Alternative stepwise adsorption process of environmental waste-based biochar for treating dental wastewater containing lead and chromium

ABSTRACT

Heavy metals from dental wastewater raised serious environmental health concerns. To address this, an environmental waste-based biochar by eggshell and fly ash was synthesized and characterized. The adsorption characteristics of lead and chromium by biochar were explored and a novel alternative stepwise adsorption process was proposed to improve the removal efficiency. Results indicated that the specific surface area was determined to be 537.6 m²/g. The maximum adsorption capacity for lead and chromium were 149.3 mg/g and 119.0 mg/g, respectively. A theoretical calculation model was developed to predict the concentration of the effluent, followed by experimental validation. The findings indicated that the concentrations of lead and chromium in the effluent remained stable, while the average removal efficiency of lead and chromium increased by 5.56% and 7.02%, respectively, compared to the conventional adsorption method. The calculated values aligned well with the experimental results, demonstrating the success of this innovative adsorption process.

KEYWORDS:

• Dental wastewater
• adsorption
• environmental waste
• lead
• chromium

Introduction

For a considerable period, metallic alloys have been widely employed in dentistry [1]. Nevertheless, the disposal of dental heavy metal wastewater, including lead, chromium, copper, and nickel compounds, during procedures such as cleaning dental instruments, disinfection, and the drainage and suction of water during dental treatments, has led to the contamination of the surrounding environment [2,3]. This contamination presents a potential threat to human health, as it can infiltrate the human body through the food chain, resulting in serious health complications like liver damage, cardiovascular disease, dermatitis, and chronic asthma [4–6]. Therefore, it is crucial to treat dental wastewater before it is discharged to minimize environmental pollution and potential risks to human health [7].

To address this issue, various treatment methods have been developed to effectively remove heavy metals from dental wastewater. One commonly used method is chemical precipitation, where chemical agents are added to the wastewater to create insoluble metal hydroxides or sulfides [8]. These solid particles can then be removed from the water through sedimentation or filtration. However, this approach has drawbacks such as expensive chemicals, difficulties in managing the resulting sludge, and the possibility of producing harmful byproducts [9]. Electrolysis has emerged as a promising technique for treating heavy metals. Through a carefully designed electrochemical process, the heavy metal ions are attracted to the electrodes, where they undergo reduction or oxidation reactions, ultimately leading to their removal from the solution [10]. Nevertheless, electrolysis is associated with high energy consumption, the requirement for costly electrodes, and constraints in treating high concentrations or intricate mixtures of heavy metals [9]. Although advanced oxidation processes have shown promising results in treating dental wastewater, it involves the use of expensive equipment and chemicals, such as hydrogen peroxide and ozone, which can significantly increase the overall cost of wastewater treatment [11]. Furthermore, membrane filtration technologies like reverse osmosis and ultrafiltration have been applied to effectively remove heavy metals and other contaminants from dental wastewater. Similarly, membrane separation can be hindered by high capital costs and membrane fouling [12].

In addition to these treatment methods, adsorption is considered a simple and effective process for treating heavy metal wastewater due to its ability to avoid producing large volumes of waste sludge, membrane backwashing effluent, or high energy consumption [13,14]. This method is particularly effective in removing low concentrations of metals and can be combined with other treatment processes for enhanced efficiency. However, its effectiveness is heavily reliant on the adsorbent’s performance. Therefore, there is growing interest in developing novel adsorbents for heavy metal removal with high adsorption capacity and selectivity [15–18].

One promising area of research in the development of novel adsorbents is the use of carbonaceous materials, which have been shown to have high surface area and reactivity, making them potentially effective adsorbents for heavy metals [19]. Another approach is the use of bio-based materials, such as chitosan and cellulose, has gained attention due to their low cost, biodegradability, and abundance in nature [20]. Additionally, industrial, agricultural, or household waste can be effectively converted into environmental waste-based biochar through appropriate processing methods [21–23].

