Qualitative and quantitative correlation of physicochemical characteristics and lead sorption behaviors of crop residue-derived chars

Accepted Manuscript Qualitative and quantitative correlation of physicochemical characteristics and lead sorption behaviors of crop residue-derived chars Yanfei Li, Xian Liu, Peizhen Zhang, Xinlei Wang, Yaoyao Cao, Lujia Han PII: DOI: Reference:

S0960-8524(18)31331-2 https://doi.org/10.1016/j.biortech.2018.09.078 BITE 20494

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

5 August 2018 12 September 2018 14 September 2018

Please cite this article as: Li, Y., Liu, X., Zhang, P., Wang, X., Cao, Y., Han, L., Qualitative and quantitative correlation of physicochemical characteristics and lead sorption behaviors of crop residue-derived chars, Bioresource Technology (2018), doi: https://doi.org/10.1016/j.biortech.2018.09.078

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Qualitative and quantitative correlation of physicochemical characteristics and lead sorption behaviors of crop residue-derived chars Yanfei Li, Xian Liu, Peizhen Zhang, Xinlei Wang, Yaoyao Cao, Lujia Han Laboratory of Biomass and Bioprocessing Engineering, College of Engineering, China Agricultural University, Box 191, Beijing 100083, China



Corresponding author. Tel.: +86 10 6273 6313; Fax: 86 10 6273 6778. E-mail: [email protected]

Abstract This study investigated the key physicochemical characteristics of char that control its ability to absorb Pb2+. Three type of crop residuederived chars and their ball milled powder were characterized using multiple approaches. The Pb2+ sorption mechanisms of biochar were caused mainly by coprecipitation reactions, which were governed by ionic minerals on chars instead of mineral crystallization (e.g., SiO2 and Al2O3), while coprecipitation reactions and π electronic interaction were the dominant mechanisms of activated carbon. Pearson analysis showed that adsorption quantity (Q) highly correlated with the cation exchange capacity (CEC) (P＜0.01)/oxygen functional groups (OFGs) (P＜0.05) and Q closely correlated with coprecipitation amount (P＜0.01)/complexation amount (P＜0.01). Linear regression equations of sorption amount and CEC (R2＞0.8)/OFGs (R2＞0.7) were established. CEC and OFGs of chars are the key factors controlled Pb2+ sorption. These results may promote the development of low-cost, engineered biochar with superior sorption qualities for environmental remediation. Keywords: biochar and activated carbon, adsorption mechanisms, qualitative and quantitative characterization, heavy metal, ball mill

1. Introduction Lead is a heavy metal pollutant in the water that requires immediate remediation because of its high toxicity, cumulative effects, and inability to be degraded(Ifthikar et al., 2017). Thus far, numerous technologies were applied for lead elimination, such as membrane separation(Doke & Yadav, 2014), ion exchange(Cavaco et al., 2007), chemical precipitation(Kurniawan et al., 2006) and adsorption(Mohan & Pittman, 2006). Of them, adsorption is considered an effective, simple and economical method for eliminating heavy metal ions(Kołodyńska et al., 2017). Therefore, diverse materials have been designed and used as adsorbents to remove heavy metal ions from wastewater in both academic research and environmental applications. Biochar is a carbon-rich solid product of biomass decomposed under heating in oxygen-limiting conditions, in a process known as pyrolysis(Oliveira et al., 2017a). Recently, the application of chars in the field of environmental remediation include pollutants removal(Teng et al., 2010; Xie et al., 2016), carbon sequestration, and soil amelioration(Gong et al., 2017a; Gong et al., 2018). Various methods and chars with application in removing heavy metals report in previous studies(Gong et al., 2017b; Suguihiro et al., 2013). It is considered to be a highly competitive adsorbent due to its functional groups, aromatic structure, surface area and porosity, elemental composition, and varied and inexpensive raw materials(Zama et al., 2017) (Oliveira et al., 2017b; Wei et al., 2018). Previous work showed that heavy metals can be adsorbed

by different mechanisms, including physical adsorption, electrostatic attraction, ion exchange, chemical precipitation, complexation reactions and electronic interactions (Inyang et al., 2016; Li et al., 2017). Lu et al.(Lu et al., 2012) reported that the Pb2+ sorption of sludge-derived biochar primarily involved about 40% coordination with organic hydroxyl and carboxyl functional groups, as well as about 60% coprecipitation or complex on mineral surfaces. Wang et al.(Wang et al., 2015) indicated that the dominant Pb2+ adsorption mechanism of peanut shell biochar and medicine residue biochar was precipitation. Additionally, the Pb2+–π interaction was strengthened, whereas the contribution of complexation was reduced with increasing pyrolysis temperature. Zhang et al.(Zhang et al., 2017) found that increased ash and specific surface area of phyllostachys pubescens biochars promoting heavy metal precipitation and complexation, while the contribution of each mechanism varied with increasing oxygen content at a low pyrolysis temperature. The above results showed that the adsorption capacity and mechanisms of Pb2+ on biochar vary among biochar produced from different feedstocks at different carbonization conditions due to its different properties. Although many studies have quantitative analysis of dominant mechanism of different biochar, a quantitative correlation analysis based on the characteristic properties and the proportion of different adsorption mechanisms were still not clear. This study focused on exploring the internal relation of physicochemical properties (e.g., microstructure, elemental composition, functional groups) and its sorption capacity. Identifying these correlations would be beneficial for a deeper understanding of how these surface properties change the biochar sorption behavior and and

