Characteristics of 21 Types of Tea Waste for Adsorbance of Ionic Dyes from Aqueous Solutions

Abstract

In this study, 21 tea types (six black, four green, three Oolong, two herb, and five medicinal) were used to remove ionic dye from wastewater, as they could be potential adsorbents for several ionic dyes without purification or activated treatment. The majority of the 21 teas could adsorb cationic (MB) and anionic (ORII) dyes, with greater suitability for cationic dyes, as well as BB54 and BR46. Black dye (KBla), a mixture of several dyes, was adsorbed to a high degree. The tea waste treatment resulted in chromaticity reduction of the dye solution and turbidity changes. Dye adsorption was greater at higher temperatures than lower ones, although the effect of temperature was not strong. The adsorptions fit the pseudo-second-order-model; therefore, they involved chemical adsorption. The tea waste had great potential for the adsorption of several types of dyes without purification or activated treatment, although the mechanisms are yet to be determined. Therefore, the physical properties and structural components of each tea type should be analyzed by comparing common or different features of tea types. Taken together, many types of tea that are consumed worldwide can be used for efficient adsorption of ionic dyes by application of the tea waste.

Introduction

Dyes have been widely used for the coloration of industrial products such as leather, plastic, textile, printing, and food.^1–5) Over 6000 pigments or dyes are registered in the Colour Index International (C.I.) database that was established by The Society of Dyers and Colourists (SDC) and The American Association of Textile Chemists and Colorists (AATCC), which classifies these dyes by their application method or chemical structure. These colorants commonly consist of synthetic aromatic structures and various functional groups that expand the conjugated system, which results in compounds that are stable and not readily biodegradable.^6–9) Therefore, even small amounts of dye color in industrial wastewater can pollute the natural water environment for a prolonged duration.^1,6,7) It also has deleterious effects on the biological life of rivers and lakes^10,11) as well as plants¹²⁾ and the health of humans.^10,13) However, dye-containing effluents are considered general waste, and dye-containing water is only regulated by the chemical oxygen demand (COD) or biological oxygen demand (BOD) standard. Therefore, the entry of dye-containing water from industries is regarded as an environmental problem in various countries and regions.^14–16) Thus, the removal of dyes and the decolorization of dye-containing effluents are important for reducing environmental pollution and the harmful effects on affected lifeforms.

Various physical, chemical, and biological treatment methods have been used to remove dyes from wastewater, including filtration, biodegradation, photocatalytic degradation, oxidation, and adsorption.^17–21) Among these techniques, adsorption is a simple and effective method for water decontamination applications.^5,22) Adsorption can be performed safely at a low cost and with a flexible design; therefore, it is likely that commonly used materials can be utilized for adsorption in various places or situations to remove harmful substances.

A variety of teas are widely consumed worldwide, such as black, green, oolong, herb, and medicinal teas. The volume of production continuously increases, and it is estimated that approximately 6 million tons of tea leaves are produced, as reported by the International TEA Committee, corresponding to an abundance of spent tea leaves. Several utilization or recycling methods have been attempted to reduce industrial waste, but most were unsuccessful. The reuse of these wastes can help reduce their negative effects on the environment.

The scope of this study was to explore the feasibility of utilizing tea waste for the adsorption of several dyes, which is a simple and low-cost application. Moreover, it could be used to categorize the adaptation of each type of tea (black tea, green tea, etc.) or each tea variety (Assam, Hojicha, etc.) for dye adsorption by comparing the properties of a large number of teas that are processed in various locations globally.

In the present study, we selected 21 types of tea waste and evaluated their capability for adsorption of several dyes (cationic dyes: methylene blue, basic blue 54, basic red 46, anionic dye: acid orange 7, and black dye: kayacryl black). In addition, the chromaticity or turbidity reduction, adsorption isotherms, adsorption kinetics, and effect of pH were investigated. The differences in the characteristics of dye adsorption between the types of tea waste and the comparisons of dye adsorption capacity on other adsorbents were also analyzed. These results provide new insights into the applications of various tea wastes as adsorbents for the removal of dyes, and they have been shown to be highly efficient and tractable.

