3.4.1. Descriptive Analysis of High-Frequency Keywords
Keywords are the high-level summarization and refinement of the article core [
78]. There are 3545 keywords provided by the authors, and 67 keywords meet the threshold of 10 occurrences. Of the keywords, 2835 only appear once, accounting for 80% of the total amounts, which reflects great diversity of the research field. The most frequent keyword, “recycling”, occurs in 343 articles, as it is included in the searching keywords in the data collection process. To eliminate the interference of repeated keywords and more accurately identify research hotspots, we exclude the homogeneous keywords that are highly similar to our search keywords, as well as the overly broad concepts concerning environmental studies. The excluded keywords and their occurrences are as follows: “recycling” (343), “waste management” (108), “waste” (88), “household waste” (69), “municipal solid waste” (50), “solid waste” (36), “solid waste management” (30), “management” (27), “waste recycling” (25), “household” (22), “analysis” (19), “environment” (18), “environmental” (17), “waste collection” (17), “household recycling” (16), “household solid waste” (15), “households” (15), “survey” (14), “municipal waste” (12), “municipal solid waste management” (10), and “recycle” (10). Some other keywords with similar meanings are merged: “sustainability” and “sustainable development”; “food waste” and “organic waste”; “recycling behavior”, “recycling behaviour”, and “behaviour”; “life cycle assessment”, “LCA”, and “material flow analysis”; “WEEE”, “e-waste”, and “electronic waste”; “source separation”, “waste separation”, “separation”, and “waste sorting”; “plastic waste”, “plastics”, and “plastic recycling”; “policy” and “environmental policy”.
Table 7 lists the high-frequency keywords after filtering. As can be seen from the list, the research field of household waste recycling has been extended to all other processes in the waste management chain, including waste generation (11 occurrences), waste collection (31 occurrences), source separation (57 occurrences), and disposal (11 occurrences). In this context, life cycle assessment, defined as “a technique to compile and analyze the environmental impacts involved in all stages of the product’s life cycle from raw material extraction stage to the disposal stage” [
79], is widely used in waste recycling studies.
Sustainable development and the circular economy are two important concepts that drive the studies of waste recycling. Household waste recycling as a part of municipal solid waste management is the foundation for a circular economy to achieve more waste prevention and better resource management. Even though various studies have discussed waste recycling in the context of the circular economy and sustainable development, the relationships between them are still blurred as these concepts have been diffuse since they were proposed [
30]
The keyword list also shows that personal recycling behavior is the most popular research object of the articles. The theory of planned behavior is widely used in relevant behavioral studies. Attitude and social norm are the most studied predictors of recycling behavior.
In all types of household waste, waste from electrical and electronic equipment (WEEE), food waste and organic waste, plastic waste, and packaging waste are mentioned the most by the authors. Waste reduction and reuse, as another two elements of the three Rs (reduce, reuse, recycle), have been discussed frequently with household recycling.
Despite the original keywords provided by the authors, our data also includes another type of keywords, namely keywords plus. Keywords plus are generated from the titles of an article’s references based upon a unique algorithm [
80]. Compared with authors’ keywords, keywords plus are more broadly descriptive but less comprehensive in presenting the content of a specific article [
81].
Figure 8 presents the yearly occurrences of the top 10 keywords plus terms. Household waste recycling has been treated mostly as a management issue, as the term “management” occurs a lot more than the other terms during the past 10 years. “Recycling behavior” draws little attention around 2010, but since then, the number of its occurrences has increased the most rapidly. On the contrary, “participation” is the only one of the 10 most relevant keywords plus terms that has declined in recent years.
3.4.2. Cluster Analysis of High-Frequency Keywords
Using multiple correspondence analysis, the conceptual structure maps of authors’ keywords and keywords plus are generated, and the keywords clusters are plotted in two-dimensional maps (
Figure 9 and
Figure 10). The closer the points representing each keyword are on the graph, the more similar the distribution of the keywords are, which means they co-occur in the articles more frequently. Moreover, the proximity of a keyword to the center point represents its popularity in the research field. Keywords around the center have received high attention from the research community, while those by the edge are less related to other research topics [
82].
In terms of authors’ keywords, we can infer from
Figure 9 that the keywords are grouped into two major clusters and three minor clusters:
(1) The first major cluster marked in red in the center of the graph involves most of the important concepts regarding the research field and is highly consistent with the topic of household waste recycling. “Behavior”, “attitudes”, and “willingness to pay” are the main research variables of this group of studies. “Life cycle assessment”, “material flow analysis”, and “reverse logistics” have also been emphasized in this cluster. Apart from the general sense of household waste, more specific types of waste, including food waste and electronic waste, have attracted considerable research interest.
(2) The second major cluster, which is marked in blue and located in the upper right part of the graph, is more related to studies on waste source separation.
(3) The first minor cluster, marked in purple on the left side of the graph, is dedicated to the study of traditional waste disposal methods, including landfill and incineration. Life cycle assessment (LCA) is a commonly used technique in these kinds of studies.
