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Preparation of three kinds of efficient sludge-derived adsorbents for metal ions and organic wastewater purification
⁎Corresponding authors. cskmust@163.com (Song Cheng), baolinxing@hpu.edu.cn (Baolin Xing)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
The sludge is pretreated to prepare the sludge bicohar by pyrolysis, which is divided into high-carbon sludge biochar and high-ash sludge biochar using flotation. The high-carbon sludge biochar and high-ash sludge biochar are used to prepare the sludge activated carbon, MgAl-layered double hydroxides (MgAl-LDO) and sludge ceramsite for Pb2+, methyl orange (MO) and ciprofloxacin (CIP) removal from wastewater, respectively. The sludge activated carbon is prepared from high-carbon sludge biochar using ZnCl2 as chemical agent in the additive of pine sawdust for MO and CIP removal with adsorption amount of the 754.05 mg/g and 635.62 mg/g, respectively. Mg-Al LDO is prepared from the leaching solution of the high-ash sludge bicohar, which can quickly remove Pb2+ from wastewater with adsorption amount of 147.89 mg/g. Mg-Al LDO has good reusability after three cycles. The MO/CIP and Pb2+ adsorption mechanism are analyzed and investigated. The sludge ceramsite prepared from the leaching residue of high-ash sludge bicohar is also used for Pb2+ removal. The above results indicate that sludge is converted into three kinds of the adsorbents for pollution removal from wastewater.
Keywords
MgAl-LDO
Sludge
Sludge activated carbon
Sludge ceramsite
Wastewater treatment
1 Introduction
Water pollution has caused global concern owe to its destruction of ecological environment (Yuan et al., 2023; Zhao et al., 2023). The Pb2+, organic dye and antibiotic wastewater are great harm to the human beings and aquatic life (Cheng et al., 2021b; Wang et al., 2022). These pollutants are not easy non-degradable in the wastewater (Wang et al., 2022a; Le et al., 2023). Pb2+ can seriously damage the nervous system, physiological systems and other organs (Cheng et al., 2021a). The organic dye will increase the chroma of receiving water, which prevents the photosynthesis in plants (Foroutan et al., 2022). The antibiotics are the organic compounds, which is one of the emerging contaminants in wastewater (Foroutan et al., 2021). Antibiotics can produce the resistant bacteria and resistance genes, which have seriously threatened to the human beings (Omer et al., 2022). These pollutions can easily migrate and accumulate in wastewater, posing a great threat to human and environment (Chakraborty et al., 2020).
The treatment methods of wastewater include chemical precipitation (Benalia et al., 2021), ion exchange (Dabrowski et al., 2004), membrane division (Li et al., 2009), adsorption (Ran et al., 2022) and phytoremediation (Awa & Hadibarata, 2020). The adsorption technology is considered as the suitable and effective treatment method among these treatment method owe to good adsorption efficiency and easy operation (Yurak et al., 2021; Zhou et al., 2022). Qin et al. (2024) prepared ZnO decorated biochar for levofloxacin removal from wastewater with adsorption capacity of 193.42 mg/g. Cheng et al., (2021a) prepared the modification crofton weed for Pb2+ removal from wastewater with the adsorption capacity of 234.60 mg/g. Cheng et al., (2022b) reported that the rhodamine B and malachite green adsorption capacities of magnetic separation photocatalyst-adsorbent are 475.49 mg/g and 646.82 mg/g, respectively. However, the price of the common adsorbent is expensive. Therefore, it is necessary to prepare the adsorbent with high adsorption capacity and low cost in practical engineering.