Experimental

Materials

Tea leaves for this study were purchased from a wholesaler (Chaoroshisouhonnpo, Japan, Shizuoka). The product state and production areas are as follows: Assam (India, GFBOP), Ceylon uva (Sri Lanka, BOP), Ceylon dimbra (Sri Lanka, BOP), Kenya (Africa, CTC), Darjeeling (India, BOP), Nilgiri (India, TFBOP), Hojicha (China, leaf), Brown rice (Japan, leaf), Green (Japan, leaf), Coarse (Japan, leaf), Pu-erh (China, leaf), Narcissus (China, leaf), Jasmine (China, leaf), White tip oolong (Taiwan, leaf), Rooibos (South Africa, superior), Rose hip (Chile, shell), Barley (Canada, roast-crush), Guava (China, roast-TB), Corn (America, roast-crush), Banaba (Philippines, roast-TB), and Mate (Brazil, cut). These teas and their serial numbers are shown in Table 1: six black, four green, three Oolong, two herb, and five medicinal teas. The pH of the solution was measured using a digital pH meter (F-73; HORIBA, Ltd., Japan).

Table 1. Types of Tea Leaves

Tea names and serial number
Black tea	Oolong tea
1. Assam	11. Pu-erh
2. Ceylon uva	12. Nacrissus
3. Ceylon dimbra	13. Jasmine
4. Kenya	14. Oriental Beauty (white tip oolong)
5. Darjeeling	Herb tea
6. Nilgiri	15. Rooibos
Green tea	16. Rose hip
7. Hojicha (roasted green tea)	Healthy tea
8. Brown rice	17. Barley
9. Sencha (green tea)	18. Guava
10. Bancha (coarse tea)	19. Corn
	20. Banaba
	21. Mate

Five types of dyes (C.I. Methylene Blue; 52015, C.I. Acid Orange 7; 15510, C.I. Basic Red 46; 110825, C.I. Basic Blue 54; 11052, C.I. Kayacryl Black; Not record, referred to as MB, OR, BR46, BB54, and KBla) were purchased from Shincoh Co., Ltd. These structure were showed in Fig. 1.

Fig. 1. Structure of Dyes Used in This Study

Preparation of Adsorbents (Tea Waste)

Tea waste was prepared from used tea leaves. The tea leaves (1.0 g) were added to 66 mL of boiling water and steeped for 3 min. The sample was filtered through a tea strainer, and the residue was cooled naturally to create the adsorbent used in this study. The entire quantity of wet and untreated adsorbent was used in a single experiment to resolve the difference in measuring weight with water adsorption and determine the value of the substance amount of the adsorbents.

Adsorption of Dyes

The adsorbent (tea waste) was added to a solution of MB, ORII, BB54, BR46, and KBla (100 mg/L, 50 mL). The suspension was shaken at 100 rpm for 24 h at 25 °C. The sample was then filtered through a 0.45 µm membrane filter, and the filtrate was analyzed with a spectrophotometer (UV-1200; Shimadzu Co., Ltd., Japan). The absorption wavelengths used for MB, ORII, BB54, BR46, and KBla were 665, 485, 603, 538, and 617 nm, respectively. The amount of dye adsorbed on the tea waste was calculated using Eq. (1), as follows:

  

(1)

where q is the amount of adsorbed adsorbate (mg/g), C₀ and C_e are the initial and equilibrium concentrations (mg/L), respectively, V is the solvent volume (L), and W is the weight of the adsorbent sample (g).

Effect of Chromaticity or Turbidity on the Adsorption of Methylene Blue by Tea Waste

The adsorbent (1.0 g) was added to a solution of MB (100 mg/L, 50 mL). The sample solution was shaken at 100 rpm at 25 °C for 24 h. The sample was then filtered through a 0.45 µm membrane filter, and the filtrate was analyzed with a chromaticity and turbidity meter (WA-PT-4DG; Kyoritsu Chemical-Check Lab. Corp., Japan). Chromaticity was indicated as the amount of decrease from the blank (chromaticity of MB). The turbidity was indicated as the value of turbidity after adsorption of MB because the value of the blank was 0.

Effect of Temperature and Initial Dye Concentration on the Adsorption of Methylene Blue, Acid Orange 7, or Kayacryl Black by Kenya Tea Waste

The adsorbent (1.0 g) was added to the solution of MB, ORII, and KBla (0.5, 1, 2, 5, 10, 20, 30, 50, 70, 100, 300, 500, 700, or 1000 mg/L, 50 mL). The sample solution was shaken at 100 rpm at 5, 25, and 45 °C for 24 h. The amount of dye adsorbed was calculated using Eq. (1).

Effect of Contact Time on the Adsorption of Methylene Blue, Acid Orange 7, or Kayacryl Black by Kenya Tea Waste

The adsorbent (1.0 g) was added to a solution of MB, ORII, and KBla (100 mg/L, 50 mL). The sample solution was shaken at 100 rpm at 25 °C for 10 min–48 h. The amount of dye adsorbed was calculated using Eq. (1).

