Adsorptive removal of acid red 18 dye from aqueous solution using hexadecyl-trimethyl ammonium chloride modified nano-pumice

Discharging untreated dye-containing wastewater gives rise to environmental pollution. The present study investigated the removal efficiency and adsorption mechanism of Acid Red 18 (AR18) utilizing hexadecyl-trimethyl ammonium chloride (HDTMA.Cl) modified Nano-pumice (HMNP), which is a novel adsorbent for AR18 removal. The HDTMA.Cl is characterized by XRD, XRF, FESEM, TEM, BET and FTIR analysis. pH, contact time, initial concentration of dye and adsorbent dose were the four different parameters for investigating their effects on the adsorption process. Response surface methodology-central composite design was used to model and improve the study to reduce expenses and the number of experiments. According to the findings, at the ideal conditions (pH = 4.5, sorbent dosage = 2.375 g/l, AR18 concentration = 25 mg/l, and contact time = 70 min), the maximum removal effectiveness was 99%. The Langmuir (R2 = 0.996) and pseudo-second-order (R2 = 0.999) models were obeyed by the adsorption isotherm and kinetic, respectively. The nature of HMNP was discovered to be spontaneous, and thermodynamic investigations revealed that the AR18 adsorption process is endothermic. By tracking the adsorption capacity of the adsorbent for five cycles under ideal conditions, the reusability of HMNP was examined, which showed a reduction in HMNP's adsorption effectiveness from 99 to 85% after five consecutive recycles.


Batch adsorption studies.
Experiments were carried out with 25 ml of dye solution at different concentrations.pH adjustment was done using NaOH and HCl 0.1 N. Samples were contacted with a desirable amount of adsorbent (0.5-3 g/l) on a shaker with 200 rpm at room temperature and a certain contact time.After adsorption, samples were centrifuged at 4500 rpm for 10 min.The studied variables in the present research were pH (3-9), adsorbent dosage (0.5-3 g/l), contact time (10-90 min) and initial dye concentration (10-70 mg/l).The amount of adsorbed dye was calculated by using the mass balance Eq. ( 1): where C 0 and Ce are the initial and final concentration of dye (mg/l), M represents the mass of adsorbents (g), and V is the volume of AR18 solution (L).
To determine the pH point of zero charge (pH ZPC ) for HMNP, 0.2 g/L of the adsorbent in 30 ml of NaCl solution (0.01M) with different initial pHs (2-4-6-8-10-12) was shaken for 24 h.then the final pH of the solutions was measured and plotted versus initial pHs.The pH in which the curve crosses the line (final pH = initial pH) is taken as the pHzpc 26 .
Analytical measurements.To obtain demanded concentration of AR18 (10-70), the first stock solution was prepared by dissolving 0.5 of dye in 1 L distilled water and other desirable concentrations were prepared from the stock solution.Concentrations of dye solution were read at λ = 507 (nm) using a UV-visible spectrophotometer (Perkin Elmer Lambda 25) (Fig. 1).Adsorption Thermodynamics.The Thermodynamic of AR18 adsorption on HMNP was evaluated at optimum conditions (pH = 4.5, adsorbent dose = 2.375 g/l, contact time = 70 min, initial AR18 concentration = 25 mg/l) and different temperatures (15-55 °C) to find the performance of adsorption process.Thermodynamics parameters of adsorption were assessed by Gibbs free energy changes (ΔG 0 ), enthalpy changes (ΔH 0 ) and entropy changes (ΔS 0 ) and sticking probability (SP*)[Eqs.2-5]: where Ea. is the activation energy (kJ/mol), T is the temperature (K), K is the sorption equilibrium constant, and β is surface coverage.
Experimental design.Experiments were designed using Design-Expert11 (Stat.Ease.Inc Minneapolis, USA) software with Response Surface Methodology (RSM).A central composite design was applied to evaluate the effect of 4 different variables on the adsorption process (pH, initial concentration, contact time, and adsorbent dosage).CCD requires centre points, axial points, and cube points.The total number of experiments can be determined by presented Eq.( 6) K represents the number of experimental variables, 2k is the cubic runs, 2k is the axial runs, and C 0 is the centre point's runs.Table 1 presents independent variables and the levels of each variable, while CCD and coded factor values are shown in Table 2. (2)

