Pilot-Scale Study of Real Domestic Textile Wastewater Treatment Using Cassia fistula Seed-Derived Coagulant

Plant-derived coagulants have exhibited a good potential in wastewater treatment due to their “green” characteristics, high coagulating-ﬂocculating activity, cost-eﬀectiveness, and biodegradability. Nevertheless, research studies have focused mainly on bench-scale experiments; pilot-scale and full-scale simulations are still limited. Herein, we ﬁrstly report a pilot-scale study of real domestic textile wastewater treatment using Cassia ﬁstula coagulant. The material characterizations using Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and dynamic light scattering (DLS) revealed that the natural gum extracted from C. ﬁstula seed possessed a rough and irregular surface containing a high molecular weight galactomannan. The bench-scale investigation was initially conducted to determine the optimal pollutant concentration, initial pH, and coagulant dosage in the coagulation-ﬂocculation process. The pilot-scale study revealed that C. ﬁstula coagulant is an eﬀective material for real textile wastewater treatment, showing percentage removal of 93.83% at a volume of 30L and a coagulant dosage of 1.17mg · L − 1 . Coagulation-ﬂocculation using C. ﬁstula seed gum could be an eﬃcient primary wastewater treatment prior to membrane or biological methods to meet Vietnamese environmental standards. The main mechanisms of textile wastewater treatment involve adsorption/bridging interactions via hydrogen bonding and electrostatic attraction between negatively charged carboxylate groups of the coagulant and positively charged pollutants.


Introduction
Water pollution has long been a global issue that has increased day by day due to the rapid pace of industrialization and population growth [1]. Watercourses have been contaminated by heavy metals, dyes, toxic anions, or organic compounds resulting from both anthropogenic activities and natural weathering processes [2][3][4]. Dyes are among important classes of pollutants in water sources [5]. ey are a necessity in dyestuffs, textiles, paper, and plastics to impart color [6].
Due to the presence of numerous hazardous chemicals, discharge of dyes into aquatic bodies without proper purification causes significant health hazards [7][8][9]. e existence of sulfur, heavy metals, nitrates, and naphthol leads to skin irritation, liver damage, or dysfunction of the kidney. Aromatic amines, formaldehyde, chlorine, and hydrocarbonbased dyeing chemicals are reported to be carcinogenic [10,11]. Moreover, dyes also cause environmental problems due to imparting an undesirable color to the water body, resulting in blocking sunlight penetration and resisting the photosynthesis and respiration of living things [12]. e use of dyes generates a considerable amount of toxic colored wastewater; however, most of them are discarded into environmental water bodies without further treatment [12]. For these reasons, the removal of dyes from different types of wastewater streams is of worldwide concern.
Focuses on specific methods to remove these toxic substances from water sources have been sought. Present treatment methods and technologies include ion-exchange, adsorption, membrane filtration, chemical oxidation, coagulation-flocculation, electrochemical methods, and biological treatments [9,10,[12][13][14][15][16]. e membrane filtration is effective for water recover and reusing, but the investment costs are often too high for small and medium industries [12,15]. e electrochemical destruction, oxidation, and ion-exchange are efficient methods of addressing toxic dyes; however, their high operating costs and energy consumption pose challenges to the application in real wastewater treatment [12]. e adsorption and coagulation-flocculation techniques are two popular methods for dye removal in industrial wastewater due to their easy of operation, cost-effectiveness, and high efficiency [10,16]. Global water industries consider coagulationflocculation to be one of the major treatment approaches to reduce cost and enhance the overall efficiency of practical treatments of wastewater [17]. It is a process of destabilizing and combining colloids, dissolved organic substances, or particles into larger aggregates [18]. ese impurities could be further settled, filtered, and removed from the bulk medium in subsequent processes [17].
Conventional coagulants used in coagulation-flocculation procedures are aluminum-based and iron-based compounds [19]. e large amounts required and their toxic leftover residues are unavoidable drawbacks of these synthesized chemicals [12,19]. e discovery and development of coagulants from natural sources are therefore significantly important. Plant-derived coagulants are promising alternative substances due to their high efficiency, environmental friendliness, and cost-effectiveness [20][21][22][23][24][25]. Among various natural coagulants, Cassia fistula seed gum has been recognized as a potential material for wastewater treatments [22][23][24]. Moreover, in Vietnam, the use of C. fistula seed has the advantages of being abundantly distributed and considered as a common solid waste [22]. To the best of our knowledge, the use of plant-derived coagulants, especially for C. fistula seed in wastewater treatment has been mainly reported at a bench scale. Pilot-scale studies for evaluating the practical applications of C. fistula-based coagulant have not been previously reported.
In this study, a natural coagulant prepared from C. fistula seed was applied in the removal of Reactive Red 195 (RR-195) and real domestic textile wastewater. e effects of pH, initial concentration of the dye, and coagulant dosage on the removal efficiency in a bench-scale simulation were investigated. A pilot-scale study was further conducted to evaluate the practical application of C. fistula coagulant in wastewater treatment to meet Vietnamese environmental standards.

