Adsorption of 4th Generation Antibiotics using Graphene

Adsorption of 4th Generation Antibiotics using Graphene

A Combined Experimental and Theoretical Study

Thaiyal Nayahi.V. A. N Lavanya. M

Department of Environmental Engineering, Department of Environmental Engineering,

University College of Engineering (BIT Campus), University College of Engineering (BIT Campus), Tiruchirapalli, India Tiruchirapalli, India

AbstractThis study explores the removal of 4th generation antibiotics like Cefclidine, Cefepime, Cefoselis, Cefluprenam, Cefozopran, Cefpirome and Cefquinome which are considered as harmful pharmaceutical pollutants. Graphene is used as an adsorbent to remove these pharmaceutical compounds. In this, graphene was produced by electrochemical exfoliation method. This synthesised graphene was used as an adsorbent material for the removal of 4th generation antibiotics from the prepared synthetic pharmaceutical sample. The effects of contact time, concentration, pH and temperature were studied. The adsorption kinetics was modelled by pseudo first and second order kinetics, Elovich and Weber and Morris intraparticle diffusion models. The rate constants for all these kinetic models were calculated and the results show that the second order kinetic models were best fitted to model the kinetic adsorption of 4th generation antibiotics. The Langmuir, Freundlich, DR isotherm and Temkin models were applied to describe the equilibrium isotherms and the isotherm constants were determined. The adsorption was studied thermodynamically, and the Gibbs free energy change (G°), enthalpy change (H°), and entropy change (S°) were calculated. Thus the study indicates graphene could be a very efficient adsorbent for the removal of 4th generation antibiotics.

KeywordsRemoval; antibiotics; graphene; kinetics; isotherms; thermodynamics.

I.INTRODUCTION

Pharmaceutical pollutants are considered as a major impending deleterious pollution that contains different groups of human and veterinary medicinal compounds that are used extensively all over the world. Due to their very low concentrations they are impalpable, cause chronic effects on ecosystems and the totality of their impacts on the aquatic environment over the long term is difficult to predict. Pharmaceutical compounds are resistant to biological degradation and retain their chemical structure long enough to do their adverse effect [1]. The most frequently found pharmaceutical pollutants in the water are antibiotics, antacids, steroids, antidepressants, analgesics, and stimulants. Cephalosporins are considered as the most commonly used antibiotics. They are grouped into "generations" by their antimicrobial properties. In that, 4th generation antibiotics are considered as the upcoming pollutant. Many fourth generation cephalosporins can cross blood brain barrier and are effective in meningitis [2]. The

fourth generation includes: Cefclidine, Cefepime, Cefluprenam, Cefoselis, Cefozopran, Cefpirome and Cefquinome. Pharmaceutical pollution doesnt seem to be harming humans yet [3], but disturbing clues from aquatic life suggests, now is the time for preventive action. Some of the methods that are used to remove pharmaceutical compounds from water stream are constructed wetland, activated sludge treatment, photocatalytic oxidation or adsorption [4-7]. Of the above mentioned methods, adsorption is the most promising method for the removal of pollutants because both water and the adsorbent could be recycled, and no by-products would be produced [8]. Hence, scientists continuously search for new types of adsorbents which can remove the pollutants efficiently.

Graphene is a new fascinating carbon material that has engrossed the attention of scientist in recent years. It is a one atom-thick, two-dimensional (2D) layer of sp2- bonded carbon. Graphene also exhibits extraordinary properties, such as excellent mechanical, electrical, thermal, optical properties and very high specific surface area. Additionally, graphene has also been used as an excellent adsorbent for different pollutants due to its large surface areas, which can form strong interactions with other pollutants [9-13]. In this study, synthesised graphene was used to adsorb a group of 4th generation antibiotics.

The effects of different adsorption conditions were studied: contact time, solution concentration, pH and temperature. Additionally, the adsorption process was studied kinetically to predict the adsorption rate in order to understand the adsorption behaviour. Then the adsorption was studied thermodynamically to understand the mechanism of adsorption and its spontaneity by calculating different thermodynamic parameters.

1. MATERIALS AND METHOD

   1. Synthesis of graphene

      The electrochemical synthesis of graphene from graphite was done by electrochemical exfoliation method. During the exfoliation process the highly oriented pyrolytic graphite (HOPG; 3 cm x 3 cm x 0.5 mm) was employed as an electrode and source of graphene for electrochemical exfoliation. Two graphite electrodes were immersed into the 0.5 M sulphuric acid (sigma Aldrich 98%) solution. Both anode and cathodes was graphite electrode. The

      electrochemical exfoliation process was carried out by applying DC bias on graphite electrode (from +10 V). Due to anodic dissolution, a few layers of graphene were exfoliated from the graphite anode. By this way the exfoliated graphene sheets were collected with a 100 nm porous filter and washed with DI water by vacuum filtration. After drying, it was dispersed in DMF solution by gentle water-bath sonication for 10 min to remove the unwanted large graphite particles produced in the exfoliation. The suspension was subjected to centrifugation at 2500 rpm. The centrifuged suspension can be used for further adsorption experiments.

