Abstract
Electrocoagulation is an electrochemical dissolution technique in which coagulating agents are generated in situ and used for treating wastewater. It could also be an effective tool for treatment of colour from textile and dye wastewaters with high removal efficiency and it rapidly removes the chromophores and by-products of dye solutions. Different metal electrode configurations were operated under galvanostatic mode to treat dye wastewater. Electrocoagulation processes were found to give excellent dye removal efficiency. Aluminium electrodes gave better removal efficiency in C.I. Disperse Blue 79 containing wastewater, whereas iron electrodes gave better removal efficiency in reactive dye Drimarene Navy HF-GN containing wastewater. A combination electrodes (ie. one iron and one aluminium electrode as anodes and one iron and one aluminium electrode as cathodes) gave high colour removal efficiency for both wastewaters containing reactive and disperse dye, and hence are found to have potential to be used in commercial electrocoagulation processes dealing with different kinds of dye effluent.
Keywords: Electrocoagulation, Dyes, Iron electrodes, Aluminium electrodes, DC current, Wastewater
Introduction
Colour plays an important role in the fashion and textile business. Our ever-growing fashion industry is conceiving new colours and designs, thereby presenting stern challenges not only for our dyers worldwide to achieve them but also for our society in terms of health, safety and environmental concerns.
In textile processing, the percentage of dye that remains unfixed to the fibre during the dyeing process and finds its way into the effluent ranges from 5-50%1. This dyeing effluent affects the aesthetics of water and also poses serious threat to aquatic life due to its interference with photosynthesis by reducing the transparency of water and also by the presence of hazardous and toxic compounds, especially azo dyes, salts, etc, which must be removed before discharging.
Several conventional methods, such as chemical coagulation and flocculation, biological treatment and tertiary treatments like adsorption, oxidation and filtration are used for colour removal but each suffers from its own limitations2. Chemical coagulation leads to a large amount of sludge formation3, whereas micro-organisms used in biological treatments are vulnerable to some commercial dyes. Tertiary treatments are often expensive and have operational problems such as regeneration and fouling of adsorbents4, clogging of membrane pores5 and generation of sludge6. Ozone treatment gives satisfactory results towards removal of direct, acid and cationic dyes but is not very effective towards disperse and vat dyes7.
Our pursuit for sustainability is leading us to various developments in wastewater treatment technology. Electrocoagulation [EC] is an emerging technology and is a combination of three foundation technologies of electrochemistry, coagulation, and flotation8. Electrocoagulation is different from the conventional chemical coagulation process as, in the EC process, coagulating agents are generated in situ; and it offers wide advantages, such as no chemical use, less and stable sludge formation, less salinity of treated water and effectiveness in a wider pH range (4-9). Also, EC can be applied in both batch and continuous processes.
Electrocoagulation is an electrochemical dissolution technique for treating wastewater, in which sacrificial metal electrodes (Al or Fe) release cations and form various metal species (monomeric and polymeric) into solution when a current is applied across these electrodes. These metal species can interact with the pollutant in different ways; for example, the metallic ionic monomeric species can neutralise the charge of the pollutants by adsorption on their surfaces (or by binding to their ionised groups) thus reducing the electrostatic interparticle repulsion to the extent that the Van der Waals attraction predominates; the metallic ionic polymeric species can bind to several particles (or molecules) of pollutant at a time; and/or the pollutants can be enmeshed into growing metallic hydroxide precipitates, or can be adsorbed on to their surfaces9. EC also liberates hydrogen gas at the cathode, which attracts the flocculated particles and floats the flocculated pollutants to the surface through natural buoyancy. Thus electrocoagulation can remove a wide range of dissolved and colloidal contaminants from wastewater.
In electrocoagulation, metals producing trivalent ions such as aluminium and iron are preferred as anodes, as trivalent ions have a higher ability to adsorb on to particles in the water than bivalent ions because they have a higher charge density. The metal ions generated hydrolyse in the electrocoagulator to produce metal hydroxide ions and neutral M(OH)3. The low solubility of the neutral M(OH)3, mainly at pH values in the range of 6.0–7.0, promotes the generation of sweep flocs inside the treated waste and the removal of the pollutants by their enmeshment into these flocs10.
Fe Electrode Reaction
In the EC process involving iron electrodes, two mechanisms have been proposed to describe the formation of H2(g) and OH- at the cathode and Fe2+/Fe3+ ions and H+(aq) at the anode11.
Al Electrode Reaction
In the EC process involving aluminium electrodes, the following reactions occur at different electrodes and in solution:
There are various other species that dimeric, trimeric and polynuclear hydrolysis products of Al can also form12,13. These Al(OH)3 flocs capture the dye molecules present in the solution by the following reaction mechanism14,9.Electrocoagulation technology is also considered to be potentially an effective tool for treatment of colour from textile wastewaters with high removal efficiency and it rapidly removes the chromophores and by-products of dye solutions16. The removal efficiency is found to be dependent on the initial pH, the dye concentration, the applied current density, and the electrolysis time in the batch model9. The chemical composition of the aqueous solution, solution temperature, type of salt used to raise conductivity, presence of chloride, electrode gap, passivation of the anode, and water flow rate also have an impact on the removal efficiency and economic durability of a given EC application17.
