*3.5. Filtration Behaviors of MG-, MM- and GG-Blocks*

Although the monomers M and G are isomers, MG-, MM- and GG-blocks displayed completely different filtration behaviors. As shown in Figure 7, the severest fouling happened in filtration of MG-blocks, and the least flux decline was observed during the filtration of MM-blocks. GG-blocks showed a fouling tendency between those of MG-blocks and MM-blocks. All these results suggest that differences lying in the molecular structures of these blocks lead to their diverse filtration properties. Combined with the TEP determination results, it can be deduced that the more TEPs formed, the less fouling that occurred during membrane filtration. TEPs possessed large sizes and easily attached on a membrane surface to form a fouling layer, which in turn prevented the serious pore clogging of membrane [28]. Thus, the least membrane fouling was recorded in the filtration of MM-blocks which showed the largest TEPs. On the contrary, the MG-blocks, which possessed the smallest size, would penetrate into the membrane pores and be absorbed onto the membrane pore walls resulting with the reduced radius and number of the effective membrane pores. The fouling propensities of alginate blocks derived from the commercial alginate decreased at the same order: MG-blocks > GG-blocks > MM-blocks, as can be seen in Figure 7b,c. These results were further evident from the FESEM observations of the fouled membrane surfaces as can be seen in Figure 8. After filtration with MM-blocks, some of a fouling layer can be observed on the membrane surface while there was very little fouling layer development on membrane surfaces after they were filtered by MG- and GG-blocks. Instead, the membrane pores seemed to be filled by foulants (Figure 8b,d). As a consequence of these results, it was found that the more TEP formed, the less serious of a fouling problem was observed in the filtration process in this study. In this study, it was found that the TEP may work as a pre-filter to reduce the amount of alginate blocks that penetrated into membrane pores; thus, mitigating the membrane fouling. The TEP themselves certainly did not lead to serious membrane fouling. It depended on the sizes of membrane pores. Figure 6 showed the TEPs gently retained by 0.05 μm filters and then freeze-dried to keep the morphology of TEPs. It could be seen that TEPs formed from the aggregation of alginate blocks and had bigger sizes than single molecules of alginate blocks. The TEP formation was the result of the intermolecular interactions between these blocks. Accordingly, in this study we deduced that TEPs possessed large sizes and easily attached on a membrane surface to form a fouling layer which in turn prevented the serious pore clogging of the membrane. In addition, the TEP level indicated the extent of intermolecular interactions of polysaccharides, which was an important factor influencing membrane fouling.

**Figure 7.** (**a**) Flux profiles of MG-, MM- and GG-blocks at 50 mg/L; (**b**) the final flux decline percentage and the (**c**) initial flux decline rates of alginate blocks derived from commercial and extracted alginates.

**Figure 8.** FESEM observations of the (**a**) clean membrane and membrane surfaces fouled by (**b**) MG-, (**c**) MM- and (**d**) GG-blocks.

It appears that this conclusion is different from the general understanding of TEPs—that they accelerate the membrane fouling [31–33]. However, it should be noted that the role of TEPs in membrane fouling is not a black and white issue. TEPs are easy to attach onto a membrane surface and promote fouling layer formation on a membrane. For the non-porous membrane and the membrane with small pores, TEPs may promote fouling layer development during membrane filtration which contributes to the filtration resistance. Differently, it turned out that the fouling layer acted as pre-filter to prevent the serious pore blocking of membranes in this study, which on the contrary, did not lead to serious fouling problems. Consequently, the TEP fouling is a complicated issue which also depends on the characteristics of membrane and the operation conditions and so on. Nevertheless, with high viscosity and big size, TEPs are prone to attach onto membranes compared to other foulants. More importantly, the TEP formation indicates the potential of polysaccharide crosslinking, which is an important issue in membrane fouling. These findings reveal that the molecular chains of polysaccharides do not exist singly, by themselves in a free state, but they usually combine together to form big three dimensional networks. The crosslinking abilities of polysaccharides vary from one to another dramatically, which in turn affects their fouling propensities. Combined with the results obtained in this study, it can be concluded that polysaccharides with different molecular compositions would have different characteristics, including their effects on membrane fouling. Therefore, future studies should pay attention to the molecule structures of foulant and the crosslinking between foulant molecules.

### **4. Conclusions**

Alginate is abundant in natural water environments and consists of three different blocks; i.e., MG-, MM- and GG-blocks. In this study, an alginate sample was first extracted from the raw seaweed and then well characterized with fractionation, FTIR and FESEM. Results confirmed the existence of alginate in the seaweed and showed that the main component in this alginate was the MM-blocks. FTIR spectra of extracted MG-, MM- and GG-blocks illustrated a similar chemical composition with commercial alginate blocks. What is more important, is that the TEP measurement showed that MG-, MM- and GG-blocks possessed different abilities in forming TEP via molecular crosslinking. Furthermore, the filtration tests of the three kinds of alginate bocks demonstrated totally different flux

profiles, indicating that they do have diverse fouling propensities. So far, many studies have employed alginate as a model foulant. However, our results reveal that alginates with different molecular compositions behave differently in membrane filtration experiments. It is a reasonable consideration that future work should include analysis of the alginate in question's molecular composition in order to obtain more reliable results.

**Author Contributions:** S.M. and L.W. conceived and designed the experiments; R.W. and S.M. performed the experiments; R.W. and X.M. analyzed the data; H.L. and M.Z. contributed reagents/materials/analysis tools; S.M. and L.W. wrote the paper.

**Acknowledgments:** This work was financially supported by grants from the National Natural Science Foundation of China (number 51808019) and this research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
