Design of hydroponic system for screening of ornamental plant species for removal of synthetic dyes using phytoremediation approach


 Most of the dyes are carcinogenic and mutagenic in nature. Plants are potential candidates to remediate textile dye wastewater from contaminated sites. The present study aimed to design an efficient hydroponic system to screen potential ornamental plant species for removal of synthetic dye solution of triarylmethane dye Methylene Blue (MB) and diazo dye Congo Red (CR). The plants selected for screening are Trachyspermum ammi, Tagetes erecta, Hibiscus rosa-sinensis, Chrysanthemum indicum, Bryophyllum fedtschenkoi, and Catharanthus roseus. The phytotreatment of dyes was done up to 40 h for two different concentrations of dyes, i.e., 10 and 20 mg L−1. Among screened plant species, the maximum decolorization was obtained from T. ammi followed by B. fedtschenkoi. Both plant species showed active growth in indigenous designed hydroponic system even after the phytoremediation process. T. ammi decolorized the MB dye 99% (10 mg L−1) and 86% (20 mg L−1) while the decolorization of the CR dye solution was up to 95% (10 mg L−1) and 84% (20 mg L−1). T. ammi was found to have maximum potential among screened plants for the removal of MB and CR dye from synthetic dye solution and can be used for phytoremediation of wastewater contaminated with synthetic dyes.


Introduction
Due to the increasing world population, there is a tremendous growth of various industries, which uses many harmful chemicals for the generation of different commodities for public demands but the side byproducts such as contaminants not only affect water bodies but also the air and soil. Dyes have a major demand and application in the textile industries for the dyeing process. About 10-15% of the azo dyes get lost in the e uent during the dyeing process (Stolz 2001) and 50% of other reactive dyes are reported for use in the textile industry which is discharged into water (Chen 2002). Azo dyes are extensively used in the dyeing process. The e uent containing dyes released into the surrounding seriously affects the environment by destroying the ecosystem, causing water pollution, and reducing light penetration for aquatic life (Imron et al. 2019). Due to textile dye wastewater, the biological oxygen demand, chemical oxygen demand, and suspended solids increase in the nearest river located beside the textile industry (Ekanayake et al. 2021). So, there is a big challenge to treat textile dyes e uent before released into water bodies.
There are many physical and chemical methods, for example, adsorption, coagulation, sedimentation, occulation, ltration, photo degradation, and chemical oxidation, for managing contamination produced by textile dyes (Khandare and Govindwar 2015). These methods relate to the high expense, low productivity, require huge space and undependable to work. Because of these issues, there is a requirement for the advancement of productive and cost-effective methods for the treatment of textile dyes. Biological methods are more effective than physical and chemical methods to treat the textile dye wastewater. Biological methods involve different enzymes, microorganisms, and plants for the removal There is no report to our knowledge where these terrestrial ornamental plants are exposed in remediation of synthetic dyes in hydroponic medium. Hence, in the present research study, a hydroponic system is designed to engrossed on the ability of terrestrial ornamental plants for decolorization of toxic, carcinogenic, and mutagenic textile dyes Methylene Blue (MB) and Congo Red (CR) dyes.

Chemicals and plant material
The triarylmethane dye, MB and a diazo dye, CR dye were used for experimentation. MB is a heterocyclic aromatic chemical compound with molecular formula C 16 H 18 N 3 SCl. The molecular weight of MB dye is 320 g mol -1 . CR dye is a diazo dye that can be synthesized by a coupling reaction containing hydroxyl, amino or other groups with an aromatic diazotized base. The chemical formula of CR dye is C 32 H 22 N 6 Na 2 O 6 S 2 and its molecular weight is 696 g mol -1 . The chemical structure of MB and CR is given in Fig. 1. MB dye and CR dyes were purchased from Sanjay lab Amritsar, India. All the chemicals used were of the highest purity and of an analytical grade. The synthetic dye wastewater was prepared at two different concentrations of 10 and 20 mg L −1 . The entire apparatus was sterilized before experimentation. Screened ornamental plants T. ammi, T. erecta, H. rosa-sinensis, C. indicum, B. fedtschenkoi, C. roseus were harvested from the Botanical Garden of Guru Nanak Dev University campus, Sathiala and Government High School, Sathiala (Punjab), India. The plants were washed completely to remove mud, dirt, and particulate matter and acclimatized for three days in distilled water. Table 1 shows the description of screened plants used for the research study.

Experimental design
A two-stage hydroponic system was designed in which two PVC pipes of 0.30 m diameter and 1m long was horizontally connected via movable stopper as shown in Fig. 2. Both the pipes were attached at an angle of 45°. The bigger holes of 0.05 m diameter were made for keeping the plants in vertical position in each pipe and small sized holes of 0.10 diameter were kept for aeration. The inlet of wastewater was kept above the top of rst pipe and nal outlet at the lower end of second pipe (Fig. 2).
Initial experiments were performed to identify the plants having the potential to decolorize the textile dyes, for which T. ammi, T. erecta, H. rosa-sinensis, C. indicum, B. fedtschenkoi, and C. roseus plants were selected. Firstly, the roots of these plants were washed with running tap water to remove adherent soil after which plants were entirely washed with distilled water. Plants were put into distilled water for hydroponic treatment (without soil) and the growth of the plants is checked up to three days. The

Results And Discussion
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).  Table 2. fedtschenkoi plant is shown in Fig. 5c and 5d respectively. The plant B. fedtschenkoi shows signi cant 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 signi cant 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. 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.  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 insigni cant for phytoremediation process. H. rosa-sinensis plant was also not considerable for survival because owers 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 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 rhizo ltration 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.

Conclusion
The design of hydroponics system shows the e cient utilization of system for screening the plants and developing the growth of the root system of plants in optimum time by using alternative two pipe system. and 84% (20 mg L −1 ) for CR dye due to its adsorption on the roots of the plant. Therefore, further research work can be focused on dye removal by using T. ammi plant based on the adsorption mechanism. In the future, adsorption mechanism explored by using different instrumental techniques such as Fourier Transform Infrared spectroscopy, Scanning Electron Microscopy, and statistical analysis can also be done with different operational parameters such as plant weights, the relative growth rate of plants, the effect of pH, etc.

Declarations
Competing interests The authors declare that there is no competing interests.
Ethics approval and consent to participate "Not applicable", as research does not report on or involve the use of any animal or human data or tissue.
Consent for publication Not Applicable.

Funding
This research work did not receive any funding.

Authors' contributions
Navjeet Kaur: Conducted the experimental studies and drafted the manuscript; Jyotsna Kaushal: Conceptualization, expert view, and overall Supervision; Pooja Mahajan: Data interpretation; Arun L. Srivastva: Suggestions and interpretation on the chemical analysis. All authors read and approved the nal manuscript.  Image depicting growth of (a) B. fedtschenkoi and (b) T. ammi before and after 2-step treatment in hydroponic system. Red circles showed the growth of roots enhanced after treatment.

Figure 4
Visual pigmentation of (a) MB (b) CR dye on roots of T. ammi after 8 h See image above for gure legend.