Dissolution enhancement and mathematical modeling of removal of residual trichloroethene in sands by ozonation during flushing with micro-nano-bubble solution

Removal of chlorine-contaminated groundwater by two-stage ozonation and biostimulation methods

Trichloroethene (TCE) contamination is a critical environmental hazard, and the substrate options for its biostimulated remediation are limited. This study applied an ozonation-and-biostimulation process to remove TCE from groundwater. The substrate used, denoted as Transferred Energy Element (TEE), was composed of natural organic materials and had a low viscosity (2.914 cP). Ten batch experiments were conducted through the application of micro-nano bubbles (MNBs) and substrates (TEE and EOS® [emulsified oil substrate]). MNBs with an average diameter of 157.5-180.8 nm effectively degraded TCE and dichloroethane within 6 min. Biostimulation using the TEE substrate effectively degraded both TCE and vinyl chloride pollutants and reached a steady state after 25 days. The two-stage dechlorination procedure with MNB treatment as the first stage enhanced TCE removal via biostimulation. MNBs reduced the TCE concentration in the first 20 min, but increased the chloride (Cl^-) concentration over the following five days (∼80 mg/L). The procedure with biostimulation as the first stage and 20 min ozonation as the second stage reduced the Cl^- concentration by ∼10 mg/L. The Cl^- concentrations rebounded after day 25 in the EOS environment. X-ray diffraction revealed that the released Na⁺ from the TEE settled with Cl^- as minerals in the soil. The novel two-stage method for TCE removal was found to be more effective than solo MNB treatment or biostimulation.

Ozone β-Cyclodextrin Inclusion Complex Characterization and Application in the Remediation of Total Petroleum Hydrocarbons

Green remediation is essential in the current practice of water resources management. In this study, a series of ozone β-cyclodextrin (O3-βCD) inclusion complexes were prepared under a selected range of different ozone concentrations, β-CD concentrations, and solution pHs to test their ozone release rates and efficiencies in the treatment of total petroleum hydrocarbons (TPH) in water. The main objectives of this study are to characterize the O3-βCD system, mathematically model its ozone release rate, and test its capability in the degradation of pollutants. From the results, it was found that by defining a set of dimensionless parameters, including β-CD to ozone molar ratio and various degrees of ozone saturation, the steady-state conditions in the O3-βCD system can be represented by a newly developed dimensionless plot. In an optimal condition, the dissolved ozone release rate of 6.8 × 10−5 mM/min can be achieved in the O3-βCD system. A mathematical model was successfully developed to estimate the ozone release rate. In the TPH removal experiments, the effects of β-CD to ozone molar ratio and ozone dosage on the removal efficiency were rigorously examined. Overall, an optimal TPH removal of nearly 90% can be achieved in the treatment of 50 mg/L of TPH in water using this inclusion complex reagent.

Nanobubble Technology Enhanced Ozonation Process for Ammonia Removal

Ozone (O3) has been widely used for water and wastewater treatment due to its strong oxidation ability, however, the utilization efficiency of O3 is constrained by its low solubility and short half-life during the treatment process. Thereby, an integrated approach using novel nanobubble technology and ozone oxidation method was studied in order to enhance the ozonization of ammonia. Artificial wastewater (AW) with an initial concentration of 1600 mg/L ammonia was used in this study. In the ozone-nanobubble treatment group, the concentration of nano-sized bubbles was 2.2 × 107 particles/mL, and the bubbles with <200 nm diameter were 14 times higher than those in the ozone-macrobubble treatment control group. Ozone aeration was operated for 5 min in both nanobubble treatment and control groups, however, the sampling and measurement were conducted for 30 min to compare the utilization of O3 for ammonia oxidation. H+ was the by-product of the ammonia ozonation process, thus the pH decreased from 8 to 7 and 7.5 in nanobubble treatment and control groups, respectively, after 30 min of operation. The fast removal of ammonia was observed in both systems in the first 10 min, where the concentration of ammonia decreased from 1600 mg/L to 835 and 1110 mg/L in nanobubble treatment and control groups, respectively. In the nanobubble treatment group, ammonia concentrations kept the fast-decreasing trend and reached the final removal performance of 82.5% at the end of the experiment, which was significantly higher than that (44.2%) in the control group. Moreover, the first-order kinetic model could be used to describe the removal processes and revealed a significantly higher kinetic rate constant (0.064 min−1) compared with that (0.017 min−1) in the control group. With these results, our study highlights the viability of the proposed integrated approach to enhance the ozonation of a high level of ammonia in contaminated water.

