Biochar derived from watermelon rinds as regenerable adsorbent for efficient removal of thallium(I) from wastewater

Process Safety and Environmental Protection 127 (2019) 257–266

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Biochar derived from watermelon rinds as regenerable adsorbent for efficient removal of thallium(I) from wastewater Huosheng Li a , Jingfang Xiong b , Tangfu Xiao b , Jianyou Long b , Qimin Wang b , Keke Li b , Ximing Liu b , Gaosheng Zhang a , Hongguo Zhang b,c,∗ a Institute of Environmental Research at Greater Bay, Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University, Guangzhou 510006, China b School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China c Guangzhou University–Linköping University Research Center on Urban Sustainable Development, Guangzhou University, 510006 Guangzhou, China

a r t i c l e

i n f o

Article history: Received 26 January 2019 Received in revised form 28 April 2019 Accepted 30 April 2019 Available online 4 May 2019 Keywords: Thallium Biochar Heavy metals Watermelon rinds Adsorption Ion exchange

a b s t r a c t Discarded watermelon rinds were used to prepare porous biochars, which act as regenerable adsorbents for treating both synthetic and real Tl-containing wastewater. The primary biochar prepared under pyrolysis temperature of 500 ◦ C was found to be the most effective for Tl(I) removal. The primary biochar had the best Tl(I) removal efficacy over a wide pH range (4–12), followed by the KOH-modified and HClmodified biochar. Maximum Tl(I) adsorption capacity reached 178.4 mg/g, which is superior to that of other biochar. Strong resistance to the interference from co-existing cations and organics on Tl(I) removal was also observed during treatment of complex synthetic and real industrial wastewater. Characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, high resolution transmission electron microscope (HRTEM), and X-ray photoelectron spectroscopy (XPS) reveal that the K- and Cl-rich primary biochar acts as regenerable amphibious ion exchange resins to perform reversible adsorption and desorption of Tl(I). The biochar derived from watermelon rinds exhibits effective Tl(I) removal performance and strong regenerability, and is a promising adsorbent for Tl(I) removal with excellent application prospects. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction In recent years, the problem of waterbodies being polluted by heavy metals has become increasingly prominent (Chen et al., 2018; Sharma et al., 2017a,b). In particular, public concerns over the water pollution by thallium (Tl) are increasing (Chen et al., 2017a). Tl is a highly toxic heavy metal, with a toxicity greater than mercury (Hg), cadmium (Cd), lead (Pb), copper (Cu), and zinc (Zn) (Zhang et al., 2009). Tl accumulation in the human body poisons the central nervous system and leads to cardiovascular diseases and fatal cases of Tl poisoning (Li et al., 2012). Tl is easily enriched within flora and fauna, threatening the safety of human beings and the ecosystem (Vanˇek et al., 2015). With the rapid development of modern industries, Tl is being widely used in the manufacture of high–tech products, such as optical devices, high–temperature superconductors, and semiconductors (Xiao et al., 2012). Although incidents involving Tl pollution

∗ Corresponding author at: School of Environmental Science and Engineering, Guangzhou University, Guangzhou 510006, China. E-mail address: [email protected] (H. Zhang).

only occur occasionally at present, sustained mining of Tl minerals and massive consumption of Tl–containing products are progressively worsening the environmental impact of Tl pollution. However, technologies for treating Tl–polluted wastewater are yet to be fully developed. In comparison to other heavy metals or metalloids, such as Hg, Cd, Pb, and As, there is a relative lack of research on the control of Tl pollution in waterbodies. Therefore, it is necessary to develop new and sustainable Tl removal technology for water/wastewater treatment (Alalwan et al., 2018). In aquatic environments, Tl exists in two oxidation states, namely Tl(I) and Tl(III). Tl(I) is stable and mobile in water, its chemical properties are similar to K(I); while Tl(III) is reactive and readily hydrolyzed in alkaline and neutral conditions, its chemical properties resemble Al(III) (Jia et al., 2018; Peter and Viraraghavan, 2005). Tl(I) is the dominant species in most water and wastewater streams, and its treatment is more difficult than Tl(III) (Li et al., 2018c). Common methods used for removing Tl(I) from wastewater include biological method (Zhang et al., 2017), oxidation–precipitation (Li et al., 2019a, b), electrochemical oxidation (Li et al., 2016; Wang et al., 2017), solvent extraction (Hassanien et al., 2011; Li et al., 2018c), ion exchange (Li et al., 2017a), and adsorption (John and Viraraghavan, 2008; Long et al., 2017). Among these techniques,

