Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption

Journal Pre-proof Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption

Jing Li, Bing Li, Haiming Huang, Ning Zhao, Mingge Zhang, Lu Cao PII:

S0048-9697(20)30349-1

DOI:

https://doi.org/10.1016/j.scitotenv.2020.136839

Reference:

STOTEN 136839

To appear in:

Science of the Total Environment

Received date:

10 September 2019

Revised date:

12 January 2020

Accepted date:

19 January 2020

Please cite this article as: J. Li, B. Li, H. Huang, et al., Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption, Science of the Total Environment (2018), https://doi.org/10.1016/j.scitotenv.2020.136839

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© 2018 Published by Elsevier.

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Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption *

Jing Li a, b, Bing Li c, Haiming Huang a , Ning Zhao a, b, Mingge Zhang a, Lu Cao a a

School of Environment and Civil Engineering, Dongguan University of Technology, Dongguan

523808, China. b

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Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

c

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Yanshan University, Qinhuangdao 066004, PR China

Department of Chemical & Materials Engineering, University of Auckland, New Zealand

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Abstract:

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Phosphate adsorption using metal-modified biochar has awakened much attention and triggered

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extensive research. In this study, the effect of lanthanum (La)-modified sludge using

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impregnation-co-precipitation was used for phosphate adsorption. Consequently, La-coated biochar at a pyrolysis temperature of 600C had the highest phosphate adsorption and the lowest

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heavy metal leaching potential. The treatment of virgin biochar with alkali before La loading was found to be beneficial for the increase of phosphate adsorption capacity. The adsorption kinetics was well depicted by the pseudo-second-order model, and indicating that intraparticle diffusion played a crucial role in the adsorption process. The good fitness between adsorption data and the Langmuir isotherm model showed a maximal adsorption capacity of 93.91 mg/g, where phosphate absorption was highly correlated to its concentration in the solution. The La-coated biochar showed high adsorption capacity when the solution pH varied from 3.0 to 6.0, and was insensitive to the coexisting chloride, nitrate, sulfate, bicarbonate and citrate. Moreover, the adsorption mechanism was further explored by using Zeta potential analysis, FTIR and XPS, indicating that 1

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the phosphate is adsorbed through electrostatic attraction in the form of the inner-sphere complexation. All the results suggested that the sludge-based biochar, as a support material for La, could serve as a promising adsorbent for phosphate in real applications. Keywords: Adsorption, phosphate, sludge biochar, lanthanum

*Corresponding author. Tel.: +86 769 22862012; fax: +86 769 22861232.

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E-mail address: [email protected] (H. Huang)

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1.

Introduction The significant use and excessive discharge of phosphorus increase nutrient concentration in

local streams, accelerates the growth of blue-green algae, and causes fish death. Such problems will continue to deteriorate and endanger human life if there were no proper solutions. What is more, the tighter phosphorus discharging limits (e.g. he permissible phosphorus discharge

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concentrations in wastewater treatment will decrease from 1 to 2 mg/L to 0.1 mg/L in the

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European Union under the Water Framework Directive, Shepherd et al., 2016) requires a substantial modification of current wastewater process. However, current phosphorus removal

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technologies such as chemical precipitation and biological treatment are unable to satisfy new

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standards (Wu et al., 2017).

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Adsorption is a preferable approach for its simplicity of design and good performance even at

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low concentrations (Loganathan et al., 2014). It is vital to develop an environmental friendly adsorbent with no secondary pollution (Li et al., 2016; Kong et al., 2018; Yuan et al., 2019). For

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example, natural materials (such as dolomite, sepiolite) (Yin et al., 2011; Boeykens et al., 2017); synthetic metal oxides and hydroxides (such as Zn-Al LDH, Fe-Mn binary oxide) (Lu et al., 2014; Hatami et al., 2018); functionalized inorganic materials (such as chitosan/La hydroxide composite aerogel beads) (Lin et al., 2018) have been examined for phosphate adsorption. In recent years, biochar with various metal oxides or hydroxides, especially magnesium, iron, aluminum and calcium, have received considerable attention because of their superior phosphate adsorption capacity. For example, the maximum phosphate adsorption capacity of biochar prepared from Mg-enriched tomato tissues could reach 100 mg/g (Yao et al., 2013). Biochar with 20% Al content exhibited an optimal phosphate adsorption capacity at 57.49 mg/g (Yin et al., 2018). The 3

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phosphate adsorption capacity of FeCl3-impregnated biochar was reported to be 111.0 mg/g (Yang et al., 2018). Besides, La-containing materials have attracted more and more attention for phosphorus adsorption due to its superior phosphate affinity, excellent selectivity, and wide operating pH range (Huang et al., 2014). Many materials, such as silica spheres (Huang et al., 2015), porous

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zeolite (He et al., 2017), tourmaline (Li et al., 2015) and magnetic mesoporous nanospheres (Chen

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et al., 2019), have been investigated as a supporter for La oxides or hydroxides. In previous literature, one gram La2O3 modified oak takes up 46.37 mg phosphorus (Wang et al., 2016), and

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La(OH)3-modified magnetic pineapple biochar absorbs 101.16 mg/g (Liao et al., 2018). To evenly

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distribute La over the surface, the preferred supporting materials should have large surface areas

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and abundant pores.

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In China, wastewater treatment plants generated over 60 million tons of sewage sludge (80% water content) each year, in which the various heavy metals, organic micropollutants and

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pathogens might threaten human health (Fytili and Zabaniotou, 2008). It is valuable to convert the sewage sludge, a hard-to-handle solid waste, into effective adsorbents due to its low cost and easy access of the material (Li et al., 2019). However, various heavy metals in sewage sludge might be bound to organic material and coprecipitate (Wang et al., 2019). Previous literature reported that most heavy metals are present in the carbon matrix in the oxidizable and residual forms after pyrolysis, especially at 600C, reducing its potential ecological risks (Jin et al., 2016). However, it is still necessary to consider the risks of sludge-based biochar during phosphorus adsorption. Therefore, this research is to develop a sewage sludge-based phosphate adsorbent. A variety of metals, including magnesium, calcium, aluminum, iron and lanthanum, have been compared to 4

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maximize the phosphate adsorption capacity under different surface treatment processes. The optimum loading condition and the method for further improving the adsorption capacity were also investigated. Meanwhile, the leaching experiments of different sludge-based biochar were performed to test whether the synthetic biochar will become a secondary pollutant or not. 2.

