Introduction

Methylene Blue (MB) and Crystal Violet (CV) are dyes that are widely used as coloring agents in various industrial fields such as the textile, paper, leather, and pharmaceutical industries. Most synthetic dyes and their degradation products have a large environmental impact, especially in aquatic environments due to their extensive use and low level of waste treatment [1]. After the dye enters the wastewater, the mixture becomes more stable and more difficult to decompose due to the complex chemical structure formed [2]. Color substances can cause an increase in Biological Oxygen Demand even though dyestuffs only make a small contribution to the total organic load in wastewater [3].

CV (C25N3H30Cl) is widely used as a dermatological agent in various commercial textile processes [4] while MB (C16H18N3SCl) is a cationic dye of aromatic hydrocarbon compound. Both are two types of cationic dyes which are stable to light and heat and difficult to decompose due to complex structures [5, 6]. Although both have many benefits in different uses, CV and MB dyes are mutagenic and toxic [7,8,9,10]. Therefore, it is necessary to reduce the concentration of these dyes in industrial waste, especially in the treatment of waste before the spread to the environment occurs.

The processing of CV and MB dyes waste has been carried out through several processes such as oxidation [11], coagulation and flocculation [12], adsorption [13, 14] and ion exchange [15]. The adsorption process is one of the right ways to treat waste containing dyes and heavy metals before being discharged into the environment, because the adsorption method is simple, relatively inexpensive, and does not cause side products that are harmful to the environment [16, 17]. The success of the adsorption process is largely determined by the suitability of the properties and types of adsorbents used. Effective adsorbents for adsorption are those having a large adsorption rate and capacity, are chemically stable, can be used repeatedly, and are environmentally friendly [1, 14].

Algae biomass naturally is a highly effective adsorbent to remove pollutants from organic compounds derived from dyes [18, 19]. However, the ability of algae to bind the chemical compounds is limited by several constraints such as small size, low specific gravity and easily damaged due to degradation by other microorganisms [20, 21]. In addition, the algae cannot be used directly in the adsorption column, because it is very soft and not granular [22, 23]. To overcome these weaknesses, various efforts have been made, among others, by immobilizing algae biomass using various supporting polymers such as silica [24].

The sol-gel process is an appropriate process for immobilizing various organic molecules, organometallics, and biomolecules in inorganic matrix. The sol-gel method is one of method applied to transform a silica surface via a homogeneous way to result in organo-silica hybrid substances. In fact, the sol-gel technique has been used to modify algae biomass to homogeneous adsorbents [25]. In this study, it was carried out immobilization of Spirulina sp. biomass with a silica matrix derived from tetraethyl orthosilicate (TEOS) producing adsorbents (termed ASS) to absorb CV and MB dyes in solution. The CV and MB dyes adsorption was studied including the effect of solution pH, contact time, and concentration of dye adsorbed by the ASS adsorbent.

Materials and methods

Materials

Spirulina sp. algae biomass which was used as an adsorbent in this study was obtained from the Lampung Marine Aquaculture Development Center, Indonesia. The chemicals used in this research were of analytical grade. TEOS, ethanol, ammonia, hydrochloric acid (HCl), sodium hydroxide, CV, MB, and CH3COONa were purchased from Pharmacopoeia European.

A stock solution of 1 g L− 1 of the CV and MB dye was prepared. For the present study, the CV or MB dye concentrations between 10 and 400 ppm were prepared from the stock solution. The CV dye showed absorbance in the λmax = 595 nm and the MB dye showed absorbance in the λmax = 664 nm (UV-Vis spectrophotometer, Agilent Cary 100). All experiments were performed using double distilled water.

Preparation of ASS adsorbent

The harvest of Spirulina sp. algae biomass was obtained from cultivation in a laboratory scale at the Lampung Marine Aquaculture Development Center, Indonesia. The algae biomass was dried to constant weight in an oven at temperature of 40 °C, then smoothed by grinding to a size of 100–200 mesh (74–149 μm).