Preparation of high efficiency and low-cost adsorbent has attracted the attention of the researchers for pollutant removal (Wang et al., 2022b; Zhang et al., 2023). The published literatures have reported various of adsorbents prepared form various of wastes such as agricultural and sidelong products, sludge and other solids waste (Florent et al., 2019; Vázquez-Durán et al., 2022). Lee et al. (2019) used the ginkgo biloba leaves as the raw material to prepare the adsorbent for Pb(II) and Cu(II) removal with good removal, respectively. The sewage sludge is the by-product of wastewater biological treatment, which is also kind of municipal solid waste. Hu et al. (2022) reported that sludge production is about 45 million every year. The sludge has many harmful substances such as heavy metals and harmful organic matter. Therefore, it should find an environmentally-friendly method to utilize the sludge to avoid polluting environment. Cheng et al., (2021c) used sludge to prepare the ZnCl2-FeCl3 modified sludge for methyl orange removal from wastewater with good result. Yan et al. (2022) reported that sludge-coconut fiber biochar can efficiently remove the ciprofloxacin, Zn and Cd from wastewater. Ma et al. (2020) prepared the modified sludge-derived biochar for CIP, norfloxacin and ofloxacin removal from wastewater with large adsorption capacity. Liu et al. (2021) used the NaOH-modified sludge to remove tetracycline from wastewater with the adsorption amount of 379.78 mg/g. Therefore, sludge can be used as the feedstock to prepare high efficient adsorbent for pollutions removal.
Pyrolysis is a common method for pretreatment of sludge. Biochar obtained from sludge pyrolysis can be used as the adsorbent for organic and inorganic pollutants removal. However, sludge has lots of inorganic mineral salt (Al2O3, Fe2O3 and SiO2), which limit its application in wastewater treatment. Therefore, the sludge biochar should be pretreated to remove the inorganic mineral salts before use. The flotation is a kind of methods for purification of minerals, which can be used to purify the sludge biochar. The sludge biochar can be divided into the high-ash sludge biochar and high-carbon sludge biochar after flotation. The high ash sludge biochar contains bivalent, trivalent metal cations and multiple anions such as calcium, aluminum, iron and magnesium, which can be used as the feedstock to prepare the layered double hydroxides (Jawad et al., 2018). The high-carbon sludge biochar can be used to prepare the sludge active carbon for pollutants removal from wastewater. The adsorbents prepared from sludge can be used in wastewater treatment.
In this work, sludge is used to prepare the sludge biochar by pyrolysis, which is divided into the high-carbon sludge biochar and high-ash sludge biochar using flotation. The high-carbon sludge biochar is used to prepare the sludge activated carbon using ZnCl2 as chemical agent to extend the pore structure of the sludge activated carbon. Mg-Al LDO is prepared from the leaching solution of the high-ash sludge bicohar. Besides, leaching residue of high-ash sludge biochar is used as the feedstock to prepare the sludge ceramsite. The innovation of this research is that the sludge is totally converted into three kinds of adsorbents, realizing the zero-emission of the sludge and avoiding the environment pollution caused from the sludge. The Pb2+, CIP and MO are used the pollution model to investigate the adsorption performance of the prepared adsorbent. The main objectives of this work are to: (1) investigate the physicochemical properties of prepared adsorbent, (2) analyze the adsorption performance of prepared adsorbent, (3) analyze the involved in adsorption mechanism of the pollution. This research work will achieve the high-value use of the sludge.
2 Experimental
2.1 Sludge separation and utilization
The sludge is pretreated by pyrolysis to obtain the sludge biochar at 600 °C for 2 h. The sludge biochar can be divided into the high-carbon sludge biochar and high-ash sludge biochar after flotation. The high-carbon sludge biochar is mixed with pine sawdust to prepare the sludge activated carbon using ZnCl2 as chemical agent. The high-ash sludge biochar can be divided into the leaching solution and leaching residue after acid-leaching treatment. The leaching solution is used as feedstock to prepare the Mg-Al LDO in the existence of the adjusting reagent (Qu et al., 2023a). The leaching residue is mixed with the coal gangue and other auxiliary materials to prepare sludge ceramsite at high temperature. The utilization process of sludge is shown in Fig. 1.
