- Research article
- Open Access
Removal of tetracycline from wastewater using pumice stone: equilibrium, kinetic and thermodynamic studies
© Guler and Sarioglu; licensee BioMed Central Ltd. 2014
- Received: 2 April 2013
- Accepted: 21 April 2014
- Published: 1 May 2014
In this study, pumice stone was used for the removal of tetracyline (TC) from aqueous solutions. It was characterized by XRD, FT-IR, SEM and BET analyses. Cation exchange capacity of pumice stone was found to be 9.9 meq/100 g. Effect of various parameters such as solution pH (2–11), adsorbent dosage (0.5-10 g/L), contact time (2.5-120 min), initial TC concentration (5–300 mg/L) and temperature (20–50°C) on TC adsorption onto pumice was investigated. Also the adsorption of TC on pumice stone was studied as a function of Na+ and Cu2+ cations changing pH from 2 to 11 using batch experiments. The best removal efficiency performance was exhibited at adsorbent dosage 10 g/L, pH 3, contact time 120 min. Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models were applied to the equilibrium data. The result has shown that the adsorption was favorable, physicochemical in nature and agrees well with Langmuir and Freundlich models. The maximum Langmuir adsorption capacity was found to be 20.02 mg/g. The adsorption behavior of TC on pumices stone was fitted well in the pseudo-second order kinetics model. Thermodynamic parameters calculated from the adsorption data at different temperature showed that the adsorption reaction was feasible, spontaneous and exothermic.
- Pumice stone
Antibiotics are used the worldwide in human and veterinary medicine for about 70 years [1–3]. TCs are the second most common antibiotic family in both production and usage in the world . TCs are relatively poorly absorbed by humans and animals. Large fractions of antibiotics including TC are excreted through urine and feces as unmodified main compound [3, 5–9].
Residues of TC are frequently detected in soil and various environmental water samples such as surface water, ground water and drinking water [1, 3, 5, 10–12]. The presence of residual antibiotics in soil and water is potentially hazardous for the bacteria and non-target organisms and these can promote the selection of genetic variants of microorganisms resulting in the occurrence of antibiotic resistant pathogens [13–17].
TC molecule is strongly polar and has three protonactive groups: a dimethylamine (C-4), a tricarbonylamide group (C-1:C-2:C-3) and a phenolic diketone groups (C-10:C-11:C-12) (Figure 1) [3, 9, 20, 21].
Many adsorbent has been used for TC removal from wastewaters; in this study the pumice stone was used for TC removal from wastewaters with comprehensive research.
TCs are highly adsorbed by several materials such as clay, montmorillonite, rectorite, palygorskite, chitosan particles, oxide minerals, humic substances, soil, activated carbon and sediments [3, 7, 15, 17, 20, 22–29].
Pumice is a light, porous and volcanic stone with a large surface area. It is generally pale in color, ranging from white, cream, blue or grey, to green-brown or black  and used as an adsorbent, filter bed and support material in water and wastewater treatment [31–33].
In this study, effects of solution pH, adsorbent dosage, temperature, Na+ and Cu2+ cations on the TC adsorption by pumice stone were investigated. The adsorption isotherms, kinetics and thermodynamics were studied by batch experiments. In addition, the experimental results were combined with XRD, FT-IR and SEM analyses in order to identify in the interaction between pumice stone and TC and the specific surface area and components of pumice stone were determined by BET and XRF analyses, respectively.
Properties of pumice stone
Specific surface area (m2g−1)
Total pore volume (cm3g−1)
Mean pore diameter (nm)
CEC (meq/100 g)
Light grey/brown granular
XRF analysis of pumice stone
% by weight
% by weight
Loss on ignition
Aim of experiment
TC conc. (mg/L)
Adsorbent dosage (g/L)
Contact time (min)
Effect of solution pH
Room temperature (20°C)
Effect of adsorbent dosage
Room temperature (20°C)
Room temperature (20°C)
Adsorption equilibrium tests
Room temperature (20°C)
Effect of temperature
Effects of 0.01 M Na+ and 0.1 mM Cu2+
Room temperature (20°C)
where C o and Ce are the initial and the equilibrium TC concentration (mg/L), V is the volume of solution (L) and m is the amount of pumice stone (g).
According to the result of XRD pattern of the pumice stone, there were three peaks at 22, 28 and 59° (Figure 2). These peaks related to the mineral dachiardite (Ca, Na, K, Al, Si, H2O). The 100 peak of the quartz is known to be at 2θ = 26.65°. This finding points out amorphous quartz substance . Similar results were also observed with the studies of other researchers [34–36]. The peaks of TC-P were observed at 28 and 31°. The changes of peaks are evidence of TC adsorption. As can be seen from XRF results, the two most important components of the pumice stone were SiO2 (69.27%) and Al2O3 (14.24%) (see Table 1). It can be concluded that the higher the silica percentage, the purer the pumice stone will be .
