Chemosphere 309 (2022) 136628
Available online 28 September 2022
0045-6535/© 2022 Elsevier Ltd. All rights reserved.
Adsorptive removal of phosphate from aqueous solutions using low-cost modified biochar-packed column: Effect of operational parameters and kinetic study
T.C. Phuong Tran
a, T. Phuong Nguyen
a, X. Cuong Nguyen
b,c,**, X.H. Nguyen
a, T.A. Hang Nguyen
d, T.T. Nguyen Nguyen
a, T.Y. Binh Vo
a, T.H. Giang Nguyen
a, T.T. Huyen Nguyen
b,c, T.D. Hien Vo
e, P. Senthil Kumar
f,i,j, Myoung-Jin Um
h, D. Duc Nguyen
e,g,*aFaculty of Environmental Engineering Technology, Hue University, Quang Tri Branch, Viet Nam
bCenter for Advanced Chemistry, Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam
cFaculty of Environmental and Chemical Engineering, Duy Tan University, Da Nang, 550000, Viet Nam
dVietnam Japan University, Vietnam National University, Hanoi, 101000, Viet Nam
eFaculty of Environmental and Food Engineering, Nguyen Tat Thanh University, Ho Chi Minh City, Viet Nam
fDepartment of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam-603110, India
gDepartment of Environmental Energy Engineering, Kyonggi University, Suwon 16227, South Korea
hDepartment of Civil Engineering, Kyonggi University, Suwon 16227, South Korea
iCentre of Excellence in Water Research (CEWAR), Sri Sivasubramaniya Nadar College of Engineering, Kalavakkam-603110, India
jDepartment of Biotechnology Engineering and Food Technology, Chandigarh University, Mohali, 140413, India
H I G H L I G H T S G R A P H I C A L A B S T R A C T
•Phosphate was successfully removed using fixed-bed modified biochar column.
•The inlet PO43−, flow rate, and bed height affected the breakthrough curve.
•The optimal conditions were 50 mg/L of PO43−, 6 mL/min flow rate, and 4.5 cm height.
•Adam–Bohart and Yoon–Nelson models were utilized to describe experimental results.
A R T I C L E I N F O Keywords:
Phosphate Adsorption Biochar Column
Operational parameters
A B S T R A C T
Adsorption in the continuous mode plays a significant role in wastewater treatment. In this study, Mimosa pigra- derived biochar modified with 2 M AlCl3 salt was used to pack a lab-scale column to eliminate PO43− from aqueous solutions. The influence of the operational factors, such as inlet PO43−
concentration (25–100 mg/L), flow rate (6–18 mL/min), and biochar bed height (1.5–4.5 cm), on the breakthrough curve was evaluated. The kinetic models of Adam–Bohart and Yoon–Nelson were utilized to analyze the experimental results. The best conditions
* Corresponding author. Faculty of Environmental and Food Engineering, Nguyen Tat Thanh University, Ho Chi Minh City, Viet Nam.
** Corresponding author. Center for Advanced Chemistry, Institute of Research and Development, Duy Tan University, Da Nang, 550000, Viet Nam.
E-mail addresses: [email protected] (X.C. Nguyen), [email protected] (D.D. Nguyen).
Contents lists available at ScienceDirect
Chemosphere
journal homepage: www.elsevier.com/locate/chemosphere
https://doi.org/10.1016/j.chemosphere.2022.136628
Received 3 June 2022; Received in revised form 24 August 2022; Accepted 25 September 2022
were determined to be the influent PO43− strength of 50 mg/L, injection speed of 6 mL/min, and column height of 4.5 cm. These results can be applied in the design of large-scale columns for the sequestration of PO43− from wastewater.
1. Introduction
Phosphorus (P) is a vital component of organisms. However, the excessive levels of P in water from agricultural and industrial activities can lead to eutrophication, which is harmful to aquatic life (Dodds et al., 2009; Phuong Tran et al., 2021). To restrict the growth of algae and thereby prevent eutrophication in receiving water bodies, the P con- centration in wastewater discharged into the environment should be less than 0.05 mg/L (Li et al., 2019a). Therefore, technologies for removing P from wastewater are of great interest.
