Evaluation of Microbial Biosorbents for Efficient Cd(II) Removal from Aqueous Solutions

The biosorption of heavy metals has become an attractive alternative to conventional methods and is considered feasible, environmentally friendly and often low-cost option. Five microorganisms (Rhodotorula sp., Cladosporium sp., Bacillus megaterium, Trichosporon sp. and Geotrichum sp.) were isolated from different environments and used for the biosorption of Cd(II) from aqueous solutions in batch mode to expand upon the existing studies and generate new data related to the main microorganisms that could be successfully applied to the removal of heavy metals from wastewaters. Considering a constant biosorbent dose (5 g/L), pH (5.4) and temperature (25 °C) and varying contact times and initial pollutant concentrations, the process efficiency and uptake capacity of the biosorbents were assessed. Statistical analysis of the experimental results revealed that a contact time longer than 24 h did not significantly increase the uptake capacity or removal efficiency of Cd(II) by B. megaterium or Geotrichum sp., which means that the available binding sites on the cell wall immediately participate in the removal of metal ions. For the other three biosorbents, increasing the contact time from 24 h to 48 h led to a significant increase in the uptake capacity and removal efficiency. A comparison of the uptake values of each biosorbent revealed that Bacillus megaterium had the highest Cd(II) uptake capacity (8.53 mg/g), followed by Trichosporon sp. (8.21 mg/g). The lowest uptake capacity, as well as the lowest efficiency after 48 h of contact, was obtained for Geotrichum sp. (0.73 mg/g and 14.97%, respectively). The results of the FTIR analysis revealed that almost all the functional groups were present on the surface of the biosorbent, but their involvement in Cd(II) biosorption differed from biosorbent to biosorbent. The phosphodiester, amide and hydroxyl groups found on the cell surface of Bacillus megaterium, Cladosporium sp., Rhodotorula sp. and Trichosporon sp. were the main groups involved in Cd(II) biosorption.