Fluoride removal from groundwater by a wide range of nanoparticles: Identification of selective nanoparticle and interfering water quality parameters

Abstract

Fluoride (F⁻) contamination in drinking water is a major problem in many parts of the world. In India, millions of people are exposed to F⁻ contamination. Thus, it is important to assess the regional groundwater quality and the performance of potential nanoparticles (NPs) in removing F⁻ under those natural groundwater conditions. The objectives of this study were (i) to assess the regional groundwater quality of F⁻ contaminated zones, (ii) to evaluate the performance of different metallic NPs in removing F⁻ and identify the promising NPs under natural groundwater conditions, and (iii) to identify the groundwater quality parameters impacting the performance of NPs. To achieve the objective, the groundwater samples are first collected from a few districts within Rajasthan, India, and then the water quality parameters are assessed. A series of experiments are conducted to evaluate the F⁻ removal efficiencies by a wide range of NPs under both de-ionized (DI) water and natural groundwater conditions. The outcome of this study indicates that the groundwater in most of the water samples in the region is unfit for consumption as it exceeds the permissible limits (Bureau of Indian Standards, BIS) for total dissolved solids (TDS, 2035 mg/L), hardness (699 mg/L), alkalinity (504 mg/L) and F⁻ (3.56 mg/L) concentration. The performance of NPs in removing F⁻ (as sorption capacity, mg/g) follows the order of nAl₂O₃ ∼ nZnFe₂O₄ > nZnO ∼ nMgO.Al₂O₃> nMgO > nCeO₂ > nLa₂O₃> nAlCeO₃> nFe₂O₃ ∼ nAl₂TiO₅ > n(CeO₂).(ZrO₂) under DI water conditions. However, under F⁻ contaminated natural groundwater, nLa₂O₃ nanoparticle shows promising performance with reasonable sorption capacity (11.12 ± 2.0 mg/g). Amongst the water quality parameters, Ca²⁺, Mg²⁺, hardness and TDS have the most adverse effect on F⁻ sorption. The lower value of F⁻ concentration in the groundwater is another factor limiting the F⁻ sorption capacities. In summary, it could be inferred that nLa₂O₃ is a promising NP, which could be applied in different forms in water filters for F⁻ removal from natural conditions. Moreover, pre-treatment of raw water for TDS and hardness reduction might be necessary. Overall, the study aims to control drinking water quality by targeting F⁻ contamination, especially under water-stressed semi-arid regions, which is aligned with the SDGs of Clean Water and Sanitation (SDG 6) and Good Health and Well–Being (SDG 3).