Removal of Hexavalent Chromium by Using Sustainable Green Materials as Low-Cost Adsorbents

Abstract

The possibility of using three types of NaOH-treated tea residues (green tea, pu-erh, and tieguanyin) as low-cost adsorbents for Cr(VI) treatment was investigated. The surface charge, composition, morphology, structure, and functional groups in the obtained biosorbents were characterized by pH_zpc, cellulose content, SEM, BET, XRD, and FTIR spectroscopy. The nonlinear pseudo-first-order, pseudo-second-order, and Elovich models were used to investigate adsorption kinetics at various initial concentrations. The adsorption processes were more consistent with the pseudo-second-order kinetic model in the range of 5-50 mg L^-1. The adsorption isotherm at 298 K was described using the nonlinear Langmuir, Freundlich, Temkin-Pyzhev, and Dubinin-Radushkevich models, indicating that the process was favorable and complex with maximum adsorption amounts of 6.15, 19.50, and 12.31 mg g^-1 for green tea, pu-erh, and tieguanyin residues, respectively. Thermodynamic analysis revealed that the adsorption was a spontaneous, endothermic process. The results demonstrated that all materials had the potential to successfully remove Cr(VI) from the aqueous solution.

1. Introduction

Heavy metal effluents discharged directly or illegally from the metallurgy, dye, electroplating, and textile industries are regarded as severe problems as a result of increasing modernization and industrialization. Depending on the types, sources, and concentrations of heavy metals, they can pose either short-term or long-term risks. Chromium is a typical heavy metal pollutant that has a bad effect on water bodies and human society. Even at low levels, chromium can harm the ecosystem and cause carcinogenic effects [1]. The most common chromium oxidation forms are Cr(VI) and Cr(III), the latter of which is a trace element required for life [2]. However, hexavalent chromium is the most harmful due to its higher oxidation state and diffusibility. Inhaling and holding Cr(VI)-containing materials can result in nasal septum perforation, asthma, bronchitis, pneumonia, laryngitis, liver inflammation, and a higher risk of bronchogenic carcinoma. Contact with Cr(VI) compounds can also cause skin allergies, dermatitis, and ulceration [3]. Many countries have been concerned for a long time about the detection and treatment of Cr(VI)-containing wastewater.

Over the years, many researchers have worked tirelessly to promote effective heavy metal removal methods such as membrane filtration, chemical precipitation, ion exchange, adsorption, electrocoagulation, and electrodialysis [4–6]. Adsorption, as one of the most promising techniques, has the advantages of high selectivity, large adsorption capacity, and simple operation, which is favored by many scientists [7–9]. Recently, the use of diverse biomass materials as alternatives to high-cost adsorbents for the treatment of hazardous compounds has become a hot research issue [10–13].

Despite the benefits of adopting these locally accessible adsorbents, there are still difficulties in utilizing different treatment technologies. First, even though some materials are affordable, they may not be available in all nations with heavy metal contamination. Secondly, the normal adsorption capability of biosorbents is not sufficient due to the limited number of active sites. Some modifications are needed to increase the materials’ ability to adsorb. Therefore, specialized knowledge and capabilities are required, which might not be appropriate for developing countries. Thus, to operate these adsorption systems, it is crucial to look for materials that are widely distributed and easy to use [14, 15].

Tea is one of the most commonly consumed beverages worldwide. Several researchers have reported that tea contains a variety of active groups and that the tea residue, a byproduct of tea consumption, also consists of various active ingredients, such as tea saponin, caffeine, purine bases, and amino acids, which may contribute to adsorption or complexation. Therefore, the utilization of tea residues has become one of the main focuses of the development of wastewater treatment technology. Recently, much published research has been carried out on the utilization of tea residue for the adsorption of various pollutants from wastewater [16–21]. China is the world’s leading tea producer and consumer, with an annual production of 2.9 million tons in 2020 [22, 23]. Thus, there is a need to convert tea residues into useful adsorbents.

In this study, the three most common types of tea residues have been particularly chosen: green tea (nonfermented), pu-erh (post-fermented), and tieguanyin (semi-fermented), which were then simply treated with NaOH. This work is aimed at estimating the effect of the alkaline treatment on the properties of tea residues, comparing the adsorption capacity of different types of tea residues for the removal of Cr(VI) from wastewater samples, and determining the possible adsorption mechanism involved, therefore providing a possible sustainable technology to the problem of wastewater treatment and low-cost adsorbent development.

2. Methods

2.1. Preparation of Adsorbents

The three different types of tea residues (green tea, pu-erh, tieguanyin) used in the experiments were collected from a local market. To improve its adsorption capacity, the tea residues were mixed in a certain solid/liquid ratio (1 : 15, 1 : 20, 1 : 40, 1 : 50 $w / v$ ) with 1 mol L^-1 NaOH at room temperature for 24 hours to ensure a complete reaction. The resulting mixture was then filtered, washed, and dried at 363 K for 10 hours. Finally, the materials were grounded and sieved to 200-400 μm for further use.

2.2. Adsorption of Cr(VI) on Modified Tea Residues

Adsorption tests were performed in a 100 mL beaker containing 50 mL of Cr(VI) solution at various initial concentrations. 0.2 g of adsorbent was added to the solution. The samples were then agitated in a magnetic stirrer at room temperature for 2 hours. The solution of 1 M HCl was used for pH adjustment. After centrifugation, the concentration of Cr(VI) in the supernatant was determined using a UV-visible spectrophotometer at $λ = 540 nm$ . Finally, each adsorbent was rinsed with 50 mL of deionized water after adsorption and separated by centrifugation. The adsorption capacity $q_{t}$ (mg g^-1) is calculated as follows: $\begin{matrix} (1) & q_{t} = \frac{(c_{0} - c_{t})}{m} \times V, \end{matrix}$ where $c_{0}$ (mg L^-1) and $c_{t}$ (mg L^-1) are the Cr(VI) ion concentrations initially and at time $t$ (min), $m$ (g) is the mass of the tea residue, and V (L) is the volume of Cr(VI) solution.

2.3. Adsorption Kinetics

In kinetic experiments, a 250 mL Cr(VI) solution with initial concentrations ranging from 5 to 50 mg L^-1 was contacted with 1 g of tea residues at 298 K and a pH of 2. The supernatant was withdrawn at different time intervals and analyzed as before. Different kinetic models were applied to fit the kinetic data (Table 1).

Table 1

The mathematical equations of kinetic models.

Kinetic model	Nonlinear form
The pseudo-first-order kinetic model (PFO)	$q_{t} = q_{e} (1 - e^{- k_{1} t})$
The pseudo-second-order kinetic model (PSO)	$q_{t} = \frac{q_{e}^{2} k_{2} t}{1 + q_{e} k_{2} t}$
The Elovich kinetic model	$q_{t} = \frac{1}{β} \ln (1 + α β t)$

$q_{e}$ and $q_{t}$ are the amounts of Cr(VI) adsorbed (mg g^-1) at equilibrium and at time $t$ (min), respectively, $k_{1}$ (min^-1) and $k_{2}$ (g mg^-1 min^-1) are the pseudo-first-order and pseudo-second-order rate constants, respectively, $α$ is the initial adsorption rate (mg g^-1 min^-1), and $β$ is the desorption constant (g mg^-1).

2.4. Adsorption Isotherm

For the equilibrium studies, 1 g of tea residues was added to 250 mL of Cr(VI) solution at initial concentrations ranging from 5 to 50 mg L^-1. The solution was agitated at different temperatures (298, 308, 323, 338, and 353 K) and a pH of 2 until equilibrium was reached (40 minutes for pu-erh, 180 minutes for tieguanyin, and 360 minutes for green tea). Four isotherm models (Langmuir, Freundlich, Temkin, and Dubunin-Radushkevich) were applied to describe the process (Table 2).

Table 2

The mathematical equations of isotherm models.