https://doi.org/10.1016/j.psep.2019.04.031 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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adsorption has many advantages such as simple operability and high efficiency (Pathania et al., 2016; Sharma et al., 2017c), and is thus the most widely studied method. However, most adsorbents have poor stability, selectivity, and regenerability. Existing adsorption methods for Tl(I) removal must address the problems including the stability, regenerability, and economics of the adsorbent material. Studies have shown that biochars have strong capacities for removal of heavy metals due to their porous surfaces, large specific surface areas (SSAs), and abundant functional groups (Chi et al., 2017; Feng and Zhu, 2018). Since the raw materials for preparing biochars are biomass wastes, the operational cost can be reduced while sustainability is enhanced (Yahya et al., 2015). There are many existing studies on the use of plant biomass wastes to treat wastewater containing heavy metals. Thirumavalavan et al. (2012) reported the successful removal of Cu, Cd, Zn, and Pb through adsorption by cellulose–based peels. Anirudhan et al. (2012) achieved effective Cd removal using banana stems. However, there have only been a few studies on the use of biomass for Tl(I) removal. Memon et al. (2008) successfully modified sawdust to achieve a Tl(I) removal efficiency as high as 98%. Zolgharnein et al. (2011) used beet pulp as an adsorbent to achieve a high Tl adsorption capacity of 185.2 mg/g. Unfortunately, the regenerability of these materials was poor. The crux for this type of research lies in the selection of an appropriate biomass as the raw material and finding the optimal preparatory conditions in order to enhance Tl(I) removal efficacy and the regenerability of the adsorbent. China’s annual output of watermelon is 7 × 107 tons, of which discarded watermelon rinds account for approximately one–third of the total amount (Chen et al., 2017b). Reports have indicated that watermelon rinds consist of pectins, cellulose, proteins, and carotenoids. These macromolecular polymers contain large amounts of hydroxyl, carboxyl, and amino functional groups, which facilitate binding with heavy metal ions (Husein et al., 2017). The effectiveness of using watermelon rinds to prepare biochars had been studied (Üner et al., 2015), and as an adsorbent for removing other heavy metal ions had been tested except for Tl(I). For example, Lakshmipathy and Sarada (2013) used dried watermelon rinds to remove Ni and Co ions in wastewater. Although the results were effective to a certain extent, the interference of co-existing ions was severe. Liu et al. (2012) used a similar method to remove Cu in water with an adsorption capacity of 5.73 mg/g. To the best of our knowledge, there has been no study on the use of biochars derived from watermelon rinds to remove Tl(I) from wastewater. Therefore, this study prepared biochars from discarded watermelon rinds and examined the impact of different preparatory methods on the characteristics of the biochars’ surface structure and their efficacy at Tl(I) removal. The biochar with the best adsorption efficacy was selected for investigation of the adsorption kinetics, isotherms, and the regeneration. Characterization techniques including transmission electron microscopy (TEM), X–ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT–IR), and X–ray photoelectron spectroscopy (XPS) were also used to characterize the composition and surface structure of the biochar derived from watermelon rinds, as well as its Tl(I) adsorption mechanism. 2. Materials and methods 2.1. Chemical reagents All chemical reagents were of analytical grade and used as received from the suppliers. HCl and KOH solutions were used to modify the biochars’ properties; HNO3 and NaOH solutions were added to the solution to adjust its pH. Tl(I) nitrate was used to prepare simulated Tl(I)–containing wastewater. NaNO3 ,

