Tetracycline Adsorption on Magnetic Sludge Biochar: Effects of pH and Natural Organic Matters

doi:10.21203/rs.3.rs-1419427/v1

Natural organic matters (NOM) are ubiquitous in the environment, but few systematic studies have examined the influence of NOM on the sorption ability of magnetic sludge biochar. In this study, magnetic sludge biochar was synthesized, characterized, and used as a sorbent for the removal of tetracycline (TC) from aqueous solutions. The effects of pH, humic acid (HA) and fulvic acid (FA) on TC adsorption by magnetic sludge biochar were studied using batch experiments. Adding HA and FA would alter adsorption behavior of TC, except its pH dependency. Results showed that relatively low concentration of dissolved HA (≤ 8 ppm) and FA(≤ 5 ppm) promoted the adsorption capacity of TC, but higher concentration competed against TC for sorption sites on surface of magnetic sludge biochar. The results of this study promote a better understanding the application of magnetic sludge biochar in real antibiotic wastewater.

adsorption

fulvic acid

humic acid

magnetic sludge biochar

tetracycline

Magnetic biochar, commonly prepared by impregnation-pyrolysis, chemical co-precipitation, solvothermal, and reductive co-precipitation^[1], has high surface area, porous structure, appreciable amounts of active sites on its surface, and good recyclability ^[2]. Furthermore, using excess sludge to prepare magnetic sludge biochar could not only reduce its preparation cost but also have great significance to solve the secondary pollution of inadequately treated sludge^[3]. Owing to its extraordinary adsorption capacity, magnetic sludge biochar has attracted much attention in the field of various pollutant removal, including antibiotics, heavy metals and dyes^{[4] [5]}.

As reported previously, natural organic matters (NOM) including humic acid (HA) and fulvic acid (FA), could be participate in the interaction between biochar and pollutants^[6]. NOM could modify the surface property of biochar and affect its adsorption behavior to various pollutants. Moreover, NOM also could result in site competition and pores blockage^[7]. Lian et al.^[8] reported HA coated biochar (prepared by straws) enhanced the adsorption of sulfamethoxazole but suppressed that of sulfanilamide on biochar. The effects of adsorbed HA on sulfonamide sorption by biochar were related to the properties of sorbate, composition and structure of HA adlayer and concentration. Luo et al. ^[7] demonstrated that low concentration of dissolved HA promoted the removal of ciprofloxacin by sludge biochar, but higher concentration of dissolved HA suppressed. Park et al.^[9] indicated that HA could change the charge of biochar, increase sorption sites resulting from less aggregation of biochar particles. In conclusion, the effect of NOM on the adsorption behavior of biochar varied greatly, which mainly depended on the properties of biochar, sorbents, NOM types and concentrations. However, very little attention has been paid on the effects of NOM on pollutants sorption by magnetic sludge biochar, especially as a function of pH in the presence of NOM.

The extensive use of antibiotics has become a serious environmental problem, which could discharge into various media^[10] and cause adverse consequences^[10b]. Among the commonly used antibiotics, tetracycline (TC) is the most widely used in aquaculture and pharmacy due to the relative low costs^{[2a] [2b]}. TC has been detected in wastewater, surface and groundwater, and might cause harm to human health and ecosystems^[11]. Therefore, in this study, TC was selected as the target organic pollutant.

The objective of this study was to investigate the sorption behavior of TC on magnetic sludge biochar as a function of pH in the presence of HA and FA. The results of this study are expected to provide a new insight into understanding how magnetic sludge biochar interacts with organic contaminants in real water environment.

2.1 Materials

The activated sludge was sampled from Gaoqiao Wastewater Treatment Plant in Zunyi, China. Tetracycline (TC, ≥ 98%), ferrous chloride and ferric chloride (≥ 99.9%) were purchased from Aladdin Industrial Corporation (USA). HA and FA were purchased from Macklin Biochemical Technology and Wengjiang Reagent Co., Ltd (Guangdong, China). All other chemicals were of analytical and used without any further purification.

