Journal of Cleaner Production 165 (2017) 221e230
Contents lists available at ScienceDirect
Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
Effect of phosphoric acid on the surface properties and Pb(II) adsorption mechanisms of hydrochars prepared from fresh banana peels Nan Zhou a, 1, Honggang Chen a, 1, Qiuju Feng b, Denghui Yao a, Huanli Chen a, Haiyan Wang c, Zhi Zhou a, *, Huiyong Li a, **, Yun Tian d, Xiangyang Lu d a
College of Science, Hunan Agricultural University, Changsha 410128, China College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China d College of Bioscience and Biotechnology, Hunan Agricultural University, Changsha 410128, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 April 2017 Received in revised form 27 June 2017 Accepted 15 July 2017 Available online 19 July 2017
The acidic environment has been confirmed to have significant influence on the biochar products through hydrothermal carbonization. To investigate the effect of H3PO4 on the surface properties and adsorption mechanisms of the biochar, fresh banana peels were used as feedstock and transformed into hydrochars under the catalysis of H3PO4 with concentration ranges from 0% to 50%wt. The addition of H3PO4 greatly impacted the physicochemical properties of the as-obtained hydrochars, such as carbonization degrees, pH values, amount of surface acidic functional groups and then further affected the adsorption mechanisms of lead ions on those hydrochars. Hydrochars catalyzed by H3PO4 with higher concentration showed higher degree of carbonization, lower value of pH and fewer amount of acidic functional groups, while lower content of H3PO4 could not completely catalyze the degradation reactions and resulted in larger quantity of intermolecular locked oxygen containing groups that could not be participated into the adsorption process. Among all six samples, hydrochar generated in 30%wt H3PO4 exhibited the best adsorption property of 241 mg g1, possibly due to the largest concentration of the acidic functional groups on the surface rather than in the intermolecular structure of the carbon. It had been confirmed that the adsorption mechanisms were the combination of surface complexation, cationic exchange and part of precipitation. Hydrochars derived from fresh banana peel via hydrothermal carbonization catalyzed by phosphoric acid could be excellent adsorbents for lead removal in aqueous environments. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Hydrochar Heavy metals removal Biochar Banana peel Adsorption mechanism Hydrothermal carbonization
1. Introduction Biochar, a carbon-rich material, is the solid post product of the carbonization of biomass resources under oxygen free or limited environment. Due to the attractive characteristics, low density, high stability, developed surface property and environmental benign nature, biochar can have widely application in pollutants amend ska et al., 2017; Tong et al., ment including heavy metals (Kołodyn 2011), organic compounds (Klasson et al., 2014; Li et al., 2014)
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (Z. Zhou),
[email protected] (H. Li). 1 Nan Zhou and Honggang Chen contributed equally to this work. http://dx.doi.org/10.1016/j.jclepro.2017.07.111 0959-6526/© 2017 Elsevier Ltd. All rights reserved.
and green house gases (Awasthi et al., 2017). Combined with abundant feedstock resources, a cost-effective conversion process and simple handing procedure, biochar has become a promising candidate to substitute the current costly adsorbents, such as activated carbon, silica and alumina in wastewater treatments. The adsorption behavior of biochar towards heavy metals is the result of the cooperation of physical attraction and chemical reaction (Tan et al., 2017). The former largely depends on the surface area and porosity, since high surface energy can strengthen the molecular and intermolecular force and high quantity of holes can apply more storing places for heavy metal ions; while the latter significantly relies on the surface functional groups and chemical fractions, due to which chemical reactions including ion exchange, complexation, precipitation, electrostatic attraction and p-interaction between
222
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
the biochar and heavy metal ions can take place. Therefore, it is very important to improve the surface property for adsorption performance development of biochars. It is well known that the composition of biomass has a great influence on the surface property of biochar, however, the surface characteristics of biochar can be significantly changed by using different carbonization methods and adjusting the reaction conditions, especially when combined with the in situ or after activation processes (Li et al., 2014). The after activation is widely used presently to improve the surface property of the products after pyrolyzation of the biomass through physical or chemical or physicochemical ways. During which, specialized reagents are commonly used and complicate apparatus are usually required, coproducing a large quantity of waste and by-products, resulting in requirement of disposal and further treatments (Xue et al., 2012). Recently, wet pyrolysis, also named hydrothermal carbonization (HTC), has attracted tremendous attentions for biochar synthesis and modification. HTC is a thermochemical process performed in water within the temperature range of 120e260 C for several hours under saturated pressure (autogenous or provided by a gas). Besides of the mild reaction environment and lower energy consumption, the overwhelm capability of HTC is the directly use of biomass with high content of moisture, which comes for most of biomass naturally obtained. The cutting of the pre-dehydration process that is required for dry pyrolysis can deeply reduce the costs in industrial applications, and the firsthand use of those high moisture-containing resources like banana peels can greatly enlarge the scope of biomass feedstock. However, most of the related literature were still utilizing dehydrated and pulverized raw materials rather than fresh biomass (Nizamuddin et al., 2016; Pellera et al., 2012), cutting the HTC out of the main market for biochars preparing. To develop the wide-scope application of HTC, increasing efforts and more investigation are urgently required. During the HTC process, the biomass is undergoing thermal degradation in water and contrariwise affects the physicochemical properties of water (Brunner, 2009), which will reduce the dielectric constant of water and turn it to be a good solvent for non-polar substances, such as organic staff under supercritical conditions. With the increase amount of the ionic species in water, acid-basecatalyzed reactions can be facilitated (Elliott, 2011). Although detailed reaction mechanisms of biomass during HTC process are yet unknown due to the complexity of natural materials (Funke and Ziegler, 2010), adding chemicals especially an acid during the HTC procedure to transform pure carbohydrates and/or biomass into functional char coal materials has been proven to be highly feasible (Fernandez et al., 2015; Lynam et al., 2011; Titirici et al., 2007). Thus, the in situ activation taken place with the pyrolysis of biochar simultaneously becomes possible and much easier. It was confirmed that adding acetic acid and salts like lithium chloride into the reacting system could effectively reduce the vapor pressure and enhance the fuel value of biochars (Lynam et al., 2011, 2012). Later, HCl, NaOH and NaCl solutions were employed to produce biochars with strengthened atrazine adsorption capability (Flora et al., 2013). It was reported that acetic acid and KOH could apply acidic and basic environment and influence the characteristics of the biochars. However, the study and utilization of transferring biomass into functional biochars through HTC is still in the embryonic stage, especially with the in situ activation by adding a specialized reagent. To accelerate the development of biochar, further studies and more efforts need to be devoted to explore the knowledge on the reaction mechanisms, the functionalization of additives, as well as application for various purposes. Phosphoric acid is found to be an efficient activating agent to improve the property of carbon based materials before and/or after the carbonization (Elmouwahidi et al., 2017; Sun et al., 2016).