Materials and methods

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2.1. Materials

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The dewatered sewage sludge was collected from the North Municipal Sewage Treatment plant, in Dongguan, China. It was air-dried at 105C until the moisture was completed removed.

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The dried sludge was then ground and passed through an 80-100 mesh. The obtained fine powder

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was then heated in a programmable tube electric furnace (BTF-1200C, Anhui BEQ Equipment

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Technology Co. Ltd., China) with a temperature increase of 5C /min, and held at the target

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temperature for 2 hours surrounded by nitrogen gas. Analytical grade magnesium chloride (MgCl26H2O), calcium chloride (CaCl22H2O), aluminum chloride (AlCl36H2O), ferric chloride

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(FeCl36H2O), La nitrate hexahydrate (La(NO3)36H2O), sodium hydrate (NaOH) and potassium dihydrogen phosphate (K2HPO4) were purchased from Macklin Co. Ltd. A 1000 mg/L phosphate stock solution was prepared by dissolving K2HPO4 into deionized water and diluted to the desired concentrations before each experiment. 2.2. Preparation of metal-bearing sludge biochar Five salt solutions, MgCl2, CaCl2, AlCl3, FeCl3 and La(NO3)3, commonly used for biochar modification for phosphate adsorption capacity improvement, were prepared with concentrations at 0.1 M. Four different modification methods were tested to determine the most effective modification strategy, using a pyrolysis temperature of 600C: 5

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(i)

1 g biochar was added into different salt solutions of 50 mL, with magnetic stirring at 100 rpm for 24 h, and then dried after filtration;

(ii)

The biochar was pretreated by 2 M HCl for 24 h and washed with deionized water until the leachate pH ranged between 6 and 7. After being dried in the oven, the following procedure was the same as those described in method (i); The biochar was treated with 2 M NaOH for 24 h. Remaining steps were the same as

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(iii)

(iv)

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mentioned in (ii);

1 g biochar was added to 50 mL salt solutions (MgCl2, CaCl2, AlCl3 and FeCl3),

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respectively. The mixture pHs were adjusted to 10 using 1 M NaOH to form metal

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precipitations, and then being agitated using magnetic stirring at 100 rpm for 24 h. During the

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same period, La precipitates were obtained by improving the pH of La nitrate solution (0.1 M)

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to 10 using 1 M NaOH. After adding 1 g of raw biochar, the mixture was set on a magnetic stirrer at 100 rpm for 24 h. Finally, biochar composites were acquired after filtration, washed

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with deionized water and dried at 80C for 12 h. Subsequently, La-coated biochar with five pyrolysis temperatures (400, 500, 600, 700, and 800C) were prepared using this method and named as La-400SS, La-500SS, La-600SS, La-700SS and La-800SS. 2.3. La-coated biochar leaching tests Leaching tests were conducted using La-coated biochar at pyrolysis temperatures of 400C, 600C and 800C. The adsorbent was added into deionized water with the solution pH being pre-adjusted from 3 to 11 by 0.1 M HCl and NaOH. After shaking on a water bath oscillator (SHZ-A, Boxun Medical-Biological Instrument Co. Ltd., Shanghai, China) for 24 h, the solution 6

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was filtered, and the concentration of phosphate and heavy metals commonly found in sewage sludge (Zn, Ni, Cr, Cu, Cd) were determined. 2.4. Methods for improving adsorption capacity of the La-coated biochar The best pyrolysis temperature was decided by considering the highest phosphate adsorption capacity and the lowest leaching amount of heavy metals. 1 g raw biochar, which was pyrolyzed at

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the optimal temperature, was treated by three different chemical reagents: (i) 20 mL hydrochloric

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acid solution (2 M); (ii) 20 mL sodium hydroxide solution (2 M); and (iii) 20 mL zinc chloride solution with the concentration of 100 g/L. Next, these three mixtures were placed on a water bath

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oscillator at 25oC for 24 h. After that, three kinds of treated biochar were filtered and rinsed with

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distilled water until the leachate pH ranged between 6 and 7; and then they were dried at 105C in

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the oven. Subsequently, the La modification process was the same as the method described in

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Section 2.2(iv), and the products treated with HCl, NaOH and ZnCl2 were named as La-SS-H, La-SS-OH and La-SS-Z, respectively, with  representing the optimal pyrolysis temperature.

2.5.1.

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2.5. Adsorption experiments

Batch adsorption tests

Batch adsorption tests were conducted in 100 mL conical flasks, which were fixed on a water bath oscillator with a constant temperature at 23  0.5oC. For each experiment, 0.025 g adsorbent was added to 50 mL phosphate solution (50 mg/L) at pH 5.4. The mixture was allowed to react for 24 h to reach equilibrium. Afterwards, the mixtures were immediately filtered through a 0.22 μm GE cellulose nylon membrane filter, and the phosphate concentration in the filtrate was analyzed by a 752 N-spectrophotometer at a wavelength of 700 nm. The phosphate adsorption amount (mg) per gram (qe, mg/g) was calculated according to Eq. (1), 7

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(1)

where, Ci and Ce (mg/L) are the phosphate concentrations at the initial and the equilibrium, respectively. V is the solution volume and M is the mass of the adsorbent. 2.5.2.

Kinetics behavior

50 mL phosphate solution (pH = 5.4, 50 mg/L) was mixed with 0.025 g adsorbent. The

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residual phosphate concentration at different times (0, 1, 2, 4, 6, 10, 14, 20, 30 h) was monitored

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and the phosphate adsorption amount (mg) per gram of the adsorbent at a different time (Qt, mg/g), was expressed, according to Eq. (2), Qt  (Ci  Ct)VM

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(2)

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where, Ct is the phosphate concentration at time t. The kinetic data were fitted by using the

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pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model.

