The removal of Pb²⁺ ion by MnFe₂O₄/waste tea leaves biochar and mechanism of adsorption

The waste tea leaves were received from a tea plantation field in Northern Thailand. The waste tea leaves were washed thoroughly with distilled water to remove dirt, and were then oven dried at 100 °C for 24 h. The surface area of biochar is important because, like some other physicochemical characteristics, it may strongly affect the reactivity and pyrolytic behavior of the biochar. There is a report that waste tea leaves with high specific surface area can be prepared in the temperature range 450 °C–500 °C [22, 23]. 'Comparatively high oxygen contents in biochar were observed with pyrolysis temperatures 400 °C–500 °C', according to Suwunwong et al [8]. Therefore, the temperature for biochar production was fixed at 500 °C in the present experiments. The biomass of waste tea leaves was measured and then they were placed in covered crucibles. Karunanayake et al [24] and Vu et al [25] report that 'the pyrolysis process was carried out by heating the covered crucibles containing waste tea leaves in a furnace under oxygen-limited conditions at the pyrolysis temperatures 500 °C with a heating rate of 10 °C min⁻¹'. To obtain BC material, the residence time was set at 2 h. 'After the pyrolysis process, the BC material was washed several times with distilled water to remove the water-soluble organic residuals, impurities and fine particles' according to Kumar et al [26]. The 20 g of BC was dried in an oven at 80 °C for 2 h. 'The BC was then ground and stored in dried and closed vessels', as reported by Kumar et al [26] and Sun et al [27]. The FMBC was prepared based on a previous study reported by Lin et al [19]. In brief, 5 g of BC was added to an aqueous solution of KMnO₄ (0.40 mol l⁻¹, 40 ml) and Fe(NO₃)₃ (0.50 mol l⁻¹, 40 ml) compounds. The mixture solution was stirred in an ultrasonic bath at room temperature for 2 h, and then was stirred in the water bath at 90 °C for 22 h in order to reduce the volume of the solution. Finally, the material was re-pyrolyzed at 500 °C with the heating rate of 10 °C min⁻¹ and the residence time of 1 h, and then washed several times with distilled water to remove impurities and fine particles. The FMBC material was separated from distilled water using a magnet. The obtained FMBC material was dried in an oven at 90 °C for 4 h.

The Total Organic Carbon (TOC) analysis was performed using a TOC analyzer (Analytik Jena AG, Germany). The elemental composition was determined using a CHNS/O analyzer (Flash 2000, ThermoScientific, Italy). The ash contents for both BC and waste tea leaves were determined by Wet Lab WI-RES-Wet lab-010 using Gravimetric method (Furnace, Carbolite CWF 11/13). The presence of 174 major mineral components in the samples was analyzed by an x-ray fluorescence (XRF) spectrometer (Zetium, PANalytical, Netherlands). The x-ray diffraction (XRD) pattern (PAN analytical, X' Pert Pro MPD) of FMBC was further recorded for confirmation of the crystalline structure of metal composites on the BC surfaces. The morphologies of BC and FMBC were imaged by a Scanning Electron Microscope (SEM, Czech Republic) and the specific surfaces by the BET (Brunauer–Emmett–Teller) method. The functional groups in the BC and FMBC were determined using a FT-IR (Vertex70, Bruker, Germany) system.

