The sewage sludge production has been increasing along with the ever-growing populations and wastewater treatment rate. Lanthanum-doped activated carbon (AC-La) was derived from municipal sludge via chemical activation and utilized for fluoride removal. Batch experiments were conducted to discuss the effect of lanthanum dosage, time and pH on the adsorption process. The results showed that 4 g/L AC-La exhibited a fluoride removal rate of 80.9% with 10 mg/L initial fluoride concentration, and the optimal pH range for adsorption was 3–10. X-ray fluorescence, scanning electron microscopy with energy-dispersive X-ray spectroscopy, Brunauer-Emmett-Teller, X-ray diffraction, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses were conducted to analyze the microstructure and chemical properties of sludge, unmodified activated carbon (AC) and AC-La. The results showed that with initial lanthanum dosage of 15 wt%, the final loading amount of La in AC-La was 13.8 wt%. After modification, the specific surface area of AC-La increased from 1.8 m2/g (sludge) to 133.0 m2/g. The removal mechanism of fluoride onto AC-La was mainly the inner-sphere complexation between lanthanum and fluoride, facilitated by exchange interaction with hydroxyls. A stability study showed that AC-La maintained a quite small dissolution and was safe in waters (La dissolution rate < 0.2‰).

  • Lanthanum-doped activated carbon (AC-La) was prepared and characterized.

  • An AC-La dosage of 4 g/L exhibited a fluoride removal of 80.9%.

  • The defluoridation mechanism was mainly the inner-sphere complexation and hydroxyls exchange.

Fluoride pollution in natural groundwater is a worldwide environmental problem, which is caused by dissolution of various fluorine-containing minerals including cryolite, fluorapatite and fluorite topaz (Yadav et al. 2018). Meanwhile, industrial wastewaters discharged from metal plating, glass production, electrochemistry, semiconductor manufacturing, brick and iron operations and coal combustion also contribute to fluoride pollution to a great extent. More than 200 million people worldwide are suffering from endemic drinking water fluorosis (Yu et al. 2018). Long-term and excess intake of fluoride may lead to various diseases of the human body such as skeletal fluorosis, heart failure, thyroid disorder, and even brain damage. The World Health Organization (WHO) recommends 1.5 mg/L as the maximum fluoride concentration in drinking water.

A wide spectrum of technologies are employed for defluoridation of contaminated water such as adsorption, reverse osmosis, membrane filtration and chemical precipitation. Among those methods, adsorption has been considered an adequate method due to its easy manipulation, excellent efficiency, comparably low cost and stable treatment. Adsorbents are crucial in the adsorption process for contaminant removal. Materials including activated alumina, activated carbon, chitosan, clays, zeolite, polymers and others are adopted for fluoride adsorption. Out of all the adsorbents, activated carbon is one kind of important adsorbent, with excellent properties including adjustable specific surface area, developed pore structure and stable chemical properties. Furthermore, utilization of by-products from agriculture and industry to produce activated carbons through chemical and physical activation methods is much more economical and eco-friendly.

As a residue of wastewater treatment plants, the sewage sludge production has been increasing along with the ever-growing population and wastewater treatment rate in recent years. Due to the existence of heavy metals and pathogenic microorganisms, the improper disposal of municipal sludge would cause serious environmental pollution and be a great threat to human health. It is necessary to handle and dispose of municipal sewage sludge properly. Accordingly, the conversion of excess municipal sewage sludge into activated carbons is an innovative way. The solid biomass of sewage sludge generated from wastewater treatment plants contains about 50–70% organic matter (Zeng et al. 2019). After pyrolysis and carbonization, most of the organic matters have decomposed and formed a channel structure in the sludge, which could greatly improve the product's adsorption capacity and suitability for contaminants removal. The feasibility of conversion of rich carbonaceous sludge into stable activated carbon has been widely studied (Gong et al. 2020).

