《1 Engineering research hotspots and engineering research focus》

1 Engineering research hotspots and engineering research focus

《1.1 Development trends of engineering research hotspots》

1.1 Development trends of engineering research hotspots

The top 10 engineering research hotspots in the field of environmental and light and textile industries engineering (hereafter referred to as environmental engineering), which includes the subfield of environmental science, environmental engineering, marine science, and climate science are summarized in Table 1.1.1. The in-depth study on traditionally engineering research hotspots includes “The chemical composition of aerosols,” “Topographic survey and soil erosion management,” “Applications of ligand-based composite adsorbents in water treatment,” “Remote sensing inversion techniques and estimation of fluxes for greenhouse gases,” “Estimation methods and models of solar radiation,” “Assessment and control of greenhouse gas emissions in wastewater treatment plants,” and “ Analytical techniques of atmospheric CH4 emissions and sources.” The emerging engineering research hotspots include “Arctic sea ice and mid-latitude climate responses,” “Regional climate response to global change,” and “Hyperspectral unmixing methods.” The annual number of core papers for individual research hotspots between 2011 and 2016 are summarized in Table 1.1.2.

(1) The chemical composition of aerosols

Please refer to Section 1.2.1.

(2) Arctic sea ice and mid-latitude climate responses

Please refer to Section 1.2.2.

(3) Hyperspectral unmixing methods

Three-dimensional (3D) hyperspectral images contain both spatial information and a wealth of spectral reflectance information with wide-band coverage, rapid non-destructive, full spectral information content, and other characteristics. Based on the abundant spectral information of hyperspectral images, the effective classification and recognition of imaging objects can be realized by image analysis and pattern recognition algorithms. Therefore, hyperspectral imaging technology has a wide range of applications in agriculture, military, land use, and environmental science. In the field of environmental science, eco

《Table 1.1.1》

Table 1.1.1 Top 10 engineering research hotspots in environmental engineering

《Table 1.1.2》

Table 1.1.2 Annual number of core papers belonging to each of the top 10 engineering research hotspots in environmental engineering

logical/environmental monitoring can be achieved through large-scale hyperspectral data; river water quality and eutrophication status can be evaluated using hyperspectral imaging; and hyperspectral remote sensing data can be used to determine aerosol thickness and composition.

Due to the complex diversity of nature and limited resolution of image sensors, one pixel may have mixed spectra of different materials. Such pixels are called mixed pixels. Mixed pixels greatly reduce the accuracy of the classification of surface features because they cannot be divided into a certain category. In order to solve the problem of mixed pixels, it is necessary to decompose the mixed pixels at the sub-pixel level through unmixing. Unmixing is the use of geometric and statistical methods and models to decompose the proportion of basic components (endmembers) and basic materials (abundance). Although there are many well-established methods, they have their own advantages and disadvantages and their applications differ. The accuracy and stability of the unmixing results have yet to be improved. Therefore, the efficient unmixing of hyperspectral images is still a popular and challenging topic in the applications of hyperspectral imaging technology.

(4) Topographic surveys and soil erosion management

Topographic surveys refer to measuring the projection position and elevation on the horizontal plane of a feature and terrain on the earth’s surface, reducing them to a certain proportion, and drawing topographic maps with symbols and notes. Soil erosion refers to the process of destruction, denudation, transportation, and deposition of soil and soil parent material under the external stress of water, wind, freeze, thaw, or gravity. The key technical issue with this research topic is to develop economical, efficient, and accurate mapping techniques, and apply them to monitoring, evaluation, and management of soil erosion. With respect to topographic surveys, aerial photogrammetry a rapidly developing filed. Notably, the applications of high-precision photographic mapping, light direction and ranging (LIDAR) mapping, and remote sensing (RS), and geographic information system (GIS) technologies based on the unmanned aerial vehicle (UAV) mapping have significantly improved the efficiency and accuracy of mapping. With the maturing UAV technology, high-precision aerial photogrammetry and mapping with UAV have gradually become the foci of current research efforts. This technique has led to the continued development of traditional terrain monitoring research and is widely used in earth science for the modeling, evaluation, and management of soil erosion; terrain modeling; and disaster prevention and mitigation. In the future, the development of economical, efficient, and accurate topographic survey techniques will remain a major research challenge in this field.