The preparation of ASS adsorbents was carried out according to the procedure of synthesis of algae-silica hybrids [26], namely reacting with 5 mL of TEOS and 2.5 mL of distilled water in a plastic bottle, then stirring with a magnetic stirrer for 30 min and added 1 M HCl to pH of 2. In another plastic container, 0.4 g of Spirulina sp. algae biomass were mixed with 5 mL of ethanol and then stirred with a magnetic stirrer for 30 min. Both solutions were then mixed and stirred using a magnetic stirrer until a gel was formed. The gel formed was filtered, then let stand for 24 h. The gel was then washed with distilled water and ethanol (60:40) to neutral pH. The gel was dried using an oven and then crushed to a size of 100–200 mesh (74–149 μm).

Characterization of ASS adsorbent

The ASS adsorbent was characterized by Fourier-transform infrared spectroscopy (FTIR) to identify its functional group (Shimadzu Prestige-21 IR). Analysis of pore volumes and specific surface area was investigated using a surface area analyzer (Quantachrome TouchWin v1.0). The constituent elements and surface morphology were identified by scanning electron microscopy-energy-dispersive-X ray spectroscopy (SEM-EDX, Zeiss MA10).

Adsorption experiments

A series of CV and MB dyes adsorption experiments in solution using the ASS adsorbent were carried out in the batch method according to procedures reported by Buhani et al. [14]. The effect of pH of the solution was studied with variations in pH of 2–10, contact time of 0–90 min, and initial concentration of 0–400 mg L− 1 at a temperature of 27 °C. The data obtained were analyzed to determine kinetics and adsorption isotherm model of the CV and MB dyes.

The quantity of CV and MB dye molecules adsorbed per mass unit of adsorbent was determined using Eq. (1). The percentage of CV and MB dyes adsorbed on ASS adsorbent was determined using Eq. (2).

$$ q=\frac{\left({C}_{o-}{C}_e\right)v}{w} $$
(1)
$$ Removal\ \left(\%\right)=\frac{\left({C}_{o-}{C}_e\right)}{C_o}x\ 100 $$
(2)

where Co and Ce (mg L− 1) is the CV or MB dye concentration before and after the adsorption process, w is the quantity of adsorbent (g), and v is the volume of solution (L), q is the amount of CV or MB dye adsorbed per unit mass (mg g− 1).

Results and discussion

Characterization of ASS adsorbent

The characterization of the ASS adsorbent was carried out using IR spectroscopy, SEM-EDX, and BET surface area method to determine the success of making ASS material from Spirulina sp. algae biomass with silica matrix from TEOS as a precursor.

From the FTIR spectra of the adsorbent shown in Fig. 1, it can be seen that the specific absorption of silica (SG) is the absorption band of 462.9 cm− 1 showing the bending vibration of the siloxane (Si-O-Si) group. The absorption band at around of 786.7 cm− 1 indicates the symmetrical stretching vibration of Si-O in siloxane. The absorption intensity at 964.4 cm− 1 comes from stretching vibrations of -Si-O- from -Si-OH and strong absorption bands at 1072.9 cm− 1 show the asymmetric stretching vibration of Si-O from siloxane (Si-O-Si) [27].

Fig. 1
figure 1

FTIR spectra of SG, Spirulina sp. algae biomass and ASS adsorbent

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In the spectrum, Spirulina sp. algae biomass is shown at absorption band of 3387 cm− 1 which indicates that the group (−OH) overlaps with N-H. The absorption shows the presence of a group (−OH) derived from polysaccharides or groups (N-H) of proteins contained in the biomass of Spirulina sp. algae. The wavenumber at 2931.8 cm− 1 shows the presence of C-H stretching vibration of aliphatic (−CH2). The carbonyl group (C=O) was detected at 1658.8 cm− 1 and 1026.1 cm− 1 showed the presence of –C-O group from the cellulose structure which is the absorption characteristic of Spirulina sp. algae biomass.

