Enzyme-catalyzed removal of phenol from refinery wastewater: feasibility studies

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.

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.

Enzymatic treatment for removal of hazardous aqueous arylamines, 4,4'-methylenedianiline and 4,4'-thiodianiline

The search for an effective and sustainable treatment method to remove the recalcitrant atom-bridged bis-anilino compounds, 4,4'-methylenedianiline (MDA) and 4,4'-thiodianiline (TDA) from water is a major challenge and focus of this study. The escalating discharge of these two toxic and carcinogenic pollutants from industrial sources may pose a serious threat to the environment. Crude soybean peroxidase (SBP), isolated from soybean seed hulls (coats), catalyzes the oxidative polymerization of these aqueous pollutants in the presence of hydrogen peroxide. The effects of several process parameters, i.e., pH, hydrogen peroxide-to-substrate concentration ratio and SBP concentration, were investigated to optimize the performance of enzymatic treatment. The minimum effective SBP concentration required for removal of MDA was 0.70 U/mL, which was higher than that of TDA (0.15 U/mL). The reaction time course to achieve ≥95% removal of these compounds from water was determined under those optimum conditions. Identification of the transformed products was performed by means of high-resolution electrospray ionization mass spectrometry. The products generally observed were protonated oxidized oxidative dimers and higher oligomers (most commonly azo-coupled products). Michaelis constant, KM, and maximum reaction velocity, Vmax, obtained from the Michaelis-Menten (M-M) model revealed that TDA had a 65-fold lower KM than MDA (indicating TDA's higher affinity for SBP), and almost 5-fold higher Vmax than MDA. A pro-forma cost analysis is presented to assess the possibility of commercialization of enzymatic treatment as an alternative to conventional/traditional treatment methods.

Removal of Selected Pharmaceuticals and Personal Care Products from Wastewater using Soybean Peroxidase

Personal care products and pharmaceuticals have been reported in various concentrations in the effluent of municipal sewage treatment plants (STP). Although they are generally found in the nanogram to microgram per liter range, many of them might have adverse health effects on humans at these concentrations. Conventional treatments applied at the STP are unable to effectively remove most of these recalcitrant compounds, thus there is a necessity for development of alternative treatment techniques. In this article, the efficiency of enzymatic treatment using soybean peroxidase in treating some commonly found micropollutants is discussed. The target compounds were, two phenolic surfactant breakdown products, nonylphenol and octylphenol, two antimicrobial agents, Triclosan and sulfamethoxazole and three phenolic steroids. The effects of the most important parameters pH, enzyme concentration and peroxide concentration have been evaluated for each compound. The treatment of synthetic wastewater was shown to be effective (≥95% removal), except for sulfamethoxazole, in concentration ranges of 10 s of µM at neutral pH with 2-5 mU/L of catalytic activity and 2-3 molar equivalents of hydrogen peroxide. The effectiveness of the treatment has also been determined for lower concentrations (6-9 nM) which approximate those in real wastewater. A matrix effect was found in the treatment of Triclosan in spiked real wastewater indicating that re-optimization of important parameters for STP treatment would be required to achieve high removal efficiency. A reverse-phase, solid-phase extraction technique was used to concentrate target analytes in real wastewater, enabling chromatographic detection by UV absorbance.

Additive Effect on Soybean Peroxidase-Catalyzed Removal of Anilines from Water

Soybean peroxidase has been shown to be effective in removal of aromatic compounds from wastewater, while the use of additives effectively reduces enzyme concentration requirement, hence overall treatment cost. Enzymatic treatment, an oxidative polymerization, was successful in removal of over 95% of both aniline and o-anisidine. The originality of this study lies in the findings that the additives, sodium dodecyl sulfate (SDS), sodium dodecylbenzenesulfonate (SDBS), Triton X-100, and sodium dodecanoate (SDOD), reduced enzyme concentration requirement, while polyethylene glycol (PEG, average molar mass of 3350 g/mol) had no effect on the required enzyme concentration. In addition, the presence of SDS also enhanced treatment by improving precipitation and color removal. These results are enabling advancement of soybean peroxidase-catalyzed treatment of anilines found in wastewaters as a new sustainable method.

