A novel aerator for enhancing the oxygen transfer rate and efficiency, named multistage vortex aerator (MVA), was developed. It uses vortex flow in repeated stages to increase the gas-liquid interfacial area and to decrease the thickness of the stagnant layer at the interface between the two phases. The basic characteristics of oxygen transfer using this aerator were investigated using the American Society of Civil Engineers standard procedure. The MVA could rapidly transfer oxygen to water to a concentration higher than 40 mg/L in 60 min owing to the effect of high purity oxygen, additional pressure induced by water and gas, and vortex flow dynamics. A gas transfer model was developed for describing the non-steady state operation of the aerator. This model is based on the mass and molar balances of oxygen in gas and water. It could successfully simulate the DO change inside the aerator. This study can help better understand the oxygen transfer mechanism and evaluate the performance of the new aerator at the various temperatures, pressures, and gas compositions found in diverse environmental systems.

Resource recovery from natural reserves is appealing and Li extraction from different brines is in the forefront. Li extraction by membranes is reviewed in the literature much more than electrochemical processes. However, a very recent review thoroughly discussed Li recovery by electrochemically switchable ion exchange (ESIX). This paper reviews Li recovery by both charge transfer processes, namely electrodialysis (ED), and electro-sorption processes, namely capacitive deionization (CDI). It also reviews ESIX with a focus on performance matrices and includes comments on the technology readiness of each separation technique. These processes exhibit promising perspectives on the separation and recovery of Li both selectively and non-selectively from simulated brine solutions and Li salt solutions. Readers are provided with guidelines to choose between the processes, depending on the applied voltage, current density, specific energy consumption and purity of recovered Li. Most electrochemical lithium capturing systems (ELiCSs) have been tested at the lab scale. Therefore, future research should be directed toward pilot-scale development and parameter optimization. Furthermore, we urge the ELiCSs research community to report information in a standard form that allows meaningful comparisons and insights into the systems.

Advanced reduction processes (ARPs) form highly reactive reducing radicals and aqueous electrons, which can reduce oxidized contaminants, by activating reducing agents. Experiments were conducted to investigate the kinetics of nitrate degradation by the sulfite/ultraviolet (UV) ARP, which has been proven to be the most effective ARP. Experimental variables were pH, UV (253.7 nm) intensity, and sulfite concentration. Rate of nitrate reduction increases with increasing pH, UV intensity, and initial sulfite concentration. The efficiency of photochemical degradation of nitrate (quantum yield based on overall reaction rate) was unaffected by UV intensity, but decreased with pH and initial sulfite concentration. Nitrite was identified as a major intermediate product of nitrate mineralization by the UV/sulfite ARP, which was simultaneously degraded along with nitrate.

This work investigates photocatalytic reduction of aqueous chlorate by using commercial P25 TiO2 in the presence of hole scavengers under simulated solar light. It compares the photocatalytic ability of P25 TiO2 to reduce chlorate in the presence or absence of a hole scavenger (methanol) with that of bismuth oxyhalides (BiOBr, TiO2-BiOBr, BiOCl, TiO2-BiOCl, and BiOI). Bismuth oxyhalides have more interest as promising photocatalysts because they have a narrow band gap and are more responsive to visible light. However, in this study, P25 TiO2 was a more effective photocatalyst for chlorate reduction in the presence of methanol than bismuth oxyhalides. Also, this study examines the effectiveness of methanol, ethanol, and formate as hole scavengers during chlorate reduction. The presence of methanol enhanced chlorate reduction more than that of formate and ethanol. Finally, this work estimates the impacts of operating parameters such as photocatalyst dose, initial chlorate concentration, solution pH, methanol concentration, and light intensity on chlorate removal and studies the reaction mechanism for chlorate reduction in the system of TiO2/chlorate/methanol (photocatalyst/contaminant/reagent) under simulated solar light. The highest chlorate removal of 97.5% was achieved using 1 g/L of TiO2 with initial chlorate concentration of 0.012 mM and methanol concentration of 2 mM at the pH of 5.4 after 180 min of radiation. The major product of chlorate photoreduction was chloride ion.

