Content. Introduction p. 3 Theory p. 4 Experiment p. 6 Data & Result p.8 Discussion p.14 Conclusion p.17 Acknowledgement p.18 Reference p.

Transcription

1 Electrocoagulation- A Potent Alternative of Wastewater Treatment 1 2 1

2 Content Introduction p. 3 Theory p. 4 Experiment p. 6 Data & Result p.8 Discussion p.14 Conclusion p.17 Acknowledgement p.18 Reference p.19 2

3 Introduction 1. Aim of the Project Industrial wastewater must be treated before being released to comply with wastewater regulations 3, aiming at improving water quality of Hong Kong by limiting the amount of chemical waste discharged to water bodies in Hong Kong. Electrocoagulation is under rapid development since it is applicable to treating a variety of effluents containing soluble substances, suspended solids, heavy metals, etc. It is known for its high particle removal efficiency 4, compact treatment facility 4 and relatively low cost 4 in removing pollutants when compared with traditional methods like filtration, precipitation and chemical coagulation. By using three water soluble dyes in this investigation, whether electrocoagulation can be efficiently used to treat soluble substances in industrial effluents in actual practice was investigated, contributing to the improvement of water quality in Hong Kong. 2. General Plan This investigation focused on the removal of soluble compounds by electrocoagulation using indigo carmine, methyl orange and methylene blue, which are all water soluble dyes. The reason why these 3 dyes were selected was that they have similar chemical structures, and by comparing the results of using methylene blue and methyl orange, whether electrocoagulation is applicable to both positive and negative substances can be determined. Besides the three dyes mentioned, sodium sulphate (Na 2 SO 4 ) was used as the electrolyte for electricity conduction during electrocoagulation. Iron plates was used as sacrificial electrodes. A colourimeter was used to determine the colour intensity of the dyes so as to monitor the concentration of the dyes at different instants of electrocoagulation. This helped check whether electrocoagulation was effective in removing the dyes used. Figure A: Indigo Carmine (C 16 H 8 N 2 Na 2 O 8 S 2 ) Figure B: Methyl Orange (C 14 H 14 N 3 NaO 3 S) Figure C: Methylene Blue (C 16 H 18 N 3 SCl) 3

4 Theory Electrocoagulation During electrocoagulation, coagulants are generated in situ. Sacrificial electrodes, and in this experiment, iron, are used. The metal anode is oxidized to form simple ions which are immediately hydrolyzed to form various hydroxides (with negative surface charge under higher ph) and polyhydroxides (with positive charge), depending on the ph of the aqueous medium 5. The products of hydrolysis of metal ions with high opposite charges neutralize the charge of ionic species and surface charge of colloid solids and reduce the electrostatic repulsion between them, allowing van der Waals forces to predominates and coagulation to occur upon approaching zero net charge 7. The negative surfaces of hydroxides are responsible for absorption of contaminants. Gases, mainly hydrogen gas formed at cathode, enable electroflotation of coagulated particles and easier collection of wastes even without filtration. Figure D: Process of electrocoagulation 6 4

5 Formation of Complex Iron Ions Two mechanisms have been proposed to explain the formation of complex iron ions. 7 The following is the chemistry of the electrocoagulation using iron sacrificial electrodes. Mechanism 1 At anode, iron is oxidized to form Fe 2+ (aq). Fe (s) Fe 2+ + (aq) 2e Fe 2+ is further oxidized to form 4Fe(OH) (aq) 3(s) 4Fe H O + O 4Fe(OH) + (aq) 2 (l) 2(g) 3(s) 8H + (aq) H + (aq) produced at anode is attracted to cathode to undergo reduction and forms hydrogen gas. 2H + (aq) + 2e H 2(g) The overall reaction is as follows: 4Fe (s) + 10H 2 O (l) + O 2 4Fe(OH) 3(s) + 4H 2(g) Mechanism 2 At cathode, water is reduced to form hydroxide ions and hydrogen. 2H 2 O (l) 2OH + H (aq) 2(g) Hydroxide ions formed are attracted to anode. At anode, iron is again oxidized to form Fe 2+ (aq). Fe(s) Fe e Fe 2+ formed reacts with (aq) OH (aq) to form insoluble Fe(OH) 2(g). The overall reaction is as follows: Fe (s) + 2H 2 O (l) Fe(OH) 2(s) + H 2(g) Due to oxidation in an electrolyte system, iron produces form of monomeric ions, Fe (OH)3 and polymeric hydroxy complex such as Fe(H 2 O) 6 3+, Fe(H 2 O) 5 2+, Fe(H 2 O) 4 (OH) 2 +, Fe(H 2 O) 8 (OH) , etc. depending on the ph of the aqueous medium. 5

