Year

2006

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining and Environmental Engineering

Abstract

Excess fluoride ion in drinking water is a serious public health problem because it has beneficial and harmful effects. When an optimum amount of 1 mg/L is present in drinking water it helps prevent decay in teeth, but long term consumption of water containing excessive fluoride (≥ 1.5 mg/L) can lead to fluorosis of the teeth and bones. There are several methods for removing fluoride, and one method that has recently received attention is electrocoagulation (EC) technology. The word "electrocoagulation" will be sometimes used with "electroflotation" and can be considered as the electrocoagulation/flotation (ECF) process. Through the process of electrolysis, coagulating agents such as metal hydroxides are produced. When aluminium electrodes are used, the aluminium dissolves at the anode and hydrogen gas is released at the cathode. The coagulating agent combines with the pollutants to form large size floes. As the bubbles rise to the top of the tank they adhere to particles suspended in the water and float them to the surface. In fact, a conceptual framework of the overall electrocoagulation process is linked to coagulant generation, pollutant aggregation, and pollutant removal by flotation and settling.

Batch experiments were designed and conducted to study the different operational parameters such as current density, electrolysis time, pH of solution, distance between electrodes, initial fluoride concentration, electrolyte conductivity, particle size, zeta potential, mass ratio of aluminium and fluoride in solution (Al3+/F- mass ratio), and ions effects (specially Ca2+ effect) on the defluoridation by EC process. In the EC process the amount of aluminium ion produced is proportional to the charge which is a product of the current supplied and electrolysis time. The charge affects fluoride removal significantly, however to avoid excessive energy consumption one must limit the charge applied. In this batch ECF process, the minimum electrolysis time required to reduce the fluoride concentration to the desirable concentration (F- =1 mg/L), is defined as the optimum detention time (dto). The results of batch experiments showed that the residual fluoride concentration reduced from 10 to 1 mg/L when dto was 55, 45, and 35 min at a 1.5, 2 and 2.5 A current range, respectively. The optimum charge values were found between 5000 -5400 C and 9500-10500 C respectively for the initial fluoride concentrations of 10 and 25 mg/L. It has been confirmed that the rate of F- removal follows a simple first order process. The batch experimental results showed that the Al3+ /F- mass ratio was between 13 and 17.5 and the defluoridation process is found to be more efficient when pH is kept constant between 6 and 8. Based on the effects of ion competition, the presence of Ca2+ ion enhances the defluoridation process.

Continuous flow experiments were also designed and conducted to investigate the effects of different parameters including current input, initial concentration of fluoride, initial pH, and flow rate on the efficient removal of fluoride. The most efficient treatment was obtained from the highest rate of charge as observed in the batch reactor. An XRD analysis of the composition of dried sludge obtained by electrocoagulation shows the formation of aluminium fluoride hydroxide complexes [AlnFm(OH)3n-m] and confirms the main mechanism for removing fluoride in both the batch and continuous flow reactors. To reduce F- concentration from 5 to 1 mg/L, it was found that the total operational cost of the current ECF process is less than % of the Nalgonda (NA) process.

The experimental results further showed that the rate constant (Kexp) depends on the independent variables or critical parameters such as the current concentration (I/V), the effect of Ca2+ concentration, distance between the electrodes (d), pH of the solution, and the initial concentration of fluoride (Co). An empirical model is developed to predict both the optimum detention time and optimum flow rate for fluoride removal. The results show good agreement between the experimental data and the predictive equation. Overall, the electrocoagulation technology using aluminium electrodes is an effective process for defluoridation of water that contains excess fluoride.

Download

Share

COinS
 

Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.