Adsorption of brilliant green dye by used-tea-powder: equilibrium, kinetics and thermodynamics studies

In recent years, the introduction of dyes, coloring agents and pigments into the water bodies from a wide range of toxic derivatives, mainly from the dyestuff manufacturing and textile industries and also from food coloring, cosmetics, paper, paints, pulp, carpet and printing industries has increased considerably (Berradi et al. 2019; Slama et al. 2021). Due to the interest of consumers in stability and fastness, dye producers are producing chemically designed dyestuffs that are more difficult to degrade by oxidizing agents, light and high temperatures after use. Unless properly treated, the dyes present in wastewater can reduce photosynthetic activity, which is due to interference of receiving streams with the transmission of sunlight and poses a serious hazard to most aquatic living organisms (Slama et al. 2021). Many dyes can cause skin diseases and have cancer-causing, mutagenetic impacts, etc. (Khan & Malik 2018). They are also responsible for esthetic pollution, eutrophication and distress in aquatic life (Khapre et al. 2021). The main reason for the release of hazardous substances in water is the oxidation and reduction of these dyes and hence demands the treatment of such coloring waste water before discharge into the water bodies (Mansoura et al. 2020).

On the basis of the structure of the dye molecules, dyes are classified as anionic (direct, acid and reactive dyes), nonionic (disperse and vat dyes) and cationic (base dyes) (Tan et al. 2015; Khapre and Jugade 2021). Among these, cationic dyes have the most dangerous and toxic effects as compared to anionic dyes in the atmosphere of receiving water and on the environment (Loqman et al. 2017). Brilliant green is an odorless cationic dye also known as diamond green G, solid green, ethyl green and basic green1 have various uses such as biological smudge, dermatological agent, veterinary medicine, an additive to poultry feed to restrain mold propagation, intestinal parasites, fungus, in textile dying, in manufacturing of inks for printing papers, etc.(Mansoura et al. 2020).

The methods used for the treatment of dye-contaminated effluents are chemical coagulation–flocculation, biological methods, oxidation processes, reverse osmosis, etc. (Qi et al. 2020). However, most of them involve a high operational cost when scaled-up to effluent treatment plant (ETP) scale. Adsorption processes using low-cost adsorbents are preferred over these methods (Pandey et al. 2022; Kaur et al. 2022; Saruchi Kumar et al. 2022). Depending upon the dye, the performance of low-cost adsorbents would vary. Biomasses and agricultural wastes such as rice husk, wheat bran, waste apricot, fly ash, peanut hull, coffee waste, peanut shell, etc. have been reported as potential adsorbents recently by various workers (Adegoke & Bello 2015).

Tea is the most common beverage in India, and so tea powder is used in every house. Used tea powder is another waste material that can be used as an adsorbent (Bansal et al. 2020). Recent literature shows the application of algal biomass for the removal of organic dyes from water bodies (Aragaw & Bogale .2021). Al-Maliki (2018) has shown that the chemical composition of algal biomass and tea waste is almost similar, including carbon, nitrogen, phosphorous, carbohydrates, as well as metal content. Considering these two aspects, it was hypothesized that the tea-waste produced in large amounts by Indian kitchens could be used for detoxification of dye-effluents.

Brilliant Green (BG) dye is a common dye used as a biological stain, dermatological agent, veterinary medicine, etc. It is known for its toxicity if it is exposed to skin or eyes, and also on ingestion. It might produce toxic gases like carbon dioxide, sulfur oxides and nitrogen oxides on decomposition that may be inhaled, causing lung toxicity (NO, NO₂) (Pandey et al. 2020; Nananwar et al. 2022). In the present study, the used-tea-powder (UTP) has been used as an excellent adsorbent for Brilliant Green (BG) dye in batch and column adsorption studies.

Tea dust was purchased from a local manufacturer (Brook Bond Red Label, product code, F12B, India). Analytical grade sodium hydroxide (NaOH) (98%), hydrochloric acid (HCl) (35% aqueous solution, B. Pt. 110°C) and Brilliant Green dye (dye content 95%) were procured from Loba Chemie, India. Double distilled water was used to prepare aqueous solutions. All of the reagents were analytical grade and did not need to be purified further.

