3.1. Characterization of materials
N2 adsorption/desorption isotherm analyzer at liquid nitrogen temperature (77 K) was used to determine the surface area, pore volume, and pore size of SB. The optimal activation ratio of carbonized sludge over ZnCl2 was found to be 1:1. The surface area, pore volume and pore size of the SB-50%ZnCl2 were estimated to be 525 m2 g− 1, 0.35 cm3 g− 1 and 8.71 nm, respectively. The isotherms belonged to Type IV in the definition of International Union of Pure and Applied Chemistry and indicated that N2 is condensed in the pores at high relative pressures. There was a hysteresis loop of the H2 style since the pores had a narrow inlet and wide hole, as shown in Fig. 1. The surface area and pore volume in this study higher than Rahman et al. [33] result (375.32 m2 g− 1 and 0.239 cm3 g− 1) and Mahmood et al. [34] result (167 m2 g− 1 and 0.16 cm3 g− 1).
The functional groups are very important characteristics of the SB, because they determine the surface properties of the SB and their quality. It can provide basic spectra of SB, especially for the determination of types and intensities of their surface functional groups. The changes in the functional groups of different Fe content and appropriate amount of DETA modified on SB were analyzed by FTIR spectroscopy, as shown in Fig. 2. The peak at 3400 cm-1 of 8%Fe-SB-DETA is significantly stronger than the peak of other materials. The -OH and N-H stretching vibration band are characteristic peaks of the amine group at about 3400 cm-1 [35]. The appearance of peaks at 2930 and 2849 cm-1 in the spectrum were attributed to C-H stretching vibration in -CH and -CH2 [35]. The band at 1620 cm-1 can be ascribed to C = C aromatic ring stretching vibration [36]. The band at 1470 − 1430 cm-1 is ascribed to C-H bending vibrations in CH3 groups [35]. C = O vibrations at 1100 − 1000 cm-1 are produced by the stretching vibration of the oxygen-containing functional group C = O bond [37]. There is a weak vibration band between 765 − 530 cm-1, which is judged as an aromatic structure. Due to the large amount of organic matter in the sludge, the chemical structure is composed of a large number of different atoms.
The aim of the XRD studies is to determine the species of SB, SB-DETA and 8%Fe-SB-DETA compounds deposited onto the biochar surface, as show in Fig. 3. The XRD pattern of biochar exhibited one broad diffraction peak corresponding to the diffraction of carbon. A peak at 2θ = 25° was observed for the SB, which was attributed to the carbon in SB. Compared with that of SB, SB-DETA and 8%Fe-SB-DETA, Fe-O typical peak was observed for 8%Fe-SB-DETA at 2θ = 36° and 57°, respectively, shown in Fig. 3 (c).
During the present studies, our aim was to look into the difference in the interaction oxidation/reduction mechanism of metal ions on SB using ECT method. Cyclic Voltammetry (CV) is used to investigate the effect of surface modification, which could provide useful information about the surface states of SB due to the presence of metal vacancy defects on the surface. Figure 4 shows the CV of 2%Fe-SB-DETA, 4%Fe-SB-DETA, 8%Fe-SB-DETA, 16%Fe-SB-DETA and 32%Fe-SB-DETA electrodes in 6 M KOH solution. The bounded area of the CV curve indicates the real active surface area of the anode and the cathode. 8%Fe-SB-DETA (0.00062 mA) shows a higher current than the 16%Fe-SB-DETA (0.00047 mA), 32%Fe-SB-DETA (0.00044 mA), 4%Fe-SB-DETA (0.00031 mA) and 2%Fe-SB-DETA (0.00023 mA) at the anode. Additionally, 8%Fe-SB-DETA (-0.0016 mA) shows a higher current than the 16%Fe-SB-DETA (-0.0012 mA), 32%Fe-SB-DETA (-0.0011 mA), 4%Fe-SB-DETA (-0.0007 mA) and 2%Fe-SB-DETA (-0.0005 mA) at the cathode. This is indicating that 8%Fe-SB-DETA has a larger active point and better oxidation reduction reaction.