Background Photodegradation of trichloroethylene (TCE) in aqueous solution under simulated solar light irradiation was studied under different experimental conditions to determine the reaction mechanism and kinetics that control TCE degradation using bismuth oxybromide (BiOBr) in the presence of sulfite. Photocatalysts were synthesized to be more responsive to visible light under simulated solar light and particular attention was focused on the reactive specie formed by reaction of the sulfite on the surface of BiOBr under simulated sunlight. Result Degradation rate of TCE was greatly enhanced by the presence of sulfite, and the enhancement increased with sulfite dose to a maximum that was retained at higher sulfite doses. Degradation rate of TCE was also affected by other factors, such as initial TCE concentration, BiOBr dose, and solution pH. In addition, the cycling performance of BiOBr was examined, and the amount of TCE degraded was almost constant over increasing cycle numbers when initial sulfite concentration was high enough to maintain a suitable sulfite concentration throughout the experiment. When TCE was degraded by BiOBr in the presence of sulfite under simulated sunlight irradiation, the major by-product measured was the non-hazardous chloride ion, and dechlorination efficiency was about 58%. Conclusion This study extended the use of a potential effective photocatalyst (BiOBr) to a semi-volatile organic contaminant (TCE), not limited to mainly focus on organic dyes, and evaluated the use of sulfite as a hole scavenger in order to enhance the degradation of TCE without needing to manipulate the structure of BiOBr. The active species being responsible for TCE degradation in BiOBr/TCE/sulfite system under simulated solar light was the sulfite radical (SO 3 ·− ), and the photocatalytic activity of BiOBr did not decrease over a number of treatment cycles when S IV dose was sufficient.

AbstractThe use of seawater as a source for potable water supply after desalination, and in once‐through industrial cooling is a common practice in many parts of the world that have limited fresh‐water resources. A disinfectant (commonly chlorine) is added to the seawater to control biofouling in desalination or cooling systems. The added chlorine reacts with bromide and other compounds present in seawater to produce a wide range of chemical oxidants. Regrettably, reactions between the residual oxidants and natural organic matter present in seawater lead to the formation of halogenated organic compounds that have detrimental effects on human health and the environment. Of particular note is the fact that brominated forms of disinfectants are more effective in producing halogenated organic by‐products than chlorinated forms. In this article, the kinetics of residual oxidants decay and formation of halogenated disinfection by‐products (DBPs) in seawater is discussed and explained in detail. A kinetic model is developed, which includes rate constants for all base reactions and stoichiometric coefficients for all halogenated (DBPs) formed by these base reactions. The model is calibrated using experimental results to obtain the different kinetic rate coefficients.

A novel hybrid system was designed for petrochemical wastewater treatment and reuse by coupling membrane distillation (MD) with a two-stage pretreatment process comprising oil/water separation and photocatalytic organics degradation. The oil/water separation process removed oil emulsions from water, while the follow-up photocatalysis process degraded dissolved oil and volatile organic pollutants. The pretreated water was then introduced to the MD process to produce distilled water. A modified stainless steel mesh/glass microfiber filter was used for removal of oil emulsions and exhibited excellent performance and durability (92% oil rejection in the first cycle and 87% after 10 consecutive cycles). In the photocatalysis stage, TiO2 P25 was used as the photocatalyst to effectively decompose the remaining organic compounds and to inactivate microorganisms under UV light irradiation. The two-stage pretreatment reached 99.5% total organic degradation and 6.02 log bacteria removal. The advantages of the hybrid system are to prevent MD membrane fouling and produce high quality distillate with low TDS and minimal volatile organics that are difficult to remove by a conventional MD process.

AbstractOxidation–reduction reactions are important processes for water treatment because they chemically change contaminants, rather than only transfer them from the water phase. However, the rates of many oxidation–reduction reactions limit their use as treatment processes. This limitation can be overcome by using highly reactive free radicals as the chemical reductant, and treatment processes that do this can be called advanced reduction processes (ARPs). Treatment processes that rely on both oxidizing and reducing radicals can be called advanced oxidation–reduction processes, and both are similar to advanced oxidation processes that apply oxidizing radicals to water treatment. This article reviews a number of applications of ARPs and summarizes the characteristics of some AORPs. ARPs have been applied to a wide range of contaminants including halogenated organics and oxidized inorganic compounds. The effects of important process variables such as incident photon flux, reagent concentration, target compound concentration, and solution pH are reviewed for ARPs, and a simple kinetic model is presented that describes much of their behavior that has been observed. The basic aspects of two important AORPs (electron beam irradiation and photocatalysis) are discussed, and a kinetic model describing the dose coefficient for electron beam treatment is presented.

Understanding adsorption–photocatalysis synergy advances solar-driven photodegradation of organic water pollutants.

The paper describes the use of advanced reduction processes (ARPs) with dithionite/UV to remove arsenic from water/wastewater. The highly reducing radicals generated by the ARP react with arsenite and arsenate to produce solid phases such as elemental arsenic or arsenic sulfides that can be readily removed. This study evaluated the effectiveness in removing arsenic at 5 different pH values (5, 6, 7, 8, and 9) as well as the mechanism of resolubilization for solids. Moderate pH values of 6, 7, and 8 were found to be the most effective for arsenic removal. Resolubilization occurred at all pH values because dithionite was consumed under UV irradiation. A simple kinetic model was applied to describe the changes in concentrations of arsenic and dithionite with time. Arsenic removal kinetics was fast at the first 10-20 min for all pH values. The highest rate constant was observed at pH 6 for arsenate removal (616 L/mol/min) and at pH 7 for arsenite removal (534 L/mol/min).

