# Trinary Component Adsorption of Methylene Blue, Methyl Orange, and Methyl Red from Aqueous Solution Using TiO2/Activated Carbon

Authors: Nguyen Thi Thanh Tu; Tran Si Thanh; Phan Tu Quy; Tran Thi Minh Ha; Phan Thi Kim Thu; Nguyen Hong Bich; Le Van Thanh Sơn; Vo Thang Nguyen; Dao Ngoc Nhiem; Pham Khac Lieu; Dinh Quang Khieu
Journal: Adsorption Science & Technology
 (2023)

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## Abstract

Porous TiO2/activated carbon (AC) material was synthesized by grafting peroxo-hydro titanium complexes to rice husk-derived activated carbon. It was found that the morphology of TiO2/AC consists of TiO2 fine particles highly dispersed on the AC matrix. The obtained TiO2/AC composites with high surface area and a red shift exhibit an excellent adsorption performance in both single and trinary system toward methylene blue (MB), methyl orange (MO), and methyl red (MR). The isotherm models including extended Langmuir, P-factor, ideal adsorbed solution theory (IAST) for Langmuir, Freundlich, and Sips models were applied to study the adsorption equilibrium data of trinary solutions. It was found that IAST for Freundlich and Langmuir models were the most suitable one to describe the adsorption of the three dyes on TiO2/AC material. The high maximum adsorption capacities (mmol g-1) in single/trinary mixture were found as 0.452/0.340 for MB; 0.329/0.321 for MO; and 0.806/2.04 for MR. Moreover, the recyclability experiments showed that the adsorbent could be reused through photocatalytic self-cleaning for at least three cycles with stable capacity. Thus, the TiO2/AC can be effectively employed for the removal of dyes from industrial textile wastewater.


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## Body

## 1. Introduction

Dye pollutants in wastewater are a critical environmental issue due to their nonbiodegradable and toxic nature which can generate serious threats to human being and marine organism. The dyeing wastewater discharged from industrial textiles is highly visible and undesirable even at low concentrations of dyes. Hence, the dyes must be effectively eliminated from the dyestuff wastewater to clear up the biological, ecological, and industrial issues. Therefore, various physical-chemistry treatment methods, including chemical ion exchange [1], membrane filtration [2], physical adsorption [3], photocatalytic [4], and biological approaches [5], have been developed to remove dyestuff from effluents. Among these, the adsorption approach has been considered as a promising method for treatment of discharged wastewater due to its easy operation, high efficiency, and economic feasibility. The nature of the adsorbent is one of the important factors deciding the effectiveness of any adsorption processes. The adsorbents with low cost, high adsorption capacity, and easy recyclability are desirable in the adsorption removal of dye molecules. The use of activated carbon prepared from rich-carbon biomasses for removing pollutants from wastewater has been intensively investigated because of its high synthesis yield, cheapness, eco-friendliness, and great adsorption efficiency [6]. However, the recycling of activated carbon material for further use usually involves organic solvents, resulting in secondary pollutants. A more eco-friendly strategy to recycle these adsorbents is photocatalytic self-cleaning activities through grafting other catalytically reactive species such as metal halides, metal oxides, and metal sulfides on various carbon matrixes. TiO2 nanoparticles is a well-known photocatalyst in advanced oxidation process which can mineralize organic pollutants under UV irradiation. However, there are some disadvantages of using TiO2 in powder form during the photocatalytic process: (i) titanium sources such as titanium alkoxides and titanium chlorides are difficult to handle due to its instability in moisture atmospheres; (ii) the fine powder TiO2 is likely to aggregate to form larger particles. These issues can be solved by using soluble-water titanium complexes instead of traditional easily hydrolyzed titanium sources. Therefore, the construction of heterojunctions by hybridizing activated carbon with water-soluble titanium complexes derived catalytically reactive TiO2 is an effective method to improve for recycling through photocatalytic self-cleaning.Although there is a great availability of research information on single-component adsorption [7–10] or binary-component adsorption [11, 12], that for complex mixture is more limited which leaves the adsorption of most industries discharge wastewater containing several components unexplainable. Gurav et al. presented the adsorption of the binary mixture containing bisphenol-A and solvent black-3 dye mixtures onto biochar by using Langmuir isotherm model [13]. Adsorption of safranine O and methylene blue dyes from single and binary systems using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich model was also reported [14]. According to our best knowledge, the study on adsorption isotherms of trinary component system is less available.In this study, we synthesized a three-dimensional TiO2/activated carbon (TiO2/AC) via peroxo-hydroxo titanium (IV) complexes and activated carbon derived from rice husks for eliminating the pollutants from aqueous solution. Three organic compounds including methylene blue (MB), methyl orange (MO), and methyl red (MR) were used as model pollutants to investigate their adsorption on TiO2/activated carbon (TiO2/AC) in single/trinary component systems using extended Langmuir, P-factor model, ideal adsorbed solution theory for Langmuir, Freundlich, and Sips models. The main objective of this work is to investigate the simultaneous removal of both anionic and cationic dyes from the single/trinary systems. Furthermore, the recyclability of the TiO2/AC was performed by the photocatalytic self-cleaning.


## 2. Experimental Methods

### 2.1. Materials

Rice husks were collected from local area (Hue city, Vietnam). All chemicals were used as received without any further purification. Anatase (TiO2; Merck, Germany, 99%), sodium (NaOH; Guangzhou, China, 97%), acid chlohydride (HCl; Guangzhou, China, 36%), hydroperoxide (H2O2 30% Guangzhou, China), methylene blue (MB, C16H18ClN3S), methyl orange (MO, C14H14N3NaO3S), methyl red (MR, C15H15N3O), and N,N- Dimethyformamide (DMF) were used in this work.


### 2.2. Synthesis of the Peroxo-Hydroxo Titanium Complexes and Activated Carbon

TiO2 was synthesized by ultrasonically assisted hydrothermal method. 0.25 grams of TiO2 powder was dispersed into a teflon flask containing 12.5 mL of 20 M NaOH solution before being processed by ultrasonic for 30 minutes. The teflon bottle was then inserted in the autoclave and kept at 130°C for 10 h. After the hydrothermal process, the autoclave was naturally cooled to room temperature. The mixture was then washed several times with 0.1 M HCl and distilled water until the pH of supernatant was neutral. The product was dried in vacuum at 100°C for 6 h. 0.25 grams obtained product was mixed with 35 mL of H2O2 at 90°C with magnetic stirring for an hour to form a yellow transparent peroxo-hydroxo titanium complex solution (0.25 grams TiO2/35 mL).The preparation of activated carbon from rice husk consists of two steps: firstly 60 grams of rice husks was soaked in 800 mL of 1 M NaOH and placed in an oven at 90°C for 24 h. The rice husks were then collected, washed, and calcined at 500°C for an hour to obtain the biochar. After that, 2 grams of biochar was mixed with 2 grams of NaOH in a Ni crucible and heated at 700°C for 2 h. The resulting product was washed with distilled water to completely remove all the excess NaOH, followed by ultrasonic treatment in DMF for 1 hour. The solid was separated by the centrifugation and dried at 100°C for 24 h to obtain the activated carbon (denoted as AC).The TiO2/AC composites with different TiO2/AC mass ratio were prepared through ultrasonic mixing of a specific volume of titanium complex solution (17.5, 35.0, 70.0, and 105.0 mL equivalent to 0.125, 0.250, 0.500, and 0.750 grams of TiO2, respectively) and 0.5 grams of AC, after sonication for 1 h the mixture is dried at 80°C for 6 hours. The resultant powders were calcined at 350°C for 2 hours in oxygen-free atmosphere. The TiO2/AC composites was denoted as TiO2/AC (0.25/1), TiO2/AC (0.5/1), TiO2/AC (1/1), and TiO2/AC (1.5/1) with the numbers in parenthesis presenting the ratio of TiO2/AC in weight. The schematic diagram of TiO2/AC synthesis is illustrated in the Scheme 1.Scheme 1
The diagram of synthesis of TiO2/AC from soluble-water titanium and rice husk-derived activated carbon.


### 2.3. Adsorption Experiments

#### 2.3.1. Adsorption Isotherm in Single Component Solution

(1) Determination of the Isoelectric Point of TiO2/AC. 10 mg TiO2/AC was dispersed into 100 mL conical flasks containing 25 mL 0.1 M NaCl, and the pHi of solution was adjusted in the range of 2-12 by adding 0.01 M HCl and 0.01 M NaOH solution. The sample was shaken in the dark for 24 h at a constant speed of 200 rpm and the pHf of the supernatant was then measured. The pH of point of zero charge was derived from the plot of ΔpH=pHf–pHiversuspHi where ΔpH=0.(2) Single Component Isotherm Studies. To study isotherm adsorption in the single component system, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 grams of TiO2/AC were added into a series of seven flasks containing 200 mL solution of individual dyes at concentration of 0.0469 mM, 0.0458 mM, and 0.0743 mM for MB, MO, and MR, respectively. These flasks were shaken in the dark for 24 hours to ensure adsorption/desorption equilibrium. The dye concentration in the supernatant was measured by UV-Vis spectrophotometry at its corresponding adsorption maxima.The adsorption capacity (qt) at a certain time and (qe) at the equilibrium time for each dye are expressed as follows:
(1)qt=Ci−CfVm,(2)qe=Ci−CeVm,where Ci (mM) and Ce (mM) are the concentration of dyes at a certain and equilibrium time in the solution, respectively; V and m are the solution volume (L) and mass of adsorbent (g).Three isotherms were used to investigate the adsorption of three dyes onto TiO2/AC, namely, Langmuir [15], Freundlich [16], Sips [17], and Temkin [18] isotherms as expressed in
(3)qe=KL.Ce1+aL,iCeLangmuirisotherm,(4)qe=aF.CebFFreudlichisotherm,(5)qe=KS.Ce1/bS1+aSCe1/bSSipsisotherm,(6)qe=RTbTlnRT.CeTempkinisotherm,where qe and Ce present adsorption capacity (mmol g-1) and concentration (mM) at equilibrium, and KL (L.g-1) and aL (L.mM-1), aF and bF,KS, aS (L.mM-1), and bS, and AT(L.g-1), bT, R (8.314x10−3J.mM−1.K−1), T (298 K), and B=RT/bT are the constants of Langmuir, Freudlich, Sips, and Tempkin models, respectively.


