Biosorption of chromium (VI) and iron (II) by acid-based modified Chlorella vulgaris and Spirulina platensis: isotherms and thermodynamics

Abstract The removal of chromium (VI) and iron (II) was investigated using acid-pretreated Chlorella vulgaris (CV) and Spirulina platensis (SP) biomass from an aqueous solution. The Brunauer–Emmett–Teller (BET) surface area of the pretreated biosorbents 8.125 and 11.215 m 2 /g for CV-H 2 SO 4 and SP-H 2 SO 4 showed improvement as compared to the untreated biosorbents 1.513 and 2.648 m 2 /g for CV-raw and SP-raw, respectively. The Fourier transform infrared analyses exhibited some new peaks on the surfaces of the biosorbents after the pretreatment such as 3288, 3083, 1155, 699, 668, and 610 cm −1 which perhaps facilitated the removal of the metal ions. The optimum biosorbent dosage was 0.4 g with the maximum removal of chromium (VI) and iron (II) being 100% at pH 6 and 3, respectively. The X-ray diffractometry analyses showed that the biosorbents were amorphous solids. The desorption study revealed 0.1 M HNO 3 as the best eluent for recovering the biosorbed metal ions with 75.5%. The thermodynamics analysis showed that the biosorption of chromium (VI) was nonspontaneous with SP-H 2 SO 4 and CV-H 2 SO 4 . At low temperature ( T  <  T eq ), the values for ΔH o , ΔS o , and ΔG o parameters are 41.98 kJ/mol , 135.28 kJ/mol/K, and 2.59, 2.20, 1.75 and 1.88 kJ/mol. The equilibrium and kinetic data were best described by the Langmuir isotherm and the pseudo-second-order kinetic model, respectively. The Freundlich isotherm model confirmed the favorability of the biosorption process with n values 3.9061, 4.7094, 3.3185, and 3.8260 . This study revealed that pretreated Chlorella vulgaris and Spirulina platensis are suitable, efficient, and eco-friendly biomass for the remediation of environmental pollutants. Graphical abstract