Schematic diagram of sludge separation process.
2.2 Preparation of adsorbent derived from sludge
2.2.1 Preparation of sludge activated carbon
10 g high-carbon sludge biochar is mixed with 10 g pine sawdust in the 200 mL distilled water. Besides, 30 g ZnCl2 is added into the above mixed solution, which is stirred for 24 h. After that, the mixture is heated in the tube furnace (Song et al., 2023). The heat temperature and heat time are 500 °C and 60 min, respectively. The residue in the tube furnace is named as the sludge activated carbon.
2.2.2 Preparation of MgAl-LDO
4.8 g NaOH and 12.72 g Na2CO3 are mixed in the 500 mL deionized water to prepare the mixed alkali solution. MgCl2 is added into high-ash sludge biochar soaked in hydrochloric acid. And then 500 mL deionized water is added to prepare the mixed salt solution. The two mixtures are mixed into the beaker, which is continuously stirred at a rotating speed of 180 r/min for 30 min. Then the mixture is placed into the three-mouth flask at 60 °C. Finally, the mixture is filtered and washed, which is dried in a vacuum drying oven at 105 °C. The dried sample is named as the MgAl-LDH. After that, MgAl-LDH is heated in the muffle furnace at 500 °C for 2 h, which is named as the MgAl-LDO (Qu et al., 2023b).
2.2.3 Preparation of sludge ceramsite
The leaching residue after acid leaching of high-ash sludge biochar is dried and crushed, which is pretreated using ball mill for 8 h at a rotating speed of 180 r/min. And then the mass ratio of the leaching residue: coal gangue: kaolin: coal powder: glass powder is 6:1:1:1:1, which is mixed evenly. The mixture is added into the granulator. The prepared pellets are uniformly placed on the iron plate and dried at 105 °C for 2 h. Dried raw pellets are pushed into the muffle furnace at 1200 °C for 10 min. The residue in the muffle furnace is the sludge ceramsite. The detailed information of the adsorption experiment is in the supporting information.
2.3 Characterizations
The surface microstructure of sample is investigated using scanning electron microscopy (SEM) (SEM, Philips XL30ESEM-TMP). The composition of the sample is analyzed by X-ray diffraction (XRD) (D/max-3B, Japan) (Zeng et al., 2023). Surface chemical properties of sample are detected by X-ray photoelectron spectroscopy (XPS) (Physical Electronics, Inc., Chanhassen, MN, USA) (Shi et al., 2024). The surface functional groups of samples are analyzed using the Fourier transform infrared spectroscopy (FTIR) (Thermo Fisher Scientific, USA).
3 Results and discussion
3.1 Characterization of sludge activated carbon and its application
3.1.1 Characterization of sludge activated carbon
Fig. 2a shows the XRD spectrum analysis of the sludge activated carbon. As Fig. 2a shown, the sludge activated carbon has the peaks of inorganic substances. The peaks of SiO2 at 26.5° and 42.5° are appeared on sludge activated carbon. Besides, the strong diffraction peak of zinc silicate is also found on sludge activated carbon. Fig. 2b shows the N2 adsorption isotherm of sludge activated carbon. As Fig. 2b shown, the type of adsorption isotherm belongs to H2 hysteresis loop, which gradually increases in the low pressure with few micropores. The curve slope of the adsorption isotherm quickly increases at P/PO ≥ 0.5, indicating that the mesoporous percentage is high (Cheng et al., 2022a). The special surface area of the sludge activated carbon is 480.18 m2/g.
XRD spectrum analysis of the sludge activated carbon (a), N2 adsorption isotherm of pore distribution of the sludge activated carbon (b), and SEM images of the sludge (c–d) and sludge activated carbon (e–f).