#### 2.3.2. Adsorption Study of Multicomponent System

(1) Effect of pH on the Simultaneous Adsorption of the Mixture of Three Chosen Dyes onto TiO2/AC Adsorbent. The influence of pH on dye absorption of TiO2/AC was studied by determining the dye uptake in a mixture of three dyes at different pH. The experiments were conducted on 200 mL solution containing 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR with the presence of 10 mg of TiO2/AC. The pH of solution was adjusted from 2 to 12 by adding 0.01 M HCl and 0.01 M NaOH solution. The samples were keep in the dark and were shaken at a constant speed of 200 rpm for 4 h to achieve adsorption equilibrium conditions. Afterwards, the samples were centrifuged at 4500 rpm for around 15 min to separate the supernatant for UV-Vis measurement at corresponding maximum absorbance wavelength to determine the residual concentrations of each dye.(2) Multicomponent Isotherm Studies. The adsorption equilibrium of multicomponent system was investigated in a similar manner in a 200 mL mixtures of 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR.The extended Langmuir, P-factor models, and ideal adsorbed solution theory (IAST) incorporating three different isotherm models of Langmuir, Freundlich, and Sips equations were used to examine the trinary dye system. Theory of these models is presented in detail in references [10, 11].A model for competitive sorption based on the Langmuir equation (extended Langmuir (EL)) was first proposed by Butler and Ockrent [19] to express the adsorption equilibrium in multicomponent systems. This isotherm is applicable as each component obeys Langmuir behavior in a single-solute system.
(7)qe,i=KL,i.Ce,i1+∑aL,iCe,i,where qe,i and Ce,i present adsorption capacity (mmol.g-1) and concentration (mM) of component i at equilibrium, and KL,i (L.g-1) and aL,i (L.mM-1) are Langmuir constants of component i.The P-factor method can be employed to investigate the competitive adsorption behavior of dyes in the trinary component system. This model presumed a simplified approach to correlate and compare between single- and multicomponents systems through the introduction of factor (P) (Equation (7)) proposed by Mckay and Al Duri [20].
(8)Pi=KL,i/aL,i,singleKL,i,mul/aL,i,trinary,where KL,i/aL,i,single=qL,i,single0 and KL,i,trinary/aL,i,trinary=qL,i,trinary0 are the maximum monolayer adsorption capacity for component i in a single and trinary systems.IfP−factor<1, the adsorption is enhanced by the existence of other components; P−factor=1, there is no noticeable interaction; and P−factor>1 indicates that the adsorption is suppress with the existence of other dyes. This model assumes that a Langmuir isotherm for each component (i), the multicomponent isotherm equation can be defined as:
(9)qe,i,trinary=1P,iKL,i.Ce,i,single1+aL,i.Ce,i,trinary,where aL,i is the Langmuir isotherm constants calculated from single solute system.The IAST is an approach to predict the adsorption isotherms of multiple components solution based on the isotherm data of each individual component considering the thermodynamic consistency of the system. [21]The Langmuir isotherm in the IAST is developed by using the spreading pressure of component,i, and ψin integral form proposed by McKay and Al Duri [20].
(10)ψ=π.ART=∫0CoqeCedCe=KLaLln1+aL.Rearranging Equations (3) and (9) [22] to obtain Equations (10) and (11).
(11)Ce,i0=expψaL0/KL,i0−1aL,i0,(12)qe,i0=KL,i0aL,i01−exp−ψaL0KL,i0.Equation (11) describes the Langmuir isotherm in the IAST at the specified spreading pressure.The Freundlich isotherm in the IAST is performed by using the spreading pressure in the form of the integral proposed by Kidnay and Myers [23] as in Equation (12).
(13)ψ=π.ART=∫0CodlnCeqedlnqedqe=qebF,(14)qe,j0=ψ.bF,jKL,i0aL,i0.The Freundlich isotherm in the IAST is as follows:(15)Ce,j0=qe,j0aF,j1/bF,j.By applying the Sips isotherm, the spreading pressure,ψ, can be expressed by [22].
(16)ψ=π.ART=∫0CoqeCedCe=qebFln1+aS.CebS,(17)Ce,j=expψaS,i0bS,i0/KS,i0−1aS,i01/bS,i.The Sips isotherm in the IAST is as follows:(18)qe,i0=KS,i0aS,i01−exp−ψaS,i0bS,i0KS,i0−1.The model’s parameters ảe by the least square method by minimizing the sum of squared deviations, SSES, by means of the numerical optimization techniques using the Solver function in Microsoft Excel. The function for minimization is
(19)SSES=∑1Nyexp−yest2,where yexp is the experimental response, and yest is the response calculated from the model.The determination coefficientR2 is obtained according to the expression
(20)R2=1−SSESSSET,where SSET is the total sum of squares equal to (∑1Nyexp−ymean2 (where ymean is the mean value of y).The AIC (Akaike’s information criteria) is used to compare two models. AIC is calculated for each model from Equation (20) [24].
(21)AIC=N·lnSSETN+2·Np+2·Np·Np+1N−Np−1,where N is the number of data points; Np is the number of parameters fit by the regression.AIC decides how well the data fit each model. The value of AIC can be positive or negative. The model with the lowestAICc score is considered the most likely correct [25].


#### 2.3.3. Recycling of Adsorbent

The recyclability of TiO2/AC was performed by photocatalytic self-cleaning. After adsorption experiments in 200 mL solution of each dyes at a concentration of 0.0469 mM for MB, 0.0458 mM for MO, and 0.0743 mM for MR in the presence of 0.2 g adsorbent, the used adsorbent, obtained from centrifugation, was desorbed by magnetic stirring under halogen lamp illumination for 2 hours. The adsorbent was then washed with distilled water and dried at 100°C for further use.


## 2.1. Materials

Rice husks were collected from local area (Hue city, Vietnam). All chemicals were used as received without any further purification. Anatase (TiO2; Merck, Germany, 99%), sodium (NaOH; Guangzhou, China, 97%), acid chlohydride (HCl; Guangzhou, China, 36%), hydroperoxide (H2O2 30% Guangzhou, China), methylene blue (MB, C16H18ClN3S), methyl orange (MO, C14H14N3NaO3S), methyl red (MR, C15H15N3O), and N,N- Dimethyformamide (DMF) were used in this work.


## 2.2. Synthesis of the Peroxo-Hydroxo Titanium Complexes and Activated Carbon

TiO2 was synthesized by ultrasonically assisted hydrothermal method. 0.25 grams of TiO2 powder was dispersed into a teflon flask containing 12.5 mL of 20 M NaOH solution before being processed by ultrasonic for 30 minutes. The teflon bottle was then inserted in the autoclave and kept at 130°C for 10 h. After the hydrothermal process, the autoclave was naturally cooled to room temperature. The mixture was then washed several times with 0.1 M HCl and distilled water until the pH of supernatant was neutral. The product was dried in vacuum at 100°C for 6 h. 0.25 grams obtained product was mixed with 35 mL of H2O2 at 90°C with magnetic stirring for an hour to form a yellow transparent peroxo-hydroxo titanium complex solution (0.25 grams TiO2/35 mL).The preparation of activated carbon from rice husk consists of two steps: firstly 60 grams of rice husks was soaked in 800 mL of 1 M NaOH and placed in an oven at 90°C for 24 h. The rice husks were then collected, washed, and calcined at 500°C for an hour to obtain the biochar. After that, 2 grams of biochar was mixed with 2 grams of NaOH in a Ni crucible and heated at 700°C for 2 h. The resulting product was washed with distilled water to completely remove all the excess NaOH, followed by ultrasonic treatment in DMF for 1 hour. The solid was separated by the centrifugation and dried at 100°C for 24 h to obtain the activated carbon (denoted as AC).The TiO2/AC composites with different TiO2/AC mass ratio were prepared through ultrasonic mixing of a specific volume of titanium complex solution (17.5, 35.0, 70.0, and 105.0 mL equivalent to 0.125, 0.250, 0.500, and 0.750 grams of TiO2, respectively) and 0.5 grams of AC, after sonication for 1 h the mixture is dried at 80°C for 6 hours. The resultant powders were calcined at 350°C for 2 hours in oxygen-free atmosphere. The TiO2/AC composites was denoted as TiO2/AC (0.25/1), TiO2/AC (0.5/1), TiO2/AC (1/1), and TiO2/AC (1.5/1) with the numbers in parenthesis presenting the ratio of TiO2/AC in weight. The schematic diagram of TiO2/AC synthesis is illustrated in the Scheme 1.Scheme 1
The diagram of synthesis of TiO2/AC from soluble-water titanium and rice husk-derived activated carbon.


## 2.3. Adsorption Experiments

### 2.3.1. Adsorption Isotherm in Single Component Solution

(1) Determination of the Isoelectric Point of TiO2/AC. 10 mg TiO2/AC was dispersed into 100 mL conical flasks containing 25 mL 0.1 M NaCl, and the pHi of solution was adjusted in the range of 2-12 by adding 0.01 M HCl and 0.01 M NaOH solution. The sample was shaken in the dark for 24 h at a constant speed of 200 rpm and the pHf of the supernatant was then measured. The pH of point of zero charge was derived from the plot of ΔpH=pHf–pHiversuspHi where ΔpH=0.(2) Single Component Isotherm Studies. To study isotherm adsorption in the single component system, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 grams of TiO2/AC were added into a series of seven flasks containing 200 mL solution of individual dyes at concentration of 0.0469 mM, 0.0458 mM, and 0.0743 mM for MB, MO, and MR, respectively. These flasks were shaken in the dark for 24 hours to ensure adsorption/desorption equilibrium. The dye concentration in the supernatant was measured by UV-Vis spectrophotometry at its corresponding adsorption maxima.The adsorption capacity (qt) at a certain time and (qe) at the equilibrium time for each dye are expressed as follows:
(1)qt=Ci−CfVm,(2)qe=Ci−CeVm,where Ci (mM) and Ce (mM) are the concentration of dyes at a certain and equilibrium time in the solution, respectively; V and m are the solution volume (L) and mass of adsorbent (g).Three isotherms were used to investigate the adsorption of three dyes onto TiO2/AC, namely, Langmuir [15], Freundlich [16], Sips [17], and Temkin [18] isotherms as expressed in
(3)qe=KL.Ce1+aL,iCeLangmuirisotherm,(4)qe=aF.CebFFreudlichisotherm,(5)qe=KS.Ce1/bS1+aSCe1/bSSipsisotherm,(6)qe=RTbTlnRT.CeTempkinisotherm,where qe and Ce present adsorption capacity (mmol g-1) and concentration (mM) at equilibrium, and KL (L.g-1) and aL (L.mM-1), aF and bF,KS, aS (L.mM-1), and bS, and AT(L.g-1), bT, R (8.314x10−3J.mM−1.K−1), T (298 K), and B=RT/bT are the constants of Langmuir, Freudlich, Sips, and Tempkin models, respectively.