Isotherm model	Nonlinear form
The Langmuir isotherm	$q_{e} = \frac{q_{m} K_{L} c_{e}}{1 + K_{L} c_{e}}, q_{e} = \frac{q_{m} c_{e}}{b + c_{e}}$ $R_{L} = \frac{1}{1 + c_{0} / K_{L}}, R_{L} = \frac{1}{1 + b c_{0}}$

The Freundlich isotherm	$q_{e} = {K_{F} c_{e}}^{1 / n}$

The Temkin-Pyzhev isotherm	$q_{e} = \frac{R T \ln A c_{e}}{b_{T}}$

The Dubinin-Radushkevich isotherm	$q_{e} = q_{m} e^{- β ε^{2}}$ $ε = R T \ln (1 + \frac{1}{c_{e}})$ $E = \frac{1}{{(2 β^{'})}^{1 / 2}}$

$c_{e}$ is the equilibrium concentration of Cr(VI) (mg L^-1), $q_{m}$ is the maximum adsorption capacity (mg g^-1), and $K_{L}$ (mg L^-1) and $b$ (L mg^-1) are the Langmuir constants. $n$ is related to the intensity of adsorption, $K_{F}$ (mg g^-1 (mg L^-1)^-1/ $^{n}$ ) is the Freundlich constant, $b_{T}$ (J mol^-1) is the Temkin-Pyzhev isotherm constant related to the heat of adsorption, $A$ (L g^-1) is Temkin-Pyzhev isotherm equilibrium binding constant, $R$ (8.314 J mol^-1 K^-1) is the universal gas constant, $ε$ (J mol^-1) is the potential of Polanyi, $β^{'}$ (mol² kJ^-2) is the Dubinin-Radushkevich isotherm constant, $T$ (K) is the absolute temperature, and $E$ (kJ mol^-1) is the mean adsorption energy.

2.5. Characterization

The point of zero charge (pH_zpc) was determined using the following procedure [24]. 0.15 g of adsorbent was added to 50 mL of a 0.01 M NaCl solution and agitated with a magnetic stirrer. The pH was then adjusted to successive initial values between 2 and 12 by adding either HCl or NaOH. After 24 hours, the final pH of the solution was measured and plotted against the initial pH. The pH_zpc was calculated as the value where $p H_{final} = p H_{initial}$ .

The amount of cellulose was measured using the method described by Liu et al. [25]. Briefly, 0.05-0.1 g of the desiccated sample was placed in a test tube. Then, 5 mL of the acetic and nitric acid mixture was added to the sample and heated in a boiling water bath for 25 minutes. The treated sample was filtered and washed several times with ionized water. After that, the sample was hydrolyzed with 10 mL of 72% H₂SO₄ and 10 mL of 0.01 mol L^-1 K₂CrO₄ in a boiling water bath for 10 minutes. Finally, 5 mL of 20% KI and 1 mL of 0.5% (C₆H₁₀O₅) $_{n}$ were added and titrated with 0.02 mol L^-1 Na₂S₂O₃. The amount of cellulose can be calculated as follows: $\begin{matrix} (2) & X = \frac{K (v_{0} - v_{1})}{(m \times 24)}, \end{matrix}$ where $X$ is the amount of cellulose, $K$ (mol L^-1) is the concentration of Na₂S₂O₃, $v_{0}$ (mL) is the amount of Na₂S₂O₃ consumed in blank samples, $v_{1}$ (mL) is the amount of Na₂S₂O₃ consumed in samples, and 24 is the equivalent number of 1 mol (C₆H₁₀O₅) $_{n}$ to Na₂S₂O₃.

The morphology was studied using scanning electron microscopy (SEM, FEI Quanta 200, USA). XRD measurements were carried out on the powder X-ray diffractometer (Bruker Phaser-D2, German). The surface functional groups of adsorbents were measured by FT-IR spectra (Nexus ThermoNicolet, USA) using KBr pellets. The surface area and pore volume were measured in a BET surface area analyzer (BSD-PM2-1203, China) at liquid nitrogen temperature (77 K).

2.6. Statistical Analysis

The Origin 7.5 software (Origin Lab, version 7.5, USA) was used to analyze and graph the experimental data.

3. Results and Discussion

3.1. Determination of pH_zpc

The pH_zpc is an important parameter in adsorption processes because it identifies the pH value at which the charges at the adsorbent’s surface shift from anionic (>pH_zpc) to cationic (<pH_zpc) [26]. Adsorption of cations is favored at pH values greater than pH_zpc, whereas that of anions is favored at pH values less than pH_zpc.

As shown in Figure 1, the values of pH_zpc obtained before and after NaOH treatment were 6.6 and 7.6 for green tea, 6.6 and 7.2 for pu-erh, and 6.3 and 7.1 for tieguanyin. This increase in pH_zpc values suggested the adsorbent surfaces were more positively charged at the experimental pH, which in turn increased the Cr(VI) anions’ adsorption attraction. These results were consistent with recent research by Anjum et al. [27], Azzaz et al. [28], Zhao and Dai [29], and Patel et al. [30].

Figure 1

The pH_zpc of green tea, pu-erh, and tieguanyin before and after NaOH treatment.

3.2. Content of Cellulose

Table 3 shows the content of cellulose in three types of tea residues. Before NaOH treatment, the cellulose content of the green tea, pu-erh, and tieguanyin residues was 8.60%, 12.27%, and 18.72%, respectively. Significantly higher cellulose content was obtained for the samples of NaOH-treated green tea (34.91%), pu-erh residues (27.34%), and tieguanyin (32.3%), indicating that alkaline treatment facilitated the removal of hemicelluloses, pectin, and lignin, which agreed with the previous results [31].

Table 3

The content of cellulose in three types of tea residues (in wt%).

	Green tea	Pu-erh	Tieguanyin
Raw tea residue	8.60	12.27	18.72
NaOH-treated tea residue	15.61	27.34	32.30
Charge	+7.01	+15.07	+13.58

3.3. SEM Analysis

Figure 2 represents the SEM images of three types of tea residues before and after NaOH treatment. In Figure 2(a), it was clearly shown that the surface of untreated green tea residue was irregular, rough, and covered with impurities like hemicellulose, lignin, and waxy elements. These impurities generally cover the cellulosic hydroxyl groups, preventing them from interacting with the adsorbate. After treatment with alkaline, the green tea residue changed significantly (Figure 2(d)), becoming cleaner and smoother as the noncellulosic components were removed from the surface. The untreated pu-erh residue flocculated into a group with many loops and humps (Figure 2(b)). The NaOH caused the fiber to begin to crack into small pieces during alkaline treatment, which resulted in an increase in surface area (Figure 2(e)). In the case of tieguanyin residue, it exhibited a compact structure that might result from hydrogen bonding, covalent O-H bonds, and Van der Waal interactions between cellulose, hemicellulose, and lignin (Figure 2(c)). The noncellulosic complexes were partially removed after alkaline treatment, exposing more porosity and surface area of the cellulose that had previously been hidden (Figure 2(f)). According to the findings, pretreatments of three types of tea residues with alkaline treatment using NaOH helped to open the cellulosic fiber and improve surface functionality, making them more accessible for interaction with Cr(VI) and enhancing adsorption capacity. The alkaline treatment promoted the dissolution of noncellulosic constituents and changed the structure and morphology of tea residues. Similar results have also been reported in previous studies [32–35].

Figure 2

SEM images of (a) green tea, (b) pu-erh, and (c) tieguanyin before NaOH treatment; (d) green tea, (e) pu-erh, and (f) tieguanyin after NaOH treatment.

(b)

(c)

(d)

(e)

(f)

3.4. BET Analysis

The impact of NaOH treatments on the surface characteristics of the tea residues was further confirmed by the nitrogen adsorption analysis. Figure 3 displays the N₂ adsorption and desorption isotherms of both original and modified tea residues. All three types of tea residues exhibited a type IV isotherm, suggesting the characteristics of nonporous and macroporous adsorbents. The desorption process showed a minor hysteresis, which could be attributed to the amorphous phase that generated macropores at the exterior surface [36].

Figure 3

Nitrogen adsorption/desorption isotherms at 77 K of the green tea, pu-erh, and tieguanyin.

