Remediation of cationic and anionic dyes from water by histidine modified mesoporous silica

ABSTRACT The organic dye is one of the most problematic water pollutants in the vicinity of leather and textile industries. Adsorption method via functional materials can be considered as an effective method for dye removal from wastewater. In present work, the synthesis of mesoporous silica nanoparticles (MSNs) functionalised with histidine (MSN-His) has been reported for simultaneous removal of methylene blue (MB) and phenol red (PR) from an aqueous solution. The properties of nanoparticles were characterised in terms of size, surface area and surface modification using a scanning electron microscope (SEM), transmission electron microscope (TEM) and Fourier transform infrared (FT-IR). The influence of various factors including the contact time, the initial concentration of the adsorbate and pH of the medium on the MB and PR adsorption efficiencies were investigated. The maximum adsorption capacity was calculated to be ca. 65 mg/g for MB in basic media, and ca. 55 mg/g for PR in acidic media. The adsorption isotherm is fitted with Freundlich for all solutions. The maximum removal efficiency was ca. 70% for MB in basic media and PR in acidic media after second cycle.


Introduction
Remediation of wastewater contaminated with toxic organic compounds has attracted attention worldwide. These organic contaminants are present in textiles, plastics, cosmetics and pharmaceuticals industrial wastes. In particular, synthetic organic dyes can be harmful to human health and they are generally considered as a human carcinogen. Many technologies have been developed to remove toxic contaminants, such as electrocatalytic/photocatalytic degradation [1,2], chemical oxidation/reduction [3], biological treatments [4,5], membrane filtration [6] and adsorption [7]. Adsorption process has been considered to be one of the most effective methods due to its high feasibility, relatively low cost and simple operation [7][8][9]. Many materials have been applied for removal toxic contaminants, including zeolites [10], activated carbon [11], clay [12,13], cellulosic derivative materials [14], metal-organic framework (MOF) [15,16] and mesoporous materials [17][18][19].
To increase the efficiency of dye adsorption, silica nanoparticles have been doped with some elements such as aluminium [40], bismuth and iron [41]. Furthermore, MSNshave been fabricated with functional groups such as amine [42][43][44][45][46] or carboxylic acid [47,48]. Adlnasab et al. synthesised amine functionalised MSNs as an adsorbent to remove alizarin yellow and phenol red from wastewater [45]. The maximum adsorption capacity of the material was 370.70 mg g −1 and 400 mg g −1 for phenol red and alizarin yellow, respectively. Shariati et al. prepared amine functionalised porous silica for removal Ponceau 4 R, Rhodamine B, Sunset Yellow and Brilliant Blue from aqueous solutions [46]. The fabricated material was able to remove more than 92% of the dyes. Deka et al. have reported the synthesis of MSNs functionalised with carboxylic acid to be used as an adsorbent for different types of dyes: methylene blue (MB), phenosafranine (PF), rhodamine B (RhB) and orange II (OII) [47]. The material showed excellent adsorption capacities for cationic dyes in comparison to MSNs, but the opposite for anionic dye.
The presence of amine and carboxylic groups in the same material may improve dye adsorption. Amino acids contain amine and carboxylic groups attached to a carbon atom [49,50]. Several studies has been reported for the modification of silica nanoparticles with amino acids for environmental applications [51][52][53]. Silica nanoparticles are functionalised with methionine and cysteine for potential Uranium (VI) remediation from water [51]. Magnetic mesoporous silica have been modified with cysteine for potential Pb (II) removal from water with high adsorption efficiency [52]. However, to the best of our knowledge, there is no work in the literature on the modification of mesoporous silica surface with histidine for dyes remediation.
To the best of my knowledge, very little work has been reported on modification of nanoparticle surface with amino acid for environmental application. In this work, mesoporous silica nanoparticles (MSNs) have been functionalised with positively charged amino acid (histidine) using coupling agents (EDC/NHS) for dyes removal from aqueous solution. MSNhistidine has been applied as an absorbent for removal anionic dye (phenol red (PR)) and cationic dye (methylene blue (MB)) from water. The adsorption of selected dyes on MSNhistidine was investigated at different pH values, initial concentration and contact time. Furthermore, the adsorption of dyes on the nanomaterials was assessed using kinetic models and adsorption isotherms. This nanomaterial has characterised by BET, IR, SEM and TEM.