Figure 5 shows the linear sweep voltammetry curves of the samples deposited at different potentials. The currents of the 8%Fe-SB-DETA (0.0027 mA) are improved after being compared with that of 2%Fe-SB-DETA (0.00007 mA) and 4%Fe-SB-DETA (0.0006 mA), indicating that the hydrogen evolution over potential with 8%Fe-SB-DETA is lower than that with 2%Fe-SB-DETA. As a result, 8%Fe-SB-DETA (0.0027 mA) has the largest currents than that the 16%Fe-SB-DETA (0.0012 mA) and 32%Fe-SB-DETA (0.0011 mA). In addition, 8%Fe-SB-DETA composite interface displays a significant change in current–voltage properties as compared to 2%Fe-SB-DETA. The result shows that the electron transfer rate of 8%Fe-SB-DETA is stronger than other materials, which is beneficial to pollutants adsorption.
The current density is tested more than 300 s and maintaining the applied voltage constant at 1.0 mV. Figure 6 shows the currents density are 0.0045, 0.0050, 0.0084, 0.0063 and 0.0060 mA for 2%Fe-SB-DETA, 4%Fe-SB-DETA, 8%Fe-SB-DETA, 16%Fe-SB-DETA and 32%Fe-SB-DETA, respectively. The current of 8%Fe-SB-DETA is 1.9 times larger than that of 2%Fe-SB-DETA, indicating that doping with iron could significantly enhance electron mobility by reducing the recombination of electron-hole pairs.
Electrochemical impedance spectroscopy is a technique to measure the internal resistance of working electrode material and determine the circuitry and corresponding resistance between the electrolyte and the electrode, like biochar carbon in this case. A sine wave of 1.0 mV amplitude was applied and the frequency was varied from 10 KHz to 10 MHz in the 6 M KOH aqueous solution. Ionic and electronic interactions cause the overall impedance of the materials. The larger value of the Z’’ (ohm) is indicative of higher resistance due to the transfer of charges. The overall resistance consists of electrical resistance, the resistance due to electrolyte, and the resistance for the carbon pore charge transfer as well as the internal resistance of the material. Result shown in Fig. 7, the 2%Fe-SB-DETA produces high the Z’’ value is indicative of higher resistance due to the transfer of charges. The smaller Z’’ value of 8%Fe-SB-DETA indicates higher charge carrier transfer efficiency. This determines the effective characteristics of materials as adsorption material. 8%Fe-SB-DETA stands out among all five samples as ideal adsorption material and better adsorption capacity.
3.2. As(III) adsorption capacity with Fe-SB-DETA
The effect of Fe loading mass onto SB-DETA is also discussed in this study. Fe ratio varies with specific dose applied by FeCl3 during the synthesis process. Different FeCl3 contents, including 2.0, 4.0, 8.0, 16.0 and 32.0 %, were evaluated with 1.5 g L-1 of 8%Fe-SB-DETA at 0.5 mg L-1 As(III) with 30 min contact time. The As(III) adsorption experiment was controlled pH 3 to obtain optimum operating conditions in this study, because the valence of As(III) was (±) neutral at pH between 2 and 8. However, the valence of As(III) was (-) negative under pH was larger than 9. In addition, the surface of 8%Fe-SB-DETA was (+) positively charged at pH between 2 and 3. However, the surface of As(III) was (-) negative under pH was larger than 4, as shown in Table 1. Therefore, there is not repulsion between As(III) and 8%Fe-SB-DETA. In this adsorption experiment, first, As(III) is oxidized to form As(V) by 8%Fe-SB-DETA. Afterward, the electrostatic interaction between As(V) and the material. At the same time, As(V) and the amino functional group on the surface of the material will also produce complexation, so that As(V) is adsorbed on the surface of the material.
Table 1
The effect of material surface charge under different pH conditions
pH | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | pHzpc |
As(III) | ± | ± | ± | ± | ± | ± | ± | - | - | - | |
As(V) | ± | - | - | - | - | - | - | - | - | - | |
8%Fe-SB-DETA | + | + | - | - | - | - | - | - | - | - | 3.87 |
Result shows the removal efficiency of arsenic significantly increases from 30–90% with increasing Fe content from 2.0 to 16.0%. However, the removal efficiency is not changed obviously from 85–90% with increasing Fe content from 8.0 to 16.0%. In contrast, the removal efficiency is decreased from 90–66% with increasing Fe content from 16.0 to 32.0%. In this study, it shows that the Fe impregnated on SB-DETA significantly can enhance the adsorption capacity for arsenic. The higher Fe content does not increase removal efficiency too much when Fe is doped on SB-DETA, maybe the pore of 8%Fe-SB-DETA is blocked. The adsorption capacity of 8%Fe-SB-DETA was calculated as 1.12, 2.23, 3.87, 4.24 and 2.83 mg g− 1 with Fe content of 2.0, 4.0, 8.0, 16.0 and 32.0 %, respectively, as shown in Fig. 8.