   2. Preparation of synthetic pharmaceutical sample

      Cefclidine, Cefepime, Cefoselis, Cefluprenam, Cefozopran, Cefpirome and Cefquinome were dissolved in distilled water for the required concentration (0.5-2.5 mg L-

      1) to obtain synthetic pharmaceutical sample for further experiments. The pH of the solution was adjusted to 7 using NaOH and HCl.

   3. Batch adsorption studies

      Batch adsorption experiments were conducted using 200 ml glass beaker with addition of 0.1 g of graphene and 100 ml of prepared synthetic pharmaceutical solutions for 7 compounds of concentration from 0.5 2.5 mg L-1. The glass beakers are then placed on a stirrer under a suspension of 200 rpm, 30°C for 90 minutes. Similarly, experiments were carried for different pH (1-13) and antibiotics concentration at 2.5 mg L-1. The concentration of antibiotic compounds in synthetic pharmaceutical solution was determined by measuring the solution at =230 nm using indirect UV method. To study the effect of temperature on adsorption, adsorption measurement was carried at different temperatures (303, 313, 323, 333, 343 and 353 K). Antibiotics uptake was calculated according to the following equation:

      Langmuir [20], Freundlich [21], D-R [22] and Temkin [23] models.

2. RESULTS AND DISCUSSION

1. Effect of pH

   The pH is one of the most important parameter that controls the adsorption efficiency and capacity. To examine this effect, a series of experiments were carried out using 2.5mg L-1 of 4th generation antibiotics containing synthetic pharmaceutical solutions. The relation between the initial pH of the solution and adsorption efficiency of pharmaceutical compounds is depicted in Fig. 1. The effect of pH on the removal of 4th generation antibiotics using 0graphene as an adsorbent was studied with nitial pH range from 1-13. The optimum 4th generation antibiotics adsorption was observed at pH range 6.08.0. We have observed that the percentage adsorption of 4th generation antibiotics increased appreciably with increase of pH from 1 to 8 and then the adsorption efficiency starts to decrease with increase in pH after 8 [14]. The lower adsorption of 4th generation antibiotics at alkaline pH might be due to the electrostatic repulsion of 4th generation antibiotics by the negatively charged graphene surface at high pH. The adsorption capacity increases with pH in the acidic range and reaches the maximum removal efficiency. This is found to be appropriate in all the 7 4th generation antibiotic compounds.

2. Effect of contact time and concentration

   In order to establish time dependence of 4th generation antibiotics adsorption under various concentrations, it is required to study the influence of contact time. From Fig. 2, it is clear that the adsorption of 4th generation antibiotics is increased with an increase in time and remains stable after the equilibrium time. The equilibrium time was 60 min for all the concentrations (0.5-2.5 mg L-1) and for all the 7 compounds.

   t

   q = (C0Ct).V

   W

   (1)

   Adsorption efficiency was calculated according to the following equation:

   Adsorption efficiency =

   CoCe × 100 (2)

   Co

   where qt is the amount (mg g -1) of antibiotics adsorbed at time t, C0 is initial concentration (mg L-1) of antibiotic compounds in aqueous solution, Ct is the concentration of antibiotic compounds at time t (mg L-1), V is the volume

   (L) of the adsorbate solution, and W is the weight (g) of dried graphene.

   After reaching the equilibrium time, the concentration of antibiotics in solution at equilibrium, Ce, was determined and the concentration in the solid phase, qe, was calculated using Equation (1). Kinetic experiment were modelled through pseudo first order equation [16] and pseudo second order [17] equation, Elovich [18] and Weber and Morris

   [19] equation. Experimental data were modelled through

   Fig. 1 The effect of initial pH on 4th generation antibiotics adsorption by graphene. Conditions: concentration= 2.5 mg L -1

   Fig. 2 The effect of agitation time and concentration of 4th generation antibiotics adsorbed on graphene.

   Conditions: concentration = 0.5-2.5 mg L 1; pH=7.0; temperature= 303 K

   The rapid adsorption is observed during the first 60 min. After 60 min, adsorption is almost found to be stagnant. This rapid initial adsorption was due to the availability of large graphene surface for antibiotic molecules at initial stages. This rapid adsorption decreases gradually until all graphene surfaces are occupied and a constant rate of adsorption is observed. The plots are single, smooth and continuous curves leading to saturation for all the 7 compounds.

3. Effect of temperature

   Temperature has an important effect on the adsorption process. As the temperature increases, rate of diffusion of adsorbate molecules across the external boundary layer and interval pores of the adsorbent particle increases [15]. Enhancement of the adsorption capacity of adsorbent (graphene) at higher temperatures may be attributed to the enlargement of pore size and/or activation of the adsorbent surface. The effect of temperature for adsorption of 4th generation antibiotics on graphene was premeditated at various temperatures (303, 313, 323, 333, 343 and 353 K). Fig. 3 illustrates that the adsorption capacity increases with increases in temperature and thus confirms the endothermic nature of adsorption process. The enhancement in uptake is attributed to better interaction between ions and adsorbent, creation of new adsorption sites and increased intraparticle diffusion at higher temperatures [15]. Therefore, the adsorption becomes more favourable in all the 7 compounds.

