The Systematic Adsorption of Diclofenac onto Waste Red Bricks Functionalized with Iron Oxides

In this study, waste red bricks were incorporated with iron oxides (goethite and hematite) and used for the removal of diclofenac (DCF) from aqueous solutions. The prepared waste red bricks were systematically characterized by XRF, XRD, BET, and SEM. The batch experiments were systematically conducted by investigating the adsorption kinetics, isotherms, thermodynamics, pH, and ionic strength effect. Results showed that the incorporation of iron oxides could enhance the adsorption capacity of DCF onto waste bricks, while the increased effect of hematite was better than goethite. DCF was adsorbed rapidly onto waste bricks, and the adsorption kinetic fitted the pseudo-second-order model perfectly, which could be attributed to the strong interaction between DCF and iron oxides. The increasing pH values decreased the adsorption capacity greatly, which may be due to the electrostatic repulsive interactions. The adsorption of DCF onto waste bricks was an exothermic reaction, and the adsorption isotherms fitted well with the Langmuir model. This study offers new guidelines for the utilization of construction waste, and shows useful methods for the elimination of micropollutants from aqueous solution.


Introduction
In the last few decades, great concern has been raised on the increasing number of emerging compounds which were detected at different levels in diverse water environments, such as wastewater, surface water, groundwater, and even drinking water [1,2].These compounds contain a lot of natural or synthetic chemicals, such as pharmaceuticals and personal care products, endocrine disrupting compounds, persistent organic pollutants, and pesticides [3].However, results showed that the conventional water treatment had little effect on the elimination of these materials, due to their ubiquitous and non-biodegradable characteristics [4].
As one of the most typical pharmaceuticals and personal care products, diclofenac (DCF) is a widely prescribed non-steroidal anti-inflammatory drug.Due to the large amount of usage (940 tons per year), DCF could enter into the environment through different pathways, such as direct disposal, hospital effluents, and wastewater treatment plant discharges [5,6].Although the concentration of DCF in environments is low, results show that it has adverse effects on aquatic life and humans [7][8][9].Therefore, it has become urgent to find new useful methods for the removal of DCF from aqueous solutions.
In recent years, many studies have been conducted for the elimination of DCF from wastewater by either batch or column experiments [10][11][12][13][14].Among these methods, adsorption remains attractive because of its advantages, such as simplicity, reliability, and cost effectiveness.Many adsorbents have been investigated for the removal of DCF from aqueous solutions, such as activated carbon, zeolite, chitosan, and graphene oxide [5,15,16].Although these materials showed extremely high adsorption of DCF, the materials' cost is also high, which restricts the application.As the key factor of the adsorption process, adsorbents should gain more attention as they would be low cost, have high adsorption capacity, and be environmental friendly.
As a developing country, urbanization has developed rapidly recently in China, and the amount of construction waste increased sharply.As one of the more common examples of construction waste, waste bricks could cause various environmental problems.The investigation of utilizing waste bricks as adsorbents has drawn great attention, due their proper surface area, micropore volume, and porous structure [17,18].Although recent studies have demonstrated that waste bricks can be used as a low-cost adsorbent for the removal of pollutants, compared with commercial adsorbents, the lower adsorption capacity inhibits their application in water treatment.Iron oxides have shown considerable promise for the removal of contaminants due to their ubiquitous properties, especially for micropollutants [19].For example, as the typical iron oxides, goethite and hematite were demonstrated to be useful adsorbents for the removal pollutants from aqueous solutions [15,20].However, most of such pure iron oxides have relatively low surface area, and the numbers of useful active sites are limited, which inhibits the use of these iron oxides in water treatment.Results also show that the impregnation of iron oxides onto conventional adsorbents could enhance the adsorption capacity and promote the utilization of iron oxides [11,15].
In this study, without dramatically modifying the original structure of the waste bricks, two typical iron oxides (goethite and hematite) were selected for the functionalization of waste red bricks, and the prepared adsorbents were used for the removal of DCF from aqueous solutions.The prepared waste bricks were characterized systematically by XRF (X-Ray Fluorescence), XRD (X-ray diffraction), BET (Brunner−Emmet−Teller), and SEM (Scanning Electron Microscope).The adsorption process of DCF onto prepared adsorbents was carried out by batch experiments and the factors affecting the adsorption, such as kinetics, pH, thermodynamics, and ionic strength, were investigated.This study could offer new guidelines for the utilization of construction waste and show useful methods for the elimination of micropollutants from aqueous solution.

