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Vol. 4. Issue 2.
Pages 109-113 (April - June 2015)
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Vol. 4. Issue 2.
Pages 109-113 (April - June 2015)
Original Article
DOI: 10.1016/j.jmrt.2014.11.004
Open Access
Study of electroflotation method for treatment of wastewater from washing soil contaminated by heavy metals
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Izabel de Oliveira da Motaa,
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izabeldamota@yahoo.com.br

Corresponding author.
, José Adilson de Castroa, Rui de Góes Casqueirab, Angelo Gomes de Oliveira Juniorb
a Doutoral Program in Metallurgical Engineering, Universidade Federal Fluminense, Volta Redonda, RJ, Brazil
b Institute of Technology, Universidade Federal Rural do Rio de Janeiro, Seropédica, RJ, Brazil
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Abstract

Electroflotation method (EFM) for treatment of synthetic solutions simulating wastewater from washing soil contaminated by drilling fluids from oil wells was investigated in this paper. Experiments were carried out to examine the effects of the operating conditions on the removal of lead, barium and zinc from solutions containing 15mgdm−3 for each metal representing a typical concentration of wastewater generated in the washing soil in this treatment. The experimental results showed that it is possible to remove these heavy metals by electrocoagulation/electroflotation (ECF) attaining 97% of removal using stainless steel mesh electrodes with a power consumption of 14kWhm−3. The optimal conditions of treatment were sodium dodecyl sulfate (SDS) in a molar ratio 3:1, current density around 350Am−2, ionic strength 3.2×10−3M, pH=10.0 and 20min of ECF. This study newly indicated that the proposed method is adequate to simultaneously treat the common heavy metals found in the drilling fluids oil wells.

Keywords:
Heavy metal removal
Remediation
Lead
Barium
Zinc
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1Introduction

Environmental pollution caused by heavy metals from different economic activities constitutes a serious risk for the environment. The oil wells drilling have been one source of soil contamination by lead (Pb), barium (Ba) and zinc (Zn), due to the composition that contains substances such as barite and galena, which are used for density control of drilling fluids.

The soil washing is a technique that separates a variety of inorganic contaminants of the clay fraction. When this remediation technique was used in contaminated soil samples from the region of Mato Rico and Santa Maria do Oeste (Paraná, Brazil), it generated a large amount of diluted wastewater whose initial concentrations of Pb, Ba and Zn ions were around 15mgdm−3 for each heavy metal. Due to their high solubility in the aquatic environments, the heavy metals, known as powerful toxic agents, can be absorbed by living organisms and may accumulate in the human body [1–4].

The traditional techniques for wastewater treatment do not show satisfactory performance for very dilute solutions (concentration below 50mgdm−3), mainly due to the low operational efficiency and high extraction costs [2,3]. In this way, an alternative method, such as the electroflotation (EFM), has been proposed as an efficient alternative treatment method that can be applied for small, medium or even large scales [5–14]. The EFM is a simple process that floats ions or solid particles, suspended or dissolved in a liquid phase, by the adhesion on tiny bubbles of hydrogen and oxygen formed on cathode and anode and moves upward in the flotation cell. The key to floatability of chemical species is the hydrophobicity, the ratio of collector to metal ion being an important factor; hence, the amount of collector used in ion flotation should be at least stoichiometric with foamy properties [7].

The electrode selection allows configuring the system for a specific process: soluble anodes, as made by iron or aluminum, generate coagulants agents in situ promoting the simultaneous electrocoagulation (EC) phenomenon. It is generally accepted that the EC process involves three stages. The first stage consists of electrode oxidation to form coagulants agents. Simultaneously, the water electrolysis occurs. At stainless steel electrodes, the following reactions take place [14,15]:

Anode:

FeFe2++2e
H2O2H++½O2+2e

Cathode:

2H2O+2e2OH+H2

In practice, the second anode reaction (Eq. (2)) must be dominant and bubbles of gas are observed at both electrodes. The second stage of EC involves destabilization. An electrochemical reduction of metal cations may occur on cathode surface and the hydroxide ions formed at the cathode increase the pH of the wastewater thereby inducing precipitation of metal ions.

Fe2++2OHFe(OH)2

However, Fe3+ ions can be formed at acidic pH and in the presence of oxygen. Depending on pH, the species Fe(OH)2+, Fe(OH)+2 and Fe(OH)3 may be present in the solution.

Fe3++H2OFe(OH)2++H+
Fe3++2H2OFe(OH)+2+2H+
Fe3++3H2OFe(OH)3+3H+

Under alkaline conditions, Fe(OH)3 and Fe(OH)4 are present in more quantity in solution and it can be expected to occur also in absorption and adsorption of heavy metals by these compounds. So, the last EC stage consists of aggregation of the destabilized phases to form flakes [16,17].

The objective of this work is to remove Pb, Ba and Zn ions of solutions that simulates wastewater from washing soil contaminated by drilling fluids from oil wells by ECF using stainless steel mesh electrodes. Parameters such as current density, ionic strength, pH and time of EF were investigated to enhance the efficiency of treatment. This paper proposes an electroflotation method that simultaneously removes the Pb, Ba and Zn in solutions with low concentrations compatible with those obtained in the washing steps of the contaminated soil treatment during the drilling wells of oil production [18,19].