how they can be optimized to enhance Pb2+ sorption. Ball milling is an optimization method that has recently been introduced into the char research field to improve its adsorption capacity (Lyu et al., 2017; Richard et al., 2016; Soares et al., 2015). This mechanical process can effectively break chemical bonding and produce fresh surfaces by fracturing material particles without changing the chemical composition. Lyu et al.(Lyu et al., 2018) prepared ball milled sugarcane bagasse biochar, which showed greater methylene blue removal capacity in water compared with unmilled biochar. Although these results are encouraging, further work is needed to explore how the variation in characteristics of ball milled chars causes changes in sorption enhancement, which can also guide the production of engineered biochars with high sorption capacities. Crop residues are easily available in large quantities. They have widespread distribution at low cost, are easy to procure and are frequently renewed. This would serve as a potential material for the production of biochar. In this work, the microstructure and chemical properties of wheat straw biochar, rice husk biochar, coconut shell activated carbon and their ball milled powders were characterized using multiple approaches. An elution test by acid was used to quantify the different formations of Pb2+ on chars, and X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) were used to qualitatively investigate the properties of chars that participate its Pb2+ adsorption. Qualitative and quantitative correlation analyses of the physicochemical properties of chars and their

proportion of Pb2+ adsorption were used to identify the key properties that control its sorption and determine how to manipulate these properties for better sorption applications of chars. 2. Materials and methods 2.1. Preparation of chars Activated carbon: Activated carbon produced from coconut shell was obtained from Xinzhiyuan Activated Carbon Co., Ltd. (Henan, China), labeled CSAC. Biochar: The crop residues of wheat straw and rice husk collected from Anhui Province and Beijing (China), respectively, were used as feedstocks for biochar. The preliminary experiments results showed that the maximal Pb2+ adsorption capacity of crop residue-derived biochar was obtained under 600 C pyrolysis temperature. Therefore, biochars were prepared by pyrolyzing the dried feedstocks in a lab-scale quartz tube furnace (GSL-1100X, Hefei Kejing, China) under N2 at a peak temperature of 600 C for 1 h with a heating rate of 20 C·min-1. These biochars were cooled to room temperature under predetermined atmospheric conditions and then ground with a ZM200 (Retsch, Haan, Germany) to pass through 20 mesh sieves. The biochars derived from wheat straw and rice husk were denoted WSBC and RHBC, respectively. Ball-milled biochar and activated carbon: The ball milled chars were produced by using a CJM-SY-B vibration ball miller (Qinhuangdao

Taiji Ring Nano Ltd., Hebei, China) with a circulating cooling system to maintain the milling process below 30 C. The 150 g chars and 1500 g ZrO2 balls (6–10 mm diameter) were mixed in a 2 L zirconia tank for an optimized ball milling time of 20 min. The ball milled powders were stored in sealed bags, denoted BMWSBC, BMRHBC and BMCSAC. 2.2. Char characterization 2.2.1 Chemical composition The C, H, N and S contents were determined using the Elementar Vario EL II (Vario Macro, Germany). The ash content was measured after burning the chars to a constant weight in a muffle furnace at 575 ± 25 C for 6 h. The ash composition was analyzed using an X-ray fluorescence spectrum (XRF, ARL ADCANT XP +, Thermo, Germany). According to ASTM standard E870-82, the O content was calculated by the subtraction method as a percentage of weight. It is the resultant of the summation of percentages carbon, hydrogen, sulfur, nitrogen, and ash subtracted from 100. (Telmo et al., 2010). 2.2.2 Structural characterization The specific surface area (SSA), pore volume (PV), micropore volume and pore diameter of chars were measured using a multipurpose Micromeritics Tristar II apparatus (Micromeritics, USA) after degassing at 105 C for 4 h. The SSA was calculated according to the Brunauer-

Emmett-Teller (BET) theory using adsorption data. The PVs were calculated according to the Barrett-Joyner-Halenda (BJH) theory using desorption isotherm leg data. 2.2.3 Properties related to adsorption The pH values were measured at a ratio of 1.0 g of char in 20 mL of deionized water after equilibrium for 24 h using an FE20 pH meter (Mettler Toledo, USA). Boehm's titration method (Boehm, 1994) was used to quantify OFGs as follows: 0.5 g of char was added into three different vials containing 25 mL of 0.02 M NaOH, Na2CO3, or NaHCO3. All vials were shaken at 160 rpm at 25 C for 24 h. After shaking, 20 mL of filtrates were collected and titrated with 0.02 M HCl. The content of carboxyl, lactones, and phenolic hydroxyl groups was calculated according to the consumption of HCl. The cation exchange capacity (CEC) was determined experimentally by mixing 1 g of samples with 50 mL of 0.01 M NH4Cl solution (Shen et al., 2015). The mixture was shaken at 300 rpm for 24 h before filtration through a 0.45 µm filter. The obtained concentrations of Na+, K+, Ca2+, Mg2+, Al3+ and Fe2+ in the filtrate were tested by inductively coupled plasma mass spectrometry (ICPMS, NexION 300, PE, USA). The CEC was calculated as the sum of the concentrations of the measured anions. XRD (XD3, Beijing Purkinje General Instrument Ltd., China) was primarily used for identifying the crystalline constituents of chars before and after Pb2+ sorption. A Cu Kα radiation source at 36 kV and 30 mA was used with scans at 2θ in the range 10°‒70° at a rate of 2°/min in 0.02° increments. Fourier transform

