Carbon Black: A Good Adsorbent for Triclosan Removal from Water

Abstract

Industrial grade carbon black was pretreated and used for adsorption of triclosan (TCS) in water, and its adsorption performance was investigated by static adsorption experiments and fixed bed adsorption experiments. The effects of pH and temperature on the adsorption capacity were investigated and the adsorption performance of carbon black and activated carbon on TCS was compared. As the pH or temperature decreased, the adsorption of TCS by carbon black increased. Carbon black had a maximum adsorption capacity of 18.62 mg/g on TCS, which was somewhat lower than the activated carbon. Furthermore, the breakthrough time increased with increasing bed height, decreasing flow rate and decreasing initial TCS concentration in the fixed bed adsorption studies. The breakthrough time was 14.33 days at a bed depth of 4.2 mm, a flow rate of 2 mL/min, and an initial TCS concentration of 0.5 mg/L TCS. The model from The Bed Depth Service Time (BDST) was well-fitting.

1. Introduction

Triclosan (TCS), a broad-spectrum bactericide, is commonly found in toothpaste, hand soap, refrigerator deodorant, and other daily necessities products [1]. It is difficult to degrade and, TCS is discharged into the environment via municipal sewage treatment systems. Sinceit is difficult to degrade, TCS has been found in a variety of water settings in recent years, including surface water, groundwater, seawater, and drinking water, thus posing a significant threat to the ecological environment and human health [2,3,4]. Physical methods, advanced oxidation technology, and biological treatment technology are the most common TCS treatment methods now being used for TCS removal [5,6,7]. These technologies, on the other hand, have drawbacks such as high energy consumption, complex operation, and poisonous byproducts. Adsorption methods for removing TCS from water have gained popularity in recent years due to their high efficiency, low energy consumption, and lack of harmful byproducts. At varied pH and ionic strength, Behera et al. evaluated the adsorption capability of activated carbon, kaolinite, and montmorillonite as adsorbents for TCS [8]. The results demonstrated that the adsorption approach removes TCS from water with reasonable effect. However, adsorbents such as activated carbon are expensive and difficult to recover, resulting in high treatment costs and resource loss [9].

Carbon black is a dark carbonaceous material created by incomplete combustion of biomass or fossil fuels, primarily in natural and manmade storage [7]. The combustion of fossil fuels creates approximately 12–24 tons of carbon black each year, while the combustion of biomass produces approximately 50–270 tons [10]. The majority role of carbon black is maintained in the soil and converted into charcoal, although very tiny carbon black particles can be transported over long distances by rivers and the atmosphere and finally deposited in the seas and surface waterways. Previous studies have found that carbon black can account for 20–50% and 5–20% of the organic carbon retained in surface water and marine sediments [11]. Carbon black particles with a hexagonal mesh structure are formed by stacking numerous microcrystals around a center, which is caused by the cyclization phenomenon that occurs when hydrocarbons are cracked at high temperatures, giving carbon black a large specific surface area and strong adsorption properties [12,13]. Carbon black is widely utilized in life production, and carbon black with low or high resistance qualities is widely used in high-tech and military industries [14], but its research in the field of water treatment has rarely been reported.

Carbon black has a high adsorption capacity and a huge natural storage capacity, as well as being simple to obtain and inexpensive. So far, there has been a lack of enough information regardingthe application of carbon black for the removal of TCS from water by using a continuous flow fixed column adsorption method. In this study, therefore, industrial grade carbon black was used to adsorb TCS in water after a simple pretreatment. The goals of this research were to: (i) adsorb TCS in water using carbon black, perform thermodynamic and kinetic fitting, and evaluate the study’s performance; (ii) investigate the effects of pH and temperature on carbon black adsorption of TCS; (iii) adsorb TCS in water using industrial grade activated carbon and compare the adsorption performance with carbon black; (iv) using fixed bed adsorption experiments, study the effect of bed height and liquid flow rate on dynamic adsorption.

2. Materials and Methods

2.1. Materials

Industrial grade carbon black and activated carbon were bought from Shijiazhuang Fiber New Material Factory (Shijiazhuang, China) and Fuchen Chemical Reagent Factory (Tianjin, China). Reagent grade TCS was purchased from Sigma-Aldrich Corporation. Chromatographically grade of N-hexane, methanol were purchased obtained from Aladdin (Shanghai, China). The rest of the materials and reagents used in this research were of analytical grade, obtained by Beijing Chemical Factory (Beijing, China). The solutions were prepared with ultrapure water.

