A characteristic study of Zea mays L. (sweet corn) cobs for synthetic dye degradation from aqueous media

The dyeing process is commonly utilized to color products and become their basic part at an industrial level. Majorly, waste waters of industries containing dyestuffs cause various environmental threats as well as damage to human health (Singh & Singh 2019). Synthetic dyes are extensively used in many technologies such as textiles, printing, paper making, leather, rubber, food processing, plastics, photography and pharmaceuticals. Round about 12% of the synthetic dyes produced annually are believed to be lost during manufacturing and processing operations and 20% of these dyes lost enter the industrial waste water. The high solubility of dyes in water results in their wide dissemination into the environment, causing drastic effects on crops, aquatic life and also to human health. The increase in occurrence of many synthetic and organic dyes/substances in natural (drinking) water has led to the importance of its purification through various techniques. Adsorption is one of the effective methods for water purification (Adeyemo et al. 2017). To overcome the contaminated water issues, different biological and physiochemical treatments are used: coagulation/flocculation, chemical precipitation, chemical oxidation, membrane processes, electrochemical and microbial degradation, reverse osmosis, photochemical oxidation and adsorption. (Ali et al. 2014; Cheruiyot et al. 2019) Among all these purification techniques, adsorption is a major industrial separation technique/method to refine effluent media. In liquid phase, it is a most efficient and less time-taking method that has been easily applied to dye removal (Rajasekhar 2014; Adeyemo et al. 2017). Various low cost adsorbents such as clay (Adeyemo et al. 2017), flyash, silica, synthetic composites (Jiang et al. 2015; Janani et al. 2019), also modified forms of wastes, have been used for dye removal but agricultural waste such as rice husk, bio-char (Wathukarage et al. 2017), Bukhulderia (Li et al. 2014), wheat bran (Sujata et al. 2019), Atrocarpous sawdust (Linda et al. 2015), banana peel, peanut hull (Tahir et al. 2016), corn stalk (Usman et al. 2019), Maracon peel (Youssef et al. 2017) and other bio-wastes were proved to be low cost sorbents for cationic and anionic dye removal (Hemant et al. 2016). Among these agriculture bio-wastes, Zea mays L. (sweet corn) cobs (ZMLC) have great potential and physiochemical properties for synthetic dye removal and serve as an effective adsorbent in aqueous media. ZMLC provides high surface area for elimination of cationic dyes (Aljeboree et al. 2019). Gentian violet (the dye discussed here) has a carcinogenic and mutagenic nature affecting both aquatic and land ecosystem. So, in degradation, de-colorization is an important consideration. In the present study, ZMLC, due to its good ability, is used for gentian violet elimination from a dye-contaminated aqueous environment. Different operations were examined that behaves as variable parameters: dye concentration, shaking time and temperature factors for adsorption treatment. The adsorbent surface was characterized by scanning electron microscopy (SEM), Fourier transform infra-red spectrometry (FT-IR) and pH at point zero charge (pH_pzc). Thermodynamics show the process is endothermic and spontaneous. Further isothermal adsorption assessment is also reported below. It is worth declaring that no previous works have addressed the biosorption of gentian violet from aqueous solutions in the following reported study.

Zea mays L. (sweet corn) cobs were collected from local farms of Sahiwal (Pakistan), washed properly to remove dirt and subsequently dried an oven operated at 100 °C for 20 hrs. Dried ZMLC was then pulverized and screened to a particle size of 200 μm. The raw ZMLC was stored in an airtight container for additional subsequent use.

Stock solution of gentian violet was prepared (1,000 mg/L) by dissolving the 1.0 g dye quantity of A.R. grade in double-distilled water. The sample solutions of different concentrations ranging from 200 to 900 mg/L were prepared by diluting a stock solution to the desired concentrations. The pH of the adsorption experiments was maintained through mineral acid and mineral base (NaOH and KOH) standard solutions. The λ_max.abs of the stock solution of gentian violet 575 nm was monitored by visible-spectrophotometer (721). The characteristic structure of gentian violet is shown in Figure 1, respectively.

The adsorbent (ZMLC) was characterized by pH at point zero charge (pH_pzc) by pH drift method, FT-IR (FT-IR 2000, PerkinElmer): fine grained material of ZMLC mixed with KBr (Merck) by 1:100 for transparent granules, and to determine the surface porosity of ZMLC, SEM analysis (JEOL-JSM-6380 LA, Japan) (Aljeboree & Ayad 2019); to observe the adsorption behavior of gentian violet towards the adsorbent a visible spectrophotometer-721 was used.

The higher the value of regression coefficient and lower SSE error values, the more reliable the model. The values of SSE error are displayed in Tables 2 and 3 with R² values. According to SSE and R² values, Temkin's isotherm and pseudo-second order kinetics are the best models to represent the adsorption experimental data.

The results of the mineral constitution of ZMLC appear in Table 1. ZMLC contained the minerals Si, K, Fe, Al and P in high proportion. These minerals contribute to defining the hydrophilic identity of ZMLC. The physiological properties of ZMLC determined that the moisture content is less than 5%, which is more favorable for the acceleration of adsorption experiments (Nwadiogbu et al. 2015).