Biochar’s high adsorption capacity, sustainability, and regenerability make them a promising solution for addressing heavy metal pollution and promoting a cleaner and healthier environment [24]. For instance, Islam produced biochar using pulp mill sludge and rice straw. The study revealed that the ability of biochar to adsorb heavy metals was primarily influenced by the type of biomass. The adsorption capacity of biochar was higher for lead (II) (109.9–256.4 mg/g) compared to nickel (II) (40.2–64.1 mg/g), cadmium (II) (29.5–42.7 mg/g), and copper (II) (18.5–39.4 mg/g) [25]. Ye obtained biochar from cattle manure and cherry wood. The maximum adsorption capacities for Pb²⁺, Cd²⁺, and Ni²⁺ were 40.8 mg/g, 124.2 mg/g, and 25.1 mg/g, respectively [26]. Curry leaf powder was prepared by the pulverization of the dried leaves for the effective removal of lead and cadmium. The removal rates were 15.7 mg/g and 12.7 mg/g for Pb and Cd at 100 mg/L of initial concentration in a single contaminant system [18].

Apart from the adsorbent, the adsorption process also plays a vital role in the approach of adsorption. Conventional adsorption processes face the disadvantage of consuming a substantial amount of adsorbent, leading to a decrease in utilization efficiency [9]. To enhance the adsorption capacity of the adsorbent, studies have been carried out to explore new adsorption processes. One such example is the work of Zhao [27], who employed powdered activated carbon as the adsorbent and implemented a countercurrent adsorption process to treat reverse osmosis concentrate. This innovative approach resulted in a remarkable reduction of 42.8% in the amount of adsorbent required, in comparison to conventional adsorption processes. Hu [28] developed a multi-stage activated carbon impregnation system, which decreased specific solute consumption and operating time by 65.11% and 85.88%, respectively. Although these new adsorption processes have reduced the amount of adsorbent used, the adsorbents in the reactor lose their adsorption capacity after becoming saturated. This accumulation hinders the complete contact between the newly added adsorbents and heavy metals, thereby affecting the progress of the adsorbent reaction. Therefore, there is a need for an improvement in this novel adsorption process.

This study aims to make a breakthrough in adsorbents and adsorption processes. To achieve this, we utilize eggshells and fly ash to produce biochar, which are derived from household and industrial waste, respectively. Compared to other materials, these wastes are generated in significant amounts, and the application of this biochar in the treatment of heavy metal wastewater enables the concept of ‘treating waste with waste’ to be realized. Despite the potentially limited adsorption capacity of our prepared biochar, we have successfully developed a novel technique that maximizes its adsorption capacity which could remove lead and chromium from dental wastewater with low dosage.

Specifically, the biochar was initially prepared using the heat activation method. Subsequently, the biochar was subjected to characterization through XRD, FTIR, SEM, and N₂ adsorption and desorption measurements. Then, the adsorption isotherm and adsorption kinetics were conducted. Moreover, investigations were carried out on the adsorption effect of biochar on binary system wastewater containing lead and chromium. The adsorption mechanism and inter-component relationships were evaluated using multi-component isothermal adsorption models. Finally, an alternative stepwise adsorption process was proposed. We established a theoretical calculation model and carried out experimental verification to further improve the utilization efficiency of the biochar. This study provided a new idea for the efficient and low-cost removal of lead and chromium from dental wastewater.

Materials and methods

Materials

AR grade lead nitrate, chromium chloride, potassium carbonate, hydrochloric acid and sodium hydroxide were purchased from Guangfu Technology (Tianjin, China). Solutions containing lead or chromium of 100 mg/L were prepared and were subsequently diluted or adjusted to the desired concentration or pH. In the alternative stepwise adsorption process experiments, the wastewater containing lead of 10.37 mg/L and chromium of 9.78 mg/L was taken from the inlet of the wastewater treatment project at Tianjin Stomatological Hospital.

Preparation of environmental waste-based biochar

The process began with fresh eggshells being placed in an oven and dried at a temperature of 105°C for 2 hours. Then, the eggshells were meticulously ground until they were completely pulverized and sifted through a 200-mesh sieve. Similarly, the fly ash was sieved through the same mesh and rinsed with deionized water to remove any soluble impurities. It was then dried in a 105°C oven for 12 hours and was mixed with the eggshell powder in a mass ratio of 3:1. This mixture was then transferred to a muffle furnace and subjected to a temperature of 800°C for 4 hours, allowing for the process of carbonization. After cooling, the biochar was obtained.

