Remediation of oil-contaminated sand with self-collapsing air microbubbles

Ultrasonics and sonochemistry: Editors' perspective

Ultrasonic waves can induce physical and chemical changes in liquid media via acoustic cavitation. Various applications have benefitted from utilizing these effects, including but not limited to the synthesis of functional materials, emulsification, cleaning, and processing. Several books and review articles in the public domain cover both fundamental and applied aspects of ultrasonics and sonochemistry. The Editors of the Ultrasonics Sonochemistry journal possess diverse expertise in this field, from theoretical and experimental aspects of acoustic cavitation to materials synthesis, environmental remediation, and sonoprocessing. This article provides Editors' perspectives on various aspects of ultrasonics and sonochemistry that may benefit students and early career researchers.

A modelling approach to explore the optimum bubble size for micro-nanobubble aeration

Bubble aeration has been widely applied in water/wastewater treatment, however its low gas utilization rate results in high energy consumption. Application of micro-nanobubbles (MNB) has emerged as a process with the potential to significantly increase gas utilisation due to their high relative surface area and high gas-liquid mass transfer efficiency. In this study, we demonstrate through calibrated models that MNB of an optimum bubble size can shrink and burst at or below the water surface enabling (1) all encapsulated gas to thoroughly dissolve in water, and (2) the bursting of nanobubbles to potentially generate free radicals. Through the understanding of MNB dimensional characteristics and bubble behaviour in water, a dynamic model that integrated force balance (i.e. buoyancy force, gravity, drag force, Basset force and virtual mass force), and mass transfer was developed to describe the rising velocity and radius variation of MNB along its upward trajectory. Unlike for conventional millimetre-sized bubbles, intensive gas dissolution of MNBs led to radius reduction for small bubbles, while a large initial radius triggers bubble swelling. The initial water depth was also crucial, where greater depth could drive the potential for bubble shrinkage so that they were more liable to contract. For example, the optimum bubble size of air (42-194 μm) and oxygen (127-470 μm) MNB that could achieve complete gas transfer (100% gas utilisation) for a range of specific water depths (0.5-10 m) were calculated. The modelling results for microbubbles (10-530 μm) were well validated by the experimental data (R²>0.85). However, the validation of the modelling results for nanobubble (<1 μm) aeration requires further study due to a lack of available empirical data. In this study, the proposed model and analysis provided new insights into understanding bubble dynamics in water and offered fundamental guidance for practitioners looking to upgrade bubble aeration system.

Assessment of bimetallic Zn/Fe0 nanoparticles stabilized Tween-80 and rhamnolipid foams for the remediation of diesel contaminated clay soil

Diesel contamination of soil due to oil spills, disposal of refinery waste, oil exploration constitutes a major environmental problem. This paper reports the remediation of diesel contaminated clay soil using Zn/Fe⁰ bimetallic nanoparticle stabilized Rhamnolipid (RMLP) and Tween-80 (TW-80) surfactant foams. Fe⁰, and Zn (x wt%)/Fe⁰ (x = 0.2, 2.0, and 10.0) bimetallic nanoparticles are synthesized by using sodium borohydride reduction method. The average particle size (from FESEM) is calculated to be 62, 57, 42 and 35 nm for the Fe⁰, Zn (0.2)/Fe⁰, Zn (2)/Fe⁰ and Zn (10)/Fe⁰ nanopowders, respectively. The highest foamability and foam stability of 109.6 and 108.5 mL, respectively are observed for the RMLP (12 mg/l) surfactant foam stabilized with 6 mg/l Zn (10)/Fe⁰ nanoparticles. The surface tension values reduce to the lowest value of 28.1 and 31.4 mN/m with the addition of 6 mg/l of Zn (10)/Fe⁰ powder in RMLP and TW-80 solutions of 12 mg/l, respectively. The maximum diesel removal efficiency of 83.8 and 59%, is achieved by RMLP (12 mg/l) foam stabilized by Zn (10)/Fe⁰ nanoparticles (6 mg/l) for the clay soil contaminated with 100 and 500 μl/g of diesel, respectively. The physicochemical properties of the nanoparticles are studied to explain the foam properties and the remediation behavior. These findings regarding the nanoparticle stabilized foams can offer a cost-effective environment friendly commercial solution for soil remediation in the future.

