Lead removal from solution by a porous ceramisite made from bentonite, metallic iron, and activated carbon

Abstract

There is increasing interest in using a porous ceramisite as the filter medium for removing heavy metals from wastewater in China given its low cost, easy preparation, large available quantities, high mechanical strength, and good performance to remove various pollutants. In this study, bentonite, metallic iron (Fe⁰), and activated carbon (AC) were used to prepare a Fe⁰/AC-ceramisite (diameter 2–4 mm) by sintering at 800 °C in a N₂ environment. The Fe⁰/AC-ceramisite achieved a high Pb²⁺ removal efficiency (>99%) at ceramisite dosages of 5–10 g L⁻¹, an initial solution pH of 3.0–5.7, and initial Pb²⁺ concentrations of 50–200 mg L⁻¹. Compared with three control ceramisites including B-ceramisite (made by bentonite only), AC-ceramisite (made by bentonite and AC), and Fe⁰-ceramisite (made by bentonite and Fe⁰), the enhanced Pb²⁺ removal by the Fe⁰/AC-ceramisite was mainly attributed to the iron–carbon micro-electrolysis induced by the addition of AC. A first-order kinetics model fit well for the Pb²⁺ removal by the Fe⁰/AC-ceramisite, and the Pb²⁺ uptake rate increased linearly with the increasing initial solution pH. Results from scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) indicated that the Pb²⁺ removal by the Fe⁰/AC-ceramisite involved multiple mechanisms including sorption, reduction (from Pb²⁺ to Pb⁰), and precipitation (in the forms of PbCO₃ and Pb₃(CO₃)₂(OH)₂).

---

Water impact

Porous ceramisites are considered as one of the most promising candidates for large-scale wastewater treatment in China because of their low cost, large available quantities, easy preparation, and high capability to remove various pollutants. However, little is known about their effectiveness and underlying mechanisms on the removal of heavy metals. In this study, a porous ceramisite (Fe⁰/AC-ceramisite) made from bentonite, metallic iron, and activated carbon was prepared in a N₂ environment, and it exhibited very good performance on aqueous Pb²⁺ removal under various conditions. SEM/EDX, XRD, and XPS analyses reveal that sorption, reduction, and precipitation were responsible for removing aqueous Pb²⁺ by the Fe⁰/AC-ceramisite. This study provides new insights for understanding the effectiveness and mechanisms of the porous ceramisite for aqueous Pb²⁺ removal and will contribute greatly to solving this significant environmental problem.

1. Introduction

Lead (Pb) contamination is a global environmental problem due to its ubiquitous presence in the environment and its damaging toxicity to human beings and ecosystems.^1–4 Increasing discharge of Pb-containing wastewater from industries such as storage batteries, automotive, lead smelting, tetraethyl-lead manufacturing, and mining is posing a great threat to water resources and public health.^5–8 In fact, Pb is listed second among 275 human toxins in the priority list of hazardous substances.⁹ According to the World Health Organization (WHO), Pb exposure is annually estimated to account for 143 000 deaths in developing regions and 600 000 new cases of children with intellectual disabilities.¹⁰ In order to protect human health, the U.S. Environmental Protection Agency (EPA) has established an action level of 15 μg L⁻¹ for Pb in public drinking water since 1991, which further highlights the need to develop cost-effective Pb removal technologies for industrial wastewater.

Various technologies such as adsorption,¹¹ ion exchange,¹² reverse osmosis,¹³ electrochemical treatment,¹⁴ oxidation/reduction methods,¹⁵ and chemical precipitation¹⁶ have been developed to remove heavy metals from water. However, the practical use of these techniques is often limited by high reagent and energy requirements, incomplete metal removal, expensive equipment, and/or the generation of hazardous waste that requires further treatment.^17,18 Take adsorption for example, although using activated carbon (AC), biosorbents (i.e. bacteria and algae), and clay minerals are very effective for heavy metal removal,^19–23 the large-scale application of these sorbents is often compromised by the lack of durability and mechanical strength for long-term metal control and difficulties in separation and recovery from the treated effluent.²⁴ Porous ceramisite offers a promising alternative to the existing methods because of its low cost, easy preparation, large available quantities, high mechanical strength, and good performance for simultaneously removing various pollutants, which enable it to provide a long-term and large-scale control for various pollutants.²⁵ Moreover, the bigger particle size of the porous ceramisite, ranging from millimeters to centimeters,^26,27 offers the advantages of easier separation from treated water and a reduced pressure drop in flow-through systems over fine particles like AC, biosorbents, and clay minerals.^28,29 In fact, porous ceramisite has been commercially available and commonly practiced as the filter medium in wastewater treatment systems, such as constructed wetlands (CWs) and biological aerated filters (BAFs), in China.^30–36 However, previous studies have been focused on its effectiveness for the removal of nitrogen, phosphorus, and organic matters.^37–41 Up to now, research on porous ceramisite being capable of removing heavy metals from water is scarce, and the underlying removal mechanism also remains largely unknown.

