Abstract

Activated carbon (AC) is the most commonly used adsorbent for water purification, although the dispersive nature of AC in aqueous solution poses a serious problem. To overcome this limitation, AC was magnetized with iron oxide using iron salts as precursor. Further to enhance its effectiveness, it was impregnated with Aliquat 336. Different characterization techniques (Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), along with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD)) were used to analyze the adsorbent. Furthermore, the value of the pH at which the overall charge on the surface of the adsorbent is neutral was found by pH drift method. The modified form of the activated carbon was used to treat the aqueous solution of bisphenol-A in the batch as well as in the continuous mode of operation. In batch mode, the data were validated using equilibrium and kinetic models, and in continuous mode, data were fitted with the Thomas, Adams-Bohart, and bed depth service time (BDST) fixed bed adsorption models. Also, the changes in Gibb's free energy, enthalpy, and entropy were estimated from the temperature study. The design of an adsorption column is proposed to treat 10,000 L/day of an industrial effluent containing BPA.

INTRODUCTION

Endocrine disrupting compounds (EDCs) are defined as ‘an exogenous agent that interferes with synthesis, secretion, transport, metabolism, binding action, or elimination of natural blood-borne hormones that are present in the body and are responsible for homeostasis, reproduction, and developmental process’ by the US Environmental Protection Agency (Diamanti-Kandarakis et al. 2009). EDCs enter the human body either through ingestion or inhalation. The interference mechanism of EDCs in living organisms is carried out in three different ways: (i) they imitate the naturally working hormones in living beings such as androgen, estrogen and thyroid hormones, (ii) they can be attached to cell receptors, preventing the binding of endogenous hormones, and (iii) they alter the production of various hormones by different organs. EDCs include a diverse range of products; these may be naturally occurring or human-made. These include pharmaceuticals, DDT and pesticides, polychlorinated biphenyls, dioxin, and dioxin-like compounds, and plasticizers like bisphenol-A (BPA). 2,2-(4,4′-Dihydroxydiphenyl) or BPA is an organic compound with phenolic groups. It is in a solid state at room temperature with less volatility, low vapor pressure, and moderate water solubility (Maffini et al. 2006). BPA finds its application as a monomer in the manufacture of polycarbonate (used in plastic toys, infant feeding bottles, food storing containers, etc.), and high performance transparent rigid plastic. These containers and toys provide a source of BPA for the environment. Store receipts, dental sealants, linings of canned foods, plastic wrap, and toilet paper are also sources of BPA in nature. BPA travels in some amount into food and beverages that are stored in such containers. The maximum use of BPA (95%) is in the production of epoxy resins and polycarbonates. The remaining 5% is used in phenolic resin, phenoplast resins, antioxidants, and PVC manufacturing. The presence of BPA in the environment can also be endorsed to anthropogenic activities. The adverse effect of exposure to BPA can be seen in the reproduction of wildlife. In males, BPA can lead to a decrease in sperm count, DNA damage, and mobility. There are increased chances of prostate cancer. In females, early puberty; infertility; malfunctioning of the thyroid gland and more chances of breast cancer are the common effects of BPA exposure. A common threat to humans includes obesity. It also affects the brain of infants in the developmental stage (Maffini et al. 2006). Water contains almost 52% of BPA, the soil contains 25%, and the remaining 23% is found in sediments. Air, biota and suspended solids comprise less than 1% of BPA (Staples et al. 1998). In Japan, the analysis of 27 wastewater treatment plants yielded that the inlet BPA concentration varied between 0.04 and 9.6 μg/L. The water in Lake Biwa of Japan was found to contain BPA in the concentration of about 1.14 μg/L. In Germany, Toronto, and the Netherlands, the BPA concentration in the wastewater treatment plants were found to be in the range of 0.15 to 12.3 μg/L, 0.16 to 28.10 μg/L, and 0.25 to 5.62 μg/L, respectively. In the USA, the concentration limit of BPA was in the range of 0.28 to 36 μg/L (Melcer & Klečka 2011). The various techniques for the elimination of BPA include reverse osmosis, membranes, nano-filtration, and ultra-filtration (Bing-zhi et al. 2008; Yüksel et al. 2013). Yoshihara and Murugananthan (Yoshihara & Murugananthan 2009) carried out anodic degradation of BPA through a boron-doped diamond electrode. Wang et al. (Wang et al. 2009) reported the degradation of BPA by photocatalytic means with the help of immobilized TiO2 accompanied by UV illumination. Elimination of BPA was also accomplished using H2O2 and ultrasonic waves (Nikfar et al. 2015). Membrane bioreactors (Chen et al. 2008) and ionic liquid membranes (Panigrahi et al. 2013) have also been analyzed for the elimination of BPA. Table S1 (available with the online version of this paper) shows the removal of BPA using various methods. The above methods incur the drawback of high cost, high energy, and large land requirement. Adsorptive removal of pollutants is hence preferred due to its inherent benefits of low cost, efficient removal of a variety of contaminants, low land requirement, possible removal of toxic substances and greater flexibility in operation. Various adsorbents have been synthesized and worked upon by researchers for removal of BPA in batch mode, but limited literature is available for removal of BPA in continuous mode of operation.

