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knowledge gap, but also an opportunity arising from the unbalance between the excellent performance reported for these cellulosic materials, frequently hydrogels or membranes (Kanmani et al. 2017; Peng et al. 2020), and the uncertainty of their applicability in real wastewaters. In any case, looking at the evolution of these keywords with time (Fig. 5b), it is clear that research on most of these topics began before 2010. From this data, it is reasonable to infer that knowing the behaviour of cellulose in aqueous dispersions, even if this knowledge was directed towards sorbents and films (Syverud and Stenius 2009), boosted the research onto the synthesis of soluble cellulose derivatives and their use as flocculants. This concept is key in Group 2, which generally involves more recent insights.
related to Materials Science are included—among others, tensile strength, cellulose fi lms (24) , mem- branes (33), and fi bers . Membranes are significantly linked with ultra fi ltration (18), in which case those membranes can even discriminate by molecular weight. Interestingly enough, even the keywords solutions , aqueous solution and spinning are often framed in the context of the preparation of different materials, as can be easily seen from a search with the string TITLE-ABS-KEY ( cellulose AND coagulation AND ( aqueous solution OR solutions OR spinning )). Some of the most relevant and recent papers match- ing this requirement report the fabrication of permeable membranes (Makarov et al. 2021), pro- tein-repellent coatings (Bracˇicˇ et al. 2021), antimicrobial packaging (Oliva et al. 2020), and composite fibres with great tensile properties (Song et al. 2018). In these cases, coagulation is not conceived as a wastewater treatment, but as a step in the production process of films or other solid products from aqueous dispersions of cellulose. This also explains the finding of the name of several chemicals as keywords. Sodium hydroxide (17) and urea (22) constitute the most popular system for the dissolution or amorphization of cellulose in aqueous media, while sulphuric acid (10) generates nanocel- lulose dispersions through the hydrolysis of the least crystalline domains of fibers. Ethanol (10) is the most widely used anti-solvent in the regeneration of cellulose from its dispersed or dissolved form. The publications with environmental aims that fall into this group, being as valuable as they undoubtedly are, do not usually address real, complex systems. Generally speaking, they target individual pollutants such as methylene blue (Hossain et al. 2021) and different metallic ions including copper(II) (Maaloul et al. 2021), lead(II) (Li et al. 2020) and chromium (VI) (Wang et al. 2020). For such purposes, the contaminant is dissolved in distilled water or in aqueous buffer solutions. On one hand, this is necessary to attain a deep understanding of the physicochemical interaction between pollutants and cellulose. On the other hand, actual wastewater samples for any manufacturing industry do not share this simplicity. Unpredictable interferences come from the presence of many different compounds, the competition between the potential targets, and the difficulties of adjusting factors such as the pH and the ionic strength. Therefore, we may identify not only a
Group 2 (green cluster): fl occulation using cellulose derivatives
Whereas in Group 1 coagulation was not framed in coagulation-flocculation treatments as much as in procedures to prepare materials, Group 2 encom- passes publications which include fl occulation as an essential feature. Whereas in Group 1 we remarked the appearance of solutions and aqueous solution , in Group 2 we found suspensions . Whereas cellulose was highlighted as the key material for membranes, films and other materials, now we must notice the high frequency of cellulose derivatives (42) since, when it comes to flocculation, the insertion of cationic or anionic functional groups is extremely useful for the aims of charge neutralization, electro- static patching, and bridging (Koshani et al. 2020). Furthermore, pH-independent ionization, such as that achieved with quaternary ammonium groups, broad- ens the applicability of the potential flocculants. The appearance of fl occulation without coagula- tion in an article does not necessarily mean that coagulation was not involved. Generally speaking, coagulation and flocculation are found simultane- ously in particle aggregation processes. Often, coagulation is deemed a constant factor and only flocculation is evaluated and mentioned, even when in the case of study there are inorganic species that act as coagulants. Cellulose derivatives may be used as flocculants in the presence of inorganic compounds such as calcium chloride, ferric sulphate or
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