The ASS adsorbent provides significant FTIR uptake, namely the emergence of a typical absorption band of SiO2 at 1087.9 cm− 1 which is Si-O asymmetrical stretching vibration on siloxane bonds while absorption at 794.7 cm− 1 is Si-O symmetrical stretching vibration of the group siloxane. Uptake at 450 cm− 1 shows the bending vibrations of Si-O-Si and the absorption peak in the area of 3400 cm− 1 indicates the –OH stretching vibration of silanol (Si-OH). The hybridization process with Spirulina sp. algae biomass is reflected by the emergence of C-H absorption band characteristics from aliphatic (−CH2) at 2931.8 cm− 1 [17]. This is confirmed by the loss of Si-O stretching vibration from Si-OH observed at 964.4 cm− 1 caused by the reduction of the silanol group due to condensation that occurs with the biomass of Spirulina sp. algae [27,28,29].

Figure 2 shows the SEM image and EDX spectrum of the material resulted from Spirulina sp. biomass hybridization with silica through a sol-gel process. The analysis results of SEM showed the surface morphology of the ASS adsorbent in the form of amorphous solids whereas in the EDX spectrum there were several elements that dominated the ASS material composition, namely Si, O, C, and H, indicating a hybridization process between the silica matrix and the Spirulina sp. algae biomass [26].

Fig. 2
figure 2

Image of SEM-EDX from the ASS adsorbent

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In Fig. 3 it can be seen that the pattern of the N2 adsorption-desorption isotherm in the ASS follows the pattern of the adsorption-desorption isotherm type combined type I and IV adsorption isotherm. The type I adsorption isotherm pattern occurs in low P/Po regions, it is seen that the form of isotherm tends to be horizontal. Type I isotherm patterns only form one layer of adsorbate and only occur in the chemisorption process. The type IV isotherm adsorption pattern occurs in the higher P/Po area with hysteresis loop H1 showing mesoporous material. The volume of adsorbates in all isotherms increased relatively sharply against (P/Po) around 0.8 which showed nitrogen capillary condensation in regular mesoporous structures. The pore size distribution of ASS derived from the adsorption branch of isotherm shows that the most probable pore size centered at 3.2 nm. The BET surface area of ASS is 90 m2 g− 1 with total pore volume 0.26 mL g− 1 while the average pore diameters are 4.7 nm which is in the mesoporous scale (2–50 nm) [30,31,32].

Fig. 3
figure 3

The nitrogen adsorption-desorption isotherms and pore size distribution of ASS adsorbent

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Influence of pH

The effect of the pH of the solution on CV and MB dyes has been studied by interacting CV and MB solutions at variations in pH 2–10 (Fig. 4). In Fig. 4, it can be observed that in CV and MB solutions there is an increase in adsorption with the increase in pH value and the optimum adsorption occurs at pH 8. CV and MB dyes are cation-based coloring agents while ASS adsorbent from Spirulina sp. biomass with silica matrix contains several functional groups such as amino, hydroxyl, and carboxyl groups derived from the algae biomass as well as silanol and siloxane groups from silica which are negative charge [24]. Therefore, at low pH there is competition between CV or MB molecules with protons found on the active site of ASS. As a result, the adsorption of CV and MB dyes on ASS adsorbents is not optimal [33]. Increasing the pH of the solution results in an increase in the amount of CV or MB dyes adsorbed, due to the increase in electrostatic interactions between CV or MB molecules [34]. At a higher pH > 8, a decrease in adsorption begins because hydroxide species (species that contain OH− ions derived from compounds present in adsorbates or adsorbents in alkaline conditions) are formed which tend to form deposits on the adsorbates and ASS adsorbents [35, 36]. In addition, silica as the matrix of ASS is less stable at high pH [25], consequently at higher pH, the active sites on the adsorbent were damaged resulting in decreased adsorption of the CV or MB dye.

