Insight into the roles of microbial extracellular polymer substances in metal biosorption

Biosorption presents a potent technology to remediate metal-contaminated aqueous environment or even to recover precious metals. Extracellular polymeric substances (EPS) are believed to play an important role in metal biosorption by microorganisms, but the reported results have been rather contradictory and the underlying mechanisms remain largely unclear so far. This review aims to clarify why large discrepancies existed for different EPS–metal systems through systematically exploring into the adsorption mechanisms and influential factors, and to offer some implications for advancing the implementation of metal biosorption technologies. The state-of-the-art methodologies for characterizing metal–EPS binding are summarized; several interaction mechanisms, including ion exchange, complexation and surface precipitation, are analyzed; the major influential factors such as EPS composition, metal species, solution chemistry and operating conditions are discussed; and lastly future research needs to advance the investigations and implementations of such biosorption processes are proposed.