Adsorption of Quaternary Ammonium Compounds onto Activated Sludge

The performance of activated sludge in the removal of tetradecyl benzyl dimethyl ammonium chloride (C14BDMA) by adsorption from aqueous solution was investigated with different PH, contact time, ionic strength and temperature. Equilibrium was achieved within 2 h of contact time. The adsorption capacity increased largely with increasing solution pH and remained constant above pH 9. The ionic strength had a negative effect on C14BDMA removal. The adsorption isotherms were analyzed by Langmuir and Freundlich isotherm models, and equilibrium partitioning data was described well by both models. Kinetics data was best described by the pseudo second-order model. Experimental results indicated that the adsorption was favorable at lower temperatures. Thermodynamic parameters, including the Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS), were also calculated. These parameters indicated that adsorption of C14BDMA onto activated sludge was feasible, spontaneous and exothermic in the temperature range of 15-35°C. The activated sludge was shown to be an effective adsorbent for C14BDMA.


Introduction
Quaternary ammonium compounds (QACs) are molecules with at least one hydrophobic hydrocarbon chain linked to a positively charged nitrogen atom with other alkyl or aryl groups being mostly short-chain substituent [1].QACs belong to the group of cationic surfactants.Because of their unique physical/chemical properties, QACs are primarily used as disinfectants, biocides or detergents, but QACs are also used as anti-electrostatics and phase transfer catalysts in a wide range of applications [2].
The widespread use of QACs causes them to be released and to accumulate in aquatic environments and wastewater treatment plants (WWTPs) [3][4].Due to their biocidal properties they may have potential impacts on organisms in activated sludge.It was revealed in literature that QACs had an adverse effect on nitrification [5] and denitrification [6].Moreover, it was reported that QACs, at a concentration of 50 mg L -1 , adversely affected the anaerobic degradation, which resulted in significantly reduced methane production and accumulation of volatile fatty acids [7].Due to the hydrophobic and electrostatic interactions, QACs can rapidly adsorb onto solids and extensively accumulate in aquatic sediments [4,[8][9][10].The adsorption of QACs has been studied previously using various adsorbents, such as coal [11], clinoptilolite [12], and activated carbon [13][14][15].
Activated sludge is a well-known biomass used for the purification of both industrial effluent and domestic wastes.Although a large number of studies have reported on biosorption of heavy metals as well as organic pollutants onto activated sludge [16][17][18], there have been few studies on the use of sludge to remove QACs [19][20][21][22].Garcia et al. [20] investigated the adsorption isotherms on activated sludge from wastewater treatment plants and surface properties in aqueous solutions of alkyl benzyl dimethyl ammonium compounds (BAC).They reported that the Langmuir and Freundlich isotherms agreed very well with experimental data, and adsorption capacity increased with increasing the alkyl chain length.
The objective of this study was to investigate the adsorption potential of activated sludge for removal of Tetradecyl benzyl dimethyl ammonium chloride (C 14 BDMA), which is one of the most commonly used QACs.Effects of different parameters, such as pH, contact time, temperature and ionic strength on the adsorption were investigated.The adsorption thermodynamics and kinetics of C 14 BDMA onto activated sludge were evaluated.

Preparation of the Adsorbent
The activated sludge used in this study was obtained from the wastewater treatment plant of the Technical and Economic Development Area (TEDA) in Tianjin, China.The activated sludge was centrifuged at 1750 g for 5 min and washed twice with DI water to remove easily suspended materials.The remaining solids were placed into flasks, immediately inactivated by autoclaving (120℃, 30 min), and then stored at 4℃ to ensure that only the adsorption effect, rather than a combined adsorption and degradation effect, was assessed.

Chemicals
Tetradecyl benzyl dimethyl ammonium chloride (C 14 BD-MA, C 23 H 42 NCl, 368.0 g mol -1 ) used in this study was purchased from Shanghai Jinchun company in China and was used without further purification.Stock solution (10 g L -1 ) of C 14 BDMA was prepared based on the active ingredient purity and concentration and was used in all experiments after dilution with DI water.