### 2.3.2. Adsorption Study of Multicomponent System

(1) Effect of pH on the Simultaneous Adsorption of the Mixture of Three Chosen Dyes onto TiO2/AC Adsorbent. The influence of pH on dye absorption of TiO2/AC was studied by determining the dye uptake in a mixture of three dyes at different pH. The experiments were conducted on 200 mL solution containing 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR with the presence of 10 mg of TiO2/AC. The pH of solution was adjusted from 2 to 12 by adding 0.01 M HCl and 0.01 M NaOH solution. The samples were keep in the dark and were shaken at a constant speed of 200 rpm for 4 h to achieve adsorption equilibrium conditions. Afterwards, the samples were centrifuged at 4500 rpm for around 15 min to separate the supernatant for UV-Vis measurement at corresponding maximum absorbance wavelength to determine the residual concentrations of each dye.(2) Multicomponent Isotherm Studies. The adsorption equilibrium of multicomponent system was investigated in a similar manner in a 200 mL mixtures of 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR.The extended Langmuir, P-factor models, and ideal adsorbed solution theory (IAST) incorporating three different isotherm models of Langmuir, Freundlich, and Sips equations were used to examine the trinary dye system. Theory of these models is presented in detail in references [10, 11].A model for competitive sorption based on the Langmuir equation (extended Langmuir (EL)) was first proposed by Butler and Ockrent [19] to express the adsorption equilibrium in multicomponent systems. This isotherm is applicable as each component obeys Langmuir behavior in a single-solute system.
(7)qe,i=KL,i.Ce,i1+∑aL,iCe,i,where qe,i and Ce,i present adsorption capacity (mmol.g-1) and concentration (mM) of component i at equilibrium, and KL,i (L.g-1) and aL,i (L.mM-1) are Langmuir constants of component i.The P-factor method can be employed to investigate the competitive adsorption behavior of dyes in the trinary component system. This model presumed a simplified approach to correlate and compare between single- and multicomponents systems through the introduction of factor (P) (Equation (7)) proposed by Mckay and Al Duri [20].
(8)Pi=KL,i/aL,i,singleKL,i,mul/aL,i,trinary,where KL,i/aL,i,single=qL,i,single0 and KL,i,trinary/aL,i,trinary=qL,i,trinary0 are the maximum monolayer adsorption capacity for component i in a single and trinary systems.IfP−factor<1, the adsorption is enhanced by the existence of other components; P−factor=1, there is no noticeable interaction; and P−factor>1 indicates that the adsorption is suppress with the existence of other dyes. This model assumes that a Langmuir isotherm for each component (i), the multicomponent isotherm equation can be defined as:
(9)qe,i,trinary=1P,iKL,i.Ce,i,single1+aL,i.Ce,i,trinary,where aL,i is the Langmuir isotherm constants calculated from single solute system.The IAST is an approach to predict the adsorption isotherms of multiple components solution based on the isotherm data of each individual component considering the thermodynamic consistency of the system. [21]The Langmuir isotherm in the IAST is developed by using the spreading pressure of component,i, and ψin integral form proposed by McKay and Al Duri [20].
(10)ψ=π.ART=∫0CoqeCedCe=KLaLln1+aL.Rearranging Equations (3) and (9) [22] to obtain Equations (10) and (11).
(11)Ce,i0=expψaL0/KL,i0−1aL,i0,(12)qe,i0=KL,i0aL,i01−exp−ψaL0KL,i0.Equation (11) describes the Langmuir isotherm in the IAST at the specified spreading pressure.The Freundlich isotherm in the IAST is performed by using the spreading pressure in the form of the integral proposed by Kidnay and Myers [23] as in Equation (12).
(13)ψ=π.ART=∫0CodlnCeqedlnqedqe=qebF,(14)qe,j0=ψ.bF,jKL,i0aL,i0.The Freundlich isotherm in the IAST is as follows:(15)Ce,j0=qe,j0aF,j1/bF,j.By applying the Sips isotherm, the spreading pressure,ψ, can be expressed by [22].
(16)ψ=π.ART=∫0CoqeCedCe=qebFln1+aS.CebS,(17)Ce,j=expψaS,i0bS,i0/KS,i0−1aS,i01/bS,i.The Sips isotherm in the IAST is as follows:(18)qe,i0=KS,i0aS,i01−exp−ψaS,i0bS,i0KS,i0−1.The model’s parameters ảe by the least square method by minimizing the sum of squared deviations, SSES, by means of the numerical optimization techniques using the Solver function in Microsoft Excel. The function for minimization is
(19)SSES=∑1Nyexp−yest2,where yexp is the experimental response, and yest is the response calculated from the model.The determination coefficientR2 is obtained according to the expression
(20)R2=1−SSESSSET,where SSET is the total sum of squares equal to (∑1Nyexp−ymean2 (where ymean is the mean value of y).The AIC (Akaike’s information criteria) is used to compare two models. AIC is calculated for each model from Equation (20) [24].
(21)AIC=N·lnSSETN+2·Np+2·Np·Np+1N−Np−1,where N is the number of data points; Np is the number of parameters fit by the regression.AIC decides how well the data fit each model. The value of AIC can be positive or negative. The model with the lowestAICc score is considered the most likely correct [25].


### 2.3.3. Recycling of Adsorbent

The recyclability of TiO2/AC was performed by photocatalytic self-cleaning. After adsorption experiments in 200 mL solution of each dyes at a concentration of 0.0469 mM for MB, 0.0458 mM for MO, and 0.0743 mM for MR in the presence of 0.2 g adsorbent, the used adsorbent, obtained from centrifugation, was desorbed by magnetic stirring under halogen lamp illumination for 2 hours. The adsorbent was then washed with distilled water and dried at 100°C for further use.


## 2.3.1. Adsorption Isotherm in Single Component Solution

(1) Determination of the Isoelectric Point of TiO2/AC. 10 mg TiO2/AC was dispersed into 100 mL conical flasks containing 25 mL 0.1 M NaCl, and the pHi of solution was adjusted in the range of 2-12 by adding 0.01 M HCl and 0.01 M NaOH solution. The sample was shaken in the dark for 24 h at a constant speed of 200 rpm and the pHf of the supernatant was then measured. The pH of point of zero charge was derived from the plot of ΔpH=pHf–pHiversuspHi where ΔpH=0.(2) Single Component Isotherm Studies. To study isotherm adsorption in the single component system, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 grams of TiO2/AC were added into a series of seven flasks containing 200 mL solution of individual dyes at concentration of 0.0469 mM, 0.0458 mM, and 0.0743 mM for MB, MO, and MR, respectively. These flasks were shaken in the dark for 24 hours to ensure adsorption/desorption equilibrium. The dye concentration in the supernatant was measured by UV-Vis spectrophotometry at its corresponding adsorption maxima.The adsorption capacity (qt) at a certain time and (qe) at the equilibrium time for each dye are expressed as follows:
(1)qt=Ci−CfVm,(2)qe=Ci−CeVm,where Ci (mM) and Ce (mM) are the concentration of dyes at a certain and equilibrium time in the solution, respectively; V and m are the solution volume (L) and mass of adsorbent (g).Three isotherms were used to investigate the adsorption of three dyes onto TiO2/AC, namely, Langmuir [15], Freundlich [16], Sips [17], and Temkin [18] isotherms as expressed in
(3)qe=KL.Ce1+aL,iCeLangmuirisotherm,(4)qe=aF.CebFFreudlichisotherm,(5)qe=KS.Ce1/bS1+aSCe1/bSSipsisotherm,(6)qe=RTbTlnRT.CeTempkinisotherm,where qe and Ce present adsorption capacity (mmol g-1) and concentration (mM) at equilibrium, and KL (L.g-1) and aL (L.mM-1), aF and bF,KS, aS (L.mM-1), and bS, and AT(L.g-1), bT, R (8.314x10−3J.mM−1.K−1), T (298 K), and B=RT/bT are the constants of Langmuir, Freudlich, Sips, and Tempkin models, respectively.


## 2.3.2. Adsorption Study of Multicomponent System

(1) Effect of pH on the Simultaneous Adsorption of the Mixture of Three Chosen Dyes onto TiO2/AC Adsorbent. The influence of pH on dye absorption of TiO2/AC was studied by determining the dye uptake in a mixture of three dyes at different pH. The experiments were conducted on 200 mL solution containing 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR with the presence of 10 mg of TiO2/AC. The pH of solution was adjusted from 2 to 12 by adding 0.01 M HCl and 0.01 M NaOH solution. The samples were keep in the dark and were shaken at a constant speed of 200 rpm for 4 h to achieve adsorption equilibrium conditions. Afterwards, the samples were centrifuged at 4500 rpm for around 15 min to separate the supernatant for UV-Vis measurement at corresponding maximum absorbance wavelength to determine the residual concentrations of each dye.(2) Multicomponent Isotherm Studies. The adsorption equilibrium of multicomponent system was investigated in a similar manner in a 200 mL mixtures of 0.0469 mM MB, 0.0458 mM MO, and 0.0743 mM MR.The extended Langmuir, P-factor models, and ideal adsorbed solution theory (IAST) incorporating three different isotherm models of Langmuir, Freundlich, and Sips equations were used to examine the trinary dye system. Theory of these models is presented in detail in references [10, 11].A model for competitive sorption based on the Langmuir equation (extended Langmuir (EL)) was first proposed by Butler and Ockrent [19] to express the adsorption equilibrium in multicomponent systems. This isotherm is applicable as each component obeys Langmuir behavior in a single-solute system.
(7)qe,i=KL,i.Ce,i1+∑aL,iCe,i,where qe,i and Ce,i present adsorption capacity (mmol.g-1) and concentration (mM) of component i at equilibrium, and KL,i (L.g-1) and aL,i (L.mM-1) are Langmuir constants of component i.The P-factor method can be employed to investigate the competitive adsorption behavior of dyes in the trinary component system. This model presumed a simplified approach to correlate and compare between single- and multicomponents systems through the introduction of factor (P) (Equation (7)) proposed by Mckay and Al Duri [20].
(8)Pi=KL,i/aL,i,singleKL,i,mul/aL,i,trinary,where KL,i/aL,i,single=qL,i,single0 and KL,i,trinary/aL,i,trinary=qL,i,trinary0 are the maximum monolayer adsorption capacity for component i in a single and trinary systems.IfP−factor<1, the adsorption is enhanced by the existence of other components; P−factor=1, there is no noticeable interaction; and P−factor>1 indicates that the adsorption is suppress with the existence of other dyes. This model assumes that a Langmuir isotherm for each component (i), the multicomponent isotherm equation can be defined as:
(9)qe,i,trinary=1P,iKL,i.Ce,i,single1+aL,i.Ce,i,trinary,where aL,i is the Langmuir isotherm constants calculated from single solute system.The IAST is an approach to predict the adsorption isotherms of multiple components solution based on the isotherm data of each individual component considering the thermodynamic consistency of the system. [21]The Langmuir isotherm in the IAST is developed by using the spreading pressure of component,i, and ψin integral form proposed by McKay and Al Duri [20].
(10)ψ=π.ART=∫0CoqeCedCe=KLaLln1+aL.Rearranging Equations (3) and (9) [22] to obtain Equations (10) and (11).
(11)Ce,i0=expψaL0/KL,i0−1aL,i0,(12)qe,i0=KL,i0aL,i01−exp−ψaL0KL,i0.Equation (11) describes the Langmuir isotherm in the IAST at the specified spreading pressure.The Freundlich isotherm in the IAST is performed by using the spreading pressure in the form of the integral proposed by Kidnay and Myers [23] as in Equation (12).
(13)ψ=π.ART=∫0CodlnCeqedlnqedqe=qebF,(14)qe,j0=ψ.bF,jKL,i0aL,i0.The Freundlich isotherm in the IAST is as follows:(15)Ce,j0=qe,j0aF,j1/bF,j.By applying the Sips isotherm, the spreading pressure,ψ, can be expressed by [22].
(16)ψ=π.ART=∫0CoqeCedCe=qebFln1+aS.CebS,(17)Ce,j=expψaS,i0bS,i0/KS,i0−1aS,i01/bS,i.The Sips isotherm in the IAST is as follows:(18)qe,i0=KS,i0aS,i01−exp−ψaS,i0bS,i0KS,i0−1.The model’s parameters ảe by the least square method by minimizing the sum of squared deviations, SSES, by means of the numerical optimization techniques using the Solver function in Microsoft Excel. The function for minimization is
(19)SSES=∑1Nyexp−yest2,where yexp is the experimental response, and yest is the response calculated from the model.The determination coefficientR2 is obtained according to the expression
(20)R2=1−SSESSSET,where SSET is the total sum of squares equal to (∑1Nyexp−ymean2 (where ymean is the mean value of y).The AIC (Akaike’s information criteria) is used to compare two models. AIC is calculated for each model from Equation (20) [24].
(21)AIC=N·lnSSETN+2·Np+2·Np·Np+1N−Np−1,where N is the number of data points; Np is the number of parameters fit by the regression.AIC decides how well the data fit each model. The value of AIC can be positive or negative. The model with the lowestAICc score is considered the most likely correct [25].


