Pentachlorophenol removal from aqueous solutions by microwave/persulfate and microwave/H2O2: a comparative kinetic study
© Asgari et al.; licensee BioMed Central Ltd. 2014
Received: 21 November 2013
Accepted: 5 May 2014
Published: 11 June 2014
Pentachlorophenol (PCP) is one of the most fungicides and pesticides used in wood protection. Poisoning from PCP may be happened in dermal absorption, and respiration or ingestion. With regard to health and environmental effects of PCP, many methods were studied for its removal. Microwave assisted other methods are environmental friendly, safety, and economical method, therefore, in this study; a modified domestic microwave assisted hydrogen peroxide (MW/H2O2) and sodium persulfate (MW/SPS) was used for PCP removal from aqueous solutions. PCP removal rate was measured under different factors such as pH, energy intensity, SPS, H2O2 concentration, Tert- butyl alcohol (TBA) and chemical oxygen demand (COD). The concentration changes of PCP were determined using spectrophotometer and HPLC spectra, respectively. The best removal PCP rate obtained in condition of pH of 11, 0.02 mol L−1 of SPS, 0.2 mol L−1 of H2O2 and energy intensity of 600 W. Moreover, COD removals in MW/H2O2 and MW/SPS process were 83% and 94%, respectively, also TBA test decreased 15% and 3% of PCP removal in MW/SPS and MW/H2O2 processes respectively. Experimental results indicated that sulfate radical was stronger than hydroxyl radical and examinations order reaction was in first order. In this study, was cleared that MW/SPS process was more effective than MW/H2O2 process in PCP removal.
KeywordsMicrowaves Pentachlorophenol Hydrogen peroxide Sodium persulfate
PCP, one of the phenolic compounds, is widely used in Wood protective industry . Exposure of this compound makes diseases such as aplastic anemia, leukemia, peripheral neuropathy and other problems related to nerve damage (neurotoxicity). This pollutant is a significant contaminant of soil, surface, and groundwater especially around wood preserving facilities [2–6]. Researchers using a mathematical model calculated that 96.5% of PCP is in soil, 2.5% in water, 1% in air, and less than 1% in suspended sediments and organisms in aquatic environments [6, 7]. Therefore, PCP removal from aqueous solutions is essential. According to previous studies is cleared that conventional treatment methods are ineffective for PCP and other refractory compounds removal, because these methods can only transfer the contaminants from one phase to another producing many environmental problems . Recently researchers have found that microwave (MW) heating in combination with hydrogen peroxide (H2O2) and sodium persulfate (Na2S2O8 or SPS)  can mineralize organic compounds successfully and completely [6, 10, 11]. The key effects of these processes is the replacement of hazardous solvents with environmentally benevolent ones . Basic of MW process is the ability of molecules or substances to absorb and transmit MW irradiation . MW irradiation is electromagnetic irradiation in the frequency range of 0.3 to 300 GHz, but laboratory microwave reactors operate at frequency of 2.45 GHz [6, 13, 14]. By breaking oxygen–oxygen bonds of H2O2 and S2O82−, MW commonly are able to dissociate H2O2 and S2O82− into OH0 and SO40 radicals and other radicals which are very powerful oxidizing species .
Similar to hydroxyl radicals, sulphate radicals react with organics by electron transfer, hydrogen abstraction, or addition mechanisms [16, 17]. According to results obtained of previous studies, SPS and H2O2 could be a good option for the MW oxidation technique. In this study, due to an environmental-friendly in addition to highly efficient method and low existence of specific work in this condition, analysis of the PCP removal by MW/H2O2 and MW/SPS under various kinds of parameters was performed and in the end, the effectiveness of MW/SPS and MW/H2O2 processes in the PCP removal was compared.
Materials and methods
Sodium salt PCP, which is the sodium salt of PCP (C6Cl5ONa) with 98% purity was used without further purification. The characteristics of the PCP included of boiling point: 309-310C0, mass molar: 288.32 g mol−1. PCP solution was prepared by dissolving PCP in NaOH solution to accelerate its dissolution [6, 18]. Hydrogen peroxide (30% w/w) and the sodium persulfate from Merck, 98% mass molar: 238.1 g mol−1 were used as oxidants.
Experimental methods and measurements
In this study different factors effects such as pH (3, 7, 11), energy intensity (180,450,600 W with optimal temperatures 80, 100, and 105C0 respectively), SPS and H2O2 dose (0.01, 0.02, 0.03, 0.04, 0.05 mol L−1), PCP concentration (100, 200, 300, 400, 500, 750, 1000 mg L−1), effect of Tert-butyl alcohol (TBA ) with 0.04 mol L−1 concentration, and COD (344 mg L−1) were determined. Changes of PCP concentration were detected using spectrophotometer according to (APHA ), and HPLC. HPLC (Part Number.WATO54275 with dimension of 4.6 mm × 250 mm and column of symmetry C18-50 μm) method was performed with an acetonitrile/water 60:40 (v/v) as mobile phase at a flow rate of 1 mL min−1 and detection wavelengths of UV was 254 nm [6, 20]. COD was detected using potassium dichromate solution as oxidizer in a strong acid medium, then by titration step using ferrous ammonium sulfate as the reducing agent and Ferroin as the indicator [6, 9].
Results and discussion
Effect of pH on PCP removal
In general and according to experimental conditions following reactions can be performed:
Under MW/SPS, the rate constants for Eqs. (1) and (2) are < 2 × 10−3 and (6.5 ± 1) × 107 M−1S−1 respectively. It is cleared that the reaction rate constant of Eq. (2) is more than Eq. (1). According these equations in all pHs and alkaline pH, both SO40− and OH0 are possibility responsible for degradation of organic contaminants, but previous studies have shown that in pHs of 3–10, amount of hydroxyl radical is more than sulfate radical and in pH > 10.5 amount of sulfate radical is more than hydroxyl [26, 27]. According these results, the difference between our work and previous studies could partly attribute to pH = 11. Results of other studies confirm that organic removal efficiency is more in alkaline pH . In similar to, under MW/H2O2 in alkaline pH, amount of OH0 and other radicals participating in PCP removal is more than other pHs (Eqs (3) to (8)) [6, 11, 28].
Effect of SPS and H2O2 concentrations on PCP removal
Effect of different energy intensity on PCP removal
Effect of radical scavenger on PCP removal
Mineralization of PCP in MW/SPS and MW/H2O2 processes and identification of oxidation intermediates
MW/SPS and MW/H2O2 processes could efficiently degrade refractory compounds at strong alkaline, via radical production. MW/SPS in PCP removal was more effective than MW/H2O2, because SPS is dissociated and activated more easily than hydrogen peroxide. Addition of SPS and H2O2 doses during MW process enhances the rate of PCP degradation, except when the radical scavenging effects of SPS and H2O2. Results obtained from radical scavenger test showed that OH° had only an initiator role, and had not a dominant role and order reaction in both of systems was in first order. Also, the microwave degradation is able to mineralize refractory compounds without any toxic byproduct. The microwave degradation has many advantages such as convenience, safety, economy and high efficiency. Accordingly these methods, especially WM/SPS, have a better prospect in future for removal of other chlorinated organic compounds such as Aldrin, Dieldrin and Lindane, in alkaline pH.
Advanced oxidation processes
Tert- butyl alcohol
Chemical oxygen demand
The authors would like to thank Hamadan University of Medical Sciences for technical and financial support of this work (9010274023).
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