Global trends and characteristics of nano- and micro-bubbles research in environmental engineering over the past two decades: A scientometric analysis

The present study has two primary goals, the first goal is to investigate a bibliometric analysis and assess the trends to evaluate the global scientific production of microbubbles and nanobubbles from 2000 to 2020. The aim is to elucidate the cornucopia of benefits the two technologies (micro and nanobubbles) can offer in environmental sciences and environmental amelioration such as wastewater treatment, seed germination, separation processes, etc. The second goal is to explicate the reason behind every chart and trend through environmental engineering perspectives, which can confer value to each analysis. The data was acquired from the Web of Science and was delineated by VOS viewer software and GraphPad Prism. Considering 1034 publications in the area of micro-and nanobubbles, this study was conducted on four major aspects, including publication growth trend, countries contribution assessment, categories, journals and productivity, and keywords co-occurrence network analysis. This article revealed a notable growth in microbubbles and nanobubbles-related publications and a general growth trend in published articles in a 20-year period. China had the most significant collaboration with other countries, followed by the USA and Japan. The most dominant categories for microbubbles were environmental sciences and environmental engineering comprising 22.5% of the total publications, while multidisciplinary subjects such as nanotechnology and nanosciences (8%) were among the dominant categories for nanobubbles. Keyword's analysis results showed that microbubbles had reached the apex since their discovery. Consequently, they are being used mostly in water/wastewater treatment or environmental improvement. On the other hand, nanobubbles are still in their infancy, and their pervasive use is yet to be fully materialized. Most of the publications are still striving to understand the nature of nanobubbles and their stability; however, a critical analysis showed that during the past two years, the trend of using nanobubbles as a cost-effective and environmentally friendly approach has already begun.

Mitigation of biofouling in agricultural water distribution systems with nanobubbles

Biofouling poses considerable technical challenges to agricultural irrigation systems. Controlling biofouling with strong chemical biocides is not only expensive and sometimes ineffective, but also contributes to environmental pollution. This study investigated the application of nanobubbles (NBs) on minimizing biofouling in agricultural irrigation water pipelines. Treatment performances were assessed using low concentration bubbles (LCB) and high concentration bubbles (HCB) together with a negative control (CK: no-NBs). 16 s rRNA gene sequencing and X-ray diffraction were used to characterize the microbial community and mineral compositions of biofilms in water emitters. Results demonstrated that NBs effectively mitigated biofouling through reducing fixed-biomass by 31.3-52.1%. A significantly different microbial composition was found in the biofilm community with reduced biodiversity. Molecular ecological network analysis revealed that NBs were detrimental to the mutualistic interactions among microbial species - destabilizing the network complexity and size, which was expressed as decreasing in extracellular polymers and biofilm biomass. Furthermore, NBs significantly decreased the deposition of carbonate, silicate, phosphate, and quartz on the pipe surfaces, leading to reductions of total content of minerals in biofilms. Therefore, this study demonstrated that NBs treatment could be an effective, and eco-friendly solution for biofouling control in agricultural water distribution systems.

Natural attenuation method for contaminant remediation reagent delivery assessment for in situ chemical oxidation using aqueous ozone

A Monitored Natural Attenuation (MNA) assessment approach typically used for contaminant remediation feasibility assessment was developed here for remediation-reagent delivery assessment. Subsurface delivery of oxidants, such as aqueous ozone (O₃) for in situ chemical oxidation (ISCO) of groundwater contaminants, is naturally attenuated by oxidant demand and reactivity. We compared mixed reactor kinetic experiments, sand column tracer transport experiments, and reactive transport modeling and assessment methods to quantify natural attenuation kinetics, aqueous O₃ solute transport, oxidant demand kinetics, and ISCO reagent delivery limitations. Sorption of aqueous O₃ to quartz sand was observed during transport of O₃ through water-saturated porous media. Pseudo 1st order decomposition rate constants of O₃ bulk attenuation with transport were comparable to mixed reactor experiments without transport, and reactive transport modeling of miscible-displacement column experiments was used to quantify each attenuation process. Aqueous ionic strength was correlated with O₃ decomposition rate constants, which was the dominant reagent delivery attenuation process. These results suggest that aqueous O₃ decomposition and oxidant delivery attenuation can be predictable upon characterization of the sediment oxidant demand and dispersion, and increasing groundwater velocity during aqueous O₃ injection can maximize transport distance for reagent delivery.

Remediation of NAPL-contaminated porous media using micro-nano ozone bubbles: Bench-scale experiments