## 2.3.3. Recycling of Adsorbent

The recyclability of TiO2/AC was performed by photocatalytic self-cleaning. After adsorption experiments in 200 mL solution of each dyes at a concentration of 0.0469 mM for MB, 0.0458 mM for MO, and 0.0743 mM for MR in the presence of 0.2 g adsorbent, the used adsorbent, obtained from centrifugation, was desorbed by magnetic stirring under halogen lamp illumination for 2 hours. The adsorbent was then washed with distilled water and dried at 100°C for further use.


## 3. Results and Discussion

### 3.1. Characterization of Materials

Crystal structures of the obtained materials were studied by XRD patterns (Figure1(a)). XRD patterns of AC exhibit the peaks at 2 theta angles of 29.45, 36.49, 39.49, 43.03, and 47.56o corresponding to (203), (213), (205), (304), and (305) Miller indexes of carbon (JCPDS-00-022-1069). XRD patterns of TiO2 and composite TiO2/AC show the characteristic peaks of anatase at 25.25, 38.15, 47.85, 54.65, and 62.75o corresponding to the diffraction plane of (101), (004), (200), (105), and (204) (JCPDS -21-1272). With increasing amount of TiO2, the intensity of diffraction peaks of anatase increases indicating that two phases of activated carbon and anatase are coexisted. The composition of titanium and carbon elements estimated by EDX is presented in Figure S1. It is found that TiO2/carbon atomic ratio in product increases with the increase of TiO2 in initial mixture, indicating that the composition of TiO2/AC could be controlled by adjusting the reaction composition. Figure 1(b) and Table 1 presents the nitrogen adsorption and desorption isotherms of AC, TiO2, and TiO2/AC composites. All the isotherms exhibit a hysteresis loop which is characteristic for type IV isotherm, confirming the existence of mesoporous materials. The hysteresis loop occurs at relatively high pressures illustrating that the mesoporous structure formed between the particles. The surface area calculated based on the BET model is 1713 m2.g-1 for AC, 124 m2.g-1 for TiO2, and 436 m2.g-1 for TiO2/AC. The surface areas of TiO2 and AC are relatively large compared with previously reported studies [26, 27] showing that the structure of TiO2 in this study is highly porous. Furthermore, the combination of TiO2 on AC matrix significantly increased the surface area of TiO2 itself.Figure 1
(a) XRD patterns, (b) nitrogen adsorption/desorption isotherms, (c) UV-Vis diffuse reflectance spectroscopy, and (d) Tauc’s plots of AC, TiO2/AC (0.25/1), TiO2/AC (0.5/1), TiO2/AC (1/1), TiO2/AC (1.5/1), and TiO2.
(a)(b)(c)(d)Table 1
Textural properties of AC, TiO2, and TiO2/AC composites.
MaterialsSurface area based on BET model (m2 g-1)Porous volume (cm3/g)Mesoarea (m2 g-1)Microarea (m2 g-1)AC17136103331380TiO21242289431TiO2/AC (0.25/1)238100108131TiO2/AC (0.5/1)494313206288TiO2/AC (1/1)436222193243TiO2/AC (1.5/1)320241182138Figures1(c) and 1(d) show the UV-Vis-DRS and their Tauc’s plot for calculating band gap energy derived from UV-Vis-DRS. No energy absorption edge of AC which can be considered as a conductor was observed. The band gap energy of TiO2 is 3.2 eV which is equivalent to previous report in the range of 3.1-3.2 eV [28]. The combination of TiO2 with AC gives rise to a red shift in the energy adsorption spectrum, corresponding a band gap energy of 1.9 -2.5 eV illustrating that these composites might have photocatalytic activity in the visible region.The morphology of TiO2 prepared from peroxo-hydroxo titanium (IV) complexes consists of fine particles of around 10 nM which can be seen to agglomerate to form larger particles, while AC exhibits the typical layered-structure. In the composite structure, TiO2 agglomerates are highly dispersed on the AC matrix. The higher TiO2 content leads to a better coverage of TiO2 on AC matrix. The distribution of TiO2 on AC was also studied by EDX mapping. Figure 2 illustrates the elementary mapping of TiO2/AC (0.5/1) sample. The extensive dispersion of Ti, C, and O elements confirms that TiO2 was well-distributed on the surface of AC. EDX spectrum also shows some impurity of Mg, Si, Na, and Ca element (of around 0.2-1% w) which might present in rice husks.Figure 2
(a) SEM observation of TiO2/AC composites and (b) EDX mapping of TiO2/AC (0.5/1).
(a)(b)


### 3.2. Adsorption Studies

#### 3.2.1. Simultaneous Determination of MB, MO, and MR by UV-Vis Spectroscopy

The dye concentration in solution was determine by UV-Vis spectroscopy at maximum wavelength of 664 nm for MB, 464 nm for MO, and 500 nm for MR. However, the spectra of three dyes are overlapped resulting in larger error for simultaneous determination of these dyes (Figure3(a)). In the work, the peaks were deconvoluted by means of OriginPro19/Deconvolution Software as illustrated in Figure 3(b). The deconvoluted UV-Vis spectrum was used to estimate the linear calibration range for dye determination. The linear range of each dye was found to be 0.0063-0.094 mM for MB, 0.015-0.138 mM for MO, and 0.019-0.167 mM for MR. The limit of detections (LODs) for MB, MO, and MR are 0.0007 mM, 0.002 mM, and 0.009 mM, respectively, which are suitable for the detection of these dyes in the adsorption study. Standard addition method is used to determine the dye in the trinary mixture. The recovery was found to be in the range of 0.9-0.99 indicating that the proposed method could be used for simultaneous determination of MB, MO, and MR (Table S1).Figure 3
(a) The UV-Vis spectra of MB, MO, and MR individual (0.0625 mM MB, 0.061 mM MO, and 0.111 mM MR) and (b) UV-Vis spectrum of trinary mixture of MB, MO, and MR as well as corresponding UV-Vis deconvulated spectra (Trinary component mixture of 0.0625 MB, 0.061 MO, and 0.111 MR, mass of adsorbent).
(a)(b)


#### 3.2.2. Adsorption Studies of Single Component System of MB, MO, and MR over TiO2, AC, and TiO2/AC

The adsorption kinetics of TiO2, AC, and TiO2/AC composites at different concentrations of dyes (0.0306–0.0743 mM) are performed in Figure S2 and Table S2. All of samples exhibit the high adsorption towards MB, MO, and MR expect only TiO2 which can adsorb MB. As can be seen from the figure, the higher the dye concentration, the greater the adsorption capacity. This can explain by the fact that the high dye concentration enhances the driving force that overcomes all the mass transfer resistance of dye between the aqueous solution and the surface of absorbent. The adsorption of dyes is fast in the earlier stage (0–100 minutes) and gradually reached the equilibrium. It is found that the time required to obtain the adsorption/desorption equilibrium was around 120 minutes for all of cases. The kinetics data were applied to the pseudo-first-order and pseudo-second-order kinetic models. The kinetic parameters of the models and R2 are presented in Table S2. The regression equation based one pseudo-first-order model provides the high values of determination coefficient (R2=0.88–0.98) and the equilibrium adsorption capacity calculated by this model (qe,cal) agrees with equilibrium adsorption capacity obtained by the experiment (qe,exp). Hence, the experimental results fit the pseudo-first-order model.In order to select the TiO2/AC composite for further study, the removal of MB using TiO2/AC as a dye model was conducted (Figure S3). The AC exhibits the highest adsorption capacity for MB; whereas, TiO2 presents a very poor adsorption capacity. The adsorption capacity and the regeneration of spent adsorbent by the photocatalytic self-cleaning are significantly dependent on the composition of TiO2 in AC. The adsorption capacity was found to be 0.865 mmol g-1 for AC, 0.211 mmol g-1 for TiO2/AC (0.25/1), 0.452 mmol g-1 for TiO2/AC (0.5/1), 0.301 mmol g-1 for TiO2/AC (1/1), 0.251 mmol g-1 for TiO2/AC (1.5/1), and only 0.2 mmol g-1 for TiO2. However, after the first recycle, the adsorption capacity of TiO2/AC(0.5/1) was the highest. Based on the balance between adsorption capacity and visible-light derived photocatalytic self-cleaning, TiO2/AC (0.5/1) catalyst was selected for further experiments. Figure S4 and Figure S5 present the relationship of equilibrium concentration and adsorption capacity of AC, TiO2/AC, and TiO2. Either Langmuir, Freundlich, or Tempkin models describe well the equilibrium data with high determination coefficients (R2=0.823–0.999) (Table 2) indicating the monolayer adsorption with different energy. The differences between the adsorption capacities of the three dyes may results from different adsorption mechanisms, which might depend on the molecular size and their electron affinities toward adsorbent surface.Table 2
The parameters of adsorption isotherms of Langmuir, Freundlich, and Temkin models for TiO2/AC (0.5/1), TiO2, and AC in single component system of MB, MO, and MR.
AdsorbentDyesLangmuir isotherm modelFreundlich isotherm modelTemkin isotherm modelqm (mmol g-1)aL (L g-1)R2bFKF (L g-1)R2KT (L g-1)bT (KJ.mol-1)R2ACMB0.8650.1390.9891.73749.8540.993589.0313.390.986TiO2/AC0.4520.0940.9971.74418.6830.998256.1923.510.997TiO20.2000.0840.9991.9329.0720.999205.2349.170.999ACMO0.5320.5390.9934.70687.2070.9914682.3028.000.992TiO2/AC0.3290.3900.9882.68637.5450.9951366.6235.120.991TiO200—00—00—ACMR1.2190.1520.9792.12964.5120.951359.848.820.982TiO2/AC0.8060.3050.9983.11675.2390.9080.805.480.823TiO200—00—00—


