Lead ions removal from aqueous solution in a novel bioelectrochemical system with a stainless steel cathode

Abstract

Heavy metal pollution, especially lead pollution in water, has been a growing concern due to the toxicity of lead to human and other beings. According to previous reports, bioelectrochemical systems (BESs) showed significant advantages in heavy metal ions removal, but have not been considered for Pb²⁺ removal. In this study, a novel BES with stainless steel cathode distinguished with traditional BESs was employed with mixed culture as biocatalyst for removing Pb²⁺ from solution. The results indicated Pb²⁺ could be effectively removed and hydrocerussite as the final product confirmed by X-ray diffraction was deposited on the stainless steel cathode. Furthermore, the principle of Pb²⁺ removal was deduced based on the experiment of the reduction of ferricyanide in the stainless steel tube-type BES. In brief, we suggested a novel low-cost approach to remove and recover Pb²⁺ from Pb²⁺-containing wastewater.

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1 Introduction

Lead, which is widely used as an industrial raw material for battery manufacturing, acid metal plating and finishing, ammunition, tetraethyl lead manufacturing, ceramic and glass industries printing, and the painting and dying industry,^1,2 has been considered as one of the most toxic contaminants discharged into the environment requiring urgent and intensive monitoring and strong policies.³ It is reported that lead is non-biodegradable and toxic to human beings, especially children.^4,5 Prolonged exposure induces lead accumulation in the body and may cause adverse effects on the brain and nervous system, heart, kidneys, blood and reproductive systems.^6–8 Numerous technologies have been developed for Pb²⁺ removal, including sorption,^9–12 filtration,^2,13,14 electrochemical technology,^3,6,15 and precipitation.^1,16 These methods are especially effective for Pb²⁺ removal, but have disadvantages including hard-recovery, high cost, requirement electricity supply and secondary pollution.

Bioelectrochemical systems (BESs) are emerging technologies which use exoelectrogens attached to electrode to catalyze oxidation reaction in bioanode and/or reduction reaction in biocathode.¹⁷ And BESs has been successfully tested for Cr⁶⁺, V⁵⁺ and Cu²⁺ treatment.^18–20 These results indicated that BESs offer a promising potential for the reductive treatment of metal pollutants as well as organics. However, few researches have been previously launched on using BESs to remove Pb²⁺ from solution. Additionally, carbon felts which were used as cathode in those studies not only make recovery very difficult, but also go against scaling up equipment for industry. Therefore, it's critical to choose proper electrode material as cathode for metal recovery and equipment scaling-up in future.

Stainless steel (SS) with high mechanical properties and long-term resistance to corrosion is a kind of commercial industrial material used in many different compositions and morphologies.²¹ Hence it offers a large range of engineering possibility for scaling up electrode. It allows solid, cost-effective, easy-to-handle equipment to be built, which is stable in the long-term and easy to maintain.²¹ Furthermore, SS has been demonstrated a promising electrode material in BES for wastewater treatment, current generation and hydrogen evolution.^21,22 However, the use of SS cathode for metal removal has never been attempted.

The objective of this study was to set up a bioelectrochemical reactor with SS cathode and investigate the feasibility of Pb²⁺ removal in this reactor. Furthermore, we conducted the experiment of ferricyanide reduction in the same reactor in order to deduce lead removal mechanism.

2 Material and methods

2.1 BESs design

The main body of BES consisted of a stainless steel cylinder (SUS304, diameter: 7.6 cm, height: 10 cm) that was concurrently used as the cathode with the effective area of 173.8 cm² in the catholyte, which was put into a Plexiglas container (10 width × 10 height × 10 depth cm). The anode was carbon felt (length: 20 cm, height: 7 cm) that was respectively washed in 1 M HCl, 1 M NaOH and deionized water until neutral before inserting into the stainless steel cylinder. The cathode and anode were fixed by titanium wire for linking external resistance (R_ext, 100 Ω) as an external load. The schematic of the reactor was used in this research was shown in Fig. 1. For observing the morphology of the attachment on the cathode, a piece of SS patch (SUS304, 1 width × 1 height × 0.1 depth cm) was adhered on the outer of stainless steel cylinder with conductive carbon double-sided tape (LG25K, NISSHIN EM Co., Ltd., Tokyo, Japan).