6 Experiment Chemicals: Sodium sulphate solution ( g dm 3 ), indigo carmine solution ( g dm 3 ) methyl orange (0.05 g dm 3 ), methylene blue (0.02 g dm 3 ), sodium hydroxide solution (2M), hydrochloric acid (2M) Apparatus Iron plates, beakers (500 ml,250 ml & 100 ml), rectifier, clips & wires, magnetic stirrer & magnetic bar, sandpaper, measuring cylinder (100 ml), multimeter, colourimeter, filter funnel, filter paper, stand and clamp Procedures: 1. Sandpaper was used to remove the oxide layer on the iron plates ml sodium sulphate and 50 ml dye solution were added into a 500 ml beaker. 3. The absorbance of the solution was measured with a colourimeter. Iron plates were put into the solution as shown in the following diagram. 4. The beaker was placed a magnetic stirrer and put a magnetic bar into the beaker. 5. Clips and wires were used to connect the plates to the terminals of the rectifer. 6. The circuit was connected to a multimeter. Set the multimeter so that it can be used to measure current (200 ma). 7. The rectifier was turned on to 12V and the stirrer. 8. Current was passed through the solution for 5 minutes. 9. The solution was filtered and the filtrate was collected with a 100mL beaker. 10. The residue collected was scraped off the filter paper with a spatula and dissolved in another 100mL beaker of water. Figure E: Experimental set up 6

7 For set ups 1, 2 and 4: test tube were filled with 4mL filtrate each. The test tubes were labelled as A, B and C.The other 3 test tubes were filled with 4mL solution with exactly the same composition of the solution in step 2. The test tubes were labelled as D, E and F mL 2.0M hydrochloric acid was added to test tubes B and E. 4mL 2.0M sodium hydroxide solution was added to test tubes C and F. 13. The absorbance of the solution was measured by colourimeter. The differences in absorbances of the solution before and after the above processes were compared. Set up Dye solution Electrode 1 Indigo carmine Iron plates 2 Indigo carmine Graphite rods 3 Methylene blue Iron plates 4 Methyl orange Iron plates 5 / Iron plates Figure F: Experimental Set up for set up 1,2 and 4 7

8 Data & Results Set up 1 : Dye solution: Indigo carmine Electrode: Iron plates Initial concentration 5.556x10 3 g dm 3 Final concentration 8.2x10 4 g dm 3 Initial transmittance (630nm) Initial absorbance (630nm) 45.6% Final transmittance (630nm) Final absorbance (630nm) 88.4% Observation at anode No observable change. Observation at cathode Colourless gas bubbles were formed. Observation before filtration Dark brown residue tended to float to the surface of the reacting mixture after magnetic stirrer was turned off. Observation after filtration Dark brown solid was collected as residue. Orange solution was collected as filtrate. Figure G: Calibration Curve For Indigo Carmine obtained from the result of set up 1 8

9 Set up 1 (con.): Figure H: Test tubes A F (from left to right) Absorbance (460nm) Filtrate collected after electrocoagulation Dye solution before electrocoagulation Original solution 9.22x x10 2 On addition of acid 1.41x x10 2 On addition of alkali 6.85x x10 1 9

10 Set up 2: Initial concentration Dye solution: Indigo carmine Electrode: Graphite rods 5.556x10 3 g dm 3 Final concentration 1.7x10 3 g dm 3 Initial transmittance Initial absorbance 45.9% Final transmittance Final absorbance 78.4% Observation at anode Colourless gas bubbles were formed. Observation at cathode Colourless gas bubbles were formed. Observation before filtration / Observation after filtration Blue solution was collected as filtrate. Figure I: Calibration curve for indigo carmine obtained from the result of set up 2 Figure J: The set up is placed on the magnetic stirrer with current passing through. The solution turns to deep blue in this stage. Set up 3: 10

