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Conversion of Fe-rich sludge to KFeS2 cluster: Spontaneous hydrolysis of KFeS2 for the effective adsorption of doxycycline
⁎Corresponding author at: Science and Technology Innovation Center for Municipal Wastewater Treatment and Water Quality Protection, Northeast Normal University, Changchun 130117, China. papermanuscript@126.com (Suiyi Zhu)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Abstract
Fe-rich sludge is a solid waste considerably generated in coagulation, Fenton, and catalytic processes for wastewater treatment, in which it is commonly disposed in landfills. However, a limited portion of sludge is recycled as polymeric ferric flocculant and iron red. Herein, the Fe-rich sludge was simulated by the hydrolysis of FeCl3 and converted to a new one-dimensional KFeS2 cluster via a one-step hydrothermal route with the addition of K2S and KOH. The results showed that in the hydrothermal process, KFeS2 cluster grew radially from 2 μm to 10 μm with the increase in KOH concentration from 2 M to 5 M. The new cluster showed a high adsorption capacity of 2933.6 mg/L for doxycycline, which is 14 times that of sludge and higher than that of hematite nanoparticles, commercial polymeric ferric flocculant and pyrite. The adsorption isotherm complied with the Langmuir model, and the adsorption kinetics fitted well with the pseudo-second-order model. During adsorption, KFeS2 cluster was completely hydrolysed to release Fe/S-bearing colloids with a considerable number of Fe-SH/Fe-OH groups for the coordination of the —NH2 group of tetracycline-type antibiotics, e.g. doxycycline. However, an inefficient removal of quinoline and p-nitrophenol was observed. With the proposed method, the used alkaline solution was completely recycled in the next round of KFeS2 synthesis without the generation of secondary waste. Such green method has potential applications in the resource utilisation of Fe-rich sludge.
Keywords
Fe-bearing precipitates
Hydrothermal conversion
KFeS2
Adsorption
Doxycycline
1 Introduction
Fe-bearing reagents, e.g. ferrous sulfate, nanoscale zero-valent iron and polymeric ferric sulfate (PFS), are commonly applied to remove contaminants from water (Ahmad et al., 2016, EPA, 2011, Li et al., 2014, Li et al., 2020a,b, Mooheng and Phenrat, 2019, Qiang et al., 2003, Zhang et al., 2019a,b). The use of such reagents generates the waste Fe-bearing sludge during mass production, with Fe content of 7.5 wt% to 45.6 wt% (Zhu et al., 2015, Zhu et al., 2020a,b), and major impurities, including Ca, Al and Si. About 25 mg/L PFS is added in a surface water plant with the project scale of 7000 m3/day, with >3 t/day sludge generated (EPA, 2011). Such waste sludge has a water content of 95–99.5% and is commonly categorised as a solid waste, followed by mechanical dewatering with the addition of cement and/or pitch before safety landfill, which is tedious and costly. In underdeveloped areas, the direct discharge of sludge into lakes and/or rivers also occurs, where the release of Fe from the sludge to water pollutes the water resources nearby (Osman and Iqbal, 2014). Therefore, a renewable route for the effective treatment of such Fe-bearing sludge should be proposed to reduce the disposal cost and control pollution.
The resource utilisation of sludge as an adsorbent for wastewater treatment is a desirable route. The sludge is rich in surface hydroxyl groups and applied as an adsorbent for the removal of contaminants, e.g. heavy metals (Ngatenah et al., 2010), antibiotics (Sun et al., 2019, Zhu et al., 2020a,b) and organic dyes (Zhu et al., 2015). In the sludge, Fe exists in the form of weakly crystallised compound, which can be converted into magnetite (Zhu et al., 2015), maghemite (Qu et al., 2019) and jacobsite (Zhu et al., 2019a,b), after hydrothermal treatment and/or calcination. The prepared product exhibits good magnetic response and can be magnetically recycled after use. However, this sludge aggregates, and after conversion into a magnetic adsorbent, the aggregates enlarge due to the dihydroxylation reaction of adjacent Fe-bearing microcrystals (Qu et al., 2019, Zhu et al., 2015, Zhu et al., 2019a,b). Thus, the sludge and the corresponding magnetic adsorbent exhibit a normal performance in the adsorption of contaminants from wastewater. Another utilisation route is to recycle the sludge as flocculant. After the sludge is dissolved in strong acids, e.g. sulfuric and hydrochloric acids, free Fe2+/Fe3+ and Al/Si impurities are released into the acid solution, where the precursor of Fe-bearing flocculant is produced (Li et al., 2020a,b, Mooheng and Phenrat, 2019). Li et al., reported the treatment of Fe-bearing sludge with Fe content of 15.5 wt% by leaching with concentrated hydrochloric acid followed by rotary evaporation, neutralisation and dilution; the prepared FeCl3 flocculants met the product standards (Li et al., 2020a,b). The Fe-bearing flocculant is hydrolysed and polymerised to form flocs and exhibits sufficient surface groups of Fe-OH to coordinate contaminants in the wastewater treatment. The drawback is the rapid hydrolysis of Fe as aggregates, resulting in a low removal efficiency.