Adsorption Experiments
Kinetic removal of C 14 BDMA was performed as follows: batch experiments were conducted using 250 mL screw-topped flasks, and the mixed liquor suspended solids (MLSS) was 250 mg L -1 .A sample of C 14 BDMA solution was then added to attain the desired initial C 14 BDMA concentrations (5, 20, 50 mg L -1 ).The flasks were sealed with stoppers and shaken at a rate of 150 rpm with a shaker at room temperature, and samples were obtained at given time intervals (0, 0.25, 0.5, 1, 2, 4 and 8 h).After centrifugation (11,410 g for 15 min), the liquid phase concentration was measured, and the mass of C 14 BDMA adsorbed to the sludge was calculated based on the difference between the total and aqueous C 14 BDMA mass.Blank experiments were carried out with C 14 BDMA solution and without adsorbent to ensure that no C 14 BDMA was adsorbed onto the walls of the flasks.
Adsorption isotherm assays were determined at initial C 14 BDMA concentrations of 10, 20, 40, 60, 80, 100, 120, 140 mg L -1 and 250 mg L -1 sludge concentration (MLSS).The flasks were shaken at a rate of 150 rpm with a shaker for 4 h.The experiments were repeated at 15, 25, and 35℃.When the sorption procedure was completed, the samples and data were treated in the same way as in the kinetic experiments.
In order to evaluate the effect of pH and ionic strength on the adsorption of C 14 BDMA onto activated sludge, the adsorption experiments were carried out over a pH range of 1-13 and the concentrations of NaCl ranging from 0 to 0.40 mol L -1 , respectively.

Analytical Methods
The concentrations of QACs in whole and centrifuged sludge samples were determined by using the previously reported modified disulfine blue (DSB) method [23].The Mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS) and pH were measured according to the Standard [24].

Effect of Contact Time
In order to establish the equilibration time for maximum uptake, the adsorption of C 14 BDMA onto activated sludge was studied as a function of contact time.
The adsorption rate was high at the beginning, and equilibrium adsorption of C 14 BDMA onto activated sludge was achieved within 2 h (Figure 1).Therefore the equilibration period of 4 h was selected for all further experiments.The rapid attainment of equilibrium was consistent with previously published reports on the adsorption of cationic surfactants onto sludge.Ismail et al. [21] investigated the batch adsorption of four tested QACs to sludge and reported the optimum equilibrium time as 4 h.All the systems studied by Garcia et al. [25] to determine the rate of BAC adsorption onto activated sludge achieved equilibrium within three hours.
Figure 1 also shows that the uptake of the C 14 BDMA increased with increasing initial C 14 BDMA concentration.Raising the initial C 14 BDMA concentration from 5 to 50 mg L -1 allows the sludge to increase the adsorption capacity from 17.61 to 148.5 mg g -1 .The results are in agreement with literature on the adsorption of organic substances onto activated sludge [26,27].This is due to an increase in the driving force of the concentration gradient as an increase in the initial adsorbate concentration.The initial concentration is an important driving force to overcome all mass transfer resistances of the C 14 BDMA between the aqueous and solid phases.Thus, a higher initial concentration would enhance the adsorption process.

Adsorption Kinetics
The adsorption kinetics data is extremely important to understand the mechanism of the adsorption and to assess the performance of the adsorbents.The pseudo firstorder adsorption [28], the pseudo second-order adsorption [29] and the intra-particle diffusion models [30] were used to fit the experimental data.
The parameters in the psuedo first and second-order models determined from the linear plots of ln (q e − q t ) versus t (Figure 2) and t/q t versus t (Figure 3) are given in Table 1.The correlation coefficients (R 2 ) for the pseudo first-order model, were relatively too low, which may be indicative of a bad correlation.The q ecal values determined from the model were not consistent with the experimental values of q e (not shown in the table).Therefore, the pseudo first-order reaction is not suitable for adsorption of C 14 BDMA onto sludge.However, the correlation coefficients for the pseudo second-order kinetic model are close to 1.0 for all cases (R 2 = 0.999), and the theoretical values of q ec were in agreement with the experimental data q e .Based on these results, the adsorption of C 14 BDMA onto activated sludge followed the pseudo second-order process rather than pseudo firstorder.The results presented in this study are comparable to a study which investigated the biosorption of cationic  clude all steps of sorption, such as external film diffusion, sorption, and internal particle diffusion, so they are surfactants by using activated carbon cloth [14].
ured q e (mg g -1 ) onsidered pseudo-models.Neither model identifies the ra-particle diffusion plot for the sorption of C c sorption mechanism, so the intra-particle diffusion model was tested in this work.According to previous studies, the intra-particle diffusion plot may represent multilinearity, indicating that two or more steps occur [31][32][33].The first portion (sharper) is the external surfaces sorption or instantaneous sorption stage.The second portion is the gradual sorption stage, where intra-particle diffusion is rate-controlled.The third portion is the final equilibrium stage, where intra-particle diffusion starts to slow down due to extremely low solute concentrations in the solution.
The int 14 BDMA is shown in Figure 4.It was observed that the data points were related by two straight lines and the plots did not pass through the origin.The deviation of the straight lines from the origin may be due to the difference in the rate of mass transfer during the initial and final stages of adsorption.The intercepts, obtain extrapolation of the linear portion of the plots, provide the boundary layer thickness.The initial curved portion is attributed to boundary layer diffusion effects or external mass transfer effects [34,35].These effects indicated that intra-particle diffusion was not the only rate-limiting step; also, other kinetic models, such as surface adsorption, may control the rate of adsorption.