Aqueous solutions of micro-nano bubbles (MNBs) containing ozone gas were injected through a NAPL-contaminated glass bead column. The glass column (15 cm × 2.5 cm) was packed with glass beads: the first 12 cm was packed with coarse glass beads while much finer glass beads were used to pack the remaining 3.0 cm of the column. Decane was used as the representative NAPL, to which an oil-soluble fluorescence tracer was added. The fluorescence tracer was considered as a constituent of the NAPL that readily reacts with ozone. Air and ozone-containing oxygen were used to generate MNB solutions, and injected through the column. In addition, H₂O₂ was introduced to the O₃-containing MNB (O₃-MNB) solution to investigate the effect of hydroxyl free radicals on the NAPL removal. An ozone gas sparging experiment was also conducted for comparison. After 72 h of O₃-MNB application, a significant mass of n-decane (27.6% of the initial mass applied) was removed from the column. H₂O₂ injection into the column during O₃-MNB application was effective in increasing the n-decane mass removal by 22%, compared to the O₃-MNB experiment. The rate of NAPL removal during O₃-MNB flushing was significant, although slower than ozone sparging. During O₃-MNB application, fast decay of fluorescence was observed; whereas, during co-injection of H₂O₂ and O₃-MNB solutions, only a slight change in the fluorescence was observed. This indicates that oxidative degradation of NAPL during H₂O₂ and O₃-MNB injection takes place only at the NAPL-water interface due to the reactivity of hydroxyl free radical, whereas ozone diffusion into NAPL induced the decay of the fluorescence tracer in the bulk NAPL. The removal characteristics during MNB application and ozone gas sparging were investigated based on the analysis of NAPL using mass spectrophotometer. When O₃-MNB and H₂O₂ were co-injected, only n-decane was detected in the NAPL; while when O₃-MNB was used for flushing, oxidative products were found in the NAPL. More hydrophilic compounds were found in the NAPL after ozone sparging. This implies different removal mechanisms depending on the kind of oxidation agent, and the state of oxidizing fluid. Based on the findings in this study, the application of O₃-MNB could be a feasible option for cleaning up NAPL-contaminated aquifers.

Nanobubble Technologies Offer Opportunities To Improve Water Treatment

Since first hypothesizing the existence of nanobubbles (NBs) in 1994, the empirical study of NB properties and commercialization of NB generators have rapidly evolved. NBs are stable spherical packages of gas within liquid and are operationally defined as having diameters less than 1000 nm, though they are typically in the range of 100 nm in one dimension. While theories still lack the ability to explain empirical evidence for formation of stable NBs in water, numerous NB applications have emerged in different fields, including water and wastewater purification where NBs offer the potential to replace or improve efficiency of current treatment processes. The United Nations identifies access to safe drinking water as a human right, and municipal and industrial wastewaters require purification before they enter water bodies. These protections require treatment technologies to remove naturally occurring (e.g., arsenic, chromium, fluoride, manganese, radionuclides, salts, selenium, natural organic matter, algal toxins), or anthropogenic (e.g., nitrate, phosphate, solvents, fuel additives, pharmaceuticals) chemicals and particles (e.g., virus, bacteria, oocysts, clays) that cause toxicity or aesthetic problems to make rivers, lakes, seawater, groundwater, or wastewater suitable for beneficial use or reuse in complex and evolving urban and rural water systems. NBs raise opportunities to improve current or enable new technologies for producing fewer byproducts and achieving safer water. This account explores the potential to exploit the unique properties of NBs for improving water treatment by answering key questions and proposing research opportunities regarding (1) observational versus theoretical existence of NBs, (2) ability of NBs to improve gas transfer into water or influence gas trapped on particle surfaces, (3) ability to produce quasi-stable reactive oxygen species (ROS) on the surface of NBs to oxidize pollutants and pathogens in water, (4) ability to improve particle aggregation through intraparticle NB bridging, and (5) ability to mitigate fouling on surfaces. We conclude with key insights and knowledge gaps requiring research to advance the use of NBs for water purification. Among the highest priorities is to develop techniques that measure NB size and surface properties in complex drinking and wastewater chemistries, which contain salts, organics, and a wide variety of inorganic and organic colloids. In the authors' opinion, ROS production by NB may hold the greatest promise for usage in water treatment because it allows movement away from chemical-based oxidants (chlorine, ozone) that are costly, dangerous to handle, and produce harmful byproducts while helping achieve important treatment goals (e.g., destruction of organic pollutants, pathogens, biofilms). Because of the low chemical requirements to form NBs, NB technologies could be distributed throughout rapidly changing and increasingly decentralized water treatment systems in both developed and developing countries.

Cyclodextrin-enhanced 1,4-dioxane treatment kinetics with TCE and 1,1,1-TCA using aqueous ozone

Enhanced reactivity of aqueous ozone (O₃) with hydroxypropyl-β-cyclodextrin (HPβCD) and its impact on relative reactivity of O₃ with contaminants were evaluated herein. Oxidation kinetics of 1,4-dioxane, trichloroethylene (TCE), and 1,1,1-trichloroethane (TCA) using O₃ in single and multiple contaminant systems, with and without HPβCD, were quantified. 1,4-Dioxane decay rate constants for O₃ in the presence of HPβCD increased compared to those without HPβCD. Density functional theory molecular modeling confirmed that formation of ternary complexes with HPβCD, O₃, and contaminant increased reactivity by increasing reactant proximity and through additional reactivity within the HPβCD cavity. In the presence of chlorinated co-contaminants, the oxidation rate constant of 1,4-dioxane was enhanced. Use of HPβCD enabled O₃ reactivity within the HPβCD cavity and enhanced 1,4-dioxane treatment rates without inhibition in the presence of TCE, TCA, and radical scavengers including NaCl and bicarbonate. Micro-environmental chemistry within HPβCD inclusion cavities mediated contaminant oxidation reactions with increased reaction specificity.