The tea powder is generally boiled in water, the aqueous extract is used for drinking purposes, while the solid residue is discarded, producing a solid waste. Hence, it was boiled multiple times to remove all the extractable contents and colors, while the residue was dried and used as an adsorbent. Tea dust was extracted with hot water multiple times until it stopped producing color. The extract was discarded and the solid residue was dried in a hot air oven at 50 °C for 24 h. The dried powder was sieved through a normal 500-micron sieve and was labeled as UTP. For future usage, the sorbent was stored in an airtight container.

To determine the functional groups of UTP, a Bruker Alpha FT-IR spectrometer (USA), working between 500 and 4,000 cm⁻¹ and averaging 23 scans for each recording, was used. The XRD pattern was recorded by an X'PERT-3 Powder X-ray diffractometer (Malvern Panalytical Ltd, UK) using a 3kW X-ray source to investigate the crystalline structure of UTP. A scanning electron microscope (SEM) image was obtained by model Carl Zeiss EVO 18, Germany and the surface morphology of UTP was observed, with simultaneous evaluation of EDX using a hyphenated instrument. An EQ-824 spectrophotometer (Equiptronics, India, 350–900 nm) with matched quartz cuvettes (Shimadzu) was used for quantitation of BG in the solution. The Shimadzu DTG-60 working up to a temperature range of 1,000 °C under a nitrogen atmosphere at a heating rate of 20 °C min⁻¹ was used for thermal stability studies on the basis of thermogravimetry and differential thermal analysis.

The XRD pattern is predicted to exhibit crystallinity. Figure 1(b) shows the XRD pattern of UTP and BG-adsorbed-UTP. It is obvious that the two XRD suggest the presence of crystalline regions. No notable change was observed in the XRD pattern after adsorption of BG. However, after dye adsorption, the reduction in peak heights of most of the peaks is an indication of a reduction in crystallinity (Bajpai & Jain 2012).

In a nitrogen atmosphere, thermogravimetric analysis (TGA) was used to investigate the material's thermal degradation profile. Figure 1(c) shows the UTP's thermogram. The sample shows three stages of weight loss. In the first stage, a mass loss of around 10% was observed around 100 °C due to moisture and trapped water evaporation. This was accompanied by endotherm on the DTA curve. When the cellulose and hemicellulose components began to break down, a second weight loss was observed between 200 and 400 °C. It was immediately followed by a third weight loss, leading to complete decomposition of the sample up to 600 °C due to lignin break down. Both of these decompositions were exothermic in nature, as shown by the DTA curve (Hu et al. 2001).

The EDS curve of UTP (Figure 1(d)) showed a considerable concentration of carbon along with calcium similar to activated carbon. SEM micrographs of UTP samples before and after dye adsorption (Figure 1(e) and 1(f)) revealed that the UTP possesses a rough surface morphology of fibers with significant porous and uneven surface structure. BG-loaded adsorbent showed uniform coverage of UTP by BG, leading to a reduction in surface porosity. This surface characteristic will substantiate the higher adsorption capacity.

The influence of pH was examined using a dye concentration of 100 mg L⁻¹, UTP dose of 100 mg, a solution volume of 25 mL and a stirring time of 60 min. The initial pH values were adjusted from 4.0 to 9.0. The results reveal that over the whole pH range, there was no substantial change in the dye elimination. The percentage removal was between 94 and 96% over the entire pH range. This means that neither the H⁺ nor the OH⁻ ions may affect dye adsorption on the adsorbent. As a result, the original pH of the dye solution, which was around 6.0, was used for the whole adsorption study (Figure 2(b)).

Increasing the amount of adsorbent from 10 to 150 mg resulted in a consistent increase in percentage removal (Figure 2(c)) for two BG concentrations of 50 and 100 mg L⁻¹. The increase in accessible surface area and the increase in the number of adsorption sites are the key reasons behind this. More binding sites are accessible for the adsorption of BG onto the UTP surface at larger adsorbent doses; however, after the optimal dosage of 100 mg, no significant change in the percentage removal was seen. As a result, 100 mg of adsorbent was used for all studies.

The kinetics studies were carried out for BG concentrations of 100 and 200 mg L⁻¹. It was observed that whereas adsorption was initially rapid, after 60 minutes, the extent of adsorption remained nearly constant (Figure 2(d)). This is due to the fact that a significant number of unoccupied surface sites were initially available for adsorption, but they gradually saturated with time. As a result, 60 minutes was chosen as the optimal time for further work.