Fig. 1 The schematic of the reactor which was used in this study.

2.2 Chemicals and BESs startup

The anodic chamber of BES₁, i.e. SS cylinder inner, was inoculated with 10 ml of mixed culture from operating MFC running on acetate and 200 ml of medium contained (for 1 L): 0.10 g KCl, 0.25 g NH₄Cl, 10.92 g Na₂HPO₄·12H₂O, 3.04 g NaH₂PO₄·2H₂O, 10 ml vitamin solution,²³ 10 ml mineral solution²³ and 2.00 g CH₃COONa with the initial pH of 7.0. Afterwards, an Ag/AgCl/KCl (sat.) electrode as a reference electrode was placed in the anode chamber, where was sparged with pure N₂ for 10 min before sealing for maintaining anaerobic condition. The cathodic chamber, i.e. SS cylinder outer, was filled with 500 ml of electrolyte containing 0.1 M NaCl, and continuously sparged with air to supply O₂ as electron acceptor. When the voltage output was stable, it indicated the BES startup successfully. An abiotic anode reactor which was similar to BES₁ except the anode wasn't inoculated with 10 ml of mixed culture was applied as the blank (BES₀).

2.3 Electrochemical test for the bioanode

After the BES run stably, a piece of carbon felt anode attached by microorganism was cut off and applied to cyclic voltammetry (CV) measurement. CV was performed on an electrochemical workstation (E550, Gaoss Union, China) in a single glass cell (50 ml) containing 40 ml fresh electrolyte with a three-electrode system. Bioanode, abiotic anode (carbon felt without culture) and treated bioanode (the bioanode was boiled for 5 min) served as the working electrodes with the same size (1 width × 1 length cm). An Ag/AgCl/KCl (sat.) and a Pt wire were used as the reference and counter electrodes, respectively. Additionally, CV was conducted in a potential range from −0.8 to 0.4 V at a scan rate of 1 mV s⁻¹. The electrolyte was the same as the anolyte of the above BESs startup. Before each experiment, the liquid and headspace of the reactor were sparged with N₂ gas for 5 min to remove the dissolved oxygen.

2.4 BES voltage output

A computer with a date collector (BRH8271, RuiBoHua, China) was used to gather terminal voltage of external resistance with or without acceptor respectively at 5 min interval for 24 h after the reactor was stable. For no electron acceptor, basal catholyte contained 4 g L⁻¹ NaNO₃ was sparged with N₂ gas for 10 min to remove the major dissolved oxygen before sealing. Oxygen and 150 mg L⁻¹ K₃[Fe(CN)₆] were added into basal catholyte as acceptor, respectively.

2.5 Pb²⁺ removal experiments

The anolyte was replaced by fresh medium and the catholyte was replaced with Pb(NO₃)₂ solution containing 40 mg Pb²⁺/L and 4 g L⁻¹ NaNO₃ after washing the cathodic chamber three times with distilled water for cleaning away interfering ions. The initial pH of catholyte was lowered to 4.00 by addition of HNO₃. 1 ml of catholyte was taken at 8 hours interval for residual Pb²⁺ concentration measurement. When test was end, the piece of SS patch adhered on the outer of stainless steel cylinder of the BES was taken out, washed with deionized water and dried in the air for scanning electron microscope (SEM) and powder scraped from SS cathode was analyzed by X-ray diffraction (XRD).

2.6 Indirect experiment for lead reduction

In order to investigate the mechanism of Pb²⁺ removal, 150 mg L⁻¹ of K₃[Fe(CN)₆] instead of Pb²⁺ were filled in the cathodic chamber. The control BES was the same as previously motioned in Pb²⁺ removal experiments except catholyte. 3 ml catholyte was taken once a day at 24 hours interval for the concentration measurement of K₃[Fe(CN)₆] and K₄[Fe(CN)₆].