Compared to other chemical activators, such as zinc chloride, hydroxide and nitric acid, H3PO4 has lower corrosivity and the asobtained products contain less harmful residues, thus environmental benign (Prahas et al., 2008). Most recently, our group discovered that fresh and dehydrated banana peels could be transferred into highly effective sorbent biochars through a facile one-step hydrothermal carbonization approach by using 20%wt phosphoric acid as the reaction medium (Zhou et al., 2017). The phosphoric acid added here in the HTC process played a key role in the dehydration of the polysaccharides and the formation of large quantity of acidic surface functional groups, which was the dominant influence parameter on the removal capacity of the asprepared biochars toward lead. It has been proven that in situ activation with wet pyrolysis of fresh banana peels into effective sorbents could be realized by using H3PO4 through HTC process. To figure out the feasible reaction mechanisms, herein, phosphoric acid with various concentrations ranging of 0%e50%wt were utilized as the reaction media to transfer fresh banana peels into biochar sorbents through hydrothermal carbonization. The pH value, the surface area, the micromorphology, the cationic ions, as well as the species and amounts of acidic functional groups of the as-obtained hydrochars were carefully investigated to illustrate the influence of the phosphoric acid on the surface properties of the hydrochars. The lead storage properties in aqueous solutions of those as-prepared hydrochars and the adsorption mechanism of the specific optimized sorbent were also studied to further demonstrate the relationship among the reaction media, the surface property and the adsorption capability. 2. Experimental 2.1. Preparation of banana peel based hydrochars Bananas were purchased from a local market, washed and peeled. The banana peels were washed with distilled water (DW) for several times to completely remove the dust and other soluble impurities. After that, the cleaned banana peels were chopped into small pieces (0.5e1 cm), and then used as the feedstock to produce hydrochars directly. Analytical pure phosphoric acid H3PO4 was purchased from Aladdin Reagent Co. Ltd. (Shanghai, China) and used as received. The phosphoric acid was diluted with double distilled water into appropriate concentrations before the synthesis process. Typically, 4 g fresh banana peel pieces (containing about 90% moisture) were added into 50 ml phosphoric acid solutions with gradient concentration from 0 to 50%wt. After soaking in the H3PO4 solution for 2 h, the mixture was transferred into a 100 ml polytetrafluoroethylene (PTFE) inner steel autoclave and heated at 230 C for 2 h. Then, the autoclave was taken out of the furnace and cooled to room temperature. The obtained product was vacuum filtered and washed with DW for several times until the washing liquid got neutral. Finally, the as-prepared sample was dried in oven at 80 C overnight. For convenience, the obtained hydrochars were abbreviated as H-x hereafter, while x stands for the concentration of H3PO4, for example, H-20 is the hydrochar produced within 20%wt H3PO4. 2.2. Characterization Scanning Electron Microscopy (SEM) was carried out to image the particle size and morphology of the samples through a JSN6380LV instrument. Elemental analyzer was performed to define the elements contents and the atomic ratios via a Vario Micro cub Elementar (Germany). The surface areas of the hydrochars were calculated based on the results of N2 sorption 77 K experiments performed in a Gemini VII 2390 Surface Area Analyzer according to
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
Brunauer-Emmett-Teller (BET) method. The pH value of the products were determined through a pH meter (Lei-ci PHS-3C, Inesainstruments Co., Shanghai, China) by examine the filtrates obtained by mixing 1 g of each sample with 20 ml of double distilled water and then shaken on a mechanical shaker for 2 h. X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) were utilized to observe the crystal structure and the component of organic groups of the samples before and after lead loading through a XRD-6000 machine (SHIMADZU, Japan) and a Spectrum 65 instrument (PE, America), respectively. The total amount of acidic oxygencontaining functional groups on the surface of as-prepared hydrochars was determined by Boehm titration. Specifically, 0.05 g of each sample was respectively added in 10 ml of 0.1 mol L1 solution of sodium hydroxide. After 24 h stirring in a closed vessel, the suspension was filtered, and then titrated by hydrochloric acid of 0.1 mol L1. Frame Atomic Absorption Spectrophotometer (FAAS) was employed to examine the adsorption amounts of lead ions via a Z-2000 instrument (HITACHI, Japan). 2.3. Adsorption experiments Lead nitrate Pb(NO3)2 with analytical purity was also purchased from Aladdin Reagent Co. Ltd. (Shanghai, China) and used as received. Stock solution of Pb2þ ions was prepared by dissolving Pb(NO3)2 in double distilled water and then diluted to appropriate concentrations. Adsorption experiments were performed in 50 mL capped glass bottle at room temperature of about 25 C under environmental pressure of 101.325 KPa. All experiments were conducted in triplicate and average results were reported, and blank samples without biochars or metal ions were run along with. Generally, 0.1 g hydrochar sample was respectively added into 25 ml of 1000 mg L1 Pb(NO3)2 solution and then shaken at 150 rpm in a mechanical shaker for 3 h. After that, the mixtures were vacuum filtered and the filtrates were analyzed directly through FAAS to determine the Pb2þ concentrations after adsorption. The influence of sorbent dose, pH, initial metal concentration and contact time on the adsorption performance of H-30 was investigated to further define the adsorption behavior of this optimized hydrochar. In brief, 0.1 g H-30 was mixed with 25 mL working solution of lead ions with concentration of 400 mg L1 and shaken for 3 h. Exception of gradient hydrochar amounts of 0.01e0.2 g, metal concentrations of 5e1000 mg L1 and adsorption durations of 0.083e24 h were respectively used for dosage, metal ion concentration and contact time investigation. The pH levels of Pb2þ-bearing solutions were adjusted by 1 M HNO3 and NaOH solutions before adding the adsorbent to perform the pH experiments. The adsorption capacity Q (mg$g1) and the removal efficiency E %, were calculated according to the following equations, respectively:
Q¼
ðC0 Ce Þ V m
E% ¼
C0 Ce 100% C0
(1)
(2)
where Ce and C0 are the equilibrium and the initial metal concentrations (mg$L1), respectively. V is the volume of solution (L) and m is the weight of added hydrochar (g). According to the adsorption test results of contact time and metal concentration, kinetic and isotherm parameters were calculated based on Pseudo-first-order and Pseudo-second-order models, Langmuir and Freundlich models. Details as listed below:
223
Pseudo first order:
Qt ¼ Qe 1 ek1 t
Pseudo second order:
Qt ¼
Qe2 k2 t 1 þ Qe k2 t
(3)
(4)
where Qt (mg$g1) is the amount of metal ions adsorbed at time t (min), k1 (min1 ) is the rate constant of Pseudo-first-order adsorption, k2 [g$(mg$min)1] is the rate constant of Pseudosecond-order adsorption, and Qe (mg$g1) is the maximum amount of Pb ions adsorbed per mass of the material at equilibrium.