Adsorption experiments

In the traditional univariate adsorption experiments, 100 mL of wastewater containing lead or chromium at various concentrations was added to several 250 mL conical flasks. A certain amount of biochar was added to the mixture, and the solution was placed in a thermostatic shaker. The mixture was then mixed at 150 rpm for a specific time at temperatures of 283 K, 298 K, and 313 K. After filtration through a 0.22 µm microfiltration membrane, the concentration of lead and chromium in the filtrate was detected.

The binary system adsorption experiment involved adding a certain amount of biochar to 100 mL of wastewater containing lead and chromium concentrations ranging from 5 mg/L to 100 mg/L. After reaching equilibrium during the adsorption reaction at a temperature of 298 K, the concentrations of lead and chromium in the filtrate were measured.

The device used for alternative stepwise adsorption was made of organic glass, with water level on the outer wall (Fig. S1). Wastewater and biochar were added through the top feeding inlet. The vent was connected to nitrogen gas to achieve pressure water discharge. The bottom of the device was equipped with a microfiltration membrane for solid-liquid separation after the adsorption cycle. The alternating stepwise adsorption experiment required three identical devices (a, b, and c) to complete the adsorption process together. The experiment consisted of a startup step and a running step and detailed procedures can be found in the supplementary information.

Analytical methods

X-ray diffraction (XRD) using DaVinci and Bruker equipment from Germany was employed to confirm the crystal structure of biochar. The surface morphology and microstructure of biochar were observed using scanning electron microscopy (SEM) with an S-4800 from Hitachi, Japan. Fourier transform infrared spectrometer (FTIR) with a Tensor 27 from Bruker, Germany was used to determine the chemical bonds. Surface Zeta potential of biochar was analyzed by Zetasizer Lab, Malvern Panalytical, UK. The specific surface area and pore structure of the sample were determined using an Automatic specific surface and porosity analyzer (BET) from Micromeritics, United States (ASAP2020M+C). The concentrations of lead and chromium were determined by an atomic absorption spectrophotometer (XplorAA) from GBC, Australia.

The adsorption effect was evaluated using removal efficiency (η, %) and adsorption capacity (q_e, mg/g), calculated as:

(1) $\eta =\frac{C_0-C_{\mathrm{e}}}{C_0}\times 100\mathrm{\%}$(1)

(2) $q_{\mathrm{e}}=\frac{(C_0-C_{\mathrm{e}})\times V}{m}$(2)

where C₀ and C_e (mg/L) represent the mass concentration of heavy metals in the wastewater before and after adsorption, respectively. V (L) is the volume of wastewater, and m is the mass of added biochar.

Results and discussion

Characteristics of biochar

Figure 1a showed that the diffraction peaks of biochar exhibit sharpness and high intensity, corresponding to peaks at 16.4°, 21.13°, 26.6°, 29.69°, 33.05°, 40.81°, 53.92°, and 60.7°. According to the International Centre for Diffraction Data (ICDD), the primary constituents of the biochar were Al₆Si₂O₁₃, SiO₂, and CaCO₃ [29]. Figure 1b illustrated a rich porous structure of biochar, which primarily consisted of pore sizes ranging from 10 nm to 30 nm. Notably, the cumulative pore volume of pores smaller than 50 nm was 0.78 (Figure 1c), indicating that the internal pores of biochar were predominantly mesopores, which was beneficial for adsorbing heavy metal ions [30]. The FT-IR spectra (Figure 3d) showed absorption peaks at wave numbers of 1079 cm⁻¹, corresponding to the stretching vibrations of the metal oxide bonds Al-O and Si-O. The peak around 554 cm⁻¹ was attributed to the stretching vibration of Al-O in [AlO₆]. The peak at 453 cm⁻¹ was due to the bending vibration of Si-O. These chemical groups were crucial for metal ion adsorption [31]. The surface potential of the biochar decreases gradually as the pH increases, reaching a zero point potential at pH 5.0 (Figure 1e). Moreover, when the pH surpassed 5.0, the adsorbent surface became negatively charged, facilitating the electrostatic attraction of cations. This property proved advantageous for the adsorption of heavy metals. The N₂ adsorption-desorption isotherm exhibited an H4 type hysteresis loop (Figure 1f), indicating that biochar was composed of crystal particles with slit holes and had significant porosity. The BET surface area and BJH adsorption average pore diameter were 537.6 m²/g and 11.6 nm, respectively, suggesting that it had a large specific surface area and more adsorption sites [32].

