An ion-imprinted imidazole-functionalized ordered mesoporous silica for selective removal of chromium(VI) from electroplating effluents

In this work, ion imprinted technology incorporated with mesoporous silica materials (MCM-41) to obtain the novel specific adsorbent, ion imprinted mesoporous silica. Cr(VI) imprinted mesoporous silica (Cr(VI)IMS) was synthesized and used for adsorption studies and waste water application. A synthesized imidazolyl silane agent act as the functional monomer in the imprinted process to build up highly ordered functionalized imprinted materials. The chemical composition, porosity, and highly ordered morphology were characterized by Fourier transform infrared spectroscopy (FTIR), solid state nuclear magnetic resonance (NMR), Brunauer–Emmett–Teller (BET) method, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), respectively. The Brunauer–Emmett–Teller (BET) surface area was 1054.51 m2 g−1 in this study. The Cr(VI)IMS showed great adsorption capacity to hexavalent chromium ions in acidic solution up to 45.6 mg g−1. Cr(VI)IMS reached the adsorption equilibrium in a short time (10 min) at acid and weak acid conditions, while most of adsorbents need more than 30 min to achieve adsorption equilibrium. Cr(VI)IMS displayed much higher adsorption capacity to Cr(VI) ions than other negative ions. The relative selectivity coefficient was 2.56, higher than those of other anions (below 1.5). After eight adsorption-regeneration cycles, the adsorption efficiency of Cr(VI)IMS still reached 92.5%. The Cr(VI)IMS was found to exhibit equivalent property after multiple cycles of experiments, indicating good repeatability and reproducibility.


Introduction
Chromium is widely used in various industries, such as plating, tanning, paint and pigment production. and metallurgy, which could possibly contaminate the environmental waters if it is not treated standardly (Gómez and Callao 2006). Chromium element in waste water is mainly existed as Cr(III) and Cr(VI). Cr(III) is an essential component having an important role in the glucose, lipid, and protein metabolism, whereas Cr(VI) poses a definite adverse impact on living organisms (Lin and Huang 2001;Saikia et al. 2011). Cr(III) can be precipitated at high pH (Petruzzelli et al. 1995), while Cr(VI) ions are highly soluble at different pHs (Li et al. 2013) and can be transported over a great distance existing in a variety of oxo-species, namely, dichromate (Cr 2 O 7 2− ), hydrochromate (HCrO 4− ), or chromate (CrO 4 2− ), which threatens environment and creature health. Above all, it is indispensable to develop efficient method to cope this problem.
Adsorption method is regarded as one of most promising techniques because of the convenience and high efficiency, which has attracted continuous attention in recent decades (Fu et al. 2019;Li et al. 2020). Activated carbon (Mohan and Pittman, 2006) and functional chitosan (Boddu et al. 2003) have been utilized to adsorb Cr(VI). Common adsorbents are seldom designed and synthesized according to the chemical structure of targets, which show low adsorption capacity and broad-spectrum adsorption.
To enhance the adsorption performance, ion imprinting technology refers to the technique of cross-linking functional monomers in the presence of template ions to obtain polymers. The imprinting cavities structurally and chemically complementary to templates in resultant polymers display specific binding ability to template ions (Mamo et al. 2020;Zhang et al. 2003). Benefit from the imprinting process, the imprinted ions are accessible to the recognition sites and captured in the imprinted polymers which can act as specific adsorbents for given targets. Base on coordination between metal ions and with lone-pair electrons, the study of cation imprinting has been developed in the past decades like Pb(II) ions (He et al. 2015;Shafizadeh et al. 2019) and Hg(II) ions (Velempini et al. 2019). The studies on anion imprinting are rarely reported, which is possibly due to the lack of recognition sites to form a strong interaction. Generally, electrostatic interaction between functional monomers and anions is a usual principle in anion imprinting (Wu 2012). Tavengwa et al. (2013) used 4-vinyl pyridine as a functional monomer to prepare magnetic hexavalent chromium imprinted polymer, which was able to selectively adsorb hexavalent chromium ion. In the study of Liu et al. (2013), chitosan was adopted as matrix material to prepare ion imprinted chitosan composite for the adsorption of hexavalent chromium in polluted water. Resorting to large specific surface area of nanomaterials, like nanoparticles and nanotubes, surface imprinted layers are thin and remarkably improve the performance of imprinted polymer, compared with the traditional micron-sized polymers. However, the polymerization process is difficult to control and the nanomaterials were easy to adhere with each other, which decreases the surface area.
Ordered mesoporous materials possess regular mesoscopic structure and tunnels, which means that the combination of ion-imprinting technology and mesoporous materials is expected to avoid the deep embedment of imprinted sites and the long adsorption time. Imprinted ordered mesoporous materials possess high adsorption capacity, high selectivity, and high adsorption rate. Grafting imprinted polymers on the mesoporous materials can lead to decrease of surface area, pore blocking, and heterogeneous distribution of binding sites (Hoffmann et al. 2006). Imprinted mesoporous silica prepared by one-step co-condensation can solve the problems mentioned above. Recently, several imprinted functionalized mesoporous materials have been synthesized by this method, focusing on the targets of Pb(II) ions (He et al. 2015), Dy(III) ions (Zheng et al. 2016), phosphopeptides (Chen et al. 2016), U(VI) and Cs(I) ions (Zhou et al. 2021), Ni(II) ions (He et al. 2018), and Pt(II) ions (Dobrzyńska et al. 2021). Imprinted mesoporous silica prepared by the co-condensation (one-step method) simplifies the synthesis process and effectively avoid the blockage of mesoporous channels. The imprinting sites were uniformly distributed in the pore wall of mesoporous materials. Hexavalent-chromium-anion imprinted mesoporous silica (Cr(VI)IMS) by using co-condensation method has not been reported yet. N-containing functional groups such as amino, imidazole, and pyridine can positively charge via a protonation process, so that polymer monomers containing such groups are often used as functional monomers for anion imprinted polymers. In this work, the functional monomer with imidazole group was used to prepare hexavalent chromium imprinted mesoporous silica via a co-condensation method. Chemical groups, porosity, and micromorphology of the imprinted mesoporous silica were characterized. The adsorption performance of materials to Cr(VI) ions was investigated by different adsorption experiments. Cr(VI)IMS was utilized to remove Cr(VI) ions from the waste water.