However, simple activated carbon derived from municipal sewage sludge is limited in pollutant removal due to its morphology structure, surface property and adsorption sites. Further modification on the raw material is essential for a better performance. Over the years, lanthanum (La) has been utilized for material modification due to its strong affinity with fluoride. Kong et al. (2019) prepared a novel three-dimensional rice-like La@MgAl nanocomposite with a fluoride adsorption capacity of 51.03 mg/g. Hernandez-Campos et al. (2018) used La-doped silica xerogels for defluoridation and found that the mechanism was attributed to the LaF3 precipitation on the composite surface. In addition, different host materials and different lanthanum species affect the ultimate adsorption behavior and mechanism. Zhang et al. (2019) synthesized three kinds of lanthanum-based nanoparticles and found that particles with La2O3·nH2O exhibited highest defluoridation ability of 28.9 mg/g compared with particles with LaCO3OH and La(OH)3. However, there is little information on La3+-modified activated carbon derived from municipal sewage sludge.

Based on the above considerations, the main purpose of this study was to synthesize sewage sludge-based activated carbon with lanthanum modification for defluoridation of solutions. The adsorption behavior was evaluated during batch experiments (effects of La3+ loading, reaction time and solution pH). To investigate the composite properties in detail, sludge, unmodified activated carbon (AC) and lanthanum-doped activated carbon (Ac-La) were characterized by X-ray fluorescence (XRF), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses. AC-La after adsorption was characterized by XPS analysis to investigate the defluoridation mechanism. ICP-MS analysis was conducted to investigate the stability and safety of AC-La composites.

Materials

Sewage sludge was collected from a municipal wastewater treatment plant in Sichuan Province (China) and had undergone traditional secondary biological treatment, gravity condensation and mechanical dewatering processes. Raw sludge was first dried in an oven (105 °C, 5 d) and then ground into powder through a 160 mesh sieve and stored for further use. The chemical reagents of zinc chloride (ZnCl2) and sulfuric acid (H2SO4) were used as activator. Lanthanum trinitrate hexahydrate (La(NO3)3·6H2O) was used as a modifier. Sodium fluoride (NaF) was selected as target adsorbate. The solution pH values were adjusted with 0.5 M HCl/NaOH solutions.

Preparation of AC and AC-La

For adsorbents preparation, the composite activator was obtained by mixing ZnCl2 solutions (6 mol/L) with H2SO4 (35 percent by weight (wt%)) at a volume ratio of 4:1. Sludge powder was immersed into the activating agent at a 1 g:4 mL mass/volume ratio, and then stirred homogeneously in an ultrasonic instrument for 9 h. After thoroughly mixing, the solution was dried in an oven (105 °C) for 1 day and transferred into a tube furnace, then carbonized at 550 °C for 60 min with 20 °C/min heating rate under the protection of N2 flow (200 mL/min). After it had cooled down to room temperature, the obtained biochar was washed with 80 °C deionized water till the pH value of filtrate was ca. 7. Then, the neutral products were dried under 105 °C for 1 d, ground and sieved through 200 mesh and then stored in a desiccator. The produced biochar products were denoted as AC. To prepare the modified adsorbent, lanthanum at a certain weight ratio of sludge powder was added and activated together with the raw material, while the subsequent treatment remained unchanged. The lanthanum-doped biochar products were denoted as AC-La.

Characterization and analytical methods

Composition of the obtained samples was measured by XRF (XRF-1800, Shimadzu). The N2 sorption–desorption isotherm (77 K) was measured by a specific surface area analyzer (SSA-4200, BJBuilder) after evacuation of samples at 150 °C for 5 h under vacuum, and the specific surface areas were measured by BET method, and the pore structure parameters were calculated by the BJH (Barrett–Joyner–Halenda) method. SEM-EDS (JSM-7500F, JEOL) was conducted to analyze the surface morphology. XRD (Empyrean, PANalytical B.V.) was used for phase identification with a Cu Kα radiation at 1.54 Å from 10° to 80° (2θ). The surface element properties were analyzed by XPS (XSAM800, Kratos) using Al Kα source. The functional groups were detected by FTIR spectroscopy (FNicolet 6700, Thermo Elemental) within 4,000–400 cm−1. The concentrations of Zn, Si, La and Al metal ions in solutions were detected using an inductively coupled plasma mass spectrometer (ICP-MS, VG PQExCell, Thermo Jarrell Ash). The pH values were tested by a pH meter (PHB-4, Rex Electric Chemical). An ion selective fluoride electrode (PF-202, Rex Electric Chemical) was used for fluoride determination.