The dye concentration was increased from 25 to 400 mg L⁻¹. At first, a high percentage of dye solution was removed (up to 125 mg L⁻¹), which further led to saturation as BG concentration increased (Figure 2(e)). This is because sorbent aggregation at high concentrations lowers the specific surface area available for dye adsorption. The adsorption impact was greatest at a concentration of 100 mg L⁻¹; hence, it was chosen for all investigations.

Most of the dye-containing effluents are also contaminated with surfactants. In order to study the effect of surfactant concentration on BG adsorption efficiency, three surfactants were selected. Anionic surfactant sodium dodecyl sulfate (SDS), cationic surfactant tetrabutyl ammonium bromide (TBAB) and nonionic surfactant Triton X-100 were added one by one to a 100 mg L⁻¹ BG solution and adsorption efficiencies were recorded under optimized conditions. It can be observed from Figure 2(f) that TBAB has a negligible effect; Triton X-100 has a moderate effect, while SDS has a remarkable effect on the adsorption efficiency of UTP. This can be attributed to ionic interactions between anionic SDS and cationic dye molecules.

Adsorption isotherms describe the interactions between adsorbates and adsorbents. The extent of affinity between the sorbent surface and sorbate molecules, as well as the surface characteristics of the adsorbent, may be determined using an adsorption isotherm.

The Langmuir isotherm coefficients are q_m (mg g⁻¹) and b (L mg⁻¹).

Table 1 shows the adsorption constants and correlation coefficients (R²) for Langmuir and Freundlich isotherms. When the R² values were compared, the Langmuir model was the best-fit for the adsorption of BG dye solution on UTP surfaces, with a correlation coefficient near 1.0. This shows that a BG monolayer has formed on the UTP surface.

Table 1

Isotherm parameters

Langmuir .			Freundlich .
(mg g−1) .	b (L g−1) .	R2 .	KF (mg g−1) .	n (g L−1) .	R2 .
101.01	5.714	0.978	8.49	1.78	0.8362

Langmuir .			Freundlich .
(mg g−1) .	b (L g−1) .	R2 .	KF (mg g−1) .	n (g L−1) .	R2 .
101.01	5.714	0.978	8.49	1.78	0.8362

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The pseudo-second-order rate constant is k₂ (g mg⁻¹ min⁻¹). The regression coefficient for log t/qt versus t is 0.9997, indicating a linear fluctuation (Figure 3(d)). The fact that the regression coefficient is near 1.0 indicates that the pseudo-second-order kinetic model is the best match to describe the dye adsorption on the used-tea-powder. It is evident that the rate-determining phase in the adsorption phenomenon is chemisorption.

Intraparticle diffusion is not the only rate-limiting step, as the plot of q_t vs t^1/2 does not pass through the origin. The slope represents the intraparticle rate constant k_int (mg⁻¹ g⁻¹min^1/2) and the non-zero intercept indicates that diffusion is not the only rate-limiting phenomenon (Figure 3(e)).

T is the absolute temperature (K), while R is the gas constant (8.314 J mol⁻¹ K⁻¹). The slope and intercept of the plot of ln K versus 1/T were used to get the values of ΔH and ΔS. The thermodynamic parameters are shown in Table 3, and the Van't-Hoff plot is presented in Figure 3(f).

At all temperatures, Gibb's free energy change was negative, indicating that the adsorption process was spontaneous. The exothermic nature of the adsorption process is indicated by the negative enthalpy change, while the enhanced randomness at the solid-solution interface during the adsorption phase is indicated by the positive entropy change.

The current study shows that UTP can be employed as an efficient adsorbent for removing BG from aqueous solutions. pH 6.0, adsorbent dose 100 mg, contact period 60 min and dye concentration 100 mg L⁻¹ were shown to be the best working conditions for the various parameters investigated. The high regression coefficient indicates that the pseudo-second-order kinetic model was used, with the Langmuir adsorption isotherm being the best fit. The thermodynamic parameters revealed that the process is spontaneous and exothermic, while positive entropy change indicates the enhanced randomness at the solid-solution interface during the adsorption phase. As indicated in Table 5, the adsorption capacity of BG onto UTP was determined to be 101.01 mg g⁻¹, which is significantly greater than most previously reported materials.

Authors are thankful to DST, New Delhi for DST-FIST grant and UGC for UGC-SAP grant.

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

The authors declare there is no conflict.