Recently, a new erdite-bearing product was synthesised with Fe-bearing sludge as the raw material (Qu et al., 2020), and it showed an ideal capability for the adsorption of tetracycline (Hu et al., 2020, Zhu et al., 2019a,b), Zn and Cu (Liu et al., 2020). In the new product, erdite has a special structure in which one Fe atom is covalent with four S atoms to form rod-shaped particles (Honma et al., 2003). The special Fe-S structure of erdite is stable in alkaline condition (Lassin et al., 2014), decomposes in neutral solutions to generate small Fe/S-bearing flocs and uses plenty of surface coordination sites for contaminant adsorption. KFeS2 exists in the form of a linear chain with an Fe/S-bearing structure and exhibits desirable application in the recycling of Fe-bearing sludge, similar to erdite. In the past, KFeS2 was used as a common reagent to prepare electrode materials in the battery industry (Guy et al., 2008, Han et al., 2020); however, the application of KFeS2 in wastewater treatment has not been reported. Notably, three important aspects should be considered in investigations to recycle the Fe-bearing sludge as KFeS2. First, the formation possibility of KFeS2 in the sludge should be investigated. Second, studies should analyse whether ferrous-bearing compounds in the sludge are involved in KFeS2 formation because Fe2+ is commonly coprecipitated with Fe3+ in sludge during coagulation, demulsification and Fenton processes (EPA, 2011, Qiang et al., 2003). Third, the hydrolysis of KFeS2 in the wastewater treatment must be verified.
Doxycycline is a common antibiotics in pharmaceutical effluents (Zaidi et al., 2019), and it is targeted as a typical contaminant to analyse the adsorption capacity of KFeS2. This compound can inhibit bacterial protein synthesis by binding to a ribosomal subunit to stop bacteria from reproducing (Chukwudi and Good, 2019). The high-value discharge of doxycycline into the environment results from antibiotic production, mainly via the continuous discharge of effluents from wastewater stations (Bousek et al., 2018, Álvarez-Esmorís et al., 2020, Zaidi et al., 2019). Although doxycycline is not closely associated with harm to the environment, recent concerns emerged regarding the antimicrobial resistance and its chronic effects on biodiversity (Chukwudi and Good, 2019, Neth et al., 2017). Hence, the effective removal of doxycycline and its derivatives from wastewater is currently of great interest to the manufacturer and water industries alike, with a focus on the intensive regulation of the water environment.
In this paper, the Fe-bearing sludge from the coagulation process was simulated and converted into the KFeS2-bearing adsorbent. The formation mechanism of KFeS2 was analysed, and the performance of KFeS2 hydrolysis in doxycycline-bearing wastewater was investigated.
2 Materials and methods
2.1 Laboratory simulation of Fe-rich sludge
The Fe-rich sludge was produced by mixing 162 g FeCl3 in 5000 mL deionised water to form a transparent solution, followed by adjusting to pH 9 by KOH. Afterwards, a brownish sludge was precipitated at the bottom of the water and collected through suction filtration by using a filter paper. A small portion of the sludge was freeze-dried at −80 °C overnight for characterisation. Another sludge was kept in a beaker and sealed with a parafilm for the KFeS2 preparation.
2.2 Hydrothermal preparation of KFeS2
About 5 mL sludge was sampled using 5 mL injection syringe and then injected into 30 mL liquid containing 2 M KOH and 1 M K2S to produce a mixture. Subsequently, the mixture was agitated for 5 min at 90 rpm and dumped into a Teflon vessel, followed by heating at 120 °C for 15 h. Afterward, black particles were formed at the vessel bottom. These particles were transferred t too plastic tube and placed in a lyophiliser (FDU-2200, EYELA, Japan) for drying at −80 °C for 24 h. The dried particles were denoted as P2. A control experiment was performed using the abovementioned method except for the increase in the KOH concentration from 2 M to 5 M. The corresponding particles were named as P5.
