Cellulose (2021) 28:9857–9871
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Frank et al. 2018). Even in the absence of chemical modifications, researchers must take into account that it is the structure, not the origin of the raw material, what determines biodegradability (Witt et al. 1999). The isolation of the crystalline domains of cellulose, as in the production of nanocrystals, hinders biodegradability in comparison to materials that keep hemicellulose and amorphous regions, which are more prone to microbial attack (Barreto et al. 2010). All things considered, biodegradation studies benefit from discussion on the supramolecular structure of the cellulosic product proposed in each time, and saline, acidic and alkaline media should be consid- ered. Finally, we suggest assessing not only the biodegradability of cellulosic flocculants, but also the toxicity of the products that result from their aerobic or anaerobic decomposition (Vikman et al. 2015).
aluminium chloride (Liu et al. 2014; Peng et al. 2020), i.e., very well-known destabilizers. Other works of similar nature actually include coagulation in the list of keywords, in the abstract or in the title (Suopaja¨rvi et al. 2013; Liu et al. 2014). Noticeably, particle aggregation may involve Ostwald ripening (25) as mechanism. Dyes (10) are, generally speaking, very popular pollutants in studies seeking to estimate the perfor- mance of a flocculation system (Kono and Kusumoto 2015). However, while treating dye containing waters is necessary, the choice of dye as model pollutant has more to do with the ease of quantitative analysis, as its concentration can be simply calculated from the absorbance at a certain wavelength, than with a social or scientific demand for those dyehouse waters. Likewise, the abundance of studies on kaolin (20), which is not the most common filler in papermaking, probably arises from its strongly negative zeta potential, ensuring the successful application of cationic cellulose derivatives (Bratskaya et al. 2006; Aguado et al. 2017). Nevertheless, many articles framed in Group 2 do not study cellulose-based flocculants for decontami- nation purposes, but within the scope of the wet end of a paper machine, as additives to the pulp stock or, more precisely, retention aids (Li et al. 2015; Aguado et al. 2017). Thus, papermaking (17), one of the major keywords, is of utmost importance, and pulp , additives and retention aids are strongly related to it. While the main goals of flocculation in wet-end chemistry are filler retention and drainage rate, it also has a decisive impact on the composition of the filtrate or white water. Like any other effluent, this stream requires treatment before being discarded, but synthetic flocculants often persist after such treatment. While biodegradability is alleged as an advantage of cellulose-derived flocculants in 16 articles, only those from the research groups of Yao and Zhang, working with polyacrylamide-grafted cellulose, include biodegradability assays in their experimental section (Zhu et al. 2016; Chen et al. 2020). Most often, however, the biodegradability of cellulose derivatives is taken for granted ignoring the fact that many chemical modifications, such as silanization, cationization and oxidation, lower the biodegradation rate or narrow down the range of suitable conditions for microorganism activity (Kargarzadeh et al. 2017;
Group 3 (blue cluster): parameters of wastewater treatments
This keyword cluster is mostly characterised by the occurrence of water treatment (39), turbidity (38), polymers (33), pH (27), particle size (24), fi ltration (23), ef fl uents (19), pollutant removal (16) and chemical oxygen demand (15), among others. There- fore, publications falling into this cluster aim at water decontamination techniques involving the use of polymers. These polymers include carboxymethyl cellulose (10) and derivatives thereof, but this derivatization, once again, often implies the use of poorly biodegradable polyacrylamides (21) (Feng et al. 2020). It is worth clarifying, however, that this term is more commonly found as a conventional flocculant, chosen for comparison purposes, than as a derivatizing agent for cellulose (Vuoti et al. 2018). In fact, charged flocculants outweigh non-ionic floccu- lants all along this search. Since the most common cellulose-based agents are carboxymethyl cellulose (anionic), cationic cellulose, cellulose sulphonate (anionic) and combinations with acrylamides (anionic or cationic), it is common to find measurements of zeta potential (12). A critical approach to these keywords demands some discussion on turbidity , the most widely used parameter to indicate contamination or removal. This can be explained because the most obvious effect of particle aggregation, caused either by coagulation- flocculation or by direct flocculation, is a reduction of
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