Materials and Methods
2.1. Chemicals. Hexane, ethanol, H 2 SO 4 , NaOH, and RR-195 were of analytical grade and used as received. Raw C. fistula seeds were collected in u Dau Mot city, Binh Duong province, Vietnam. e real textile wastewater was collected from Nam Tan Uyen Industrial Park, Binh Duong province, Vietnam.

Preparation of Natural Gum from C. fistula Seed.
e raw C. fistula seeds were dried under direct sunlight, and then the whole seeds were ground into powder. e extraction was performed using hexane as a solvent in a Soxhlet system. e fibrous mass was produced by ethanol precipitation after 48 h. e solid material was washed with distilled water and dried at 50°C in air for 2 h to obtain C. fistula seed gum.

Material Characterization.
e surface morphology of the as-synthesized gum was monitored by SEM using a Hitachi S4800 scanning electron microscope. e FTIR spectra were recorded using a FTIR/NIR spectrometer (Frontier/PerkinElmer, USA). A Malvern Zetasizer Nano-ZS dynamic light scattering instrument was employed to determine the zeta potential of C. fistula seed gum in aqueous solution.

Coagulation-Flocculation Process.
e coagulationflocculation process of dye and textile wastewater treatment was performed using a jar test apparatus. It was equipped with a series of six beakers containing 1 L of waste solution for each test run. e pH of the mixture was adjusted using 1 M H 2 SO 4 or 1 M NaOH. e experiment was conducted for 5 min with initial rapid mixing (coagulation) at 200 rpm after the addition of each coagulant(s), followed by a slow mixing stage for 5 min at 20 rpm (flocculation). e floc was allowed to settle for 30 min. After obtaining two layers, the supernatant was collected and examined for the reduction in color. e removal efficiency of color was calculated using equation (1) as follows: where C 0 and C e are the initial and equilibrium concentration of dye in solution, respectively. A pilot-scale treatment of dye and real textile wastewater with C. fistula seed gum was carried out to evaluate its practical application. A simulation diagram of the pilot-scale system is shown in Figure 1. A 30-L volume of pollutants was used for each test run. e removal process consisted of three consecutive stages similar to the jar test: rapid mixing (200 rpm, 5 min), slow mixing (20 rpm, 5 min), and quiescent gravity separation (30 min). e supernatant was then collected and examined for validation purposes. e removal efficiency of the pilot-scale treatments was also calculated by equation (1).

