# Wastewater Treatment by Polymeric Microspheres: A Review
## 1 Introduction  
Water pollution caused by heavy metal ions and dyes is causing serious environmental problems. Heavy metal ions and dyes such as chromium ion (Cr(VI)) and methylene blue (MB) used in various industries are soluble in aquatic conditions [1; 2]. In an aquatic environment, they can be easily consumed and can cause severe health problems, including carcinogenicity and mutagenicity. Various methods have been used for the detoxification and removal of heavy metal ions and dyes, including extraction, flocculant, electrocatalytic oxidation, and membrane separation [3]. Among them, adsorption is an efficient method due to its gentle operating condition, cost-effectiveness, and the fact that it does not derive secondary pollution [4]. Currently, powder, films, and lamellar forms are used, but they have low removal efficiency due to insufficient active sites and low surface area. Activate carbon (AC) is used widely for commercial purpose, but it has drawbacks in that its removal efficiency toward hydrophobic dyes, colloidal dyes, and some macromolecules are low, and it is expensive. Thus, hollow microspheres became promising adsorbents for their high surface area and is convenient to recover after usage [5]. Polymer microspheres can not only enhance the stability and mechanical property of the adsorbents by providing network or protection for other materials but can also enhance the adsorption capacity. To enhance the adsorption capacity, various modifications are put into place. Polymers can provide more active sites and functional groups used for the adsorption. Nanomaterials such as TiO\({}_{2}\) are used as photocatalyst as they can operate in mild conditions, and they use renewable energy sources [6; 7; 8]. Various polymers are used to prepare adsorbents toovercome the conventional adsorbents by enhancing the adsorption capacity, recyclability, non-toxicity, cost-effectiveness, and unique application. Thus, eco-friendly, efficient, and cost-effective polymeric microspheres presents their potential as adsorbents for industrial wastewater treatment. Vinyl polymer-based magnetic nanocomposites are used as catalysts for the removal of dyes and heavy metal ions from aqueous solution. Inorganic/organic nanocomposites can be incorporated into a polymer matrix such as polystyrene (PS), which is easy to handle, economical, and of uniform size [9]. Current studies display low adsorption efficiency for a low initial concentration. Thus, research is done to detect in surface waters at ng/L and ug/L [10]. Poly(glycidyl methacrylate) (PGMA) is used due to its high mechanical strength, pH resistance, and the high reactivity of the epoxy group with adsorbate [11]. Magnetic adsorbents are used to enhance the removal of dyes. They have zero energy separation, do not cause secondary pollution, and have high adsorption efficiency, fast separation speed, and good recyclability. These magnetic nanoparticles present on the surface of porous vinyl polymer microsphere can easily bind with the adsorbate. Coating with diazo resin (DR) can even protect Fe\({}_{3}\)O\({}_{4}\) in various pH environment and can enhance the adsorption capacity [12]. PDA has been used as microsphere or coating for microsphere for adsorption of organic dyes and heavy metal ions. Dopamine (DA) can spontaneously self-polymerize under weak alkaline environment at room temperature [13]. PDA has excellent adsorbing capacity, and has abundant functional groups like hydroxyl and amine that can bind both organic and inorganic pollutants though various types of interactions. However, adsorbents made of pure PDA display low adsorption capacity [14; 15]. These properties can be improved by various types of modifications. The biomimetic, hydrophilic, eco-friendly PDA inspired from mussels can be coated on various substrate and nanoparticles. Additionally, PDA can be synthesized into diverse sizes by controlling the pH of the environment [16; 17].