Calcium oxide addition and ultrasonic pretreatment-assisted hydrothermal carbonization of granatum for adsorption of lead


 Hydrochar is a promising adsorbent for the removal of heavy metals, but the low surface area limits the removal efficiency and practical application. Therefore, improving the surface area of the hydrochar is critical to increasing the adsorbent removal. In this study, the ultrasonic pretreatment of biomass (10‒30 min) and CaO addition (5‒15%) were adopted to assist the hydrothermal carbonization (HTC) of granatum at 220°C. The properties of the modified hydrochar and the removal efficiency for Pb2+ in the aqueous solution were investigated. Results showed that the porosity of hydrochar was obviously improved by the CaO addition, and the largest surface area of 21.86 m2·g−1 was obtained during HTC with 15% CaO addition. Meanwhile, the functional groups of ‒OH and C═O increased and the pH of the hydrochar increased from weakly acidic to alkaline by CaO addition. The Pb2+ adsorption capacity of raw hydrochar was 10.03 mg·g−1, and it was enhanced by 80.76‒171.58% after CaO addition. The ultrasonic pretreatment of granatum had little effect on the characteristics of hydrochar except to improve the surface area from 8.27 to 9.06 m2·g−1, resulting in a 1.30‒6.78% increase in the adsorption capacity.


Introduction
With the rapid pace of global industrialization and urbanization, large quantities of heavy metals were disposed into the environment, causing them to accumulate in water bodies to levels that exceed the environmental quality standards [1]. Water quality in China continues to deteriorate, and the heavy metal that accumulates along the food chain threatens public health and the ecological environment. For example, due to the multitude of sources and the possibility of serious harm to human health, especially to the nervous and reproductive systems, lead (Pb 2+ ) is ranked as a precedent-controlled contaminant. Adsorbent removal is a common method to remove heavy metals from wastewater, but most adsorbents are expensive and not very environment-friendly [2]. Biochar has gained much attention as it can remove toxins from contaminated water [3]. However, the pyrolysis of biomass results in the production of exhaust gas and tar, which may potentially result in environmental damage [4,5]. Consequently, there remains an urgent need for greener and more ecofriendly adsorbents to remove heavy metals from contaminated water.
Hydrochar is the solid carbonaceous adsorbent produced by hydrothermal carbonization (HTC) of biomass. Hydrochar can retain higher contents of H and O than biochar, resulting in a higher density of surface functional groups [6,7]. Additionally, less exhaust gas is produced during HTC, and the wastewater can be treated by anaerobic digestion [8]. Surface functional groups of the carbon-based adsorbents play an important role in the removal of heavy metals from wastewater. Hydrochar has shown considerable promise in heavy metal removal owing to several advantages [9]. However, the low surface area of hydrochar weakens the removal effect and precludes further application of hydrochar. Some methods have been developed for hydrochar activation, such as microwave assistance and alkali modification, but these are limited by high energy consumption and complexity of operation [10][11][12].
It was reported that the porosity of the hydrochar could be considerably enhanced by the addition of CaO during HTC [13,14]. However, not much is known about the heavy metal removal capacity of hydrochar modified by CaO and the effect of CaO addition on the functional groups of hydrochar. In addition, the porous structure of the biomass may be improved by ultrasonic pretreatment [15], a combination of ultrasonic and H 2 O 2 synergistic treatment is adopted for hydrochar activation [16], and the application of ultrasonic pretreatment-assisted HTC is also performed for hydrochar modification in this study.
The objectives of this study are as follows: (1) to assess the effects of HTC assisted by CaO addition and ultrasonic pretreatment of biomass on the characteristics of hydrochar; (2) to explore the Pb 2+ removal efficiency of such hydrochar; and (3) to investigate the optimal CaO additive content for hydrochar modification.

Materials and reagents
Dry granatum (pomegranate husk) was used as the material for hydrochar preparation, which was obtained from the Pomegranate Research Center at Zaozhuang University. The granatum was ground using a blender (CPEL-23, China) until the powder passed through a 100-mesh sieve. Analytical-grade chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China). The Pb 2+ standard solution (1 mg·mL −1 , in 1% HNO 3 ) was purchased from Guobiao Testing and Certification Co., Ltd (Beijing, China).