There are many methods for removing P from wastewater, including biological treatment (Yu et al., 2021), adsorption (Liu et al., 2020), chemical precipitation (Ahmed et al., 2017), and ion exchange (Guida et al., 2021). Although biological P removal has been applied exten- sively in wastewater treatment processes (Li et al., 2019b), it has many shortcomings. This method results in a substantial amount of P-con- taining sludge, which poses environmental risks and requires expensive sludge disposal (Ma˜nas et al., 2011). Chemical precipitation is also a standard method for P removal. However, its actual application is usu- ally limited by the high consumption of chemicals to control the pH and large amounts of sludge (Xu et al., 2015).
The removal of P from wastewater by adsorption is feasible, rela- tively simple, inexpensive, and efficient (Tan et al., 2015; Zhang et al., 2019; Su et al., 2021). Among various adsorbents, biochars produced under oxygen-limited or oxygen-free conditions often demonstrate a high removal efficiency, low cost, and environmental friendliness (Mohan et al., 2014). Because biochars typically have porous structures, large surface areas, and many surface functional groups (Wu et al., 2020), they can absorb P and other contaminants (Xiang et al., 2020;
Ghodszad et al., 2021). In particular, there have been studies on modi- fying biochar with metal catalysts to improve the P adsorption capacities of pristine biochars. For example, the P adsorption capacity of Ca–flour-biochar (Ca(OH)2: flour ratio of 2:1) was 314.22 mg/g, which
was higher than that of the original biochar (48.44 mg/g) (Wang et al., 2018). Additionally, while the P adsorption ability of the original bio- char was limited to the range of 1.88–2.78 mg/g, that of the MgO-modified biochar reached 28.20–29.22 mg/g. The results indi- cated that virgin biochars had surface areas of only 0.26–8.82 m2/g, total volumes of 0.06–0.07 cm3/g, and pore diameters of 1.04–3.69 nm, while those of the Mg–biochars were 22.02–28.07 m2/g, 0.12–0.15 cm3/g, and 17.21–22.23 nm, respectively (Oginni et al., 2020). In our previous study,Mimosa pigra-derived biochar modified with 2 M AlCl3
salt (MBC) exhibited a high PO43− adsorption capacity, reaching 70.6 mg PO43−/g in the batch experiments, which was approximately 13.8 times higher than that of the unmodified biochar (5.1 mg PO43−/g) (Phuong Tran et al., 2021). Because surface-modified biochar has more pores than the unmodified biochar, the larger surface area and enhanced surface functional groups increase the adsorption efficiency (Qu et al., 2022). Mimosa pigra is an invasive species (Hawwal et al., 2021) that threatens the development of other plant species (Suksungworn et al., 2021). Thus, utilizing this tree as a water treatment material benefits the environment. Furthermore, the pyrolysis of the cellulose component in M. pigra hardwood trees is more effortless than that of softwood (Lin et al., 2022) and potentially produces favorable biochar for heavy metal adsorption (Jiang et al., 2017) and phosphate (Arbelaez Breton et al., 2021). However, batch adsorption is not practical for actual wastewater treatment, because it requires ample space. Presently, few studies have been conducted on adsorption in the continuous mode and focus on kinetic studies and optimization of operational parameters (Ding et al., 2016).
To better understand the practical applicability of MBC as a P adsorbent, this study evaluated the P treatment performance of MBC in packed column tests. The influence of the inlet P strength, flow rate, and biochar bed height on the adsorption performance of the column was explored. The experimental results were fitted to two standard kinetic models for column tests: Adam–Bohart and Yoon–Nelson.
2. Materials and method 2.1. Materials
Mimosa pigra trees were collected in the field, and after removing the bark and leaves, the trunks were used as biomass to produce the biochar.
The biochar-modifying agent used was aluminum chloride hexahydrate (97% AlCl3⋅6H2O). All the chemicals used in this study were high-purity reagents purchased from Xilong Chemical Co. Ltd. These included so- dium hydroxide (99% NaOH), hydrochloric acid (37% HCl), PO43−
containing sulfuric acid (98% H2SO4), nitric acid (68% HNO3), ammo- nium persulfate (98% (NH4)2S2O8), ammonium molybdate (99%
(NH4)6Mo7O24⋅4H2O), zinc chloride (98% ZnCl2⋅2H2O), potassium dihydro phosphate (99.5% KH2PO4), ascorbic acid (99% C6H8O6), and potassium antimony tartrate (99% K(SbO)C6H4O⋅5H2O).