FESEM and TEM.
While HMNP has become more agglomerated with a smooth surface and is extremely porous, showing more accessible sites for dye adsorption (Fig. 3b), NP exhibits sharp edges and a rough surface texture in FESEM images (Fig. 3a).The image taken following the adsorption procedure demonstrates how dye molecules filled the pores and surfaces of the HMNP.(Fig. 3c).Particle size can be understood from the TEM image of nano pumice in Fig. 4, and it is also clear that the particles are agglomerated and have a semi-polygonal form.
EDAX analysis.Elemental constituent of HMNP material determined by Energy-dispersive X-ray spectroscopy (EDX).From the results in Fig. 5, it is obvious that the major contents are Si and oxygen, with 42.3% and 41.7%, respectively; other elements are Al, Fe, Ca, K, and Cl.
BET. BET (Microtrac Bel Corp, BElSORP Mini) analysis (N 2 gas adsorption method) was used to calculate total pore volume, specific surface area and average pore diameter of NP and HMNP.The results of BET are shown in Table 4.The increased surface area by modification of Nano-pumice (from 1.49 to 10.27) is aligned with previous studies 32,33 .The pore size distribution calculated by the BJH method is displayed in Fig. 6.As it can be seen, HMNP pore size distribution is between 1 and 100 nm, and most of the particle's pore sizes are 2-50 nm which shows the mesoporous size of the sorbent.

FTIR Analysis.
To obtain the functional groups of the pumice sample, the Fourier transform infrared analysis was performed (PerkinElmer, Spectrum Two) while the analysis range was between 400 and 4000 cm −1 .Figure 7 shows the FTIR spectra of NP, HMNP, and after adsorption, HMNP Peaks at 3415-3423 cm −1 were related to water molecules 18 .1049 cm −1 and 1060 cm −1 have appeared in NP, HMNP and after adsorption HMNP, which are related to Si-O and Si-Al stretching vibration 34 .1625-1641 cm -1 showing stretch vibration of band O-H.Si-O-Al band was located at 779-786 cm -1 while near 466 cm -1 bending vibration of the Si-O-Si band was identified 23 .The band around 587-621 cm -1 is associated with the bending vibration of Fe-O3 20 .In HMNP and after adsorption HMNP, two new peaks were observed at 2928 and 1384 cm -1 related to C-H band and C=O, respectively 35,36 .The peak at 2033 cm -1 found after adsorption of HMNP may be due to the C-O bond.