3
T concentrations in the range of 10-140 mg L -1 at 25℃.The sludge concentration (MLSS) was 250 mg L -1 and contact time was 4 h.
The equilibrium data was d Freundlich [37] isotherm models.The Langmuir parameters, q m , which is the maximum amount of adsorption and K L , which is the Langmuir adsorption constant, obtained from the equation of plot of q e versus C e (Figure 5), were found to be 368.3mg g -1 and 0.047 L mg -1 , with a correlation coefficient (R 2 ) of 0.984; results are listed in Table 2.The fitted results indicate that the Langmuir isotherm model was applicable to describing the C 14 BDMA adsorption equilibrium by inactivated sludge.
The Fre gure 5.The values of K F which is the constant related to the adsorption capacity obtained in this study are higher than previously reported values [21,38].The K F and n Table 2. Isotherm constan onto activated sludge at 25℃.
g g 25 3 2 0 0. 9.17  2. The value of n varies with the heterogeneity of the adsorbent and should be less than 10 and higher than 1 for a favorable adsorption process.The Freundlich constant 1/n obtained in this study was smaller than 1, which indicated that the adsorption process was favorable under studied conditions.According to the correlation coefficient (0.918), the adsorption pattern of C 14 BDMA onto was also well fitted with the Freundlich isotherm models.This may be due to both homogeneous and heterogeneous distribution of active sites on the surface of the sludge.This observation was in agreement with that of Garcia et al. [20] mentioned above.However, equilibrium partitioning data obtained from the adsorption of cationic surfactants onto municipal sludge [21] and activated carbon cloth [14] were described by the Freundlich isotherm model; these results may be caused by differences in the absorbents rather than the adsorbates.

3
T investigated under isothermal conditions in the temperature range of 15-35℃.Figure 6 shows the temperature dependence of C 14 BDMA adsorption onto activated sludge.The experimental results indicate that the magnitude of C 14 BDMA adsorption is closely related to the solution temperature.The adsorption capacity decreased when temperature rose from 15-35℃, indicating that C 14 BDMA uptake was favored at lower temperatures.Literature has shown many cases in which the increase in temperature caused a decrease in the adsorption capacity of different adsorbates [39,40].This might be due to the weakening of adsorptive forces between the active sites of the adsorbent and adsorbate species and between the vicinal molecules of the adsorbed phase.
In order to gain insight into the mechan the adsorption process, thermodynamic parameters for the present system were calculated at different temperatures.The variations in the enthalpy (ΔH 0 ), and entropy (ΔS 0 ) were calculated were calculated from the slope and intercept of the plot between ln K d versus 1/T (Figure 7).K d is the distribution coefficient and T is the absolute temperature (K).ΔG 0 can be calculated using the Gibbs Free Energy Equation [41].According to Table 3, the  n process was feasible and spontaneous.In addition, the free energy increased from -3.336 to -2.648 kJ mol -1 with an increase of temperature from 15-35℃, showing a decrease in spontaneity.
The negative value of 0 ture of adsorption, also supported by the decrease in value of C 14 BDMA uptake with the rise in temperature.

T (℃)
ΔG 0 (kJ mol -1 ) ΔH 0 (kJ mol -1 ) ΔS 0 (J mol -1 K -1 ) 15 -3.336 hich means that C 14 BDMA sorption to activated sludge corresponds to a decrease in entropy.The negative value of ΔS 0 suggests the decreased randomness at the solid/ liquid interface during adsorption.This finding indicates that the changes associated with molecular ordering actually impede the sorption process.C 14 BDMA is relatively hydrophobic (Log Kow = 3.91) [43], which suggests that hydrophobic interactions may cause sorption to be entropically driven; the current results are in conflict with this notion.Similar results were observed in 17α-Ethinylestradiol (EE 2 ) adsorption onto activated sludge [44].Hydrophobic interactions are hypothesized to result in greater overall system entropy because of the decrease in the ordering of water molecules surrounding the hydrophobic compounds of interest.

