1 Introduction

Contamination of environment has attracted more attention in recent years [1]. Various studies have been performed to sampling, determination, chemical and electrochemical removal or degradation of environment pollutants for avoiding the irreparable complications of these hazardous materials on the ecosystem cycle [2,3,4,5]. Among the contaminant issue, industrial wastewaters are in great concern [5,6,7,8,9]. Discharge of industrial wastewater without any pretreatment or proper treatment led to the contamination of water body [10, 11]. Among the industrial wastewaters, concerns regarding dye-containing waters were more widespread [12,13,14,15]. It is estimated that about 7 × 105 tons of dyestuff produced annually [16]. Industries such as textiles, pharmaceuticals, foods, pigments, cosmetics, paints, and ceramics used dyes to color their products [17]. Several reports have shown that approximately 15% of the dyes were lost in wastewater during dyeing operation [18]. Due to complex molecular structures, these dyes are considered non-biodegradable, a property makes them more stable and hard to biodegrade [19]. The presence of dyes led to the pollution of water bodies. Along with being aesthetically unpleasing, these compounds also prevent sunlight penetration into water and consequently lead to serious changes in water ecosystems. They are also harmful to living organisms, leading to mutagenic and cancerogenic changes [20]. Several chemical, physical, and biological treatment methods have been considered for the removal of dyes from aqueous solutions. Coagulation-flocculation, precipitation, AOPs (advanced oxidation processes), ion exchanges, electrochemical techniques, adsorption, and membrane separation can be mentioned as treatment approach for removal of dyes [17, 19, 21]. High cost of chemicals and reagents, production of sludge, membrane fouling, and sophisticated operation can be addressed as main drawback of abovementioned approaches [22,23,24,25]. It is thought that adsorption has considerable potential for the removal of synthetic dyes from wastewater. Adsorption was considered to be cost-effective and technically feasible for removal of dyes from wastewaters [17, 26]. Application of adsorption for dye removal has been reported in several studies. For example, in a study conducted by Bentahar et al. [27], natural clay was applied for removal of methylene blue, crystal violet, and Congo red from binary and ternary systems. Mouni et al. [19] reported the application of Kaolin in removal of methylene blue from aqueous solutions. Result of Rajasekhar showed that application of corncob as adsorbent has a good performance in removal of malachite green from aqueous solution [28]. Due to high adsorption capacity, its pore structure and presence of different surface oxygen functional groups at its surface, activated carbon is one of the most widely used groups of adsorbents for the removal of a wide range of pollutants from wastewater [26, 29,30,31,32]. However, the main drawback of activated carbon-based adsorption processes is price of carbon that significantly influences the total cost of an adsorption process system [17, 26]. In recent years, researches have focused on the utilization of available low cost and efficient precursors for the production of activated carbons. In comparison to activated carbon, usage of agricultural by products as low-cost alternative adsorbent has attracted more attention [33]. During the last decade, several types of agricultural waste such as malt bagasse [34], Ficus carica [35], Pterospermum acerifolium shells [36], cashew nut shell [37], almond shell, banana peel [38], sugarcane bagasse [39], cotton wove waste [40], tea factory waste [41], coconut shell [42], and Luffa acutangula [33] have been used for the removal of dyes and heavy metals from waste water.

Rhodamine B (RhB) is a basic cationic dye. Transient mucous membrane and skin irritation are likely to occur in long-term exposure to RhB. According to the literature, RhB is a potential mutagen. It could disturb the aquatic life via obstructing light penetration and oxygen transfer [17]. Therefore, there is an urgent need to removal of RhB from aqueous solution.

The removal of RhB from aqueous solution has been recently studied by Laysandra et al. for bentonite-titanium dioxide composites [17], Ptaszkowska-Koniarz et al. for modified carbon xerogels [20], Wang et al. for SnS2 nanostructure [43], and Khamparia and Jaspal for Argemone mexicana [12]. In Kermanshah province as study area, corn is cultivated in a large area. However, harvested corn stalks can be considered as the main part of the agricultural waste flow. Recycling this type of waste as low-cost adsorbent in the field of wastewater treatment can be attractive. Regarding the removal of RhB, some previous studies applied corncob activated carbon as adsorbent. To the best of our knowledge, no study has been reported regarding removal of RhB by stalk corn activated carbon (SCBAC).