Langmuir isotherm:
Freundlich isotherm:
Qe ¼
Qmax KL Ce 1 þ KL Ce 1
Qe ¼ KF Ce n
(5)
(6)
where Qe (mg$g1) is the equilibrium adsorption capacity, Ce (mg$L1) is the equilibrium concentration after the adsorption; Qmax (mg$g1) is the maximum sorption capacity, KL is a Langmuir constant, KF is the Freundlich constant, and 1/n (dimensionless) is the intensity of adsorption. The kinetics and adsorption isotherms were fitted through non-linear regression by using Origin Pro 8.0 (OriginLab, USA). 3. Results and discussion 3.1. Effect of H3PO4 on physicochemical properties Fig. 1 gives the photographs of the products and the SEM images that depict the micro morphologies and the particle sizes of the asprepared hydrochars. As is seen, hydrochar carbonized within pure water (H-0) displayed a dark brown color and appeared in a thick film morphology, which had a similar size with the chopped banana peel pieces. The film appearance and the large size indicated that the hydrochar prepared through this way might not be totally carbonized and still hold the original structure of the raw banana peel feedstock. Reversely, crushed powders with much darker color were obtained for other hydrochars that produced by using phosphoric acid as media. It is well informed that organic acids (e.g., acetic, lactic, propenoic, levulinic, and formic acids) will be formed during the decomposition process of saccharides and can lead to the further catalyzation of the degradation (Sevilla and Fuertes, 2009; Sun and Li, 2004b). The much smaller size and dark color of the product demonstrated that the adding of H3PO4 could efficiently catalyze the transformation of the feedstock and then samples with higher degree of carbonization could be obtained. Accordingly, the SEM images of sample H0 exhibit that this sample had irregular morphologies, large particle sizes and rough surfaces, further demonstrating that fresh banana peels could not be completely carbonized in pure water under 230 C for 2 h. Particles with similar morphologies but smaller sizes were observed in the SEM images of H-10, and H-20 was consisted of scattered pieces combined with small particles, indicating that a majority of the polymers had been degraded. The particle sizes of the hydrochars decreased progressively with the increasing of the H3PO4 concentration for sample H-30, H-40 and H-50, suggesting that the biomass like fresh banana peels could be carbonized thoroughly through HTC process with the help of acid like H3PO4. It has been reported that protons can be an effective catalyst to significantly accelerate the decomposition and dehydration of the biomass (Sun and Li, 2004a). However, since fresh banana peels with high content of moisture were used directly as the starting material, the large quantity of H2O inside the plant cell and the molecular could
224
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
Fig. 1. SEM images and sample pictures of FBP based biochars: (a) (g) H-0, (b) (h) H-10, (c) (i) H-20, (d) (j) H-30, (e) (k) H-40, (f) (l) H-50.
dilute the concentration of the acid and then impede the functionality of the protons. More regular sample particles with narrow dimensions were produced when higher content of phosphoric acid was used, further evidencing that the completely chain scission and degradation of the polysaccharides and other organics might only be happened with certain amount of initial phosphoric acid medium. Based on the percentage of the origi-nal precursor weight that remains after HTC process (calculated after removing the 90% weight of water), the overall solid yields of six hydrochars were calculated, as presented in Table 1, combined with the data of the chemical elemental compositions, the corresponding atomic ratios, the BET surface areas and the original pH values. Hydrochar produced in pure water had the highest yield of about 39.59%, while much lower solid yields around 27% were obtained for other five ones that carbonized in H3PO4, which should be attributed to the
higher degrees of carbonization of those samples. Parallel situations had been found by Lu et al. (2014). who transferred cellulose into biochar through HTC process with the application of HCl or H2SO4. Combined with our results, the catalyst characteristics of acids during the HTC reactions can be confirmed. The overall solid yields of H-30, H-40 and H-50 were slightly larger than those of H10 and H-20, presumably due to the crosslinking reactions promoted by phosphoric acid which can help retaining some low molecular species into solid phases (Vernersson et al., 2002). The elemental compositions and the corresponding atomic ratios of the six hydrochars shown in Table 1 further evidenced the conclusion obtained above, since sample H-0 had a lower carbon content of about 63.02%, while the carbon percentages in hydrochars H-10 to H-50 were all above 67%. Besides, both of the contents of oxygen and hydrogen dropped with the adding of H3PO4, resulting in lower ratio of O/C and H/C. It is well known that
Table 1 Overall solid yields, elemental compositions, atomic ratios, BET surface areas and pH values of the hydrochars. Sample
Yieldsa (%)
H-0 H-10 H-20 H-30 H-40 H-50
39.59 26.28 25.53 28.86 29.17 28.89
a
± ± ± ± ± ±
1.24 2.10 1.02 1.33 1.52 2.31
Elements composition (%)
Atomic ratio
C
H
O
N
O/C
H/C
(O þ N)/C
63.02 67.18 67.44 67.57 68.45 69.13
6.42 5.90 5.89 5.98 6.09 5.84
23.59 20.09 20.29 20.50 20.67 21.01
0.73 0.37 0.46 0.42 0.24 0.65
0.374 0.299 0.300 0.301 0.302 0.304
0.102 0.088 0.087 0.089 0.089 0.084
0.39 0.33 0.34 0.31 0.30 0.31
Yield % was calculated after removing 90% weight of water.