11 Dye solution: Methylene blue Electrode: Iron plates Initial concentration 6.667x10 3 g dm 3 Final Concentration 1x10 3 g dm 3 Initial transmittance 13.5% Final transmittance 73.4% Initial absorbance Final absorbance Observation at anode No observable change. Observation at cathode Colourless gas bubbles were formed. Observation before filtration Dark brown residue tended to float to the surface of the reacting mixture after magnetic stirrer was turned off. Observation after filtration Dark brown solid was collected as residue. Blue solution was collected as filtrate. Figure K: Calibration Curve For Methylene Blue obtained from the result of set up 3 11

12 Set up 4: Dye solution: Methyl orange Electrode: iron plates Initial concentration 1.667x10 2 g dm 3 Final Concentration 7x10 3 g dm 3 Initial transmittance (460nm blue) Initial absorbance (460nm blue) 10.0% Final transmittance (460nm blue) 1 Final absorbance (460nm blue) 39.1% Observation at anode Colourless gas bubbles were formed. Observation at cathode Colourless gas bubbles were formed. Observation before filtration Dark brown residue tended to float to the surface of the reacting mixture after magnetic stirrer was turned off. Observation after filtration Dark brown solid was collected as residue. Orange solution was collected as filtrate. Figure L: Set up 4 on the magnetic stirrer before the experiment start. The colour of the solution is orange. Figure M: Over a period of time, the colour of the solution becomes brown and with brown solid. Figure N: Calibration Curve For Methyl Orange obtained from the result of set up 4 12

13 Set up 4 (con.): Absorbance (green 565nm) and observation Filtrate collected after electrocoagulation Dye solution before electrocoagulation Original solution 9.66x10 3 Orange solution On addition of acid 7.73x10 2 Orange solution turns pink On addition of alkali 3.49x10 3 Solution remains orange. 8.33x10 3 Orange solution 9.69x10 2 Orange solution turns pink. 1.10x10 2 Solution remains orange. Set up 5 : Solution: Na 2 SO 4 Initial concentration g dm 3 Electrode: Iron plates Initial transmittance 99.1% Final transmittance 98.2% Initial absorbance 3.93x10 3 Final absorbance 7.89x10 3 Observation at anode Colourless gas bubbles were formed. Observation at cathode Colourless gas bubbles were formed. Observation of the overall resultant solution Dark brown solid particles tended to float to the surface of the reacting mixture after magnetic stirrer was turned off. Observation of the filtrate and residue Dark brown solid was collected as residue. Colourless solution was collected as filtrate. 13

14 Discussion 1) Electrocoagulation coagulated dyes and separated them from their aqueous solution, giving it an advantage over direct oxidation. The absorbance of the dye solutions decreased after electrocoagulation using iron sacrificial electrode. This indicated that the concentrations of the dyes in the solutions after electrocoagulation decreased. Electrocoagulation took place and some of the dyes in the solutions were removed. After filtration, the residues collected contained a mixture of brown sludge (products formed from oxidation of iron electrodes and hydrolysis of iron ions) and sludge with the colour of the original dye. When the residues were added to water and stirred thoroughly, the residue redissolved to form a solution with the colour which resembled the original dye solution. This proved that part of the residue was from the dye. The process of electrocoagulation separated the dye from the solutions by coagulating the dyes but not oxidizing and chemically destroying the dyes. For the other set ups using graphite as electrodes, decolourization took place but no residue was formed. The dye might be oxidized to form gases which escaped and colourless soluble compound. The study demonstrated the advantage of electrocoagulation which separates dyes from their aqueous solution rather than chemically destroying them as direct oxidation does. Coagulating pollutants ensure that the targeted pollutants are thoroughly removed. Take permanganate (MnO 4 ) as an example. By direct oxidation, permanganate ions (MnO 4 ) can only be reduced to form manganese(ii) ions (Mn 2+ ), which still heavily pollute nearby water bodies if released in a large amount. Although the effluent is decolourized, the soluble products of direct oxidation by electrolysis may still be harmful and is not suitable to be directly released. On the other hand, by electrocoagulation, the targeted chemicals are sure to be removed as coagulants without forming other harmful soluble products from the regarded chemicals. 2) Electrocoagulation is applicable to both positively charged and negatively charged soluble substances The aim of using the 3 dyes was to prove that electrocoagulation is applicable to both positively charged and negatively charged soluble substances. Using indigo carmine in set up 1 was to prove that electrocoagulation is workable. In the experiment, the absorbance of indigo carmine decreased, showing that its concentration decreased. It could be deduced that indigo carmine was separated from solution by coagulation. Methyl orange has a negatively charged part in its structure which gives it its colour, like indigo carmine. In set up 4, the absorbance of the methyl orange solution decreased after electrocoagulation and filtration. This indicated that methyl orange was separated from solution by coagulation. It was proved that electrocoagulation is also applicable to separation of other negatively charged soluble substances from its aqueous solution. Methylene blue has a positively charged part in its structure which gives it its blue colour. In set up 3, the absorbance of the methylene blue solution decreased after electrocoagulation and filtration. proving that electrocoagulation is applicable to positively charged soluble substances as well. This indicated that methyl orange was separated from solution by coagulation. It was proved that electrocoagulation is also applicable to separation of positively charged soluble substances from its aqueous solution. 14