Batch experiments

NaF was dissolved in deionized water to prepare 100 mg/L fluoride solutions, which were then diluted to 10 mg/L. Adsorbents at a concentration of 4 g/L were added into 50 mL solution at a particular pH value (2–11) in a 150 mL conical flask. Then the mixture was put in the incubator (150 rpm) for a specific time (0–90 min). Subsequently, the supernatant was filtered out through a 0.22 μm membrane for fluoride determination by electrode. All experiments were performed at least three times under room temperature and the standard deviation was controlled within 2.5%.

Stability experiments

To evaluate the stability, 4 g/L AC-La was put into 50 mL deionized water in a 150 mL conical flask for a predetermined time (1 h–7 d) under room temperature. Half of the samples were kept still, and the others were placed in an incubator shaker (150 rpm) for continuous agitating. Subsequently, the supernatant was filtered out through a 0.22 μm membrane at different time intervals. Zn, Si, La and Al ion concentrations of the filtrates were then determined by ICP-MS analysis.

Adsorption study

La3+ loading

The effect of La3+ loading amounts on AC-La for fluoride adsorption performance was investigated at operational condition of pH = 7.1, 4 g/L adsorbent dosage, 10 mg/L fluoride concentration and 60 min reaction time. As seen from Figure 1, the fluoride removal rate was remarkably improved with the increase of La3+ loading amount on AC-La. The unmodified AC exhibited a low fluoride adsorption rate of only 12.1%, and AC-La with 50 wt% La3+ loading exhibited a very high fluoride adsorption rate of 91.4%. There was a significant fluoride removal increment with La3+ addition from 0 to 15 wt%, and then it became stable with further increasing of La3+ loading from 15 to 50 wt%. The total number of sites available for La3+ attachment on a certain amount of activated carbon was limited and thus the removal rate kept steady. Considering the comprehensive adsorption and preparation economy, AC-La with 15 wt% La3+ loading was selected for further study and AC-La in the following text all refers to AC-La with 15 wt% La3+ loading.

Figure 1

Effect of La3+ loading amounts on fluoride adsorption by AC-La.

Figure 1

Effect of La3+ loading amounts on fluoride adsorption by AC-La.

Close modal

The constituent contents of inorganic elements in raw sludge, AC and AC-La determined by XRF are summarized in Table 1. As listed in Table 1, the inorganic fractions of the three materials were mainly C, O and Si. Compared to the raw sludge, concentrations of C and O were slightly decreased in AC and AC-La, while nitrogen had completely disappeared after the carbonation process. Meanwhile, the content of Zn was obviously increased due to ZnCl2 activation. To prepare AC-La, 15 wt% La3+ was added into the raw sludge as a ratio calculation, and the final loading amount of La in AC-La was 13.8 wt%. It proved that La was effectively loaded during the preparation process.

Table 1

XRF analysis for sludge, AC and AC-La composites

ElementsSludge (weight%)AC (weight%)AC-La (weight%)
39.0 33.4 33.7 
36.1 29.1 24.5 
Si 7.4 8.0 7.1 
3.3 – – 
Al 3.2 3.5 3.2 
3.1 2.9 2.5 
0.9 3.6 3.4 
Fe 2.3 0.8 1.0 
Ca 1.5 0.1 0.5 
1.4 0.7 0.7 
Zn – 16.6 8.8 
La – – 13.8 
ElementsSludge (weight%)AC (weight%)AC-La (weight%)
39.0 33.4 33.7 
36.1 29.1 24.5 
Si 7.4 8.0 7.1 
3.3 – – 
Al 3.2 3.5 3.2 
3.1 2.9 2.5 
0.9 3.6 3.4 
Fe 2.3 0.8 1.0 
Ca 1.5 0.1 0.5 
1.4 0.7 0.7 
Zn – 16.6 8.8 
La – – 13.8 

–: not detected.