#### 3.2.3. Adsorption Isotherms of Trinary Component System for MB, MO, and MR over TiO2/AC (0.5/1)

(1) Effect of pH on the Adsorption of TiO2/AC (0.5/1). The pH effect on the adsorption of trinary component mixture by TiO2/AC is presented in Figure 4(a). The MB uptake was found to increase from 0.188 to 0.889 mmol g-1 for an increase in pH from 2 to 5. At higher pH values (5–11), the MB uptake remains almost constant. The point of zero charge of TiO2/AC obtained by the pH drift method was found at pH=6.4 (Figure 4(b)). At higher pH (>6.4), the surface of TiO2/AC may become negatively charged, which promoted the adsorption of the positively charged MB cations (pKa of MB=3.8 [29]) through electrostatic attraction. This, however, cannot explain its constant adsorption by TiO2/AC at all the studied pH values. There might be another mode of adsorption, e.g., ion exchange which is similar to the adsorption of MB onto wheat shells [30] or malachite green onto treated sawdust [31].Figure 4
(a) pH effect on dye adsorption capacity; (b) The point of zero charge estimated by pH drift method.
(a)(b)In contrast, the uptake of MO and MR decreases with the rise of pH. When pH increases from 2 to around 6, the adsorption capacity decreases from 0.344 to 0.124 mmol g-1 for MO and from 0.582 mmol g-1 to 0.452 mmol g-1 for MR. The MO or MR uptake keep unchanged with further pH increasing. Higher adsorptions at lower pH values could be due to protonation properties of the TiO2/AC. At pH below the point of zero charge (pHs<6.4), the positive charges at the surface of TiO2/AC favoured the adsorption of anionic MO and MR (pKa of MO=3.8 [32] and pKa of MR=5.1 [33]). A similar trend of adsorption behavior has also been reported with some different adsorbents [34].In general, the mechanism of dye adsorption is related to the interaction between the adsorbent and the adsorbate in solution. This interaction can occur in many different ways such as hydrogen bonding, electrostatic interaction,π-π stacking interaction, ion-exchange, hydrophobic bonding, and pores and chemical bonding [16, 25, 37, 62, 63] as illustrated in the Scheme 2.Scheme 2
The diagram of interaction mechanisms of MB, MR, and MO with TiO2/AC.


#### 3.2.4. Equilibrium Studies

The equilibrium data of trinary component system were fitted to the extended Langmuir model (EL), the Langmuir model using P-factor models (P-Langmuir model) and Langmuir, Freundlich, and Sips models using IAST (denoted L-IAST, F-IAST, and S-IAST, respectively) as shown in Figure5. As can be seen, except EL model, the other four models are able to adequately correlate the equilibrium data. Although, the determination coefficient (R2) is widely used to estimate the goodness of fit for the models with an equal number of experimental points and of the fitting parameters, Akaike information criterion (AIC) is preferred for comparing the models with different parameters. The smaller AICs are, the better the model fit the data. As can be seen from Table 3, it is little difficult to estimate how the AICs are small because there are three values of AICs for each model. Relatively, it can be seen from Table 3 that the AICs are possibly rearranged from low to high based on the sum of AICs as follows: -160.92 (-58.37, -61.23, and -40.92) for F-IAST; -155.67 (-55.34, -58.86, and -41.47) for L-IAST; -152.67 (-54.34, -57.86, and -40.47) for P-factor-Langmuir model; -145.99 (-50.99, -57.76, and -37.24) for S-IAST; and –39.1(-22.96, -11.83, and -4.31) for EL suggesting both F-IAST model were the best to fit experimental data of all three chosen dyes followed by L-IAST. The Langmuir maximum sorption capacities based on L-IAST model in trinary dyes system were found to be lower than those obtained from the single-dye system, due to the presence of competing dyes. The influence of competitive dyes on the sorption of specific dyes onto the TiO2/AC is analyzed by using P-factor. The ratios of qL,MR,0/qL,MB0andqL,MR,0/qL,MO0 are >1, indicating the predominant MR adsorption over MB and MO in both single and trinary systems. The P-values of QL,MR,single0/QL,MR,trinary0=0.806/2.04<1 prove that the sorption process of MR was promoted by the existence of other dyes, while that of qL,MB,single0/qL,MB,trinary0=0.450/0.340>1 presents that the MB adsorption was hindered by the existence of competing dyes. The ratio of qL,MO,single0/qL,MO,trinary0=0.329/0.321=~1 reveals that the MO adsorption is independent on the existence of other dyes. The presence of MB and MO seems to enhance significantly the adsorption capacity of TiO2/AC toward MR to ~2.5 times. MB contains a positively charged amine group while MR and MO contain negatively charged carboxyl and sulfite groups, respectively. It is possible that the MB moleculars adsorbed on TiO2/AC promote the MR adsorption via electrostatic interaction and π-π stacking interaction. The mechanism of completive adsorption of the components is not clear and need to study further.Figure 5
The plot of adsorption isotherms for different models.Table 3
The parameters of EL, P-factor, and IAST-sorption isotherm (Langmuir, Freundlich, and Sips),R2, and AICc values.
DyeExtended-Langmuir modelR2AICsqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0 (L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)MB2.27468.8230.2630.576-48.16MO0.6077.03127.950.900-37.04MR0.8065.9882.080.674-29.50P-factor-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.39426.6567.630.916-54.34MO0.32433.10101.90.996-57.86MR1.2816.905.400.918-40.47IAST-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.34023.2267.630.917-55.34MO0.32132.67101.860.998-58.86MR2.0410.995.400.919-41.47IAST-Freudlich modelDyeaF,MBaF,MOaF,MRbF,MBbF,MObF,MRR2AICsMB1.050.430.996-58.37MO0.8430.350.998-61.23MR6.400.890.987-40.92IAST-Sips modelDyeKS,MB0KS,MO0KS,MR0aS,MB0aS,MO0aS,MR0bS,MB0bS,MO0bS,MR0R2AICsMB1.270.440.4660.943-50.99MO1.110.600.400.999-57.76MR16.6517.50.950.945-37.24Table3 reveals that TiO2/AC can adsorb as much as approximately two third its own weight (total 763.77 mg MB+MO+MR/1000mg) at ambient temperature. The adsorption capacity of TiO2/AC was compared with the reported materials for the dye adsorption. The adsorption of MB, MO, or MR in the literature is mostly in single component system (Table 4). One can see that the reported adsorbents include carbon nanotubes, activated carbon, and biopolymers as well as one metal organic framework. However, most of these materials only exhibited lower adsorption capacities than the TiO2/AC adsorbent or even the MOF, MIL-53(Fe), just possessed an adsorption capacity of 0.668 mM.g-1. Meanwhile, the adsorption capacity in single system/trinary system of TiO2/AC is 0.452/0.340 for MB, 0.329/0.321 for MO, and 0.806/2.04 for MR. This indicates that the potential using TiO2/AC to remove dyes is indeed promising.Table 4
A comparison of maximum adsorption capacity based on Langmuir isotherm model of TiO2/AC with those of other adsorbents reported.
AdsorbentsAdsorption capacity for MB (mmol g-1)Adsorption capacity for MO (mmol g-1)Adsorption capacity for MR (mmol g-1)ReferencesWalnut shell-derived activated carbon0.251[37]Kaolinite0.062[38]Wasted biomass-derived activated carbon0.048[34]Pistachio shell-derived activated carbon0.430[39]TiO2/rice husk-derived activated carbon∗0.452/0.340The present workMultiwalled carbon nanotubes0.157[40]Diaminoethane sporopollenin biopolymer0.014[41]Hypercrosslinked polymeric adsorbent0.217[42]Calcined-layered double hydroxides0.611[43]TiO2/rice husk-derived activated carbon∗0.329/0.321The present workApple fruit shell-derived activated carbon1.615[44]Ag@Fe nanoparticles0.646[45]MIL-53(Fe)0.680[46]TiO2/rice husk-derived activated carbon∗0.806/2.04The present work∗adsorption capacity in single system/trinary system.


#### 3.2.5. Recyclability of Adsorbent

The regeneration and reuse of the adsorbent are one of critical criteria for its application in practice. Conventional recycle process is progressed by resining the used adsorbents with organic solvents (ethanol, benzene, and methanol) [35] and heating in limited oxygen environment [36] to elute dyes, resulting in the secondary pollutants and extra cost. Since TiO2/AC could act as a photocatalytic material, the used TiO2/AC could be regenerated by self-cleaning when exposed to visible light. Figure 6(a) shows the change in the adsorption capacity of TiO2/AC after three regenerations by photocatalytic self-cleaning. It can be seen that after the first-cycle regeneration, TiO2/AC still possessed a comparable adsorption capacity, suggesting that the photocatalytic self-cleaning was a feasible and effective technique to remove dyes from the used TiO2/AC. After three reused cycles, TiO2/AC could still exhibit 96% of its initial adsorption capacity. Figure 6(b) presents the XRD patterns of the reused TiO2/AC. It was found that there is a slight decrease in intensity but the characteristic peaks of anatase phase still remain unchanged after the recycle process indicating that TiO2/AC is stable for three recycles.Figure 6
(a) Variation of adsorption capacity and (b) XRD patterns of TiO2/AC and reused TiO2/AC.
(a)(b)