2.2. Preparation and Characterization of Carbon Black

Initially, 100 g of carbon black was added into 1 L of ultrapure water under continuously heating at 100 °C for 1 hour. After that, carbon black was repeatedly collected by filtration and dried. Ethanol was then added, and the mixtures were vigorously shaken (500 rpm) for 1 h. Carbon black was collected, washed byultrapure water, and dried for further use in experiments.

Fourier transform-infrared (FTIR, Nicolet 6700) techniques was used to record the functional groups of the samples. Scanning electron microscopy (SEM, SSX-55) techniques was used to observe carbon black’s surface morphology. Nano ZS 90 zetasizer (Malvern, UK) was used to measure the zeta potential.

2.3. Static Adsorption Experiments

In the static adsorption experiments, the adsorption kinetics of TCS on carbon black was examined by stirring 100 mL of TCS solution at concentrations ranging from 0.5 to 2 mg/L with 20 mg carbon black at 500 rpm.

Batch pH experiments were carried out by stirring 100 mL of TCS solution at concentrations ranging from 0.5 to 5 mg/L with 20 mg carbon black at 500 rpm, at a pH range of 3 to 9. The pH values of the aqueous solution were adjusted by HCl and NaOH (1.0 M).

Batch experiments were performed at pH = 7. The initial TCS concentrations were controlled at 0.5–5 mg/L and 20mg of carbon black was added. The samples were stirred with a shaking rate of 500 rpm at temperatures changing from 15 to 45 °C.

The adsorption performance of TCS on carbon black and activated carbon were compared by stirring 100 mL of 2 mg/L of TCS solutions with 2 mg Carbon black at 500 rpm for 5 min at room temperature The pH was adjusted to 7.

The adsorption capacity (q_e) of carbon black for TCS was calculated using Equation (1) [15]:

q_e = (C₀ − C_e) × V/m

(1)

where, C₀ and C_e represents the initial and equilibrium TCS concentration (mg/L), respectively. m (g) is the weight of carbon black, and V (L) is the volume of TCS solution.

2.4. Fixed Bed Column Experiment

A SPE column (10 cm × 1.6 cm i.d.) packed with adsorbent was used for the fixed bed column experiment. Quartz was used at the top and bottom of the column ensurethe solution distribution was uniform and to prevent the outflow of carbon black. The TCS solution was passed through the fixed bed column downward at a specific flow rate by an automated solid phase extraction equipment. A seriesof effluent samples were collected at regular intervals and then analyzed.

The total adsorption amounts of TCS (q_total (mg)) and the maximum adsorption capacity (q_e (mg/g)) when reach the exhaustion point at C_t/C₀ = 0.7 were determined by Equations (2) and (3):

$$q_{total}=Q/1000\times {{\displaystyle \int }}_{t=0}^{{t=t}_e}(C_0-C_t)dt$$

(2)

$$q_e=\raisebox{1ex}{$q_{total}$}\!\left/ \!\raisebox{-1ex}{$m$}\right.$$

(3)

In order to study the effect of column bed depth on TCS removal, 20, 50 and 100 mg carbon black was picked in the fixed bed column with the 500 μg/L of TCS solution was passed through the fixed bed column from the bottom to top. The effect of flowrate on TCS removal was conducted by packing 20 mg carbon black in the fixed bed column with the 500 μg/L of TCS solution flow downward at a specific flow rate (1.5 mL/min, 2 mL/min and 2.5 mL/min).

In all of the experiments, the pH was adjusted to 7 by HCl and NaOH (1.0 M) and temperature was set at 25 °C.

2.5. Analysis for TCS

In this experiment, 8 steps are followed to analyze TCS. The detailed information is stated below [16]: 1 mL sample, 10 mL of 0.1 mol/L sodium tetraborate and 1 mL of acetic anhydride are put in a conical flask, vigorouslyshaking them for 3-seconds, adding 2 mL of n-hexane and moderate anhydrous sodium sulfate, vigorously shaking them for 1minute, toppling the solution from the conical flask to colorimetric tube, removing the lower solution, adding 10mL sodium tetraborate into the conical flask, then adequately shaking it and decanting it into the colorimetric tube. Eventually, we shookthe mixture up and down, after calming for 5 min, 1 mL of the sample is abstracted from upper layer and analyzed by a PerkinElmer Clarus 680 Gas Chromatography armed with a DB-5 capillary column (30 m × 0.25 mm × 0.25 μm) and an electron capture detector.