(5) Remote sensing inversion techniques and estimation of fluxes for greenhouse gases

Remote sensing technology for atmospheric methane (CH4) includes space-based and ground-based spectral detection techniques. There are some observation methods for ground-based greenhouse gases, such as Fourier transform spectrometry, grating spectrometry, and LIDAR. For green house gas detection techniques, the tendency of research and development in the future lies towards higher spectral and spatial resolution. Currently, the UK, Germany, France, and the USA are trying to develop a technique based on infrared laser heterodyne spectroscopy. This technique offers many advantages including high spectral resolution, high sensitivity, small volume, and ability to detect carbon dioxide (CO2) and CH4 through remote sensing. Therefore, the research focus of this project is to delve deeper into the development of the traditionally available techniques.

(6) Assessment and control of greenhouse gas emissions in wastewater treatment plants

Nitrous oxide (N2O) is the third largest contributor to global warming after CO2 and CH4. The warming effect of N2O is 350 times that of CO2. N2O has a significant impact on ozone depletion and causes serious damage to ecosystems. Wastewater treatment is an important source of N2O emissions, and the problem of N2O production in the traditional process of sewage denitrification has attracted the attention of many scholars. It is widely acknowledged that N2O is the intermediate product or by-product of nitrification and denitrification. The total amount of N2O emissions has increased with the widespread use of the sewage denitrification process.

This research topic focuses on emissions, reaction mechanisms, temporal variation, control strategies, and factors influencing biological nitrification and denitrification in sewage treatment processes. Key areas of research include the influence of sewage treatment processes and operation modes on N2O emissions, application of mathematical modeling in the prediction of N2O emissions during sewage treatment, the effect of pH on N2O loss and nitrogen removal, and the effect of inorganic carbon on the interaction of microorganisms within the biofilm in the process of nitrification/ammonia oxidation. The influence of different technologies on N2O emission characteristics in the process of sewage denitrification will continue to remain an important research direction in the future.

(7) Applications of ligand-based composite adsorbent in water treatment

Please refer to Section 1.2.3.

(8) Estimation methods and models of solar radiation

Solar radiation is a major source of energy for physical, biological, and chemical processes on the earth’s surface, and also the basic driving force dictating the behavior of the weather and climate. However, due to the restrictions in funds, technology, and environmental conditions, compared to conventional meteorological observation data, such as temperature, humidity, and precipitation, only a few weather stations in China provide solar radiation data, which fails to satisfy research requirements. Different mechanisms and empirical models have been established by researchers from all over the world. Empirical models utilize probability theory and statistical theory as the foundation to build the relationship between solar radiation and meteorological factors, but there are uncertainties in the estimation of solar radiation. Therefore, the research focus of this project is to delve deeper into the development of the traditionally available techniques.

(9) Analytical techniques to determine atmospheric CH4 emissions and sources

After CO2, CH4 is the most important greenhouse gas, and plays a significant role in global warming and chemical destruction of the ozone layer. CH4 emission sources can be divided into anthropogenic and natural sources. Anthropogenic sources include rice fields, fossil fuel combustion, landfills, and livestock. Natural sources include wetlands, termites, seas, and geological formations. Estimation of global CH4 sources is based on the observation of CH4 fluxes but the constraints of time, space, and effects of environmental factors on CH4 emissions increase the uncertainty of estimations. In the study of biological emission sources, different industries carry out research on emissions from rice fields, wetlands, and livestock but studies on urban emissions are lacking. Therefore, it is necessary to strengthen the research on CH4 observations and source estimation techniques to obtain more accurate CH4 source distribution and flux data.

(10) Regional climate response to global change

Please refer to Section 1.2.4.

《1.2 Understanding of engineering research focus》

1.2 Understanding of engineering research focus

1.2.1 The chemical composition of aerosols

China has faced serious atmospheric pollution problems in recent years, especially in autumn and winter. Aerosols in the atmosphere not only influence air quality but also have a significant effect on cloud formation and precipitation. Atmospheric aerosols include primary particles, directly emitted from human activities, and secondary species, converted from gases emitted to the atmosphere. Research on transformation processes, dry and wet deposition, chemical scavenging processes of different types of pollutants in atmospheric aerosols (including cloud, fog, and haze aerosols), and the degradation due to hazardous materials in aerosols to the environment, climate, and human health can provide a theoretical and scientific basis for solving the problem of atmospheric pollution and climate change. Data on the chemical composition of aerosol particles during heavy pollution events in autumn and winter, the composition of oxygenated organic aerosols, and the formation of aerosol chemical species will provide help form the public perception of pollution risks and the government’s response.