Fig. 2c–f shows the SEM images of the sludge and sludge activated carbon. The sludge activated carbon has fibrous and irregular pore structure, forming many irregular protrusions compared to sludge. This result is similar to the “sponge” wrapped around sludge activated carbon. ZnCl2 is a kind of the chemical agent to extend the pore structure of sludge activated carbon in the preparation process. The surface of sludge activated carbon becomes loose and forms a certain of pore after ZnCl2 activation, which contributes to pollutions removal from wastewater.
3.1.2 MO and CIP adsorption on sludge activated carbon
3.1.2.1 MO and CIP adsorption kinetics
Pseudo-first-order and pseudo-second-order kinetic model are used to analyze MO and CIP adsorption process (Cheng et al., 2022b). As Fig. 3 shown, MO and CIP removal quickly increase at the beginning, and then gradually reach adsorption equilibrium. Table 1 lists the fitting results of MO and CIP adsorption data fitting adsorption kinetics models. As Table 1 shown, the correlation coefficient R2 of the pseudo-second-order kinetic is larger than the pseudo-first-order at 200–400 mg/L, indicating that pseudo-second-order kinetic can accurately describe MO and CIP adsorption process. This result indicates that MO and CIP adsorption process belong to the chemical adsorption. The efficiency of adsorption system is related to the adsorption site of the sludge activated carbon (Li et al., 2022a). It does not depend on the concentration of MO and CIP in the solution.
MO (a) and CIP (c) adsorption data fitting Pseudo-first-order kinetics, and MO (b) and CIP (d) adsorption data fitting Pseudo-second-order kinetics model.
Pollution
Model
Parameters
Initial concentration (mg/L)
200
300
400
MO
Pseudo-first-order
R2
0.977
0.837
0.796
k1
0.044
0.082
0.106
Qe,c (mg/g）
170.61
264.93
333.30
Pseudo-second-order
R2
0.991
0.985
0.986
k2
0.032
0.007
0.014
Qe,c (mg/g）
189.54
282.87
351.09
CIP
Pseudo-first-order
R2
0.689
0.639
0.541
k1
0.097
0.126
0.191
Qe,c (mg/g）
166.01
205.01
259.27
Pseudo-second-order
R2
0.911
0.925
0.897
k2
0.008
0.009
0.001
Qe,c (mg/g）
178.24
217.68
271.82
3.1.2.2 MO and CIP adsorption isotherm
MO and CIP adsorption process are further analyzed using the Langmuir and Freundlich isotherm models. Table 2 lists the fitting results. As Table 2 shown, MO and CIP adsorption on sludge activated carbon conform to the Langmuir and Freundlich adsorption isotherm model. However, the R2 values of MO and CIP adsorption data fitting the Langmuir model are large compared to the Freundlich model, indicating that Langmuir model is more consistent with MO and CIP adsorption process. As Table 2 shown, MO and CIP adsorption amount are increase as adsorption temperature increases. This result indicates that MO and CIP adsorption on sludge activated carbon are endothermic process. Therefore, MO and CIP adsorption amount of sludge activated carbon could be improved by increase of the adsorption temperature in the actual adsorption process. As Table 2 shown, the MO and CIP adsorption amount of the sludge activated carbon are 754.05 and 635.62 mg/g, respectively, according to Langmuir model calculation. Fig. 4 shows the MO and CIP adsorption data fitting the Langmuir and Freundlich adsorption isotherm models. As Fig. 4 shown, MO and CIP adsorption amount gradually increase until adsorption equilibrium as MO and CIP concentration increase at different adsorption temperature. Table S3 lists the adsorption capacity of similar adsorbents for MO and CIP removal from wastewater. The comparison result indicates that sludge activated carbon has promising potential in MO and CIP wastewater treatment.
