Comparative study of the effective removal of hexavalent chromium via calcium alginate and calcium alginate/Ulva fasciata composite

Abstract The present study revealed for the first time the removal of hexavalent chromium Cr(VI) with a high efficiency using an eco-friendly composite beads of alginate with the green alga Ulva fasciata compared to calcium alginate beads. This conclusion was reached in this study from the comparison of the removal efficiency of Cr(VI) by the well-known biosorbent material, calcium alginate (CA) with the studied calcium alginate/Ulva fasciata (CA/UF) biocomposite beads. The characterization of the prepared beads was achieved using several techniques such as Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray (EDAX). The Brunauer, Emmett, Teller (BET) and Barrett, Joyner, Halenda (BJH) analyses were also employed which indicated a three-fold increase in the surface area, a mean pore diameter of 2.3-fold and a total pore volume of 14-fold for calcium alginate/Ulva fasciata composite compared to calcium alginate beads. The results of batch experiments demonstrated the fast complete removal of Cr(VI) by the CA/UF composite compared to the maximum removal (75%) by CA. The Dubinin–Radushkevich isotherm model reflected the physical pore volume filling of Cr(VI) rather than adsorption on the pore walls, giving an adsorption mean free energy (2.24 kJ/mol) for CA/UF greater than that of CA (0.13 kJ/mol). Furthermore, Brunauer–Emmett–Teller isotherm reflected the multilayer adsorption for CA and CA/UF. Flory–Huggins isotherm model showed the more spontaneous adsorption for CA/UF than CA, with negative Gibbs free energy ΔG 0 values of − 4.76 and − 6.91 kJ/mol, respectively. Whereas, Temkin isotherm model showed a higher adsorption binding energy of Cr(VI) on CA/UF than CA beads. In this study, the Langmuir model of Cr(VI) adsorption on CA/UF beads was the least applied among all studied adsorption isotherm models, which also revealed the multilayer adsorption mechanism of Cr(VI) ions. The Intra-particle diffusion model was applied for CA/UF composite beads, and this application suggested that the intra-particle diffusion is a part of the rate-limiting steps. The regeneration study showed a decrease in the adsorption efficiency of CA/UF composite from 97.4 to 82.3% for three consecutive cycles