Effect of pH on Adsorption of Methylene Blue, Acid Orange 7, or Kayacryl Black by Kenya Tea Waste

The adsorbent (1.0 g) was added to a solution of MB, ORII, and KBla (100 mg/L, 50 mL) at different pH values (pH 2–12). The pH of the solution was adjusted by adding 0.1 mol/L hydrochloric acid or sodium hydroxide. The sample solution was shaken at 100 rpm at 25 °C for 24 h. The amount of dye adsorbed was calculated using Eq. (1).

Results and Discussion

Properties of Dye Adsorption by 21 Types of Tea Waste

The amounts of cationic (MB) and anionic (ORII) dyes adsorbed on the 21 types of tea waste are shown in Fig. 2. The results indicate that many of the tea wastes had great capacity for MB adsorption, despite their poor ORII adsorption ability. The capacity of MB adsorption showed little difference among tea waste varieties (Assam, Ceylon, etc.) or types (black, green, Oolong, etc.). In addition, the amounts of several other cationic dyes adsorbed on these tea wastes are shown in Fig. 3. The difference of each tea waste variety was clear in terms of the capacity for adsorption of cationic dyes, and black tea waste had a greater potential for cation dye adsorption than the other classifications. Kenya tea waste had the greatest amount of cation dye adsorption of BB54, BR46, and KBla. Therefore, the mechanisms of dye adsorption on the Kenyan tea waste were investigated.

Fig. 2. Amount of Methylene Blue (MB) or Acid Orange 7 (ORII) Adsorbed on Tea Leaves

Fig. 3. Amount of Kayacryl Blue GGSL-ED (BB54), Kayacryl Red GRL-ED (BR46), or Kayacryl Black R-ED (KBla) Adsorbed on Tea Wastes

In this study, tea types with high consumption worldwide are included in 21 types of tea waste. Therefore, these can be utilized as adsorbents for dye decontamination in various areas and situations. Chromaticity and turbidity are recognized as important indicators of the practicality of dye contamination. Table 2 shows the amount of reduction in chromaticity or turbidity after adsorption of MB by 21 types of tea waste. These results demonstrate that the amount adsorbed and level of chromaticity reduction were not directly correlated, but the majority of tea waste types had the potential to reduce chromaticity after adsorption of MB. In addition, the turbidity changed little before and after the adsorption of MB.

Table 2. Amount of Reduction in the Chromaticity or Turbidity after Adsorption of Methylene Blue

Chromaticity
1. Assam	225	8. Brown rice	340	15. Rooibos	960
2. Ceylon uva	355	9. Sencha	510	16. Rose hip	335
3. Ceylon dimbra	345	10. Bancha	435	17. Barley	70
4. Kenya	130	11. Pu-erh	75	18. Guava	580
5. Darjeeling	165	12. Nacrissus	665	19. Corn	215
6. Nilgiri	205	13. Jasmine	370	20. Banaba	760
7. Hojicha	100	14. Oriental Beauty	565	21. Mate	435
Turbidity
1. Assam	0.2	8. Brown rice	0.9	15. Rooibos	0.2
2. Ceylon uva	0.4	9. Sencha	0.6	16. Rose hip	0.4
3. Ceylon dimbra	0.5	10. Bancha	0	17. Barley	6.1
4. Kenya	0.1	11. Pu-erh	1.2	18. Guava	0.6
5. Darjeeling	0.3	12. Nacrissus	0.3	19. Corn	0
6. Nilgiri	0.5	13. Jasmine	0.2	20. Banaba	0.5
7. Hojicha	0.2	14. Oriental Beauty	0.2	21. Mate	0.4

Adsorption Isotherms of Three Dyes on Kenya Tea Waste

To investigate the mechanisms of dye adsorption on tea waste, quantitative analysis and isotherms of MB, ORII, and KBla on tea waste are presented in Fig. 4. The amount of adsorbed MB decreased with low temperature, ORII did not change substantially with temperature, and KBla was higher at 25 °C than at other temperatures. To estimate the appropriate correlations for the equilibrium data in the design of adsorption systems, two common isotherm equations were tested in the present study: the Langmuir and Freundlich models.^23,24) The two models are presented below.

  

(2)

  

(3)

Fig. 4. Adsorption Isotherm of Methylene Blue, Acid Orange 7, or Kayacryl Black R-ED on Kenya Tea Leaves at Different Temperatures

where W_s is the monolayer adsorption capacity, a is the Langmuir constant, log k is the relative adsorption capacity of the adsorbent, and 1/n is a constant related to adsorption intensity. C_e and q_e are the equilibrium concentration of the adsorbate and the equilibrium adsorption capacity, respectively.