4. Adsorption kinetics model

   The studies of adsorption equilibrium are important in determining the effectiveness of adsorption; however, it is also necessary to identify the types of adsorption mechanism in a given system. In this study we used four different models to predict the adsorption kinetics of 4th generation antibiotics on graphene. In the present study, four kinetic models, namely, pseudo first order, pseudo second order, Elovich and Weber and Morris intraparticle diffusion models were examined to obtain the rate constants, equilibrium adsorption capacity and adsorption mechanism at different concentrations of all the 7 compounds of 4th generation antibiotics.

   Fig. 3 The effect of adsorption efficiency of 4th generation antibiotics on graphene at various temperatures (303, 313, 323, 333, 343 and 353 K).

   Conditions: concentration=2.5 mg L -1; pH=7.0.

   3.36

   		qe (exp) (mg g -1)	FIRST ORDER KINETICS			SECOND ORDER KINETICS
   			qe (Cal) (mg g -1)	K1 (min -1)	R2	qe (Cal) (mg g -1)	K2 (min.g mg -1)	R2
   	0.5	0.39	5.88	0.1124	0.5749	0.37	0.07733	0.9898
   	1.0	0.77	3.31	0.0713	0.4930	0.74	0.04678	0.9911
   Cefclidine	1.5	1.16	3.93	0.0612	0.4362	1.11	0.02257	0.9913
   	2.0	1.54	3.47	0.0401	0.3157	1.45	0.01138	0.9818
   	2.5	1.94	4.78	0.0559	0.3738	1.9	0.01584	0.9979
   	0.5	0.37	2.87	0.09476	0.5036	0.36	0.1071	0.9959
   	1.0	0.73	3.41	0.07719	0.5045	0.71	0.04806	0.9959
   Cefepime	1.5	1.085	3.31	0.06153	0.4352	1.125	0.03171	0.9997
   	2.0	1.46	4.37	0.05739	0.4182	1.41	0.02280	0.9963
   	2.5	1.815	3.58	0.05158	0.3232	1.81	0.03703	0.9941
   	0.5	0.415	3.69	0.09446	0.5565	0.4	0.04848	0.9801
   	1.0	0.82	5.87	0.09661	0.5131	0.79	0.06826	0.9929
   Cefluprenam	1.5	1.22	10.1	0.09617	0.4693	1.16	0.02465	0.9922
   	2.0	1.64	5.51	0.07279	0.4187	1.6	0.03540	0.9946
   	2.5	2.04	6.68	0.07176	0.3031	2.01	0.03465	0.9949
   	0.5	0.43	0.09666	0.5362	0.42	0.08485	0.9945
   	1.0	0.86	6.57	0.10635	0.5262	0.83	0.1033	0.9998
   Cefoselis	1.5	1.28	7.52	0.09875	0.4647	1.25	0.0559	0.9938
   	2.0	1.72	7.44	0.08667	0.4597	1.69	0.04925	0.9969
   	2.5	2.13	8.62	0.08302	0.4703	2.07	0.03415	0.9963
   	0.5	0.35	3.93	0.1052	0.5515	0.34	0.09589	0.9947
   	1.0	0.69	5.09	0.1029	0.5424	0.7	0.1069	0.9980
   Cefozopran	1.5	1.026	21.9	0.4118	0.5554	1.001	0.1107	0.9986
   	2.0	1.37	6.75	0.09742	0.5187	1.34	0.08652	0.9991
   	2.5	1.703	3.51	0.05769	0.4122	1.705	0.05517	0.9998
   	0.5	0.33	6.979	0.1232	0.55174	0.31	0.07	0.9716
   	1.0	0.667	9.82	0.1122	0.55249	0.63	0.04652	0.9892
   Cefpirome	1.5	0.981	11.47	0.107	0.54424	1.2	0.03947	0.9931
   	2.0	1.322	12.07	0.1025	0.49591	1.3	0.0303	0.9899
   	2.5	1.63	13.04	0.0991	0.47589	1.6	0.02467	0.9888
   	0.5	0.34	6.55	0.1273	0.57609	0.33	0.1553	0.9958
   	1.0	0.675	9.21	0.1141	0.54812	0.64	0.0635	0.9934
   Cefquinome	1.5	1.011	12.07	0.1051	0.5404	0.95	0.02896	0.9885
   	2.0	1.362	11.12	0.1008	0.5035	1.3	0.02521	0.9890
   	2.5	1.68	14.75	0.0961	0.49159	1.67	0.01626	0.9839

   Table 1 A comparison between the experimental and calculated qe values for different concentrations in first and second order adsorption kinetics of 4th generation antibiotics on graphene, temperature of 303 K and pH 7

   1. First order lagergren model.

      The first order Lagergren model is generally expressed as follows [16]:

      dt

      d qt = k1(qe qt) (3)

      where qe and qt are the adsorption capacities at equilibrium at time t (mg g -1) and the adsorption capacities at time t (min) respectively, and k1 (min) is a rate constant of first order adsorption. The integrated form of the above equation with the boundary conditions (t = 0 to t and qt = 0 to qt) is rearranged to obtain the following time dependence function:

      log (q q ) = log q k1 t (4)

      e t t

      2.303

      The experimental data were analyzed initially with the

      first order Lagergren model. The plot between log (qe – qt) vs. t should give the linear relationship from which k1 and qe can be determined by the slope and intercept, respectively (eqn (4)). The computed results are presented in Table 1. The results show that the theoretical qe (cal) value doesnt agree with the experimental qe (exp) values for all

      Fig. 4 Adsorption kinetics of 4th generation antibiotics adsorption by graphene for the pseudo-second order model.