Characterizations of C. fistula Seed Gum
3.1.1. FTIR. Functional groups in the C. fistula seed gum were identified by FTIR in the range of 4000 to 450 cm −1 (Figure 2(a)). e broad bands at 3448 cm −1 and 2935 cm −1 could be attributed to the −OH stretching of carboxylic acid or alcohol and the C-H stretching of aliphatic structures, respectively [26]. e absorption bands at 1423 cm −1 and 1642 cm −1 could represent C�C (aromatic) and adsorbed water molecules, respectively [21,23].
e vibrations of C-O in C-OH bonds could be observed in the broad band between 960 cm −1 and 1180 cm −1 . e peak at 876 cm −1 and 814 cm −1 were related to the presence of anomeric deformation of β-d-mannopyranose and α-d-galactopyranose, respectively [27], indicating the presence of galactomannans in the C. fistula seed gum in this study. Galactomannans are neutral polysaccharides, consisting of a skeleton with straight chain of (1 ⟶ 4)-β-d-mannopyranose units and single (1 ⟶ 6)-α-d-galactopyranose attached to main chain unit [28]. ey can be extracted from the endosperm of the seeds of certain leguminous plants, such as Senna tora [26], Cassia obtusifolia [29], C. fistula [27], and Cassia grandis [28].
ese polysaccharides have been recognized to be promising natural coagulants for wastewater treatment possessing advantages of posing minimal health risk to living organisms and forming biodegradable sludge [30].

SEM.
e surface morphology of C. fistula seed gum displayed an irregular shape (Figure 2(b)). e presence of fibrous networks was observed, with rough surface and porosity, providing greater specific surface area and more adsorption sites. As a result, these properties might benefit the coagulation-flocculation process by improving the adsorption and bridging ability [20].

DLS.
e average zeta potential of C. fistula seed gum measured by DLS in aqueous solution was −15.7 mV (Figure 3). is relatively high negative potential value indicated a stable and dispersed gum in aqueous solution. is can be attributed to negative-negative repulsion of carboxylate groups (COO − ) on the surface of the seed gum as also observed in other polysaccharides derived from Alyssum homolocarpum seed gum [31] and chia seed gum [32].

Dye and Real Textile Wastewater Treatments.
e coagulation-flocculation processes for dye and real textile wastewater treatments were performed using C. fistula seed gum.
e prefeasibility studies to screen out important factors influencing the removal efficiency were carried out on a jar test apparatus. e pilot-scale coagulation-flocculation plant was further studied to simulate full-scale wastewater treatment.

Screening Experiments at Bench Scale for Optimal Initial Concentration of RR-195, Dosage, and pH Conditions.
e effect of initial concentration of RR-195 on decolorization is demonstrated in Figure 4. e removal efficiency dropped when initial concentration of RR-195 increased.
is could be explained by the natural coagulant being exhausted when the initial concentration of RR-195 increased, due to the saturation of pollutant on its surface. erefore, a higher dosage of coagulant was required to treat pollutants at higher initial concentration in order to provide a greater number of sites for adsorption, bridging, or electrostatic attraction. pH is a critical variable affecting removal efficiency in the coagulation-flocculation process. e experimental studies on the effect of pH were therefore carried out in the initial pH range of 1-11 ( Figure 5). e removal of RR-195 and treatment of real textile wastewater by C. fistula seed gum was found to be dependent on pH, with different behaviors. For RR-195, the removal efficiency was much higher at lower pH. RR-195 is an anionic dye, and the natural coagulant possesses a more negatively charged surface at higher pHs due to deprotonation. Hence, electrostatic repulsion would be seen at higher pH. Moreover, the possibility that C. fistula seed gum performed better at lower pH was attributed to the protonation of the coagulant that promoted better adsorption and bridging through hydrogen bonding. For textile wastewater, containing multiple textile dyes, inorganic salts, and organic additives [33], removal by coagulation would be more complicated. Adsorption/bridging and electrostatic attraction might occur simultaneously during this coagulation-flocculation process. In the pH range of 1-7, the removal efficiency for real textile wastewater displayed a variation tendency similar to that of RR-195 due to a better adsorption through H-bonding. When the pH was increased from 7 to 11, the removal of color was improved.
is can be attributed to the fact that more negative charges were created on the surface of the coagulant at higher pH due to a partial deprotonation, resulting in an enhancement of electrostatic attraction between cationic dyes and the negatively charged coagulant.
Coagulant dosage is another important factor in the coagulation-flocculation process. e study of this factor could be of benefit not only for economic evaluation purposes but also to prevent the excessive use of coagulant during wastewater treatment [30]. e effect of dosage of C. fistula seed gum on the removal of RR-195 and textile wastewater is presented in Figure 6. e removal efficiency for both types of pollutant initially increased with the increase in coagulant dosage and decreased slightly with a further increase in dosage. Under low coagulant dosage, the collision between coagulant and pollutants was insufficient to destabilize pollutants, leading to lower treatment efficiency [34]. However, excess coagulant dosage led to the restabilization of coagulant flocs, resulting in low effectiveness of the solid-liquid separation process [35].