The present study aimed to prepare magnetic activated carbon loaded with Aliquat 336, and to use it for the treatment of an aqueous solution containing BPA by adsorption. The batch (equilibrium and kinetic) and column (fixed bed) studies were conducted to analyze the performance of the prepared adsorbent on BPA removal efficiency. Also, with the help of these data, design parameters of the adsorption column were proposed.

MATERIALS AND METHOD

Materials

Commercially available activated carbon (mesh size = 300) was purchased from CDH, India. Ferric chloride (anhydrous, 96%) and NaOH (98%) were provided by Qualigens Fine Chemicals, India. Nitric acid (69%) and iron sulfate heptahydrate (98%) were obtained from Merck, India. Aliquat 336 (99%) and bisphenol-A (99%) were purchased from Sigma Aldrich, USA.

Synthesis of adsorbent

Magnetic activated carbon (m-AC) was prepared in the same way as explained in the earlier studies (Datta et al. 2016, 2017). Initially, oxidation of 50 g of AC was carried out with 200 ml of HNO3 (20%) for 6 h at 343 K under constant stirring in a magnetic stirrer (Remi Lab, 5MLH, India). Carbon particles were separated from the solution, washed with deionized water, and dried at 353 K. 1 M ferric chloride and 0.5 M of ferrous sulfate were mixed with AC (mass ratio of (Fe+2 + Fe+3):AC = 4:1). The solution containing AC and iron salts solution was put on the stirrer at 353 K for 4 h by maintaining a pH between 10 and 12 by NaOH. The solution was then aged for 2 h at a temperature of 353 K. A magnet was used to separate prepared magnetized AC. Finally, m-AC was washed with distilled water and then ethanol followed by 4 h drying at 353 K. The incorporation of Aliquat 336 (A336) into m-AC was done by following the procedure described by Aranda et al. (Aranda et al. 2012). 100 ml of 20% A336 in ethanol was mixed with m-AC, and shaken at room temperature. After filtration of A336 impregnated m-AC (A-m-AC), it was oven dried at 343 K for 4 h.

Batch and column experiments

In batch experiments, a water bath shaker (RSB-12, Remi Lab, India) maintained at a temperature, T of 303 K was used. 10 mg/L of BPA solution (Co in mg/L) was taken with 20 g/L of A-m-AC, and the data were collected at different time (t) intervals. Dosage of A-m-AC was varied from 4 g/L to 30 g/L in a 10 mg/L BPA solution. Temperature and pH were varied from 303 K to 333 K, and 2 to 12, respectively. The equilibrium concentration of BPA (Ce, mg/L) in the aqueous phase after separation was measured by UV-VIS spectroscopy (Evolution 220, Thermo Fisher Scientific, USA) at a wavelength of 276 nm. Removal efficiency (in %) and adsorption capacity (qe, mg/g) of A-m-AC were calculated using Equations (1) and (2), respectively. 
formula
(1)
 
formula
(2)

V (ml) is the total aqueous volume, and m (g) is the dosage of the adsorbent.

A glass column of 30 cm length and 0.8 cm internal diameter was used with external circulating water to maintain a constant temperature. The filled adsorbent was supported by glass wool below and above the packing. Experiments were carried out by changing column length (z = 0.5, 1 and 2 cm), the inlet concentration of BPA (Co = 20, 40, and 50 mg/L), flow rate (Q = 2, 3 and 5 mL/min), and column temperature (T = 303, 313, and 323 K). Initially, the column was operated with deionized water, and then BPA solution was passed through the column. BPA concentration in the treated water sample received from the bottom of the column was analyzed at regular time intervals.