      Conditions: concentrations = 0.5-2.5 mg L -1; pH = 7.0; temperature= 303 K

      concentrations and compounds studied and with a poor correlation co-efficient.

   2. Second order lagergren model

      The Lagergren second order kinetic model is generally expressed as follows [17]:

      dt

      dqt = k2 (qe qt)2 (5) where k2 is rate constant of second order adsorption.

      The integrated form of eqn (5) with the boundary condition

      (t = 0 to t) and (q = 0 to qt) is

      favourably explain the 4th generation antibiotics adsorption on graphene.

   3. Elovich model

      The Elovich model equation is generally expressed as follows [18]:

      t

      dqt = exp(q ) (8)

      dt

      The simplified form of Elovich (eqn (6)) is

      qt = 1 ln (ab) + 1 ln (t) (9)

      b b

      1 1

      (q q ) =

      q + k2 t (6)

      where a is the initial adsorption rate (mg g -1 min-1) and

      e t e

      Eqn (6) can be rearranged and linearized as

      b is the desorption constant (g mg -1). The Elovich model was tested for the 4th generation antibiotics adsorbed kinetic values. A plot between qt vs. ln (t) should yield a

      =

      +

      t 1 t

      qt k2qe2 qe

      (7)

      linear relationship with the slope of (1/b) and an intercept

      of 1/b ln (ab) and values of a and b are calculated using (eqn (9)). Table 2 depicts the results obtained from the

      where qe and qt are the amount of 4th generation

      antibiotics adsorbed on graphene at equilibrium time t and at time t (min) respectively, and k2 is the rate constant for the second order kinetic model. The kinetic data were fitted to the second order Lagergren model (eqn (7)). The equilibrium adsorption capacity, qe (cal) and k2 were determined from the slope and intercept of plot of t/qt vs. t (Fig. 4) and are tabulated in Table 1. The plots were found to be linear with good correlation coefficients. The theoretical qe (cal) values agree well with the experimental qe (exp) values. This implies that in all the 7 compounds and in all the concentrations, the second order models are well-suited with the experimental datas and can be used to

      Elovich equation. The lower regression value shows the inapplicability of this model.

   4. Weber and Morris intraparticle diffusion model

      With the aim of nearing into the mechanisms and rate controlling steps affecting the kinetics of adsorption, the kinetic results were analysed by the intraparticle diffusion model to elucidate the diffusion mechanism, whose model is expressed as follows [19]:

      POLLUTANT	ELOVICH MODEL			INTRAPARTICLE DIFFUSION MODEL
      	a (mg g -1 min -1)	b (mg g-1 )	R2	kp (mg g -1 min-1/2)	R2
      Cefclidine	0.3350	1.9395	0.9680	0.186	0.8866
      Cefepime	0.4508	2.2905	0.9680	0.1426	0.8866
      Cefluprenam	0.4406	2.0029	0.9301	0.1609	0.8235
      Cefoselis	0.4896	1.9630	0.9391	0.1645	0.8364
      Cefozopran	0.8931	2.9985	0.9817	0.1095	0.9096
      Cefpirome	0.2024	2.0651	0.9541	0.1575	0.8711
      Cefquinome	0.1630	1.8337	0.9665	0.1803	0.9081

      Table 2 Elovich and Intraparticle diffusion model for 4th generation antibiotics at temperature 303 K, temperature 2.5 mg L -1 and pH 7.

      qt = kp

      t12 + C (10)

      concentration at a constant temperature, and the resulting function is called adsorption isotherm [20].

      where C is the intercept and kp is the intra-particle diffusion rate constant (mg g -1 min-1/2), which can be evaluated from the slope of the linear plot between qt vs. t1/2. The intercept of the plot reflects the boundary layer effect. The larger the intercept, the greater contribution of the surface adsorption in the rate controlling step. If the regression of qt vs. t1/2 is linear and passes through the origin, then intraparticle diffusion is the sole rate-limiting step. Lower and higher values of kid illustrate an

      enhancement in the rate of adsorption and better adsorption

      In this, the widely used Freundlich, Langmuir, DR isotherm and Temkin models are applied to simulate and understand the adsorption mechanism of 4th generation antibiotics at various concentrations.