2Methods

The flotation cell used in this study was designed with 13cm×13cm×13cm acrylic cube and peripherals. Fig. 1 shows the experimental apparatus composed by a continuous current (CC) supply 0–30V/0–15A connected to the soluble electrodes through an isolated copper wire. The electrodes which correspond to S=40cm2 were assembled with a stainless steel wire (0.4mm diameter) disposed on handmade acrylic frame, located on the flotation cell bottom. The distance between both electrodes was e=10mm.

Fig. 1.

Experimental EF unit and stainless steel mesh electrodes.

(0.09MB).

All the runs were carried out at a room temperature (28°C). In each run, 1dm3 of stocked synthetic wastewater was added to the flotation cell. The collector (SDS) was added in the correct ratio as well as the pH regulators and bubbly agent (ethanol). The CC font was turned on and the procedure was initiated. After flotation time (specific for each test) the samples were collected and taken to atomic absorption analysis. It is important to report that the initial pH varies along the experiments, being necessary small adjustments to keep the desired during the experiments.

All experiments were carried out on bench scale and the chemicals used were listed as follows: lead II sulfate (PbSO4, VETEC, Brazil), barium sulfate (BaSO4, ISOFAR, Brazil), zinc sulfate anhydrous (ZnSO4, VETEC, Brazil), sodium sulfate (Na2SO4, VETEC, Brazil), sodium dodecil sulfate (C12H25NaO4S, B’HERZOG, Brazil), sulfuric acid (H2SO4, VETEC, Brazil), sodium hydroxide (NaOH, VETEC, Brazil), ethyl alcohol (C2H6O, B’HERZOG, Brazil) and distilled water. All chemicals were of analytical grade.

The sampling method was carried out for different time intervals, current densities and concentrations, depending on the runs. The samples were collected within the solution suitable for atomic absorption analysis technique. The foam was not analyzed.

3Results and discussions

EF is reported to be strongly dependent on pH. This trend is confirmed by Fig. 2 that shows the effect of the pH on the Pb removal. In fact, when the pH varies, heavy metals species take different reactivity. The efficiency of the removal of heavy metal during the treatment was evaluated by its removal efficiency (η) given by Eq. (8)[15–26]:

where Co and C are, respectively, the concentrations ions before and after the treatment.

Fig. 2.

Pb2+ species fraction diagram ([Pb2+]=15mgdm−3).

(0.04MB).

The Pb2+speciation diagram (Fig. 2) was constructed using the MEDUSA software [24]. This diagram indicates the predominance of Pb2+ specie in the pH <6.0 range which allows the use of an anionic collector; therefore, SDS collector was chosen. Fig. 2 also shows the formation of the anionic species complex in very basic pH (above 12). Consequently, as SDS is an anionic collector, the floatability above pH 10.0 was not studied which is corroborated with previous literature data [27].

In EF tests, stainless steel mesh electrodes were used, and EF involved in situ generation of coagulant. The iron ions generation in solution occurs initially with Fe+2. In alkaline conditions, Fe+2 species can hydrolyze and in acid conditions Fe+3 ions may be formed. Therefore, the study of the pH effect on Pb removal initially on pH=4.0 was investigated, as shown in Fig. 3.

Fig. 3.

Removal of Pb as a function of the pH and the time (Pb:SDS ratio=3, ionic strength 3.2×10−3M, 0.1%, v/v ethanol and 350Am−2).

(0.09MB).

Fig. 3, as expected, shows that the electrolytic process is also influenced by time. After 5min it was possible to obtain above 94% removal of Pb. The EF kinetics is strongly influenced by the amount of bubbles dispersed within the solution. Since the tests were conducted under the same current density and the same ionic strength, the amount of bubbles remained theoretically constant during the runs. In spite of high process efficiency under all conditions examined, from the environmental standpoint, 20min were enough to attain the concentration, according to Brazilian environmental laws, that stands the maximum concentrations of Pb, Zn and Ba for effluent discharge are 0.5mgdm−3, 5.0mgdm−3 and 5.0mgdm−3, respectively. In the literature, the study of individual heavy metal ions is well established [3–5]. However, the removal efficiency of heavy metal ions in wastewater containing several ions usually presents quite different trends [22–30]. In this sense, this paper presents novelty. For the Pb, Ba and Zn ions simultaneously the final efficiency is higher compared with other methods and lower when compared with individual ions, depending on the characteristics of coagulation and charge interactions [22–26].

