Microbial preparation of magnetite/reduced graphene oxide nanocomposites for the removal of organic dyes from aqueous solutions

Magnetic graphene materials are good adsorbents for pollutant removal from aqueous solutions. However, most chemical methods for the synthesis of magnetic adsorbents generally require the use of toxic reductants, proceed under rigorous conditions, and thus cause negative impacts on the environment. Here, an eco-friendly method applying the reduction activity of microbial cells was developed to prepare magnetite/reduced graphene oxide (MRGO) nanocomposites for adsorptive removal of organic dyes. The biogenic MRGO was characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and vibrating sample magnetometry. Factors (adsorbent dosage, contact time, solution pH and ionic strength, etc.) affecting the adsorption process were investigated. The adsorption behavior of MRGO fits well with a Langmuir isotherm and pseudo-second-order kinetic model. The maximum adsorption capacity of MRGO for methylene blue at 303 K was determined to be 144.9 mg g^-1, which is higher than those of many other chemically synthesized magnetic graphene materials. Thermodynamic analysis revealed the spontaneous and endothermic nature of the adsorption process. Moreover, the MRGO adsorbent could be regenerated through a Fenton-like reaction and reused effectively in five successive runs. The biogenic MRGO has great potential for the treatment of wastewater containing dyes and other environmental pollutants.