Temperature-dependent carbothermally reduced iron and nitrogen doped biochar composites for removal of hexavalent chromium and nitrobenzene

Synthesizing new composites with better efficiency and stability from abundant sources offers an appealing prospect for removal of heavy metals and organic pollutants to address water pollution concerns. In this study, iron (Fe) and nitrogen (N) doped biochar (BC) composites were investigated for hexavalent chromium (Cr(VI)) and nitrobenzene (NB) removal. Specifically, temperature-dependent Fe1-N1-BC1 composites were synthesized with different mass ratios of FeCl3 as Fe, melamine as N, and pinewood sawdust (PWS) as BC precursors via carbothermal reduction at 300, 500, 700, and 900 °C. Detailed characterizations indicating low-temperature incorporated superior N and high-temperature incorporated more Fe species in Fe1-N1-BC1 composites. The Cr(VI) and NB were removed with Fe1-N1-BC1, and experimental results attested with standard isotherm and kinetic models. The N species (pyridinic-N, pyrrolic-N, graphitic-N, quaternary-N, and oxidized-N) and Fe species (Fe3C, Fe0, Fe 2p 3/2, and Fe 2p 1/2) in temperature-dependent Fe1-N1-BC1 composites participated in pollutants removal. Mass ratio of 1:1:1 of Fe, N, and BC was effective, and under optimum conditions, low-temperature (Fe1-N1-BC1-300) and high-temperature (Fe1-N1-BC1-700) composites completely removed Cr(VI) and NB, respectively. In comparison to pinewood sawdust BC (PWBC) and composite of FeCl3 with PWS (Fe-BC), melamine with PWS (N-BC), and FeCl3 with melamine (Fe-N), the Fe1-N1-BC1 was more efficient due to synergistic effect of Fe, N, and BC for better pollutant interaction and removal. Effect of different factors on pollutants removal with Fe1-N1-BC1 was studied to reveal removal mechanism insights. Additionally, Fe1-N1-BC1 showed better efficiency in simultaneous removal of Cr(VI) and NB, stability for > 4 months, reusability, and applicability for multiple pollutants. This study offers a benign approach for synthesizing and applying Fe1-N1-BC1 for pollutants removal and exploring reaction mechanisms.