All experiments were repeated three times at about 25 °C with the same anolyte except special declaration, while fresh anolyte was feed in anodic chamber at the end of every experiment. And results are presented as the mean values from the replicate analyses.

2.7 Chemical analysis and calculation

The concentration of lead, iron and chromium were measured by flame atomic absorption spectrophotometer (AAS, Z-2300, Hitachi, Japan). The concentration of K₃[Fe(CN)₆] was analyzed by spectrophotometer (TU-1810, Purkinje General, China) with a characteristic absorption at 420 nm. Additional, K₄[Fe(CN)₆] was assumed the product reduced from K₃[Fe(CN)₆], the concentration of which could be confirmed by KMnO₄ measurement with an absorption peak at 535 nm according to the reaction eqn (3):

3Fe(CN)⁴⁻₆ + MnO⁻₄ + 4H⁺ = 3Fe(CN)³⁻₆ + MnO₂(S) + 2H₂O (1)

Then, the concentration of K₄[Fe(CN)₆] was calculated by formula as follow:

where n = 3, is the number of oxidation of K₄[Fe(CN)₆] by 1 mol KMnO₄; ΔC is the change of concentration of KMnO₄ (mg L⁻¹); V₁ is the volume of KMnO₄ solution added into reaction system (ml); V₂ is the volume of sample of catholyte (ml); M_K4[Fe(CN)6] = 368, is molecular weight of K₄[Fe(CN)₆]; M_KMnO4 = 158, is molecular weight of KMnO₄.

2.8 SEM measurement

The morphology of precipitates on the electrode was examined with SEM (S4800, Hitachi, Japan). Two adjacent sites (one was covered with the attachment, the other not) were chosen for energy dispersive X-ray spectroscopy (EDS, IE250, Oxford, Britain) under the same condition (acceleration voltage of 20 KV with spacing of 15 mm).

2.9 XRD measurement

The crystal structure of precipitates on the electrode was examined with XRD at an acceleration voltage of 5.0 kV. XRD patterns were recorded with Bragg-Brentano geometry in a XPERT PRO DX2500 diffractometer. Data collection was carried out at room temperature using monochromatic Cu KR1 radiation in the 2θ region between 10° and 80°, step size 0.03°.

3 Results and discussion

3.1 Electrocatalytic activity of the anode

After the BES run stably, cyclic voltammetry was performed to detect the electrocatalytic activity of the abiotic electrode (carbon felt without culture, (a)), bioanode (b) and treated bioanode (the bioanode was boiled for 5 min, (c)) with synthetic organic wastewater as electrolyte. When the abiotic electrode (i.e. carbon felt without culture) was used as working electrode, no oxidation–reduction peak was observed (Fig. 2a), showing no electrochemical activity. However, two strong oxidation peaks were respectively appeared at −0.30 V and −0.35 V when bioanode attached with mixture culture served as the working electrode (Fig. 2b). It was suggested that biofilm cells on the anode could transfer electrons from substrate to electrode. But CV of the sterilized bioanode (boiled for 5 min) had no peaks, the shape of which was very similar to that of the abiotic electrode (Fig. 2c). These results demonstrated that an established biofilm which were responsible for the direct electron transfer from electron donor source to electrode had formed on the anode in the BES.

Fig. 2 Cyclic voltammograms of the anode. (a) anode without a biofilm, (b) the anode with a biofilm and (c) the anode with a sterilized biofilm.

3.2 BES voltage output

Fig. 3 indicated different electron acceptor impacted on the terminal voltage of external resistance (18.20 ± 3.99 mV for no electron acceptor, 29.74 ± 1.71 mV for oxygen and 69.87 ± 6.06 mV for K₃[Fe(CN)₆], respectively) in this BES. Furthermore, the output lowered with the decrease of K₃[Fe(CN)₆] concentration. In control case the voltage was maintained at 5.75 ± 4.64 mV with or without electron acceptors. Although ionic connection wasn't developed between SS cylinder inner and outer in our research, the result that electron acceptor influenced on output was similar to conventional BES.²⁴ That hinted there must be some relationship between inner and outer of the SS, but an appropriate principle wasn't found to explain the phenomenon. However, the BES output was fairly low in relation to traditional BES, so in this research waste treatment was more important destination than current generation.