In order to study adsorption of a heavy metal by BC and FMBC, the removal of Pb²⁺ ions by BC and FMBC was screened with 10 mg l⁻¹ Pb²⁺ (Aldrich, USA) solution at 30 min adsorption time. The adsorption isotherms and kinetics of Pb²⁺ removal by FMBC were determined in batch experiments. The final concentrations of Pb²⁺ ions in the sample solutions after the adsorption process were determined with the ICP-OES technique (ICP-OES PerkinElmer, Optima 2000 DV, USA). The effects of contact time on adsorption of Pb²⁺ by FMBC were investigated. A 10 ml sample of the 100 mg l⁻¹ Pb²⁺ solution was transferred to a 15 ml centrifuge tube along with 0.1 g of the FMBC adsorbent. The solution was agitated at 180 rpm in a thermostatic shaker water bath for a controlled time from 5 to 120 min at 25 °C. The FMBC powder was separated from the sample solution using a magnet. The final concentrations of Pb²⁺ ions in the supernatant were determined with ICP-OES as the Pb²⁺ contents left in the solution. A 10 ml mixed solution sample compared with Pb²⁺ solution was transferred to a 15 ml centrifuge tube along with 0.1 g of the FMBC adsorbent with the adsorption time of 30 min, at 25 °C. The % removal and adsorption capacity (${q}_{e}$) of FMBC on removal of Pb²⁺ was determined with the ICP-OES technique. The equilibrium capacity, to remove Pb²⁺ ions by FMBC from the aquous solution, can be described by an adsorption isotherm in which the ratio of Pb²⁺ ion adsorbed or that remains in solution at equilibrium at a fixed temperature' as proposed by Freundlich [30] and Langmuir [31]. In this study, the Pb²⁺ adsorption was measured at optimum conditions, at 25 °C. Wan et al [28] suggested that 'the Langmuir equation can be used to estimate the maximum adsorption capacity, with complete monolayer coverage of the biochar surface'. In addition, the empirical Freundlich model is used to estimate the adsorption intensity by an adsorbent, in which the model assumes that the adsorbed molecules interact with a heterogeneous multilayer surface [29, 30].

The adsorption kinetics experiments were run in order to investigate the adsorption dynamics of the Pb²⁺ ion uptake rate, which is the rate limiting step in adsorption of metal ions by FMBC surfaces. In this work, 'pseudo-first order and pseudo-second order model types were selected for representing the adsorption kinetics of Pb²⁺ ion by FMBC, at 25 °C', as proposed by Mahmoud [31] and Simonin [36].

The pseudo-first order kinetic model is given by equation (1).

$$\begin{eqnarray}&&\mathrm{log}\,({{\rm{q}}}_{{\rm{e}}}\mbox{--}{{\rm{q}}}_{{\rm{t}}})={\mathrm{log}q}_{{\rm{e}}}-\,\displaystyle \frac{{{\rm{k}}}_{1}}{2.303}{\rm{t}}\end{eqnarray} \tag{ 1 }$$

The adsorption kinetics data were also analyzed in terms of the pseudo-second order model, given by equation (2).

$$\begin{eqnarray}&&\displaystyle \frac{t}{{{\rm{q}}}_{{\rm{t}}}}=\displaystyle \frac{1}{{{\rm{k}}}_{2}\,{{{\rm{q}}}_{{\rm{e}}}}^{2}}+\displaystyle \frac{1}{{{\rm{q}}}_{{\rm{e}}}}{\rm{t}}\end{eqnarray} \tag{ 2 }$$

The initial adsorption rate h (mg/g·min) is given in equation (3).

$$\begin{eqnarray}&&h\,={k}_{2}\,{{{\rm{q}}}_{{\rm{e}}}}^{2}\end{eqnarray} \tag{ 3 }$$

And the equations (2) and (3) can be written as equation (4).

$$\begin{eqnarray}&&\displaystyle \frac{{\rm{t}}}{{{\rm{q}}}_{{\rm{t}}}}=\displaystyle \frac{1}{{\rm{h}}}+\displaystyle \frac{1}{{{\rm{q}}}_{{\rm{e}}}}{\rm{t}}\end{eqnarray} \tag{ 4 }$$

