Efficient microwave degradation of humic acids in water using persulfate and activated carbon

Environmental Chemistry Letters - Humic acids are complex mixtures of organic molecules of different sizes, molecular weights and functional groups such as phenols, carboxyls, quinones and amino...     

Electro-oxidation of the dye azure B: kinetics,mechanism, and by-products

In this work, the electrochemical degradation of the dye azure B in aqueous solutions was studied by electrochemical advanced oxidation processes (EAOPs), electro-Fenton, and anodic oxidation processes, using Pt/carbon-felt and boron-doped diamond (BDD)/carbon-felt cells with H₂O₂ electrogeneration. The higher oxidation power of the electro-Fenton (EF) process using BDD anode was demonstrated. The oxidative degradation of azure B by the electrochemically generated hydroxyl radicals (^?OH) follows a pseudo-first-order kinetics. The apparent rate constants of the oxidation of azure B by ^?OH were measured according to pseudo-first-order kinetic model. The absolute rate constant of azure B hydroxylation reaction was determined by competition kinetics method and found to be 1.19?×?10⁹ M^?1 s^?1. It was found that the electrochemical degradation of the dye leads to the formation of aromatic by-products which are then oxidized to aliphatic carboxylic acids before their almost mineralization to CO₂ and inorganic ions (sulfate, nitrate, and ammonium). The evolution of the TOC removal and time course of short-chain carboxylic acids during treatment were also investigated.     

Ferrate(VI) enhanced photocatalytic oxidation of pollutants in aqueous TiO<Subscript>2</Subscript> suspensions

Background, aim and scope  

Photocatalytic oxidation using UV irradiation of TiO₂ has been studied extensively and has many potential industrial applications, including the degradation of recalcitrant contaminants in water and wastewater treatment. A limiting factor in the oxidation process is the recombination of conduction band electrons (e ⁻ _cb) with electron holes (h_vb⁺) on the irradiated TiO₂ surface; thus, in aqueous conditions, the presence of an effective electron scavenger will be beneficial to the efficiency of the oxidation process. Ferrate (FeO₄²⁻) has received much recent attention as a water treatment chemical since it behaves simultaneously as an oxidant and coagulant. The combination of ferrate [Fe(VI)] with UV/TiO₂ photocatalysis offers an oxidation synergism arising from the Fe(VI) scavenging of e ⁻ _cb and the corresponding beneficial formation of Fe(V) from the Fe(VI) reduction. This paper reviews recent studies concerning the photocatalytic oxidation of problematic pollutants with and without ferrate.     

Nanoplastics are potentially more dangerous than microplastics

Environmental Chemistry Letters - Nanoplastics are probably much more dangerous for living organisms than microplastics&nbsp;because they are more abundant and reactive. They can potentially...     

Reactivity of chlorine dioxide with amino acids, peptides, and proteins

Amino acids, proteins, and peptides are found ubiquitously in waters. They can form harmful byproducts during water treatment by reaction with disinfectants. Chlorination and chloramination of water containing natural organic matter is known to result in the production of toxic substances, often referred to as disinfection byproducts. The main advantage of using chlorine dioxide (ClO₂) over other known chlorine-containing disinfectants is the minimization of the formation of harmful trihalomethanes. Because ClO₂ is a promising alternative to other chlorine-containing disinfectants, the chemistry of ClO₂ interactions with amino acids, proteins, and peptides should be understood to ensure the safety of potable water supplies. Here, we present an overview of the aqueous chemistry of ClO₂ and its reactivity with amino acids, peptides, and proteins. The kinetics and products of the reactions are reviewed. Only a few amino acids have been reported to be reactive with ClO₂, and they have been found to follow second-order kinetics for the overall reaction. The rate constants vary from 10^?2 to 10^7?M^?1?s^?1 and follow an order of reactivity: cysteine?>?tyrosine?>?tryptophan?>?histidine?>?proline. For reactions of histidine, tryptophan, and tyrosine with ClO₂, products vary depending largely on the molar ratios of ClO₂ with the specific amino acid. Products of ClO₂ oxidation differ with the presence or absence of oxygen in the reaction mixture. Excess molar amounts of ClO₂ relative to amino acids are associated with the production of low molecular weight compounds. The oxidation of the biochemically important compounds bovine serum albumin and glucose-6-phosphate dehydrogenase by ClO₂ suggests a denaturing of proteins by ClO₂ by an attack on tryptophan and tyrosine residues and relates to the inactivation of microbes by ClO₂.     

Depollution of indigo dye by anodic oxidation and electro-Fenton using B-doped diamond anode

Hazardous wastes are generated in the synthesis of dyes and pigments applied in industries. Efficient methods are thus needed to clean wastewaters. Here, we use anodic oxidation and electro-Fenton with B-doped diamond anode to degrade the synthetic dye indigo in aqueous sodium dithionite. Results show the near-complete mineralization of the dye within 80 min at 500 mA. Mineralization was faster by electro-Fenton than anodic oxidation. The second-order rate constant (k) for the reaction of indigo with ^·OH was measured as 4.03 × 10⁹ M^?1 s^?1 at pH 3.0 and was compared with the rate constants of reactions between dyes and ^·OH. The results clearly demonstrate that both electro-Fenton and anodic oxidation can be used to depollute dyes in textile effluent with high efficiency and low cost. The main oxidant, ^·OH, being a non-selective reagent, the method could be applied to degrade other organic pollutants.     

Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons

• Regulation of redox conditions promotes the generation of free radicals on HM. • HM-PFRs can be fractionated into active and inactive types depending on stability. • The newly produced PFRs readily release electrons to oxygen and generate ROS. • PFR-induced ROS mediate the transformation of organic contaminants adsorbed on HM. The role of humic substance-associated persistent free radicals (PFRs) in the fate of organic contaminants under various redox conditions remains unknown. This study examined the characterization of original metal-free peat humin (HM), and HM treated with varying concentrations of H₂O₂ and L-ascorbic acid (VC) (assigned as H₂O₂-HM and VC-HM). The concentration of PFRs in HM increased with the addition of VC/H₂O₂ at concentrations less than 0.08 M. The evolution of PFRs in HM under different environmental conditions (e.g., oxic/anoxic and humidity) was investigated. Two types of PFRs were detected in HM: a relatively stable radical existed in the original sample, and the other type, which was generated by redox treatments, was relatively unstable. The spin densities of VC/H₂O₂-HM readily returned to the original value under relatively high humidity and oxic conditions. During this process, the HM-associated “unstable” free radicals released an electron to O₂, inducing the formation of reactive oxygen species (ROS, i.e., ^•OH and ^•O₂⁻). The generated ROS promoted the degradation of polycyclic aromatic hydrocarbons based on the radical quenching measurements. The transformation rates followed the order naphthalene>phenanthrene>anthracene>benzo[a]pyrene. Our results provide valuable insight into the HM-induced transformation of organic contaminants under natural conditions.     

Reduction of ferrate(VI) and oxidation of cyanate in a Fe(VI)-TiO2-UV-NCO- system

The aqueous photocatalytic degradation of cyanate (NCO(-)), which is a long-lived neurotoxin formed during the remediation of cyanide in industrial waste streams, was studied in the ferrate(VI)-UV-TiO2-NCO(-) system. Kinetics measurements of the photocatalytic reduction of ferrate(VI) were carried out as a function of [NCO(-)], [ferrate(VI)], [O(2)], light intensity (I(o)), and amount of TiO2 in suspensions at pH 9.0. The photocatalytic reduction rate of ferrate(VI) in the studied system can be expressed as -d[Fe(VI)]/dt=kI(o)(0.5) [NCO(-)] [TiO2]. The rate of photocatalytic oxidation of cyanate with ferrate(VI) was greater than the rate in the analogous system without ferrate(VI). The possibility of involvement of reactive ferrate(V) species for this enhancement was determined by studying the reactivity of ferrate(V) with NCO(-) in a homogeneous solution using a premix pulse radiolysis technique. The rate constant for the reaction of ferrate(V) and NCO(-) in alkaline medium was estimated to be (9.60+/-0.07) x 10(2) M(-1) s(-1), which is much slower than the ferrate(VI) self-decomposition reaction (k approximately 10(7) M(-1) s(-1)). An analysis of the kinetic data in the Fe(VI)-UV-TiO2-NCO(-) system suggests that ferrate(V) is not directly participating in the oxidation of cyanate. Possible reactions in the system are presented to explain results of ferrate(VI) reduction and oxidation of cyanate.     

Oxidative transformations of environmental pharmaceuticals by Cl₂, ClO₂, O₃, and Fe(VI): kinetics assessment

Several pharmaceuticals have been detected globally in surface water and drinking water, which indicate their insufficient removal from water and wastewater using conventional treatment methods. This paper reviews the kinetics of oxidative transformations of pharmaceuticals (antibiotics, lipid regulators, antipyretics, anticonvulsants, and beta-blockers) by Cl(2), ClO(2), O(3), and ferrate(VI) (Fe(VI)O(4)(2-),Fe(VI)) under treatment conditions. In the chlorination of sulfonamide antibiotics, HOCl is the major reactive Cl(2) species whereas in the oxidation by Fe(VI), HFeO(4)(-) is the dominant reactive species. Both oxidation processes can oxidize sulfonamides in seconds at a neutral pH (t(1/2)≤ 220 s; 1 mg L(-1) HOCl or K(2)FeO(4)). The reactivity of O(3) with pharmaceuticals is generally higher than that of HOCl (k(app,pH 7) (O(3))=1-10(7)M(-1)s(-1); k(app,pH 7) (HOCl)=10(-2)-10(5)M(-1)s(-1)). Ozone selectively oxidizes pharmaceuticals and reacts mainly with activated aromatic systems and non-protonated amines. Oxidative transformation of most pharmaceuticals by O(3) occurs in seconds (t(1/2)≤ 100 s; 1 mg L(-1) O(3)) while half-lives for oxidations by HOCl differ by at least two orders of magnitude. Ozone appears to be efficient in oxidizing pharmaceuticals in aquatic environments. The limited work on Fe(VI) shows that it can also potentially transform pharmaceuticals in treatment processes.