Layered Double Hydroxide-Modified Biochar Reduces Transport of Extracellular Antibiotic Resistance Genes in Saturated Porous Media

Extracellular antibiotic resistance genes (eARGs) can move with pore water through soils and sediments. In this study, Mg-Al layered double hydroxide (LDH)-modified biochar (MPC) and sodium-dodecyl-sulfonate-treated LDH biochar (SPC) were evaluated in water-saturated quartz-sand columns. Microscopy, elemental mapping, and diffraction were consistent with an Mg-Al-containing layered component on the modified biochars, while spectroscopy and zeta-potential measurements showed changes in surface composition and electrokinetic charge. Extracellular DNA (eDNA) effluent recovery decreased from approximately 70% in the unamended control to 52% with MPC and 43% with SPC, and the eARG breakthrough curves followed the same treatment order. Electrostatic attraction and metal-related interactions are possible contributors to these responses. Increasing NaCl or CaCl2 concentration reduced eDNA plateau concentrations in the tested series. Humic-acid and fulvic-acid preconditioning increased subsequent eDNA and eARG effluent recovery, with a larger response to humic acid. The results identify material-dependent transport responses and their sensitivity to solution chemistry in a controlled porous-medium model. The input–output deficit represents DNA unrecovered from the effluent during the collection interval.