There should be a strict standard for the experiment: the GC temperature program set out at 150 °C for 1minute, climbing to 280 °C at a speed of 20 °C per minute, holding for 2minutes. The whole processing lasted ten minutes. The carrier and makeup gas were highly pure (>99.999%) nitrogen at 1.5 and 30.0 mL/min, respectively. The injector and detector temperatures were set at 250 °C and 280 °C, respectively. The limit of quantification is 0.2 μg/L.

3. Results

3.1. Structural and Morphology of Carbon Black

SEM was used to observe the surface morphology as well as the structural characteristics of the carbon black powder, and the results are presented in Figure 1. The carbon black is mostly present in the structure of aggregates and agglomerates, as shown in the figure, and the pretreatment carbon black has good dispersion.

Energy dispersive -ray spectroscopy (EDX) analysis was used to determine the elemental composition of carbon black, and the results indicated the carbon black is mainly composed of C and O atoms, with C atoms accounting for 76.12% of the total.

Figure 2 depicts the zeta potential of carbon black surfaces at various pH levels. The isoelectric point of carbon black is pH_iep = 4.4. The surface of carbon black is positively charged when the pH is less than 4.4. The surface of carbon black is negatively charged when the pH is greater than 4.4. At the same time, as the pH value rises, the potential of carbon black decreases.

3.2. Effect of pH

The effect of pH on the adsorption performance of carbon blacks for TCS adsorption was investigated. The initial pH values of the solutions were adjusted to the range of 3–9, and the initial concentrations of TCS solutions were 0.5, 1, 2 and 5 mg/L, and 0.02 g of carbon black was added. The results are shown in Figure 3. The greater the initial concentration of TCS solution, the more the pH value influences the equilibrium adsorption capacity of carbon black on TCS. The adsorption capacity of carbon black on TCS was stronger at a higher TCS concentration when the solution was acidic. As described in previous work [17], triclosan (pKa = 8.14) does not dissociate at acidic pH, thus minimizing electrostatic repulsive interactions and improving the adsorption capacity; moreover, the increase in the final solution to alkaline pH led to the dissociation of triclosan, converting to a negative ion. When the pH of the solution is 3, the carbon black surface is positively charged, and the TCS in the solution is primarily in the TCS- form, resulting in an electrostatic gravitational attraction between them that aids the adsorption reaction. As the pH of the solution rises, the surface of carbon black becomes more negatively charged, and TCS remains in its original state in the solution, the electrostatic force between carbon black and TCS reduces, lowering the adsorption capacity. In comparison to the solution pH = 3, the equilibrium adsorption of carbon black on TCS at a solution concentration of 5 mg/L and pH = 9 dropped from 25.14 mg/g to 15.55 mg/g. When the concentration of TCS was less than 1.0 mg/L, the pH value had no effect on the adsorption of carbon black on TCS.

3.3. Effect of Temperature

The effect of temperature on the adsorption performance of carbon black was investigated by injecting 0.02 g of carbon black into 100 mL of 5 mg/L TCS solution at set temperatures of 15 °C, 30 °C and 45 °C, pH 7. The results are shown in Figure 4, the equilibrium adsorption capacity of carbon black on TCS gradually decreased with the increase in the experimental temperature. The equilibrium adsorption capacity of carbon black on TCS decreased from 25.52 mg/g to 19.63 mg/g when the solution concentration was 5 mg/L and the reaction temperature was 45 °C. This is because the adsorption of carbon black on TCS is an exothermic process, and the adsorption reaction is affected by temperature changes. Due to the progressive increase in temperature, the TCS molecules originally adsorbed on the surface of carbon black would seek to migrate into the solution, causing some TCS to be desorbed and the adsorption effectiveness to be lowered. In addition, as the initial concentration of TCS solution increases, the equilibrium adsorption capacity of carbon black on TCS is steadily increased by temperature, as shown in Figure 4. This is because the adsorption of carbon black on TCS is an exothermic process, the adsorption reaction will be affected as the temperature rises. The TCS molecules that were originally adsorbed on the surface of carbon black will migrate into the solution due to the gradual increase in temperature, causing some of the TCS to be removed from the solution and the adsorption effectiveness to be lowered. In addition, as the initial concentration of TCS solution increases, the effect of temperature on the adsorption capacity of carbon black on TCS gradually increases.