The electrocoagulation process has not been commercially successful due to the issue of electrode reliability and the lack of a systematic approach to electrocoagulation reactor design and operation8. However, this process is emerging and offers a lot of potential, especially for decentralised water-treatment facilities.
Both aluminium and iron, when used as anodes, offer advantages as well as limitations over each other. Wastewater treated with iron electrodes tends to develop a brown colour due to formation of ferric hydroxide, which often affects the aesthetics of water, and iron has higher electrode consumption than aluminium during electrocoagulation process18. Aluminium, on the other hand, has high coagulation efficiency but consumes more power than iron electrodes during the electrocoagulation process18.
In this paper, we have investigated the efficiency of different electrode materials and their combination in the removal of disperse and reactive dyes from synthesised dye wastewater. We attempt to see whether a combination of iron and aluminium electrodes can be a better alternative to iron or aluminium electrodes, which are currently used individually.
Experimental
Materials
Dye wastewaters were prepared from commercially available disperse and reactive dyes. For all dye wastewaters trials, the dye concentration was maintained at 250 mg/l. Disperse dye wastewater contained dye C.I. Disperse Blue 79 and Reactive dye wastewater contained Drimarene Navy HF-GN. Sodium Chloride was added to the dye wastewater to increase the conductivity to around 2000 µS/cm. Initial pH of dye wastewater prepared was adjusted to neutral (7.0 ± 0.2) using sodium carbonate and acetic acid.
Apparatus and Instruments
All the electrocoagulation experiments were conducted in a 5L borosil-make glass beaker. There were four electrodes used in each configuration. All the electrodes were made from plates with dimensions of 120mm x 100mm x 6mm. There were three different electrodes configurations used, ie. 1. Iron electrodes (two iron electrodes as anodes and two iron electrodes as cathodes); 2. Aluminium electrodes (two aluminium electrodes as anodes and two aluminium electrodes as cathodes); and 3. Combination electrodes (one iron and one aluminium electrode as anodes and one iron and one aluminium electrode as cathodes).
All the cathodes and anodes were in parallel connection with the spacing between electrodes maintained at 5mm. A spacing of 25mm was maintained between bottom of glass beaker and the electrodes in order to allow magnetic stirring of dye wastewater to take place. The electrodes were connected to a digital DC power supply (Gwinstek PSW 80-27; 80 V, 27 A) and all experiments were operated in galvanostatic mode. The electrodes were connected as monopolar electrodes in parallel connection, ie. both anodes connected to the positive terminal and both cathodes connected to the negative terminal of the DC power supply. This electrode mode connection was selected as it offers the most cost-effective solution in terms of process economy, as studied by Kobya et al10.
A UV-vis spectrophotometer (GBC UV/VIS 918) was used to measure dye concentration. pH and conductivity were measured by research-grade meter (Hanna HI4522, USA) and Chemical Oxygen Demand (COD) was measured using Lovibond COD Vario MD 200 instrument. Turbidity of dye wastewater was measured by turbidity meter (Hanna HI 88713, USA) and reported in Nephelometric Turbidity Units (NTU).
Methods
All the electrodes were washed before each trial in a freshly prepared solution containing 300 cm3 HCl solution (35%) and 600 cm3 of hexamethylenetetramine aqueous solution (2.80%) for 5 min, to remove all impurities from the electrode surfaces, and further with distilled water10. A magnetic stirring rate of about 350rpm was maintained throughout the experimental trial. In each trial, the dye wastewater volume used was 3.5L and the total time duration of the trial was 10 mins unless noted otherwise. The DC power supply provided the desired constant current (4A and 8A) to the electrodes by varying the voltage, which was recorded. All experiments were carried out at room temperature (30°C ± 2°C).
Conductivity of solution plays an important role in electrolytic processes and high conductivity can help achieve high current at lower voltages to keep power consumption low. Therefore sodium chloride was added to the dye wastewater to increase the conductivity to around 2000 µS/cm. pH was kept initially at neutral for all trials, as high colour removal efficiency is reported at neutral media15 and also the size of hydrogen bubbles liberated at cathodes is minimum around neutral pH and thus is helpful in the flotation process19.
Dye removal was estimated by measurement of absorbance of initial and final dye wastewater after each trial in the UV-vis spectrophotometer. Absorbance of dye solution was measured at wavelength of maximum absorbance (λmax).
Chemical Oxygen Demand (COD) was tested using standard test method ISO 15705:2002, also called the sealed tube method. The samples were first digested (oxidised) at 150°C for 2 hrs and then the COD was measured by photometric method using Lovibond COD Vario MD 200 instrument.