Pollution
Isotherms
Parameters
Temperature (°C)
30
40
50
MO
Langmuir
R2
0.993
0.993
0.990
Qmax (mg/g)
726.57
745.65
754.05
Freundlich
R2
0.986
0.987
0.986
n
0.661
0.63369
0.618
kF
6.800
8.481
9.446
CIP
Langmuir
R2
0.983
0.981
0.984
Qmax (mg/g)
618.37
624.32
635.62
Freundlich
R2
0.967
0.982
0.978
n
0.446
0.434
0.430
kF
19.854
22.148
22.863
MO (a) and CIP (c) adsorption data fitting the Langmuir model, and MO (b) and CIP (d) adsorption data the Freundlich model.
3.1.2.3 Adsorption mechanism
FT-IR spectra of the sludge activated carbon before and after MO and CIP adsorption are shown in Fig. 5a. As Fig. 5a shown, the peaks of the 3420, 2925 and 1626 cm−1 correspond to the —OH stretching vibration peak, C—H stretching vibration and C⚌C stretching vibration, respectively. The peak position of the C⚌C stretching vibration peak has changed to the 1605 cm−1 after MO adsorption. This result indicates that it exists the π-π conjugate reaction between sludge activated carbon and benzene ring of MO. The fluorine in CIP molecule has a strong ability of electron absorption, which can bond with C atom. Therefore, it can be acted as the π-electron acceptor to form the π-π accumulation with aromatic C⚌C. Besides, surface chemical groups such as C⚌O and —OH can also bind with MO and CIP, realizing MO and CIP removal from wastewater.
FT-IR spectra (a) and C1s spectra analysis of before (b) and after MO (c) and CIP (d) adsorption of sludge activated carbon.
C1s spectra analysis of the sludge activated carbon before (a) and after MO (b) and CIP (c) adsorption are shown in Fig. 5b. C1s spectrum has three peaks, which are O—C⚌O, C—O and C—C/C⚌C group, corresponding to the binding energies of the 289.92 eV, 286.12 eV and 284.80 eV, respectively. O—C⚌O/C—O peak area decrease by 0.43 % and 2.20 % after MO adsorption, respectively. While, O—C⚌O peak area also decreases after CIP adsorption. This result indicates that O—C⚌O/C—O group involve in MO and CIP adsorption.
Sludge activated carbon has well-developed pore structure, which also contributes to MO and CIP adsorption. In summary, the adsorption mechanism of MO and CIP includes physical adsorption of pore in the sludge activated carbon, π-π conjugate reaction and surface functional groups complexation.
3.2 Characterization of MgAl-LDO and its application
3.2.1 Characterization of MgAl-LDO
XRD spectra of the MgAl-LDH and MgAl-LDO are shown in Fig. 6a. As Fig. 6a shown, the characteristic diffraction peaks of the MgAl-LDH are obvious appeared at peak of the 11.86°, 23.63° and 35.09°. This result indicates that MgAl-LDH is successfully prepared using the leaching solution of high-ash sludge bicohar. Characteristic diffraction peak of the MgAl-LDH is similar with the MgAl-LDO. N2 adsorption isotherm analysis of the MgAl-LDO and MgAl-LDH is shown in Fig. 6b. The adsorption isotherm curve is the typical of non-rigid aggregates of H3 type. The specific surface area of the MgAl-LDH and MgAl-LDO is 17.59 m2/g and 52.31 m2/g, respectively. The specific surface area of the MgAl-LDO increases after heat treatment. MgAl-LDO has large specific surface area, contributing to pollutants removal.
XRD spectra of MgAl-LDH and MgAl-LDO (a), N2 adsorption–desorption isotherms and pore size distribution curves of MgAl-LDH and MgAl-LDH (b), SEM (c–d) and TEM (e–f) images of MgAl-LDH, and SEM (g–h) images of MgAl-LDO.
