Application of aluminum-modified food waste biochar as adsorbent of fluoride in aqueous solutions and optimization of production using response surface methodology

Highlights

• Aluminum impregnated biochar from food waste (Al-FWB) synthesized for fluoride removal.

• Pyrolysis temperature and time, and aluminium content were optimized using RSM.

• Maximum adsorption capacity of Al-FWB was 123.4 mg/g.

• High buffering-capacity of Al-FWB enables high removal efficiency over a wide pH.

Abstract

Adsorption of fluoride from aqueous solutions by aluminum-impregnated biochar derived from food waste (Al-FWB) was studied. The individual and interactive effects of various factors on fluoride adsorption, including pyrolysis temperature and time, and aluminum content, were investigated. The optimum conditions for the synthesis of Al-FWB, predicted through a Box–Behnken-based response surface methodology model, which were as follows: a temperature of 315 °C, time of 0.65 h (39 min), and an aluminum content of 5.89%. Batch experiments were conducted to assess the feasibility of using Al-FWB for fluoride removal, and its mechanism. The Langmuir isotherm model and pseudo-second-order kinetics proved to be the best fit for the equilibrium data, with a maximum adsorption capacity of 123.4 mg/g. Thermodynamic results revealed a spontaneous endothermic reaction for fluoride adsorption. The Al-FWB showed a superior removal efficiency (91.4%) in a wide pH range (5–11) due to its pH buffering capacity during the adsorption process. The influence of co-existing anions on the fluoride adsorption was as follows: PO₄³‾ > SO₄²‾ > HCO₃‾ > NO₃‾. Therefore, Al-FWB can be used as an effective adsorbent to remove fluoride from aqueous solutions.

Introduction

The concentration of fluoride in fresh water varies significantly, depending on the presence of local fluoride-containing minerals. Fluoride contamination in groundwater has caused many human casualties, with only approximately 2.5 billion people having access to safe and potable water [1]. Fluoride ion at low concentrations of between 1.0 and 1.5 mg/L is necessary and beneficial for human health, especially in promoting the development of strong bones and the formation of dental enamel [2]. The World Health Organization (WHO) claims that the concentration of fluoride in drinking water should not exceed 1.5 mg/L, as chronic ingestion can cause severe diseases such as dental and skeletal fluorosis [3]. However, a high concentration of fluoride in water resources is prevalent in many locations worldwide, from 0.01 to 3 mg/L in fresh water, 1.0–35 mg/L in groundwater [4,5], and 100 to 6500 mg/L in typical wastewater stemming from the manufacturing process of aluminum fluoride [6]. It has been estimated that more than 200 million people in the world consume in excess of 1 mg/L of fluoride from drinking water [7].

Therefore, several methods such as adsorption [8], precipitation [9], electrode/sorption [10], ion exchange [11], and reverse osmosis [12] have been tested to successfully remove excessive fluoride from water, including wastewater. The most widely-used technique for fluoride removal from drinking water is adsorption because of the method's high efficiency, low cost, and simple operation [4]. According to the WHO and US Environmental Protection Agency, one of the best adsorbents to use in fluoride adsorption is activated alumina because it is effective and widely available. It can also remove a number of other toxic elements that often coexist with fluoride in groundwater, including arsenic and selenium [13]. However, in spite of its wide use, it is fairly expensive, and its adsorption capacity has been significantly impacted by pH and the presence of major anions such as hydrogen phosphate, bicarbonate, sulfate, silicate, and chloride [14]. Therefore, it is essential to search for equally effective adsorbents that are more affordable than activated alumina and environmentally friendly, while not requiring any costly additional pre-treatment stage in the fluoride removal process.

Food waste is the largest waste stream of municipal solid waste (MSW) in the Republic of Korea, accounting for approximately 23% of MSW, which translates to 48,499 ton/day [15]. Food waste in Korea has a high moisture content and high salinity, requiring additional pre-treatment and recycling methods [16]. Several techniques are employed worldwide to process waste into value-added products and uses, including compost, biogas, animal feed, fertilizer, and chemicals [17,18].

Biochar is a carbon-rich and stable carbon-dominant product that is obtained through high-temperature processes from solid by-product (biomass) under oxygen-limited conditions [19,20]. It has the potential of reducing greenhouse gas emissions by approximately 870 kg of CO₂ equivalent (CO₂-e) per ton of dry feedstock throughout its life cycle [21]. The application of biochar derived from food waste will address the problem of excessive amounts of food waste ending up in landfill sites, as well as produce a value-added product that can be utilized to remove contaminants from aqueous solutions. Biochar is also a good adsorbent for water and wastewater treatment because of high porosity, low cost, environmental friendliness, good stability, good physical/chemical surface characteristics, and easy synthesis methods [22]. Biochar derived from food waste is green and sustainable because raw material can be readily obtained in bulk quantities from abundantly available waste biomass and the making of biochar converts waste into value-added products.

Recently, there has been much interest in the modification of biochar, referred to as “engineered/modified biochar”, by either transforming the production process or loading the feedstock with chemical agents aimed at enhancing its adsorption capacity [23,24]. Modification of the biochar surface can be achieved through different methods, such as steam activation [25,26], heat treatment [27], acidic modification [[28], [29], [30]], alkaline modification [[31], [32], [33], [34]] and impregnation [[35], [36], [37], [38]]. Biochar modified through impregnation produces new composites with single or multiple metal oxides, metal hydroxides, and metal elements, and can significantly improve its functionality [23,24,36,37,39,40].

Carbothermal conversion of biomass into metal–biochar composites improves carbon pore structure, increases surface zeta potential, ensures uniform distribution of metals, and enriches surface functional groups such as hydroxyl, carboxyl, and carbonyl, thus optimizing the chemical composition for more active sorption sites [41]. Fluoride ion has a strong affinity for metal ions including Al(Ⅲ), Fe(Ⅲ) and La(Ⅲ) because of its high electronegativity and small ionic size [42,43]. Research has demonstrated that the presence of aluminum (Al) hydroxide/oxide on the surface of adsorbents can increase the selectivity and level of fluoride removal in aqueous solutions [44,45]. This study examines a method for the synthesis of aluminum-impregnated food waste biochar (Al-FWB) as a novel adsorbent that can be used to treat fluoride-containing wastewater and drinking water.

The objective of this study is to investigate the optimization of fluoride capacity of Al-FWB by using a response surface methodology (RSM) with independent variables such as temperature, reaction time, and impregnated dosage of Al. Experimental and model analyses for kinetic, equilibrium, and thermodynamic adsorption of fluoride by Al-FWB investigate the characteristics of Al-FWB and the mechanisms of fluoride adsorption by Al-FWB. The effect of various experimental parameters such as adsorbent dose, pH, and ion composition are explored to assimilate their impact on fluoride removal efficiency from aqueous solutions.