Materials and Chemicals
The diclofenac sodium salt (>99%) was purchased from Sigma Aldrich Chemical Co.(Madison, WI, USA), and used without further purification.The stock solution of DCF was prepared at 1 g/L, and was diluted to the desired concentrations before the experiments.The chemical formula of diclofenac sodium is C 14 H 10 Cl 2 NNaO 2 , and the molecular weight is 318.14.The protonation constant (pK a ) of DCF is 4.15.Methanol and acetonitrile were HPLC (High Preussuer Liquid Chromatography) grade and obtained from Fisher Scientific Corp.All other chemicals, such as hydrochloric acid, nitric acid, FeSO 4 •7H 2 O, NaHCO 3 , Fe(NO 3 ) 3 •9H 2 O, NaCl, KOH, and NaOH were all of analytical grade.Ultrapure water (MilliQ) was used in all experiments.

The Preparation of Adsorbents
In this study, three different kinds of waste red bricks were prepared: pickling red brick particles (PRBP), goethite-coated brick particles (GCBP), and hematite-coated brick particles (HCBP).The method of preparation of GCBP and HCBP was carried out according to Schwertmann [21].
PRBP: The waste red bricks were firstly collected at Beijing University of Civil Engineering and Architecture.Then, the bricks were broken into grain and sieved to the size of 0.5 to 1.0 mm.The obtained particles were then soaked in acid solution (HCl 6 mol/L) at 90 ± 1 • C for 24 h.The slurry was filtered through 0.45 microns polytetrafluoroethylene membrane filter and washed with ultrapure water several times, dried at 105 ± 1 • C, and the final PRBP was obtained.
GCBP: First, 0.05 mol/L FeSO 4 solution were prepared by adding FeSO 4 •7H 2 O into ultrapure water after N 2 was bubbled through for 30 min.Second, 1 mol/L NaHCO 3 solution was added into the solution where the N 2 was replaced by air.Third, a certain amount of PRBP was added into the solution, and the slurry was stirred continuously for 48 h.The solution pH values were maintained at 7.0 ± 0.1 by NaHCO 3 buffer solution.Oxidation was completed when the color of the suspension changed from green-blue to ocherous.The resultant product was then filtered by a 0.45 microns polytetrafluoroethylene membrane filter, and washed thoroughly until a clear supernatant was obtained.The GCBP was finally obtained by freeze-drying.
HCBP: First, 0.2 mol/L Fe(NO 3 ) 3 solution was prepared by adding certain Fe(NO 3 ) 3 •9H 2 O into preheated ultrapure water.Second, 1 mol/L KOH and 1 mol/L NaHCO 3 solutions were added into the mixture.Third, a certain amount of PRBP was added into the solution and the mixture was held in a closed polyethylene flask at 90 • C for 48 h.The resultant product was cooled to room temperature, filtered, and washed thoroughly with ultrapure water until a clear supernatant was obtained.The HCBP was finally obtained also by freeze-drying.

Characterization Techniques
The surface morphology of waste red bricks was examined by scanning electron microscopy (SEM, Limited S-4800, Hitachi, Tokyo, Japan).The X-ray powder diffraction (XRD) patterns of bricks were obtained by an X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Germany) with Cu Kα radiation.The chemical analysis of waste red bricks was carried out using an X-ray fluorescence spectrometer (XRF, XRF-1800, Shimadzu, Tokyo, Japan).N 2 adsorption isotherms were collected at 77 K on a Micromeritics (ASAP 2020 HD88, Mike, Atlanta, GA, USA), and the pore volume and pore size distribution were calculated using a BJH (Barret-Joyner-Halenda) method.The bricks were digested with acid mixture using a constant temperature electric heating plate, and Fe (III) contents was determined by atomic absorption spectrophotometry (Z-2010, Hitachi, Tokyo, Japan).