Pilot-Scale Study of Real Textile Wastewater Treatment.
After performing the bench-scale study using a jar test apparatus, a pilot-scale test was carried out to simulate full-scale textile wastewater treatment in a more detailed way. ese pilot-scale experiments were designed based on the allowable pollution parameters of Vietnamese environmental standards. For the color factor, the national technical regulation on the limits for industrial wastewater (QCVN 40:2011/BTNMT) discharging into the water sources serving or not serving tap water supply are 50 ppm (Column A) and 150 ppm (Column B), respectively [36,37]. From an initial color concentration, the required percentage removals to ensure these two values for textile wastewater were calculated (Table 1). e dosages of C. fistula needed for the pilot-scale study were further approximated based on the linear plot of percentage removal and dosage found in the bench-scale experiments (Figure 7). After performing the pilot-scale study, the final color concentrations obtained were 85.33 ppm and 195.33 ppm for column A and column B, respectively. ese results slightly exceeded the regulated limits. However, this is reasonable as the coagulationflocculation method is used mainly as a pretreatment prior to membrane or biological treatments [38,39]. Highstrength wastewater is treated primarily by this technique to reduce the cost of the secondary stage by reducing the  Journal of Chemistry load of pollutants, the number of secondary treatment units required, or eliminating biological process [39,40]. ese results indicated that C. fistula seed gum is an effective and promising material for textile wastewater treatment using a coagulation-flocculation process.

Proposed Mechanism of Coagulation-Flocculation
Process. Depending on the properties of the surface sites, charges, type of pollutant, and solution chemistry of the aqueous phase, coagulation-flocculation might involve one or more mechanisms [41]. In this study, the results obtained supported the hypothesis that adsorption/bridging and electrostatic attraction should be the predominant mechanisms for dye-based wastewater treatment. e proposed mechanisms are illustrated in Figure 8. In aqueous media, C. fistula coagulant contains long polymeric chains exposing carboxyl (-COO -/COOH) and hydroxyl (-OH) functional groups. e negatively charged surface of the coagulant is created by the partial deprotonation of the carboxylic functional groups. ese negative charges could provide the adsorption sites for the cationic dye pollutants present in real textile wastewater. On the other hand, as the C. fistula coagulant could also remove anionic RR-195 effectively at  [36,37]. b e national technical regulation on the limits for industrial wastewater (QCVN 40:2011/BTNMT) discharging into the water sources not serving tap water supply [36,37]. c Required percentage removals to ensure national technical regulation on the limits for industrial wastewater.    low pH, the bridging mechanism through hydrogen interaction would also be involved in the coagulation-flocculation process. is interaction can be driven via hydrogen bonding between the hydroxyl groups of the coagulant and the N−, O−, and/or S−containing groups of the dyes.

Conclusions
e C. fistula-derived coagulant was successfully formed with rough and irregular surface. e screening experiments at bench-scale revealed that the removal efficiency was strongly dependent on coagulant dosage, initial pH, and pollutant concentration. e pilot-scale study operated to simulate the full-scale treatment revealed that the coagulation-flocculation process using C. fistula seed gum was an effective primary method with the percentage removal of 93.83%. e mechanism of dye-based wastewater treatment using C. fistula seed gum is mainly driven by adsorption and bridging interaction via hydrogen bonding and electrostatic attraction between opposite charges of the coagulant and dyes.
is study indicates that natural coagulant derived from C. fistula seed exhibited a great potential for real domestic textile wastewater treatment. It can be developed to provide an economical, efficient, and environmentally friendly wastewater treatment.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare no conflicts of interest.