CaCl2 , MgSO4 , ethylenediaminetetraacetic acid disodium salt (C10 H14 N2 Na2 O8 , Na2 -EDTA), diethylenetriaminepenlaacetic acid (C14 H23 N3 O10 , DTPA), and sodium humate (C9 H8 Na2 O4 , HA) were used to observe the impact of co-existing metal ions and organic matter on the biochar’s efficacy at Tl(I) removal. 2.2. Preparation of the biochars Discarded watermelon rinds were washed before being dried at 85 ◦ C for 24 h. Next, the samples were crushed using a plant–type crusher for 3 min before these were passed through a 100–mesh sieve. The samples were then pyrolyzed at a temperature of 500 ◦ C for 1 h. After being cooled to room temperature, the samples were ground before being passed through a 100–mesh sieve again to obtain the primary biochar. The aforementioned method was repeated to create other biochars, but with the pyrolysis temperatures set to 400 ◦ C and 600 ◦ C instead. The primary biochars prepared at 400 ◦ C, 500 ◦ C, and 600 ◦ C are denoted as PB–400, PB–500, and PB–600, respectively. The primary biochars were modified by being subjected to acid or alkali treatments. Specifically, 5 g of the PB–500 was mixed with 0.1 M HCl and stirred for 12 h. After clarification, the supernatant was discarded while the residue was rinsed with water three times. Upon drying, acid–modified biochars (MB–HCl) were obtained. Immediately after the previous step, the acid–modified biochars and solid KOH were placed in deionized water at a mass ratio of 1:2 before being stirred for 10 h. The aforementioned drying method was used before pyrolysis at 500 ◦ C for 1 h to obtain alkali–modified biochars (MB–KOH). 2.3. Adsorption experiments Unless otherwise stated, the conditions for the adsorption experiments were as follows: the initial pH of the reaction was 6.5, adsorbent dosage was 2 g/L, reaction temperature was 25 ◦ C, oscillation speed was 200 rpm, and reaction time was 30 min. All experiments were conducted in triplicates, and the results were expressed with the average and the standard deviation values. The influence of various factors, including the adsorbent type, adsorbent dosage, reaction pH, reaction temperature, co–existing ions, and organic complexes, were investigated by changing each variable with the other conditions fixed. In order to select the optimum biochar type, different adsorbents (PB–500, MB–HCl, and MB–KOH) at different dosages of 1, 2, 3, 4, 5, and 6 g/L were used; in addition, different adsorbents at different pH of 2, 4, 6, 8, 9, 10, 11 and 12 were also tested. This enabled determination of the best biochar type and its optimum adsorbent dosage and reaction pH. After selection, PB–500 was used for the experiments of co-existing cations and organics, kinetics and isotherms studies. Primary biochar was also used for the treatment of industrial wastewaters from two zinc oxide production plants. 2.4. Experiments on regenerability and reusability Desorption experiments were also performed to determine the reuse conditions of the biochar. HNO3 , HCl, H2 SO4 , or H3 PO4 were tested for desorption of Tl. The desorption time was 5 min. In addition, five consecutive adsorption-desorption cycles were also conducted to examine the reusability and regenerability of the biochar. 2.5. Analytical method The metal concentrations were determined using an atomic absorption spectrometer (ICE-3000, Thermo Scientific, USA). Inductively coupled plasma-mass spectrometry (ICP–MS, NexION 300,

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Fig. 1. SEM images of different biochars, including (a) PB–400, (b) PB–500, (c) PB–600, (d) MB–HCl, and (e) MB–KOH; and (f) the XRD patterns of the prepared biochar.

PerkinElmer, USA) was used to determine Tl contents lower than 0.500 mg/L with a detection limit of 0.01 ␮g/L. A scanning electron microscope (SEM; JSM–7001F, JEOL, Japan) equipped with an energy dispersive X–ray spectrometer (EDS) was used to study the morphology of the adsorbents. TEM (JSM-7001F, JEOL, Japan) was used to analyze the fine structure of the adsorbent. An SA3100 (Beckman Coulter, Inc.) specific surface area (SSA) analyzer was used to determine the SSA of the biochars. The XRD was carried out on a PW3040/60 (PANalytical, Netherlands) analyzer. FT–IR spectra were recorded using a Tensor27 (Bruker, Germany) spectrometer. XPS (ESCALAB 250Xi, Thermo Fisher, UK) analysis was carried out to determine the elemental compositions of the adsorbents. All binding energy values are referenced to 284.6 eV of the C 1s peak. 3. Results and discussion Our preliminary experiments indicate that watermelon rindsderived biochar had the best Tl removal performance among five types of adsorbent (commercial activated carbon, and biochar derived from watermelon rinds, orange rinds, durian rinds and pomelo rinds) at different adsorbent dosage (Fig. S1). In particular, the watermelon rinds-based biochar removed 98.4% Tl even at a low adsorbent dosage of 0.5 g/L, while others had poor Tl removal (<46.7%) at the same experimental conditions. Therefore, watermelon rinds were selected as the sources to synthesize biochar for systematic study on Tl removal. 3.1. Characterization of the biochars Among the biochars prepared under different pyrolysis temperatures, the sample prepared at 500 ◦ C was more porous (Fig. 1b). In contrast, the biochars prepared at 400 ◦ C (Fig. 1a) and 600 ◦ C (Fig. 1c) had finer intra–pores and no apparent intra–pore structure, respectively. The intra–pore structure of the latter biochar could have collapsed due to the high temperature. Both the PB–500 (Fig. 1b) and MB–HCl (Fig. 1d) had apparent intra–pore structures, but the former had more intra–pores and a multi–layered pore structure. The MB–KOH had relatively smooth surfaces (Fig. 1e) and less apparent pore structures compared to PB–500. This might be due to the collapsing of the pore structure from the prolonged pyrolysis or the corrosive effects of KOH (Husein et al., 2017).

