Transport and Retention of Acid-Modified Biochar Nanoparticles and Their Role in Co-Transport and Remobilization of Lead in a Saturated Sand Column

Abstract

Biochar nanoparticles can act as carriers of pollutants in the groundwater, posing a threat to the environment. This study explored the transport and retention behavior of wood-based (NWBCs) and corn-residues-based (NCBCs) acid-modified biochar nanoparticles produced at different pyrolysis temperatures (at 400 °C and 700 °C). The effects of feedstock type, pyrolysis temperature, and input concentration on the mobility of nanoparticles in a saturated sand column were evaluated. Additionally, sequential transport and co-transport experiments were conducted to assess the nanoparticles' ability to remobilize pre-adsorbed Pb²⁺ and the transport of the nanoparticle-Pb²⁺ complex. HYDRUS-1D simulations using depth-dependent and Langmuirian models were applied to evaluate nanoparticles' retention. Nanoparticles produced at the pyrolysis temperature of 400 °C were more mobile than those produced at 700 °C. The highest nanoparticle mobility was observed when acid-modified wood-based biochar nanoparticles produced at 400 °C (NWBC400) were applied at an input concentration of 100 mg L⁻¹, while the lowest mobility was observed at an input concentration of 300 mg L⁻¹. The sequential transport and co-transport experiments revealed that NWBC400 quickly removed the pre-adsorbed Pb²⁺ from the sand. Also, Pb²⁺ in the metal-nanoparticle complex was highly mobile. Moreover, depth-dependent retention was detected as the dominant process describing nanoparticles' retention. As biochar nanoparticles increased the Pb²⁺ mobility in the porous media, adopting policies eliminating such conditions is essential for the environment. Also, understanding and managing biochar nanoparticle mobility can help protect water resources and public health from pollution risks.