X-Ray Absorption Spectroscopy Analysis of Lead Species Adsorbed on Various Oxides from High pH Solution

Lead dissolved in water must be removed in order not to cause diseases, espe-cially from high pH aqueous solution. Various oxides having high specific surface area are often applied to remove lead in water media. To improve removal ability for lead species, it is necessary to understand the adsorbed structure of lead species on oxides. At first, the adsorption behavior of lead from high pH solution in the presence of Ca 2+ and Na + was compared. Lead and calcium species were adsorbed up to the monolayer, and the adsorption isotherm was analyzed as Langmuir-type adsorption. In the presence of Ca 2+ , the amount of removed lead was reduced. To clarify this influence of Ca 2+ , X-ray absorption spectroscopy was adopted. It was for the first time revealed that lead species at pH > 12 and pH < 10.5 differed, and that lead species adsorbed on various oxides had a similar structure.


Introduction
In our daily life, a large amount of wastes such as domestic garbage, used plastics, and papers, are disposed. These wastes, called municipal wastes, are incinerated, which leave ash to be withdrawn from the bottom of the incinerator and fly ash to be collected by the electric dust collector. The fly ash contains signifi-How to cite this paper: Miyake, T., Honma, T., Arimatsu, H., Fukunishi, H., Liu, H., Sano, M., Kakutani, Y., Okada, M., Shimizu, K., Yoshida, S. and Suetsugu, K. (2020) X-Ray Absorption Spectroscopy Analysis of Lead Species Adsorbed on Various Oxides from High pH Solution. Journal of Environmental Protection, 11, 807-820. cant amounts of poisonous elements such as lead and cadmium, and causes problems when buried [1] [2] [3] [4]. This is because calcium hydroxide solution is sprayed onto the fly ash to neutralize hydrogen chloride and sulfur oxides.
Then, when the fly ash is buried and rain reaches the buried ash, the pH of the eluent can reach as high as pH 12, which leads to dissolution of lead. To prevent dissolution of lead, the fly ash is solidified into cement or is chemically treated using chelate compounds [5]. The former method necessitates a special facility and is costly. In the latter method, chelate compounds are expensive and gradually degrade after months, releasing lead. Thus, a better and preferably permanent treatment for lead from the fly ash is needed.
There have been many reports to make heavy metals immobilize with SiO 2 and Al 2 O 3 [6] [7] [8] [9] [10]. Studies on adsorption using mesoporous materials have also been reported [11]- [22]. However, these methods cannot be adopted for lead removal because SiO 2 , Al 2 O 3 , and most of the mesoporous materials dissolve into high pH aqueous solution. Mesoporous manganese oxide having a large surface area has been reported [23] [24] [25]. Papers dealing with lead removal using manganese oxide are reported; however, no papers adopted pH higher than neutral to the best of our knowledge. Manganese oxides do not dissolve under high pH conditions caused by the sprayed calcium hydroxide and are not expensive.
In this paper, at first we studied lead removal from a high pH 12.4 aqueous solution using mesoporous manganese oxide as an adsorbent. Then, the lead species adsorbed under high pH of 12.4 on various oxides was analyzed especially with X-ray absorption spectroscopy.

Preparation of Mesoporous Manganese Oxide
Into 50 mL of distilled water, 10 mmol of potassium permanganate was dissolved. Then, 3.3 mmol of maleic acid in 50 mL of distilled water was added as a reducing agent and the mixture was stirred for 1 h. After aging for 24 h at room temperature, the resulting black precipitate was washed with distilled water. After drying at 70˚C overnight, the mesoporous manganese oxide (Meso-Mn) was obtained by calcination in air at 300˚C for 2 h.

Removal of Lead
Typically, into 100 mL of the prepared lead solution, a 0.20 g portion of Meso-Mn was added, and the mixture was stirred with a Teflon-coated stirrer bar for 1 min to 240 h at room temperature. The removal was carried out in a sealed glass Erlenmeyer flask, preventing dissolution of CO 2 into the alkaline solution.
After adsorption of lead, Meso-Mn was centrifuged and the concentrations of lead and calcium species in the supernatant were analyzed with Inductively Coupled Plasma (ICP) spectroscopy (ICPS-7510, Shimadzu Corp., Japan). When the influence of co-existing calcium was investigated, NaOH was used instead of Ca(OH) 2 to control pH.

Characterization
The powder X-ray diffraction (XRD) patterns were obtained with RMT-18kWHFVE (Rigaku, Japan) with Cu Kα-radiation at 40 kV and 20 mA. N 2 adsorption-desorption isotherms were obtained with BELSORP-mini2 (MicrotracBEL, Japan), and the pore size distribution (BJH method) and BET surface area were calculated based on the adsorption-desorption isotherms. The elemental analysis was carried out using ICP. The analysis sample was prepared by dissolving 0.050 g of a solid before or after removal of lead in 20 mL of 10 mol•L −1 HNO 3 with a few drops of 30 wt% H 2 O 2 aqueous solution. Before measurement, the solution was diluted to a desired concentration range. The zeta potential was measured by the laser Doppler method with Zetasizer Nano-Z (ZEN2600, Malvern Instruments Inc., United Kingdom). For the measurement, 0.020 g of powder sample was dispersed in distilled water. The pH of the suspension was arranged with aqueous HCl, NaOH or Ca(OH) 2 solution. X-ray photoelectron spectra (XPS) were recorded by JEOL JPS-9000MX (JEOL, JAPAN) with Mg Kα-radiation at 10 kV and 10 mA, and the peak position was calibrated with Pt 4f7/2 at 70.9 eV as an internal standard.
X-ray absorption fine structure (XAFS) and X-ray absorption near edge spectrum (XANES) were obtained at 14B2 beamline at SPring-8, Japan. Silicon (311) crystal was used as a monochromator. For the measurement of Pb L III edge of high Pb concentration samples, the transmission mode was adopted. For the low concentration samples, Pb L III edge spectra were collected by the fluorescence mode using 19-element Ge semi-conductivity detector. Radial structure function was Fourier transformed for the XAFS vibration between wave number k of 2 and 8 Å −1 . Powder samples were pelletized together with boron nitride and solution samples were packed in polyethylene bag.