      1. Langmuir isotherm

         The Langmuir model assumes monolayer coverage on the adsorbent. The linearized form of the Langmuir adsorption isotherm model is as follows [21]:

         with improved bonding between the pollutant and the

         1 = 1

         ( 1 ) + 1

         (11)

         adsorbent particles, respectively. However, the linear plots at each concentration did not pass through the origin. This indicates that the intra-particle diffusion was not only rate controlling step. The results are presented in Table 2.

         The tables 1 and 2 depict the computed results obtained from first order, second order, Elovich and Weber and Morris intraparticle diffusion. From the tables, it is found that the adsorption follows the second order model rather than the other models. Furthermore, the calculated qe values agree well with the experimental qe values for the second order kinetics model, concluding that the second order kinetics equation is the best fitting kinetic model for all the 7 compounds.

5. Adsorption isotherm

   The quantity of adsorbate that can be taken up by an adsorbent is a function of both the characteristics concentration of adsorbate and temperature. The characteristics of the adsorbate that are of importance include: solubility, molecular structure, molecular weight, polarity and hydrocarbon. Generally, the amount of material adsorbed is determined as a function of the

   qe Kaqm Ce qm

   where qe is amount adsorbed (mg g -1) at equilibrium concentration Ce (mg L-1), qm is the Langmuir constant representing maximum monolayer adsorption capacity (mg g -1) and Ka is the Langmuir constant related to energy of adsorption.

   The Langmuir isotherm constants Ka and qm were calculated from the slope and intercept of the plot between 1/qe vs. 1/Ce (Fig. 5). The Langmuir model parameters and the statistical fits of the adsorption data to this equation are given in Table 3. As from Table 3, the higher regression coefficient confirmed that the Langmuir isotherm best represented the equilibrium adsorption of 4th generation antibiotics to grapheme at various concentrations. The excellent fit of the Langmuir isotherm to the experimental data at all temperatures were studied, confirmed that the adsorption is monolayer; adsorption of each molecule had equal activation energy and that adsorbentadsorbate interaction was negligible.

   POLLUTANT	Langmuir adsorption Isotherm				Freundlich adsorption isotherm
   	qm (mg g -1)	Ka (L mg -1)	R2	RL	Kf (L mg -1)	N	R2
   Cefclidine	23.040	0.1556	0.9987	0.9763	3.344	1.019	0.9985
   Cefepime	14.720	0.1974	0.9990	0.6696	2.510	1.092	0.9965
   Cefluprenam	12.485	0.4020	0.9981	0.4987	4.208	1.058	0.9981
   Cefoselis	35.65	0.1746	0.9993	0.6961	5.688	1.032	0.9978
   Cefozopran	12.899	0.1853	0.9995	0.6834	2.106	1.054	0.9998
   Cefpirome	89.92	0.0217	0.9982	0.9484	1.898	1.021	0.9982
   Cefquinome	45.43	0.1853	0.9996	0.8948	2.069	1.012	0.9992

   Table 3 Langmuir and Freundlich adsorption isotherm for 4th generation antibiotics at temperature 303K and pH 7.

   Fig. 5 A Langmuir plot (1/qe vs. 1/Ce) for 4th generation antibiotics adsorption by graphene. Conditions: pH = 7.0; temperature = 303 K and concentration = 0.5-2.5 mg L -1

   The essential characteristics of the Langmuir isotherm can be expressed as the dimensionless constant RL [21]:

   log qe = log Kf + 1 log Ce (13) where kf is the Freundlich constant related to adsorption

   n

   capacity (L mg -1), n is the energy or intensity of

   adsorption, Ce is the equilibrium concentration of nitrate (mg L-1). The values of kf and 1/n obtained from the intercept and slope from a plot between log qe vs. log Ce, are shown in Fig. 6. The values are specified in Table 3. The values of 1/n was less than unity, suggesting that 4th generation antibiotics was adsorbed favourably by graphene at all concentrations studied.

   Table 3 shows that the adsorption of 4th generation antibiotics onto graphene had a higher regression coefficient for determination of Langmuir isotherm and Freundlich isotherm. The dimensionless constant RL was calculated from Eqn. (12). The RL values were found to lies between 0 and 1 for all the concentrations. The n values lies between 1 and 10 for all the concentrations. From the table 3, this study obeys both Langmuir and Freundlich

   L

   R = 1

   1+KaCo

   (12)

   isotherm.

   c. DubininRadushkevich isotherm

   where RL is the equilibrium constant and it indicates the type of adsorption, Ka is the Langmuir constant and Co is the various concentrations of 4th generation antibiotics. RL gives a qualitative measure of the favourability of the adsorption process; if RL greater than 1, it indicates unfavourable adsorption and if RL is between 0 to 1, it indicates the favourable adsorption. From the study RL ranges from 0 to 0.97. This suggests that the adsorption is favourable.

