Removal of acid yellow 17 dye from aqueous solutions using activated water hyacinth (Eichhornia crassipes)

In the world, numerous industries including textile, leather, plastics, paper, food, cosmetics, etc., use dyes and pigments to paint their products. The colored wastewaters of these industries are harmful to aquatic life in rivers and lakes, reducing light penetration and also enhancing extremely virulent metals in the environment (Ozacar 2002).

Dyes can be classified into natural and synthetic colorants. The synthetic dyes are characterized by their variety of colors and fastness, which makes them more widely used than natural dyes (Scholz 2019). Dyes can also be classified as cationic (basic) dyes, anionic (acid, direct, and reactive) dyes, and non-ionic (vat and disperse dyes) (Khan et al. 2018). Dye-containing wastewater is usually composed of dissolved solids, acids, bases, color, and toxic compounds. Color is noticeable at very low concentrations. Therefore, it needs to be treated before it is discharged to the environment.

The various methods for dye removal can be classified into three categories including biological, physical, and chemical methods such as chemical oxidation, electrochemical oxidation, photo degradation, biological treatment, membrane filtration, ion exchange, adsorption, precipitation, and coagulation (Ferrarini et al. 2016). Among these wastewater treatment techniques, adsorption is superior to other techniques for water reuse in terms of initial cost, flexibility, and simplicity of design, ease of operation, and insensitivity to toxic pollutants (Alemu et al. 2019). It also does not result in the formation of harmful substances (Mousa & Taha 2015). Adsorption is a natural process by which molecules of a dissolved compound collect on and adhere to the surface of an adsorbent solid. Commercially available activated carbon is effective in removing various types of contaminants (li et al. 2007; Owalude & Tella 2016), but it costs a lot and there is a continuous search for low-cost adsorbents prepared from agricultural or industrial wastes such as rice husk, sawdust, coir pith, wheat husk, ash, egg shell, orange fruit peels, peas waste, cassava peel, and eucalyptus bark (Ashraf et al. 2013; Ukanwa et al. 2019).

Acid yellow 17 (AY-17) dye is a reactive and anionic dye having a general structure shown in Figure 1. It is usually used in the textile, printing, soap, detergent, and cosmetic industries (Jedynak et al. 2019). It can damage the respiratory, nervous, and cardiovascular systems in humans and animals. Its thermal degradation can produce oxides of nitrogen and carbon that are toxic to living organisms. Moreover, it can cause mutagenic and tumorigenic effects in yeast, bacteria, and mammals (Choy et al. 2000).

Water hyacinth is a free-floating plant on aquatic bodies. It also grows in marshy areas in abundance. It is an exotic species with rapid and invasive proliferation resulting in choking of water bodies. This plant is very common in East African countries including Kenya, Uganda, and Ethiopia. It has been studied as an important biosorbent for the treatment of dyes, trace metals, and other physicochemical parameters (total suspended solids, total dissolved solids, biological oxygen demand, and chemical oxygen demand) (Nath et al. 2014; Priya & Selvan 2017; Yi et al. 2019).

The aim of this study was to evaluate the adsorption of AY-17 dye from aqueous solutions using activated water hyacinth (AWH) root under different operational conditions, such as adsorbent dose, pH, contact time, and initial dye concentrations. The equilibrium isotherm and adsorption kinetics models of AY-17 dye onto the AWH were also investigated.

Powder of AY-17 dye (dye content 60%, chemical formula: C₁₆H₁₀Cl₂N₄Na₂O₇S₂), H₃PO₄ (≥ 85%, Merk), HCl (37% concentration, Fisher Chemical), NaOH (98%, Fine Chemical Industries) used to adjust the pH of the solution during batch adsorption experiment), distilled water for the solution preparation and rinsing purposes. All the chemicals used were of an analytical grade. Water hyacinth root (collected from Lake Tana) was used for the preparation of activated carbon. KBr pellet was used for FT-IR spectra development of the adsorbent.

Water hyacinth was collected from Lake Tana, Ethiopia. The stem parts were cut off and the roots were collected and then washed thoroughly with distilled water to remove all the earthly impurities. Then it was dried in the open air for 6 days. The dried roots were crushed by using mortar and pestle. The crushed samples were chemically activated with mol L⁻¹ phosphoric acid (≥ 85%) for 24 hours and were dried by using an oven (PH 030A) to the next carbonization step. The activation/carbonization was performed at 600 °C for 2 hours using a furnace (Nabertherm B180, Germany). The activated samples were then washed with distilled water repeatedly to remove the acid from the surface of activated carbon, then the samples were oven-dried at 105 °C and grinded and sieved between 1 and 1.7 mm mesh and stored in clean containers.

A Fourier transform infrared spectrometer (Spectrum 65 FT-IR, PerkinElmer) was used to illustrate the functional group of activated carbon made from water hyacinth before and after adsorption. It was recorded in the mid infrared region (4,000–400 cm⁻¹) using a pressed KBr pellet technique. SEM (SEM – JEOL, JSM 6360 LV) was used to investigate the surface morphology of the AWH adsorbent using a voltage of 15 kv and 1–3 nA beam current.