RESULTS AND DISCUSSION

Characterization

Fourier transform infrared spectroscopy (FT-IR)

In FT-IR (Figure 1), the broad peaks between 3,500 and 3,200 cm−1 indicated stretching vibrations of -OH and alcoholic groups in A-m-AC (Tatarchuk et al. 2019). The peak at 554.26 cm−1 refers incorporation of iron oxide (Fe3O4) inside the adsorbent (Tiwari et al. 2019). After 5 adsorption cycles, the magnetic properties of the adsorbent were not affected. In the oxidized AC, a prominent peak at 1,536.70 cm1 is due to the existence of = CO functional group (Anyika et al. 2017). Two peaks of –CH3 group at 2,923 and 2,854 cm−1 are present in A-m-AC after impregnation, adsorption, and regeneration. The peaks at 1,455 cm-1 and 1,337 cm-1 are due to the vibrations of the quaternary ammonium group (Cui et al. 2013).

Figure 1

FT-IR spectrum of oxidized, m-AC, A336 impregnated, used, and regenerated AC.

Figure 1

FT-IR spectrum of oxidized, m-AC, A336 impregnated, used, and regenerated AC.

Field emission scanning electron microscopy (FESEM) and energy dispersive spectroscopy (EDS)

The irregularities present on the rough surface of AC act as sites for adsorption, Figure 2 (Bhatia et al. 2018). Deposited iron oxide on the surface of AC gives a cloudy appearance to m-AC. After impregnation of A336, more irregularities with a more dense structure are observed on the surface of A-m-AC. In used A-m-AC, heavy loading of BPA molecules was seen on the A-m-AC. In EDS, %carbon decreased to 20.29% (m-AC) from 34.29% (oxidized AC) as iron oxide was added (Bhatia et al. 2018). A peak of chlorine in A-m-AC indicates successful impregnation of A336. BPA adsorption increased oxygen content and after regeneration this amount decreased.

Figure 2

FE-SEM and EDS images of (a) A-m-AC, (b) BPA adsorbed A-m-AC, and (c) regenerated A-m-AC.

Figure 2

FE-SEM and EDS images of (a) A-m-AC, (b) BPA adsorbed A-m-AC, and (c) regenerated A-m-AC.

X-ray diffraction (XRD)

The phase in which iron is impregnated in the adsorbent can be seen from the XRD analysis. The XRD graph shown in Figure 3 depicts that iron is predominantly present as magnetite (Fe3O4) in m-AC, and the magnetic properties of A-m-AC are not affected by the impregnation of A336, and even after regeneration (Figure 3), ascertaining the stable magnetism in AC.

Figure 3

XRD plots of (a) A-m-AC, and (b) regenerated A-m-AC.

Figure 3

XRD plots of (a) A-m-AC, and (b) regenerated A-m-AC.

Brunauer–Emmett–Teller (BET)

The surface area and pore volume of adsorbent (MAC + A336) and used adsorbent were determined using BET analysis (NOVAtouch LX2, Quantachrome Instruments). The results are shown in Table 1. It was observed that after adsorption, there is an increase in the surface area and pore volume. This may be due to the opening of the pores at the time of treatment of the water solution containing BPA.

Table 1

BET results for MAC + A336 and used adsorbent

S. no. Adsorbent Surface area (m2/g) Pore volume (m3/g) 
1. MAC + A336 86.75 0.0425 
2. Used MAC + A336 223.19 0.2600 
S. no. Adsorbent Surface area (m2/g) Pore volume (m3/g) 
1. MAC + A336 86.75 0.0425 
2. Used MAC + A336 223.19 0.2600 

Point of zero charge analysis of A-m-AC

The pH drift method was adapted to determine the neutral pH point experimentally (Balistrieri & Murarry 1981), which was found to be 7.9 (Figure 4).

Figure 4

Finding out point of zero charge of A-m-AC (dosage = 3 g/L; NaCl = 0.01 M; NaF = 0.01 M; t = 48 h; and T = 303 K).

Figure 4

Finding out point of zero charge of A-m-AC (dosage = 3 g/L; NaCl = 0.01 M; NaF = 0.01 M; t = 48 h; and T = 303 K).