Based on N₂ adsorption and desorption analysis, the specific surface areas and average pore diameters were calculated. Table 4 shows that after alkali treatment, pu-erh and tieguanyin tea residues greatly increased in specific surface area and average pore diameter. The specific surface of pu-erh and tieguanyin tea residues was 5.75 and 3 times that before modification, respectively. The average pore diameter decreased by 21.3%, 25.73%, and 8.67% for green tea, pu-erh, and tieguanyin tea residues, respectively. The cracks and holes in the treated tea residues’ outer and cross-sectional surfaces had an effect on surface areas. NaOH treatment resulted in the formation of mesopores. This outcome matched the findings of an earlier SEM analysis of the study’s sample materials. While NaOH treatment increased the surface areas of pu-erh and tieguanyin residues, it had the reverse effect on green tea residue, which might be a result of the partial framework collapsing. As a result, the surface area of treated green tea was not increased following NaOH treatment [37, 38].

Table 4

The BET results of three types of tea residues before and after NaOH treatment.

	Green tea	NaOH-treated green tea	Pu-erh	NaOH-treated pu-erh	Tieguanyin	NaOH-treated tieguanyin
BET surface area (m² g^-1)	0.56	0.41	0.28	1.61	0.36	1.08
The average pore diameter (nm)	14.65	11.53	23.90	17.75	15.45	14.11

3.5. XRD Analysis

Further studies were performed in order to better understand the impact of the alkaline treatment on the crystalline structure of the tea residues. The phase and crystallinity of the untreated and NaOH-treated samples were evaluated using X-ray diffraction (XRD). Typical diffraction patterns are shown in Figure 4.

Figure 4

XRD patterns of (a) green tea, (b) pu-erh, and (c) tieguanyin before and after NaOH treatment.

As seen in Figure 4, the XRD patterns of the raw and modified tea samples showed significant amorphous regions. Two broader peaks at 16.54° and 20.46° for green tea, 16.3° and 19.2° for pu-erh, and 15.98° and 19.7° for tieguanyin appeared in the untreated tea residues, respectively. These peaks correlated with the cellulose I structure and are consistent with the high levels of lignin and hemicellulose that contributed to the amorphous phase [35].

The XRD patterns after modification demonstrated that alkali treatment not only noticeably affected the position, shape, and intensity of the main peaks in treated samples but also caused some peaks to become slightly more defined. Some corresponding peaks shifted to the $2 θ = {21.28}^{\circ}, {21.52}^{\circ}, and {21.8}^{\circ}$ for green tea, pu-erh, and tieguanyin, respectively. These distinct peaks might indicate the transition of cellulose I into cellulose II [39].

Additionally, the characteristic diffraction peaks of green tea and pu-erh gradually weakened. The peaks of the alkali-treated tieguanyin residue, however, were sharper and larger than those of the untreated samples, indicating that the amorphous regions were largely destroyed following the alkaline treatment, which also resulted in an increase in crystallinity [40]. This conclusion was in line with the earlier cellulose examination of samples used in the study.

3.6. FTIR Analysis

FTIR analysis was performed to evaluate the characteristics of functional groups in the tea residues before and after alkali treatment. Figure 5 shows a comparison between FTIR spectra obtained for raw tea residues and NaOH-treated samples. Typically, the broad, strong peaks at 3440-3063 cm^-1 indicated the existence of surface hydroxyl groups (-OH). The peaks at 2922-2852 cm^-1 corresponded to the aliphatic C-H group [41]. The peaks at 1736-1653 and 1656-1653 cm^-1 were assigned to C=O stretching and N-H bending, which were the bonds of hemicellulose, lignin, and amino acids. The peaks located at 1537-1517 cm^-1 indicated the secondary amine group [42]. The peaks that appeared at 1456, 1400, and 1386-1367 cm^-1 were attributed to N-H bending, phenyl groups, and -CH₃ bending. Besides, the peaks at 1240-1235, 1160-1106, and 1028-1067 cm^-1 were related to -SO₃ stretching, C-O-C groups, and C=O groups, respectively. Overall, the tea residues, which consisted of cellulose, hemicellulose, and lignin components, were confirmed by the existence of alkanes, esters, aromatics, ketones, amines, and alcohols [43].

Figure 5

FTIR spectra of (a) green tea, (b) pu-erh, and (c) tieguanyin before and after NaOH treatment.

In contrast, it was clear that the NaOH-treated tea residues showed some changes compared to the spectra of the raw tea residues. As shown in Figure 5, the carbonyl groups (C=O) and hydroxyl groups (-OH) increased significantly in all NaOH-treated samples, indicating a high ionic exchange capacity to uptake Cr(VI) from the solution. In addition, the fact that treating tea residues with NaOH changed the surface functional group was also evident by the shift in peaks shown in Table 5. Furthermore, the absorption bands of the aliphatic C-H groups, C=O stretching, C-O-C groups, and the secondary amine group were not detected when compared to those cellulose samples due to the hydrolysis of carbonyl groups and the elimination of noncellulosic constituents in alkaline solutions [44].

Table 5

The surface functional group shift in peaks.

	Wave number range (cm^-1) before treatment	Wave number range (cm^-1) after treatment	Assignment
Green tea residue	3444	3416	-OH or N-H
	3130	3128	-OH or N-H
	2923	2923	Aliphatic C-H
	2852	—	Aliphatic C-H
	1736	—	C=O
	1653	1637	C=O
	1400	1401	Phenyl groups
	1386	—	Aliphatic C-H group
	1235	—	–SO₃ stretching/P=O, C-O
	1150	—	C-O-C
	1064	1071	C-O

Pu-erh residue	3441	3441	-OH or N-H
	3312	3328	-OH or N-H
	3063	—	-OH or N-H
	2923	2923	Aliphatic C-H
	2854	—	Aliphatic C-H
	1655	1678	C=O
	1517	—	Amide II of proteins, N-H, C-N
	1456	1456	N-H bending, C-H
	1367	—	Aliphatic C-H group
	1106	—	C-O-C
	1028	1025	C-O

Tieguanyin residue	3443	—	-OH or N-H
	3335	3330	-OH or N-H
	2922	2922	Aliphatic C-H
	2853	2853	Aliphatic C-H
	1736	—	C=O
	1656	1655	C=O
	1537	—	Amide II of proteins, N-H, C-N
	1456	1459	N-H bending或C-H
	1380	—	Aliphatic C-H group
	1240	—	–SO₃ stretching/P=O, C-O
	1160	—	C-O-C
	1067	1022	C-O

3.7. The Effect of Tea Residues: NaOH (S/L) Ratio on Cr(VI) Adsorption

The effect of NaOH modification on adsorbent adsorption performance was also investigated (Figure 6). In all three cases, the presence of NaOH initially increased the adsorption capacity and removal efficiency of Cr(VI). At a 1 : 40 ratio, green tea (87.76%, 2.15 mg g^-1) and pu-erh (90.67%, 2.36 mg g^-1) showed the highest removal efficiency and adsorption capacity. However, the highest removal efficiency (98.72%) and adsorption capacity (2.34 mg g^-1) of tieguanyin residue were observed at a ratio of 1 : 20.

Figure 6

The effect of tea residues: NaOH (S/L) ratio on Cr(VI) adsorption onto tea residues.

It is worth noting that the removal efficiencies of Cr(VI) for NaOH-treated tea residues were higher than for raw tea residues. This was explained by the fact that the alkali treatment altered the chemical composition of the tea by producing more O-Na sites that could bond with Cr(VI) molecules. Similar results have been observed in other research [28].

3.8. Adsorption Kinetics

Figure 7 depicts the change in the adsorption capacity versus the contact time for three different types of tea residues with an adsorbent dosage of 1 g and a 250 mL Cr(VI) solution of 10 mg L^-1 at 298 K. The adsorption capacity for each tea residues increased rapidly in the first 100 minutes and tended to level off afterward. It is worthy of note that both green tea and tieguanyin residues had comparable adsorption kinetics, whereas the adsorption of pu-erh was faster initially, and the contact time required for adsorption equilibrium was much shorter for pu-erh residue than that for green tea and tieguanyin residues. In addition, the $q_{t}$ values of pu-erh were consistently higher than those of green tea and tieguanyin at every time interval, demonstrating that it was the best adsorbent for Cr(VI).

Figure 7

Kinetic curves for Cr(VI) adsorption onto tea residues.

