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Cellulose (2021) 28:9857–9871
c Fig. 5 Clusters of keywords that appear with a frequency greater than 5 ( a ), and evolution of such keywords over time in the period 2005–2020 ( b ). The color scale indicates the frequency according to the year of publication. (For interpre- tation of the references to color in this figure, the reader is referred to the web version of this article)
seen as China is the country with most publications and citations. Table 1 comes to show that in China there are not only many researchers with an interest in the topic, but also that they have authored many publications. Among the institutions, the Chinese universities of Zhejiang Sci-Tech University and Wuhan University stand out, with several authors on the list. Japan is also present in the table, and the presence of Spain and Finland is also noteworthy. The two latter countries count with some very prolific authors, suggesting that in these countries publica- tions originated from a few research groups; the lack of out-standing authors in the USA or India seems to suggest the opposite, that is, publications originated from many disperse research groups. Figure 4 presents the nine Scopus- and JCR- indexed journals with most publications in this area of study. The journals Carbohydrate Polymers , Journal of Applied Polymer Science and Cellulose constitute the top three journals with most publica- tions, with more than 15 each, followed closely by Journal of Membrane Science , with more than 10 publications. The aforementioned journals belong to the first quartile of the area of study that encompasses the publications considered in the year 2019, except for Journal of Applied Polymer Science , which belongs to the second quartile, although it maintains a high h index, over 150 at the end of 2020 (SJR).
Their impact factors cannot be related to the number of publications, in general terms. In any case, in the same range of impact factors, the number of publi- cations is higher for the journal whose scope addresses the topic more specifically.
Analysis of keywords
A thoughtful evaluation of keywords implied revising carefully the data, manually discarding vague or unrelated terms. Likewise, synonyms were removed or merged. Figure 5 displays a network of the main keywords in publications dealing with cellulose- based coagulation and/or flocculation. The resulting map contains 169 terms, of which the most frequent are cellulose , fl occulation and coagulation . Each node in Fig. 5 presents a keyword in such way that terms in bigger rectangles are more frequent than those in smaller ones. In Fig. 5a, keywords are classified in groups (or clusters) attending to the relations among them, which are shown as lines joining terms that tend to appear in the same work. Broad lines mean a more frequent coexistence than thin lines. Thence 5 clusters were identified. Furthermore, the evolution of their relevance over the period of study (2005–2020) is depicted in Fig. 5b. It is not meaningless that the term fl occulation was found with higher frequency than coagulation . Although less frequent, water- soluble polyelectrolytes, including those derived from biopolymers such as cationic cellulose and cellulose sulphonate, may be capable of “direct flocculation” (Chong 2012; Lee et al. 2014), thus not needing a previous destabilizing step, i.e., not needing to rely on the coagulation-flocculation process.
Group 1 (red cluster): materials and target pollutants
The most remarkable keywords in the red cluster were cellulose (274 appearances) and coagulation (127). This is also the cluster in which most terms
Fig. 4 Journals with most publications on the use of cellulose as coagulant and flocculant, highlighting their impact factor (as of 2020)
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