Batch study

Time study

The removal rate is very fast in the initial 5 min, and the system gradually reached equilibrium after 120 min with 93.10% efficiency (Figure 5(a)). Pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion model (IPD), and Elovich models were fitted to kinetic data to obtain the model parameters (Table 2). The kinetic data were closely matched with the PSO model for BPA adsorption (R2 = 0.991) on A-m-AC, showing chemisorption is the rate determining step.

Table 2

Kinetic model equations with their parameters

PFO:

(Lagergren 1898
qm (mg/g) = 0.465
k1 (1/min) = 0.083
R2 (-) = 0.883 
PSO:

(Ho & McKay 1999
qm (mg/g) = 0.470
k2 (mg/g/min) = 1.384
R2 (-) = 0.999 
IPD:

(Findon et al. 1993
c (mg/g) = 0.254
kin (mg/g/min1/2) = 0.023
R2 (-) = 0.535 
Elovich model:

(Aharoni & Tompkins 1970
ae (mg/g/min) = 13.425
be (g/mg) = 21.496
R2 (-) = 0.860 
PFO:

(Lagergren 1898
qm (mg/g) = 0.465
k1 (1/min) = 0.083
R2 (-) = 0.883 
PSO:

(Ho & McKay 1999
qm (mg/g) = 0.470
k2 (mg/g/min) = 1.384
R2 (-) = 0.999 
IPD:

(Findon et al. 1993
c (mg/g) = 0.254
kin (mg/g/min1/2) = 0.023
R2 (-) = 0.535 
Elovich model:

(Aharoni & Tompkins 1970
ae (mg/g/min) = 13.425
be (g/mg) = 21.496
R2 (-) = 0.860 
Figure 5

(a) Effect of time, (b) adsorption isotherm, (c) effect of temperature, (d) effect of pH, (e) effect of salt concentration, and (f) regeneration.

Figure 5

(a) Effect of time, (b) adsorption isotherm, (c) effect of temperature, (d) effect of pH, (e) effect of salt concentration, and (f) regeneration.

Effect of A-m-AC dosage

Adding more adsorbent (from 4 to 20 g/L) in the solution of BPA increased %removal (from 81.22% to 93.37%, Table 3), but with a decrease in the adsorption capacity (from 2.03 mg/g to 0.47 mg/g). The increased in the percentage removal was observed because of the increase in the number of active sites, which reduces the competition among various BPA molecules. The decrease in adsorption capacity can be attributed to limited availability of BPA as the number of active sites increased, hence reducing the number of BPA molecules competing for a single active site.

Table 3

Influence of A-m-AC amount on the adsorption of BPA (10 mg/L) at 303 K

m (g/L) 10 16 20 30 
qe (mg/g) 2.03 0.84 0.56 0.47 0.27 
Removal (%) 81.22 84.53 90.06 93.37 83.43 
m (g/L) 10 16 20 30 
qe (mg/g) 2.03 0.84 0.56 0.47 0.27 
Removal (%) 81.22 84.53 90.06 93.37 83.43 

Study of BPA initial concentration

The studies were conducted by changing Co in the range of 10 mg/L to 50 mg/L with 20 mg/L of A-m-AC at 303 K showing a decreasing trend in the removal (from 90.91% to 68.81%). Three equilibrium isotherm (Langmuir, Freundlich, and Temkin) models were applied to match the equilibrium data, and estimated values of model parameters are represented in Table 4 with predicted values of qe as shown in Figure 5(b). The Freundlich isotherm model provided the finest match with the equilibrium data (R2 = 0.991) indicating that the adsorbent surface is heterogeneous, and there is an exponential distribution of active sites and their energies. A fixed adsorbent amount provides a fixed number of active sites. Thus, an increase in the number of BPA molecules with increased interaction gave lower removal.