Adsorption experiments

To study the effect of reaction time on fluoride removal, the agitation time was varied from 0 to 90 min at operational condition of pH = 7, 4 g/L AC-La and 10 mg/L fluoride concentration. Obtained results are shown in Figure 2(a). Initially, fluoride adsorption increased rapidly with the increasing reaction time and reached 64.4% within 5 minutes, and then gradually increased to ca. 80.9% at 1 h. Fluoride removal became stable and insignificantly changed after 1 h due to active sites on AC-La surface becoming exhausted over time.

Figure 2

Effect of time (a) and pH (b) on fluoride adsorption by AC-La.

Figure 2

Effect of time (a) and pH (b) on fluoride adsorption by AC-La.

Close modal

Initial solution pH plays an important role in defluoridation by affecting surface charges of adsorbents and existence forms of adsorbates. Zhang et al. (2017) found that higher pH led to the deprotonation of zirconium-iron oxide nanoparticles and reduced phosphate adsorption. To estimate the favorable pH range, initial pH of fluoride solution was varied from 2 to 11 at operational condition of 4 g/L adsorbent dosage, 10 mg/L fluoride concentration and 60 min reaction time. As shown in Figure 2(b), fluoride removal efficiency increased drastically from 12.1% to 87.3% as pH changed from 2 to 3, and then was stable at ca. 78% within pH 4–9, while around pH 10–11, a decline of the fluoride removal efficiency (71.0% to 49.2%) was observed. Favorable pH range for fluoride adsorption onto AC-La was from 3 to 10; thus the adsorbent was suitable in both acid and base conditions, implying the potential applicability of the produced carbon for different kinds of fluoride wastewater treatment. High fluoride adsorption (87.3%) on AC-La at pH 3 could be due to the strong attractive forces between the positive charges on the carbon surface protonated by H+ ions and the negative charges of fluoride ions, whereas the low defluoridation efficiency (12.1%) at pH 2 was probably due to the excess H+ hindering the movement of F ions, which reduced the fluoride affinity on the adsorbent surface. The sharp decrement of fluoride adsorption on AC-La at pH 11 was due to the excessive OH ions that competed with fluoride for the active sites on the carbon surface. Low defluoridation rates at both strong acid and base conditions have also been reported by other research (Mullick & Neogi 2018).

Characterizations

BET and SEM analysis

The morphology structures of the adsorbent are important factors affecting the adsorption process. Therefore, pore structure parameters of sludge, AC and AC-La are given in Table 2. As listed in Table 2, raw sludge had a relatively poor pore structure. However, compared to sludge, there were considerable changes in surface area and total pore volume of AC and AC-La. The specific surface area of AC-La was 131.2 m2/g greater than that of raw sludge. Meanwhile, the average pore of AC and AC-La was about eight times smaller than that of the sludge. It confirmed that the chemical activation by ZnCl2 and H2SO4 deeply improved textural properties of both AC and AC-La, which was due to the fact that during carbonization the evaporation of activator had released spaces occupied previously (Kumar & Jena 2017). In comparison to AC, the specific surface and the pore volume of AC-La were slightly reduced by 22.6 m2/g and 0.063 cm3/g, respectively, which was caused by surface pore occupation and La combination on the activated carbon (Vences-Alvarez et al. 2015). Despite similar pore structure and specific surface area, AC exhibited a comparatively low defluoridation rate (Figure 1). It was concluded that the fluoride adsorption onto AC-La did not depend on pore structure and surface areas.

Table 2

BET results of sludge, AC and AC-La composites

SampleSpecific surface area (m2/g)Total pore volume (cm3/g)Micropore volume (cm3/g)Mesopore volume (cm3/g)Average pore diameter (nm)
Sludge 1.8 0.018 0.000 0.018 20.1 
AC 155.6 0.217 0.047 0.169 2.8 
AC-La 133.0 0.154 0.014 0.140 2.3 
SampleSpecific surface area (m2/g)Total pore volume (cm3/g)Micropore volume (cm3/g)Mesopore volume (cm3/g)Average pore diameter (nm)
Sludge 1.8 0.018 0.000 0.018 20.1 
AC 155.6 0.217 0.047 0.169 2.8 
AC-La 133.0 0.154 0.014 0.140 2.3 

