Enhanced bacterial capture in hematite-modified biochar amended coral sand filters: Lab-scale validation and DFT...

Tropical coral island soils exhibit high porosity and poor water retention, enabling rapid pathogen leaching into fragile freshwater lenses that serve as the primary drinking source for island residents, creating significant public health risks through groundwater contamination. To address this critical vulnerability, we engineered hematite-modified biochar (HBC) enhanced coral sand filters where merely 2 wt% HBC amendment achieved exceptional bacterial retention exceeding 99 % for both E. coli and B. subtilis, representing a dramatic improvement over the 11.6 % retention in untreated sand. Mechanistically, HBC simultaneously reduces electrostatic repulsion through surface charge neutralization quantified by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, shifting interaction energies from 388 kT repulsion to net attraction, while Density Functional Theory (DFT) calculations revealed hematite's atomic-scale chemisorption dominance via stable Fe-O-P coordination bonds with bacterial extracellular components, evidenced by adenosine monophosphate adsorption energy reaching -4.97 eV. Mathematical modeling through HYDRUS-1D's two-site kinetic framework confirmed irreversible adsorption dominance under continuous flow conditions. Crucially, HBC maintained over 98 % retention efficiency against humic acid interference, diverse real water matrices, and prolonged high bacterial loads up to 10⁷ cells per milliliter, establishing this amended filtration system as a robust solution for protecting island freshwater resources against pathogenic contamination.