2.5.3.

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Expressions and descriptions for those models are shown in Table S1. Isotherm models, and effect of initial phosphate concentrations

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Isotherm models fitting and the effect of initial concentrations were investigated by changing initial phosphate concentrations (50, 100, 150, 200, 250, 300 mg/L) at pH 5.4, with 0.025 g adsorbent dosage. The phosphate adsorption amount at equilibrium was determined according to Eq. (1), and phosphate removal efficiency was calculated according to Eq. (3). R(%)  (Ci  Ce)Ci  100%

(3)

The experimental data were fitted by both the Langmuir and the Freundlich models. The expressions for these models are also listed in Table S1. 2.5.4.

Effect of competitive anions and solution pH

In order to explore the effect of competitive anions on phosphate adsorption, 5 common 8

Journal Pre-proof anions Cl−, NO3−, SO42−, HCO3− and CH3COO− with various concentrations (50, 100, 200, 400 mg/L) were selected. The solution pH was fixed at 5.4, which is consistent with the blank experiment. Also, the effect of solution pH on phosphate adsorption capacity was investigated with varying initial pH from 3 to 12, which was adjusted through using 0.1 M HCl and NaOH in 50 mL phosphate solution at a concentration of 50 mg/L.

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2.6. Characterization and analytical method

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In this study, phosphate concentrations in solution were determined by the Mo-SS anti-spectrophotometric method (752 N-spectrophotometer; China). Concentrations of heavy

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metals were determined by atomic absorption spectrophotometer (Thermofisher ICE3500;

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America). The BET surface area (SBET), total pore volume (TPV), and average pore size (APS)

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were measured by N2 adsorption at 77 K on a BK100A surface area analyzer (China). Additionally,

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the morphology of biochar at different pyrolysis temperatures before and after the modification was characterized, and types of elements in La-coated biochar were detected by using scanning

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electron microscopy (SEM-EDS; JSM―6701FJE0L1; Japan). Zeta potential of the modified biochar was determined by a zeta potential analyzer (Zetasizer Nano ZSE, Britain). Fourier Transform Infrared Spectroscopy (FTIR) was carried out for analysis, which was recorded by using Thermo Fisher Scientific Nicolet iS5 (America). Besides, the chemical compositions of the materials were analyzed using an XPS (PHI 5600, Physical Electronics Inc., USA) with Al Ka radiation (1486.6 eV). 3.

Results and discussion

3.1. Effect of different metal-loaded biochar Fig. 1 displays the phosphate adsorption amount using biochar modified by different metals 9

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under various treatment methods. As can be seen in Fig. 1A, direct sludge biochar addition into metal salt solutions have little effect on phosphate adsorption, suggesting that direct immersion could not effectively load metal ions onto the biochar. Furthermore, the pretreatment process through acid and alkali conditioning were carried to increase the porosity of the biochar and to explore the effect of porosity on the ion loading. As a consequence, the phosphate adsorption

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capacities of Al- and Fe-modified biochar were enhanced after acid treatment, but there was less

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improvement from biochar loaded by Mg, Ca and La (Fig. 1B). However, the adsorption capacity of the biochar modified by five metals, respectively, after alkali treatment of virgin biochar as

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biochar surface properties and functional groups.

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shown in Fig. 1C, all increased. Therefore, metal ions loading might be closely related to the

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It is evident that Al-modified biochar has the highest phosphate adsorption capacity among

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the five metals after both acid and alkali treatments. However, its adsorption capacity is much lower than other Al-based absorbents, such as Al-doped biochar derived from poultry manure (the

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maximum P adsorption capacity was 701.65 mg/g) and sugarcane straw (the maximum P adsorption capacity was 758.96 mg/g) (Novais et al., 2018). It suggests that the sludge-based-biochar is not good as a support material for loading metal ions by immersion method when comparing with biochar prepared from other biomasses. Conversely, the La hydroxide coated biochar shows strong phosphate adsorption capability, demonstrating the feasibility of loading the La group onto sludge biochar for efficient phosphate adsorption. Fig. 1 here 3.2. Effect of pyrolysis temperature on phosphate adsorption by La-coated biochar The effect of La-coated biochar, using sludge biochar pyrolyzed at different temperatures, on 10

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phosphate adsorption is shown in Fig. 2. As can be seen from the figure, the phosphate adsorption capacity enhanced slightly with an increase in the pyrolysis temperature ranging from 400C to 600C. When the pyrolysis temperature continued to increase over 600C, the phosphate adsorption capacity decreased slightly. The value of P was lower than 0.01 through univariate analysis, demonstrating that the difference in adsorption capacity among different temperatures

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was statistically significant.

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SEM was used to characterize biochar at different pyrolysis temperatures of 400C, 600C and 800C, and the results are shown in Fig. 3a, b and c. It could be observed that the morphology

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of 400SS appeared to be rough and composed of block structures with many cracks. However, the

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surface structure of 600SS was much smoother, with only small particles scattered sporadically.

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Compared to 400SS, the morphology of 600SS was more regular and hierarchical. Further, the

increases accordingly.

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surface morphology of 800SS was directly covered with many compact particles, and the porosity

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The surface characteristics and pore structures of raw biochar at different pyrolysis temperatures might be responsible for the varying adsorption capacity. Previous studies reported that higher temperature would contribute to the increase of activation energy for microporosity development, leading to the formation of additional micropores (Yavari et al., 2017). What is more, the release of CO2 at elevated temperatures might lead to secondary reactions between CO2 and carbon atoms in a biochar matrix, and therefore opening its occlusive pores and increasing the porosity (Thines et al., 2017). The BET adsorption-desorption curves of the biochars with different pyrolysis temperatures are shown in Fig. 3d, proving that the biochar surface areas increase with the pyrolysis temperature. Besides, more hydroxyl groups were decomposed at elevated pyrolysis 11

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temperature. And organic fatty hydrocarbon was decomposed into methane, carbon dioxide and other gases or aromatic structures when heating to higher pyrolysis temperature (Lu et al., 2013). The biochar at higher temperature contains less O and H bonds (Takaya et al., 2016; Saadat et al., 2018), which will decrease the effectiveness of O- and H-carrying functional groups. Overall, the surface texture of 600SS was more suitable for the attachment and dispersion of La compounds.

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Therefore, La-600SS was the best for phosphate adsorption and was thus selected for further

Fig. 2 here

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Fig. 3 here

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evaluations.

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3.3. Phosphate and heavy metals leaching from La-coated biochar of different pyrolysis

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temperatures

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Leaching of the La-coated biochar pyrolyzed at 400C, 600C and 800C were investigated and are shown in Fig. 4a. The concentration of phosphate was almost zero when the initial pH was

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lower than 5, while more phosphate ions were present in solution with higher pH. The presence of mineral variscite/berlinite, amorphous Ca3(PO4)2 in the sludge and biochar have been proved by Li et al. (2018) through combining the experimental results together with theoretical simulation of mineral P. In a previous study, phosphate existing in the sludge and biochar could be extracted by HCl and NaOH solution for the existence of amphoteric P-containing phases, which was different from the result of this study (Li et al., 2018). This phenomenon could be attributed to the intervention of La. According to the reported research, La-loaded materials usually have a higher adsorption capacity for phosphate in an acidic environment, but the phosphate adsorption capacity will be greatly weakened in an alkaline environment (Wang et al., 2016). Therefore, phosphate 12

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released by the biochar at lower pH is more likely to be adsorbed by the La-coated biochar, resulting in the low amount of leaching in an acidic environment. The P concentration gradually increases with the increase of solution pH due to the decrease in phosphate adsorption capacity of the La-coated biochar in an alkaline condition. Moreover, the phosphate concentration increases with the elevated pyrolysis temperature. A conclusion has been obtained by Li et al. (2018) that

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the extraction capacity of phosphate is gradually increased from the sludge to 600SS but declines

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afterwards. In the current study, the leaching of phosphate from La-400SS is the least, followed by La-600SS. The result, that leaching of phosphate from La-800SS is higher than La-600SS, which

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might be due to the lower phosphate adsorption capacity of La-800SS.

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The leaching behaviour of heavy metals in biochar pyrolyzed at different temperatures also

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have been interpreted. Under a varying pH from 3 to 11, the nickel concentration increases slightly

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with increasing temperature and the leaching amount at different conditions all lower than 0.1 mg/L, as shown in Fig. 4b. As can be seen in Fig. 4c, the leaching of cadmium at different

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pyrolysis temperatures is similar, regardless of the solution pH change. Zinc prefers to be leached out under acidic conditions, and the amount of leaching zinc would decrease with the pyrolysis temperature, as displayed in Fig. 4d. The maximum cadmium and zinc leaching amount are both less than 0.04 mg/L, which occur for using La-400SS. The leaching of copper (in Fig. 4e) also decreases with increasing pH, in which La-400SS shows higher leaching potential (0.7 mg/L) than La-600SS and La-800SS. As for chromium (in Fig. 4f), varying pH significantly affects the leaching efficiency, while pyrolysis temperature shows negligible impact. The leaching regulation and amount are consistent with previous literature, stating that most heavy metals can exist in the biochar matrix stably after pyrolysis, leading to a low potential threat to the environment (Jin et al., 13

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2016). Therefore, there is no need to worry about heavy metal leaching when using La-600SS for future applications. Fig. 4 here 3.4. Effect of chemical treatments on phosphate adsorption Fig. 5 displayed the effect of HCl, NaOH and ZnCl2 solution treated La-600SS on phosphate

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adsorption. It could be seen that the adsorption capacity of La-600SS-OH was improved when

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comparing with other biochar. The phosphate adsorption capacity of La-600SS-OH was significantly different from those of other La-coated materials, while there was no obvious

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difference among La-600SS, La-600SS-H and La-600SS-Z. Generally, chemical reagents might

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improve the microporosity by reducing its volume and creating new pores (Takaya et al., 2016).

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As is known, acidic surface functional groups could be improved by oxidation or acid treatment.

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Such modification could enhance the black carbon cation exchange property (Takaya et al., 2016). Conversely, alkali treatment might increase the number of hydroxyl groups, resulting in ligand

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exchange between -OH and the phosphate (Samadi, 2006; Sarkhot et al., 2013), and is responsible for the increase of phosphate removal rate when using La-600SS-OH. Similar conclusions could also be drawn from Fig. 5. Furthermore, comparing with 600SS, the BET surface area of 600SS-H increased from 11.63 m2/g to 75.85 m2/g, while that of 600SS-OH increased to 61.80 m2/g (see Table S2). The increased surface area might be due to the demineralization caused by HCl and NaOH (Mahmoud et al., 2012; Yakout, 2015; Takaya et al., 2016), which was known to occur after alkali and acid treatment. However, the phosphate adsorption capacity of 600SS-H with the highest BET surface area was lower than that of 600SS-OH, indicating that the adsorption process is more like chemisorption instead of physical adsorption. 14

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The biochar treated by ZnCl2 solution before La loading, shows a similar BET surface area to that of La-600SS, and has no noticeable effect on phosphate removal. Generally, there is a strong dehydrator function and porogen potential of ZnCl2 due to substantial external localized decomposition (Park et al., 2015). In terms of porosity development, biochar benefited from ZnCl2 treatment at temperatures of <500C (Takaya et al., 2016). In the published report, the BET

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surface area of the sludge biochar could increase from 1.53 m2/g to 184.41 m2/g when the

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dewatered sludge powders were soaked in ZnCl2 solution, and then the impregnated samples were pyrolyzed in 550°C (Yang et al., 2018). However, the results in this study demonstrate that it is of

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no use for the porosity improvement by soaking after pyrolysis process.

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3.5. Characterization of La-coated biochar

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Fig. 5 here

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The morphology of La-600SS-OH is displayed in Fig. 6(a). The La-coated biochar is covered by a large amount of emerging filamentous particles with random distribution, while the surface of

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600SS presented in Fig. 3(b) was relatively neat. In addition, higher La concentration shown in Fig. 6(b) represents that La compounds assuredly exist on the biochar surface. Moreover, the BET surface area, total pore volume and average pore size of La-600SS-OH are all lower than those of the 600SS-OH, which might be caused by the coverage of La compounds on the biochar’s surface. Fig. 6 here 3.6. Kinetics behavior The phosphate adsorption of La-600SS-OH increases gradually, and reached equilibrium after 24 h. Its kinetic-depicted curves are shown in Fig. 7, and the kinetic parameters are listed in Table 1. As can be seen, the experiment data fit with the pseudo-second-order kinetic model (R2 = 15

Journal Pre-proof 0.99) better than the pseudo-first-order kinetic model (R2 = 0.96), indicating that chemisorption or chemical bonding between phosphate and the active sites of the absorbent might be the dominant process of phosphate adsorption (Huang et al., 2014). Besides, qe (71.81 mg/g) calculated from the pseudo-second-order model was close to the experimental data of 71.25 mg/g. Similar conclusions were obtained from La-loaded oak biochar (Wang et al., 2016) and La(OH)3-modified magnetic

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pineapple biochar (Liao et al., 2018).

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In addition to the commonly used pseudo-first-order and pseudo-second-order kinetic models, the intraparticle diffusion model was also applied to describe the adsorption process. According to

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the relationship between the phosphate adsorption amount at time t (Qt, mg/g) and parameter of

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t0.5, whether the adsorption process was closely related to the intraparticle diffusion or not could

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be determined. Previous research indicated that the slow absorption rate of La-coated biochar was

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because of the penetration by diffusion inside the adsorbent, where more time is required (Zelmanov and Semiat, 2015; Yang et al., 2018; Saadat et al., 2018). In this study, the

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experimental data fit well with the intraparticle diffusion model (R2  0.91), indicating that intraparticle diffusion process played a crucial role in phosphate adsorption when using the La-coated biochar.

Table 1 here Fig. 7 here 3.7. Isotherm models, and effect of initial phosphate concentrations The specific relationship between the amount of adsorbate on the surface of the adsorbent and concentration in the solution can be expressed by adsorption isotherm (Wang et al., 2016), which could be described by the Langmuir and the Freundlich models. The Langmuir adsorption 16

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model is based on the assumption of monolayer adsorption, where (i) all surface sites are equivalent (i.e., free of defects), and (ii) adsorption to one site is independent of adjacent sites occupancy condition (Lalley et al., 2016). The Freundlich adsorption model is applicable for nonideal adsorption on heterogeneous surfaces with multilayer sorption (Lalley et al., 2016). As a consequence, the R2 calculated by the Langmuir model was higher than Freundlich (as shown in

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Table 2), indicating that the phosphate adsorption on La-600SS-OH was caused by a monolayer

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homogenous adsorption process, which is consistent with other La-modified adsorbents (He et al., 2017; Wang et al., 2016; Liao et al., 2018). The maximum adsorption capacity calculated by the

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Langmuir model was 93.91 mg/g. Furthermore, the phosphate adsorption capacities of different

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La modified materials are displayed in Table 3. The comparison reveals that the adsorption

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capacity of the La-coated biochar in this study was relatively high. Further, dimensionless

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separation factor RL, a Langmuir parameter, could also be used to predict the affinity between

RL 

1 1  k LCe

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phosphate and La-600SS-OH according to Eq. (4).

(4)

RL1, RL1 and 0RL1 represent the unfavorable, linear and favorable phosphate sorption (Wang et al., 2016), respectively. In this study, the values of RL are all far lower than 1 when the initial concentration changed, indicating La-600SS-OH was the favorable sorption. As a result, phosphate adsorption capacity could be enhanced. That may be attributed to a vital driving force provided by the initial concentration to resist the mass transfer resistance between solutions and adsorbents (Wang et al., 2016). However, it is observed in Fig. 8 (b) that the removal efficiency decreased when the initial phosphate concentrations increased from 25 to 300 mg/L. The phosphate removal efficiency could reach 98.89% at the initial concentration of 25 17

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mg/L, indicating the efficient phosphate adsorption ability at a lower concentration. But the phosphate removal efficiency is only 40.08% when the initial phosphate concentration increased to 100 mg/L. Table 2 here Table 3 here

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Fig. 8 here

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3.8. Effect of competitive anions with different concentrations

Foreign anions in the solution usually affect the phosphate adsorption and should be

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considered in practical application. Therefore, the effects of different anions with varying

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concentrations were investigated, and the results are shown in Fig. 9. As can be seen, even at

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relatively high concentrations, Cl−, NO3−, SO42−, HCO3− and CH3COO−, had nearly no influence

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on phosphate adsorption. In the range of competitive anions concentration tested, the phosphate adsorption capacity has almost no change. That is to say, La-coated biochar is able to overcome

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the interference between phosphate and impure anions. This is consistent with other La-based materials, such as magnetic La(OH)3/Fe3O4 nanocomposites (Liao et al., 2018) and La hydroxides (Xie et al., 2014). Such advantage makes La-loaded biochar a suitable adsorbent comparing with Mg and Ca modified ones, which would be easily affected by HCO3− (Yao et al., 2013) and SO42− (Kong et al., 2018). It is widely accepted that the combination of phosphate and the adsorbent was from the formation of inner-sphere and outer-sphere complexes with La hydroxides (Fu et al., 2018). The outer-sphere complex is formed by electrostatic interaction and, therefore, is highly sensitive to electrolytes addition. Moreover, the inner-sphere complex, being formed by direct coordination with surface groups, is hardly interfered by coexisting anions (Xie et al., 2014). 18

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Therefore, La-600SS-OH has superior selective adsorption performance for phosphate, which is beneficial in practical applications. Fig. 9 here 3.9. Adsorption mechanism 3.9.1.

