# Notes on Common Misconceptions in Microplastics Removal from Water
## 3 Microplastics Removal from Water
### Coagulation/Flocculation  
Coagulation/flocculation is a chemical-based water treatment method used for the removal of dissolved or colloidal particles from water before phase separation (sedimentation, filtration). Coagulation is a process based on charge neutralization (the negative charge of suspended particles is neutralized by a positively charged coagulant), leading to the formation of insoluble aggregates (flocs). Flocculation is a process stage of coagulation at which gentle agitation is applied to boost the particles aggregation. This leads to the formation of large agglomerates that can already be well separated via sedimentation or filtration [37]. Several studies have focused on microplastic particles removal from water via coagulation. [PERSON] et al. investigated coagulation of PE microparticles combining with ultrafiltration (FeCl\({}_{3}\cdot\)6H\({}_{2}\)O and AlCl\({}_{3}\cdot\)6H\({}_{2}\)O used as coagulating agents). It was observed that aluminium \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \begin{tabular}{c} **Energetically** \\ **Expensive** \\ \end{tabular} & \begin{tabular}{c} **Applicable on** \\ **Particle Size** \\ \end{tabular} & \
\begin{tabular}{c} **Removal via** \\ **Separation** \\ \end{tabular} &
\begin{tabular}{c} **Removal via** \\ **Destruction** \\ \end{tabular} \\ \hline Coagulation/flocculation & No & mm-\(\upmu\)m & Yes & No \\ Degradation (e.g., photo) & Yes & mm & No & Yes \\ Separation/flotation & No & mm-\(\upmu\)m & Yes & No \\ Separation/filtration & No & mm-\(\upmu\)m & Yes & No \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Groups of methods applicable on microplastics removal. salts were more suitable for the removal of PE particles. The smaller the PE particles, the higher the removal efficiency. This was explained via the zeta potential values of flocs and PE microparticles--zeta potential of Al flocs was at positive values (0.52 \(\pm\) 0.14 mV), while zeta potential of PE microplastic particles was negative, at pH \(\geq\) 4. Therefore, easier capture of smaller microparticles in coagulated flocs and their consequent removal with separated sludge occurred. The highest removal efficiency achieved was 37% for particles smaller than 0.5 mm. However, this efficiency was increased to 61% after the addition of 15 mg/L of anionic PAM (polyacrylamideamide) [38]. At neutral pH, positively charged flocs were formed; anionic PAM therefore increased removal efficiency via charge neutralization. The removal efficiency increase caused by the addition of PAM was attributed to denser flocs formation, compared to the flocs formed without the addition of PAM. It resulted in increased adsorption capacity via stronger bridging effects and stronger sweep floculation. The same phenomenon was observed in another study by the same research group [39]. Subsequently, the PE particles were completely removed by ultrafiltration through a membrane (pore size of 30 nm), where the particles coagulated with aluminium salts slightly clogged the membrane. Another positive effect was observed by [PERSON] et al., where microplastics removal was enhanced by the presence of an anionic surfactant, but nonionic surfactant lead to worse removal efficiency [36]. In addition, a negative effect of nonionic surfactants was observed by [PERSON] et al. The authors determined that the presence of nonionic surfactants helps microplastic particles to \"stealth\" before aluminum flocs during the coagulation process [40]. Coagulation processes are often combined with sand filters and other water treatment techniques that enhance microplastics removal efficiency [28; 41]. It was described that coagulation/floculation methods might be sufficiently efficient in microplastics removal; however, no model solution studies are presented. These cases described real effluents treated in WWTPs (wastewater treatment plants). Membrane filtrations might be a very efficient method with absolute removals, but they have very limited capacity; further, the cost of membranes operation would be unbearably high.