In our previous work, more than 97% of aqueous Pb²⁺ was removed using an Fe₂O₃-ceramisite prepared by bentonite, metallic iron powder (Fe⁰), and AC at 800 °C.²⁹ However, this ceramisite was prepared under air conditions, leading to the oxidation of Fe⁰ into ferric oxide (Fe₂O₃) and AC into carbon dioxide (CO₂). Thus, the oxidized Fe⁰ and AC played no contribution to the Pb²⁺ removal. In order to keep the reactivity of Fe⁰ and AC for the Pb²⁺ removal, in this study, a new ceramisite (named as “Fe⁰/AC-ceramisite”) was prepared by the same raw materials at 800 °C under N₂ conditions rather than under air conditions to prevent the oxidation of Fe⁰ and AC. Previous studies have demonstrated the effectiveness of Fe⁰ and AC in the removal of various contaminants,^19,20,42,43 and the couple of Fe⁰ and AC is believed to exhibit an enhanced performance owing to the iron–carbon internal electrolysis reaction.^44–46 For example, the Fe⁰/AC couple was capable of removing 100% of As(V) from aqueous solution, whereas only about 60% and 5% of As(V) could be removed by Fe⁰ alone and AC alone, respectively, under the same conditions.⁴⁴ Owing to the presence of Fe⁰ and AC, the removal performance of Pb²⁺ by Fe⁰/AC-ceramisite was expected to be enhanced compared with the Pb²⁺ removal by the Fe₂O₃-ceramisite. To verify this hypothesis, a series of controlled experiments were conducted under varying conditions, including different ceramisite dosages, initial solution pHs, and initial Pb²⁺ concentrations. The combined effect of the AC and Fe⁰ on the Pb²⁺ removal was studied by comparing the Pb²⁺ removal efficiencies of Fe⁰/AC-ceramisite and three control ceramisites, B-ceramisite (made by bentonite only), AC-ceramisite (made by bentonite and AC), and Fe⁰-ceramisite (made by bentonite and Fe⁰). The Fe⁰/AC-ceramisite was characterized using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM/EDX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) to reveal the removal pathways of Pb²⁺ by the Fe⁰/AC-ceramisite.

2. Materials and methods

2.1 Materials

Detailed information of the raw materials including bentonite, Fe⁰, and AC was described in our previous work.²⁹ Lead nitrate (Pb(NO₃)₂), (Beijing Chemical Plant, China) was used to prepare the Pb²⁺ stock solution (1000 mg L⁻¹) with nanopure water (Millipore Milli-Q). Pb²⁺ working solutions (50, 100, and 200 mg L⁻¹) were obtained by diluting Pb²⁺ stock solution with nanopure water. The initial pH of the Pb²⁺ working solution (3.0, 3.8, 4.7, and 5.7) was adjusted by adding 0.1 M HNO₃ and/or NaOH. All the chemicals used in this study were analytical grade.

2.2 Ceramisite preparation

The production process of the Fe⁰/AC-ceramisite included mixing the raw materials, pelletizing, drying, sintering, and cooling. 20% w/w of Fe⁰, 6% w/w of AC, and 74% w/w of bentonite were well mixed with an appropriate amount of distilled water, and then spherical pellets were manually made, with diameters ranging from 2 to 4 mm. These pellets were then oven dried at 105 ± 5 °C for 3 h, and sintered in an electric tube furnace. The tube furnace temperature was initiated at 25 °C, ramped at 10 °C min⁻¹ to 400 °C and held for 20 min, then ramped at 5 °C min⁻¹ to 800 °C and held for 1 h. Afterwards, the tube furnace gradually cooled down to room temperature within 6 h, and the obtained Fe⁰/AC-ceramisite was kept dry in sealed storage bags prior to use. Three control ceramisites including AC-ceramisite (made by 6% w/w of AC and 94% w/w of bentonite), Fe⁰-ceramisite (made by 20% w/w of Fe⁰ and 80% w/w of bentonite), and B-ceramisite (made by 100% w/w of bentonite) were prepared following the same procedure described above. A N₂ environment was supplied during the whole preparation process to prevent oxidation of the Fe⁰ and AC.

2.3 Pb²⁺ uptake experiments

Batch experiments were conducted on a thermostatic shaker at 130 ± 10 rpm (20 ± 0.5 °C) to investigate the Pb²⁺ removal performance by Fe⁰/AC-ceramisite. The solution pH was recorded at the end of the experiments. To investigate the effect of the ceramisite dosage on the Pb²⁺ removal, 0.15, 0.25, 0.35, 0.50, 0.60, and 0.70 g of Fe⁰/AC-ceramisite were added into six conical flasks each containing 50 mL of 200 mg L⁻¹ Pb²⁺ solution with an initial solution pH of 4.7. After 20 h, the supernatants were sampled and filtered through a 0.45 μm membrane filter (Millipore), and the residual Pb²⁺ concentrations were determined using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Teledyne Leeman labs prodigy ICP).