2.2. Instruments
The biomass was pyrolyzed to produce biochar in a muffle furnace (FX-14, DAIHAN Scientific, Korea). After washing, the biomass and biochar were dried in an oven (ThermoStable ON-50, DAIHAN Scienti- fic, Korea). The surface functional group combustion of the material was analyzed by Fourier transform infrared analysis (FTIR) in the mid-IR region wavelength (4000–400 cm−1) at a resolution of 4 cm−1. The characteristics of the biochar surface were evaluated using a scanning electron microscope (Hitachi S-4700 SEM, Japan). A micromeritics analyzer was used to identify the textural properties of the biochar Symbolic meanings in formulas
C0 (mg/L) inlet PO43− concentration
Ct (mg/L) outlet PO43− concentration column cross-sectional area (cm2)
E (%) absorbate removal efficiency
F (cm/min) linear velocity as determined by the flow rate (cm3/ min) through the
H (cm) bed depth
kAB (L.mg-1.min−1) the Adam–Bohart rate constant kYN (1/min) Yoon–Nelson rate constant
m (g) mass of biochar
N0 (mg/L) Removal capability per unit volume of packed adsorption column
Q (mL/min) flow rate
R2 The coefficient of determination t (min) time
τ (min) time required for 50% PO43− breakthrough ttotal (min) total column working time
tb (min) breakthrough time at time Ct =0.1C0
Vb (mL) volume of solution that flowed through the column until the breakthrough time
(ASAP 2020 V3⋅00H, USA). The pH measurements were conducted using a pH meter (HQ40D, Hach, USA). PO43− content analysis using the ascorbic acid method was performed at 880 nm by UV–Vis molecular absorption spectroscopy (Hitachi U-2910, Japan). The machines were inspected and evaluated twice a year. The experimental model was operated in the laboratory at room temperature (25 ±2 ◦C). The sam- ples were analyzed immediately after sampling, which was regularly accompanied by the analysis of blank and standard samples. The sam- ples were analyzed in three replicates, and the measurements were averaged. The data were processed using Microsoft Excel and Origin.
2.3. Synthesis of modified biochars
Biomass treatment: The leaves and bark of the M. pigra trees were obtained, finely chopped, and naturally dried outdoors for three days.
The samples were then ground into small pieces of debris (>2 mm) and washed. The biomass was dried in an oven at 105 ◦C for 24 h.
Modifying biochar: In a previous study, we investigated the engineered biochar with the best absorptive capacity and selected that made from M.
pigra and AlCl3 salt (MBC) (Tran et al., 2021). The MBC was prepared using the following steps. First, a 2 M AlCl3 solution was prepared by dissolving AlCl3⋅6H2O in distilled water. Next, the prepared material was soaked in the solution at a ratio of material mass to solution volume of 1 g:7 mL (Yin et al., 2018). The solution was continuously mixed for 6 h at 100 rpm (Jar test apparatus) and then filtered through a 0.45 μm pore size filter. The solid residue was then dried at 80 ◦C (~48 h).
Subsequently, the pretreated material was packed with zinc foil and pyrolyzed in a furnace. The furnace was set up at a heating rate of 10 ◦C/min until it reached 500 ◦C, and the pyrolysis lasted 2 h. Finally, the biochar was washed twice, parched at approximately 103–105 ◦C for 24 h, and ground into tiny particles (<0.25 mm).
2.4. Column adsorption experiments
A transparent PVC column with an inner diameter of 2 cm and height of 15 cm was built as a fixed-bed column to elucidate the phosphate adsorption potential using dynamic modeling. There is no standard design for adsorption columns, and previous studies have reported different column parameters, for example, diameter ×height of 1.8 × 20 cm (Pan et al., 2019), 1 ×30 cm (Jung et al., 2017a), and 4 ×20 cm (Ye et al., 2019). The bottom of the column contained small holes with a diameter of 3 mm to drain the solution. To prevent the material from drifting and losing adsorbent, the filter column was set up with 5 layers from the bottom to the top as follows: (i) a 0.5-cm thick layer of 4–7 mm small rocks to prevent the materials in the upper layers from escaping the column, (ii) a 1-cm thick layer of 1–1.2 mm coarse sand to immo- bilize the fine sand and coal, (iii) a 3-cm thick layer of fine sand to prevent the biochar from drifting, (iv) a biochar layer of varying heights for different experiments, and (v) a 1-cm thick waterproof glass wool layer to maintain the even distribution of water on the column’s surface and to reduce the movement of the biochar into the lower layers.