Fig. 3 Voltage continuous output in the BES. Different electron acceptors (oxygen and 150 mg L⁻¹ K₃[Fe(CN)₆) were applied to detect the effect on voltage and for observing background voltage testing was conducted under no electron acceptor.

3.3 Pb²⁺ effective removal in the BES with SS cathode

Fig. 4 showed the concentration of Pb²⁺ decreased gradually from 39.21 mg L⁻¹ to 1.29 mg L⁻¹ with the removal rate of 0.53 mg L⁻¹ h⁻¹ and the removal efficiency of 97% for 72 h in the experimental BES. In control case, there was almost no change of Pb²⁺ concentration from 38.57 mg L⁻¹ to 37.33 mg L⁻¹. In addition, tiny metal-brilliant crystals appeared on the smooth surface of SS cathode after the test. However, there was very little deposit on the SS surface of the control group (Fig. A1†). Furthermore, the SEM pictures demonstrated that polygonal solid evenly covered the surface of SS cathode in the BES (Fig. 5A), whereas the same or analogous solid wasn't found in the control case (Fig. 5B). In order to explore the composition of solid sediment on SS, EDS was carried out at two adjacent sites (Fig. 5C) and the results indicated that the element of lead was appeared in attachment on the electrode after BES run 72 h in experimental group compared to control case (Fig. 5D and E).

Fig. 4 Changes of Pb²⁺ concentration. Pb²⁺ concentration during 72 h operation with the initial concentration of 40 mg L⁻¹ Pb⁻²⁺ and 4 g L⁻¹ of NaNO₃ in BES₁ (with inocula, ■) and BES₀ (without inocula, □). The slope of regression equation represents the average removal rate.

Fig. 5 SEM pictures and EDS spectrum. Scanning electron micrographs of the SS patch adhered on the outer of SS cylinder in BES₁ (A) and BES₀ (B) after 72 h operation. Two adjacent sites with (spectrum 1) and without (spectrum 2) attachment were chose for EDS test (C). EDS spectra (D and E) show composition of the entire particle.

3.4 Hydrocerussite as the final product for Pb²⁺ removal

To further confirm what the final product is, the solid sediment on SS was scraped for XRD analysis. The results showed that eight intense peaks appeared at 2θ = 11.3°, 19.8°, 24.6°, 27.1°, 34.2°, 40.4°, 54.0° and 71.5°, corresponding well with the most intense peaks of hydrocerussite (Pb₃(CO₃)₂(OH)₂ (PDF no. 013-0131)) (Fig. 6). Additionally, Chemical tests had verified when molar mass of Na₂CO₃ was insufficient in relation to Pb(NO₃)₂, the product was cerussite (Fig. A2A†). On the contrary, the product was hydrocerussite when Pb(NO₃)₂ was lacking (Fig. A2B†). According to composition of hydrocerussite, hydroxyl was indispensable in the formation of hydrocerussite.²⁵ However, it was noted that lead ion was abundant in solution and acidic condition was maintained in this study, which could eliminate the possibility of the direct formation of hydrocerussite. Therefore, it was interesting that how the process happened from lead ions to hydrocerussite.

Fig. 6 XRD powder pattern. XRD powder pattern for particulate matter on SS cathode in BES₁ and the standard XRD pattern for hydrocerussite (Pb₃(CO₃)₂(OH)₂ (PDF no. 13-0131)). Peak labels: d = 7.84 at 11.3°, d = 4.47 at 19.8°, d = 3.61 at 24.6°, d = 3.27 at 27.1°, d = 2.63 at 34.2°, d = 2.23 at 40.4°, d = 1.70 at 54.0° and d = 2.31 at 71.5°.

