Advanced Oxidation Processes (AOPs)

Advanced Oxidation Processes

• Advanced oxidation processes (AOPs) are being increasingly researched, with diverse process variants and materials tested in labs.
• An inconsistency in experimental approaches hinders the identification, comparison, and scaling of the most promising AOPs.
• This tutorial review aims to streamline future studies by providing guidance for comparable and scalable oxidation experiments to aid the development of new advance oxidation solutions and materials.
• Developments in catalytic, ozone-based, radiation-driven, and other AOPs are reviewed.
• Future perspectives and research needs are outlined.
• Basic rules and key parameters for lab-scale evaluation, including suitable probe compounds and scavengers for measuring reactive species are proposed due to standardization of experimental procedures not being available in most AOPs.
• A two-phase assessment approach is suggested: (i) basic research/proof-of-concept (TRL 1-3) and (ii) process development in the intended water matrix, incorporating cost comparison with established processes using comparable, scalable parameters such as UV fluence or ozone consumption (TRL 3-5).
• Demonstration of the new process (TRL 6-7) is discussed briefly.
• Important research tools for a thorough mechanistic process evaluation and risk assessment are noted.
• Screening for transformation products should combine chemical logic with complementary tools (mass balance, chemical calculations).

Introduction to Advanced Oxidation Processes

• AOPs use in situ generated highly reactive radicals to oxidatively degrade contaminants.
• Hydroxyl radical (OH) is the major radical formed in most AOPs.
• Reactive oxygen species (ROS), persulfate radicals, carbonate radicals and solvated electrons may be involved in AOPs and affect process kinetics and product formation.
• Sulfate and chlorine radical-induced oxidations are often referred to as AOP-like processes.
• AOPs are applied in drinking water and wastewater treatment, water reuse, brine/leachate treatment, and groundwater remediation.
• These are mainly used to degrade organic contaminants but also for reduction of natural organic matter, disinfection, or as pre-treatment to improve downstream processes.
• Radicals, including OH, can be generated for AOPs in various ways, broadly classified into ozone-based, radiation-driven, catalytic, and other AOPs.
• Research activity and interest has been growing due to the AOP diversity.
• Various processes have been tested at pilot scale, while others are being explored and developed at lab scale, beyond AOPs established at full scale.
• Various alternative process combinations, from centralized treatment to point-of-use-scale, and reactor designs for catalytic or radiation-driven AOPs have been developed.
• A large array of water contaminants, including emerging contaminants, has been investigated.
• The prospective applicability of newly developed AOPs needs critical evaluation.
• Established AOP variants are few, developed from lab-to pilot- and full-scale implementation.
• Many novel AOPs' suitability for water treatment is debatable due to material toxicity, lack of long-term stability, or high energy demand.
• Critical information that would allow a sound evaluation of efficiency in real water matrices, including chemical and energy demand is often lacking in AOP studies with new materials.
• An example could be rate constants of target contaminants only but without further information on energy input, oxidant dose, or target containment reactivity.
• Systematic approaches are needed for objective assessment of new AOPs for contaminant oxidation to streamline research efforts.

Review Structure

• This review aims to guide researchers in developing innovative solutions and materials for advanced oxidation, divided into five sections.
• Section 2 reviews the status, potential, and research needs for various AOPs.
• Section 3 evaluates the electrical energy per order (EEO) concept as a metric to assess and compare AOP efficiency.
• Section 4 provides guidance on selecting probe compounds and scavengers for oxidation experiments.
• Section 5 describes experimental methodologies for systematic evaluation of AOPs at the laboratory scale.
• Section 6 outlines a systematic approach for evaluating novel AOPs.

Conventional and Emerging Oxidation Processes

• There is an immense variety of AOPs proposed and tested to generate radicals in water.
• Established AOPs operating at full scale include ozone- and UV-based approaches, but Fenton-based processes are widely established for industrial wastewater treatment.
• The most widely applied AOPs typically also provide the highest energy efficiencies.
• Less efficient AOPs might still provide suitable solutions for specific applications.
• Advantages and limitations of individual AOPs, future research needs, and potential areas for application are discussed.
• Advances in catalytic AOPs, novel concepts/materials for ozone-based and radiation-driven AOPs, and new alternative solutions for in situ radical generation are reviewed.

