6 Advanced Oxidation Technologies for Wastewater Treatment

Advanced oxidation processes (AOPs), as a branch of physical-chemical treatment, are widely used as a pretreatment step for toxic and refractory industrial wastewater because of their high treatment efficiency and their ability to thoroughly destroy toxic pollutants. They have gradually become a hot topic in water treatment research. Current AOPs mainly include chemical oxidation, electrochemical oxidation, wet oxidation, supercritical water oxidation, and photocatalytic oxidation.

01 Chemical Oxidation

Chemical oxidation is often used as a pretreatment before biological treatment. Generally, under the action of a catalyst, a chemical oxidant is used to treat organic wastewater to improve its biodegradability, or to directly oxidize and degrade the organic matter in the wastewater to stabilize it.

1. Fenton oxidation. Originating in the mid-1890s and proposed by French scientist H.J. Fenton, under acidic conditions H2O2 is effectively catalyzed by Fe2+ ions to oxidize tartaric acid, and was later applied to the oxidation of malic acid. For a long time, the generally accepted principle of Fenton was the use of ferrous ions as a catalyst for hydrogen peroxide, producing hydroxyl radicals via the reaction Fe2+ + H2O2 → Fe3+ + OH- + ·OH, mostly under acidic conditions. Among chemical oxidation methods, the Fenton method shows certain advantages in treating some refractory organics (e.g., phenols, anilines). With deeper research, ultraviolet (UV) light, oxalate, etc. have been introduced into the Fenton method in recent years, greatly enhancing its oxidation capacity.

2. Fenton-like oxidation. Fenton-like reactions refer to a class of reactions in which, besides Fe(II), Fe(III), iron-containing minerals, and other transition metals such as Co, Cd, Cu, Ag, Mn, and Ni can accelerate or replace Fe(II) to catalyze H2O2. Studies show that homogeneous catalysts such as Fe3+ and Mn2+, as well as heterogeneous catalysts such as iron powder, graphite, and iron/manganese oxide minerals, can also decompose H2O2 to produce ·OH. Because their basic process resembles that of the Fenton reagent, they are called Fenton-like systems. For example, using Fe3+ instead of Fe2+ reduces the chance of ·OH being reduced by Fe2+ (since Fe2+ is generated in situ), improving ·OH utilization. Adding certain complexing agents (e.g., C2O4^2-, EDTA) to the Fenton system can increase the removal rate of organics.

3. Ozone oxidation. The ozone oxidation system has a high redox potential and can oxidize most organic pollutants in wastewater, so it is widely used in industrial wastewater treatment. Ozone can oxidize many organics in water, but the reaction is selective, and ozone cannot completely decompose organics into CO2 and H2O; the products are often carboxylic-acid-type organics. Moreover, ozone is chemically very unstable, especially in non-pure water, with oxidative decomposition on a timescale of minutes. In wastewater treatment, ozone oxidation is usually not a standalone unit but is combined with enhancement means such as photocatalytic ozonation, alkali-catalyzed ozonation, and heterogeneous catalytic ozonation. In addition, coupling ozone oxidation with other technologies is also a research focus, e.g., ozone/ultrasound and ozone/biological activated carbon adsorption.

02 Electrochemical Catalytic Oxidation

This technology originated in the 1940s and has the advantages of a wide application range, high degradation efficiency, simple energy requirements, easy automation, and flexible application modes. Electrochemical catalytic oxidation can be used both as a pretreatment to improve the biodegradability of refractory wastewater and as an advanced treatment for refractory phenolic wastewater; under optimized pH, temperature, and current intensity, phenol can be almost completely decomposed. For high-concentration, refractory, toxic, and hazardous phenol-containing wastewater, traditional biological and physical-chemical methods lose their advantages, and chemical oxidation is hindered from widespread use by its high cost; electrochemical catalytic oxidation is increasingly favored. However, it also has problems such as power consumption, mostly precious-metal electrode materials, high cost, and anode corrosion, and the micro-kinetic and thermodynamic research guiding its application is still incomplete.

03 Wet Oxidation

Wet oxidation, also known as wet combustion, is an effective method for treating high-concentration organic wastewater. Its basic principle is to pass air under high temperature and high pressure so that the organic pollutants in the wastewater are oxidized. According to whether a catalyst is used in the process, it can be divided into wet air oxidation and catalytic wet air oxidation.

1. Wet air oxidation (WAO). The WAO process was first developed and industrialized by Zimpro Inc. of the United States, which has applied WAO to the treatment of toxic and hazardous industrial wastewater such as olefin production wash liquor, acrylonitrile production wastewater, and pesticide production wastewater. WAO technology passes air under high temperature (125–320°C) and high pressure (0.5–20 MPa), directly oxidizing and degrading macromolecular organics in the wastewater into inorganic or small-molecular organics. Using WAO for pretreatment of dimethoate production wastewater, the removal rate of organophosphorus reaches 95% and that of organic sulfur reaches 90%. Zimpro's WAO has high efficiency and short reaction time, but requires high-temperature and high-pressure conditions, large equipment investment, and harsh operating conditions, making it difficult for ordinary enterprises to adopt. Therefore, catalytic wet air oxidation, which uses catalysts to lower reaction temperature and pressure or shorten residence time, has received extensive attention and research in recent years.