Adsorption Experiments
The batch experiment was conducted to investigate the adsorption behavior of DCF onto PRBP, GCBP, and HCBP.All batch experiments were conducted in the dark to inhibit the potential photodegradation.In this study, all the tests were performed in duplicate, and the values listed in the figures were calculated by the average.
In the kinetics experiments, three sets of 500 mL solutions were prepared with the DCF concentration of 5 mg/L.The PRBP, GCBP, and HCBP were then added into the solution with a dosage of 2 g/L.The mixed solution was stirred continuously with a speed of 170 rpm, and the solution pH values were maintained at 7.0 ± 0.1 by 0.1 mol/L HCl or NaOH solutions.The samples were taken at appropriate time intervals, filtered through a 0.22 µm polytetrafluoroethylene membrane filter, and the concentration of DCF in the solution was analyzed.
Adsorption isotherms and thermodynamics experiments were conducted by adding 0.06 g PRBP, GCBP, and HCBP into 30 mL solutions with the DCF concentration of 1-40 mg/L.The solution pH values were maintained at 7.0 ± 0.1, and the temperature was maintained at 298, 308, and 318 K.The mixed solution was stirred continuously with a speed of 170 rpm for 24 h to reach equilibrium.Other experimental conditions were the same as in the kinetics experiments.
The effect of pH values and ionic strength were conducted by adding 0.06 g PRBP, GCBP, and HCBP into 30 mL solutions with the DCF concentration of 5 mg/L.The solution pH was adjusted and maintained to desired values in the range of 4.0-9.0.The ionic strength effect was studied with different ionic strength (0.001-0.2 mol/L NaCl), and the solution pH was maintained at 7.0 ± 0.1.Other experimental conditions were the same as in the kinetics experiments.
The concentrations of DCF were determined by Ultra Performance Liquid Chromatography (UPLC, Acquity, Waters, CA, USA) at a wavelength of 265 nm, using a C 18 reverse-phase column (100 mm × 2.1 mm and 1.7 µm particle size).The mobile phase consisted of 20% ultrapure water (0.01%CH 3 COOH) and 80% methanol at a flow rate of 0.4 mL/min.

The Characteristics of PRBP, GCBP, and HCBP
The results of XRF, BET, pore size, and Fe content are illustrated in Table 1.As shown in XRF, the waste red bricks consist of Si 2 O, Al 2 O 3 , Fe 2 O 3 , and others.After functionalization with the iron species, the main composition of waste red bricks was mainly unchanged, while the amount of coated iron in GCBP and HCBP was 3.18 and 2.29 mg/g, respectively.Furthermore, although the specific surface area of GCBP and HCBP decreased from 10.42 to 6.93 and 2.43 m 2 /g, the average pore size increased from 4.52 to 5.35 and 5.80 nm, which may be more useful for the removal of pollutants from aqueous solutions.These results were consistent with the study of impregnated iron species onto activated carbon [22].The N 2 adsorption/desorption curves and corresponding BJH pore size distribution of PRBP, GCBP, and HCBP were presented in Figure 1.According to the IUPAC (International Union of Pure and Applied Chemistry) classification, it has been observed that all three N 2 adsorption/desorption curves belonged to type IV isotherms with hysteresis loops between adsorption and desorption branches, indicated the presence of a porous structure on the adsorbent [1].Besides, compared with PRBP, the higher P/P 0 and hysteresis of GCBP verified the larger pore size, which was shown in Table 1.All these results showed that the main structure of waste red bricks was maintained after the incorporation of iron species.