Fig. 6c–h shows SEM-TEM images of the MgAl-LDH and MgAl-LDO. As Fig. 6c–d shown, MgAl-LDH has good morphology with the platelet-like nanosheet structure. The lamellar structure of the MgAl-LDH stacks tight with the uniform structure, indicating that MgAl-LDH has high purity. Fig. 6e shows the cross-section of MgAl-LDH with the diameter of 77.27 nm. The fringe widths of MgAl-LDH are 0.376 nm and 0.227 nm, which are consistent with the peaks (0 0 6) and (0 1 5) of PDF#54-1030 of MgAl-LDH standard card, respectively (Khatem et al., 2015). As Fig. 6g–h shown, MgAl-LDO appears the irregular polygon structure after heat treatment.
3.2.2 Pb2+ adsorption on MgAl-LDO
3.2.2.1 Pb2+ adsorption kinetics
Pb2+ adsorption kinetics process is further analyzed using the pseudo-first-order and pseudo-second-order model (Shi et al., 2021). The fitting results of Pb2+ adsorption on MgAl-LDO are listed in Table 3. Pb2+ adsorption data fitting the pseudo-first-order and pseudo-second-order model is shown in Fig. 7. As Fig. 7 shown, Pb2+ adsorption amount is general increase as adsorption time increases. Pb2+ adsorption on MgAl-LDO quickly reaches the adsorption equilibrium before 80 min due to fact that MgAl-LDO has lots of active site. Subsequently, Pb2+ adsorption on MgAl-LDO is slow increase. It can be explained that the active sites of MgAl-LDO are occupied and little active site is available for Pb2+ adsorption. R2 value of the pseudo-first-order is low compared to the pseudo-second-order of 0.984. This result indicates that Pb2+ adsorption behavior conforms the pseudo-second-order model. The calculated values of qe and qcal are close to the adsorption result based on pseudo-second-order. Therefore, Pb2+ adsorption on MgAl-LDO can be more accurately analyzed by the pseudo-second-order (Li et al., 2022).
Model
Parameters
Initial concentration (mg/L）
200
300
400
Pseudo-first-order
R2
0.981
0.982
0.982
k1
0.006
0.008
0.012
Qe,c (mg/g）
133.17
142.97
150.15
Pseudo-second-order
R2
0.984
0.985
0.986
k2
0.001
0.001
0.001
Qe,c (mg/g）
195.74
195.99
181.82
Pb2+ adsorption data fitting the Pseudo-first-order kinetics (a) and Pseudo-second-order kinetics (b) models.
3.2.2.2 Pb2+ adsorption isotherm
Two kinds of adsorption isotherm models such as Langmuir and Freundlich models are used to analyze Pb2+ adsorption on MgAl-LDO. Pb2+ adsorption data fitting the adsorption isotherm models is listed in the Table 4. As Table 4 shown, the R2 values of the Langmuir and Freundlich models are 0.975–0.987 and 0.991–0.997, respectively. This result demonstrates that Freundlich model better describes Pb2+ adsorption process than Langmuir model. Due to large size of Pb ion, only a small amount of Pb ion can enter the interlayer, and most of the Pb ion are adsorbed on the surface of the hydrotalcite nanosheet. The Pb2+ adsorption amount of the MgAl-LDO is 147.89 mg/g, according to Langmuir model calculation. Pb2+ adsorption amount of MgAl-LDO is increase as adsorption temperature increases, indicating that Pb2+ adsorption on LDO is endothermic process. The fitting result of the adsorption isotherm models is shown in Fig. 8. Table S4 lists the adsorption capacity of similar adsorbents for Pb2+ removal from wastewater. The comparison result indicates that MgAl-LDO has promising potential in Pb2+ wastewater treatment.
Model
Parameters
Temperature (°C)
30
40
50
Langmuir
R2
0.975
0.979
0.987
Qmax (mg/g)
147.89
153.96
172.7
kL
0.006
0.008
0.007
Freundlich
R2
0.997
0.995
0.991
n
0.337
0.288
0.329
kF
0.847
0.503
0.849
Pb2+ adsorption data fitting the Langmuir (a) and Freundlich (b) models.