   1. Freundlich isotherm

   Freundlich model is an empirial model allowing for multilayer adsorption on adsorbent. The linearized logarithmic form and the Freundlich constants can be expressed as follows [22]:

   DubininRadushkevich isotherm assumes that the characteristic of adsorption curve is related to the porous structure of the adsorbent and apparent energy of adsorption. This model is given by [23]:

   qe = qs exp(B2) (14) where is the Polanyi potential equal to RT ln (1 +

   1/Ce), B is related to the free energy of sorption and qs is the DubininRadushkevich (DR) isotherm constant. The linearized form is

   POLLUTANT	DUBININ RADUSHKEVICH ISOTHERM			TEMKIN ADSORPTION ISOTHERM
   	E (KJ mol -1)	qs (mg g -1)	R2	B ( J mol -1)	Kt (L g -1)	R2
   Cefclidine	0.04524	6.447	0.9918	0.9170	12.03	0.9119
   Cefepime	0.04489	5.317	0.9916	0.8466	10.32	0.9999
   Cefluprenam	0.04390	7.281	0.9928	0.9357	15.82	0.9132
   Cefoselis	0.04710	169.7	0.9973	1.0061	19.24	0.9338
   Cefozopran	0.04430	33.48	0.9929	0.7899	9.042	0.9266
   Cefpirome	0.04301	4.469	0.9959	0.7806	7.957	0.9397
   Cefquinome	0.04321	4.749	0.9934	0.8089	8.312	0.9299

   Table 4 Dubinin Radushkevich Isotherm and Temkin adsorption isotherm for 4th generation antibiotics at temperature 303K and pH 7

   C

   ln qe = ln qs 2BRT ln (1 + 1 ) (15)

   e

   The constant B gives the mean free energy of adsorption per molecule of the adsorbate when it is transferred from the solid from infinity in the solution and the relation is given as

   E = 1

   2B

   (16)

   The DR model, which does not assume a homogeneous surface or a constant adsorption potential as the Langmuir model, was also used to test the experimental data. It was applied to distinguish between physical and chemical adsorption of 4th generation antibiotics. The plots between ln qe vs. ln (1+1/Ce) gives a straight line at all concentrations as shown in Fig. 7. The values of constants qe and B thus obtained are given in Table 4. The value of regression coefficient was much lower than those of the other two isotherms at all studied concentrations. Therefore, in all of the cases, the DR equation represented the least fit to experimental data than the other isotherm equations. The constant B gives an idea of the mean sorption energy, E, which is defined as the free energy transfer of 1 mol of solute from infinity of the surface of the adsorbent and can be calculated using the relationship in eqn (14). If the magnitude of E is < 8, then it is physical adsorption; if it ranges from 8-16 it is chemical adsorption. Thus from the analysis, E value ranges between 0 and .05 in all 7 compounds, hence it is physical adsorption.

   Fig. 6 A Freundlich plot (ln qe vs. ln Ce) for 4th generation antibiotics adsorption by graphene. Conditions: pH = 7.0; temperature = 303 K and concentration = 0.5-2.5 mg L -1.

   d. Temkin isotherm

   Temkin is used for determining heat adsorption value and binding energy value. The linear form of the Temkin isotherm equation is represented by the following equation [24]:

   qe = B ln Kt + B ln Ce (17) where B = RT/b, T is the absolute temperature in Kelvin,

   R the universal gas constant (8.314 J K-1 mol), 1/b is the

   Fig. 7 A Dubinin-Radushkevich plot (ln qe vs. ln 1+1/Ce) for 4th generation antibiotics adsorption by graphene. Conditions: pH = 7.0; temperature = 303 K and concentration = 0.5-2.5 mg L -1.

   Temkin constant related to the heat of sorption (kJ mol -1)

   linear relationship from which k1 and qe; k2 and 1/qe are determined respectively using the slope and intercept.

   The computed results are represented in Table 5. From first order, the results show that the theoretical qe (cal) value doesnt agree with the experimental qe (exp) values at all temperatures studied with a poor correlation co- efficient.

   So, the experimental data were fitted further with a second order Lagergren model. From second order, the plots were found to be linear with good correlation coefficients as shown in Fig. 9. The theoretical qe (cal) values agree well with the experimental qe (exp) values. This implies that the second order model is in good agreement with the experimental data and can be used to favourably explain the 4th generation antibiotics adsorption on graphene at various temperatures.

   Thermodynamic behaviour of 4th generation antibiotics

   on graphene was evaluated by the thermodynamic parameters viz., Gibbs free energy change (G°), enthalpy (H°), and entropy (S°). These parameters were calculated using the following equations [16, 25]:

   which indicates the adsorption potential (intensity) of the adsorbent, Kt the equilibrium binding constant, and the constant B is related to the heat of adsorption. Values of B

   ln K2

   = ln C Ea

   RT

   (18)

   and Kt were calculated from the plot of qe vs. ln Ce as shown in Fig. 8. The values of B and Kt thus obtained are

   ln K = Ea ( 1 1 ) (19)

   c

   R T1 T2

   given in Table 4.

   ln K

   = S H

   (20)

   c R RT

   Fig. 8 A Temkin plot (qe vs. ln Ce) for 4th generation antibiotics adsorption by graphene. Conditions: pH = 7.0; temperature = 303 K and concentration = 0.5-2.5 mg L-1