Instruments
Fourier transform infrared spectroscopy (4000-400 cm −1 ) was conducted by a PerkinElmer Spectrum Two Fourier transform spectrophotometer. 13 C NMR spectra were collected on a Bruker AVANCE III HD 400 superconducting NMR spectrometer. Small angle X-ray diffraction (XRD) inspections were recorded on a Bruker D8 Advance Scattering system using Cu K α (λ = 1.5405 Å). Transmission electron microscopy images and scanning electron microscopy images were conducted on a HITACHI H-7650 transmission electron microscope and a HITACHI SU8010 field emission scanning electron microscope, respectively. N 2 adsorption-desorption measurements were carried out by using Micromeritics ASAP-2020 surface area and porosimetry analyzer. All anion concentrations were inspected by Shimadzu UV-1750 spectrophotometer.

Preparation of mesoporous silica MCM-41
Adopted to the method published (Chen et al. 2016), 200 mg CTAB and 58 mg NaOH were dissolved in 100 mL deionized water and stirred for 20 min at 80 °C. Then, 5 mmol TEOS was added into the mixture sequentially and continued stirred for 5 h. The mixture was then filtered and washed with deionized water and ethanol to remove the unreacted chemicals. Surfactant CTAB was eluted by HCl-ethanol solution (1:9, v/v).

Preparation of CPTES-Imi
The synthesis method of CPTES-Imi was referred to Adam's method (Adam et al. 2013). A total of 0.4 mmol CPTES and 0.4 mmol Imi were mixed and stirred in nitrogen atmosphere for 48 h at 90 °C.

Preparation of Cr(VI)IMS
Cr(VI) imprinted mesoporous silica (Cr(VI)IMS) was synthesized by hydrolysis of CPTES-Imi (functional monomer) and TEOS (Fig. 1). Firstly, 115 mg K 2 Cr 2 O 7 was added into 5 mL deionized water, followed by adding 0.4 mmol functional monomer. After pre-reacting for 2 h, 200 mg CTAB and 58 mg NaOH were dissolved in 100 mL deionized water by stirring for 30 min at 80 °C. A total of 5 mmol TEOS was added into the mixture dropwise. After stirring for 4 h, the solid product was filtered and washed by deionized water, ethanol, and HCl-ethanol solution (1:9, v/v) separately numerous times to extract chromium(VI) ion and the surfactant. The non-imprinted mesoporous silica (NIMS) was prepared with the same procedure except that no K 2 Cr 2 O 7 was added into the initial mixture. Cr(VI)IMS and NIMS were dried at for 12 h at 100 °C under vacuum for characterization or further adsorption experiments.