infrared spectroscopy (FTIR, Spectrum 400, Perkin Elmer, USA) was used to determine OFGs on the surfaces of chars before and after Pb2+ sorption. The FTIR data were recorded in the 4000–400 cm−1 region using KBr pellets with 64 scans at 4.0 cm−1 resolution. 2.3. Batch sorption experiment Pb2+ standard solution (10 g/L) was prepared by dissolving the analytical reagent lead nitrate (Pb(NO3)2), which contained 0.01 M NaNO3 as a background electrolyte, and diluting to 50-500 mg/L. Solution pH will influence the surface charge of the chars and the existing form of Pb2+. H+ and H3O+ will compete with Pb2+ for adsorption sites on the surface of chars with the lower pH. When the pH value exceeded the upper limit of the microprecipitation of Pb2+, most of the Pb2+ in the liquid formed insoluble metal oxides or ammonia oxide which leaded to the inability to adsorb. Therefore, based on the previous results, the sorption amount reached the maximum when pH value was around 5 (Lu et al., 2012; Tan et al., 2018). The pH of all tested solutions was adjusted to 5±0.05 by adding 0.1 M HNO3 or NaOH. A total of 50 mg chars were added into vials containing 50 mL of 100 mg/L Pb2+ solution. All vials were shaken at 160 rpm at 25 C for 24 h. For sorption kinetics, different vials containing 100 mg/L Pb2+ solutions were shaken for different times (0.5 h, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 8 h, 16 h, and 24 h). For the sorption isotherms, the initial Pb2+ concentration was 50 mg/L, 100 mg/L, 150 mg/L, 200 mg/L, 300 mg/L, 400 mg/L, and 500 mg/L. When the adsorption experiment was completed, the filtrates were collected with a disposable needle tube through 0.45 μm filters (within 3 seconds). The

concentration of Pb2+ in the filtrate was detected by using an atomic absorption spectrometer (a Vario 6 series, Analytik Jena, Germany). The detection limit was 10µg/L” 2.4. Statistical analysis of the sorption experiment data The adsorption capacity Q (mg/g) was calculated according to the following equation: Q=

(𝐶0 −𝐶𝑒 )∗𝑉

(1)

𝑚

where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium Pb2+ concentrations, respectively, V (mL) is the volume of the solution and m (g) is the weight of chars. The following two kinetic models were used to fit the adsorption kinetic data of Pb2+ sorption on chars. Pseudo-first-order kinetic model 𝑞𝑡 = 𝑞𝑒 (1 − 𝑒 −𝑘1 𝑡 ) Pseudo-second-order kinetic model

(2)

𝑘 𝑞2 𝑡

𝑞𝑡 = 1+𝑘2 𝑒𝑞

2 𝑒𝑡

(3)

where 𝑞𝑒 (mg/g) is the equilibrium adsorption amount, 𝑞𝑡 (mg/g) is the adsorption amount at time t, 𝑘1 (h-1) is the pseudo-first-order adsorption rate constant, and 𝑘2 (g/(mg·h)) is the pseudo-second-order adsorption rate constant. Langmuir (Eq.(4)) and Freundlich (Eq.(5)) models were used to fit the adsorption isotherm data of Pb2+ sorption on chars. Langmuir model 𝑄𝑒 =

𝐾𝐿 𝑄𝑚 𝐶𝑒 1+𝐾𝐿 𝐶𝑒

(4)

Freundlich model 1

𝑄𝑒 = 𝐾𝐹 𝐶𝑒𝑛

(5)

where 𝑄𝑒 (mg /g) is the amount of adsorbed Pb2+ at equilibrium, 𝑄𝑚 (mg /g) is the maximum absorption capacity, 𝐶𝑒 (mg /L) is the Pb2+ concentration in solution at equilibrium, 𝐾𝐿 (L /mg) is the adsorption coefficient of the Langmuir model, which indicated the extent of affinity

between the adsorbent and adsorbate, n is the Freundlich parameter related to the adsorption intensity, and 𝐾𝐹 (mg /g) is the adsorption coefficient of the Freundlich model, which indicated the adsorption capacity. 2.5. Quantitative analysis of different mechanisms for Pb2+ sorption The contribution of different adsorption mechanisms for Pb2+ on chars can be calculated according to relative previous studies (Ho et al., 2017; Wang et al., 2015; Zhang et al., 2017). In this study, the adsorption amount of Pb2+ includes physical adsorption (Qp), metal ion exchange (Qi), mineral precipitation (Qm), OFGs complexation (Qc) and electronic interactions of π (Qπ). Q is the total amount of Pb2+ sorption on chars, which can be calculated as: Q=Qp+Qi+Qm+Qc+Qπ