To explore pH at point zero charge (pH_pzc) on surface of ZMLC was found to be 3.15 (Figure 2). pH_pzc was determined by using the pH-drift method (Nasiruddin & Sarwar 2007). At 3.15 pH, the net surface charge of ZMLC becomes zero and ZMLC provides more electro-chemical potential for gentian violet adsorption in bulk fluid form. The ZMLC surface exhibits a relationship between pH_pzc and adsorption capacity of adsorbent and highest percentage dye adsorption takes place at 3.15 pH. The results show that sorption of gentian violet is more beneficent at pH less than point of zero charge (1–3 pH), while it is less favorable higher than pH_pzc (4–12 pH). This behavior investigates that, due to greater electrostatic force of attraction between ZMLC and oxygen containing surface functional groups, gentian violet molecules rapidly adjusted over the adsorbent surface. Whereas, at pH > pH_pzc adsorption was lewer due to less attraction between protonated oxygen of the ZMLC surface and the dye (Jawad et al. 2018).

The pattern for adsorption of gentian violet onto ZMLC due to the availability of active binding sites and bonds on the adsorbent surface was examined by FT-IR spectral analysis. The adsorbent was chemically composed of lignin, cellulose, hemicelluloses and plentiful collections of hydroxyls like tannins (Oluyori et al. 2018; Qaisar et al. 2019). The lignocellulose portion of the adsorbent follows the ion-exchange mechanism with respect to the corresponding dye (gentian violet). The hydrocarbon sites actively react with the cationic portion of the dye (Sobhy & Mahmoud 2015). FT-IR spectra of native ZMLC source (a) and loaded with gentian violet dye (b) illustrated by Figure 3 to various functional groups with respect to wavenumber (cm⁻¹). The peaks at 3,750 and 3,772 cm⁻¹ symbolized the –OH phenolic group growing from cellulose and the lignin portion of ZMLC. At 2,800 cm⁻¹ the peak that expressed the stretching vibrations of N-H and O-H functional groups indicates the presence of nitrogen in the methylene and hydrocarbons in ZMLC surface. The 1,810 and 1,780 cm⁻¹ peaks show the existence of –CH₂ of the aliphatic portion randomly distributed on the ZMLC surface with C = O bond stretching. The bands between 1,600–920 cm⁻¹ give stretching of C = C bonds of alkenes, alkynes and olifenes. The given peaks in Figure 3 are the result of functional groups, stretching behavior, collection of dye and also proposed the density location of gentian violet molecules on the ZMLC surface. The adsorption of the ZMLC surface area exhibits continuous spectrum peaks in FT-IR analysis demonstrating that adsorption is due to the electrostatic force of attraction that is the result of physiosorption, which may be weak or strong, no chemical correlation proceeded. (Aljeboree et al. 2019) Trough regions appeared as the system absorbs energy from the surroundings but the FT-IR spectra of the ZMLC surface revealed that it requires less energy for adsorption, thus there is less difference between native and loaded ZMLC peaks. Moreover, after gentian violet adsorption the presence of new IR-bands is due to many functional groups undergoing red or blue shifting (Figure 3(b)) in the finger print region and proposes the presence of dye molecules adsorbed on the ZMLC surface.

Scanning electron microscopy provides information about the surface morphology of ZMLC and also gives the fundamental physical features. An SEM diagram of the adsorbent (ZMLC) was examined (Figure 4). The good pore size ability before adsorption (Figure 4(a)) and loaded with dye (Figure 4(b)) after equilibrium show the pore filling by the adsorbent. Figure 4(a) determines that the ZMLC surface is crowded with hoops and lumps that are clearly visible, then after equilibrium (Figure 4(b)) dye molecules have created a film-like mass on the surface of the adsorbent that hides the porosity. The resulting pore structures of the ZMLC provide suitable surface area for gentian violet removal from aqueous media (Aljeboree & Ayad 2019).

pH is an abundant property that controls the whole adsorption process and adsorption capacity of the adsorbent. It is not only affecting the surface charge of the adsorbent but also the degree of ionization of dye molecules present in solution and dissociation of active bonding site functional groups on ZMLC. It is observed that cationic dyes are absorbed quickly at lower pH due to the positively charged adsorbent surface. In the present adsorption system, pH is measured from 1 to 12; the adsorption capacity for gentian violet is highest at 3.0 pH as shown in Figure 5. The adsorption is more favorable at pH_pzc of ZMLC due to the massive electrostatic attractive force between the dye and the negatively protonated charged surface of the cobs.