This can be explained by the fact that, initially, the active sites on the tea residues surface would be occupied due to the complexation of the -OH and C=O groups with the metal ions; the adsorption was predominated by chemisorption, whereas at later times, the adsorption developed slowly, possibly indicating adsorption by diffusion. However, these processes are also governed by particle size and the point of zero charge.

The nonlinear pseudo-first-order, pseudo-second-order, and Elovich models were used to fit the experimental data for three types of tea residues (Figure 8), and the results obtained are shown in Table 6. It can be inferred from Table 6 that the pseudo-second-order kinetic model fitted the kinetic data better than the other two models for three types of tea residues, with $R^{2}$ values greater than 0.95. Moreover, the calculated equilibrium adsorption amount $q_{e, cal, 2}$ was comparable to the experimental values $q_{e, \exp}$ . The kinetic rate constant $k_{2}$ for pu-erh ( $5.49 \times 10^{- 2} g m g^{- 1} \min^{- 1}$ ) was greater than for green tea ( $1.05 \times 10^{- 2} g m g^{- 1} \min^{- 1}$ ) and tieguanyin ( $1.52 \times 10^{- 2} g m g^{- 1} \min^{- 1}$ ), which agreed with the experimental results. In conclusion, the pseudo-second-order model was better suited for predicting Cr(VI) adsorption onto three different types of tea residues, indicating the role of chemical adsorption in this process [7, 45].

Figure 8

Adsorption kinetics for Cr(VI) adsorption onto tea residues: (a) pseudo-first-order and pseudo-second-order kinetic model and (b) Elovich kinetic model.

(b)

Table 6

Parameters of pseudo-first-order, pseudo-second-order, and Elovich models for Cr(VI) adsorption onto tea residues at different initial concentrations.

	Green tea residue	Pu-erh residue	Tieguanyin residue
$q_{e, \exp}$ (mg g^-1)	2.32	2.37	2.1
Pseudo-first-order
$q_{e, cal, 1}$ (mg g^-1)	2.08	2.32	1.99
$k_{1}$ (min^-1)	$2.16 \times 10^{- 2}$	$8.99 \times 10^{- 2}$	$2.72 \times 10^{- 2}$
$R^{2}$	0.8948	0.9787	0.9203
Pseudo-second-order
$q_{e, cal, 2}$ (mg g^-1)	2.42	2.53	2.26
$k_{2}$ (g mg^-1 min^-1)	1.05 × 10^-2	5.49 × 10^-2	1.52 × 10^-2
$R^{2}$	0.9511	0.9836	0.9727
Elovich
$α$ (mg g^-1 min^-1)	0.11	3.03	0.17
$β$ (g mg^-1)	1.90	2.89	2.22
$R^{2}$	0.9815	0.8271	0.9888

The Elovich model was also used to look into the chemisorption hypothesis for the heterogeneous system in more detail. It turned out that the Elovich model was discovered to be the most suitable for green tea and tieguanyin, indicating that these adsorption processes may occur on heterogeneous surfaces and that chemisorption may be involved in Cr(VI) adsorption onto these two tea residue surfaces [46]. Some carbonate groups that were present on the shell surface may have played a role in forming bonds with the Cr(VI) ion molecules. The outcome and the FTIR analysis results were in good agreement.

3.9. Adsorption Isotherms

To further analyze the interaction between Cr(VI) and tea residues, the equilibrium adsorption data for different initial concentrations at 298 K were analyzed using different isothermal adsorption models, and the fitting results are shown in Figure 9 and Table 7.

Figure 9

Adsorption isotherms for Cr(VI) adsorption onto tea residues at 298 K: (a) Langmuir adsorption isotherm, Freundlich adsorption isotherm, and Temkin-Pyzhev adsorption isotherm and (b) Dubinin-Radushkevich adsorption isotherm.

(b)

Table 7

Parameters of different isotherms at 298 K.

	Green tea residue	Pu-erh residue	Tieguanyin residue
Langmuir
$q_{m}$ (mg g^-1)	6.15	19.50	12.31
$b$ (L mg^-1)	0.29	0.18	0.14
$R_{L}$	0.07-0.41	0.10-0.53	0.13-0.6
$R^{2}$	0.8954	0.9949	0.9260
Freundlich
$K_{F}$ (mg g^-1 (mg L^-1)^-1/ $^{n}$ )	1.67	3.54	2.04
$n$	2.48	2.06	1.94
$R^{2}$	0.9903	0.9830	0.9872
Temkin-Pyzhev
$b_{T}$ (kJ mol^-1)	3.01	0.81	1.49
$A$ (L g^-1)	16.06	5.66	5.76
$R^{2}$	0.8976	0.9530	0.8797
Dubinin-Radushkevich
$E$ (kJ mol^-1)	1.36	1.13	1.54
$q_{m}$ (mg g^-1)	4.04	14.09	7.06
$R^{2}$	0.7271	0.9378	0.7246

It was observed that the Langmuir model provided a good description of the interaction between pu-erh residue and Cr(VI) in terms of the values of $R^{2} > 0.99$ . The maximum adsorption amounts calculated from this model were 6.15, 19.50, and 12.31 mg g^-1 for green tea, pu-erh, and tieguanyin, respectively, revealing that pu-erh had the highest adsorption capacity. And the values $R_{L}$ in all cases were between 0 and 1, indicating a favorable process and a strong relationship between the tea residues and Cr(VI) [47].

As compared to other models, the Freundlich model showed the best degree of fit, as indicated by values of $R^{2}$ of 0.9903, 0.9830, and 0.9872 for green tea, pu-erh, and tieguanyin, respectively. Similar conclusions have been recorded previously [48–51]. The distribution of pore sizes and the presence of functional groups on the surfaces of the tea residues contributed significantly to their high degree of heterogeneity. The heterogeneous nature is also consistent with the results of the Elovich kinetic analysis. Additionally, the heterogeneity parameter $n$ indicates the favorability of adsorption. If $n$ is between 1 and 10, the adsorption is favorable. Table 7 shows that the values of $n$ in all three cases were greater than one, indicating that the adsorption of Cr(VI) onto the three types of tea residues was favorable [10, 17].

The average adsorption energy $E$ calculated from the Dubinin-Radushkevich isotherm model was less than 8 kJ mol^-1, implying that physical adsorption took place at 298 K [28, 52, 53]. Thus, the adsorption of Cr(VI) onto pu-erh was a complex process that involved physical and chemical adsorption, as well as monolayer and multilayer adsorption. But owing to the low values of $R^{2}$ , this model might not adequately describe the adsorption processes of green tea and tieguanyin.

For the Temkin-Pyzhev isotherm model, the $R^{2}$ values were greater than 0.95 for the pu-erh residue, indicating that electrostatic interaction was an important mechanism during the adsorption process. The magnitude of $b_{t}$ (adsorption heat) indicated that the adsorption process was feasible with high Cr(VI) coverage on the surface of biosorbents, and this type of tea residue had strong chromium affinities. Similar results have been reported before [54, 55]. However, for the green tea and tieguanyin, the Temkin fittings appeared to deviate from the experimental data.

In general, among the four studied isotherms, the Freundlich isotherm provided the greatest match to the experimental results, as shown in Table 7. The findings suggested that the adsorption of Cr(VI) onto the three tea residues might occur on the heterogeneous active sites present on the adsorbent surfaces.

3.10. Adsorption Thermodynamics

The following equations can be used to calculate the thermodynamic parameters of chromium adsorption by tea residues: $\begin{matrix} (3) & {Δ G}^{⊖} = {Δ H}^{⊖} - T {Δ S}^{⊖}, \\ {Δ G}^{⊖} = - R T \ln K_{e} = - R T \ln \frac{q_{e}}{c_{e}}, \\ \ln \frac{q_{e}}{c_{e}} = - \frac{{Δ H}^{⊖}}{R T} + \frac{{Δ S}^{⊖}}{R}, \end{matrix}$ where $K_{e}$ is the distribution coefficient of the adsorbate.