Statistical analysis.
The validity of linear, 2FI, Quadratic and Cubic models was assessed.The Quadratic model was selected with an insignificant lack of fit (0.6656), confirming that the model is valid with Adjusted R 2 = 0.9898 and Predicted R 2 = 0.9787.Data for all fitted models are shown in Table 5.As can be seen from Table 6, different variables have their effects on dye adsorption.In this study, initial dye concentration showed the most impact on dye adsorption efficiency with an F-value of 1354.54.On the other hand, time had the lowest effect.Also, the interaction between A and C significantly affects adsorption due to its bigger F value among all other interaction variables.The quadratic function of D also showed the highest effect on dye adsorption compared with three others (A 2 , B 2 , and C).
Figure 8a displays predicted vs. actual efficiency in AR18 removal from solution by HMNP, which shows a good correlation between obtained experimental efficiency and predicted efficiency by software.In Fig. 8b, the residual amount of each run is shown, indicating a small difference between them (the highest and the lowest amounts were between 2 and − 2).Interactive effect of pH and adsorbent dosage.According to Fig. 9a,c and the negative coefficient of pH, dye removal effectiveness declines as pH rises.AR18 adsorption increases dramatically at pH < 4.2 but gradually at pH > 4.2.The existence of more surface positive charges on the adsorbent at lower pHs and negative charges on the dye molecules, and the resultant electrostatic sorption between them, can be used to explain why AR18 removal is higher at acidic pHs 37 .The calculated pH ZPC value for HMNP was 5.6.It implies that the sorbent's surface is positively charged when the pH of the solution is lower than pH ZPC, and adsorbent surfaces become negatively charged at pH levels above pH ZPC value, which causes dye ions to repel one another and reduce AR18 adsorption.Whereas at pH = 5.6, surface charges are zero 38 .As seen from Fig. 9a,c, increasing the  www.nature.com/scientificreports/adsorbent dosage increased the effectiveness of dye removal.On the other hand, adding more HMNP (0.5-3 g/l) increased the adsorption efficiency.It is most likely because more sites for dye adsorption can be provided with higher dosages.This outcome is consistent with earlier research 39 .
Interactive effect of time and initial concentration.In Fig. 9b, d, The removal efficiency increases from 40 to about 100% as the initial concentration of AR18 decreases, particularly from 45 to 10 mg/l.It also increases when the sorbent has more time to contact the dye molecules, from 10 to 90 min.In the time range of 10-90 min, the impact of contact time was examined.It was discovered that as the contact time grew, more dye was absorbed.It makes sense that as the amount of time increases, more dye molecules have a chance to adsorb on the surface of the HMNP.This result is in agreement with other studies 40 .Initial dye concentration is the primary factor affecting the effectiveness of dye adsorption and significantly impacting the absorption rate.Initial dye concentration has a reverse effect on dye removal.While the initial concentration increases, dye removal decreases.A possible explanation is adsorbent's free sites are occupied when the concentration is high 41 .Isotherm studies.To realize the nature of the interaction between dye molecules and HMNP adsorption isotherms is necessary 42 .In the present study to model the relationship between adsorbed dye on the adsorbent and remained dye in solution, Langmuir, Freundlich, Temkin and Dubinin-Radushkevich models were used (Their plots are shown in Fig. 10), obtained parameters and constants are shown in Table 7. Isotherm models give a better understanding of the adsorption mechanism.To carry out isotherm's studies, all parameters were at their optimized conditions with pH = 4.5, adsorbent dose = 2.375 g/l, contact time = 70 min and initial AR18 concentration in the range of 10-70 mg/l at room temperature.To corroborate the fitted model, the correlation coefficients were used.Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich are expressed by following equations (Eqs.7-10) respectively in Table 8.Langmuir model is valid for the monolayer adsorption of a liquid on a homogenous adsorbent surface 43 .Where q max (mg/g) is the maximum adsorption capacity of the adsorbent, C e is defined as equilibrium concentrations, and b is the adsorption rate constant related to the energy of adsorption (l/mg), the larger b value depicts a larger affinity of adsorbent to the pollutant 44 .From obtained data, the maximum sorption capacity and adsorption energy for HMNP were 12.84 mg/g and 2.64 l/mg, respectively.Additionally, the high b value (2.64) points out the strong binding of AR18 on the HMNP surface.
Freundlich model uses for adsorption on heterogeneous surfaces with the interaction between adsorbed molecules and describes heterogeneous systems 45 .K F is the Freundlich constant (l/g) is the adsorption or distribution coefficient, and the 1/n F value indicates the degree of non-linearity between solution concentration and the adsorption process 46 .Furthermore, 1/n F > 1 demonstrates cooperative adsorption, while 1/n F < 1 implies a normal Langmuir adsorption 47 .The result of experimental data from the Freundlich model showed 1/n F > 1 (0.159), which reveals that the adsorption process of AR18 removal follows a normal L-type Langmuir adsorption.Besides, the coefficient 1/n (generally 0-1) indicates the favourable adsorption of the adsorbate to adsorbent 42 .Temkin isotherm model considers the effects of indirect adsorbent-adsorbate interaction on adsorption isotherms and heat of adsorption 42 .BT = (RT)/b T , T is the absolute temperature (Kelvin), R is the universal gas constant (8.314J/mol K), b is the heat of adsorption constant, and A T (L/g) is the binding constant 46 .
The D-R isotherm model is used to identify the nature of the adsorption process as physical.Where ϵ is Polanyi potential, β is a constant for the free energy of adsorption 48 .The affinity between AR18 and HMNP can be estimated by the R L constant, which is dimensionless.b (L/mg) is the Langmuir constant and C 0 (mg/L) is the AR18 concentration.The value of R L shows the nature of adsorption as follows 49 : 0 < R L < 1 favorable, R L > 1 unfavorable, R L = 1 linear, R L = 0 irreversible.The calculated R L value is between 0.003 and 0.03.As all of these values are 0 < R L < 1, it can be understood that pumice has favourable adsorption.
In this study, Langmuir had a greater R 2 value than other models, and it was obtained at 0.9962; for D-R, this value was 0.85; for Temkin, it was 0.71, and for Freundlich, it was 0.64, which means that the adsorption isotherms are in good agreement with Langmuir model and AR18 sorption on HMNP is a monolayer.Table 7 displays isotherm model parameters and constants for AR18 removal by HMNP.
Kinetic studies.Kinetic studies were conducted to understand the adsorption mechanism and dye uptake rate.Pseudo-first-order, Pseudo-second-order, and Intra-particle diffusion models were used to analyze adsorption kinetics and Eqs.11-13 present models, respectively (Table 8) in which q e (mg/g) is the amount of adsorbed dye on the adsorbent at equilibrium conditions, q t (mg/g) is the amount of adsorbed dye at any time.K 1 (min −1 ), K 2 (g/mg .min) and K dif (mg/ g•min 0.5 ) are the rate constants of pseudo-first-order, second-order and