A kinetic model for the competitions was applied to understand the reductive dechlorination of tertiary DNAPL mixtures containing PCE, TCE, and 1,1,1-TCA. The model assumed that the mass transfer rates were sufficiently rapid that the target compounds in the solution and the DNAPL mixture were in phase equilibrium. Dechlorination was achieved using either a mixture of Fe(II), Fe(III), and Ca(OH)2 (Fe(II/III)-L) or a mixture of Fe(II) and Portland cement (Fe(II)-C). PCE in the DNAPL mixtures was gradually reduced and it was reduced more rapidly using Fe(II)-C than Fe(II/III)-L. A constant total TCE concentration in the DNAPL mixtures was observed, which implied that the rate of loss of TCE by dechlorination and possibly other processes was equal to the rate of production of TCE by PCE dechlorination. On the other hand, 1,1,1-TCA in the DNAPL mixtures was removed rapidly and its degradation rate by Fe(II/III)-L was faster than by Fe(II)-C. The coefficients in the kinetic model (ki, Ki) were observed to decrease in the order 1,1,1-TCA>PCE>TCE, for both Fe(II/III)-L and Fe(II)-C. The concentrations of target compounds in solution were the effective solubilities, because of the assumption of phase equilibrium and were calculated with Rault's Law. The concentration changes observed were an increase and then a decrease for PCE, a sharp and then gradual increase for TCE, and a dramatic decrease for 1,1,1-TCA. The fraction of initial and theoretical reductive capacity revealed that Fe(II)-C had ability to degrade target compounds.

This study investigates the removal of arsenite (As(III)) from water using dithionite activated by UV light. This work evaluated the removal kinetics of As(III) under UV light irradiation as affected by dithionite dose and light intensity, and characterized the nature of the precipitated solids using XPS and SEM-EDS. Photolysis of dithionite was observed by measuring dithionite concentration using UV absorbance at 315nm. This study also investigated the effect of UV light path length on soluble As concentrations to understand resolubilization mechanisms. Total soluble As concentrations were observed to decrease with reaction time due to reduction of arsenite to form solids having a yellow-orange color. The removal mechanism was found to be reductive precipitation that formed solids of elemental arsenic or arsenic sulfide. However, these solids were observed to resolubilize at later times after dithionite had been consumed. Resolubilization of As was prevented and additional As removal was obtained by frequent dosing of dithionite throughout the experiment. As(III) removal is attributed to photolysis of dithionite by UV light and production of reactive radicals that reduce As(III) and convert it to solid forms.

This study investigates removal of selenite [Se(IV)] by reductive precipitation during treatment with an Advanced Reduction Process (ARP) that uses dithionite activated by ultraviolet (UV) irradiation. Our screening experiments evaluated a number of ARP and found that the dithionite/UV ARP was most effective in removing soluble selenite. Furthermore, this work considers effects of operating conditions such as dithionite dose, solution pH, initial selenite concentration, and light intensity on reduction of Se(IV). Selenite [Se(IV)] was completely removed in 120min when initial Se(IV) concentration was 0.023mM, dithionite dose was 1mM, and the initial pH was approximate to 4.5. Higher dithionite doses, lower pH, and higher incident UV irradiance increased soluble Se(IV) removal. Selenium in the solids was effectively removed from solution by conversion to solids that were removed by filtration. Scanning emission microscopy/energy dispersive X-ray spectroscopy (EDS), X-ray diffraction, and X-ray Photoelectron Spectroscopy results showed that Se(IV) was reduced by the dithionite/UV ARP to form solids identified as elemental Se or as a compound composed of both Se and S (e.g., SemSn). Elemental Se was the primary solid, especially at higher initial Se(IV) concentrations. In the dithionite/UV ARP, rapid removal of soluble Se(IV) at low pH is attributed to photolysis of dithionite or a dithionite decomposition product (e.g., bisulfite, metabisulfite, thiosulfate, and trithionate) that is initially present in the dithionite solution.

This article reports on the application of an advanced reduction process (ARP) that combines ultraviolet (UV) irradiation and dithionite to remove selenium from solution. Batch kinetic removal of selenite (Se(IV)) and selenate (Se(VI)) with the dithionite/UV ARP at different pH were evaluated under anaerobic conditions and a removal mechanism was proposed. Resolubilization of selenium was observed for all pH (7, 8, and 9) after 15 to 20min, which was the time when dithionite was completely consumed. A hypothesis was proposed that selenium first was reduced to elemental Se by radicals formed from photolysis of dithionite. Then, it was resolubilized by reaction with oxidizing radicals formed by photolysis of dithionite degradation products. A kinetic model for dithionite photolysis and a second-order model for precipitation of selenium were developed to describe selenium removal. Experimental data were used in nonlinear regressions to estimate quantum yields and second-order rate constants. Lowest quantum yields were observed at pH 8 for experiments with both selenite and selenate. The rate of selenite removal from solution was not affected by pH, but the rate of selenate removal was faster at pH 8 than at pH 7 and 9.