3.3. Mass balance experiment
Mass balance experiment was conducted to understand the mass before and after arsenic adsorption in the adsorption experiment, as shown in Fig. 9. The adsorption of As was studied with 20 mg L− 1 As(III) solution with using 43 g L− 1 of 8%Fe-SB-DETA material at pH 3 for 30 min contact time. The remaining concentrations and mass of As(III) and As(V) in 0.2 L solution after adsorption are 1.2 ppm and 0.24 mg; 2.3 ppm and 0.46 mg, respectively. In this specific adsorption, As(III) is oxidized to form As(V) by 8%Fe-SB-DETA. This result has been proved by XPS analysis results that Fe3+ reduced to Fe2+ after As(III) adsorption. Afterward, the iron oxyhydroxides particles located on the SB-DETA surface are capable of replacing the -OH ligand of As(V) molecules, forming mono or bi-dentate complexes allowing them to be attached to the surface. In addition, the 8%Fe-SB-DETA powder was subjected to a dissolution experiment, and the test results showed that the dissolution concentrations and mass of As(III) and As(V) were 6.8 ppm and 1.36 mg; 9.4 ppm and 1.88 mg, respectively. Result show the total dissolved concentration after arsenic adsorption is 3.94 mg. The concentration of arsenic is lost 0.06 mg, which is equivalent to 1.5 ± 0.2%. This is caused by the loss of 8%Fe-SB-DETA powder during the dissolution test and filtration process.
3.4. The adsorption configurations of arsenic on Fe-SB-DETA
In order to understand the adsorption mechanism of amine functional group and metal functional group complexes for arsenic adsorbed on the surface of SB and Fe-SB-DETA. Therefore, this study was used DFT to verify the arsenic adsorption mechanisms by SB and Fe-SB-DETA composite material. In this research, single layer structure is used to simulate carbon surface models of SB, as shown in Fig. 10 (a). Figure 10 (b) shown that the arsenic was adsorbed onto SB surface with Eads values of -27.3 kJ mol− 1, indicating a physical adsorption between arsenic and SB surface. This result has proved electrostatic between the positively charged surface of the SB and the anionic arsenic. The results for the adsorption of arsenic on the Fe-SB-DETA surface display all the obtained conformations in Fig. 10 (c-d) and Table 2. It also shows that the bond type, bond distance and adsorption energies. Adsorption configurations were considered in Fig. 10 (c-d) order to achieve the most stable arsenic adsorption configuration. The more monodent conformations have been identified, where the interaction of dopant O with the unprotonated O atom belonging to an As-O bond is formed. The bond distances of N-O and Fe-O are 1.84 and 1.42 Å, respectively, which are shorter for the compounds in compliance with the increased adsorption energies (-93.5 and − 246.3 kJ mol− 1), indicating a strong chemical adsorption between arsenic and Fe-SB-DETA surface.