SBET (m2/g)
pH
45.27 36.85 31.65 31.54 30.91 28.80
5.38 4.73 4.62 4.56 4.35 4.31
± ± ± ± ± ±
0.02 0.03 0.03 0.02 0.01 0.02
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
dehydration reaction is one of the main mechanisms for the removal of O and H from biomass, the decrease of these two ratios indicating that the reaction degree of the dehydration of banana peels was improved by the phosphoric acid. High amount of oxygen and hydrogen in the H-0 sample suggested that there were large quantity of oxygen-containing groups beside/inside the carbon skeleton, which are in accordance with the results of FTIR analysis that discussed later. Furthermore, the content of carbon slightly grew up with the increase of the concentration of phosphoric acid, possibly due to the complex effects of H3PO4 on catalyzing carbonization of organics and retaining low molecular weight species into the solid phase. The physical adsorption, which can have effects on the storing performance of biochars, is significantly depends on the surface energy and porous structure. Thus, the surface areas of six hydrochars were determined by examining the adsorption and desorption amount of nitrogen, results also shown in Table 1. Unlike the most popular carbon based adsorbent, activated carbon, those hydrochars derived from fresh banana peels had very low surface areas of several tens, from which could be concluded that physical adsorption should not be the main mechanism for the attracting behavior of those hydrochars toward lead. Thus, the adsorption properties of those hydrochars should be dominated by chemical reactions, which are greatly affected by the pH values. The pH values of all samples were then been detected and results turned out to be weak acidic. It is well known that biochars prepared though pyrolysis usually exhibit weak alkaline characteristics (Kambo and Dutta, 2015), but hydrochars produced here via HTC procedure turned out to be acidic even without phosphoric acid. Similar situations had been reported in previous literature, and the reason should be the formation of several low molecular acids because of the dehydration and degradation of polysaccharides, due to which the reactions would be accelerated and then enormous acidic oxygen containing groups would be formed and retained on the surface of the char products (Sun and Li, 2004a). This might also explain the phenomenon that the pH values of the as-prepared hydrochars continuously dropped with the adding amount of phosphoric acid. Suitable quantity of protons might appropriately catalyze the chemical degradation reactions, promoting the formation of acidic surface oxygen containing functional groups (OFG), such as hydroxyl and carboxyl. But over dehydration and fully degradation might transfer the biomass into soluble low molecular species, and then reduce the solid yields and lose surface OFGs. Besides, too many protons would impede the chemical reactions, such as surface complexation, ion exchange and precipitation, hampering the heavy metals storing capacity of ad ska et al., 2012). Therefore, the concentration of sorbents (Kołodyn the initial acidic catalyst should be optimized. 3.2. Effect of H3PO4 on adsorption mechanisms FTIR were carried out to get further insight into possible functional groups presented on the main structure of the as-prepared hydrochars before and after lead adsorption, as well as commercialized glucose for comparison, and the obtained spectra are shown in Fig. 2. Fig. 2(a) summarizes the FTIR spectra of the hydrochars obtained under heterogeneous operational conditions and the pure glucose. For H-10 to H-50, independently of various concentrations of phosphoric acid applied, all spectra were consisted of same IR bands, indicating that those five hydrochars had a similar chemical nature. However, the one obtained from fresh banana peels in pure water, exhibited a slightly different pattern. In acid media cases, strong and broad signal corresponding to the stretching vibration of hydroxyl groups at around 3450 cm1 were detected, which should be H-bonding hydroxyl groups from car-
225
boxyls, phenols and chemisorbed water. Bands assigned to eCH2 or CH3 groups in carboxylic acid located at approximately 2920 cm1 and bands represents the C]O (CeO) stretching vibration of carboxyl groups posited at about 1700 cm1 were also observed, all of which confirms the existence of large quantity of acidic oxygen containing functional groups (OFG), such as hydroxyl and carboxyl. It is reported that the surface OFGs are the key factors that significantly impact the chemical adsorption reactions like surface complexation and ion exchange (Rajapaksha et al., 2016). Thus, large quantity of surface OFGs can greatly improve the attracting property of the hydrochar adsorbents. The possibly reaction equations are as follow: eCOOH þ M2þ / eCOOMþ þ Hþ
(7)
eReOH þ M2þ / eReOMþ þ Hþ
(8)
where M is the heavy metal ions. However, the contents of OFGs had been largely reduced with the increasing of the phosphoric acid, possibly due to the over dehydration process undergone during the HTC with massive protons (Nizamuddin et al., 2017), implying that the adsorption capability might be weakened for hydrochars generated under highly acidic environment. In water media case, much stronger and broader peaks indexed to hydroxyl and carboxyl were found, confirming the existence of abundant acidic oxygen functional groups within the carbon structure, which is in agreement with the results of elements composition and atomic ratio. However, bands with sharp peaks corresponding to aromatic C]C groups were also been detected in the FTIR spectrum of H-0, indicating that the original organics might not be totally degraded and several aromatic carbon nucleus remained in the hydrochar construction. This assumption was proved by comparing the spectrum with the one of pure commercialized glucose, from which similar figure with parallel bands was detected, giving another evidence that FBP hydrochar generated in water had low carbonization degree and maintained large part of the saccharides. Thus, despite the hydrochar obtained in H2O medium had the highest amount of OFGs, those groups might be locked in the chain structure of the non-completely carbonized organic products and could not participate into the complexation or ion exchange reactions with lead ions, which was further evidenced by the FTIR spectra of Pb-loaded H-0 sample that gave almost the same size of peaks located in similar positions without obvious decrease or dismiss (Fig. 1(b)). On the contrast, as shown in Fig. 2 (b), peaks indexed to eOH and eCOOH groups had been greatly narrowed and shorted after the adsorption for all the H3PO4 catalyzed hydrochars, indicating that the ion exchange and surface complexation had largely taken part in the reactions between the hydrochars and metal ions. Wang et al. (2015). had investigated the adsorption mechanisms of lead ions on pyrolytic biochars derived from various feedstock and made a conclusion that precipitation with mineral fragments should be the dominant mechanism for the lead removal. To clarify the contribution of precipitation in the lead storing performance of the as-generated hydrochars herein, XRD experiments were carried out with 2q range of 10 e60 to determine the crystal structures of the samples before and after adsorption. Fig. 3 displays the XRD patterns of the HTC products with and without lead loading. As shown in Fig. 3(a), broad peaks around 22 that can be indexed to the disordered graphite (002) diffraction plane (Nautiyal et al., 2016) were depicted for acid catalyzed hydrochars, combined with no other obvious peaks corresponding to impurities, demonstrating that the charcoal products prepared from fresh banana peels via HTC process were basically amorphous carbon materials even with the catalytic activation of phosphoric acid. However,
226
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
Fig. 2. FTIR spectra of pure (left) and lead loaded (right) hydrochars.