Catalytic AOPs

• Homogenous and heterogeneous catalysts can be used to generate reactive species by activating radical precursors or increase radical formation from oxidants.
• The section focuses on dark catalytic processes (i.e., without light as radical initiator).
• The most widely studied and applied homogeneous catalytic oxidation system is the Fenton reaction, in which H2O2 reacts with dissolved ferrous iron (Fe(II)) to generate OH.
• Critical aspects for Fenton applications include a narrow operation range determined by maximum catalytic activity at pH = 2.8-3.0, rapid consumption of free radicals by excess Fe(II), and costs for iron-containing sludge disposal.
• To avoid these bottlenecks, the application of chelating agents including humic substances and iron-free Fenton-like systems have been explored.
• A great deal of attention has been paid to both natural and synthetic solid catalysts, engineered nanomaterials, and single-atom catalysts.
• Many of these materials possess high catalytic activity at the laboratory bench scale and promise AOP operation under neutral pH conditions.
• Efforts should be directed towards producing standardized, regenerable, cost-effective, and sustainable catalysts with high physical/chemical stability, and testing their suitability/long-term performance for water treatment.
• Standardized testing procedures of catalytic materials should be developed, allowing to rank different materials and support the selection of specific technology for specific applications.
• Challenges concerning reactor design need to be addressed to allow for high contaminant-to-surface mass transfer and contaminant degradation efficiency, while at the same time minimizing catalyst loss and operational costs .
• Innovative catalysts show promise to broaden AOP applications towards enhanced in situ chemical oxidation or decentralized point-of-use water treatment, and tackle specific treatment goals for the removal of recalcitrant contaminants.

Ozone-Based Advanced Oxidation

• In every ozonation process," are produced by the interaction of ozone with the water matrix, (particularly with organic matter.
• "Indirect" reaction compared to the "direct" reaction of ozone with contaminants is a reference commonly made when discussing contamination by OH.
• The production of OH can be enhanced by the addition of H2O2 in the peroxone process, pH elevation, catalytic ozonation, or ozone photolysis. UV/O3 is considered separately.
• In the peroxone process, ozone swiftly reacts with the H2O2 anion (HO2-) yielding 0.5 mol OH for every mol reacted ozone.
• This process is frequently employed to reduce formation of undesired bromate or to neutralize excess ozone which further leads to additional OH formation.
• At elevated pH, ozone may react with hydroxide ions to produce OH, slow reaction rate which restricts ozonation at elevated pH towards a limited range of source and wastewaters.
• Ozonation homogeneous and heterogenous catalysis relies on the breakdown of ozone by transition metal ions or solid metal oxide catalysts with a preference for iron- and manganese-based materials, as well as activated carbon and other carbon-based materials.
• Heterogeneous catalytic ozonation can remove pollutants through interfacial reactions at the catalyst surface.
• An advantage of aqueous phase, OH generated from ozone decomposition at the catalyst surface is does not necessitate additional metal ions to be added or eliminated.
• Understanding of the driving mechanisms for catalytic ozonation is still limited because catalysts undergo transformations leading to some causing ozone decay without significant OH production, others with varying catalytic activity.
• Mechanisms of catalytic ozonation with activated carbon-based materials are still under investigation.
• Future studies on catalytic ozonation should adhere to standardized procedures that allow for benefits in radical generation in comparison to ozonation alone (see section 5), with mid-to long-term catalytic performance analysis.

Radiation-Driven AOPs

• The energy of electromagnetic radiation is utilized to form radicals.
• UV water treatment has been applied for several decades for water disinfection using mercury (Hg) lamps.
• Light emitting diodes (LEDs) have quickly evolved in the UV range and show promise to increasingly replace Hg-containing irradiation sources.
• Radiation-driven AOPs can be distinguished into homogeneous and heterogeneous processes, utilizing a similar array of radical precursors for homogeneous processes including H2O2, chlorine and chlorine dioxide, peroxydisulfate (S2O32-), ozone, and the photo-Fenton process.
• The most commonly employed processes are UV/H2O2 and UV/chlorine.
• The UV/H2O2 process is used in drinking water treatment and potable reuse schemes to remove trace contaminants that are difficult to remove otherwise and can be advantageous when ozone-based processes would result in undesired bromate formation from naturally occurring Br.
• UV/H2O2 demands a higher energy input compared to conventional ozonation and the peroxone process.
• UV/chlorine (HOCl) is increasingly used with and without the addition of chloramines in full-scale potable reuse schemes.
• Reactions of these reactive species in water are discussed in more detail elsewhere.
• UV/chlorine is mostly applied as the last treatment step for simultaneous degradation of pollutants and the provision of disinfectant residuals, with limited application in organic-rich and ammonia-containing waters due to formation of toxic by-products.
• Sulfate radicals react more substrate specific than OH.
• The UV/S2O8 benefits from reduced oxidant scavenging by the water matrix, but less reactive compounds are more difficult to eliminate compared to the UV/H2O2 process.
• Therefore, the UV/S2O process may provide tailored solutions towards individual substances or substance groups rather than broadband treatment envisioned in most AOP applications.
• In addition, residual sulfate ions can be undesirable in some applications.
• At the same dissolved molar concentration, ozone results in an efficient photolysis into OH is 20 times more efficient than H2O2 photolysis.
• The UV/O3 process is typically not competitive with alternative solutions due to the combination of two energy-intensive processes.
• The major obstacle of the photo-Fenton process is that Fe(III) precipitates in water at pH > 5 and, hence Fenton processes need to be operated at acidic conditions.
• Review articles on photo-Fenton, including solar photo-Fenton are available.
• Other developments are related to using complexing agents to operate at pH 7 and using less costly reactor designs.