Microbubbles for Effective Cleaning of Metal Surfaces Without Chemical Agents

Traditional cleaning methods involving surfactants and ultrasound generate large amounts of wastewater. Microbubbles offer a more eco-friendly technology for interface cleaning. Here, we explored the efficiency of microbubbles for cleaning oil from metal surfaces. Air microbubbles at concentrations as high as 10⁶ particles/mL were generated by hydrodynamic cavitation. Under optimal conditions, cleaning efficiencies for the removal of oil from carbon-steel and stainless-steel surfaces were 78.5 and 49.8% after 15 min, respectively, compared to only 6.5 and 9.9% without microbubbles. Additionally, combining microbubble treatment with the ultrasonic method achieved a higher efficiency than ultrasonic cleaning alone, achieving an efficiency of 85.5% after 3 min compared to 69.0%. The mechanism of microbubble cleaning was determined using a fluorescence observation system, and a model was established to describe the cleaning process. The use of microbubbles produced less emulsified oil wastewater because the oil that attaches to the microbubble surface floats with the bubbles to the surface of the cleaning water, where it can be removed, allowing for water recycling. This novel microbubble cleaning technology, which both enhances cleaning efficiency and reduces wastewater production, represents a viable and eco-friendly option for degreasing processes.

Ultrafine bubble water mitigates plant growth in damaged soil

Water containing ultrafine/nano bubbles (UFBs) promoted the growth of tomato (Solanum lycopersicum) in soil damaged by cultivation of tomato in the previous year or bacterial wilt-like disease and also promoted the growth of lettuce (Lactuca sativa) when lettuce was grown in the soil damaged by repeated cultivation of lettuce. On the other hand, UFB supply did not affect plant growth in rock wool or healthy soil. Furthermore, the growth of lettuce was not affected by UFB water treatment in the soil damaged by the cultivation of tomato. UFB water partly suppressed the growth of the pathogen of bacteria wilt disease, Ralstonia solanacearum in vitro. These data suggest that UFB water is effective to recover the plant growth from soil damage.

Exploring the effect of ultrasonic power, frequency, and load toward remediation of oil-contaminated beach and oilfield sands using ANOVA

Despite the potential shown by previous investigations on the use of ultrasound for the remediation of oil-contaminated sand, the influence and interactions among ultrasonic parameters and oily sand are unclear, leading to possible ineffective treatment and high-power consumption. In order to improve the process efficiency, this work analyzes the effects of ultrasonic power, frequency, and load toward the cleaning of crude oil-contaminated sand, using two different sample positions and sand types. Crude oil-contaminated beach sand and produced sand from offshore oil well were used as samples. They were cleaned in custom-made ultrasonic bath reactor for 10 min with power from 30 to 120 W, frequency covering 25-60 kHz, and sand load of 10-100 g. With experimental design consisting multiple factors and levels, the interactions between factors in all possible combinations were determined using ANOVA (n = 210). From p-value based at 95% confidence interval and extensive F test, the three most significant factors were the sand type, the ultrasonic frequency, and the interaction between sand type and frequency. The best setting for suspended samples involved high frequency of 60 kHz, whereas bottom samples preferred low frequency at 28 kHz. This finding was justified when the acoustic pressure attenuation, standing wave pattern, and surface pitting/cracking were found in correlation with the cleaning results. Overall, the maximum treatment under ultrasonic bath solely gained around 60%, improvable by hybrid cleaning with other techniques such as chemical, biological, mechanical, and thermal.