Many countries in the world consider aerosol pollution as the focus of atmospheric pollution research. Many disciplines with a long-term research direction study the influence of aerosols on health, environment, and climate, and the emission characteristics and secondary transformation characteristics of aerosols.

The review of 50 core papers on aerosol chemical composition revealed that these studies were cited as many as 2766 times, with a per paper citation frequency of 55.32, mostly from 2013 to 2015. Frequently cited papers account for 24.00% of all papers (Table 1.1.1). The number of patents is relatively small. Almost all 50 core studies were conducted in the USA and China, and the citation frequency per paper for both countries is 44.14 and 43.93, respectively (Table 1.2.1). The Chinese Academy of Sciences published 40% of the papers, which were cited866 times (Table 1.2.2). The core papers are mainly about aerosol composition, their physical and chemical properties, and sources, and to a lesser extent, about formation mechanisms, transformation processes, and wet and dry deposition. Studies on aerosol formation mechanisms and transformation processes can not only provide a technical basis for the reduction of aerosol formation but also a scientific basis for efficient government decision-making. Therefore, it is crucial to strengthen research in these two aspects.

In order to control and improve environmental quality without affecting economic development, aerosol chemical composition, formation mechanisms, and transformation processes need in-depth scientific research. Aerosol chemistry has become an important direction with regard to environmental improvement and economic development.

When analyzing the engineering research focus of “The chemical composition of aerosols,” the top three countries or regions that published the highest number of core papers are the USA (29), China (29), and Switzerland (14), and the top three countries or regions with highest average citations are Germany (49.13), Spain (48.00), and the USA (44.14) (Table 1.2.1). Among these countries or regions, China and the USA extend the highest level of cooperation to each other, followed by Germany, Switzerland, and England (Figure 1.2.1).

The top three institutions that published the highest number of core papers are the Chinese Academy of Sciences (20), the Paul Scherrer Institute (14), and Aerodyne Research, Inc. (12), and the top three cited institutions per core paper are University of Colorado (82.86), French Alternative Energies and Atomic Energy Commission (CEA) (55.33), and Aerodyne Research, Inc (54.75) (Table 1.2.2). The institutions with the highest number of published papers in China include the Chinese Academy of Sciences, Nanjing University, Nanjing University of Information Engineering, and Peking University (Table 1.2.2).The Chinese Academy of Sciences has a tie-up with Nanjing University of Information Engineering, Nanjing University, and University of California-Davis (Figure 1.2.2).

The major research institutions with regard to the citing core papers include the Chinese Academy of Sciences, Nanjing University, Nanjing University of Information Engineering, and Peking University in China; and Aerodyne Research, Inc. and University of California, Davis in the USA; and the Paul Scherrer Institute in Switzerland (Table 1.2.3 and Table 1.2.4).

1.2.2 Arctic sea ice and mid-latitude climate responses

The Arctic is one of the most sensitive areas to global climate change. The average temperature rise in the last 30 years is twice the rate of global warming, and this phenomenon is known as the “Arctic amplification.” Since the 1970s, global temperatures have increased and have had a profound impact on the Arctic, decreasing the Arctic sea ice coverage. By 2012, the area of Arctic sea ice has reduced to less than 40% of the original. With the influence

《Table 1.2.1》

Table 1.2.1 Major producing countries or regions of core papers on the engineering research focus “The chemical composition of aerosols”

《Table 1.2.2》

Table 1.2.2 Major producing institutions of core papers on the engineering research focus “The chemical composition of aerosols”

Note: CEA stands for French Alternative Energies and Atomic Energy Commission.

of global warming, internal positive feedback is the key process for the Arctic amplification, which not only leads to polar climate change but also has a significant effect on the global climate, resulting in many extreme weather and climate events. One of these typical events occurred at 11 a.m. on December 30, 2015, when a powerful storm from the Atlantic moved into the Arctic Ocean. At that time, the temperature near the North Pole was approximately the same as that in Beijing. Affected by this event, the Arctic temperature had broken the record of 0 °C, nearly 30 °C higher than the normal temperature in winter. The event also led to extreme cold wave events in China and other mid-latitude regions.