3
F sorption onto activated sludge.As shown, the removal of C 14 BDMA from aqueous solution was strongly affected by solution pH.The amount of C 14 BDMA adsorbed onto activated sludge was found to increase from 78.66 to 199.7 mg g -1 in the pH range of 1-9 and attained a constant value above pH 9.
The lower adsorption at lo e presence of excess protons in solution competing with cations for the adsorption, and electrostatic repulsion exists between the positively charged surface and the positively charged C 14 BDMA molecule.pH ZPC is the required pH value to give a zero net surface charge on the adsorbent.When pH is lower than pH ZPC , the number of negatively charged adsorbent sites decreases, and the number of positively charged surface sites increases, which favors the adsorption of anions [45].Moreover, the cations in solution may compete with C 14 BDMA for sorption sites, resulting in the reduction or inhibition of the binding of C 14 BDMA.Therefore, the sorption of

C moval efficiency.
In general, adsorp pzc.For pH values above the pH ZPC , the cell walls will have a negative net charge, which promotes electrostatic attractions between positively charged cations and negatively charged binding sites.Similar observations are shown in literature for cetyl trimethyl ammonium bromide (CTAB) in a pH range of 2-10 using coal samples, as reported by Mishra and Panda [11].

3
In types of electrolytes that have significant effects on the adsorption process, so it is important to evaluate the effects of ionic strength on the removal of C 14 BDMA from aqueous solutions.
Figure 9 shows 4 BDMA adsorption.Results showed that an increase in ionic strength led to a decrease of C 14 BDMA adsorption onto activated sludge.As the concentration of NaCl increased from 0 to 0.40 mol L -1 , the amount of C 14 B-DMA uptake decreased from 158.1 to 102.3 mg g -1 .Similar trends were also reported in studies on the sorption of 17α-ethinylestradiol onto activated sewage sludge [46] and the sorption of dyes onto metal hydroxide sludge [47].
The a unds decrease in the presence of inorganic salts; this is c Kre effect of NaCl on C 14 BDMA would be evident with the increase of ion strength, resulting in the reduction of C 14 BDMA concentration in aqueous solution.This led to a new distribution equilibrium between water and sludge phases and decreased the adsorption of C 14 BDMA onto the activated sludge [46,48].Moreover, Na + and positively charged C 14 BDMA molecules competed for the same binding sites on the biosorbent surface.With the increase of ionic strength, the number of active sites available for C 14 BDMA adsorption decreases.

4
In sorption rate was high at the beginning and the equilibrium time was 2 h.The kinetics of C 14 BDMA adsorption onto activated sludge followed the pseudo second-order model.The curves obtained for the Langmuir and Freundlich isotherm models described the experimental equilibrium data well, indicating both heterogeneous and homogeneous distributions in the active site on the surface.The adsorption of C 14 BDMA onto activated sludge decreased with increasing temperature.Thermodynamic parameters, including the Gibbs free energy (ΔG 0 ), enthalpy (ΔH 0 ), and entropy (ΔS 0 ), indicated that adsorption of C 14 BDMA onto activated sludge was feasible, spontaneous and exothermic in the temperature range of 15-35℃.The adsorption of C 14 BDMA on activated sludge was found to be low in acidic solution but increases with an increase in pH and remains constant in the neutral and alkaline pH regions.As the concentration of NaCl increased, the amount of C 14 BDMA uptake decreased.The experimental results indicated that the activated sludge could be used as an alternative, inexpensive and effective material to remove high amounts of C 14 BDMA from wastewater.

Figure 2 .
Figure 2. Linearized pseudo first-order kinetic model for adsorption of C 14 BDMA onto activated sludge at various initial concentration.(MLSS: 0.25 g L -1 ; temperature: 25℃).(Error bars represent one standard deviation of the means).

Figure 4 .
Figure 4. Intra-particle diffusion model for adsorption of

. 4 .
plots, along with the regression cor-Effect of Adsorption Temperature he effect of temperature on the adsorption isotherm was isms involved in c relation coefficients, are given in Table

Figure 6 .
Figure 6.C 14 BDMA adsorption isotherm onto activated sludge at different temperatures.(Error bars represent one standard deviation of the means).

Figure 7 .
Figure 7. Van't Hoff plot for the adsorption of C 14 BDMA

. 5 .
is useful for distinguishing phy-ΔS ) was -33.94 J mol K , w Effect of pH igure 8 shows the pH dependence of C 14 BDMA adwer pH may be attributed to th E sisorption and chemisorption.Physisorption is typically associated with heats of adsorption in the 5-20 kJ mol -1 range, while chemisorption is typically associated with much larger ΔH 0 values (100-400 kJ mol -1 )[42].The current results suggest that adsorption processes were primarily physisorption.The entropy change (

Figure 8 . 6 .
Figure 8.Effect of pH for the adsorption of C 14 BDMA onto

Figure 9 .
Figure 9.Effect of ionic strength on the adsorption of

Table 1 . Pseudo first-order and pseudo second-order ad- sorption rate constants of C 14 BDMA onto activated sludge at different initial concentration.
ec