Fungal biosorption - an alternative to meet the challenges of heavy metal pollution in aqueous solutions

The removal of heavy metal from the environment, especially wastewater, is now shifting from the use of conventional methods to the use of biosorption, which may be defined as the binding and concentration of selected heavy metal ions or other molecules on to certain biological material. Although most biosorption research concerns metal and related pollutants, including radionuclides, the term is now applied for particulates and all manner of organic pollutants as well. Such pollutants can be in gaseous, soluble and insoluble forms. Biosorption is a physical process carried out through mechanisms such as ion exchange, surface complexation and precipitation. It is a property of both living and dead organisms (and their components) and has been heralded as a promising biotechnology for pollutant removal from solution. Various biomasses such as plant products (tree bark, peanut skin, sawdust, plant weeds etc.) have been tested for metal biosorption with very encouraging results. In this comprehensive review, biosorptive ability of fungal biomass toward heavy metals is emphasized. A detailed description of adsorption properties and mode of action of fungal biosorbents is offered in order to explain the heavy metal selectivity displayed by these biosorbents. The cell structure and cell wall of the fungal cell is evaluated in terms of metal sequestration. The parameters influencing the passive uptake of pollutants are analysed. The binding mechanism is discussed, including the key functional groups involved in the process. Quantification of metal-biomass interactions is fundamental to evaluation of potential implementation strategies; hence sorption isotherms and sorption kinetics, as well as models used to characterize fungal biosorbent sorption, are reviewed. Despite the continuing dramatic increase in published research on biosorption, there has been little or no exploitation in an industrial context. Thus, the current status and future directions regarding biosorption at an industrial level are discussed. A systematic comparative review of the literature, based on the metal-binding capacity of fungal biomass under different conditions, is also provided. The problems associated with fungal biosorption are analysed and suitable remedies are discussed. Thus, this article reviews the achievements and current status of fungal biosorption technology and hopes to provide insights into future research.