1 Introduction

The most essential and leading solvent used in textile industries is water. Textile industries are water-intensive in each of their production steps and thus, consume enormous quantities of water. Water is used mainly as a solvent for chemicals and dyes, as a means to transfer dyes and chemicals to fabric, and also as a cleansing and rinsing medium [1]. Textile factories in India alone consume around 425,000,000 gallons of water daily to produce clothing materials. But, more importantly, these factories release 90–95% of used water as effluents at the end that are frequently dumped direct into nearby waterways [2]. While this being a just a figure, perhaps the larger problem lies in the inconsiderable pollution of water, particularly in the developing countries. Inadequate facilities for effluent discharge and a lack of strict regulations by governmental organizations have led to water contamination to a vast extent [3]. Textile wastewaters have a wide range of pollutants that contaminate external water bodies. These pollutants include non-biodegradable organics and inorganics such as dyes, metals, surfactants, phenols, pesticides, etc. [4]. Among these, dyes are the most dangerous chemicals that have been adversely affecting the environment. The physico-chemical effects of dyes in water are: (i) increased concentration of total suspended solids (TSS), (ii) elevated levels of biochemical oxygen demand (BOD) and chemical oxygen demand (COD), (iii) high level of total dissolved solids (TDS) (iv) change in pH (v) colored water, and (vi) unpleasant pungent odor [5,6,7]. Increased levels of COD and BOD in textile effluents lead to the death of many aquatic life due to the depletion of oxygen. Higher TDS and TSS levels block the entry of light, thereby deteriorating water quality and making it unfit for plants and animals’ survival. This contamination has also led to increased rates of cancer, respiratory diseases, and skin infections among workers working in textile industries [8, 9].

Acid violet 90 (AV) is an aromatic azo compound that is formed when the aryl diazonium cation is converted to another aryl ring that contains an electron-releasing group [10]. The diazonium salts are more stable when the coupling reactions are carried out at lower temperatures. Azo dyes derived from oxidation of hydrazines and benzidines are carcinogenic, and mutagenic and lead to severe human organ damage [11, 12]. These dyes are metal-complex dyes formed by the combination of a metal ion (usually chromium, nickel, cobalt, iron, and copper) with a single or double dye molecules. The high water-solubility of these dyes makes their removal quite difficult [13]. Therefore, azo dye pollution is a serious problem that needs to be addressed urgently [14, 15]. Several methods have been employed to remove azo dyes from effluents but these are outdated, cost-ineffective, generate excess toxic sludge, have high maintenance costs, huge energy, and reagent requirements, and often lead to incomplete removal of contaminants, etc.

Adsorption is a widely used technique which is a more rapid and efficient process and also has the ability to remove a wide range of organic and inorganic compounds from water [16,17,18,19]. However, the high cost of chemical adsorbents and the amount of toxic substances they release has encouraged researchers to find viable alternatives that are cheaper, natural, biodegradable, biocompatible, and less harmful.

Bio-adsorbents are non-conventional adsorbents derived from biological materials for the effective removal of organic and inorganic pollutants from water. For dye removal various biosorbents have been used so far such as wheat flour, Graham flour, cellulose, Penaeus indicus shrimps, Ganoderma lucidum, Spirulina platensis, etc. [20]. Based on this rationale, in this work, we reported the adsorption properties of Zingiber officinale powder for the removal of acid violet 90 dye from aqueous solutions. Ginger (Zingiber officinale) belongs to the family of Zingiberaceae and is the most commonly consumed dietary condiments in the world [21]. It is also widely used as a traditional herbal medicine and an additive to enhance the taste of food. India is the largest producer of ginger with its production greater than 275,000 million tonnes per year [22]. Due to its worldwide and substantial availability, its various parts are used extensively in the research field such as its extract containing several water-soluble phyto-constituents is used for synthesizing nanoparticles viz., AgNPs [23], and its treated waste material is used as a biosorbent for removal of dyes [22, 24,25,26]. In this regard, we investigated the ginger powder for the treatment of water containing AV 90 dye. The novelty of this work is that based on the perusal of literaturethere is no study has been reported for the removal of AV 90 dye with ginger root powder. The effects of various experimental parameters such as pH of the solution, initial concentration of dye, contact time, and temperature on the uptake process were investigated through the batch process of adsorption. The non-linear regression method was used to determine the most appropriate isotherm and kinetic models for this system.

2 Materials and methods

2.1 Materials

Zingiber officinale (ginger) was obtained from the local vegetable shop. Acid violet 90 (AV) was supplied from Synthesia Company Czech Republic and was not purified prior to use. All other chemicals such as sodium hydroxide, sodium carbonate, nitric acid, HCl, ethyl alcohol, and acetone were all laboratory-grade chemicals (Merck, Germany) and used as received (Table 1).

