Ultra-trace mercury immobilization and methylation suppression by nitrogen-enriched activated carbon

Immobilizing bioavailable mercury in paddy soils suppresses methylation and prevents rice accumulation. Conventional remediation fails to address ultra-trace bioavailable Hg in porewater (typically <100 ng L⁻¹) amid dissolved organic matter and ionic interference. Here, we report a nitrogen-enriched activated carbon functionalized with in situ-grafted porous graphitic carbon nitride for mercury immobilization. This material features facile synthesis, high surface area, abundant active sites, and exceptional stability. Density functional theory calculations reveal that amine-heptazine sites achieve Hg binding energies of −89.47 to −95.01 kcal mol⁻¹—fourfold stronger than thiol-Hg bonds (−22.37 kcal mol⁻¹). The engineered carbon reduces aqueous Hg to <1 ng L⁻¹, outperforming thiol-functionalized activated carbon by two orders of magnitude. Efficacy persists under pH variations, dissolved organic matter, and ionic interference, with structural/functional stability maintained after 30-day exposure to corrosive media. Geobacter sulfurreducens PCA pure-culture experiments demonstrate immobilized mercury resists microbial methylation (94.3% less methylatable Hg than conventional activated carbon). Soil incubation under simulated flooded conditions confirms 96.6% porewater Hg, 97.1% porewater methylmercury, and 93.9% soil methylmercury reduction at 1 wt% loading. Notably, 0.01 wt% achieves efficacy matching standard activated carbon, demonstrating outstanding mercury immobilization.