Effective removal of Pb²⁺ from oral medical wastewater via an activated three-dimensional framework carbon (3D AFC)

Abstract

Oral medical wastewater with heavy metal ions (such as plumbum, Pb²⁺) is regarded as the main pollutant produced in the oral cavity diagnosis, and the treatment process can pose a serious threat to human health. The removal of Pb²⁺ from oral medical wastewater facing major difficulties and challenges. Therefore, it is of great significance to take effective measures to remove Pb²⁺ by using effective methods. A new activated three-dimensional framework carbon (3D AFC), regarded as the main material to remove Pb²⁺ in the oral medical wastewater, has been fabricated successfully. In this experiment, the effects of 3D AFC absorbing Pb²⁺ under different conditions (including solid-to-liquid ratio, pH, ionic strength, contact time, and initial concentration, etc.) were discussed. And the result revealed that the adsorption kinetics process of Pb²⁺ on 3D AFC conformed to the pseudo-second-order model and the adsorption isotherm conformed to the Freundlich model. Under the condition that pH = 5.5 and T = 298 k, the calculated maximum adsorption capacity of 3D AFC for Pb²⁺ was 270.88 mg/g. In practical application, it has strong adsorption ability for Pb²⁺ in oral medical wastewater. Thus, 3D AFC shows promise for Pb²⁺ remove and recovery applications because of high adsorption capacity for Pb²⁺ in oral medical wastewater due to its high specific surface area, outstanding three-dimensional network structure.

Introduction

Oral medical wastewater containing a large amount of heavy metal ions will be generated in the process of oral diagnosis and treatment (including the removal of old prosthesis, tooth preparation, prosthesis trial wearing and the use of some metal instruments), such as cobalt ion (Co²⁺), chromium ion (Cr²⁺), molybdenum ion (Mo²⁺), silicon ion (Si²⁺), cadmium ion (Cd²⁺), and plumbum ion (Pb²⁺). If the oral medical wastewater containing these heavy metal ions was discharged into the groundwater system through the water system of the dental chair without any special treatment, it will not only increase the difficulty of water purification, but also threat to human health, cause many diseases, such as anemia, kidney dysfunction, brain tissue damage and even death (Shilu et al. 2000). Pb²⁺, one of the main heavy metal ion pollutants in oral medical wastewater, has the concentration as high as 4–20 mg/g (Bhattacharjee et al. 2003), far higher than the limit of Pb²⁺concentration in the water released by the World Health Organization (0.01 mg/L) (Rotimi et al. 2011). Therefore, it is of great significance to take effective measures to remove Pb²⁺ in oral medical wastewater.

Currently, various methods can be adopted to remove Pb²⁺ in oral medical wastewater, including ion exchange (Pember et al. 2016), liquid–liquid extraction (Soniya et al. 2015), membrane filtration (Li et al. 2016), biosorption (Oguntimein et al. 2015), electrodialysis (Deghles et al. 2016), electrocoagulation (Ghanbari et al. 2015), and adsorption technology (Zhu et al. 2018; Ghasemi et al. 2018). However, most of them have some disadvantages, such as high operating cost, low processing efficiency, and the possibility of secondary pollution like toxic products (Zhu et al. 2018), thus limiting their applications in practice. Adsorption technology, however, has become one of the most popular and promising method due to the properties of low cost, high adsorption efficiency, and convenient operation (Zhu et al. 2018). For instance, porous hydroxyapatite-carbon composite material (Zhu et al. 2018) and a new Sawdust/MNP/PEI nanocomposite material (Ghasemi et al. 2018) have been successfully used for the removal of Pb²⁺ in aqueous solution.

