Mechanistic Insight of Biochar-layered Double Hydroxide Composite for Ecient Removal of Aqueous Pb(II) and Cu(II)

To explore the heavy metal adsorption mechanisms by biochar-based materials, MgAl-layered double hydroxide (MgAl-LDH) was loaded on a commercial coconut shell biochar (BC) to obtain the composite (BC-LDH) for adsorptive decontamination of Pb(II) and Cu(II) in water. The removal capacities of BC-LDH (294 and 38.6 mg/g) was higher than BC (13.3 and 11.4 mg/g) and LDH (126 and 22.7 mg/g) for aqueous Pb(II) and Cu(II). The pseudo-second-order equation and the Langmuir model could better illustrate the adsorption process, respectively. The interaction mechanisms between BC-LDH and heavy metals were classied as mineral precipitation (Qpre), cation exchange and isomorphic replacement (Qexc), electrostatic attraction (Qele), and surface complexation (Qcom) according to the characterization results. The quantitative analysis indicated that the contributions of the above mechanisms of BC-LDH for Pb(II) and Cu(II) followed the order of Qpre > Qele > Qexc and Qpre > Qexc > Qele, respectively. Compared with BC, the Qpre increased and Qele decreased signicantly, certied the combination of BC and LDH provided a kind of viable composite in heavy metal wastewater treatment.


Introduction
With the rapid progress of the economy, the heavy metal wastewater was discharged increasingly. Heavy metals can be adsorbed and accumulated in plants and animals easily which causes severe toxicity to human organs at a low concentration through food chains (Mohamed Khalith et al., 2021). Therefore, reducing heavy metal pollution is urgent and imperative.
The combination of BC and LDH with synergetic effect has exhibited signi cant improvement in physicochemical properties and adsorption characteristics for pollutants (Chang et al., 2020). The composite material could avoid the limitation of conventional materials in water treatment, like high-cost, low-selectivity, and lower reusable rate. Moreover, the BC, as an ideal support substance, provided wide reactive areas for metal ions and prevented LDH aggregation (Gholami et al., 2020). In the previous works, the metal ions have been removed by some composites containing BC and LDHs (Wang et al., 2020;Yi et al., 2020). Chen et al. (2018) decorated NiAl-LDH to the porous carbons activated by H 3 PO 4 and found the removal capacity of Cr(VI) reached 271.5 mg/g. Xie et al. (2019) synthesized magnetic MgFe-LDH loaded carbon spheres. The maximum adsorption capacities added up to 3.66 mmol/g for Cu(II) and 5.33 mmol/g for Pb(II). Although the results of these studies all indicated that BC-LDH composite material enhanced the adsorption ability of toxic metals, the interaction mechanisms need to be further explored.
Generally, LDHs adsorbed metal ions by the formation of precipitation, complexation with hydroxyl groups, and isomorphism of elements between structure layers (Zhang et al., 2020). BC could remove heavy metals via mineral precipitation, electrostatic attraction, ion exchange, and complexation of functional groups . Taking Pb(II) adsorbed by BC as an example, the contribution of these four mechanisms was different because the BC was prepared from multiple sources at different temperatures. For the largest contribution, most of the researchers believed that the dominant mechanism was precipitation which accounted for 70.36-89.03% Xiao et al., 2020).
However, the BC pyrolyzed from wheat straw mainly adsorbed Pb(II) via ion exchange up to 85.12% (Cao et al., 2019). Up to now, the quantitative analysis of heavy metal removal mechanisms is mainly focused on BC materials and there is little research on other materials, such as BC-LDH composite.
In this work, the MgAl-LDH was selected as a typical LDHs to prepare BC-LDH composite to investigate the adsorption mechanisms for aqueous Cu(II) and Pb(II). The qualitative and quantitative interaction mechanisms were performed and discussed in detail. The adsorption isotherm and kinetics were also conducted by batch equilibrium experiments.

Synthesis of BC-LDH
The BC powder was bought from Jiangsu Yiqing Activated Carbon Co., LTD, China which was made up of coconut shells carbonized at 900 o C. The other reagents were acquired from Tianjin Damao Reagent Factory.
Solution A contained 1.250 mol/L Mg(II) (Mg(NO 3 ) 2 ·6H 2 O) and 0.625 mol/L Al(Ш) (Al(NO 3 ) 2 ·9H 2 O) and solution B composed of NaOH and Na 2 CO 3 (4.125 mol/L OH − and 1.251 mol/L CO 3 2− ) were prepared rstly. The coconut shell biochar (3 g) was dispersed in a four-necked ask using distilled water. Then the solution A and solution B were added drop by drop according to the co-precipitation method of LDHs. The pH value was maintained at 10.0 ± 0.1. After the addition, the mixture was continuously stirred for 12 h and then statically crystallized for another 6 h. The temperature was kept at 60 o C during the preparation process. The precipitate was obtained by centrifuging, washing and desiccating at 80 o C. The MgAl-LDH was also synthesized following the above procedure without BC.