Freundlich
Log q e = Log K f + (1/n F ) log C e log (qe) vs. log (Ce) Ln q e vs. ϵ 2 K = slop (10)   8 q max = intercept Kinetic Pseudo-first-order ln(q e − q t ) = − k 1 t + ln(q e ) log(q e − q t ) vs. t 9 q e = intercept 1/q e = Slope Pseudo-second-order t/q t = 1/(k 2 qe) 2 + (t/qe) t/q t vs. t 1/(k 2 q e 2 ) = Intercept (12) Intra-particle diffusion models, respectively.In this study, Pseudo-first-order describes the uptake rate based on adsorption capacity.The obtained data didn't align with this model due to its low q e compared to the calculated q e and the low R 2 .The R 2 value for the Pseudo-second order was obtained at 0.999, revealing that the adsorption process is best fitted into this model.Furthermore, the calculated qe value in the Pseudo-second-order model (qe cal = 10.834mg/g) is closer to the experimental qe value (qe exp.= 10.36 mg/g).These results are in agreement with Gomez 50 Kuczajowska-Zadrożna 51 and Zhang 52 .The rate of adsorption site occupation is assumed to be proportional to the square of the number of empty sites by pseudo-second-order kinetic theory.Table 9 displays Kinetic model parameters and constants for AR18 removal by HMNP, and Fig. 11 represents kinetic model plots.
Thermodynamic studies.Temperature is an important parameter in the adsorption process.Thermodynamic studies were carried out at five different temperatures to understand the effect of temperature on dye removal efficiency.Table 10 indicates that increasing temperature increase k and q e , which suggests that AR18 removal by HMNP can have higher efficiency at a higher temperature.The obtained values of Gibbs free energy changes (ΔG 0 ), enthalpy changes (ΔH ͦ ) and entropy changes (ΔS ͦ ) are presented in Table 10.ΔH 0 has a positive value (33.59 (kJ/mol)), meaning the adsorption process is endothermic.In other words, by increasing temperature, the removal efficiency increases since heating the active sites of adsorbents to high temperatures strengthens the bonds between the adsorbate molecules.Negative ΔG 0 (between -1.978 and -6.938 kJ/mol) indicated the spontaneous nature of dye removal.The positive amount of ΔS° (0.117 kJ/mol) can ascertain the increased randomness at the solid/liquid interface 53 .These results are in agreement with previous studies 54 .www.nature.com/scientificreports/Adsorption mechanism.Ion exchange, physisorption, and chemisorption are the three main divisions of the adsorption mechanism.The term "physisorption mechanism" refers to surface adsorption that doesn't interfere with the adsorbent's electronic orbitals or the adsorbate.Van der Waals interactions, electrostatic interactions, hydrogen bonds, diffusion, and hydrophobic interactions could all be involved.The opposite scenario is the chemisorption mechanism which involves valence and electronic orbital forces between the absorbent and adsorbate.It produces an irreversible chemical connection to the adsorbent's surface.Complex formation, chelation, covalent bonding, redox reaction, and proton displacement can all be part of the mechanism behind the chemisorption process [53][54][55] .The ΔH 0 value can be used to determine the physicochemical characteristics of adsorption; when it is between 0 and 20 kJ/mol, adsorption is physisorption; between 20 and 80 kJ/ mol, both physisorption and chemisorption occur; and between 80 and 400 kJ/mol, the adsorption is followed by chemisorption [56][57][58] .According to the calculated ΔH 0 (Table 10), the adsorption type for HMNP is physical-chemical adsorption.Due to the positively charged surface of HMNP in low-pH solutions, AR18 removal increases.The opposing charges on the molecules of AR18 and AR18 bring about electrostatic attraction between HMNP and dye.However, the outcome indicates that the AR18 and HMNP have no attraction for one other at high pH.As a result, at high pH, the elimination of AR18 molecules is reduced.The findings thus imply that chemisorption may be the mechanism of AR18 elimination in a low-pH solution.The elimination process may involve physisorption at high pH levels.Alternatively, surfactants can be utilized to improve the adsorption capacity of mineral adsorbents.HDTMA is one of the most often utilized surfactants for modification.The interaction of mineral absorbents with the hydrophobic tails of HDTMA ions, which replaces the Na + cation on the surface of the absorbent and causes the adsorbent's surfaces to be positively charged, causes the increase in adsorption capabilities.Because of the electrostatic interaction between the adsorbate and the surfactant-modified adsorbent, anionic dyes could be adsorbed 59,60 .
Reusability study of HMNP.For economic reasons, the reusability of the chosen adsorbent plays an important role in studies.By tracking the adsorption capacity of the adsorbent for five cycles under ideal conditions, the reusability of HMNP was examined.Desorption was carried out by eluting the AR18 adsorbed on HMNP with a 0.5 M NaOH solution following each run of adsorption.The modified pumices' good recyclability for AR18 adsorption is demonstrated in Fig. 12, which depicts a reduction in HMNP's adsorption effectiveness from 99 to 85% after five consecutive recycles.The reusability test results revealed that the prepared HMNP sorbent shows no considerable loss in its efficiency even after five cycles.Some previous studies on different contaminants like Antimony and phosphate prove this 61,62 .
Comparison of adsorbent with other reported adsorbents.The adsorption capacity of this study was compared with other pumice adsorbents reported by other researchers.Other studies have investigated the removal of  different pollutants by pumice, and their maximum adsorption capacities (Q max ) are listed in Table 11.In most studies, the maximum adsorption capacity has occurred at acidic.

