Physical states of treatment
The variations in temperature, pH, ORP, and DO during the exposure times were recorded to show the conditions of the selected floating plant of S. molesta to remove MO dye from water. For the three MO concentrations of 5, 15, and 25 mg/L, temperature and pH were almost equal in both treatments (with plants and without plants are shown in Table 1). The experiments were carried out under lab condition with a temperature range of 25–27 ± 0.5 °C. The solution pH in the glass vessel with plants and without plant were in the range of pH 6.3–7.3, demonstrating the suitable pH needed for growth and activity of S. molesta, especially for dye removal. According to a study conducted by Yaseen and Scholz37, pH has no obvious influence in phytoremediation with S. molesta. However, Khataee et al.2 determined that a pH value of 6.5 was the optimal decolourization pH. In this experimental the pH and temperature reported values were in natural environment which is satisfactory for the MO decolorization process.
For DO measurement, it can be observed that the effect of S. molesta on the physical conditions varied between 4 and 5 mg/L for the three MO concentrations (5, 15, and 25 mg/L). The initial ORP of the glass vessels with plants was 7 ± 10 mV, which then decreased to (− 50) ± 10 mV, showing a significant influence of the plant, which plays an important role in the performance of decolourization, with a decrease in dissolved oxygen and lower ORP values due to the activity of the bacteria. In contrast, for the glass vessel without plant the DO and ORP were even lower, ranging between (3–4) ± 0.5 mg/L and (50 to − 50) ± 10 mV due to the MO dye and the absence of the plant.
Plant growth was observed for 10 days of MO exposure. As illustrated in Fig. 2, the wet and dry weight of S. molesta can grow well in all concentrations, while better growth was physically observed (Fig. 3) for lower MO concentrations (5 mg/L). As shown by the results, all of the plants had increases in wet weight after exposure to the MO dye contaminants. The wet weight increased from 8 ± 0.5 g to 14 ± 2 g in different MO dye concentrations of aqueous solution. At the end of 10-day exposure, the leaves of S. molesta remained green which contributed to the removal performance during the operation period, confirming the capability of S. molesta to treat dye-contaminated water. According to the two-way ANOVA, there was a significant difference (p < 0.05) in wet weight with different MO concentrations, as depicted in Fig. 2. However, no statistically significant difference was found for dry weight at different MO concentrations (p > 0.05) (Fig. 2). The F-value for Levene’s test is 0.011 with p = 0.989, leading to retain the null hypothesis (no difference) for the assumption of homogeneity of variance. According to Dhir, Sharmila and Saradhi38, the roots of Salvinia species were able to store high amount of heavy metals than its leaves.
The relative growth rate, which is dependent on wet weight, was affected by three concentration values of MO dye (5, 15, and 25 mg/L) compared to the corresponding plant control. As shown in Fig. 4, when the concentration of MO dye increased, there was a clear decrease in RGR after a 10-day period due to the effect of the plant when exposed to the dye. Statistical analysis results showed that the RGR had significant difference (p < 0.05) within the MO concentrations, confirming that all MO concentrations can grow and be regenerated although RGR decreases. Based on the dye concentration as tabulated in Table 2, the ratio of dye content to the fresh biomass of plants can be calculated as mg dye per fresh wet weight of plants to determine the amount of biomass required to remove certain amount of dye in water. According to Al-Badawi et al.39 findings, the ratio of plant numbers to the total mass of contaminant should be calculated to determine the phytotoxicity effects of the contaminant concentration. The same principle can be used if this plant were to be applied in real wastewater of different concentrations. In this work, the ratio was determined as the amount of initial dye content to the fresh wet weights since floating plants were used. From Table 2, a ratio of 0.67 mg MO mass / g of fresh plants would be the maximum ratio to be used to quantify the amount of required plant biomass for future application in different dye concentration of wastewater or in pilot scale application.
Hence, the overall findings indicate that S. molesta has potential used in phytoremediation and can be applied in wastewater treatment ponds with low pollutant concentrations, which is in agreement with the conclusion obtained by Muthunarayanan et al.40, who used Eichhornia crassipes to decolourise dyes in textile effluent. In addition, S. molesta had demonstrated to remove 31% of ammonia after 12 day of exposure41.