Influence of shaking time on adsorption of gentian violet with ZMLC was measured at three different dye concentration: 600, 700, 800 mg/L, 3.0 pH, 0.6 g ZMLC amount and shaking time varying from 5 to 120 min (Figure 6). The time duration at which the adsorption culture medium attains equilibrium was measured, after which no further more effective percentage of dye uptake occurs. 30 min equilibrium time was determined from the plot of dye uptake versus contact time (min) with equilibrium sorption capacity. From the results, 700 mg/L gentian violet at 30 min contact time serve as better conditions for dye removal in less time in bulk fluid form. This contact time is very important for the highest dye uptake from aqueous media, even from wastewater at the industrial level. Figure 6 explains the relationship between adsorption capacity and contact time at different concentration in bulk form. However, adsorption capacity remains constant throughout variable contact time durations due to similar availability of active sites of ZMLC in the adsorption culture medium but percentage adsorption varies from 78 to 96% for 600 mg/L, 76–95.5% for 700 mg/L and 76.2–93.8% in the case of 800 mg/L concentration. This explains that even at very high concentration of gentian violet in adsorptive medium, ZMLC provides an actively large surface area for dye extraction from aqueous medium.

The adsorption equilibrium data related to dye concentration (200–1,000 mg/L) at three variable temperatures 283, 293, 303 K was further applied to the four adsorption isothermal models: Langmuir, Freundlich, Temkin, Dubinin-Radushkevich, respectively. Freundlich and Temkin's isotherm proved to be the best adsorption isotherms related to experimental data on the basis of the R² and SSE error values given in Tables 2 and 3.

The values of mean free energy are given in Table 2. From these values it is concluded that the heterogeneous, macro porous ZMLC surface on gentian violet sorption provides physiosorption adsorption mechanism properties of adsorbent.

The plotted graphs at variable concentrations revealed that the pseudo-second order model is the more defined model for gentian violet adsorption on ZMLC on the basis of R² values.

The adsorption of gentian violet dye on ZMLC at 700 mg/L dye concentration was carried out using optimized conditions at specific designed temperatures varying from 273 to 333 K and the results are shown in Figure 14 (Appendix). It was observed that dye adsorption decreases at higher temperatures such as at 323 and 333 K.

H^o was calculated by plotting the graph between ln K_c versus 1/T as shown in Figure 15 (Appendix) and the values of thermodynamic parameters are presented in Table 5. The positive values of Gibbs free energy indicate that the thermodynamic process for gentian violet removal is non-spontaneous in nature and endothermic due to positive values of enthalpy change. Moreover, the increase in temperature and increase in numeric values of proposed that adsorption of gentian violet on ZMLC surface is favorable at both lower as well as higher temperatures. Similarly, negative values of entropy change represent decrease in the random degree of freedom of dye molecules.

0.1–1.0 g of adsorbent weight was examined by keeping other batch parameters constant. At first, percentage adsorption increases rapidly because of accessibility of ZMLC porous sites during the course of experimental determination. After 0.6 g, no further gentian violet was removed from dye solution and was said to be constant. This phenomenon demonstrated the lower availability of active bonding sites of ZMLC for the same concentration of dye in bulk form. Figure 16 (Appendix) proposes the characteristic effect of ZMLC weight to remove excess amount of gentian violet in an adsorption medium.

Reusability of the adsorbent is an important feature of waste water treatment. Experimental analysis was performed to study desorption from ZMLC pre-adsorbed surface by using basic media. Mineral base potassium hydroxide (KOH) solution with variable normality was utilized in batch medium. Results are placed in Table 6, which determine that an increase in base concentration increases the percentage of desorption from pre-treated ZMLC with gentian violet and the highest value of percentage desorption was 92.5, respectively.

The application of the developed adsorption procedure on tap water was examined to remove 700 mg/L gentian violet concentration by applying optimized batch conditions. From the results (Table 7), 84–96.5% adsorption efficiency was recorded and adsorption equilibrium was attained at 30 min shaking time duration. On the basis of this study, it is concluded that abundantly available ZMLC shows maximum potential for gentian violet removal from aqueous medium even from perforated water, which can also be used for safe ejection of highly toxic synthetic dye from industrial effluents.

ZMLC has effective and suitable adsorption characteristics for the removal of hazardous, carcinogenic synthetic dye gentian violet from an aqueous environment. The adsorption experimental data were well fitted to Freundlich, Temkin's as well as the D-R isotherm on the basis of regression coefficient analysis at three distinctive temperatures 283 K (0.995, 0.991, 0.985), 293 K (0.970, 0.912, 0.942), 303 K (0.980, 0.96, 0.958) and the SSE values show inverse behavior in accordance with each isothermal R² value. Dye adsorption on ZMLC well obeyed the pseudo-second order kinetic model rate expression on a regression coefficient basis at three concordant concentrations 600, 700 and 800, having 1.0 value at each concentration of adsorption replication and SSE shows values in the range of 10⁻⁹ to 10⁻¹¹, which is an insignificant error. The dye uptake slightly decreases at higher temperatures (323, 333 K) and thermodynamics justified that the adsorption mechanism is endothermic due to positive values of enthalpy change. On the basis of this analysis, it is concluded that ZMLC, an abundantly available agriculture waste, proved to be an eco-friendly, more efficient and less cost adsorbent with significant potential for removal of gentian violet in aqueous media even from tap and polluted waste water effluents and can be recycled in a basic environment.

The authors wish to acknowledge the efforts of Department of Chemistry, University of Sahiwal, Pakistan and the supervisor for providing the reagents and apparatus used during the course of this assessed work.

There is no conflict of interest by the authors.

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