3.2.2.3 Adsorption mechanism
XRD spectra of the MgAl-LDO before and after Pb2+ adsorption are shown in Fig. 9a. As Fig. 9a shown, the characteristic peaks of the MgAl-LDO are weakened after Pb2+ adsorption. Besides, the characteristic peaks of Pb3(CO3)2(OH)2 appear at 9.846°, 20.884°, 24.640°, 27.080°, 34.155° and 40.396°. This result indicates that Pb2+ forms carbonate precipitation, realizing Pb(II) removal from wastewater. Zhang and Yuntao (2018) reported that Pb2+ forms the Pb3(CO3)2(OH)2 on the biochar/hydrotalcite composite, which is similar with our work.
The XRD spectra of MgAl-LDO before and after Pb2+ adsorption (a), SEM images of MgAl-LDO (b–c) and Pb2+ adsorption on MgAl-LDO (d–e) and EDS images of the Pb2+ adsorption on MgAl-LDO (f–g).
As Fig. 9b-–e– shown, the peaks of the MgAl-LDO before and after Pb2+ adsorption almost changed. However, the surface of the MgAl-LDO after Pb2+ adsorption is rough, which just likes the “potato chip” shape. The surface of the MgAl-LDO has C, Mg, Al and Pb elements based on EDS mapping images (Fig. 9f–g), indicating that Pb2+ is successful adsorbed on the MgAl-LDO.
Fig. 10 shows the XPS spectra of the MgAl-LDO before and after Pb2+ adsorption. As shown in Fig. 10a, the sign of the Pb4f is found on the MgAl-LDO after Pb2+ adsorption. Besides, Al, Mg, O and C elements are appeared on the MgAl-LDO. The atomic ratio of Mg/Al of the MgAl-LDO decreases from 0.84 to 0.75 after Pb2+ adsorption. This result indicates that Mg2+ may be replaced with Pb2+, demonstrating that it is existence of cation exchange between Mg2+ and Pb2+. Therefore, cation exchange also contributes to Pb2+ removal from wastewater. In addition, the spectrum of the C1s has three peaks of the C—C, C—OH and O—C⚌O groups. However, the peak location and peak area of the C1s spectrum have changed after Pb2+ adsorption. For instance, the peak area of C—OH and O—C⚌O groups decrease by 13.47 % and 5.43 %, respectively. The reason might be that surface oxygen-containing functional groups bond with Pb2+ to form O—Pb group, realizing Pb2+ removal from wastewater. As Fig. 10d shown, two peaks at 138.17 and 143.02 eV are appeared on the MgAl-LDO after Pb2+ adsorption, which are attributed to the Pb4f7/2 and Pb4f5/2, respectively. This result indicates that it might form the OH—Pb, O—Pb and COO—Pb group, realizing Pb2+ removal from wastewater. The Be center of PbCO3 is 138.7 eV, while the Be center of Pb(OH)2 is 137.3 eV based on the report of the Park et al. (2007). The peaks location of the PbCO3 and Pb(OH)2 are within the scope of the 138.17 and 143.02 eV, respectively, indicating Pb2+ might form PbCO3 and Pb(OH)2. This result is consistent with XRD analysis. The summary of Pb2+ adsorption mechanism is shown in Fig. 10e.
XPS scan spectrum (a) and C1s XPS spectra of MgAl-LDO before and after Pb2+ adsorption (b-d) and Pb4f of MgAl-LDO after Pb2+ adsorption (e).
3.2.2.4 Reusability of the MgAl-LDO
The reusability result of the MgAl-LDO is shown in Fig. 11. Thiourea and Pb2+ can generate the complex cation
(Pb2+ +
=
. Therefore, thiourea is used as the desorption agent in the reusability experiment. As Fig. 11 shown, the Pb2+ removal decreases after three cycles. The slight decrease of Pb2+ removal may be due to the incomplete desorption of Pb2+ adsorbed on the MgAl-LDO or the slight destruction of layered structure of MgAl-LDO. The results indicates that MgAl-LDO has good reusability after three cycles.