(6)

The amount of different adsorption mechanisms can be calculated by using the following steps: Step 1: A total of 100 mg of sample after Pb2+ adsorption was mixed with 20 mL of deionized water and shaken for 24 h at 160 rpm. Physical adsorption refers to the removal of Pb2+ by diffusional movement of Pb2+ to char pores without formation of chemical bonds(Inyang et al., 2016). The adsorption process occurred in aqueous solution and some dissoluble inorganic compound was hardly immobilized onto chars

after Pb2+ sorption. Thus, the dissoluble inorganic ions were not affected by deionized water washing process. The amount of Pb2+ washed by deionized water contributes to the physical adsorption mechanism, where Qp can be obtained. Step 2: A 1 M HCl solution was used to demineralize the sample after step 1. In this acid-wash process, the minerals containing Pb formed by ion exchange (Qi) and precipitation (Qm) were removed. The contribution of OFGs was not included. The adsorption amount of Pb2+ after step 2 is defined as Qr, where Qi and Qm can be described as: Qi+Qm=Q-Qp-Qr

(7)

Step 3: The quantitation limit of inductively coupled plasma mass spectrometry (ICP-MS, NexION 300, PE, USA) for Na+, K+, Ca2+, Mg2+ was 50µg/L, 5µg/L, 5µg/L and 5µg/L, respectively. The content of Na+, K+, Ca2+, Mg2+ in dissolving liquid was above 120.8µg/L, 117.2µg/L, 94.7µg/L and 38.4µg/L, respectively. This metal ions can be detected by ICP-MS (Zhou et al., 2018). Thus, the total concentration of this metal ions displaced by ion exchange after adsorption can be calculated, where Qi can be obtained and Qm is calculated as: Qm=Q-Qp-Qr-Qi

(8)

Step 4: The adsorption amount of OFGs complexation (Qc) was calculated by a decrease in the pH before and after step 2 based on the following reaction modes:

COOH+Pb2++H2O→ –COOPb++H3O+

–OH+Pb2++H2O→–OPb++ H3O+

(9)

(10)

and Qπ is calculated as: Qπ=Qr-Qc

(11)

2.6. Statistical analysis The data in the tables and figures were shown as the replicated mean ± standard deviation. The analysis of variance (ANOVA) based on Duncan’s multiple comparison test and the correlation between the physicochemical properties of chars and adsorption mechanisms based on Pearson’s method were obtained by using SPSS 20. 3. Results and discussion 3.1 Comparison of physiochemical properties of different chars Table 1 shows the physiochemical properties of different biochars and activated carbon 3.1.1 Element composition

In terms of element composition, Table 1 shows that the content of C element in WSBC and CSAC was higher than in RHBC, and there was no significant difference in the content of C element between WSBC and CSAC. The content of H element followed the order WSBC > RHBC > CSAC. Both the H/C values of WSBC and RHBC were higher than that of CSAC, and there was no significant difference between the H/C values of WSBC and RHBC. This suggested that CSAC contained more unsaturated C and its degree of aromatization was higher than that of WSBC and RHBC. The order of O/C is WSBC > CSAC > RHBC, indicating that WSBC may contain more OFGs and has strong polarity. 3.1.2 Ash content and ash composition Among these chars, RHBC had the highest ash content, which was 43.82% and much higher than WSBC and CSAC. The ash content of WSBC and CSAC was approximately 25%, and there was no significant difference between the two. The ash composition of different chars varied. The main components of RHBC ash was SiO2, with a content of 92.78%; the content of SiO2 in the CSAC ash was 61.02%, and the content of SiO2 in the WSAC ash was 37.10%. The content of K2O in the WSBC ash was 33.18%, much higher than that in RHBC and CSAC. The content of Cl, Ca, S, Mg, P and Na elements in the WSBC ash were all higher than that in the RHBC and CSAC. The content of Al and Fe elements in the CSAC ash were all higher than that in the WSBC and RHBC. 3.1.3 Microstructures

Table 1 showed that compared with WSBC and RHBC, CSAC exhibited a high specific surface area (SSA) and good porous structure. The SSA of CSAC was 458.64 m2/g, which is approximately 32 times that of WSBC and 2.3 times that of RHBC. The pore volume (PV) of CSAC was 0.227 cm3/g, which is approximately 23 times that of WSBC and 2.4 times of RHBC. The pore diameter of CSAC was 3.198 nm, which is approximately 1/2 and 3/4 that of WSBC and RHBC, respectively. Ball-milling treatments significantly increased the SSA of different chars. The SSAs of BMWSBC, BMRHBC, and BMCSAC were approximately 3.8 times, 1.1 times, and 1.5 times those of WSBC, RHBC and CSAC, respectively. Moreover, the PV, including the micropore volume, of different chars was significantly increased by ball-milling. The PVs of BMWSBC, BMRHBC and BMCSAC were approximately 3, 1.2 and 1.4 times those of their respective precursors. The pore diameter of BMWSBC was approximately 2-fold higher than that of WSBC. Ball-milling improved the microstructure of different chars, of which the improvement in WSBC was the most significant. 3.1.4 Properties related to adsorption Table 1 shows that the content of OFGs in different chars was in the following order: WSBC > RHBC > CSAC and that the content of carboxyl groups in each sample was higher than that of lactone groups and phenol hydroxyl groups. The content of OFGs in WSBC was approximately 1.7- and 3.6-fold higher than RHBC and CSAC, respectively. The CEC value of WSBC was 14.6 (cmol/kg), which is