Heterogeneous Photocatalytic AOPs

• Research on photocatalysis for degradation of refractory pollutants has been intense since the discovery of photocatalytic water splitting.
• Heterogeneous photocatalysis employs a range of semiconducting catalysts, the most widely researched being TiO2 and ZnO.
• Despite much research on photocatalysis in the last decades, including large-scale demonstration on the use of solar spectrum UV-A and UV-B radiation, photocatalysis is rarely applied in water treatment beyond lab and pilot scale.
• There is an ongoing quest for new photocatalysts, with a broad variety of synthesis approaches taken, with the aim to either increase quantum efficiency or to expand the useable wavelength range to access a larger part of the electromagnetic spectrum.
• Limitations and barriers to industrial uptake of photocatalysis include low energy efficiency, complex reactor design, and catalyst immobilization.
• One example is the ability to generate reductive conditions to treat oxyanions or certain heavy metals.

Lamp Technology

• UV LEDs are radiation sources based on semiconductors such as gallium nitride (GaN).
• Pilot- and full-scale application in water treatment has been tested primarily for disinfection and photocatalysis in the UV-A range.
• The acquisition costs of UV LEDs with wavelengths <300 nm are currently still high, while the service life, radiant power, and energy efficiency are limited compared to visible spectrum LEDs.
• Alternative lamp technologies include Xe Excimer lamps with a peak emission at 172 nm emission and, more recently, LP-Hg arcs at 185 nm.
• At 185 nm, almost 90% of the photons are absorbed in the first 5 mm of the optical pathlength in water.
• Therefore, the treatment of large volumes of water requires the development of reactors that go beyond thin-film reactors and enable efficient use of 185 nm photons without excessive pumping energy or reactor construction costs.
• UV/chlorine process regarding the formation of halogenated oxidation by-products.
• Analogously, in waters with high nitrate concentrations, nitrate photolysis may generate potentially harmful nitrogenous oxidation by-products.
• This is an aspect that has not yet been thoroughly investigated for the VUV process.

Other AOPs

• The term "other AOPs" encompasses a broad and diverse array of processes designed to generate radicals in water for treatment purposes.
• Other than heat-activated persulfate activation and electrochemical oxidations, these methods generally require a higher energy input.
• Techniques like ultrasound, plasma treatment, supercritical water oxidation, and electrochemical oxidation are presently being explored for the removal of resistant poly- and perfluoroalkyl substances (PFAS).
• Application of ultrasound in water generates oscillating gas bubbles, pyrolysis of water vapour yields OH and H radicals.
• Local accumulation of reactive species leads to comparably high energy demand that is not readily tested.
• Plasma is a gas-like state of matter in which a significant portion of molecules or atoms are ionized making plasma highly conductive and a potential initiator of reduction and oxidation reactions.
• Cascading effects of accelerated electrons impacting on carrier gas molecules and water matrix compounds leads to a plethora of reactive, the contribution of each reactive species is highly substance and system specific.
• The thermal activation of persulfate is a viable method for in situ chemical oxidation, typically employed to remediate contaminated groundwater and soil.
• Advantage of a pronounced drop in pH and robust radical scavenging effects, particularly when chloride and bicarbonate are concurrently present.
• Supercritical water oxidation (SCWO) has proven to be an effective method for degrading various types of waste at both laboratory and full-scale levels.
• In electrochemical advanced oxidation processes, OH are directly generated from water oxidation at the electrode surface.
• Due to the efficient conversion of electrical energy into "OH and secondary oxidants, electrical AOPs are often considered environmentally-friendly treatment solutions.

Benchmarking AOPs with Electrical Energy per Order

• A significant advantage of AOPs and one factor that sets them apart from many other waste treatment technologies is that the majority of AOPs are electrically driven. Electrical energy per order is proposed as most importance figure of merit for comparing AOPs.
• Electric energy required to decrease the concentration of a target contaminant by 90%, i.e., by one order of magnitude.
• There are variations of the EEO for systems with high contaminant concentration and for solar-driven systems.
• In contrast, the electrical energy dose (EED)defined as "electrical energy kWh consumed per unit volume of water treated as operational metric is not applied to compare different AOPS.
• Thereby, the EEO has been proven useful for the initial assessment and comparability of the energy efficiency across different AOPs, additional relevant experimental and water quality parameters are required to allow for appropriate comparison.