Fig. 3. XRD patterns of pure (left) and lead loaded (right) hydrochars.
broad peaks corresponding to small molecular organic chemicals like ammonium acetate, ammonium tartrate were detected in the pattern of water medium sample H-0, certifying the noncompletely dehydration and degradation of the biomass feedstock, which is in agreement with the results obtained above. Small amount of calcium carbonates and magnesium phosphate were found in the as-obtained hydrochars, and those minerals disappeared after the adsorption of lead, further confirms the cationic exchange reactions between the hydrochar adsorbents and lead ion adsorbate. No notable shift of the C (002) peak was found after the lead adsorption, as shown in Fig. 3(b), suggesting that the adsorption process had no significant impact on the carbon structure of the hydrochars. Nevertheless, small peaks corresponding to hydrocerussite (Pb3(CO3)2(OH)2) and cerussite (PbCO3) were detected, indicating the formation of lead containing precipitations
during the adsorption procedure, from which it can be concluded that sedimentation reactions had participated between the hydrochars and the lead ions. However, the lead carbonates had poor crystal constructions according to the low peak intensities, demonstrating that precipitation should not be the main adsorption mechanism of the FBP derived hydrochars. To investigate the exactly adsorption mechanism of the FBP based hydrochars, the cations concentrations, the content of the acidic functional groups as well as the pH values of the metal ion solutions were analyzed before and after the adsorption. As presented in Table 2, all concentrations of Kþ, Naþ, Mg2þ and Ca2þ were increased for all six samples after lead loading, indicating that cationic ions had been released during the adsorption process. Besides, the pH values of the solutions and the amounts of acidic functional groups had been reduced after the adsorption, depicting
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
227
Table 2 Content of cationic ions and acidic functional groups of the pure and lead loaded hydrochars and the pH values of the solution before and after the adsorption. Biochar
Content of cationic ions (mmol/g) Kþ
H-0 H-0-Pb
D H-10 H-10-Pb
D H-20 H-20-Pb
D H-30 H-30-Pb
D H-40 H-40-Pb
D H-50 H-50-Pb
D
0.022 0.024 0.002 0.016 0.026 0.010 0.018 0.026 0.008 0.018 0.028 0.011 0.019 0.025 0.006 0.023 0.025 0.002
Naþ ± 0.0001 ± 0.0028 ± 0.0006 ± 0.0030 ± 0.0039 ± 0.0024 ± 0.0002 ± 0.0065 ± 0.0019 ± 0.0033 ± 0.0018 ± 0.0023
0.025 0.025 0.000 0.000 0.002 0.002 0.000 0.004 0.004 0.000 0.003 0.003 0.000 0.001 0.001 0.002 0.003 0.001
± 0.0032 ± 0.0042 ± 0.0025 ± 0.0012 ± 0.0011 ± 0.0018 ± 0.0001 ± 0.0020 ± 0.0004 ± 0.0001 ± 0.0001 ± 0.0043
Mg2þ
Ca2þ
0.005 ± 0.0011 0.012 ± 0.0016 0.007 0.006 ± 0.0001 0.023 ± 0.0019 0.017 0.006 ± 0.0007 0.0019 ± 0.0001 0.013 0.004 ± 0.0026 0.021 ± 0.0028 0.017 0.005 ± 0.0003 0.019 ± 0.0025 0.014 0.008 ± 0.0013 0.015 ± 0.0022 0.007
0.016 0.056 0.040 0.018 0.077 0.059 0.015 0.076 0.061 0.011 0.080 0.069 0.013 0.058 0.045 0.020 0.057 0.037
that enormous protons had been released from the surface acidic groups of hydrochars for heavy metal ions exchange. Thus, the dominant adsorption mechanism of lead on those hydrochars should be ion exchange and surface complexation. Among all six hydrochars, the one obtained in 30%wt phosphoric acid presented the largest amount of surface acidic functional groups. Simultaneously, highest alteration of the increased protons, decreased acidic groups, and exchanged cationic ions after lead loading had been detected for this sample, suggesting that high quantity of surface complexation and cationic exchange reactions had taken place on the surface of this hydrochar, which would greatly promote the adsorption performance. This assumption is in accordance with the results of adsorption experiments discussed in next paragraph. Fig. 4 represents the adsorption capacities of the as-prepared hydrochars by examining the reduced concentration of lead ions after the addition of 0.1 g hydrochar into 25 ml of 1000 mg L1 Pb(NO3)2 solution. As shown in Fig. 4, all hydrochar samples exhibited good storing capabilities toward lead ions. For the one synthesized in pure water (H-0), a high capacity of 136.4 mg g1 was detected, but even higher capacities were observed for hydrochars generated within phosphoric acid, possibly due to the abundant surface functional groups formed during the synthesis
Fig. 4. Adsorption capacities of Pb2þ on Hydrochars.
Content of acidic functional groups (mmol/g) ± 0.0011 ± 0.0042 ± 0.0006 ± 0.0008 ± 0.0002 ± 0.0027 ± 0.0056 ± 0.0094 ± 0.0018 ± 0.0084 ± 0.0055 ± 0.0099
2.638 1.976 0.662 2.562 1.928 0.634 2.638 1.452 1.186 2.867 1.452 1.415 2.638 1.566 1.072 2.562 1.376 1.186
± 0.32 ± 0.18 ± 0.12 ± 0.45 ± 0.24 ± 0.14 ± 0.36 ± 0.32 ± 0.48 ± 0.54 ± 0.22 ± 0.18
pH of the solution
6.29 ± 4.93 ± 1.36 6.19 ± 4.75 ± 1.44 6.16 ± 4.61 ± 1.55 6.17 ± 4.15 ± 2.02 6.13 ± 4.31 ± 1.82 5.98 ± 4.33 ± 1.65
0.03 0.02 0.01 0.01 0.03 0.01 0.03 0.02 0.02 0.02 0.02 0.01
process and the outstanding cationic exchange capabilities of the hydrochars. The storing amount of Pb2þ by those acidic hydrochars continuously increased with the rise of H3PO4 content, and then reached the maximum value of 241 mg g1 when 30%wt initial concentration of H3PO4 was employed. This value is much higher than other banana peel based adsorbents (Annadurai et al., 2003; Anwar et al., 2010; Castro et al., 2011) and HTC with phosphoric acid seems to be a promising way to transform banana peels into efficient hydrochar adsorbents. However, the adsorption performance of the hydrochars started to withdraw when higher volume of 40%wt and 50%wt phosphoric acid were applied, indicating that 30%wt might be the optimized concentration for effective hydrochar adsorbent production from fresh banana peels through hydrothermal carbonization process. This conclusion is in agreement with the assumption that summarized from the results of the aforementioned analysis, and the extraordinary performance of H30 should be attributed to the abundant surface functional groups, the enormous bonding cations, and part of mineral species. Previous studies on the mechanisms of the formation of biochar microspheres from polysaccharides, such as cellulose and semicellulose reported a nucleation growth process following the LaMer model (Sevilla and Fuertes, 2009; Sun and Li, 2004a). Basically, those polysaccharides will be hydrolyzed into monosaccharides and then soluble organic compounds, during which kinds of organic acids like acetic, lactic, propenoic, levulinic, and formic acids will be formed due to the decomposition of the monosaccharides. The large quantity of organic acids will then rapidly decrease the pH of the system and considerately accelerate the degradation reactions as catalysts. As a result, the concentrations of small pieces of aromatic clusters will increase, and when they reached the critical line, a burst nucleation process will take place to form a nucleus. Surface chemical species like reactive oxygen functionalities (hydroxyl, carbonyl, carboxylic, etc.) play important roles in the formation process of the carbon nuclei, thus abundant surface functional groups will be attached on the outer surface of the core-shell biochar microsphere structures. In the current case, the initial acidic environment brought by the phosphoric acid medium will excite the dehydration of the polysaccharides at the first place and then catalyze the decomposition during the whole process. Besides, the existence of proton can help the materialization of oxygen containing functional groups, which are the main force for the formation of core-shell carbon microsphere as demonstrated above. Finally, hydrochars with various