In the present work, performance of RhB adsorption by SCBAC was investigated. The effect of practical variables such as contact time, pH, initial dye concentrations, and adsorbent dosages on the adsorption of RhB on to the SCBAC was examined. In addition, kinetics, adsorption isotherm models, and thermodynamic studies were carried out to evaluate experimental data.

2 Material and method

2.1 Chemicals and reagents

RhB, sodium hydroxide (NaOH, 96%), sulfuric acid (H2SO4, 98%), and phosphoric acid (H3PO4, 99%) were analytical grade and were purchased from Merck. Stock solution of 1000 mg L−1 of RhB was prepared by dissolving 1.00 g of dye in 1 L distilled water. Solutions of different concentrations were obtained by diluting the stock solution.

2.2 Preparation of adsorbent

The stalk corn was collected from agriculture lands around Kermanshah, Iran, and was washed with distilled water to remove sand and dirt. The material was dried at 100 °C in an oven for 24 h to remove moisture. Then, it was cut into small pieces (2 mm). The activation was performed by mixing of row stalk corn with phosphoric acid (1 N) in a ratio of 1:10 (w/w) for 24 h in laboratory temperature. This mixture was then placed in an oven in 100 °C for 24 h for further drying. The dried mixture was carbonized in a muffle furnace in absence of air firstly in 500 °C for 30 min and after that in 700 °C for 15 min. Nitrogen (N2) was used as a protective gas. The obtained carbon then cooled, pulverized, and sieved to 50-mesh size. Regarding the removal of any impurity, the obtained activated carbon was washed several times with deionized water until the pH of the residual solution reached 6–7 [44].

2.3 Rhodamine B adsorption experiments

Batch adsorption experiments were conducted to evaluate the removal performance of RhB in aqueous solutions. The effect of parameters such as initial solution pH, adsorbent amount, initial RhB concentration, and contact time on the adsorption process was investigated. The experimental studies were perused with 100 mL solution of desired concentration (10–50 mg L−1), contact time of 30–110 min, and adsorbent dosages at the ranges of 0.5–2.5 g L−1 in 250 mL conical flask agitated on a thermostated shaking incubator at 120 rpm. In addition, the effect of pH was studied in the range 3–11 and was adjusted using 0.5 M H2SO4 and NaOH. At the end of mentioned contact times, sampling was done to determine the adsorbate concentration using double beam UV–visible spectrophotometer (Systronics 2203). The concentration of RhB was measured at a wavelength of 554 nm. The amount of adsorbed RhB at equilibrium was calculated by the following equation [45]:

$$qe={\left({C}_{0}-{C}_{e}\right)}^{*}V/m$$
(1)

where qe (mg g−1) implies the equilibrium adsorption capacity of RhB adsorbed per gram of the SCBAC, C0 and Ce (mg L−1) the initial and equilibrium RhB concentrations, respectively, V the volume of the RhB solution (L), and m the SCBAC mass (g). The dye removal percentage was calculated using the following relationship:

$$\%Dye Removl=\frac{{C}_{0}-{C}_{e}}{{C}_{0}}$$
(2)

Each experiment was conducted in duplicate under identical conditions.

2.4 Adsorption equilibrium

An adsorption isotherm is equations for description of adsorbate equilibrium between solid and liquid interfaces [46]. The adsorption isotherm can be useful to illustrate the interaction of the adsorbate with the adsorbent. The adsorption isotherm correlations are important for evaluation of adsorbent distribution on solid/liquid interface and for the estimation of adsorbent uptake capacity. In the present work, Langmuir and Freundlich isotherms were used to identify the mechanisms of adsorption process.