Effect of solution pH and zeta potential

of

The effect of solution pH on the adsorption capacity is shown in Fig. 10(a). When the pH was

ro

lower than 6.0, the phosphate adsorption capacity remains at a high level. Such influence might be due to the charge properties of the adsorbent surface and the presence of phosphate ions in the

-p

solution (Totlani et al., 2012). When the solution pH increased from 3.0 to 6.0, the dominant

re

species of phosphate in solution was H2PO4−, which was easier to combine with La (Fu et al.,

lP

2018). Meanwhile, the zeta potential tests (Fig. 10(b)), showed that the point of zero charge (pzc)

na

for the adsorbent was at 6.99. When the pH was lower than 6.99, the surface of La-600SS-OH exhibited positively charged via getting protonated to form La-OH2+, further bonding with

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phosphate ions through electrostatic attraction (He et al., 2017). Otherwise, the surface of the adsorbent showed negative chargeability, which was not conducive for phosphate adsorption. Also, phosphate in the solution might be captured by ligand exchange, and was inferred by the increase of equilibrium solution pH. The protonation process of the adsorbent, electrostatic attraction and ligand exchange could be described as follows (Fytili and Zabaniotou, 2008): 600SS-OH-La-OH + H+  600SS-OH-La-OH2+ 600SS-OH-La-OH2+ + H2PO4−  (600SS-OH-La-OH2)+( H2PO4)− 600SS-OH-La-OH + H2PO4−  600SS-OH-La-H2PO4 + OH− 2 600SS-OH-La-OH + HPO42−  (600SS-OH-La)2-HPO4 + 2OH− 19

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While with the increase of the solution pH, electrostatic repulsion between phosphate anion and the adsorbent was enhanced because of the deprotonation, causing the adsorption efficiency decrease (Wang et al., 2016). Besides, a large number of hydroxyl ions were released at higher solution pH, which would compete with phosphate for the active sites on the La-600SS-OH surface, hence, further decrease in the adsorption capacity (Wang et al., 2016, Liu et al., 2013).

FTIR analysis

ro

3.9.2.

of

Fig. 10 here

FTIR spectra of the adsorbent before and after phosphate adsorption are displayed in Fig. 11.

-p

Unlike other materials loaded with La, the peak at around 1068 cm−1 correspond to the P-O

re

asymmetric stretch vibration of PO43− group, and peaks at 616 cm−1 and 540 cm−1 correspond to

lP

the bend vibration of O-P-O, which are observed before and after phosphate adsorption (Fu et al.,

na

2018). These bonds are believed to be from the phosphate present in the sludge biochar, in the form of mineral variscite/berlinite and amorphous Ca3(PO4)2 (Li et al., 2018), but the intensity of

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peaks after adsorption increased, suggests that the extra phosphate has been successfully adsorbed onto La-60SS-OH. During the same period, it was documented that the characteristic bands for the vibration of La−OH at around 674 cm−1 disappeared after adsorption (Fu et al., 2018). The peak related to the vibration of −OH from La-OH at 1384 cm−1 was weakened, which probably resulted from the exchange of −OH from La−OH with phosphate to form inner-sphere complexation (Liao et al., 2018). Fig. 11 here 3.9.3.

XPS study

The XPS spectra of La-600SS-OH before and after phosphate adsorption were displayed in 20

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Fig. 12A. The sharp peak centered at 836.2 eV relatively corresponded to the La element, suggesting the La compound was loaded onto the sludge biochar surface. The appearance of P 2p with the binding energy at 133.9 eV after adsorption demonstrates the successful adsorption of phosphate by La-600SS-OH. The distinguished peaks of La 3d before and after phosphate adsorption are reported in Fig. 12C. The representative peaks of La 3d5/2 for La-600SS-OH are

of

located at 835.5 and 839.1 eV, and peaks of La 3d3/2 are centered at 852.3 eV and 856.0 eV. After

ro

adsorption, the double peaks of La 3d5/2 shift to 836.1 and 839.6 eV, and the peak positions of La 3d3/2 are at 853.0 and 856.5 eV. The binding energy of 3d5/2 and 3d3/2 shifting to higher values

-p

(0.50.7 eV) are observed clearly. This phenomenon indicates the possible electron transfer in the

re

valence band of La 3d and the formation of La-O-P inner-sphere complexation (Liao et al., 2018).

lP

Based on the above, it can be concluded that phosphate is bound to the La-coated biochar by

na

electrostatic attraction at a lower pH and in the form of monodentate and bidentate inner-sphere surface complex through ligand exchange with hydroxide. The possible phosphate adsorption

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mechanism of La-coated biochar is presented in Fig. 13.

4.

Fig. 12 here Fig. 13 here

Conclusion A highly efficient phosphate adsorbent, where La compounds were loaded onto the sludge

biochar, was prepared and tested. The results indicated that the La-600SS-OH had the highest phosphate adsorption capacity (93.91 mg/g). The adsorption process was well described by the pseudo-second-order and the Langmuir isothermal adsorption model. The adsorbent had better performance at a pH range of 3.0 to 6.0, while other common anions in wastewater had a 21

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negligible effect on the phosphate adsorption. Besides, heavy metals in sludge biochar are less likely to be leached out and therefore would not cause secondary pollution. All experiments indicated that the adsorption process could be ascribed to electrostatic attraction, and the inner-sphere complexation between La and phosphate through ligand exchange. However, the re-usability of the adsorbent in this study can’t be evaluated accurately because of the mass loss

of

during solid-liquid separation, which is supposed to be solved further.

ro

Acknowledgments

This work was financially supported by the Natural Science Foundation of Hebei Province

-p

(Grant nos. E2018203293) and Qinhuangdao Science and Technology Research and Development

re

Plan (Grant nos. 201801B031).

lP

Reference

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Chen, L., Li, Y.Z., Sun, Y.B., Chen, Y., Qian, J.S., 2019. La(OH)3 loaded magnetic mesoporous nanospheres with highly efficient phosphate removal properties and superior pH stability. Chem. Eng. J. 360, 342-348. Fu, H.Y., Yang, Y.X., Zhu, R.L., Liu, J., Usman, M., Chen, Q.Z., He, H.P., 2018. Superior adsorption of phosphate by ferrihydrite-coated and lanthanum-decorated magnetite. J. Colloid Interf. Sci. 530, 704-713. Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new methods—a review. Renewable Sustainabe Energy Rev. 12, 116-140. 22

