# Wastewater Treatment by Polymeric Microspheres: A Review
## 3 Natural Polymer
### Alginate  
Algae that adsorb heavy metal ions are mostly used in the form of SA as adsorbents. Alginate is naturally abundant, can be obtained from seaweed, and is biodegradable and hydrophilic. It is modified in various ways; for instance, it can be mixed with zeolite and made into bead form. The efficiency of alginate as an adsorbent is summarized in Table 6.  
Ferriferous oxide@hydrous cerium oxide (Fe\({}_{3}\)O\({}_{4}\)@HCO) was encapsulated into sodium alginate microbead (SAB) by an improved co-precipitation method to remove antimony (Sb(III)) ions within a concentration range of 5-60 mg/L [56]. The magnetic microsphere can be easily retrieved from aqueous solution by a magnetic separation. The introduction of SA resolves the poor precipitation and easy hardening issues of prior Fe\({}_{3}\)O\({}_{4}\)@HCO adsorbent as SA easily complexes with divalent cations, which can make gel forms. The adsorption kinetics followed the PSO model and the adsorption isotherm followed the Langmuir model, indicating that the mechanism was the result of cooperation of chemisorption (ion exchange) and physisorption (diffusion reaction). Surface hydroxyl groups of SAB whose presence was confirmed by FTIR results were the major source for the adsorption. Initial and pH and temperature had an influence on adsorption performance, and microbeads did not collapse within a pH range of 3-7. The environmentally friendly and cost-effective main components (Fe\({}_{3}\)O\({}_{4}\) and HCO) and the facile preparation of SAB show the potential of the microsphere as adsorbent for Sb(III) in large scale. Similarly, a hyperbranched polyamide-functionalized sodium alginate (HA@SA) microsphere was prepared for the removal of Sb(III) and followed the PSO and the Langmuir models [57]. A HA@SA microsphere was fabricated by grafting 1.0 g of HA on the surface of SA microsphere. SEM image showed that the microsphere had average diameter of 3.0 nm and surface roughness increased after the adsorption of Sb(III). The optimal adsorption was conducted at pH 5.0. and the maximum Sb(III) adsorption capacity was 195.7 mg/g, which was 1.16 times higher than SA microspheres without HA. The adsorption process was a homogeneous single-layer adsorption which was controlled by chemisorption and was an exothermic and spontaneous reaction. The HA@SA microsphere maintained its adsorption capacity of 90% after recycling for eight adsorption-desorption cycles, even at low flow rate and concentration in practical usage. A zeolite/alginate microsphere was fabricated for Ni ions removal from aqueous solution [58]. The SA microsphere improves the adsorption of zeolite adsorbent by immobilizing the zeolite powder due to its hydrophilic property. The adsorption ability of the SA-modified microsphere is supported by many characterizations. The FT-IR results shows that Zeolite particles reacted with alginate polymer through hydrogen bonding. XRD pattern show the crystalline phase is the same as that of zeolite, but in a lower intensity due to the amorphous phase. SEM micrographs shows the folds and groves of the microsphere that enhance its adsorption capacity. The microsphere showed a removal efficiency of 98% within 150 min. The kinetic study showed that the adsorption followed the PSO model, indicating that the metal ion