Surface modification of pumice enhancing its fluoride adsorption capacity: An insight into kinetic and thermodynamic studies

Highlights

• A cheap geomaterial called pumice is identified and used.

• Modification of pumice using chemical agents improved the surface area and hence the defluoridation capacity.

• Desorption of fluoride up to 100% is achieved which proves the effective reusability of the pumice material.

Abstract

The present research contribution is pertaining to the surface modification of natural pumice (NP) using aqueous solution of magnesium chloride (MgCl₂) and hydrogen peroxide (H₂O₂) and exploring the fluoride uptake capacity between the natural and modified pumice materials. The effect due to hydrogen peroxide, an oxidant, towards modification of pumice surface was greater than the magnesium chloride solution and showed a substantial increase in the specific surface area of 53.11 m² g⁻¹ and 41.63 m² g⁻¹ respectively for hydrogen peroxide modified pumice (HMP) and magnesium chloride modified pumice (MGMP) as compared to that of NP of 2.34 m² g⁻¹. The extent of surface modification with enhanced porosity in MGMP and HMP was apparent from the recorded SEM patterns. XRD and FTIR studies of surface modified pumice did not show any structural distortion. In this contribution, the fluoride uptake capacity of NP was initially studied and then compared with the modified pumice adsorbents, MGMP and HMP. From the results of various kinetic models, pseudo-second-order fit well with the fluoride sorption kinetics conducted at different initial fluoride concentrations. The experimental adsorption isotherm complied with Freundlich type with K_F less than 1 ascertains the dominance of multisite adsorption on the surfaces of NP, HMP and MGMP. Thermodynamic parameters viz., ΔG°, ΔH° and ΔS° reveal that the fluoride adsorption process was feasible and endothermic nature associated with increased randomness. Desorption potential of NP of 88% was improved to 98% and 100% for the modified MGMP and HMP adsorbents respectively.

Introduction

Due to the abundance of fluorine in the earth crust, fluoride compounds are usually found in groundwater at low concentrations [1]. Some fluoride compounds, such as sodium fluoride and fluorosilicates, dissolve easily into ground water as they move through gaps and pore spaces between rocks. Apart from fluoridated water, fluoride is ingested from other sources such as pesticides, post-harvest fumigants, air, food, salt, medications, toothpaste, dental restorations, and health supplements. Children aged 8 years and younger, when exposed to fluoride greater than 1.5 mg L⁻¹ have serious health problems such as skeletal fluorosis, mottling of teeth and lesions of endocrine glands, thyroid, liver and some other organs. Exposure to excessive consumption of fluoride over a lifetime may lead to increased likelihood of bone fractures leading to pain and tenderness, osteosclerosis (brittle bones and calcified ligaments), cancer and neurological impairment in human beings [2]. Another emerging area of interest is the interaction between fluoride and iodine resulting in a functional iodine deficiency. Iodine is required for the proper functioning of many organs of the body and reduced tissue iodine levels, possibly through the inhibition of mammary gland deiodinases by fluoride, may be a factor in the development of breast cancer [3]. World Health Organization (WHO) suggested a globalized standard for fluoride in the range of 0.5–1.5 mg L⁻¹. But the peoples’ consumption of groundwater with fluoride greater than the permissible level seems inevitable either due to lack of awareness or water scarcity. Those suffering from fluorosis complain of fatigue. Typically the bones of the backbone, neck, hands or legs of the affected person become fragile and lead to deformity. As dental or skeletal fluorosis has no permanent treatment, the only possible remedy is prevention by having the fluoride intake within safe limits. The various available defluoridation technologies, such as coagulation/chemical precipitation [4], [5], adsorption/ion exchange [6], membrane filtration [7], [8], electrolysis [9], electro coagulation [10], fluidized bed crystallization [11], nano-filteration [12] and adsorption [13], [14] have been developed for the fluoride removal from wastewater. Mohapatra et al. [15] and Miretzky and Cirelli [16] reviewed about the removal of fluoride using chitosan composites and various materials of natural and synthetic origins respectively. In the recent years, low cost materials are of great interest to carry out the defluoridation process both in the natural and modified forms [17], [18]. Geomaterials are low cost adsorbent resources offering frequent applications to water and wastewater treatments. They are mostly available in the local sources and the requirement for processing them is minimal. Geomaterials such as fired clay [19], ando soil [20], bentonite and charfine [21], tertiary soil [22], kaolinite [23], titanium rich bauxite [24] and surface tailored zeolites [25], [26], [27], [28], [29] were used as adsorbents for defluoridation. The uptake capacity (mg g⁻¹) of these geomaterials was inferred in the following order: Ando soil (5.51) > Titanium rich bauxite (3.7–4.1) > Charfine (0.95) > Bentonite (1.15) > Kaolinite (0.67) > Fired clay (0.20–0.29) > Tertiary soil (0.15).

In the series of geomaterials, a porous and amorphous material which consists mainly of SiO₂ is pumice. Apart from its traditional applications in construction industry [30], [31], a possible extension with wide scope was further studied by the researchers in the field of wastewater treatment. The natural and modified pumice were explored to be a better adsorbent for organic and inorganic water pollutants in the recent years [32], [33], [34], [35], [36], [37], [38], [39], [40]. In continuation to the study conducted on defluoridation capacity of pumice functionalized by cationic surfactant [41], the present contribution is attempted with a focus on the fluoride uptake capacity of low cost, naturally occurring and chemically modified pumice materials. The real notion of the present research work was to explore and compare the defluoridation capacities of the natural (NP) and surface modified pumice materials (HMP and MGMP).