Phenolic Compounds In Aqueous Solution Research Articles

Enhancement of integrated nano-sensor performance comprised of electrospun PANI/carbonaceous material fibers for phenolic detection in aqueous solutions

The detection of trace levels of organic residue in water samples is a key health issue. This manuscript describes the fabrication of integrated nano-sensors composed of electrospun microfibers consisting of a nanocomposite of carbonaceous materials (CNMs) containing polyaniline (PANI) and polycaprolactone (PCL) for phenolic detection in aqueous solutions. The morphology of the resulting microfiber composite was characterized by scanning electron microscopy. It revealed elongated fibers with a highly interconnected web-like pattern in the presence of reduced graphene oxide (rGO). Shorter microfibers were observed in the composite filled with multi-walled carbon nanotubes (MWCNTs), whereas large agglomerates were formed upon the incorporation of single-walled CNTs (SWCNTs) and graphene 300 (G300). Comparative analysis showed that the PANI/CNM sensors exhibited the best electrochemical properties, in particular in the presence of rGO and MWCNTs, where greater electrical conductivity was achieved, i.e., 4.33 × 10−3 and 7.22 × 10−4 S/cm, respectively, as compared to the PANI-PCL sensor (3.79 × 10−4 S/cm). All the PANI/CNM sensors exhibited high sensitivity. Notably, PANI/rGO was found to have a detection limit of 8.34 × 10−3 µM for aminophenol. All the sensors exhibited good selectivity in the presence of interference to detecting phenolic compounds in aqueous solutions, thus confirming their value for industrial applications.

Retarding Oxidative and Enzymatic Degradation of Phenolic Compounds Using Large-Ring Cycloamylose.

The phenolic compounds (PCs) abundant in fruits and vegetables are easily browned by oxygen and browning enzymes, with subsequent destruction of nutrients during food processing and storage. Therefore, natural anti-browning additives are required to control these reactions. The aim of the present study was to investigate the feasibility of cycloamylose (CA) complexation as a way to improve stability of PCs against oxidation and browning enzymes. The complex was prepared by reacting enzymatically produced CA with a degree of polymerization of 23–45 with PCs in aqueous solution. No significant differences were observed between the PCs and their CA complexes in 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical scavenging experiments. However, the reduction rate of their antioxidant activity was clearly reduced in the presence of CA for as long as 4 weeks. At the studied concentrations, the activity of polyphenol oxidase on all of the tested PC species was inhibited in the presence of CA, although this effect was less evident as the substrate concentration increased. The higher the CA concentration added to apple juice, the lower the variation in the total color difference (ΔE*) during storage, confirming that CA could be used as an effective natural anti-browning agent. Our study is the first to study the potential of CA as a natural material for browning control. The results obtained will provide useful information for active food applications requiring oxidative stability in fruit products.

Development and characterization of pH-indicator films based on cassava starch and blueberry residue by thermocompression

Due to the concern about the inadequate disposal of synthetic and non-biodegradable plastic packaging, a great number of researches pointing at the substitution of these materials by natural and biodegradable sources have been growing in recent years. Another important category recognizes as intelligent packaging has also received more prominence. The study intended to achieve the matter aforementioned developing starch-based films by thermocompression without and with blueberry residue as pH change indicator. Moisture content, water solubility, water vapor permeability, oxygen permeability, morphology, color, and mechanical properties of the films were analyzed. Moreover, analysis of color change and compounds migration were conduct in the films with blueberry. The residue insertion led to the formation of less compact films with high oxygen permeability. Despite the irregular dispersion of the blueberry residue on the polymer matrix identified by optical microscopy, the films showed equal values of water vapor permeability, hydrophilicity, and mechanical properties when compared to the samples without the residue. In addition, they exhibited visually perceptible color changes in the pH range between 2 and 12, which was confirmed by colorimetric analysis since the ΔE94∗ values between the samples were greater than 3. The compounds migration after film immersion in aqueous simulant solution (acetic acid) showed a higher intensity in comparison to the samples immersed in a fatty simulant solution (ethanol) due to the anthocyanins hydrophilicity. Therefore, the film with blueberry has potential as intelligent packaging. Besides that, it present potential as active packaging due to the migration of phenolic compounds in aqueous solutions.