## 3.1. Characterization of Materials

Crystal structures of the obtained materials were studied by XRD patterns (Figure1(a)). XRD patterns of AC exhibit the peaks at 2 theta angles of 29.45, 36.49, 39.49, 43.03, and 47.56o corresponding to (203), (213), (205), (304), and (305) Miller indexes of carbon (JCPDS-00-022-1069). XRD patterns of TiO2 and composite TiO2/AC show the characteristic peaks of anatase at 25.25, 38.15, 47.85, 54.65, and 62.75o corresponding to the diffraction plane of (101), (004), (200), (105), and (204) (JCPDS -21-1272). With increasing amount of TiO2, the intensity of diffraction peaks of anatase increases indicating that two phases of activated carbon and anatase are coexisted. The composition of titanium and carbon elements estimated by EDX is presented in Figure S1. It is found that TiO2/carbon atomic ratio in product increases with the increase of TiO2 in initial mixture, indicating that the composition of TiO2/AC could be controlled by adjusting the reaction composition. Figure 1(b) and Table 1 presents the nitrogen adsorption and desorption isotherms of AC, TiO2, and TiO2/AC composites. All the isotherms exhibit a hysteresis loop which is characteristic for type IV isotherm, confirming the existence of mesoporous materials. The hysteresis loop occurs at relatively high pressures illustrating that the mesoporous structure formed between the particles. The surface area calculated based on the BET model is 1713 m2.g-1 for AC, 124 m2.g-1 for TiO2, and 436 m2.g-1 for TiO2/AC. The surface areas of TiO2 and AC are relatively large compared with previously reported studies [26, 27] showing that the structure of TiO2 in this study is highly porous. Furthermore, the combination of TiO2 on AC matrix significantly increased the surface area of TiO2 itself.Figure 1
(a) XRD patterns, (b) nitrogen adsorption/desorption isotherms, (c) UV-Vis diffuse reflectance spectroscopy, and (d) Tauc’s plots of AC, TiO2/AC (0.25/1), TiO2/AC (0.5/1), TiO2/AC (1/1), TiO2/AC (1.5/1), and TiO2.
(a)(b)(c)(d)Table 1
Textural properties of AC, TiO2, and TiO2/AC composites.
MaterialsSurface area based on BET model (m2 g-1)Porous volume (cm3/g)Mesoarea (m2 g-1)Microarea (m2 g-1)AC17136103331380TiO21242289431TiO2/AC (0.25/1)238100108131TiO2/AC (0.5/1)494313206288TiO2/AC (1/1)436222193243TiO2/AC (1.5/1)320241182138Figures1(c) and 1(d) show the UV-Vis-DRS and their Tauc’s plot for calculating band gap energy derived from UV-Vis-DRS. No energy absorption edge of AC which can be considered as a conductor was observed. The band gap energy of TiO2 is 3.2 eV which is equivalent to previous report in the range of 3.1-3.2 eV [28]. The combination of TiO2 with AC gives rise to a red shift in the energy adsorption spectrum, corresponding a band gap energy of 1.9 -2.5 eV illustrating that these composites might have photocatalytic activity in the visible region.The morphology of TiO2 prepared from peroxo-hydroxo titanium (IV) complexes consists of fine particles of around 10 nM which can be seen to agglomerate to form larger particles, while AC exhibits the typical layered-structure. In the composite structure, TiO2 agglomerates are highly dispersed on the AC matrix. The higher TiO2 content leads to a better coverage of TiO2 on AC matrix. The distribution of TiO2 on AC was also studied by EDX mapping. Figure 2 illustrates the elementary mapping of TiO2/AC (0.5/1) sample. The extensive dispersion of Ti, C, and O elements confirms that TiO2 was well-distributed on the surface of AC. EDX spectrum also shows some impurity of Mg, Si, Na, and Ca element (of around 0.2-1% w) which might present in rice husks.Figure 2
(a) SEM observation of TiO2/AC composites and (b) EDX mapping of TiO2/AC (0.5/1).
(a)(b)


## 3.2. Adsorption Studies

### 3.2.1. Simultaneous Determination of MB, MO, and MR by UV-Vis Spectroscopy

The dye concentration in solution was determine by UV-Vis spectroscopy at maximum wavelength of 664 nm for MB, 464 nm for MO, and 500 nm for MR. However, the spectra of three dyes are overlapped resulting in larger error for simultaneous determination of these dyes (Figure3(a)). In the work, the peaks were deconvoluted by means of OriginPro19/Deconvolution Software as illustrated in Figure 3(b). The deconvoluted UV-Vis spectrum was used to estimate the linear calibration range for dye determination. The linear range of each dye was found to be 0.0063-0.094 mM for MB, 0.015-0.138 mM for MO, and 0.019-0.167 mM for MR. The limit of detections (LODs) for MB, MO, and MR are 0.0007 mM, 0.002 mM, and 0.009 mM, respectively, which are suitable for the detection of these dyes in the adsorption study. Standard addition method is used to determine the dye in the trinary mixture. The recovery was found to be in the range of 0.9-0.99 indicating that the proposed method could be used for simultaneous determination of MB, MO, and MR (Table S1).Figure 3
(a) The UV-Vis spectra of MB, MO, and MR individual (0.0625 mM MB, 0.061 mM MO, and 0.111 mM MR) and (b) UV-Vis spectrum of trinary mixture of MB, MO, and MR as well as corresponding UV-Vis deconvulated spectra (Trinary component mixture of 0.0625 MB, 0.061 MO, and 0.111 MR, mass of adsorbent).
(a)(b)


### 3.2.2. Adsorption Studies of Single Component System of MB, MO, and MR over TiO2, AC, and TiO2/AC

The adsorption kinetics of TiO2, AC, and TiO2/AC composites at different concentrations of dyes (0.0306–0.0743 mM) are performed in Figure S2 and Table S2. All of samples exhibit the high adsorption towards MB, MO, and MR expect only TiO2 which can adsorb MB. As can be seen from the figure, the higher the dye concentration, the greater the adsorption capacity. This can explain by the fact that the high dye concentration enhances the driving force that overcomes all the mass transfer resistance of dye between the aqueous solution and the surface of absorbent. The adsorption of dyes is fast in the earlier stage (0–100 minutes) and gradually reached the equilibrium. It is found that the time required to obtain the adsorption/desorption equilibrium was around 120 minutes for all of cases. The kinetics data were applied to the pseudo-first-order and pseudo-second-order kinetic models. The kinetic parameters of the models and R2 are presented in Table S2. The regression equation based one pseudo-first-order model provides the high values of determination coefficient (R2=0.88–0.98) and the equilibrium adsorption capacity calculated by this model (qe,cal) agrees with equilibrium adsorption capacity obtained by the experiment (qe,exp). Hence, the experimental results fit the pseudo-first-order model.In order to select the TiO2/AC composite for further study, the removal of MB using TiO2/AC as a dye model was conducted (Figure S3). The AC exhibits the highest adsorption capacity for MB; whereas, TiO2 presents a very poor adsorption capacity. The adsorption capacity and the regeneration of spent adsorbent by the photocatalytic self-cleaning are significantly dependent on the composition of TiO2 in AC. The adsorption capacity was found to be 0.865 mmol g-1 for AC, 0.211 mmol g-1 for TiO2/AC (0.25/1), 0.452 mmol g-1 for TiO2/AC (0.5/1), 0.301 mmol g-1 for TiO2/AC (1/1), 0.251 mmol g-1 for TiO2/AC (1.5/1), and only 0.2 mmol g-1 for TiO2. However, after the first recycle, the adsorption capacity of TiO2/AC(0.5/1) was the highest. Based on the balance between adsorption capacity and visible-light derived photocatalytic self-cleaning, TiO2/AC (0.5/1) catalyst was selected for further experiments. Figure S4 and Figure S5 present the relationship of equilibrium concentration and adsorption capacity of AC, TiO2/AC, and TiO2. Either Langmuir, Freundlich, or Tempkin models describe well the equilibrium data with high determination coefficients (R2=0.823–0.999) (Table 2) indicating the monolayer adsorption with different energy. The differences between the adsorption capacities of the three dyes may results from different adsorption mechanisms, which might depend on the molecular size and their electron affinities toward adsorbent surface.Table 2
The parameters of adsorption isotherms of Langmuir, Freundlich, and Temkin models for TiO2/AC (0.5/1), TiO2, and AC in single component system of MB, MO, and MR.
AdsorbentDyesLangmuir isotherm modelFreundlich isotherm modelTemkin isotherm modelqm (mmol g-1)aL (L g-1)R2bFKF (L g-1)R2KT (L g-1)bT (KJ.mol-1)R2ACMB0.8650.1390.9891.73749.8540.993589.0313.390.986TiO2/AC0.4520.0940.9971.74418.6830.998256.1923.510.997TiO20.2000.0840.9991.9329.0720.999205.2349.170.999ACMO0.5320.5390.9934.70687.2070.9914682.3028.000.992TiO2/AC0.3290.3900.9882.68637.5450.9951366.6235.120.991TiO200—00—00—ACMR1.2190.1520.9792.12964.5120.951359.848.820.982TiO2/AC0.8060.3050.9983.11675.2390.9080.805.480.823TiO200—00—00—


### 3.2.3. Adsorption Isotherms of Trinary Component System for MB, MO, and MR over TiO2/AC (0.5/1)

(1) Effect of pH on the Adsorption of TiO2/AC (0.5/1). The pH effect on the adsorption of trinary component mixture by TiO2/AC is presented in Figure 4(a). The MB uptake was found to increase from 0.188 to 0.889 mmol g-1 for an increase in pH from 2 to 5. At higher pH values (5–11), the MB uptake remains almost constant. The point of zero charge of TiO2/AC obtained by the pH drift method was found at pH=6.4 (Figure 4(b)). At higher pH (>6.4), the surface of TiO2/AC may become negatively charged, which promoted the adsorption of the positively charged MB cations (pKa of MB=3.8 [29]) through electrostatic attraction. This, however, cannot explain its constant adsorption by TiO2/AC at all the studied pH values. There might be another mode of adsorption, e.g., ion exchange which is similar to the adsorption of MB onto wheat shells [30] or malachite green onto treated sawdust [31].Figure 4
(a) pH effect on dye adsorption capacity; (b) The point of zero charge estimated by pH drift method.
(a)(b)In contrast, the uptake of MO and MR decreases with the rise of pH. When pH increases from 2 to around 6, the adsorption capacity decreases from 0.344 to 0.124 mmol g-1 for MO and from 0.582 mmol g-1 to 0.452 mmol g-1 for MR. The MO or MR uptake keep unchanged with further pH increasing. Higher adsorptions at lower pH values could be due to protonation properties of the TiO2/AC. At pH below the point of zero charge (pHs<6.4), the positive charges at the surface of TiO2/AC favoured the adsorption of anionic MO and MR (pKa of MO=3.8 [32] and pKa of MR=5.1 [33]). A similar trend of adsorption behavior has also been reported with some different adsorbents [34].In general, the mechanism of dye adsorption is related to the interaction between the adsorbent and the adsorbate in solution. This interaction can occur in many different ways such as hydrogen bonding, electrostatic interaction,π-π stacking interaction, ion-exchange, hydrophobic bonding, and pores and chemical bonding [16, 25, 37, 62, 63] as illustrated in the Scheme 2.Scheme 2
The diagram of interaction mechanisms of MB, MR, and MO with TiO2/AC.