Mesoporous Silica Nanoparticles(MSNs)
In 250 mL flask, 1 g of CTABwas dissolved in 150 mL DI water. Ammonium hydroxide (3.5 mL) was added to the aqueous solution at 40°C under stirring. A mixture (25 mL) of n-hexane: TEOS (4:1) was added drowsily in the solution. After 18 h, the solid was filtrated and washed with DI water and ethanol. The solid (1.5 g) was dispersed in a mixture of concentrated HCl (5 mL) and methanol (90 mL) at 80°C. The solid was centrifuged and washed with ethanol.
In 25 mL flask, 0.5 g of MSN-AP was suspended in a mixture of DCM (5 mL) and pyridine (5 mL). Succinic anhydride (0.5 g) was added to the reaction mixture and sonicated for 20 min. The suspension was stirred at 25°C overnight. MSN-COOH was separated and washed with DMF.

Histidine -functionalised MSNs (MSN-His)
MSN-COOH (200 mg) was suspended in DMF (2 mL) and sonicated for 15 min. To the suspension, 200 mg of EDC and 200 mg of NHS were added and sonicated for 20 min. The reaction mixture was stirred at 25°C overnight. The solid was separated and washed with DMF.
MSN-NHS (100 mg) was suspended in DMF (10 mL) and sonicated for 15 min. To the suspension, 50 mg of histidine and 200 µL triethylamine were added at 25°C overnight. MSN-His was separated and washed with DMF and ethanol.

Adsorption Studies
The behaviour of MB and PR adsorption in MSN-His was investigated at different pH values, initial concentrations and contact time. Different concentrations of MB and PR (50,80, 100, 120, 150 mg L −1 ) were prepared in DI water. In each experiment, 10 mg of MSN-His were suspended in 10 mL of dyes solution and stirred (150 r/min) at 25°C. The solid was separated by centrifugation. MB and PR content were measured by UV-Vis spectrophotometer.

Measurement and Characterisation
BET surface area: prior to analysis, the samples were degassed at 130°C for 12 h. Micromeritics Gemini 2375 volumetric analyser was utilised to estimate the material surface area using N 2 physisorption isotherms. The surface areas were calculated by Brunauer-Emmett-Teller (BET) model at a relative pressure (P/P o ) of 0.05-0.25. The total pore volume was estimated from N 2 adsorbed at a relative pressure of 0.99. Barrett-Joyner-Halenda (BJH) model was applied to estimate the average pore size distribution.
Scanning Electron Microscopy (SEM) Samples SEM images were taken by JEOL JSM-6380 LA instrument. Transmission Electron Microscopy (TEM): TEM images for samples were acquired by JEOL JEM-1230 instrument. For the preparation of the TEM sample, the nanoparticles were dispersed in high-purity ethanol via ultrasonic equipment. Two to three droplets of a dilute ethanol solution of the nanoparticles were placed onto a copper mesh (2000 square mesh) on a carbon film. The samples were dried under vacuum. FTIR Spectroscopy: the structural group composition of samples was studied by Thermo Scientific Nicolet IS10 in KBr pellets in the region of 4000-400 cm −1 . UV-Vis spectrophotometer: UV spectra of MB and PR was obtained by Shimadzu (UV-2600).