6. Thermodynamic parameters

   1. Adsorption kinetics at various temperatures

Using pseudo first and second order Lagergren equations (eqns. (2) & (5)), the rate constants are obtained at various temperatures (303, 313, 323, 333, 343 and 353 K) and at a constant concentration (2.5 mg L-1). The plot between log (qe-qt) vs. t (1st order) and t/qe vs. t (2nd order) will give a

G = RT ln Kc (21)

where C is the constant of equation (g mg min -1), E is the energy of activation (J mol-1), Kc is the equilibrium constant, R is the gas constant and T is the temperature in

1. Fig. 10 shows that the rate constants vary with temperature according to eqn (17). The value of ln k2 is obtained from the second order kinetics with varying temperature and with a constant concentration. The ln k2 value is shown in Table 6. The activation energies Ea are calculated for 4th generation antibiotics from the slope of fitted equation. The ln Kc is obtained from eqn. (19) at various temperatures.

   The enthalpy change (H°) and entropy change (S°) were obtained from the slope and intercept of the vant Hoff linear plots of ln Kc versus 1/T (Fig. 11) (eqn (20)). A positive value of enthalpy change (H°) indicates that the adsorption process is endothermic in nature and the negative value of change in internal energy (G°) shows the spontaneous adsorption of 4th generation of antibiotics on the adsorbent. Positive value of entropy (S°) change shows the increased randomness of the solution interface during the adsorption of 4th generation antibiotics on the adsorbent (Table 6). The free energy change is obtained from eqn (21). The values of Kc and G° are presented in Table 6. From

   POLLUTANT	TEMPERATURE (K)	qe (exp) (mgg -1)	FIRST ORDER KINETICS			SECOND ORDER KINETICS
   			qe (Cal) (mg g -1)	K1 (min -1)	R2	qe (Cal) (mg g -1)	K2 (min.g mg -1)	R2
   	303	1.94	4.78	0.05587	0.3738	1.89	0.02840	0.9971
   	313	1.99	3.98	0.05654	0.3473	1.98	0.04049	0.9975
   Cefclidine	323	2.04	5.08	0.08259	0.4074	2.02	0.07404	0.9998
   	333	2.09	10.02	0.12110	0.5742	2.07	0.11860	0.9898
   	343	2.14	4.1	0.09126	0.5672	2.13	0.16040	0.9997
   	353	2.19	3.53	0.08816	0.6621	2.19	0.20780	0.9995
   	303	1.81	3.58	0.05159	0.6621	1.81	0.0330	0.9994
   	313	1.84	7.82	0.09099	0.5036	1.80	0.06096	0.9974
   Cefepime	323	1.86	8.79	0.10255	0.5045	1.854	0.08936	0.9994
   	333	1.89	8.32	0.10868	0.4352	1.85	0.1333	0.9995
   	343	1.91	6.01	0.02503	0.4182	1.90	0.1877	0.9996
   	353	1.94	6.61	0.1093	0.3232	1.92	0.2288	0.9994
   	303	2.04	6.68	0.07176	0.3030	2.01	0.03465	0.9949
   	313	2.06	11.93	0.10609	0.4727	2.02	0.05864	0.9982
   Cefluprenam	323	2.09	8.11	0.09961	0.5272	2.07	0.0976	0.9988
   	333	2.11	10.8	0.12411	0.5334	2.10	0.1509	0.9998
   	343	2.14	6.46	0.11271	0.5920	2.01	0.2475	0.9996
   	353	2.17	7.98	0.13566	0.6082	2.16	0.3236	0.9999
   	303	2.13	8.86	0.08302	0.4703	2.07	0.03415	0.9963
   	313	2.15	20.4	0.12337	0.5313	2.12	0.05864	0.9987
   Cefoselis	323	2.17	21.9	0.13233	0.5889	2.16	0.0976	0.9993
   	333	2.20	21.6	0.14469	0.6233	2.17	0.15090	0.9998
   	343	2.22	7.59	0.12155	0.5717	2.22	0.24750	0.9992
   	353	2.25	9.42	0.1320	0.5992	2.24	0.32360	0.9999
   	303	1.70	3.50	0.0576	0.4122	1.70	0.05401	0.9998
   	313	1.72	3.75	0.0648	0.4851	1.726	0.07732	0.9985
   Cefozopran	323	1.75	8.94	0.1046	0.5713	1.721	0.1081	0.9989
   	333	1.77	5.11	0.0907	0.5596	1.77	0.1556	0.9996
   	343	1.80	13.81	0.1357	0.5797	1.79	0.1989	0.9998
   	353	1.82	14.05	0.1440	0.6055	1.81	0.2809	0.9998
   	303	1.63	13.0	0.0991	0.4758	1.6	0.02467	0.9888
   	313	1.65	11.8	0.1091	0.5214	1.61	0.06146	0.9969
   Cefpirome	323	1.68	10.2	0.1157	0.5366	1.66	0.1253	0.9995
   	333	1.70	9.31	0.1250	0.5951	1.7	0.2421	0.9998
   	343	1.73	7.66	0.1318	0.6077	1.72	0.4172	0.9991
   	353	1.75	7.10	0.1420	0.6420	1.75	0.6689	0.9999
   	303	1.68	14.7	0.0961	0.4915	1.67	0.0162	0.9839
   	313	1.70	13.9	0.1031	0.5242	1.64	0.0356	0.9960
   Cefquinome	323	1.73	13.4	0.1076	0.5619	1.7	0.06263	0.9973
   	333	1.75	11.5	0.1162	0.5926	1.72	0.12202	0.9982
   	343	1.78	9.86	0.1215	0.6113	1.76	0.20481	0.9994
   	353	1.80	8.86	0.1284	0.6031	1.80	0.2958	0.9997

