Amino-functionalized mesoporous MCM-41: an efficient adsorbent for the removal of chromium (III) ions from aqueous solution

The discharge of heavy metals such as lead, copper, zinc, manganese, cadmium, and chromium in water is of great concern as they may have adverse impacts on the environment and pose serious human health risks (Gupta et al. 2011; Saleh & Gupta 2012; Kumar & Chawla 2014; Chawla et al. 2015; Kumar et al. 2015). Chromium is one of the heavy metals which exists in different oxidation states with chromium (III) and (VI) as the most abundant species. Chromium (III) is likely to be oxidized to chromium (VI) after water treatment. Chromium enters into the environment through natural processes as well as human activities. Chromium metal is often added to steel during the preparation of stainless steel due to high corrosion resistance and hardness of chromium (Erdam et al. 2004). Chromium containing leather waste mainly consists of collagen and chromium (III) complexes (Mahmood et al. 2012). Chromium compounds are also used in textile, plating and dyeing industries. Sanyala et al. (2015), in a recent study, concluded that the aquatic ecosystems in the Ranaghat–Fulia region of West Bengal, India were greatly contaminated by chromium due to a number of handloom textile factories in the nearby area. The study also raised concerns regarding potential human health hazards through consumption of fish contaminated with chromium. The National Pollutant Release Inventory database (NPRI 2012) also listed 358 facilities that reported release of chromium compounds across Canada in 2012.

Chromium (III) is vital for normal operation of human vascular and metabolic systems as well as combating diabetes. The in vitro studies explained that chromium (III) has the possibility to cause DNA damage in cell culture systems (Eastmond et al. 2008; Khan et al. 2012). The WHO (2008) recommended the limit of chromium in drinking water be 0.05 mg/L. Chromium (VI) is the most dangerous form of chromium as it enters the blood stream and may cause health problems including: allergic reactions, skin rashes, respiratory problems, nose irritations and nosebleed, ulcers, weakening of immune system, alteration of genetic material, lung cancer, kidney and liver damage, and may even cause death of the individual (Mohan et al. 2006; Wang et al. 2014; Ghosh et al. 2015).

Many techniques, such as ion exchange, reverse osmosis, nano-filtration, precipitation, and adsorption have been used for the removal of chromium from water but are expensive and involve heavy instrumentation (Shaalan et al. 2001; Kozlowski & Walkowiak 2002; Ozaki et al. 2002; Rengaraj et al. 2002; Covarrubias et al. 2008; Golbaz et al. 2014; Kim et al. 2015). Adsorption is the most simple and economic method and involves simple laboratory equipment for the removal of chromium from water. Several studies have reported the application of various natural as well as synthetic adsorbents for the removal of various metal ions from water (Kumar & Chawla 2014; Chawla et al. 2015). For instance, a low-cost activated carbon (ATFAC) was synthesized from coconut shell fibers and utilized for chromium (III) removal from water/wastewater and evaluated for its adsorption capacity in comparison to commercially available activated carbon fabric cloth (ACF) (Mohan et al. 2006). The maximum adsorption capacities of ATFAC and ACF at 298 K were 12.2 and 39.6 mg/g, respectively. Chromium (III) adsorption increased with an increase in temperature and followed the pseudo-second-order rate kinetics. The biosorption of chromium (III) ions from aqueous solution was investigated using lichen (Parmelinatiliaceae) biomass (Uluozlu et al. 2008). The monolayer biosorption capacity of Parmelinatiliaceae biomass for chromium (III) ions was found to be 52.1 mg/g with pseudo-second-order kinetics. In another report, amine-based polymer, aniline formaldehyde condensate (AFC) was applied as an adsorbent for removal of chromium (III) ions in an aqueous environment (Kumar et al. 2009). Adsorption increased with pH and a maximum removal of 80% was reported at pH 6. Formation of a multi-dentate coordinate bond between [CrOH]²⁺ and and the deprotonated amino group (–NH₂) of AFC was suggested as a possible mechanism for removal of chromium (III) in an aqueous environment. Adsorption of chromium (III) on AFC followed the Langmuir isotherm model with maximum monolayer coverage of 30.77 mg/g (Kumar et al. 2009).