The influent PO43− solution, prepared from KH2PO4, was contained in a 1-L container located above the column. The PO43− solution was fed into the column in the downflow direction via a speed control device, which was a valve mounted on a water pipe to regulate speed. The speed was determined by counting the drops per unit of time. The water level in the inlet tank was maintained at a stable height. The adsorption columns were operated continuously under different conditions of initial PO43− strength, flow volume, and column height (i.e., biochar) at 1 atm and 25 ±2 ◦C. Wastewater samples were collected from the bottom of the column at various times. The water samples were filtered through a device of pore size 0.45 μm. The control experiments were performed by (i) passing the PO43− solution through a column without biochar and (ii) passing distilled water through the column with biochar. All the ex- periments were repeated three times.
Continuous column experiments were performed under various
process parameters: inlet PO43−concentration (C0 =25–100 mg/L, H =3 cm, Q =6 mL/min), flow rate (Q =6–18 mL/min, H =3 cm, C0 =50 mg/L), and bed height (H =1.5–4.5 cm, Q =6 mL/min, C0 =50 mg/L).
These experimental parameters were referenced from our previous publication (Phuong Tran et al., 2021) and previous studies on phos- phate adsorption columns (Li et al., 2013; Jung et al., 2017a; Gizaw et al., 2022). A diagram of the continuously packed column is illustrated in Fig. 1. The operational parameter values are listed in Table 1.
2.5. Analysis method
The PO43− strengths of the samples were analyzed by the ascorbic acid method at 880 nm (EPA, 1978) using a UV–Vis spectrophotometer (Standard Method, 1999). The test results were calculated according to Equations (1)–(4).
The column adsorption capacity (qtotal, mg) is the total mass of PO43−
retained by the column:
qtotal= Q 1000
∫
t=ttotal
t=0
(Co− Ct)dt (1)
The saturated adsorption capacity of the biochar in the column (qeq (exp), mg/g) is expressed as follows:
Fig. 1. Schema of continuous packed bed for PO43− treatment experiments.
Table 1
The operating parameters of PO43− removal column experiments.
Experiment Inlet strength
(mg/L) Flow rate (mL/
min) Bed height
(cm) Effect of inlet PO43¡
strength 25 6 3
50 6 3
100 6 3
Effect of flow rate 50 6 3
50 12 3
50 18 3
Effect of bed height 50 6 1.5
50 6 3
50 6 4.5
qeq(exp)=qtotal
m (2)
The total mass of PO43− entering the column:
mtotal=Co.Q.ttotal
1000 (3)
The efficiency of column in treating the amount of substance adsorbed through the column:
E=qtotal
mtotal
⋅100% (4)
Adsorption kinetics were determined to test the PO43− absorption capacity of the biochar in the column at different times (t). The sampling time of the adsorption column was at 10 min intervals until Ct =Co. Various mathematical models have been applied to describe and predict the trend of adsorption kinetics in the column, the most popular of which are the Bohart–Adams and Yoon–Nelson models. The Adams–Bohart model is based on the assumption that there is no reverse process and that the removal rate at one site is proportional to both the concentration of the adsorbate and the residual capacity of the adsor- bent (Chu, 2020). This model is expressed in linear (Gouran-Orimi et al., 2018) and nonlinear (Chu, 2020) forms as follows:
ln (Ct
C0
)
=kAB.C0.t− kAB.N0.H
F (5)
Ct
C0
= 1
1+exp(
kAB.N0.HF− kAB.C0.t) (6) The Yoon–Nelson model hypothesizes that the decrease in the ab- sorptivity capacity per adsorbate particle is proportionate to the adsorption capability and breakthrough of the adsorbate (Ayoob and Gupta, 2007). This model does not consider the detailed features of the adsorbate, adsorbent, or dimensions of the adsorption bed. It is expressed in a linear form (Chu, 2020) and converts the calculation to nonlinear forms as follows:
ln ( Ct
C0− Ct
)
=kYN.(t− τ) (7)
Ct
C0
= exp(kYN.t− kYN.τ)
1+exp(kYN.t− kYN.τ) (8)
3. Results and discussion
3.1. Physicochemical properties of biochar
The FTIR spectrum of the modified biochar is illustrated in Fig. 2a.