To investigate the effect of the initial Pb²⁺ concentration on the Pb²⁺ removal kinetics, 2.0 and 4.0 g of Fe⁰/AC-ceramisite were added into six conical flasks each containing 400 mL of 50, 100, and 200 mg L⁻¹ Pb²⁺ solutions with an initial solution pH of 4.7. To investigate the effect of the initial solution pH on the Pb²⁺ removal kinetics, 4.0 g of Fe⁰/AC-ceramisite was added into four conical flasks each containing 400 mL of 100 mg L⁻¹ Pb²⁺ solutions with initial solution pHs of 3.0, 3.8, 4.7, and 5.7. At certain reaction time intervals, the supernatants were sampled and filtered for residual Pb²⁺ concentration measurement. The soluble Fe concentrations in the supernatants were also measured using the ICP-AES during the removal process of Pb²⁺ by Fe⁰/AC-ceramisite.

To investigate whether AC and Fe⁰ had a combined effect on the Pb²⁺ removal, 0.50 g of B-ceramisite, AC-ceramisite, Fe⁰-ceramisite, and Fe⁰/AC-ceramisite were added into four conical flasks each containing 50 mL of 100 mg L⁻¹ Pb²⁺ solution with an initial solution pH of 4.7. After 20 h, the supernatants were sampled and filtered for residual Pb²⁺ concentration measurement.

2.4 Characterization methods

The compressive strength of the Fe⁰/AC-ceramisite was determined according to the previously described procedure.³⁹ The specific surface areas of Fe⁰/AC-ceramisite, Fe⁰, and AC were analyzed by using a BET surface area analyzer (Micromeritics ASAP 2010, U.S.A.); element compositions and micrographs of the surface of the Fe⁰/AC-ceramisite before and after removing the Pb²⁺ were characterized using a Quanta 200FEG SEM/EDX; crystalline phases of the Fe⁰/AC-ceramisite before and after removing the Pb²⁺ were determined using a Rigaku Dmax-2400 XRD analyzer; the detailed information about these three measurements was provided in our previous work.²⁹ The surface composition and the valences of the elements of Fe⁰/AC-ceramisite before and after removing Pb²⁺ were investigated using an Axis Ultra XPS with monochromatized Al Kα radiation. Binding energies were calibrated using the C 1s hydrocarbon peak at 284.80 eV to compensate for surface charge effects. Prior to the SEM/EDX, XRD and XPS analyses, the ceramisite samples after the Pb²⁺ removal were taken out from the liquid and dried at room temperature in a N₂ atmosphere. For the XRD and XPS analyses, the dried ceramisite samples were further ground into powder (<200 mesh).

2.5 Data analysis

The removal efficiency R (%) and Pb²⁺ uptake capacity q (mg g⁻¹) of the tested ceramisites were calculated by eqn (1) and (2), respectively. The first-order kinetic model expressed by eqn (3) was used to determine the Pb²⁺ uptake rate k (h⁻¹).where C₀, C_t, and C_e (mg L⁻¹) are Pb²⁺ concentrations at the beginning, time t, and equilibrium, respectively. V (L) is the solution volume and m (g) is the dry weight of the tested ceramisites. The uptake rate constant k (h⁻¹) is the slope of the plot of −ln(C_t/C₀) versus time.

3. Results and discussion

3.1 Characterization of the Fe⁰/AC-ceramisite

Among clays, bentonite is considered as a main candidate for the removal of heavy metals and organic contaminants due to its good adsorption capacity, low cost, and abundant availability.^47–49 In particular, the composite of bentonite and Fe⁰/iron oxides exhibited very good removal performance for heavy metals.^48,50–52 However, the application of these composite materials in flow-through systems is not feasible or even possible due to the high pressure drops.⁵³ Besides, after wastewater purification, the composites would become a viscous sludge with a high content of water, which is very difficult for the dewatering process.⁵⁴ Follow-up treatments have to be considered before disposal of these slurries with high concentrations of heavy metals, thus resulting in increased cost for heavy metals treatment.²⁹ To overcome these issues, in this study, bentonite was used as a supporting material to prepare a porous ceramisite with large diameters of 2–4 mm, which could be easily separated and recovered from treated water. The synthesized Fe⁰/AC-ceramisite also exhibited a good mechanical strength (6.9 ± 0.3 MPa), meeting China’s standard for filter materials for wastewater treatment (GB 2838-81, compressive strength ≥ 6.5 MPa). Besides, the Fe⁰/AC-ceramisite had a specific surface area of 106.6 m² g⁻¹, which was higher than the raw materials, including AC (81.4 m² g⁻¹) and Fe⁰ (0.763 m² g⁻¹). SEM/EDX (Fig. 5a and d) and XRD (Fig. 6) analyses revealed the presence of AC and Fe⁰ in the Fe⁰/AC-ceramisite, demonstrating that the N₂ environment was effective for protecting the AC and Fe⁰ from oxidation during the preparation process. Montmorillonite (at 2θ of 19.7°, 29.6°, 34.7°, and 61.8°)^55,56 and quartz (at 2θ of 20.9°, 26.4°, and 50.2°)^56,57 exhibited small/insignificant peaks, indicating they had poor crystallinity. This could be explained as the calcination at 800 °C breaking the crystal structure of bentonite, making it become amorphous in XRD.⁵⁸ A similar result was observed by Y. Turhan et al.⁵⁹ who reported that the peaks of the calcinated bentonite were completely disappeared at 650 °C.