2.3 Application of KFeS2 in doxycycline removal
Doxycycline was targeted as the model organic pollutant to determine the adsorption performance of synthesised KFeS2-bearing products. Doxycycline is a hydrophilic molecule of C22H24N2O8 with a molar mass of 444 g/mol and a low log Kow value of −0.02 (Álvarez-Esmorís et al., 2020). However, this compound is amphoteric with three pKa values of 3.5, 7.7 and 9.5 (Brigante and Avena, 2016). At a low pH (<3.5), doxycycline was fully protonated as DCH3+ (Fig. 1). With the pH increase, deprotonation reaction occurred at the hydroxyl groups of the C-3 position to form zwitterionic DCH2±. When the solution pH was higher than 7.7, the deprotonation step continued at the hydroxyl groups of O-10/O-12 ketophenolic position to form negative DCH−. However, at a high pH of 9.5, the final deprotonated step involved the dimethylamino group with the generation of negative DC2− (Gao et al., 2012).
Molecular structure of protonated doxycycline.
The adsorption experiment was performed as follows. In brief, 0.2 g raw sludge was dispersed in a solution containing 10 mg/L doxycycline, followed by shaking at 60 rpm for 2 h. Afterwards, the added sludge was separated. About 1 mL supernatant was collected, and the doxycycline concentration in the supernatant was detected by high-performance liquid chromatography (LC-16, Shimazu, Japan). Control experiments were performed by varying the doxycycline concentrations (10–2000 mg/L) following the above steps, and accordingly, the maximum adsorption capacity (qm) of raw sludge was determined. The raw sludge was replaced with P2 and P5 to determine the qm values of P2 and P5, respectively.
3 Results and discussion
3.1 KFeS2 cluster growth in the presence of KOH
The raw sludge was an irregular block with Fe content of 31.4 wt% and showed weak peaks of FeOOH, revealing that Fe oxides were weakly crystallised in the sludge. When the sludge was treated with the addition of 1 M K2S and 2 M KOH, the product P2 was a short rod with 2 μm length and 200 nm diameter, respectively, and exhibited sharp peaks of KFeS2 and hematite, suggesting the conversion of the weakly crystallised FeOOH into KFeS2 and hematite (Figs. 2 and 3(a)). With the increment in the KOH concentration from 2 M to 5 M, the prepared P5 exhibited typical KFeS2 and hematite peaks, but its length grew radially to 10 μm. This finding indicated the radial growth of KFeS2 rods to clusters at high KOH concentration. However, without KOH, only the peaks of sulphur and pyrite were recorded (Fig. S1), demonstrating that KOH was important for KFeS2 and hematite formation.
Scanning electron microscopy (SEM) images of (a) raw sludge and KFeS2 prepared with (b) 2 and (c) 5 M KOH.
(a) X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) (b) Fe2p and (c) S2p spectra of the raw sludge, P2 and P5 and (d) Fe concentration in the supernatant after dissolution of the raw sludge and Fe2+-bearing sludge at pH 8 and 13.
