Large accumulation of plastic wastes, projected to exceed 12000 Mt by 2050, requires immediate treatments. To address these problems, the upcycling and valorization of plastic wastes based on the principle of chemical degradation have attracted much attention from all relevant fields. Here, we developed a novel conversion strategy to design adsorbents for water treatment based on the aminolysis process of waste plastic PET bottles. The aminolysis of PET using diethylenetriamine and triethylenetetramine resulted in the formation of terephthalamide oligomers with abundant amino groups. Depending on the different water solubility of terephthalamide oligomers, two types of adsorbents were designed and the adsorption characteristics were studied using a model system of industrial dye. Water-soluble oligomeric products of PET aminolysis prepared at higher amine-to-PET ratios were cross-linked in water using ethylene glycol diglycidyl ether (EGDE) to form a hydrogel adsorbent with a swelling ratio of ca. 70. On the other hand, the water-insoluble aminolysis products prepared at lower amine-to-PET ratios were co-precipitated with Fe3O4 nanoparticles to produce a magnetic microparticles adsorbent showing a highly superparamagnetic property. Due to the presence of abundant amino groups and aromatic moieties, both of adsorbents were tested for removal of anionic dyes (Congo Red, CR) in aqueous media. Their maximum adsorption capacities for CR removal were comparable and found to be ca. 511 and 780 mg/g for hydrogel and magnetic microparticles adsorbent, respectively. Magnetic microparticles adsorbent provided a rapid kinetics with dye removal by 70-80% of the saturation adsorption capacity within a few minutes, which was dramatically, more than hundreds of times, faster than that of hydrogel adsorbent, due to faster mass transfer to adsorption cites that is not limited by slow diffusion in hydrogel. The proposed here material design addresses simultaneously the problem of plastic waste utilization and preparation of cost-effective and efficient adsorbents for environmental cleaning purposes.