The calculated parameters are presented in Table 3. The adsorption isotherm data fit with the Langmuir isotherms (R²: 0.66–0.99), but did not completely fit the Freundlich isotherms (R²: 0.03–0.84). However, dye adsorption occurred readily when 1/n < 2 in several prior studies; cationic dyes on activated clay (0.37–0.96), anionic dye on bentonite (1.61), and black dye on zinc oxide (1.34), which was consistent in this study (0.32–1.66). These results indicate that dyes were easily adsorbed on tea waste in the range of 5–45 °C.

Table 3. Freundlich and Langmuir Constants for the Adsorption of Dyes on Kenya Tea Leaves

Dyes		Freundlich model			Langmuir model
		1/n	Log KF	R2	KL (L/mg)	Qm (mg/g)	R2
MB	5 °C	0.66	1.34	0.27	0.29	22.7	0.81
	25 °C	0.40	2.07	0.14	7.73	101	0.74
	45 °C	0.95	2.04	0.68	1.09	400	0.97
KBla	5 °C	1.11	0.45	0.60	0.08	6.33	0.80
	25 °C	1.18	0.40	0.29	0.01	75.8	0.92
	45 °C	2.50	1.45	0.84	0.03	25.2	0.84
ORII	5 °C	1.66	0.81	0.19	0.09	0.20	0.66
	25 °C	0.32	1.55	0.03	0.12	2.85	0.70
	45 °C	0.41	1.82	0.05	0.03	8.89	0.99

Adsorption Kinetics of Three Dyes on Kenya Tea Waste

Figure 5 shows the effect of contact time on the adsorption of MB, KBla, or ORII by Kenya tea waste. To reach equilibrium, these dyes required approximately 12 h. ORII was adsorbed most rapidly, followed in order by KBla and MB; in contrast, the maximum adsorption capacity was highest with MB, followed in order by KBla and ORII. The kinetics of these adsorptions was investigated using pseudo-first-order and pseudo-second-order models.²⁵⁾ The two models are presented below.

  

(4)

  

(5)

Fig. 5. Effect of Contact Time on the Adsorption of Methylene Blue, Acid Orange 7, or Kayacryl Black on Kenya Tea Leaves

●: MB, △: KBla, ■: ORII.

where q_t is the amount of solute adsorbed per unit weight of adsorbent (mg/g) at time t (min) of agitation, q_e is the amount of dye adsorbed (mg/g), and K₁ and K₂ are the rate constants of the pseudo-first-order and pseudo-second-order models (1/h and g/mg/h) for the adsorption process.

The kinetic parameters and correlation coefficients are presented in Table 4. The values of q_e, cal of the pseudo-second-order model were much closer to the values of q_e, exp for tea waste than those of the pseudo-first-order model. The correlation coefficients for the pseudo-second-order model (0.98–0.99) are higher than those of the pseudo-first-order model (0.89–0.98). These results suggest that the dyes are chemisorbed to the tea waste.

Table 4. Kinetic Parameters for the Pseudo-first-order and Pseudo-second-order Models

Sample	qe,exp (mg/g)	Pseudo-first-order model			Pseudo-second-order model
		qe,exp (mg/g)	k1 (1/h)	R2	qe,exp (mg/g)	k2 (g/mg/h)	R2
MB	99.7	47.1	0.46	0.98	100	2.80 × 10−7	0.99
KBla	58.3	55.1	0.23	0.92	60.6	1.00 × 10−5	0.99
ORII	24.9	17.5	0.09	0.89	26.4	1.24 × 10−4	0.98

Effect of pH of Three Dyes Adsorbed on Kenya Tea Waste

The pH of the solution affects the ionization of the dye and changes the surface properties of the tea waste during the chemisorption process. Therefore, the effect of pH on the adsorption capacity was investigated for the initial pH solution in the range of 3 to 12, and the results are shown in Fig. 6. The adsorption capacity of the dyes was sustained in the solution with an initial pH value in the range of 3 to 10, which indicated that tea waste was applicable over a wide pH range. The chemisorption failed at very high pH values, so the biological constituents of tea waste, which play important roles in the adsorption of dyes, were probably denatured by strong alkalinity. These adsorption trends were similar to those of cationic (MB), anionic (ORII), or other (Kbla) dyes; therefore, the dye adsorption on tea waste was impervious to the fluctuation of pH values that were not strongly alkaline.