228
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
surface properties will be obtained, as illustrated in Fig. 5. However, phosphoric acid with different concentrations had different influences: lower concentration might not enough to effectively catalyze the hydrothermal reactions due to the huge amount of moisture in the fresh raw banana peels; while higher concentration might over catalyze the reaction and lead to the formation of soluble small molecular fragments and then causing oxygen and hydrogen lost. A concentration of 30%wt seems to be the optimized one with the current reaction conditions based on the analysis results discussed above. 3.3. Adsorption performance of H-30 To further understand the adsorption performance of the H-30, the adsorption behavior under the effects of sorbent dose, pH, initial ion concentration and contact time were investigated, results shown in Fig. 6. As one of the most affective parameters that effects the adsorption process, adsorbent dose is the key to determine the storage capacity of the adsorbate with a given initial concentration (Chen et al., 2011). Thus, the effect of sorbent dose on the removal of lead was first studied by adding gradient amounts of H-30 into Pb2þ solutions with an initial concentration of 400 mg L1. As displayed in Fig. 6(a), the adsorption capacity of H-30 decreased from 53.89 mg g1 to 5.74 mg g1 when the dosage increased from 0.01 g to 0.1 g, which is a normal situation happens to most of the adsorbents in former literature since settled amount of metal ions was employed. On the contrary, the remove efficiency of the hydrochar sample towards the heavy metal increased from 71.3% to 89.6% with the adding of adsorbent, demonstrating that the hydrochars obtained from fresh banana peels through hydrothermal carbonization with the phosphoric acid medium are effective adsorbents for heavy metals' pollution treatment. The removal efficiency reached the equilibrium of about 90% with the adsorbent amount of 0.1 g, thus this content was settled for the rest adsorption studies of effects from pH, metal concentration and reaction time. Since the adsorption mechanism of the FBP derived hydrochars are mainly surface complexation and cationic exchange combined with part of precipitation, the adsorption property should be strongly affected by the initial pH of the metal solutions, due to which the degree of ionization, the surface charge of the adsorbent and especially the speciation of the heavy metals are dominated ska et al., 2012). The pH study was carried out by adjusting (Kołodyn the proton concentration with HCl and NaOH before adding the H30 biochar, and the range was set between 2 and 7 since Pb2þ ions started to precipitate with the hydroxyl ions when the pH value came larger than 7. As shown in Fig. 6(b), nearly no uptake of lead ions was found at pH 2, and low removal efficiency of 30% was observed for pH 3, giving the information that weak adsorption behavior of H-30 at lesser pH, which was also reported in previous
literature (Lu et al., 2012). The removal efficiency jumped into 63% with one point adding of pH and quickly achieved the equilibrium of 92% when the pH reached 6. The pH research results demonstrated that large quantity of proton could affect the adsorption significantly, indicating the Hþ ions can compete for the surface active sites on the hydrochars and impede the formation of compounds of lead simultaneously, confirming that the adsorption mechanism of the FBP based hydrochar should be chemical reactions like surface complexation, ion exchange and precipitation. The influence of contact time on the adsorption was also studied by mixing 0.1 g hydrochar sample with Pb2þ solution and two kinetic models, Pseudo-first-order (PF-order) and Pseudo-seconderorder (PS-order), were utilized to analysis the kinetics quantitatively. As illustrated in Fig. 6(c), typical kinetic curve was obtained for the sample H30, which rapidly increased at the initial stage of about 30 min and then arrived equilibrium within 3 h. The fast adsorption ability of this sample indicates that the hydrochar derived from fresh banana peels with 30%wt phosphoric acid as the HTC reaction medium can be a designable adsorbent for heavy metals removal. Based on the experimental data, the fitting line according to the PF-order and PS-order were obtained and the corresponding parameters were calculated, as presented in Table 3. Apparently, the values of the regression coefficient (R2) calculated based on the PS-order kinetic model (R2 ¼ 0.909) is larger than the one that obtained from the PF-order kinetic model (R2 ¼ 0.828), demonstrating that the adsorption data fitted the PS-order model better than the PF-order model (Table 3). Previous literature confirms that PF-order model relies on the hypothesis that the diffusion of adsorbate dominants the adsorption velocity, while PSorder model depends on the assumption that chemical sorption such as bonding forces through sharing or ion/electrons exchange between adsorbate and adsorbent limits the step rate (Hydari et al., 2012). The fitting result indicated that rate-limiting step is the chemical sorption between the adsorbate and the adsorbent, further granted that the fasten behavior of FBP derived hydrochars toward lead ions were chemical sorption processes. Fig. 6(d) demonstrates the results of isotherm adsorption experiments, which were carried out in ion solutions at different initial concentrations ranging from 5 to 1000 mg L1. It can be seen that the binding capacity of sample H-30 boosted considerably with the enlarging of the metal ion amounts in the initial stage, and then the speed of the growth slowed down and became moderately with continuous rising of the metal concentration. No obvious equilibrium was observed within the analysis scope, implying that even better adsorption capacity could be obtained when higher content of lead ions was applied, which gave another evidence of the outstanding adsorption performance of FBP based hydrochars. Typical models of Langmuir and Freundlich were utilized to evaluate the adsorption isotherm quantitatively, based on which the
Fig. 5. Schematic illustration of the HTC process of the hydrochars.