Batch adsorption experiments
The experiments were implemented in a centrifuge tube containing 50 mg BC-LDH or BC with 20 mL heavy metal solution. The tube was put into a thermostatic oscillator and shaken at 200 rpm and 25 o C. Then the supernatant liquid was obtained by centrifuging at 8000 r/min, ltered through 0.45 µm poresized membranes and injected into an atomic ame adsorption spectrometer (AA-7000, Shimadzu, Japan) to get the concentration of heavy metals. The difference value of heavy metals between the initial and equilibrium solutions was applied to calculate the total removal capacity (Q, mg/g). The experiment was conducted three times and the detailed experimental conditions are listed in the gure notes.

Adsorption mechanistic study
The XRD, XPS, FT-IR, and zeta potential techniques with relevant instruments (Table S1) were applied to characterize BC-LDH material before and after Cu(II) and Pb(II) adsorption. The morphology structure of BC-LDH was detected by SEM with EDS and N 2 adsorption/desorption experiment. The quantitative experiments of different mechanisms were also conducted. The adsorption mechanisms of BC for heavy metals usually involve physical adsorption (Qphy), mineral precipitation (Qpre), cation exchange (Qexc), electrostatic attraction (Qele) and complexation with functional groups (Qcom). Then the contributions of each mechanism were calculated and the speci c procedure is listed in Text S1. Figure 1a displays the XRD pattern of BC, LDH, and BC-LDH. The main peak in BC at 2θ of 24.00° was observed which was related to the interlayer condensation of carbon. This indicated the amorphous nature of BC (Adorna et al., 2020). The apparent peaks of MgAl-LDH at 2θ = 11.53° (003), 23.40° (006), 34.64° (012), 38.87° (015), 46.03° (018), 60.79° (110), and 62.04° (113) were typical peaks of LDHs . BC-LDH also showed all the diffraction peaks of MgAl-LDH, indicating that the high purity and good crystal shape of LDH with the combination of BC. However, no obvious diffraction peak of BC was found.

Properties of BC-LDH
To demonstrate the functional groups of BC-LDH, the materials were analyzed by FT-IR. In Fig. 1b, the broad adsorption bands at 3410 and 1356 cm − 1 corresponded to the hydroxyl vibrations from the layers or water molecules, respectively (Tang et al., 2018). The peaks existing at 760 − 500 cm − 1 were assigned to the M-O or O-M-O vibrations (M represents Mg and Al) (Fu et al., 2021). For BC, the carboxyl (-COOH) vibrations occurred weakly at 1702 cm − 1 . The C-O and C = C vibrations were observed at 1158 and 1576 cm − 1 , respectively (Xu et al., 2017). To further investigate the functional groups, XPS was used to characterize BC-LDH and BC. The O, as well as C elements, were the basic constituents of BC-LDH and BC (Fig. S1). The C element was rich in BC-LDH and the four main peaks at 282.70, 284.60, 286.10, and 288.34 eV were ascribed to the inorganic carbide, C = C/C-C, C-O-C and O = C-O groups, respectively ( Fig. 1c) (Yu et al., 2019). The deconvolution of O 1s spectra (Fig. 1d) revealed the presence of -OH at 531.81 eV for BC-LDH, H-O-C at 532.81 eV for BC (Swaidan et al., 2021), and another peak at 530.96 eV for BC was C = O group (Kamran and Park, 2020). The main elements of Mg and Al in BC-LDH were also identi ed. The Al 2p and Mg 1s spectra at 74.45 and 1303.88 eV (Fig. S2) indicated the formation of Al 2 O 3 and MgO in BC-LDH (Huang et al., 2018a).
The microstructure and morphology of BC-LDH and BC were shaped by the SEM technique. As re ected in the SEM images, the surface morphology BC was compact and relatively smooth (Fig. 1e). After coprecipitation of MgAl-LDH, the small particles could be seen on the surface of BC-LDH and the surface roughness increased (Fig. 1f). Moreover, the EDS and elemental mapping analysis showed that the Mg, Al, O, and C elements existed and distributed homogeneously on the BC-LDH surface (Fig. S3). The purchased BC had a larger BET surface area of 1195 m 2 /g, while BC-LDH had a smaller value of 394.8 m 2 /g (Table S2 and Fig. S4). This illustrated that the MgAl-LDH particles may be assembled into the pores of BC during the preparation process and occupied part of the surface area. The isotherms models of Langmuir and Freundlich (Eq. S5 and S6 in Text S2) were applied to further explore the adsorption mechanisms and the correlation coe cients (R 2 ) and isothermal parameters were determined ( Table 1). The removal process of Cu(II) and Pb(II) by BC-LDH was matched with the Langmuir model well (R 2 > 0.99), while that of BC tted with the Freundlich model well (R 2 > 0.97). The q m values of BC-LDH were 38.6 and 294 mg/g for Cu(II) and Pb(II), respectively, which were larger than other biochar-based or LDH materials (Table 2). Moreover, BC-LDH can be obtained from the commercial biochar via the simple co-precipitation method. This suggested that the composite containing BC and LDH was effective in the wastewater treatment containing heavy metals.

