Batch study of cadmium biosorption by carbon dioxide enriched aphanothece sp. dried biomass

Abstract

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 Aphanothece sp. cultivars (A0, A8, and A15) were used to eliminate cadmium (Cd2+) 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 m2/g. Biosorption of Cd2+ was pH and concentration dependent. Sorption was spontaneous (G =-8.39 to-10.88 kJ/mol), exothermic (H=-41.85 to-49.16 kJ/mol), and decreased randomness (S =-0.102 to-0.126 kJ/molK) 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 (k2 = 2.79 x 10-2, 3.96 x 10-2, and 4.54 x 10-2 g/mgmin). 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 (qmax, D-R = 9.74 x 10-4, 4.79 x 10-3, and 9.12 x 10-3 mol/g and mean free energy of 8.45, 11.18, and 11.18 kJ/mol) on the monolayer and homogenous surface (qmax, Langmuir 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 Cd2+ (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.