Chromium adsorption on surface activated biochar made from tannery liming sludge: A waste-to-wealth approach

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Chromium adsorption on surface activated biochar made from tannery liming sludge: A waste-to-wealth approach

Md. Abul Hashem

^a,

* , Sofia Payel

^a

, Sadia Mim

^a

, Md. Anik Hasan

^a

, Md. Shahruk Nur-A-Tomal

^a

, Md. Aminur Rahman

^b,c

, Majher I. Sarker

^d

aDepartment of Leather Engineering, Khulna University of Engineering&Technology (KUET), Khulna 9203, Bangladesh

bDepartment of Public Health Engineering (DPHE), Zonal Laboratory, Jashore 7400, Bangladesh

cGlobal Centre for Environmental Remediation (GCER), College of Engineering, Science and Environment, The University of Newcastle, Callaghan, NSW 2308, Australia

dSustainable Biofuels and Co-Products Research Unit, Eastern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, Wyndmoor, PA 19038, USA

Received 4 March 2022; accepted 25 July 2022 Available online 6 September 2022

Abstract

In a beamhouse, liming plays a key role in the removal of hair/wool and epidermis, but problems are created when waste liming sludge is discharged to the environment. The treatment of tannery wastewater is another major challenge to the industry. In this study, thermally-activated biochars derived from liming sludge were studied for their effective adsorption of chromium (Cr) from the tannery wastewater. The thermally activated biochars (B500, B550, B600, and B650) were prepared at different temperatures from the liming sludge. Their characteristics before and after the treatment were investigated using Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, Bru- nauereEmmetteTeller, and scanning electron microscopy analyses. The related functional groups (CeH, OeH, CeN, and ¼CeO) and chromium adsorption capacity were determined according to the surface morphology, element contents (C, O, Ca, Na, Al, Mg, and Si), surface area (5.8e9.2 m²/g), pore size (5.22e5.53 nm), and particle size (652e1 034 nm) of the experimental biochars. The biochar originated at 600C from the tannery liming sludge (B600) had a greater surface area with a chromium adsorption capacity of 99.8% in comparison to B500, B550, and B650 biochars. This study developed an innovative way of utilizing liming sludge waste to minimize the pollution load and wastewater treatment cost in the tannery industry.

creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Adsorption; Waste recycling; Biochar application; Thermal modification; Tannery; Solid waste

1. Introduction

The leather sector is one of the largest industries in several Asian countries, including China, India, Bangladesh, and Pakistan (Bharagava and Mishra, 2018). It is the second largest contributor to Bangladesh's economy, accounting for 1.0810⁹USD of export revenue (Hong, 2018). Given that

leather processing produces a massive amount of liquid and solid waste, it is also a major pollution-creating industry (Payel et al., 2021). Processing one ton of raw hide or skin generates 30e35 m³of wastewater and 850 kg of solid waste (Yoseph et al., 2020). This tannery waste contains both inorganic and organic pollutants, as well as toxic heavy metals (Chowdhary et al., 2017), creating serious environmental problems and eventually endangering public health. More than 250 chemicals have been used in leather processing, with chromium salt (basic chromium sulphate) being used most widely (Tadesse and Guya, 2017). As a result, the content of chromium (Cr) in tannery effluent is greater than that of any

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*Corresponding author.

E-mail addresses: [email protected], [email protected] (Md. Abul Hashem).

Peer review under responsibility of Hohai University.

https://doi.org/10.1016/j.wse.2022.09.001

creativecommons.org/licenses/by-nc-nd/4.0/).

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other chemicals (Kokkinos et al., 2019). During the chrome tanning process, only 60% of Cr salt is taken up by the pelt, while the rest of the salt is washed away in the effluent (Wang et al., 2016). During tanning, Cr remains in trivalent form, but it may exist in both oxidation states: trivalent Cr(III) and hexavalent Cr(VI) in the effluent (Pinakidou et al., 2021).

Compared to Cr(III), Cr(VI) is more mobile, toxic, and soluble (Ertani et al., 2017), as well as more dangerous to human health (Yahya et al., 2020a). Depending on various parameters such as organic matter, pH, temperature, concentration, and environmental conditions, Cr(VI) and Cr(III) can be interchangeable (Bashir et al., 2020). Due to its mobility, Cr(VI) may leach from topsoil to groundwater and be washed away by rainwater, thereby polluting the terrestrial and aquatic environment. Cr(VI) also has detrimental effects on seed germination and plant growth. The crops cultivated in the vi- cinity of Cr-affected areas should be tested before being consumed because Cr may be present in the edible parts of crops. Due to the bio-accumulation of heavy metals like Cr, they can enter the food chain, affecting people's health (Hashem et al., 2020). To mitigate these problems, the tannery effluent must be treated before being discharged to the environment.