3.4. Comparison with PAC

The adsorption of TCS by carbon black and activated carbon was experimentally investigated as activated carbon is commonly utilized as an adsorbent material in water pollution remediation [18]. A 100 mL solution of TCS at a concentration of 2 mg/L was added with 0.02 g each of pretreated carbon black and activated carbon, and the mixture stirred at 500 r/min at 20 °C and pH = 7. The results showed that the equilibrium adsorption capacity of carbon black for TCS was 18.62 mg/g and the equilibrium adsorption capacity of activated carbon for TCS was 20.43 mg/g, with a difference of less than 10%. At the same time, carbon black has significant economic advantages such as easy availability and low cost, suggesting the possibility of its potential application in water treatment in the future.

3.5. Adsorption Isotherms and Kinetics

The equilibrium adsorption capacity of carbon black on a low concentration of TCS was investigated at 20 °C and pH = 7 by adding 0.02 g of carbon black to 100 mL of TCS solutions with initial concentrations of 0.5, 1.0 and 2.0 mg/L, respectively. The results are shown in Figure 5. The adsorption of carbon black on TCS rapidly occurred within the initial 0.25 min, and the adsorption process could be basically completed within about 0.5 min, when the adsorption amount reached 95% of the saturation adsorption capacity. The maximum adsorption capacity no longer changed when the reaction time was continued.

In the initial stage of the reaction, the adsorption rate of carbon black is the fastest because the TCS concentration in the reaction system is high and there are sufficient adsorption sites on the surface of carbon black. The adsorption sites given by carbon black steadily diminish as reaction time grows, while the adsorption amount gradually increases, and the reaction eventually reaches equilibrium. The adsorption of TCS by carbon black increases when the initial concentration of TCS solution increases, owing to an increase in the mass transfer driving force between the solid and liquid phases, as well as an increase in the rate of TCS diffusion to the carbon black surface. The saturation adsorption of TCS by carbon black reached about 18.65 mg/g at a starting concentration of 2 mg/L, and the TCS removal rate could approach 96.2%.

The Langmuir (Equation (4)) and Freundlich (Equation (5)) adsorption isotherm models were used to fit the adsorption of TCS on carbon black [19,20]. The isotherms are shown in Figure 6, and the fitted parameters are shown in

Table 1. Linear fitting parameters of CB adsorption isotherm equation.

Langmuir			Freundlich
qm (mg/g)	KL (L/mg)	R2	n	KF (mg/g) (L/mg)1/n	R2
634,121.47	0.43	0.90	0.66	631.16	0.99

.

$$q_m=\frac{q_e{K}_L{C}_e}{1+K_L{C}_e}$$

(4)

where C_e (mg/L) is the equilibrium concentration of TCS, q_e (mg/g) is the amount of TCS adsorbed at adsorption equilibrium, q_m (mg/g) is the maximum adsorption amount, and K_L (L/mg) is the Langmuir constant.

$$q_m=K_F{C}_e^{\frac{1}{n}}$$

(5)

where K_F ((mg/g) (L/mg)1/n) and n are Freundlich constants.

According to the coefficients (R²) in Table 1, the Freundlich isotherm model (R² = 0.99) can better represent the adsorption process of carbon black on TCS in water than the Langmuir isotherm model (R² = 0.99). Combined with the basic assumptions of the Freundlich isotherm adsorption model at low concentrations, it is clear that the adsorption process of carbon black on TCS mainly occurs on relatively inhomogeneous surfaces, which are surface adsorption. Simultaneously, the surface adsorption sites are extremely reactive and prone to electron transfer, implying the presence of chemisorption.

