Metronidazole removal in powder-activated carbon and concrete-containing graphene adsorption systems: Estimation of kinetic, equilibrium and thermodynamic parameters and optimization of adsorption by a central composite design.
Summary (4 min read)
- Pharmaceuticals constitute a diverse group of organic compounds and are considered to be one of the most important emerging contaminants in the recent years.
- Antibiotics and/or their metabolites were found to have longer stability or half-life in aqueous and soil environments.
- Due to the increasing interest in the removal of antibiotics from aqueous systems, several researchers explored the possibility of MNZ removal by electrochemical process, photocatalysis, electro-catalytic reduction, membrane process, Fenton process and coupled electro-reduction-biological treatment, etc.
- The disposal of construction and demolition wastes has become one of the biggest concerns throughout the world, as it is generated in huge quantities.
- The application of concrete prepared with the addition of graphene for water purification especially for antibiotics' removal has not been investigated.
2.1. Preparation of adsorbent
- PAC was supplied by Merck, India, and it was used without any modification.
- CG (2%w/w, 1– 2 mm size) was obtained from the Building Technology and Construction Management laboratory of IIT Madras, India.
- Prior to the adsorption experiments, the CG specimen was crushed, and the particles retained between ASTM sieve Nos. 8 and 16 were collected and washed several times with tap water followed by distilled water.
- Subsequently, the particles were air-dried and stored in an airtight container for further use.
- The surface morphology of the adsorbents before and after adsorption was analyzed by using a scanning electron microscope (GENESIS-2100 SEM, EmCrafts, South Korea).
2.2. Preparation of adsorbate
- MNZ (C6H9N3O3) of analytical grade supplied by Sigma-Aldrich was used for the preparation of stock solution (1,000 mg/L).
- The prepared stock solution was placed in a volumetric flask, sealed and stored in a refrigerator.
- At regular time intervals, the samples were withdrawn from the flasks and analyzed for MNZ concentration.
- The experiments were conducted for a pseudo-equilibrium time obtained from the kinetic study.
- Using the experimental data, MNZ adsorption capacity qe (mg/g) at equilibrium was calculated as given in Eq. (1): where C0 and Ce are MNZ concentrations at the start of the experiment and at equilibrium (mg/L), respectively.
2.4. Quantification of MNZ concentration
- After the adsorption study, an aliquot of MNZ solution was collected, centrifuged and filtered.
- Subsequently, the filtered sample was analyzed using high-performance liquid chromatography (HPLC) fitted with a UV–Vis variable wavelength detector (Dionex UltiMate 3000).
- The column was operated at a reverse phase mode at a wavelength of 254 nm using acetonitrile:water (60:40) as a mobile phase.
- The pump was operated at a flow rate of 1 mL/min.
- The HPLC analysis results were used to calculate the MNZ removal as per Eq. (2):.
2.5. Determination of rate constants
- The rate of adsorption was determined using the adsorption equilibrium data and equilibrium models (i.e., the pseudo-first-order and pseudo-second-order models).
- The increase in time reduces the distance of equilibrium, while the distance disappears at equilibrium, i.e., qe qt = 0..
- Equation (4) depicts the expression for the pseudo-first-order model in a linearized form: where qe and qt are adsorption capacities (mg/g) at equilibrium and at various times (t), respectively.
- K1 is the pseudo-first-order rate constant (min−1) that was obtained by plotting a graph of ln(qeqt) versus t. 5 Pseudo-second-order model.
- The rate of pseudo-second-order reaction depends on the amount of adsorbate adsorbed on the adsorbent.
2.6. Adsorption isotherm
- An adsorption isotherm represents the relationship between the amount of adsorbate adsorbed onto the surface of an adsorbent and the concentration of adsorbate in the solution at a constant temperature under the equilibrium condition..
- The Langmuir isotherm model assumes that the adsorption is monolayer and takes place at specific homogeneous sites on the adsorbent.
- The Langmuir isotherm is shown in Eq. (7): Slope and intercept of the plot ln(qe) vs ε2 give KDR constant related to the mean free energy of adsorption (mol2/kJ2) and maximum adsorption capacity (qm), respectively, where ε indicates the Polanyi potential given by Eq. (16).
- Equations (18) and (19) represent the Elovich isotherm model and its linearized form, respectively.
2.7. Estimation of thermodynamic parameters
- Adsorption thermodynamics play a vital role in understanding adsorption mechanisms, i.e., physisorption or chemisorption.
- Thermodynamic parameters such as change in free energy ΔG0), enthalpy (ΔH0) and entropy (ΔS0) were used to determine the spontaneity, heat of change and randomness in the adsorption of MNZ using Eqs. (20)–(22), respectively: ΔG0 is related to the change in enthalpy and entropy given by Eq. (21):.
- The value of kc can be calculated using Eq. (23): The slope and intercept of the plot ln(kc) versus (1/T) give ΔH0 and ΔS0, respectively.