One gram of the AY-17 dye powder was dissolved in distilled water to prepare the stock solution (1,000 mg/L), which was kept in dark-colored glass bottles. This dye stock solution was used to prepare intermediate and working solutions for batch adsorption experiments.

Batch experiments were conducted in a series of 250 mL jar containers. In all of the experiments 100 mL of AY-17 dye solution was added and agitated by using a rotary incubator shaker at 200 rpm (Excella E-24 Model). The pH of the solution was adjusted with 0.1 mol L⁻¹ HCl or 0.1 M NaOH before adding the adsorbent. It was measured using an electronic pH meter (Jenway 430 Model). The experiments were carried out at room temperature.

To evaluate dye removal efficiency and capacity of the adsorbent optimization experiments were done. The effects of adsorbent dose, contact time, pH, and dye initial concentrations were investigated by varying any one of the process parameters and keeping the other parameters constant. All the experiments were performed in duplicate and the mean values were reported to investigate the influence of contact time (10–150 min), pH (2–11), adsorbent dosages (1–6 g/L), and initial dye concentration (50–300 mg/l). In this study 100 mL of the AY-17 dye solution with different concentrations (50, 100, 150 200, 250, and 300 mg/L) and a known amount of adsorbent were added into each jar at different conditions. It was shaken continuously at a constant oscillation at room temperature. Then the solution was filtered by using Whatman No. 41 filter paper (0.45 μm pore diameter). The amount of dye that might be retained in the filter paper was not taken into consideration. The removal efficiency of the dye was analyzed using a UV-VIS spectrometer at an adjusted wavelength of 402 nm.

The FT-IR spectra of AWH before and after adsorption of AY-17 dye is shown in Figure 2. The FT-IR spectrum of AWH was recorded in the range of 400–4,000 cm⁻¹ helped to identify the functional groups on the material surface. These functional groups help to bind the dye molecules onto its surface (Figure 2(a)). The appearance of a peak around 3,418 cm⁻¹ represents OH stretching vibrates suitable for inter-and intra-molecular hydrogen bonding of polymeric compounds such as carboxylic acids, alcohols, or phenols as in hemicellulose, and cellulose (Samiey & Ashoori 2012). The peak observed at 2,371 cm⁻¹ owing to C = O stretching vibration of the carboxylic acid. A signal produced at 1,600 cm⁻¹ is attributed to the C-C stretch vibration of carboxylates (Liu et al. 2006; Sundari & Ramesh 2012). The bands observed at 1,094 and 1,160 cm⁻¹ indicates C-O stretch vibrations of an ester carboxyl acids, phenols, and alcohols (Segneanu et al. 2012). In this range the peaks appeared may be the presence of phosphate groups in the AWH sample, such as P = OOH (hydrogen bonded, P+ −O − (in acid phosphate esthers), P-H (in phosphine group) (Zhang et al. 2017). The FT-IR band at 500 and 766 cm⁻¹ indicates = C-H bending vibrations. After adsorption, a shift was observed on the characteristic peaks of FT-IR spectra, which indicates the biosorption of AY-17 dye onto the AWH surface (Figure 2(b)).

SEM is widely used to study the morphological feature and surface characteristics of the adsorbent material (Elkady et al. 2015). In this study, the surface structures of AWH root before and after activation were analyzed using SEM as shown in Figure 3(a) and 3(b), respectively. The scanning electron micrographs enable the direct observation of the surface microstructures of the adsorbent (Nwankwo et al. 2018). The micrographs of the AWH showed more ridged pores than the non-activated water hyacinth. This might be the micropore formation via the insertion of P-O-P from H₃PO₄ used for activation into the C lattice in water hyacinth root (Zhao et al. 2017). Thus, it indicates a better possibility of the acid yellow dye to be adsorbed onto the surface AWH (Figure 3(a) than the non-activated biomass of water hyacinth root (Figure 3(b)).

The pH of the system can affect the adsorptive uptake of adsorbate molecules most probably due to its influence on the surface properties of the adsorbent and ionization or dissociation of the adsorbate molecule (Santhi et al. 2010). The effect of pH on the adsorption of AY-17 dye on AWH is shown in Figure 4. It was observed that pH influences the AWH surface dye-binding sites and the dye chemistry in water. The highest dye removal efficiency was observed at pH 2 (92%). The figure shows the adsorption of AY-17 dye on AWH root was decreased to 91.2%, with increasing pH to 11. This value is in agreement with the effect of pH on the removal of Congo red dye on the surface of halloysite-magnetite-based composite (Ferrarini et al. 2016). This indicates the effect of pH on the change of surface property is low. The removal of AY-17 dye may be based on the reactions of different functional groups on the dye (-OH, -N = N-, two anionic parts) with the different functional groups (-C ≡ C-H, -C = O, -C-O, -OH) on the AWH. The presence of ligands on the dye helps to react with the surface of AWH both in acidic and basic conditions forming a complex that helps for the removal of the dye in aqueous solution.

