Preparation of Activated Carbon from the Wood of Paulownia tomentosa as an Efﬁcient Adsorbent for the Removal of Acid Red 4 and Methylene Blue Present in Wastewater

: Paulownia tomentosa, a woody plant that is widely found in Pakistan and in other regions of the world, was used as a raw material to prepare activated carbon using chemical and physical activation methods. Adsorption of the dyes- acid red 4 and methylene blue onto the prepared activated carbon were analyzed by batch experiments. The impacts of different adsorption parameters such as pH, temperature, contact time, initial dye concentration and adsorbent dosage were also evaluated. Equilibrium data were ﬁtted into various isotherm models such as: Langmuir, Temkin and Freundlich. High regression values were achieved with Langmuir isotherm model. Different kinetic adsorption models such as pseudo-ﬁrst-order, pseudo-second-order and intra-particle diffusion model models were applied. The adsorption kinetics was found to be best-ﬁtted into pseudo-second-order kinetic model. The optimum pH for acid red 4 was around 1 while for methylene blue it was 8. The optimum adsorbent dosage was 0.3 g for both dyes used. The activation energy (E a ) values were 30.57 and 3.712 kJ/mol, respectively for acid red 4 and methylene blue while the enthalpy ( ∆ H) and entropy ( ∆ S) values were correspondingly as 24.88/1.1927 kJ/mol and − 2843.32/ − 0.329 J · mol/K for the mentioned dyes. The experimental result showed that the prepared activated carbon was the best in the removal of acid red 4 and methylene blue from aqueous media and therefore, could be preferably used as cheap adsorbent in wastewater treatment.


Introduction
The requirement of clean water for drinking and other purposes is a direct need in this modern world as the available sources of drinking water are often contaminated with organic and inorganic pollutants throughout the world [1]. Rapid industrialization is the main cause of water pollution as water is used as solvent in different industrial processes. Textile industry is considered to be one of main contributor of water pollution as water is used there as solvent in fiber processing as well as to remove unnecessary chemicals from fabrics [2]. It is predicted that about 40-65 L of textile wastewater is produced per kg of cloth manufactured. These wastewaters are loaded with a number of synthetic chemicals, which are mostly incompatible for natural degradation processes leading to severe hazardous effects on environment and human health [3,4]. Color in water bodies is an unwanted product that leads to toxicity of water bodies apart from interferences in photosynthesis process [5,6]. Today, more than 3000 dyes are being used in the textile industries [7,8]. Synthetic colors are resolute mixtures having general applications in various areas such as in: paper, plastic and textile industries, etc. [9]. Acid red 4 is an acidic dye, which is used in dyeing nylon, wool, silk, paper, leather, and in the production of inks [10]. Acidic dyes are highly carcinogenic and can cause dysfunctions in many human body organs like kidneys, brain, liver, reproductive system and nervous system [11]; hence, the removal of these dyes from wastewater and water bodies is essential due to their adverse effects on environment [12].
Different wastewater treatment processes, like coagulation, flocculation, biodegradation, adsorption, and oxidation, are used to remove color residues from water [13]. None of them is 100% efficient, however, among them adsorption is a more effective process [14]. The significant benefit of adsorption for the control of water contamination is its low starting cost, straightforwardness and simple recovery of the adsorbent and adsorbate materials [15]. A number of adsorbents are used, however, among them activated carbon (AC) is preferably used due to its high surface area and because the fact that it can be prepared from low-cost raw materials [16]. Plant waste materials may offer an inexpensive source for their use in the AC production. These materials after carbonization and activation, can attain a high surface area ranging from 300-1200 m 2 /g [17]. Different plants waste biomass has been converted into activated carbon and have been used to remove different organic and inorganic pollutants from wastewaters.
Paulownia tomentosa is a woody plant found in Pakistan, China, India and other regions of the world that can grow over 30 m tall and has large heart-shaped leaves ranging from 10-20 cm in width and 15-30 cm in length with a 10-20 cm long petiole. The leaves are often preceded by pale violet to purple shaded tubular flowers similar to a foxglove. It is an ornamental plant grown in parks and gardens. The plant produces large quantities of biomass in terms of weathered twigs and leaves that needs to be utilized in a meaningful way [18][19][20].
In the connection with previous findings, the present study was designed to prepare activated carbon from Paulownia tomentosa using physiochemical activation methods. The prepared activated carbon was used as an adsorbent for the removal of the textile dyes acid red 4 (AR4) and methylene blue (MB) from water.