Fig. 4
figure 4

Effect of the solution pH on the adsorption of CV and MB dyes by the ASS adsorbent (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, contact times of 60 min, and temperature of 27 °C)

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Effect of contact times

The effect of the interaction times of CV and MB dyes on the ASS adsorbent was studied by interacting CV or MB solutions to the ASS adsorbent with contact times varying from 0 to 90 min (Fig. 5). From Fig. 5, it can be observed that the adsorption of CV and MB dyes is relatively fast. At the first 30 min the adsorption increased very sharply, and then a slight increase in the adsorbed dyes and eventually reached a constant at 60–90 min time. At this stage the adsorption process is estimated to have reached equilibrium.

Fig. 5
figure 5

Experimental data and the fitted non-linear forms of pseudo-second-order (PSO) kinetics models for the adsorption of CV and MB dyes onto ASS (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, pH of 8, and temperature of 27 °C)

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Effect of contact times of CV and MB solutions on ASS found in Fig. 5 was further analyzed to find out its kinetic model using the pseudo-first-order (Eq. 3) and pseudo-second-order (Eq. 4) kinetics models [7, 27, 37].

$$ \log \left({q}_e-{q}_t\right)=\log \kern0.6em {q}_t\frac{k_1}{2.303}t $$
(3)
$$ \frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e} $$
(4)

The results of the analysis using the two kinetic models show that the CV and MB dye adsorption kinetics models on the ASS adsorbent tend to follow the pseudo-second-order kinetic model (Fig. 5). This can be seen from the value of the linear regression coefficient (R2) in the pseudo- second-order kinetic model greater than the pseudo-first-order kinetic model (Table 1). In addition, in Fig. 5, it can be seen that the experimental data of the amount of CV and MB dyes adsorbed (q) have a pattern that is relatively the same as the data of non-linear forms of pseudo-second-order kinetics models.

Table 1 Kinetic parameters for the adsorption of CV and MB dyes on ASS adsorbent (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, pH of 8, and temperature of 27 °C)
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Adsorption isotherm

The effect of the concentration of the adsorbate plays an important role in determining the amount of adsorbate adsorbed. This is because an increase in the concentration of the adsorbate will increase its interaction with the active site of adsorbent. In this study the initial concentrations of CV and MB dyes varied from 0 to 400 mg L− 1 which resulted in an interaction between the adsorbate and the adsorbent in the equilibrium state as shown in Fig. 6. From Fig. 6, it can be observed that there is an increase in the quantity of CV and MB dyes adsorbed with increasing concentration used. The adsorption of CV and MB dyes increased sharply at low concentrations and gradually increased at high concentrations. This shows that the ASS adsorbent has a high adsorption affinity for CV and MB dyes. The increase in adsorption is related to the number of active sites available on the surface of the adsorbent [38, 39].

Fig. 6
figure 6

Effect of the equilibrium concentration (Ce) of CV and MB dyes on the amount of adsorbates adsorbed (qe) (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, contact time of 60 min, pH of 8, and temperature of 27 °C)

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The pattern of isotherm adsorption of CV and MB dyes on the ASS adsorbent was analyzed using the Langmuir (Eq. 5), Freundlich (Eq. 6), Dubinin-Raduskevich (DR) (Eq. 7), and Temkin (Eq. 8) adsorption models [40,41,42]. The linear equation of each model is shown in Fig. 7. A general linear form of saturated monolayer Langmuir isotherm represented as:

$$ \frac{C_e}{q_e}=\frac{1}{q_m{K}_L}+\frac{C_e}{q_m} $$
(5)
Fig. 7
figure 7

The linear equations of the adsorption isotherm models of the CV and MB dyes on the ASS adsorbent with the adsorption isotherm model of (a) Langmuir, (b) Freundlich, (c) Dubinin-Raduskevich, and (d) Temkin

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where qe (mg g− 1) is the amount of adsorbed adsorbate per unit weight of adsorbent and Ce (mg L− 1) is unadsorbed adsorbate concentration in solution at equilibrium. The constant KL (L mg− 1) is the Langmuir constant and qm (mg g− 1) gives the maximum monolayer adsorption capacity. A plot Ce/qe versus Ce gives a slope and interceps 1/qm and 1/qmKL repectively.