Results & Discussion
Removal of disperse dyes
Effect of Electrode Configuration on Removal of Disperse Dyes
All three electrode configurations showed colour removal efficiency in excess of 99%, as shown in Table 1. Disperse dyes are non-ionic dyes that are relatively insoluble in water at room temperature and have only limited solubility at higher temperatures20; thus, compared to soluble dyes, it is relatively easy to separate disperse dyes from dye wastewater. Aluminium electrodes offered better and faster colour removal than iron and mixed electrodes. Dye wastewater treated with iron electrodes shown a slight brown tint due to formation of ferric hydroxides21, although later most of it had settled down along with the sludge. Similarly Chemical Oxygen Demand (COD) removal was also higher in the case of aluminium electrodes and a maximum of 57% was achieved at 8A current. Turbidity also came down from 1315 NTU in initial disperse dye wastewater to nearly 1-5 NTU in treated wastewater in all electrode configurations. A combination of electrodes also was able to reduce maximum colour, high chemical oxygen demand and high turbidity at a lesser voltage requirement than aluminium electrodes. No brown tint of iron oxide was within visual observable range in combination-electrode-treated wastewater.
Effect of Current on Removal of Disperse Dye
The coagulation rate significantly increased with an increase in current and complete separation of dye molecules from dye wastewater took place in around 6-8 min. at 8A, compared to 12-15 min. at 4A, in all three electrode configurations. Current is the most crucial parameter for the electrocoagulation technique and hence an increase in current leads to an increase in the rate of release of ions and to a higher rate of hydrogen bubble formation22.
Removal of reactive dyes
Effect of Electrode Configuration on Removal of Reactive Dyes
It was clearly observed that iron electrodes are more effective than aluminium electrodes for removal of reactive dyes from reactive dye wastewater. With iron electrodes, colour removal efficiency of 98.3% and 99.5%, at 4A and 8A respectively, was achieved, whereas with aluminium, maximum colour removal achieved was 43.7% at 8A current. However, with combination electrodes, ie. two Al and two Fe, colour removal was 97.6% at 8A current. Similarly, iron electrodes led to higher reduction in chemical oxygen demand of dye wastewater compared to the other two configurations, ie. aluminium electrodes and combination electrodes (Table 2). A maximum of 61.5% reduction in chemical oxygen demand (COD) was possible with iron electrodes at 8A current.
Effect of Current on Removal of Reactive Dyes
All the results showed that there is an increase in colour removal efficiency and a significant increase in the rate of coagulation with an increase in current. With iron electrodes, the colour-removal efficiency increased from 98.3% to 99.5% when current was increased from 4A to 8A. This change was observed in only 10 min. in the case of 8A current, compared to 15 min. with 4A current. Beyond this point, the removal of colour did not significantly change over time.
Similar results were obtained with aluminium electrodes and combination electrodes, with colour-removal efficiency increased from 37.8% to 43.7% and 95.3% to 97.6% respectively at higher current. Similar results were also obtained for reduction in chemical oxygen demand (COD) of dye wastewater, as shown in the Table 2. Increase in current leads to higher rate of release of metal ions and hence higher rate of coagulation. Another reason for the high rate of separation of reactive dyes from dye wastewater at high current is the higher production of hydrogen bubbles, which entrap the formed microflocs and help in flotation, thus leading to effective flocculation.
Effect on pH of Dye Wastewater
An increase in pH was observed from neutral (7.0 ± 0.2) to (8.4± 0.4) during electrocoagulation trials. It can be explained by the fact that electrocoagulation process produces hydroxides, ie. Al(OH)3 or Fe(OH)3 in solution, which lead to increase in pH23.
Effect of Electrode Configurations on Voltage during Galvanostatic Mode of Operation
The amount of voltage required to achieve constant current varied with different electrode configurations. Aluminium required a higher voltage to achieve the same current and hence led to higher power consumption. No significant change in the conductivity of dye wastewater was observed during the entire trials and it was constant around 2080 µS/cm.
Conclusion
Electrocoagulation processes are found to give excellent dye removal efficiency, as high as 99.8% in the case of disperse dye and up to 99.5% in the case of reactive dye investigated. In the case of disperse dye, the turbidity removal efficiency was 99.9% when aluminium electrodes were used and clear treated water was obtained. Electrocoagulation also gave higher chemical oxygen demand removal efficiency, up to 57% in the case of disperse dye with aluminium electrodes and up to 61.5% in the case of reactive dye with iron electrodes.
Combination electrodes gave satisfactory results, giving higher colour removal efficiency and lesser brown tint than iron electrodes in the case of disperse dyes. Similarly, they gave higher colour removal efficiency at lower voltage (ie. lower power) than aluminium electrodes in the case of reactive dyes. Thus, combination electrodes (ie. one iron and one aluminium electrode as anodes and one iron and one aluminium electrode as cathodes) are found to have potential to be used in commercial electrocoagulation processes dealing with different kinds of dye effluent and offer scope for further investigation with other classes of dyes and validation in pilot-scale trials.
Acknowledgement
This study is a part of an R&D project “To Develop an effective and eco-friendly electroflocculation technique to treat wastewater effluent with high FOGs (Fats, Oils & Grease), metals and organic loads for the woollen industry”, sponsored by the Ministry of Textiles, Government of India.
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