Analysis of Research Hotspots in the Field of Reverse Osmosis Desalination

. Reverse osmosis technology in the field of seawater desalination has become a hot topic in recent years. In this study, 2,507 literatures in WOS database from 2013 to 2022 were analyzed using Citespace visualization technology. It is found that the number of literatures published and their citation frequency on reverse osmosis desalination are on the rise globally. At present, research hotspots mainly focus on forward osmosis, interfacial polymerization, draw solution, reverse osmosis, brine discharge and boron removal. Future research hotspots will focus more on ammonia carbon dioxide, layer, draw solution and economic analysis. Through bibliometric analysis method, this study provides a quantitative review of relevant literature for the development of reverse osmosis desalination field, and has an in-depth understanding of the development and evolution of this field.


Introduction
Water is a precious natural resource for human beings, but the fresh water resources of the earth only account for 2.5% of the total water. Of this, more than 70 percent is frozen in the ice caps of the North and South Poles, and 87 percent is inaccessible due to inaccessible mountain glaciers and frozen snow. The only real freshwater resources that can be used are rivers, lakes and groundwater, which account for about 0.26 percent of the Earth's total water. Nine countries account for 60% of the world's freshwater resources, while 40% of countries and regions, accounting for 80% of the world's population, are seriously short of water. By 2025, it is expected that 3 billion people worldwide will be short of water, involving more than 40 countries and regions [1] . In the 21st century, fresh water has become a precious and scarce resource. Freshwater resources are not only a resource problem, but also a major strategic issue related to the sustainable development and long-term stability of a country's economy and society.
Seawater desalination is a method to convert seawater into fresh water by desalting technology, which plays an important role in increasing the total amount of water resources, improving the structure of water resources and enhancing the security of water resources. It is a key strategic choice to deal with the crisis of fresh water resources [2] . Desalination technology has the advantages of not flooding the land, not migrating, not competing for water and not being affected by climate. At the same time, it also has the characteristics of stable water supply, high-quality water source and protection from climate impacts, which is conducive to the harmonious development of man and nature [3] . As an alternative and incremental technology of fresh water resources, seawater desalination is more and more recognized and supported by countries all over the world. It is regarded as one of the feasible solutions and important ways to deal with the global water shortage in the future. At present, the mainstream technologies used commercially in seawater desalination include reverse osmosis (RO), low temperature multi-effect (MED) and multistage flash (MSF) [4] . Among them, reverse osmosis technology has become the leading technology in the current seawater and salt water desalination market because of its low construction cost, low energy consumption and better adaptability [5] .
Reverse osmosis technology, also known as reverse osmosis technology, is a technology that uses pressure difference as a driving force to separate solvent from solution through membrane separation operation. For treating seawater, the reverse osmosis technique allows fresh water to be obtained on the low pressure side of the membrane and water with a higher concentration on the high pressure side. At present, the research of reverse osmosis technology for seawater desalination at home and abroad mainly focuses on three aspects, the first is the research of reverse osmosis desalination membrane technology; The second is for the high pressure pump research; The third is the research of energy recovery device [6] . On this basis, this paper reviews the research hotspots of reverse osmosis technology in the past ten years, and has an in-depth understanding of the research hotspots of reverse osmosis technology in seawater desalination, which helps us to master the frontier dynamics and future development direction of the technology, and then guide the research and application practice of seawater desalination technology [7] . In addition, the exploration of the research hotspot of reverse osmosis technology, but also help to promote the exchange and cooperation between academia, accelerate the further improvement and development of technology, improve the efficiency and sustainability of seawater desalination. In the context of global freshwater resource shortage, in-depth research on reverse osmosis technology and its hot issues has important academic and social significance for solving the problem of global water shortage, ensuring safe drinking water for human and promoting sustainable development [8] .

Data Collection
The core collection of Web of Science (SCI, SSCI, A&HCI, ESCI, CPCI-S, CPCI-SSH, IC and CCR-EXPANDED) is taken as the data source, advanced retrieval is used, the language type is all languages, and the document type is "Article". Search on the topic "reverse osmosis seawater desalination" (TS=(" reverse osmosis seawater desalination ")), The time span is set as a custom year range 2013-2022. The search date was 2023-03-12, and a total of 2507 literatures were retrieved.