The Characteristics of PRBP, GCBP, and HCBP
The results of XRF, BET, pore size, and Fe content are illustrated in Table 1.As shown in XRF, the waste red bricks consist of Si2O, Al2O3, Fe2O3, and others.After functionalization with the iron species, the main composition of waste red bricks was mainly unchanged, while the amount of coated iron in GCBP and HCBP was 3.18 and 2.29 mg/g, respectively.Furthermore, although the specific surface area of GCBP and HCBP decreased from 10.42 to 6.93 and 2.43 m 2 /g, the average pore size increased from 4.52 to 5.35 and 5.80 nm, which may be more useful for the removal of pollutants from aqueous solutions.These results were consistent with the study of impregnated iron species onto activated carbon [22].The N2 adsorption/desorption curves and corresponding BJH pore size distribution of PRBP, GCBP, and HCBP were presented in Figure 1.According to the IUPAC (International Union of Pure and Applied Chemistry) classification, it has been observed that all three N2 adsorption/desorption curves belonged to type IV isotherms with hysteresis loops between adsorption and desorption branches, indicated the presence of a porous structure on the adsorbent [1].Besides, compared with PRBP, the higher P/P0 and hysteresis of GCBP verified the larger pore size, which was shown in Table 1.All these results showed that the main structure of waste red bricks was maintained after the incorporation of iron species.The XRD patterns of PRBP, GCBP, and HCBP are shown in Figure 2.After functionalization with hematite and goethite, compared with the results of PRBP, the main diffraction patterns of bricks were almost identical, which suggested the main structure and composition of waste red bricks were nearly unchanged.However, the XRD patterns of GCBP show several new well-defined characteristic peaks of (110), ( 120), ( 111), (121), and (151), suggesting the presence of goethite structure on the adsorbent, which indicated that goethite was successfully incorporation into the red bricks [23].In the same way, the new peaks of (012), (110), and (024) in the patterns of HCBP indicated the successful incorporation of hematite onto waste red bricks [19].These results were associated with the increasing quantity of deposited iron oxide in Table 1.
The XRD patterns of PRBP, GCBP, and HCBP are shown in Figure 2.After functionalization with hematite and goethite, compared with the results of PRBP, the main diffraction patterns of bricks were almost identical, which suggested the main structure and composition of waste red bricks were nearly unchanged.However, the XRD patterns of GCBP show several new well-defined characteristic peaks of (110), (120), ( 111), (121), and (151), suggesting the presence of goethite structure on the adsorbent, which indicated that goethite was successfully incorporation into the red bricks [23].In the same way, the new peaks of (012), (110), and (024) in the patterns of HCBP indicated the successful incorporation of hematite onto waste red bricks [19].These results were associated with the increasing quantity of deposited iron oxide in Table 1.Representative SEM images of PRBP, GCBP, and HCBP are depicted in Figure 3. Figure 3a showed that the virgin red bricks had a relatively irregular and rough surface, small cracks and pores.After the incorporation of hematite and goethite, Figure 3b,c show a similar disordered structure, but unevenly distributed fine particles are observed on the surface, which may be attributed to the immobilization of goethite and hematite onto the solid surface.Furthermore, both goethite and hematite did not change the surface properties and main structure of waste red bricks.

The Adsorption Kinetics of DCF onto PRBP, GCBP, and HCBP
Figure 4 showed the adsorption kinetics of DCF onto PRBP, GCBP, and HCBP.For all three adsorbents, DCF was adsorbed rapidly in the first 60 min, followed by a relatively slow process, and finally achieved equilibrium at 9 h.The high adsorption rate could be attributed to the roughness, Representative SEM images of PRBP, GCBP, and HCBP are depicted in Figure 3. Figure 3a showed that the virgin red bricks had a relatively irregular and rough surface, small cracks and pores.After the incorporation of hematite and goethite, Figure 3b,c show a similar disordered structure, but unevenly distributed fine particles are observed on the surface, which may be attributed to the immobilization of goethite and hematite onto the solid surface.Furthermore, both goethite and hematite did not change the surface properties and main structure of waste red bricks.
The XRD patterns of PRBP, GCBP, and HCBP are shown in Figure 2.After functionalization with hematite and goethite, compared with the results of PRBP, the main diffraction patterns of bricks were almost identical, which suggested the main structure and composition of waste red bricks were nearly unchanged.However, the XRD patterns of GCBP show several new well-defined characteristic peaks of (110), (120), ( 111), (121), and (151), suggesting the presence of goethite structure on the adsorbent, which indicated that goethite was successfully incorporation into the red bricks [23].In the same way, the new peaks of (012), (110), and (024) in the patterns of HCBP indicated the successful incorporation of hematite onto waste red bricks [19].These results were associated with the increasing quantity of deposited iron oxide in Table 1.Representative SEM images of PRBP, GCBP, and HCBP are depicted in Figure 3. Figure 3a showed that the virgin red bricks had a relatively irregular and rough surface, small cracks and pores.After the incorporation of hematite and goethite, Figure 3b,c show a similar disordered structure, but unevenly distributed fine particles are observed on the surface, which may be attributed to the immobilization of goethite and hematite onto the solid surface.Furthermore, both goethite and hematite did not change the surface properties and main structure of waste red bricks.

The Adsorption Kinetics of DCF onto PRBP, GCBP, and HCBP
Figure 4 showed the adsorption kinetics of DCF onto PRBP, GCBP, and HCBP.For all three adsorbents, DCF was adsorbed rapidly in the first 60 min, followed by a relatively slow process, and finally achieved equilibrium at 9 h.The high adsorption rate could be attributed to the roughness,