The accelerated melting of sea ice in the Arctic and the dramatic reduction in the sea ice extent will also have an impact on the global economy. Research shows that navigation time in the Arctic Channel is getting longer. By 2030, the airworthiness time may reach 120 d per year. The possible effect on the huge value of commercial shipping has drawn the attention of all stakeholders. In addition, the melting of sea ice has increased the potential for oil exploitation in the Arctic. The rich oil, gas, and mineral resources in the Arctic have attracted attention from many countries or regions. The change in the Arctic sea ice

《Figure 1.2.1》

Figure 1.2.1 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “The chemical composition of aerosols”¹

¹ In the figure, the nodes refer to the countries or regions, the size of the nodes refers to number of papers, the connecting line between nodes refers to papers published based on research cooperation, and the thickness of the connecting line indicates the number of papers based on research cooperation. These are the same in full text.

《Figure 1.2.2》

Figure 1.2.2 Collaboration network of the major producing institutions of core papers on the engineering research focus “The chemical composition of aerosols”

《Table 1.2.3》

Table 1.2.3 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “The chemical composition of aerosols”

extent is not only a climate issue but has also brought about a new focus on engineering exploitation techniques in the 21st century.

Although the Arctic sea ice covers only a small part of the global ocean, it plays an important role in Earth’s weather and climate. Sea ice has relatively high albedo, which forms a feedback effect in the global climate system. At the same time, the thermal conductivity of sea ice is very low and can block atmospheric heat and water vapor exchange in sea-ice-covered areas, affecting the atmospheric circulation system and weather conditions, leading to large-scale extreme weather events. Therefore, the study of the interaction between the Arctic sea ice reduction and the atmospheric circulation in the Northern Hemisphere is very important and necessary to reveal the role of Arctic sea ice in climate change. One of the important missions of Arctic science is to investigate the mechanisms underlying these positive feedback processes. Major scientific problems related to Arctic amplification mainly concern atmosphere-ice-sea interactions, sea ice being the most active factor. To solve these problems, the changes in the structure of the sea ice should be determined, 

《Table 1.2.4》

Table 1.2.4 Major producing institutions of core papers that are cited by core papers on the engineering research focus “The chemical composition of aerosols”

and thermodynamic characteristics of the change should be quantitatively expressed. Thus, factors such as melting pool, lateral melting, and snow and sea ice drift should be fully considered. The ocean is a key source of energy for changes in the Arctic. In winter, sea ice spreads over the ocean, creating an insulating layer that prevents much heat from escaping from the ocean to warm the air. Ocean currents also bring heat from warmer regions into the Arctic Ocean. Therefore, we need to understand the distribution of ocean heat flux (the mechanism of energy storage and release) and the effect of the changes in freshwater and thermocline structure on sea-air coupling.

The rapid changes in the Arctic sea ice extent have attracted significant attention. Implemented research projects include WCRP (World Climate Research Programme), CliC (Climate and Cryosphere), ACSYS (Arctic Climate System Study), SHEBA (Surface Heat Budget of the Arctic Ocean Experiment), LOIRA (Land-Ocean Interactions in the Russian Arctic), JWACS (Joint Western Arctic Climate Study), DAMOCLES (Developing Arctic Modeling and Observing Capabilities for Long-term Environmental Studies), and NABOS (Nansen and Amundsen Basins Observational System). These projects have focused on the Arctic and global climate change using onsite observations and numerical simulations. The response to the changes in sea ice extent on climate change was investigated through the research and development of the atmosphere-ocean-sea-ice coupling model. Among them, launched in 1993, ACSYS, one of the important subprograms of WCRP, mainly studies the effect of the Arctic climate change (primarily reflected in the reduction of sea ice) on the global environmental system.