Visualization Analysis
In this study, CiteSpace (version 6.1.R6) was used to conduct a visual analysis of 2507 literatures from 20 years to 2022. The time span was 2013-2022, and the interval was 1 year. Country/regional cooperation network analysis, keyword network analysis, keyword cluster analysis, and outburst word analysis. The data selection was Top25(top 25 keywords, countries, institutions, etc., in terms of annual quantity), and the data selection threshold of cooperation network analysis of research institutions was c=2, cc=2, ccv=20(citation frequency of literature was 2). Select the clipping mode of Pathfinder, cut and cut the Network structure to highlight important features, and select Cluster Viewstatic and Show Merged Network to display the visualization analysis mode to present the final analysis results. The research analyzes the hot spots and trends from the time analysis of the number of articles published and cited frequency, the cooperation network analysis of countries/regions and institutions, and the analysis of keywords and emergent words.  Through the Citespace analysis of the issuing countries/regions and research institutions (Table 1), the main research countries/regions (FIG. 2) and institutions (FIG. 3) in the field of seawater desalination reverse osmosis can be obtained and their main cooperative relationships can be understood. A total of 88 nodes and 331 links were obtained, indicating that at least 88 countries were involved in the research in this field. Although China ranked first in the world in the number of articles published in the past 10 years, its centrality was only 0.08, indicating less cooperation with China and other countries in this field. While the United States, though less than China in the number of articles published, had the highest centrality among all countries. Australia, which ranked fifth in the number of publications, also had a centrality of 0.16, much higher than China. China still needs to strengthen international exchanges and cooperation with relevant research institutions in the United States, Australia and other countries in the field of seawater desalination reverse osmosis research.

Keyword network and emergent word analysis
Select the node type as Keywords in Citespace software, merge the similar/synonymous keywords in the obtained results, and cluster the keyword network map. Finally, two indexes of module value Q and average contour value S were obtained according to Citespace to evaluate the effect of the map (Q value is 0.4~0.8, which can be regarded as meeting the requirements; The S value is 0~1, the higher the value, the clearer the research topic within the cluster [9] ). After analysis, Q value of the keyword clustering map is 0.5057, S value is 0.7147, and the research topic within each cluster is clear, indicating that the relationship map meets the requirements. The high frequency keywords mainly have "reverse osmosis", "seawater desalination", "forward osmosis", "optimization", "energy", "removal", "recovery" and "membrane "Cut", "nanofiltration" and "energy consumption", etc. Based on this, the main research topics in the field of reverse osmosis desalination can be divided into 6 themes ( Table 2). brine discharge brine discharge, water bodies, water quality, discharge of waste liquids, # 6 boron removal reverse osmosis, energy production, irrigation water, boron removal, oilfield produced water, precipitation processes Emergent words are a method of keyword analysis of Citespace, referring to the terms that appear suddenly in a period of time and have a fast frequency growth rate, reflecting the dynamic changes of research in this field, exploring the development and research trend of a certain discipline [10] . Through the analysis of the main outburst words of seawater desalination reverse osmosis technology, it can be found that, ammonia carbon dioxide, layer, draw solution, draw solution and economic analysis have longer duration and higher research heat than other breakout words. renewable energy, energy efficiency and temperature are the current hot research words, which are the latest research frontiers and hot fields.

Conclusion
Based on the network information visualization analysis of Citespace software, this paper analyzed the number of publications and citations in the research field of reverse osmosis seawater desalination in the past ten years, systematically understood the countries and institutions studied, and clarified the current research hotspots and key word connections of root architecture through keyword analysis. Based on the above bibliometric research, it is shown that: • Research on reverse osmosis desalination has been a hot field in the past ten years. Although the number of published papers in 2018, 2019 and 2022 dropped slightly, the overall number of published papers showed an increasing trend, indicating significant research achievements in this field.
• Keyword clustering can be divided into 6 fields: forward osmosis, interfacial polymerization, draw solution, reverse osmosis, brine discharge and boron removal. In the future, the in-depth research in this field can focus on the above topics, looking for innovation points and research breakthrough points in the field. According to the analysis of breakout words, "ammonia carbon dioxide" is the main research direction of reverse osmosis desalination, which has a long duration and a high breakout intensity. And "renewable energy" is the key word lasting up to now with high emergent intensity. Researchers can focus on both areas.