Fluoride (F−) contamination in water is a global health concern, threatening the well-being of millions. This study investigated the role of ZnCl2/FeCl3-rice husk-modified biochar (Zn-BC and Zn/Fe-BC) in treating F−-contaminated surface and groundwater under the influence of varying solution chemistry, co-existing ions, and biochar-amended through column transport experiments. Modified biochar showed maximum F− adsorption, 99.01% and 91.90% using Zn/Fe-BC and Zn-BC, respectively, than 85.87% using raw biochar (R-BC). Raw/modified biochars were characterized with FESEM-EDAX, FTIR, XRD, particle size, surface area, electro-kinetic potential, and point of zero charge analyses. Langmuir and pseudo-second-order kinetic could explain that F−-biochar interactions are dominated by chemisorption at ambient temperature while physisorption at higher temperatures. The influence of salt concentrations and co-occurring ions reduced F− sorption using Zn/Fe-BC. Increased salt strengths led to reduced electrophoretic mobility of biochar particles, i.e., biochar–biochar particles attract each other and increase the hydrodynamic diameter, which ultimately reduces the active sites on biochar for F− adsorption. Co-transport and deposition of biochar and F− in saturated porous media revealed lower mobility of biochar, and maximum F− adsorption was observed at 10 mM salt strength. Biochar transport is governed by electrostatic interactions, whereas F− transport mainly occurs through chemisorption. In rural areas, hand pumps and tube wells are generally used as source of potable water for drinking and cooking purposes; thus, biochar-mediated sand columns can be utilized for defluoridation. Thus, Zn/Fe-BC can be utilized as a potential bio-adsorbent for F−-contaminated natural surface and groundwater with optimum preparation and treatment costs. Display omitted •Mass transfer rates are observed for fluoride–biochar interactions.•pHpzc and electro-kinetic potential values govern fluoride (F) adsorption.•Co-transport and retention of biochar and F investigated in saturated porous media.•Influence of background solution and ionic strength are critical for F removal.