The understanding of Arctic sea ice and atmospheric-ocean interactions has not been translated into the physical essence of sea ice change. The results of the existing studies attributed the changes in Arctic sea ice to global climate change and positive feedback processes it caused in the Arctic. The studies suggest that these processes make the ocean absorb more heat and strengthen the interactions among the atmosphere, ocean, and ice, and result in further decreases in sea icecover. However, it is not easy to determine these processes because most of them involve the physical processes of ocean and sea ice changes, which are not clearly understood. As a result, research on changes in the Arctic has not progressed much in recent years. In order to improve this situation, it is necessary to study the physical processes in the Arctic in depth, focusing on new and degenerative processes caused by Arctic warming. In addition, studying the physical processes based only on the assimilation data and reanalysis is far from sufficient. It is necessary to carry out large-scale field observations to obtain on-site data from the Arctic to understand the physical processes.

The analysis of the core papers and the main producing countries or regions (Table 1.2.5) shows that the USA, England, and Germany are the top three countries or regions with regard to the number of papers published on this research topic, followed by Australia, Japan, Korea, China, Norway, Switzerland, and Finland. The USA has published 61.22% of the core papers, indicating that the USA has a greater research interest in this field. England comes second, accounting for 30.61% of all papers on the subject. The total citation reflects the subsequent effects of the study; the ranking is consistent with the proportion of core papers by country or region. Although China’s rank with regard to the total number of papers on the index is not the highest, the average citation of each paper from China ranks first (157.00 times).

The University of Exeter in England and the National Center for Atmospheric Research and Rutgers State University in the USA are the top three institutions publishing core papers, followed by the University of Melbourne (Australia), the National Oceanic and Atmospheric Administration (the USA), the University of Colorado (the USA), the Potsdam Institute for Climate Impact (Germany), the Colorado State University (the USA), the National Institute of Polar Research (Japan), and the Atmospheric and Environmental Research, Inc. (the USA) (Table 1.2.6). Six institutions out of ten are located in the USA as well. Chinese research institutions do not appear within the top 10 rankings, indicating that most research activities are conducted by other institutions.

The main research partnerships are among the USA, England, Germany, and Australia, while China has a relationship only with the USA (Figure 1.2.3). Except for the National Institute of Polar Research (Japan), there is extensive communication among the top 10 institutions, which illustrates the importance of cooperation and communication in scientific research on the Arctic (Figure 1.2.4).

Arctic sea ice reduction has a major impact on the climate of mid-latitude regions, including the USA, Europe, and China. Its impact on the USA has been thoroughly researched, and studies of the impact on Europe have also yielded some accomplishments. China’s studies in the Arctic began in the 1990s; despite starting late, the activities have developed rapidly.

China established the Arctic Yellow River Station in 2004 and carried out seven Arctic scientific surveys in 1999, 2003, 2008, 2010, 2012, 2014, and 2016. In response to worldwide attention on the exploitation of the Arctic, China’s research in the Arctic has developed rapidly.

On December 10, 2013, ten Arctic research institutes from China and the Nordic countries (Iceland, Denmark, Finland, Norway, and Sweden) signed an agreement in Shanghai and announced the establishment of the ChinaNordic Arctic Research Center (CNARC).

On December 20, 2016, construction of an icebreaker was started in the Jiangnan Shipyard. It is the first independently built Chinese polar scientific expedition icebreaker. The ship is expected to be completed in 2019 and then become part of a polar expedition fleet with the Snow Dragon. With these developments, China’s abilities with regard to polar oceanographic surveys and support have improved.

In December 2016, China launched the first fully owned overseas satellite ground station near the North Pole in Kiruna, Sweden. With this station, China has made great technical progress and is able to access remote sensing data much more quickly. This development is of vital significance for rapid response to natural disasters and will promote international cooperation in this field.

The literature analysis shows that the number of core papers published by Chinese scientists in this research field rank among the top 10 (seventh), but most of them have been published by different research institutions; therefore, China fails to enter the top 10 list of the core paper producing institutions. The top 10 countries or regions and institutions ranked by the number of producing citing core papers on sea ice change are listed in Table 1.2.7 and Table 1.2.8. The USA, England, and Germany and their research institutions appear at the top of the list, while China is missing from the list (Table 1.2.7 and Table 1.2.8). China’s two core papers (Table 1.2.5) focuses on the relationships between Arctic sea ice ablation and winter snowfall and between Arctic sea ice ablation and extreme weather events in the Northern Hemisphere. The main research institutions in China are the Chinese Academy of Sciences and Beijing Normal University.

