Sorption and transformation of arsenic by extracellular polymeric substances extracted from Synechocystis sp. PCC6803.

Cyanobacteria widely distribute in the aqueous ecosystem and produce abundant extracellular polymeric substances (EPS), yet little is known about how the quantity and composition of cyanobacterial EPS change upon As exposure, and what are functions of these complex biopolymers in the As sorption and transformation processes. Here we extracted the EPS from Synechocystis sp. PCC6803, characterized their properties, quantified their components upon exposure to arsenite (As(III))/arsenate (As(V)) treatments, and investigated As binding and speciation as affected by the levels of EPS and solution pH. The total binding sites, zeta potential and reducing power of EPS were 17.47 mmol g-1, -19.72 mV and 1.71. The amounts of EPS increased by 22-65.3% and 13.8-39% when the cells were treated with 10-500 μM As(III) and As(V) respectively. The As removal was influenced by the EPS doses and solution pH, with 52.8% at pH 8.5 for As(III) and 49.5% at pH 4.5 for As(V) at 300 mg L-1 EPS. In addition, As speciation was transformed with the addition of EPS. As(V) and As(III) respectively accounted for 4.9-20.3% and 6.5-26.7% of the total dissolved As after the EPS were added (100-300 mg L-1) to the As(III) and As(V) solutions. Fourier transform infrared spectroscopy (FTIR) and three-dimensional excitation-emission fluorescence spectra (3D-EEM) revealed that As was bound to functional groups such as C═O, ─NH, and ─OH in the EPS via surface complexation/hydrophobic interactions. Taken together, this study demonstrated that the EPS extracted from Synechocystis were capable to bind and transform As and could be potentially applied to remove or detoxify As in solutions.