In the present study, biosorption of Cr(VI) from synthetic and real industrial effluents by biomass of dried sludge as an effective and eco-friendly biosorbent was evaluated. The fresh and spent biosorbent was characterized by scanning electron microscopy, Fourier-transform infrared spectroscopy, energy-dispersive X-ray, and Brunauer–Emmett–Teller (BET) techniques. These structural and morphological characterizations designated that prepared biosorbent has potent adsorptive functional groups (oxygen-containing functional groups), heterogeneous, and a porous surface. Moreover, the BET result indicated that the biomass of dried sludge with a surface area of 8.79 m2/g can provide a suitable surface for the biosorption of Cr(VI). The key experimental parameters were investigated to examine their feasibility in the maximum removal of Cr(VI). Furthermore, kinetic models were assessed, providing the best fitting for the experimental data. For evaluating the mechanism of the biosorption process, isotherms were also conducted under optimum adsorption conditions. The post characterization results confirmed the adsorption of Cr species onto the sludge surface. The maximum biosorption efficiency of Cr(VI) was found at 83.96% and at initial Cr(VI) concentration of 50 mg/L, time of 100 min, biosorbent dosage of 8 g/L, and pH of 5. Kinetic study showed the biosorption process follows the pseudo-second-order kinetic model (R2 = 0.9973 and k2 = 1.8 × 10–3 g/mg min), which suggests the adsorption process involves a chemisorption mechanism. The half-life of Cr(VI) removal in the biosorption process was 0.6 h. Also, the isotherm study revealed that Cr(VI) biosorption process was best fitted with the Redlich–Peterson model (R2 = 0.9993) with a maximum Cr ion sorption capacity of 9.082 mg/g. This result confirms that chromium removal occurs on a biosorbent with the homogenous surface by monolayer adsorption. Besides, the Gibbs free energy (ΔG°) value obtained from thermodynamic equilibrium reaffirmed that the prepared biosorbent possesses a large capacity for biosorption of Cr(VI). Finally, the results promulgated that biomass of dried sludge can be applied as an effective and practical biosorbent for the removal of Cr(VI) from industrial wastewater.
