Treatment of highly acidic wastewater containing high energetic compounds using dimensionally stable anode
Graphical abstract
Introduction
A number of industries such as ammunition industries/labs, pharmaceutical industries, mining sites, steel industry, electroplating and phosphorous industries discharge highly acidic effluents. This highly acidic wastewater causes severe environmental problems as it is highly toxic and carcinogenic. The low pH effluent engenders several problems in the effluent treatment as well as in the water body in which it is discharged. The low pH causes the heavy metal and other contaminants to dissolve, which otherwise bond with negative hydroxide ion to form dense, insoluble, metal hydroxides which are removed easily by filtering or settling. Due to this, the treatment of effluent becomes more tedious. It also affects the aquatic life adversely as such low pH doesn’t allow aquatic life to sustain. Also, highly acidic medium breaks organic matter and kills any microbial growth which may help to treat the water naturally.
Many chemical industries, in particular, those manufacturing high energetic materials discharge wastewater with low pH due to the presence of high amount of nitric acid. The effluents generally have very low BOD to COD ratio which basically indicates their poor biodegradability. The wastewater can be of various varieties but some common constituents are nitro-aromatic compounds like hexamethylene, other nitrate and nitrites, waste acid like sulphuric acid, nitric acid, phosphoric Acid, etc. Other constituents may include hardness, sulphates, ammonia, fluorides, phosphates, silica, metals (Al, Fe, Mn, etc.), etc.
Conventional treatment methodology involving physio-chemical and biological pathways are generally rendered ineffective for such kind of wastewaters. Treatment methodologies reported in the literature to treat the acidic wastewater include chemical addition, advanced oxidation processes (AOP) [1], catalytic peroxidation [2], Fenton method [3], electrochemical oxidation [4], [5], electro-Fenton method [6], sono-chemical [7] and photochemical processes [7]. The basic mechanism seen in these processes include degradation of pollutant by utilizing electrons as a vector. Electrochemical oxidation treatment methodology has direct or indirect treatment pathways. During direct type, the pollutant is degraded directly upon adsorption on the electrode surface, whereas, in indirect type, the oxidant (OH, OOH, OCl, etc.) is produced in the bulk of wastewater which further degrades pollutant in the wastewater [8], [9], [10]. Electrode characteristics like electrode morphology, dimensional stability, oxygen evolution, etc. impact majorly on the efficiency of the process.
Electrodes used in electrochemical wastewater treatment are classified majorly as: sacrificial electrodes and dimensionally stable anode (DSA) electrodes. Sacrificial electrodes like aluminium or stainless steel are cheap and easily available; however, these electrodes easily get consumed in the electrochemical cell, in turn, producing sludge which act as secondary pollutant and require further handling. On the other hand, DSA type electrodes are highly stable and resistant electrodes and don’t create any secondary pollutant, however, they are costlier. A number of DSA electrodes such as titanium and tantalum coated with RuO2 [11], [12], IrO2 [12], SnO2 [13], PbO2, boron-doped diamond (BDD) [14], Pt/Ti [15], Ti/Pt and Ti/Pt/Ir [16], TiO2-NTs/SnO2-Sb [17], TiO2-NTs/Sb–SnO2/PbO2 [18], RuO2/IrO2/TiO2 [19], etc. have been used by various researchers for the treatment of diverse wastewaters [20]. Ti/RuO2 is one of the highly stable electrodes which works efficiently under highly acidic conditions. It has high reusability and can be used for large number of experimental runs [21]. Being an active DSA, it provides its adsorption sites for radicals to get absorbed and degrade the pollutant over its surface area [22]. It also favours (Cl) and oxychloro (OCl) radicals generation which also facilitates in degradation of pollutant both at anode as well as bulk of system [5], [23], [24].
Scavengers of HO, for example, Cl−, Br−, SO42−, carbonates, organic matter, etc. majorly affect the pollutant degradation efficiency of the overall process in more than one way [25], [26]. Chen et al. [17] studied the degradation of highly acidic explosive wastewater by applying electro-catalytic technique coupled with anoxic–oxic biodegradation using TiO2-NTs/SnO2-Sb. Degradation of water contaminated with RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) along with other nitrates using Fenton process was practiced by Tanvanit et al. [27] and Zoh and Stenstrom [28]. Rodgers and Bunce [29] also reviewed for variety of techniques been applied to degrade water contaminated with nitroaromatic explosives, for example, TNT (2,4,6-trinitrotoluene), DNB (dinitrobenzene), DNT (dinitrotoluene), TNB (1,3,5-trinitrobenzenene), picric acid (2,4,6-trinitrophenol) and tetryl (methyl-N,2,4,6-tetranitroaniline). These all works reported, prove that all these technologies are well able to reduce the strong acidic, carcinogenic and toxic behavior of such wastewaters. However, it may be noted that no studies have been reported on treatment of such high acidic and toxicity wastewater.
The present study is based on the electrochemical treatment of actual highly acidic wastewater from a nearby industry using an electro-oxidation method using a DSA electrode namely ruthenium oxide coated titanium (Ti/RuO2). The wastewater was highly acidic with extremely high COD. The overall process was optimized over various parameters like current density (A/m2), electrode gap and H2O2 dosage. Moreover, COD degradation, specific energy consumption, and operating cost were calculated for the overall treatment process. Also, the characterization of wastewater was done using various methods including gas chromatography/mass spectroscopy (GC/MS), ion chromatography (IC), and Fourier transform infrared spectroscopy (FTIR). Also, the morphology of the electrode was studied using field emission scanning electron microscope (FE-SEM) and X-ray powder diffraction (XRD).
Section snippets
Materials
All chemicals used for the experiment were of analytical grade. MilliQ water was used for preparing different solutions required for the study at room temperature. Ti/RuO2 electrodes used for electrochemical treatment were procured from Titanium Tantalum Products Limited, Gowriwakam, Chennai, India. Tributyl phosphate, used as an extractant for GC/MS study, was purchased from Hi-Media laboratories, limited. The physical and chemical characteristics of wastewater are presented in Table 1.
Experimental setup
Characterization of wastewater
In order to characterize the wastewater, various analysis like GC/MS, IC and FTIR was used. Fig. 2a shows the FTIR spectra of the wastewater obtained, which can provide some evidence of the functional groups present in the wastewater. The presence of stretch extending from 3750 to 2475 cm−1 is due to OH group and vibrations due to intermolecular and intramolecular hydrogen bonding. The stretch, indicating the presence of alcohols (free or H-bonded) and carboxylic acids, is due to free as well as
Conclusions
In the present study, electrochemical (EC) degradation of wastewater by titanium coated with ruthenium oxide (Ti/RuO2) was studied. On the basis of results and discussion presented, some major conclusions were drawn. The optimum condition for the EC treatment by Ti/RuO2 was found to be: current density ≈ 750 A/m2, treatment time ≈ 150 min, temperature = 50 °C and electrode gap ≈ 1 cm. At the optimum condition, COD removal efficiencies without and with the use of H2O2 ( ≈ 323.43 mM) was found to be: ≈41.83%
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