(4) The second cluster, marked in green on the right side of the graph, is mainly related to waste sorting, recycling behavior, attitude, and research carried out in China.
(5) The last minor cluster, marked in yellow on the bottom right, is mainly related to the reuse of plastic waste and the application of the theory of planned behavior.
Overall,
Figure 9 shows the distribution pattern of authors’ keywords, demonstrating that household waste recycling, as the core research topic, is surrounded by other related topics such as source separation, reuse, and final disposal.
The co-occurrence of the keywords plus is more uniformly distributed. As can be seen from
Figure 10, five clusters with similar scales are generated using multiple correspondence analysis:
(1) The first cluster, marked in red, focuses more on the technical and engineering aspects. High-frequency keywords that dominate this cluster are life cycle assessment (LCA), incineration, recovery, systems, and energy.
(2) The second cluster, marked in blue, focuses on electronic waste (WEEE) and sustainability. The research areas of the studies in this cluster are mainly China and other developing countries.
(3) The third cluster, marked in green, is dedicated to household solid waste management. Collection, consumption, willingness to pay, performance, and disposal are important topics in this group of studies.
(4) The fourth cluster, marked in yellow, is related to numerous studies that apply the theory of planned behavior. This popular theory in the field of pro-environmental behavior is not only applicable to recycling behavior but also widely used in studies of source separation.
(5) The last cluster, marked in purple, is mainly related to concepts and topics under social psychology. Top contributing keywords in this cluster are motivation, norms, incentives, attitude, program, and determinants. The United Kingdom (UK) also appears in this cluster, indicating that many of the studies from the perspective of social psychology are contributed by British scholars.
3.4.3. Thematic Evolution Analysis
The clusters of keywords obtained from the co-word analysis are considered as themes of our research field [
83]. We further created a thematic map (
Figure 11) based on co-word network analysis and clustering. The thematic map, also referred to as the strategic diagram [
84,
85], describes two parameters (“centrality” and “density”) that characterize the themes in a two-dimensional space. Centrality measures to what extent a network interacts with other networks. The centrality of a theme represents the strength of its external connections to other themes and can be used as an indicator to measure the influence of the theme in the entire research field. Density measures the strength of internal ties among all the keywords within a theme. Therefore, the density of a given theme represents its development.
Based on their centrality and density, the themes distributed in four quadrants in
Figure 11 are defined by the following four categories [
83,
84,
86,
87]:
Motor themes in the upper-right quadrant. Such themes are both fully developed and vital to the research field.
Specialized and peripheral themes in the upper left quadrant. Given that these themes have a relatively higher density but lower centrality, they are isolated and have limited influence on the field despite their distinctive internal development.
Emerging or declining themes in the lower left quadrant. The themes of this category are weakly developed and marginal to the research field.
Basic and transversal themes in the lower right quadrant. They are not yet fully developed, but they have a very important position in the field of research.
According to the strategic diagram generated using authors’ keywords from our data, we can observe eight main themes with different levels of density and centrality. What stands out is that different types of waste differ greatly in their positions in the figure. Electronic waste (e-waste), belonging to the basic and transversal themes, has a much higher centrality but lower density compared with other types of waste. Greywater has the lowest centrality, implying that this specialized theme is relatively peripheral and marginal in our field of research. The theme represented by “circular economy”, “life cycle assessment”, and “material flow analysis” has its centrality and density both above the average line. However, the upper right part of the strategic diagram is still vacant, indicating that motor themes still need to be found in future studies.
In the last part of this section, we perform thematic evolution analysis and map the results with a Sankey diagram (
Figure 12). The Sankey diagram helps us clarify the quantity and direction of thematic flow and conversion relationships between the themes [
88].
The timespan of our data is evenly divided into three slices with two cutting years, 2000 and 2010. The nodes in the diagram represent the main research themes generated form the co-word network analysis in each time slice. The text labels next to the nodes indicate the core keywords of the themes as well as the time slices. The number of keywords included in each theme is represented by the size of the corresponding node. Themes from adjacent time slices are connected by streamlines when they share the same keywords. The width of the streamlines is proportional to the number of keywords shared by the connected themes and indicates the relevance between them.
From
Figure 12, we can see that as the research progresses, the pattern of research themes has gradually shifted from decentralization to uniformity. In the early stage (1991–2000), various themes are dominated by specialized and unrelated keywords, such as “incinerators”, “chemical composition”, “contingent valuation”, “incentives”, and “behavior”. In the second stage (2001–2010), the methodology of life cycle assessment and the theory of planned behavior have been established, developed, and widely used in the research field. Under the theme of “management”, numerous interdisciplinary studies have emerged from isolated themes, including “contingent valuation”, “incentives”, “solid waste”, “behavior”, “attitudes”, and “socioeconomic based survey” in the first stage. In the third stage (2011–2020), the comprehensiveness of the research field is further enhanced, as different themes from the former time slices merge again into new themes.