Immobilized soybean hull peroxidase for the oxidation of phenolic compounds in coffee processing wastewater

Chitosan beads were prepared, using glutaraldehyde as a crosslinking agent for the immobilization of soybean hull peroxidase (SBP). The activity of free and immobilized SBP was studied. The optimum pH was 6.0 for both the free and immobilized enzyme; however, enzyme activity became more dependent on the temperature after immobilization. This study evaluated the potential use of immobilized and free enzyme in the oxidation of caffeic acid, of synthetic phenolic solution (SPS) and of total phenolic compounds in coffee processing wastewater (CPW). Some factors, such as reaction time, amount of H2O2 and caffeic acid were evaluated, in order to determine the optimum conditions for enzyme performance. Both enzymes showed a potential in the removal of caffeic acid, SPS and CPW, and immobilized SBP had the highest oxidation performance. The immobilized enzyme showed a potential of 50% in the oxidation of caffeic acid after 4 consecutive cycles.

Pyrolysis behaviors and kinetics of refining and chemicals wastewater, lignite and their blends through TGA

Co-pyrolysis behaviors of refining and chemicals wastewater solid (RS) and Huolinhe lignite (HL) were investigated via thermogravimetric analysis (TGA). The thermal degradation process of RS and the blends proceeded in three stages, while two stages for HL. The increased percentage of RS in the blends reduced the characteristic temperature (Ti, Tp, Tf) and residual mass (Mr), while raised the characteristic reaction rate (Rp, Rv) and comprehensive devolatilization parameter (D). The results indicated that there existed some inhibitive interactions between RS and HL. The activation energies were calculated by using the Friedman and Starink method. The activation energy of RS increased first and then decreased with conversion degree, and the variation wasn't as great as that of the blends and lignite. No matter which conversion degree is, the activation energy decreased with the percentage of RS in the blends increasing. The minimum value was obtained by blending 75wt.% RS.

Crude soybean hull peroxidase treatment of phenol in synthetic and real wastewater: enzyme economy enhanced by Triton X-100

Soybean peroxidase (SBP)-catalyzed removal of phenol from wastewater has been demonstrated as a feasible wastewater treatment strategy and a non-ionic surfactant, Triton X-100, has the potential for increasing the enzyme economy of the process. Systematic studies on the enzyme-surfactant system have been lacking as well as demonstration of its applicability to industrial wastewater. This paper addresses those two gaps, the latter based on real wastewater from alkyd resin manufacture. The minimum effective Triton X-100 concentrations for crude SBP-catalyzed phenol conversion (≥95%) over 1-10 mM showed a linear trend. To illustrate translation of such lab results to real-world samples, this data were used to optimize crude SBP needed for phenol conversion over that concentration range. Triton X-100 increases enzyme economy by 10- to 13-fold. This treatment protocol was directly applied to tote-scale (700-1000 L) treatment of alkyd resin wastewater, with phenol ranging from 7 to 28 mM and total organic carbon content of >40 g/L, using a crude SBP extract derived from dry soybean hulls by simple aqueous elution. This extract can be used to remove phenol from a complex industrial wastewater and the process is markedly more efficient in the presence of Triton X-100. The water is thus rendered amenable to conventional biological treatment whilst the hulls could still be used in feed, thus adding further value to the crop.

Horseradish peroxidase: modulation of properties by chemical modification of protein and heme

Horseradish peroxidase (HRP) is one of the most studied enzymes of the plant peroxidase superfamily. HRP is also widely used in different bioanalytical applications and diagnostic kits. The methods of genetic engineering and protein design are now widely used to study the catalytic mechanism and to improve properties of the enzyme. Here we review the results of another approach to HRP modification-through the chemical modification of amino acids or prosthetic group of the enzyme. Computer models of HRPs with modified hemes are in good agreement with the experimental data.