In order to test whether the BC and FMBC derived from waste tea leaves are suitable for use as eco-friendly adsorbents that remove Pb²⁺ ions from wastewater, or even for soil amendment and fertilizer, the chemical components, especially the heavy metal components in BC and FMBC derived from waste tea leaves were characterized before adsorption studies. The total organic carbon (TOC), ash and the elemental (CHNOS) components in waste tea leaves, BC, and FMBC, were determined and are presented in table 1. Moreover, the physical characteristics and chemical compositions of BC and FMBC materials were determined with SEM, elemental analysis, XRF and FT-IR techniques. The mineral components in FMBC compared to the raw material waste tea leaves are presented in table 2. The results in tables 1 and 2 show that organic matter (C, H, N, O and S) is the main component in both BC (93.31%) and FMBC (61.17%). The obtained BC material derived from waste tea leaves prepared at 500 °C pyrolysis temperature at the heating rate 10 °C min⁻¹, is suitable for carbon sequestration process or for use as fertilizer with the carbon content of 64.14% and the total organic carbon content of 57.89%. Regarding FMBC, the carbon, hydrogen, and oxygen contents were decreased due to dehydration and decarboxylation. Some carbon, hydrogen and oxygen can be lost by releasing volatiles C_xH_yO_z and C_xH_y. According to Liu et al [32] 'the high oxygen contents in biochar were observed which may benefit the surface complexation of oxygen-containing group, on biochar, with Pb²⁺ ions in adsorption process'. Figure 1 shows the XRD pattern of FMBC compared to the XRD pattern of MnFe₂O₄ spinel structure [33, 34] corresponding to the structure reported by Vaez-Zadeh and Mohammadi [35]. Based on the high contents of Mn and Fe in XRF results, at 17.11 and 16.73% contents, and the XRD pattern of FMBC, the Fe–Mn binary oxide as a MnFe₂O₄ spinel phase particles was successfully deposited on the surfaces of BC.

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SEM images of BC and FMBC are seen in figure 2. For BC the SEM images exhibit rough surface morphology with porosity (figures 2(a) and (b)), and figure 2(c) shows a mesoporous surface with various sizes of small canals in the BC material. In figure 2(d), the SEM image shows a composite with Mn_xFe_3−xO₄ powder on BC surfaces. Moreover, various sizes of spinel phase crystals of Mn_xFe_3−xO₄ compound are observed on the surface of BC, corresponding to the structure reported by Vernekar et al [36]. The surface areas and the porosity types of FMBC and BC are summarized in table 3. BC and FMBC have mesoporous structures with average pore diameters of 4.44 and 9.59 nm, respectively. Based on the N₂ adsorption, the BET mesopore volume estimates for BC and FMBC were 0.012 and 0.058 cm³ g⁻¹, respectively. The surface area was increased to 24.38 m² g⁻¹ by the modification of BC to FMBC, from that for baseline BC (8.06 m² g⁻¹). Li et al 2018 [19] indicate that this benefits Pb²⁺ ion adsorption on adsorbent, and that the surface chemistry of functional groups on the modified biochar is significantly related with the heavy metal ion adsorption [24, 25, 37]. According to Guo et al [38] and Chen et al [39] 'the main functional groups in FMBC are quite similar to those of BC, but some functional groups, such as Fe–O and Mn–O bonds, were formed during the development of Fe–Mn-biochar process by adding the MnFe₂O₄ particle composites on the biochar surface'. Based on the peak at around 540–680 cm⁻¹ as shown in figure 3, that was identified as the peak of Mn–O and Fe–O bonds from Fe–Mn–O composite, or attributed to the bonds that formed between the oxygenic functional groups on the biochar surface and iron or manganese, which might be considered as the Fe–O–H or Mn–O–H bonds [40–42]. The C–H bending peak for BC and FMBC were observed at 876 and 869, respectively. The C–O stretching vibrational peak at approximately 1404 and 1260 cm⁻¹ of BC was weaken and shifted to 1378 and 1144, respectively, after the MnFe₂O₄ was successful composited to BC surface. From the literature, a broad stretching vibrational peak at approximately 3429 cm⁻¹ was attributed to the hydroxyl groups (–OH) on the FMBC surfaces, and a larger amplitude of stretching vibrations was observed in Fe–Mn-biochar due to the binding of Mn or Fe ions with –OH group [40–42].

