# Enhanced Specific Mechanism of Separation by Polymeric Membrane Modification--A Short Review
## 5 Adsorptive Membranes
### Selective Properties  
One of the significant advances of adsorptive membranes is the flexible decoration of their surface by several types of adsorbents. The adequately chosen adsorbent is decided regarding the particular selective properties of AM. For AM preparation, the organic and inorganic adsorbents could be applied. AMs have found great application in removing ions from aqueous solutions, especially heavy metal recovery. Polyethene films by radiation grafting of acrylamide had 6.2 mmol/g adsorption ability to Hg\({}^{2+}\) and could reject 99% Hg\({}^{2+}\) of a 200 ppm solution [84]. The grafted hyper-branched poly(amidoamine); HYPAM) in 0.45 m polytetrafluoroethylene (PTFE) microfiltration membrane obtained a maximum adsorption capacity of 1.49 g/m\({}^{2}\) to Cu\({}^{2+}\), 37.4% rejection of 50 ppm Cu\({}^{2+}\) solution, and pure water flux of 2556 L/m\(\cdot\)h\(\cdot\)bar [85]. The next example of the grafted porous membrane is the porous chitosan membranes with immobilised histidine. This membrane had the maximum adsorption capacity for Cu(II) ion of 3.0 mmol/g [86]. Furthermore, the incorporation of Cibacron blue F3 GA in different polymers was used to increase the selective properties of AMs. The microporous poly(vinyl butyl) membranes immobilised Cibacron blue F3 GA had adsorption capacities of 16.8 mmol/m\({}^{2}\) for Zn\({}^{2+}\), 7 mmol/m\({}^{2}\) for Cu\({}^{2+}\), 34.2 mmol/m\({}^{2}\) for Pb\({}^{2+}\), and 22.2 mmol/m\({}^{2}\) for Cd\({}^{2+}\), and could reach adsorption equilibrium in 15 min [87]. [PERSON] et al. reported that PVDF adsorptive membranes grafted with caffe acid had an adsorption capacity of 0.23 mmol/g for Cs\({}^{+}\) and the selectivity factor of 8.46 for Cs\({}^{+}\) versus Li\({}^{+}\)[88]. The next series of membranes are used for fabrication the inorganic adsorbents. The mainly used adsorbents are Al\({}_{2}\)O\({}_{3}\)[89; 90], ZnO [91], TiO\({}_{2}\)[92], Fe\({}_{2}\)O\({}_{3}\)[93], manganese oxide (spinel-type) [94], montmorillonite [95], zeolites [96], Co-Fe\({}_{2}\)O\({}_{3}\)[97], activated carbon and their modifications, nanotubes [98], and graphene oxide [99]. Figure 6: Types of adsorptive membranes. Reproduction from [75]. The MMM prepared with zeolite had the best adsorptive capacity for heavy metal ions. The PSF/zeolite MMM had adsorption capacities of 682 mg/g for Pb\({}^{2+}\) and 122 mg/g for Ni\({}^{2+}\), and the removal efficiencies of 91% Pb\({}^{2+}\) from a 100 ppm feed and 42% Ni\({}^{2+}\) from a 50 ppm feed [96]. The MMMs prepared with HMO and montmorillonite had a better adsorptive capacity to heavy metal ions. The PES/HMO MMM had an adsorptive capacity of 204 mg/g for Pb\({}^{2+}\). The chitosan/montmorillonite membrane cross-linked by glutaraldehyde possessed the maximum capacity of 193 mg/g for palladium(II) at optimum pH 2 and was able to reach adsorption equilibrium in 90-100 min. [95]. The PSf/GO MMM had the maximum adsorption capacities of 79 mg/g for Pb\({}^{2+}\), 75 mg/g for Cu\({}^{2+}\), 68 mg/g for Cd\({}^{2+}\), 17.5 mg/g for Ni\({}^{2+}\), and 154 mg/g for Cr\({}^{6+}\). Therefore, the PSF/GO MMM had a rejection of 90-96% to the ions mentioned above and a pure water flux of 15.42 L/m\(\cdot\)h\(\cdot\)bar [99]. The particular properties strongly depend on the applied adsorbent, and AMs could be found in an extensive range of selective processes to transport a particular rejection element. Furthermore, the particular properties could be associated with the sorption mechanism based on the EDLs and sorption in micro- and mesopores. Water is a significant natural resource for humans. As such, wastewater containing heavy metals is seen as a grave problem for the environment. Currently, adsorption is one of the standard methods used for both water purification and wastewater treatment. Adsorption relies on the physical and chemical interactions between heavy metal ions and adsorbents.