The Adsorption Kinetics of DCF onto PRBP, GCBP, and HCBP
Figure 4 showed the adsorption kinetics of DCF onto PRBP, GCBP, and HCBP.For all three adsorbents, DCF was adsorbed rapidly in the first 60 min, followed by a relatively slow process, and finally achieved equilibrium at 9 h.The high adsorption rate could be attributed to the roughness, high porosity, and surface area of the bricks, which enhanced the mass transfer of DCF in the adsorption process.Furthermore, the chemical structure of waste red bricks may offer abundant useful adsorption sites for DCF removal.
high porosity, and surface area of the bricks, which enhanced the mass transfer of DCF in the adsorption process.Furthermore, the chemical structure of waste red bricks may offer abundant useful adsorption sites for DCF removal.To investigate the adsorption process further, the adsorption kinetics were interpreted by the pseudo-first-order and pseudo-second-order model represented by Equations ( 1) and ( 2), respectively [24].
where Qe and Qt (μg/g) are the adsorption capacity at adsorption equilibrium and time; and k1 (μg/mg/h) and k2 (μg/h) are the kinetic rate constants of the pseudo-first-order model and pseudo-second-order model, respectively.The kinetic parameters and correlation coefficients were listed in Table 2. Comparing the pseudo-first and pseudo-second modes reveal that the pseudo-second-order model was more suitable for describing the adsorption of DCF onto PRBP, GCBP, and HCBP (r 2 > 0.96), which indicated that the adsorption of DCF onto waste red bricks may be chemisorption.Furthermore, results also showed that the K2 and qe followed the order of HCBP > GCBP > PRBP, which suggested that the incorporation of iron species enhanced the adsorption rate and capacity of DCF onto waste red bricks.These results were consistent with the adsorption of DCF onto other adsorbents, such as goethite and surfactant-modified zeolite [19,20].

Effect of pH on the Adsorption of DCF onto PRBP, GCBP, and HCBP
As illustrated in Figure 5, the solution pH values influenced the adsorption of DCF onto PRBP, GCBP, and HCBP remarkably.For all three bricks, the influence of pH values were similar, To investigate the adsorption process further, the adsorption kinetics were interpreted by the pseudo-first-order and pseudo-second-order model represented by Equations ( 1) and ( 2), respectively [24].
where Q e and Q t (µg/g) are the adsorption capacity at adsorption equilibrium and time; and k 1 (µg/mg/h) and k 2 (µg/h) are the kinetic rate constants of the pseudo-first-order model and pseudo-second-order model, respectively.The kinetic parameters and correlation coefficients were listed in Table 2. Comparing the pseudo-first and pseudo-second modes reveal that the pseudo-second-order model was more suitable for describing the adsorption of DCF onto PRBP, GCBP, and HCBP (r 2 > 0.96), which indicated that the adsorption of DCF onto waste red bricks may be chemisorption.Furthermore, results also showed that the K 2 and q e followed the order of HCBP > GCBP > PRBP, which suggested that the incorporation of iron species enhanced the adsorption rate and capacity of DCF onto waste red bricks.These results were consistent with the adsorption of DCF onto other adsorbents, such as goethite and surfactant-modified zeolite [19,20].