Several physicochemical and biological techniques for Cr removal from tannery wastewater have been evaluated in the literature, including membrane filtration (Mohammed and Sahu, 2019), chemical precipitation (Zapana et al., 2020), electroplating (da Silva et al., 2020), and ion exchange (Bibi et al., 2018). The disadvantages of these methods are (1) in- efficiency, (2) generation of secondary pollutants, and (3) cost- ineffectiveness (Bibi et al., 2018). Recently, adsorption techniques have become more popular because they are efficient and cost-effective (Nasrollahzadeh et al., 2018). Non-toxic activated carbon material can be used as an adsorbent because it has a porous structure combined with a vast surface area (Hashem et al., 2020).Yahya et al. (2020b) used cobalt ferrite-activated carbon to eradicate chromium from tannery wastewater with an efficiency of 98.2%.Rafique et al. (2021) investigated the performance of wood waste-derived activated carbon in removing chromium from contaminated soil. Guo et al. (2021)reported that activated carbon derived from cassava sludge could remove 99.04% of Cr(VI) from an aqueous solution. However, after wastewater treatment, handlings such a large volume of tannery sludge also poses a serious challenge (Agustini et al., 2018). In leather processing, more than 70% of the sludge comes from the liming and unhairing processes (Kanagaraj et al., 2020). Several techniques, such as fertilizer production (Mirmohamadsadeghi et al., 2019), incineration (Yang et al., 2020), and landfilling (Srivastava and Chakma, 2022), have been used to manage the sludge. However, all these strategies either tend to create secondary pollutants and/or require high maintenance costs (Verma and Sharma, 2020).

In this study, thermally activated biochar derived from liming sludge was evaluated for effective adsorption of Cr from tannery wastewater. The physical properties of the biochars synthesized at different temperatures were characterized with different analytical tools before and after the treatment.

The adsorption behaviors were analyzed and compared using the linear and nonlinear equations of pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetics.

2. Materials and methods

2.1. Wastewater collection

After liming and chrome tanning operations, the samples of wastewater were collected in plastic containers from a local tannery situated in Khulna, Bangladesh.

2.2. Biochar from liming sludge

First, sludge was collected from the liming wastewater through the coagulation process. The coagulant, aluminum sulfate (Al₂(SO₄)₃), was gradually added to the liming wastewater through continuous stirring and maintenance of a pH value of 8.5e9.0. At this pH, the colloid and suspended solids of the wastewater tended to settle. After 24 h of settling, sludge was collected from the bottom of the drum, dried in the sun, and placed in an oven heated to 105C. The dried sludge was pulverized, homogenously mixed, and thermally activated in a muffle furnace (Thermo Scientific) at 500C, 550C, 600C, and 650C with a rate of 5C per minute, and the samples were designated as B500, B550, B600, and B650, respectively. This continued in limited oxygen conditions for 3 h. The prepared biochars were cooled to room temperature.

Afterwards, they were sieved with an 80-mesh sieve and stored for further experimental studies. Fig. 1 shows the thermally activated biochars derived from the liming sludge.

2.3. Characterization of biochars

The thermally activated biochars (B500, B550, B600, and B650) derived from the tannery liming sludge were investigated using a Fourier transform infrared (FT-IR) spectrometer (PerkinElmer, USA) before and after the treatment. The recorded FT-IR spectra were within a wavenumber of 400e4 000 cm¹ for the biochar placed on KBr discs to identify the functional groups presented in the molecule. The thermally activated pure and Cr-loaded biochars were analyzed for metal content with an energy dispersive X-ray (EDX) de- tector (Sigma HV, Carl Zeiss Microscopy Ltd.), with the samples being coated with gold. The surface morphologies of pure and Cr-loaded biochars were evaluated using a scanning electron microscope (SEM) (JEOL JSM-6490, USA) operating at 3 kV on the gold-coated samples. Images of the biochar surface were collected from SEM at an accelerating voltage of 15 kV with magnification at a level of 2 500 times. Surface area, average particle size, pore diameter, and pore volume of the thermally activated biochars (B500, B550, B600, and B650) were determined using BrunauereEmmetteTeller (BET) analysis. This was achieved using the Micromeritics Gemini 2375 and Gemini V Particle Analytical tools. Nitrogen (N₂) gas was passed through the samples for 3 h at 110C.

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2.4. Chromium adsorption experiments with thermally activated biochars

In an Erlenmeyer flask, 60 mL of chrome tannery wastewater was individually treated with 1.20 g of the thermally activated biochars (B500, B550, B600, and B650). At room temperature, the flasks were shaken in an orbital shaker (GFL, Model-3017, Germany) at 120 r/min for 60 min. Filtration was carried out after 15 h of settling. The chromium content and pH of the filtrate were measured using an atomic absorption spectroscope (Spectra240FS AA, Agilent, USA) and a pH meter (BT-675, BOECO, Germany), respectively. Each test was carried out in triplicate.

2.5. Biochar dose optimization

For dose optimization, 0.20 g, 0.45 g, 0.70 g, 0.95 g, 1.20 g, 1.45 g, and 1.70 g of B600 biochar were respectively added to seven separate Erlenmeyer flasks. Each flask contained 60 mL of chrome tannery wastewater. These flasks were shaken in an orbital shaker for 60 min at 120 r/min and then allowed to settle for 15 h. The chromium content and pH of the super- natant were determined. The test was conducted in triplicate.

2.6. Kinetic analysis

For kinetic analysis, 24 g of B600 biochar was added to 1 000 mL of chrome tannery wastewater and shaken at 120 r/min using an GFL orbital shaker. Samples of 50 mL of wastewater were collected at predetermined time intervals of 1.0 h, 2.0 h, 3.0 h, 4.0 h, 5.0 h, and 6.0 h. After immediate filtration, the chromium content of the filtrate was measured using an atomic absorption spectroscope.

The Lagergren equation (Lagergren, 1898) is one of the most popular kinetic models used to explain the liquidesolid biochar system according to biochar capacity. The pseudo- first-order (PFO) Lagergren's kinetic model (Lagergren, 1898) is expressed as

dq_t

dt ¼k1ðqeqtÞ ð1Þ

whereq_tis the adsorbate at timet;q_eis the adsorbate at the equilibrium condition; andk₁is the PFO model rate constant.

The pseudo-second-order (PSO) kinetic theory was pro- posed byBlanchard et al. (1984), developed byHo (1995), and finally refined by Azizian (2004). The PSO equation is expressed as

dq_t

dt ¼k₂ðq_eq_tÞ² ð2Þ

wherek2is the PSO model rate constant.Table 1lists different linear and nonlinear transitions of Eqs.(1) and (2).

3. Results and discussion

3.1. Impact of activation temperature of biochar on yield and chromium adsorption efficacy

The thermally-activated biochars produced at different temperatures were compared in terms of yield and capacity to adsorb chromium from the wastewater. The initial chromium concentration, pH, electrical conductivity, and total dissolved solids of the wastewater were found to be 3 109.27 mg/L, 3.8, 59.3 mS/cm, and 40.23 g/L, respectively. The capacities of chromium removal using biochars B500, B550, B600, and B650 with a dosage of 1.2 g were 99.84%, 99.89%, 99.89% and 99.89%, respectively. This indicated that the variation of activation temperature for biochar preparation did not affect chromium adsorption capacity. However, a high temperature led to a low yield. For the activated biochars B500, B550, B600, and B650, their yields were 51.07%, 49.56%, 47.28%, and 43.59%, respectively. Therefore, more impurities and organic matter in the liming sludge were incinerated at higher temperatures, resulting a lower yield.

3.2. Adsorbent characterization 3.2.1. FT-IR analysis

Fig. 2 shows the FT-IR spectra of pure and Cr-loaded activated biochars of B500, B550, B600, and B650.Tables 2 and 3show the peak wavenumbers and functional groups of pure and Cr-loaded thermally activated biochars at different temperatures. As shown in Fig. 2, at each temperature, the band shifted after the adsorption process with a change in peak value. However, maximum shifts were found inFigs. 2(b) and (d) with the bands shifting from 1 040 cm¹to 1 106 cm¹and Fig. 1. Thermally activated biochars of B500, B550, B600, and B650.

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from 1 113 cm¹ to 1 288 cm¹, respectively. The shifting occurred due to the adsorption in which the biochar acted as an anchoring or a binding site for chromium ions (Farghali et al., 2013). For all biochars, the peaks shifted to higher wavenumbers after adsorption, possibly because of the interaction

between biochars and chromium ions (Setshedi et al., 2014).

As shown inTable 2, different functional groups existed in the pure biochars including CeH bending and CeO, S¼O, and NeO stretching. The peaks detected at 673 cm¹, 852 cm¹, 877 cm¹, and 842 cm¹ indicated the stretching of¼CeH

Table 1

Nonlinear and linear equations of PFO and PSO models.

Kinetic model Linearity Equation Plot Equation number

PFO1 Linear

lgðqeqtÞ ¼lgqe k1

2:303t lgðqeqtÞversust (3)

PFO2 Linear lnðqeqtÞ ¼lnqek1t lnðqeqtÞversust (4)

PFO3 Nonlinear qt ¼qeð1e^k¹^tÞ qtversust (5)

PFO4 Nonlinear Ct

C0 ¼1msqeð1e^k¹^tÞ C0

Ct/C0versust (6)

PSO1 Linear 1

qeqt ¼ 1

qeþk2t (qeeqt)¹versust (7)

PSO2 Linear t

qt

¼ 1 k2q²_eþ1

qe

t t/qtversust (8)

PSO3 Linear 1

qt ¼ 1 qeþ 1

k2q²_et

1/qtversus 1/t (9)

PSO4 Linear qt ¼qe qt

k2qet

qtversusqt/t (10)

PSO5 Linear qt

t ¼k2q²_ek2qeqt qt/tversusqt (11)

PSO6 Nonlinear

qt ¼ k2q²_et 1þk2qet

qtversust (12)

PSO7 Nonlinear Ct

C0 ¼1 msq²_ek2t C0ð1þqek2tÞ

Ct/C0versust (13)

Note: Ctis the Cr concentration at timet(mg/L);C0is the initial Cr concentration (mg/L); andmsis the mass of the biochar in the solution (g/L).

Fig. 2. FT-IR spectra of thermally-activated pure and Cr-loaded biochars.

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bonds from the alkene group. CeO vibration was detected in all biochars. However, OeH stretching was found at B500, B600, and B650 biochars, and B600 and B650 exhibited significant bending (Teshale et al., 2020).

Table 3 summarizes the Cr-loaded peak positions and functional groups of the thermally activated biochars. A distinctive band over C¼C stretching was noticeable, with regard to the biochars (Zhang et al., 2019). The shifting at 1 106e1 297 cm¹indicated CeO stretching on the adsorbent (Teshale et al., 2020). The existence of CeH, OeH, and NeO indicated the possibility of physical adsorption, complex for- mation with the functional groups, and reaction with the

biochar surface. This result agreed with the adsorption studies byArim et al. (2018) andLi et al. (2017).

3.2.2. EDX analysis

Fig. A.1 shows the side-by-side energy dispersive X-ray (EDX) analyses of the pure and Cr-loaded biochar samples of B500, B550, B600, and B650. The chemical compositions of the biochars after adsorption specified the presence of Cr that was absent in pure biochars. This confirmed the chromium adsorption capacity of the thermally-activated biochars.Figs.

A.1(a), (c), (e), and (g) indicate the presence of C, O, Ca, Na, Pd, Mg, and other trace elements. The presence of C and O in ample amounts indicated the presence of cellulose or hemi- cellulosic compounds, which might come from the protein materials found in the liming sludge. Regarding the after- adsorption peaks,Figs. A.1(b), (d), (f), and (h) show the presence of Si, O, Na, Al, and P, proving that the chemical com- ponents were involved in adsorption (Rambabu et al., 2020).

Fig. A1 confirms that Cr was adsorbed on the thermally- activated liming sludge biochars.