In general, the USA, Germany, and England have published the largest number of core papers. Although the number of core papers published by Chinese scientists ranks within the top 10 and the papers are cited frequently (Table 1.2.5), there is no leading research institution on the list. Korea and Japan also ranked among the top 10 in several lists (Table 1.2.5, Table 1.2.6, and Table 1.2.8) given their close attention to research in this field. China’s research focuses mainly on the impact of Arctic sea ice on East Asia and China’s climate rather than the global climate. Therefore, even though more papers have been published, the number of publications that cited the papers about sea ice change is relatively small.

Moreover, there is close cooperation among the topranked countries or regions, which shows the importance of international cooperation in research in the Arctic. The

《Table 1.2.5》

Table 1.2.5 Major producing countries or regions of core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

《Table 1.2.6》

Table 1.2.6 Major producing institutions of core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

《Figure 1.2.3》

Figure 1.2.3 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

《Figure 1.2.4》

Figure 1.2.4 Collaboration network of the major producing institutions of core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

《Table 1.2.7》

Table 1.2.7 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

《Table 1.2.8》

Table 1.2.8 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Arctic sea ice and mid-latitude climate responses”

Chinese government has recently set the goal of becoming a maritime power. Maintaining and expanding interests in the Arctic is an important part of becoming a maritime power. It requires serious scientific research and surveys in the Arctic as well as abundant funds. China needs to build and consolidate bilateral and multilateral international platforms to promote multi-level communication and cooperation in this field.

1.2.3 Applications of ligand-based composite adsorbent in water treatment

Heavy metals are a group of harmful pollutants in the water environment. Due to their persistence and bio-enrichment and bio-accumulation effects, heavy metals pose threats to the environmental and ecological health. Wastewater discharge containing excess heavy metals into surface water or groundwater leads to pollution in the aquatic and soil environments. Through intake and enrichment by plants and animals, heavy metals could be transferred to the human body via water and food, which increases cancer risks. With growing concerns over public health and the ecological threat induced by heavy metals, the maximum allowable regulated contaminant levels for heavy metals in wastewater, drinking water, and natural waters are becoming increasingly stringent worldwide. Such stringency in standards and water shortages underscore the need for the complete removal of heavy metals from water to ensure drinking water safety, protect the aquatic/ecological environment, and improve water utilization efficiency. Adsorption is one of the most efficient technologies to remove heavy metals from water due to its easy operation and low cost. Moreover, various types of reactors can be designed for adsorption to fulfill the practical demand of different scales, from point-of-use (POU) to full-scale applications.

The development of efficient adsorbents is the key to improving adsorption technologies. Complexation is an important type of specific interaction between heavy metals and organic/inorganic ligands. Through the immobilization of ligands onto adsorbents, various ligand-based composite adsorbents have been developed for selective removal of heavy metals from water. On the one hand, efforts have been made to design and synthesize ligands with high selectivity and high affinity for heavy metals. On the other hand, the adsorbents of multiple functions, such as optical sensing, photocatalysis, and antibacterial features, have been developed.

This research focus covered 40 core papers, which have been cited 34.83 times on average; 40.00% of these are consistently cited papers, showing a rising trend. Thus, these articles have a high impact on the related studies. The average publication year of these articles is 2014.08, indicating rapid citation and impact (Table 1.1.1). The numbers of articles containing the topics “ligand,” “composite,” “conjugate,” “fibrous,” and “complex” are 28, 23, 18, 18, and 14, respectively. These articles are mostly on the removal of Cu(II), Pb(II), Cs(I), As(V), Pd(II), Ce(III), Co(II), and the lanthanides. Eight and seven core papers refer to the removal of Cu(II) and Pb(II), respectively. Five core papers study the removal of Cs(I), which is radioactive and present in nuclear waste. In addition, several core papers are about the multi-functional materials for separation of heavy metals, optical sensing, antibacterial features, and photocatalysis. The ligands of interest in those core papers mainly include N-substituted heterocycles, S-containing ligands, azo-based ligands, Schiff bases, crown ethers, and molybdophosphates. Most ligands are of complicated structures; three typical structures are listed in Figure 1.2.5. Ligands (a) and (b) could possess visible light absorption or fluorescence feature due to their large conjugation structures. Ligand (c) could selectively sequester Cs(I) through size effect. These ligands are immobilized on hosts such as mesoporous silica to synthesize the ligand-based composites as adsorbents, which can be separated from water after adsorption.