Effect of pH on the Adsorption of DCF onto PRBP, GCBP, and HCBP
As illustrated in Figure 5, the solution pH values influenced the adsorption of DCF onto PRBP, GCBP, and HCBP remarkably.For all three bricks, the influence of pH values were similar, indicated that the adsorption mechanism was not changed after the functionalization of goethite and hematite.With the increase in solution pH values from 4.0 to 9.0, the adsorption capacity of DCF decreased, apparently, from 303.20, 330.50, and 345.70, to 11.03, 12.05, and 32.05 µg/g, for PRBP, GCBP, and HCBP, respectively.
Water 2018, 10, x FOR PEER REVIEW 7 of 13 indicated that the adsorption mechanism was not changed after the functionalization of goethite and hematite.With the increase in solution pH values from 4.0 to 9.0, the adsorption capacity of DCF decreased, apparently, from 303.20, 330.50, and 345.70, to 11.03, 12.05, and 32.05 μg/g, for PRBP, GCBP, and HCBP, respectively.The observed results were associated with the characteristics of DCF and surface properties of the bricks.As the pKa value of DCF was 4.15, with the solution pH values increased from 4.0 to 9.0, and most of the DCF exhibited a negative charge.As shown in Figure 6, the charge of all bricks was mainly negative for all pH values.After the incorporation of iron oxides, the zeta potential of bricks increased, and the charge followed by the order of HCBP > GCBP, which may be attributed to the higher zero charge of hematite (8.1 vs. 6.9).With the increase in solution pH, the positive charge of adsorbents decreased, the negative charges increased, and the electrostatic repulsion between DCF and adsorbents increased, which induced the decreased adsorption capacity.Furthermore, previous reports suggested that the low solubility of DCF in acidic medium and the decrease in DCF adsorption may be attributed to the increased mobility and solubility of DCF under alkaline conditions [25].The observed results were associated with the characteristics of DCF and surface properties of the bricks.As the pK a value of DCF was 4.15, with the solution pH values increased from 4.0 to 9.0, and most of the DCF exhibited a negative charge.As shown in Figure 6, the charge of all bricks was mainly negative for all pH values.After the incorporation of iron oxides, the zeta potential of bricks increased, and the charge followed by the order of HCBP > GCBP, which may be attributed to the higher zero charge of hematite (8.1 vs. 6.9).With the increase in solution pH, the positive charge of adsorbents decreased, the negative charges increased, and the electrostatic repulsion between DCF and adsorbents increased, which induced the decreased adsorption capacity.Furthermore, previous reports suggested that the low solubility of DCF in acidic medium and the decrease in DCF adsorption may be attributed to the increased mobility and solubility of DCF under alkaline conditions [25].
Water 2018, 10, x FOR PEER REVIEW 7 of 13 indicated that the adsorption mechanism was not changed after the functionalization of goethite and hematite.With the increase in solution pH values from 4.0 to 9.0, the adsorption capacity of DCF decreased, apparently, from 303.20, 330.50, and 345.70, to 11.03, 12.05, and 32.05 μg/g, for PRBP, GCBP, and HCBP, respectively.The observed results were associated with the characteristics of DCF and surface properties of the bricks.As the pKa value of DCF was 4.15, with the solution pH values increased from 4.0 to 9.0, and most of the DCF exhibited a negative charge.As shown in Figure 6, the charge of all bricks was mainly negative for all pH values.After the incorporation of iron oxides, the zeta potential of bricks increased, and the charge followed by the order of HCBP > GCBP, which may be attributed to the higher zero charge of hematite (8.1 vs. 6.9).With the increase in solution pH, the positive charge of adsorbents decreased, the negative charges increased, and the electrostatic repulsion between DCF and adsorbents increased, which induced the decreased adsorption capacity.Furthermore, previous reports suggested that the low solubility of DCF in acidic medium and the decrease in DCF adsorption may be attributed to the increased mobility and solubility of DCF under alkaline conditions [25].

The Adsorption Isotherms of DCF onto PRBP, GCBP, and HCBP
Figure 7 showed the adsorption isotherm of DCF onto PRBP, GCBP, and HCBP.For all three adsorbents, the adsorption capacity of DCF increased with the increasing equilibrium concentration.Furthermore, compared with PRBP, GCBP and HCBP had a higher adsorption capacity for nearly all equilibrium concentrations, which suggested that the functionalization of iron oxide was beneficial for the removal of DCF from aqueous solution.
Water 2018, 10, x FOR PEER REVIEW 8 of 13