3.2 Pb²⁺ removal performance of the Fe⁰/AC-ceramisite

3.2.1 Effect of the ceramisite dosage on the Pb²⁺ removal. The effect of the ceramisite dosage on the removal of Pb²⁺ by the Fe⁰/AC-ceramisite is shown in Fig. 1. With the increase of ceramisite dosage from 3.0 to 14.0 g L⁻¹, the removal efficiency for Pb²⁺ increased from 67.8% to 99.9%, while the Pb²⁺ uptake capacity decreased from 45.2 to 14.3 mg g⁻¹. The higher removal efficiency obtained at elevated ceramisite dosage was because the increasing ceramisite provided a larger amount of active sites for Pb²⁺ ions, which coincided with previous studies.⁶⁰ In order to balance the high removal efficiency and the good uptake capacity of Pb²⁺, a ceramisite dosage of 10 g L⁻¹ was chosen for further studies, unless otherwise stated, since the Pb²⁺ removal efficiency was greater than 99.5% when the ceramisite dosage was higher than 10 g L⁻¹.

Fig. 1 Effect of the ceramisite dosage (3–14 g L⁻¹) on the removal of Pb²⁺ by the Fe⁰/AC-ceramisite.

3.2.2 Effect of initial Pb²⁺ concentration on removal kinetics. As shown in Fig. 2a and b, the Fe⁰/AC-ceramisite was very effective for Pb²⁺ removal with the residual Pb²⁺ concentration being less than 0.6 mg L⁻¹ and the Pb²⁺ removal efficiency being higher than 99% at initial Pb²⁺ concentrations of 50–200 mg L⁻¹ and ceramisite dosages of 5 and 10 g L⁻¹. Our Pb²⁺ data fit the first-order kinetic model very well with regression coefficient (R²) values higher than 0.9782 under all conditions (Fig. 2c and d). A higher Pb²⁺ uptake rate constant (k) was obtained at a lower initial Pb²⁺ concentration and higher ceramisite dosage. For example, at an initial Pb²⁺ concentration of 50 mg L⁻¹ and ceramisite dosage of 10 g L⁻¹, the Pb²⁺ uptake rate constant (k) was 1.349 h⁻¹, while at an initial Pb²⁺ concentration of 200 mg L⁻¹ and ceramisite dosage of 5 g L⁻¹, remarkably k decreased to 0.0699 h⁻¹. This result indicated that a prolonged reaction time was needed, especially at higher initial Pb²⁺ concentrations and lower ceramisite dosages in order to achieve a good Pb²⁺ removal efficiency.

Fig. 2 Effect of the initial Pb²⁺ concentration (50–200 mg L⁻¹) on the removal kinetics of Pb²⁺ by the Fe⁰/AC-ceramisite at ceramisite dosages of 5 g L⁻¹ (a) and 10 g L⁻¹ (b); panels (c) and (d) show the Pb²⁺ data fitting using the a first-order kinetic model at a ceramisite dosage of 5 g L⁻¹ (R² ranging from 0.9880 to 0.9943) and 10 g L⁻¹ (R² ranging from 0.9782 to 0.9969), respectively; and (e) equilibrium solution pH at different initial Pb²⁺ concentrations.

At the initial Pb²⁺ concentrations of 50, 100, and 200 mg L⁻¹, an elevated solution pH was also observed (Fig. 2e), which increased from 4.7 to 8.2, 7.9, and 6.0 at a ceramisite dosage of 5 g L⁻¹ and to 9.9, 9.7, and 8.7 at a ceramisite dosage of 10 g L⁻¹, respectively. The increased solution pH could be attributed to the reactions between Fe⁰, water, and dissolved O₂ (eqn (4) and (5)),⁶¹ in which H⁺ was consumed by Fe⁰ in the Fe⁰/AC-ceramisite:

2Fe⁰ + 4H⁺ + O₂ → 2Fe²⁺ + 2H₂O (4)

Soluble Fe was also detected during the Pb²⁺ removal process (shown in Fig. S1 and S2†), further demonstrating that reactions (4) and (5) were involved in the Pb²⁺ removal. The increased solution pH would lead to a more negatively charged ceramisite surface, enhancing the removal of the positively charged Pb²⁺ ions through electrostatic attraction.^62,63 Besides, the primary forms of Pb in solution would convert from free Pb²⁺ ions at pH < 7 to Pb(OH)⁺ and Pb(OH)₂ at pH 7–10,⁶⁴ indicating that precipitation would also contribute to Pb²⁺ removal (confirmed by XRD analysis in Fig. 6).