Figure 1. Characteristics of biochar for (a)XRD patterns, (b) SEM images, (c) pore size analysis, (d) FTIR spectra (e) Zeta potential of the surface, and (f) nitrogen adsorption/desorption isotherms.

Single-component adsorption

Optimal dosage and adsorption time

At lower biochar dosages, the removal efficiencies of lead and chromium increased proportionally with the dosage increment. At a dosage of 0.5 g/L, the removal efficiencies of lead and chromium reached 97.1% and 84.7% respectively, after which they remained relatively stable (Figure 2a). It could be observed that both lead and chromium reached adsorption equilibrium within 20 minutes, with corresponding removal efficiencies of 96.7% and 84.1%, respectively (Figure 2b). These findings indicated that the biochar utilized in this study exhibited rapid adsorption properties for lead and chromium, with significantly shorter equilibrium times compared to other reported adsorbents [33]. This type of biochar, which exhibits rapid adsorption characteristics, enhances the efficiency of wastewater treatment and is well-suited for our proposed novel adsorption process in the following section.

Figure 2. The fundamental adsorption data for biochar on lead and chromium (a) removal efficiency changed with dosage (b) removal efficiency changed with adsorption time.

Adsorption kinetics

Kinetic analysis was crucial in the design of an effective adsorption process and aided in comprehending the limiting steps and mechanisms behind adsorption [34]. Based on the experimental adsorption data of lead and chromium, the pseudo-first-order kinetic model was excluded from further discussion due to its poor R² values (0.65 and 0.77, Table S1) and the lack of agreement between the experimental and theoretical adsorption capacities of biochar (Figure 3b). In contrast, both the intraparticle diffusion and pseudo-second-order models (Table S2, Figures 3a, 3c) effectively represented the current experimental data, yielding R² values of 0.94 and 0.94 for lead, and 0.92 and 0.99 for chromium, respectively. Based on these findings, it seems that either the pseudo-second order or intraparticle diffusion models would be suitable for describing the adsorption of lead and chromium to biochar [35,36]. The results obtained from fitting the intra-particle diffusion model revealed that the adsorption process of lead and chromium by biochar comprised two distinct stages (Figure 3a). The first stage involved an exceedingly rapid trend of metal ions adhering to the biochar surface and spreading into the pores due to the abundance of available adsorption sites. However, with increased contact time, the active sites got occupied, and the lower concentration of heavy metals made mass transfer challenging, eventually leading to adsorption saturation [37]. From Figure 3c, it could be observed that the experimental data fit better with the pseudo-second order kinetics model, with a coefficient of determination (R²) close to 1.0 (Table S1). These results were consistent with the adsorption of heavy metals by biochar prepared from pulp mill sludge and straw [25].

Figure 3. Adsorption characteristics of lead and chromium by biochar (a) intra-particle diffusion model, (b) pseudo-first-order kinetic model and (c) pseudo-second-order kinetic model.

Adsorption isotherm

The classical Langmuir model and Freundlich model [43, 44] were used to fit the experimental data, and the results showed that the Langmuir model was in good agreement with the adsorption data (Figure 4). The isotherms demonstrated that biochar adsorbs lead and chromium in a monolayer fashion, facilitated by surface pores. Analysis of the Langmuir model parameters (Table S2) suggested that lowering the temperature from 313K to 283K enhanced the adsorption of lead and chromium by biochar. This was evidenced by an increase in the saturated adsorption capability for lead from 114.9 mg/g to 149.3 mg/g, and for chromium from 74.63 mg/g to 119.0 mg/g. These adsorption capacities were higher than those achieved using biochar made from canola straw [38] and biochar prepared from banana peel [39].