### 3.2.4. Equilibrium Studies

The equilibrium data of trinary component system were fitted to the extended Langmuir model (EL), the Langmuir model using P-factor models (P-Langmuir model) and Langmuir, Freundlich, and Sips models using IAST (denoted L-IAST, F-IAST, and S-IAST, respectively) as shown in Figure5. As can be seen, except EL model, the other four models are able to adequately correlate the equilibrium data. Although, the determination coefficient (R2) is widely used to estimate the goodness of fit for the models with an equal number of experimental points and of the fitting parameters, Akaike information criterion (AIC) is preferred for comparing the models with different parameters. The smaller AICs are, the better the model fit the data. As can be seen from Table 3, it is little difficult to estimate how the AICs are small because there are three values of AICs for each model. Relatively, it can be seen from Table 3 that the AICs are possibly rearranged from low to high based on the sum of AICs as follows: -160.92 (-58.37, -61.23, and -40.92) for F-IAST; -155.67 (-55.34, -58.86, and -41.47) for L-IAST; -152.67 (-54.34, -57.86, and -40.47) for P-factor-Langmuir model; -145.99 (-50.99, -57.76, and -37.24) for S-IAST; and –39.1(-22.96, -11.83, and -4.31) for EL suggesting both F-IAST model were the best to fit experimental data of all three chosen dyes followed by L-IAST. The Langmuir maximum sorption capacities based on L-IAST model in trinary dyes system were found to be lower than those obtained from the single-dye system, due to the presence of competing dyes. The influence of competitive dyes on the sorption of specific dyes onto the TiO2/AC is analyzed by using P-factor. The ratios of qL,MR,0/qL,MB0andqL,MR,0/qL,MO0 are >1, indicating the predominant MR adsorption over MB and MO in both single and trinary systems. The P-values of QL,MR,single0/QL,MR,trinary0=0.806/2.04<1 prove that the sorption process of MR was promoted by the existence of other dyes, while that of qL,MB,single0/qL,MB,trinary0=0.450/0.340>1 presents that the MB adsorption was hindered by the existence of competing dyes. The ratio of qL,MO,single0/qL,MO,trinary0=0.329/0.321=~1 reveals that the MO adsorption is independent on the existence of other dyes. The presence of MB and MO seems to enhance significantly the adsorption capacity of TiO2/AC toward MR to ~2.5 times. MB contains a positively charged amine group while MR and MO contain negatively charged carboxyl and sulfite groups, respectively. It is possible that the MB moleculars adsorbed on TiO2/AC promote the MR adsorption via electrostatic interaction and π-π stacking interaction. The mechanism of completive adsorption of the components is not clear and need to study further.Figure 5
The plot of adsorption isotherms for different models.Table 3
The parameters of EL, P-factor, and IAST-sorption isotherm (Langmuir, Freundlich, and Sips),R2, and AICc values.
DyeExtended-Langmuir modelR2AICsqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0 (L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)MB2.27468.8230.2630.576-48.16MO0.6077.03127.950.900-37.04MR0.8065.9882.080.674-29.50P-factor-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.39426.6567.630.916-54.34MO0.32433.10101.90.996-57.86MR1.2816.905.400.918-40.47IAST-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.34023.2267.630.917-55.34MO0.32132.67101.860.998-58.86MR2.0410.995.400.919-41.47IAST-Freudlich modelDyeaF,MBaF,MOaF,MRbF,MBbF,MObF,MRR2AICsMB1.050.430.996-58.37MO0.8430.350.998-61.23MR6.400.890.987-40.92IAST-Sips modelDyeKS,MB0KS,MO0KS,MR0aS,MB0aS,MO0aS,MR0bS,MB0bS,MO0bS,MR0R2AICsMB1.270.440.4660.943-50.99MO1.110.600.400.999-57.76MR16.6517.50.950.945-37.24Table3 reveals that TiO2/AC can adsorb as much as approximately two third its own weight (total 763.77 mg MB+MO+MR/1000mg) at ambient temperature. The adsorption capacity of TiO2/AC was compared with the reported materials for the dye adsorption. The adsorption of MB, MO, or MR in the literature is mostly in single component system (Table 4). One can see that the reported adsorbents include carbon nanotubes, activated carbon, and biopolymers as well as one metal organic framework. However, most of these materials only exhibited lower adsorption capacities than the TiO2/AC adsorbent or even the MOF, MIL-53(Fe), just possessed an adsorption capacity of 0.668 mM.g-1. Meanwhile, the adsorption capacity in single system/trinary system of TiO2/AC is 0.452/0.340 for MB, 0.329/0.321 for MO, and 0.806/2.04 for MR. This indicates that the potential using TiO2/AC to remove dyes is indeed promising.Table 4
A comparison of maximum adsorption capacity based on Langmuir isotherm model of TiO2/AC with those of other adsorbents reported.
AdsorbentsAdsorption capacity for MB (mmol g-1)Adsorption capacity for MO (mmol g-1)Adsorption capacity for MR (mmol g-1)ReferencesWalnut shell-derived activated carbon0.251[37]Kaolinite0.062[38]Wasted biomass-derived activated carbon0.048[34]Pistachio shell-derived activated carbon0.430[39]TiO2/rice husk-derived activated carbon∗0.452/0.340The present workMultiwalled carbon nanotubes0.157[40]Diaminoethane sporopollenin biopolymer0.014[41]Hypercrosslinked polymeric adsorbent0.217[42]Calcined-layered double hydroxides0.611[43]TiO2/rice husk-derived activated carbon∗0.329/0.321The present workApple fruit shell-derived activated carbon1.615[44]Ag@Fe nanoparticles0.646[45]MIL-53(Fe)0.680[46]TiO2/rice husk-derived activated carbon∗0.806/2.04The present work∗adsorption capacity in single system/trinary system.


### 3.2.5. Recyclability of Adsorbent

The regeneration and reuse of the adsorbent are one of critical criteria for its application in practice. Conventional recycle process is progressed by resining the used adsorbents with organic solvents (ethanol, benzene, and methanol) [35] and heating in limited oxygen environment [36] to elute dyes, resulting in the secondary pollutants and extra cost. Since TiO2/AC could act as a photocatalytic material, the used TiO2/AC could be regenerated by self-cleaning when exposed to visible light. Figure 6(a) shows the change in the adsorption capacity of TiO2/AC after three regenerations by photocatalytic self-cleaning. It can be seen that after the first-cycle regeneration, TiO2/AC still possessed a comparable adsorption capacity, suggesting that the photocatalytic self-cleaning was a feasible and effective technique to remove dyes from the used TiO2/AC. After three reused cycles, TiO2/AC could still exhibit 96% of its initial adsorption capacity. Figure 6(b) presents the XRD patterns of the reused TiO2/AC. It was found that there is a slight decrease in intensity but the characteristic peaks of anatase phase still remain unchanged after the recycle process indicating that TiO2/AC is stable for three recycles.Figure 6
(a) Variation of adsorption capacity and (b) XRD patterns of TiO2/AC and reused TiO2/AC.
(a)(b)


## 3.2.1. Simultaneous Determination of MB, MO, and MR by UV-Vis Spectroscopy

The dye concentration in solution was determine by UV-Vis spectroscopy at maximum wavelength of 664 nm for MB, 464 nm for MO, and 500 nm for MR. However, the spectra of three dyes are overlapped resulting in larger error for simultaneous determination of these dyes (Figure3(a)). In the work, the peaks were deconvoluted by means of OriginPro19/Deconvolution Software as illustrated in Figure 3(b). The deconvoluted UV-Vis spectrum was used to estimate the linear calibration range for dye determination. The linear range of each dye was found to be 0.0063-0.094 mM for MB, 0.015-0.138 mM for MO, and 0.019-0.167 mM for MR. The limit of detections (LODs) for MB, MO, and MR are 0.0007 mM, 0.002 mM, and 0.009 mM, respectively, which are suitable for the detection of these dyes in the adsorption study. Standard addition method is used to determine the dye in the trinary mixture. The recovery was found to be in the range of 0.9-0.99 indicating that the proposed method could be used for simultaneous determination of MB, MO, and MR (Table S1).Figure 3
(a) The UV-Vis spectra of MB, MO, and MR individual (0.0625 mM MB, 0.061 mM MO, and 0.111 mM MR) and (b) UV-Vis spectrum of trinary mixture of MB, MO, and MR as well as corresponding UV-Vis deconvulated spectra (Trinary component mixture of 0.0625 MB, 0.061 MO, and 0.111 MR, mass of adsorbent).
(a)(b)


## 3.2.2. Adsorption Studies of Single Component System of MB, MO, and MR over TiO2, AC, and TiO2/AC

The adsorption kinetics of TiO2, AC, and TiO2/AC composites at different concentrations of dyes (0.0306–0.0743 mM) are performed in Figure S2 and Table S2. All of samples exhibit the high adsorption towards MB, MO, and MR expect only TiO2 which can adsorb MB. As can be seen from the figure, the higher the dye concentration, the greater the adsorption capacity. This can explain by the fact that the high dye concentration enhances the driving force that overcomes all the mass transfer resistance of dye between the aqueous solution and the surface of absorbent. The adsorption of dyes is fast in the earlier stage (0–100 minutes) and gradually reached the equilibrium. It is found that the time required to obtain the adsorption/desorption equilibrium was around 120 minutes for all of cases. The kinetics data were applied to the pseudo-first-order and pseudo-second-order kinetic models. The kinetic parameters of the models and R2 are presented in Table S2. The regression equation based one pseudo-first-order model provides the high values of determination coefficient (R2=0.88–0.98) and the equilibrium adsorption capacity calculated by this model (qe,cal) agrees with equilibrium adsorption capacity obtained by the experiment (qe,exp). Hence, the experimental results fit the pseudo-first-order model.In order to select the TiO2/AC composite for further study, the removal of MB using TiO2/AC as a dye model was conducted (Figure S3). The AC exhibits the highest adsorption capacity for MB; whereas, TiO2 presents a very poor adsorption capacity. The adsorption capacity and the regeneration of spent adsorbent by the photocatalytic self-cleaning are significantly dependent on the composition of TiO2 in AC. The adsorption capacity was found to be 0.865 mmol g-1 for AC, 0.211 mmol g-1 for TiO2/AC (0.25/1), 0.452 mmol g-1 for TiO2/AC (0.5/1), 0.301 mmol g-1 for TiO2/AC (1/1), 0.251 mmol g-1 for TiO2/AC (1.5/1), and only 0.2 mmol g-1 for TiO2. However, after the first recycle, the adsorption capacity of TiO2/AC(0.5/1) was the highest. Based on the balance between adsorption capacity and visible-light derived photocatalytic self-cleaning, TiO2/AC (0.5/1) catalyst was selected for further experiments. Figure S4 and Figure S5 present the relationship of equilibrium concentration and adsorption capacity of AC, TiO2/AC, and TiO2. Either Langmuir, Freundlich, or Tempkin models describe well the equilibrium data with high determination coefficients (R2=0.823–0.999) (Table 2) indicating the monolayer adsorption with different energy. The differences between the adsorption capacities of the three dyes may results from different adsorption mechanisms, which might depend on the molecular size and their electron affinities toward adsorbent surface.Table 2
The parameters of adsorption isotherms of Langmuir, Freundlich, and Temkin models for TiO2/AC (0.5/1), TiO2, and AC in single component system of MB, MO, and MR.
AdsorbentDyesLangmuir isotherm modelFreundlich isotherm modelTemkin isotherm modelqm (mmol g-1)aL (L g-1)R2bFKF (L g-1)R2KT (L g-1)bT (KJ.mol-1)R2ACMB0.8650.1390.9891.73749.8540.993589.0313.390.986TiO2/AC0.4520.0940.9971.74418.6830.998256.1923.510.997TiO20.2000.0840.9991.9329.0720.999205.2349.170.999ACMO0.5320.5390.9934.70687.2070.9914682.3028.000.992TiO2/AC0.3290.3900.9882.68637.5450.9951366.6235.120.991TiO200—00—00—ACMR1.2190.1520.9792.12964.5120.951359.848.820.982TiO2/AC0.8060.3050.9983.11675.2390.9080.805.480.823TiO200—00—00—