3.2.3 Effect of the initial solution pH on the Pb²⁺ removal kinetics. As shown in Fig. 3a, the Pb²⁺ removal efficiency by the Fe⁰/AC-ceramisite remained over 99.5% despite the initial solution pH increasing from 3.0 to 5.7, indicating that the Fe⁰/AC-ceramisite was very effective for Pb²⁺ removal in this pH range. The Pb²⁺ uptake rate constant (k) decreased linearly from 1.760 h⁻¹ (R² = 0.9681) at pH 5.7 to 0.323 h⁻¹ (R² = 0.9879) at pH 3.0 (Fig. 3b and c), suggesting that the lower initial solution pH reduced the Pb²⁺ removal rate. This result agreed well with other reports, indicating that the removal rate of some pollutants was pH-sensitive⁶⁰ and thus an optimal solution pH was needed for achieving the best removal performance. Similar to Fig. 2e, an elevated solution pH was also observed at the end of this experiment, and it increased to 9.1, 9.6, 9.7, and 10.2 for initial solution pHs of 3.0, 3.8, 4.7, and 5.7, respectively. Hence, precipitation was expected to be involved in the removal of Pb²⁺ by the Fe⁰/AC-ceramisite at an initial solution pH of 3.0–5.7. These results further demonstrated the effectiveness of the Fe⁰/AC-ceramisite in treating Pb²⁺-containing solutions at different initial solution pHs.

Fig. 3 Effect of the initial solution pH (3.0–5.7) on the removal kinetics of Pb²⁺ by the Fe⁰/AC-ceramisite (a); panel (b) shows the Pb²⁺ data fitting using a first-order kinetic model (R² ranging from 0.9681 to 0.9879); (c) linear regression between the Pb²⁺ uptake rate constant (k) and the initial solution pH; and (d) equilibrium solution pH at different initial solution pHs.

3.2.4 The combined effect of AC and Fe⁰ on Pb²⁺ removal. As shown in Fig. 4, the Fe⁰/AC-ceramisite achieved the highest Pb²⁺ removal efficiency (99.5%) compared with the B-ceramisite (4.8%), AC-ceramisite (7.8%), and Fe⁰-ceramisite (39.6%). Assuming that there were no combined effects between AC and Fe⁰ on the Pb²⁺ removal, the individual Pb²⁺ uptake capacities of Fe⁰, AC, and bentonite would be 17.9, 5.5, and 0.5 mg g⁻¹ based on the their mass percentages and the Pb²⁺ removal efficiencies of these three control ceramisites. Thus, the Pb²⁺ uptake capacity of the Fe⁰/AC-ceramisite would theoretically be 4.3 mg g⁻¹, which was much lower than its obtained Pb²⁺ uptake capacity (10.0 mg g⁻¹). This discrepancy indicated that the combined effect between Fe⁰ and AC was likely involved in the enhanced Pb²⁺ removal by the Fe⁰/AC-ceramisite. According to previous studies, the combined effect was attributed to the iron–carbon micro-electrolysis reaction, in which Fe(II), hydroxyl, and atomic hydrogen were released, enhancing the reactivity of Fe⁰ for the removal of heavy metals and organic contaminants.^44–46

Fig. 4 Comparison of Pb²⁺ removal efficiencies of the Fe⁰/AC-ceramisite, B-ceramisite, AC-ceramisite, and Fe⁰-ceramisite.

3.3 Pb²⁺ removal mechanisms

3.3.1 SEM/EDX and XRD analyses. To investigate the removal mechanisms of Pb²⁺, the SEM/EDX and XRD techniques were applied to characterize the Fe⁰/AC-ceramisite before and after the removal of Pb²⁺. As shown in Fig. 5b and c, two new mineral phases containing Pb were developed on the surface of the Fe⁰/AC-ceramisite after Pb²⁺ removal compared with the original ceramisite (Fig. 5a). The corresponding EDX spectra (Fig. 5f and g) revealed that the Pb mass percentages in the prismatic and cuboid crystals were 79.1% and 73.9%, which were close to the mass percentages of Pb in Pb₃(CO₃)₂(OH)₂ (80.1%) and PbCO₃ (77.5%), respectively. The slight discrepancy could be due to the emergence of some other elements (such as Si, Al, and Fe) during the formation of these Pb-containing crystals, therefore reducing the Pb concentration after the reaction. XRD analysis (Fig. 6) further confirmed the presence of PbCO₃ and Pb₃(CO₃)₂(OH)₂, suggesting that Pb²⁺ could be removed from water by precipitation of PbCO₃ and Pb₃(CO₃)₂(OH)₂. This result coincided well with previous studies^56,65 and the carbonates were previously reported to originate from atmospheric CO₂ ^56,66,67 and/or impurities such as calcite⁶⁵ present in the bentonite. Besides, Pb²⁺ may also be removed from solution via sorption by AC, as shown in Fig. 5d; although no Pb-containing crystals were observed, 10.6% Pb was present on the AC by EDX analysis. Compared to the XRD pattern of the Fe⁰/AC-ceramisite before Pb²⁺ removal, Fig. 6 shows the presence of Pb⁰, which is located at 55.39° and 62.39°,⁴² suggesting that the Fe⁰/AC-ceramisite could also remove Pb²⁺ from solution by the reduction of the Pb²⁺ to Pb⁰. This was evidenced by the standard reduction potential of Pb²⁺/Pb⁰ (−0.13 V) being higher than that of Fe²⁺/Fe⁰ (−0.41 V),⁵¹ leading to the occurrence of the electrochemical reaction between Pb²⁺ and Fe⁰. In the meantime, Fe⁰ was oxidized into iron oxides such as γ-FeOOH and Fe₃O₄ after removing Pb²⁺ (Fig. 6).