Adsorption kinetics
Adsorption kinetics are usually applied to assess the reaction rate and the adsorption properties. As shown in Fig. 3, the removal capacity was enhanced signi cantly along with the increase of time and then tended to equilibrium. For BC-LDH, the adsorption equilibrium time was 100 and 180 min for Cu (II) and Pb(II), respectively, and that of BC and LDH were both 180 and 200 min. The pseudo-rst-order (PFO) and pseudo-second-order (PSO) models (Eq. S7 and S8 in Text S2) were employed to depict the adsorption kinetic data. The R 2 values of PSO were entirely greater than that of PFO (Table 3). This indicated that it was preferable for PSO to explain the adsorption behaviors. Therefore, the adsorption of Pb(II) and Cu(II) onto the active sites of BC-LDH and BC were chemical reactions mainly involving ion exchange, surface complexion, and precipitation according to the hypothesis of PSO (Ho and McKay, 1998).

Adsorption mechanisms
To clarify the interaction mechanisms between BC-LDH and heavy metals, a series of characterizations were performed and discussed in detail. Despite no apparent changes were observed in the FT-IR spectra of BC-LDH after heavy metal adsorption (Fig. S5), the differences were obviously occurred in the XPS spectra and XRD patterns before and after adsorption and applied to elucidate the interaction mechanisms qualitatively. The quantitative analysis of each mechanism was also conducted.

Mineral precipitation
Biochar can release anions, such as OH − , SO 4 2− , CO 3 2− , and PO 4 3− . The previous reports have testi ed the precipitation between the heavy metals and anions (Bandara et al. 2020) or surface groups of LDH materials (Li et al., 2021). Figure 4a illustrated that all the characteristic XRD peaks of BC-LDH after Cu(II) and Pb(II) loading were not shifted. This indicated the structure of BC-LDH was not destroyed during the adsorption process. After adsorption of Pb(II), BC-LDH had some distinctly characteristic peaks. The new peaks were attributed to the Pb 3 (CO 3 ) 2 (OH) 2 based on the PDF card of 13-0131 (Zhang et al., 2020).
Nonetheless, there was unapparent changes in the XRD pattern of BC-LDH adsorbed Cu(II).
Coincidentally, Zhang et al. (2021) also did not detect the typical XRD peaks of Cu(II) after adsorbing onto cow manure biochar. To further explore the mineral precipitation mechanisms, the XPS survey spectra were used (Fig. S6). It could be seen that Cu 2p and Pb 4f spectra were distinctly presented in BC-LDH after adsorption. As shown in Fig. 4b, the three new peaks were discovered in the spectrum of Cu 2p. The binding energy at 953.82 eV was Cu 2p 1/2 , which meant the emergence of Cu(OH) 2 (Huang et al., 2018b).
Furthermore, the Pb 4f 5/2 was also observed at the binding energy at 143.06 eV corresponding to the Pb(II) oxidation state such as Pb 3 (CO 3 ) 2 (OH) 2 (Cheng et al., 2020).
The above XRD and XPS analysis indicated that the precipitation was involved in the adsorption mechanisms of BC-LDH (Fig. 4a, b) and BC (Fig. 4c, d). Moreover, the contribution ratio to the total removal amount of BC-LDH reached 76.26% for Pb(II) and 45.0% for Cu(II) according to the quantitative analysis (Fig. 5). This suggested that the mineral precipitation dominated the removal performance of BC-LDH. As for BC, the contribution ratios were 65.55% and 16.14%, respectively. Compared with BC, the increased contribution of precipitation of BC-LDH may be due to the loading of MgAl-LDH on BC.