15 3) Addition of acid and alkali ruled out the possibility that decolourization took place due to ph change Two of the dyes studied, indigo carmine and methyl orange, can be used as ph indicators since they show colour change upon ph change. Methyl orange turns from orange to red as ph drops below 3.1, and it turns from orange to yellow as ph rises above 4.4. Indigo carmine turns from blue to yellow as ph increases to exceed 13. To rule out the possibility that the dye solutions decolourize due to ph change, hydrochloric acid was added to filtrate from the resultant dye solution. Assume indigo carmine underwent colour change due to increase in ph, addition of hydrochloric acid lowered ph, and hence indigo carmine should be able to restore its original colour, should the colour change result from increase in ph. But from results, the filtrate remained pale blue, showing that decolourization of indigo carmine was not ph change. However, addition of acid and alkali to methyl orange solution before and after electrocoagulation failed to provide useful information in order to reach a conclusion which rules out the possibility that the dye solutions underwent colour change due to ph change. This was because methyl orange can have 3 different colours under different ph. Addition of acid turned both the solutions before and after electrocoagulation from orange to red, and the colour intensities of the 2 solutions showed little difference. In future studies, the filtrate of methyl orange solutions after electrocoagulation and the solution before electrocoagulation can be added to buffer solution with ph approximately the same as that of the solution before electrocoagulation. If the filtrate does not restore its original colour intensity such that it has a different absorbance as that of the solution before electrocoagulation in a buffer solution, the decolourization of methyl orange in electrocoagulation is not due to ph change. 4) Stirring speeds up electrocoagulation During trials, current passed through methyl orange solution electrolyzed with iron electrodes without stirring. The solution (i.e. methyl orange) showed almost no observable colour change over the 10 minute time interval. However, when current was passed through the same set up with stirring, the solution soon showed observable decolourization, and much more solid was formed on the solution surface. It was deduced that stirring sped up electrocoagulation. This might be because that polymeric ions formed at the anode were able to spread across the reacting mixture, rather than concentrating around the anode and slowly diffusing to other portions of the reacting mixture. Effective polymeric ions responsible for electrocoagulation were able to come in contact with more dye particles to coagulate them. The frequency of collisions between them was higher and number of effective collisions per unit time increased, thereby speeding up electrocoagulation. 15

16 Limitations: The study was conducted in a small laboratory scale, using a beaker (500 ml), with a magnetic stirrer stirring the reaction mixture thoroughly to speed up decolourization. In industrial scale, the gas bubbles (H2 at cathode) formed would bring the coagulant up to the surface so that pollutants can be separated effectively without the use of filters. However, in this study, the coagulants did not concentrate on the surface of the reacting mixture effectively. One explanation was that the effect of current generated by the stirrer in a beaker with relatively small volume was rather significant. The current easily disturbed the coagulated solids and affects the sedimentation of the solid residue easily, as it kept swirling them up within the mixture and suspending around, instead of allowing them to coalesce on the surface at once. Scattered small portions of the residue could not be collected efficiently. Also, using a beaker as the tank for electrocoagulation could not showcase one of the major advantages of electrocoagulation avoiding the use of filters, especially for very small coagulated particles, in order to save maintenance costs of damaged filters. In an industrial set up, there is a water exit at the bottom of the electrocoagulation tank to discharge treated water, leaving the coagulated solid floating on the reacting mixture behind. In school laboratory, it was difficult to access tailor made electrocoagulation tanks with outlets at the top and at the bottom. Here is a simplified illustration of the industrial set up for electrocoagulation. Iron plates which serve as sacrificial anode were placed vertically in the water tank. The metal tank serves as cathode to conduct electricity as well as to contain the reacting mixture. The waste which contains sludge formed from electrocoagulation can be expelled at the outlet on the top, whereas the solution with pollutants removed can be expelled at the bottom. 16