Figure 4. Adsorption isotherm model for (a) Langmuir, (b) Freundlich.

Binary-component adsorption

The application of a single-component adsorption constant could not describe the interactions between components in a multi-component system. Therefore, in this section, an interaction term n_i was introduced into the classical Langmuir isothermal adsorption model to obtain the M-Langmuir isothermal adsorption model, as well as the extended Langmuir isothermal adsorption model (E-Langmuir) for fitting adsorption data of binary systems [40]. The model equations were shown as:

(3) $q_{\mathrm{e},\mathrm{i}}=\frac{Q_{\mathrm{m},\mathrm{i}}{b}_{\mathrm{L},\mathrm{i}}({C_{\mathrm{e},\mathrm{i}}/n}_{\mathrm{i}})}{1+\sum _{j=1}^N{b}_{\mathrm{L},\mathrm{j}}({C_{\mathrm{e},\mathrm{j}}/n}_{\mathrm{j}})}$(3)

(4) $q_{\mathrm{e},\mathrm{i}}=\frac{Q_{\mathrm{m}\mathrm{a}\mathrm{x}}{b}_{\mathrm{i}}{C}_{\mathrm{e},\mathrm{i}}}{1+\sum _{j=1}^N{b}_{\mathrm{j}}{C}_{\mathrm{e},\mathrm{j}}}$(4)

where, n_i is the Langmuir constant of i in binary-component systems, which is affected by the concentration of other components in the mixture; q_e,i (mg/g) presents the adsorption amount of the biochar when the i component is equilibrium; Q_m,i (mg/g) is the maximum adsorption capacity of the i component derived from the single-component system; b_L,i (L/mg) is the Langmuir constant for one-component systems i; C_e,i (mg/L) is the concentration of adsorbate at the equilibrium of the i components; Q_max (mg/g) is the maximum adsorption capacity of the biochar; b_i (L/mg) is the extended Langmuir constant for the i component.

The correlation coefficient R² for the fitting of the M-Langmuir isotherm adsorption model of biochar on lead and chromium in a binary system was 0.841, and n_i was less than 1 (Table S3), indicating that the adsorption process of biochar on lead and chromium binary component wastewater did not conform to the M-Langmuir isotherm adsorption model.

The fitting of the E-Langmuir isotherm model yielded a correlation coefficient R² value of 0.982 (Table S3), indicating a strong relationship between the observed and predicted values. Moreover, the biochar’s maximum adsorption capacity was found to be 66.39 mg/g, which closely aligned with the experimental data. This further supported the conclusion that the adsorption process of lead and chromium components by the biochar adhered to the E-Langmuir isotherm model. The E-Langmuir model assumed uniform distribution of adsorption sites on the adsorbent surface, with components in the solution competing for the same sites. Additionally, each component exhibited conformity with its corresponding Langmuir isotherm model for single-component adsorption [40].

Based on the above conclusions, the E-Langmuir model could be employed to obtain the calculated values for the adsorption amounts of each component in the binary system. By comparing the adsorption experimental data of each component at different concentrations with the calculated values, the accuracy of the isotherm model could be verified, and the competitive relationship between each component could be determined [41].

In the lead-chromium binary system, the calculated values were represented by the 3D response surface generated using the E-Langmuir isotherm adsorption model. A comparison between the experimental values and the calculated values revealed that a significant portion of the experimental data aligned with the 3D response surface, demonstrating a consistent trend in adsorption changes. Specifically, it could be observed that as the concentration of coexisting chromium increases, the adsorption amount of lead by biochar decreased (Figure 5a). Similarly, the adsorption amount of chromium also decreased as the concentration of coexisting lead increased (Figure 5b). However, taking into consideration the changes in adsorption amount alongside the convexity or concavity of the surface, it could be concluded that the competitive adsorption effect of chromium on lead was more pronounced [42].