Results and Discussion
MSN-His was prepared in five steps. MSNs were obtained by the condensation reaction of TEOS in basic solution in the presence of CTAB and n-hexane, as expander agent. CTAB was removed by ion exchange method, to allow reaction in the inner surface. MSNs was reacted withAPTES to obtain aminopropyl groups conveniently bonded on the MSNs surface. To obtain MSN-COOH, MSN-AP reacts with succinic anhydride. To improve the coupling efficiency, EDC was used in conjunction with NHS to activate carboxylic groups on MSN-COOH. The primary amine groups in histidine can react with NHS-esters on the surface, producing MSN-His (Figure 1).
Bare MSNs were characterised using SEM and TEM techniques. SEM image exhibited that the nanoparticles are almost spherical in shape particles with size ranging between   Figure 2B shows TEM image for MSNs. It can be seen that the size of fabricated MSNs was estimated to be 320 nm, with a spherical shape. Moreover, the MSNs have well-ordered porous with an average size of approximately 5.5 nm.
To confirm the modification process, FT-IR techniques were used to identify the organic molecules attached to the MSNs surface ( Figure 3). In all sample spectra, peaks at 1200-1100 cm −1 and ~810 cm −1 were observed, assigned Si-O bands stretching of the silica network. Peaks of C-H groups in MSN-AP were appointed at ~2900 cm −1 and ~1430 cm −1 . For MSN-COOH, a new peak appears at ~1700 cm −1 , which was assigned to -C = O stretching of the carbonyl group in carboxylic acid. After surface derivatisation  Nitrogen adsorption/desorption isotherm for the fabricated nanoparticles is shown in Figure 4. According to the physisorption data, Type IV isotherm is observed for all samples, which is typically for the ordered mesoporous materials. The hysteresis loops of MSN-AP and MSN-His were observed to be relatively lower pressure than bare MSNs. As shown in Table 1, the surface area, pore size and pore volume decrease, confirming the modification process. The pore size of bare MSNs was estimated to be ca. 5, which is in a good agreement with TEM image.
To estimate the molecules anchored to MSNs surface, the elemental analysis was used to measure percentages of carbon, hydrogen and nitrogen in bare MSNs, MSNs-AP, MSN-COOH and MSN-His, as shown in Table 2. It is clear that there is an increase in the percentage of C, H and N elements as the surface of MSNs is modified, indicating the successful attachment of the functional groups in the surfaces.  MSNs-His surface has zwitterionic, anionic or cationic character, depending on solution pH values. MSNs-His has zwitterionic character at intermediate pH. At pH above the pKa of amines, carboxylic acid groups ionise while the amine groups deprotonate, resulting anionic character surface. On the other hand, the amine groups protonate while the carboxylic acid groups are no longer ionised at pH below the pKa of carboxylic acid groups, leading to have cationic character surface.
Different initial concentration of dyes (50, 80, 100, 120, 150 mg L −1 ) were used to investigate the removal of MB and PR from aqueous solution by MSN-His at 25°C. According to equation (1), the adsorption capacity (q e ) was calculated: q e is adsorption capacity at equilibrium in mg. g −1 . C 0 and C e are initial concentration (mg. L −1 ) and concentration at equilibrium (mg. L −1 ) for selected dyes, respectively. V represents the volume of selected dyes solution (L) and m is the mass of MSN-His (g). After modification with histidine, carboxylic and amine groups are present on the surface of MSN-His. Depending on the pH values, the surface of MSN-His can be a cationic, zwitterionic, or anionic character. Amino groups are protonated when the pH becomes below their pKa. A surface with zwitterionic character is obtained at intermediate pH. At pH below 1.5, the carboxylic acid groups are no longer ionised; therefore, a cationic character surface is obtained. When the surface becomes cationic character, the PR adsorption reaches the maximum adsorption capacity and the MB adsorption reaches the minimum adsorption capacity. In contrast, at high pH value, the carboxylic groups are no longer ionised and the amine groups become deprotonated, resulting in an increase in the capacity adsorption of MB and reduce the amount of PR adsorbed on the surface, as presented in Figure 5.
In Figure 6, it was observed that as the concentration of dyes increased, the amount of dyes adsorbed increased at different pH values. In basic media, the amount of MB adsorbed on MSN-His was the heights, whereas PR was the lowest, due to the interaction between negative charge surface of MSNs-His and MB (cationic dye), and repulsion force between the surface and PR (anionic dye). In contrast, the amount of PR adsorbed was the heights in acidic media, while MB was the lowest, due to the positive charge on MSN-His surface. The effect of contact time on the selected dyes adsorption on MSN-His was investigated at different exposure time. As shown in Figure 7, the amount dyes adsorbed on MSN-Hissurface increased as the contact time increase during 90 min, which may assign to the accessible adsorption sites. In basic media, the amount of MB adsorbed reached ca. 65 mg. g −1 in 100 min, comparing to 25 mg. g −1 in acidic media at the same time. On the other hand, the maximum amount of PR adsorbed was ca. 50 mg. g −1 in acidic media comparing to ca. 30 mg. g −1 in basic media in 90 min. At intermediate pH, the maximum amount adsorbed of both dyes was ca. 30 mg. g −1 at 100 min.
A number of studies has been reported in literatures about the removal of MB and PR from aqueous solutions using different materials. Table 3. shows the performance of MSN-His, comparing to different materials reported in the literatures.
Three different isotherm models have been applied to explain dyes adsorption on MSN-His. Langmuir model isotherm characterized by:  C e represented yes concentration (mg. L −1 ) and q e is the adsorption capacities at equilibrium (mg. g −1 ). q m represents the maximum dyes adsorption capacities (mg. g −1 ). k l is the Langmuir constant (L.mg −1 ) [64]. Freundlich model isotherm characterizes by: 1/n represents the adsorption intensity. K F is the Freundlich adsorption constant ((mg. g −1 )/ (mgL −1 ) 1/n ) [65]. Temkin model isothermcharacterizes by: R represents gas constant (8.314 J −1 mol −1 K −1 ). b T and k T are a constant related to the heat of adsorption and the Temkin isotherm constant (L −1 g −1 ), respectively, T is the absolute temperature in Kelvin. Dyes adsorption isotherms on MSN-His surface is presented in Figures 1S and 2S, and the relevant parameters are reported in Table S1. The adsorption equilibrium can be fitted with Freundlich model in both dyes and different pH solutions rather than Langmuir or Temkin models. This result suggests that dyes adsorption occurs heterogeneity and prefers to occupy the stronger binding sites.
Pseudo first-and second-order kinetic models have been fitted for MB and PR adsorptions on MSN-His surface. The pseudo-first-order equation is: log q e À q t ð Þ ¼ log q e ð Þ À k 1 2:303 t (5) q t is the adsorption capacity at time t (mg. g −1 ). k 1 is the pseudo-first-order adsorption rate coefficient (L.min −1 ). The pseudo-second-order equation is: k 2 is the pseudo-second-order adsorption rate coefficient (g.mg −1 .min −1 ) [66].
To understand the adsorption mechanism of selected dyes on MSN-His surface, two kinetics models are used. As shown in Figures 3S and 4S, linear relationships can be expressed based on pseudo-first-order and pseudo-second-order kinetics models. The parameters of q e , k 1 , k 2 and R 2 are presented in Table S2. In intermediate pH, the adsorption mechanism of both dyes are fitted with the pseudo-first-order model. MB adsorption in acidic media and PR adsorption in basic media can also be fitted with the pseudo-first-order model. However, the correlation coefficient values (R 2 ) are closed to 1 when PR adsorption in acidic media and MB adsorption in basic media are plotted with the pseudo-second-order model, comparing to the pseudo-first-order model.
Regeneration performance of proposed adsorbents for dyes removal was evaluated. Dyes@MSN-His His was washed several times with acidic and basic aqueous solution under ultrasonication; then, the solid was separated and dried at 80°C overnight. MSN-His was used again for dyes removal (150 ppm) at different pH values of solutions. As shown in Figure 8, the adsorption performance of MSN-His decreased after the cycling process. The maximum removal efficiency was ca. 70% for MB in basic media and PR in acidic media after second cycle