In recent years, much work has been done on the adsorption of heavy metals on mesoporous silica materials. These are mesoporous molecular sieves having a pore size between 2 and 50 nm. The widely used representatives of this family include MCM-41, MCM-48, and MCM-50. Of these, MCM-41 is the best known and most widely studied silica solid. It exhibits high specific surface areas, high crystallinity, high thermal stability, uniformity of hexagonal cylindrical pores, narrow pore distribution, and regulation of pore diameter from 1.5 to 10 nm (Aguado et al. 2009). The structures of mesoporous materials include amorphous silica containing an ample amount of silanol groups. The groups on the surface of these materials are weakly acidic which makes them an effective adsorbent and helps in the removal of dyes and heavy metal ions from their aqueous solution (Anbia et al. 2010; Wu et al. 2012; Gupta et al. 2013). The modified MCM-41 was used as adsorbent for the adsorption of organic dyes and metal ions from their aqueous solutions (Mittal et al. 2009, 2010; Qin et al. 2009; Northcott et al. 2010; Idris et al. 2011; Parida et al. 2012). There are two methods for surface modification of mesoporous silica materials, post-grafting and direct synthesis or one pot co-condensation. The one pot co-condensation method is dominant as it enables a higher and more homogenous coverage of moieties at the surface of mesoporous silica. Considering the important properties of functionalized silica-based materials, the present work aimed at the preparation of low-cost and more efficient amino-functionalized mesoporous material (NH₂-MCM-41) based on surfactant cetyltrimethylammonium bromide (CTAB) by the co-condensation method and its application as an adsorbent for the removal of chromium (III) metal ions from aqueous solution.

Tetraethylorthosilicate (TEOS) 98% used for the preparation of mesoporous silica material was of reagent grade and purchased from Sigma-Aldrich, India. The surfactant, CTAB with 99% purity was obtained from Merck and employed as a structure directing agent. The organoalkoxysilane selected for the functionalization process was 3-aminopropyltriethoxysilane (APTES) of 99% purity, obtained from Sigma-Aldrich, India. The source of metal ions for conducting the adsorption experiments was chromium sulfate 99.99% (Sigma-Aldrich, India). Sulfuric acid 98% (LOBA Chemie, India) and sodium hydroxide 97% (Sigma-Aldrich, India) were used for adjusting pH while carrying out the pH experiment. The apparatus was washed thoroughly first with chromic acid of AR grade then with double distilled water.

A mixture of 2 g of CTAB, 7 mL of 2 N NaOH and 80 mL of water was heated for 30 min at 80 °C. To the clear solution, 10 mL of TEOS and 1.34 mL of APTES were then added for the preparation of NH₂-MCM-41 (as-such). After the addition, white precipitates were formed after 3 minutes' stirring at 300 rpm. The reaction temperature was maintained at 80 °C for 2 hrs. The precipitates were filtered, washed and dried in a hot-air oven for 24 hrs. The as-such material (1 g) was acid extracted with a mixture of 100 m of ethanol and 1 m of concentrated HCl for removing entrapped surfactant. The material thus obtained was dried and stored in a clean vial for use as an adsorbent for the removal of chromium (III) ions from aqueous solution.

A stock solution of Cr₂(SO₄)₃.12H₂O of 1,000 ppm was prepared by dissolving 11.69 g in 1 L of double distilled water. This solution was stored in a volumetric flask. The chromium solutions of different concentrations were prepared by diluting the stock solution. The adsorption experiments were carried out in batch mode by shaking the appropriate amount of NH₂-MCM-41 with chromium (III) solution (50 mL) and the process of adsorption was studied systematically in terms of pH (3–8), contact time (30 min to 12 hrs), adsorbent dose (0.5 g/L to 2 g/L), and initial concentrations (20 mg/L to 300 mg/L) at different temperatures (293 K, 298 K, 303 K, 313 K) in the reagent bottles at 150 rpm. After shaking the bottles, NH₂-MCM-41 was separated from the solution by filtration and the final concentration of chromium (III) was obtained by recording the absorbance of supernatant solution by using an absorption flame emission spectrophotometer (AA-6200, Shimadzu).

All the experiments were carried out in triplicate and statistical analysis of data was performed using Microsoft Office Excel 2007 by applying regression analysis and one-way analysis of variance (ANOVA). Regression analysis gives R² value with value ranging from 0 to 1. An R² = 1 indicates that the regression line perfectly fits the data. One-way ANOVA was applied to check the significance of experimental data in terms of f-ratio.