Kinetic models can be used to fit the whole adsorption process and to better investigate the adsorption mechanism of adsorbent materials, among which the commonly used kinetic models are the proposed primary and secondary kinetic models. The proposed primary kinetic model (Equation (6)) and the proposed secondary kinetic model (7) were used to describe the adsorption process [21,22,23]. The linear fitting results are shown in Figure 7, and the fitted parameters are listed in

Table 2. Model parameters of adsorption kinetics of CB on TCS in water.

	Pseudo-First-Order			Pseudo-Second-Order
C0 (mg/L)	k1 (1/min)	qe (mg/g)	R2	k2 (g/(mg·min))	qe (mg/g)	R2
0.5	0.15	0.78	0.89	32.74	2.76	1
1	0.11	0.54	0.42	11.31	5.68	1
2	0.15	0.53	0.58	7.86	18.61	1
5	0.16	0.42	0.62	8.07	35.21	0.98

.

$$ln(q_e-q_t)=ln{q}_e-k_{1}t$$

(6)

$$t/q_t=1/k_2{q_e}^2+t/q_e$$

(7)

where q_t (mg/g) and q_e (mg/g) are the amount of TCS adsorbed at time t and the amount adsorbed at adsorption equilibrium, respectively. k₁ (1/min) is the rate constant of quasi primary and k₂ (g/(mg·min)) is the rate constant of quasi secondary.

As can be seen from Table 2, for different initial concentrations of TCS, the fitted secondary kinetic equation (R² = 1) provides a better fit for the adsorption kinetics than the fitted primary kinetic equation (R² < 0.90) and the q_e values obtained from the fitted secondary kinetic equation are essentially the same as the experimental data. This indicates that the physical adsorption between carbon black and TCS mainly occurs on the surface of carbon black and diffuses to each adsorption site, with chemisorption occurring through electron sharing or exchange.

3.6. Column Studies

3.6.1. Effect of Column Bed Depth

The breakthrough curves were studied by passing a 0.5 mg/L TCS solution through adsorption columns with bed depths of 0.9 mm, 2.2 mm, and 4.2 mm at a flow rate of 2 mL/min from top to bottom (0.02 g, 0.05 g and 0.1 g). The results are shown in Figure 8. The adsorption impact of carbon black on TCS and the running time of the fixed bed are affected by the height of the adsorption bed. The penetration duration increases as the adsorption bed height increases; for adsorption bed heights of 0.9, 2.2, and 4.2 mm, the penetration time is 3.33, 8 and 14.33 d, respectively, and the penetration end point is reached at 6, 13 and 18 d. This can be explained by the fact that with the increase in bed height, the residence duration of the TCS solution going through the fixed bed extended, allowing more TCS to be absorbed by carbon black. The figure also shows a rightward shift of the breakthrough points as the adsorption bed height increases, as well as a gradual decrease in the slope, which is due to the increased distance between the carbon black and the TCS mass transfer region, which improves the adsorption reaction. The calculated parameters for different conditions are shown in the

Table 3. Calculation parameters for different bed heights.

H (mm)	Q (mL/min)	C0 (mg/L)	ta (d)	tb (d)	Qe (mg/g)	h (%)
0.9	2	0.5	3.33	6	244.56	66 ± 4
2.2	2	0.5	8.00	13	231.04	67 ± 6
4.2	2	0.5	14.33	18	218.62	59 ± 6
0.9	1.5	0.5	5.00	9	252.70	56 ± 3
0.9	2	0.5	3.33	6	244.56	66 ± 4
0.9	2.5	0.5	1.33	2.33	221.17	69 ± 7
0.9	2	0.2	5.00	12	226.71	43 ± 4
0.9	2	0.5	3.33	6	244.56	66 ± 5
0.9	2	1.0	1.33	3	353.60	71 ± 6

.

3.6.2. Effect of Initial TCS Concentration

Figure 9 represents the carbon black adsorption breakthrough curves for various TCS solution concentrations by passing TCS solutions with concentrations of 0.2, 0.5, and 1.0 mg/L through an adsorption column with a bed height of 0.9 mm at a given flow rate of 2 mL/min. The efficacy of the fixed bed adsorption column systems is affected by the concentration of the TCS solution, as shown in Figure 10. For initial TCS concentrations of 0.2, 0.5, and 1.0 mg/L, the breakthrough time were 5, 3.33,1.33 days, respectively. With the increase in the initial TCS concentration, the breakthrough times were decreased, the breakthrough points showed a significant leftward trend, and the breakthrough curves were seen to have a sharper appearance. This is because when the TCS solution concentration increases, the higher concentration gradient increases the mass transfer driving force of TCS on the carbon black surface during the adsorption process, allowing TCS to quickly cover more adsorption sites and the carbon black to reach saturation. The carbon black surface can no longer absorb the remaining TCS in the solution in time.

