Millimeter-scale elemental sulfur and nanoscale zero-valent iron composites enhance sulfur autotrophic denitrification: performance and mechanistic investigation

Abstract

Sulfur autotrophic denitrification (SADN) requires no external organic carbon and has low operating costs. However, its slower denitrification kinetics than heterotrophic denitrification remains a key bottleneck limiting its application for nitrogen removal from low carbon-to-nitrogen (C/N) wastewater. Herein, elemental sulfur (S⁰) and nanoscale zero-valent iron (nZVI) were integrated into uniform millimeter-scale composite pellets (nZVI/S⁰) to enhance SADN. The nZVI/S⁰-amended group not only attained an average total nitrogen (TN) removal efficiency of 96.0% but also exhibited rapid TN removal at an average rate of 3.80 mg N/(L·h), far exceeding the rates observed in the groups supplemented with S⁰ (1.17 mg N/(L·h)) or nZVI (0.08 mg N/(L·h)). In the nZVI/S⁰ system, both nZVI and S⁰ could serve as electron donors, contributing to the enhanced denitrification performance. Specifically, nZVI in nZVI/S⁰ markedly increased the relative abundance of the core sulfur autotrophic denitrifier Thiobacillus and the relative abundances of denitrification genes, sulfur oxidation genes, and electron transfer-related genes. Additionally, it promoted the expression of genes related to denitrification and sulfur oxidation, enhanced the activities of key denitrifying enzymes (including nitrate reductase Nar and nitrite reductase Nir), and elevated the electron transfer activity in the system. Meanwhile, integration within the sulfur matrix significantly reduced the microbial toxicity of nZVI, retarded its rapid passivation, and suppressed the ammonium-forming side reaction caused by direct nitrate reduction by nZVI. This study provides a new approach for improving SADN and achieving efficient nitrogen removal from low C/N wastewater.

Graphical abstract

Keywords

Sulfur autotrophic denitrification / Elemental sulfur / Nanoscale zero-valent iron / Low C/N wastewater

Highlight

	● nZVI/S0 effectively enhanced sulfur autotrophic denitrification (SADN).
	● nZVI enriched functional microorganisms and genes of SADN.
	● nZVI enhanced denitrification gene expression and enzyme activity.
	● nZVI accelerated electron transfer.
	● S0 encapsulation reduced nZVI toxicity, passivation, and NH4+ formation.

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