Interrupted amperometry on solid electrodes: Determination of hydroquinone and phenol

The new method, interrupted amperometry, is proposed to increase the sensitivity of amperometric measurements. Analytical possibilities of interrupted amperometry on solid electrodes are studied on an example of the determination of phenolic compounds in aqueous solutions.

Separation of phenolic compounds with modified adsorption resin from aqueous phase products of hydrothermal liquefaction of rice straw

Hydrothermal liquefaction can be used to convert rice straw into an aqueous phase product that contains valuable phenolic compounds. In experiments, commercial adsorption resin XAD-4 was modified by a benzene ring – α,α′-dichloro-p-xylene (DCX) – in order to separate the phenolic compounds from the aqueous phase product; and, the optimal conditions for separation were explored. The results showed that, after modification of the resin, its adsorption capacity improved by 50%, due to increases in surface area, pore volume and micropore volume. The selectivity of the resin increased when the benzene ring was introduced as the ring formed hydrogen bonds with the compounds. The optimal conditions for separation were desorption agent of 40%, 45% and 55% ethanol solution, a flow rate of 2.5–5mL/min, and a ratio of the sample volume to the column volume was 1:1. The total content of phenolic compounds in aqueous solution increased from 18% to 78% after separation.

Photooxidation of phenolic compounds in aqueous solution with a vacuum-UV lamp

The decomposition of phenolic compounds (phenol, p-chlorophenol, and p-nitrophenol) in wastewaters by VUV direct photolysis and VUV/H2O2 process with a 185 nm Vacuum-UV lamp in a batch photoreactor was studied under various solution pH values and purging gases (N2 and O2). The photolytic properties of these phenolic compounds were found to be highly dependent on the solution pH. Experimental results for the VUV alone system revealed that photolytic rates of organics by purging O2 were apparently larger than those by purging N2 and the removal of organics was found to be above 95%. For the 185 nm/VUV/H2O2 process, the reaction rates were significantly raised compared to those by direct photolysis.

Photocatalytic oxidation of phenolic compounds using zinc oxide and sulphate radicals under artificial solar light

Photocatalytic degradation of phenolic compounds in aqueous solution was investigated in TiO2/persulphate/UV–vis and ZnO/persulphate/UV–vis light. It is found that ZnO exhibits higher activity than TiO2 in photocatalytic degradation of phenol under simulated solar light. The oxidation of phenol in either ZnO/UV–vis or persulphate/UV–vis system occurs at a similar rate while the degradation will be significantly enhanced with ZnO/persulphate/UV–vis system. The kinetics of phenol oxidation in ZnO/persulphate/UV–vis system follows the first order model. Several parameters influencing the reaction rate were also investigated including power intensity, phenol, ZnO and persulphate concentrations. It is shown that stronger light intensity will increase phenol oxidation. An optimal concentration of ZnO and persulphate will be required for maximum degradation of phenol. For phenol, dichlorophenol and trichlorophenol, photocatalytic activity in ZnO/persulphate/UV–vis shows an order of phenol > dichlorophenol > trichlorophenol.

The bromination kinetics of phenolic compounds in aqueous solution

The purpose of this study was to investigate the bromination kinetics of selected phenolic compounds in aqueous solutions over the pH range of 5–11. The experiment results indicated that the reaction of hypobromous acid with the phenoxide ions controlled the overall reaction rate, whereas the reaction between hypobromite ion and the phenoxide ions and the reaction between hypobromous acid and the undissociated phenolic species were considered to be negligible respectively in the pH range of 7–9. The apparent second-order rate constants of the reaction of hypobromous acid with the phenoxide ions ranged from 7.9 × 10 6 M −1 s −1 for 3-chlorophenol to 6.5 × 10 8 M −1 s −1 for 3-methoxyphenol, respectively. The Hammett correlation could be successfully used to estimate the reactivity of bromine with substituted phenols and the linear regression was log( k 2) = −2.85б + 8.00. The rate constants of the reaction of bromine with phenol-like organic compounds were about three orders of magnitude higher than with chlorine and two to three orders of magnitude lower than with ozone.