Among the 40 core papers, 25 (62.5%) were published in Chemical Engineering Journal, indicating the popularity of this topic. Japan contributed the largest number of articles (31 core papers) within the list of major countries or regions (Table 1.2.9), accounting for 77.50% of core papers and 85.49% of total citations. Saudi Arabia, Egypt, Bangladesh, and India contributed 5 to 12 core papers, and the other countries or regions published only one core paper. Among the top 10 institutions (Table 1.2.10), Japan Atomic Energy Agency ranked first with 27 core papers, which

《Figure 1.2.5》

Figure 1.2.5 Three typical structures of ligands studied in the core papers regarding ligand-based composite adsorbents

《Table 1.2.9》

Table 1.2.9 Major producing countries or regions of core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”

《Table 1.2.10》

Table 1.2.10 Major producing institutions of core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”

accounted for 67.5% of core papers and 75.33% of total citations. King Saud University and National Institute for Materials Science contributed 12 and 8 core papers, respectively. The other institutions in Table 1.2.10 contributed 2 to 5 core papers.

A total of 30 out of the 40 core papers were contributed by M. R. Awual from Japan Atomic Energy Agency. M. R. Awual published 26 of these as the first author and corresponding author, two as the corresponding author, and two as co-author. The 30 core papers have been cited 1176 times, amounting to 39.20 times per article on average. Moreover, among the 436 papers citing the above 30 core papers, 49 were from the same author and 387 were published by other researchers, indicating the leading position of author M. R. Awual in the present research focus. Among the 12 core journal articles of other corresponding authors, four were contributed by M. Naushad from King Saud University, three by S. A. El-Safty from the National Institute of Materials Sciences in Japan (of which two were with M. R. Awual as co-author), and three were contributed by D. Pathania from Shoolini University in India. Yuxiang Li from Southwest University of Science and Technology in China and A. Kumar from Shoolini University in India both contributed one core papers. It can be seen from their cooperative history (not limited to the core papers) that M. Naushad and M. R. Awual cooperatively published 11 articles in 2015—2016, S. A. El-Safty and M. R. Awual cooperatively published 10 articles in 2011—2013, while D. Pathania and M. Naushad cooperatively published 17 articles in 2014—2017. The cooperative networks are shown in Figure 1.2.6 and Figure 1.2.7. The role of the other countries or regions and institutions in the figures is mainly limited to participating in the above core papers. The distribution of the main contributing countries or regions (Table 1.2.11) and institutions (Table 1.2.12) with regard to the papers citing the above core papers is similar to those of the core papers (Table 1.2.9 and Table 1.2.10), indicating the citations are primarily generated by the main contributing research groups, and the present research focus is being led by the institutions listed in this paragraph.

China contributed one core papers, the corresponding author being Prof. Yuxiang Li from Southwest University of Science and Technology. The paper focused on the removal of cesium (Cs) using a composite ion exchanger of silica matrix impregnated with ammonium molybdophosphate. According to the results of the literature search, hundreds of articles from China have been published on topics related to this engineering research focus. However, the number of citations is relatively lower than the 40 core papers, suggesting the possibly lower popularity of this engineering research focus. Nevertheless, the 40 core papers were selected through data mining using the keyword “clustering analysis,” which is subject to limitations of whether the related papers belong to this research focus. On the other hand, various ion-exchanger-based nanocomposites and chelating resins have been developed by Chinese institutions to remove heavy metals from water. Such composite materials have appropriate hydrodynamic properties for practical applications and

《Figure 1.2.6》

Figure 1.2.6 Collaboration network of the major producing countries or regions of core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”

《Figure 1.2.7》

Figure 1.2.7 Collaboration network of the major producing institutions of core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”

《Table 1.2.11》

Table 1.2.11 Major producing countries or regions of core papers that are cited by core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”

《Table 1.2.12》

Table 1.2.12 Major producing institutions of core papers that are cited by core papers on the engineering research focus “Applications of ligand-based composite adsorbent in water treatment”