The raw sludge, P2 and P5 were also characterised using XPS (Fig. 3(b) and 3(c)). For the Fe 2p curves, an extensive peak was observed at 710.5 eV, and it belonged to the Fe-O bond of Fe oxides in the sludge (Liu et al., 2020). However, with the addition of KOH, a new peak appeared at 708.4 eV, and it was affiliated to the structural Fe in the (FeS2)nn− bond of KFeS2 (Bronold et al., 1991, Zhang et al., 2019a,b). For the S 2p curves, four peaks at 160.3, 161.2, 163.2 and 167.4 eV belonged to the structural S in (FeS2)nn− group (Bronold et al., 1991), S2−, elemental S and sulfate, respectively, demonstrating that KFeS2 was generated from the FeOOH in the sludge (Fig. 3(d)). Fig. 4(a) shows the Raman spectra of the raw sludge, P2 and P5. For the sludge, a strong peak, which is characteristic of FeOOH, appeared at 148 cm−1 (Dunnwald and Otto, 1989, Gui and Devine, 1993). With hydrothermal treatment, the peak disappeared, whereas two new peaks were observed at 332 and 363–365 cm−1. These new peaks corresponded to the symmetry and antisymmetric stretching modes of the Fe-S4 tetrahedron (Thomas et al., 1970), a characteristic (FeS2)nn– structure of KFeS2. In addition, two broad peaks at 972 and 1421 cm−1 were recorded in the two products, and they were assigned to hematite (Gui and Devine, 1993, Mazzetti and Thistlethwaite, 2002). Fig. 4(b) shows the infrared (IR) spectra of these products. The sludge comprised peaks of the adsorbed/lattice water (1627 cm−1), Fe-OH (1014 and 1396 cm−1) and stretching of Fe-O (465 cm−1) (Liu et al., 2018). After the hydrothermal process, the P2 and P5 spectra showed that the peaks of Fe-O shifted slightly to 466–469 cm−1, whereas that of Fe-OH disappeared. A new peak appeared at 547–550 cm−1, and it belonged to the stretching vibration of Fe-S bond (Wang et al., 2021), in agreement with the formation of KFeS2. Moreover, three new peaks at 998–999, 1127 and 1224–1227 cm−1 signalled the presence of the adsorbed sulfate (Galembeck and Alves, 1995, Rupa Ranjani et al., 2021). From the transmission electron microscopy (TEM) image in Fig. 5, the low-magnification images confirmed that fibre-sharp particles were distributed in the clusters of P2 and P5. The high-resolution images showed that the particles were highly crystallised with the interplanar distances of 0.564 and 0.282 nm, which matched well with the (0 2 0) and (2 2 0) facets of KFeS2 (JCPDS No. 45-1008), respectively, revealing that the fibre-sharp KFeS2 was generated in P2 and P5.
(a) Raman and (b) Fourier transform infrared spectroscopy (FTIR) spectra of the sludge, P2 and P5.
TEM images of (a) P2 and (b) P5.
3.2 Formation mechanism of KFeS2 and hematite
Fe-bearing minerals in the sludge were stable at room temperature and contained numerous surface hydrogel groups. However, at high temperature, the conjunction reaction between the adjacent Fe-OH of the Fe-bearing microcrystal occurred, in which a water molecule was released, and a stable Fe-O-Fe bond was formed. Such reaction accelerated with the temperature increase, and thereby, well-crystallised hematite was generated (Fig. 6), leading to the polymerisation of Fe-bearing microcrystals. The dehydration in the conjunction reaction intensified at high-alkaline conditions, which accelerated the formation of hematite and the aggregates of sludge particles.
Conversion of sludge to KFeS2 in the K2S/KOH-bearing solution.
When K2S and KOH were introduced to the hydrothermal system, numerous HS− and OH− were generated, which steadily adjusted the solution to pH > 13.5. Consequently, the conversion of FeOOH to hematite occurred. However, in the solution, the free OH− diffused to the FeOOH surface, where it attacked the surface structural Fe of FeOOH to generate Fe(OH)4−. Thereby, free Fe(OH)4− was released into the solution. Fig. 3(d) shows that 0.15 mg/L Fe was produced in the supernatant at pH 8, and this phenomenon was affiliated with the dissolution of Fe3+-bearing compounds in the raw sludge. However, the Fe concentration increased to 3.9 mg/L in the supernatant at pH 13 due to the formation of Fe(OH)4−. At high alkaline solution, the Gibbs energy of Fe(OH)4− formation at room temperature was −201.97 kcal/mol (Diakonov et al., 1999), in accordance with the spontaneous conversion of Fe on the sludge to Fe(OH)4−. Thus, Fe(OH)4− was generated continuously. The free Fe(OH)4− has abundant hydroxyl groups, which can be replaced by HS− to form Fe(OH)3HS−. Subsequently, two new Fe(OH)3HS− groups were spontaneously polymerised to FeS2Fe(OH)4− with two water molecules released to the solution. As the polymerisation reaction continued, the (FeS2)nn− group was generated. The free charge was neutralised by free K+ in the solution, where the interspace tunnel was also occupied by free K+, resulting in the KFeS2 crystal in the form of one-dimensional rods (Fig. 6). With the increase in KOH concentration from 2 M to 5 M, the OH− concentration also increased, which accelerated the generation and the release of Fe(OH)4− from the sludge surface. Accordingly, the polymerisation reaction proceeded with the supplementation of abundant Fe(OH)4−, leading to the radial growth of the KFeS2 cluster (Fig. 3).
