## 3 Applications of Crosslinked PEI
### Adsorption of Heavy Metals on a PEI Coating and Release
\begin{table}
\begin{tabular}{l l l l l l} \hline \hline
**PEI/Composite** & \begin{tabular}{l} **Crosslinker** \\ **Type** \\ \end{tabular} & **Analytes Removed** & \begin{tabular}{l} **Removal Efficiency** \\ **(mg g\({}^{-1}\))** \\ \end{tabular} & \begin{tabular}{l} **Adsorption Isotherm/** \\ **Kinetics** \\ \end{tabular} & **Ref.** \\ \hline PEI/cellulose & - & \begin{tabular}{l} La\({}^{3+}\) \\ Eu\({}^{3+}\) \\ Er\({}^{3+}\) \\ \end{tabular} & 84.5 & & & \\  & & \begin{tabular}{l} Eu\({}^{3+}\) \\ 120.3 \\ \end{tabular} & 101.8 & - & [74] \\ \hline PEI/diatomaceous earth & Glutaraldehyde & Cu\({}^{2+}\) & 46.9 & - & [75] \\ \hline Alginate/PEI composite & - & \begin{tabular}{l} Cr\({}^{6+}\) \\ \end{tabular} & 431.6 & - & [77] \\ \hline cellulose nanofibrils/ & - & \
\begin{tabular}{l} Cu\({}^{2+}\) \\ Pb\({}^{2+}\) \\ \end{tabular} & 175.44 & - & [78] \\ \hline Montmorillonite/sodium & & & & \\ alginate/carboxylylated & Ca\({}^{2}\) &
\begin{tabular}{l} Cu\({}^{2+}\) \\ \end{tabular} & 203.99 & - & [79] \\ \hline Chitosan/PEI aerogel & ECH & Cr\({}^{6+}\) & 445.29 & - & [76] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._fighting potential fouling [85]. However, the use of copper-based paints will likely increase the copper concentration in the marine water via the gradual leaching of copper from the paint, which has become a threat to the life of sea inhabitants. Copper concentrations that exceed 3.1 ppb (the U.S. federal standard) affect various life stages of marine organisms, including mussels, oysters, scallops, sea urchins, and crustaceans [86]. While the use of copper is necessary to avoid/minimize the impact of fouling, it should be performed in such a way that the impact of copper to the marine environment is avoided. This is possible through the controlled release and adsorption of Cu from the paint. In this regard, a study was conducted on Cu\({}^{2+}\) uptake in a PEI coating and its electrochemically initiated release on a need basis (Scheme 12). In the work, a thin film of PEI/HPMC was deposited on a conducting carbon cloth substrate using a spray coating technique. The coating was then crosslinked by immersing the coated material in a dilute glutaraldehyde solution. The HPMC additive is sparingly soluble in water, which leaches out from the coating upon keeping it in water to yield a porous structure. Copper ions were adsorbed on the coating by dipping the crosslinked-PEI-coated carbon cloth in an artificially prepared copper solution. Once copper adsorption was completed, the release of the stored Cu\({}^{2+}\) was investigated by electrochemical means by applying different current densities to the coating. The amounts of Cu\({}^{2+}\) released were measured to be 4.1, 11.8, and 19.9 mg cm\({}^{-2}\) L\({}^{-1}\) for 1, 2, and 5 mA cm\({}^{-2}\) of applied current, respectively. However, 20.6 mg cm\({}^{-2}\) L\({}^{-1}\) of copper was measured at an applied current of 15 mA cm\({}^{-2}\), which is almost similar to the amount of copper collected from 5 mA cm\({}^{-2}\) of applied current. The release of the adsorbed copper was also achieved by an acid treatment (2% HNO\({}_{3}\)), resulting in the release of 12.5 mg cm\({}^{-2}\) L\({}^{-1}\) copper within 20 min and 18.1 mg cm\({}^{-2}\) L\({}^{-1}\) after 24 h, which is slightly lower than the chronopotentiometric release at 5 mA cm\({}^{-2}\). The regenerated coatings were also reusable for the subsequent adsorption of copper ions. Scheme 12 shows the preparation of the crosslinked PEI coating, uptake of copper ions, and release of Cu\({}^{2+}\)[32]. In conclusion, the use of a thin coating of crosslinked PEI on a conducting carbon cloth/other substrate to adsorb and release ions in a controlled way has been reported in the literature. The idea of the electrochemical-triggered release of ions such as copper stems from the need to control the amount of copper that goes into the environment from antifouling coatings of marine infrastructure.