Fig. 5 SEM/EDX results of the Fe⁰/AC-ceramisite before (a and e) and after (b and f, c and g, d and h) Pb²⁺ removal. For the EDX spectra, only the top four major elements were listed.

Fig. 6 XRD patterns of the Fe⁰/AC-ceramisite before and after Pb²⁺ removal.

3.3.2 XPS analysis. The XPS technique was employed to identify the valence states of Fe and Pb in the Fe⁰/AC-ceramisite. As shown in Fig. 7a, the elements O, Fe, Mg, C, Si, and Al were observed in the Fe⁰/AC-ceramisite samples both before and after Pb²⁺ removal, whereas the element Pb could be only detected in the Fe⁰/AC-ceramisite sample after Pb²⁺ removal. From the obtained high-resolution Pb 4f spectrum (Fig. 7b), two peaks centred at 141.5 eV (Pb 4f_5/2) and 136.8 eV (Pb 4f_7/2) were observed. According to ref. 42 and 68, these peaks could be assigned to the Pb oxidized state Pb²⁺, which was in good agreement with our SEM/EDX and XRD analyses that PbCO₃ and Pb₃(CO₃)₂(OH)₂ were formed on the Fe⁰/AC-ceramisite after Pb²⁺ removal. The high-resolution Fe 2p spectra (Fig. 7c) of the Fe⁰/AC-ceramisite after the Pb²⁺ removal showed two peaks centred at 726.2 eV (Fe 2p_1/2) and 712.5 eV (Fe 2p_3/2), which were in close agreement with Fe²⁺/Fe³⁺ at 725.2 eV and Fe³⁺ at 710.7 eV, respectively.⁶⁹ This observation implied that the Fe⁰ in the Fe⁰/AC-ceramisite after the Pb²⁺ removal was enveloped by a layer of iron oxides such as γ-FeOOH and Fe₃O₄ (confirmed in Fig. 6). Although both Pb⁰ and Fe⁰ were observed by XRD analysis, neither of them appeared in the XPS patterns, which may result from the 10 nm detection depth of XPS.

Fig. 7 XPS patterns of the Fe⁰/AC-ceramisite before and after Pb²⁺ removal: (a) wide-scan survey; (b) high-resolution XPS spectrum of Pb; and (c) high-resolution XPS spectra of Fe.

Based on the results described above, the Pb²⁺ removal mechanism by the Fe⁰/AC-ceramisite involves three components: one is that Pb²⁺ ions in aqueous solution are absorbed by the AC in the Fe⁰/AC-ceramisite since the AC can absorb the metal ions;⁷⁰ one is that the Pb²⁺ ions can precipitate in the forms of PbCO₃ and Pb₃(CO₃)₂(OH)₂ on the surface of the Fe⁰/AC-ceramisite; another is that some of the absorbed Pb²⁺ can be reduced by Fe⁰ to Pb⁰. Through these three pathways, the Fe⁰/AC-ceramisite could effectively remove Pb²⁺ ions from the aqueous solution and thus it could be considered as a promising filter medium for the existing wastewater treatment systems in China.

4. Conclusions

In this study, a novel porous ceramisite, Fe⁰/AC-ceramisite, was prepared with bentonite, Fe⁰, and AC by sintering at 800 °C in a N₂ environment. Batch experiments have demonstrated its effectiveness in removing Pb²⁺ ions from water under various environmental conditions. The Pb²⁺ removal efficiency was enhanced by the addition of AC in the Fe⁰/AC-ceramisite due to the iron–carbon micro-electrolysis reaction, and the Pb²⁺ uptake rate was influenced by initial Pb²⁺ concentration, initial solution pH, and ceramisite dosage. The results obtained from SEM/EDX, XPS, and XRD indicated that the Fe⁰/AC-ceramisite was capable of removing Pb²⁺ ions via reduction, sorption, and precipitation. The synthesized Fe⁰/AC-ceramisite offers a great potential for the Pb²⁺ removal from wastewater.

Acknowledgements

We acknowledge the two anonymous reviewers for their thorough, insightful, and constructive comments that have significantly improved the paper. This work was supported by the National Natural Science Foundation of China (no. 21077002 and 41201532). Li Yuan acknowledges support from the China Scholarship Council (CSC) visiting scholarship.