The –OH spectral band at a wavenumber of approximately 3350 cm−1 (Melanie et al., 2015) appeared because of the establishment of Al(OH)3 on the surface of the MBC when it was modified with the AlCl3 salt. The peaks at 2360 cm−1 and 2341 cm−1 indicate the presence of CO2
(Oancea et al., 2012). The peak at 1700 cm−1 is attributed to the asymmetric oscillation of C––O on the biochar (Guan et al., 2007). The band at wavenumber 1579 cm−1 indicates the presence of stretched C–C aromatic rings (Bell et al., 2019). Al2O3 and Al(OH)3 nanoparticles appear on the MBC, as shown by the stretched Al––O bond in the nanocrystal at 1075 cm−1 (Ram, 2001). The CO32− ions are present at wavenumbers of 872 cm−1 (Fleet, 2009) and 1411 cm−1 (El-Gamal et al., 2017) because Al3+ions promote the generation of aluminum carbonate compounds. Thus, it can be said that the functional groups contain abundant oxygen on the MBC surface. The presence of negatively charged oxygen functional groups indicates the presence of a positively charged ion, Al3+, on the MBC surface. Al3+is the key factor for better PO43--adsorption properties of the MBC than that of the unmodified biochar.
The morphologies of the MBC are shown in Fig. 2b. A rough and rod- shaped surface predominated the MBC on a large nanoscale. The nanostructured modified biochar surface helps prevent the aggregation Fig. 2. The FTIR spectrum (a) and SEM micrographs (b) of the MBC.
Fig. 3.Effect of inlet PO43− strength on the breakthrough curve (H =3 cm, Q = 6 mL/min).
of particles in the material structure, creating a greater density of adsorption centers and significantly improving the adsorption capacity, as well as the adsorption efficiency and stability in harsh adsorption environments (Manyangadze et al., 2020). The textural properties and pore structure were expressed through the parameters: SBET =255.85 m2/g, SLangmuir =346.76 m2/g, Smicro =82.40 m2/g, Sextenal =173.44 m2/g, Vpore =0.28 cm3/g, Vmicro =0.04 cm3/g, and Dpore =4.35 nm.
They demonstrated that nanostructures appeared on the MBC surface.
The pHpzc value of the MBC was 7.80.
3.2. Effect of operating conditions on phosphate adsorption by the column 3.2.1. The influence of PO43− concentrations
The breakthrough curves illustrate the effect of the inlet PO43− con- centration on the PO43− adsorption by the column. Fig. 3 shows the breakthrough curves of the varying inlet PO43− strengths (25–100 mg/L) at a fixed height of 3 cm and flow rate of 6 mL/min. When the PO43−
concentration was increased, the saturation time and breakthrough point were shorter. The saturation times were 22 h, 18 h, and 6 h with the input PO43− strengths of 25 mg/L, 50 mg/L, and 100 mg/L, respec- tively. The column uptime decreased with the increasing adsorbate concentration. This trend occurred because when the column height and solution flow rate were fixed, the higher inlet PO43− concentration led to a more significant amount of adsorbate captured by the column.
Moreover, the diffusion of PO43− into the capillary pores of the MBC increased because the increased concentration gradient resulted in a reduced mass transfer (Gourdon and Casamatta, 1991).
This result is similar to the findings of Manjunath and Kumar (2021)
in a study on the co-sorption of antibiotics and nutrients using activated carbon from Prosopis juliflora. As the concentrations of the adsorbates, such as metronidazole, phosphate, or nitrate, were increased from 25 to 100 mg/L, the volume of solution treated, breakout time, activity time, and saturation time of the column were all reduced. Similarly, Pan et al.
(2019) studied perchlorate adsorption using magnetic biopolymer resins and reported that the breakthrough volume decreased from 150 to 0 mL with increase in NO3− concentration from 50 to 100 mg/L.