Degradation of estrogens by laccase from Myceliophthora thermophila in fed-batch and enzymatic membrane reactors

Several studies reported that natural and synthetic estrogens are the major contributors to the estrogenic activity associated with the effluents of wastewater treatment plants. The ability of the enzyme laccase to degrade these compounds in batch experiments has been demonstrated in previous studies. Nevertheless, information is scarce regarding in vitro degradation of estrogens in continuous enzymatic bioreactors. The present work constitutes an important step forward for the implementation of an enzymatic reactor for the continuous removal of estrone (E1) and estradiol (E2) by free laccase from Myceliophthora thermophila. In a first step, the effect of the main process parameters (pH, enzyme level, gas composition (air or oxygen) and estrogen feeding rate) were evaluated in fed-batch bioreactors. E1 and E2 were oxidized by 94.1 and 95.5%, respectively, under the best conditions evaluated. Thereafter, an enzymatic membrane reactor (EMR) was developed to perform the continuous degradation of the estrogens. The configuration consisted of a stirred tank reactor coupled with an ultrafiltration membrane, which allowed the recovery of enzyme while both estrogens and degradation products could pass through it. The highest removal rates at steady state conditions were up to 95% for E1 and nearly complete degradation for E2. Furthermore, the residual estrogenic activity of the effluent was largely reduced up to 97%.

Soybean peroxidase-catalyzed removal of an aromatic thiol, 2-mercaptobenzothiazole, from water

This paper demonstrates, for the first time, the capability of soybean peroxidase (SBP), an enzyme, for catalyzing the removal of an aromatic thiol, 2-mercaptobenzothiazole (MBT), from aqueous solution. The optimum pH for enzymatic conversion of MBT in aqueous buffer was found to be in the range 6.0 to 9.0. The optimum hydrogen peroxide (H2O2): MBT stoichiometry was 0.6. In terms of standard units (U) of catalytic activity, the minimum SBP concentration required for 95% conversion of 1.0 mM MBT in 3 hours was found to be 0.9 U/mL. The presence of polyethylene glycol at 50 mg/L can reduce the enzyme concentration required for the same conversion by 3-fold. It is proposed that these findings should be the basis for viable and cost-effective treatment of MBT in industrial wastewater and/or process water.

Peroxidase-mediated polymerization of 1-naphthol: impact of solution pH and ionic strength

Peroxidase-mediated oxidation has been proposed as a treatment method for naphthol-contaminated water. However, the impact of solution chemistry on naphthol polymerization and removal has not been documented. This research investigated the impact of pH and ionic strength on peroxidase-mediated removal of 1-naphthol in completely mixed batch reactors. The impact of hydrogen peroxide to 1-naphthol ratio and activity of horseradish peroxidase was also studied. Size exclusion chromatography was used to estimate the molecular weight distribution of oligomeric products, and liquid chromatography/mass spectrometry was used to estimate product structure. Naphthol transformation decreased with ionic strength, and substrate removal was lowest at neutral pHs. Solution pH influenced the size and the composition of the oligomeric products. An equimolar ratio of H(2)O(2):naphthol was sufficient for optimal naphthol removal. Polymerization products included naphthoquinones and oligomers derived from two, three, and four naphthol molecules. Our results illustrate the importance of water chemistry when considering a peroxidase-based approach for treatment of naphthol-contaminated waters.