Table 4

Values of equilibrium isotherm constants for the adsorption of BPA

Langmuir:

(Clarke & Irving Langmuir 1916
qm (mg/g) = 1.561
KL (L/mg) = 1
R2 = 0.952 
Freundlich:
(Freundlich 1906
KF [(mg/g) (L/g)1/n] = 1.593
n= 1.2
R2 = 0.991 
Temkin:
(Temkin 1941
qm (mg/g) = 0.426
KT (L/mg) = 2.489
R2 = 0.927 
Langmuir:

(Clarke & Irving Langmuir 1916
qm (mg/g) = 1.561
KL (L/mg) = 1
R2 = 0.952 
Freundlich:
(Freundlich 1906
KF [(mg/g) (L/g)1/n] = 1.593
n= 1.2
R2 = 0.991 
Temkin:
(Temkin 1941
qm (mg/g) = 0.426
KT (L/mg) = 2.489
R2 = 0.927 

Influence of temperature

Figure 5(c) depicts the effect of varying temperature on BPA removal. A fall in the %removal was seen from 93.37% to 62.98% as the rise in temperature enhanced the total energy of BPA molecules, making them separate from the surface of A-m-AC, and having an exothermic effect. The following formulas were used to determine the thermodynamic parameters of BPA adsorption. 
formula
(3)
 
formula
(4)

Gibbs free energy (ΔG°) in the standard state at a constant temperature was calculated (Equation (3)) and plotted against temperature (Equation (4)) to obtain the standard change in enthalpy and entropy of adsorption. A negative value of ΔH° (=− 62.59 kJ mol-1) confirms that the adsorption process is exothermic, and a positive value of ΔS° (=150.22 J mol-1 K-1) shows that there is increased disorder in the system.

Solution pH effect

Aqueous solution pH was adjusted by 0.1 M HCl or 0.1 M NaOH in the range of 2 to 12. 25 ml BPA solution was taken in 100 ml conical flask with 20 mg/L of A-m-AC at different pH values (Figure 5(d)). Maximum adsorption happened in the acidic pH by bisphenolate anion.

Effect of interfering ions on BPA adsorption

Effect of ions on the adsorption of BPA was analyzed using NaCl and Na2SO4 in a concentration ranging from 1 g/L to 50 g/L (Figure 5(e)). In both cases, percent removal decreased with an increase in the salt concentration, depicting an inhibitory effect of salt ions on the removal of BPA (Thanhmingliana et al. 2014).

Regeneration of used A-m-AC

Regeneration and reusability of adsorbent become a significant parameter in determining the possible use of adsorbent in practical situations. Reusability of adsorbent is currently under the focus of researchers, as depicted from literature (Mironyuk et al. 2019). The used adsorbent was regenerated for 5 cycles using 1 M NaOH solution in batch mode at room temperature over 2 h, and studied for its performance with 10 mg/L BPA solution and with 20 mg/L of regenerated adsorbent at 303 K. The same cycle of regeneration and adsorption was studied for 5 times, observing a significant decrease in the performance of the adsorbent from 93.37% to 55.8% (Figure 5(f)).

Effect of column parameters in fixed bed adsorption

Increased bed lower down the effluent BPA concentration (Ct) at a particular time (Co = 20 mg/L, Q = 3 mL/min, and T = 303 K, Figure 6(a)) (Goel et al. 2005). A greater adsorbent amount ensures higher saturation capacity of the column. Different shapes of the curve were obtained, with a declining slope at increased bed height.

Figure 6

Breakthrough curves at different (a) bed depths, (b) flow rates, (c) inlet concentrations and (d) temperatures.

Figure 6

Breakthrough curves at different (a) bed depths, (b) flow rates, (c) inlet concentrations and (d) temperatures.

An early saturation was achieved at 5 mL/min higher flow rates as, with increasing flow rate, the mass transfer rate of BPA from the bulk to solid phase was facilitated (z = 0.5 cm, Co = 20 mg/L, and T = 303 K, Figure 6(b)). At 2 mL/min, a significant value of the breakthrough time was observed with the highest %removal attributed to more residence time being available for BPA adsorption.

Changing Co from 20 mg/L to 40 mg/L decreased the breakthrough time; that is, less time was required for the bed to become saturated (Q = 3 mL/min, z = 0.5 cm, and T = 303 K, Figure 6(c)). The increased amount of BPA molecules available for adsorption could be the reason for early saturation of the bed. With further increase in Co from 40 to 50 mg/L, there is continuous adsorption and desorption observed, indicating that at a particular amount of the adsorbent, the column is not suitable to be used for the adsorption of BPA.

A 10° increase in T from 303 to 313 K resulted in a decrease in the removal of BPA, depicting that at higher temperature column studies may not be suitable for BPA adsorption (Co = 20 mg/L, z = 0.5, and Q = 3 mL/min, Figure 6(d)). Similar results were also obtained in the batch experiments. Further increase in the temperature from 313 to 323 K favors desorption of BPA molecules from the surface of the adsorbent, providing a lower removal rate.