2.4.1 Langmuir model

The Langmuir model, simplest and widely used model, is based on the assumption that adsorption takes place in monolayer form without any interaction between adsorbed molecules. Based on this model, adsorption energies onto the surface are uniform [47,48,49,50]. The Langmuir equation is written as (Eq. 3):

$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{q}_{m}b}+\frac{{C}_{e}}{{q}_{m}}$$
(3)

where qm is maximum adsorption capacity (mg g−1) and b is adsorption equilibrium (L mg−1).

$${R}_{l}=\frac{1}{1+b{C}_{0}}$$
(4)

RL, can be expressed in terms of separation factor, indicates the adsorption nature to be unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1), or irreversible (RL = 0).

2.4.2 Freundlich isotherm

The Freundlich isotherm assumed that there is no restriction to the formation of a monolayer adsorption and can be used to illustrate heterogeneous adsorbent surface without uniform distribution of the heat of adsorption [51,52,53]. It can be represented by the following equation:

$$\mathrm{Ln qe}={\mathrm{lnK}}_{\mathrm{f}}+1/{\mathrm{nlnC}}_{\mathrm{e}}$$
(5)

where KF and n are Freundlich constants, which related to the sorption capacity and the sorption intensity of the system. The magnitude of the term (1/n) provides an indication of the favorability of the sorbent/adsorbate systems [35, 54].

2.5 Adsorption kinetic

Batch adsorption experiments were carried out by addition of well-known amount of adsorbent to the liquid solution. The kinetic experiments were conducted by taking the samples from the solution. The kinetic experiments were performed for 10 mg L−1 of RhB and pH of 3. The adsorbent dosage was fixed as 2.5 g L−1. In the present work, two well-known adsorption kinetic models were applied: the pseudo-first-order Lagergren equation and the pseudo-second-order rate equation. The pseudo-first-order equation, proposed by Lagergren, is a method for analyzing the kinetics of adsorption process which can be displayed its linear form as (Eq. 6) [55, 56]:

$$\mathrm{Ln}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{Ln qe}-{\mathrm{k}}_{1}\mathrm{t}$$
(6)

where qe (mg g−1) and qt (mg g−1) are values of the adsorbed adsorbate at equilibrium and at time t, respectively, and k1 (min−1) is the rate constant of pseudo-first-order adsorption.

The pseudo-second-order kinetics model can be defined as (Eq. 7):

$$\frac{t}{{q}_{t}}=\frac{q}{{k}_{2}{q}_{e}^{2}}+\frac{1}{{q}_{e}}t$$
(7)

In this regard, k2 (g mg min−1) shows the rate constant of the pseudo-second-order equation rate constant.

3 Result and discussion

3.1 Surface functional group of adsorbent

The SEM micrograph of SCBAC is shown in Fig. 1. The porous surface of activated carbon could be observed by this micrograph. The identification of functional groups on the adsorbent is useful toll in figure out of chemical structures of the adsorbents. FTIR analysis was applied to identify some characteristic functional groups of activated carbon (FCBAC) (Fig. 2). The peak observed in the region around 3450 cm−1 corresponds to O–H stretching of the hydroxyl groups. The distinct peak at 1650 cm−1 is related to C = C stretching vibration in aromatic rings [57]. The peak identified at 1275 cm−1 represents C–N stretching. The presence of C–O groups is indicated by the peak appeared at 1110 cm−1. The peak at 825 cm−1 is due to C–H functional group [58].

Fig. 1
figure 1

SEM of prepared SCBAC

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Fig. 2
figure 2

FT-IR spectrum of SCBAC

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3.2 Influence of initial solution pH

One of the most important factors in the adsorption process is solution pH [19, 59]. The effect of solution pH generally depends on the ions present in the reaction mixture as well as electrostatic interactions with the adsorption surface. In the present study, the adsorption of RB was studied as function of pH of 3, 7, and 11.