Without the addition of KOH, only K2S was hydrolysed to HS− and OH−, and the solution pH was below 10 after the reaction. A small amount of Fe (0.15 mg/L at pH 8) was detected in the solution (Fig. 3(d)), which retarded the formation of the KFeS2 crystal. Thus, free HS− was directly diffused to the sludge surface via electronic adsorption, where one electron was transferred from S to structural Fe, which triggered the redox reaction between the Fe of the sludge and HS−, with pyrite being the final product (Fig. 4).
In comparison with the Fe3+-bearing sludge, the Fe2+-bearing sludge was in the form of an irregular block but did not release high amounts of free Fe (<0.1 mg/L) into the solution (Fig. 3d). After hydrothermal treatment, the Fe2+-bearing sludge was converted into a mixture of KFeS2, arcanite, 2Fe(OH)SO4 and K2S2O3 in aggregated form (Fig. 7(a) and 7(b)). In the Fe2+-bearing sludge production, dissolved oxygen was present in the solution, and a small portion of Fe2+ was oxidised to Fe3+ (Li et al., 2020a,b), and this Fe3+ was involved in the formation of KFeS2. However, no rod nor cluster was observed (Fig. 7b), demonstrating that the involvement of Fe2+ in the KFeS2 formation was retarded.
(a) XRD pattern and (b) SEM image of the product synthesised with Fe2+-bearing sludge.
3.3 Doxycycline adsorption
The sludge and prepared products were employed to adsorb doxycycline (Fig. 8). The adsorption data of doxycycline were fitted by a non-linear Langmuir model, and the relative equation (Eq. (1)) was expressed as follows:
Adsorption isotherm of doxycycline by (a) raw sludge, P2 and P5 in comparison with that by (b) pyrite, PFS and hematite particles.
Table 1 summarises the adsorption parameters. The regression coefficient (R2) of hematite was 0.928, whereas those of other adsorbents were > 0.99. This finding confirmed that the sludge, P2, P5 and other three common adsorbents had energetically homogeneous surface for the adsorption of doxycycline.
Model
Parameters
sludge
P2
P5
Pyrite
PFS
hematite
Langmuir
R2
0.993
0.999
0.998
0.999
0.998
0.928
qm (mg/g)
273.2
2811.8
2933.6
303.9
784.6
194.6
KL
0.006
0.037
0.02
0.018
0.017
0.001
The raw sludge had a low qm of doxycycline (273.2 mg/g), which was approximately 1/14 that of P5 (Fig. 8a). The qm of P5 was 2933.6 mg/g, which was slightly high compared with that of P2, suggesting that the radial growth of KFeS2 as cluster particles is profitable. Fig. 8b shows that the qm of P5 was also higher than those of pyrite (303.9 mg/g), hematite (194.6 mg/g) and PFS (784.6 mg/g). This finding demonstrates that in P2 and P5, the formed KFeS2 played a key role in doxycycline adsorption.
P5 showed a desirable adsorption capacity of doxycycline, and accordingly, its adsorption kinetic of doxycycline was also investigated. As shown in Fig. 9, the adsorption data fitted well with the pseudo-second-order model (Eq. (2)), and both the experimental adsorption capacities of doxycycline (qt) were in agreement with the calculated values of pseudo-second-order model (Table 2). Thus, the adsorption of doxycycline on P5 was controlled by chemisorption.
Adsorption kinetics of doxycycline on P5. In the figure, the experiment was performed at the doxycycline concentration of 2000 mg/L at the initial pH 5.5.
Model
Parameters
P5
Pseudo-second-order kinetic
R2
0.997
K1 (×10−3 g/mg·min)
0.091
qt (mg/g)
1988.8
The used P5 was regenerated with 15% NaCl solution at pH 5 for 48 h (named as Method A) or calcinated at 450 °C for 2 h (named as Method 2). The results showed that P5 was regenerated easily using NaCl solution as the desorption agent. However, the removal efficiency of doxycycline decreased to 28.5%. After being calcinated at 450 °C, the regenerated P5 showed a low removal efficiency of doxycycline (53.2%). In addition, with the continued regeneration process, the removal efficiency gradually decreased to 27.1% for the second round and 18.1% for the third round (Fig. 10) due to the aggregation of Fe-bearing compounds in the calcination process. These findings indicate that P5 cannot be feasibly reused.