Hydrochar preparation and modification
Stainless-steel cylindrical reactors (MMJ-200, OM Labtech, Japan) of 100 mL capacity were used to perform the HTC experiment. Five grams of granatum and 40 mL of distilled water were loaded into each reactor. Ultrasonic pretreatment of the granatum and CaO addition during HTC were used to prepare the modified hydrochar, respectively. An ultrasonic cleaner (KQ-500E, Kunshan Ultrasonic Instruments Co. LTD, China) was used to carry out the ultrasonic pretreatment of granatum, and the pretreatment durations were 10, 20, and 30 min, respectively. The weight of the added CaO was 0.25, 0.5, and 0.75 g, respectively, corresponding to the granatum content of 5%, 10%, and 15%. The HTC of untreated granatum without CaO addition was designed as the control group.
The reactors were heated to 220°C and maintained at the same temperature for 4 h using an electric heating oven (DHG-9023A, Yiheng, Shanghai, China). After being cooled to room temperature, the solid product was separated from the liquid using vacuum filtration (FY-1H-N, VALUE, Zhejiang, China) through a 0.22-μm membrane. The hydrochar was washed with distilled water and dried at 105°C for 24 h. The dried hydrochar was stored in enclosed plastic bags for further use. Hydrochar produced from the control group, ultrasonic pretreatment (10, 20, and 30 min), and CaO addition (5%, 10%, and 15%) at 220°C were denoted as H220, H220-U10 (U20 and U30), and H220-Ca5 (Ca10 and Ca15), respectively.

Adsorption experiments
The adsorption experiments were carried out on a shaking table with constant stirring of 150 rpm at 25°C for 4 h, using 50 mL conical flasks in which 0.1 g hydrochar and 25 mL of Pb 2+ adsorption solution (200 mg·L −1 , pH = 6.05) are placed. The adsorption solution was prepared with 5 mL of Pb 2+ standard solution (1 g·L −1 ) and 20 mL of ultrapure water. Each group of adsorption experiments was repeated thrice. After that, the mixtures were filtered and the equilibrium concentration of Pb 2+ in the aqueous solutions was determined using an atomic adsorption spectrometry (Z-2000, Japan). The final pH of the solution was measured after adsorption, and no pH adjustment was adopted during the adsorption. The adsorption capacity of hydrochar (Q e , mg·g −1 ) was calculated as follows: where C 0 (mg·mL −1 ) and C e (mg·mL −1 ) represent the initial solution concentration and equilibrium solution concentration, V (mL) represents the volume of adsorption solution, and M (g) represents the hydrochar content.

Characterization methods
The Brunauer-Emmett-Teller (BET) surface area and pore diameter were measured using an automatic nitrogen adsorption analyzer (JW-BK, JWGB SCI. & TECH., Beijing, China). The hydrochar morphology was analyzed using a scanning electron microscope (SEM, JSM-7800F, Japan). The pH value of hydrochar was measured using a pH meter (PHS-3D, INESA, Shanghai, China) in deionized water at a 1:10 ratio of hydrochar to water. The crystal structure of the hydrochar was examined by X-ray powder diffractometer (XRD) with XRD-6000 (Shimadzu, Japan). The functional groups of the hydrochar were determined using Fourier transform infrared (FTIR) spectroscopy (Nicolet IS50, Thermo, USA) with KBr.
3 Results and discussion

BET surface area and SEM analysis
The pore properties and pH value of the hydrochar after HTC under different conditions are shown in Table 1. The BET surface area of the hydrochar was 8.27 m 2 ·g −1 in the control group; it was enlarged to 8.43-9.06 m 2 ·g −1 after ultrasonic pretreatment and to 14.15-21.86 m 2 ·g −1 after CaO addition. The low surface area of the hydrochar was because HTC was a mild and inadequate carbonization reaction [17]. Moreover, the surface area of the hydrochar increased with the increase in CaO content. Figure 1 displays the morphological structure of the representative hydrochar from different HTC conditions. The hydrochar from the control group and ultrasonic pretreatment exhibited flake or flocculent structures with small pores, while spherical structures were developed on the surface of the modified hydrochar by CaO addition. Additionally, the pH value of the hydrochar was free from the influence of ultrasonic pretreatment while it was increased after CaO addition because of the alkalinity of CaO. It was obvious that CaO addition could improve the porous structure of the hydrochar more than the ultrasonic pretreatment during HTC, possibly because the porosity of hydrochar was related to the disintegration of the biomass and the ash content in the original biomass, and the hydrolysis of biomass was accelerated under alkaline environments during HTC; the decomposition of organic matters was facilitated by the presence of CaO during the HTC process [18,19].