Experimental
MB and AR4 dyes were used as the adsorbates that were subjected to adsorption on activated carbon prepared from the wood of Paulownia tomentosa. Both dyes AR4 and MB were used without any further purification (purchased from Sigma Aldrich, Munich, Germany). Other chemicals used, such as HCl, HNO 3 , and NaOH were also purchased from Sigma Aldrich. The chemical structures of the selected dyes and the compounds' names, chemical formulae and solubility features are given in Figure 1a,b and Table 1 below.

Instrumentation
The prepared adsorbent was characterized by a surface area analyzer (NOVA2200e, Quantachrome, Boynton Beach, FL, USA), scanning electron microscopy (SEM, JSM5910, JEOL, Tokyo, Japan) and EDX (INCA200/Oxford Instruments, Buckinghamshire, UK) at the University of Peshawar, Pakistan. The dye concentration was determined by measuring absorbance using a double beam UV-vis spectrophotometer (UV-1800, Shimadzu Scientific Instruments Inc, Tokyo, Japan) at a wavelength of 508 and 665 nm for AR4 and MB, respectively. Adsorption experiments were conducted in a thermostatic water-bath shaker. The pH measurements were carried out using pH meter.

Surface Area Determination
To calculate surface area, Brunauer-Emmett-Teller (BET)-N 2 adsorption analysis were performed manometrically using the Quantachrome NOVA 2200e system at −196 • C and the relative pressure (P/P o ) range: 0.05-0.85. BET equation was used to obtain the surface area. The Dubinin-Radushkevich (DR) method and the NovaWin2 data analysis software was used to obtained pore size distribution.

Preparation of Activated Carbon
Paulownia tomentosa wood was used as raw material for the preparation of activated carbon. It was collected from Peshawar, Pakistan. A schematic presentation of the experimental setup is given in Figure 2.
The wood was liberated from the outer shell and afterwards homogenized into small pieces. The pieces were then washed and afterwards held in a laboratory fume hood for 24 h for the removal of water. After drying, wood pieces carbonized in a furnace under a nitrogen atmosphere. The carbonized pieces were then crushed through a sifter (150-180 nm).
The following parameters were determined: where, q t is the amount of dye adsorbed (mg/g), %R is dye removal efficiency, C 0 is the initial dye concentration (mg/L), c t is dye concentration after adsorption, V is the volume of dye solution (mL), while W is the weight of adsorbent (g). The calibration curves of the experimental measurements performed on the dyes are given Figure 3.   Equilibrium adsorption experiments were conducted on a number of solutions ranging from 50-1000 mg/L by applying 0.3 g of prepared adsorbent. The pH of these samples was adjusted to 2 and 8 for AR4 and MB, respectively and after addition, the mixtures were stirred for one hour. The adsorbent was then separated from the mixture through filtration and remaining concentrations were determined using UV/VIS spectrophotometer. Kinetic studies were performed on 100 mg/L solutions, while remaining parameters were similar as mentioned above. Similarly, effects of adsorbent dosage, pH (2-12) and temperature were determined for 100 mg/L solutions keeping other parameters same as mentioned before.

Energy Dispersed X-ray (EDX) Measurements of AC
The EDX is an analytical technique used for the analysis of elemental composition and chemical characterization of samples. The EDX spectra of activated carbon showed that on the surface of all the AC samples, carbon and oxygen were mostly present. Carbon in the samples had a greater ratio than that of oxygen. The EDX spectra also showed the presence of other elements, such as calcium, magnesium, potassium and chlorine. The EDX spectra of AC is shown in Figure 5.