The breakthrough (∼50% or Ct/Co = 0.5) adsorption capacity of the column can be calculated using Equation (5) (Goel et al. 2005). 
formula
(5)
Similarly, the exhaustive capacity (qe) of adsorbent in the column was also calculated using the same formula with the replacement of exhaustion time (Ct/Co = 1) by breakthrough time. The values of qb and qe were calculated for 2 mL/min solution flow rate, 0.5 g adsorbent and 20 mg/L BPA concentration at 303 K, and were found to be 14.40 and 45.60 mg/g, respectively. Whereas when the flow rate was 5 mL/min (other conditions remaining the same) the values of qb and qe were found to be 1 mg/g and 3 mg/g, respectively.
The following three models, Thomas (Unuabonah et al. 2010, Equation (6)), Adams Bohart (Faizal et al. 2014, Equation (7)) and bed depth service time (BDST, Taylor et al. (2013), Equation (8)) were used to describe the adsorption kinetics in the fixed bed. 
formula
(6)
 
formula
(7)
 
formula
(8)
where kTh = Thomas model constant; qo = equilibrium adsorption capacity; w = adsorbent amount; kAB = Adams Bohart constant, Uo = linear velocity; No = saturation concentration, kBDST = rate constant; and Cb = exit BPA concentration at breakthrough.

The values of model parameters for column studies are shown in Table 5, with the predicted values shown in Figure 6(a)–6(d). There is a variation in the predicted values of Ct/Co observed at higher Co and higher temperature due to the simultaneous adsorption-desorption of BPA molecules from the adsorbent surface. Therefore, these models could not fit the experimental data accurately. The BDST model fitted at two different ratios of Ct/Co = 0.90, and 0.40 for the experimental conditions of Co = 20 mg/L, z = 2 cm, Q = 3 mL/min, and T = 303 K.

Table 5

Estimated values of column model parameters

Co Thomas model
 
Adams Bohart model
 
mL/min Cm mg/L kTh (L/mg/min) qo (mg/g) R2 kAB (L/mg/min) No (g/L) R2 (-) 
20 303 0.000776 7.864 0.93 0.000327 2.776 0.93 
20 303 0.001461 1.814 0.87 0.000422 1.469 0.82 
0.5 20 303 0.006884 0.797 0.98 0.001182 0.522 0.74 
20 303 0.000450 19.78 0.96 0.000200 5.158 0.86 
20 303 0.017250 0.772 0.94 0.003750 0.339 0.92 
40 303 0.000425 −1.580 0.82 0.000125 3.683 0.90 
50 303 0.000980 1.567 0.78 0.00016 3.003 0.63 
20 313 0.001700 1.553 0.92 0.000350 1.586 0.96 
20 323 0.000100 −5.340 0.01 0.000200 1.721 0.09 
Bed depth service model 
Ct/Co KBDST (L/mg/min) No (g/L) R2 
0.9 0.111 0.996 0.94 
0.4 0.0081 0.0721 0.97 
Co Thomas model
 
Adams Bohart model
 
mL/min Cm mg/L kTh (L/mg/min) qo (mg/g) R2 kAB (L/mg/min) No (g/L) R2 (-) 
20 303 0.000776 7.864 0.93 0.000327 2.776 0.93 
20 303 0.001461 1.814 0.87 0.000422 1.469 0.82 
0.5 20 303 0.006884 0.797 0.98 0.001182 0.522 0.74 
20 303 0.000450 19.78 0.96 0.000200 5.158 0.86 
20 303 0.017250 0.772 0.94 0.003750 0.339 0.92 
40 303 0.000425 −1.580 0.82 0.000125 3.683 0.90 
50 303 0.000980 1.567 0.78 0.00016 3.003 0.63 
20 313 0.001700 1.553 0.92 0.000350 1.586 0.96 
20 323 0.000100 −5.340 0.01 0.000200 1.721 0.09 
Bed depth service model 
Ct/Co KBDST (L/mg/min) No (g/L) R2 
0.9 0.111 0.996 0.94 
0.4 0.0081 0.0721 0.97 

Evaluating adsorption column design parameters

Various design parameters of the fixed bed adsorption column were calculated as suggested by Ozdemir et al. (Ozdemir et al. 2017), as given in Table 6.

