-analysis.

Immobilized enzymes and cell systems: an approach to the removal of phenol and the challenges to incorporate nanoparticle-based technology

The presence of phenol in wastewater poses a risk to ecosystems and human health. The traditional processes to remove phenol from wastewater, although effective, have several drawbacks. The best alternative is the application of ecological biotechnology tools since they involve biological systems (enzymes and microorganisms) with moderate economic and environmental impact. However, these systems have a high sensitivity to environmental factors and high substrate concentrations that reduce their effectiveness in phenol removal. This can be overcome by immobilization-based technology to increase the performance of enzymes and bacteria. A key component to ensure successful immobilization is the material (polymeric matrices) used as support for the biological system. In addition, by incorporating magnetic nanoparticles into conventional immobilized systems, a low-cost process is achieved but, most importantly, the magnetically immobilized system can be recovered, recycled, and reused. In this review, we study the existing alternatives for treating wastewater with phenol, from physical and chemical to biological techniques. The latter focus on the immobilization of enzymes and microorganisms. The characteristics of the support materials that ensure the viability of the immobilization are compared. In addition, the challenges and opportunities that arise from incorporating magnetic nanoparticles in immobilized systems are addressed.

Free and immobilized biocatalysts for removing micropollutants from water and wastewater: Recent progress and challenges

Enzymatic conversion of micropollutants into less-toxic derivatives is an important bioremediation strategy. This paper aims to critically review the progress in water and wastewater treatment by both free and immobilized enzymes presenting this approach as highly efficient and performed under environmentally benign and friendly conditions. The review also summarises the effects of inorganic and organic wastewater matrix constituents on enzymatic activity and degradation efficiency of micropollutants. Finally, application of enzymatic reactors facilitate continuous treatment of wastewater and obtaining of pure final effluents. Of a particular note, enzymatic treatment of micropollutants from wastewater has been mostly reported by laboratory scale studies. Thus, this review also highlights key research gaps of the existing techniques and provides future perspectives to facilitate the transfer of the lab-scale solutions to a larger scale and to improve operationability of biodegradation processes.

Roles and applications of enzymes for resistant pollutants removal in wastewater treatment

Resistant pollutants like oil, grease, pharmaceuticals, pesticides, and plastics in wastewater are difficult to be degraded by traditional activated sludge methods. These pollutants are prevalent, posing a great threat to aquatic environments and organisms since they are toxic, resistant to natural biodegradation, and create other serious problems. As a high-efficiency biocatalyst, enzymes are proposed for the treatment of these resistant pollutants. This review focused on the roles and applications of enzymes in wastewater treatment. It discusses the influence of enzyme types and their sources, enzymatic processes in resistant pollutants remediation, identification and ecotoxicity assay of enzymatic transformation products, and typically employed enzymatic wastewater treatment systems. Perspectives on the major challenges and feasible future research directions of enzyme-based wastewater treatment are also proposed.

Applications of Biocatalysts for Sustainable Oxidation of Phenolic Pollutants: A Review

Phenol and its derivatives are hazardous, teratogenic and mutagenic, and have gained significant attention in recent years due to their high toxicity even at low concentrations. Phenolic compounds appear in petroleum refinery wastewater from several sources, such as the neutralized spent caustic waste streams, the tank water drain, the desalter effluent and the production unit. Therefore, effective treatments of such wastewaters are crucial. Conventional techniques used to treat these wastewaters pose several drawbacks, such as incomplete or low efficient removal of phenols. Recently, biocatalysts have attracted much attention for the sustainable and effective removal of toxic chemicals like phenols from wastewaters. The advantages of biocatalytic processes over the conventional treatment methods are their ability to operate over a wide range of operating conditions, low consumption of oxidants, simpler process control, and no delays or shock loading effects associated with the start-up/shutdown of the plant. Among different biocatalysts, oxidoreductases (i.e., tyrosinase, laccase and horseradish peroxidase) are known as green catalysts with massive potentialities to sustainably tackle phenolic contaminants of high concerns. Such enzymes mainly catalyze the o-hydroxylation of a broad spectrum of environmentally related contaminants into their corresponding o-diphenols. This review covers the latest advancement regarding the exploitation of these enzymes for sustainable oxidation of phenolic compounds in wastewater, and suggests a way forward.