- To determine the thermodynamics' parameters, batch adsorption studies were carried out at 10 mg/L MNZ concentration, at pH 7 and under three different temperatures (i.e., 293, 303 and 313 K).
2.8. Optimization of adsorption conditions by a central composite design
- The effects of pH (x1), temperature (x2) and adsorbent dosage (x3; either CG or PAC), i.e., effect of independent variables, on MNZ adsorption were evaluated by using a CCD.
- A three-factor full factorial CCD was constructed using Minitab 16 and the design is shown in Table 1.
- The effect of independent variables on MNZ adsorption was evaluated by calculating MNZ removal (Y1; in %) and Gibbs free energy of adsorption Y2; in J/mol).
- Using the experimental design, the experiments (Runs 1–20) were conducted in a batch mode with continuous shaking at an initial MNZ concentration of 10 mg/L for predetermined equilibrium time.
- The center point was repeated six times to ensure the reproducibility of the experimental outcomes.
2.9. Desorption study
- MNZ desorption from the adsorbents was evaluated using the adsorbents recovered (after carefully decanting the supernatant) at the end of the experiments from the equilibrium study.
- The flasks containing the adsorbents recovered from various systems (i.e., initial MNZ concentrations of 1, 5, 10, 50 and 100 mg/L) were added with 100-mL distilled water and kept in a temperature-controlled incubator shaker at 25°C for 24 h.
- The supernatant was withdrawn from flasks at the end of 24 h and analyzed for MNZ concentration.
- The difference in MNZ concentration, i.e., MNZ adsorbed in the adsorbents and in the supernatant after 24 h, was considered as an irreversible portion of MNZ from the adsorbent due to chemisorption.
3. Results and discussion
- 1. Effect of contact time and kinetics of MNZ adsorption 9.
- In the PAC system, the calculated qe was found to be 9.88 mg/g at the end of 5 min and no significant change was observed thereafter.
- From the plots and Table 2, it was observed that the pseudo-second-order model was found to fit well the data of kinetic study.
- This shows that MNZ adsorption on both PAC and CG follows the second-order kinetics (R2 of 1.00 and 0.997 for PAC and CG, respectively).
- On the other hand, the coefficient of determination for the pseudo-first-order model 10 was found to be very less for both PAC (0.368) and CG (0.264) adsorption systems.
3.2. Equilibrium study
- The MNZ adsorption in PAC was fairly well fitted to all the isotherms (R2 > 0.9), whereas the Freundlich isotherm was found to be more suitable to estimate the maximum MNZ adsorption capacity on CG (Supporting information, 4S–8S).
- On the other hand, the value, i.e., R2, was higher for the Freundlich isotherm for CG systems.
- The constant B obtained from the Temkin isotherm shows that heat of adsorption was higher for PAC (13.65) compared to CG (3.12).
- The aforementioned observations, i.e., values of , RL and E, indicate that chemisorption was responsible for MNZ adsorption on both PAC and CG.
3.3. Outcomes of CCD experiments
- 3.1 Effect of pH, temperature and adsorbent dosage on MNZ removal.
- The outcomes of CCD experiments are shown in Table 1.
- It can be observed that the increase in adsorbent dosage at any pH range shows an increase in MNZ removal (Figs. 3a and 4a).
- A second-order quadratic model was obtained using the CCD data as shown in Eqs. (24) and (25), which gives the empirical relationship between the independent variables (x1, x2 and x3) and the MNZ removal (dependent variable, i.e., Y) for PAC and CG, respectively.
- These equations can be used to calculate a set of combinations of x1, x2 and x3 for a predetermined Y value, which will be useful in real-time operations.
- In Figure 5c, ΔG0 was more negative at 40°C and adsorbent dosage of 1,000 mg/L in the PAC system, whereas ΔG0 decreased as adsorbent dosage decreased and temperature showed much less significant effect on DGo value in the CG system (Fig. 5d).
3.4. Correlation of MNZ adsorption and thermodynamic parameters
- The correlation between temperature and MNZ removal was analyzed by estimating the thermodynamic parameters using Eqs. (20)–(23).
- The thermodynamic parameters can also be calculated based on the Langmuir constant for organic compounds with weak charges like MNZ as suggested by Liu..
- The negative ΔG0 value for PAC indicates that the adsorption process is thermodynamically feasible and spontaneous in nature, whereas the positive ΔG0 value for CG indicates the nonspontaneous nature of MNZ adsorption.
- The positive ΔH0 value for both PAC and CG indicates that the adsorption process is endothermic in nature.
- The adsorption process in the solid–liquid system is a combination of two processes, i.e., desorption of previously adsorbed water molecules from adsorbent surface and adsorption of adsorbate species.
3.5. Desorption study
- The desorption study helps to explain the mechanism of adsorption process.[48–50].
- An adsorbate's weak association with the adsorbent could be removed by water (i.e., universal solvent), which is an indication of weak bonds during the adsorption process.
- Figure 6 shows the percentage of MNZ desorbed from the surface of PAC and CG after 24 h.