3.2.2. The influence of the bed height
The height of the MBC layer was altered to determine the change in the P adsorption capacity of the column, thus identifying the optimal bed height. The effects of the MBC layer heights of 1.5, 3, and 4.5 cm on PO43− adsorption in the column with a constant inlet strength (50 mg/L) and flow into the column (6 mL/min) are shown in Fig. 4.
The increase in the MBC layer height led to an increase in the breakthrough and saturation times; therefore, the PO43− removal per- centage improved (Table 2 and Fig. 4). As the height of the MBC layer expanded from 1.5 to 3 and 4.5 cm, the saturation time increased from 4 h to 15–23 h. This trend was similar to that of the adsorption capacity (qtotal). The qtotal was 49.79, 145.53, and 254.50 mg at column heights of 1.5, 3.0, and 4.5 cm, respectively. Taking the MBC height at 1.5 cm for comparison, when the height was increased by two and three times, the qtotal value increased 2.2 and 5.1 times, respectively. This indicates that the height of the adsorbent in the packed column is proportional to the phosphate adsorption efficiency. This is because a greater MBC layer height results in more active adsorption sites. Increasing the height of the adsorbent layer led to a longer retention time of the adsorbate, a Fig. 4. Effect of column height on the breakthrough curve of the PO43− ions
sorption on the MBC (C0 =50 mg/L, Q =6 mL/min).
Table 2
Calculation results of process parameters for PO43− sorption on the MBC-packed bed column.
Operating conditions Adsorption parameters
H (cm) Q (mL/min) C0 (mg/L) tb (hour) ttotal (hour) Vb (mL) V total (mL) qtotal (mg) mtotal (mg) E (%)
3 6 25 6.5 19 2340 6840 89.06 171 52.08
3 6 50 6 15 2160 5400 145.53 270 53.90
3 6 100 – 6 – 2160 41.36 216 19.15
1.5 6 50 0.5 8 180 2880 49.79 144 34.58
3 6 50 6 15 2160 5400 145.53 270 53.90
4.5 6 50 9 23 3240 8280 254.50 414 61.47
3 6 50 6 15 2160 5400 145.53 270 53.90
3 12 50 1.2 8 864 5760 96.91 288 33.65
3 18 50 – 5 – 5400 61.67 270 22.84
Fig. 5.Effect of flow rate (C0 =5 mg/L, H =3 cm) on PO43− sorption break- through curve.
larger volume of solution to be treated, and a longer operating time of the column. Consequently, the column tended to become saturated more slowly. Manjunath and Kumar (2021) investigated the effect of changes in column height on the adsorption of metronidazole, PO43−, and NO3−
using activated carbon derived from Prosopis juliflora. It was concluded that increasing the column height from 5 to 15 cm (the inlet adsorbate concentration and inflow rate were kept constant) resulted in an in- crease in the time to reach saturation and the removal efficiency. A similar result was reported by Mekonnen et al. (2021a) for the removal of PO43− using leftover coal.
3.2.3. The effect of the flow rate
The influence of different flow rates on the PO43− sorption by the MBC-packed bed column is illustrated by the breakthrough curves in Fig. 5. These experiments were carried out by varying the flow rates from 6 to 18 mL/min while maintaining an invariable inlet PO43− solu- tion concentration (50 mg/L) and MBC layer height (3 cm). The increase in the flow rate of the PO43− solution in the packed column (6–12 and 18 mL/min) reduced the breakthrough time (from 6 to 1.3 and 0 h).
Similarly, the saturation time of the column was shortened from 18 to 9 and 6 h, respectively.
The calculated results in Table 2 show that an increase in the flow rate led to an increase in the amount of adsorbate provided to the col- umn. However, the actual adsorption capacity of the column and the percentage of adsorbed PO43− were reduced. These phenomena are due to the increasing inlet strength, which enlarges the adsorbate mass
through the column to the capillaries (i.e., adsorbent). Because the active sites on the adsorbent surface were rapidly saturated, the break- through time was reduced (Oguz and Ersoy, 2014). In addition, with an increase in the flow rate, the adsorbate had less time to diffuse from the surface into the biochar capillaries (Manjunath and Kumar, 2021), reducing the external resistance of the adsorbent surface (Cavas et al., 2011) and enhancing the mass transfer time for the adsorption of PO43−
on the adsorption sites. Furthermore, an increased adsorbent layer height implies an enlarged adsorbent mass in the column, thereby increasing the surface area and binding sites (Oguz and Ersoy, 2014).