To further characterize the surface morphological structure of products, SEM-EDS analysis was conducted for all materials (raw sludge, AC, AC-La) and results are displayed in Figure 3 and Table 3. As shown in Figure 3, raw sludge had a smooth surface and comparatively fewer pores. As compared to the raw sludge, numerous fragmented particles were distributed on both AC and AC-La surfaces, forming numerous tiny pores. AC and AC-La showed an irregular rough surface and much developed pore structure. These results were validated quantitatively by the BET analysis (Table 2). Table 3 shows the amounts of different elements in mass and atomic percentages for all the samples. After carbonation, the concentration of C and O in AC and AC-La decreased slightly, while Zn concentration increased significantly. These results were consistent with XRF analysis (Table 1). The lanthanum concentration on the characterized carbon surface area was 28.7 wt% as listed in Table 3, also confirming the effective loading of lanthanum on AC-La.

Table 3

EDS analysis of sludge, AC and AC-La composites

ElementsSludge
AC
AC-La
weight%atomic%weight%atomic%weight%atomic%
C K 51.9 66.2 37.2 60.8 32.8 60.9 
O K 23.3 22.3 18.2 22.3 15.8 22.0 
Al K 6.4 3.6 4.0 2.9 5.3 4.3 
Si K 10.0 5.5 3.8 2.6 4.5 3.6 
P K – – – – – – 
S K – – – – – – 
Ca K 1.7 0.7 0.3 0.2 0.6 0.3 
Fe K 6.3 1.7 5.7 2.0 1.3 0.5 
Zn L 0.4 0.1 30.8 9.2 11.0 3.8 
La L – – – – 28.7 4.6 
ElementsSludge
AC
AC-La
weight%atomic%weight%atomic%weight%atomic%
C K 51.9 66.2 37.2 60.8 32.8 60.9 
O K 23.3 22.3 18.2 22.3 15.8 22.0 
Al K 6.4 3.6 4.0 2.9 5.3 4.3 
Si K 10.0 5.5 3.8 2.6 4.5 3.6 
P K – – – – – – 
S K – – – – – – 
Ca K 1.7 0.7 0.3 0.2 0.6 0.3 
Fe K 6.3 1.7 5.7 2.0 1.3 0.5 
Zn L 0.4 0.1 30.8 9.2 11.0 3.8 
La L – – – – 28.7 4.6 

–: not detected.

Figure 3

SEM images of sludge (a), AC (b) and AC-La (c).

Figure 3

SEM images of sludge (a), AC (b) and AC-La (c).

Close modal

XRD analysis

XRD analysis of raw sludge, AC and AC-La was further employed to determine the phase structures of these materials (Figure 4). Crystalline peaks were observed in all samples. The characteristic peaks at 20.90, 26.65, 36.55, 39.49, 45.79, 50.15 and 59.97° are ascribed to the (100), (011), (110), (102), (021), (112) and (211) planes of SiO2 (JCPDS card 79–1906), respectively. Obviously, SiO2 is the dominant crystallographic texture in all the samples. SiO2, as a common mineral in sewage sludge, mainly comes from the pipeline through sewage transportation (Gong et al. 2020). It is noteworthy that no lanthanum-related peaks were detected on AC-La, indicating that the lanthanum structure on AC-La was amorphous and lanthanum was thoroughly dispersed on AC-La (Liu et al. 2016).

Figure 4

XRD patterns of raw sludge, AC and AC-La.

Figure 4

XRD patterns of raw sludge, AC and AC-La.