Figure 5. E-Langmuir 3D response surface and experimental values in the lead-chromium system (a) adsorption of lead, (b) adsorption of chromium.

Alternative stepwise adsorption process

Although the conventional adsorption approach exhibited good removal effect in removing lead and chromium, there was still room for improvement in the adsorption capacity of the added adsorbent. In addition, the adsorbent accumulated in the reactor lost its adsorption capacity after reaching saturation, which hindered the full contact between the new adsorbent with adsorption capacity and the adsorbent. This lack of contact was not conducive to the adsorption reaction. Consequently, to enhance the adsorption efficiency, this section aimed to refine the conventional adsorption approach and proposed an innovative alternative stepwise adsorption process.

The optimal dosage of biochar for adsorbing lead and chromium was 0.5 g/L. Therefore, based on the alternating graded adsorption process, it was determined that when the volume of wastewater (V) was 200 mL, the value of m was 0.1 g. Since the optimal adsorption time was 20 minutes, it was divided into two stages of adsorption, with each stage lasting 10 minutes, i.e. t was 10 minutes.

Considering that the adsorption of lead by biochar followed the Langmuir isotherm adsorption equation, substituting the relevant parameters into the equation yields:

(5) $q_{\mathrm{e}}=\frac{l{q}_{\mathrm{m}}{C}_{\mathrm{e}}}{1+l{C}_{\mathrm{e}}}=\frac{97.1C_{\mathrm{e}}}{1+0.68C_{\mathrm{e}}}$(5)

In the startup stage, wastewater was added to devices a and b, reaching a volume of 200 mL. Additionally, 0.1 g of biochar was added and stirred for 10 minutes. After the reaction was completed, the concentration in both devices a and b was denoted as C₁, as shown in Equation (6)(6) $\frac{C_0-C_1}{m/V}=\frac{97.1C_1}{1+0.68C_1}$(6) :

(6) $\frac{C_0-C_1}{m/V}=\frac{97.1C_1}{1+0.68C_1}$(6)

By substituting the known conditions, the value of C₁ could be calculated. The wastewater in device a was discarded, but to retain the biochar, 160 mL of wastewater was discharged and replaced with 160 mL of initial wastewater, resulting in a lead concentration of C₀’ in device a.

The wastewater in device b was then filtered through a membrane into device c, reducing the lead concentration to C₁. Simultaneously, 0.1 g of new biochar was added to device c for a first-stage adsorption reaction with the low-concentration wastewater. The biochar that had been used once in device a underwent a second-stage adsorption reaction with the high-concentration wastewater of C₀’, while the saturated biochar in device b was removed.

After 10 minutes, the wastewater in device a was filtered into b, reducing the concentration to C₁’. The wastewater in device c was filtered and separated to obtain the first cycle effluent with a concentration of C₂. At this point, the biochar in device a had accumulated two stages of adsorption, allowing for the calculation of its adsorption capacity and the value of C₂ using the following equations:

(7) $\frac{C_0-C_1}{m/V}+\frac{{C_0}^{\prime }-{C_1}^{\prime }}{m/V}=\frac{97.1{C_1}^{\prime }}{1+0.68{C_1}^{\prime }}$(7)

(8) $\frac{C_1-C_2}{m/V}=\frac{97.1C_2}{1+0.68C_2}$(8)

The operational phase followed the startup phase. In device c, 200 ml of lead-containing wastewater with a concentration of C₀ was added, resulting in an initial lead concentration of C₀’“. This wastewater underwent a second-stage adsorption reaction with the biochar that had been used once. In device b, 0.1 g of fresh biochar was added and it underwent a first-stage adsorption reaction with the lead wastewater of concentration C₁”. Simultaneously, the saturated biochar in device a was removed. After 10 minutes, the wastewater in device c was filtered and entered device a, resulting in a reduced concentration of C₁’’. The wastewater in device b was filtered and separated, yielding the second cycle effluent with a concentration of C₂’ that could be calculated using equation 14.