References

1. G. Flora, D. Gupta and A. Tiwari, Interdiscip. Toxicol., 2012, 5(2), 47–58.
2. M. I. Khan, N. Islam, A. A. Sahasrabuddhe, A. A. Mahdi, H. Siddiqui, M. Ashquin and I. Ahmad, J. Hazard. Mater., 2011, 189(1), 255–264.
3. D. A. Gidlow, Lead toxicity, Occup. Med., 2004, 54(2), 76–81.
4. L. Yuan, W. Zhi, Y. Liu, S. Karyala, P. J. Vikesland, X. Chen and H. Zhang, Environ. Sci. Technol., 2015, 49(2), 824–830.
5. T. Bahadir, G. Bakan, L. Altas and H. Buyukgungor, Enzyme Microb. Technol., 2007, 41(1), 98–102.
6. R. Shanmugavalli, P. Shabudeen, R. Venckatesh, K. Kadirvelu, S. Madhavakrishnan and S. Pattabhi, Uptake of Pb(II) ion from aqueous solution using silk cotton hull carbon: an agricultural waste biomass, J. Chem., 2006, 3(4), 218–229.
7. M. M. Matlock, B. S. Howerton and D. A. Atwood, Ind. Eng. Chem. Res., 2002, 41(6), 1579–1582.
8. J. R. Peralta-Videa, M. L. Lopez, M. Narayan, G. Saupe and J. Gardea-Torresdey, Int. J. Biochem. Cell Biol., 2009, 41(8), 1665–1677.
9. ATSDR (Agency for Toxic Substances and Disease Registry), Priority List of Hazardous Substances, http://www.atsdr.cdc.gov/SPL/, Accessed 7 May, 2014.
10. WHO (World Health Organization), Lead poisoning and health, http://www.who.int/mediacentre/factsheets/fs379/en/, Accessed October, 2014.
11. M. Kobya, E. Demirbas, E. Senturk and M. Ince, Bioresour. Technol., 2005, 96(13), 1518–1521.
12. A. Da̧browski, Z. Hubicki, P. Podkościelny and E. Robens, Chemosphere, 2004, 56(2), 91–106.
13. H. Ozaki, K. Sharma and W. Saktaywin, Desalination, 2002, 144(1), 287–294.
14. M. Hunsom, K. Pruksathorn, S. Damronglerd, H. Vergnes and P. Duverneuil, Water Res., 2005, 39(4), 610–616.
15. S. Chen, W. Chen and C. Shih, Water Sci. Technol., 2008, 58(10), 1947–1954.
16. M. M. Matlock, B. S. Howerton and D. A. Atwood, Water Res., 2002, 36(19), 4757–4764.
17. O. Chaalal, A. Y. Zekri and R. Islam, Energy Sources, 2005, 27(1–2), 87–100.
18. E. Malkoc, Y. Nuhoglu and M. Dundar, J. Hazard. Mater., 2006, 138(1), 142–151.
19. M. Corapcioglu and C. Huang, Water Res., 1987, 21(9), 1031–1044.
20. K. Pyrzyńska and M. Bystrzejewski, Colloids Surf., A, 2010, 362(1), 102–109.
21. M. Pazirandeh, B. M. Wells and R. L. Ryan, Appl. Environ. Microbiol., 1998, 64(10), 4068–4072.
22. T. A. Davis, B. Volesky and A. Mucci, Water Res., 2003, 37(18), 4311–4330.
23. E. Alvarez-Ayuso and A. Garcia-Sanchez, Clays Clay Miner., 2003, 51(5), 475–480.
24. G. M. Gadd, J. Chem. Technol. Biotechnol., 2009, 84(1), 13–28.
25. G. Xu, J. Zou and G. Li, Water Res., 2009, 43(11), 2885–2893.
26. S. Z. Yuan, H. Lu, J. Wang, J. T. Zhou, Y. Wang and G. F. Liu, Process Biochem., 2012, 47(2), 312–318.
27. C. Zhang, K. Wang, S. Tan, X. Niu and P. Su, Chem. Ecol., 2013, 29(8), 668–675.
28. B. Pan, Q. Zhang, W. Zhang, B. Pan, W. Du, L. Lv, Q. Zhang, Z. Xu and Q. Zhang, J. Colloid Interface Sci., 2007, 310(1), 99–105.
29. L. Yuan and Y. Liu, Chem. Eng. J., 2013, 215, 432–439.
30. W. Zhi, L. Yuan, G. Ji and C. He, Environ. Sci. Technol., 2015, 49(7), 4575–4583.
31. W. Zhi and G. Ji, Water Res., 2014, 64, 32–41.
32. W. Zhi and G. Ji, Sci. Total Environ., 2012, 441, 19–27.
33. G. Ji, R. Wang, W. Zhi, X. Liu, Y. Kong and Y. Tan, Distribution patterns of denitrification functional genes and microbial floras in multimedia constructed wetlands, Ecol. Eng., 2012, 44, 179–188.
34. W. Weiliang, Procedia Environ. Sci., 2012, 12, 79–86.
35. S. Cao, W. Chen and Z. Jing, Afr. J. Biotechnol., 2014, 11(16), 3825–3831.