Activated carbon, with large specific surface area (250–3000 m²/g), high stability, especially no degradation risk in aqueous solution, has been acknowledged as one of the most popular and widely used adsorbent in the treatment of polluted wastewater in the world. There have been many reports discussing the application of activated carbon adsorbents in the removal of Pb²⁺, but the adsorption capacity varies. For example, the activated carbon before functionalization has the limited adsorption capacity for Pb²⁺, being 20.3 mg/g only (Asuquo et al. 2017). The MOS₂@Kaolin type activated carbon adsorption material (Yuan et al. 2020) greatly improved the adsorption capacity of Pb²⁺. A new type of protonated graphite carbon nitride and acid activated montmorillonite (g-C3N4/Mt) composite material was fabricated by Wan et al. (Wan et al. 2019), and the adsorption capacity of Pb²⁺ was 124.2 mg/g. A dual-ecological pistachio wood-derived activated carbon prepared by a two-stage process (PWAC-2) had an adsorption capacity of 190.2 mg/g for Pb²⁺ (Sajjadi et al. 2019). In a word, though the activated carbon adsorption materials are fabricated by different methods, they all possess good adsorption capacity for Pb²⁺. However, the removal of Pb²⁺ from oral medical wastewater has not been studied. And these activated carbon adsorption materials reported have more or less disadvantages, such as demanding preparation conditions, complicated fabricating processes, and existing difficulty in mass production, hence limiting the applications in practice. In our previous experiments, a new method for the large-scale preparation of three-dimensional framework carbon (3D FC) by directly calcining the sodium citrate without any additional carbon source, template or catalyst was reported in clean energy field, and this method could making the whole process simple and inexpensive (Yang et al. 2018). But, the 3D FC prepared by this method do not contain oxygen-containing functional groups, and it is great significance to evaluate whether the activated 3D FC (3D AFC) can be used for the removal of Pb²⁺ from oral medical wastewater.

So, in this study, for the first time to explore the 3D AFC to the removal of Pb²⁺ in oral medical wastewater and discuss the influencing factors like ionic strength, pH value, contact time and solid–liquid ratio on the adsorption of Pb²⁺ on the 3D AFC surface.

Materials and methods

Material

Sulfuric acid (98%) and nitric acid (70%) were purchased from Sinopharm Chemical Reagent Network. Pb(NO₃)₂ was purchased from Aladdin. And the pH meter model was pHS-3C (Shanghai, China). The Ultraviolet and visible spectrophotometry (UV–VIS) adsorption spectrometer model was UV-1800, MAPADA (Shanghai, China).

The fabrication and activation treatment of 3D FC

The fabrication of 3D FC was referred to the methods previously reported by our research group (Yang et al. 2018). The specific experimental steps were as follows: The sodium citrate was calcinated at 1000 °C for 1 h (The heating rate was 5 °C/min) under the protection of nitrogen. And the black product obtained from the above steps was cleaned alternately with deionized water and ethanol at 80 °C until the residual sodium, salt and impurities were removed. Finally, it was dried at 100 °C, hence obtaining the 3D FC. The activation steps of 3D FC were as follows: 2.0 g 3D FC sample was weighed and mixed in sulfuric acid (98%) and nitric acid (70%) with a volume ratio of 1:3, and then it was poured into the reactor. The sample was kept at 100 °C for 2 h. After cooling to room temperature, it was fully cleaned with a large amount of deionized water until pH value reached to 7. When the drying process was completed, the activated 3D FC was obtained, marking as 3D AFC.

Batch adsorption experiment

The 1 g/L Pb²⁺ standard solution was fabricated using Pb(NO₃)₂ as the raw material, and all the batch adsorption experiments were completed in polyethylene centrifuge tubes. HCl or NaOH solutions with different concentrations (0.001, 0.01, 0.1, 1.0, and 2.0 mol/L) were selected to adjust the pH value of the solution. 0.0020 g 3D AFC (m/V = 0.4 g/L) was added into the 10 mL polyethylene centrifuge tube and pH value was adjusted to 5.5, ensuring that the total volume of each sample was 5 mL by adding deionized water. Meanwhile, the sample was placed in a thermostatic oscillator for 12 h and filtered. Next, the filtered supernatant was taken and the concentration of Pb²⁺ ion was determined by flamless atomic absorption spectrophotometry (FAAS). Finally, the effects of solid–liquid ratio, contact time, pH value, ionic strength and initial concentration on the adsorption behavior of Pb²⁺ on 3D AFC surface were evaluated. The adsorption percentage and adsorption capacity of Pb²⁺ were calculated according to formulas (1) and (2).

$${\text{Adsorption}}(\% ) = \frac{{C_{0} - C_{{\text{e}}} }}{C0} \times 100\%$$

(1)

$$qe = \frac{{C_{0} - C_{{\text{e}}} }}{m} \times V \times 238$$

(2)

In the equations, C₀ and C_e represent the initial and equilibrium concentrations (mg/L) of Pb²⁺ in aqueous phase, respectively. m and V designate the weight (g) of the 3D AFC and the solution volume (L), respectively.

