Removal of 2,4-dichlorophenoxyacetic acid in aqueous solution by pulsed corona discharge treatment: Effect of different water constituents, degradation pathway and toxicity assay
Introduction
With growing population and high demand of food source, the use of organic pesticides in modern agriculture has been tremendously increased. Nevertheless, health and environmental problems associated with the use of pesticides cannot be overlooked. Pesticides applied to the soil and crops may leach to the groundwater or reach to surface water through rain water runoff (Senthilnathan and Philip, 2010). These organic pesticides are often found in ground and surface water and their concentrations often exceeds the drinking water standards (Ccanccapa et al., 2016, Cruzeiro et al., 2016, Mekonen et al., 2016, Rajendran et al., 2011).
2,4 dichlorophenoxyacetic acid (2,4-D) is the most commonly utilized herbicides because of its low cost and high efficiency against broadleaf weed species (EPA, 2005). It is sparingly soluble in water (667 mg/L) and often detected in surface (≈6 μg/L) and drinking water (2.2–3.2 μg/L) sources (Mekonen et al., 2016, Teklu et al., 2015). Recently, some study reported the cardio-toxicity and geno-toxicity of 2,4-D on zebra fish (Li et al., 2017). In general, it posses medium to high risk to aquatic species (Teklu et al., 2015). Its neurotoxicity, acute toxicity and its endocrine disruptive potential cannot be neglected (EPA, 2005). The regulatory standard for the 2,4-D in drinking water is 30 μg/L and 70 μg/L as per Indian standard (BIS 10500:2012) and US standards (USEPA, 2017), respectively. The traditional technologies such as biodegradation, micro-filtration, adsorption and membrane separation used for the removal of pesticides from drinking water source, only do a phase transfer of pollutants and also not sufficient enough to produce treated water to meet drinking water standard (Lafi and Al-Qodah, 2006, Poyatos et al., 2010). Advanced oxidation processes (AOPs) like Fenton (Chen et al., 2015); photo-Fenton (Schenone et al., 2015), UV/Vis-photocatalysis (Akpan and Hameed, 2011, Bandala et al., 2007, Senthilnathan and Philip, 2010), and UV/H2O2 (Nienow et al., 2008) have been applied for the degradation of several classes of recalcitrant pesticides. Though these advanced technologies have been proven to remove these pesticides effectively from water, worldwide researchers are trying to trade-off between energy consumption and degradation efficiency. Recent advancement in water treatment technology, plasma technology has been emerging as the potent technology for the rapid removal of these recalcitrant compounds (Foster, 2017). Like other AOPs, plasma technology also generates hydroxyl radicals and other reactive species. Plasma generation using dielectric barrier discharges (DBP) are often employed for the removal of micro-pollutant from water (Hijosa-Valsero et al., 2013, Magureanu et al., 2011). Pulsed corona discharges are another method for the plasma generation in the air (Locke and Thagard, 2012). Needle–plane geometry are often used for the removal of various micro-pollutants from the water (Gerrity et al., 2010, Panorel et al., 2013).
In the last decade, plasmas generated by different types of electrical discharges have been used for the treatment of various micro-pollutants from water. However, the information on the effects of different water constituents on the degradation process is limited. In drinking water source, different water constituents such as carbonates, bicarbonates, sulphates, nitrates, chlorides and humic acids are often present, which can quench the plasma generated hydroxyl radicals during the treatment process (Jiang et al., 2014, Singh et al., 2016a, Singh et al., 2016b). Also the information about the fate of pollutant degradation in context of complete mineralization and detoxification under plasma treatment process is extremely limited. Many a time it is reported that the intermediates generated from parent compounds are more toxic (Drzewicz et al., 2004, Lapertot et al., 2008). The conventional water treatment methods such as chlorine or ozone oxidation also lead to formation of carcinogenic by-products such as trihalomethanes, haloacetic acid in water (Zhang et al., 2005, Harvey, 2011, Tian et al., 2013). Therefore, a complete assessment of the degradation pathway along with toxicity evaluation is also required.
The present study focussed on the degradation of 2,4-D by the application of pulsed corona discharge. The objectives of this study are, (i) to investigate the effect of environmental parameters like carbonates, nitrate, sulphate, chloride, humic acid, pH etc. and operating parameters such as voltage and frequency on 2,4-D degradation (ii) to investigate the extent of mineralization of 2,4-D by performing the TOC analysis and mass spectroscopy, (iii) to propose the degradation pathway of 2,4-D in plasma treatment system, and (iv) to evaluate the ecotoxicity of 2,4-D and its degradation products.
Section snippets
Materials
Analytical grade 2,4-D (99.9% purity) was procured from Sigma Alderich and analytical grade NaCl, NaCO3, NaNO3, Na2SO4 and humic acid from Merk India, were used for this study. All the stock solutions were prepared in MilliQ water (Millipore, USA) and dilutions were made with distilled water.
Experimental set-up
The pesticide degradation experiments were carried out in a multiple-pin plane configured pulsed corona discharge reactor (Fig. 1). Seven multiple pin made up of tungsten was served as high voltage
Effect of voltage and frequency on 2,4-D degradation
The oxidation of organic compounds in plasma processes is highly dependent on energy input and the energy input is dependent on applied pulsed voltage and frequency (Eqs. (3), (4)). Therefore, the effect of applied pulsed voltage and frequency was studied for the degradation of 2,4-D. Fig. 2 shows the degradation profile of 2,4-D as a function of time where pulse voltage and frequency were varied. A faster degradation rate of 2,4-D was achieved at higher voltage and frequency. Earlier in our
Conclusion
Complete degradation of 2,4-D was achieved in 6 min of plasma treatment with a yield of 0.9 g/kWh. A detailed investigation was carried out to understand the effects of pH and different water constituents on 2,4-D degradation efficiency/yield. In acidic pH condition, a better degradation efficiency and yield was achieved. The effects of nitrate, sulphate and chloride were insignificant, while degradation efficiency was significantly decreased in presence of CO32ˉ ions and humic acid. The first
Acknowledgement
Financial support by Department of Science and Technology (DST) (grant number: SERB/MOFPI/021/2015), India is gratefully acknowledged.
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