The most characteristic peaks of pumice stone and TC-P are 800, 1700 and 3500 cm−1. The peak at 800 cm−1 in FT-IR spectra of the pumice stone (Figure 3) may have resulted from the Si-O bending strength vibrations of the amorphous quartz. The peak at 1700 cm−1 can be assigned to amide I or C = O amide stretching and the amide group played an important role in TC adsorption. The other peak was observed around 3500 cm−1. This peak indicates the OH stretching vibrations of the adsorbed water (moisture) by the pumice stone from the outside environment [34, 37]. These peaks were also determined with prior studies [34, 37, 38].
As can seen be from FT-IR spectra of TC-P, the peak at 1700 cm−1 of TC-P were observed different from the natural pumice stone. The band changes of amide I or C = O amide groups at 1700 cm−1 demonstrated that TC was adsorbed to the pumice stone with cation exchange and surface complexation .
The SEM images of natural pumice stone and TC-P indicated that the surface of pumice stone had a large porous surface and external surface of pumice stone was covered by TC, respectively (Figure 4). In addition, the specific surface area, total pore volume and mean pore diameter of pumice stone were 11.88 m2g−1, 0.0410 cm3g−1 and 13.813 nm, respectively.
Effects of solution pH and Na+ and Cu2+ cations on adsorption
Adsorption mechanisms depending on pH values
Studied pH range
2.0 < pH < 3.0
3.0 < pH < 7.0
Cation exchange and surface complexation
The cation exchange mechanism between cations of TC and positively charged pumice stone surface was dominated at low pH values. A surface complexation mechanism for zwitter anion of TC was important, which was accompanied with proton uptake on pumice stone [19, 23, 42]. Physical mechanisms such as van der Waals forces attraction and hydrogen bonding between polar TC groups and acidic groups on the surface of pumice stone may also contribute to surface complexation mechanism in TC adsorption [22, 23]. The decrease in TC removal under alkaline conditions may be due to competition of excess hydroxyl ions with anion TC for active sites on pumice stone . The pHPZC values of pumice stone in literature ranged between 6.9 and 9.3 . The PZC value found in study is compatible with literature.
In order to determine the effects of Na+ and Cu2+ ions on the TC adsorption, 0.1 mM Cu2+ and 0.01 M Na+ cations were simultaneously adsorbed with TC at initial concentration of 50 mg/L. The results of TC adsorption by pumice stone in the presence of Na+ and Cu2+ was nearly similar. However, the presence of Cu2+ slightly increased TC adsorption on pumice stone at low pH (pH < 5). As a transitional metal cation, Cu2+ could form strong complexes both with TC and pumice stone [1, 39]. Therefore, Cu2+ may role as a bridging ion on TC adsorption. The presence of Cu2+ had only minor effects on TC adsorption on pumice stone at high pH due to the formation of CuOH and Cu(OH)2. Similar observations were reported on the TC adsorption onto soil and sediment in the presence of copper . On the other hand, the presence of Na cation decreased on TC adsorption. This condition can be explained as the results of an electrostatic competition between TC cations and Na+ cations for adsorption to the same binding sites on surface of pumice stone. Similar results were also observed by other researchers [3, 4].
The adsorption partition coefficient is expressed with respect to the CEC because interactions of the TC with specific charge sites on surface of the pumice stone were anticipated . The adsorption partition coefficients (KD) were relatively high at pH 2 and 3 and decreased at pH 4–9 and remained nearly constant between pH 9–11. The results of adsorption capacity (qe) and adsorption partition coefficients (KD) are compatible with each other and KD and qe values decreased with increasing pH value. In this case, the pH value was selected as 3 in further studies.
Effect of adsorbent dosage on adsorption
Kinetics of TC adsorption onto pumice stone
where qe (mg/g) and qt (mg/g) are the amounts of adsorbed TC on pumice stone at equilibrium and time t (min), C is the intercept and k 1 (min-1), k 2 (g/mg.min) and kd (mg/g.min0.5) are the rate constant of pseudo first-order kinetic model, pseudo second-order kinetic model and intraparticle diffusion model, respectively. A straight line of log (q e -q t ) versus t suggest the applicability of pseudo first-order kinetic model. q e and k 1 can be determined from the intercept and slope of the plot, respectively. The plot t/q t versus t should give a straight line if pseudo second-order kinetics are applicable and q e and k 2 can be determined from the slope and intercept of the plot, respectively. The intraparticle diffusion model rate constat (kd) and C can be evaluated from the slope and intercept of the linear plot of qt versus t0.5, respectively [44, 45]. According to this intraparticle diffusion model, the plot of qt versus t0.5 should be linear (C = 0) if intraparticle diffusion is involved in the overall adsorption mechanism and the intraparticle diffusion is the sole rate controlling step of the process. If this line did not through the origin (C ≠ 0), there are intraparticle diffusion and boundary layer effect in adsorption process. As the intercept value (C) increase, the effect of surface sorption in the rate controlling step increase .
