An empirical model for defluoridation by batch monopolar electrocoagulation/flotation (ECF) process
Introduction
Most groundwaters have low or acceptable concentration of fluoride (<1.5 mg/l) in the world [1]. In groundwater, the natural concentration of fluoride depends on the geological, chemical and physical characteristics of the aquifer, the porosity and acidity of the soil and rocks, the temperature, the action of other chemical elements, and the depth of wells. Due to these variables, the fluoride concentrations in groundwater can range from less than 1 mg/l to more than 35 mg/l. In India and Kenya, concentrations up to 38.5 and 25 mg/l have been reported, respectively. The total number of people affected is not known, but an estimate would number in the tens of millions. In 1993, 15 of India's 32 states were identified as an endemic for fluorosis. A study by UNICEF shows that fluorosis is endemic in at least 27 countries across the globe [1]. These countries are: Algeria, Argentina, Australia, Bangladesh, China, Egypt, Ethiopia, India, Iran, Iraq, Japan, Jordan, Kenya, Libya, Mexico, Morocco, New Zealand, Palestine, Pakistan, Senegal, Sri Lanka, Syria, Tanzania, Thailand, Turkey, Uganda, and United Arab Emirates. In Australia, the fluoride concentration was recorded 13 mg/l in a water bore near Indulkana region; however, it is not used for human consumption [2].
Health impacts from long-term use of drinking water with a high fluoride concentration have been summarized in Table 1 [3], [4]. The maximum acceptable concentration of fluoride in water is 1.5 mg/l. Fluoride also can be found in industrial wastewaters, such as in glass manufacturing industries [5] and in high concentrations in semiconductor industries [6]. The discharge of these wastewaters without treatment into the natural environment may also contribute to groundwater contamination. To control fluoride concentrations in drinking water, several treatment options exist. A number of defluoridation processes, such as adsorption [7], chemical precipitation [8], electrodialysis [9], and electrochemical methods [10], [11] have been tested globally. In the precipitation technology, alum or combination of alum and lime are added to water with low and high concentrations of fluoride, respectively. Fluoride is then removed by flocculation, sedimentation and followed by filtration. Using chemical coagulants for precipitation is one of the most essential processes in conventional water and wastewater treatment. However, the generation of large volumes of sludge, the hazardous waste categorization of metal hydroxides, and high costs associated with chemical treatment have made chemical coagulation less acceptable compared to other processes.
An effective process that produces less waste sludge and that could replace the conventional chemical coagulation, increase the process efficiency and that can be retrofitted to existing facilities would be highly desirable. A promising process is electrocoagulation/flotation (ECF), which is an electrochemical technique, in which a variety of unwanted dissolved particles and suspended matter can be effectively removed from an aqueous solution by electrolysis. More recently ECF has been suggested as an alternative to conventional coagulation [12]. Some researchers [13], [14], [15], [16], [17], [18] have in fact demonstrated that electrocoagulation using aluminium electrodes (as anodes) is effective for defluoridation in water and industrial wastewater treatment. It has been suggested that the electrocoagulation process for fluoride removal does not require a substantial investment [10]. Hu et al. [14] reported that defluoridation efficiency in the ECF system was almost 100% in solutions without co-existing anions (Cl−, NO3−, SO4−). Shen et al. [15] reported that the combination of electrocoagulation (EC) and electroflotation (EF) process was successfully applied in treating wastewater-containing fluoride.
The previous results showed that the defluoridation process is more efficient when pH is kept constant between 6 and 8 during experiments. So, the pH was kept constant between this range in each run and the effects of initial pH have not been significant. Electrolysis time (t) determines the rate of dissolution of Al3+ ions, as it strongly depends on the current value in the ECF process. Faraday's law can be used to describe the relationship between current value, volume of reactor and the amount of aluminium, which goes into solution. In batch ECF process, a minimum electrolysis time is required to reduce the fluorine concentration to the NHMRC and ARMCANZ [3], and WHO [4] drinking water guidelines (0.5 < F− ≤ 1.5 mg/l) and is called the detention time (dt). The main aim of this research is firstly to develop an empirical model using critical parameters such as current concentration (I/V), electrode distance (d), and initial fluoride concentration (C0) on evaluation of the rate constant (K) for fluoride removal by a monopolar ECF process. Secondly, to determine the optimal detention time (dto) required to achieve a desirable fluoride concentration.
Section snippets
Fundamentals of ECF
In the ECF process, when aluminium electrodes are used, the aluminium dissolves at the anode (Eq. (1)) and hydrogen gas is released at the cathode (Eq. (2)). During the dissolution of Al at the anodes various aqueous aluminium species are produced, which depend on the solution chemistry. The aluminium species act as a coagulant by combining with the pollutants to form large size flocs. Interactions occurring within an electrocoagulation reactor are shown in Fig. 1. The electrolytic dissolution
Bench scale batch ECF apparatus
A laboratory batch electrocoagulation reactor was designed and constructed to the dimensions shown in Fig. 3. In the electrochemical cell, five aluminium (purity of Al 95–97%, Ullrich Aluminium Company Ltd., Sydney) plate anodes and cathodes (dimension 250 mm × 100 mm × 3 mm) were used as electrodes. Fig. 3 also shows the electrode arrangement where three aluminium cathodes were interspersed with two aluminium anodes. The electrodes were connected using a monopolar configuration in the
Effect of t and I/V
In most electrochemical processes, current (I) and electrolysis time (t) are the most important parameters for controlling the reaction rate in the reactor. Current not only determines the coagulant dosage but also the mixing rate within electrocoagulation. Electrolysis time (t) determines the rate of dissolution of Al3+ ions, as it strongly depends on the current value [10], [13]. It is important to determine the Al3+ dose achieved within the EC process. The higher the aluminium dose the
Analysis of data
Based on the experimental results, presented in Fig. 6, Fig. 7, the rate constant (Kexp) may depend on the current concentration (I/V), initial fluoride concentration (C0), and electrodes distance (d) for a constant temperature and pH. The Kexp can be expressed as:A multiple regression analysis is performed using the SPSS statistical package. The results are shown in Table 3 and present a high degree of correlation (R2 = 0.99) for the following equation:
Conclusion
An empirical model is developed to relate the critical parameters such as current concentration, electrode distance and initial fluoride concentration with the rate constant (K) for fluoride removal using monopolar ECF process. Based on the operational parameters, an empirical equation is given to calculate the optimal detention time for fluoride removal. The results show good agreement between the experimental data and the predictive equation. It is also found that there is no significant
Acknowledgements
The financial support provided by the Ministry of Health of the Iranian Government and Qazvin University of medical sciences is gratefully acknowledged by the first author. The assistance provided by the University of Wollongong's Sustainable Water and Energy Research Group, the Environmental Engineering laboratory senior technical staff, Joanne George and Norm Gal, is much appreciated. The assistance provided by Stephen Rainow from Nganampa Health Council for supplying the high fluoride bore
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