Using the experimental data at 298-353 K, the ${Δ H}^{⊖}$ and ${Δ S}^{⊖}$ were calculated from the slope and intercept by plotting $\ln q_{e} / c_{e}$ against $1 / T$ , and the ${Δ G}^{⊖}$ was estimated at different temperatures. The results obtained are shown in Figure 10 and Table 8.

Figure 10

Thermodynamics for Cr(VI) adsorption onto tea residues.

Table 8

Thermodynamic parameters for Cr(VI) adsorption onto tea residues.

Adsorbents	${Δ H}^{⊖}$ (kJ mol^-1)	${Δ S}^{⊖}$ (J mol^-1 K^-1)	${Δ G}^{⊖}$ (kJ mol^-1)
Adsorbents	${Δ H}^{⊖}$ (kJ mol^-1)	${Δ S}^{⊖}$ (J mol^-1 K^-1)	298 K	308 K	323 K	338 K	353 K
Green tea residue	7.62	27.22	-0.49	-0.76	-1.17	-1.58	-1.99
Pu-erh residue	4.39	157.17	-2.97	-4.55	-6.9	-9.26	-11.62
Tieguanyin residue	36.3	128.19	-1.9	-3.18	-5.1	-7.03	-8.95

As can be seen from Table 8, the positive values of ${Δ H}^{⊖}$ for each tea residues suggested that the adsorption processes were endothermic. Moreover, when the ${Δ H}^{⊖}$ value is between -40 and -600 kJ mol^-1, the adsorption process is generally driven chemically. Accordingly, the experimental results revealed that in the case of green tea, in the present experimental range, chromium adsorption might involve physical adsorption due to electrostatic attraction, hydrogen bonding, and ligand group exchange. As for pu-erh and tieguanyin residues, the process might involve chemical adsorption, which confirmed the previous analysis.

At different temperatures, ${Δ G}^{⊖}$ values for the three types of adsorbents were negative, suggesting the spontaneity nature of the adsorption of Cr(VI) onto green tea, pu-erh, and tieguanyin residues. And they became more negative with increasing temperature, meaning the adsorption is more favorable at higher temperatures. This result was consistent with other reports obtained by Villabona Ortíz et al. (2020) [56]. Furthermore, the fact that the absolute values of ${Δ G}^{⊖}$ were less than 40 kJ mol^-1 showed that the adsorption in this study appeared to be a physicochemical sorption process instead of a simple physical or chemical adsorption process [57].

The increase in disorder at the solid-liquid interface during the adsorption process was reflected in the positive values of $Δ S^{⊖}$ , indicating a high affinity of Cr(VI) for the adsorbents [58, 59]. This endothermic and disordered nature of tea residue-chromium adsorption could be due to a possible hexavalent chromium reduction reaction as well as physical adsorption in this adsorption process, which was consistent with previous research [60, 61].

3.11. Possible Adsorption Mechanism

According to FTIR research, the three types of tea residues were found to contain C=O bonds, carboxylic acids, amines, phenolics, and amide groups, all of which have been shown to be highly effective at removing Cr(VI) ions from wastewater. The existence of C=O bonds can create hydrogen bonds, and amino and oxygen groups, for instance, can form complexes with Cr(VI) ions, increasing the electrostatic attraction with tea residues [57, 62, 63].

Additionally, pH will have a significant impact on adsorption behavior. The ion speciation of Cr(VI) and the surface charge of the adsorbents are both impacted by the pH of the solution. The experiment’s findings showed that at pH 2, when the majority of the adsorbent’s functional groups were protonated and became more positively charged, there was the greatest amount of adsorption. Furthermore, in the aqueous phase, Cr(VI) can take on a number of anionic forms, such as chromate (CrO₄ ^2-), dichromate (Cr₂O₇ ^2-), and hydrogen chromate (HCrO₄ ^-) (Figure 11) [64, 65]. Within the experimental pH range (below pH_zpc), the solution was dominated by negatively charged Cr₂O₇ ^2-, HCrO₄ ^- ions, which were more readily attached to the positively charged functional groups (-OH, -COOH) on the biosorbent due to electrostatic affinity [17, 19, 49].

Figure 11

Distribution of Cr(VI) species.

However, when the pH increased above the pH_zpc of the adsorbent, surface protonation declined, the competition between the chromate and OH species became stronger, and the electrostatic attraction between the Cr(VI) species and the surface of the adsorbent decreased, demonstrating a reduction in Cr(VI) removal efficiency and adsorption capability. Furthermore, a low pH promotes the reduction of Cr(VI) ions to Cr(III) ions via the acidic and phenolic groups in the tea residues [66, 67].

According to SEM and BET results, NaOH treatments increased specific surface area in comparison to untreated tea residues. Generally, the relationship between increased surface area and enhanced metal uptake was shown to be positive. Increased surface area improved the adsorptive nature of tea residues or increased the number of active binding sites.

Based on the kinetic and isotherm analysis, it was hypothesized that for green tea, adsorption was mostly the result of physical interactions, while for pu-erh and tieguanyin, adsorption was a combination of both physisorption and chemisorption. The main mechanisms controlling the adsorption of Cr(VI) by tea residues were complexation, hydrogen bonding, electrostatic attraction, and reduction reactions.

3.12. Comparisons with Other Different Tea Residue Adsorbents

A comparison of the adsorption characteristics of other tea residues for Cr(V) is presented in Table 9. As can be observed in Table 9, different varieties of tea residues have been found to be effective according to their remarkable adsorption capabilities. The adsorption mechanisms mainly included reduction reactions, complexation, coprecipitation, and ion exchange. It can also be noticed that the three types of tea residues have comparable adsorption capabilities to other adsorbents, implying that they have the potential to remove Cr(V) from aqueous solutions as low-cost adsorbents. Nevertheless, as a result of the differences in structure, composition, and modification methods, the adsorption capacity and mechanism might be different for the adsorption of chromium and other contaminants by different types of tea residues, which needs to be further studied in order to achieve the aim of residue control by residue.

Table 9

Comparison of adsorption capacities of tea residues with other adsorbents for Cr(VI) adsorption.

No.	Adsorbents	Optimized conditions	The maximum adsorption capacity (mg g^-1)	References
1	Green tea waste	pH of 2, contact time of 60 minutes, and initial Cr(VI) concentration of 100 mg L^-1	33.9	[17]
2	Tea waste	pH of 3.9, adsorbent dosage of 6 g L^-1, contact time of 240 minutes, and temperature of 303 K	23.3-73.3	[41]
3	Bleached-tea waste	pH of 2, adsorbent dosage of 2.5 g L^-1, contact time of 24 hours, and initial Cr(VI) concentration of 100 ppm	80	[42]
4	Black tea waste	pH of 10 and adsorbent dose of 14 g L^-1	10.64	[48]
5	Mixed tea waste	pH of 2, adsorbent dose of 2 g L^-1, contact time of 180 minutes, temperature of 303-323 K, initial Cr(VI) concentration of 10-30 mg L^-1, and agitation speed of 250 rpm	94.34	[49]
6	Spent tea leaves	pH of 10, adsorbent dose of 2 g L^-1, contact time of 60 minutes, and initial Cr(VI) concentration of 25.6 mg L^-1	3.168	[68]
7	Spent tea leaves	pH of 3, initial Cr(VI) concentration of 50 mg L^-1, adsorbent dose of 0.7625 g, and contact time of 100 min	47.98	[69]
8	Tea waste	Contact time of 60 minutes, room temperature, initial Cr(VI) concentration of 152 ppm, and agitation speed of 100 rpm	5.20	[70]
9	Nanoceria-loaded tea waste	pH of 3, adsorbent dose of 0.8 g, temperature of 333 K, and initial Cr(VI) concentration of 10 mg L^-1	32.15	[71]
10	Red and black tea waste	pH of 2, temperature of 298 K, contact time of 60 minutes	1.4-3.3	[72]
11	Green tea residue	pH of 2, adsorbent dose of 2 g L^-1, contact time of 40 minutes for pu-erh, 180 minutes for tieguanyin, and 360 minutes for green tea	6.15	Present study
	Pu-erh residue		19.50
	Tieguanyin residue		12.31

4. Conclusion

In this paper, the adsorption of Cr(VI) from the aqueous solution was completed by using three types of tea residues (green tea, pu-erh, and tieguanyin). Characterization analysis (pH_zpc, cellulose content, SEM, BET, XRD, and FTIR) showed that the alkaline treatment, which was inexpensive and environmentally friendly, was a very efficient step to remove hemicellulose, lignin, wax, and pectin from the tea residues. Kinetic studies revealed that all biosorbents followed the pseudo-second model for Cr(VI) adsorption, with pu-erh having the highest rate constant in all three cases. The equilibrium data fit well with the Freundlich isotherm model. The maximum Cr(VI) adsorption capacities of green tea, pu-erh, and tieguanyin residues were found to be 6.15, 19.50, and 12.31 mg g^-1, respectively. Thermodynamic parameters were predicted and indicated that the adsorption was spontaneous and endothermic. Based on this research, the three types of tea residues can serve as appealing adsorbents for the adsorption of wastewater containing Cr(V) ions.