Table 6

Values of fixed bed column design parameters

Parameters Ct/Co = 0.5
 
Ct/Co = 0.7
 
Z
 
0.5 cm 1 cm 2 cm 0.5 cm 1 cm 2 cm 
tz (min) 13 50 75 15 45 
hz (cm) 0.440 0.869 1.163 0.084 0.232 0.677 
Uz (cm/min) 0.034 0.017 0.015 0.028 0.016 0.015 
%Saturation 64.51 69.56 72.041 87.63 83.68 75.74 
Parameters Ct/Co = 0.5
 
Ct/Co = 0.7
 
Z
 
0.5 cm 1 cm 2 cm 0.5 cm 1 cm 2 cm 
tz (min) 13 50 75 15 45 
hz (cm) 0.440 0.869 1.163 0.084 0.232 0.677 
Uz (cm/min) 0.034 0.017 0.015 0.028 0.016 0.015 
%Saturation 64.51 69.56 72.041 87.63 83.68 75.74 
The time required by the adsorption zone to move the height equal to its height (tz), and through the entire bed along the column after it has become established, (te): ; the rate of movement of the adsorption zone along the bed,  
formula
 
formula

The fraction of bed still having the capacity of adsorbing the adsorbate molecule, ; the percent saturation of the column at breakthrough, . Here, Vs = volume of effluent collected between the breakthrough and exhaustion time; Ve = total volume collected till saturation; hz = height of the adsorption zone; h = total height of the bed; Sz = amount of solute removed between the breakthrough and exhaustion; Smax = total amount of solute removed till exhaustion, Vb = total volume of water treated to breakthrough.

Scaling of laboratory fixed bed column

The following data were considered for the scale-up of the column:

Diameter of column, d = 0.008 m, z = 0.02 m, Q = 2 mL/min, density of A-m-AC, ρ = 250 kg/m3, cross-sectional area of the bed, S = πd2/4 = 5.026 × 10−5 m2. Filtration flux, F = Q/S= 0.502 m3/m2/min.

For treating 10,000 L/day (=6.94 × 10−3 m3/min) of industrial effluent, the specifications of column required:

Area of column, A = Q/F = 6.94 × 10−3/0.502 = 0.0138 m2

Hence, diameter, D = 0.133 m

Bed volume, V = A × z = 0.0138 × 0.02m3

Contact time with the adsorbent, τ = V/Q = 0.5 min

Height of packing, H = τQ/A = 0.5 × 0.502 = 0.251 m

Volume of scaled-up column, Vp = πD2H/4 = 3.48 × 10−3 m3

Mass of the adsorbent required = Vp × ρ = 0.87 kg.

Therefore, a column of 0.133 m diameter and 0.0251 height with 0.87 kg of adsorbent will be required to treat 10,000 L/day of industrial effluent.

Possible adsorption mechanism

The adsorption of BPA onto A-m-AC is preferred in the acidic pH range as concluded from the effect of pH on adsorption. Also, the pHzc value of the adsorbent is 7.9, which indicates that below the pH value of 7.9, the surface of the adsorbent is positively charged, which is responsible for the attraction of bisphenolate anions present in the aqueous medium to the adsorbent surface. A possible adsorption mechanism is shown in Figure S1 (available with the online version of this paper).

CONCLUSION

In this work, Aliquat 336 incorporating magnetic activated carbon was successfully developed using the co-precipitation method. FT-IR gave the peaks of iron and chloride confirming the incorporation of magnetic properties and Aliquat 336, respectively. The presence of magnetite from iron was ascertained from XRD. The pH drift method was applied to evaluate the point of neutral charge of the adsorbent (=7.9). 120 min equilibrium time was sufficient for the adsorption of BPA. Increased dosage gave higher removal of BPA. The Freundlich model best fitted the equilibrium data and the pseudo-second-order for kinetic data. In the column study, at a lower BPA concentration, the Thomas model fitted the data better, and at higher BPA inlet concentration and higher bed temperature, the Adams Bohart model fitted the data. A column with 0.133 m diameter, 0.251 m height and 0.87 kg of A-m-AC will be required to treat 10,000 L/day of an industrial effluent containing BPA.

ACKNOWLEDGEMENTS

We acknowledge the Material Research Centre, Malaviya National Institute of Technology, Jaipur for providing necessary facilities for the characterization using FT-IR, FE-SEM, EDS, XRD, and BET.

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Supplementary data