Reuse of the precipitate of hydrolysed P5 for doxycycline. In the figure, the experiment was performed at the initial concentration of 2000 mg/L at pH 5.5 at room temperature.
3.4 Characterisation of used KFeS2 rods
After adsorption, the raw sludge was almost unchanged and showed a weakly crystallised FeOOH block (Fig. 11(a) and (b)). However, for P2 and P5, the rod-shaped KFeS2 disappeared, and irregular aggregates were observed (Fig. 11(b) and (c)). Accordingly, the sharp peaks of KfeS2 were absent, and only peaks of hematite were observed, along with the weak peaks of sulfur from the oxidation of structural S in KFeS2 (Fig. 12a). This finding suggests that KFeS2 was spontaneously decomposed, and hematite was stable during doxycycline adsorption. The raw sludge, P2 and P5 were also characterised using XPS after adsorption (Fig. 12(b) and 12(c)). For the Fe 2p spectra, the peak of Fe-O bond was also recorded in the sludge spectrum. However, after adsorption, the peak of structural Fe in the (FeS2)nn− group in the spectra of P2 and P5 disappeared, in agreement with the decomposition of KFeS2; a new peak was recorded at 710.2 eV, in reference with the structural Fe in the Fe—S/Fe—O group (Li et al., 2008, Simonetti et al., 2006). For the N 1s spectra, two typical peaks at the binding energies of 399.5 and 402 eV were affiliated to the —NH2 group and the —N— bond of doxycycline, respectively (Yang et al., 2018). After calculating the relative ratio of peak intensity (short for I), the I402/I399.5 of P2 was 0.67, which is close to that of P5 (0.7). This result indicates that the adsorbed doxycycline on P2 and P5 exhibited a disordered structure, which was conducive to doxycycline adsorption.
SEM images of (a) sludge, (b) P2 and (c) P5 after doxycycline adsorption.
(a) XRD pattern and XPS (b) Fe 2p and (c) N 1s spectra of the sludge, P2 and P5 after doxycycline adsorption.
The Raman spectra showed that the typical peaks of Fe-S4 tetrahedron disappeared in the used P2 and P5 after adsorption (Fig. 13), which was in agreement with the spontaneous hydrolysis of KFeS2 in the solution. Moreover, the broad peaks of hematite were not observed. Instead, two tall peaks appeared at 1337–1341 and 1569–1570 cm−1 in the D and G bands, respectively, and they overlapped with the hematite peaks. The D band belonged to the aromatic structure of the sp3 bonded carbons, whereas the G band was due to first-order scattering of the stretching vibration mode E2g observed for sp2 carbon domains. This phenomenon demonstrated the existence of adsorbed doxycycline on hydrolysed KFeS2 of P2 and P5. The FTIR spectra of P2 and P5 after adsorption displayed peaks at 470 and 550 cm−1, which demonstrated that Fe-O and/or Fe-S were in the hydrolysed product. The hydrolysed P2 and P5 exhibited adequate -SH bond, which commonly corresponds to the peak of 1590 cm−1 (Wang et al., 2021). However, after adsorption, the peak shifted slightly to 1598 cm−1, probably because of the coordination reaction between the -SH bond and —NH2 group of doxycycline. In addition, a peak was observed at 1448–1450 cm−1, and it was caused by the bending vibration of —CH3 in doxycycline (Tang et al., 2021). The peaks of the adsorbed sulfate weakened and shifted to 1033–1036, 1110 and 1219–1221 cm−1, in accordance with the variation in the adsorption sites of the adsorbed sulfate on the hydrolysed KFeS2.
(a) Raman and (b) FTIR spectra of P2 and P5 after doxycycline adsorption.
3.5 Adsorption mechanism
The sludge comprised weakly crystallised FeOOH, which had a special structure consisting of one Fe coordinated with 5.2 hydroxyl groups (Jianmin, 1994). Thus, the sludge contained plenty of hydroxyl groups for the coordination of heavy metals and cationic organic compounds (Ngatenah et al., 2010, Zhu et al., 2015, Zhu et al., 2020a,b). Doxycycline occurs in three forms in solutions, namely, cationic at pH < 3.5, zwitterionic at pH 3.5–7.7 and anionic at pH > 7.7 (Brigante and Avena, 2016). When the sludge was dispersed in the doxycycline-bearing solution, the zwitterionic doxycycline was attached to the sludge surface, in which the —NH2 group of doxycycline spontaneously reacted with the hydroxyl group of the sludge, with the generation of stable Fe—O—NH3+— ligand. This phenomenon led to the adsorption of doxycycline on the sludge surface. In the conversion of FeOOH into hematite, the Fe-bearing microcrystals aggregated to produce hematite particles, which decreased the number of functional hydroxyl groups (Jianmin, 1994), whereas hematite showed a low doxycycline adsorption.