Table 1 The BET results for different temperature pyrolyzed biochars and different modified biochars. Biochars

SSA (m2 /g)

Pore volume (cm3 /g)

Pore diameter (nm)

PB–400 PB–500 PB–600 MB–HCl MB–KOH

3.127 4.751 3.872 14.180 13.763

0.015 0.016 0.004 0.031 0.145

1.220 2.457 1.220 2.741 3.419

The SSAs of the biochars were not high as compared to other biochars (Thirumavalavan et al., 2012) and were ranked in the following order: MB–HCl > MB–KOH > PB–500 > PB–600 > PB–400 (Table 1, Fig. S2). In terms of the laser particle size analysis, the particle size of biochars exhibited some signs of particles aggregation in water (Fig. S3). The particle size of modified biochars was finer than that of the PB–500 due to corrosion by HCl or KOH, thus leading to higher SSAs. The primary biochars were more porous than the modified biochars, because the porous structure formed by the metal oxides could be damaged by the treatment of HCl or KOH. The main elements of the primary and modified biochars were C and O (Table 2). There was also a certain amount of K and Cl, and small amounts of N, S, and Fe. Among these, the MB–HCl biochars had the highest C contents, but the lowest proportion of O, because the acid had eroded the metal oxides and dissolved the ionic compounds in the biochars. Notably, the MB–KOH had the highest K contents (19.99%), followed by the PB–500 (5.14%), PB–400 (3.31%), PB–600 (2.86%), and MB–HCl (0.00%). The XRD patterns reveal that KCl crystals occurred in the primary biochars prepared at the pyrolysis temperature from 400 ◦ C to 600 ◦ C (Fig. 1f). There were no characteristic XRD peaks in the MB–HCl, suggesting that the MB–HCl was amorphous and that the crystals were dissolved and washed out after acid treatment. The peaks of K2 CO3 ·1.5H2 O crystals were observed in the MB–KOH, which implies that the CO2 in the air might have contributed to the formation of K2 CO3 . The PB–500 and the MB–KOH have distinctive crystals, with identified lattice fringe primarily belonging to (200), and (321) planes corresponding to the d–spacing of 0.315 nm for KCl (JCPDS No. 41–1476) and 0.272 nm for K2 CO3 ·1.5H2 O (JCPDS No. 11–0655), respectively (Fig. 2, Fig. S4). The lattice fringes were

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Table 2 The EDS determined elemental contents of different temperature pyrolyzed biochars and different modified biochars. Elements

PB–400 (At%)

PB–500 (At%)

PB–600 (At%)

MB–HCl (At%)

MB–KOH (At%)

C N O S Cl K Fe

55.47 8.31 32.53 0.00 0.38 3.31 0.00

66.05 4.73 23.06 0.15 0.87 5.14 0.00

62.36 13.73 20.76 0.07 0.22 2.86 0.00

79.27 8.31 12.34 0.04 0.04 0.00 0.00

37.08 0.12 42.54 0.15 0.10 19.99 0.02

Fig. 2. TEM images: (a) and (b) for primary biochar-500; (c) and (d) for HCl-modified biochar; (e) and (f) for KOH-modified biochar.

hardly observed in PB–400 and PB–600, indicating that these two biochars were crystal–poor (Fig. S4). Overall, the primary biochars pyrolyzed at different temperature have similar morphology and physicochemical structure; whereas the KOH or HCl modified biochars have distinctive morphology and structural properties from the primary ones. 3.2. Comparison of the biochars’ performance in Tl(I) removal Our preliminary tests show that an increase in primary biochars dosage led to an improvement in Tl(I) removal, and that effective Tl(I) removal was achieved over a wide pH range from 2 to 12 (Fig. S5). Further, the PB–500 was found to be the best adsorbent because it had the highest Tl(I) removal. The minimum adequate biochar dosage was 2 g/L, since the adsorption efficiency remained above 96% when the adsorption saturation point had been reached (Fig. S5). The Tl(I) removal efficacy of different modified biochars (PB–500, MB–HCl, and MB–KOH) under different adsorbent dosages and reaction pH were investigated (Fig. 3). Nearly complete Tl(I) removal (> 99%) via PB–500 and very low Tl(I) removal (< 10%) by MB–HCl was observed at all dosages studied (1–6 g/L). An increase in MB–KOH dosage led to a progressive improvement ˜ at a biochar dosage in Tl(I) removal, until reaching a plateau (97%) of 4 g/L (Fig. 3a). Therefore, PB–500 was the best biochar for Tl(I) removal, followed by the MB–KOH and MB–HCl. The excellent Tl(I) removal via PB–500 over a wide range of adsorbent dosage reveals that the PB–500 as an adsorbent had excess binding sites, even at a dosage of 1 g/L to bind all the Tl when the initial Tl(I) concentration was 10.25 mg/L. Such that no further enhancement on Tl removal was observed with increasing biochar dosage. The comparable Tl removal by MB–KOH might be due to the ion exchange