2. Catalytic wet air oxidation (CWAO). CWAO adds a suitable catalyst to the traditional wet oxidation process so that the oxidation reaction can be completed under milder conditions and in a shorter time, thereby reducing reaction temperature and pressure, improving oxidation and decomposition capacity, accelerating reaction rate, shortening residence time, and accordingly reducing equipment corrosion and operating costs. The key issue of CWAO is a highly active and easily recoverable catalyst. CWAO catalysts are generally divided into three categories: metal salts, oxides, and composite oxides. According to the form of the catalyst in the system, CWAO can be further divided into homogeneous and heterogeneous catalytic wet air oxidation.

In homogeneous CWAO, since the catalyst (mostly metal ions) is a soluble transition-metal salt existing in ionic form in the wastewater, it catalyzes the oxidation of organics by initiating free-radical reactions of the oxidant at the ionic or molecular level and continuously regenerating. Because the catalyst acts independently at the molecular or ionic level, molecular activity is high and oxidation is effective. However, because the catalyst exists in ionic form, it is difficult to recover and reuse from the wastewater and easily causes secondary pollution. In heterogeneous CWAO, an insoluble solid catalyst is added to the reaction system; its catalytic action occurs on the catalyst surface, and the specific surface area greatly affects the degradation rate. Because the solid catalyst does not dissolve or leach, activation, regeneration, and recovery are easy, so its application prospects are very broad.

04 Supercritical Water Oxidation

Supercritical water oxidation is an enhancement and improvement of wet air oxidation, successfully developed by MODAR Inc. of the United States in 1982. Its principle is to use supercritical water as a medium to oxidize and decompose organics. Also using water as the liquid phase and oxygen in air as the oxidant, the reaction occurs under high temperature and high pressure. The improvement lies in exploiting the properties of water in the supercritical state: its dielectric constant decreases to approximate that of organics and gases, so that gases and organics are completely soluble in water, the phase interface disappears, a homogeneous oxidation system is formed, the inter-phase mass-transfer resistance present in wet oxidation is eliminated, and the reaction rate increases. Moreover, because the independent activity of oxidative free radicals is higher in a homogeneous system, the oxidation degree is also improved. Supercritical water is a good solvent for organics and oxygen; organics undergo homogeneous oxidation in oxygen-rich supercritical water very rapidly—at 400–600°C, the structure of organics is destroyed within a few seconds, the reaction is complete and thorough, and organic carbon and hydrogen are fully converted into CO2 and H2O. Because of its rapid reaction and thorough oxidation, supercritical water oxidation has attracted increasing attention. How to lower the reaction temperature and pressure or shorten the residence time through catalysts is a research hotspot. Currently, most commonly used catalysts are those for wet catalytic oxidation; finding catalysts with broad-spectrum catalytic performance for supercritical water oxidation is a difficulty in its promotion.

05 Photocatalytic Oxidation

Photocatalytic oxidation was developed on the basis of photochemical oxidation, which is the oxidative degradation of organic pollutants under visible or ultraviolet light. Part of the near-ultraviolet light (290–400 nm) in the natural environment is easily absorbed by organic pollutants; in the presence of active substances, strong photochemical reactions occur, degrading the organics. However, due to reaction-condition limitations, photochemical oxidative degradation is often incomplete and easily produces various aromatic organic intermediates, which is a problem that photochemical oxidation needs to overcome. Since Carey et al. first used TiO2 to photocatalytically degrade biphenyl and chlorinated biphenyls in 1976, the research focus of photocatalytic oxidation has shifted to the degradation of organic pollutants using TiO2 as a catalyst. Because photocatalytic oxidation equipment is simple, the reaction conditions are mild, operation is easy to control, oxidation capacity is strong, and there is no secondary pollution—combined with the high chemical stability, non-toxicity, and low cost of TiO2—TiO2 photocatalytic oxidation is a novel water treatment technology with broad application prospects.

06 Ultrasonic Oxidation

The development of sonochemistry has drawn increasing attention to its application in water and wastewater treatment. The driving force of ultrasonic oxidation is acoustic cavitation: when ultrasound of sufficient intensity (15 kHz–20 MHz) passes through aqueous solution, in the negative-pressure half-cycle the acoustic pressure amplitude exceeds the liquid's internal static pressure and cavitation nuclei in the liquid expand rapidly; in the positive-pressure half-cycle the bubbles burst due to adiabatic compression, lasting about 0.1 μs. At the moment of bursting, a local high-temperature and high-pressure environment of about 5000 K and 100 MPa is generated, along with a strong shock micro-jet at a velocity of 110 m/s. Ultrasonic oxidation uses magnetostrictive or piezoelectric ultrasonic transducers to generate ultrasound via electromagnetic conversion. In laboratories, radiation-plate ultrasonic instruments, probe-type instruments, and NAP reactors are more commonly used. Ultrasonic oxidation has mild reaction conditions, usually at room temperature, low equipment requirements, and is a pollution-free green treatment technology with broad application prospects.

Recommended reading: Ozone Advanced Catalytic Oxidation–Flotation Integrated Equipment (CDOF).