Abstract
The presence of excessive amounts of fluoride than prescribed standards has been reported in various sources of domestic water supply around the slopes of Mount Meru and other parts in Tanzania. Efforts to remove the excessive fluoride have been carried out using various technologies. In this study, electrocoagulation experiments were carried out to assess its efficiency on fluoride removal. The fluoride concentration tested ranged from 1.37 to 48 mg/L in both synthetic and natural waters. The voltage applied in the electrocoagulation (EC) process ranged from 0 to 50 V while maintaining pH values of 4 to 9. The representative experimental results for the Ngarenanyuki river water with initial fluoride concentration of 29.5 mg/L accomplished a removal efficiency of 90% at an optimal electrolysis time of 30 min, an applied voltage of 30 V and an optimal pH of 6. The method showed efficient fluoride removal in water to allowable limits by World Health Organization (WHO) and Tanzania Bureau of Standards (TBS) (1.5 mg/L). Despite the voltage applied (30 V), the pH at neutrality remained unchanged thus making the process more efficient. At this voltage (30 V) the process has been reported previously also to have the capability of disinfecting the water and hence rendering such water safe for use.
Keywords
Introduction
Depending on its concentration, fluoride in water can have both beneficial and harmful effects on the environment and human health. The concentration of fluoride around 1 mg/L in drinking water can help in teeth decay prevention [29]. However, the long-term consumption of water containing an excess of fluoride can lead to fluorosis of teeth and bones [28,29]. Globally, fluoride has become a source of some health concern when consumed either at very low or very high concentrations. A fluoride concentration of 1.5 mg/L is the amount recommended by the World Health Organization [28] and the Tanzania Bureau of Standards [26] in drinking water. Exposure to lesser concentrations than this recommended amount can lead to tooth decay; whiles prolonged exposure to greater concentrations does have harmful effects such as dental fluorosis, retarded growth of children, and skeletal defects [17,29].
The contamination of water with fluoride can occur naturally in regions with geological deposits of marine origin and at the foot of volcanic mountains such as the mount Meru considered in this present study [30]. Furthermore, groundwater gets polluted due to deep percolations from intensively cultivated fields due to various ecological factors either natural or anthropogenic, liquid and solid wastes from industries, disposal of hazardous wastes, sewage disposal, etc. Drinking water in Tanzania is reported to have a fluoride concentration of 20 mg/L and beyond in some areas under the Mount Meru slopes [5]. This is way above the recommended levels making defluoridation of drinking water necessary for human health.
Various methods for defluoridation from water such as adsorption, coagulation and flocculation, ion exchange, reverse osmosis, chemical sedimentation, electrodialysis, electrocoagulation (EC), and nanofiltration have been under investigation [5,11,21,27]. Chemical coagulation requires a huge amount of chemicals and produces large volumes of sludge which makes it very costly and undesirable. Other membrane filtration technologies such as electrodialysis, reverse osmosis, and nanofiltration requires energy, high maintenance costs and produces a very concentrated sludge [2,22,27]. The Bone char defluoridation technique is the method that is widely applied in Tanzania but it requires high maintenance cost and social-cultural issues which limits its application [18].
Quite recently, there have been numerous studies concentrated on electrocoagulation (EC), which is an effective process used to destabilize and remove finely dispersed particles like fluorides from water and wastewater. Electrocoagulation is a simple electrochemical process in which voltage is applied across two or more metal electrodes (aluminum or iron) in an EC reactor containing water and coagulants in the form of metal cations, which are generated in-situ by the dissolution of the sacrificial anode. [13]. Mixing of the coagulant leads to flocculation and precipitation that results in the removal of dissolved and suspended pollutants. During the same process (see fig.SM2a), hydrogen is generated at the cathode and oxygen at the anode, resulting in further removal of pollutants by flotation. The reactions that occur on aluminum electrodes are illustrated (see fig.SM2a) in equations 1?3:
The oxidation reaction that takes place at the anode,(1)Al–3e? ? Al3+
The reduction reaction that takes place at the cathode,(2)2H2++2e? ? H2
The hydrolysis reaction,(3)Al3+ +3H2O ? Al (OH)3 + 3H2+
Aluminum ions react with water to generate solid aluminum hydroxides. Flocks are formed by the precipitates that combine with water contaminants, which depend on the metal hydroxides formed by hydrolysis [12]. These coagulants aggregate suspended particles and adsorb the fluorides (F) to form the aluminum complexes. Al(OH)3 flocks are believed to adsorb F strongly as indicated in Eq. (4).(4)Al (OH)3 +xF ?Al (OH)3?x Fx+xOH
In comparison, coagulant salt is added in conventional coagulation and pollutants are removed by flocculation, and settling only. Electrocoagulation has several other advantages, such as electro-oxidation, higher efficiency, in situ generation of coagulant ions, lower sludge production, no mechanical parts, and low operation and maintenance costs. Moreover, it is a self?sustained technique that does not require filter/ membrane exchange since it can operate with automatic backwash unlike other techniques applied in Tanzania. In conventional coagulation (CC), the pH of the water decreases, requiring post-coagulation neutralization. Conversely, in EC, the pH of the water can increase, decrease, or remain neutral depending on the nature of the contaminant and initial pH [1,6,24]. The floc formed by EC is relatively large and contains less bound water. They also are more stable and, therefore, responsive to filtration [4]. Electrocoagulation has been widely applied for the purification of wastewater and leachate, but its use in the field of drinking water is relatively limited. Electrocoagulation has been used to remove a wide variety of pollutants from water and wastewater, including suspended solids, fluoride, chromium, nitrate, phosphate, arsenic, hardness, algae, oils and greases, dissolved organic carbon, several types of dyes, and for desalination.
The Electrocoagulation technique requires good maintenance of the aluminum anodes to ensure effective fluoride removal. Furthermore, EC has also been shown to be effective in disinfection as well. According to [7] it shows that electrocoagulation using aluminum? electrodes has been effective in disinfection with a significant removal efficiency of 98% with an increase in time at an applied voltage of 100?200 mV. The E.coli eliminating performance of 99.8% was also observed at an electrolysis time of 2 min at an applied voltage of 4 V [15] which provide safe drinking water.
This study aimed at investigating and optimizing the Electrocoagulation process for fluoride removal from selected sources of domestic water supply around the Mount Meru areas in Arusha- Tanzania (see fig.SM1).
*Read the full-text article online at https://www.sciencedirect.com/science/article/pii/S2468227621000417
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