Removal of COD from landfill leachate by an electro/Fe2+/peroxydisulfate process
Introduction
An increasing amount of solid waste is generated on account of the growing population, industrialization, living standards, which has resulted in the deterioration of environment. Sanitary landfilling is now the most common way to dispose solid waste. The decomposition of waste and percolation of rainfall in the landfill will produce landfill leachate, which is a highly polluted wastewater containing large amounts of organic pollutants and other contaminants. The removal of organic matter based on chemical oxygen demand (COD) and so on is the usual prerequisite before discharging the leachates into natural water bodies [1], [2]. Therefore, various technologies have been proposed for the treatment of landfill leachate, among which advanced oxidation processes (AOPs) are regarded as one of the most effective techniques to remove recalcitrant organic pollutants from landfill leachate [1], [3], [4], [5]. Generally, AOPs are defined as the oxidation processes that generate hydroxyl radicals in sufficient quantity to affect leachate treatment [3], [6]. Similar to hydroxyl radicals, sulfate radicals have a high redox potential (E0 = 2.6 V), and sulfate radical-based AOPs have been applied to the treatment of landfill leachate [7], [8], [9], [10]. Basically, sulfate radicals can be generated by the activation of peroxymonosulfate (PMS) or peroxydisulfate (PDS) with transition metal, heat, UV or ultrasound (US) [11], [12], [13], [14], [15], [16]:
Sun et al. [7] reported that a 56.9% of COD removal efficiency was achieved by Co2+ activated PMS process. Over 90% COD was removed from the leachate when thermal persulfate oxidation was employed [8]. Recently, persulfate was combined with other oxidants such as ozone for the treatment of stabilized leachate and 72% of COD removal efficiency was obtained [9], [10].
Because of its being inexpensive and nontoxic, ferrous iron has been widely used to mediate the activation of PMS or PDS [13], [17], [18]:
However, the slow regeneration of Fe2+ after conversion to ferric ion is usually considered as one of the drawbacks of iron-mediated advanced oxidation processes [19]. This problem can be solved by coupling electrochemical process and iron activated PMS/PDS process, and ferrous ions are electro-regenerated via cathodic reduction of ferric ions [20]:
As an analogue to electro-Fenton process with a sacrificial iron anode, a novel “Electro–Fe(II)/Oxone” process was put forward and applied to the degradation of 2,4,5-trichlorophenoxyacetic acid in aqueous solution [19]. This process was further enhance by introducing the UV light [21], [22]. Based on Fered-Fenton process, another novel “electro/iron (Fe2+, Fe3+)/PDS (PMS)” process was proposed and evaluated in our previous studies. It exhibited better performance than iron (Fe2+, Fe3+)-catalyzed activation of PDS (PMS) process in the removal of Acid Orange 7 [20], bisphenol A [23] and clofibric acid [24]. Unfortunately, this process has not been applied to the real wastewater treatment. In the iron activated persulfate process, the organics could be removed by the oxidation with reactive radicals generated from the activation of persulfate and by the coagulation with iron salts. Nevertheless, only the overall COD removal was concerned in the transition metal activated persulfate process and the contribution of oxidation and coagulation to COD removal has not been investigated. COD is one of the most important wastewater characteristics and the dichromate reflux method of COD determination is widely used [25]. Like H2O2 in Fenton process, the residual persulfate would interfere with COD measurement. Therefore, an empirical correlation between persulfate concentration and COD value was determined first in this study. Then the electro/Fe2+/peroxydisulfate method was applied to the leachate treatment. The overall COD removal efficiencies as well as COD removal by oxidation and by coagulation under various operating conditions such as initial pH, peroxydisulfate concentration, Fe2+ concentration and current density were investigated.
Section snippets
Leachate collection and characterization
Landfill leachate samples were collected with polyethylene bottles from a municipal sanitary landfill located in Wuhan, China. Samples were stored in a refrigerator at 4 °C in accordance with the Standard Methods [25]. Its characteristics were pH 9.5, COD 1900 mg/L, NH4+–N 2150 mg/L, Cl− 3822 mg/L and total Fe 78.9 mg/L.
Chemical reagents
Sodium peroxydisulfate (Na2S2O8), ferrous sulfate heptahydrate (FeSO4·7H2O), sulfuric acid (H2SO4), sodium hydroxide (NaOH), potassium dichromat (K2Cr2O7), silver sulfate (Ag2SO4),
Effect of initial pH
The initial pH values investigated for electro/Fe2+/peroxydisulfate process were 3.0, 6.0 and 9.0, respectively when PDS concentration was 62.5 mM, Fe2+ concentration was 15.6 mM and current density was 13.89 mA/cm2. As can be seen in Fig. 1, the highest COD removal was achieved at pH0 3.0. This is similar to the result observed in Fenton reaction. When initial pH was adjusted to 6.0, the applied ferrous ion would form Fe2+ complexes in the initial stage of the reaction, which may retard the
Conclusions
The results presented in this study demonstrate that electro/Fe2+/peroxydisulfate process, as one of sulfate radical-based AOPs, is effective to degrade organic pollutants in landfill leachate concentrate. The comparative study indicated that oxidative degradation was the main mechanism to remove COD from leachate in electrochemical oxidation, while the major COD removal was attributed to coagulation in Fe2+/peroxydisulfate process. The contribution of both oxidation and coagulation to COD
Acknowledgements
This research was supported by the National High-Tech R&D Program (863 Program) of China (Grant No. 2008AA06Z332), Wuhan Science and Technology Bureau, China (Grant No. 201060723313), Project of Innovation and Entrepreneurship Training of National Undergraduate (Grant No. 1210486038) and Human Settlements and Environment Commission of Shenzhen Municipality.
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