with K+ for Tl+ because MB–KOH is rich in K+ , which plays a key role in stabilization of Tl(I) by natural minerals (Wick et al., 2017) via ion exchange. In contrast, the acid treatment of MB–HCl causes the loss of most metal oxides and exchangeable metal ions as well as organic residues that have abundant ligands to bind heavy metals (Sun et al., 2015). Therefore, the MB–HCl has low affinity for Tl(I). PB–500 was also effective in Tl(I) removal over a wide pH range from 4 to 12 (Fig. 3b). Comparable Tl(I) removal performance was found for MB–KOH. For the MB–HCl, the adsorption efficiency increased with an increase in reaction pH. Tl(I) removal was 20–65% when pH = 2–9, exceeded 90% only when pH > 10 and reached the maximum at pH 11. The performance of Tl(I) removal can thus be ranked in the following order: PB–500 > MB–KOH > MB–HCl. Under acidic conditions (pH < 4), all biochars had poor Tl(I) removal due to the electrostatic repulsion between positively charged adsorbent surface and the Tl+ ions (Husein et al., 2017). Under alkaline conditions (pH > 10), the adsorbent surface becomes negatively charged and is prone to adsorb metal cations (Long et al., 2017) such as Tl+ herein. In addition, the hydroxylation of the adsorbent surface becomes stronger under alkaline conditions, and thus surface complexation between the O H group and Tl+ could also contribute to the Tl(I) removal (Li et al., 2018b). The excellent Tl(I) removal by biochar derived from the watermelon rinds under a wide pH range suggests that other mechanisms other than electrostatic attraction contribute to the Tl(I) removal. Thus, the primary biochar prepared at 500 ◦ C was selected for the follow–up study. 3.3. Tl(I) adsorption by the selected primary biochar 3.3.1. Impact of co-existing ions, organic matter and temperature Low levels (<0.01 mol/L) of co–existing metal ions (NaNO3 , MgSO4 , and CaCl2 ) did not obviously inhibit the Tl(I) adsorption

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Fig. 3. Tl(I) removal by PB–500, MB–HCl, and MB–KOH: (a) at different biochar dosages of 1–6 g/L (initial Tl(I) solution of 10.25 mg/L, reaction time of 30 min) and (b) at different reaction pH from 2 to 12 (biochar dosage of 2 g/L, initial Tl(I) solution of 10.25 mg/L, reaction time of 30 min).

Fig. 4. Effect of (a) co–existing cations and (b) organics on Tl(I) removal; (c) the kinetic and (d) the isotherms of Tl adsorption using PB-500; (e) Tl removal from high strength industrial wastewater with PB-500; and (f) Tl removal from low strength industrial wastewater with PB-500.

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Fig. 5. (a) Effect of eluent concentration on Tl(I) desorption (PB–500 dosage of 2 g/L, initial Tl(I) solution of 10.25 mg/L, adsorption time of 30 min (25 ◦ C), and then desorped for 5 min); and (b) the regeneration performance of the PB–500 for Tl(I) removal (using 0.1 mol/L HNO3 for 5 times).

(Fig. 4a). The increase in the co–existing metal ion concentration from 0.01 to 1.0 mol/L caused a decreased Tl(I) removal. CaCl2 had the strongest adverse impacts on Tl(I) adsorption, followed by MgSO4 and NaNO3 . Previous studies had shown that co–existing divalent ions inhibit Tl(I) removal more than monovalent metal ions due to ion competition for binding sites (Huangfu et al., 2015; Li et al., 2017b, a), which is consistent with this study. Nearly complete Tl(I) removal was observed in the presence of low concentrations (<100 ␮mol/L) of organics (EDTA, DTPA and HA), indicating that low level of these organics has little effect on Tl(I) removal (Fig. 4b). At a high dosage of organic matter (500 ␮mol/L), EDTA had a significant inhibitory effect on the adsorption process (Fig. 4b), with Tl removal decreased by 40.7% compared to zero addition of organic matter. The high dosage of EDTA might have contaminated the biochar surface and thus inhibited the adsorption to a certain extent. Previous studies have shown that DTPA has a strong inhibitory effect on Tl removal when using Fe–Mn binary oxides (Li et al., 2017b), however, it is not a concern in this study. This is because the biochar could adsorb the DTPA as well, since the TOC concentration is low (data not shown). In addition, the possible ion exchange between K(I) and Tl(I) seems to contribute to the Tl(I) removal, which is not likely affected by these external organic matter. Regardless of the adsorption temperatures, the Tl(I) removal efficacy was consistently above 95% (Fig. S6). Between 5 ◦ C and 45 ◦ C, the adsorption efficiency changed from 98.5% to 98.3%, which is statistically insignificant. Thus, the reaction temperature had effectively no effect on the adsorption of Tl(I) under the conditions studied. The high resistance to co–existing cations and organics, and the stable performance under different temperature during Tl(I) removal suggests that the biochar derived from watermelon–rinds is an excellent adsorbent for Tl(I) removal from wastewater.