The characterization of amino-functionalized mesoporous MCM-41 and adsorption of chromium (III) ions were investigated by various techniques. The results obtained are discussed in the following sections.

X-ray diffraction (XRD) is a technique commonly used for identification of the structure of crystalline materials and analysis of unit cell dimensions. This technique allows detailed characterization of crystalline samples, determination of unit cell dimensions, and quantitative determination of modal amounts of minerals in a sample.

Analysis of the surface of adsorbent was carried out using Micromeritics ASAP-2020. The surface area of adsorbent was determined by Brunauer, Emmett, and Teller (BET) N₂ sorption procedure with liquid N₂ at −195.7 °C. The BET surface area of NH₂-MCM-41 was found to be 155.1 m²/g.

The adsorption potential of NH₂-MCM-41 for chromium (III) removal from aqueous solution was investigated by varying experimental conditions such as pH, initial metal ion concentration, contact time, adsorbent dose, and temperature. The results of adsorption studies thus obtained are discussed in the following sections.

Contact time is a very important parameter which governs the process of adsorption and is one of the most effective factors in the batch adsorption process. In order to optimize the contact time for maximum adsorption capacity of the adsorbent, NH₂-MCM-41 for chromium (III), the effect of contact time on the adsorption of chromium (III) by NH₂-MCM-41 was investigated at adsorbent dose of 1 g/L for 50 ppm chromium (III) solution from 0.5 hr to 12 hrs. Rapid adsorption of chromium (III) occurs in the first 30 minutes which led to the removal of 85% chromium (III). Further, it was observed that the percentage removal of chromium (III) increased from 85% to 92% in the next 2 hrs. Thereafter, the adsorption of chromium (III) increased marginally until it reached the equilibrium state. Hence, the optimum contact time of 2 hrs was maintained in further studies. Qin et al. (2006) explained in a study that the transfer rate of metal ions to the surface of sorbent was faster in the early stage, which led to high adsorption, and afterwards, adsorption becomes almost constant due to the lesser diffusion rate of metal ions into the intra-particles pores of the adsorbent.

Adsorbent dose plays a significant role in adsorption as it refers to the availability of adsorption sites. The effect of adsorbent dose was examined by varying the adsorbent dose from 0.25 g/L to 2 g/L for 50 ppm chromium (III) solution for contact time of 0.5 hr to 2 hrs. It was observed that the percentage removal of chromium (III) increased to 92% on increasing the dose of NH₂-MCM-41 from 0.25 g/L to 1 g/L for contact time of 2 hrs. The increase in chromium (III) adsorption with increasing adsorbent dose is due to increase in adsorbent surface area and availability of more adsorption sites. Study revealed that the adsorption sites remain unsaturated up to adsorbent dose 1 g/L and the number of available sorption sites increases. However, further increase in the adsorbent dose barely improved the percentage removal of metal ion. Therefore, the optimum dose of 1 g/L was fixed for further experiments. This may be attributed to overlapping of sorption sites as a consequence of overloading of adsorbent particles (Kumar & Gayothri 2009).

The effect of initial chromium (III) ion concentration was studied by taking various concentrations of chromium (III) solution ranging from 20 mg/L to 300 mg/L with optimum dose of 1 g/L and contact time of 2 hrs. It was observed that the adsorption of chromium (III) decreased with increasing concentration of chromium in the aqueous solution. At low concentration of metal ion, more of the amine sites were available to bind with metal ions whereas at high metal ion concentration, the numbers of amine binding sites available are less with respect to the metal ions in the solution. This results in unfavorable conditions and hence, the adsorption of metal ions decreases. Shroff & Vaidya (2011) explained that increase in metal ions concentration caused more concentration gradient that led to a higher likelihood of collision among metal ions and the active adsorption sites, thereby increasing adsorption capacity. With further increases in metal ion concentration, the adsorption capacity remained constant because of saturation of the active adsorption sites.

Adsorption isotherms are quite helpful for determining the distribution of metal ions between solution and sorbents. Langmuir adsorption isotherm, Freundlich adsorption isotherm, and Temkin adsorption isotherm models were applied for adsorption of chromium (III) on NH₂-MCM-41 to evaluate the isotherm constants.