In contrast to FeOOH and hematite, the KFeS2 was metastable in neutral solution and spontaneously hydrolysed to small rods and further to fine Fe/S-bearing colloid (Fig. 14). The hydrolysis of KFeS2 occurred slowly in comparison with those of pure FeCl3 and/or PFS. As shown in Fig. S2(a), hematite and KFeS2 peaks were observed in the hydrolysed P2 after the doxycycline adsorption for 20 mins, in accordance with the slow hydrolysis of KFeS2 in the solution. The newly formed product of KFeS2 was a fine colloid with a hydrodynamic radius of about 0.5 μm (Fig. S2(b)) under constant stirring at 110 rpm, polymerised gradually in the form of flocs after stirring and exhibited abundant Fe-SH/Fe-OH groups. In comparison with the Fe-OH group, the new Fe-SH group was a typical Lewis base and had good affinity to complex with the —NH2 group of doxycycline because S has a larger atomic radius than O and shows strong electronegativity in the form of -S− (Chen et al., 2011, Prashanth et al., 2021). Thus, the flocs exhibited a desirable doxycycline adsorption capacity.
Possible route of KFeS2 rod hydrolysis for doxycycline adsorption.
3.6 Potential application in environmental pollution control
The synthesised product P5 showed a higher removal efficiency of doxycycline compared with P2 and the Fe-bearing sludge. Thereby, P5 was applied in the adsorption of quinoline, p-nitrophenol and the derivatives of doxycycline (Fig. 15). The removal efficiency of doxycycline was nearly 100%, similar to those of oxytetracycline, chlortetracycline and minocycline, indicating that P5 was efficient in the adsorption of tetracycline-type antibiotics. However, the adsorption efficiencies were 2.1% and 1.8% for quinoline and p-nitrophenol, respectively. This behaviour demonstrated that Ps-0.5 has a high selectivity in the adsorption of tetracycline-type antibiotics in comparison with those of quinoline and p-nitrophenol. When P5 was added to the solution, it spontaneously hydrolysed to form the Lewis base of Fe-SH. Thereafter, quinoline and p-nitrophenol were negatively charged in the solution and showed no attachment to the hydrolysed product of KFeS2 via electrostatic repulsion, leading to a low removal efficiency. However, tetracycline-type antibiotics were zwitterionic in the solution, and easily cooperated with Lewis base of Fe-SH from KFeS2 hydrolysis, resulting in the high level of removal efficiency.
Removal efficiencies of quinoline, p-nitrophenol and tetracycline-type antibiotics on P5. For this figure, the experiment was performed with the initial concentration of 20 mg/L at pH 5.5 at room temperature.
In this study, a strong alkaline solution was employed for KFeS2 synthesis, in which the added S2− and OH– were consumed in the hydrothermal reaction. However, after the hydrothermal treatment, the used solution comprised portions of S2− and OH–, and they can be recycled for the next synthesis with supplementary KOH and K2S. Without the supplementary KOH and K2S, the rod-shape KFeS2 crystal was observed (Fig. 16), but the length was shorted in comparison with that of P5. When KOH and K2S were added to the residual supernatant, cluster-sharp KFeS2 crystals appeared, similar to those of P5. This finding indicates that the residual supernatant can be completely recycled in the subsequent round for KFeS2 synthesis.
SEM images of KFeS2-bearing products synthesised by the recycling of supernatant (a) without and (b) with the addition of 0.5 M KOH and K2S.
