| Summary: | The conventional method for cadmium removal in aqueous solutions (1−100 mg/L) is ineffective and inefficient. Therefore, a batch biosorption reactor using a local freshwater microalga (originating from an urban lake, namely, Situ Rawa Kalong-Depok) as dried biosorbent was tested. Biosorbent made from three kinds of cyanobacterium <i>Aphanothece</i> sp. cultivars (A0, A8, and A15) were used to eliminate cadmium (Cd<sup>2+</sup>) ions in aqueous solution (1−7 mg/L). The biosorbents were harvested from a photobioreactor system enriched with carbon dioxide gas of 0.04% (atmospheric), 8%, and 15% under continuous light illumination of about 5700−6000 lux for 14 d of cultivation. Produced dried biosorbents had Brunauer−Emmet−Teller (BET) surface area ranges of 0.571−1.846 m<sup>2</sup>/g. Biosorption of Cd<sup>2+</sup> was pH and concentration dependent. Sorption was spontaneous (ΔG = −8.39 to −10.88 kJ/mol), exothermic (ΔH = −41.85 to −49.16 kJ/mol), and decreased randomness (ΔS = −0.102 to −0.126 kJ/mol. K) on the interface between solid and liquid phases when the process was completed. The kinetic sorption data fitted best to the pseudo-second-order model (k<sub>2</sub> = 2.79 × 10<sup>−2</sup>, 3.96 × 10<sup>−2</sup>, and 4.54 × 10<sup>−2</sup> g/mg.min). The dried biosorbents of A0, A8, and A15, after modeling with the Langmuir and Dubinin−Radushkevich isotherm models, indicated that cadmium binding occurred through chemisorption (<i>q<sub>max</sub></i><sub>, D-R</sub> = 9.74 × 10<sup>−4</sup>, 4.79 × 10<sup>−3</sup>, and 9.12 × 10<sup>−3</sup> mol/g and mean free energy of 8.45, 11.18, and 11.18 kJ/mol) on the monolayer and homogenous surface (<i>q<sub>max</sub></i><sub>, Langmuir</sub> of 12.24, 36.90, and 60.24 mg/g). In addition, the results of SEM, EDX, and FTIR showed that there were at least nine functional groups that interacted with Cd<sup>2+</sup> (led to bond formation) after biosorption through cation exchange mechanisms, and morphologically the surfaces changed after biosorption. Biosorbent A15 indicated the best resilient features over three cycles of sorption−desorption using 1 M HCl as the desorbing eluent. These biosorbents can be a potent and eco-friendly material for treating aqueous wastewater.
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