approximately 2.3- and 7.4-fold higher than RHBC and CSAC, respectively. Ball-milling treatment increased the amount of OFGs in chars. The OFGs of BMCSAC increased significantly, which is approximately 3.4 times higher than that of WSBC. In addition, the pH value of the chars decreased after ball milling. The pH values of BMWSBC, BMRHBC and BMCSAC decreased by 0.61, 0.72, and 0.66, respectively, which also suggests that ball milling introduced more OFGs to the surface of chars, which is consistent with a study by Munkhbayar(B. Munkhbayar, 2013) and Lyu(Honghong Lyu & Hua Huang, 2018). Notably, ball milling changed the chars as described above but did not affect the CEC value. 3.2 Adsorption behavior 3.2.1 Adsorption kinetics The adsorption kinetics curves of different chars are shown in Fig. 1, and the kinetic parameters of the adsorption kinetic model are shown in Table 2. The results showed that the squared regression coefficients (R2) of the pseudo-second-order kinetic model are much higher than that of the pseudo-first-order kinetic model, which indicated that the pseudo-second-order kinetic model preferably described the Pb2+ adsorption process on chars. Several studies indicated that the formation of chemical bonds is the main influencing factor of pseudo-second-order kinetics(Chen et al., 2011; Mohan et al., 2007), Therefore, the adsorption process of Pb2+ in chars may be mainly controlled by chemical

adsorption processes. The rate constant of the pseudo-second-order model follows the order CSAC > RHBC > WSBC. Table 2 show that the time when WSBC, RHBC and CSAC reach the adsorption equilibrium is 16 h, 8 h and 6 h, respectively. The equilibrium adsorption amount order is WSBC > RHBC > CSAC. This indicated that the Pb2+ adsorption rate of CSAC is the largest, whereas the Pb2+ adsorption capacity is the lowest and WSBC is the opposite of CSAC. After ball milling, the Pb2+ adsorption amount and rate of chars were increased to varying degrees, and the adsorption equilibrium time was shortened, which is attributed to the above changes in the physiochemical properties of the ball milled powder. 3.2.2 Adsorption isotherm The adsorption isotherm of chars is shown in Fig. 2., and the isotherm parameters of the adsorption isotherm are shown in Table 3. The Langmuir adsorption equation assumes that the adsorbate is distributed in a single layer of molecules on the surface of the adsorbent, whereas the Freundlich adsorption equation describes multilayer adsorption under nonuniform surface conditions(Wang et al., 2018). The squared regression coefficients (R2) of the Langmuir and Freundlich equation are greater than 0.96, which indicated that both of these models can describe the adsorption behavior of Pb2+ on chars. As the initial concentration of the Pb2+ solution increased, the Pb2+ adsorption amount of the chars significantly increased (P > 0.01).

The values of Qm and KL of the chars were increased after ball milling, indicating that ball-milling introduced more effective adsorption sites and increased the affinity of chars to Pb2+. In the Freundlich model fitting results, the index n of different chars is greater than 2.15 (1/n < 0.5), indicating that the Pb2+ adsorption of chars is the dominant adsorption process. Both the n and KF values of the ball milled powder were greater than those without ball milling, indicating that the ball milling treatment improved the adsorption capability of the chars. 3.3 Analysis on Pb2+ adsorption mechanism of chars The proportion of five adsorption mechanisms of different chars is shown in Fig. 3. The physical adsorption amount (Qp) of Pb2+ in 6 chars accounted for less than 1% of the total adsorption amount (Q). Thus, the adsorption mechanism of Pb2+ was mainly chemical adsorption, which was consistent with the results of the adsorption kinetics. The precipitation adsorption amount (Qm) of WSBC, RHBC and CSAC accounted for 70.60%, 83.60% and 62.46% of Q, respectively, indicating that coprecipitation adsorption with minerals(Wang et al., 2018) was the dominant adsorption mechanism. The adsorption amount of the complexation (Qc) with OFGs(Ding et al., 2014) and the π electronic interaction (Qπ) with the unsaturated carbon bond (e.g., C=C, C≡C)(Cui et al., 2016) is next to Qm. The ion exchange adsorption amounts (Qi) of WSBC, RHBC and CSAC were 3.12, 2.46 and 0.26 m2/g, respectively. The contribution of ion exchange was the smallest of the chemical adsorption reactions. In summary, the adsorption mechanisms of