2.2 Preparation of magnetic sludge biochar

Firstly, 5 mL 1 mol/L HCl was added into 500 mL sludge (suspended solid concentration: 14.0 g/L). In order to destroy the cell wall, the agitated mixture (400 rpm, 20 min) was transferred to an ultrasonic cell crusher and pretreated under 59 KHz for 15min, and then dried at 105 ℃ overnight. Secondly, magnetic sludge biochar was prepared according to the reference^[12]. For forming Fe₃O₄ particles on sludge biochar’s surface, 5.28 g FeCl₂ and 13.50 g FeCl₃ were dissolved in 500 mL water, and then 50 g dried sludge was added into the aqueous solution. Subsequently, 2 mol/L of NaOH solution was added into the above mixture until the pH of suspension reached 10.0 under vigorous stirring conditions. The solid phase was separated by centrifugation at 4000 rpm, vacuum dried at 70 ℃. At last, the composite was pyrolyzed at 400 ℃ for 4 hours in a tubal furnace under N₂ atmosphere to prepare magnetic sludge biochar.

2.3 Characterization of magnetic sludge biochar

The morphology of magnetic sludge biochar was characterized by Scanning Electron Microscope (SEM, Scios, FEI Company, Hillsboro, USA). The specific surface area and pore size of magnetic sludge biochar were determined by BSD-PS (Beishide Instruments, China). Surface functional groups of the magnetic sludge biochar were obtained by a FTIR analyzer (VERTEX70 spectrometer, Bruker Co. Germany). The zeta potentials of magnetic sludge biochar were determined using Malvern Zetasizer (Nano ZS90, Malvern, UK). The iron weight ratio in the magnetic biochar was measured by Atomic Absorption Spectroscopy (AAS, 990SUPER, Beijing Purkinje General Instrument Co., Ltd.).

2.4 Batch sorption experiment

To examine influence of HA and FA on magnetic sludge biochar adsorbing TC, the sorption experiments were conducted in dark by adding 0.07 g magnetic sludge biochar samples into 100 mL of 200 mg/L TC solutions at room temperature 25℃(the final dissolved HA and FA concentration was 0, 5, 8, 12 and 15 ppm, respectively), then agitated in the incubator shaker at 150 rpm. To investigate the effect of pH on the adsorption behavior of TC in above conditions, the initial pHs of TC solution were adjusted in the range of 2.0–8.0 with 0.2 mol/L HCl or NaOH. At certain time intervals, 2.0 mL of the suspension was sampled and separated magnetically. Then, the concentrations of TC, HA, and FA in the supernatant was determined by UV-vis (UV-vis, CARY 300, Agilent, USA) and a total organic carbon (TOC) analyzer (Shimadzu 3201, Japan).

3.1 Characterization of magnetic sludge biochar

The morphology of magnetic sludge biochar was characterized by SEM (Fig. 1A). The surface morphology of magnetic biochar is similar to that of activated carbon with a large number of irregular Fe₃O₄ particles are distributed on its surface. SEM-EDX results (Fig. 1B) confirmed the existence of Fe species, moreover, the atom and weight ratio of iron on the surface of magnetic sludge biochar was 2.8% and 7.1%, respectively. Meanwhile, the iron weight ratio in the magnetic sludge biochar bulk by AAS analysis was 7.8%, which was consistent with that of EDX result. The above results indicate that Fe₃O₄ exists in both the surface and the interior of the sludge biochar.

The surface and pore features of as-prepared magnetic biochar were detected by nitrogen adsorption desorption test. As depicted in Fig. 2A, the isotherm curves are consistent with the IUPAC classification type IV curve and H₃ type hysteresis loop. The S_BET was calculated as 93.41 m²/g, and the average pore size was 9.017 nm. The isotherm has no obvious inflection point in the low relative pressure region, indicating that the magnetic biochar has a low proportion of micropore. The lack of a saturated adsorption platform in the medium relative pressure area indicates that the biochar has irregular pores and morphologies.

Figure 2B exhibited the surface zeta potentials of magnetic sludge biochar. The isoelectric point (pH_IEP) of magnetic biochar was estimated to be 5.7, when pH < 5.7, the surface of magnetic sludge biochar carried positive charge, and the surface displayed negative charge when pH > 5.7.

In order to reveal the functional groups of the as-prepared magnetic biochar, FTIR analysis was performed (Fig. 2C). The strong adsorption peak at 1,037 cm^− 1 corresponded to the C-O-C stretching vibration band ^[21]. The presence of the peaks at 3,389 and 1,633 cm^− 1 indicated that the magnetic biochar possessed a significant amount of -OH groups on its surface^[13]. The peak at 470 cm^− 1 was ascribed to Fe-O stretching bond^[14]. Finally, the magnetic separation property was investigated by a separation test(Fig. 2D), and the results showed that the biochar can be completely separated from TC and MS suspensions within 120 s of application of magnets.