3.5 Metal lead as intermediate for Pb²⁺ removal

Based on the experimental facts of precipitation dried in air in this research, we hypothesized that hydrocerussite was spontaneously formed from metal lead with oxygen, water and carbon dioxide²⁶ according to the chemical equations as follow:

6Pb + 3O₂ + 2H₂O + 4CO₂ = 2Pb₃(CO3)₂(OH)₂ (3)

If the above hypothesis was true, metal lead would be the important intermediate generated from lead ion reduced by electron in the novel BES with SS cathode. Unfortunately, metal lead was too difficult to determine because of its active chemical property in air, so that an indirect test of using ferricyanide instead of Pb²⁺ as electron acceptor was applied in the same reactor described above to identify the Pb²⁺ reduction was possible in this reactor.

The experimental phenomena apparently showed the color of the catholyte contained K₃[Fe(CN)₆] changed from yellow to colorless after five days operation of BES. Furthermore, Fig. 7A indicated the concentration of K₃[Fe(CN)₆] in solution gradually decreased from 151.67 mg L⁻¹ to 10.67 mg L⁻¹ for five days, and the concentration of K₄[Fe(CN)₆] was up to 141.14 mg L⁻¹ at the end. However, the concentration of K₃[Fe(CN)₆] was from 157.00 mg L⁻¹ to 125.83 mg L⁻¹ in the control reactor without inocula. In thermodynamics, it's possible that Fe(CN)₆³⁻ react with the content, included iron, manganese, nickel and chromium, consisted of SS to form Fe(CN)₆⁴⁻. However, Fe³⁺, Mn²⁺, Fe²⁺ and Ni²⁺ respectively precipitate with Fe(CN)₆³⁻ and Fe(CN)₆⁴⁻ (Fig. A3†), to test iron and chromium concentration could deduce the reaction motioned above happened or not. The result iron kept 32.77 ± 0.54 mg L⁻¹ and no chromium detected (Fig. 7B) meant the transformation of Fe(CN)₆³⁻ to Fe(CN)₆⁴⁻ wasn't caused by Fe(CN)₆³⁻ reaction with SS. So the reactor used in this research could result in the reduction of Fe(CN)₆³⁻ to Fe(CN)₆⁴⁻. By analogy, Pb²⁺ was reduced to metal lead in this device possibly. Especially, the cathode potential reached the potential for Pb²⁺ reduction (Fig. A4†). But the deduction about Pb²⁺ reduction in this reactor will need to be further verified in future. Additionally, what material resulted in reduction reaction will be studied necessarily.

Fig. 7 Changes of ion concentration in catholyte. (A) Changes of the concentration of potassium ferricyanide (■) and potassium ferrocyanide (▼) in BES₁ and concentration of potassium ferricyanide in BES₀ (□) as a function of duration under 150 mg L⁻¹ K₃[Fe(CN)₆] and 4 g L⁻¹ NaNO₃ at about 25 °C for five days. (B) Changes of the concentration of iron and chromium in the catholyte of BES₁.

4 Conclusions

In this research, the results indicated Pb²⁺ could be effectively removed and hydrocerussite as the final products were deposited on the stainless steel cathode in this BES. Furthermore, metal Lead as intermediate was critical for Pb²⁺ removal. Subsequently, the metal lead was oxidized with carbon dioxide, oxygen and water into hydrocerussite. The general electrochemical reaction can be expressed as follow:

CH₃COO⁻ + 2H₂O + 4Pb²⁺ = 2CO₂ + 7H⁺ + 4Pb (4)

And the general reaction in this work can be described as follow:

3CH₃COO⁻ + 10H₂O + 12Pb²⁺ + 6O₂ + 2CO₂ = 21H⁺ + 4Pb₃(CO₃)₂(OH)₂ (5)

In brief, we suggested a novel low-cost approach to remove and recover Pb²⁺ from Pb²⁺-containing wastewater.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51074149).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06398h

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