Sequential biosorptive-degradative remediation of methylene blue from polluted soil and wastewater by a newly isolated Bacillus safensis SMAH biomass: optimization, kinetics, isotherms and thermodynamics assessments

The rising necessity for available environmentally sustainable and low-cost effective soil remediation technology has driven our efforts to conduct this research investigation. A laboratory-based experimental approach has been designed and applied to assess the effective action of a new bacterial biomass for the biosorption-biodegradation of methylene blue dye (Mbd) from contaminated soil and wastewater. A dried biosorbent of Bacillus safensis strain SMAH biomass (BS-SMAH-B), was isolated from tannery wastewater, and identified by 16 S rRNA sequencing. The BS-SMAH-B performance was initially screened using synthetic Mbd solutions under controlled laboratory condition and successively followed by validation in real contaminated soil matrices. The screening experiments showed a high removal efficiency of 97%. However, when applied to real soil samples, the efficiency decreased to 82.05%, reflecting the complexity of the soil matrix. The BS-SMAH-B morphology and composition revealed uniform particles with average pore diameter (12.1 nm). The FT-IR spectral analysis of BS-SMAH-B confirmed numerous active functional groups including –COOH, –OH and –NH2. The elemental composition of C (51.72%), O (27.10%), N (14.97%), was investigated and assessed by the EDX analysis. The zeta potential (–25 mV) of BS-SMAH-B was identified to verify the surface negative sites which are therefore, capable of enhancing the biosorption process of cationic dyes as Mbd. Thermodynamic and kinetic analyses of Mbd biosorption from soil using the BS-SMAH-B revealed that the process is endothermic and taken place spontaneously. The biosorption behavior conformed to the pseudo-second-order, outlining chemisorption as the rate-limiting step. Furthermore, the equilibrium values were well correlated to Langmuir and Freundlich models, with a qmax 10.81 mg/g. To further elucidate the adsorption mechanism and removal efficiency, kinetics models including Elovich, Avrami, Dubinin–Radushkevich (D–R), and Temkin were also applied, providing a more comprehensive understanding of the biosorption characteristics of Mbd. BS-SMAH-B exhibited high performance toward Mbd removal from real Mbd-polluted soil samples via two-step mechanism, involving the initial Mbd biosorption from soil surface by BS-SMAH-B, followed by the biodegradation step in which Mbd molecules were broken down into smaller and harmless products.