3.2 Adsorption of TC on magnetic sludge biochar in the absence of NOM

3.2.1 Effect of initial pH of TC

Figure 3 exhibited that the sorption capacity of TC raised apparently as the initial solution pH increasing, and the maxim adsorption amount occurred at pH = 6. In aqueous solutions, TC will undergo protonation-deprotonation reactions and present different ionic species including cation (TC⁺), molecule (TC⁰) and anions (TC⁻ and TC²⁻) as the dissociation constants (pKa) of TC are 3.3, 7.7, and 9.7 (shown in Fig. S1) ^[3]. Apparently, when pH ≤ 3.0, the adsorption capacity of TC was very limited, due to both surface of prepared magnetic biochar and TC are positively charged^[15]. As pH increasing, the adsorption capacity sharply increased, and the adsorption performance of TC on magnetic biochar was maximized at pH = 6 (193.3 mg/g). Although the zeta potential of magnetic biochar was very small and TC mainly became neutral, the maximum adsorption capacity could be explained by the π-π electron donor-acceptor and hydrogen bonding interactions^[16]. whereas, when pH increased far from 6, both magnetic sludge biochar and TC (TC⁻ and TC²⁻) were negatively charged, leading to a reduction of adsorption.

3.2.2 Effect of magnetic biochar dose

It was obvious that the adsorption capacity of TC increased with the rising of magnetic biochar dose (Fig. 4). The higher dose of magnetic biochar resulted in higher adsorption capacity of TC due to more available active sites. Specifically, the adsorption capacity of TC reached the maximum as the dosage of adsorbent was 0.6 g·L^− 1, and decreased for further addition.

3.3 Effect of HA and FA on the adsorption behavior of TC

Figure 5 and Fig. 7 presented the effect of HA and FA in different concentrations on TC adsorption by magnetic sludge biochar. Overall, adding HA or FA to magnetic sludge biochar varied the adsorption behavior of TC significantly, but hardly affected its global regularity of pH dependency. HA and FA contain functional groups including carboxylic, phenolic and aromatic groups^[17], which make them carry negative charge over the whole pH range ^[18]. Therefore, HA and FA could be adsorbed on the surface of magnetic sludge biochar and change its surface properties or result in competition for surface sorption sites, so as to affect the adsorption behavior of TC. As shown in Fig. 6 and Fig. 8, dissolved HA and FA concentration at the end of experiment partly reduced, which indicated that dissolved HA and FA was partially adsorbed onto the magnetic sludge biochar over the pH range from 2.0 to 8.0. However, just simple electrostatic interaction mechanism could not completely account for the pH dependence of the sorption, because when pH > pH_PZC (5.7), HA, FA and magnetic sludge biochar were all negatively charged, but as shown in Figs. 6 and 8 partial adsorption still occurred. Therefore, we suspected that the functional groups of HA and FA interacted with the components of magnetic sludge biochar which was responsible for the adsorption of HA and FA.

Low concentration of dissolved HA (8 ppm) promoted the sorption capacity of TC from 193.3 to 260.1 mg/g, then dramatically decreased to 95.4 mg/g with further increasing dissolved HA to 15 ppm, which could be caused by surface sorption sites competition of higher dissolved concentration HA. Compared with the HA samples, FA has less significantly influence in adsorption behavior of TC. Specifically, the presence of FA (5 ppm) enhanced the sorption capacity of TC from 193.3 to 220.6 mg/g, while higher concentration of dissolved FA inhibited. Moreover, HA and FA showed a major effect on the adsorption of TC at pH ≤ 6, and little effect at higher pHs. When pH ≤ 3, the adsorption capacity of TC increased dramatically in the presence of HA and FA, which could be attributed to adsorbed HA and FA on the surface of magnetic sludge biochar that could change its surface properties and make the repulsive force decreased. As the pH increased, the functional groups of absorbed HA and FA could enhance the π-π electron donor-acceptor and hydrogen bonding interactions. As a result, the sorption capacities of TC were enhanced. When pH above 6, the dominant form of tetracycline was anion, and HA, FA, and magnetic sludge biochar were more negatively charged, the amount of HA and FA adsorbed on magnetic sludge biochar decreased, which resulted in little effect of HA and FA on TC adsorption process.