Experimental for Pb²⁺ selectivity in oral medical wastewater

The oral medical wastewater was extracted randomly from the Hospital of Stomatology, Lanzhou University, check and measure the composition and concentration of the main polluted metal ions, that is, Cd²⁺ 0.00235 mg/L, Co²⁺ 2.678 mg/L, Cr²⁺ 0.00268 mg/L, Pb²⁺ 0.00835 mg/L, Sr²⁺ 2.6515 mg/L. And then, a total volume of 5 mL mixed solution was adopted, and the pH value was adjusted to 5.5 at room temperature. 3D AFC material sample of 0.002 g was placed in the sample of the oral medical wastewater, fully oscillated, and fully adsorbed for 48 h. The sample solution was taken to analyze the remained concentrations of each ions.

Characterization

The morphology structure and chemical composition of 3D AFC were characterized by scanning electron microscopy (SEM, Hitachi s4800), X-ray diffraction (XRD, panalytical x'pert pro, Cuk α), and Fourier-transform infrared spectroscopy (FTIR, spectrum 100 Perkin Elmer, USA) before and after Pb²⁺adsorption. The concentration of Pb²⁺ in solution before and after the adsorption was measured by UV–Vis adsorption spectrometer (uv-1800, mapada, Shanghai, China).

Results and discussions

Characterization of the 3D AFC before and after Pb²⁺ adsoprtion

The morphology of 3D AFC was characterized by SEM. As shown in Fig. 1, the SEM results of the fabricated 3D AFC show that the ultra-thin and curved carbon sheets are interconnected to form a porous three-dimensional network structure, and the pore structure size is 50–80 nm. In addition, good three-dimensional pore network structure is conducive to improve the adsorption performance (Cao et al. 2020).

Fig. 1

The SEM results of 3D AFC for low magnification (a) and high magnification (b)

The chemical composition of 3D AFC before and after the adsorption of Pb²⁺ was characterized by XRD and FTIR (Fig. 2), respectively. In the XRD results (Fig. 2a), a large peak appears at 2θ = 26.3° before the adsorption of Pb²⁺, corresponding to amorphous carbon (Prabhakar et al. 2020; Zhang et al. 2015). After the adsorption of Pb²⁺, the peaks at 2θ = 23.2°, 36.1°, 39.5°, 43.2°, 47.5° and 48.6° correspond to the standard diffraction card (jcpds6-0452) (Tartaja et al. 2001), respectively, indicating that surface precipitation might be one of the mechanisms for 3D AFC to adsorb Pb²⁺ (Farzin et al. 2020; Azouaou et al. 2014). In the FTIR results (Fig. 2b), before the adsorption of Pb²⁺, a strong broad peak at 3448.53 cm⁻¹ corresponds to the stretching vibration peak of –OH (Azouaou et al. 2014; Ai et al. 2018a,

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Funding

We would like to thank the Chinese Stomatological Association Western Stomatology Clinical Research Fund Project (CSA-W2018-07) and National Natural Science Foundation of China (20190431) for their financial supports.

Author information

1. Fuxiang Song and Na Wang these authors and institutions have equally contributed to this paper.

Authors and Affiliations

1. School of Nuclear Science and Technology of Lanzhou University, Lanzhou, 730000, Gansu, China

   Fuxiang Song, Na Wang, Zhen Zhang & Liu Bin

2. School/Hospital of Stomatology of Lanzhou University, Lanzhou, 730000, Gansu, China

   Fuxiang Song, Na Wang, Zezhou Hu, **aoxue Mai, Weibo Jie & Liu Bin

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Correspondence to Fuxiang Song, Weibo Jie or Liu Bin.

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Song, F., Wang, N., Hu, Z. et al. Effective removal of Pb²⁺ from oral medical wastewater via an activated three-dimensional framework carbon (3D AFC). Appl Water Sci 11, 157 (2021). https://doi.org/10.1007/s13201-021-01486-2

• Received: 09 February 2021

• Accepted: 23 August 2021

• Published: 04 September 2021

• DOI: https://doi.org/10.1007/s13201-021-01486-2

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