Surface Area Determination
Different techniques, such as BET, BJH, Langmuir and DR are used to calculate surface area of different samples. However, among them, the BET surface area (m 2 /g) analysis method is the most common and extensively used method. The BET equation is expressed as follows: where: W = Weight of gas adsorbed, P/P 0 = Relative pressure, W m = weight of adsorbate constituting a monolayer coverage, C = related to the energy of adsorption in the first adsorbed layer. The values of calculated parameters are summarized in Table 2.

Dosage Effect on Removal of Acid Red 4 and Methylene Blue
Dosage studies are important, because they determine the removal efficiency of a given adsorbent for a given pollutant and may also be used to predict the cost of the adsorbent per unit of solution to be treated. Hence, this effect was enumerated and it was found that the dye removal increased significantly as the adsorbent dosage was increased. The effect of dose of adsorbent on the percentage removal of AR4 and MB dyes is shown in Figure 6a,b. For both dyes the optimum dosage was found to be 0.3 g, which was used in all the subsequent experiments.

Effect of pH on Removal of Acid Red 4 and Methylene Blue
The pH is an important parameter that affects the interactions between the adsorbent and adsorbate and the effect of pH on the adsorbate uptake may be highly, moderately or slightly significant depending on chemical structures of both dyes as it affects the degree of ionization. Figure 7a,b show the effect of pH on adsorption of the selected dyes. The observed behaviours of the selected dyes towards the prepared adsorbent differ from each other, which is not surprising as one dye is negatively charged, while the other is positively charged and the tested pH ranges used were on acidic side for acid red 4, while methylene blue was in the basic form. For acid red the optimum pH for removal was around 1, while for methylene blue it was 8.

Adsorption Kinetics
It is necessary to consider the adsorption kinetics to understand the precise mechanism of the adsorption process. Apart from the determination of the adsorption mechanism, it is also helpful to understand the adsorbate-adsorbent interactions and adsorption characteristics. For this purpose, 0.3 g AC was added to series of 30 mL flasks containing 100 mg/L AR4 and MB solutions. These flasks were shaken for different intervals of time at temperatures of 293, 303 and 313K.
The equilibrium times of AR4 and MB adsorption on the prepared AC are given in Figure 8a,b, respectively, at three different temperatures. The equilibrium was reached in 5 min. for both dyes. Initially, the adsorption sites available were high in numbers and as fast adsorption occurred, number of available adsorption sites decreased to a steady state. A number of kinetics models were used to explain the kinetics of adsorption process (Figures 8 and 9). The different models applied in this study are given as follows [21][22][23]: Pseudo-first order equation log q e − q t = log q e − k 1 2.303 Second-order Lagergren equation t q e = 1 k 2 q 2 e + 1 q e t (5) Intra-particle diffusion model The pseudo first and second order kinetics parameters are enumerated in Table 3,  while Table 4 shows the intraparticle diffusion model parameters at different temperatures.

Isotherm Studies
In order to find out the surface properties and affinity of the adsorbent for adsorbates, different isotherm models were applied. The isotherm study is important for the description of how the adsorbates interact with the adsorbent and gives information about the adsorption capacities. The surface of the adsorbent may be occupied by the adsorbate with the formation of monolayer or multilayer. To determine different isotherm parameters, the Langmuir, Freundlich and Temkin models were applied.

Langmuir Isotherm
In order to explain the sorption of selected dye onto the prepared adsorbent Langmuir model was used. Basic assumptions of this model are; the sorption process takes place at specific homogenous sites of the adsorbent and it only forms a monolayer on the surface. The linear form of this model can be given as follows [24][25][26]: where q e, C e, K L and Q m are the adsorbed amount of dye adsorbed at equilibrium (mg/g), concentration of dye solution at equilibrium (mole/L), Langmuir constant (mole/g) and the maximum adsorption capacity (mg/g), respectively. A plot of C e /q e, versus C e (from slope and intercept) enabled us to determine the value of K L and Q m (Figure 10a,b) and their values along with the correlation coefficient R 2 are given in Table 5. The Langmuir isotherm provided the best correlation coefficient R 2 value and hence, the best fit for the experimental data.