Results and Discussion
The preparation and characterization of mesoporous manganese oxide (Me-  [26]. In brief, Meso-Mn was amorphous (XRD) and mesoporous (inter-particle mesopores of about 5 nm) and had a high BET surface area of approximately 250 m 2 •g −1 .

Removal of Lead at pH 12.4
The adsorption isotherm was studied in order to understand the mechanism of adsorption and the maximum amount of adsorption. When lead adsorption was carried out at pH 12.4 controlled with NaOH, a Langmuir-type adsorption phenomenon was observed ( Figure 1) with a monolayer adsorption capacity of lead of 4.01 mmol•g −1 . At pH 12.4, the concentration of co-existing Na + was varied by adding NaNO 3 , and the influence of the concentration of Na + is indicated in    and SiO 2 removed lead most. As SiO 2 also adsorbed a large amount of Ca 2+ , Meso-Mn was the most selective for the lead removal. In addition, the dissolution of SiO 2 and Al 2 O 3 themselves was confirmed for SiO 2 and Al 2 O 3 . Thus, Meso-Mn was the best choice for the removal of lead at a high pH of 12.
In summary for the adsorption of lead from the high pH solution, manganese oxide was the most promising adsorbent. The Langmuir-type adsorption was observed for both lead and calcium. Lead and calcium competed for the same adsorption site. It was suggested that lead and calcium formed a specific complex on Meso-Mn.

Lead Species in the Solution
The same XANES spectra, and therefore the same radial structure function, were observed for the lead species in solution between pH 4 and 10.5 ( Figure 6).
However, different XANES and XAFS spectra were observed at pH 12.4 ( Figure   7). Up to pH 10.5, it is suggested from the coefficient of complex formation [27] T. Miyake et al.   that lead exists as Pb 2+ , Pb(OH) + and Pb(OH) 2 (Electronic supplementary information Figure S1). On the other hand, Pb OH − is suggested at pH 12.4.
Thus, it was revealed that when lead existed as cationic or neutral species, the structure of the first coordination sphere was the same. This is the first XANES information reported for lead in alkaline media. For the XANES spectra at pH 12.4, different spectra were obtained when Ca(OH) 2 and NaOH were used as the pH controller ( Figure 8). This may be because Ca(OH) + in the Ca(OH) 2 solution (Electronic supplementary information Figure S2

Lead Species Adsorbed on Oxides
When the concentration of lead was 3000 ppm in the presence of Ca(OH) 2  Thus, the behavior of lead in the highly alkaline conditions was analyzed for the first time and this information is very important to develop excellent materials to remove lead in water.

Adsorbed Species
First, the species of lead and calcium in the solution at different pH were estimated by the coefficient of complex formation [27] (Electronic supplementary   information Figure S1, Figure S2). As mentioned in 3. Pb OH − , and a positive surface bearing Ca(OH) + . This is in accordance with the results by XANES in Figure 8.
To directly investigate the adsorbed lead species, Meso-Mn was dried at 80˚C overnight after the adsorption of lead, and analyzed with XRD. No distinct diffraction pattern related to lead species could be observed, even though the amount of lead was sufficiently high to be detected with XRD (data not shown).
This may be because the lead species existed in the form of a monolayer. Even if lead existed as Pb(OH) 2 , the stability of Pb(OH) 2 has been questioned [28], and observing the diffraction pattern of Pb(OH) 2 may be difficult.
The valence state of lead was studied with XPS of Pb 4f 7/2 ( Figure 11) [29]. As far as we know, there are no reports by EXAFS on the adsorbed lead species under high pH conditions.
As discussed above, we do not have any decisive idea for the adsorbed lead species at this moment. However, considering the equimolar adsorption of lead and calcium, the existence of Pb-O bond and the valence of lead to be 2+, we Figure 11. XPS spectra for Pb 4f adsorbed on Meso-Mn. pH control: Ca(OH) 2 for Ca-Pb and NaOH for Na-Pb.

Conclusions
Lead adsorption on oxides from a high pH aqueous solution was studied and manganese oxide was the most promising adsorbent. It was suggested that lead and calcium species competed for the same adsorption site on manganese oxide and the amount of removed lead reached 1.8 mmol•g −1 under the co-existence of calcium hydroxide. The structure of lead species in the solution was different at pH 12.4 and at pH lower than 10.5. The adsorbed lead species was not made clear; however, a double hydroxide composed of lead and calcium like CaPb(OH) 4 was estimated. This information is very important to develop materials to remove lead in high pH water from the buried waste.
Based on the knowledge obtained, more promising material would be prepared by a monolayer loading of manganese oxide on a support of higher surface area than about 300 m 2 •g −1 .