3.6.3. Effect of Flow Rate

The effect of flow rate on the adsorption performance of carbon black on TCS was investigated by flowing a 0.5 mg/L TCS solution through an adsorption column with a bed height of 0.9 mm at flow rates of 1.5 mg/L, 2 mg/L, and 2.5 mg/L, respectively. The breakthrough curves are shown in Figure 10. As the flow rate of TCS solution increases, the breakthrough time of the adsorption bed becomes shorter. For flow rates of 1.5, 2, and 2.5 mL/min of TCS feed water, the breakthrough times were 5, 3.33, and 1.33 d, respectively, indicating that the breakthrough times were inversely proportional to the flow rates.

It is also observed that the breakthrough point significantly shifts to the left and the breakthrough curve becomes steeper as the flow rate increases. This can be explained by the fact that the increase in flow rate corresponds to a reduction in the distance of the mass transfer process, resulting in a shorter interaction time between carbon black and TCS [24], and the mass transfer force between carbon black surface and TCS does not change substantially, causing the carbon black reaches saturation in a short time. As can be shown, accelerating the influent flow rate is not beneficial to the removal of TCS by carbon black in fixed bed adsorption systems.

3.7. Breakthrough Curves Models Analysis

In fixed bed adsorption experiments, the bed height is an important factor affecting the adsorption efficiency and operating cost, and the relationship between the fixed-bed operating time and the bed height can represent with the BDST model [25]. Its Equation (8) is as follows:

$$t_e=N_{0}h/(C_{0}v)-1/(k_B{C}_0)\times \ln (C_0/C_t-1)$$

(8)

where C₀ is the influent concentration, C_t (mg/L) is the effluent concentration at time t, t_e (d) is the service time at the set breakthrough point C_t/C₀ = 0.6, N₀ (g/L) is the adsorption capacity of carbon black per unit volume of adsorption bed, h (mm) is the adsorption bed height. v (mm/min) is the flow rate, and k_B (L/(d·mg)) is the BDST model constant.

The BDST fit is based on the fixed bed adsorption data, with the results shown in Figure 11 and the model parameters shown in

Table 4. Error of fitting parameters and penetration time of BDST model.

H	v	(t0.6) Exp	C0	N0	(t0.6) Cal	ε
(mm)	(mm/min)	(d)	(mg/L)	(mg/L)	(d)	(%)
0.9		2.95			2.97	0.67
2.2	6.37	7.42	0.5	77.87	7.48	0.80
4.2		14.30			14.33	0.21

. Figure 11 shows that the BDST model’s fitted curve has a high correlation coefficient R² of 0.9998, indicating that the relationship between the adsorption bed height and breakthrough time in this device can be characterized and predicted by the BDST model. When the initial concentration of TCS is 0.5 mg/L and the flow rate is 2 mL/min, Table 4 displays the theoretical and experimental breakthrough times for different bed heights, as well as their errors. It can be seen that the error between the theoretical breakthrough time calculated by the model and the experimental value is less than 1%, indicating that the BDST model is capable of accurately predicting and fitting the breakthrough curve.

4. Conclusions

Industrial grade carbon black was pretreated and characterized using FTIR, SEM, and zeta potential measurements. The adsorption of carbon black on TCS rapidly occurred in the first 0.5 min of the reaction and could reach 98% of the saturation adsorption capacity. Carbon black had a maximum adsorption capacity of 18.62 mg/g on TCS, which was roughly 10% lower than activated carbon. The adsorption process was more consistent with the Freundlich isothermal model and the pseudo-second-order kinetic model. The fixed bed adsorption results showed that the breakthrough time increased with the increase in bed height, the decrease in initial TCS solution concentration, and the decrease in flow rate. The BDST model was used to analyze the data of fixed-bed adsorption, and the results revealed that the correlation coefficients R² fitted by the BDST models were 0.99, indicating that BDST model had fitting effects.