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
229
Fig. 6. Effects of parameters on Pb2þ removal performance of H-30: (a) adsorbent dosage, (b) initial pH, (c) contact time, (d) metal concentration, and the fitting lines obtained according to corresponding equations.
linear forms were fitted and the correlation coefficients (R2) and constants were calculated, as presented in Table 3. For Langmuir isotherm model, the correlation coefficient R2 is 0.925, which is to some extent lower than that obtained from the Freundlich model (R2 ¼ 0.962). Apparently, the Fruendlich fits the isotherm data better than the Langmuir model dose, illustrating that the attracted lead ions on the surface of hydrochar H-30 should be multilayer adsorption, which is in accordance with the conclusion that the adsorption mechanism of FBP derived hydrochars should be the combination functionality of surface acidic groups complexation, cationic exchange and few amount of precipitation. Based on the Langmuir constant KL and the initial concentration
Table 3 Kinetic and isotherm parameters of Pb2þ on the H-30 biochars. Models
Parameters
H30
Pseudo-first-order
k1 (min1) Qe,cal (mg$g1) R2 k1 (g$mg1$min1) Qe,cal (mg$g1) R2 Qe,cal (mg$g1) KL (L$mg1) R2 n KF (L$mg1) R2
17.812 ± 4.60 14.799 ± 0.57 0.828 1.970 ± 0.47 15.010 ± 0.47 0.909 356.17 ± 48.89 0.004 ± 0.001 0.925 2.075 ± 0.22 10.856 ± 3.50 0.962
Pseudo-second-order
Langmuir
Freundlich
of heavy metal ions C0 (mg$L1), the dimensionless separation parameter RL can be calculated according to the following equation (Amosa, 2016; Gorgievski et al., 2013):
RL ¼ 1=ð1 þ KL C0 Þ
(9)
RL is usually employed to describe the characteristics of the adsorption isotherm of the Langmuir model, according to which whether the sorption process is a thermodynamically favorable process or not can be determined (Amosa et al., 2016). Specifically, the condition is unfavorable when RL > 1; linear conditions prevail when RL ¼ 1; the condition is favorable when 1 > RL > 0; and the reaction is irreversible when RL ¼ 0. In our case, the value of KL is 0.004, from which the values of RL can be determined in the range of 0e1, implying that the Pb2þ adsorption on FBP derived hydrochars are thermodynamically favorable processes. Besides, the value of RL is negatively correlated with the initial concentration of Pb2þ, suggesting that the adsorption capacity can be further enhanced, which is evidenced by the calculated Qmax of 356 mg g1 (given in Table 3).
4. Conclusion Fresh banana peels were employed as biomass feedstock to prepare biochars via hydrothermal carbonization within phosphoric acid of various concentrations. The initial amount of the H3PO4 had a significantly influence on the surface properties including carbonization degrees, pH values, contents of surface
230
N. Zhou et al. / Journal of Cleaner Production 165 (2017) 221e230
acidic functional groups of the as-prepared hydrochars, and then further impacted the adsorption mechanism of lead ions on those hydrochars. Lower content of H3PO4 was not enough to promote the degradation to form surface OFGs, while higher content of H3PO4 might cause the lose of hydrogen and oxygen. Hydrochar generated in 30%wt H3PO4 exhibited the best adsorption property among all six samples, possibly due to the largest quantity of the acidic functional groups formed on the surface of the carbon with appropriate catalyzation. The adsorption behavior of those hydrochars were confirmed to be the combination functionality of surface complexation, cationic exchange and part of precipitation. It can be concluded that hydrochars derived from fresh banana peel via hydrothermal carbonization catalyzed by phosphoric acid could be excellent adsorbents for lead removal in aqueous environments. Acknowledgement The authors would like to thank financial support from China Postdoctoral Science Foundation (2016M592427), Chinese National Natural Science Foundation (21402063), Natural Science Foundation of Hunan Province (2016JJ3065, 2015JJ6091), Hunan Agricultural University (13YJ02, 14YJ05) and Hunan Provincial Key Laboratory for Germplasm Innovation and Utilization of Crop (15KFXM20). References Amosa, M.K., 2016. Sorption of water alkalinity and hardness from high-strength wastewater on bifunctional activated carbon: process optimization, kinetics and equilibrium studies. Environ. Technol. 37, 2016e2039. Amosa, M.K., Jami, M.S., Alkhatib, M.A.F.R., 2016. Electrostatic biosorption of COD, Mn and H2S on EFB-based activated carbon produced through steam pyrolysis: an analysis based on surface chemistry, equilibria and kinetics. Waste Biomass Valoriz. 7, 109e124. Annadurai, G., Juang, R.S., Lee, D.J., 2003. Adsorption of heavy metals from water using banana and orange peels. Water Sci. Technol. 47, 185e190. Anwar, J., Shafique, U., Waheed-uz-Zaman, Salman, M., Dar, A., Anwar, S., 2010. Removal of Pb(II) and Cd(II) from water by adsorption on peels of banana. Bioresour. Technol. 101, 1752e1755. Awasthi, M.K., Wang, M., Chen, H., Wang, Q., Zhao, J., Ren, X., Li, D.-s., Awasthi, S.K., Shen, F., Li, R., Zhang, Z., 2017. Heterogeneity of biochar amendment to improve the carbon and nitrogen sequestration through reduce the greenhouse gases emissions during sewage sludge composting. Bioresour. Technol. 224, 428e438. Brunner, G., 2009. Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processes. J. Supercrit. Fluids 47, 373e381. Castro, R.S.D., Caetano, L., Ferreira, G., Padilha, P.M., Saeki, M.J., Zara, L.F., Martines, M.A.U., Castro, G.R., 2011. Banana peel applied to the solid phase extraction of copper and lead from river water: preconcentration of metal ions with a fruit waste. Ind. Eng. Chem. Res. 50, 3446e3451. Chen, X., Chen, G., Chen, L., Chen, Y., Lehmann, J., McBride, M.B., Hay, A.G., 2011. Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution. Bioresour. Technol. 