Bio-inspired materials for defluoridation of water: A review

Highlights

• Fluoride is essential for potential health benefits however, excess is hazardous.

• In India, over 9% of the population are affected with high fluoride in ground water.

• Current defluoridation methods are expensive, less efficient or pH dependent.

• Deflu oridation of water using bio-inspired material could be good alternative.

• Fluoride removal efficiency can be enhanced by chemical modification of sorbent.

Abstract

The polluted water sources pose a serious issue concerning the various health hazards they bring along. Due to various uncontrolled anthropogenic and industrial activities, a great number of pollutants have gained entry into the water systems. Among all the emerging contaminants, anionic species such as fluoride cause a major role in polluting the water bodies because of its high reactivity with other elements. The need for water remediation has led the research community to come up with several physicochemical and electrochemical methods to remove fluoride contamination. Among the existing methods, biosorption using bio and modified biomaterials has been extensively studied for defluoridation, as they are cheap, easily available and effectively recyclable when compared to other methods for defluoridation. Adding on, these materials are non-toxic and are safe to use compared to many other synthetic materials that are toxic and require high-cost design requirements. This review focuses on the recent developments made in the defluoridation techniques by biosorption using bio and modified biomaterials and defines the current perspectives of fluoride removal specifically using biomaterials.

Introduction

On a global prospect, the water bodies contribute to over 70% of the total earth’s surface, out of which only 0.002% remains fit for human consumption (Alrumman et al., 2016). Owing to its limited availability and increased need, preserving the water resources remains a tough challenge. A number of factors contribute to groundwater contamination including the geochemical processes, agricultural wastes, industrial effluents, and household wastes. Besides, the uncontrolled urbanization, industrialization and unskilled utilization of water have led to the degradation of water quality bringing them down from the WHO standards for safe drinking water. Among various pollutants that get into the water bodies, heavy metal ions and toxic anions such as fluoride are considered to be major sources of pollutants due to their hazardous risk factors associated (Chouhan and Flora, 2010; Wasana et al., 2017).

Fluoride, on the other hand, is a versatile anion being essential for dental health at lower concentrations and hazardous at higher concentrations. About 0.5–1.0 mg/L of fluoride is beneficial to dental health and exceeding the limit of 1.5 mg/L is harmful to human beings (Lennon et al., 2004). At lower concentrations, it is beneficial for the development of teeth and is used in the treatment of osteoporosis (Lizneva et al., 2018). However, chronic consumption leads to fluorosis, kidney problems and also may lead to cancer (Ghosh and Mukhopadhyay, 2019). People who consume drinking water containing fluoride concentrations above 1 mg/L are likely to be affected by dental fluorosis. If water with fluoride concentrations above 3 mg/L is consumed over a period of time, skeletal fluorosis is likely to be developed. Fluoride concentrations in this range are also said to cause gastrointestinal problems. Thus, the WHO has declared a permissible limit of 1.5 mg/L for the level of fluoride in drinking water (Cotruvo, 2017; Nath, 2019). In addition to this, fluoride contamination has shown its adverse effects on the ecosystem, especially to the plants, algae and animals (Zuo et al., 2018). Though some parts of the world have adopted the practice of adding fluoride to the drinking water to maintain its minimum concentration for its potential benefits, many regions around the world have shown fluoride concentration in groundwater as high as 30 mg/L (Fig. 1, a) (Yadav et al., 2018). India is one among the fluoride affected countries where more than 19 states are suffering from a high level of fluoride contamination in groundwater. The most affected areas are Delhi, Gujarat, Haryana, Punjab, and Rajasthan, Andhra Pradesh, Karnataka, Tamil Nadu, and Telangana. It has been predicted that the total number of people affected could be around 120 million, which counts to around 9% of the population. The most affected states in India have been predicted in Fig. 1, b (Podgorski et al., 2018).

Owing to the alarming rise in the extent of fluoride contamination the research community has come up with different techniques to detect and remove fluoride (Madhuprasad et al., 2012; Kigga and Trivedi, 2014; Madhuprasad et al., 2014; Madhuprasad and Trivedi, 2014b, a; Bhat et al., 2016; Patil et al., 2018). Defluoridation involves the complete removal of excess of fluoride to make it potable and fit for consumption. Various methods have been reported and practiced to remove fluoride from drinking water in the recent years (Mobeen and Kumar, 2017; Dubey et al., 2018a, 2018b; John et al., 2018; Mukherjee and Halder, 2018; Grich et al., 2019; Nabbou et al., 2019; Castañeda et al., 2020). Different defluoridation techniques along with their advantages and limitations have been listed in Table 1.

Though the above-mentioned methods show a higher efficiency of defluoridation, they have practical operational problems such as the requirement of high-cost instruments, adjusting and maintaining the optimal conditions, labor charge, and creation of secondary wastage. However, it has been observed that the adsorption technique is best suited for defluoridation due to a wide choice of cost-effective adsorbents. Besides, the usage of biomaterials as adsorbents could effectively reduce the cost of defluoridation even further. The adsorption process depends on the interaction between adsorbate and adsorbent which could be due to either a physical or a chemical phenomenon and via electrostatic, covalent and hydrogen bondings. Further, the process is also affected by selectivity to Fˉ ions, particle size, pore size, surface area and reusability of adsorbents. Scientists around the world are giving much attention to the development of this technique and to make the process of defluoridation as well as anion removal much simpler and economical (Park et al., 2010; Othman and Mohd-Asharuddin, 2012; Loganathan et al., 2013; Gupta et al., 2015; Joshi, 2018; Manna et al., 2018; Yadav et al., 2018). In addition, there are many reports concerned on soil sorbents (Waghmare and Arfin, 2015b; Wambu et al., 2016) and biosorbents (Mohan et al., 2014b; Bhatnagar et al., 2015). However, all these reports are diversified and it is important to coalesce these reports for a better understanding of the defluoridation techniques. Consequently, in this manuscript, we are reviewing important reports where the bio-inspired material based defluoridation techniques have been developed. Thus by reviewing major developments, we give special focus on the use and exploration of more biomaterials as they can be ideal candidates in water purification technologies. Also, this review encourages the research communities to come up with ideas to chemically modify these adsorbents where these materials can exhibit enhanced removal efficiencies.