9866
Cellulose (2021) 28:9857–9871
general, for all ecosystems. Further research is necessary, thus, not only to identify greener alterna- tives to the functionalization with acrylamides with no major loss of performance, but also to broaden the spectrum of contaminants to be efficiently removed.
light scattering, i.e., an increase of transparency of the liquid phase. Moreover, legislation and standards on water quality make a prominent use of turbidity as a key indicator (Boyd 2015). However, the absence of pathogens is also a legal requirement in drinking water and, despite the usefulness of flocculation to generate microbial flocs that could be physically separated, the extent of disinfection is seldom reported. In this direction, particle size and chemical oxygen demand are undoubtedly helpful to monitor and evaluate the extent of flocculation, but as unspecific as turbidity . The three of them are quantitative indicators. Therefore, it is difficult to judge if cellulose-based flocculation, as a decontam- ination technique, is successful when it comes to the removal of emergent pollutants, which are increas- ingly raising concern among lawmakers. This bibliometric finding encourages the use of chro- matography coupled to mass spectrometry (or another kind of detector for qualitative purposes), which is available in most laboratories, to track the removal efficiency for each compound in a real system (Ma et al. 2021). Most likely, in the elimination of persistent contaminants, particle aggregation complements an advanced technique (Suarez et al. 2009), since wastewater treatment plants are typically unable to target such contaminants through filtration, flotation or sedimentation only. Instead, the pollutant may be adsorbed onto activated carbon (6), oxidized with ferric chloride (5), or exchanged with acrylic resins (7). In those cases, like when working with biological membranes or ultrafiltration devices, coagulation- flocculation generally precedes the advanced tech- nique (Zahrim et al. 2011), freeing those methods from suspended solids. Publications included in Group 3 tend to study more real wastewater streams, frequently industrial waste (5) or sewage (8), than those in the previous keyword groups. This is clearer in the most recent publications (Vuoti et al. 2018). As for the pollutants targeted, there is a wide variety of them, remarkably encompassing phenols (5), azo dyes (5) and metal ions (5), but not addressing pharmaceuticals and other emerging pollutants (PhEPs) which include pharmaceutical compounds and their metabolites, hormones, steroids, personal care products, etc., contained in water at concentrations below ppm level, with harmful effects on human health and, in
Group 4 (yellow cluster): biological applications
A relatively low number of publications featuring cellulose-based flocculation, or even where cellulose and flocculation are only tangentially related, show biological or biochemical terms: protein (6), enzyme (5), metabolism (11), nonhuman (17), human (7), bacteria (8) and animal (11), along with animal experiment . Less intuitively, ion-exchange chro- matography (5) must also be understood in the frame of biological macromolecules, since it has been applied as a high-performance separation technique for protein samples. However, that method is less commonly used in recent papers (Fig. 5b), giving way to affinity chromatography as a more selective technique. In these cases, a cellulose derivative, such as cellulose acetate phthalate, is used as microbicide or as another kind of biological agent for disease prevention or treatment of animal organs (Otten et al. 2005). Then, the role of flocculation across this cluster ranges from recovering an animal protein from an aqueous suspension (e.g., livestock waters) to synthesizing bacterial cellulose.
Group 5 (purple cluster): micro- and nano- technological applications
In what should be labelled as another minor cluster, many keywords are related to micro- and nanotech- nology: microscopy (5), micro fi ltration (6), microorganisms (7), nanocrystals (5), nanoparticles (7). In publications which belong to Group 5, interestingly, flocculation may be something to be avoided, rather than promoted. This is the case of Li et al.’s (2017) cellulose films with silver nanoparti- cles, whose antimicrobial potential is hindered by flocculation (Li et al. 2017). Ultimately, there is a wide variety of applications fitting this context. Nonetheless, the main one is harvesting microalgal biomass by means of floccu- lants, which is indicated by the appearance of harvesting (7), biofuels (6) and microalgae (6), three
123
Made with FlippingBook Online document maker