The synthesis cost of KFeS2 is an important parameter for its involvement in wastewater treatment. The synthesis of 1 t KFeS2 needs 0.64 t potassium sulphide, 0.12 t potassium hydroxide, 15 h high-temperature steam for heating and 6 kW·h power for drying, with a total cost of US$890.1 (Table 3). This process comprises the recycling of used alkaline solution and does not generate secondary liquid waste. In addition, ferric trichloride is used as the Fe source in the process, and it can be replaced by waste Fe-rich sludge, which not only reduces the synthesis cost of KFeS2 but also eliminates the potential pollution of such Fe-rich sludge. Thus, the synthesis cost of KFeS2 was relatively lower than the reported methods with chemical Fe-bearing reagents as raw materials (Boon, 2015, Galembeck and Alves, 1995). By using KFeS2-bearing product (e.g. P5) to treat doxycycline-bearing wastewater, the cost approximates US$0.37/m3, which is lower than that of widely reported adsorbents, e.g. graphene nanosheet (Rostamian and Behnejad, 2017), metal–organic framework (Xiong et al., 2019), molybdenum disulfide (Chao et al., 2014) and commercial active carbon (Mansour et al., 2018, Xiang et al., 2019), indicating that the application of KFeS2 for doxycycline removal is economically acceptable.
Reagent and processing
Cost
Usage per ton
Subtotal cost (US$/ton)
Recycling of used alkaline solution for KFeS2 synthesis
Ferric Trichloride
86.6 US$/ton
1.02 ton
88.3
Supplementary Potassium Sulphide
864.2 US$/ton
0.64 ton
553.1
Supplementary Potassium Hydroxide
733.3 US$/ton
0.12 ton
87.9
Pulping
0.23 US$/kW⋅h
0.5 kW⋅h (total 1 h)
0.1
Hydrothermal process (Steam Heating)
8.5 US$/h
15 h
127.5
Drying
0.23 US$/kW⋅h
6 kW⋅h (total 20 h)
33.1
Total cost
890.1
With such method, a portion of Fe-rich sludge, e.g. cold-rolling sludge, heavy-metal-bearing electroplating sludge and Al/Si-bearing groundwater sludge, comprised weakly crystallised Fe-bearing compounds. The sludge can be recycled as Fe resource to synthesise KFeS2. This process not only reduces the production of waste Fe-rich sludge but also develops a new product of wastewater treatment. Other sludges include red mud and calcinated mineral slag and contain considerable amounts of crystallised Fe-bearing minerals, e.g. andradite, melanite, pyreneite etc. Such crystallised Fe minerals do not release Fe in strong alkaline solutions and are thereby not involved in the formation of KFeS2. In summary, the abovementioned process can serve as an alternative strategy for the resource utilisation of Fe-rich sludge.
4 Conclusion
The Fe-bearing sludge is commonly generated in the coagulation, Fenton and catalyst process. With the method, such sludge can be recycled as KFeS2-bearing product via a facile hydrothermal route. During the conversion of sludge to KFeS2, three important reactions occurred. First, the surface Fe of the sludge was dissolved as Fe(OH)4− in the presence of 2 M KOH. The dissolution intensified with the addition of 5 M KOH. Second, one hydroxyl group of Fe(OH)4− was replaced by free HS−, and as the replacement reaction continued, a number of intermediate Fe(OH)3HS− was produced. Third, the polymerisation reaction of Fe(OH)3HS− transpired to release water molecules, and (FeS2)nn− was the final product. In the hydrothermal system, well-crystallised hematite was also formed from the combination of the adjacent FeOOH microcrystals. Thus, a mixture product of KFeS2 and hematite was generated.
The conversion of the sludge to KFeS2-bearing mixture had two remarkable merits, one of which is the reduced cost and space of sludge disposal. In water treatment plant industry, this expenditure was generally considered in the treatment cost of wastewater in the past but can now regarded as a deduction, effectively saving the operation expense of the water treatment plant. The other merit is the formation of rod-shaped KFeS2-bearing products, which exhibited a desirable hydrolysis performance in the neutral solution and good adsorption capacity for doxycycline. The qm of the new adsorbent for doxycycline was 2933.6 mg/g, which is higher than that of raw sludge, pyrite, hematite and PFS. Thus, the new KFeS2-bearing product has a desirable treatment efficiency in doxycycline-bearing wastewater.
CRediT authorship contribution statement
Yu Chen: Project administration. Zhihua Wang: Data curation. Dongxu Liang: Validation. Yanwen Liu: Validation. Hongbin Yu: Supervision. Suiyi Zhu: Writing - original draft. Leilei Zhang: Software.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (Grant No. 2019YFE0117900), the National Natural Science Foundation of China (Grant Nos. 51878134 and 52070038) and the Science and Technology Program of Jilin Province (Grant No. 20190303001SF).
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.2021.103173.
Appendix A
Supplementary material
The following are the Supplementary data to this article:
