UMTP and Department of Chemistry, University of Beira Interior, 6201-001 Covilhã, Portugal

Abstract

The electrochemical treatment of leachate samples from a Portuguese intermunicipal sanitary landfill was carried out using combined electrocoagulation/anodic oxidation processes. The electrocoagulation (EC) was performed with iron consumable anodes, at different initial pH, with and without stirring, at different applied potentials. In the anodic oxidation (AO) assays, a boron-doped diamond anode was used and applied current densities of 10 to 30 mA/cm² were tested. The influence of the experimental conditions of the electrocoagulation pretreatment on the anodic oxidation performance was also assessed. In the EC assays the lowest iron and energy consumptions per mass of organic load removed were obtained at initial pH of 4 in the unstirred assays. In the combined treatments, the highest average current efficiency in the anodic oxidation was obtained for the samples with EC pretreatment performed at pH of 6. For the combined treatment with EC pretreatment run at natural pH of 8.6, an increase in AO current density led to a decrease in average current efficiency.

Keywords: landfill leachate, electrochemical treatments, electrocoagulation, anodic oxidation, BDD.

Introduction

The generation of municipal solid waste grows exponentially in urban society as a consequence of population growth, concentration of population in urban centres and new patterns of consumption. Sanitary landfills are the primary method currently used for municipal solid waste disposal in many countries [1]. One of the main problems generated by this common practice is the production of leachate [1]. It is a very complex wastewater containing different heavy metals, organic and inorganic compounds, some of them refractory and toxic, that possesses color and odor [1,2]. There is great concern about the contamination caused by landfill leachate, mainly because it is particularly difficult to treat. Thus, it is very important to apply reliable and effective treatment technology. Conventional treatments (biological or physical-chemical) are not sufficient to reach the level of purification needed to eliminate the negative impact of landfill leachate on the environment. Membrane processes have proved to be a good solution to achieve full purification. However, these processes are restricted by treatment costs and membrane fouling [3]. Due to their effectiveness and ease in operation, electrochemical methods have recently received significant attention for wastewater treatment and several studies have reported the application of these methods in wastewater treatment [4-7].

Electrocoagulation (EC) is one of the electrochemical methods applied in the treatment of many types of wastewaters. A number of reports have described its application for wastewater treatment, including landfill leachate [8-12]. These studies have shown that electrocoagulation is a promising method to remove heavy metals, arsenic, dyes and other persistent pollutants that are hardly removed by conventional treatments [13]. Ilhan et al. [14] investigated the application of electrocoagulation to leachate treatment using aluminium and iron electrodes and showed that the electrocoagulation process has higher treatment performance than the chemical coagulation process and it can be successfully used as a step of a combined treatment.

The generation of metallic ions by EC is very dependent on the applied potential and on the characteristics of the wastewater, namely, the pH and the conductivity. The main processes that take place in the electrolytic system when iron anodes are used can be described by reactions (1) to (4). At the anode, the Fe²⁺ is present in the wastewater due to the oxidation of Fe according to reaction (1):

At the cathode, H₂ gas is formed from the reduction of proton in acidic medium (reaction 2) or from water reduction in alkaline medium (reaction 3). In both cases, the pH increases during electrolysis and the Fe²⁺ ion can react and form iron hydroxides (reaction 4):

In solution, iron ions hydrolyse and, depending on the medium pH, different hydroxide mononuclear complexes may be formed. The insoluble Fe(OH)₂ and Fe(OH)₃ flocs have strong affinity for colloids, dispersed particles and ionic species and cause flocculation, which generates bigger particles. The formed flocs can be removed by sedimentation or flotation [4,13].

Another promising electrochemical method used in wastewater treatment is the electrochemical oxidation that, in most of the existing studies, is applied as a post-treatment method. Deng and Englehardt [15] present an overview of electrochemical oxidation processes used to treat landfill leachates. Different types of anode materials (i.e. TiO₂/RuO₂, PbO₂/SnO₂, Ti/Pt, Ti/PbO₂ and boron- doped diamond (BDD)) and the effect of several operating factors have been assessed [16,17], being the best results obtained with BDD anodes. Panniza and Cerisola [18] published a review of the results presented in the literature with this electrode in the past decade. This material has unique chemical, electrochemical and structural stabilities that allow its use at high potentials, where most organic pollutants can be oxidised [19]. More recently, Urtiaga et al. [20] reported the combination of activated sludge treatment with Fenton oxidation and electrooxidation with BDD anodes, at pilot scale, to treat a landfill leachate, where almost complete removal of the organic matter and ammonium nitrogen was achieved. A combination of biochemical treatment and electrochemical oxidation on a BDD electrode was also proposed, with promising results [21]. It was also described in the literature the application of electrochemical treatment to a stabilized leachate from a semi-aerobic sanitary landfill, using graphite carbon electrodes in the presence of sodium sulphate as electrolyte; high biochemical oxygen demand (BOD), chemical oxygen demand (COD) and color removals were achieved [22]. Although the results demonstrate that, under appropriate conditions, electrochemical oxidation can significantly eliminate organic contaminants, ammonia and color from leachate, the formation of undesirable oxidation by-products, such as nitrate anions and chlorinated organic compounds, has been detected [23]. Enormous advances have taken place in the development of electrochemical oxidation processes for landfill leachate; however more studies need to be done in order to optimize the efficiency of these processes.

The aim of this work was to evaluate the application of electrocoagulation and anodic oxidation as a combined treatment for a leachate from an intermunicipal sanitary landfill. The electrocoagulation was used as a first step, in order to remove colloidal and suspended particles, and it was followed by anodic oxidation to eliminate the remaining dissolved persistent organic compounds. In the electrocoagulation assays different experimental conditions were tested, in order to study their influence on the removal of organic matter: the initial pH of the samples that was varied in the ideal pH range for the application of electrocoagulation technique [13]; the existence of stirring, to find out also how it influences the time needed to electroprecipitate the suspensions; and the applied potential, to see if an increase in the rate of iron oxidation could lead to a more favourable energetic consumption.

Materials and methods

Leachate samples

The leachate samples used in this study were collected from an intermunicipal sanitary landfill, in two different seasons of the year, at the beginning of summer (L1) and at the beginning of winter (L2). The in situ treatment performed at the sanitary landfill comprises a biological step, followed by an ultrafiltration operation. The leachate samples were collected in the stabilization lagoon before the biological treatment, and kept refrigerated, in order to maintain its initial characteristics.

Analytical determinations

The samples collected at the leachate treatment plant and those from the electrochemical assays were analyzed, according to the procedures described in the standard methods [24], for the following parameters: Chemical oxygen demand, using closed reflux dichromate titrimetric method; Dissolved solids (DS); Suspended solids (SS); Total organic carbon (TOC) or dissolved organic carbon (DOC) (determined after filtration with a 1.2 μm filter), measured using a TOC analyser, Shimadzu TOC-V CPH. UV-Visible absorption spectrophotometric analyses, with absorbance (Abs) measurements from 200 to 800 nm, were also done, using a Shimatzu UV-1800 spectrophotometer. Measurements of pH and conductivity were carried out with a Mettler-Toledo pH-meter and a Mettler-Toledo conductimeter, respectively.

Electrochemical assays

The cell used in the electrocoagulation study contained an iron anode, with an immersed area of 20 cm², a stainless steel cathode of 16 cm² and the distance between them was 2.3 cm. The useful cell volume varies between 150 and 250 mL and experiments were conducted in batch mode. The electrodes were dipped into a vessel containing 150 mL of leachate. A constant voltage in the electrochemical cell was maintained using a DC power supply (GW, Lab DC, model GPS-3030D, 0~30V, 0~3 A). In order to calculate energy consumptions, current intensities were registered. All experiments were conducted at room temperature (22-25 °C) with no addition of background electrolyte. All the assays were run at least twice. In the cases where removals of COD (or DOC) of both assays differ from more than 10%, assays were repeated, thus guaranteeing that standard deviations of the results presented for these parameters are less than or equal 10%. The results presented are the medium of all the experiments performed.