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The removal efficiency by FMBC was screened with Pb²⁺ ions in an aqueous solution. The percentage removal of Pb²⁺ by FMBC as adsorbent was compared to BC for various masses of the adsorbent, as shown in figure 4(a). The increasing removal of Pb²⁺ by FMBC (to 98%) confirms that Fe–O and Mn–O bonds in Fe–Mn binary oxide-loaded biochar play an important role in the chemical adsorption of Pb²⁺ ions by FMBC.

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The pH of the adsorption solution affected the removal of Pb²⁺ ions by Fe–Mn binary oxide/biochar. The pH can modify adsorbent surface charge and ionization of heavy metal. Figure 4(b) shows the change of the adsorption efficiency of Pb²⁺ ions with pH gradually increased from 3 to 7, and it reached a steady value at pH 6 until pH 7. At pH 6 to 7 the removal efficiency of Pb²⁺ ions was by around 95%. However, the adsorption of Pb²⁺ ions gradually decreased with further increasing pH (>pH7). An increased pH is useful to hydrolysis, since the electronic structure of Pb²⁺ ions is more prominent than those produced by free hydroxides. At pH 7 the adsorption sites of carbon are saturated. On increasing the solution pH, the Pb²⁺ adsorption competes with the ion exchange of protons to binding sites, and the precipitation or creation of hydroxide complexes of Pb²⁺ ion might improve removal efficiency. Thus, the highest adsorption capacities were observed in solutions of pH 7.0 (figure 4(b).

The contact time is an important factor influencing the adsorption of Pb²⁺ from solution onto FMBC at 25 °C as shown in figures 4(c) and (d). The removal in terms of adsorption capacity (q_e ) of Pb²⁺ ions by FMBC increased consistently with time and increased rapidly at the initial time of adsorption (0–15 min). From the kinetics study, the removal of Pb²⁺ by FMBC reached equilibrium at around 30 min of adsorption. 'The adsorption isotherm models can be used to describe the interaction of the Pb²⁺ ion on the FMBC surface, the maximum adsorption capacity (${q}_{\max }$) of the FMBC material as well as the dynamic equilibrium of adsorption system' according to Tharaneedhar et al [43]. From the experimental results in table 4, the Freundlich isotherm is gave the best fit with the R² value of 0.99. According to this fitted model, the Pb²⁺ ions are possibly adsorbed in multiple layers on FMBC under a non-ideal reversible adsorption process, based on the derivation/assumptions of the Freundlich isotherm [44]. The kinetics of Pb²⁺ ion adsorption by FMBC were well fit with the pseudo-second order kinetic model with R² value of 1, as shown in table 4. Fan et al [45] suggested that 'the pseudo-second order model can confirm the process of Pb²⁺ ion chemisorption on the FMBC surface in which the liquid film diffusion, intra-particle diffusion and surface adsorption are contributed in the adsorption mechanisms involving electrostatic attraction, hydrogen bonding, ligand exchange in complexation'.

Table 4. Isotherm and kinetic constants of different models for the adsorption of Pb²⁺ ions by FMBC at 25 °C.

Isotherm model			Kinetics model
Langmuir	${k}_{L}$ (l mg−1)	21.495	Pseudo-	qe, exp (mg g−1)	9.85
	${q}_{max}$ (mg g−1)	25.00	first order	qe, cal (mg g−1)	2.35
	R2	0.91		k1 (1 min−1)	0.0976
				R2	0.89
Freundlich	${k}_{F}$ (l g−1)	1.1148	Pseudo-	qe, cal (mg g−1)	9.85
	N	1.0953	second order	k2 (g mg−1 min−1)	0.1455
	R2	0.99		h	14.1243
				R2	1