Conclusion
In the present study, HMNP adsorbent has been synthesized for the adsorption of dye-containing industrial wastewater.CCD did prediction and optimization of the AR18 removal process with RSM.From obtained ANOVA results, it was understood that the AR18 initial concentration has the highest effect on the adsorption process while contact time has the lowest.The maximum adsorption capacity of HMNP was 12.84 mg/g with C 0 = 25 mg/l, adsorbent dosage 2.375 g/l and pH = 4.5.The Langmuir isotherm equilibrium model was found best fitted in this study, and adsorption kinetic data showed a good agreement with the pseudo-second-order.The adsorption process is defined to be endothermic and random due to the Positive ΔH o and ΔS o values.Moreover, negative ΔG can be considered evidence for the spontaneous nature of HMNP.In comparison to other adsorbents, HMNP has a low maximum adsorption capacity.Due to its easy accessibility, abundance, non-toxicity, and eco-friendliness, HMNP can be considered a useful adsorbent for low concentrations of AR18 despite its low adsorption capability.Nonetheless, it might be able to adsorb other pollutants more effectively.
www.nature.com/scientificreports/Insignificant terms (p values > N 0.05) were dismissed for the Development of the regression model equation:Determining optimal settings.Using the numerical optimization method from software, the maximum efficiency (Eff = 99%) was determined to occur at pH = 4.5, adsorbent dose = 2.375 g/l, contact time = 70 min and initial AR18 concentration = 25 mg/l.But the best AR18 removal efficiency was achieved at 98.8% in practice with the mentioned conditions.

Figure 9 .Figure 10 .
Figure 9. 2D and 3D and contour plots showing the effect of pH and adsorbent dose (a, c), initial concentration of AR18 and time (b, d).

Table 1 .
Independent variables and the levels of each variable.

Table 4 .
BET analyzes pore volume and average pore diameter of Nano-pumice.SorbentSpecific surface area (m

Table 5 .
Fitted models data for AR18 removal.

Table 6 .
ANOVA for Quadratic model.

Table 7 .
Isotherm and Kinetic models parameters and constants for AR18 removal by HMNP.

Table 8 .
Kinetic and isotherm models parameter.

Table 9 .
Kinetic models parameters and constants for AR18 removal by HMNP.

Table 10 .
The values of thermodynamic parameters of AR18 adsorption onto the HMNP.

Table 11 .
Comparison of maximum adsorption capacity of pumice in adsorbing various contaminants.