Footnotes

Data Availability

Data are available on request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors would like to thank the Hubei Key Laboratory of Purification and Application of Plant Anti-Cancer Active Ingredients for the financial support. This study was supported by the Scientific Research Project of Hubei Provincial Department of Education (Grant No. D20223001).

References

Sharma

Singh

S. P.

Parakh

S. K.

Tong

Y. W.

Health hazards of hexavalent chromium (Cr (VI)) and its microbial reduction

Bioengineered 2022 13

6012467

3 4923 4938

10.1080/21655979.2022.2037273

35164635

Balali-Mood

Naseri

Tahergorabi

Khazdair

M. R.

Sadeghi

Toxic mechanisms of five heavy metals: mercury, lead, chromium, cadmium, and arsenic

Frontiers in Pharmacology 2021 12

6012467

643972

10.3389/fphar.2021.643972

33927623

Prasad

Yadav

K. K.

Kumar

Gupta

Cabral-Pinto

M. M.

Rezania

Radwan

Alam

Chromium contamination and effect on environmental health and its remediation: a sustainable approaches

Journal of Environmental Management 2021 285, article 112174

6012467

10.1016/j.jenvman.2021.112174

Chadha

Selvaraj

S. K.

Vishak Thanu

Cholapadath

Abraham

A. M.

Zaiyan

Manoharan

Paramsivam

A review of the function of using carbon nanomaterials in membrane filtration for contaminant removal from wastewater

Materials Research Express 2022 9, article 012003

6012467

10.1088/2053-1591/ac48b8

Nnam

Ahj

Zaa

Fly ash modified magnetic chitosan-polyvinyl alcohol blend for reactive orange 16 dye removal: adsorption parametric optimization

International Journal of Biological Macromolecules 2021 189

6012467

464 476

Nur-E-Alam

Mia

M. A.

Ahmad

Rahman

M. M.

An overview of chromium removal techniques from tannery effluent

Applied Water Science 2020 10

6012467

9 10

10.1007/s13201-020-01286-0

Shu

Cui

Shi

Wang

Luo

Guo

Almond shell-derived, biochar-supported, nano-zero-valent iron composite for aqueous hexavalent chromium removal: performance and mechanisms

Nanomaterials 2020 10

6012467

2 198

10.3390/nano10020198

31979270

Din

S. U.

Khan

M. S.

Hussain

Imran

Haq

Hafeez

Zain-ul-Abdin Rehman

Chen

Adsorptive mechanism of chromium adsorption on siltstone-nanomagnetite-biochar composite

Journal of Inorganic and Organometallic Polymers and Materials 2021 31

6012467

4 1608 1620

10.1007/s10904-020-01829-7

Dong

Liang

Yang

Zhao

Porous biochar derived from waste distiller’s grains for hexavalent chromium removal: adsorption performance and mechanism

Journal of Environmental Chemical Engineering 2023 11

6012467

3 110137

10.1016/j.jece.2023.110137

10.

Jamil

Zeeshan

Khan

S. R.

Saeed

Synthesis and two-step KOH based activation of porous biochar of wheat straw and waste tire for adsorptive exclusion of chromium (VI) from aqueous solution; thermodynamic and regeneration study

Journal of Water Process Engineering 2023 53

6012467

103892

10.1016/j.jwpe.2023.103892

11.

Hariharan

Harini

Sandhya

Rangabhashiyam

Waste Musa acuminata residue as a potential biosorbent for the removal of hexavalent chromium from synthetic wastewater

Biomass Conversion and Biorefinery 2023 13

6012467

1297 1310

12.

Hashem

M. A.

Momen

M. A.

Hasan

Nur-A-Tomal

M. S.

Sheikh

M. H.

Chromium removal from tannery wastewater using Syzygium cumini bark adsorbent

International journal of Environmental Science and Technology 2019 16

6012467

3 1395 1404

10.1007/s13762-018-1714-y

2-s2.0-85047218483

13.

Badessa

T. S.

Wakuma

Yimer

Bio-sorption for effective removal of chromium(VI) from wastewater using Moringa stenopetala seed powder (MSSP) and banana peel powder (BPP)

BMC Chemistry 2020 14

6012467

1 71

10.1186/s13065-020-00724-z

33303014

14.

Senanu

L. D.

Kranjac-Berisavljevic

Cobbina

S. J.

The use of local materials to remove heavy metals for household-scale drinking water treatment: a review

Environmental Technology and Innovation 2023 29

6012467

103005

10.1016/j.eti.2023.103005

15.

Huang

D. L.

J. G.

Xue

W. J.

Z. H.

Chen

Deng

Guo

X. Y.

Megamerger of biosorbents and catalytic technologies for the removal of heavy metals from wastewater: preparation, final disposal, mechanism and influencing factors

Journal of Environmental Management 2020 261

6012467

109879

10.1016/j.jenvman.2019.109879

32148248

16.

Inbaraj

B. S.

Sridhar

Chen

B. H.

Removal of polycyclic aromatic hydrocarbons from water by magnetic activated carbon nanocomposite from green tea waste

Journal of Hazardous Materials 2021 415

6012467

125701

10.1016/j.jhazmat.2021.125701

34088189

17.

Timbó

Kandawa-Schulz

Amuanyena

M. O.

Kwaambwa

H. M.

Adsorptive removal from aqueous solution of Cr(VI) by green moringa tea leaves biomass

Journal of Encapsulation and Adsorption Sciences 2017 7

6012467

2 108 119

10.4236/jeas.2017.72008

18.

V. T.

Dao

M. U.

H. S.

Tran

D. L.

Doan

V. D.

Nguyen

H. T.

Adsorption of Ni(II) ions by magnetic activated carbon/chitosan beads prepared from spent coffee grounds, shrimp shells and green tea extract

Environmental Technology 2020 41

6012467

21 2817 2832

10.1080/09593330.2019.1584250

2-s2.0-85062330789

30767655

19.

Katha

P. S.

Ahmed

Alam

Saha

Rahman

M. S.

Efficiency analysis of eggshell and tea waste as low cost adsorbents for Cr removal from wastewater sample

South African Journal of Chemical Engineering 2021 37

6012467

186 195

10.1016/j.sajce.2021.06.001

20.

Kadhum

S. T.

Alkindi

G. Y.

Albayati

T. M.

Eco friendly adsorbents for removal of phenol from aqueous solution employing nanoparticle zero-valent iron synthesized from modified green tea bio-waste and supported on silty clay

Chinese Journal of Chemical Engineering 2021 36

6012467

8 19 28

10.1016/j.cjche.2020.07.031

21.

Çelebi

Gök

Adsorption capability of brewed tea waste in waters containing toxic lead(II), cadmium (II), nickel (II), and zinc(II) heavy metal ions

Scientific Reports 2020 10

6012467

1 17570

10.1038/s41598-020-74553-4

33067532

22.

Cai

Zou

Liu

Xie

Kuo

Buyukada

Evrendilek

Thermal degradations and processes of waste tea and tea leaves via TG-FTIR: combustion performances, kinetics, thermodynamics, products and optimization

Bioresource Technology 2018 268

6012467

715 725

10.1016/j.biortech.2018.08.068

2-s2.0-85051927535

30145379

23.