Freundlich adsorption isotherm illustrates that the adsorption process occurs on heterogeneous surfaces with multilayers [43]. The linear equation of the Freundlich isotherm model is shown as follows:

$$ \log \kern0.3em {q}_e=\log \kern0.3em {K}_F+\frac{1}{n}\log \kern0.2em {C}_e $$
(6)

The intercept KF is Freundlich constant, n is Freundlich exponent obtained from the plot of log qe versus log Ce. The Freundlich exponent n values were between 1 and 10.

The DR adsorption isotherm model can be used to explain the adsorption that occurs on a homogeneous and heterogeneous surface. The DR adsorption isotherm model can be expressed as Eq. (7).

$$ \ln \kern0.3em {q}_e={q}_{DR}-2{B}_{DR} RT\kern0.2em \ln \left(\mathrm{l}+\mathrm{l}/{C}_e\right) $$
(7)

where, R is the gas constant (8.314 kJ mol− 1), T is the absolute temperature (K), qDR and BDR are the DR isotherm constants in mg g− 1 and mo 2 kJ− 2 respectively. Plot log qe vs. ln(1 + 1/Ce) will produce slope and intercept.

Temkin adsorption isotherm model can also be expressed in the form of linear equation as Eq. (8):

$$ {q}_e=\frac{RT}{b_{Te}}\ln \left({K}_{Te}{C}_e\right) $$
(8)

where KTe (L mg− 1) is Temkin constant, bTe shows the heat of adsorption, T (K) is temperature. A plot of qe versus ln Ce are able to determine the constant KTe. The Temkin adsorption isotherm model explains that the heat of adsorption decreases linearly as adsorbate-adsorbent interactions increases [44].

The results of data analysis from the completion of linear equations in each model of adsorption isotherm in Fig. 7 in the form of adsorption parameters shown in Table 2.

Table 2 Adsorption isotherm parameters of CV and MB dyes on ASS adsorbent (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, contact time of 60 min, pH of 8, and temperature of 27 °C)
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From Table 2, it can be observed that the parameters of the linear equation regression coefficient (R2) of CV and MB dyes adsorption by ASS for the Freundlich adsorption isotherm model (at interaction time of 60 min, interaction pH of 8 and temperature of 27 °C) are 0.995 and 0.975, respectively. This shows that the adsorption isotherms of CV and MB dyes are more in accordance with the models of Freundlich adsorption. The Freundlich adsorption isotherm illustrates that the adsorption process of the CV and MB dyes occurs on heterogeneous surfaces with multilayers [43]. Furthermore, in the Langmuir adsorption isotherm model, it is assumed that on the surface of the adsorbent there are a certain number of active sites that are proportional to the surface area, the surface of the adsorbent is uniform and the adsorption process is monolayer [45, 46]. Thus it can be stated that the adsorption process of CV and MB dyes that occur in the ASS adsorbent involves physical and chemical adsorption [47]. The physical adsorption occurs through pores contained in the ASS adsorbent while the chemical adsorption occurs through active sites derived from the active groups of Spirulina sp. algae biomass (−OH, −N-H, and -C = O) and silica matrix (−Si-OH, −O-Si-O-).

The ability of ASS adsorbent to absorb CV and MB dyes in a single adsorption process (Figs. 4, 5, 6) shows that CV dye is relatively absorbed more compared to MB. This is in line with the kinetics data found in Table 1 which shows the rate constant (k2) of the pseudo-second-order kinetics model in CV dyes greater than that of MB dye. The data are also supported by simultaneous adsorption data of CV and MB dyes on the ASS adsorbent shown in Fig. 8. The data show that the ASS adsorbent is relatively more suitable for the CV dye, if competed simultaneously. In the simultaneous adsorption process which is binary adsorption, there are three possible interactions that occur, namely: synergism, antagonism, and non-interaction [48]. Therefore, the difference in size and molecular structure between CV and MB dyes causes a difference in reactivity to the ASS adsorbent when competed together. This is in line with the studies that have been reported on CV and MB dyes removal from aqueous solutions using the fly ash-based adsorbent material-supported zero-valent iron [49].

