Sulfur source promotes the biosorption and bioprecipitation of Cd in purple non-sulfur bacteria

Sulfur availability is an important factor affecting the tolerance and removal capability of heavy metals (HMs), but the roles of exogenous sulfur source in this process have rarely been reported in microorganisms. In this study, the effects of sulfur salts on Cd(II) bioremediation and the possible mechanism by Rhodopseudomonas palustris SC06 were investigated. R. palustris SC06 belonging to purple non -sulfur bacteria (PNSB) was isolated based on the Cd(II) tolerance and removal capability as well as their growth. Sodium sulfide (Na2S) was selected from six sulfur slats, and the synergistic effect with R. palustris SC06 could removed 94 % Cd(II) from aqueous solution containing 120 mg L-1 Cd(II) at 72 h. The functional groups including C--O, N-H/C-N, -SO3, -S-O, and -S-S- on the cell surfaces played the key role in the adsorption of Cd(II); among those, -S-O and -S-S- activated by Na2S. Na2S application significantly increased the content of Cd and S elements in R. palustris SC06, thereby clearly forming the bioprecipitation of CdS particles. Moreover, the content of cysteine, glutathione, and total sulfhydryl of R. palustris SC06 with the addition of Na2S were greatly upregulated under Cd(II) exposure, and the yield of extracellular polymeric substances (EPS) was also significantly improved, reaching up to 4437.30 mg g EPS-1, which was 17.0 % higher than that of the control. Hence, these results indicate that Na2S plays the vital role in adsorption and bioprecipitation of Cd(II) in PNSB, which provide a great potential strategy for the management of Cd(II) contaminated wastewater.