The Adsorption Isotherms of DCF onto PRBP, GCBP, and HCBP
Figure 7 showed the adsorption isotherm of DCF onto PRBP, GCBP, and HCBP.For all three adsorbents, the adsorption capacity of DCF increased with the increasing equilibrium concentration.Furthermore, compared with PRBP, GCBP and HCBP had a higher adsorption capacity for nearly all equilibrium concentrations, which suggested that the functionalization of iron oxide was beneficial for the removal of DCF from aqueous solution.To investigate the adsorption isotherms of DCF further, the Langmuir and Freundlich isotherms were used to illustrate the adsorption behavior, and the models were presented in Equations ( 3) and (4), respectively.The Langmuir isotherm model is based on the assumption that each adsorption site can hold only one adsorbate molecule.The Freundlich isotherm model is commonly used to describe the adsorption characteristics for heterogeneous surface [26].
where Ce is equilibrium concentration (mg/L), Qmax is the maximum adsorption capacity of DCF onto bricks (μg/g), b is a measure of the energy of adsorption, Kf is the Freundlich adsorption constant, and n is related to the adsorption intensity.As listed in Table 3, compared with the Freundlich model, the Langmuir model fitted the adsorption isotherms of DCF onto three bricks better (r 2 > 0.97), which suggested that the adsorption of DCT onto bricks was probably monolayer molecular adsorption.In addition, compared with the Qmax of PRBP (187.9 μg/g), the HCBP seems to be more efficient for the removal of DCF (292.3 μg/g), whereas the GCBP shows a Qmax of 220.5 μg/g.The adsorption capacity mainly depends on the chemical structure of red bricks and the coated iron oxides.Although the process of incorporation decreased of specific surface area of bricks (showed in Table 1), the adsorption efficiency was enhanced owing to the interaction between iron oxides and DCF.Furthermore, 1/n is a constant indicative of adsorption intensity or surface heterogeneity.As shown in Table 3, the values of 1/n for all three bricks were less than 1.0, indicating that the adsorption DCF onto bricks was favorable.These results all indicate that waste red bricks could be used as potential adsorbents for the removal of DCF from aqueous solutions.To investigate the adsorption isotherms of DCF further, the Langmuir and Freundlich isotherms were used to illustrate the adsorption behavior, and the models were presented in Equations ( 3) and (4), respectively.The Langmuir isotherm model is based on the assumption that each adsorption site can hold only one adsorbate molecule.The Freundlich isotherm model is commonly used to describe the adsorption characteristics for heterogeneous surface [26].
where C e is equilibrium concentration (mg/L), Q max is the maximum adsorption capacity of DCF onto bricks (µg/g), b is a measure of the energy of adsorption, K f is the Freundlich adsorption constant, and n is related to the adsorption intensity.As listed in Table 3, compared with the Freundlich model, the Langmuir model fitted the adsorption isotherms of DCF onto three bricks better (r 2 > 0.97), which suggested that the adsorption of DCT onto bricks was probably monolayer molecular adsorption.In addition, compared with the Q max of PRBP (187.9 µg/g), the HCBP seems to be more efficient for the removal of DCF (292.3 µg/g), whereas the GCBP shows a Q max of 220.5 µg/g.The adsorption capacity mainly depends on the chemical structure of red bricks and the coated iron oxides.Although the process of incorporation decreased of specific surface area of bricks (showed in Table 1), the adsorption efficiency was enhanced owing to the interaction between iron oxides and DCF.Furthermore, 1/n is a constant indicative of adsorption intensity or surface heterogeneity.As shown in Table 3, the values of 1/n for all three bricks were less than 1.0, indicating that the adsorption DCF onto bricks was favorable.These results all indicate that waste red bricks could be used as potential adsorbents for the removal of DCF from aqueous solutions.

Effect of Ionic Strength on DCF Adsorption
To explain the adsorption mechanism of DCF onto PRBP, GCBP, and HCBP further, the influence of ionic strength on DCF adsorption were conducted, and the results were listed in Figure 8. Results showed that the increasing concentration of NaCl affected the adsorption of DCF greatly, and the influences could be divided into two regions.With the NaCl concentration increased from 0 to 0.02 mol/L, the adsorption capacity increased from 74.1, 114.9, and 138.3, to 123.8, 211.9, and 222.4 µg/g, for PRBP, GCBP, and HCBP, respectively.When the NaCl concentration increased further, the adsorption capacity of DCF decreased significantly.The enhanced adsorption of DCF onto bricks may be attributed to the charge screening effect, which can reduce the repulsion between DCF and bricks that increased the adsorption capacity greatly.However, when NaCl concentration increased further (>0.02mol/L), abundant Cl − in the solution could also be adsorbed onto the surface of adsorbents, and occupied part of the useful adsorption site, which decreased the adsorption capacity of DCF greatly [27].According to the results of others, the non-specific adsorption is usually influenced greatly by the change of background electrolyte, whereas the specific adsorption is not [28].In this study, the adsorption of DCF onto three bricks was influenced by the concentration of NaCl greatly.Thus, the adsorption of DCF onto waste red bricks could be non-specific adsorption.

Effect of Ionic Strength on DCF Adsorption
To explain the adsorption mechanism of DCF onto PRBP, GCBP, and HCBP further, the influence of ionic strength on DCF adsorption were conducted, and the results were listed in Figure 8. Results showed that the increasing concentration of NaCl affected the adsorption of DCF greatly, and the influences could be divided into two regions.With the NaCl concentration increased from 0 to 0.02 mol/L, the adsorption capacity increased from 74.1, 114.9, and 138.3, to 123.8, 211.9, and 222.4 μg/g, for PRBP, GCBP, and HCBP, respectively.When the NaCl concentration increased further, the adsorption capacity of DCF decreased significantly.The enhanced adsorption of DCF onto bricks may be attributed to the charge screening effect, which can reduce the repulsion between DCF and bricks that increased the adsorption capacity greatly.However, when NaCl concentration increased further (>0.02mol/L), abundant Cl − in the solution could also be adsorbed onto the surface of adsorbents, and occupied part of the useful adsorption site, which decreased the adsorption capacity of DCF greatly [27].According to the results of others, the non-specific adsorption is usually influenced greatly by the change of background electrolyte, whereas the specific adsorption is not [28].In this study, the adsorption of DCF onto three bricks was influenced by the concentration of NaCl greatly.Thus, the adsorption of DCF onto waste red bricks could be non-specific adsorption.