Table 1 Chemicals used for adsorption of AV 90 dye onto Zingiber officinale biosorbent
Full size table

2.2 Preparation of Zingiber officinale biosorbent

Ginger (Zingiber officinale) rhizomes/roots were washed several times with distilled water to get rid of any impurities stuck to the roots. The outer thick layer of the rhizomes was removed and was grated with a grater to obtain rhizomes in the form of flakes. The ginger flakes thus obtained were placed on a glass plate and covered with gauze to prevent any sort of contamination. The ginger flakes containing glass plates were kept under natural sunlight for complete dryness. After about three weeks, the ginger flakes get dried completely. The ginger flakes were ground to obtain a powder with different molecular sizes. The ground sample was screened through different size sieves to separate granulates and particles with a size range of 50–125 μm were separated. The root powder was mixed with an excess quantity of distilled water (1:10) under vigorous stirring at 60 °C for 30 min. The extracts were cooled, filtered by Whatman No.1 filter paper to remove the soluble extract, and stored at room temperature for experimental use.

2.3 Characterization of prepared Zingiber officinale biosorbent

The X-ray diffraction (XRD) pattern of the prepared biosorbent was measured in the continuous scanning mode on PANalytical diffractometer system (X’Pert PRO) using Cu K radiation (λ = 1.5406 A0) in the 2θ range of 20 to 60°. To determine the vibrational frequencies of the functional groups attached to the surface of the biosorbent, Fourier transform infrared (FTIR) spectra was recorded on a Perkin–Elmer Spectrum 1000 spectrophotometer over 4000–400 cm−1 using the KBr disk technique. The BET surface area, total pore volume, and average pore radius were measured using the Brunauer-Emmett-Teller (BET) method. The pore size distributions of the samples were obtained by using the Barrett-Joyner-Halenda (BJH) method(QuantachromeTouchWin) (Table 2). To investigate the surface morphological as well as elemental analysis of the prepared biosorbent, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis was performed using Scanning Electron Microscope (JEOL-JSM-5600). The point of zero charges (pHpzc) of the biosorbent was evaluated as follows:100 ml of 0.01N NaCl solutions were prepared and added to eleven (11) different Erlenmeyer flasks. The pH values of the flasks were respectively tuned(from pH 2.0 to pH 12.0) using either 0.01N HCl or 0.01N NaOH solution. The initial pH of the solutions was measured with a pH meter and noted as pH initial. After the constant value of pH initial had been reached, 0.1 g of adsorbent sample was added to each Erlenmeyer flask. The solution pH was measured after 24 h and noted as pH final. Then, a plot of the change in pH, ΔpH (pHfinal-pHinitial)against pH initial was generated and the pHpzc of the sample was marked off at the point where the ΔpH is zero.

Table 2 Physical parameters of Zingiber officinale biosorbent
Full size table

2.4 Batch adsorption experiment

The stock solution of AV 90 dye was prepared by dissolving the requisite amount of dye in distilled water. The solutions of the required concentrations were prepared by dilution of the stock solution with their pH maintained. In an equilibrium experiment, a certain amount (0.02 g) of Zingiber powder and 100 ml of AV 90 solution were placed in a 125 ml glass stoppered flask and stirred in a thermostatic shaker operated at 200 rpm at 30 °C for a pre-defined time. After experiments, the flasks containing samples were withdrawn from the shaker and centrifuged to separate the biosorbent from the solution. The residual concentration of the supernatant solution was measured with a Shimadzu UV/Visible Spectrophotometer. The equilibrium adsorption capacity of dye adsorbed onto adsorbent, qe (mg/g), and percent removal of dye were calculated by the following relationships:

$${q}_e\frac{\left({C}_o-{C}_e\right).V(l)}{W}$$
(1)
$$\mathrm{Removal}\%=\frac{C_o-{C}_e}{C_o}x100\%$$
(2)

where Co and Ce are the initial and final concentrations of AV 90 (mg/L), V is the volume (L) of dye, and W(g) is the weight of the adsorbent. All adsorption experiments were carried out in duplicate and the corresponding mean values were reported.

