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

Highlights

• Environmental sciences and chemistry were the dominant categories for microbubbles.

• Nanotechnology and physical chemistry were dominant for nanobubbles.

• Half of the most productive countries were among the developed and high-income countries.

• Keyword analysis revealed “water treatment” as the focus of microbubbles.

• Nanobubbles are still being investigated for their unique characteristics such as stability.

Abstract

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.

Introduction

The availability of water resources in densely populated countries across the globe is a severe issue. In this regard, some of the cities that have made international headlines due to water shortage are Cape Town in South Africa, Bangalore in India, Jakarta in Indonesia, and Sao Paulo in Brazil (Oki and Quiocho, 2020). Limited land area to reserve rainfall exacerbates the problem since a large amount of water is squandered as it cannot be captured or retained (Tortajada, 2006; Wen et al., 2011). Unbridled urbanization, anthropogenic activities, and the perpetual increase in the world's population have strained water resources, thereby necessitating improved discharge quality (Drangert and Sharatchandra, 2017; Fan et al., 2019). If not adequately treated, wastewater can be detrimental to aquatic and terrestrial ecosystems' health (Lee et al., 2019; Li et al., 2019; Wan et al., 2020; Xu et al., 2020). In terms of quantity, municipal secondary effluent is large and has the potential to be used as a sustainable source for reuse (Fan et al., 2019; Larsen et al., 2016; Sun et al., 2016).

According to a recent report, in 2018, the Chemical Abstract Service listed 143 million chemical products (American Chemical Society, 2020; Azuma et al., 2019) which will inevitably find their way into wastewater treatment plants to be treated for reuse (Tiedeken et al., 2017; Verlicchi et al., 2010). Emerging contaminants (ECs), which are also known as emerging organic contaminants or contaminants of emerging concerns (CECs) (Bilal et al., 2019; Castillo-Zacarías et al., 2021), represent a group of natural and synthetic chemicals in water bodies that are able to adversely affect ecosystems and human health (Geissen et al., 2015). The list of compounds and chemicals in this group is significantly large and getting perpetually larger by incorporating new commercial substances and disposal of chemicals currently in widespread use. Moreover, with the state-of-the-art techniques to analyze the water and detect low concentrations of emerging contaminants, new ECs have been recently identified (Gasperi et al., 2014; Rodriguez-Narvaez et al., 2017). Some major categories of ECs include pesticides, pharmaceuticals, and personal care products (PPCPs), disinfection by-products, industrial chemicals, endocrine disrupting compounds (EDCs), artificial sweeteners and food additives, nanomaterials, sunscreen, and UV filters, flame retardants, and Perfluorooctanoic Acid (PFOA) (NORMAN, 2020; Richardson and Ternes, 2014). These chemicals can negatively impact various water resources (Alygizakis et al., 2016; Azuma et al., 2019; López-Serna et al., 2013). Therefore, urgent actions are needed to analyze and remediate such compounds in water bodies as potable water is becoming scarce across various parts of the globe. Even though conventional methods are proven to be effective, the concept of “biodegradability” constrains the treatment of certain types of wastewater, making it an arduous task to achieve standard discharge quality. Consequently, wastewater treatment plants require sustainable technologies (Jabesa and Ghosh, 2016). In this regard, micro and nano-bubbles possess the capacity to solve this issue and, therefore, are attracting a lot of attention owing to their possible applications in different sectors (Agarwal et al., 2011; Gurung et al., 2016).

Micro and nanobubbles are air bubbles with dimensions of 10–15 μm and less than 200 nm, respectively. These bubbles possess unique characteristics that make them efficient in water treatment. Unlike macro bubbles that reach the surface with a high velocity and then burst, microbubbles rise with much lower velocity, and they burst midway. Upon bursting, ozonated microbubbles release hydroxyl radicals, reacting with the contaminants and degrading them (Yasui et al., 2019b). At the final moment of the violent bubble collapse, the bubble temperature increases significantly to 7200 K, even though the endothermic heat of O₃ considerably cools the bubble. Most oxygen (O) atoms are immediately consumed inside a bubble by the chemical reaction with water vapor H₂O + O→2OH. In addition, more than 10⁷ OH (hydroxyl) radicals are produced inside the bubble, as mentioned in the above chemical reaction (Temesgen et al., 2017a), rendering them ideal for disinfection and contaminant degradation. Fig. 1 elucidates the process of bubble burst and radical generation.

Nanobubbles can stay afloat in the water's body for hours to several days if generated correctly. Nanobubbles have three unique properties aside from high durability; first, they have Brownian motion, which means they can cover a larger surface area when rising to the surface or remaining in the water (Sekiguchi et al., 2011); as a result, they are capable of bringing back suspended solids or oil droplets (fat particles) to the surface more efficiently. Second, they possess a very large surface area; if a typical bubble has a 0.1 mm to 0.24 m² surface area ratio, nanobubbles have 0.24 nm to 240 m², which is approximately 1000 times more surface per milliliter. This results in higher mass transfer, and third, in the presence of surface-active agents (surfactants), nanobubbles can acquire electrostatic charge (various charges based on the type of surfactant). Consequently, nanobubbles can surround contaminants, similar to forming a micelle (Attard, 2003), providing a unique opportunity to remove contaminants from the body of the water.

Though nano and microbubbles' technology has long been investigated, there is a lacuna of knowledge, especially when it comes to the application in water treatment. Only a few literature reviews have considered nanobubbles and their utilization in water treatment or other environmental improvements. Based on the Web of Science database (Clarivate, 2020), in the last four decades (from 1978 to 2020), only 17 review papers have been published on nanobubbles, and among those, only two papers assessed nanobubbles role in water treatment (Atkinson et al., 2019; Temesgen et al., 2017b). This, indeed, calls for both review and bibliographic publications that researchers can use to have a better perspective toward nano and microbubbles. Consequently, it is difficult for scholars and researchers to read all the publications in this field. Moreover, to the best of the authors' knowledge, no bibliometric research about nano and microbubbles has been done over the past 42 years. In this study, the authors have strived to provide a general overview of nano and microbubbles research trends to assess their characteristics and their utilization in environmental remediation.

The overarching aim of this study is to conduct a scientometric analysis of nano- and microbubble research from 2000 to 2020, considering the increasing attention given to this topic. Multiple key areas were assessed, including keywords, authors, countries, major journals, number of publications, citation, and keywords co-occurrence network. In order to visualize the results, social network analysis (SNA) was employed (Macías-Quiroga et al., 2020; Zhuang et al., 2013).