Close modal

FTIR analysis

The surface functional groups are crucial to the adsorptive ability of activated carbons. FTIR spectra of raw sludge, AC and AC-La are shown in Figure 5. All the materials represented a typical stretching vibration around 3,413 cm−1 assigned to adsorbed water, while this peak of AC and AC-La showed a slight decline compared to the raw sludge because of pyrolysis. As compared to the peak of raw sludge at 1,651 cm−1 assigned to bending vibration of hydroxyls, weakening peaks around 1,624 cm−1 of AC and AC-La were also observed due to the pyrolysis process (Xie et al. 2019a). Moreover, bands around 1,040 cm−1 and 795 cm−1 assigned to Si–O–Si were present in all materials (Shen et al. 2020). Raw sludge presented additional functional groups including the stretching vibration of C=C (around 1,410 cm−1) and aliphatic C–H (around 2,925 and 2,856 cm−1), suggesting decomposition of some organic compounds during the pyrolysis process on AC and AC-La (Xie et al. 2019b). It is noteworthy that AC and AC-La showed a very similar FTIR spectrum, but large different fluoride removal behavior. The result showed that the fluoride adsorption process on AC-La was not dominated by those surface functional groups. In addition, compared to sludge and AC, AC-La presented a new peak around 615 cm−1 ascribed to M-O (possible La-O) (Kong et al. 2018; Manjunatha et al. 2019).

Figure 5

FTIR spectra of raw sludge, AC and AC-La.

Figure 5

FTIR spectra of raw sludge, AC and AC-La.

Close modal

XPS analysis

The chemical interaction in and properties of raw sludge, AC, AC-La and AC-La after fluoride adsorption were characterized by XPS investigations. As shown in Figure 6, the peaks for O KLL, O 1s, N 1s and C 1s are all found for the four samples in the wide spectra. Six peaks for zinc atom were observed in AC and AC-La composites due to the activation process. For AC-La, six new peaks appearing at 1,129, 888, 852, 836, 197 and 103 eV were assigned to La 3p, La LMM, La 3d3/2, La 3d5/2, La 4p and La 4d, respectively, which confirmed the successful sequestration of La on the adsorbent. After adsorption, the peak at 685.38 eV assigned to F 1s spectrum was much larger than 684.5 eV of NaF, suggesting the effective adsorption and strong interaction between fluoride and AC-La (Xie et al. 2019a).

Figure 6

XPS spectra of sludge, AC, AC-La and AC-La after fluoride adsorption.

Figure 6

XPS spectra of sludge, AC, AC-La and AC-La after fluoride adsorption.

Close modal

The spectra of C 1s, O 1s, F 1s and La 3d of AC-La and AC-La after adsorption are shown in Figure 7. From AC-La and AC-La after adsorption, the peaks of La 3d3/2 and La 3d5/2 shifted to a higher binding energy, showing that La transformed to a higher positive oxidation state. This was due to the electron transfer from valence band of the ligand form to the 4f orbital of La atom, illustrating formation of a new La-complexation (He et al. 2017). Thus, the shifts of 0.2–0.7 eV indicated strong affinity via inner-sphere complexation interaction between AC-La and fluoride (Fu et al. 2018).

Figure 7

XPS spectra of La 3d, F 1s, C 1s, O 1s on AC-La and AC-La after adsorption.

Figure 7

XPS spectra of La 3d, F 1s, C 1s, O 1s on AC-La and AC-La after adsorption.

Close modal

Spectra of C 1s in AC-La was deconvoluted into four peaks (Figure 7). The peaks at 288.87/288.91, 287.06/287.32, 285.73/285.93 and 284.51/284.53 eV were attributed to O–C=O, C=O, C–O and C–C/C=C functional bands, respectively (Wang et al. 2017; Lingamdinne et al. 2019). Obviously, the relative area of these peaks had changed insignificantly after adsorption.

In the case of O 1s spectra, there were three main peaks at 533.18/533.18, 532.09/532.09 and 531.16/531.21 eV ascribed to adsorbed water (H2O), metal hydroxides or hydroxyl groups (–OH), and metal oxides groups (M–O), respectively (He et al. 2017; Elanchezhiyan et al. 2019). Observably, the peak area ratio of M–O increased from 24.67% to 26.81% while that of –OH decreased from 61.00% to 54.82% after fluoride adsorption. The hydroxyl functional groups were assumed to be involved in the adsorption process and led to the exchange of hydroxyls by fluoride (Wang et al. 2018; Chaudhary et al. 2019). Based on the above, the removal mechanism of fluoride onto AC-La was mainly the inner-sphere complexation between lanthanum and fluoride, facilitated by exchange interaction with hydroxyl groups.