Inactivation of enzyme laccase and role of cosubstrate oxygen in enzymatic removal of phenol from water

Research was conducted to evaluate the potential use of laccase and its susceptibility to inactivation in an alternative enzyme-based treatment technology to remove parent phenol from buffered distilled water. Enzymatic oxidative polymerization of phenol with laccase was carried out in continuously stirred batch reactors. The reaction products were insoluble polymers, which precipitated out of the solution once their solubility limits were exceeded. The findings demonstrated that the polymeric products had significant effects on enzyme activity consumption and subsequent phenol removal. Enzyme species present in the reaction vessel were classified into enzyme remaining in the solution (type 1) and enzyme adhering to the precipitate polymers (type 2). Type 1 enzyme was more efficient in removal of phenol from solution compared with type 2. Subsequent filtration enhanced the phenol removal by removing type 2 enzyme adhering to the polymer particles and decelerating enzyme inactivation. The study also investigated the effects of available dissolved oxygen, provided through aeration and hydrogen peroxide addition, on phenol removal. Aeration and hydrogen peroxide addition increased the dissolved oxygen concentration, but had no effect on the progress curve for phenol removal.

Enzymatic treatment of sulfonated aromatic amines generated from reductive degradation of reactive azo dyes

Anaerobic degradation, an effective treatment process of textile industry effluent, generates sulfonated aromatic amines, which are carcinogenic, mutagenic, and resistant to microbial degradation. These aromatic amines can be effectively removed by oxidative polymerization catalyzed by peroxidase enzyme. The amines, generated in this study from the anaerobic reduction by zero-valent iron of two reactive azo dyes (Reactive Red 2 [RR2] and Reactive Black 5 [RB5]), were successfully removed (90%) by Arthromyces ramosus peroxidase (ARP). For better understanding of the process, enzymatic treatment of two model compounds, diphenylamine (DPA) and 2-amino-8-naphthol-3,6-disulfonic acid (ANDSA), were also studied. Diphenylamine has a similar diarylamine bond as RR2. The ANDSA has a similar structure as the dye reduction products. The secondary amine bond in DPA and RR2 were oxidized by ARP. Enzymatic reaction of sulfonated aromatic amines generated soluble colored compounds, which were removed by coagulant. Optimum reaction parameters were also determined.

Laccase-catalyzed removal of bisphenol-A from water: protective effect of PEG on enzyme activity

The feasibility of using the enzyme laccase to treat synthetic wastewater containing bisphenol-A (BPA) was examined. Optimization of pH, laccase concentration, polyethylene glycol (PEG) as an additive for >95% conversion and precipitation of BPA over 3 h of reaction period was determined through colorimetric assay and HPLC. PEG reduced enzyme inactivation, allowing a 5.2-fold reduction in the amount of laccase required for >95% removal of BPA in the range of 0.1-1 mM over 3 h. The fate of PEG after the reaction was also monitored. Linear relationships were found between the concentration of BPA (0.1-1 mM) and the optimum concentrations of laccase and PEG. Little PEG remained in the solution when up to 75 mg/L of PEG was used to treat 0.5 mM BPA. Beyond this level, PEG concentration increased linearly in the supernatant. It is inferred that an interaction between PEG and the polymeric products resulted in the protection of laccase.

Removal of aqueous phenol using immobilized enzymes in a bench scale and pilot scale three-phase fluidized bed reactor

The main objective of this work was to investigate the removal of aqueous phenol using immobilized enzymes in both bench scale and pilot scale three-phase fluidized bed reactors. The enzyme used in this application was a fungal tyrosinase [E.C. 1.14.18.1] immobilized in a system of chitosan and alginate. The immobilization matrix consisted of a chitosan matrix cross-linked with glutaraldehyde with an aliginate-filled pore space. This support matrix showed superior mechanical properties along with retaining the unique adsorptive characteristics of the chitosan. Adsorption of the o-quinone product by the chitosan reduced tyrosinase inactivation that is normally observed for this enzyme under these conditions. This approach allowed reuse of the enzyme in repeated batch applications. For the bench scale reactor (1.2-l capacity) more than 92% of the phenol could be removed from the feed water using an immobilized enzyme volume of 18.5% and a residence time of the liquid phase of 150 min. Removal rates decreased with subsequent batch runs. For the pilot scale fluidized bed (60 l), 60% phenol removal was observed with an immobilized enzyme volume of 5% and a residence time of the liquid phase of 7 h. Removal decreased to 45% with a repeat batch run with the same immobilized enzyme.