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Sulfur autotrophic denitrification is a feasible option for nitrate removal from wastewater. Element sulfur (S^o) can be utilized as an electron donor to enhance denitrification processes, with S^o metabolism playing an important role. The general concern with the use of S^o as an electron donor is the conversion and transfer of S^o, since S^o hardly dissolves in water. The conversion of S^o and the transfer of intermediates in S^o-based biofilms was investigated by applying a negative potential in a denitrification reactor with a S^o-carbon black cathode. The nitrate reduction rate was greatly increased, while nitrite accumulation was limited. When the cathode was prepared with equal volumes of element sulfur and carbon black powder, the nitrogen removal rate could reach 1.03 ± 0.00 g/m²/d. Half-order rates still applied to the reactions with applied potential, indicating that S^o-based denitrification was the dominant …