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Hatami, H., Fotovat, A., Halajnia, A., 2018. Comparison of adsorption and desorption of phosphate on synthesized Zn-Al LDH by two methods in a simulated soil solution. Appl. Clay Sci. 152, 333-341. He, Y.H., Lin, H., Dong, Y.B., Wang, L., 2017. Preferable adsorption of phosphate using lanthanum-incorporated porous zeolite: Characteristics and mechanism. Appl. Surf. Sci. 426,

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Zelmanov, G., Semiat, R., 2015. The influence of competitive inorganic ions on phosphate

na

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agglomerates. J. Water Process. Eng. 5, 143-152.

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Table captions Table 1 Kinetic model parameter on phosphate adsorption by La-600SS-OH. Table 2 Isotherm model parameters on phosphate adsorption by La-600SS-OH. Table 3 Comparison of the phosphate adsorption capacities of different La-coated adsorbents.

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Figure captions

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Fig. 1. Effect of different metals and treatment methods on phosphate adsorption capacity. Fig. 2. Phosphate adsorption capacity of La-modified biochar at different pyrolysis temperatures.

-p

Fig. 3. The SEM morphologies of raw biochar pyrolyzed by different temperatures (a) 400C, (b)

re

600C, (c) 800C, and (d) BET adsorption-desorption curves of the biochars.

lP

Fig. 4. The leaching tests of phosphate and heavy metals in La-coated biochar: (a) phosphate, (b)

na

nickel, (c) cadmium, (d) zinc, (e) cuprum, (f) chromium.

methods.

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Fig. 5. Phosphate adsorption capacities of La-modified biochar treated by different chemical

Fig. 6. The SEM-EDS analysis of the La-coated biochar: (a) SEM, (b) EDS. Fig.

7.

Phosphate

kinetics

adsorbed

by

La-600SS-OH:

(a)

pseudo-first-order

and

pseudo-second-order models. (b) intra-particle diffusion model. Fig. 8. Phosphorus adsorption isotherm analysis of La-600SS-OH: (a) Langmuir and Freundlich models. (b) effect of initial phosphate concentrations. Fig. 9. Effect of competitive anions on the phosphate adsorption. Fig. 10. (a) Effect of solution pH on phosphate adsorption and equilibrium pH. (b) Zeta potential of La-600SS-OH at different pH. 29

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Fig. 11. FT-IR analysis before and after phosphate adsorption. Fig. 12. XPS spectra of (A) wide scan, (B) P 2p, (C) La 3d before and after phosphate adsorption by La-600SS-OH. Fig. 13. Schematic illustration of the possible phosphate adsorption mechanism on the La-coated

lP

re

-p

ro

of

biochar.

Table 1

na

Table 1 Kinetic model parameter on phosphate adsorption by La-600SS-OH. Parameter 1

Parameter 2

R2

k1 = 0.88

qe = 66.85

0.96

Error values

0.13

1.84

-

Pseudo-second-order

k2 = 0.02

qe = 71.81

0.99

Error values

0.001

0.97

-

Intra-particle diffusion

kid = 6.89

A = 38.72

0.91

Error values

1.19

2.96

-

Model

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Pseudo-first-order

30

Journal Pre-proof Table 2 Table 2 Isotherm model parameters on phosphate adsorption by La-600SS-OH. Parameter 1

Parameter 2

R2

Langmuir

kL = 0.18

Qmax = 93.91

0.98

Error values

0.01

0.65

-

Freundlich

kF = 55.97

n = 10.63

0.93

Error values

3.42

1.44

-

re

-p

ro

of

Model

lP

Table 3

Table 3 Comparison of the phosphate adsorption capacities of different La-coated adsorbents. Qmax (mg/g)

Reference

La(OH)3/Fe3O4 nanocomposites

83.5

Wu et al. (2017)

La(OH)3-modified exfoliated vermiculites

79.6

Huang et al. (2014)

La-loaded mesoporous silica spheres

44.8

Huang et al. (2015)

lanthanum-incorporated porous zeolite

14.8

He et al. (2017)

La-modified tourmaline

108.7

Li et al. (2015)

La(OH)3 loaded magnetic mesoporous nanospheres

54.2

Chen et al. (2019)

hydroxyl–iron–lanthanum doped activated carbon

29.44

Liu et al. (2013)

nano-porous palygorskite with lanthuman hydroxide

109.63

Kong et al. (2018)

lanthanum doped lignocellulosic wastes biochars

36.06

Xu et al. (2019)

hydrated lanthanum oxide-modified diatomite

58.7

Wu et al. (2019)

La-coated sludge-based-biochar

93.91

This study

Jo ur

na

Materials

fiber

31

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Figure 1

A

Soaked in metal salt solution after acid treatment of sludge biochar

ro

50 40

-p

30

60 50

Ca

C

Al

La

Soaked in metal salt solution after alkali treatment of sludge biochar

40

Jo ur

30

Mg

Ca

Al

Fe

La La

D

Hydroxide co-precipitation

Al

20

Mg

10

Fe

lP

0 70

Mg

re

20 10

P adsorbed (mg/g)

Soaked in metal salt solution directly

na

P adsorbed (mg/g)

60

B

of

70

Ca

Fe

La

Mg

Ca

Al

Fe

0

Fig. 1. Effect of different metals and treatment methods on phosphate adsorption capacity.

32

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Figure 2

80

of

60

ro

50 40

-p

30

re

20 10 0 400℃

lP

Adsorption capacity (mg/g)

70

500℃

600℃

700℃

800℃

na

Pyrolysis temperature

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Fig. 2. Phosphate adsorption capacity of La-modified biochar at different pyrolysis temperatures.

