Elsevier

Chemical Engineering Journal

Volume 250, 15 August 2014, Pages 76-82
Chemical Engineering Journal

Removal of COD from landfill leachate by an electro/Fe2+/peroxydisulfate process

https://doi.org/10.1016/j.cej.2014.03.114Get rights and content

Highlights

  • An electro/Fe2+/peroxydisulfate process was used to remove COD from landfill leachate.

  • An empirical correlation between persulfate concentration and COD value was obtained.

  • COD removal by oxidation and coagulation was quantitatively determined.

  • Effects of pH0, current density, Fe2+ and PDS dosage on COD removal were investigated.

  • 62.2% COD was removed at 60 min, [PDS]:[Fe2+] = 62.5 mM:15.6 mM, pH 3 and j = 13.89 mA/cm2.

Abstract

Electrochemical method was used to enhance COD removal during treatment of landfill leachate by ferrous ion activated peroxydisulfate (PDS) process. The contribution of oxidation and coagulation to COD removal was quantitatively determined. The effects of initial pH, current density, PDS and ferrous ion dosage on COD removal were investigated. The COD removal by oxidation increased with PDS concentration and current density, but decreased with initial pH. There existed an optimal Fe2+ dosage for the oxidative degradation of leachate. The COD removal by coagulation increased with Fe2+ dosage, but decreased with current density. The overall COD removal efficiency was significantly improved with respect to ferrous ion activated PDS 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]:S2O82-+Mn+SO4-+SO42-+M(n+1)+HSO5-+Mn+SO4-+OH-+M(n+1)+S2O82-+heat/UV/US2SO4-HSO5-+heat/UV/USSO4-+OH

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]:Fe2++S2O82-Fe3++SO42-+SO4-HSO5-+Fe2+Fe3++SO4-+OH-

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]:Fe3++e-Fe2+

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.

References (47)

  • G.J. Price et al.

    Sonochemical acceleration of persulfate decomposition

    Polymer

    (1996)
  • G.J. Price et al.

    Ultrasonically enhanced peroxydisulfate oxidation of polyethylene surfaces

    Polymer

    (1996)
  • G.P. Anipsitakis et al.

    Transition metal/UV-based advanced oxidation technologies for water decontamination

    Appl. Catal. B: Environ.

    (2004)
  • M.G. Antoniou et al.

    Degradation of microcystin-LR using sulfate radicals generated through photolysis, thermolysis and e-transfer mechanisms

    Appl. Catal. B: Environ.

    (2010)
  • S.N. Su et al.

    Degradation of amoxicillin in aqueous solution using sulphate radicals under ultrasound irradiation

    Ultrason. Sonochem.

    (2012)
  • A. Rastogi et al.

    Sulfate radical-based ferrous-peroxymonosulfate oxidative system for PCBs degradation in aqueous and sediment systems

    Appl. Catal. B: Environ.

    (2009)
  • A. Rastogi et al.

    Effect of inorganic, synthetic and naturally occurring chelating agents on Fe(II) mediated advanced oxidation of chlorophenols

    Water Res.

    (2009)
  • Y.R. Wang et al.

    Degradation of 2,4,5-trichlorophenoxyacetic acid by a novel electro–Fe(II)/oxone process using iron sheet as the sacrificial anode

    Water Res.

    (2011)
  • J. Wu et al.

    Degradation of Acid Orange 7 in aqueous solution by a novel electro/Fe2+/peroxydisulfate process

    J. Hazard. Mater.

    (2012)
  • Y.R. Wang et al.

    Photo-assisted degradation of 2,4,5-trichlorophenoxyacetic acid by Fe(II)-catalyzed activation of oxone process: the role of UV irradiation, reaction mechanism and mineralization

    Appl. Catal. B: Environ.

    (2012)
  • Y.R. Wang et al.

    Photo-assisted degradation of 2,4,5-trichlorophenol by electro–Fe(II)/oxone® process using a sacrificial iron anode: performance optimization and reaction mechanism

    Chem. Eng. J.

    (2013)
  • H. Lin et al.

    Degradation of bisphenol A in aqueous solution by a novel electro/Fe3+/peroxydisulfate process

    Sep. Purif. Technol.

    (2013)
  • H. Lin et al.

    Degradation of clofibric acid in aqueous solution by an EC/Fe3+/PMS process

    Chem. Eng. J.

    (2014)
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