# Wastewater Treatment by Polymeric Microspheres: A Review
## 2 Synthetic Polymer  
As Figure 1 represents, microsphere adsorbents derived from polymers are fabricated to purify wastewater. Both synthetic and natural polymers are used for the preparation of microsphere adsorbent. Various synthetic polymers were used, such as graphene oxide, sodium silicate, polymeric resin. Summaries of the efficiencies for each adsorbent are listed in Table 1.  
A porous inorganic polymer microsphere was fabricated via suspension dispersion solidification method for removal of Pb\({}^{2+}\) or other heavy metal ions from wastewater [24]. SEM analysis confirmed that the microsphere had a diameter of around 100 \(\upmu\)m. It was added with a foaming agent for formation of pore structures, which enhanced the BET surface area from 79.80 m\({}^{2}\)/g to 100.9 m\({}^{2}\)/g and decreased the pore diameter to 7 nm. Due to its small particle size and large surface area, the microsphere had a high adsorption capacity of 629.21 mg/g for Pb\({}^{2+}\). In addition, adsorption reached equilibrium within 5 h due to the high adsorption rate and low diffusion resistance. During the adsorption, the adsorbates moved through the pores of the microsphere to the center, so all the active sites were used. The adsorption process followed the pseudo-second-order (PSO) model, indicating the chemical adsorption. It followed the Langmuir model and the D-R adsorption model, suggesting the monolayer's adsorption, which was due to a combination of chemical and physical adsorption. It could be easily regenerated by using EDTA-2 Na solution with 51% of regeneration ratio. Figure 1: Schematic representation of wastewater treatment. A multi-functional HGM@MPD-ALS adsorbent with inorganic core and surface modified with organic functional groups was fabricated to treat dyeing wastewaters [25]. The adsorbent was synthesized by co-grafting m-phenylenediamine (MPD) and sodium allyl sulfonate (ALS) on the surface of a hollow glass microsphere (HGM). Alkali corrosion increased the surface roughness and -Si-OH groups of HGM, and functional groups were grafted onto the surface of HGM. The obtained microsphere had amphoteric attraction abilities for anionic and cationic dyes, acid green 25 (AG25), and fuchsin (BF) dyes, respectively. The adsorption followed the PSO kinetic model and the Langmuir isotherm model. The maximum adsorption capacity for AG25 was 454.55 mg/g and 588.24 mg/g for BF: XPS peaks were analyzed to examine the adsorption mechanisms (which were electrostatic attraction, \(\pi\)-\(\pi\) stacking, and hydrogen bonds), which were assumed to be the interaction forces of adsorption. From the hollow structure of HGM, the material has self-floating ability; thus, adsorbents over 95.7% were adsorbed at water surface through a self-floating process within 12 min. This was very helpful for the solid-liquid separation process and recycling of adsorbent, which are very important adsorbent properties for a longer lifetime. Another organic-inorganic hybrid of \(\beta\)-cyclodextrintrin-graphene oxide aerogel microspheres (CD-GAM) was fabricated to remove dyes and organic micropollutants from water [26]. Figure 2 represents the schematic representation of removal of pollutants. CD-GAM was prepared via the electrospraying method combined with freeze-casting and successive mild self-assembly. The TGA curve of CD-GAM showed gradual slope of weight loss below 100 \({}^{\circ}\)C due to loss of water but had rapid loss near 300 \({}^{\circ}\)C due to CD loss. Adsorption for various adsorbates was possible due to graphene oxide and \(\beta\)-cyclodextrintrin that were integrated into aerogel microsphere. The adsorption capacities for dyes were 439 mg/g for MB, 388 mg/g for rhodamine B (RhB), 234 mg/g for acid red 87, and 167 mg/g for methyl orange (MO). Additionally, it showed great adsorption capacities for organic micropollutants aromatic model compounds and pharmaceuticals; they were 17 mg/g for