Sample preparation for characterization
Exactly 1 mg of sample was put into approximately 200 mg dry KBr powder that was dried under an IR lamp. The mixture was ground and mixed evenly in an agate mortar. The mixture was pressed into a transparent sheet before FT-IR test. Before small angle XRD test, around 500 mg of dry sample was pressed in sample tank by smooth glass. In nitrogen adsorption desorption analysis, 100 mg dry sample was quickly transferred to the bottom of the sample tube after vacuum dry. The sample tube was quickly installed on the instrument and the test starts immediately.

Adsorption experiments
Adsorption experiments were conducted by adding 10 mg Cr(VI)IMS or NIMS into 10 mL solution at 25 °C. All experiments were carried out in triplicate. The concentration of Cr(VI) was measured by spectrophotometric method after reacted with diphenylcarbazide. To investigate the effect of pH of solution on adsorption ability of Cr(VI)IMS or NIMS to Cr(VI) ions, Cr(VI)IMS or NIMS was added into Cr(VI) solutions (200 mg L −1 ) at different pH (2.0-10.0) with 3 h shaking. The respective solution pH was adjusted using 0.5 mol L −1 HCl/NaOH solutions.
The equilibrium adsorption experiments were conducted in a Cr (VI) solution (initial concentration: 5, 10, 20, 50, 100, 150, 200, 300, and 400 mg L −1 ) at pH 3.0 with 3 h shaking. The binding capacity Q was calculated by Q = (C 0 -C e )V/m, where C 0 and C e represented the initial concentration and equilibrium concentration of Cr(VI), while V and m were the volume of solutions and the mass of adsorbents. Imprinting factor (IF) is defined as IF = Q I /Q N , where Q I and Q N represent the binding capacity of Cr(VI)IMS and NIMS.
In the adsorption kinetic study of Cr(VI)IMS, we selected Cr(VI) solutions of two levels concentration (50 mg L −1 and 200 mg L −1 ) at pH 3.0 and measured the concentration of Cr(VI) ions at different adsorption time.
Dihydrogen phosphate and nitrate were chosen for comparison to study the adsorption selectivity of Cr(VI)IMS. Cr(VI)IMS was dispersed in the solution containing 1 mmol L −1 Cr(VI), H 2 PO 4 − , or NO 3 − at pH 3.0. The concentration of dihydrogen phosphate and nitrate were measured by

Reusability and reproducibility
For the reusability test of Cr(VI)IMS, Cr(VI)IMS adsorbed Cr(VI) in the solution of 200 mg L −1 . Then, Cr(VI)IMS was eluted by 2 mol L −1 HCl and dried at 85 °C for regeneration. The Cr(VI)IMS endured 8 adsorption-regeneration cycles. Five batches of Cr(VI)IMS synthesized with the same method were utilized for reproducibility study.

Removal of Cr(VI) in industrial samples
The wastewater sample was collected from Zhaoqing Dawang Electroplate Industrial Park (Guangdong, China) with clean polyethylene bottles. The untreated sample of pH 4.0 contained 22 mg L −1 Cr(VI). It was then filtered by 0.45 μm membrane. Different amounts of Cr(VI)IMS were added into 500 mL wastewater with 2 h shaking.