The obtained results showed that by increasing of pH from 3 to 11, dye removal was decreased (Fig. 3). A possible explanation for this could be the interaction of the positively charged (cationic) RhB molecules with the negatively charged surface of the adsorbent via electrostatic interactions. These results are in agreement with those obtained by others. For example, Mohammadi et al. [60] evaluated the removal of RhB from aqueous solution using palm shell-based activated carbon. Result of their study showed the positive effect of acidic pH on adsorption of RhB. In addition, Cheng et al. investigate the effect of graphene oxide/silicalite-1 composites on adsorption of RhB. They claimed that in acidic environment, the fabricated adsorbent has high performance in removal of RhB [18]. In the point of zero charge (pHzpc), the positive and negative electrical charges found at the adsorbent surface are balanced. The influence of pHzpc on adsorption process is seen in three ways: (1) The adsorbent surface has a negative charge if pHzpc < pH, (2) the adsorbent surface has a positive charge if pHzpc > pH, and (3) if pHzpc = pH, the adsorbent surface has natural [61]. In the present study, the pHpzc was determined via the salt addition method [62]. The pHpzc of the prepared adsorbent was determined 5.9. At pH 3, the surface of prepared activated carbon is positively charged because when the pH < pHpzc, the surface of adsorbent becomes positively charged and therefore attracts the ionic form of RhB dye led to enhanced adsorption at pH lower than pHpzc value [63]. At pH above 3, due to deprotonated form of the COOH groups on the RhB, transformation from a cationic form to a zwitterionic form can occur which in turn leads to an electrostatic repulsion between RhB and the negatively charged SCBAC, thus decreasing its adsorption capacity [18].

Fig. 3
figure 3

Effect of pH on RhB adsorption on the SCBAC (adsorbent dosage = 1.5 g L−1, contact time = 70 min)

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3.3 Effect of adsorbent dosage

Effect of adsorbent dosage in removal of RhB from aqueous solution at optimum pH 3 was investigated. The applied adsorbent dosage was varied from 0.5 to 2.5 g L−1 for the dye concentrations from 10 to 50 mg L−1. As depicted in Fig. 4, removal of RhB shows increasing trend by increase of adsorbent dosages. Increase in amount of RhB adsorption by increasing of SCBAC dosage can be related to the increase of surface area and increase of reaction sites. The results obtained in this study are consisted with that obtained by El-Sayed et al. [26], Hayeeye et al. [64], Rangabhashiyam and Balasubramanian [36], and Rajasekhar [28] who reported that increasing of adsorbent dosage led to the increase of adsorbed adsorbate.

Fig. 4
figure 4

Effect of adsorbent dosage on removal of RhB (pH = 3, contact time = 70 min)

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3.4 Effect of initial concentration of dye

The effect of initial dye concentration on removal of RhB at various adsorbent dosages and optimum pH of 3 was studied. As shown in Fig. 5, the removal efficiency of RhB was decreased by increasing of dye concentration. The removal of RhB at various dosages of adsorbent was decreased from 38.6 to 24.2%, 57.3 to 5.8%, and 89.6 to 45% respectively by increasing of dye concentration from 10 to 50 mg L−1.

Fig. 5
figure 5

Effect of initial dye concentration on removal of RhB

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By increasing of initial dye concentration, the active sites of adsorbent were occupied by adsorbate. In addition, the competition of adsorbate molecules for the adsorbent binding sites can be increased [36]. The obtained results are consistent with the previously conducted study [26, 36].

3.5 Effect of contact time

One of the critical parameters that pointedly affect the performance of dye removal is the contact time between adsorbent and adsorbate. Fig. 6 illustrates the effect of contact time on the extent of RhB removal. The percentage removal rate for RhB varied from 65.5 to 62.6%, 41.6 to 36%, and 34.3 to 30.2% for concentrations of 10, 30, and 50 mg L−1, respectively. Looking at Fig. 6, it is apparent that the maximum RhB removal for all the dye concentrations was attained at the first 30 min of experiment. After that time, removal rate was decreased slightly. This can be due to the fact that at the beginning of the experiments, the available vacant adsorption sites are high and by elapsing the time, the amount of vacant sites decreased and thereby it is difficult to occupy these sites [65].

