Treatment of petroleum refinery wastewater by ultrasound-dispersed nanoscale zero-valent iron particles
Highlights
► Combined effect of ultrasound and NZVI on complex wastewater treatment was explored. ► The NZVI produced were characterized using SEM and XRD. ► The operating parameters influence was analyzed. ► The kinetics of treatment was analyzed.
Introduction
Petroleum refineries use large quantity of water for various purposes and generate significant quantities of wastewater. The characteristics of wastewater generated strongly depend on the process configuration. Contaminants like cyanide, oil, phenols, benzene, sulfide, ammonia and heavy metals are present in refinery effluents. In addition, chemicals dosed to control corrosion and biofouling are also found [1]. This wastewater is normally rich in biological oxygen demand (BOD) and chemical oxygen demand (COD) [2].
Widely used treatment methods include Fenton oxidation [3], photocatalytic degradation [4], ozone treatment [5], and electrochemical methods [6]. In the recent past, advanced oxidation processes (AOPs) have been used to improve the biodegradability of industrial effluents through the generation of highly reactive hydroxyl radicals (OH) which are used for the oxidization of organic pollutants [7]. Sonolysis is one of the AOPs used for wastewater treatment [8] and sonochemical decomposition of organic pollutants results from the formation and collapse of high-energy cavitation bubbles. Due to the bubble’s collapse, an enormous increase in temperature and pressure occurs, which results in a number of pyrolytic reactions [9].
Nanoparticles are materials having a size in the range of 1–100 nm. Iron oxide, titanium dioxide, fullerenes and carbon nanotubes have been made into nanoparticles [10], [11]. Nano zero valent iron (NZVI) is an effective reducing agent and catalyst for various applications in environmental remediation. The heterogeneous reaction using ZVI involves five steps: (i) mass transfer of the reactant to the ZVI surface from the bulk solution; (ii) adsorption of the reactant on the ZVI surface; (iii) chemical reaction at the ZVI surface; (iv) desorption of the reaction product from the ZVI surface; and (v) mass transfer of the product into the bulk solution [12]. Treatment of wastewater using nano-scale iron particles represents a new generation of environmental remediation and this provides cost-effective solutions to some important environmental problems [13]. Su and Puls [14] used zero valent ion and tin for the reduction of trichloroethene and reported that both reductive elimination and hydrogenolysis were involved in the reduction of TCE. The results showed that the reduction was controlled by chemical reaction-limited kinetics rather than by mass transport of the TCE to the metal surface. Song and Carraway [15] studied the reduction of eight chlorinated ethanes using nanosized zero-valent iron. They reported reductive β-elimination as the major pathway for the chlorinated ethanes possessing α,β-pairs of chlorine atoms to form chlorinated ethenes, which subsequently reacted with nanosized iron. Reductive alpha-elimination and hydrogenolysis are also reported as concurrent pathways for compounds possessing chlorine substitution on one carbon only, forming less chlorinated ethanes. Pradhan and Gogate [16] studied the removal of p-nitrophenol using hydrodynamic cavitation, either operated individually or in combination with H2O2 and conventional Fenton process. They reported that the removal observed with venturi was higher than with the orifice plate in combination with Fenton chemistry. Dorathi and Palanivelu [17] carried out the degradation experiments on p-chlorophenol using both ultrasound and hypervalent iron and concluded that the sono-assisted ferrate degradation method was more effective than the simple ferrate method. Khoobdel et al. [18] studied the combined action of sonochemical and UV irradiation for the treatment of carbaryl (Carcinogenic compound). The sample was treated in an ultrasound reactor with three different frequencies. The highest degradation of carbaryl was achieved at 130 kHz compared to 35 kHz. The combination of ultrasound and UV irradiation was considerably more effective than when UV or ultrasound was operated individually. Gopinath et al. [19] studied the sonochemical degradation of Congo red. The results showed that the initial dye concentration and pH of the dye solution influenced the % decolorization and low initial values resulted in high % decolorization. Liang et al. [20] reported on the reduction of nitrite by ultrasound dispersed nanoscale zero-valent iron particles and showed that NZVI could be an efficient reductant. Mu et al. [21] used NZVI to reduce nitrobenzene in aqueous solutions. The scope of the present study is to treat petroleum refinery wastewater sonochemically in the presence of NZVI. NZVI particles were synthesized from ferrous sulfate, and were characterized using scanning electron microgram (SEM) and X-ray diffraction (XRD).
Section snippets
Wastewater
The petrochemical wastewater was collected from an industry located at Manali, Chennai. The initial characterization shows that the wastewater had a COD of 40,000 mg/l and a pH of 5.4. This wastewater was stored in a deep freezer and used for further studies.
Preparation of NZVI
The NZVI particles were synthesized by the well-known liquid phase reduction method [22], [23]. 10 mmol (2.78 gm) of FeSO4·7H2O was dissolved in 100 ml of an aqueous solution of ammonium persulfate ((NH4)2S2O8). 1.85 g of sodium borohydrate (NaBH
Sonolysis
The % COD remaining with respect to time of exposure to ultrasound irradiation is shown in Fig. 1. The results show clearly that the % degradation of wastewater increases with an increase in time and almost 40% of degradation was achieved in 60 min. Generally prolonged exposure of wastewater to ultrasound may enhance the generation of oxidative species in water. This is initiated by the hemolytic cleavage of water molecules by pyrolytic reactions, which may be represented as follows:
Conclusion
NZVI particles prepared through a liquid-phase reduction method are almost spherical and exhibit higher surface area available for reactions. Reduction test results indicate that ultrasonication can accelerate the reduction of organic pollutants present in wastewater when used with NZVI. The degradation of organic pollutants present in wastewater with NZVI under sonication followed first-order reaction kinetics. Experiments carried out under various initial pH levels revealed that the optimum
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