Nitrogen-doped transition-metal biochars from wheat straw and coffee grounds for aqueous Cr(VI) removal

Nitrogen-doped transition-metal-modified biochars were prepared from wheat straw and spent coffee grounds for Cr(VI) removal from acidic aqueous solutions. Based on the predefined material-design strategy and preliminary screening, nZVI/N-KWBC(Nano-scale zero-valent iron supported nitrogen-doped KOH-modified wheat straw biochar) and Co/N-KCBC(Cobalt-loaded nitrogen-doped KOH-modified coffee-ground biochar) were selected as representative wheat-straw-derived Fe-based and coffee-ground-derived Co-based composites, respectively. Both materials showed optimal Cr(VI) removal at pH 2.0, with adsorption capacities of 41.29 and 43.82 mg·g − 1 at 50 mg·L − 1 Cr(VI), respectively. The equilibrium and kinetic data were best described by the Langmuir and pseudo-second-order models, respectively. Combined with spectroscopic evidence, these results suggest that adsorption and redox reactions jointly contributed to Cr(VI) removal. The Langmuir maximum capacities at 25 °C were 109.77 mg·g − 1 for nZVI/N-KWBC and 145.50 mg·g − 1 for Co/N-KCBC. Mechanistic analyses suggested that Cr(VI) removal involved electrostatic attraction, surface complexation, reduction to Cr(III), and immobilization. Cr(VI) removal by nZVI/N-KWBC appeared to involve a stronger contribution from Fe 0 /Fe 2+ -mediated reduction, while Co/N-KCBC involved Co–N x /N sites and Co-assisted electron transfer.