102, 8877e8884. Elliott, D.C., 2011. Hydrothermal Processing, Thermochemical Processing of Biomass. John Wiley & Sons, Ltd, pp. 200e231. n-García, E., Pe rez-Cadenas, A.F., Maldonado-Ho dar, F.J., Elmouwahidi, A., Bailo Carrasco-Marín, F., 2017. Activated carbons from KOH and H3PO4-activation of olive residues and its application as supercapacitor electrodes. Electrochim. Acta 229, 219e228. Fernandez, M.E., Ledesma, B., Rom an, S., Bonelli, P.R., Cukierman, A.L., 2015. Development and characterization of activated hydrochars from orange peels as potential adsorbents for emerging organic contaminants. Bioresour. Technol. 183, 221e228. Flora, J.F.R., Lu, X., Li, L., Flora, J.R.V., Berge, N.D., 2013. The effects of alkalinity and acidity of process water and hydrochar washing on the adsorption of atrazine on hydrothermally produced hydrochar. Chemosphere 93, 1989e1996. Funke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels, Bioprod. Biorefining 4, 160e177. Gorgievski, M., Bo zi c, D., Stankovi c, V., Strbac, N., Serbula, S., 2013. Kinetics, equilibrium and mechanism of Cu2þ, Ni2þ and Zn2þ ions biosorption using wheat straw. Ecol. Eng. 58, 113e122. Hydari, S., Sharififard, H., Nabavinia, M., Parvizi, M.R., 2012. A comparative
investigation on removal performances of commercial activated carbon, chitosan biosorbent and chitosan/activated carbon composite for cadmium. Chem. Eng. J. 193e194, 276e282. Kambo, H.S., Dutta, A., 2015. A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications. Renew. Sustain. Energy Rev. 45, 359e378. Klasson, K.T., Uchimiya, M., Lima, I.M., 2014. Uncovering surface area and micropores in almond shell biochars by rainwater wash. Chemosphere 111, 129e134. ska, D., Krukowska, J., Thomas, P., 2017. Comparison of sorption and Kołodyn desorption studies of heavy metal ions from biochar and commercial active carbon. Chem. Eng. J. 307, 353e363. ska, D., Wne˛ trzak, R., Leahy, J.J., Hayes, M.H.B., Kwapin ski, W., Hubicki, Z., Kołodyn 2012. Kinetic and adsorptive characterization of biochar in metal ions removal. Chem. Eng. J. 197, 295e305. Li, J., Li, Y., Wu, Y., Zheng, M., 2014. A comparison of biochars from lignin, cellulose and wood as the sorbent to an aromatic pollutant. J. Hazard. Mater. 280, 450e457. Lu, H., Zhang, W., Yang, Y., Huang, X., Wang, S., Qiu, R., 2012. Relative distribution of Pb2þ sorption mechanisms by sludge-derived biochar. Water Res. 46, 854e862. Lu, X., Flora, J.R.V., Berge, N.D., 2014. Influence of process water quality on hydrothermal carbonization of cellulose. Bioresour. Technol. 154, 229e239. Lynam, J.G., Coronella, C.J., Yan, W., Reza, M.T., Vasquez, V.R., 2011. Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresour. Technol. 102, 6192e6199. Lynam, J.G., Toufiq Reza, M., Vasquez, V.R., Coronella, C.J., 2012. Effect of salt addition on hydrothermal carbonization of lignocellulosic biomass. Fuel 99, 271e273. Nautiyal, P., Subramanian, K.A., Dastidar, M.G., 2016. Adsorptive removal of dye using biochar derived from residual algae after in-situ transesterification: alternate use of waste of biodiesel industry. J. Environ. Manag. 182, 187e197. Nizamuddin, S., Baloch, H.A., Griffin, G.J., Mubarak, N.M., Bhutto, A.W., Abro, R., Mazari, S.A., Ali, B.S., 2017. An overview of effect of process parameters on hydrothermal carbonization of biomass. Renew. Sustain. Energy Rev. 73, 1289e1299. Nizamuddin, S., Mubarak, N.M., Tiripathi, M., Jayakumar, N.S., Sahu, J.N., Ganesan, P., 2016. Chemical, dielectric and structural characterization of optimized hydrochar produced from hydrothermal carbonization of palm shell. Fuel 163, 88e97. Pellera, F.-M., Giannis, A., Kalderis, D., Anastasiadou, K., Stegmann, R., Wang, J.-Y., Gidarakos, E., 2012. Adsorption of Cu(II) ions from aqueous solutions on biochars prepared from agricultural by-products. J. Environ. Manag. 96, 35e42. Prahas, D., Kartika, Y., Indraswati, N., Ismadji, S., 2008. Activated carbon from jackfruit peel waste by H3PO4 chemical activation: pore structure and surface chemistry characterization. Chem. Eng. J. 140, 32e42. Rajapaksha, A.U., Chen, S.S., Tsang, D.C.W., Zhang, M., Vithanage, M., Mandal, S., Gao, B., Bolan, N.S., Ok, Y.S., 2016. Engineered/designer biochar for contaminant removal/immobilization from soil and water: potential and implication of biochar modification. Chemosphere 148, 276e291. Sevilla, M., Fuertes, A.B., 2009. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem. e A Eur. J. 15, 4195e4203. Sun, X., Li, Y., 2004a. Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angew. Chem. 116, 607e611. Sun, X.M., Li, Y.D., 2004b. Colloidal carbon spheres and their core/shell structures with noble-metal nanoparticles. Angew. Chem. Int. Ed. 43, 597e601. Sun, Y., Li, H., Li, G., Gao, B., Yue, Q., Li, X., 2016. Characterization and ciprofloxacin adsorption properties of activated carbons prepared from biomass wastes by H3PO4 activation. Bioresour. Technol. 217, 239e244. Tan, X.-f., Liu, S.-b., Liu, Y.-g., Gu, Y.-l., Zeng, G.-m., Hu, X.-j., Wang, X., Liu, S.-h., Jiang, L.-h., 2017. Biochar as potential sustainable precursors for activated carbon production: multiple applications in environmental protection and energy storage. Bioresour. Technol. 227, 359e372. Titirici, M.M., Thomas, A., Yu, S.-H., Müller, J.-O., Antonietti, M., 2007. A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chem. Mater. 19, 4205e4212. Tong, X.-j., Li, J.-y., Yuan, J.-h., Xu, R.-k., 2011. Adsorption of Cu(II) by biochars generated from three crop straws. Chem. Eng. J. 172, 828e834. Vernersson, T., Bonelli, P.R., Cerrella, E.G., Cukierman, A.L., 2002. Arundo donax cane as a precursor for activated carbons preparation by phosphoric acid activation. Bioresour. Technol. 83, 95e104. Wang, Z., Liu, G., Zheng, H., Li, F., Ngo, H.H., Guo, W., Liu, C., Chen, L., Xing, B., 2015. Investigating the mechanisms of biochar's removal of lead from solution. Bioresour. Technol. 177, 308e317. Xue, Y., Gao, B., Yao, Y., Inyang, M., Zhang, M., Zimmerman, A.R., Ro, K.S., 2012. Hydrogen peroxide modification enhances the ability of biochar (hydrochar) produced from hydrothermal carbonization of peanut hull to remove aqueous heavy metals: batch and column tests. Chem. Eng. J. 200e202, 673e680. Zhou, N., Chen, H., Xi, J., Yao, D., Zhou, Z., Tian, Y., Lu, X., 2017. Biochars with excellent Pb(II) adsorption property produced from fresh and dehydrated banana peels via hydrothermal carbonization. Bioresour. Technol. 232, 204e210.