Fig. 8
figure 8

Simultaneous adsorption of CV and MB dyes on ASS adsorbents (adsorbent dose of 50 mg, volume of adsorbate of 20 mL, concentrations of the adsorbate mixture are 50–200 mg L− 1, contact time of 60 min, pH of 8, and temperature of 27 °C)

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Reuse of adsorbents

The ability of adsorbents to be used repeatedly in the adsorption process is one of the important factors needed to determine the effectiveness of an adsorbent. In this study, repeated use of ASS adsorbent in adsorbing CV and MB dye solutions was studied by absorbing CV and MB dyes adsorbed sequentially using water and 0.1 M HCl solution as an eluent (Fig. 9). From Fig. 9 it can be observed that the use of 3 repetitions of ASS adsorbents did not reduce the amount of dyes adsorbed significantly (% of the dye adsorbed > 80%) but the capability of the adsorbent decreased in the 4th adsorption repetition. Desorption of CV and MB dyes with water was carried out to release CV and MB dyes adsorbed through physical interactions which illustrated the contribution of the trap mechanism in the adsorbent pores [53]. Furthermore, desorption with HCl solution is effective as an eluent for releasing CV and MB molecules from adsorbents by substituting protons against CV or MB molecules as organic cations [54].

Fig. 9
figure 9

Reuse the ASS adsorbent on the adsorption of CV and MB dyes (adsorbent dose of 50 mg, adsorbate concentration of 200 mg L− 1, volume of adsorbate of 20 mL, contact time of 60 min, pH of 8, and temperature of 27 °C)

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If the results of this study are compared with other adsorbents to adsorb CV and MB dyes (Table 3), it can be stated that the ASS adsorbent has advantages because the CV and MB dyes adsorption processes are dominated by physical interactions, consequently CV and MB dyes adsorbed are easy to be released. So that the impact of its use on processing of dye waste in industry will be more effective because the adsorbent can be used repeatedly. Economic factors are one of the main considerations in planning of waste treatment systems in the industry. The cost of using algae biomass will be cheaper than synthetic adsorbent because its availability in nature is quite abundant. Besides that the remaining adsorbent that has been used do not cause problems in the environment because the ASS adsorbent contains algae that is easily degraded.

Table 3 Comparison of several adsorbents for CV and MB dyes
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Conclusions

The ASS adsorbent derived from hybridization of Spirulina sp. algae biomass with silica as matrix for removal of CV and MB dyes in the solution was successfully carried out. The CV and MB dye kinetics models on the ASS adsorbent follow the pseudo-second-order kinetics and the Freundlich adsorption isotherm model. The ASS adsorbent is an effective adsorbent for removing CV dyes in solution compared to MB dye, both in single adsorption process and simultaneous adsorption. The findings from this research are summarized as follows:

  1. 1.

    The FTIR spectra show that the hybridization process occurs between the Spirulina sp. algae biomass and silica as the supporting matrix.

  2. 2.

    The analysis results of surface morphology with SEM show the surface morphology of the ASS adsorbent in the form of amorphous solid.

  3. 3.

    In the EDX spectrum there are several elements that dominate the material composition of ASS, namely the elements of Si, O, C, and H, indicating that hybridization between the silica matrix and the Spirulina sp. algae biomass.

  4. 4.

    The BET surface area of ASS is 90 m2 g− 1 and the total pore volume of ASS is 0.26 mL g− 1 while the average pore diameter is 4.74 nm.

  5. 5.

    The isotherm adsorption pattern of CV and MB dyes follows the Freundlich adsorption isotherm, and the use of 3 repetitions of ASS adsorbents does not reduce the amount of dye adsorbed significantly (% of the dye adsorbed > 80%).