The effect of adsorbent dose on adsorption was studied using different adsorbent doses in the range of 1–6 g. An increase of adsorbent dose from 1 to 4 g in 100 mL of dye solution, % of removal of AY-17 dye on AWH was increased from 75.9% to 90.37%. The reason behind increasing the adsorption with increasing adsorption dose from 10 to 60 g/L is the greater availability of adsorption sites for the AY-17 dye solution. Figure 5 shows that the dye removal percentage was very low at the beginning. But this increased with increasing adsorbent dose. The maximum removal efficiency of AY-17 dye onto the surface AWH was 94.2%. After this dose, the increment of dosage doesn't show any significant change in the percentage of removal of the dye.

The results from experimental data indicated that the percentage of removal of AY-17 dye increased with the contact time. The effect of contact time on the percentage of AY-17 dye removal from synthetic solution is shown in Figure 6. The observed contact time was in the range between 10 and 150 minutes. In the first 30 min of contact time the rate of reaction was fast or the percentage removal of the AY-17 dye was rapid and thereafter it proceeds slowly and finally attains saturation at 120 min.

The effect of initial AY-17 dye concentration in the range 50–300 mg/L onto the AWH adsorbent was studied. As it can be seen from Figure 7, the percentage of dye removal decreased with an increase in an initial dye concentration. This might be due to the saturation of the available active sites on the AWH adsorbents. At a pH of 2, the percentage of AY-17 dye removal decreased from 92.26 to 75.34% as the initial dye concentration increased from 50 to 300 mg/L.

The relationship between the amount of a substance adsorbed at constant temperature and its concentration in the equilibrium solution is termed as the adsorption isotherm. To optimize the design of dye adsorption system, it is important to establish the most appropriate correlations of the equilibrium data of each system. There are two most commonly used isotherm models to analyze equilibrium data of solute between adsorbent and solution. These are the Langmuir and the Freundlich isotherm models. The parameters obtained from models provide important information on the adsorption mechanisms and the surface properties and affinities of the adsorbent.

The correlation coefficient (R²) from the linear Langmuir isotherm model is 0.953 (Figure 8(a)). This shows that the Langmuir isotherm fits the experimental data very well and confirms the monolayer coverage of the dye onto the AWH (q_max = 11.8 mg/g) and also the homogeneous distribution of active sites on the adsorbent.

The Freundlich model was used to evaluate the adsorption of AY-17 dye onto AWH. The linear plot of the Freundlich model log q_e vs. log C_e showed a correlation coefficient (R²) of 0.6266 (Figure 8(b)). The KF and 1/n values are obtained from Equation (7) were 1.563 and 0.491, respectively. This indicated AWH produced a favorable condition for adsorption of AY-17 dye on its surface (Mallampati & Valiyaveettil 2013).

Comparing the two isotherm models, it was found that the data were better fitted with Langmuir isotherm model (R² = 0.953) than the Freundlich isotherm model (R² = 0.6266) (Table 1). The R_L value of this study was 0.038 which is between 0 < R_L < 1. This indicates that the adsorption process of AY-17 dye onto AWH root powder is favorable.

Adsorption kinetic models are used to investigate the mechanism of sorption and potential rate-controlling steps to select optimum operating conditions for the full-scale batch process. The kinetic parameters, which are helpful for the prediction of adsorption rate, give important information for designing and modeling the adsorption processes (Santhi et al. 2010).

Determination of best-fit kinetic models is the most common way to predict the optimum adsorption kinetic expression (Kumar & Sivanesan 2006). Pseudo-first-order and pseudo-second-order kinetic models were used to investigate our study.

The kinetics of AY-17 dye adsorption onto AWH adsorbent were analyzed using pseudo-first-order and pseudo-second-order kinetic models in the study. The correlation coefficient (R²) of the pseudo-first-order and pseudo-second-order kinetic models in this study are 0.6219 and 1.00, respectively (Table 2; Figure 9(a) and 9(b)).

The calculated q_e value of pseudo-second-order kinetic model agreed well with the experimental data obtained in the lab. This implies that the pseudo-second-order kinetic model is best to describe the adsorption kinetics of AY-17 dye onto AWH.

The removal of AY-17 dye using AWH was compared with other adsorbents reported in literature. The removal capacities with their experimental settings are shown in Table 3. The AWH indicated good removing potential of AY-17 dye in aqueous solution. The availability of WH in tropical lakes and simple preparation might be important for both the treatment of AY-17 contaminated wastewater and management of invasive WH species on aquatic bodies.

This study was conducted to analyze the removal efficiency of the AWH adsorbent for the removal of AY-17 dye from textile wastewater using aqueous solutions. Batch studies were carried out under some limited operating parameters including pH, contact time, adsorbent dosage, and initial dye concentration. At optimum conditions, AWH root showed a removal efficiency of 92.26% AY-17 dye from aqueous solution. The results obtained from this study indicated high removal efficiency of AY-17 dye in aqueous solutions. The material used for adsorption is obtained from the collected water hyacinth root around lakes, which is cheap and locally available. Therefore, the water hyacinth root activated carbon can be used as a potential sorbent to AY-17 dye in the textile wastewater to reduce its burden on the environment.

The authors are grateful to Bahir Dar University for financial support of this study.

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

The authors declare there is no conflict.