WSBC, RHBC and CSAC were mainly coprecipitation reactions, followed by π electronic interactions and OFGs complexation, and small amount of ion exchange adsorption. The Qm of the three ball milled chars increased by 9.55-14.36 mg/g compared with the unmilled chars. Ball milling increased the Qc by 2.09-5.08 mg/g and the Qπ by 1.25-11.13 mg/g. Of them, the Qπ of BMCSAC was greatly increased compared with CSAC. In addition, ball milling had little effect on the Qi. The increased Q of ball milled chars was due to the increase in Qm, Qπ and Qc by improving the microstructure and surface properties of chars. Notably, the different adsorption mechanism of two types of biochar followed the order Qm > Qc > Qπ > Qi, whereas the adsorption mechanism of activated carbon follows the order Qm > Qπ > Qc > Qi. This may be due to more OFGs contained in the biochar, which provides more complexing adsorption sites for Pb2+, and the higher degree of aromatization of activated carbon, which exposed more unsaturated carbon bond adsorption sites for Pb2+. XRD was used to characterize the inorganic minerals on the surfaces of chars before and after the adsorption Pb2+. Crystalline calcite (CaCO3), calcium hydroxide (Ca(OH)2), quartz (SiO2), aluminum oxide (Al2O3) and amorphous silica were the main inorganic minerals detected on chars before Pb sorption. Strong evidence was obtained from XRD patterns suggesting the formation of hydrocerussite (Pb3(CO3)2(OH)2),

lead phosphate (Pb3(PO4)2), cerussite (PbCO3) and alamosite (PbSiO3) on Pb-loaded chars, whereas some minerals, including SiO2 and Al2O3 may not participate in Pb2+ coprecipitation. Yang et al., demonstrated that mineral released anions (e.g., CO32- and PO43-) in biochars were bound with Pb(II) to form precipitates(Yang et al., 2016). The large amount of Pb mineral precipitates suggested that the dissolved anions (e.g., CO32-, SiO32-, OH-and PO43-) provided by the minerals on chars contribute to forming precipitates. Additionally, the organic functional groups on the surfaces of chars before and after Pb2+ adsorption were also characterized using FTIR. By comparing the FTIR spectrum before and after Pb2+ adsorption, it was observed that the bands at 1695 cm-1 and 1298 cm-1 corresponding to the C=O in carboxyl group or ketone(Guo et al., 2014) and phenolic-OH(Yuan et al., 2015), respectively, were weakened in the Pb2+ loaded chars. These OFGs may be involved in complexing with Pb2+(Wang et al., 2015). By comparing the unmilled and ball milled chars, the peak of the phenolic-OH at 3453 cm–1 corresponding to -OH stretching vibration (Yuan et al., 2015), and the peaks at 997 cm-1 and 973 cm-1 corresponding to C-O stretching vibration(Shen et al., 2017) became more apparent, confirming that ball milling introduced additional OFGs as active adsorption sites to the surface of chars. In addition, the peak at 1594 cm-1 representing C=C groups(Guo et al., 2014) and the peaks at 878 cm-1 and 796 cm-1 corresponding to aromatic -CH groups(Uchimiya et al., 2010) were strengthened after ball milling. This demonstrated that ball milling exposed unsaturated bonds(Honghong Lyu & Hua Huang, 2018) for enhanced π electronic interactions with Pb2+.

3.4 Correlation analysis of physicochemical properties of chars and its amount of Pb2+ adsorption 3.4.1 Pearson correlation analysis To identify the key properties of chars that govern their effectiveness in removing Pb2+, the correlation analysis results of the properties and Pb2+ adsorption mechanism are shown in Table 4. The SSA of chars was composed of the outside surface area and the inner pore surface area(Ji et al., 2016). Thus, the SSA had a significantly positive relationship with PV (r=0.99). The SSA and PV were negatively related to Qm and Qi and had no correlation with Q. This can be explained by the finding that controlled adsorption is a chemical characteristic not of the microstructure, but of the same char and that improving its microstructure, such as by ball milling, helps its adsorption capacity. The OFGs of chars were positively related to the Qm (r=0.85), Qc (r=0.88) and Q(r=0.91). This correlation was confirmed by the FTIR analysis showing that the stretching vibration of OFGs in chars was weakened after Pb2+ adsorption, and the acid elution test also demonstrated the existence of OFG complexation in Pb2+-loaded chars. The CEC is another key property of chars due to its highly positive relationship to Qm (r=0.95), Qi (r=0.99) and Q (0.93). Moreover, the ash content in chars have poor correlations with Qm and Q. Wang et al.(Wang et al., 2015) also indicated that the ash content in biochars have a poor correlation with the sum of Qm and Qi. Based on the correlation analysis and XRD results, the predominant coprecipitation resulted from the dissolved anions provided by minerals in the chars instead of mineral crystallization