Decolorization of methyl orange dye
Removal of dye was determined at different concentrations with 150 g of S. molesta. The adsorption of the three MO dye concentrations within 10 days varied greatly compared to the control dye (without plants) (Fig. 5). After a 10-day exposure of S. molesta to 5, 15, and 25 mg/L MO dye, the concentrations of the dye decreased to 2.9, 8.95, and 20.92 mg/L compared to the glass vessel without plants, in which the MO dye decreased to 4.6, 10.5, and 23.12 mg/L, respectively. The MO decolorization efficiencies for different concentrations (5, 15 and 25 mg/L dyes) were 42, 20, and 15%, respectively, indicating the dye can be removed effectively by S. molesta. The decolorization of the MO dye in the contaminant control was only 12, 7, and 6% for 5, 15, and 25 mg/L dye concentrations, respectively, providing evidence for the significant role of the plants in the removal of MO dye. Extending treatment time may lead to better dye removal achievement. Study on different type of aquatic plants indicated same trend as obtained by Yaseen and Scholz37, who utilised floating plant of Lemna minor in microcosms as a polishing stage for treatment of 10 mg/L dye in textile wastewater with 53% removal. Statistical analysis confirmed that there was a significant difference (p < 0.05) in MO removal within the different concentrations for each day, as shown in Fig. 5. The decolourization in the control glass vessel might be due to the microbe activities and photodegradation25.
Comparing the results of the current study with previous studies, a study by Li et al.42 showed a new novelty for the decolorization of the azo methyl orange (MO) dye in aqueous solution through a new clay-supported nanoscale zero-valent iron with 99.1% decolorization over 50 min. Ornamental plants of Tagetes patula, Aster amellus, Portulaca grandiflora and Gaillardia grandiflora, grown for one month on the ridges of wetland which was irrigated with textile wastewater, could reduce the ADMI values in soil by 18, 25, 40, 47, 66 and 73% as observed within 30 days, respectively43. Plants of Fimbristylis dichotoma and Ammannia baccifera had decolorized 50 mg/L methyl orange up to 91% and 89% after 60 h exposure, respectively. Whereas, when applied together as a consortium, MO decolorization of 95% was achieved within 48 h of exposure44. Also, Chandanshive et al.45 observed that Salvinia molesta had the capability to degrade azo dye, Rubine GFL, up to 97% at a concentration of 100 mg/L within 3 days using 6,072 g of root biomass.
As shown in Table 3, there was a significant interaction between treatment, time and MO concentrations with removal of MO efficiency by S. molesta [F(72) = 59.603, p < 0.05]. While, there was no significant interaction between time and MO concentrations with plant growth, F(18) = 2.359, p > 0.05.
Fourier transform infrared (FTIR) spectroscopy analysis for phytotransformation
The FTIR spectrum of the control dye (Fig. 6A) varied significantly from the spectrum of MO decolourization at different concentrations by S. molesta (Fig. 6C–E), and it was found that S. molesta that grow in distilled water (Fig. 6B). The results support the phytotransformation of the dye into different metabolites. The FTIR spectrum of the MO dye (Fig. 6A) shows the presence of different high peaks at 3,445.7 cm−1 for the secondary amides (N–H stretch), 1,600.6 cm−1 for the aromatic compounds (C–C stretch), 1,113.5 cm−1 for aliphatic amines (C–N stretch), and 693.6 cm−1 for alkyl halides (C–Cl stretch).
The FTIR spectra of MO phytotransformation at different concentrations by S. molesta (Fig. 6C–E) show major peaks at 3,338.8 cm−1 for secondary amides (N–H stretch); 2,917 cm−1 for alkanes (C–H stretch); 2,849.2 cm−1 for aldehydes (H–C = O: C–H stretch); 1616.5 cm−1 for amines (N–H bend); and 1,033.8 cm−1 for alcohols, carboxylic acids, esters, and ethers (C–O stretch). While the FTIR spectra of the S. molesta plant in distilled water (Fig. 6B) showed peaks at 3,572 cm−1 for alcohols (O–H), 2,920.9 cm−1 for alkanes (C–H stretch), 1611.8 1 cm−1 for amines (N–H bend), 1,413.4 cm−1 for aromatics (C–C stretch), and 1,034 cm−1 for aliphatic amines (C–N stretch), indicating substantial changes in the peak position in comparison to the exposed plants with MO dye (Fig. 6C–E). The disappearance of a peak that was present in the spectrum of the dye indicates that the MO dye bond split46.