3.2.3. SEM analysis

Fig. 3 shows the SEM images of the thermally-activated biochars derived from the liming sludge. The pure biochars (Figs. 3(a), (c), (e), and (g)) were more porous and rougher than the used biochars (Figs. 3(b), (d), (f), and (h)). There was a cloud-like material in the pure biochars, possibly originating from the oxidation of the biochar during thermal activation (Yahya et al., 2020a). However, there was a significant change in the surface morphology (Figs. 3(b), (d), (f), and (h)) after adsorption. The Cr-loaded biochars showed the accumulation of particles on their surface. The pore size and uniformity of the Cr-loaded biochars were also different from the surface texture of the pure biochars. This is a common phenomenon in metal- loaded biochars. Comparison between the pure and Cr-loaded biochars at different temperatures clearly demonstrated that all biochars prepared from the liming sludge became less porous after adsorption of Cr. This indicated that Cr was successfully adsorbed by the thermally-activated B500, B550, B600, and B650 biochars derived from the liming sludge.

3.2.4. Surface area analysis

Table 4illustrates the specific surface areas, pore volumes, pore sizes (diameters), and average particle sizes of the thermally-activated biochars derived from the liming sludge.

The specific surface areas of B500, B550, B600, and B650 were 5.80 m²/g, 9.12 m²/g, 9.20 m²/g, and 6.27 m²/g, respectively.

The specific surface area of B600 was the greatest at 9.20 m²/g.

The surface area of organically-modified montmorillonite clay was 9.794 m²/g (Setshedi et al., 2014). Higher surface areas were reported in several studies on chemically-modified adsorbents (Xu et al., 2021;Zeng et al., 2021;Kumar and Jena, 2017). However, in this study, the biochars were modified only by thermal activation without any chemical addition or treatment. The total pore volumes of the biochars ranged from 0.000 70 cm³/g to 0.000 94 cm³/g. The pore sizes of B500, B550, B600, and B650 were 5.53 nm, 5.22 nm, 5.30 nm, and

Table 3

Peak wavenumbers and functional groups of Cr-loaded biochars B500, B550, B600, and B650.

Biochar Wave number (cm¹) Vibration

B500 880 CeH bending

1 000 C¼C bending

1 297 CeO and CeN stretching

1 627 C¼C stretching

B550 857 C¼C bending

1 106 CeO stretching

B600 854 ¼CeH bending

1 627 C¼C stretching and NeH bending

854 ¼CeH bending

Table 2

Peak wavenumbers and functional groups of pure activated biochars B500, B550, B600, and B650.

Biochar Wave number (cm¹) Vibration

B500 673 C¼C and¼CeH bending

850 CeCl stretching

1 050 COeOeCO, S¼O, and CeO stretching

1 509 NeO stretching

3 458 OeH stretching

1 040 COeOeCO and S¼O stretching

1 506 CeN, NeO stretching

B600 877 CeH bending

1 069 CeO and S¼O stretching

1 505 NeO stretching

1 579 C¼C stretching

B650 662 CeCl stretching

842 ¼CeH bending

1 052 S¼O stretching

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5.27 nm, respectively. The maximum average particle size of the biochars was 1 034 nm for B500, and the minimum value was 652 nm for B600.Table 4clearly demonstrates that the average particle size varied at different temperatures. The BET results indicated that the pore volume, pore size, and surface area varied significantly with the thermal activation temperature, most likely because different organic and inorganic elements in the liming sludge were torrefied differently at various temperatures.

3.3. Selection of optimum dosage

Biochar dosage was optimized in terms of Cr adsorption from the wastewater with the highest capacity. The best chromium removal efficiency (99.69%) was obtained using 1.20 g of B600 in 60-mL Cr-containing wastewater.Fig. 4shows the Cr removal efficiencies with different adsorbent dosages. The Cr removal efficiency was improved when the biochar dosage was increased up to 1.2 g. After that, it reached equilibrium.Fig. 4 also shows the change in pH due to the addition of biochar doses in the suspensions. pH increased with the increase of the biochar dosage to 1.2 g. After that, it reached an equilibrium state. The increase in pH might have been caused by the hydroxyl groups in the adsorbents found in FT-IR analysis. Given that the removal efficiency and pH did not change substantially with biochar dosages above 1.2 g, the best possible biochar dosage was considered to be 1.2 g for 60 mL of wastewater.

3.4. Kinetics analysis

Kinetics analysis was conducted to explain the adsorption rate and adsorbateebiochar reaction mechanism (Guimar~aes et al., 2020). However, the transformation of nonlinear to linear equations might change the kinetic parameters, due to a shift in the error structure (Show et al., 2021).Fig. 5shows the linear (Eqs. (3) and (4) for PFO1 and PFO2, respectively) and nonlinear forms (Eqs. (5) and (6) for PFO3 and PFO4, respectively) of the PFO model. Table 5 shows the kinetic parameters of the PFO models. PFO1 and PFO2 obtained the same rate constant (k₁), and both models had the same determination coefficient value of 0.891 1.