(i.e., SiO2 and Al2O3). This explain why the adsorption capacity of WSBC was greater than the high ash content of RHBC. This finding was due to more SiO2 crystals in the ash of RHBC that could not participate in the formation of the precipitate, whereas the ash of the WSBC consisted of water-soluble ions promoting coprecipitation with these minerals. Furthermore, although there was no significant difference in the ash content between CSAC and WSBC, the Pb2+ adsorption amount of CSAC is lower because of more Al2O3 and SiO2 crystals in the ash of CSBC. The Qm, Qc and Qi have positive correlations with each other. Based on the above correlation analysis, the OFGs and CEC properties of chars control the adsorption capacity, and an increase in these properties for chars will enhance their Pb2+ adsorption performance. 3.4.2 Quantitative correlations among physicochemical properties of chars and their Pb2+ adsorption amount The relationships between Qm and CEC, and Qm and OFGs are shown in Fig. 4a and Fig. 4b and can be elaborated by Qm=4.097×CEC+6.101 (R2=0.945) and Qm=77.556×OFGs-1.260 (R2=0.722), respectively. Based on these equations, it could be inferred that CEC values were good predictors of the Pb2+ adsorption amount by precipitation. The CEC value is the total amount of released cations from minerals in chars, which is consistent with the amount of minerals released as dissolved anions. The relationship between Qc and OFGs, and Qc and CEC is shown in Fig. 4c and Fig. 4d, respectively, and can be described by Qc=16.614×OFGs-2.306 (R2=0.844) and Qc=0.805×CEC+0.008 (R2=0.816). It could be concluded that Qc showed an increasing trend as the OFGs or CEC increased. The relationship between CEC and Qi is

shown in Fig. 4e and can be calculated by Qi=0.233×CEC-0.171 (R2=0.993). The linearity demonstrated that the CEC value in chars is a good predictor of Qc. The positive relationship between Q and CEC shown in Fig. 4f can be elaborated by Q=4.534×CEC+19.085 (R2=0.866). The positive relationship between Q and OFGs shown in Fig. 4g can be elaborated by Q=111.665×OFGs-2.448 (R2=0.824). The positive relationship among Qm, Qc and Qi shown in Fig. 4h can be elaborated by Qm=10.505×Qi+1.318×Qc+12.902 (R2=0.919). Based on the above correlations and equations, it can be inferred that the CEC and OFG properties in chars can predict its Pb2+ adsorption capacity and that increasing the value of OFGs and CEC can enhance the Pb2+ adsorption amount. 4. Conclusion This study showed that (i) coprecipitation reactions were the dominant Pb2+ adsorption mechanism of crop residue-derived chars, followed by π electronic interactions and complexation; (ii) ball milling treatment increased the Qm, Qc and Qπ by introducing more minerals, OFGs and unsaturated carbon bonds to the surface of chars, respectively; and (iii) the property of chars controlling its Pb2+ adsorption was chemical characteristics, not its microstructure. The amount of minerals that could release dissolved anions (e.g., CO32-, SiO32-, OH-and PO43-) influenced the adsorption amount of the precipitate. Quantitative equations illustrating the key properties, including CEC and OFGs, controlled its Pb2+ sorption.

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Figure 1. Adsorption kinetics for Pb2+ on different chars. qt (mg/g) is the adsorption amount of Pb2+ at time t h. t (h) is the adsorption time.

Figure 2. Adsorption isotherms for Pb2+ on different chars. Qe (mg/g) is the equilibrium adsorbed concentration of adsorbed Pb2+. Ce (mg/L) is the equilibrium concentration of Pb2+.

Figure 3. The proportion of five adsorption mechanisms of different chars.

Figure 4. Quantitative correlations among physicochemical properties of chars and their Pb2+ adsorption amount

Table 1. Physiochemical properties of different chars. WSBC Elements

65.13±0.68B

C(%)

C

H(%)

1.70±0.08 6.98±0.79 1.12±0.03

S(%)

0.57±0.02

H/C O/C Ash and ash composition

50.63±0.97A

-

B

-

0.026±0.000

B

0.107±0.002

C

1.37±0.05

B

3.45±0.52

A

0.48±0.03

B

0.25±0.02

A

CSAC

-

67.26±1.74B

-

1.06±0.1

-

0.027±0.000

-

0.068±0.001

A

-

B

BMCSAC -

A

-

5.10±0.86

B

-

0.39±0.01

A

-

C

B

-

BMRHBC

0.76±0.05

-

0.016±0.001

A

-

-

0.076±0.002

B

-

-

25.43±1.04

A

-

B

-

-

24.50±0.94

A

SiO2(%)

37.10±0.28

A

-

92.78±0.05

-

61.02±0.11

K2O(%)

33.18±0.11C

-

2.93±0.02B

-

1.13±0.01A

-

Cl(%)

9.34±0.04C

-

0.03±0.00B

-

0.01±0.00A

-

-

1.03±0.01

A

-

1.34±0.02

B

-

1.00±0.01

B

0.88±0.01

A

-

0.47±0.01

B

0.29±0.00

A

-

0.81±0.00

B

0.67±0.00

A

-

Ash(%)

CaO(%) SO3(%) MgO(%) P2O5(%) Na2O(%) Al2O3(%) Fe2O3(%) Microstructures

-

-

C

N(%)

RHBC

-

C

O(%)

BMWSBC

2

C

6.03±0.09 6.3±0.26

C

43.82±1.25

C

C

2.91±0.05

-

C

1.88±0.03

-

0.97±0.01

C

1.17±0.02

B

0.75±0.02

B

SSA(m /g)

14.37±0.75

PV(cm3/g)