Influence of membrane, solute and solution properties on the retention of phenolic compounds in aqueous solution by nanofiltration membranes

The objective of this study was to investigate the physico-chemical parameters that determine the retention of phenolic compounds in aqueous solution by nanofiltration membranes. To this purpose, fundamental properties of two membranes (NF90 and NF270) were determined from their observed performance. Molecular weight cut-offs (MWCO) were determined using crossflow filtration of different molecular weights of polyethylene glycol (PEG). Their mean pore size was also estimated by the steric hindrance pore (SHP) model using neutral organic solutes (saccharides). The salt retention sequence of membranes was determined using single aqueous solutions of Na 2SO 4, NaCl and CaCl 2. Retention of phenolic compounds by NF270 was lower than that of NF90, because of its larger pore size. The influence of the solute adsorption upon the NF90 membrane retention was described by the molecular hydrophobicity of organic compounds. Finally, the effect of solution chemistry upon the membrane retention of phenol and resorcinol was investigated at different pH values. Near the isolelectric point of the membranes, minimum organic retention and maximum flux were observed due to membrane electroneutrality.

Peroxidase Conjugate of Cellulose Nanocrystals for the Removal of Chlorinated Phenolic Compounds in Aqueous Solution

Peroxidase Conjugate of Cellulose Nanocrystals for the Removal of Chlorinated Phenolic Compounds in Aqueous Solution

Photocatalytic degradation kinetics of di- and tri-substituted phenolic compounds in aqueous solution by TiO2/UV

In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO2/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1–0.5mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir–Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k 0.0083mMmin−1 and K 9.03mM−1. For TBP and TMP the values of k and K were obtained as 0.0040mMmin−1, 19.20mM−1, and 0.0017mMmin−1, 51.68mM−1, respectively. Degradation rate constant of DBP was similar as DCP (0.0029mMmin−1 for DBP and 0.0031mMmin−1 for DCP) whereas adsorption equilibrium constants differed (48.40mM−1 for DBP and 30.52mM−1 for DCP). K and k of DMP found as 83.68mM−1 and 0.0019mMmin−1, respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28mM−1 which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied.

Enhancement of nitrophenol adsorption in the presence of anionic surfactant and the effect of the substituent position

Adsorption behavior of phenol, ortho-nitrophenol (ONP), meta-nitrophenol (MNP) and para-nitrophenol (PNP) on a kaolinite-type clay has been investigated by means of spectrophotometric method at 298 K. Adsorbability of the phenolic compounds in aqueous solution decreases in the following order; ONP > phenol > MNP ≅ PNP. Influence of an anionic surfactant, sodium dodecylsulfate (SDS), on adsorption behavior of phenol and nitro-substituted phenols onto kaolinite is studied by determining their adsorption isotherms. Experimental evidence has shown that phenol does not adsorb to the surface of kaolinite in the presence of SDS due to large repulsive, electrostatic barrier between negatively charged phenol and negatively charged adsorptive SDS micelles or hemimmicelles. The adsorption capacity of kaolinite for the aromatic nitro compounds in the SDS system is markedly increased compared with the adsorption onto bare kaolinite surface especially at high-concentration region. Adsorption results were interpreted by comparing isotherms of phenolic compounds and SDS. It is shown that bilayer structure of SDS forms even at low SDS concentrations. In this region, PNP molecules are adsorbed via adsolubilisation whereas penetration of ONP and PNP is difficult because of their structural properties and electrical interactions. However, adsolubilisation phenomenon dominates adsorption process of all nitrophenol compounds at higher concentrations. Data show that adsorption capacities of the adsorbates in the presence of SDS follow the order ONP > MNP > PNP. The differences in the adsorption capacities depending on position of nitro group on phenol ring are correlated with the capacity of SDS. The equilibrium isotherms of phenols have been analyzed using Freundlich and Langmuir equations.