Characterization of radicals in oxidation processes

• Measuring the reactive species formed in radical-based processes is complex due to their short life-time with a range of microseconds( OH and SO4) to milliseconds(e.g., carbonate radicals and superoxide radicals).
• Although, these short-lived reactive species can be measured, it requires specialized equipment.
• Instead of specialized equipment, one alternative is to use chemical probes as diagnostic tools in AOP research such as to determine formed reactive species.
• Chemical Probes are divided into with 3 focus: Reactive type and exposure, reactive quantification, excited triplet state quantification.
• There are numerous chemical probes to detect reactive species in water that is reviewed, most specific is to identify "OH. It is significant to separate the Quenches used from scavengers present in real water matrices with high effects on AOP.

Set-ups and methodologies for laboratory-scale AOP experiments

• The transmission of information has been limited knowledge due to experiments unorganized or not comparable. Initial Feasibility tests to confirm Reactive species formation in pure or nonpure water with nonstandard experimental set up is used to support the claim that for cost effectness and energy demand a experiment with comparable and scalable information needs to be presented.
• Matrix effects influence are reported because this impacts the oxidation process.

General Aspects Summary

• Matrix are known to compete with radical precursors. Matrix scavenging with highly reactive cause less and produce secondary radicals. Depending on the targeted usage matrix would need to include organic, in-organic, nitrate, ammonia.
• Synthetic Waters, buffer used should be considered from effects that can be involved.
• The levels of chemicals are needed to be listed

UV-Based Processes

• Fluence-based evaluation focus on using set-ups that allow assessing photochemical reactions as a function of the fluence in the reactor as basis for comparing photochemical studies.
• In evaluation source must be known and have the correct wave length dependent, radiation properties. By these parameters, proper fluence rate has been displayed.

Advantages and disadvantages of laboratory setups

• From a chemical engineering point of view, we can distinguish batch experiments in continuously stirred tank reactors and experiments in flow-through reactors.
• A qCB consists of a (i) light source, (ii) an optical system, (iii) a shallow, typically round photoreactor whose surface is homogeneously illuminated with the resulting parallel rays.
• Due to simplicity the pathlength of photons, as well its attenuation measured are well-defined creating easily calculated results.
• The second type of photoreactor are flow-through systems, systems is characterized by biodosimetry often provided by the manufacturer are often conducted at research Labs.

###Ozone-Based Processes

• Transfer scalability, transfer rate is a necessary measurement to make up transferable data for ozone based AOP. Measurements, the consumption and decay of ozone are both necessary for transferable data.
• For good measurement, the level of radical formation requires the appropriate measurement that yields against conventional data.
• For setup pure measurements involving a gas phase, requires total mass balance which includes the remaining gases after measurements.

Catalytic AOPs

• The test needs is a series of experiments that will require a the correct setup.
• This includes characterization of dissolved components, the catalyst, and water quality.

Evaluation

• When relating to the reactions activity, it may be connected to catalyst concentration that determines the other metrics.
• There can be problems that include many other characteristics that don't include catalyst information which is not completely understood.

Advantages

• CSTR are recommended initially due to its ability to control of mass transfer. Test are recommended to directly compare its functionality.

AOP

• Experiments are often are required to scale to bench so to create a sustainable data reading including mass drivers.

Assessment of New Concepts and Materials for Advanced Oxidation

• New concepts in this context refer to the development of new AOPs, but also to new materials (e. g., catalysts) or reactor designs that improve the performance of an existing AOP.
• One key of research focus is on proof of a scalable and transferable product that includes cost and initial performance reading.

Prefeasibility Assessment

• Recommended to consider new material or procedure based on known literature. Criteria that must occur: Stability under water treatment, potential toxic chemicals.
• Robust materials that is nontoxic are to be considered and used during testing, although specialized material is fine under water research and must have consideration when applied.
• High material numbers must is known to require high cost while non complex has less material limits (and less testing requirement), which must have a appropriate balance.

Proof of Concept

Test that create a appropriate product/treatment reading. -If a direct measurement is needed for the creation of new test designs its is often possible with well study setups described, it is important that measurements use the same area under the same conditions such has PH and temperature.

• The details are figured out by understanding the correct testing variables.
• If direct testing is not a process, the confirmation should follow oxidative effects on any degradation and identify major species involved.