Fig. 6
figure 6

Effect of contact time on adsorption of RhB onto SCBAC

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3.6 Adsorption kinetics

In the present work, two adsorption kinetic models were conducted: the pseudo-first-order Lagergren equation and the pseudo-second-order rate equation were both fitted with the experimental data [19]. The pseudo-first-order equation is based on the assumption that physisorption limits the rate of adsorption of the particles onto the adsorbent, while, in the pseudo-second-order model, chemisorption is considered as the rate-limiting mechanism of the process [66]. The result showed that the experimental data is better fitted to a pseudo-second-order model (R2 = 1) than to a pseudo-first-order model (R2 = 0.87), for the adsorption of RhB onto SCBAC (Table 1, Fig. 7).

Table 1 Pseudo-first-order and pseudo-second-order kinetic parameters of RhB adsorption onto SCBAC
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Fig. 7
figure 7

Pseudo-first-order (a) and pseudo-second-order (b) kinetic models for RhB adsorption onto SCBAC (ads. dosage = 2.5 g L−1; T = 25C; pH = 3)

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The obtained data are in line with that obtained by others [18, 19, 55] who reported that the kinetic of adsorbate adsorption on different adsorbent follows the pseudo-second order. Based on the obtained results, it can be concluded that the adsorption of RhB onto SCBAC was a chemisorption process relating valence force through sharing or electron exchange between adsorbate and adsorbent species [55].

3.7 Adsorption isotherm

The Langmuir and Freundlich isotherm models were used to test the equilibrium data of RhB adsorption on SCBAC. The plot for Freundlich and Langmuir isotherm is shown in Fig. 8. The parameters obtained from these isotherm models are illustrated in Table 2. It was found that the “qm,” maximum capacity of the adsorbent and the most significant parameter of the Langmuir isotherm, was 5.3 mg g−1. In the present work, the RL was 0.347 which imply the favorability of the RhB adsorption onto SCBAC (0 < RL < 1). In a study conducted by Nethaji et al. [55], removal of Cr (VI) was investigated by corncob activated carbon coated with nano-sized magnetite particles. Their results showed that the obtained qm was varied from 52.24 to 57.37 mg g−1. Compared to the present work, the difference between the obtained qm can be related to the difference in the nature of the pollutant, different activation method, as well as further modification of the used adsorbent.

Fig. 8
figure 8

Langmuir (a) and Freundlich (b) adsorption isotherm curves for the adsorption of RhB onto SCBAC

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Table 2 Langmuir and Freundlich isotherm parameters of SCBAC
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The favorability of the adsorption process was also tested by Freundlich equation. In this study, the sorption intensity of the system (n) was 2.63, which confirm the favorability of the process (0 < n < 10). The correlation coefficient of Freundlich model is better than that of Langmuir model, so Freundlich isotherm model was found to be the best fit for the studied system. The obtained results are in contrast with the results found by Kamarehie et al. [67] who showed that Langmuir isotherm provides a better fit for the adsorption of alizarin red S onto the PAC/γ-Fe2O3. According to the assumptions of the Freundlich isotherm, therefore, it can be concluded that the prepared SCBAC had heterogeneous surface.

The sorption capabilities of the prepared activated carbon in this study regarding RhB were compared with some adsorbents reported previously (Table 3). It is obvious that SCBAC has lower removal ability of RhB than other adsorbents previously mentioned.

Table 3 Comparison of the adsorption capacities on different adsorbents for RhB
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4 Conclusion

Evaluation of adsorption capacity of SCBAC was done through batch mode studies. The maximum removal of RhB (89.6%) was attained at optimum pH 3 and 2.5 g of adsorbent. The sorption isotherm of RhB by SCBAC was found to be best fitted by the Freundlich model with acceptable R2 = 0.9. Kinetic study showed that adsorption of RhB on SCBAC followed pseudo-second-order model and the maximum adsorption capacity of SCBAC for RhB is 5.3 mg g−1. Based on the results derived from this study, SCBAC has low uptake capacity for higher concentration of RhB. Application of this adsorbent is recommended for low concentration of RhB. Prepared SCBAC showed that can be utilized as economical and effective adsorbent for the adsorption of dyeing pollutants, like RB, from aqueous solutions.