(9) $\frac{C_1^{\prime }-C_2^{\prime }}{m/V}=\frac{97.1C_2^{\prime }}{1+0.68C_2^{\prime }}$(9)

Then, 200 mL of lead-containing wastewater with a concentration of C₀’’“ was added to device b, and 0.1 g of biochar was added to device a. Since the biochar in device c had accumulated two stages of adsorption, its adsorption capacity was calculated as Equation 15, from which the value of C₁”’ could be obtained.

(10) $\frac{C_1-C_2}{m/V}+\frac{{C_0}^{{\prime }^{\prime }}-{C_1}^{{\prime }^{\prime }}}{m/V}=\frac{97.1{C_1}^{\prime \prime }}{1+0.68{C_1}^{\prime \prime }}$(10)

The running cycle and calculation method for each cycle’s effluent concentration remained the same as described above in the running phase.

The adsorption process of chromium in the alternating graded adsorption process was similar to that of lead. The experimental values for each cycle could be obtained through adsorption experiments. By comparing the differences between the calculated and experimental values, the adsorption efficiency of the alternating stepwise adsorption process for lead and chromium could be predicted.

From the comparison of the calculated and experimental values for 10 cycles (Figure 6), it was evident that the effluents of the alternating stepwise adsorption process remained stable, with average concentrations of 0.29 mg/L and 0.85 mg/L for lead and chromium, respectively, and average removal rates of 97.2% and 91.3%. However, the experimental values for the effluent in each cycle were higher than the theoretical values, which might be due to a small amount of biochar adhering to the stirrer or the inner wall of the device, preventing it from fully participating in the adsorption reaction.

Figure 6. Calculation and experimental values of lead and chromium adsorption using biochar alternative stepwise adsorption process.

Figure 6 also indicated that the experimental values of lead and chromium in the effluent for the initial two cycles were much higher than the theoretical values. This disparity arose from the lack of accumulated biochar in the device during the start-up stage. Additionally, it took time for the solid biochar to be evenly dispersed in the solution after being added. Consequently, the adsorbent and the adsorbate did not come into sufficient contact, thereby hindering the realization of the complete adsorption process.

Compared with conventional adsorption process, the alternating stepwise adsorption process reduced the effluent concentration of lead and chromium by 0.425 mg/L and 0.556 mg/L respectively and increased the average removal efficiency by 5.56% and 7.02% respectively. The dental wastewater underwent an alternative stepwise adsorption process to treat it, resulting in an effluent that complied with the water pollutant discharge standards set for Chinese medical institutions, with regards to the concentration of lead and chromium.

Conclusions

An environmental waste-based biochar was prepared using eggshell and fly ash blended with high temperature carbonization method. The prepared biochar showed rapid and efficient removal of lead and chromium from dental wastewater. This was made possible by the presence of mesopores within internal pores, a substantial specific surface area, an abundance of adsorption sites, and beneficial chemical groups within the biochar. Both the pseudo-second order and intraparticle diffusion models are appropriate for describing the adsorption of lead and chromium onto biochar in single component wastewater. The maximum adsorption capacity were 149.3 mg/g and 119.0 mg/g for lead and chromium respectively. In the lead-chromium binary component system, the E-Langmuir model can be used to calculate the adsorption amounts of each component. As one heavy metal’s concentration increased, the adsorption of biochar to the other heavy metal decreased. The competitive adsorption effect of chromium on lead is more prominent. An innovative alternative stepwise adsorption process was developed to maximize the capacity of biochar and enhance the efficiency of heavy metal removal. Furthermore, a precise theoretical value calculation model was incorporated to accurately predict the concentration of the effluent.

Author contributor

Tian Wang is a clinical dentist, M.D. You Zhang is a physician, B.S. Guihong Li is a clinical dentist, B.S. Huiru Zou is a clinical dentist, M.D.

Acknowledgements

The authors thank Tianjin Key Medical Discipline (Specialty) Construction Project [TJYXZDXK-078D; 2022YTZX11] for funding this study.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/26395940.2023.2288877

Additional information

Funding

The work was supported by the Tianjin Key Medical Discipline (Specialty) Construction Project [TJYXZDXK-078D; 2022YTZX11].