Optimum conditions of zero-valent iron nanoparticle stabilized foam application for diesel-contaminated soil remediation involving three major soil types

Stability of foam, enhanced by nano zero-valent iron (nZVI) and its optimized constituents, may have significant potential for effective treatment of soil contaminated with diesel oil-a major environmental problem. The optimum diesel removal efficiency from distinct types of soil accomplished by the unique application of such foams as well as the optimum conditions of the foaming constituents have not been reported in literature so far. Hence, in this work, the removal of diesel contaminant from different soil types (desert, coastal, clay soil) is optimized, and the optimized results are reported for the first time, using response surface methodology (RSM), for alkylpolyglucoside phosphate (APG-Ph) foam, stabilized by nZVI. The effect of concentrations of APG-Ph (0.02, 0.04, 0.06, 0.08, and 0.1 volume %) and nZVI (2, 3, and 3.5 mg/l) on diesel removal efficacy from soil is studied using Box-Behnken design (BBD) of response surface methodology (RSM). Maximum diesel removal efficiency obtained at a concentration of 0.1 volume % APG-Ph foam with 3.5 mg/l nZVI for desert, coastal, and clay soil is 94.6, 95.3, and 57.5%, respectively. The optimum concentrations of APG-Ph and nZVI are found to be 0.98 volume % and 0.8 mg/l, respectively. Validation of this optimal condition experimentally results in highest removal efficiency of 98.3, 97.2, and 75.9% for desert, coastal, and clay soil respectively. This is in good agreement with the predicted values by RSM (98.67, 97.57, and 76.85%). The maximum diesel removal efficiency predicted at optimal concentration of APG-Ph and nZVI is significantly larger than the results reported in literature in last three years.

Micro-bubbles enhanced removal of diesel oil from the contaminated soil in washing/flushing with surfactant and additives

Diesel removal of contaminated soil by washing/flushing was enhanced with micro-bubbles and selected surfactants based on their solubilization properties and decontamination capacities. The influencing factors were studied to aim for increasing washing/flushing efficacy. The mixture solution of saponin and cyclodextrin increased the removal efficiency significantly compared to the single-agent solution flushing with an increasing range of 20%-31%. Meanwhile, micro-bubble enhancement increased over 20% of the diesel removal for the sandy soil flushing. As the flushing process may cause soil eroded, the TDS and soil solute in flushing solution were measured to evaluate the circulation time. The 90 min flushing time ensured the cleaning goal and reserved the soil solute by circulation flushing. The soil solute, especially the electron acceptor (NO₃^-) , was remained in the soil, which was highly demanded for residual diesel biodegradation of loam soil. It is concluded that mixed agents, circulation of flushing solution, and micro-bubbles increased the diesel removal, and the circulation flushing could be very promising in practical applications.

The role of bulk micro-nanobubbles in reagent desorption and potential implication in flotation separation of highly hydrophobized minerals

Micro-nanobubbles (MNBs) generated during hydrodynamic cavitation (HC) have been extensively studied in mineral processing field in the past two decades. Many researchers have claimed that MNBs can effectively promote the collection of fine particles in flotation, while studies on MNBs assisted mineral separation are rare. In this study, the role of bulk MNBs in desorbing flotation reagent was investigated, with the aim of illustrating the potential effects of MNBs on minerals separation. The results showed that bulk MNBs could efficiently remove the sodium oleate (NaOl) from diaspore surfaces, reducing the residual concentration of NaOl on solids, which was more significant when the amount of NaOl pre-adsorbed was relatively small. Furthermore, lower residual concentration of NaOl on solids caused by MNBs cleaning made the particles less hydrophobic and flocs more friable. Given that gangue entrapment in flocs was one of the main limits for high-selective flotation, the roles of MNBs in enhancing reagent desorption and associated flocs breakup and reorganization probably contribute to higher separation efficiency of different minerals, which was confirmed by the flotation results of diaspore/kaolinite mixture.

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.

Progress in ultrasonic oil-contaminated sand cleaning: a fundamental review

Steady efforts in using ultrasonic energy to treat oil-contaminated sand started in the early 2000s until today, although pilot studies on the area can be traced to even earlier dates. Owing to the unique characteristics of the acoustic means, the separation of oil from sand has been showing good results in laboratories. This review provides the compilation of researches and insights into the mechanism of separation thus far. Related topics in the areas of oil-contaminated sand characterizations, fundamental ultrasonic cleaning, and cavitation effects are also addressed. Nevertheless, many of the documented works are only at laboratory or pilot-scale level, and the comprehensive interaction between ultrasonic parameters towards cleaning efficiencies may not have been fully unveiled. Gaps and opportunities are also presented at the end of this article.

