## 3 HCL Polystyrene Based Nanocomposites
### Nanocomposites Containing Magnetic Nanoparticles
The obtained HCL nanocomposite materials, characterized by SSA close to 1150 m\({}^{2}\)/g, an average pore size 3.3 nm, and mesopore/micropore volume ratio of about 3.7, showed fast adsorption kinetics and a large adsorption capacity towards nitrofurazone (180 mg/g) and oxytetracycline (200 mg/g). For these systems, the reusability was also investigated, evaluating the decrease of their adsorption capability toward tetracycline after several adsorption/desorption cycles. Results showed that nanocomposites that were obtained by resins with wide pore structure (average pore size 8.4 nm) had only a 4.94% decrease in adsorption efficiency after ten reuses, whereas nanocomposites that were obtained by resins with a narrow pore size (3.3 nm) displayed a significant decrease, about 23% in adsorption capacity, after the same number of cycles. This phenomenon was ascribed to accumulation of target molecules in small pores that caused pore blockage. [PERSON] et al. [82; 83] obtained HCL resins loaded with preformed magnetite nanoparticles by simple stirring the commercial HCL polystyrene Diapak P-3 (BioKhimMak ST), previously dissolved in methanol, with the nanoparticles in an electromechanical vibromixer for 40 min. The so obtained magnetic adsorbent was then separated by the application of an external magnetic field, washed, and dried. Through this method, magnetic nanocomposite sorbents containing up to 30 wt % of magnetite were obtained. Even at the highest nanofillers loadings, the adsorption capability of the nanocomposite towards oxytetracycline was negligibly affected. Nevertheless, increasing the relative amount of nanofiller, a gradual detrimental effect on the degree of desorption of oxytetracycline using a methanol/acetonitrile solution was recorded. Using the same conditions, the system containing 5 wt % of magnetite showed a degree of desorption of 95%, whereas this value decreased to 50% for the sample  
Figure 6: (**a**) TEM micrograph of Fe\({}_{3}\)O\({}_{4}\) nanoparticles embedded in a fragment of commercial hyper-crosslinked polystyrene MN200 (Purolite Int., UK); (**b**) histogram and differential function of magnetite nanoparticles size distribution (200 particles from 12 nanocomposite fragments). Reprinted by permission from [PERSON] (Berlin, Germany) [76], copyright 2014.  
containing 30 wt % of magnetite. This phenomenon was attributed to the oxytetracycline/magnetite interactions [82]. The same materials were also tested for the removal of sulfonamides from river water and milk samples, showing a recovery efficiency >85% [83]. In general, the approach based on the in-situ synthesis of magnetic nanoparticles showed a better distribution of the nanoparticles within the porous structure of the resins, with no relevant agglomeration phenomena evidenced. This is clearly shown in Figure 5. On the contrary, the ex-situ approach revealed in some cases agglomeration phenomena of Fe\({}_{3}\)O\({}_{4}\) nanoparticles on the surface of the resin beads [77]. For what concerns the large-scale applicability of these systems, even if some results were reported on their reusability and fouling resistance [80], their long-term efficiency still needs a deeper evaluation. Moreover, recently, [PERSON] et al. [84] developed magnetic polystyrene/ethylene diamine tetraacetic acid (PS/EDTA) magnetic microsphere. The maximum adsorption capacity of these systems towards tetracycline was estimated at 166 mg/g. Even if the obtained materials are not based on HCL PS, they are to be mentioned because, in this case, magnetic Fe\({}_{3}\)O\({}_{4}\) nanoparticles were embedded in preformed PS/EDTA microspheres by direct mixing with ferric ammonium oxalate followed by a solvothermal treatment, an approach still unexplored for polystyrene based porous systems.