Fig. 6 shows the linear and nonlinear forms of the PSO model (Eqs. (7) through (13)). As shown in Table 5, all PSO models obtained slightly different regression coefficient values. The PSO models can be grouped in the following descending order in terms of R²: PSO1 (0.990 5), PSO3 (0.984 0), PSO4 (0.982 9), PSO5 (0.982 9), and PSO 2 Fig. 3. SEM images of thermally-activated pure and Cr-loaded biochars.

Table 4

BET surface analysis of B500, B550, B600, and B650.

Biochar Specific surface area (m²/g)

Pore volume (cm³/g)

Pore size (nm)

Average particle size (nm)

B500 5.80 0.000 70 5.53 1 034

B550 9.12 0.000 94 5.22 658

B600 9.20 0.000 72 5.30 652

B650 6.27 0.000 73 5.27 956

Fig. 4. Changes in Cr removal efficiency and pH of wastewater with different biochar dosages in 60-mL wastewater.

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(0.971 3). The calculatedqevalues of PSO3, PSO4, and PSO5 were the closest to the experimental qe value (112.27 mg/g).

Overall, the PSO models outperformed the PFO models in simulating the adsorption kinetics. This indicated that the

removal of Cr from the chrome-containing wastewater was caused by the physicochemical interactions between adsorbate and biochar (Payel et al., 2021). The occurrence of the interactions might be caused by hydrogen bonding, electrostatic Fig. 5. PFO models in linear and nonlinear forms used to fit experimental data.

Table 5

Parameters of linear kinetic models.

Linear model Experimental equation R² qe(mg/g) k1(h¹) k2(g/(mg$h))

Experimental value Calculated value

PFO 1 y¼ 0.143 5xþ0.645 9 0.891 1 112.27 4.425 0.330 4

PFO 2 y¼ 0.330 5xþ1.556 2 0.891 1 112.27 4.741 0.330 5

PSO 1 y¼0.210 9xþ0.039 4 0.990 5 112.27 4.742 0.009

PSO 2 y¼0.008xþ0.002 3 0.971 3 112.27 434.780 1.130

PSO 3 y¼0.000 4xþ0.008 9 0.984 0 112.27 112.360 0.198

PSO 4 y¼ 0.044 2xþ112.34 0.982 9 112.27 112.340 0.201

PSO 5 y¼ 22.258xþ2 501.2 0.982 9 112.27 112.370 0.198

Fig. 6. PSO models in linear and nonlinear forms used to fit experimental data.

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interaction, or chelation formed by hydroxyl or carboxylic/

carboxylate groups. Similar findings were reported by Bai et al. (2020), Yahya et al. (2020a), and Nigam et al. (2019) based on their data analyses.

4. Conclusions

In this study, the biochars B500, B550, B600, and B650 were prepared from tannery liming sludge by the thermal activation process, and they were used to remove Cr from tannery wastewater. The thermally-activated B600 biochar exhibited the maximum Cr removal efficiency of 99.8%. The adsorption kinetics were explained by both PFO and PSO models. The determination coefficient showed that the PSO models outperformed the PFO models in fitting the experimental data. The BET analyses showed that the specific surface areas of the biochars changed with the activation temperature, and the maximum surface area was found to be 9.20 m²/g for B600. The FT-IR analyses revealed that the peaks of functional groups shifted due to the adsorption of chromium. The EDX analyses identified the presence of Cr after adsorption, while Cr was absent in pure biochars. The SEM analysis also indicated that the adsorption process led to a change in the surface morphology of the biochars. The pure biochars were more porous than the Cr-loaded biochars, indicating that some Cr content accumulated on the biochar surface. Overall, tannery liming sludge can potentially be utilized to reduce the pollution load originating from chrome containing wastewater in tan- neries. Utilization of liming sludge for wastewater treatment will be greatly beneficial to promote the waste management policy ofwaste recycling for waste treatmentin Bangladesh.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgements

The authors are grateful to the Committee for Advanced Studies and Research (CASR) at Khulna University of Engi- neering and Technology in Khulna, Bangladesh for the necessary support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.wse.2022.09.001.

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