Fluoride-based rejected by membranes having pore areas

Fluoride-based health effects are a major environmental problem in Sri Lanka and also many other regions of the world. The recommended desirable fluoride limit of drinking water is 1.0 mg L-1 and permissible limit is 1.5 mg L-1 according to WHO standards. Recently, the recommended level has been brought down to 0.7 ppm. When the fluoride concentration is more than permissible limit, it causes many health problems including the Chronic Kidney Disease (CKD) prevailing in the North-Central Province and nearby areas. The drinking well waters of this region contains excess fluoride up to 3 ppm in some well waters. Recent study in the Moneragala District of Uva Province of Sri Lanka revealed that drinking waters of patients with glomerular membrane damage contain far in excess of fluoride, in some wells up to 8 ppm. Other suspicious species such as Cd and As are well below the recommended levels and there is hardly any difference between the levels in waters consumed by CKDU patients (Sample) and the healthy people (Control) living in nearby areas but there is a clear difference in fluoride levels with a mean of  2.02 ppm in the sample areas which is three times higher than that of recommended value (0.7 mg l-1) whereas the mean fluoride content of control area (0.39 ppm) is half that of recommended maximum. This study clearly revealed that there is a direct correlation between the glomerular basement membrane damage and fluoride in drinking waters 1. Also, some industrial effluents contain excess fluoride ions as high as 20-30 ppm. As such, fluoride removal from drinking waters has become a very important issue.

Various nano-technological methodologies have already been developed to remove some pollutants from contaminated waters. By performing molecular dynamics simulations, Heiranian et al. 2 showed that a nanopore in a single-layer molybdenum disulfide can effectively reject ions and allow transport of water at a very high rate. They have found that more than 88% of ions are rejected by membranes having pore areas ranging from 20 to 60 Å2 with a water flux of two to five orders of magnitude larger than that of other known nanoporous based  membranes. Pore chemistry is shown to play a significant role in modulating the water flux. Pores with only molybdenum atoms on their edges lead to higher fluxes, which are ?70% greater than that of graphene nanopores. MoS2 membranes show much better performance than traditionally used reverse osmosis filter membranes. Despite these advantages, these technologies are unaffordable due to their high cost and hence development of filters based on these materials for controlling CKDU in Sri Lanka is not realistic. On the other hand, filters developed using low-cost and readily available materials such as burnt bricks, sand etc. are not 100% efficient in removing fluoride in waters used by people living in CKDU endemic areas. As such, we thought of developing low-cost and highly-efficient filter medium to remove fluoride from drinking waters. In this research, we developed a novel method to prepare highly porous nanoparticles of calcium carbonate in its spherical vaterite polymorph and we studied their efficacy in removing fluoride from aqueous solutions. We find that these nanoparticles are highly efficient in removing fluoride from water containing ppm levels of fluoride; levels that are comparable to those present in well waters of CKDU affected areas. In this communication, we reveal our study on fluoride removal from water using vaterite nanoparticles. 

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