Fig. 6. Effect of pH Solution on the Adsorption of Methylene Blue, Acid Orange 7, or Kayacryl Black on Kenya Tea Leaves

●: MB, △: KBla, ■: ORII.

Comparison of Adsorption Capacities for Various Dyes onto Different Adsorbents

To estimate the practicality of tea waste for dye adsorption, comparisons of the dye adsorption capacity and treatment features are summarized in Tables 5, 6.^26–43) The data show that tea waste has great potential for the adsorption of several types of dyes without purification or intervening treatment.

Table 5. Capacities of Different Adsorbents for Methylene Blue Adsorption

Dye	Type of adsorbents	Feature of treatment	Adsorption capacity (mg/g)	Reference
Methylene Blue	Mixed tea waste from tea factory (china)	Purification and dry	113	26)
Methylene Blue	Mixed tea waste from cafe (Ireland)	Purification and dry	111	27)
Methylene Blue	Spend tea leaves from tea shop (Muthanna)	Purification and dry	62.2	28)
Methylene Blue	Tea waste from household (Bangladesh)	Purification and dry	85.2	12)
Methylene Blue	Domestic tea waste (Iranian black tea)	Magnetic-np loaded	119.1	29)
Methylene Blue	Banana peel	—	20.8	30)
Methylene Blue	Wheat shells	—	16.6	31)
Methylene Blue	Coffee husk	—	90.1	32)
Methylene Blue	Almond shell-AC	—	1.33	33)
Methylene Blue	Hezelnut shell-AC	—	8.82	34)
Methylene Blue	Fly ash	—	75.5	35)
Methylene Blue	ZnO-NRs-AC	—	89.3	36)
Methylene Blue	AG-NPs-AC	—	71.4	37)
Methylene Blue	Kenya tea waste	Untreated and wet	101	This study

Table 6. Comparisons of Dye Adsorption Capacity on Various Adsorbents

Dye	Type of adsorbents	Feature of treatment	Adsorption capacity (mg/g)	Reference
Congo Red	Tea leaves from tea plant (Iran)	Purification and dry	106.4	14)
Basic Red 46	Actibated cray (AC)	Purification and dry	93.5	4)
Basic Red 46	Fe graphite	CVD process	46.73	38)
Basic Red 46	Kenya tea leaves	Untreated and wet	82.8	This study
Basic Blue 54	Extracellular polymeric substance (EPS)	Purification	2.0	1)
Basic Blue 54	Kenya tea leaves	Untreated and wet	63.55	This study
Acid Orange 7	Material of institute lavoisier (Fe-53)	Purification and dry	115.1	39)
Acid Orange 7	Spent brewery grains	Dry	28.54	40)
Acid Orange 7	Ant’s head fine-grained tea waste	Acid and calcined	41.66	41)
Acid Orange 7	Kenya tea waste	Untreated and wet	32.2	This study
Reactive Black 5	Fly ash from thermal power plant	—	7.93	42)
Reactive Black 5	Powdered activated carbon (PAC)	—	58.8	42)
Reactive Black 5	Cattle bone char	—	23.28	43)
Kayacryl Black R-ED	Kenya tea waste	Untreated and wet	75.8	This study

Conclusion

In this study, we have presented the potential of tea waste as an adsorbent for the removal of several ionic dyes without purification or activated treatment. The majority of the 21 types of tea waste had the capacity to adsorb cationic (MB) and anionic (ORII) dyes, and these adsorbates were particularly suitable for cationic dyes, as well as BB54 and BR46. Additionally, black dye (KBla), which is a mixture of several dyes, was adsorbed by these tea wastes to a high degree. The treatment of tea waste for dye adsorption resulted in chromaticity reduction of the dye solution and turbidity changes. The amount of dye adsorbed at room (25 °C) or high (45 °C) temperature was greater than that at low temperature (5 °C), although the effect of temperature on dye adsorption was not strong. The adsorption of dyes fit a pseudo-second-order model; therefore, these were chemical adsorptions. Furthermore, the tea waste used in this study has great potential for the adsorption of several types of dyes without purification or activated treatment. However, these adsorption mechanisms were not elucidated in this study. The adsorption mechanisms should be investigated by analyzing the physical properties and structural components of each tea type to compare common or different features of tea types. Taken together, many types of tea, which are consumed as black, green, oolong, herb, or medicinal tea globally, can be used for efficient adsorption of ionic dyes with simple application as tea waste.

Acknowledgments

This work was financially supported by JSPS KAKENHI Grant Number JP19K20485 (to T. N.).

Conflict of Interest

The authors declare no conflict of interest.

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