Nanozymes for Environmental Pollutant Monitoring and Remediation

Nanozymes are advanced nanomaterials which mimic natural enzymes by exhibiting enzyme-like properties. As nanozymes offer better structural stability over their respective natural enzymes, they are ideal candidates for real-time and/or remote environmental pollutant monitoring and remediation. In this review, we classify nanozymes into four types depending on their enzyme-mimicking behaviour (active metal centre mimic, functional mimic, nanocomposite or 3D structural mimic) and offer mechanistic insights into the nature of their catalytic activity. Following this, we discuss the current environmental translation of nanozymes into a powerful sensing or remediation tool through inventive nano-architectural design of nanozymes and their transduction methodologies. Here, we focus on recent developments in nanozymes for the detection of heavy metal ions, pesticides and other organic pollutants, emphasising optical methods and a few electrochemical techniques. Strategies to remediate persistent organic pollutants such as pesticides, phenols, antibiotics and textile dyes are included. We conclude with a discussion on the practical deployment of these nanozymes in terms of their effectiveness, reusability, real-time in-field application, commercial production and regulatory considerations.

An accomplished procedure of horseradish peroxidase immobilization for removal of acid yellow 11 in aqueous solutions

Horseradish peroxidase (HRP) characteristics were improved by two techniques, Na-alginate entrapment and glutaraldehyde crosslinking prior to alginate entrapment, in order to enhance the stability, functionality and removal of dyes in waste water. Free, entrapped and crosslinked-entrapped enzymes were compared by activity assays, which indicated the optimum temperature is 25 °C and pH 4.0-5.0. Kinetics results showed that alginate entrapment and crosslinking prior to entrapment increased V_max and did not cause any significant decrease in K_m. The thermal resistance of the free enzyme was short-term, zero residual activity after 250 min, while the immobilized enzymes preserved more than 50% of their activity for 5 h at 60 °C. Immobilized HRP was resistant to methanol, ethanol, DMSO and THF. The storage stability of free HRP ended in 35 days whereas entrapped and crosslinked-entrapped HRPs had 87 and 92% residual activity at the 60th day, respectively. HRP was used in the decolorization of azo dye Acid yellow 11 and total decolorization (>99%) was obtained using crosslinked-entrapped HRP. Reusability studies presented the improvement that crosslinked-entrapped HRP reached 74% decolorization after 10 batches. The results demonstrated that the novel immobilized HRP can be used as an effective catalyst for dye degradation of industrial waste effluents.

Development of a Surfactant-Containing Process to Improve the Removal Efficiency of Phenol and Control the Molecular Weight of Synthetic Phenolic Polymers Using Horseradish Peroxidase in an Aqueous System

To reduce phenolic pollutants in the environment, many countries have imposed firm restrictions on industrial wastewater discharge. In addition, the current industrial process of phenolic resin production uses phenol and formaldehyde as the reactants to perform a polycondensation reaction. Due to the toxicity of formaldehyde and phenolic pollutants, the main purpose of this research was to design a green process using horseradish peroxidase (HRP) enzymatic polymerization to remove phenols and to produce formaldehyde-free phenolic polymers. In this study, the optimal reaction conditions, such as reaction temperature, pH, initial phenol concentration and initial ratio of phenol, and H₂O₂, were examined. Then, the parameters of the enzyme kinetics were determined. To solve the restriction of enzyme inactivation, several nonionic surfactants were selected to improve the phenol removal efficiency, and the optimal operation conditions in a surfactant-containing system were also confirmed. Importantly, the molecular weight of the synthetic phenolic polymers could be controlled by adjusting the ratio of phenol and H₂O_2. The content of biphenols in the products was almost undetectable. Collectively, a green chemistry process was proposed in this study and would benefit the treatment of phenol-containing wastewater and the production of formaldehyde-free phenolic resin in the future.

Degradation of phenol by coal-based carbon membrane integrating sulfate radicals-based advanced oxidation processes

Phenol, as a representative organic pollutant in aquatic environments, has posed a serious threat to humans and ecosystem. In this work, a novel integration system combined coal-based carbon membrane with sulfate radicals-based advanced oxidation processes (SR-AOPs) was designed for degradation of phenol. The integrated system achieved 100% removal efficiency under the optimal condition (peroxydisulfate dosage is 0.2 g/L, at alkaline condition with 2 mL/min flow velocity). The quenching experiments revealed that the efficient removal of phenol by the integrated system were attributed to the co-existence of radical and nonradical mechanisms. This study proposes a green and efficient technique for the removal of phenol.

Protecting Enzymatic Activity via Zwitterionic Nanocapsulation for the Removal of Phenol Compound from Wastewater

Horseradish peroxidase (HRP) holds great potential in wastewater treatment. However, its instability in harsh environments remains a major issue. Various immobilization technologies were developed to retain enzyme stability at the cost of its effectiveness. We demonstrate that zwitterionic encapsulation of HRP retained both protein stability and activity to a large degree. In a water treatment study, encapsulating HRP into a zwitterionic nanogel resulted in a three-fold increase in the catalytic oxidation efficiency of phenol molecules. In addition, zwitterionic nanocapsules exhibited the best performance when compared with nanocapsules made from other hydrophilic polymers. These results indicated that zwitterionic HRP nanocapsules hold great potential in the decontamination of organic pollutants from wastewater.