33

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Jo ur

na

lP

re

-p

ro

of

Figure 3

Fig. 3. The SEM morphologies of raw biochar pyrolyzed by different temperatures (a) 400C, (b) 600C, (c) 800C, and (d) BET adsorption-desorption curves of the biochars.

34

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Figure 4

(a) 0.30

(b)0.12

0.15 0.10 0.05 0.00

0.08

of

0.20

La-400SS La-600SS La-800SS

0.10

0.06 0.04

ro

La-400SS La-600SS La-800SS

Ni concentration (mg/L)

P concentration (mg/L)

0.25

0.02

4

6

pH

8

10

12

2

4

6

8

10

12

pH

lP

0.03

La-400SS La-600SS La-800SS

0.01 0.00

4

6

pH

8

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2

(e) 0.7 0.6 0.5 0.4

10

La-400SS La-600SS La-800SS

0.03 0.02 0.01 0.00

12

2

4

6

pH

8

10

12

(f) 0.5 La-400SS La-600SS La-800SS

Cr concentration (mg/L)

0.02

Zn concentration (mg/L)

re

(d) 0.04

na

Cd concentration (mg/L)

(c)0.04

Cu concentration (mg/L)

-p

0.00

2

0.3 0.2 0.1 0.0

0.4 0.3 0.2 La-400SS La-600SS La-800SS

0.1 0.0

2

4

6

pH

8

10

12

2

4

6

pH

8

10

12

Fig. 4. The leaching tests of phosphate and heavy metals in La-coated biochar: (a) phosphate, (b) nickel, (c) cadmium, (d) zinc, (e) cuprum, (f) chromium.

35

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Figure 5

80

of

60

ro

50 40

-p

30 20

re

P adsorbed (mg/g)

70

10

l H l -OH -ZnC 2 600SS 0SS-H S-OH S-ZnC 2 SSS S S S 0 S 6 La 600 600 a-600S La600 LaL

na

600

lP

0

methods.

Jo ur

Fig. 5. Phosphate adsorption capacities of La-modified biochar treated by different chemical

36

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Figure 6

Jo ur

na

(b)

lP

re

-p

ro

of

(a)

Fig. 6. The SEM-EDS analysis of the La-coated biochar: (a) SEM, (b) EDS.

37

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Figure 7

(a)80

of

Qt (mg/g)

70 60 50 40 30 20 10 0 -10 5

(b)75

15 20 Time (h)

lP

70 65

Qt (mg/g)

10

25

30

re

0

-p

ro

Experimental data Pseudo-first-order Pseudo-second-order

na

60

1 91

55

Jo ur

50

Y=

+ 7X

5 .71 38

45

6.8

45 40

1.0

Fig.

7.

Phosphate

kinetics

1.5

adsorbed

2.0

2.5 t^(0.5)

by

3.0

La-600SS-OH:

pseudo-second-order models. (b) intra-particle diffusion model.

38

3.5 (a)

4.0 pseudo-first-order

and

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Figure 8

95

of

90 85

ro

80

Experimental data Freundlich Langmuir

70 0

(b) Phosphate removal efficiency (%)

-p

75

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na

lP

100 90 80 70 60 50 40 30 20 10 0

50 100 150 200 250 300 Equilibrium concentrations (mg/L)

re

Adsorption capacity (mg/g)

(a)

0

50

100

150

200

250

300

Initial concentrations (mg/L)

Fig. 8. Phosphorus adsorption isotherm analysis of La-600SS-OH: (a) Langmuir and Freundlich models. (b) effect of initial phosphate concentrations.

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na

lP

re

-p

ro

of

Figure 9

Jo ur

Fig. 9. Effect of competitive anions on the phosphate adsorption

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Figure 10

12

70

of

60

10

ro

50 40

-p

30 20 10 2

3

4

5

6

7

8

9

9 8

Equilibrium pH

11

re

Adsorption capacity (mg/g)

(a)80

7

6 10 11 12 13

Solution pH

lP

(b) 40

na

20 10 0

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Zeta potential (mV)

30

pHpzc= 6.99

-10 -20 -30 -40

2

4

6

8

10

12

pH

Fig. 10. (a) Effect of solution pH on phosphate adsorption and equilibrium pH. (b) Zeta potential of La-600SS-OH at different pH.

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Figure 11

1068

1384 1553

3447

-p

ro

Transmittance

of

616 540

674

re

1000

1500

2000

lP

500

Before adsorption After adsorption

2500

3000

Wavenumber (cm-1)

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na

Fig. 11. FT-IR analysis before and after phosphate adsorption

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3500

4000

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Figure 12

La 3d

O 1s C 1s

200

400

ro

-p

0

La 3d

O 1s

600

re

P 2p C 1s

A

of

Intensity (CPS)

La-600SS-OH La-600SS-OH after P adsorption

800

1000

1200

lP

Binding Enegry (eV)

P 2p

Intensity (CPS)

na Jo ur

Intensity (CPS)

a

b

a: La-600SS-OH b: La-600SS-OH after adsorption 126

128

La 3d

B

130

132

134

136

138

C

La 3d3/2

La 3d5/2

a shifts b a: La-600SS-OH b: La-600SS-OH after adsorption

140

142

828

834

840

846

852

858

864

Binding Enegry (eV)

Binding Energy (eV)

Fig. 12. XPS spectra of (A) wide scan, (B) P 2p, (C) La 3d before and after phosphate adsorption by La-600SS-OH

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lP

re

-p

ro

of

Figure 13

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biochar.

na

Fig. 13. Schematic illustration of the possible phosphate adsorption mechanism on the La-coated

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Journal Pre-proof Declaration of interests ☒ 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.

Jo ur

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lP

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-p

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Jo ur

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-p

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Graphical abstract

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Highlights  The effect of La-modified sludge-based-biochar for phosphate adsorption was explored.  Sludge-based-biochar at pyrolysis temperature of 600C was more conducive.  The maximal phosphate adsorption capacity of La-coated was calculated to be 93.91 mg/g.  The La-coated biochar was insensitive to the coexisting anions.

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 The acidic condition was preferable for phosphate adsorption by La-coated biochar.

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