3 Results and discussion

3.1 Structural and morphological characterization of Zingiber officinalebiosorbent

The presence of various functional groups attached to the surface of native biosorbent prior to the adsorption and post-adsorption was investigated using FT-IR and the corresponding spectrum is presented in Fig. 1. The band at region 3000–3500 cm−1 represents the O−H stretching vibration of the glucose molecule in the cellulose structure. This group is responsible for the formation of either intra- or inter-molecular hydrogen bonds, based on their linkage alignment within the glucose molecule. The native Zingiber officinale sample exhibited an absorption peak at 3269 cm−1, indicating the presence of bonded and free hydroxyl groups, due to the O–H groups’ vibrations (carboxylic acid, phenol, or alcohols) of cellulose, lignin, hemicellulose, and adsorbed water [27]. The peak at 2929 cm−1 is associated with the stretching vibration of the C–H groups [19]. The dual bands at 1634 and 1335 cm−1 correspond to the CH2 stretching of an aromatic ring from the hemicellulose and O-H group from phenolic compounds. The peak at 1057 cm−1 indicates the stretching vibration of C−O groups attributed to carboxylic acid, alcoholic, phenolic, ether, and ester groups of lignin and hemicelluloses components [19]. The minute peak at 848 cm−1 is due to the bending vibration of =C–H groups present on the surface of the Zingiber officinale biosorbent [22, 25, 26, 28].

Fig. 1
figure 1

FTIR of A native Zingiber officinale, B AV 90 dye-loaded Zingiber officinale

Full size image

After the uptake of AV 90 dye, a minor shift in the peaks 3906 to 3935, 3869 to 3896, 3839 to 3844, 3824 to 3793, 3804 to 3761, 3755 to 3703, 3698 to 3666, 1634 to 1620, and 1335 to 1320 cm−1. The slight difference between the pre-and post-biosorption wavenumbers is a consequence of the adsorptive interaction between the acid violet 90 dye molecules and the Zingiber officinale surface [29, 30].

The X-ray diffraction spectra of native Zingiber officinale granules before and after adsorption of Acid violet 90 dye are presented in Fig. 2. The XRD of native Zingiber officinale granules shows characteristic crystallinity peaks of an A-type pattern. The native Zingiber granules had a semi-crystalline structure with sharp diffraction peaks at 15.07°, 17.16, 18.05°, 22.88°, 26.52°, and 39.34° (2θ), which indicated a typical pattern like starch A-type [31,32,33]. The degree of crystallinity was 44%. The inter- and intra-molecular hydrogen bonds were responsible for the crystalline structure [34]. As can be seen in Fig. 2, native Zingiber granules dyed with acid violet 90 showed a similar profile to native ones, but the diffraction peaks at 17.16 and 22.88 were split in addition to a disappearance of the diffraction peak at 39.30° (2θ).

Fig. 2
figure 2

XRD diffractogram of native Zingiber officinale and AV 90 dye-loaded Zingiber officinale

Full size image

For the topographical aspect of the biosorbent before and after adsorption, electron microscopic (SEM) analysis was carried out and the corresponding micrographs have been presented in Fig. 3. The SEM pictures of native Zingiber officinale biosorbent revealed the irregular and rough surface having prominent pores. These pores get filled with the AV 90 dye molecules upon uptake from the solution displaying the adherence of dye molecules on the surface of the Zingiber officinale biosorbent. On comparing the EDX spectra of the biosorbent before and after the uptake process (Fig. 4), the presence of new peaks for the elements S and Cr that are the constituents of the AV 90 dye confirmed the adsorption of dye molecules over the surface of Zingiber officinale biosorbent.

Fig. 3
figure 3

SEM images of a native and b AV 90 dye-loaded Zingibero fficinale biosorbent

Full size image
Fig. 4
figure 4

EDX images of a native and b AV 90 dye-loaded Zingiber officinale biosorbent

Full size image

The determination of pHzpc is helpful in the evaluation of the nature of the adsorbent surface and the adsorption of contaminants over the surface as pH >pHzpc favors the uptake of cationic species and the reverse favors the anionic species from the aqueous medium. The pHzpc of the Zingiber officinale biosorbent was found as 7.8 (figure not shown).

3.2 Application of Zingiber officinale biosorbent for adsorption of acid violet

The prepared Zingiber officinale biosorbent was employed for the removal of AV 90 dye from the aqueous solutions and the effect of various parameters governing the adsorption process is investigated and discussed in detail in the subsequent sections.

3.2.1 Effect of initial pH on adsorption capacity

The aqueous chemistry (i.e., the surface charge of adsorbate and adsorbent, degree of ionization of adsorbate in the solution, and dissociation of functional groups present on the adsorbent surface) and the binding capacity of the adsorbent towards the adsorbate is entirely governed by the pH of the solution. Thus, solution pH plays an important role in any adsorption process. The effect of pH was studied by conducting the experiment with 100 mL of initial AV concentration of 30 mg/L, adsorbent dose of 0.2 g/L, contact time of 2 h, and varying pH from 2.0 to 10.0 at the temperature of 30 °C. The effect of the initial solution pH on the adsorption of AV is shown in Fig. 5.

