Adsorption kinetics of Cd(II) from aqueous solutions onto live vs. dead biosorbents of Sphingomonas sp. GX_15

Abstract

In this study, the adsorption capacity and removal efficiency of Cd(II) via the live and dead biosorbents of Sphingomonas sp. GX_15 were studied using a batch experiment. The adsorption characterization was investigated using scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) analysis. The optimal adsorption conditions were determined to be pH of 6.0, contact time of 60 min and dosage of 1.0 g/L. The highest removal efficiency and maximum biosorption capacity of the live and dead biomasses were 78.35% and 90.99%, and 28.33 mg/g and 26.60 mg/g, respectively. R2 values (0.9582 vs. 0.8468 and 0.9784 vs. 0.9831) implied that the data obtained from the dead pellets were better fitted to the Langmuir isotherm model while the Freundlich isotherm model was better fitted for the live pellets. The parameters of KL (0.0875 vs. 0.0646), RL (0.0541 vs. 0.0719) and n (3.8640 vs. 2.7218) in the two isotherm models showed that the adsorption process of dead cells demonstrated a higher affinity and was easier and stronger than that of the live cells. The pseudo-second-order kinetic model was a better fit for depicting the adsorption process, meaning that the rate-limiting step might be chemisorption. The rate parameter of k2 (0.0465 vs. 0.0294) showed that the adsorption was faster for the dead biosorbent. The positive parameters of ΔH° and ΔS° indicated that the adsorption process was endothermic and was increasing in the randomness degree. SEM analysis revealed that the cell surface changed from invaginated to plump, confirming the metals adsorbed into the cell wall. Additionally, the FT-IR spectra study demonstrated that the O–H, N–H, C–H, C = O, C–N, COO-, and –SO3 groups were involved in Cd(II) binding. In sum, we strongly suggest that the dead cells of Sphingomonas sp. GX_15 can act as alternative novel biosorbents for Cd-contaminated soil or water.