The effect of temperature was studied by adding optimum adsorbent dose of 1 g/L to aqueous solutions of chromium (III) of appropriate concentrations at different temperatures, i.e., 293 K, 298 K, 303 K, and 313 K. The reagent bottles were stirred at 150 rpm in a temperature controlled orbital shaker at the appropriate temperature. From the study, it was observed that the adsorption capacity of sorbents increased with increasing the temperature, indicating the endothermic nature of adsorption.

The thermodynamic parameters recorded for the adsorption of chromium (III) on NH₂-MCM-41 are shown in Table 4. It can be seen from the data that the value of K_D (distribution ratio) is increasing with rising temperature, also indicating the endothermic nature of adsorption. The enthalpy (ΔH⁰) and the entropy changes (ΔS⁰) for the adsorption processes were calculated to be 8.9 KJ/mol and 0.039 KJ/mol/K, respectively. The positive value of ΔS⁰ indicated that there is an increment in the randomness in the system solid–solution interface during the adsorption process. This increase of disorder in the solid–solution could be the result of extra translational entropy gained by the water molecules previously adsorbed onto adsorbent but displaced by metal ions. The positive value of ΔS⁰ reflected the affinity of the NH₂-MCM-41 for chromium (III) ions. In addition, the positive value of ΔH⁰ indicated that the adsorption was endothermic. The negative values of ΔG⁰ at various temperatures indicated the spontaneous nature of the adsorption process.

The value of square of regression correlation coefficient (R²) was determined for both the plots and calculated q_e values allowing one to check the agreement between the predicted and experimental capacity of adsorbent towards chromium (III) ions. R² of the developed pseudo-second-order reaction kinetics was found to be very close to unity compared to pseudo-first-order rate equation (Table 5) and calculated q_e value was found to be in good agreement with the experimental q_e value, which confirmed that the kinetics of adsorption of chromium (III) using NH₂-MCM-41 was best explained by the pseudo-second-order model rather than the pseudo-first-order model (Figure 9(a) and 9(b)). This could be explained by the adsorption of chromium on adsorbent occurring through the chemisorption mechanism which involves the valence forces through sharing or exchange of electrons.

It is important to point out that contamination of water bodies by chromium due to industrial discharge is a serious issue as chromium has been detected in environmental water samples. It may enter the body through ingestion of contaminated water or fish from contaminated water and may pose unknown risks that are not well explored to date. There are many knowledge gaps regarding human health implications due to chromium toxicity because of insufficient in vivo studies. Sanyala et al. (2015), in a recent study, concluded that the aquatic ecosystems in the Ranaghat–Fulia region of West Bengal, India were greatly contaminated by chromium due to the number of handloom textile factories in the nearby area. There is an urgent need for cheap and efficient remediation methods as well as more in vivo toxicity studies being required in order to ensure the presence of safer concentrations of chromium ions in aquatic bodies. The present study is very useful for practical application as it provides a very economic method for industrial effluent treatment.

Chromium contamination has been a serious environmental issue for many years and many research groups have attempted to find various adsorbents for the removal of chromium ions. The compiled information of different types of sorbents used for the removal of chromium (III) ions in aqueous environments is presented in Table 6. It is clear from the table that all the sorbents already reported in the literature for the removal of chromium (III) ions from aqueous solution showed adsorption capacity less than 50 mg/g except for activated carbon synthesized from corncob waste by Correa et al. (2013). In other words, it can be concluded that NH₂-MCM-41 possesses superior or comparable adsorption properties and can be applied as a promising adsorbent for removal of chromium ions from water.

In the present study, the amino-functionalized MCM-41 prepared by the co-condensation method was successfully prepared and applied as an effective adsorbent for the removal of chromium (III) ions from aqueous solutions. The Langmuir isotherm model is best fitted with the experimental data with high values of regression coefficient (R² = 0.996–0.998) compared to the Freundlich and Temkin models, indicating monolayer adsorption of chromium ions on the surface of NH₂-MCM-41. The maximum Langmuir adsorption capacity was 83.33 mg/g. Kinetics of the adsorption process can be best explained by a pseudo-second-order model rather than pseudo-first-order model. The thermodynamic calculation showed that adsorption of the chromium ions on the surface of NH₂-MCM-41 is an endothermic process of a spontaneous nature. The results obtained in batch mode studies showed that NH₂-MCM-41 can be used as a viable and economic adsorbent for the removal of chromium (III) ions from aqueous solution.

The authors are grateful to the Centre of Emerging Life Sciences, Guru Nanak Dev University, Amritsar for providing the instrumentation facilities.