To further investigate the competing role of HA and FA, the adsorption behaviors of HA and FA on magnetic sludge biochar were performed, and the results were shown in Fig.S2. Compared with the concentration of the residual HA and FA in the presence of TC (Fig. 6 and Fig. 8), the adsorption capacity of HA and FA on magnetic sludge biochar almost unchanged with relatively low concentrations (HA ≤ 8 ppm, FA ≤ 5 ppm) as well as pH < 3.0 or pH > 6.0. Nevertheless, the adsorption capacity of HA and FA decreased greatly at pH 3–6 with higher concentration. All these results (Fig. 6, 8, and S2) supported those higher concentrations of dissolved HA and FA at pH 3–6 mainly leading to the competition for surface sorption sites. The adsorbed HA and FA could on the one hand occupy active sites on the magnetic sludge biochar surface, on the other hand repel the approaching TC molecules sterically, thus exhibiting competing behavior against the sorption of TC.

HA and FA could efficaciously affect not only the adsorption capacity but also the adsorptive speed of TC. As shown in Fig. 9, when the initial HA concentration was below 8 ppm, the adsorption speed was improved and the adsorption equilibrium achieved at 12 h, while, with further increase of dissolved HA to 15 ppm, the adsorption speed was decreased and the adsorption equilibrium achieved at 18 h. Compared with the HA samples, FA affected adsorption speed of TC less significantly, but the influence trend was similar to HA.

3.4 Adsorption performance of TC on magnetic sludge biochar in coexistence of HA and FA system

Considering the co-existence of HA and FA in some actual water bodies, the adsorption performance of TC in coexistence system was also conducted. As shown in Figure S3, the adsorption capacity and adsorptive speed were intermediate when HA and FA coexisted. Remarkably, the adsorptive equilibrium was remained.

The effect of raw materials, preparation method of biochar, NOM, adsorbate as well as the effect of NOM on adsorption behavior were summarized in Table 1. These findings suggested that the effects of dissolved NOM (model: HA and FA) on organic contaminants adsorption by biochar were greatly dependent on the properties of sorbate, biochar, types and concentration of NOM.

Table 1

The effect of NOM on adsorption behavior of organic contaminants by biochar
Raw materials	Preparation method	NOM	Adsorbate	Effect	Reference
crop straws	pyrolysis	HA	sulfamethoxazole sulfonamide	declined (2.5–30 ppm) improved (> 30 ppm) improved (2.5–30 ppm)	^[8]
				declined
				(> 30 ppm)
pine needles wheat straw sludge	pyrolysis	HA HA	polychlorinated biphenyls ciprofloxacin	enhanced (10 ppm) promoted (10 ppm)	^[19] ^[7]
sludge	pyrolysis	HA FA	tetracycline	improved (≤ 8 ppm) improved (≤ 5 ppm)	This work

In summary, municipal sewage sludge was chosen as the raw material and magnetic sludge biochar was successfully synthesized as adsorbent for TC adsorption. Considering the complex influence factors in nature system, the adsorption behavior of TC in the absence or presence of HA and FA was investigated. The adsorption of TC on magnetic sludge biochar was greatly enhanced in the presence of low concentration HA and FA, but higher concentration of dissolved HA and FA inhibited. The results obtained from this study help us to better understand the environmental behavior of antibiotics removal by magnetic sludge biochar in water treatments and natural systems. Moreover, the excellent performance of magnetic sludge biochar gives a new aspect of sewage sludge reutilization.

Author Contributions

Yuanhui Wu contributed to the conception of the study, experiment and manuscript preparation; Dan Long performed the experiment and participated in data analysis; Fanian Yang contributed to the experiment; Suxing Luo conceived of the study, designed the study, coordinated the study and helped draft the manuscript. All authors reviewed the manuscript.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51561033 and 22166011), the Special Key Laboratory of Electrochemistry for Materials of Guizhou Province (No. QJHKYZ [2018]004), the Zunyi City-School Joint Fund (No. HZ [2021]198), the Research Fund for the Doctoral Program of Zunyi Normal University (No. BS [2019]13) and BS [2019]38).

