ELECTROCHEMICAL TREATMENT OF DYE BEARING WASTEWATER

Abstract

Textile industries, although provide clothes which are one of the basic needs of human beings, however, they are one of the largest exploiters of water and other complex chemicals during various stages of processing such as preparation, dyeing, finishing, sizing and other operations during manufacturing of textile materials. In India, a large number of cotton, composite and spinning mills are located in Ludhiana, Kanpur, Coimbatore, Mumbai, Surat, and Ahmadabad. Textile industries widely use organic dyes in finishing and coloring processes. About 20% of these dyes enter into the aquatic environment through discharge of effluents from treatment plants. Many synthetic dyes, also called as triarylmethane dyes are derived from triphenylmethane by substitution, and are well known for higher color intensity, brilliant shades and low light fastness. These cationic dyes have positive charge and are used for dying fabrics which have negative charge such as wool, silk, nylon, cotton, etc. Triphenylmethane dyes such as basic green 4 (malachite green) on photo–oxidation via nascent oxygen break into various N–de–alkylated primary and secondary amine derivatives which are similar to carcinogenic aromatic amines. Discharge of colored effluents containing dyes reduces the photosynthetic activity in aquatic environment, thus reducing the quantity of oxygen available in the water for use by the aquatic life. Different treatment technologies have been examined for dye removal. These include biological, physical and chemical methods and their various combinations. Biological treatment of dying wastewater is cheaper than other methods, but it cannot be applied to most textile wastewaters due to the toxicity of most commercial dyes to the organisms used in the biological treatment methods. Physico-chemical methods such as coagulation, adsorption, chemical oxidation, advanced chemical oxidation, photocatalytic degradation, Fenton’s processes, etc. are most widely researched methods for textile wastewater treatment. Advantages and disadvantages of each method have been extensively reviewed in literature. In the last one decade, electrochemical (EC) treatment methods have received great attention for treatment of various types of wastewaters including dye bearing wastewater. EC method is comparatively inexpensive and is characterized by its easy operation, reduction in sludge volume and equipment costs. EC method utilizes metal electrodes as anode and cathode. Anode produces the coagulant via dissolution of electrodes by electrolytic reaction. Finally the coagulant turns into precipitates in the form of metal hydroxides in the appropriate pH range. Electrode potential, surface properties of electrode, type of electrolyte, and type of iii transient intermediates formed affect the EC degradation of dyes. A lot of literature is available in EC treatment of dye bearing wastewater in the open literature. However, a number of research gaps were identified from the exhaustive literature review in the present study. Variation in zeta potential during the interactions between Al ions generated from anode dissolution and cationic dye helps to understand the mechanism of removal. Only scarce studies are reported in the literature explaining the removal mechanism of EC process in relation to change in zeta potential during variation of current density (j), initial dye concentration (Co) and initial pH (pHo). Very few studies are available in open literatures which try to elucidate mechanism of degradation of chemical structure of organic dye into smaller one during degradation processes. Most of these studies are, however, based on catalytic degradation, biological degradation and advance oxidation processes. No study is reported in the literature on mineralization of any dye during EC treatment. Local dyers in small towns produce huge amount textile printing dye-bath effluent (DBE) during dying of textiles, woollens, etc. in small dye bath, and discharge them without any treatment to open channels. Our research group previously carried out parametric optimization of parameters for EC treatment of DBE using SS and Al electrodes separately. However, there was a further need to study the treatment of DBE with SS and Al in different anode-cathode combinations (Al–Al, Al–SS, SS–SS and SS–Al). It is also important to study the change in zeta potential and colloid particle size distribution (PSD) of colloids in the solution during the treatment. Despite small amount of sludge generation in the EC method as compared to conventional coagulation and biological methods, still the sludge containing the electrode material needs to be disposed off. Only scarce studies are reported on preparation of useful material from different solid wastes but not with EC sludge. Nano-composite materials (NCMs) possessing meso- and micro-pores have received much attention in wastewater treatment in recent years. A study of literature on preparation of NCMs shows that there is a probability of conversion of EC residues to NCMs which can further be used as catalysts for dye degradation. Based on the research gaps identified, following aims and objectives were set for the present work: 􀂙 To perform multi-step optimization of operating parameters using Plackett-Burman (PB) design, steepest accent/descent method and Box-Behnken (BB) design for EC iv treatment of synthetic textile wastewater (STW) containing basic dyes with aluminum electrode. 􀂙 To study the effect of zeta potential during EC treatment with aluminum electrode of basic green 4 dye with variation in current density, pH, and initial dye concentration and to study the performance in terms of color, COD and total organic carbon (TOC) removal efficiencies; and energy consumption (ENC) and electrode consumption (ELC). 􀂙 To identify intermediates formed at various treatment time by mass spectroscopic and chromatographic techniques and to explain degradation mechanism by different possible schemes. 􀂙 To perform EC treatment of actual (real) DBE using aluminum (Al) and stainless steel (SS) electrodes in various anode-cathode combinations in a batch EC reactor and to study the effect of operating parameters on the removal efficiency in terms of color, COD, TOC and turbidity; and other performance parameters such as ENC and ELC 􀂙 To convert EC sludge generated by aluminium and stainless steel electrodes into NCMs and NMs by thermally heating method and to characterize the NCMs and use them as catalyst for degradation of dyes. Circular shape batch reactor having working volume of 1.0 litre was used to conduct the EC treatment experiments. Magnetic stirrer was used to agitate the dye wastewater. One pairs of Al, SS and metal oxides coated electrodes having thickness of 1.5, 2.5 and 4.0 mm, respectively, each having dimensions of 10 cm × 9 cm with inter-electrode spacing of 1 cm were used. STW was synthetically prepared using basic orange 30, basic violet 16 and basic green 4 dyes as per the method reported in the literature using chemicals such as carboxymethyl cellulose, starch, acetic acid, NaOH, H2SO4, Na2CO3, NaHCO3 and glucose which are commonly found in textile mill wastewater. Multi-step optimization of EC treatment of STW by aluminum electrode was then carried out. A multi-step procedure was applied to screen and optimize the factors. First, PB design was employed to screen most significant three factors among a largest number of parameters. Afterwards, method of steepest ascent and BB design were used to determine the optimum levels of the factors that significantly influence the COD and color removal efficiencies. At the optimum operating conditions of current density=185.30 A/m2, time=190 min and pH~5, more than 70.5% COD and 98.2% color v removal efficiencies was observed. Electro-coagulation and electro-flotation were found to be the main reasons for COD and color removal in STW. In this part of the study, the degradation mechanism of basic green 4 dye during EC treatment with aluminum electrode was investigated. Zeta potential was measured with changes in operating variables such as j, pHo, and Co. At the optimum condition, 82.4% COD, 63.5% TOC and 99.4% color removal efficiencies were observed for Co=100 mg/L within 45 min with j=117.64 A/m2 and pHo=6.2. The ELC and ENC values at the optimum conditions were 0.16 kg Al/kg COD and 2.48 kWh/kg COD removed. It was found that the zeta potential of the dye solution was always negative during the treatment. The decolorization rate increased with an increase the pH from acidic to alkaline pH due to conversion of dye to leuco form. Zeta potential study helped to identify the conditions of maximum interaction between the hard acid (Al ions and hydroxides) and basic dye. More ever, since basic green 4 dye itself is positively charged, maximum removal of dye was found to occur at the lowest value of negative zeta potential (-15.2 mV at pH≈6.2), that is, when the concentration of positively charged colloids in the solution were least. It seems that overall the removal was due to adsorption on neutral aluminum hydroxide. UV–visible and Fourier transform infrared (FTIR) spectroscopy, high performance liquid chromatography (HPLC), gas chromatography–mass spectroscopy (GCMS) and high resolution mass spectroscopy (HRMS) analysis showed that the degradation occurred via the cleavage of conjugated structure and N–de–methylation. The intermediates products identified included hydroxymethylated intermediates during the N–de–methylation of the dye; and N, N, N’, N’–tetramethyl–4, 4'–diaminobenzophenone; 4, 4'–bis–aminobenzophenone and N– methyl–para–aminophenol after cleavage of conjugated triphenylmethane ring (Fig. 4). Generation of active species such as hydrogen peroxide, ozone and chlorinated oxidizing compounds was observed during the EC treatment process; and that the basic green 4 dye degradation occurred via •OH radical attack. Present study also investigates the EC treatment of actual DBE with different combination of Al and SS electrodes as anode and cathode. Effects of j and pH with different anode-cathode combinations (Al–Al, Al–SS, SS–SS and SS–Al) were studied. The change in zeta potential with current density at different time intervals, and the change in colloid particle diameters at different pH gave information regarding potential stability of colloidal suspension. Color, COD, TOC and turbidity removal efficiencies, residual zeta potential, vi average colloid particle diameter, ELC and ENC values after treatment were in the following order: SS–SS > SS–Al > Al–Al > Al–SS. It was found that higher value of j produced higher amount of metallic cationic species which in turn increase the color and COD removal efficiencies because of which zeta potential of the treated DBE moves toward positive value. Charge neutralization and sweep coagulation by respective hydroxides allowed the different particles at come together by van der Waals interaction and adsorption mechanism, respectively. These mechanisms increased the particle size at optimum pH which later on settles to the bottom and cause highest color and COD removal efficiencies. At high pH (>9), ClO─ formed via secondary reactions of chlorine direct oxidizes the colloidal matter present in DBE. Maximum color, COD, TOC and turbidity removal efficiencies were found to be 99.90%, 82.50%, 68.8% and 98.8%, respectively, at j=117.64 A/m2 and pH=8.5 with SS–SS electrode combination. FTIR, powder X-ray diffraction (PXD), Field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy of X-rays (EDX) and thermogravimetric analysis (TGA) techniques were used to characterize the solid residues (sludge) obtained during EC treatment. FESEM and EDX also used for determine the morphology of various types of electrodes. The Barrett–Joyner–Halenda (BJH) method was used to determine the pore size distribution. Brunauer–Emmett–Teller (BET) surface area of sludge was found to be in the following order: Al–Al > Al–SS > SS–SS >SS–Al. However, the trend was opposite for BJH pore volume and pore diameter. All the anode-cathode combinations sludge was meso-porous in nature. Thermo-gravimetric analysis showed that the heating value of sludges was about one-tenth of the Indian coal. Solid waste (sludge) generated during EC treatment of dye wastewater with SS and Alelectrodes was recycled by heating the solid waste at different temperatures under controlled condition to produce nano composite and nanomaterials materials (NCMs and NMs), respectively. Characterization by PXD, FE-SEM and EDX, TEM, BET and X-ray photoelectron spectroscopy (XPS) confirmed that NCMs synthesized from SS-EC sludge contains iron, chromium, nickel and oxygen in the form of α-Fe2O3 (metal: oxygen= 40:60), (Fe,Cr,Ni)2O3 and trevorite NiFe2O4, (Ni,Fe,Cr) (Fe,Cr,Ni)2O4 (metal: oxygen = 43:57). Similarly, active aluminum oxide nanoparticles (NPs) in different nano-crystalline forms γ-, α- and β-alumina’s (Al2O3) were obtained from Al-EC sludge when incinerated at different temperatures. Degradation studies using the recycled NCMs and NMs on dye wastewater showed high removal efficiency and good adaptability.