Table 2
Adsorption energies of arsenic adsorbed onto Fe-SB-DETA
Pollutant | Bond | Bond distances (Å) | Eads (KJ mol− 1) |
As(III) | C-O (b) | 3.52 | -27.3 |
N-O (c) | 1.84 | -93.5 |
Fe-O (d) | 1.42 | -246.3 |
3.5. Mechanism discussion of arsenic adsorption on 8%Fe-SB-DETA
The mechanism for the adsorption of arsenic by 8%Fe-SB-DETA material has been proposed with the results obtained from the characteristics analysis, experimental data and kinetic adsorption. The reaction mechanism is divided into four pathways, as presented in Fig. 11. In the first pathway, first, As(III) is oxidized to form As (V) by 8%Fe-SB-DETA at pH 3, as shown in the formula (3). Afterward, the iron oxyhydroxides particles located on the 8%Fe-SB-DETA surface are capable of replacing the -OH ligand of As(V) molecules, forming mono or bi-dentate complexes allowing them to be attached to the surface [38–40], as shown in the formula (4). The second pathway is the interaction of amino functional groups on the surface of 8%Fe-SB-DETA with the As(V) molecule produces a complexation interaction, causing As(V) to be adsorbed on the surface of the material, as shown in the formula (5). In the third pathway, the attachment of As(V) ions onto the 8%Fe-SB-DETA via electrostatic interactions, the surface of the material is positively charged and the As(V) ion is negative charged produces electrostatic adsorption under acidic conditions. In this specific adsorption, the coordination only occurs in charged molecules that were previously attracted to the surface of the 8%Fe-SB-DETA, or in those molecules that have energy enough to counteract the electrostatic repulsion with the 8%Fe-SB-DETA surface. Therefore, in 8%Fe-SB-DETA with a negative surface charge, electrostatic repulsion between the surface and As(V) anions occurs. If these ionic molecules have difficulties in overcoming these electrostatic forces they cannot interact to a higher extent with iron particles and hence adsorption occurs to a lesser extent [38]. The fourth pathway is the attachment of As(III) and As(V) ions into the pore of 8%Fe-SB-DETA material via physical adsorption, which may be attributed to Van der Waals forces.
M-FeOOH + H3AsO3 = M-FeO + H2AsO4− + 2H+ (3)
M-FeOOH + H2AsO4− + H+ = M-FeO-H2AsO4 + H2O (4)
M-NH3 + H2AsO4− = M-NH2-H3AsO4 (5)
In order to understand whether As(III) is oxidized to As(V), this study uses XPS to detect the material before and after adsorption. The valence state and relative content of As(III) after adsorption was studied with 1.5 g L− 1 of SB-DETA and 8%Fe-SB-DETA, 0.5 mg L− 1 As(III) solution, pH 3 and 30 min contact time. XPS peak differentiation-imitating analysis of the As3d spectrum was conducted. The core level spectrum of As3d of the SB-DETA sample reveals the binding energy peaks at 44.3 eV and 45.5 eV corresponding to As(III) and As(V) core levels, respectively. The percentages of As3+ (79.15%) and As5+ (20.85%) relative to the total Fe were calculated, mainly As3+ valence state, as shown in Fig. 12 (a). Figure 12 (b) can be seen that the relative content of As3+ after adsorption on the surface of 8%Fe-SB-DETA was reduced to 39.65%; meanwhile, that of As5+ increased to 60.35%.
To determine the valence state and relative content of Fe the reaction, XPS peak differentiation-imitating analysis of the Fe2p spectrum was conducted, as shown in Fig. 12 (c, d). The core level spectrum of Fe2p of the 8%Fe-SB-DETA sample reveals the binding energy peaks at 714.5 eV and 728.1 eV corresponding to Fe2p3/2 and Fe2p1/2 core levels, respectively. The photoelectron peaks observed at 714.3 eV and 717.8 eV corresponded to the binding energies of Fe2p3/2, which suggested that the SB-DETA was covered by a layer of iron oxides/hydroxides of iron, such as FeO, Fe3O4 and Fe(OH)3. The percentages of Fe3+ and Fe2+ relative to the total Fe were calculated. By comparing Fig. 12 (c) and (d), it can be seen that the relative content of Fe3+ after the reaction was reduced from 58.28–21.88%; meanwhile, that of Fe2+ increased from 41.72–78.12%. This result is consistent with the mass balance experiment, Fe3+ (21.88%) are reduced to Fe2+ (78.12%) and As3+ (39.65%) are oxidized to As5+ (60.35%).
In order to understand whether there is complex adsorption between 8%Fe-SB-DETA and arsenic after modification. The functional group changes of 8%Fe-SB-DETA before and after the adsorption of arsenic under different initial concentrations, 1.5 mg L-1 dosage at pH 3 and 30 min, are shown in Fig. 13. The intensity of the characteristic peaks at 3400 cm-1 and 1040 cm-1 was significantly reduced when increased of the arsenic concentration. This result proved the C-NH3 function group on the surface of 8%Fe-SB-DETA has generated complexation with the arsenic.