Magnetic sodium alginate (SA)-based biosorbent Fe₃O₄@SA-Ca gel beads were synthesized by droplet polymerization using Ca2+ ions as crosslinking agent, and characterized by Scanning Electron Microscopy (SEM). Fe₃O₄@SA-Ca polymer was used for the removal of Direct Orange 26 (DO-26) dye from water. The ratio of raw materials and some important conditions affecting the adsorbent performance were carefully examined. The adsorption kinetics, isotherms and thermodynamics were investigated. When the concentrations of SA, CaCl2 and Fe3O4 solutions were 1.9, 10 and 10 g/L, respectively, the black Fe₃O₄@SA-Ca gel beads with about 3 mm diameter were successfully prepared by curing 6h at room temperature. The polymer gel exhibits ultra-high adsorption capacity of 1252 mg/g and removal efficiency of 96.2 % for DO-26 under the conditions of 0.05 g polymer dosage, 2600 mg/L of dye initial concentration, solution pH 2.0 and 90 min of the adsorption time at 298 K. The dye removal efficiency can be all over 92 % in a wide pH range of 2.0 to 10.0. The adsorption process completely accorded with the pseudo-second-order rate model at different temperatures (298 ~ 328 K). The dye adsorption behavior was well in line with Freundlich model. The thermodynamic study indicated that the adsorption reaction was of spontaneous and exothermic nature. SEM analysis showed that the Fe₃O₄@SA-Ca gel beads appeared uneven surface with irregular folds and grooves. A high-cost-effective magnetic biosorbent, Fe₃O₄@SA-Ca can super effectively remove direct dyes from high-concentration wastewater in a wide range of acidity, and can be easily separated and recovered from water after adsorption without secondary pollution, and would have a good application prospect.

A novel calcium alginate (SA/Ca) gel pellet adsorbent was prepared by ion exchange reaction and polymerization between sodium alginate (SA) and Ca2+ in the solution, and characterized by SEM, EDS, FTIR and XRD. Taking acid orange II ( AO II) dye as the adsorption object, the influence of some important conditions about preparation and adsorption on SA/Ca properties was discussed in detail. The adsorption kinetics were studied. The results show that SA/Ca adsorbent has good adsorption effect on AO II dye under the conditions of the mass concentration ratio 2 : 5 of SA and CaCl2 solutions and reaction time of 2h at 25°C. When the adsorbent dosage is 4g/L and initial dye concentration of natural pH is 1000 mg/L, the adsorption equilibrium can basically reach in 180 min at 25°C with the adsorption capacity of 225mg/g and more than 90% of removal efficiency. The adsorption kinetics fully fit pseudo-second order model. The characterization results indicate that the SA powder is composed of various irregular particles, while dry SA/Ca solid are all pellets with diameter of about 1mm. The surface of inhomogeneous spheres is stacked with obstacles, and folds and ravines coexist. Ca2+ and most Na+ in SA molecule carried out ion exchange and formed a stable network-structure polymer SA/Ca gel pellets, which can effectively remove acid dyes from wastewater, and are easy to be separated after adsorption. As a biopolymer adsorbent with simple preparation method, environmental friendliness and non-toxic side effects, SA/Ca has good development potential and application prospects.