Fig. 5
figure 5

Effect of pH on adsorption capacity of Zingiber officinale biosorbent for AV 90 dye (reaction conditions: dye conc.=30 mg/l; adsorbent conc.= 0.2 g/L; contact time =2h; temperature= 30 °C; particle size range= 50−125 μm)

Full size image

The result showed that higher adsorption capacity was recorded at acidic pH. Meanwhile, the adsorption capacity progressively abated with a decrease in the acidity of the adsorbate solution This observation is explained by the fact that at decreasing acidity, the electrostatic repulsion between the anionic character of the Acid violet 90 dye and those on the adsorbent surface [35, 36].

It is evident from the figure that the amount of dye adsorbed on the surface of the Zingiber biosorbent declined with the elevation in the pH of the solution. This behavior can be explained by taking into account the pHzpc of the adsorbent as well as the solution pH. At lower solution pH (pH<pHzpc) the surface of the adsorbent would be positively charged due to the protonation of surface hydroxyl groups facilitating the dye adsorption due to electrostatic attraction between the dye molecules and the biosorbent. However, when the pH of the solution increases (pH >pHzpc) the adsorbent surface become negatively charged, and thus the adsorption of AV which is an anionic dye diminished resulting in a decline in the adsorption rate [37, 38]. As the adsorption was maximum at pH 2.0 hence it was regarded as an optimum pH and all experiments were carried out by maintaining this pH.

3.2.2 Effect of adsorbent dose on adsorption capacity

The amount of adsorbent is another important parameter that affects the adsorption process. To assess the effect of the adsorbent dose on the adsorption of AV, the experiment was conducted by varying the dose from 0.2 to 2 g/L and keeping other experimental conditions constant (Fig. 6).

Fig. 6
figure 6

Effect of adsorbent dose on adsorption capacity of Zingiber officinale biosorbent for AV 90 dye (reaction conditions: dye conc.= 30 mg/L; pH = 2; contact time =2h; temperature= 30 °C; particle size range= 50−125 μm)

Full size image

The figure shows that the dye uptake decreases with an increase in adsorbent concentration. At lower concentrations of adsorbent, the number of active sites available on the surface is higher while an increase in concentration leads to the filling up of the pores of the adsorbent leading to aggregation of particles and thereby decreasing the available surface area for dye uptake. The optimum adsorbent dose was estimated as 0.2 g/L and it was maintained throughout the experiments. A similar pattern of an inverse relationship between the adsorbent concentration and adsorption capacity was previously reported [39, 40].

3.2.3 Effect of contact time and initial concentration on adsorption capacity

The uptake of any pollutant is highly dependent upon its initial concentration in the aqueous medium. The adsorption capacity of biosorbent as a function of contact time with two different initial dye concentrations (20 mg/L and 40 mg/L) is shown in Fig. 7.

Fig. 7
figure 7

Effect of contact time and initial dye concentration on adsorption capacity of Zingiber officinale biosorbent for AV 90 dye (reaction conditions: adsorbent conc. = 0.2 g/L; pH = 2; temperature= 30 °C; particle size range= 50−125 μm)

Full size image

It was observed that the rate of uptake was quick in the beginning up to 10 min and becomes steady thereafter and finally adsorption equilibrium was attained within 10 min. The higher availability of active binding sites on the biosorbent surface for the dye molecules resulted in a faster rate of uptake in the beginning which get saturated in due course of time and uptake slows down in later stages. It was also observed that the adsorption capacity of biosorbent was higher for increased dye concentration (40 mg/L) that may be due to the utilization of all the accessible binding sites for adsorption at higher initial dye concentration [38]. Also, fast kinetics was recorded such that about 26.05 mg/g and 63.16 mg/g of the maximum biosorption capacity obtained at 20 and 40 mg/L initial dye concentration, respectively was achieved within the first 10 min.

3.2.4 Effect of temperature on adsorption capacity

To examine the effect of temperature on the adsorption capacity of biosorbent, the experiment was performed in different temperature zones from 30 to 70 °C with an initial AV 90 dye concentration of 20 mg/L and keeping other experimental conditions constant (Fig. 8).

Fig. 8
figure 8

Effect of temperature on adsorption capacity of Zingiber officinale biosorbent for AV 90 dye (reaction conditions: dye conc. = 20 mg/L; adsorbent conc. = 0.2 g/L; pH = 2; particle size range= 50−125 μm)

Full size image

It was observed that the AV 90 dye uptake on the biosorbent declines with the rise in temperature suggesting the exothermic nature of the uptake process. This trend can be accounted for by the fact that higher temperature weakens the vander Waals’ force and the H-bonding between the adsorbate and the adsorbent resulting in decreased interaction and thus decreased uptake. For the present system, the maximum uptake of AV 90 onto Zingiber officinale biosorbent was achieved at 30 °C. Similar findings were also reported by other research groups that low temperatures favored the uptake of the dye onto various materials [38, 41].