Characterization
We utilized FT-IR and solid-state 13 C NMR to confirm the functional chemical groups in Cr(VI)IMS. Figure 2a is the IR spectra of Cr(VI)IMS. Curves 1 and 2 represented the IR absorption signals of MCM-41 and Cr(VI)IMS, respectively. Three peaks at 1078 cm −1 , 941 cm −1 , and 795 cm −1 indicated the silica matrix. The bending vibration of O-H of silica materials was displayed at 1632 cm −1 (Wang et al. 2014). The broad band at 3454 cm −1 of curve 1 might attribute to the existence of Si-OH. However, some water was firmly adsorbed in porous microstructure. The spectra peaks of  (2); d N 2 adsorption-desorption isotherms of Cr(VI)IMS water overlapped the spectral peaks of Si-OH at 1632 cm −1 and 3454 cm −1 . The weak adsorption peak of curve 2 at 1577 cm −1 was ascribed from the vibration of imidazole C-N bond (Kryszak et al. 2017). The solid-state 13 C NMR spectra of Cr(VI)IMS were exhibited in Fig. 2b. The peaks at 8 ppm, 24 ppm, and 50 ppm of Cr(VI)IMS corresponded to C1, C2, and C3, while signals at 121 ppm and 134 ppm were assigned to the C atoms in imidazole ring (Li et al. 2007). The FT-IR and NMR spectra demonstrated that the functional monomers have poly-condensed into Cr(VI)IMS.
The mesoporous features of Cr(VI)IMS were investigated with N 2 adsorption-desorption experiment. The isotherm of Cr(VI)IMS was of the type IV (Fig. 2d), suggesting that the pores of Cr(VI)IMS were in the mesoporous range. The specific surface area (1054.51 m 2 g −1 ) and pore volume (1.16 m 3 g −1 ) were calculated with Bnmauer-Enunett-Teller (BET) method. The BJH-determined average pore size distribution was 3.45 nm, with pore sizes largely in 2.44-3.63 nm range. The test results corresponded to the characteristics of mesoporous silica MCM-41 ( Fig. S1; Table 1).
The morphology and microstructure of Cr(VI)IMS was further observed by SEM and TEM. Cr(VI)IMS mainly existed as dispersed microspheres with particle diameter of 100-400 nm in SEM images (Fig. 3a). TEM images (Fig. 3b) demonstrated the ordered 2D hexagonal symmetrical structure of mesoporous channels with pore diameter of 2-3 nm inside Cr(VI)IMS, which basely corresponded with the result of N 2 adsorption-desorption experiment.

Effect of pH on adsorption capacity of Cr(VI)IMS
The acidity of solution poses obvious effects to the absorption performance of Cr(VI)IMS. During the imprinting process, the interaction between imidazole groups and Cr(VI) can be concluded as electrostatic attraction and imprinting effects. In aqueous environment, electrostatic attraction dominates the whole adsorption process. As displayed in Fig. 4a, at low pH, Cr(VI)IMS showed high absorption ability. Cr(VI)IMS achieved the largest binding capacity (45.6 mg g −1 ). The protonation of imidazole groups of Cr(VI)IMS were positively charged at low pH. With the increase of pH values, deprotonation increased simultaneously, which can lead to intense decrease of adsorption capacity (Amara and Kerdjoudj 2003). While the pH value of the solution was higher than 7, OH − was able to neutralize the positive charge of adsorbents, which caused the binding capacities of Cr(VI)IMS obviously decreased. We used imprint factor (IF) value to estimate the specific adsorption ability of Cr(VI)IMS. The calculation formula of IF value is IF = Q I /Q N , where Q I and Q N represent adsorption capacity of Cr(VI)IMS and NIMS, respectively. With the increase of pH values, IF value increased. The imidazole groups of Cr(VI)IMS and NIMS gradually deprotonated, but NIMS did not show imprinting effects to Cr(VI) ions, so adsorption ability of NIMS decreased more remarkably. Bichromate can be seen as a combination of two chromates, so they possess common similar parts. Due to electrostatic attraction and imprinting effects, Cr(VI)IMS showed higher adsorption ability to bichromates and chromates than NIMS at the same condition. Above all, the adsorption abilities of Cr(VI)IMS was mainly determined by the extent of protonation of imidazole groups. The imprinted sites of Cr(VI)IMS exhibited higher specificity to Cr(VI) ions at higher pH, while Cr(VI)