XRD pattern
The XRD patterns of different hydrochar are shown in Figure 2. The hydrochar showed four weak diffraction peaks in the control group. Three of these peaks (44°, 65°, and 78°) were ascribed to the amorphous structure of aromatic carbon because of the inadequate carbonization reaction during HTC, while the peak at 29-30°C was consistent with the trace amount of elemental Ca in granatum [20,21]. The stronger peaks of aromatic carbon and weaker peaks of Ca 2+ were observed for the hydrochar from HTC with ultrasonic pretreatment, and the peak intensity of aromatic carbon was basically reinforced with the increase in the duration of ultrasonic treatment. It may be because C, H, and O were major elements while Ca was a trace element in granatum, and the element distribution in granatum became more uniform after ultrasonic pretreatment [22]. The hydrochar modified by CaO addition exhibited a crystal structure dominated by Ca 2+ , and there was no distinct difference in the peak intensity of Ca 2+ with increasing CaO content. It was suggested that the structure of CaO was much stronger than that of aromatic carbon, and the crystal structure of Ca 2+ was free from the influence of CaO dosage.

FTIR analysis
The FTIR spectra of hydrochar for different HTC conditions are shown in Figure 3. Four peaks related to the stretching vibration were found on the surface of the hydrochar because of the dehydration and aromatization during HTC, which were ascribed to the -OH (3,440 cm −1 ),  [19]. It has also been reported that the formation of ketone organics (C]O) was promoted under alkaline environments during HTC [25]. However, the intensity of the functional groups showed a non-obvious reinforcement with the CaO content increasing from 5% to 15%, which was similar to the previous results [13]. This was possible because the decomposition of biomass reached dynamic equilibrium under low alkaline conditions at a certain temperature.

Adsorption capacity of Pb 2+
Pb 2+ adsorption by hydrochar is considered to be the result of multiple mechanisms, including physical adsorption by porous structures, metalπ interaction with aromatic C]C bonds, and metal complexation with oxygencontaining functional groups [31]. In addition, the solution pH has important effects on the Pb 2+ adsorption. The adsorption of Pb 2+ was improved with the increase in pH values, since it favors the competition with H + and the complexation with functional groups [32]. However, it was also reported that a high pH (>7.0) was unfavorable for the adsorption because of the reduction of Pb 2+ mobility and the formation of Pb precipitates [33]. Hence, the optimum pH value for Pb 2+ adsorption is in the range of 4.0-7.0 [34].  Table 2, and the adsorption performance of modified hydrochar in this study was lower than the results of other studies because of the difference in the modification conditions. The FTIR spectra of the hydrochar after adsorption are shown in Figure 5. The characteristics of -OH reinforced after adsorption, proving the contribution of functional  LDHlayered double hydroxides. groups to Pb 2+ adsorption. Only the surface area of the hydrochar was slightly improved after ultrasonic pretreatment, and hence, the enhancement of the Pb 2+ adsorption capacity could be attributed to providing points for extra adsorption on the surface of the hydrochar. An overt linear relationship between the surface area and the adsorption capacity of the hydrochar is presented in Figure 6. The surface area and the adsorption capacity of hydrochar were increased by 1.93-9.55% and 1.30-6.78%, respectively, after ultrasonic pretreatment. It suggested that the adsorption capacity of Pb 2+ was increased by 0.71 mg·g −1 for each increase of 1 m 2 ·g −1 in the surface area of the hydrochar. On the other hand, although the pH value of hydrochar was not conducive to improving the capacity of Pb 2+ adsorption after the CaO addition, the increment in the surface area (71.10-164.33%) and the intensified functional groups achieved an increase of 80.76-171.58% for the Pb 2+ adsorption capacity.

Conclusion
Granatum hydrochar prepared by CaO addition and ultrasonic pretreatment-assisted HTC were characterized and investigated for the adsorption of Pb 2+ from water. The properties of the hydrochar are little affected by the ultrasonic pretreatment of biomass, while it is obviously improved by CaO addition. The surface area of the hydrochar was enlarged with the CaO content increasing from 5% to 15%, and the functional groups of the hydrochar were also reinforced. As a result of the performance enhancement, the Pb 2+ adsorption capacity of hydrochar was enhanced more than twice. The results suggested that CaO addition is a promising method for improving the properties of hydrochar, which will be beneficial for the application of hydrochar in environmental restoration.
Funding information: Authors state no funding is involved.

Conflict of interest:
Authors state there is no conflict of interest.
Data availability statement: Some or all data, models or code generated or used during the study are available from the corresponding author by request.