3.3.2. Kinetics and isotherms During the first 10 min after adsorption began, the adsorbate caused strong concentration gradient and the adsorbent had sufficient adsorption sites on its surface (Kumar et al., 2015; Mittal et al., 2016), so that the adsorption rate was high and the adsorbent rapidly captured the Tl(I) (Fig. 4c). Higher initial Tl(I) concentration resulted in slower Tl(I) adsorption equilibration; it took 5, 10, and 20 min to reach equilibrium under initial Tl(I) concentration of 20, 50, and 100 mg/L, respectively. For a given dosage of adsorbent, the binding sites were fixed, and thus the higher initial Tl(I) concentration required more adsorbent to provide more active binding sites. With an increase in the initial Tl(I) concentration, a higher qe coefficient was observed (Table S1), while lower k2 constant was found due to slower adsorption rate (Zhang et al., 2018). The Tl(I) adsorp-

tion via PB–500 was more consistent with the pseudo–second order kinetic model rather than the pseudo–first order kinetic one (Table S1), since the R2 values of the former (0.90–0.97) were higher than those of the latter (0.80–0.90). Many other studies on the adsorption of heavy metal ions by other biochars show that the pseudo–second order kinetic model was the best in describing the heavy metals adsorption process (Ali et al., 2016; Sabermahani et al., 2016). With increasing initial Tl(I) concentrations, the amount of Tl(I) adsorbed increased, peaked, and then remained stable (Fig. 4d). The Langmuir model had the highest goodness of fitting among the three isotherm models (R2 value: 0.932 (Langmuir) > 0.930 (Temkin) > 0.868 (Freundlich), Table S2), suggesting that this adsorption is more close to monolayer adsorption (Naushad et al., 2016; Sharma et al., 2017b). The maximum adsorption capacity was 178.4 mg/g, which is higher than that of most adsorbents such as sawdust (13.2 mg/g) (Memon et al., 2008), and alginate–Prussian blue (103.0 mg/g) (Vincent et al., 2014), and is close to that of modified beet pulp (185.2 mg/g) (Zolgharnein et al., 2011), indicating the good adsorptive ability for Tl(I) among various adsorbents (Table S3). 3.3.3. Tl(I) removal from real wastewater The effluent Tl(I) concentrations gradually decreased with increases in the biochar dosage (Fig. 4e and f). For low strength wastewater, the effluent Tl(I) concentration was treated to below the local discharge limit (<5 ␮g/L) at the biochar dosage of 10 g/L. For the high strength wastewater, a maximum removal of 87% was obtained at the highest biochar dosage, such that additional treatment is needed for this high strength wastewater. Given the complexity of actual wastewater, the dosage of biochar used should probably be higher than that for simulated wastewater with simple components. In summary, biochars derived from watermelon rinds have good efficacy at removing Tl(I) from actual industrial wastewater with complex properties, and have promising application prospects. 3.4. Desorption and reuse Eluents including acid (HNO3 , HCl, H2 SO4 , and H3 PO4 ) and alkali (NaOH) solutions, were tested for Tl desorption solutions (Fig. S7). Alkali solutions were unable to elute Tl from the adsorbent because the alkaline conditions in turn rendered the negatively charged adsorbent surface more prone to capture the Tl ions via electrostatic attraction (Fig. S7). Acid solutions, especially HNO3 and HCl were found to be good eluents for Tl desorption. The HNO3 concentration at 0.1 mol/L appeared to be the best, since low eluent concentration is not sufficient, while an excess of eluent does no good for Tl desorption. Therefore, HNO3 at a concentration of

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Fig. 6. EDS-based elemental mapping of the adsorbent before and after adsorption of Tl(I) (Before adsorption: a, b, c, d, and e; After adsorption: f, g, h, i, j, and k. Red: C, purple: O, blue-green: Cl, green: K, purplish-red: Tl). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