- A maximum of 3.5% and 1.7% MNZ desorption was observed in the PAC and CG systems, respectively.
- Strong acidic solvents (HCl, H2SO4 and HNO3), strong basic solvents (NaOH) and organic acids (CH3COOH) may promote the MNZ recovery and the regeneration of adsorbents.
- Batch-mode MNZ adsorption experiments were carried out using PAC and CG.
- Adsorption of MNZ on both PAC and CG followed the pseudo-second-order kinetic model.
- The increase in the system's temperature (20–40°C) ensured complete removal of MNZ at a much lower dosage of PAC.
- On the other hand, the MNZ removal was non-spontaneous and endothermic on CG.
- As a whole, PAC and CG with more graphene content could be useful in treating water and wastewater containing MNZ and other similar compounds.
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Frequently Asked Questions (15)
Q1. What have the authors contributed in "Metronidazole removal in powder-activated carbon and concrete- containing graphene adsorption systems: estimation of kinetic, equilibrium and thermodynamic parameters and optimization of adsorption by a central composite design" ?
Metronidazole ( MNZ ) removal by two adsorbents, i. e., concrete-containing graphene ( CG ) and powder-activated carbon ( PAC ), was investigated via batch-mode experiments and the outcomes were used to analyze the kinetics, equilibrium and thermodynamics of MNZ adsorption.
Q2. Why are construction and demolition wastes a major concern?
Due to rapid and excessive urbanization, construction and demolition wastes have become a major concern in the context of urban solid waste management.
Q3. What is the main reason for the use of graphene in wastewater treatment?
In the recent years, graphene and graphene composites have been used as adsorbents to treat water and wastewater containing heavy metals, organic dyes and antibiotics.
Q4. What is the chemical composition of MNZ?
MNZ has very high solubility (9.8 g/L) and molecular diffusivity (8.48 × 106 cm2/s) in water, and is expected to be highly mobile in aqueous systems.
Q5. What solvents can promote the MNZ recovery?
strong acidic solvents (HCl, H2SO4 and HNO3), strong basic solvents (NaOH) and organic acids (CH3COOH) may promote the MNZ recovery and the regeneration of adsorbents.
Q6. Why was it reported that construction and demolition wastes were dumped illegally?
it was reported that the wastes were dumped illegally on land or in natural drainages in most developing countries due to shortage of space for dumping.
Q7. what is the qe of the equilibrium adsorption capacity?
Where qe is the equilibrium adsorption capacity (mg/g), qm is the Langmuir isotherm constant representing monolayer adsorption capacity (mg/g), Ce is the equilibrium concentration of MNZ in the solution (mg/L) and KL is the Langmuir constant.
Q8. What is the slope and intercept of the plot ln(qe/Ce)?
The slope and intercept of the plot ln(qe/Ce) versus qe give maximum adsorption capacity (qm) and Elovich equilibrium constant (KE in L/mg), respectively:Adsorption thermodynamics play a vital role in understanding adsorption mechanisms, i.e., physisorption or chemisorption.
Q9. What is the main reason why construction and demolition wastes are being used in the wastewater treatment process?
this process has been used extensively in the wastewater treatment process owing to its ease in operation, low cost, absence of by-product formation, regeneration potential and sludge-free operation when compared to other treatment methods.
Q10. What is the way to calculate the MNZ removal?
The suitable operational conditions/combinations, i.e., adsorbent dosage, temperature and pH, for almost complete MNZ removal by PAC were analyzed using the response optimizer function in Minitab.
Q11. How much graphene could be used in treating water and wastewater containing MNZ?
As a whole, PAC and CG with more graphene content could be useful in treating water and wastewater containing MNZ and other similar compounds.
Q12. What is the importance of comparing modified concrete with graphene?
At the same time, it is essential to compare the performance of modified concrete with graphene with commercially available adsorbents including PAC which has huge importance in the application of modified concrete for water purification applications.
Q13. What are some examples of graphene oxide membranes for the removal of trace and gross organic?
To mention a few, iron– aluminum oxide–graphene oxide composite for fluoride removal, graphene oxide for removing diclofenac and sulfamethoxazole antibiotics, polysaccharide-modified graphene oxides for the removal of cationic dyes (Methylene blue, Rhodamine 6G) and anionic dyes (Orange II, Acid fuchsin), and graphene oxide membranes for Cu2+, Cd2+ and Ni2+ removal are some examples.
Q14. What is the common method of removal of phosphorous from water and wastewater?
Several investigations in the past reported the application of cementitious materials in the removal of various pollutants from water and wastewater including phosphorous by composite cement mortars; fecal coliforms and phosphorous by pervious geopolymer concrete; p-chloronitrobenzene by the cementitious catalytic membrane with ozonation;
Q15. How much MNZ removal was observed in the kinetic study?
On the other hand, only 86.7% of MNZ removal was observed in the kinetic study when the experiment was conducted at 10 mg/L MNZ, pH 7 and at 25°C with 1,000 mg/L PAC dosage.