3.2.4. Kinetic study of the column adsorption
It is assumed that the PO43− adsorption capacity of the MBC modeled using Adams–Bohart exhibits the same trend between the residual vol- ume and the initial PO43− concentration. Figs. 6 and 7 and Table 3 pre- sent the results of fitting the kinetic models to the experimental data for PO43− adsorption. Table 3 indicates that the No values increased, while kAB decreased with increasing the column height from 1.5 to 4.5 cm.
However, kAB and No exhibited opposite trends with an increase in the PO43− strength or flow rate. Thus, when the flow rate or initial concen- tration of the PO43− solution through the column was increased, the sorption column quickly became saturated, and the adsorption capacity was diminished. This occurred because there was an increase in the adsorption positions when the adsorbent layer height was increased, and the adsorbate concentration increased proportionally to the mass transfer kinetics by prolonging the contact time between the PO43− ions Fig. 6. Experimental and calculative breakthrough curves based on Bohart–Adams model under various conditions: various PO43− strengths, column height =3 cm, flow rate =6 mL/min (a); different bed heights, inlet PO43− strength =50 mg/L, flow rate =6 mL/min (b); different flow rates, inlet PO43− strength =50 mg/L, column height =3 cm (c).
and MBC (Gouran-Orimi et al., 2018). Meanwhile, the increased flow rate contributed to the mobility of the PO43− ions and reduced the retention time of these ions in the column, increasing the kAB value as the flow scale was increased. The experimental data of PO43− adsorption onto MBC were consistent with those predicted by the Adams–Bohart model with R2 >0.9.
The optimal column conditions from previous phosphate adsorption studies were relatively different. In a phosphate adsorption column fil- led with biochar electrochemically modified with calcium–alginate beads (1 cm in diameter), Jung et al. (2017a) revealed that the highest efficacy was achieved at a layer height of 15 cm, a flow rate of 2.5
mL/min, and an initial phosphate concentration of 30 mg/L. A coal-packed column with particle sizes of 0.5–2.5 mm and 2.6 cm in diameter achieved the largest adsorption capacity at the adsorbent height of 8 cm, flow rate of 1 mL/min, and concentration of 25 mg/L (Mekonnen et al., 2021a). The optimal height of the adsorption material layer in this study was lower than that in prior studies. This is because the particle size of the materials used in our study was small (less than 0.25 mm), which caused flow obstruction when operating at a height, unlike the aforementioned studies.
When assuming that the decrease in the adsorption of PO43− onto MBC is proportional to the absorptivity potential of the MBC and the Fig. 7. Experimental and calculative breakthrough curves based on Yoon–Nelson model under different conditions: different inlet PO43− strengths, bed height =3 cm, flow rate =6 mL/min (a); different bed heights, inlet PO43− strength =50 mg/L, flow rate =6 mL/min (b); and different flow rates, inlet PO43− strength =50 mg/L, bed height =3 cm (c).
Table 3
Predicted parameters of Adams–Bohart and Yoon–Nelson models for PO43− adsorption.
Experimental conditions Adams-Bohart Yoon-Nelson
H (cm) Q (mL/min) C0 (mg/L) kAB (x10−4) (L.mg-1.min−1) N0 (x103) (mg/L) R2 KYN (x10−3) (1/min) τexp (min) τcal (min) R2
3 6 25 1.46 14.84 0.98 7.29 600 621 0.98
3 6 50 3.26 11.25 0.99 16.30 463 471 0.99
3 6 100 6.11 1.34 0.98 30.88 80 56 0.98
1.5 6 50 3.69 1.59 0.97 18.47 68 66 0.97
3 6 50 3.26 11.25 0.99 16.30 463 471 0.99
4.5 6 50 1.43 20.19 0.99 7.13 836 845 0.99
3 6 50 3.26 11.25 0.99 16.30 463 471 0.99
3 12 50 4.18 3.54 0.98 20.92 139 148 0.98
3 18 50 6.14 1.33 0.98 30.68 51 56 0.99
capability breakthrough of PO43−, Table 3 indicates that KYN declined with increase in the MBC layer height and KYN increased with increase in the PO43− concentration or flow rate. As explained above, the higher the adsorbent layer height, the lower the mass transfer rate owing to the increase in residence time, thus resulting in a diminished KYN. In contrast, the higher the flow rate and strength of the adsorbate, the greater the mobility of the adsorbate, thereby leading to an increased KYN (Gouran-Orimi et al., 2018). The τ value increased with increasing the MBC layer height and decreased with increasing the flow rate or inlet PO43− strength. The trial τ and modeled τ values of the Yoon–Nelson model were almost identical. Moreover, R2 > 0.9 indicates that the Yoon–Nelson model is suitable for forecasting the experimental data of PO43− adsorption onto the MBC-packed bed column.