Sonolysis of natural phenolic compounds in aqueous solutions: degradation pathways and biodegradability

The sonochemical degradation in water of p-coumaric acid and p-hydroxybenzaldehyde, compounds that are commonly found in effluents of agricultural origin, was investigated using an 80 kHz horn-type sonicator. Experiments were performed at initial concentrations varying between 10 and 100 mg L −1, an applied power varying between 75 and 150 W and liquid bulk temperatures varying between ambient and 70°C. At the conditions in question, substrate removal was found to increase with increasing power and decreasing initial concentration and temperature. Addition of 1-butanol as a radical scavenger at a concentration of 1000 mg L −1 nearly completely suppressed p-coumaric acid degradation. Conversely, addition of radical promoters (Fe 2+ ions or H 2O 2) had a positive effect on removal, thus implying that degradation proceeds predominantly via hydroxyl radical-induced reactions. Reaction intermediates were determined using liquid and gas chromatography and a reaction network for p-coumaric acid degradation is suggested. Shake flask tests with activated sludge were performed to assess the aerobic biodegradability (in terms of chemical oxygen demand removal) before and after sonochemical treatment. At the conditions under consideration, the use of ultrasound enhanced the aerobic degradability of the substrates in question.

Determination of phenols by flow injection and liquid chromatography with on-line quinine-sensitized photo-oxidation and quenched luminol chemiluminescence detection

An on-line quinine-sensitized photo-oxidation with quenched chemiluminescence (CL) detection method is developed for phenols using flow injection (FI) and liquid chromatography (LC). This detection method is based on the decrease of light emission from the luminol CL reaction due to the photo-oxidation of phenols that scavenge the photogenerated reactive oxygen species (e.g. singlet oxygen ( 1 O 2 ) and superoxide (O 2 −)). On-line photo-oxidation is achieved using a coil photo-reactor made from fluoroethylene–propylene copolymer tubing ( 3048 mm×0.25 mm i.d.) coiled around a mercury UV lamp. A buffer of pH 7 and a concentration of 350 μM for quinine sulfate are determined optimum for the sensitized photo-oxidation. Using a carrier system flow rate of 60 μl/min, calibration curves taken by FI for 10 phenolic compounds in aqueous solutions showed this decreasing sensitivity order: 4-chlorophenol, phenol, 4-nitrophenol, 3-hydroxy- l-kynurenine, 2-nitrophenol, salicylate, 3-nitrophenol, catechol, 2,4-dinitrophenol, and 2,4-dichlorophenol. This detection method using two tandem coil photo-reactors is also applied for the LC separation of phenol, 4-nitrophenol and 4-chlorophenol on an octadecyl (C18) silica LC column using acetonitrile-H 2O (40:60, v/v) as a mobile phase. The quenched CL detection limits (about 1 μM or 20 pmol) for phenol and 4-chlorophenol are comparable to those for UV detection at 254 nm. Some selectivity in the quenched CL detection is evident by no interference in the FI phenol response even when benzaldehyde and phenethanol concentrations are 8 and 15 times that of phenol.

Effects of activated carbon on the reactions of combined chlorine with phenols

In the presence of ammonia, prechlorination in drinking water treatment results in contact of combined chlorine (monochloramine) with activated carbon, which is used to remove organic compounds from water. Monochloramine reacts very slowly with phenolic compounds in aqueous solution, giving low yields of chlorinated phenols. When monochloramine reacts with phenols adsorbed on granular activated carbon (GAC), however, several oxidized products, principally hydroxylated biphenyls, are formed. Some of the hydroxylated biphenyls are chlorinated (hydroxylated PCBs). Their formation is particularly important because of their potential toxicity. Such compounds are major reaction products from chlorophenol, but they are also formed in small amounts from nonchlorinated phenols. Most of the monochloramine-GAC-phenolic compound reaction products are also produced in similar reactions with free chlorine, indicating that similar reaction mechanisms (free radical mechanisms) take place on carbon's surface. No organic compounds are produced from the reaction of monochloramine with GAC alone.

Determination of Phenolic Compounds by HPLC

Details are given of a procedure for separation and identification of phenolic compounds in aqueous solution by high-performance liquid chromatography. It can also be applied to non-aqueous samples after preliminary isolation of a polar fraction containing the phenolic compounds.