Adsorption isotherm study
The adsorption isotherm experiments were carried out at the optimized pH condition (pH = 3); as the initial concentration of Cr(VI) ions increased, the adsorbing capacity increased gradually (Fig. 4b). When the initial concentration of Cr(VI) ions was 200 mg L −1 , Cr(VI)IMS reached saturated adsorption (45.6 mg g −1 ), which was much higher than that of NIMS (19.3 mg g −1 ). The IF value was 2.36, which indicated that Cr(VI)IMS exhibited high affinity to Cr(VI) ions, compared with NIMS. We used Scatchard model to analyze the results of adsorption isotherm experiments (Gao et al. 2011). The Scatchard equation is Q e /C e = (Q max -Q e )/K D , where C e , Q max , and Q e represent equilibrium concentration of Cr(VI) ions, apparent maximum adsorption capacity, and equilibrium adsorption capacity. K D is the equilibrium dissociation constant. The results of the linear regression Scatchard scatter plots of Cr(VI)IMS displayed two separate straight lines with different slopes, suggesting two types of recognition sites (specific binding and nonspecific binding) existed in the Cr(VI)IMS (Li et al. 2014). On the other hand, the scatter plots could be modeled as one straight line, indicating that there was one kind of binding sites NIMS. (Fig. 5; Table 2). The Langmuir adsorption model was also used to assess the adsorption thermodynamic properties (Fig. S2). The expression is C e /Q e = C e /Q max + 1/(Q max b), where C e , Q max , and Q e represent equilibrium concentration of Cr(VI) ions, the maximum amount of adsorption and equilibrium adsorption capacity. And b is the constant in the Langmuir adsorption model. The adsorption behavior basically conformed to the Langmuir adsorption model (correlation coefficient R 2 = 0.9878), indicating that surface adsorption was rather uniform. The maximum amount of adsorption Q max and the experimental amount Q Exp are 55.9 mg g −1 and 45.6 mg g −1 , respectively.

Adsorption kinetic
We studied the adsorption kinetic to Cr(VI) ions of Cr(VI) IMS at the concentration of 50 mg g −1 and 200 mg g −1 (Fig. 6). At low concentration, it took Cr(VI)IMS 7 min to achieve equilibrium. For 200 mg g −1 , the adsorption equilibrium time of Cr(VI)IMS was 10 min. The results suggested that higher concentration led to longer absorption time for more Cr(VI) ions to enter the imprinted cavities. The recognition sites in nanoscale pore walls of large surface area accelerate the mass transportation. Compared with the ion imprinted polymers of classic structure, the functionalized mesoporous micromorphology remarkably shorten the adsorption time of Cr(VI)IMS to Cr(VI) ions (Bayramoglu and Arica 2011).
Kinetic studies were explored by two equations including the pseudo first-order and the pseudo second-order model. The fitting plots of kinetic equations are shown in Fig. S3a and S3b. The pseudo first-order kinetic equation is q t = q e (1-exp(− k 1 t)), and the pseudo second-order kinetic equation is q t = k 2 qe 2 t/(1 + k 2 q e t), where t, q t , and q e represent adsorption time, the amount of adsorption, and equilibrium adsorption capacity. k 1 and k 2 are adsorption rate constants of the pseudo first-order and the pseudo second-order kinetics, respectively. The related parameters are presented in Tables 3 and 4. It is obvious that the pseudo-first-order model was better than the pseudo-second-order model to describe adsorption kinetics as shown by the responding regression coefficient (R 2 ). Figure 7 was the results of selective experiments. The functional chemical groups of Cr(VI)IMS were organized during the imprinting process and fixed in the imprinted silica. Because of the existence of complementary imprinted cavities for Cr(VI) ions, the Cr(VI)IMS exhibited obviously selective binding ability to Cr(VI) ions. H 2 PO 4 − and NO 3 − are common anions. And they were adsorbed by positive charged Cr(VI)IMS and NIMS via electric attraction. Distribution coefficient K = Q/C e , where Q and C e were the adsorption capacity of adsorbents and the concentration of residual ions. We used selective coefficients R SF = K I /K N to estimate the selectivity of three Cr(VI)IMS, where K I and K N represented distribution coefficient of Cr(VI)IMS and NIMS. The R SF values of Cr(VI)IMS (2.56) were much higher than other two anions (Table 5), which could signified H 2 PO 4 − and NO 3 − were hard to be captured by the recognition sites due to their differences with Cr(VI) ions. On the other hand, functional monomers were distributed randomly in the materials and showed nonspecific adsorption to anions.    Reusability plays a significant role in the application of adsorbents. To estimate the reusability of Cr(VI)IMS, adsorption-regeneration cycling experiments were conducted on the same batch of Cr(VI)IMS. After 8 cycles, the adsorption efficiency of Cr(VI)IMS still reached 92.5%, showing their excellent stability and reusability (Fig. 8a). Reproducibility is another important property of adsorbent. Five batches of Cr(VI)IMS exhibited no obvious difference in adsorption capacities (Fig. 8b).