3.3 Thermodynamic analysis for uptake of AV 90 onto Zingiber officinale biosorbent

In order to explore the nature and feasibility of uptake of AV 90 dye onto Zingiber officinale biosorbent, thermodynamic parameters viz., Gibb’s free energy change (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were determined at various pre-defined temperatures. The Gibb’s free energy change, ΔG° was calculated using Eq. (3), the equilibrium constant was estimated by Eq. (4), and the values of enthalpy change, ΔH° and entropy change, ΔS° of the adsorption process was determined from the slope and intercept of the Van’t Hoff plot Eq. (5) [42, 43].

$$\varDelta {G}^{{}^{\circ}}=- RTlnK{{}^{\circ}}_e$$
(3)
$${K}_e^0=\frac{1000\times {K}_L\times Molecular\; Weight\; of\; the\; Adsorbate\times {\left[ Adsorbate\right]}^0}{\upgamma}$$
(4)
$$\mathit{\ln}\;{K}_e^0=\frac{\varDelta {S}^0}{R}-\frac{\varDelta {H}^0}{RT}$$
(5)

where, \({K}_e^0\) is the thermodynamic equilibrium constant, [Adsorbate]0 is the standard concentration of adsorbate in mol/L, γ is the activity coefficient, KL is K of best-fitted isotherm model, T is the absolute temperature in K and R is the universal gas constant (8.314 J/mol K). The thermodynamic parameters and their corresponding values are tabulated in Table 3.

Table 3 Thermodynamic parameters for adsorption of AV 90 dye onto Zingiber officinale biosorbent at different temperatures
Full size table

The negative values of ΔG° at all the temperatures confirmed the feasibility and spontaneity of the uptake process of AV 90 dye onto Zingiber officinale biosorbent. The negative value of ΔH°(−409.59 kJ/mol) indicated the exothermic nature of the uptake process. The negative value of ΔS° (−47.42 J/K/mol) indicated the good affinity of AV 90 dye molecules towards the Zingiber biosorbent and increased disorderness on the biosorbent surface after the uptake process. The findings reported in this study were also supported by other research groups for the removal of dyes with various materials [44, 45].

3.4 Adsorption isotherms modeling for uptake of AV 90 onto Zingiber officinale biosorbent

Adsorption isotherms explain the forces governing the interaction between the adsorbate and the adsorbent and therefore play an important role in designing any adsorption system. The equilibrium data of the uptake process of AV 90 onto Zingiber biosorbent was analyzed with two parameter isotherms viz., Langmuir, Freundlich, and Tempkin; and three-parameter isotherms viz., Sips and Toth isotherm models to get the most appropriate model explaining the equilibrium data [45].

3.4.1 Langmuir isotherm model

It describes the monolayer adsorption of adsorbate on the surface of adsorbent at finite active sites without having any sort of interaction amid adsorbed molecules. Once the saturation is reached, no further adsorption takes place [46, 47]. The non-linear form of the Langmuir equation is as follows:

$${q}_e=\frac{q_0b{C}_e}{\left(1+b{C}_e\right)}$$
(6)

where, qe is the amount of dye adsorbed onto Zingiber officinale biosorbent at equilibrium (mg/g) and Ce is the equilibrium concentration of dye in the solution (mg/L), q0(maximum adsorption capacity; mg/g) and b (adsorption energy; L/mg) are Langmuir constants.

The values of the Langmuir isotherm constant, aL (l/mg), and the Langmuir constant, KL (l/g) are 0.034 and 11., respectively indicating a reasonable adsorbent-adsorbate interaction [40, 41].

3.4.2 Freundlich isotherm model

It describes the multilayer adsorption of adsorbate over the heterogenous surface of adsorbent having the non-uniform distribution of heat of adsorption [48]. The non-linear Freundlich equation is used in the following form:

$${q}_e={K}_f{C}_e^{{~}^{1}\!\left/ \!{~}_{n}\right.}$$
(7)

where, qe is the amount of dye adsorbed onto Zingiber officinale biosorbent at equilibrium (mg/g) and Ce is the equilibrium concentration of dye in the solution (mg/L), Kf (adsorption capacity of the adsorbent; mg/g (L/mg)1/n and n (energy of adsorption; g/L) are Freundlich constants.

The value of n, was 1.55, indicating that the adsorption of acid violet 90 ions onto Zingiber officinale is favorable; however, a value of 1/n < 1 suggests a slight suppression of adsorption at lower equilibrium concentrations.