0.1 mol/L was selected as the best eluent. Most previous studies on effective adsorbent regeneration used HNO3 or HCl solution as an effective eluent (Li et al., 2018b), in which the regeneration mechanism is the reversible ion exchange between adsorbent and metal ions. The cyclic adsorption–desorption experiments show that the adsorption efficiency decreased slightly but still remained > 95% (Fig. 5b). The average Tl(I) desorption efficiency was kept around 80% during five consecutive regeneration tests. The results indicate good stability and reusability of the PB–500 for Tl(I) removal from wastewater. In most past studies on adsorbent reuse for Tl(I) removal, the desorption efficiency was low and the adsorbent was not sufficiently stable. Memon et al. (2008) studied the use of modified sawdust for Tl(I) adsorption and desorption, but no in–depth study of the adsorbent’s regenerability and stability has been conducted. Zhang et al. (2018) achieved good Tl(I) removal efficacy with titanium peroxide, but the regenerability of this was not good. In contrast, the biochar in this study has good regenerability and stability, and thus has good application potential for Tl(I) removal from wastewater. 3.5. Adsorption mechanism The EDS analyses show that the biochar did not contain Tl prior to its adsorption of Tl(I) from solution (Table S4). After adsorption, the Tl content increased to 1.7% (Table S4), and the elemental mapping (Fig. 6) also obviously shows the prominent brightness of Tl, indicating that the biochar was indeed effective in adsorbing Tl(I). K and Cl contents both decreased after adsorption, while N content slightly increased (Table S4). It can be inferred that the surface of the biochar derived from watermelon rinds contains not only activatable K(I) but also activatable anions, such as Cl– and NO3 – . The EDS mapping (Fig. 6) indicates a clear decrease in elemental content of K (in green color) and Cl (in blue-green color), since their brightness significantly decreased after adsorption (Fig. 6), which is consistent with the EDS analyses in Table S4. It appears that the K(I) from the biochar served as exchangeable cations to replace Tl(I), while

the Cl− from the biochar reacted with Tl(I) to form TlCl precipitate, which is an insoluble compound in solution, thus achieving effective removal of Tl(I). The NO3 – from the TlNO3 can exchange with the Cl− from solution, leading to an increase in N content (Table S4). The XRD analyses (Fig. 7a) show that the PB–500 contained crystalline KCl (JCPDS No. 41–1476) before adsorption, which became TlCl (JCPDS No. 06–0486) after Tl adsorption. Both K(I) and Tl(I) are monovalent cations with very similar ionic radii and chemical properties (Peter and Viraraghavan, 2005). It is highly likely that ion exchange between K(I) and Tl(I) occurred during adsorption (Martin et al., 2018), which is supported by the data from the EDS analyses (Fig. 6, Table S4). The abundant exchangeable Cl− from PB–500 reacted with some of the Tl(I) to form TlCl, which was then precipitated due to its low solubility (Li et al., 2005) and eventually adsorbed to the porous biochar material. Other Tl(I) might have been removed via surface complexation. After cyclic regeneration, KNO3 (JCPDS No. 32–0824) was found in the regenerated biochar. This is probably due to NO3 – in the HNO3 eluent having replaced the Cl– from the used biochar, which is also evidenced by the data from the EDS analyses (Table S4), such that NO3 – was then sorbed to the regenerated biochar and formed KNO3 with pristine K(I). The functional groups on the surface of the MB–HCl were less abundant compared to those on the surface of the PB–500 and MB–KOH (Fig. 7b), due to acid modification having dissolved the metals oxides and metal ions from the biochar. Consequently, the adsorption and removal efficacies of the PB–500 and MB–KOH were significantly better than that of the MB–HCl. The characteristic peaks of the 500–PB and MB–KOH were similar, and there were pronounced CO2 adsorption peaks at 2350 cm–1 (Lenza and Vasconcelos, 2001). The hydroxyl peak of the MB–KOH was the strongest at 3414 cm–1 (Long, 2010; Sharma et al., 2018a), because the K2 CO3 ·1.5H2 O observed in the XRD pattern (Fig. 1f) could generate the strongest intensity of the O H group. Similar observation on MB–KOH was also obtained by Husein et al. (2017). While the weaker hydroxyl peak was observed after the PB–500 desorption due to the exotic oxyacid radical group. The C O stretching vibra-

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Fig. 7. Characterization of the PB–500 before adsorption, after adsorption and desorption: (a) XRD patterns; (b) FT–IR spectra; (c) XPS spectra of survey; (d) XPS Tl 4f core level; (e) XPS O 1s core level; and (f) XPS Cl 2p core level.

tions at 1655 cm–1 , which is likely due to aldehydes, ketones, and carboxylic acids (Wallace et al., 2013), became weaker after adsorption owing to adsorption of Tl that reduces the signals. In–plane bending of methylene or methyl group vibrations were observed at 1387 cm–1 (Yang and Qiu, 2010). The strong C O stretching vibrations at 1060 cm–1 may be due to alcohol, phenol, and ester functional groups in the biochar (Martins et al., 2015). The XPS spectra show that Tl peaks appeared after adsorption and disappeared after desorption, suggesting the successful adsorption of Tl during adsorption and the effective regeneration after desorption (Fig. 7c). This is consistent with the EDS analyses. The characteristic XPS spectra of the Tl 4f core level have two symmetrical Tl 4f7/2 and Tl 4f5/2 peaks with a spin–orbit splitting ˜ eV (Kothari et al., 2002). The Tl 4f peaks can be split into of 4.4 sub–peaks at 118.5 eV (Rumble et al., 2010), and 119.0 eV (Wan et al., 2014), which correspond to Tl2 O (27.9%), and Tl(I) (72.1%), respectively (Fig. 7d). There is no valence change on Tl during the adsorption via the PB–500.

