The utility of hydroponic system was optimized by using two sets of plants- one set in upper pipe where treatment of synthetic water was screened by different plants and in lower pipe, growth of roots of screened plant were observed in treated water after treatment (Fig. 3). The decolorization results of each plant were compared with the abiotic and biotic control dye solution. The roots of plants were found to have dye pigmentation in comparison to biotic control via physical examination (Fig. 4). The results of different batch experiments for decolorization of dyes with respect to time are shown in Fig. 5. It has been observed from Fig. 5 that the decolorization percentage of dye increases with increase in time. The same pattern of dye decolorization has been reported by various researchers (Reemaet al. 2014; Kaushal and Mahajan 2015; Al-Baldawi et al. 2020). For instance, the different Green HE4B dye concentrations were reduced to varying extent during 48 h of contact by G. pulchella and maximum decolorization was observed at 48 h in each concentration (Kabra et al. 2011). All these decolorization results and the impact of synthetic dye wastewater on the growth of plant used for screening are summarized in Table 2.
Figure 5a and 5b show the decolorization pattern of MB and CR dye by utilizing T. ammi plant. Out of six screened plants, excellent decolorization of MB was observed in the case of T. ammi plant. The decolorization of 10 and 20 mg L−1 MB was 99 and 86%, respectively. Plant growth was normal after adsorption of the dye into the roots. The decolorization (%) of CR dye by T. ammi shown in Fig. 5b clearly indicates the admirable efficiency of T. ammi to decolorize the CR. The percentage decolorization of 10 and 20 mg L−1 of CR dye was 95 and 84%, respectively. The plant remains survived after the adsorption of dye into the roots. However, percentage decolorization decreases with an increase in concentration. These outcomes show that T. ammi plant is an outstanding plant to decolorize the azo dye CR and triarylmethane dye MB at a lower concentration. The decolorization of MB and CR dye by using B. fedtschenkoi plant is shown in Fig. 5c and 5d respectively. The plant B. fedtschenkoi shows significant decolorization of triarylmethane dye MB having a percentage decolorization of 85% (10 mg L−1) and 69% (20 mg L−1). The response of B. fedtschenkoi plant towards the removal of a toxic azo dye, CR was also observed as significant for textile wastewater treatment. The B. fedtschenkoi decolorized the CR dye 77 and 70% for 10 and 20 mg L−1 dye concentrations, respectively. It was observed that plant parts remained active after adsorption of the dye and were able to remove more dye concentration than 20 mg L−1. These results proved that B. fedtschenkoi plant has a good tendency to decolorize synthetic wastewater of CR azo dye as well as triarylmethane dye MB.
Figure 5e and 5f show the decolorization of MB and CR respectively by using C. indicum. The percentage decolorization obtained for 10 and 20 mg L−1 MB dye concentrations were 87 and 70% respectively. Initially plant leaves became dried, later stems and roots of the plant also showed dryness after the removal of dyes. The plant becomes died after treatment with higher dye concentrations. However, the MB color removal by this plant was acceptable yet plant endurance was not significant for the treatment of triarylmethane dye, MB. The results with CR dye synthetic wastewater revealed only 44 and 42% decolorization at 10 and 20 mg L−1 concentrations, respectively. Wilting of the plant takes place after treatment of CR dye. The plant was not able to treat dye concentration higher than 20 mg L−1. Hence, C. indicum is not suitable for the phytotreatment of CR synthetic dye wastewater.
T. erecta plant was also used for a screening test to remove MB dye from synthetic wastewater. It was observed that plant had the more capacity to decolorize the triarylmethane dye, MB in comparison to CR dye. Fig. 5g and 5h show the decolorization of MB and CR dye, respectively. The decolorization for 10 and 20 mg L−1 MB dye wastewater was 84 and 68% respectively. After decolorization, the MB dye plant shows withering. Initially, the leaves become dry then subsequently stems and roots. Due to these conditions, the plant was no more active for treatment with more MB dye concentrations than 20 mg L−1. The percentage decolorization was observed 67 and 66% for 10 and 20 mg L−1 CR dye concentrations, respectively. Though the plant can decolorize the azo dye, CR and MB but T. erecta plant dryness after removal of the toxic dye makes it unsuitable for the treatment of synthetic dye wastewater.
Figure 5i and 5j show the decolorization of MB and CR respectively by H. rosa-sinensis plant. The decolorization obtained were 86 and 71% from the 10 and 20 mg L−1 MB concentrations respectively and 41 and 39% decolorization at 10 and 20 mg L−1 CR dye solution. It indicates the potential of H. rosa-sinensis for MB synthetic dye wastewater decolorization. But the toxicity of dye effects on plant growth results in its inability to remove dye concentrations than 20 mg L−1.
Figure 5k and 5l show the percentage decolorization of MB and CR dye respectively by C. roseus. The decolorization percentage obtained for MB 10 and 20 mg L−1 was 35 and 34% respectively and 48 and 43% for CR 10 and 20 mg L−1 respectively. In the case of C. roseus plant, it is found that the plant remains active after dye removal however plant removal efficiency is quite slow for both the dyes. It was observed that plant could not effectively decolorize the synthetic wastewater up to 40 h.
Hence, the results obtained from the screening experiments clearly indicate that the maximum percentage decolorization obtained from the T. ammi plant followed by B. fedtschenkoi and both plants also remain active after removal both MB and CR dyes. C. indicum and T. erecta plants also show their potential for decolorization of synthetic dye wastewater however, their survival rate makes them insignificant for phytoremediation process. H. rosa-sinensis plant was also not considerable for survival because flowers wither after dye removal. The plant C. roseus can bear the toxic impact of dyes but the rate of decolorization is quite slow for both MB and CR dyes.
In the literature, the removal of MB and CR was reported by a few researchers by using the phytoremediation technique as shown in Table 3. E. crassipes successfully removed MB dye (50 mg L−1) in 20 days up to 98% (Tan et al. 2016) while L. minor (2 g) was exposed to 50 mg L−1 of MB dyes for 24 h decolorization of 81% (Imron et al. 2019). In another study, 98% decolorization has been reported for L. minor in 144 h at 10% concentration and authors claimed it as a phytoremediation agent to remove MB dye from wastewater (Reema et al. 2011). Another aquatic species Azolla pinata also reported in literature for removal of MB dye (Al-Baldawi et al. 2018). In literature, MB remediation is mostly reported by using aquatic plant species. In the present research work, ornamental plant T. ammi plant showed the decolorization up to 99 (10 mg L−1) and 86% (20 mg L−1) for MB dye in 40 h experiment only. Hence, T. ammi plant has been proven to be more effective than E. crassipes and L. minor. Again, for phytoremediation of CR dye, C. vulgaris (Mahajan and Kaushal 2013) and P. stratiotes (Mahajan and Kaushal 2019) aquatic species are reported for maximum decolorization 95 and 90% respectively. In the present study, T. ammi exhibited the maximum decolorization up to 95 and 84% at 10 and 20 mg L−1 CR dye concentrations respectively and remained active after the decolorization process. However, it has been observed that the dye was found to be adsorbed on the roots of T. ammi plant possibly due to the rhizofiltration process, and hence plant could be able to provide maximum decolorization. Therefore, T. ammi plant acts as a potential candidate for future research where it can be used as a phytoremediator for decolorization of dye wastewater.