36. M. Wu, Q. Li, X. Tang, Z. Huang, L. Lin and M. Scholz, Int. J. Environ. Anal. Chem., 2014, 94(6), 618–638.
37. M. Hartman, O. Trnka and M. Pohorelý, Ind. Eng. Chem. Res., 2007, 46(22), 7260–7266.
38. Y. Liu, F. Du, L. Yuan, H. Zeng and S. Kong, J. Hazard. Mater., 2010, 178(1), 999–1006.
39. J. L. Zou, G. R. Xu, K. Pan, W. Zhou, Y. Dai, X. Wang, D. Zhang, Y. Hu and M. Ma, Sep. Purif. Technol., 2012, 94, 9–15.
40. G. Ji, Y. Zhou and J. Tong, Environ. Eng. Sci., 2010, 27(10), 871–878.
41. M. Ji, W. Liu, J. Zhou and L. Sun, A Study of Pre-Treatment of Slightly-Polluted Water with Biological Ceramsite Filter Tank, Gongye Yongshui Yu Feishui, 2003, 34(6), 22–25.
42. X. Zhang, S. Lin, Z. Chen, M. Megharaj and R. Naidu, Water Res., 2011, 45(11), 3481–3488.
43. S. M. Ponder, J. G. Darab and T. E. Mallouk, Environ. Sci. Technol., 2000, 34(12), 2564–2569.
44. X. Dou, R. Li, B. Zhao and W. Liang, J. Hazard. Mater., 2010, 182(1), 108–114.
45. L. Fan, J. Ni, Y. Wu and Y. Zhang, J. Hazard. Mater., 2009, 162(2), 1204–1210.
46. B. Liang, Q. Yao, H. Cheng, S. Gao, F. Kong, D. Cui, Y. Guo, N. Ren and A. Wang, Environ. Sci. Pollut. Res., 2012, 19(5), 1385–1391.
47. M. Jiang, X. Jin, X. Lu and Z. Chen, Desalination, 2010, 252(1), 33–39.
48. M. Ranđelović, M. Purenović, A. Zarubica, J. Purenović, B. Matović and M. Momčilović, J. Hazard. Mater., 2012, 199, 367–374.
49. D. Tomašević, G. Kozma, D. V. Kerkez, B. Dalmacija, M. Dalmacija, M. Bečelić-Tomin, Á. Kukovecz, Z. Kónya and S. Rončević, J. Nanopart. Res., 2014, 16(8), 1–15.
50. L. Shi, X. Zhang and Z. Chen, Water Res., 2011, 45(2), 886–892.
51. Y. Zhang, Y. Li, J. Li, G. Sheng, Y. Zhang and X. Zheng, Chem. Eng. J., 2012, 185, 243–249.
52. Y. Li, J. Li and Y. Zhang, J. Hazard. Mater., 2012, 227, 211–218.
53. L. Z. Pillon, Interfacial properties of petroleum products, CRC Press, 2007, p. 205.
54. H. Wright and J. Kitchener, J. Colloid Interface Sci., 1976, 56(1), 57–63.
55. E. Eren and B. Afsin, J. Hazard. Mater., 2008, 151(2), 682–691.
56. H. Zhang, Z. Tong, T. Wei and Y. Tang, Appl. Clay Sci., 2012, 65, 21–23.
57. S. Hashemian, H. Saffari and S. Ragabion, Water, Air, Soil Pollut., 2015, 226(1), 1–10.
58. B. Ritherdon, C. R. Hughes, C. D. Curtis, F. R. Livens, J. F. Mosselmans, S. Richardson and A. Braithwaite, Appl. Geochem., 2003, 18(8), 1121–1135.
59. Y. Turhan, Z. G. Alp, M. Alkan and M. Doğan, Microporous Mesoporous Mater., 2013, 174, 144–153.
60. M. Hou, C. Ma, W. Zhang, X. Tang, Y. Fan and H. Wan, J. Hazard. Mater., 2011, 186(2), 1118–1123.
61. R. Crane and T. Scott, J. Hazard. Mater., 2012, 211, 112–125.
62. H. Elliott and C. Huang, Water Res., 1981, 15(7), 849–855.
63. O. Abollino, M. Aceto, M. Malandrino, C. Sarzanini and E. Mentasti, Water Res., 2003, 37(7), 1619–1627.
64. S. Yang, D. Zhao, H. Zhang, S. Lu, L. Chen and X. Yu, J. Hazard. Mater., 2010, 183(1), 632–640.
65. A. Nakano, L. Li, M. Ohtsubo, A. Mishra and T. Higashi, Environ. Technol., 2008, 29(5), 505–514.
66. F. Bouchelaghem and N. Jozja, Eng. Geol., 2009, 108(3), 286–294.
67. R. Herrera-Urbina and D. Fuerstenau, Colloids Surf., A, 1995, 98(1), 25–33.
68. C. D. Wagner and G. Muilenberg, Handbook of x-ray photoelectron spectroscopy: a reference book of standard data for use in x-ray photoelectron spectroscopy, Perkin-Elmer Corp., 1979.
69. H. Peng, Z. Mo, S. Liao, H. Liang, L. Yang, F. Luo, H. Song, Y. Zhong and B. Zhang, Sci. Rep., 2013, 3, 1765.
70. A. Netzer and D. Hughes, Water Res., 1984, 18(8), 927–933.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ew00091b

---