Desorption-adsorption cycle of the MgAl-LDO.
3.3 Characterization of sludge ceramsite and its application
XRD analysis of the sludge ceramsite is shown in Fig. 12a. As Fig. 12a shown, sludge ceramsite mainly has 3Al2O3·2SiO2, SiO2 and Al2O3 with the high peak intensity. These three minerals constitute the structural skeleton and pore structure of the sludge ceramsite. N2 adsorption isotherm curve of the sludge ceramsite is shown in Fig. 12b. As Fig. 12b shown, the adsorption isotherm curve of sludge ceramsite belongs to the H3 type, which is the characteristic curve of macroporous solid materials. There is no obvious absorption peak in the low pressure area, and the interaction between sludge ceramsite and nitrogen is weak. A large number of pores of sludge ceramsite are gradually filled by N2 molecules with increasing in pressure. The N2 adsorption on the surface of sludge ceramsite generally increases as adsorption pressure increases. Therefore, the N2 adsorption on sludge ceramsite is multi-layer adsorption with underdeveloped micropores and a large proportion of large pores. The average pore size of the sludge ceramsite is 5.83 nm. The surface of sludge ceramsite is rough, which has lots of the interconnected pores (Fig. 12c–d). Theses pores are formed in the preparation process of the sludge ceramsite.
XRD spectrum of sludge ceramsite (a), N2 adsorption–desorption isotherms and pore size distribution curves of sludge ceramsite (b), SEM images of sludge ceramsite (c–d).
The adsorption capacity of sludge ceramsite is limited for the wastewater treatment. Therefore, sludge ceramsite is modified to improve its adsorption capacity. The sludge ceramsite is immersed in 1 mol/L sodium hydroxide solution for alkali modification. As Fig. 13a shown, Pb2+ adsorption amount of sludge ceramsite after alkali modification is 1.437 mg/g, which is increase with increasing in adsorption temperature. The SEM-EDS images of the sludge ceramsite after Pb2+ adsorption are shown in Fig. 13b-–d–. As shown in Fig. 13d, the sludge ceramsite is composed of the Al, Si, O and Pb elements, indicating that Pb2+ is successfully adsorbed on the sludge ceramsite.
SEM images of sludge ceramsite (a-b), EDS image of sludge ceramsite adsorption Pb2+ (c) and Pb2+ adsorption capacity of sludge ceramsite (d).
4 Conclusion
The sludge activated carbon, MgAl-LDO and sludge ceramsite are successfully prepared from sludge for pollutant removal from wastewater. The MO and CIP adsorption on sludge activated carbon can be analyzed using the Langmuir isotherm and pseudo-second-order model with MO adsorption amount of 754.05 mg/g and CIP adsorption amount of 635.62 mg/g, respectively. Mg-Al LDO can quickly remove Pb2+ from wastewater with adsorption amount of 147.89 mg/g. Pb2+ adsorption mechanism indicates that mineral precipitation and functional group complexation contribute to Pb2+ removal from wastewater. Mg—Al LDO also has good reusability after three cycles, which has promising potential in actual application. The prepared sludge ceramsite can be also used as adsorbent for Pb2+ wastewater treatment after modification. The Pb2+ adsorption ability of the sludge ceramsite should be improved in the future.
Acknowledgments
The authors would like to express their gratitude to the Specialized Research Fund for the National Natural Science Foundation of China (51974110, 52074109, 52274261 and 21966019), Natural Science Foundation of Henan Province (232300420298), the Fundamental Research Funds for the Universities of Henan Province (NSFRF220417) for financial support.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Appendix A
Supplementary material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2024.105671.
Appendix A
Supplementary material
The following are the Supplementary data to this article:Supplementary data 1
Supplementary data 1