With the aim of optimizing experimental conditions, different applied potentials, between 2 and 6 V, were tested for L1 samples, at natural pH of 8.5. For L2 samples, several different initial pH conditions were also studied, namely, natural pH of 8.6 and pH of 4, 6 and 10, with applied potentials between 4 and 6 V. All pH adjustments were done by the addition of concentrated NaOH or H₂SO4 solutions. To study the effect of stirring in the process efficiency, all the experimental conditions mentioned above were tested with (600 rpm) and without stirring. After various elapsed times, the electrocoagulated samples started to precipitate. When massive precipitation was no longer observed (10 to 50 minutes after starting the assay), current was turned off and the treated effluent was allowed to settle for 30 min and then the supernatant liquid was collected for the analytical determinations. Between experiments, the electrodes were washed, first with tap water and then with distilled water, to remove the sludge adsorbed on the electrode surface.

The combined treatment, electrocoagulation followed by anodic oxidation (AO), was performed with 450 mL of leachate at the following electrocoagulation conditions: natural pH of 8.5, without stirring, for L1 samples; pH 6 and 8.6 (natural), without stirring, for L2 samples. After the EC treatment, which took between 45 and 60 min, the formed flocs were allowed to settle, and the suspension was decanted and subjected to filtration, in order to eliminate most of the suspended matter from the liquid mixture. This filtration avoids the unnecessary energy consume that would be used in the anodic oxidation of the organic matter present in the smaller flocs. The filtrate was then subjected to the anodic oxidation treatment.

Anodic oxidation experiments were conducted in batch mode, for 8 h, with imposed current densities of 10, 20 and 30 mA cm^-2, using 200 mL of the electrocoagulated effluent. The useful cell volume varies between 100 and 300 mL. A BDD anode, with an immersed area of 20 cm², and a stainless steel cathode, with identical area, were used, being the distance between them 2 cm. A GW, Lab DC, model GPS-3030D (0~30V, 0~3A), was used as power supply. Experiments were conducted at room temperature (22-25 °C) and samples were collected regularly, to perform the analytical determinations. All the AO were repeated twice and the values presented are the mean of the results obtained. Standard deviations for the samples collected in both assays, hourly, were calculated and they were always lower than 5%. Between experiments, the cell was cleaned with a NaCl solution 0.1 M, for 10 min, using an applied current density of 30 mA cm^-2, and rinsed with 3 × 500 mL of distilled water.

Results and discussion

Samples characterization

Although samples were collected in different seasons, COD is similar for L1 and L2. The most significant differences were observed in DOC content, being DOC value for L1 more than twice the value for L2. Since DOC is determined in the filtrate, the DS and conductivity are also higher for L1 samples.

Electrocoagulation assays

When the results of all the assays performed with L2 samples are compared, we may draw the following conclusions: although initial pH of 4 gives the highest load removals, the final solutions present very low pH and very high conductivity; for initial pH 10, the load removal is poor and final pH and conductivity are high; only assays run at pH of 8.6 present a reduction in the conductivity; regarding the iron consumption, there is a tendency to a decrease in the iron needed to promote the flocculation with the increase in initial pH; in most assays performed at equal initial pH and applied potential, the energy consumption increases with stirring; regarding the influence of the applied potential, in most of the cases, an increase in the applied potential leads to a decrease in the time needed to form flocs with critical size (ECtime), i.e., with enough mass to precipitate.

The specific removals in COD are very dependent on the time necessary to observe the flocculation, and, in general, this time increased with stirring. Regarding the influence of initial pH, the most economical solutions, in terms of iron and energy consumptions, are obtained in the unstirred assays, at pH 4 and 8.6, by this order. However, according to literature, at pH lower than 5 the amount of iron dissolved from the anode is higher than the one expected from Faraday's law due to chemical attack of the anode by protons [25]. This way, the results obtained at this pH may be due to an extra quantity of dissolved iron that can be used in the precipitation of the organic matter.

Anodic oxidation assays

Since in the results obtained by EC the COD removal efficiencies were relatively low, further treatment is required in order to eliminate dissolved organic compounds. In this work, it was proposed to treat the leachate using combined processes, an EC pretreatment followed by anodic oxidation using a BDD anode. The EC's pretreatment conditions chosen for the combined treatment were: pH of 8.6, because it does not require addition of chemicals; no stirring, because it leads to quicker settling; applied potential of 5 V, since it is the applied potential that led to a higher COD+DOC removal.