Application in real water samples
The concentration of Cr(VI) of the filtered waste water was 22.1 mg L −1 . The water samples of the same volume were treated by different amount of adsorbent. The adsorption performance to industrial waste water was shown in Table 6. Lower residual concentration can be obtained, treated by larger amount of adsorbent. Using 6.0 g L −1 of Cr(VI)IMS, the residual concentrations of Cr(VI) ions was 0.22 mg L −1 , which was lower than discharge standard of China (the maximum allowable emission concentration of Cr(VI) of GB8978-1996 is 0.5 mg L −1 ).

Comparison of adsorption performance of other adsorbents
Activated carbon is one of conventional adsorbents for heavy metal ions treatment. Activated carbon can sourced from   lots of materials and easily available. The cost is lower than synthesized adsorbent, like Cr(VI)IMS, but the adsorption performance of Cr(VI)IMS includes adsorption capacity, adsorption and desorption time, selectivity, and regeneration superior. On the other hand, synthesized specific adsorbent with robust structure can be reused many times and be developed for recycling chromium, while activated carbon show broad spectrum adsorption (Koodyńska et al. 2016) and is hard to be regenerated (Ranganathan 2000). The solid waste residue after adsorption can cause another environmental problem. Considering the cost of economy and environment, synthesized specific adsorbent is a promising research direction. The synthesized adsorption performance, several adsorbents for removal of Cr(VI) ions was summarized in Table 7. Fe 3 O 4 50%-PANI@GO hybrid composite (adsorbent 1), imidazole functionalized mesoporous SBA-15 (adsorbent 2), and ion-exchange resin (adsorbent 3) are three kinds of nonspecific anion adsorbents. The adsorption capacities of three kinds of adsorbents are higher than that of Cr(VI)IMS. However, the equilibrium time is much longer. Three kinds of adsorbents were not designed for adsorption of Cr(VI), so they have not shown high selectivity to Cr(VI). After 5 cycles regeneration, the adsorption capacity of Fe 3 O 4 50%-PANI@GO hybrid composite has decreased remarkably, while Cr(VI)IMS with relatively robust structure has shown reliable reusability. Cr(VI)IMS was designed for specific adsorption of Cr(VI) and showed high selectivity to Cr(VI), compared with adsorbent 1, 2, and 3. Cr(VI) imprinted polymers (adsorbent 4) and magnetic Cr(VI) imprinted nanocomposites (adsorbent 5) mentioned in Table 7 have been applied imprinted technology, so they have performed well in selective adsorption but adsorption time is longer than that of Cr(VI)IMS. Compared with adsorbent 4 and 5 synthesized by traditional method, Cr(VI)IMS possessed robust microporous structure. The adsorption capacity can reach 45.6 mg g −1 , and the adsorption equilibrium time can be shorten to 10 min. Due to the orderly porous microstructure of Cr(VI)IMS, Cr(VI) ions could quickly access to binding sites embedded in the pore walls. Cr(VI) imprinted polymers were synthesized by traditional free radical polymerization or sol-gel method. Lots of binding sites were encapsulated in the polymers. Cr(VI) ions take longer to diffuse into the polymers.
According to the comparison of adsorption capacities, adsorption time, and selectivity, it can be concluded that ion imprinting technology enhances the selectivity of adsorbents and Cr(VI)IMS prepared by co-condensation method exhibited the advantages of fast adsorption rate, high binding capacity and specific adsorption.

Conclusions
In this study, we developed Cr(VI) imprinted mesoporous silica synthesis method which combined the advantages between mesoporous material with ion imprinted technology. The positive charged imidazolyl groups were be integrated into the mesoporous silica successfully, proved by FT-IR and NMR and showed strong electrostatic attraction to anions. Compared to NIMS and other traditional adsorbents, Cr(VI)IMS exhibited specific adsorption to Cr(VI) anions. Benefited by highly ordered MCM-41 mesoporous structure, adsorption capacity of Cr(VI)IMS can reach 45.6 mg g −1 . Cr(VI)IMS reached the adsorption equilibrium in a short time (10 min) at acid and weak acid conditions, while most of adsorbents need more than 30 min to achieve adsorption equilibrium. The adsorption efficiency is significantly higher than other traditional imprinted adsorption materials. The Cr(VI) concentration of wastewater is lower than emission standard limit (0.5 mg L −1 ) after treatment by the optimized amount of Cr(VI)IMS. The combination of imprinted technology and highly ordered mesoporous silica synthesized by one-step co-condensation shows a prospective method for development of specific and efficient pollutant adsorbents.