Novel FeOOH and Fe2O3 co-grafted TiO2 photocatalysts were synthesized and tested for bisphenol A (BPA) degradation in water under simulated solar and visible light irradiations. The grafting temperature was simply controlled to adjust the FeOOH and Fe2O3 compositions, with 55°C being the optimal temperature that led to the highest BPA degradation. Co-grafted FeOOH and Fe2O3 on TiO2 is advantageous over mono-grafted or un-grafted TiO2, because FeOOH promotes BPA adsorption and Fe2O3 enhances photocatalytic activity by facilitating the interfacial charge transfer. Findings from this study contribute to the fabrication of efficient, low-cost photocatalysts for water pollutant removal.

A new class of water treatment processes called advanced reduction processes (ARPs) combine activation methods, such as ultraviolet (UV) irradiation, with reducing reagents to produce highly reactive reductants. This article reports on the application of ARPs to immobilize arsenic and selenium. Batch screening experiments were conducted under anaerobic conditions to evaluate the effectiveness of different ARPs in removing arsenic and selenium and to identify the most promising ARP for this purpose. The combination of sulfite with UV irradiation was not able to effectively remove arsenic and selenium. Although the ferrous iron/UV ARP was able to continuously remove all target compounds, the dithionite/UV ARP was observed to be the most promising method for removal of arsenic and selenium. However, resolubilization of both target compounds was observed with the dithionite/UV ARP. The dithionite/UV ARP rapidly removed soluble arsenic and selenium by reducing them to solid phases (As4S4, elemental Se). This was followed by resolubilization that was probably caused by oxidation of the solids by sulfite radicals or by reaction of sulfite with elemental selenium to form soluble species.

Water reuse is an alternative strategy targeting water shortages. In this study, electron beam (eBeam) irradiation was investigated as a method for removing bromate and perfluorooctanoic acid (PFOA) from a synthetic water designed to simulate a treated wastewater intended for potable water reuse. In the absence of oxygen, an exponential model was able to relate bromate concentration to absorbed dose. However, a more complex model was needed to describe PFOA defluorination, so a model was developed that assumed formation of one partially defluorinated intermediate and this model was used to describe the relationship between free fluoride concentration and absorbed dose. Nitrate negatively affected the removal of bromate and the dose constant was inversely proportional to the nitrate concentration as predicted by a simple model that assumes the presence of radical scavengers. In contrast, the presence of nitrate improved the degradation of PFOA, possibly due to formation of oxidizing radicals or by other reactions of nitrate degradation products. Fulvic acid and alkalinity exerted negligible influences on bromate removal. Fulvic acid dampened the defluorination efficiency, probably due to the scavenging of oxidizing radicals such as the hydroxyl radical (OH). Alkalinity was found to accelerate PFOA defluorination, possibly because of the formation and reactivity of the carbonate radical (CO3−). As pH increased from 5.0 to 7.3, the dose constant for bromate removal increased from 0.45kGy−1 to 0.69kGy−1, but it barely changed when pH was further increased to 9.0. In the presence of oxygen, both contaminants were degraded less efficiently and showed more complex patterns of degradation. Pretreatment to remove dissolved oxygen would probably be needed to apply eBeam in practice for degradation of bromate and PFOA.

Chlorate is one of the disinfection byproducts that are formed when chlorine/chlorine dioxide is used as a primary disinfectant. This study investigated the removal of chlorate by photochemical degradation using an advanced reduction process, which is a treatment method that combines a reducing agent with an activating method to generate reducing radicals. The effectiveness of combinations of reducing agents and three UV light sources having a peak output at 254, 365, and 312 nm were evaluated for chlorate removal. Dithionite irradiated by broad-band UVB lamp having the peak energy at 312 nm showed the highest chlorate removal. In pursuit of finding the optimum advanced reduction process conditions, the environmental process variables including pH, reducing agent dose, and light intensity were investigated. Dithionite/UV-B advanced reduction process was effective in weakly acidic conditions (pH < 5), and chlorate removal occurred in two steps. The first was an initial rapid decrease in chlorate concentration that occurred before initiating UV irradiation and was attributed to reaction with dithionite decomposition products. The second step was a slow decrease during UV irradiation that is caused by radicals produced by photolysis of the products of dithionite decomposition. The major product of chlorate destruction was chloride, with negligible amounts of chlorite produced.