Fluoride (F) contaminated ground water poses a serious public health concern to rural population with unaffordable purification technologies. Therefore, development of a cost-effective, portable, environment and user-friendly defluoridation technique is imperative. In the present study, we report on the development of a green and cost-effective method that utilizes Fe₃O₄ and Al₂O₃ nanoparticles (NPs) that were synthesized using jojoba defatted meal. These NPs were impregnated on to polyurethane foam (PUF) and made into tea infusion bags. The Al₂O₃ NPs-PUF displayed a higher water defluoridation capacity of 43.47 mg g^-1 of F as compared to 34.48 mg g^-1 of F with Fe₃O₄ NPs-PUF. The synthesized Al₂O₃-PUF infusion bags removed the F that was under the permissible limit of 1.5 mg L-¹. The sorption experiments were conducted to verify the effect of different parameters such as pH, contact time, size of PUF and initial F concentration. The different properties of adsorbent were characterized using a combination of FESEM, EDX, XRD and FTIR techniques, respectively. The calculated total cost per NPs-PUF pouch developed is as low as US $0.05, which makes the technology most suitable for rural communities. This paper will be beneficial for researchers working toward further improvement in water purification technologies.

Water scarcity is considered as a major crisis of the 21^st century. It is reported that in 2015 approximately 663 million people lack access to safe drinking water worldwide¹. Fluoride (F) is a major contributor to the world water crisis, affecting about 200 million people worldwide. It is reported that around 24 countries are severely affected by high F concentration in drinking water². The rural population is more prone to F contamination as in some places, the available techniques are neither acquainted nor affordable. The fluorosis is reported more prevalent in rural population due to excess F contaminated water inevitably consumed by the rural population^{3, 4}. F is known to cause mottled enamel, osteoporosis, crippling skeletal fluorosis, thyroid imbalance, growth retardation, kidney imbalance, types of morbidity and in severe cases leading to mortality⁵. Several methods have been developed to efficiently remove F from water, including nanofiltration, reverse osmosis (RO), coagulation, electrocoagulation, electrochemical oxidation, ion exchange and adsorption^6,7,8,9. Till date, the defluorinated water at the community level in the outreach areas is far away due to its high cost and complex treatment modalities.

After water, tea infusions are the most popular beverages consumed worldwide by communities¹⁰. It is well-known that tea plants can accumulate F, for example, in 1930 it is reported that Camellia sinensis (tea plant) is a hyperaccumulator plant of F¹¹. F concentrations above permissible limit were reported in tea drinks of India (1.55–3.21?mg?L^?1), China (1.60–7.34?mg?L^?1), Kenya and Tibet (2.59?mg?L^?1)^12,13,14. The cost and effectiveness of the defluoridation techniques are still not satisfactory and thus required further improvements.

Among the reported techniques, adsorption is considered more advantageous for the rural population as it is inexpensive, rapid, easy to operate and highly efficient¹⁵. Several traditional adsorbents were reported such as activated carbon, zeolites and bone char but nanostructural materials proved highly efficient for F removal because of their high surface-to-volume ratio^16,17,18. Various techniques have been known for nanomaterials production, such as reverse micelles, microwave, electrochemical, nonelectrochemical and green synthesis technique^{19,20,21,22,23}. For the synthesis of nanomaterials, green chemistry route proved beneficial as compared to the chemical methods in term of its cost-effectiveness, environment-friendly and scalable properties²⁴. Yet another challenge the researchers are facing is the separation of NPs from suspension after adsorption is the fact that NPs suspension form fine colloids in aqueous solution. To solve this problem, several researchers have used nanomaterials impregnated on support matrices. Diverse studies have been conducted using nanomaterials support matrices such as poly (acrylic acid) (PAA), polyurethane (PU) and poly (vinyl alcohol) (PVA)²⁵. Recently, polyurethane foams (PUF) has been widely utilized and found promising in various water filtration systems because of its outstanding features of high-temperature resistance, UV resistance, enhanced mechanical property, abrasion resistance, easy availability and low cost^{26, 27}. The impregnation of nanomaterials on the matrices can be achieved through various processes, such as surface nucleation, blending and dip coating^{25, 28}. Dip coating technique was proved to be more favorable for impregnation due to advantages of its low cost and easy handling.

Recently, Al₂O₃/bio-TiO₂ nanocomposite (ABN) and Al₂O₃/bio-TiO₂ nanocomposite impregnated into electrospun TPU nanofiber membrane (ABN/TPU-NFM) have been developed for defluoridation of water samples. The adsorption capability of the developed adsorbent reported was 1.9?mg?g^?1. However, the adsorption capacity of material developed was relatively low, expensive and uneasy and unsuitable for rural areas²⁹.

Therefore, the present research for defluoridation is focused on the development of easily affordable technology for the rural population. Here, Jojoba (Simmondsia chinensis) seed meal was utilized for green synthesis of NPs. The seed meal was obtained as a waste byproduct during the oil extraction process which we utilized for NPs synthesis. None of the available literature focused on the F removal using Fe₃O₄ and Al₂O₃ NPs impregnated PUF. Also, very few reports are available on the F removal from tea infusions, there is requirement to design cost-effective defluoridation technology for tea infusions. Kinetic and isothermic parameters were illustrated in order to describe the F adsorption mechanism. The interactions between F ions and adsorbent were analyzed by a combination of physico-chemical methods, such as field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and Fourier transformed infrared (FTIR). Based on our experimental data, we demonstrated the successful F adsorption in samples and its mechanism through following three steps: (a) Green synthesis of Fe₃O₄ and Al₂O₃ NPs using novel defatted jojoba seed meal; (b) These NPs impregnated in polyurethane foams (PUF) and tested for defluoridation of water and tea samples; (c) Al-PUF tea infusion bag like pouches were found to be relatively more efficient in defluoridation of tea infusions. Overall, this work provides an inexpensive tool toward addressing public health and safety, especially at the resource-limited rural areas to mitigate health risks associated with ground water F contaminations. …

*Read the full article online at https://www.nature.com/articles/s41598-017-08594-7