The adsorption of Pb²⁺ on FMBC surfaces takes place with various mechanisms and in multiple steps with both physical and chemical interactions. The adsorption process is mainly involving external liquid film diffusion, surface adsorption, intraparticle diffusion and complexation. In this work, the mechanism of adsorption process can be characterized and explained based on the SEM-EDS related with the FT-IR pattern. The kinetic and isotherm adsorption results can also explain the adsorption mechanisms of electrostatic interaction, surface complexation, and ion exchange for the removal of Pb²⁺ ions by FMBC. The potential mechanism between FMBC and Pb²⁺ includes electrostatic interaction, surface complexation, ion exchange and other processes. The EDS analysis indicated that Mn, Fe, and O were present in the FMBC powder after Pb²⁺ adsorption compared to pristine surface of FMBC. The Pb was presented in the FMBC powder after Pb²⁺ adsorption. The mass percentages of elements were recorded at three points. According to the results of EDS spectrum (figure 5), the means of mass percentages (%) of C, O, Mn, and Fe of the FMBC before and after adsorption Pb²⁺ were 52.78, 31.47, 7.50, 5.58 and 56.76, 28.49, 6.42, 4.94, respectively, and the means of mass percentages (%) of Pb of the FMBC after adsorption of Pb²⁺ was 1.20%. This pattern confirmed that Pb ions were adsorbed on FMBC surfaces.

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With the results of experiments, the mechanism can be elucidated as follows:

3.3.1. Surface complexation and ion exchange

The binding of metal ion Pb²⁺ on FMBC can be explained by surface complexation. Many oxygen-containing groups existed on the surface of FMBC, such as MnFe₂O₄ and MnO₂. The FT-IR analysis confirmed that the interaction between FMBC and Pb²⁺ involved surface complexation (figure 6). For the FMBC sample, the bands of Fe–O and Mn-O (from MnFe₂O₄ and amorphous MnO₂) are from stretching vibrations of MnFe₂O₄. The functional groups on FMBC such –COOH, –OH and –C=O participate in Pb²⁺ absorption related with the shift of wavenumber as shown in table 5. This mechanism was confirmed by the decrease of pH after adsorption process. Ion exchange is an important mechanism participating in the chemisorption of Pb²⁺ ions on FMBC surfaces. The ions of Fe³⁺ and Mn²⁺ were observed in the leachate water after adsorption process confirming that the adsorption of Pb²⁺ by FMBC is also related to ion exchange and complexation, which led to the release of the observed cations.

$$\begin{eqnarray}&&-{\rm{COOH}}+{{\rm{Pb}}}^{2+}+{{\rm{H}}}_{2}{\rm{O}}\to -{{\rm{COOPb}}}^{2+}+{{\rm{H}}}_{3}{{\rm{O}}}^{+}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&-{\rm{OH}}+{{\rm{Pb}}}^{2+}+{{\rm{H}}}_{2}{\rm{O}}\to -{{\rm{OPb}}}^{+}+{{\rm{H}}}_{3}{{\rm{O}}}^{+}\,\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&{\rm{FMBC}}-{{\rm{Fe}}}^{3+}+{{\rm{Pb}}}^{2+}\to {\rm{FMBC}}-{{\rm{Pb}}}^{2+}+{{\rm{Fe}}}^{3+}\,\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{\rm{FMBC}}-{{\rm{Mn}}}^{2+}+{{\rm{Pb}}}^{2+}\to {\rm{FMBC}}-{{\rm{Pb}}}^{2+}+{{\rm{Mn}}}^{2+}\end{eqnarray} \tag{ 8 }$$

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Table 5. Functional groups of FMBC before and after adsorption of Pb²⁺ ions.

	FMBC, cm−1		FMBC-Pb2+, cm−1
Assignment	Before adsorption	After adsorption	Difference, cm−1
Bonded –OH	3398	3385	−13
C=O stretching, aromatic C=C, C=O	1580	1578	−2
C–O	1380	1367	−13
Fe–O	586	572	−14