Stability of AC-La

When solid-waste-derived activated carbons are applied in real water treatment, the stability of adsorbent deserves much attention. To evaluate the leaching properties of AC-La, 4 g/L AC-La was placed in deionized water for a period of time; then sample solutions were collected for Zn, Si, La and Al concentration analysis. The results are listed in Table 4 and show that time span from 1 h to 7 d and operational condition (standing or stirring) had little impact on the leaching concentrations for all four elements. The leaching levels of Si (1.1–2.6 mg/L), La (0.12–0.04 mg/L) and Al (0.01 mg/L) were extremely low and can be ignored. Although Zn exhibited a relatively high leaching level (57.7–69.6 mg/L), it was still below the corresponding upper limits of 100 mg/L of Zn leaching according to the national hazardous materials identification standard of China (GB 5085.3-2007). This demonstrated that AC-La could be used as a stable and safe adsorbent in water treatment. It should be noted that the theoretical addition of La was 552 mg/L (13.8%*4 g/L = 552 mg/L), while La maintained a quite small dissolution amount (<0.12 mg/L, dissolution rate < 0.2‰) in either standing or stirring conditions. Related studies revealed that soluble heavy metal compounds were converted to insoluble matter after sludge was made into activated carbon (Li et al. 2016). The activated carbon preparation process successfully immobilized La onto raw sludge and obtained an efficient adsorbent for fluoride removal in aqueous solutions.

Table 4

Leaching content (mg/L) determined from ICP analysis of water samples

ConditionTimeZnSiLaAl
Standing 1 h 60.9 1.2 0.11 0.01 
4 h 62.8 1.5 0.11 0.01 
1 d 64.3 1.4 0.04 <0.01 
2 d 65.6 1.7 0.07 <0.01 
7 d 69.6 2.6 0.04 <0.01 
Stirring 1 h 57.7 1.1 0.12 0.01 
4 h 62.1 1.3 0.10 0.01 
1 d 62.4 1.5 0.07 <0.01 
2 d 64.7 1.7 0.09 0.01 
7 d 68.2 2.6 0.04 <0.01 
ConditionTimeZnSiLaAl
Standing 1 h 60.9 1.2 0.11 0.01 
4 h 62.8 1.5 0.11 0.01 
1 d 64.3 1.4 0.04 <0.01 
2 d 65.6 1.7 0.07 <0.01 
7 d 69.6 2.6 0.04 <0.01 
Stirring 1 h 57.7 1.1 0.12 0.01 
4 h 62.1 1.3 0.10 0.01 
1 d 62.4 1.5 0.07 <0.01 
2 d 64.7 1.7 0.09 0.01 
7 d 68.2 2.6 0.04 <0.01 

La-AC derived from municipal sewage sludge for fluoride removal was prepared by chemical activation. Batch adsorption experiments showed that 4 g/L AC-La exhibited a fluoride adsorption rate of 80.9% with 10 mg/L initial concentration, and the adsorptive process achieved equilibrium in 60 min. Favorable pH range for fluoride adsorption onto AC-La was from 3 to 10. XRF results showed that with initial dosage of 15 wt%, the final loading amount of La in AC-La was 13.8 wt%, demonstrating that lanthanum was successfully adhered onto the composite. SEM and BET analysis showed that both AC and AC-La had developed excellent and similar textural properties compared to raw sludge. XRD analysis revealed that SiO2 was the dominant crystallographic texture in AC-La and the structure of lanthanum was amorphous. XPS analysis showed the removal mechanism of fluoride onto AC-La was mainly the inner-sphere complexation between lanthanum and fluoride, enhanced by exchange interaction with hydroxyl groups. Furthermore, AC-La was quite safe with little leaching amounts and good stability in aqueous solutions (La dissolution rate < 0.2‰). It was favorable to utilize this La-doped sewage sludge-derived activated carbon for fluoride wastewater treatment.

This work was supported by the Experimental Technology Funding Project of Sichuan University (No. SCU203023).

All relevant data are included in the paper or its Supplementary Information.

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