   Table 5 A comparisons between the experimental and calculated qe values for different temperatures in first and second order adsorption kinetics of 4th generation antibiotics on graphene at various temperatures, concentration 2.5 mg/L and pH 7

   Fig. 9 Second order kinetic plot between different concentrations of 4th generation antibiotics for various temperatures vs. time. Conditions: pH= 7.0

   POLLUTANT	TEMPERATURE (K)	Kc	G° (KJ mol -1)	H° (KJ mol -1)	S° (J mol -1 K)
   	303	1.2146	-0.4899
   	313	1.6110	-1.2410
   Cefclidine	323	2.5198	-2.4818	116.47	35.03
   	333	3.8370	-3.7228
   	343	5.7013	-4.9639
   	353	8.2837	-6.2051
   	303	1.1853	-0.4282
   	313	1.5172	-1.0848
   Cefepime	323	2.2436	-2.1701	101.86	30.627
   	333	3.2404	-3.2549
   	343	4.5801	-4.3400
   	353	6.3502	-5.4251
   	303	1.1853	-0.5993
   	313	1.5172	-1.5181
   Cefluprenam	323	2.2436	-3.0513	142.49	42.841
   	333	3.2404	-4.5545
   	343	4.5801	-6.0726
   	353	6.3502	-7.5909
   	303	1.2685	-0.5292
   	313	1.7921	-1.3406
   Cefoselis	323	3.1186	-2.6813	125.87	37.846
   	333	5.1815	-4.0221
   	343	8.4101	-5.3629
   	353	13.283	-6.7037
   	303	1.1617	-0.3776
   	313	1.444	-0.9563
   Cefozopran	323	2.0388	-1.9129	89.79	27.001
   	333	2.8193	-2.8695
   	343	3.8255	-3.8260
   	353	5.1018	-4.7825
   	303	1.3528	-0.7612
   	313	2.0982	-1.9285
   Cefpirome	323	4.2038	-3.8562	181.06	54.442
   	333	8.0841	-5.7860
   	343	14.954	-7.7138
   	353	26.733	-9.6435
   	303	1.3904	-0.6791
   	313	1.9371	-1.7206
   Cefquinome	323	3.6020	-3.4413	161.61	48.593
   	333	6.4527	-5.1619
   	343	11.174	-6.8828
   	353	18.755	-8.6034

   Table 6 Thermodynamic parameters for adsorption of 4th generation antibiotics

   Fig. 10 Plot between ln k vs. 1/T. Conditions: concentration: 2.5 mg L -1

   and pH= 7.0.

   Fig. 11 Plot between ln Kc Vs 1/T. Conditions: concentration= 0.5 mg L – 1 and pH= 7.0.

   the table, it is found that the negative value of G° indicates the spontaneous nature of adsorption.

   1. CONCLUSION

The adsorption of Cefclidine, Cefepime, Cefoselis, Cefluprenam, Cefozopran, Cefpirome and Cefquinome by high surface area graphene from aqueous solution was studied. This study shows that electrochemically exfoliated graphene is the most promising adsorbent which removes 4th generation antibiotic compounds effectively. The effect of different factors such as contact time, concentration, pH and temperature was studied. The adsorption capacity increases with increase in temperature indicating the endothermic nature of the adsorption process. The adsorption was studied kinetically and the experimental data were best fitted using the pseudo-second order kinetic model, which provided excellent correlation coefficients and agreement between the experimental adsorption capacities and the calculated one and it shows that the equilibrium is achieved within 60 min. The adsorption isotherm could be well fitted with Langmuir and Freundlich adsorption isotherm models. In D-R isotherm model, E value is < 8. Hence it indicates the process is

physical adsorption. The thermodynamics study of the adsorption process showed the spontaneity of the adsorption since the G° values were negative, the adsorption process was exothermic in nature with negative H° values and the positive value of S° shows the increased randomness of the solution interface during the adsorption of 4th generation antibiotics on the adsorbent.

ACKNOWLEDMENT

We would like to thank Dr. S.Vasudevan, Principal Scientist, CECRI for his visionary guidance and constructive ideas for the execution of our project. We are also grateful to Ms.M.Vishnupriya, M.E., (Ph.D)., Assistant Professor, Department of Environmental Engineering, Park College of Technology, Coimbatore for her insightful advices and genuine support. Our special thanks to Mr. Ramakrishnan Kamaraj, Senior Research Fellow for his full support and cooperation to complete the project successfully.

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