Data availability statement

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

Confict of interest

The authors declare no competing interests.

a) Z. Feng, R. Yuan, F. Wang, Z. Chen and H. Chen, Science of The Total Environment 2020, 765, 142673; b) C. Lai, F. Huang, G. Zeng, D. Huang, L. Qin, M. Cheng, C. Zhang, B. Li, H. Yi and S. Liu, Chemosphere 2019, 224, 910-921; c) C. Lei, Y. Sun, D. Tsang and D. Lin, Environmental Pollution 2017, 10.
a) Y. C. A, C. H. A, D. D. A, Y. L. B and L. L. A, Ecotoxicology and Environmental Safety 197; b) Zeyou, Chen, Wei, Zhang, Gang, Wang, Yingjie, Zhang, Yanzheng and Gao, Environ.sci.technol 2017.
L. T. A. C, J. Y. A. C, Y. P. B, G. Z. A. C, Y. D. A. C, J. W. A. C, X. R. A. C, S. Y. A. C, B. P. A. C and H. F. A. C, Chemical Engineering Journal 2018, 336, 160-169.
L. Wang, J. M. Wang, R. Zhang, X. G. Liu, G. X. Song, X. F. Chen, Y. Wang and J. L. Kong, Rsc Advances 2016, 6, 42876-42884.
a) R. Bian, J. Zhu, Y. Chen, Y. Yu, S. Zhu, L. Zhang and M. Huo, RSC Advances 2019, 9; b) Xiao-Hu, Dai, Shi-Jie and Yuan, Environmental Science Nano 2017.
G. Cornelissen and O. Gustafsson, Environmental Pollution 2006, 141, 526-531.
K. Luo, Y. Pang, Q. Yang, D. Wang, X. Li, L. Wang, M. Lei and J. Liu, Chemosphere 2019, 231, 495-501.
F. Lian, B. Sun, X. Chen, L. Zhu, Z. Liu and B. Xing, Environmental Pollution 2015, 204, 306-312.
C. M. Park, J. Han, K. H. Chu, Y. Al-Hamadani, N. Her, J. Heo and Y. Yoon, Journal of Industrial & Engineering Chemistry 2017, 48, 186-193.
a) V. K. Sharma, N. Johnson, L. Cizmas, T. J. Mcdonald and H. Kim, Chemosphere 2016, 150, 702-714; b) Y. Chen, F. Wang, L. Duan, H. Yang and J. Gao, Journal of Molecular Liquids 2016, 222, 487-494.
a) R. Acosta, V. Fierro, D. Martinez, D. Nabarlatz and A. Celzard, Chemosphere 2016, 149, 168-176; b) Y. Luo, L. Xu, M. Rysz, Y. Wang, H. Zhang and P. Alvarez, Environmental Science & Technology 2011, 45, 1827-1833.
J. Wang, Z. Liao, J. Ifthikar, L. Shi, Z. Chen and Z. Chen, RSC Advances 2017, 7, 18696-18706.
E. A. Ferreiro and S. Bussetti, Colloids & Surfaces A Physicochemical & Engineering Aspects 2007, 301, 117-128.
a) S. Kong, Y. Wang, H. Zhan, M. Liu, L. Liang and Q. Hu, Journal of Geochemical Exploration 2014, 144, 220-225; b) T. Chen, Z. Zhou, S. Xu, H. Wang and W. Lu, Bioresource Technology 2015, 190, 388-394.
Y. Liu, Q. Yang, R. Feng, W. Xu and D. Wei, New Journal of Chemistry 2019, 43.
W. A. Jia, A. Yl, A. Jl, A. Yz, Y. B. Hui and C. Yp, Chemosphere 236, 124254-124254.
H. Wang, Y. N. Dong, M. Zhu, X. Li, A. A. Keller, T. Wang and F. Li, Water Research 2015, 80, 130-138.
S. Yu, J. Liu, Y. Yin and M. Shen, Journal of Environmental Sciences 2018, 63, 198-217.
W. Yu, L. Wang, G. Fang, H. Herath and D. Zhou, Environmental Pollution 2013, 172, 86-93.

No competing interests reported.

SI.docx

Tetracycline Adsorption on Magnetic Sludge Biochar: Effects of pH and Natural Organic Matters

Status:

Version 1

Abstract

Figures

1 Introduction

2 Materials And Methods

2.1 Materials

2.2 Preparation of magnetic sludge biochar

2.3 Characterization of magnetic sludge biochar

2.4 Batch sorption experiment

3 Results And Discussion

3.1 Characterization of magnetic sludge biochar

3.2 Adsorption of TC on magnetic sludge biochar in the absence of NOM

3.2.1 Effect of initial pH of TC

3.2.2 Effect of magnetic biochar dose

3.3 Effect of HA and FA on the adsorption behavior of TC

3.4 Adsorption performance of TC on magnetic sludge biochar in coexistence of HA and FA system

4 Conclusions

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1