Kinetic parameters of the TC adsorption
According to the fitted linear regression plots, the experimental data are well fitted to the pseudo-second order kinetic model with higher value correlation coefficient (R2 > 0.999) compared to pseudo-first order kinetic model. Kinetics of TC adsorption on pumice stone followed the pseudo-second order model, suggesting that the adsorption rate limiting step may be chemisorptions and the adsorption of TC occurs probably via surface complexation reactions at specific adsorption sites [20, 30, 47]. In addition, present study, the plots indicated that the intraparticle diffusion model was not the sole rate controlling step due to did not pass through the origin (C ≠ 0). This indicated that both intra particle diffusion and boundary diffusion affected the TC adsorption on pumice stone.
The value of RL indicates the type of the isotherm to be irreversible (RL = 0), favorable (0 < RL > 1), linear (RL = 1) or unfavorable (RL > 1).
where, k F (L/g) is the Freundlich adsorption constant related to the adsorption capacity. n is the adsorption intensity. The 1/n values were between 0 and 1 indicating that the adsorption was favorable at studied conditions.
If E value is between 8 and 16 kJ/mol, the adsorption process follows by chemical ion-exchange and if E < 8 kJ/mol, the adsorption process is likely physical adsorption.
Langmuir, Freundlich and D-R adsorption isotherm parameters of TC
The correlation coefficients of Langmuir (R2 0.978), Freundlich (R2 0.994) and D-R (R2 0.992) models well fitted the adsorption data. The TC adsorption is compatible with both Langmuir and Freundlich isotherms. The applicability of both Langmuir and Freundlich isotherms on the adsorption of TC onto pumice stone shows that adsorption occurs under monolayer and heterogeneous surfaces. From Langmuir model, the maximum adsorption capacity (Qm) was determined to be 20.02 mg/g. Using the Freundlich model, kF and 1/n were found to be 0.99 and 0.52, respectively. The 1/n heterogeneity value was between 0 and 1 indicating that the TC adsorption on the pumice stone was favorable at studied conditions.
The calculated RL values range between 0 and 1, indicating that the TC adsorption on pumice stone is favorable.
The mean free energy (E; kJ/mol) values calculated from the D-R isotherm model were also determined between 8 and 16 kJ/mol. It was indicating that the adsorption is mainly a chemical process occurs through cation exchange.
where ∆G is the Gibbs free energy change, R is the universal gas constant (8.314 J/mol K), T is the temperature (K) and K C (qe/Ce) is the equlibrium constant.
Thermodynamic parameters of TC adsorption on pumice stone
The value of ΔH and ΔG were negative, indicating that the TC adsorption process is feasible, spontaneous and exothermic. The positive value of ∆S shows the increased in randomness at the solid/liquid interface during the adsorption process.
The analysis of ions in aqueous solution after TC adsorption
The amounts of ions passing from pumice stone into TC solution
Total ion amount
According to Table 9, total ion amount in aqueous solution after TC adsorption at optimum conditions is 31.78 mg/L. The amount of adsorbed TC by pumice stone at optimum conditions is 37.09 mg/g. In this case, a major part of TC adsorption on pumice stone realized with cation exchange between ions (Ca2+, Mg2+, Na+, K+ and Fe3+) in pumice stone with TC antibiotics.
In this study, pumice stone was used as a new adsorbent for TC antibiotics removal from the aqueous solution. XRD, FT-IR and SEM data show that TC species adsorbed onto pumice stone. The adsorption mechanism of TC on pumice stone was cation exchange and surface complexation. The adsorption process is pH dependent and the optimum pH was found to be 3. The adsorption isotherm data for pumice stone could be fitted well by both Langmuir and Freundlich isotherm models. The maximum adsorption capacity (Qm) was found to be 20.02 mg/g. TC adsorption reached equilibrium within 90 min and adsorption kinetics fitted well to the pseudo-second order kinetic model. In the presence of Na+ cations, the TC adsorption decreased with increasing pH. The presence of Cu2+ was facilitated TC adsorption on pumice stone at low pH. Also, the TC adsorption onto pumice stone is found to be feasible, spontaneous and exothermic from thermodynamic studies.
The band changes of amide I or C = O amide groups in FT-IR spectra of TC adsorption on pumice stone was demonstrated that TC was adsorbed to the pumice stone with cation exchange and surface complexation.
The pumice stone was found to be an efficient adsorbent for the removal of TC antibiotics from aqueous solution. However, the effect of pretreatment for enhances the adsorption capacity of pumice stone can be examined in further studies.
We would like to thank Rresearch and Application Center of Erciyes University (TAUM), for their help in XRD, FTIR and SEM analysis.
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