Co-transport and deposition of fluoride using rice husk-derived biochar in saturated porous media: Effect of solution chemistry and surface properties

Abstract

Fluoride (F−) contamination in water is a global health concern, threatening the well-being of millions. This study investigated the role of ZnCl2/FeCl 3-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 fluoride 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 are generally applied for drinking and cooking; 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.