The O 1s peaks can be spit to three peaks with 530.5, 531.6, and 532.5 eV (Fig. 7e), which correspond to the metal oxides, hydroxyl group and H2 O, respectively (Li et al., 2013). However, the metal oxides–related oxygen reduced by 3.2% and further reduced by 9.5% again after desorption. This is probably because the metal oxides dissolved in water and other metal oxides further dissolved with acid during desorption. The relative amount of hydroxyl group–related oxygen decreased by 38.9% after adsorption but increased by 8.2% after desorption. The adsorbed water–related oxygen increased by 42.0% after adsorption and further increased slightly by 1.3% after desorption. This variation can be explained by the formation of TlCl after adsorption and KNO3 formation after desorption (Fig. 7a). The peaks of C 1s XPS spectra can be split into three subpeaks, including C–C, C = O, and C–O at 284.6 eV, 287.9 eV, and 286.2 eV (Fig. S8a), respectively, which are similar to a previous study (Stankovich et al., 2007). Little change in the shape of the C 1s XPS spectra was observed, indicating that the dominant C skele-

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efficacy when applied to real industrial wastewater. Therefore, the primary biochar prepared from watermelon rinds is an excellent, regenerable adsorbent for treating wastewater containing Tl(I). Conflicts of interest None. Acknowledgements The research was supported by the National Natural Science Foundation of China (51808144, 51778156, 51208122, 51678562, U1612442), the Science and Technology Program of Guangzhou (201906010037, 201707010256, 201804010281), and the Guangzhou Education Bureau (1201630390). Appendix A. Supplementary data

Fig. 8. The mechanism on reversible adsorption-desorption for Tl removal using watermelon rinds–derived biochar.

ton structure was stable, which is favorable to the reuse of primary biochar for Tl removal. Evident Cl 2p peaks were observed (Fig. 7f), owing to the presence of KCl in the PB–500 (Fig. 1f). After adsorption or desorption, Cl 2p peaks disappeared because of the washout of KCl into the solution. The crystalline TlCl observed by XRD after adsorption might be present in small amounts, such that the Cl 2p peaks were not identified during XPS analysis. Interestingly, K 2p peaks were observed for all biochar samples (Fig. S8c). After adsorption, the K 2p peaks became skewed and their intensity decreased due to the capture of Tl. After desorption, the shape and intensity of K 2p peaks were restored to almost their pre–adsorption states (Mo et al., 2015). It seems that the ion exchange between K(I) and Tl(I) took place during adsorption. The K(I) in the biochar likely have been produced by acid (HNO3 ) treatment, which implies that the new K(I) might have originated from the corrosion of the outer surface of the biochar. N 1s peaks appeared after adsorption because of the adsorption of NO3 – from the synthetic TlNO3 solution (Fig. S8b). The intensity of N 1s peaks became stronger after desorption due to the further capture of NO3 – from the eluent (HNO3 solution), which is supported by the crystalline KNO3 observed in the XRD spectra (Fig. 7a). Therefore, the mechanism of Tl(I) removal by biochar derived from watermelon rinds is mainly through ion exchange between the pristine K(I) on its surface and Tl(I) (Fig. 8). In addition, the Cl− on the surface of biochar can react with Tl(I) to form precipitate of TlCl, which is then captured by the mesopores of the biochar; this is another mechanism for Tl removal. Surface complexation between the O H group and Tl(I) may also contribute to the Tl(I) removal. The regeneration of biochar is due to the reversible ion exchange between H+ and Tl(I). This nature renders the watermelon rinds–derived biochar stably regenerable and effective for Tl(I) removal. 4. Conclusions The primary biochar pyrolyzed at temperature of 500 ◦ C has the best Tl(I) removal efficacy among the various biochar prepared. The maximum adsorption capacity of 178.4 mg/g is clearly stronger than that of similar biochar adsorbent. The biochar could effectively remove Tl(I) from wastewater via ion exchange between Tl(I) and the pristine K(I) or the O H groups on the biochar surface. Moreover, the biochar can be stably reused, and has good Tl(I) removal

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