3.3. Potential application and future research
The results of this work indicate that the MBC-packed column is a useful method to adsorb PO43− from water. This also demonstrates that a fixed-bed column using MBC can be accomplished, which overcomes the disadvantages of the batch adsorption process. The best adsorption conditions were found to be an inlet PO43− of 50 mg/L, injection speed of 6 mL/min, and material height of 4.5 cm. The operating conditions of the MBC-packed absorption column can be used to extend the pilot-scale phosphate adsorption experiments. Furthermore, the initial results of this study may benefit the scale-up of phosphate treatment systems. In addition, the outcome of the kinetic models can be used to evaluate and predict PO43− sorption on the MBC-packed bed column.
In terms of the material structure, it was evident that when the material was crushed, it readily blocked the flow, particularly with a high layer height. However, in this case, the No value was enhanced because of the extended contact surface. The phosphate adsorption column in this study using MBC produced an No value of 20.19 x 103 mg/
L, which is higher than those of many studies with different material structures, such as biochar electrochemically modified with cal- cium–alginate beads with an No of 2104 mg/L (Jung et al., 2017b), leftover coal with an No of 2602 mg/L (Mekonnen et al., 2021b), and magnesia–pullulan with an No of 11,178 mg/L (Ye et al., 2019). Thus, adsorption columns using mixed materials can help reduce the flow blockage and increase the amount of MBC in the column. This will contribute to increasing the efficiency and practical applications of adsorption columns in wastewater treatment. Furthermore, a pilot-scale experiment on phosphate treatment using MBC or mixed materials with the proposed operating parameters is recommended.
4. Conclusion
This study demonstrated that MBC can be an effective adsorbent for removing PO43− from solutions in a lab-scale packed bed. The PO43−
sorption capacity was improved by increasing the bed height and reducing the PO43− concentration and flow rate. The best adsorption conditions were found to be an inlet PO43− strength of 50 mg/L, an in- jection speed of 6 mL/min, and a material height of 4.5 cm. The kinetic models of Bohart–Adams and Yoon–Nelson are suitable for evaluating the PO43− sorption on the MBC-packed bed column.
Author contribution statement
T. Cuc Phuong Tran: Conceptualization, Writing - original draft, T.
Phuong Nguyen: Data curation, Formal analysis, X. Cuong Nguyen:
Conceptualization, Supervision Methodology, Writing - review & edit- ing, X.H. Nguyen: Writing - review & editing, T. An Hang Nguyen: Data curation, Formal analysis, T. Thao Nguyen: Data curation, Formal analysis, T. Yen Binh Vo: Writing - review & editing. T. Hoai Giang Nguyen: Formal analysis, Writing - review & editing, T. T. Huyen Nguyen: Data curation, Formal analysis, T. Dieu Hien Vo: Writing - re- view & editing, Senthil Kumar P: Funding acquisition, Writing - review
& editing, MyoungJin Umg: Funding acquisition, Conceptualization,
Writing - review & editing, D. Duc Nguyen: Conceptualization, Super- vision Methodology, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
This research was funded by Hue University, Vietnam, under the grant number DHH 2019-13-07. In addition, T. Thanh Huyen Nguyen was funded by Vingroup JSC and supported by the Master’s, PhD Scholarship Program of Vingroup Innovation Foundation (VINIF), Institute of Big Data, code VINIF.2021. ThS.74.
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