Conclusion
In this work, mesoporous silica has been synthesised with relatively high surface area, 950 m 2 .g −1 , and 280 nm particle size. The surface of the nanoparticles was decorated with aminopropyl groups, followed by reaction with succinic anhydride. Histidine was successfully attached to the MSN surface via EDC/NHS coupling reaction. The characteristic of MSN-His is zwitterionic at intermediate pH, cationic below the pKa and anionic above the pKa. When the concentration of MB and PR increased, the amount of adsorbed dyes increased. The maximum amount of MB was ca. 65 mg. g −1 in basic media, whereas PR reached ca. 55 mg. g −1 in acidic media, due to the interaction between the surface charge and dyes. For all solutions, the adsorption isotherm was found to be fitted with the Freundlich model. The kinetics dyes adsorption was fitted with the pseudo-first-order model in intermediate pH, and in acidic media for MB adsorption and PR adsorption in basic media. PR adsorption in acidic media and MB adsorption in basic media were fitted with the pseudo-second-order model. The maximum removal efficiency was ca. 70% for MB in basic media and PR in acidic media after second cycle.

Data availability
(1) SEM and TEM were utilized to support the findings that the prepared materials are MSNs.
(2) FTIR spectra and elemental analysis were used to support the findings that the functional groups were successfully anchored onto the surface. (3) The adsorption of MB and PR on MSN-His vary with pH, supporting the finding that the dyes are affected by a surface charge on the surface. (4) Adsorption isotherm models were used to support the findings that all data can be fitted with Freundlich model. (5) The kinetics of MB and PR adsorptions were used to support the findings that data can be fitted with pseudo-first-order pseudo and second-order kinetic model, depending on the pH.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This research received no external funding.