3.4.3 Sips isotherm model

This model is a combination of the Langmuir and the Freundlich isotherm models and is expected to effectively explain the heterogeneous surfaces. Based on the concentration of the adsorbent it takes up the form of either of the two models. At low concentrations, it follows Freundlich isotherm, whereas, it reduces to Langmuir isotherm at higher concentrations. The non-linear form of the equation is given as [49, 50]:

$${q}_e=\frac{q_m{\left({K}_S{C}_e\right)}^{m_s}}{1+{\left({K}_S{C}_e\right)}^{m_s}}$$
(8)

where, qe is the amount of adsorbed onto Zingiber officinale biosorbent at equilibrium (mg/g) and Ce is the equilibrium concentration of dye in the solution (mg/L),qm is the maximum adsorption capacity (mg/g), KS is the Sips equilibrium constant related to adsorption energy and ms is Sips model exponent characterizing the heterogeneity of the system.

3.4.4 Toth isotherm model

It is useful in describing the adsorption processes in heterogeneous systems. This model assumes that the most of the adsorption sites on the surface of the adsorbent have less adsorption energy than the maximum adsorption energy [50, 51].

$$\frac{q_e={q}_T{K}_T{C}_e}{{\left[{\left(1+{K}_T{C}_e\right)}^{\beta}\right]}^{{~}^{1}\!\left/ \!{~}_{\beta }\right.}}$$
(9)

where, qe is the amount of dye adsorbed onto Zingiber officinale biosorbent at equilibrium (mg/g) and Ce is the equilibrium concentration of dye in the solution (mg/L),qT(the maximum monolayer adsorption capacity; (mg/g) and KT(the adsorption energy; L/mg) are the Toth constants.

The comparison between experimental isotherm data and the theoretical non-linear fitting of isotherm plots for two parameter isotherms (Langmuir, Freundlich, and Tempkin); and three-parameter isotherms (Sips and Toth) for the uptake of AV 90 dye onto the Zingiber biosorbent are shown in Fig. 9(a−b). The estimated values of two and three isotherm parameters coupled with their coefficient of determination and error functions with the least normalized sum are depicted in Table 4.

Fig. 9
figure 9

Comparison between a two-parameter models and experimental data and b three-parameter models and experimental data obtained for adsorption of AV 90 onto Zingiber officinale biosorbent (reaction conditions: adsorbent conc. = 0.2 g/L; pH = 2; temperature= 30 oC; particle size range= 50−125 μm)

Full size image
Table 4 Isotherm constants and error analysis of two-parameter models for the adsorption of AV 90 dye onto Zingiber officinale biosorbent at 30 °C
Full size table

3.5 Adsorption kinetics for uptake of AV 90 dye onto Zingiber officinale biosorbent

The kinetic analysis of any uptake process furnishes information regarding the rate-determining pathway (chemical reaction, diffusion, and mass transfer), the surface adsorption rate of the adsorbate to the adsorbent, the quantity of species adsorbed (adsorption capacity) and its applicability at industrial scale. The kinetic equations, pseudo-first-order (PFO), pseudo-second-order (PSO), intra-particle diffusion model, and Bangham model were applied to the experimental kinetic data to get the best fit kinetic model [52,53,54].

3.5.1 Pseudo-first-order and pseudo-second-order kinetic models

The pseudo-first-order model assumes the kinetics of the adsorption process to be physical in nature and diffusion-controlled whereas, the pseudo-second-order model assumes that the rate of adsorption is chemical in nature i.e., controlled by sharing of electrons between the adsorbate and the adsorbent [55].

Then on-linear expressions for pseudo-first-order and pseudo-second-order are outlined as follows:

$${q}_t={q}_e\left(1-\mathit{\exp}\left(-{k}_1t\right)\right)$$
(10)
$${q}_t=\frac{k_2{q}_e^2t}{1}+{k}_2{q}_e^t$$
(11)

where,qe and qt(mg/g) are the amounts of dye adsorbed on Zingiber biosorbent at equilibrium and at any time, t, respectively, k1 (min-1) and k2 (g/mg/min) are rate constants of pseudo-first-order and pseudo-second-order model equations.

It is clear from the data in Table 5 that the pseudo-first-order and pseudo-second-order models suitably described the kinetic data at both initial dye concentrations. Upon further inspection, it is found that the values of the rate constants (k1 and k2) for both models decrease with increasing dye concentration. According to Hameed and Khairy [56], the larger the rate constant, the shorter the time required to attain a fractional uptake within the biosorption system. Hence, the attainment of specific fractional uptake was faster in 20 mg/L than in the 40 mg/L initial dye concentration system in both models. In this regard, two opposing effects can be safely opined regarding an increasing initial dye concentration, namely, the higher the initial concentration, the more the competition between the dye molecules for vacant sites, and the better the rate of solute mass transfer across the biosorbent interface.