Previous works on ion flotation have indicated that the use of specific stoichiometric ratios of collectors plays an important role in the removal performance [11]. The heavy metal/collector ratio has not been the object of this work, the selected ratio was based on the work of Martins et al. [21], which concluded that the highest removal efficiency of Pb by dissolved air flotation can be obtained using 1:3 metal/collector (SDS) ratio, for the same initial concentration of Pb:15mgdm−3, which is justified by the similarities of the experiments. Similar results can be found elsewhere [26–30]. When collector/heavy metal ratio is low, a very thin and unstable foam layer was formed at the top of the cell [1]. As shown in Fig. 3, theoretically the pH=8.0 is the best point for the Pb ECF during the first 20min of flotation. Similar pH condition was indicated by Martins et al. [21] and depending on the ions interactions the recommended electroflotation time may change [18–26].

Fig. 4 shows the effect of the current density on Pb removal by ECF. Although almost constant, an average removal of 92% was achieved. Comparing the results obtained in this study with similar ones described by other authors [22,25–28], it can be observed that the current density did not prove to give meaningful results in the studied range. The electrochemical reaction was found to be limited, probably due to diffusion, resulting in a linear relationship between the current density and the Pb removal, as shown in Fig. 5. The increase of tension promoted an increase in bubbles swarm, nevertheless insufficient to increase the Pb removal.

Fig. 4.

Removal of Pb as a function of the current density (Pb:SDS ratio=3, pH=8.0, 20min of EF, 0.1%, v/v ethanol and ionic strength 3.2×10−3M).

(0.1MB).
Fig. 5.

Removal of Pb as a function of the ionic strength (Pb:SDS ratio=3, pH=8.0, 0.1%, v/v ethanol, 20min of EF and 350Am−2).

(0.08MB).

The ionic strength (I) is a measure of the electrical potential of the solution. Generally, in ECF process, the increase in ionic strength causes an expansion in conductivity, and this usually leads to a gain in removal efficiency [26,30]. The ionic strength is defined by Eq. (9):

where Ci is the molal concentration of each ion, for dilute solutions it is possible to use the molar concentration as a good approximation, and zi is the charge on each ion in solution.

The experimental results of the effect of added supporting electrolyte on Pb removal are shown in Fig. 5 where the ionic strength was agreed with sodium sulfate. There has been a slight decrease in removal efficiency with increasing ionic strength of the system. This can be explained, probably, by the competition of ions with other ions to adhere with the collector. This trend is confirmed by previous work concerned with water treatment [5,7,25].

The tests for Pb removal by ECF showed that there are certain conditions where the ECF technique was able to produce results in agreement with Brazilian standard emission limits. The optimum conditions for removing Pb from a solution with 15mgdm−3 as an initial concentration is current density of 350Am−2, 3.2×10−3M of ionic strength, 20min of flotation time, Pb:SDS ratio=3 and pH=8.0.

Using the above optimal conditions found in the Pb ECF, with the exception of pH, which was evaluated again, the ECF technology was studied on Pb, Ba and Zn removal from a synthetic effluent simulating wastewater from washing soil contaminated by drilling fluids. Initial concentrations of Pb, Ba and Zn equal to 15mgdm−3 for each heavy metal were employed, close to the true effluent concentrations obtained by soil washing. The results are shown in Fig. 6. The pH=10.0 is suggested as the best pH for the simultaneous removal of metals under operating conditions studied.

Fig. 6.

Removal of Pb, Ba and Zn as a function of the pH (Heavy metal:SDS ratio=3, ionic strength 3.2×10−3M, 20min of EF and 350Am−2).

(0.07MB).

Under the experimental conditions studied, it has been found that the voltage increased linearly with the current applied on electrodes. One of the most important parameters, that affect the application of ECF technique for wastewater treatment, was the power consumption. The energy (E) consumed during the electrolytic process (kWhm−3) could be calculated by Eq. (10)[27]:

where U is the applied voltage (V), i is the current (A), t is the time flotation (hour) and V is the volume of the effluent treated (m3).

The specific power consumption achieved in this ECF process was calculated as 14kWhm−3, which represents a relative low cost of the treatment when compared with other methods [2–6].

4Conclusions

The performance of batch ECF technique for the treatment of synthetic solutions simulating wastewater from washing soil contaminated by drilling fluids from oil wells was investigated. The effects of different parameters, including the pH, the electrolysis time, the current density and the supporting electrolyte dosage were evaluated. Some parameters as pH and ECF time investigated were very important for design optimal conditions of the treatment. However, some variables tested, such as ionic strength and current density, were not proved to be very important under the conditions studied.

The experimental results revealed that it was feasible to remove up to 97% of the Pb, Ba and Zn by ECF method with a total energy consumption of 14KWhm−3. The optimum conditions found in this work were as follows: current density 350Am−2, ionic strength 3.2×10−3M, pH=10.0 and 20min of ECF, 0.1% ethanol (as a foamy) and molar ratio 3:1 SDS:heavy metal. The investigation of an efficient new method to treat effluents which contains simultaneously Pb, Ba and Zn is proposed. The overall removal to attain the Brazilian laws was compatible with operation time feasible in the industrial scale with relative low cost. This technique is expected to be also efficient for other effluents which contain other heavy metals with small adjustments on the operating conditions.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors acknowledge the financial support from Conselho Nacional de Desenvolvimento Tecnológico (CNPq) and Fundação Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ), Brazil.

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Copyright © 2014. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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