In India, most of the work on defluoridation process is done using batch mode. Here, in the present study the main focus is on electrolytic defluoridation by continuous process which is more efficient and less time consuming in comparison to the batch process. The methodology applied in the present study is illustrated here.
In its simplest form, an electro-coagulating reactor is made up of an electrolytic cell with one anode and one cathode. When connected to an external power source, the anode material electrochemically corrodes due to oxidation, while the cathode is subjected to passivation. A feed tank of 200 L (A nestable tank) capacity was filled with raw water of initial fluoride concentrations of 5, 8, 6 and 10 mg/L. The initial pH of raw water was adjusted around 6.5-6.9 by adding concentrated HCl. A peristaltic pump of maximum efficiency of 1L/min was used to maintain the flow rate accordingly for a continuous process. The flow rate was adjusted by setting the rpm (rotation per minute) on the digital display.
Experiments were performed in a reactor consisting of plastic bucket of 19 L capacity. A direct current (DC) by stabilized power supply (TESTRONIX 34C, Volt and Ampere Digital Display) was applied to the terminal electrodes in which electrical current was controlled by a variable transformer. Constant current was maintained during each run by appropriately adjusting the impressed cell voltage from a regulated DC power supply. A bucket of 52 cm height and diameter of 44 cm was used as a settling tank in the lab scale experiment for electrolytic defluoridation by continuous process.
Aluminum plates were cut from a commercial grade aluminum sheet (99% purity) of 2 mm thickness each with a dimension of 100 mmx180 mm and an effective area of 180 cm2 on each side. Distance between the electrodes was 5 mm. Monopolar configurations with three aluminium plate electrodes were used. Central plate was connected to anode and two end plates were connected to cathode. The electrodes were designed with a surface area to volume ratio of 7.2 m2 / m3 which is within cited range of 6.9 – 43 m2 / m3 as reported by P.K. Holt, 2003.
Raw water of various fluoride concentrations was prepared by diluting the stock solution in tap water. The experimental procedure was divided into two major steps i.e. start up batch process and Continuous treatment process. Each run was conducted using 19 litre of raw water in the reactor for start-up batch process and rendering it upto the desired limit of 1 mg/L by providing proper detention time. Detention time (t) is calculated by using Eq. 1 based on Faraday’s law:
m =K. i. t (1)
m = Weight of aluminium dissolved (g)
K = Electro-chemical constant i = Current (Ampere) t = Time of electrolysis Value of m can be calculated by;
m= (C0 – Cf) x volume x (Al / F) (2)
C0 = Initial fluoride concentration = 5 mg/l Cf = Fluoride conc. in treated water = 1 mg/L Volume = Volume of treated water Al/F ratio = 3 (At pH 6)
All experiments were conducted at room temperature in the range 26 – 28 C. Concentrated hydrochloric acid was used for pH adjustment to 6.0-6.5 which is the optimum range for efficient electrocoagulation process. Current was varied from 2.0A to 3.0A, however, it was held constant for each run by appropriately adjusting the impressed cell voltage from a regulated DC power supply. The Ion Selective Electrode method was used to determine concentration of fluoride and in the treated water, residual aluminium estimation was carried out by Eriochrome cyanine R method.