Wang

Tang

Liu

Yao

Zhao

Chen

Adsorption of Cr (VI) from water by modified tea residue biochar

Shangdong Chemical Industry 2022 51

6012467

21 5 10

24.

Valerio Cárdenas

Martínez-Herrera

Velázquez-Vargas

D. L.

De la Cruz-Burelo

Determination of the point of zero charge of pineapple (Ananas comosus L.) peel and its application as copper adsorbent

Agro Productividad 2021 14

6012467

53 59

10.32854/agrop.v14i11.2009

25.

Liu

Wen

J. F.

Jiao

Y. R.

Wang

Y. L.

Determination of cellulose, hemicellulose and lignin content in abandoned fruit core shell

Journal of Yulin University 2022 32

6012467

2 6 9

26.

Carvalho

J. T.

Milani

P. A.

Consonni

J. L.

Labuto

Carrilho

E. N.

Nanomodified sugarcane bagasse biosorbent: synthesis, characterization, and application for Cu(II) removal from aqueous medium

Environmental Science and Pollution Research 2020 28

6012467

24744 24755

27.

Anjum

Mazari

S. A.

Hashmi

Jatoi

A. S.

Abro

Bhutto

A. W.

Mubarak

N. M.

Dehghani

M. H.

Karri

R. R.

Mahvi

A. H.

Nasseri

A review of novel green adsorbents as a sustainable alternative for the remediation of chromium (VI) from water environments

Heliyon 2023 9

6012467

5, article e15575

10.1016/j.heliyon.2023.e15575

37153391

28.

Azzaz

A. A.

Jellali

Akrout

Assadi

A. A.

Bousselmi

Optimization of a cationic dye removal by a chemically modified agriculture by-product using response surface methodology: biomasses characterization and adsorption properties

Environmental Science and Pollution Research 2017 24

6012467

11 9831 9846

10.1007/s11356-016-7698-6

2-s2.0-84990946172

27726078

29.

Zhao

Dai

Tetracycline adsorption mechanisms by NaOH-modified biochar derived from waste Auricularia auricula dregs

Environmental Science and Pollution Research 2022 29

6012467

6 9142 9152

10.1007/s11356-021-16329-5

34498178

30.

Patel

Gupta

Mondal

Modeling and optimization of process parameters of MB dye adsorption using waste-derived chemically activated biosorbents 2022

Biomass Conversion and Biorefinery

10.1007/s13399-022-02693-w

31.

Taşar

Ş.

Özer

A comparative study of hemicellulose isolation with hot water, alkaline, and delignification methods from tea leaf brewing waste

Biomass Conversion and Biorefinery 2022 12

6012467

7 2501 2514

10.1007/s13399-020-00978-6

32.

Indolean

Burcă

Măicăneanu

Adsorptive removal of malachite green from model aqueous solutions by chemically modified waste green tea biomass

Acta Chimica Slovenica 2017 64

6012467

2 513 521

10.17344/acsi.2017.3271

2-s2.0-85021172694

33.

Wen

Yin

Peng

Zhang

Using H₂O₂ to selectively oxidize recyclable cellulose yarn with high carboxyl content

Cellulose 2019 26

6012467

4 2699 2713

10.1007/s10570-018-2217-1

2-s2.0-85059532239

34.

Barman

D. N.

Haque

M. A.

Hossain

M. M.

Paul

S. K.

Yun

H. D.

Deconstruction of pine wood (Pinus sylvestris) recalcitrant structure using alkali treatment for enhancing enzymatic saccharification evaluated by Congo red

Waste and Biomass Valorization 2020 11

6012467

1 10

35.

Salahuddin

N. A.

Abdelwahab

M. A.

Akelah

A. M.

Elnagar

Adsorption of Congo red and crystal violet dyes onto cellulose extracted from Egyptian water hyacinth

Natural Hazards 2021 105

6012467

2 1375 1394

10.1007/s11069-020-04358-1

36.

Allanas

Rahman

A. M.

Arlin

E. M.

Prasetyanto

E. A.

Study surface area and pore size distribution on synthetic zeolite X using BET, BJH and DFT methods

Journal of Physics: Conference Series 2021 2019, article 012094

6012467

10.1088/1742-6596/2019/1/012094

37.

Boonsuk

Sukolrat

Bourkaew

Kaewtatip

Chantarak

Kelarakis

Chaibundit

Structure-properties relationships in alkaline treated rice husk reinforced thermoplastic cassava starch biocomposites

International Journal of Biological Macromolecules 2020 167

6012467

130 140

10.1016/j.ijbiomac.2020.11.157

38.

Ateş

Effect of alkali-treatment on the characteristics of natural zeolites with different compositions

Journal of Colloid and Interface Science 2018 523

6012467

266 281

10.1016/j.jcis.2018.03.115

2-s2.0-85056124335

29626764

39.

Njeh

Hamza

Bouaziz

Abdelhédi

Abdelmouleh

Isolation, characterization and methylene blue adsorption: application of cellulose from olive sawdust

Korean Journal of Chemical Engineering 2022 39

6012467

3 760 774

10.1007/s11814-021-0931-0

40.

Kassa

H. S.

Jabasingh

S. A.

Mohammed

S. A.

Baek

Park

Extraction and characterization of cellulose nanocrystals from anchote (Coccinia abyssinica) bagasse

Macromolecular Research 2022 30

6012467

11 776 782

10.1007/s13233-022-0088-7

41.

Nigam

Rajoriya

Singh

S. R.

Kumar

Adsorption of Cr (VI) ion from tannery wastewater on tea waste: kinetics, equilibrium and thermodynamics studies

Journal of Environmental Chemical Engineering 2019 7

6012467

3, article 103188

10.1016/j.jece.2019.103188

2-s2.0-85067179986

42.

Annath

Chen

Mangwandi

Upcycling tea waste particles into magnetic adsorbent materials for removal of Cr(VI) from aqueous solutions

Particuology 2023 80

6012467

115 126

10.1016/j.partic.2022.11.017

43.

Bondarev

Popovici

Călin

Mihai

Sȋrbu

Doukeh

Black tea waste as green adsorbent for nitrate removal from aqueous solutions

Materials 2023 16

6012467

12 4285

10.3390/ma16124285

37374469

44.

Karn

Thakur

S. J.

Shrestha

Extraction and characterization of cellulose from agricultural residues: wheat straw and sugarcane bagasse

Journal of Nepal Chemical Society 2022 43

6012467

1 93 98

10.3126/jncs.v43i1.47034

45.

Alvarez-Galvan

Minofar

Futera

Francoeur

Jean-Marius

Brehm

Yacou

Jáuregui-Haza

U. J.

Gaspard

Adsorption of hexavalent chromium using activated carbon produced from Sargassum ssp.: comparison between lab experiments and molecular dynamics simulations

Molecules 2022 27

6012467

18, article 6040

10.3390/molecules27186040

36144787

46.

Abdoul

H. J.

M. H.

Prieto

Yue

H. B.

Ellis

Clark

J. H.

Budarin

V. L.

Shuttleworth

P. S.

Efficient adsorption of bulky reactive dyes from water using sustainably-derived mesoporous carbons

Environmental Research 2023 221, article 115254

6012467

10.1016/j.envres.2023.115254

36634890

47.

Ajmani

Shahnaz

Narayanan

Narayanasamy

Equilibrium, kinetics and thermodynamics of hexavalent chromium biosorption on pristine and zinc chloride activated Senna siamea seed pods

Chemistry and Ecology 2019 35

6012467

4 379 396

10.1080/02757540.2019.1584614

2-s2.0-85062516008

48.

Nur-E-Alam

Abu Sayid Mia

Ahmad

Mafizur Rahman

Adsorption of chromium (Cr) from tannery wastewater using low-cost spent tea leaves adsorbent

Applied Water Science 2018 8

6012467

5 1 7

10.1007/s13201-018-0774-y

49.

Cherdchoo

Nithettham

Charoenpanich

Removal of Cr(VI) from synthetic wastewater by adsorption onto coffee ground and mixed waste tea

Chemosphere 2019 221

6012467

758 767

10.1016/j.chemosphere.2019.01.100

2-s2.0-85060913073

30684773

50.