## 3.2.3. Adsorption Isotherms of Trinary Component System for MB, MO, and MR over TiO2/AC (0.5/1)

(1) Effect of pH on the Adsorption of TiO2/AC (0.5/1). The pH effect on the adsorption of trinary component mixture by TiO2/AC is presented in Figure 4(a). The MB uptake was found to increase from 0.188 to 0.889 mmol g-1 for an increase in pH from 2 to 5. At higher pH values (5–11), the MB uptake remains almost constant. The point of zero charge of TiO2/AC obtained by the pH drift method was found at pH=6.4 (Figure 4(b)). At higher pH (>6.4), the surface of TiO2/AC may become negatively charged, which promoted the adsorption of the positively charged MB cations (pKa of MB=3.8 [29]) through electrostatic attraction. This, however, cannot explain its constant adsorption by TiO2/AC at all the studied pH values. There might be another mode of adsorption, e.g., ion exchange which is similar to the adsorption of MB onto wheat shells [30] or malachite green onto treated sawdust [31].Figure 4
(a) pH effect on dye adsorption capacity; (b) The point of zero charge estimated by pH drift method.
(a)(b)In contrast, the uptake of MO and MR decreases with the rise of pH. When pH increases from 2 to around 6, the adsorption capacity decreases from 0.344 to 0.124 mmol g-1 for MO and from 0.582 mmol g-1 to 0.452 mmol g-1 for MR. The MO or MR uptake keep unchanged with further pH increasing. Higher adsorptions at lower pH values could be due to protonation properties of the TiO2/AC. At pH below the point of zero charge (pHs<6.4), the positive charges at the surface of TiO2/AC favoured the adsorption of anionic MO and MR (pKa of MO=3.8 [32] and pKa of MR=5.1 [33]). A similar trend of adsorption behavior has also been reported with some different adsorbents [34].In general, the mechanism of dye adsorption is related to the interaction between the adsorbent and the adsorbate in solution. This interaction can occur in many different ways such as hydrogen bonding, electrostatic interaction,π-π stacking interaction, ion-exchange, hydrophobic bonding, and pores and chemical bonding [16, 25, 37, 62, 63] as illustrated in the Scheme 2.Scheme 2
The diagram of interaction mechanisms of MB, MR, and MO with TiO2/AC.


## 3.2.4. Equilibrium Studies

The equilibrium data of trinary component system were fitted to the extended Langmuir model (EL), the Langmuir model using P-factor models (P-Langmuir model) and Langmuir, Freundlich, and Sips models using IAST (denoted L-IAST, F-IAST, and S-IAST, respectively) as shown in Figure5. As can be seen, except EL model, the other four models are able to adequately correlate the equilibrium data. Although, the determination coefficient (R2) is widely used to estimate the goodness of fit for the models with an equal number of experimental points and of the fitting parameters, Akaike information criterion (AIC) is preferred for comparing the models with different parameters. The smaller AICs are, the better the model fit the data. As can be seen from Table 3, it is little difficult to estimate how the AICs are small because there are three values of AICs for each model. Relatively, it can be seen from Table 3 that the AICs are possibly rearranged from low to high based on the sum of AICs as follows: -160.92 (-58.37, -61.23, and -40.92) for F-IAST; -155.67 (-55.34, -58.86, and -41.47) for L-IAST; -152.67 (-54.34, -57.86, and -40.47) for P-factor-Langmuir model; -145.99 (-50.99, -57.76, and -37.24) for S-IAST; and –39.1(-22.96, -11.83, and -4.31) for EL suggesting both F-IAST model were the best to fit experimental data of all three chosen dyes followed by L-IAST. The Langmuir maximum sorption capacities based on L-IAST model in trinary dyes system were found to be lower than those obtained from the single-dye system, due to the presence of competing dyes. The influence of competitive dyes on the sorption of specific dyes onto the TiO2/AC is analyzed by using P-factor. The ratios of qL,MR,0/qL,MB0andqL,MR,0/qL,MO0 are >1, indicating the predominant MR adsorption over MB and MO in both single and trinary systems. The P-values of QL,MR,single0/QL,MR,trinary0=0.806/2.04<1 prove that the sorption process of MR was promoted by the existence of other dyes, while that of qL,MB,single0/qL,MB,trinary0=0.450/0.340>1 presents that the MB adsorption was hindered by the existence of competing dyes. The ratio of qL,MO,single0/qL,MO,trinary0=0.329/0.321=~1 reveals that the MO adsorption is independent on the existence of other dyes. The presence of MB and MO seems to enhance significantly the adsorption capacity of TiO2/AC toward MR to ~2.5 times. MB contains a positively charged amine group while MR and MO contain negatively charged carboxyl and sulfite groups, respectively. It is possible that the MB moleculars adsorbed on TiO2/AC promote the MR adsorption via electrostatic interaction and π-π stacking interaction. The mechanism of completive adsorption of the components is not clear and need to study further.Figure 5
The plot of adsorption isotherms for different models.Table 3
The parameters of EL, P-factor, and IAST-sorption isotherm (Langmuir, Freundlich, and Sips),R2, and AICc values.
DyeExtended-Langmuir modelR2AICsqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0 (L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)MB2.27468.8230.2630.576-48.16MO0.6077.03127.950.900-37.04MR0.8065.9882.080.674-29.50P-factor-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.39426.6567.630.916-54.34MO0.32433.10101.90.996-57.86MR1.2816.905.400.918-40.47IAST-Langmuir modelDyeqL,MB0 (mmol g-1)qL,MO0 (mmol g-1)qL,MR0 (mmol g-1)KL,MB0 (L.g-1)KL,MO0 (L.g-1)KL,MR0(L.g-1)aL,MB0 (L.mM-1)aL,MO0 (L.mM-1)aL,MR0 (L.mM-1)R2AICsMB0.34023.2267.630.917-55.34MO0.32132.67101.860.998-58.86MR2.0410.995.400.919-41.47IAST-Freudlich modelDyeaF,MBaF,MOaF,MRbF,MBbF,MObF,MRR2AICsMB1.050.430.996-58.37MO0.8430.350.998-61.23MR6.400.890.987-40.92IAST-Sips modelDyeKS,MB0KS,MO0KS,MR0aS,MB0aS,MO0aS,MR0bS,MB0bS,MO0bS,MR0R2AICsMB1.270.440.4660.943-50.99MO1.110.600.400.999-57.76MR16.6517.50.950.945-37.24Table3 reveals that TiO2/AC can adsorb as much as approximately two third its own weight (total 763.77 mg MB+MO+MR/1000mg) at ambient temperature. The adsorption capacity of TiO2/AC was compared with the reported materials for the dye adsorption. The adsorption of MB, MO, or MR in the literature is mostly in single component system (Table 4). One can see that the reported adsorbents include carbon nanotubes, activated carbon, and biopolymers as well as one metal organic framework. However, most of these materials only exhibited lower adsorption capacities than the TiO2/AC adsorbent or even the MOF, MIL-53(Fe), just possessed an adsorption capacity of 0.668 mM.g-1. Meanwhile, the adsorption capacity in single system/trinary system of TiO2/AC is 0.452/0.340 for MB, 0.329/0.321 for MO, and 0.806/2.04 for MR. This indicates that the potential using TiO2/AC to remove dyes is indeed promising.Table 4
A comparison of maximum adsorption capacity based on Langmuir isotherm model of TiO2/AC with those of other adsorbents reported.
AdsorbentsAdsorption capacity for MB (mmol g-1)Adsorption capacity for MO (mmol g-1)Adsorption capacity for MR (mmol g-1)ReferencesWalnut shell-derived activated carbon0.251[37]Kaolinite0.062[38]Wasted biomass-derived activated carbon0.048[34]Pistachio shell-derived activated carbon0.430[39]TiO2/rice husk-derived activated carbon∗0.452/0.340The present workMultiwalled carbon nanotubes0.157[40]Diaminoethane sporopollenin biopolymer0.014[41]Hypercrosslinked polymeric adsorbent0.217[42]Calcined-layered double hydroxides0.611[43]TiO2/rice husk-derived activated carbon∗0.329/0.321The present workApple fruit shell-derived activated carbon1.615[44]Ag@Fe nanoparticles0.646[45]MIL-53(Fe)0.680[46]TiO2/rice husk-derived activated carbon∗0.806/2.04The present work∗adsorption capacity in single system/trinary system.


## 3.2.5. Recyclability of Adsorbent

The regeneration and reuse of the adsorbent are one of critical criteria for its application in practice. Conventional recycle process is progressed by resining the used adsorbents with organic solvents (ethanol, benzene, and methanol) [35] and heating in limited oxygen environment [36] to elute dyes, resulting in the secondary pollutants and extra cost. Since TiO2/AC could act as a photocatalytic material, the used TiO2/AC could be regenerated by self-cleaning when exposed to visible light. Figure 6(a) shows the change in the adsorption capacity of TiO2/AC after three regenerations by photocatalytic self-cleaning. It can be seen that after the first-cycle regeneration, TiO2/AC still possessed a comparable adsorption capacity, suggesting that the photocatalytic self-cleaning was a feasible and effective technique to remove dyes from the used TiO2/AC. After three reused cycles, TiO2/AC could still exhibit 96% of its initial adsorption capacity. Figure 6(b) presents the XRD patterns of the reused TiO2/AC. It was found that there is a slight decrease in intensity but the characteristic peaks of anatase phase still remain unchanged after the recycle process indicating that TiO2/AC is stable for three recycles.Figure 6
(a) Variation of adsorption capacity and (b) XRD patterns of TiO2/AC and reused TiO2/AC.
(a)(b)


## 4. Conclusions

TiO2/AC materials were synthesized from water soluble titanium hydroperoxide complex and activated carbon from rice husks. The obtained material has high surface area and excellent adsorption capacity to methylene blue, methyl orange, and methyl red compared to the other materials reported. The Freundlich model and Langmuir model in the ideal adsorbed solution theory provide the best prediction to the trinary mixture data in comparison to the extended Langmuir model; Langmuir model using P-factor and Sips model in ideal-adsorbed solution theory. The TiO2/AC exhibits the excellent reusability by visible light-driven photocatalytic self-cleaning, and their adsorption efficiency was maintained even after three cycles; the change in structure and adsorption capacity of TiO2/AC was insignificant, proving that this material is durable and has potential for practical applications.



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