Cation exchange and isomorphic replacement
Cation exchange is critical to the adsorption mechanism of BC for heavy metals and it can occur between the aqueous metal cations and the cations released from biochar materials. In this work, the concentrations of Na(І), Ca(II), Mg(II), and K(І) in the aqueous solutions all increased after adsorption by BC-LDH (Table S3), which explained the occurrence of ion exchange.
The isomorphic substitution is one of the characteristics of LDH materials. The XPS data were used to calculate the atomic ratio of Mg/Al of BC-LDH. With the procedure of isomorphic substitution of Mg element with aqueous Cu(II) or Pb(II), the molar ratio of Mg/Al of BC-LDH reduced from 1.94 to 1.62 and 1.83, respectively. The replacement of LDH layer element was interconnected with the characteristics of aqueous metal ions, such as ionic charge and radius (Huang et al.,2018a). In the case of MgAl-LDH, this reaction will occur more easily when the ionic radius of metal is closer to Mg (0.066 nm). In consequence, Pb (0.12 nm) is more di cult to undergo isomorphic substitutions than Cu (0.072 nm) (Ravuru et al., 2019).
As shown in Fig. 5, the cation exchange and isomorphic replacement was another key mechanism of Cu(II) adsorption by BC-LDH and BC. The contributions were 44.18% and 45.87%. Moreover, the Qexc of BC for Cu(II) was 4.89 mg/g and that of BC-LDH increased to 13.03 mg/g, suggesting the isomorphic replacement contributed to these mechanisms. By contrast, the contributions of Qexc for Pb(II) adsorption by BC-LDH and BC were only 9.95% and 3.35%, respectively.

Electrostatic attraction
The surface charge of adsorbent is the vital factor impacting the removal performance of functional material for aqueous metals (Ahmed et al., 2021). The negative charge of material can easily remove metal cations through electrostatic attraction and the zeta potential values link with the surface charge. For BC, the negative zeta potentials at the pH range of 3-12 decreased after heavy metal adsorption (Fig.   6a), demonstrating the important role of electrostatic attraction during the adsorption process. The quantitative analysis also identi ed this and the electrostatic attraction effect accounted for 29.82% (Pb(II)) and 28.52% (Cu(II)). After loading of MgAl-LDH, the surface charge of BC-LDH changed from negative to positive (Fig. 6b). Then the contribution of electrostatic attraction decreased sharply (accounted for 12.46% and 7.67%) and the amounts were only 19.7 and 2.26 mg/g for Pb(II) and Cu(II), respectively.

Complexation with functional groups
In general, the oxygen-containing functional groups (OFGs) of BC could chelate with heavy metals in aqueous solution and it showed great signi cance in removing contaminants from water. However, the contribution of the complexation is usually lower than that of precipitation (Li et al., 2018;Zhang et al. 2021). The XPS methods and quantitative analysis in this work also con rmed this. After adsorbing, the areas of C 1s spectra of BC-LDH and BC changed signi cantly (Fig. 7). The O = C-O ratio decreased by 8.79% and 3.00% while the C-O-C ratio decreased by 7.20% and 4.24% (Table S4), demonstrating the role of the complexation of Pb(II) and Cu(II) with OFGs. The contributions of the complexation were only 1.01% for Pb(II) and 3.08% for Cu(II). Then the ability of complexation with OFGs of BC-LDH for heavy metals was weak. Moreover, the contributions of physical adsorption were also lower than 0.5% and could be ignored.

Conclusion
In this work, the biochar from commercial coconut shell was combined with MgAl-LDH to improve the adsorption capability for aqueous heavy metals and to investigate the intercalation mechanisms qualitatively and quantitatively. After loading MgAl-LDH, BC-LDH had all the typical XRD patterns of LDHs and the speci c surface area decreased from 1195 to 394.8 m 2 /g. Moreover, the maximum adsorption ability of BC-LDH was signi cantly improved compared to BC and LDH. The adsorption data can be described by the pseudo-second-order kinetic equation and the Langmuir isothermal model. Among the mechanisms of Pb(II) and Cu(II) adsorption by BC-LDH, the precipitation reaction was dominated and the contributions were 76.26% for Pb(II) and 45.0% for Cu(II). The cation exchange and isomorphic replacement was another main mechanism of Cu(II) and the contribution was 44.18%. The electrostatic attraction and surface complexation was also involved in the adsorption process. However, the contributions of surface complexation and physical adsorption were less than 3.1%. Compared with BC, the higher adsorption capacity of BC-LDH for Pb(II) and Cu(II) and the increased precipitation contribution certi ed that the combination of BC and LDH can provide a viable candidate material for heavy metals removal.

Declarations
Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.    and 5.69 for Pb(II), C0 = 100 mg/L for Cu(II)) and 400 mg/L for Pb(II), t = 5 -600 min).

Authors' contributions
Page 18/20 Figure 4 XRD patterns of BC-LDH (a) and BC (c) before and after heavy metal adsorption, and XPS spectra of Cu 2p and Pb 4f in BC-LDH (b) and BC (d).
Page 19/20 Figure 5 Capacities and contribution ratios of the adsorption mechanisms of BC-LDH and BC for Pb(II) (a) and Cu(II) (b).