Zhang

Liu

Wang

Zhang

Xiao

Removal of Cr(VI) from aqueous solution by rice-husk-based activated carbon prepared by dual-mode heating method

Carbon Resources Conversion 2023 6

6012467

2 76 84

10.1016/j.crcon.2023.01.003

51.

Yilmaz Camoglu

Ozdes

Duran

Adsorption behaviour of EDTA modified magnetic Fe₃O₄ coated brewed tea waste on Cr(VI) removal

Chemistry Africa 2023 6

6012467

2 921 931

10.1007/s42250-022-00545-8

52.

Abdel Salam

Abukhadra

M. R.

Adlii

Insight into the adsorption and photocatalytic behaviors of an organo-bentonite/Co₃O₄ green nanocomposite for malachite green synthetic dye and Cr(VI) metal ions: application and mechanisms

ACS Omega 2020 5

6012467

6 2766 2778

10.1021/acsomega.9b03411

32095700

53.

Tattibayeva

Tazhibayeva

Kujawski

Zayadan

Musabekov

Peculiarities of adsorption of Cr (VI) ions on the surface of Chlorella vulgaris ZBS1 algae cells

Heliyon 2022 8

6012467

9, article e10468

10.1016/j.heliyon.2022.e10468

36105478

54.

Jalees

M. I.

Farooq

M. U.

Basheer

Asghar

Removal of heavy metals from drinking water using Chikni Mitti (kaolinite): isotherm and kinetics

Arabian Journal for Science and Engineering 2019 44

6012467

7 6351 6359

10.1007/s13369-019-03722-z

2-s2.0-85068782050

55.

Khalil

Bilal Shakoor

Ali

Rizwan

Nasser Alyemeni

Wijaya

Adsorption-reduction performance of tea waste and rice husk biochars for Cr(VI) elimination from wastewater

Journal of Saudi Chemical Society 2020 24

6012467

11 799 810

10.1016/j.jscs.2020.07.001

56.

Villabona Ortíz

Tejada-Tovar

C. N.

Ortega Toro

Adsorption thermodynamics of Cr(VI) removal by using agro-industrial waste of oil palm bagasse and plantain peels

Revista Ingenieria E Investigacion 2020 40

6012467

3 22 28

10.15446/ing.investig.v40n3.83709

57.

Neolaka

Y. A.

Lawa

Naat

J. N.

Riwu

A. A.

Iqbal

Darmokoesoemo

Kusuma

H. S.

The adsorption of Cr(VI) from water samples using graphene oxide-magnetic (GO-Fe₃O₄) synthesized from natural cellulose-based graphite (kusambi wood or Schleichera oleosa): study of kinetics, isotherms and thermodynamics

Journal of Materials Research and Technology 2020 9

6012467

3 6544 6556

10.1016/j.jmrt.2020.04.040

58.

Saigl

Al-Dafeeri

Selective separation of chromium (VI) from aqueous solutions by adsorption onto truffle peels as novel biomass

International Journal of Environmental Analytical Chemistry 2021 103

6012467

928 946

59.

Abdel-Galil

E. A.

Hussin

L. M.

El-kenany

W. M.

Adsorption of Cr(VI) from aqueous solutions onto activated pomegranate peel waste

Desalination and Water Treatment 2021 211

6012467

250 266

10.5004/dwt.2021.26588

60.

Altintiga

Karakasb

Basokb

Altundagc

Adsorption behaviour of Cr(VI) on activated carbon: isotherm, kinetic, and thermodynamic studies

Desalination and Water Treatment 2021 217

6012467

221 231

10.5004/dwt.2021.26888

61.

Naseem

Bibi

Arif

Waseem

Haq

Azra

M. N.

Liblik

Zekker

Adsorption and kinetics studies of Cr (VI) by graphene oxide and reduced graphene oxide-zinc oxide nanocomposite

Molecules 2022 27

6012467

21 7152

10.3390/molecules27217152

36363976

62.

Campagnolo

M. A.

Celso Gonçalves

A. C.

Schwantes

Dragunski

Demetrio

Deminski

L. H.

Cr(total) removal using chicken feathers derived materials: a laboratory study with adsorption-precipitation in electroplating effluents

Separation Science and Technology 2022 57

6012467

12 1910 1925

10.1080/01496395.2021.2008439

63.

Yang

Cui

Yang

Ren

Yang

Wang

Chitosan-modified magnetic carbon nanomaterials with high efficiency, controlled motility, and reusability-for removal of chromium ions from real wastewater

Environmental Science and Pollution Research 2023 30

6012467

17 51271 51287

10.1007/s11356-023-25302-3

36809614

64.

Marinho

B. A.

Cristóvão

R. O.

Boaventura

R. A.

Vilar

V. J.

As(III) and Cr(VI) oxyanion removal from water by advanced oxidation/reduction processes-a review

Environmental Science and Pollution Research 2018 26

6012467

2203 2227

65.

Stern

C. M.

Hayes

D. W.

Kgoadi

L. O.

Elgrishi

Emerging investigator series: carbon electrodes are effective for the detection and reduction of hexavalent chromium in water

Environmental Science: Water Research & Technology 2020 6

6012467

1256 1261

66.

Yan

Chen

Influence of pH on Cr(VI) reduction by organic reducing substances from sugarcane molasses

Applied Water Science 2019 9

6012467

3 1 8

10.1007/s13201-019-0940-x

67.

Fei

Guan

Huang

Xiao

Zhang

The pH-sensitive sorption governed reduction of Cr(VI) by sludge derived biochar and the accelerating effect of organic acids

Journal of Hazardous Materials 2022 423, article 127205

6012467

10.1016/j.jhazmat.2021.127205

34844345

68.

Mia

A. S.

Ali

Nur-E-Alam

R. M.

Alam

Removal of heavy metals from tannery wastewater by using sawdust and spent tea leaves as an adsorbent

Physical Science & Biophysics Journal 2020 4

6012467

1, article 000139

10.23880/PSBJ-16000139

69.

Dakhil

I. H.

Adsorption of chromium (VI) from aqueous solutions using low cost adsorbent: equilibrium and regeneration studies

Journal of Engineering 2023 19

6012467

11 1406

10.31026/j.eng.2013.11.04

70.

Mariana

Mulana

Adsorption of heavy metal Cr (VI) using biosorbent of tea dregs: experimental and modeling

Proceedings of the 5th Annual International Conference Syiah Kuala University in Conjunction with the 8th International Conference of Chemical Engineering on Science and Applications 2015 93 99

71.

Sahu

U. K.

Zhang

Huang

Mandal

Sahu

S. K.

Sahu

M. K.

Patel

R. K.

Nanoceria-loaded tea waste as bio-sorbent for Cr(VI) removal

Materials Chemistry and Physics 2022 290

6012467

126563

10.1016/j.matchemphys.2022.126563

72.

Majeed

Z. N.

Farhan

S. A.

Dadoosh

R. M.

Mohammed

H. H.

Using spent tea residue to elimination of chromium in aqueous liquid

Journal of Physics: Conference Series 2020 1664, article 012082

6012467

127205

10.1088/1742-6596/1664/1/012082

Removal of Hexavalent Chromium by Using Sustainable Green Materials as Low-Cost Adsorbents

Abstract

1. Introduction

2. Methods

2.1. Preparation of Adsorbents

2.2. Adsorption of Cr(VI) on Modified Tea Residues

2.3. Adsorption Kinetics

2.4. Adsorption Isotherm

2.5. Characterization

2.6. Statistical Analysis

3. Results and Discussion

3.1. Determination of pHzpc

3.2. Content of Cellulose

3.3. SEM Analysis

3.4. BET Analysis

3.5. XRD Analysis

3.6. FTIR Analysis

3.7. The Effect of Tea Residues: NaOH (S/L) Ratio on Cr(VI) Adsorption

3.8. Adsorption Kinetics

3.9. Adsorption Isotherms

3.10. Adsorption Thermodynamics

3.11. Possible Adsorption Mechanism

3.12. Comparisons with Other Different Tea Residue Adsorbents

4. Conclusion

Footnotes

Data Availability

Conflicts of Interest

Acknowledgments

References

3.1. Determination of pH_zpc