Table 5 Isotherm constants and error analysis of three-parameter models for the adsorption of AV 90 dye onto Zingiber officinale biosorbent at 30 °C
Full size table

3.5.2 Intraparticle diffusion kinetic model

The determination of the rate controlling/determining step in the uptake process improves the understanding of the mechanism involved in the adsorption of adsorbate by the adsorbent. In any adsorption process there are three definite steps that describe the rate (1) intraparticle or pore diffusion, where adsorbate molecules diffused into the interior of the adsorbent, (2) liquid film or surface diffusion, where the adsorbate is transported from the bulk solution to the external surface of the adsorbent, and (3) adsorption on the interior sites of the sorbent. Thus, the kinetic data of the experiments were also analyzed by the intraparticle diffusion model as follows [53, 55]:

$${q}_t={k}_{id}{t}^{0.5}+C$$
(12)

where, qt is the adsorption capacity at any time, t (min), C (mg/g) is the intraparticle diffusion constant related to the boundary layer thickness and kid is the diffusion rate constant (mg/g min1/2). The regression of qt versus t0.5 was not linear and did not pass through the origin shows that intraparticle diffusion is not the sole rate-determining step (Fig. 10). Probably, some other mechanisms such as surface as well as external diffusion are responsible for the uptake of AV 90 dye onto Zingiber officinale biosorbent.

Fig. 10
figure 10

Comparison between intra-particle model and experimental data obtained for adsorption of AV 90 onto Zingiber officinale biosorbent (reaction conditions: adsorbent conc. = 0.2 g/L; pH = 2; temperature= 30 oC; particle size range= 50–125 μm])

Full size image

3.5.3 Bangham kinetic model

To evaluate the possibility of adsorption to be pore-diffusion controlled the kinetic data was analyzed with Bangham’s pore diffusion model [50, 54]:

$${q}_t={q}_e\left[1-\mathit{\exp}\left(-{k}_b{t}^n\right)\right]$$
(13)

where, qe and qt (mg/g) are the amounts of dye adsorbed on Zingiber officinale biosorbent at equilibrium and at any time, t, respectively, kb and n are constants.

The comparison between experimental kinetic data and the theoretical non-linear fitting of kinetic plots for the uptake of AV 90 dye onto the Zingiber officinale biosorbent is shown in Fig. 11. The values of estimated kinetic parameters coupled with their coefficient of determination and error functions with the least normalized sum are tabulated in Table 6.

Fig. 11
figure 11

Comparison between kinetic models and experimental data for adsorption of AV 90 onto Zingiber officinale biosorbent (reaction conditions: dye conc.= 20 and 40 mg/L; adsorbent conc. = 0.2 g/L; pH = 2; temperature= 30 oC; particle size range= 50−125 μm)

Full size image
Table 6 Kinetic model constants and error analysis for adsorption of AV 90 dye onto Zingiber officinale biosorbent
Full size table

3.6 Statistical validity of the isotherm and kinetic models

3.6.1 Error analysis

The assessment of best-fit isotherm and kinetic models based on linear regression analysis of experimental data is although the most feasible tool but is associated with inherent biasness. It is also inappropriate for isotherm models having more than two parameters. Hence, the non-linear regression analysis of the experimental data and the data obtained from predicted models is done. This method of analysis employed the error analysis using the solver add-in of Microsoft Excel [52, 54, 57]. During error analysis, the error distribution between the experimental data and the data obtained from predicted isotherm and kinetic models was minimized using error functions to determine the most accurate model for isotherm and kinetic data for this study. In this study, to optimize the isotherm and kinetic parameters the common error functions used were: Average relative error (ARE), Average percentage error (APE%), Sum of Absolute Error(EABS), Sum of Squares of Error (ERRSQ/SSE), Hybrid fractional error function (HYBRID), Marquardt’s Percent Standard Deviation (MPSD) and Nonlinear chi-square test (χ2) (Table 7). The best-fit isotherm model is selected on the basis of a higher coefficient of regression (R2) and lower error function values while for the best-fit kinetic model, the closeness between experimental and theoretical qe values is also considered [54, 57,58,59,60,61,62].

Table 7 List of non-linear error functions used in this study
Full size table

3.6.2 Best fit isotherm model

Upon non-linear regression analysis of all the isotherm models namely Langmuir, Freundlich, Sips, and Toth, the experimental data were best fitted to the Freundlich isotherm model. The value of n in the Freundlich model (Table 4) is 1.546, satisfying the condition of favorable adsorption (i.e., 0 <n< 10). Also, it shows the highest R2 value, as well as the lowest error functions values (Tables 4 and 5) indicating that it gave the best overall fit to the data explaining the equilibrium adsorption of AV 90 dye onto Zingiber officinale biosorbent. Based on the correlation coefficient (R2) and error function values considered in this study, the four isotherm models applied to the experimental data in this study followed the following order of fitness: Freundlich > Sips > Langmuir > Toth.