Comparison of zero-valent iron and iron oxide nanoparticle stabilized alkyl polyglucoside phosphate foams for remediation of diesel-contaminated soils

Stable surfactant foam might play a vital role in the effective remediation of diesel oil contaminated soil-a major environmental hazard. This paper, first of its kind, is reporting the remediation of diesel-contaminated desert soil, coastal soil and clay soil by aqueous alkylpolyglucoside phosphate (APG-Ph) surfactant foams stabilized by Fe⁰ and Fe₃O₄ nanoparticles. Zero-valent iron (Fe⁰, ∼28 nm) and iron oxide (Fe₃O₄, ∼20 nm) nanoparticles are synthesized by liquid-phase reduction and precipitation methods, respectively. The effect of these nanoparticles on foamability, foam stability, surface tension and remediation of diesel-contaminated soils are examined at various concentrations (volume %) of alkylpolyglucoside phosphate (APG-Ph) surfactant and nanoparticles (mg/l). The maximum values of foamability and foam stability recorded for 0.1 vol % APG-Ph foam stabilized by 3.5 mg/l Fe⁰ are 108.3 and 110.4 mL, respectively. At the same conditions, the Fe₃O₄ results in 99.4 and 87.5 mL, respectively, depicting the better performance of Fe⁰. Reduction in surface tension of 0.1 vol % APG-Ph solution (50.75 mN/m) with the addition of 3.5 mg/l Fe⁰ (9.51 mN/m) and Fe₃O₄ (19.45 mN/m) nanoparticle is observed. Both the nanoparticles enhance remediation. The foam formed with 0.1 vol % APG-Ph and stabilized by 3.5 mg/l Fe⁰ shows the maximum diesel removal efficiency of 95.3, 94.6, and 57.5% for coastal soil, desert soil and clay soil, respectively. On the other hand, Fe₃O₄ (3.5 mg/l) stabilized APG-Ph foam of the same concentration shows merely 76.0, 79.6 and 51.6% diesel removal efficiency for coastal soil, desert soil, and clay soil, respectively. The rate of diesel removal by zero-valent iron and iron oxide nanoparticle stabilized foams are found to be well described by the first order kinetic model. Higher foamability, foam stability, and reducing capacity accompanying lower surface tension, compared to those of the Fe₃O₄ nanoparticle stabilized foam, could explain higher diesel removal efficiency of the Fe⁰ nanoparticle stabilized foam.

Remediation of oil spill-contaminated sands by chemical-free microbubbles generated in tap and saline water

Oil spill-contaminated sands caused by marine oil spillage could severely impact aquatic and terrestrial ecosystems. In this work, self-collapsing microbubbles (MBs)-based method was explored as a chemical free approach for remediation of oil spill-contaminated sands. Tap water and saline water with 32 g/L of sodium chloride were employed as the media to generate MBs. Results showed that almost all oils were removed from the oil spill-contaminated sands after the 40-min treatment with MBs generated in tap water. The oil removal efficiency of MBs in saline water was slightly lower than that in tap water, while more than 90% of oil removal efficiency was still achieved after the 40-min treatment. The analyses by FTIR and UV spectra further confirmed the oil removal from oil spill-contaminated sands by MBs. Consequently, self-collapsing MBs showed a great potential as an environmentally friendly technology for high-efficiency remediation of oil spill-contaminated sands.

Introduction to environmental engineering

Nowadays we can easily say that environmental engineering is truly an interdisciplinary science. Combining biology, ecology, geology, geography, mathematics, chemistry, agronomy, medicine, economy, etc. environmental engineering strives to use environmental understanding and advancements in technology to serve mankind by decreasing production of environmental hazards and the effects of those hazards already present in the soil, water, and air. Major activities of environmental engineer involve water supply, waste water and solid management, air and noise pollution control, environmental sustainability, environmental impact assessment, climate changes, etc. And all this with only one main goal – to prevent or reduce undesirable impacts of human activities on the environment. To ensure we all have tomorrow.