Aa

b

-

0.11±0.01

A

0.29±0.00

A

0.18±0.00

A

-

Ba

0.35±0.00

B

-

25.75±0.00

-

C

217.03±4.36

7.54±0.17 b

C

458.64±27.02

Ca

663.60±45.45b

53.82±5.82

198.11±2.61

0.010±0.001Aa

0.030±0.006b

0.094±0.002Ba

0.113±0.005b

0.227±0.012Ca

0.326±0.020b

6.072±0.535Ca

13.040±3.427b

4.708±0.252Ba

7.882±1.797b

3.198±0.039Aa

3.210±0.060a

Pore diameter (nm)

Micro-PV (cm3/g) Properties

Carboxyl

related to

(mmol/g)

adsorption

Lactones (mmol/g)

0.009±0.001Ba

0.013±0.002b

0.009±0.002Ba

0.027±0.005b

0.040±0.002Aa

0.061±0.002b

0.24±0.02Ba

0.36±0.02b

0.20±0.03Ba

0.34±0.01b

0.08±0.02Aa

0.25±0.02b

0.15±0.01Ca

0.23±0.02b

0.09±0.02Ba

0.16±0.02b

0.02±0.01Aa

0.11±0.01b

0.18±0.03Ca

0.26±0.03b

0.04±0.01Aa

0.12±0.02b

0.06±0.00Ba

0.19±0.01b

0.57±0.02Ca

0.84±0.03b

0.33±0.01Ba

0.62±0.05b

0.16±0.01Aa

0.55±0.04b

14.60±1.21Ca

15.07±0.89a

6.27±0.87Ba

6.44±0.92a

1.97±0.45Aa

2.13±0.48a

10.28±0.16Ba

9.67±0.11b

10.03±0.14Ba

9.31±0.10b

7.20±0.10Aa

6.54±0.08b

Phenolic hydroxyl (mmol/g) OFGs (mmol/g) CEC (cmol/kg) pH

Data are shown as mean value ± standard deviation, A-C a-b

Means followed by different superscripts in the same row are significant difference at P<0.01,

Means followed by different superscripts are significant difference about before and after ball milling at P<0.01.

Table 2. Kinetic parameters of the pseudo-first-order and pseudo-first-order model for Pb2+ adsorption onto different chars. Sorbent

Q(mg/g)

WSBC

Pseudo-first-order

Pseudo-second-order

qe(mg/g)

k1(h-1)

R2

qe(mg/g)

k2(g/(mg·h))

R2

78.43

77.20

1.37

0.868

79.90

0.03

0.993

BMWSBC

97.63

95.86

2.34

0.825

97.91

0.05

0.991

RHBC

36.73

35.84

2.02

0.824

37.10

0.10

0.996

BMRHBC

54.93

54.03

2.64

0.831

55.47

0.11

0.994

CSAC

17.53

17.02

2.21

0.820

17.66

0.25

0.987

BMCSAC

42.87

42.66

3.57

0.877

43.32

0.26

0.971

Table 3. Adsorption isotherms parameters of Langmuir and Freundlich model for Pb2+ adsorption onto different chars. Sorbent

Langmuir

Freundlich

Qm(mg/g)

KL(L/mg)

R2

n

KF(L/mg)

R2

WSBC

164.23

0.007

0.984

2.44

11.49

0.990

BMWSBC

210.90

0.008

0.991

2.49

14.59

0.968

RHBC

73.50

0.011

0.993

2.67

9.14

0.993

BMRHBC

103.99

0.012

0.991

2.87

10.84

0.996

CSAC

38.98

0.013

0.984

2.92

4.30

0.997

BMCSAC

78.86

0.014

0.993

3.45

11.86

0.974

Table 4. Correlation analysis results of physicochemical properties of chars and its Pb2+ adsorption mechanism. SSA

PV

H/C

Ash%

OFGs

SSA

1.00

PV

0.99b

1.00

H/C

-0.87

-0.89

1.00

Ash%

-0.06

-0.09

0.53

1.00

OFGs

-0.44

-0.43

-0.14

-0.14

1.00

CEC

-0.89a

-0.89 a

-0.22

-0.22

0.69

a

a

Qm

-0.84

-0.84

-0.11

-0.11

0.85

0.95b

a

a

-0.62

-0.61

-0.42

-0.42

0.88

Qπ

0.38

0.39

-0.87

-0.87

0.46

Qi

-0.91a

-0.91a

-0.14

-0.14

0.68

-0.71

-0.70

0.69

-0.25

a

Correlation is significant at the 0.05 level.

b

Correlation is significant at the 0.01 level.

0.91

Qm

Qπ

Qc

Qi

Q

1.00 a

Qc

Q

CEC

a

1.00 0.91a

1.00

0.07

0.08

0.46

1.00

0.99b

0.96b

0.88a

0.04

1.00

b

b

b

0.33

0.92

0.90

0.93

0.97

0.98

1.00

a

43

Highlights: 1) Crop residue chars and their ball milled powder were studied by multiple approach. 2) Coprecipitation dominated Pb sorption followed by π interaction and complexation. 3) Pb sorption amount highly correlated with coprecipitation and ion exchange amount. 4) CEC and OFGs were two key characteristics governing chars’ Pb sorption ability. 5) Linear regression equations were established of sorption amounts and CEC /OFGs.

44