3.6.3 Best fit kinetic model

The non-linear regression analysis of the kinetic data of AV 90 dye uptake onto Zingiber biosorbent was done using four models namely, pseudo-first-order, pseudo-second-order, intraparticle diffusion, and Bangham. According to the results summarized in Table 6, the correlation coefficient of all the models is close, but the error function values are quite different indicating a better fitting of experimental data to the kinetic model. On the basis of lower values of error functions and the closeness between calculated and theoretical qe values, the pseudo-second-order kinetic model provides the best fitting to the experimental data. It indicated that the adsorption of AV 90 dye was chemical in nature, i.e., chemisorption. The overall fitness of the models based on the results summarized in Table 6, the four models can be ordered as pseudo-second-order > Bangham > intraparticle diffusion > pseudo-first-order.

Further, the plot of intraparticle diffusion (Fig. 10) suggested that the adsorption was not controlled by intraparticle diffusion only, instead surface diffusion and external diffusion also participated in the uptake process. The plateau in the plot suggested slow diffusion of adsorbate into the inner pores of the adsorbent. Furthermore, higher R2 values evaluated from the fitting analysis (Table 5) demonstrated that the uptake process is governed by pore-diffusion. This observation was confirmed by Bangham plot (Fig. 11) where the experimental points are aligned on a line and the correlation coefficient is closer to the unity i.e., (R2 = 0.997).

3.6.4 Proposed mechanism of uptake of AV 90 dye onto Zingiber officinale biosorbent

Understanding the adsorption mechanism of acid violet 90 onto Zingiber officinale makes it easier to remove dyes during wastewater treatment. The removal of acid violet 90 from aqueous solution onto Zingiber officinale may be described by two steps; firstly, electrostatic attraction between the positively charged groups of Zingiber officinale occurs in solution at optimal pH (4.0) and the negatively charged (-SO3-- ) groups of acid violet 90. The second is the hydrogen bonding between the –OH group of Zingiber officinale and the O atom of the acid violet 90 dye molecule. The third is intra-particle diffusion control involving (a) movement of the adsorbate from the bulk solution to the adsorbent surface, (b) diffusion of the adsorbate through the boundary layer to the adsorbent surface, and (c) intra-particle diffusion of the dye molecules into the internal pores of the adsorbent. Finally, pi-pi stacking interactions

3.6.5 Comparison of adsorption capacity of Zingiber officinale biosorbent with other adsorbents for AV 90 dye

The maximum adsorption capacity for acid violet 90 onto Zingiber officinale, according to the Langmuir isotherm model, was 342.69 mg g−1. This relatively large adsorption capacity value is an indication of the strong affinity between the Zingiber officinale and the acid violet 90 molecules [37, 63,64,65,66,67] as compared to the result of the previous studies which are shown in Table 8.

Table 8 Comparison of adsorption capacity of Zingiber officinale biosorbent with other adsorbents for AV 90 dye
Full size table

4 Conclusion

The present study revealed the effectiveness of Zingiber officinale biosorbent for the effective uptake of AV 90 dye from an aqueous medium. The operational parameters have influenced the overall adsorption process. The optimum adsorption (342.68 mg/g) was achieved at pH 2, adsorbent dose 0.2 g/L, contact time 10 min, and 40 mg/L dye concentration at a temperature of 30 °C. The thermodynamic parameters ΔH° (−456.35 kJ/mol), ΔS° (−58.50 J/K/mol) and ΔG° (7.83 × 10−3 kJ/mol) indicated the exothermic, spontaneous, and feasible process of dye uptake. The isotherm of the process was well explained by Freundlich isotherm model based on the higher R2 and lower error function values and it followed the order of fitness: Freundlich > Sips > Langmuir > Toth. The pseudo-second-order model best explained the kinetics of the uptake process based on higher R2 lower error function values and the closeness of calculated and theoretical qe values. The intraparticle diffusion was not the only rate-determining step, besides Bangham plot suggested the involvement of pore diffusion in the rate-determining step. The chemical nature of adsorption was also supported by the equilibrium time i.e., within 10 min of the process adsorption equilibrium was attained. Based on the adsorption capacity displayed by the Zingiber officinale biosorbent it can be concluded that this biosorbent is highly efficient for removal of the AV 90 dye and can be used as an alternative candidate for removing other organic pollutants from aqueous medium.