Freundlich Isotherm Model
This model describes non-ideal and reversible adsorption behavior that is not restricted to the formation of a monolayer. This model can be applied to the adsorption on heterogeneous surfaces (multilayer adsorption) and non-uniform distribution of adsorption heat and affinities of the sites over the adsorbent surface. This model can be expressed as follows: lnq e = lnKf + 1 n ln C e (8) where q e, C e, Kf, and 1/n are the amount of adsorbate adsorbed at equilibrium, equilibrium concentration of dye solution, adsorption capacity at unit concentration and adsorption intensity, respectively. It should be noted that when 1/n = 0, the type of isotherm will be irreversible and when 0 < 1/n < 1 adsorption will be favorable and when 1/n > 1, the adsorption process will be unfavorable.
A plot of ln q e versus C e enables us to determine the values of n and K f from the slope and intercept (Figure 10c,d). The values of these parameters are given in Table 5.

Temkin Isotherm Model
In the linear form this isotherm can be given as follows: q e = B 1 lnK T + B 1 lnC e (9) where: B 1 and K T are heat of adsorption and equilibrium binding constant (L/mg), respectively. A plot q e versus lnC e enable us find out the values of these constants (Figure 10e,f) and their values are given in Table 5. According to Table 5, the R 2 values are higher for Langmuir isotherm rather than Freundlich and Temkin models.

Thermodynamic Study
The thermodynamic parameters for the adsorption of AR4 and MB onto prepared AC were calculated using following equations: where, In these equations, ∆G • , ∆H • and ∆S • , are the changes in Gibbs free energy, enthalpy and entropy, respectively. A plot lnK d versus 1/T enabled us to determine the values of ∆H • and ∆S • , that are usually obtained from slope and intercept of the resultant plot. The values as given in Table 6.
The values of Gibbs free energy (∆G • ) were negative at all temperatures indicating the feasibility of the adsorption process and its spontaneous nature. The observed decrease in ∆G • values with increase in temperature suggests that the higher temperature would facilitate the adsorption process. The negative value of ∆H • indicates the exothermic nature of the process and the positive value of entropy shows the increase in randomness at the dye/adsorbent interfaces and feasibility of the process. AC as an adsorbent is claimed to have high adsorption potential as it has high adsorption capacity as compared to other adsorbents reported in literature [27][28][29][30][31][32]. Table 7 shows a comparison of present adsorbent with reported ones. Previous studies have emphasized the role of the effects of pH and parameters on the solubility and performance of nanomaterials, which can be used similarly in following studies [33][34][35][36][37].

Conclusions
An efficient adsorbent was prepared from Paulownia tomentosa, which was characterized by surface area analysis, SEM and EDX. The adsorption of acid red 4 and methylene blue was examined by equilibration time and kinetic studies and adsorption isotherm studies at different temperatures (293, 303, and 313 K) were evaluated using batch experiments. SEM data showed pores of various sizes and shapes on the prepared adsorbent surface where dye residues could favorably be attached. EDX spectrum of prepared AC showed the presence of carbon and oxygen atoms along with other elements. The equilibration time for acid red 4 and methylene blue dye were almost 5 min, indicating the quick adsorption of the selected dyes onto the surface of adsorbent. Langmuir and pseudo-second order models, respectively, fitted well the isotherm and kinetics data. The intraparticle diffusion model showed the adsorption of selected dyes was a diffusion-controlled process. Funding: This research was funded by project nr T190087MIMV and European Commission, MLTKT19481R "Identifying best available technologies for decentralized wastewater treatment and resource recovery for India, SLTKT20427" Sewage sludge treatment from heavy metals, emerging pollutants and recovery of metals by fungi and by project KIK 15392 and 15401 by European Commission.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study is available on request from the corresponding author.