17 Further investigation: 1) Application of electrocoagulation on heavy metal ions and colloidal solutions This study only centred around soluble dyes. It is reported that electrocoagulation can be applied to treat various species, such as heavy metal ions 8, emulsified colloids 4 and suspended solids 4. Heavy metal ions were not studied in this experiment. First, many coloured heavy metal ions have positions lower than iron and aluminium in the reactivity series and displacement reaction would occur during electrocoagulation. This might disable electrocoagulation before the solution was significantly decolourized when the surface of the metal sacrificial electrodes is completely replaced by the concerned heavy metal. Sacrificial metal electrodes would have to be replaced much more frequently, leading to a waste of material. However, if electrocoagulation is applicable to heavy metal ions, it is occasionally preferable to conventional chemical precipitation by addition of alkalis, as the process can be carried without filters. Application of electrocoagulation on colloidal solutions can be further studied in school laboratory though. To prove that electrocoagulation is a feasible method to treat colloidal solutions, milk, which contains emulsified butterfat globules and proteins aggregated with minerals, and sodium chloride solution with suspended sulphur solids formed from mixing sodium thiosulphate and hydrochloric acid, can be used in the study.if electrocoagulation can be proved feasible for treating colloidal solution, it can become a useful water treatment method for colloidal solutions, especially those containing suspended solids with very small diameter, as they cause significant damage the filters and the maintenance cost of filters for traditional filtration methods is high. Regarding the use of electrocoagulation on organic colloidal solutions like milk, bacteria digestion is one of the common ways to decompose these matters, but the solution needs to be sterilized afterwards to remove the bacteria. The use of electrocoagulation to remove these organic matters saves chemicals that needed to be added to sterilize the resultant solution, and prevents the sterilizers from polluting nearby water bodies when released. 2) Cost effectiveness of electrocoagulation over chemical precipitation or chemical coagulation It has been argued that one of the reasons why electrocoagulation is more cost effective than chemical precipitation by addition of bases is that electrocoagulation generates coagulants in situ and does not involve addition of chemicals, and electrocoagulation does not require filters which need to be maintained. Although maintenance costs of filters and costs of additives can be saved, electrocoagulation does have its own drawbacks in the aspect of cost efficiency. First, sacrificial electrodes are oxidized and dissolved in the treated effluents that they need to be regularly replaced. Second, electrocoagulation requires electricity. Future studies may compare the costs of electrocoagulation, chemical precipitation and chemical coagulation, in order to find out the best way for factories to treat wastewater. Conclusion From the study, it was concluded that electrocoagulation is applicable to removal of positively charged and negatively charged soluble substances. 17

18 Acknowledgement This project was technically supported by our teacher in charge Mr Chung Ming Long, Mr Tse Hon Kam and laboratory technician Mr Fung Chi Lap. We wish to thank them here sincerely. Teacher in charge: Mr Tse Hon Kam Chemistry Teacher Mr Chung Ming Long Chemistry Teacher Laboratory Technician: Mr Fung Chi Lap Reference 1 [cover page photo] TurkFreeZone: Dye Chemicals 2 [cover page photo] CivilDigital: Pollution Control In Dye Industry control in dye industry/ 3 Sewage Services (Trade Effluent Surcharge) Regulation 4 Dr. Carlos Alberto Martínez Huitle ( ): Direct and indirect electrochemical oxidation of organic pollutants, p.37 5 Fuat Ozyonar, Bunyamin Karagozoglu (2009): Operating Cost Analysis and Treatment of Domestic Wastewater by Electrocoagulation Using Aluminum Electrodes, Satish.I. Chaturvedi (2013): Electrocoagulation: A Novel Waste Water Treatment Method, International Journal of Modern Engineering Research (IJMER), p Satish.I. Chaturvedi (2013): Electrocoagulation: A Novel Waste Water Treatment Method, International Journal of Modern Engineering Research (IJMER), p.93 7 Satish.I. Chaturvedi (2013): Electrocoagulation: A Novel Waste Water Treatment Method, International Journal of Modern Engineering Research (IJMER), p.95 8 Riyad H. Al Anbari A, Jabar Albaidani B, Suuad Mahdi Alfatlawi B, and Thikra Aissa Al Hamdani C (2008): Removal of heavy metals from industrial water using electro coagulation techniques, Twelfth International Water Technology Conference, IWTC Alexandria, Egypt, p.1 18