Adsorption Thermodynamics
In order to investigate the adsorption mechanism further, the temperature influences on the adsorption process were conducted at 298, 308, and 318 K, and the results were shown in Figure 9.With the temperature increase from 298 to 318 K, the adsorption capacity of DCF on all three bricks decreased, suggesting that the adsorption process was exothermic, and the adsorption was more favorable at lower temperature.The results were consistent with other studies, which the adsorption occurred on different types of pollutants onto clay minerals and soils [26].

Adsorption Thermodynamics
In order to investigate the adsorption mechanism further, the temperature influences on the adsorption process were conducted at 298, 308, and 318 K, and the results were shown in Figure 9.With the temperature increase from 298 to 318 K, the adsorption capacity of DCF on all three bricks decreased, suggesting that the adsorption process was exothermic, and the adsorption was more favorable at lower temperature.The results were consistent with other studies, which the adsorption occurred on different types of pollutants onto clay minerals and soils [26].To investigate the adsorption process of DCF onto bricks further, the thermodynamic parameters were calculated by Equations ( 5) and ( 6) [29].
where Kd is the distribution coefficient, ΔH o is the change of enthalpy (kJ/mol), ΔS o is the change of entropy (J/mol/K), ΔG o is the change of Gibbs free energy (kJ/mol), T is the absolute temperature in Kelvin (K), and R is the gas constant (8.314J/mol/K).
As shown in Table 4, the negative ΔH for PRBP, GCBP, and HCBP was −28.64, −34.70, and −17.98 kJ/g, respectively, confirming the interaction between DCF and brick was an exothermic process.Furthermore, the negative values of ΔS were consistent with the fact that the randomness decreased during the adsorption of DCF onto waste red bricks.Simultaneously, the adsorption capacity of GCBP and HCBP was higher than PRBP at different temperatures, which suggested the functionalizations of goethite and hematite have good affinity for DCF removal.To investigate the adsorption process of DCF onto bricks further, the thermodynamic parameters were calculated by Equations ( 5) and ( 6) [29].
where K d is the distribution coefficient, ∆H o is the change of enthalpy (kJ/mol), ∆S o is the change of entropy (J/mol/K), ∆G o is the change of Gibbs free energy (kJ/mol), T is the absolute temperature in Kelvin (K), and R is the gas constant (8.314J/mol/K).
As shown in Table 4, the negative ∆H for PRBP, GCBP, and HCBP was −28.64, −34.70, and −17.98 kJ/g, respectively, confirming the interaction between DCF and brick was an exothermic process.Furthermore, the negative values of ∆S were consistent with the fact that the randomness decreased during the adsorption of DCF onto waste red bricks.Simultaneously, the adsorption capacity of GCBP and HCBP was higher than PRBP at different temperatures, which suggested the functionalizations of goethite and hematite have good affinity for DCF removal.

Conclusions
In this study, the adsorbents were prepared by incorporating goethite and hematite onto waste red bricks, and used for the removal of DCF from aqueous solution.Results showed that goethite and hematite were successfully incorporated onto red bricks, and the main structure of bricks was maintained.The adsorption process of DCF onto three bricks was fast and fitted the pseudo-second-order model.The adsorption isotherms fitted well with Langmuir isotherm and the Q max of PRBP, GCBP, and HCBP were 187.9, 220.5, and 292.3 µg/g, which could be attributed to the strong interaction between DCF with goethite and hematite.The pH values affected the adsorption behavior greatly, and when the solution pH values increased, the adsorption capacity of DCF onto bricks apparently decreased.When the NaCl concentration increased, the adsorption capacity of DCF increased, firstly, followed by a great decrease, which suggested that the adsorption of DCF onto waste red bricks could be non-specific adsorption.Adsorption thermodynamics indicated that the adsorption process was exothermic, and the adsorption was more favorable at lower temperatures.

Table 3 .
The adsorption isotherm parameters of DCF.

Table 3 .
The adsorption isotherm parameters of DCF.

Table 4 .
The thermodynamic parameters of DCF adsorption.