Abstract

The objective of this study is to investigate the sorption of methyl parathion (MP) pesticide from aqueous solution using Typha australis leaf powder as an alternative low cost biosorbent. The effects of various parameters such as adsorbent dosage and contact time are studied using the batch technique. Kinetics models (pseudo-first and pseudo-second orders) and isotherms models (Langmuir and Freundlich) are used to fit the experimental data by linear and non-linear methods. The results show that the pseudo-second order kinetics model is the best for describing the adsorption of MP by Typha australis for all initial MP concentrations. The equilibrium data fit well with the Langmuir model, suggesting the existence of monolayer adsorption of MP. Typha australis leaf powder can be considered as a new useful low cost natural biosorbent for pesticide clean up operations in aquatic systems.

NOTATION

     
  • C

    equilibrium pesticide concentration in solution

  •  
  • ΔG

    free Gibbs energy

  •  
  • K

    model coefficient

  •  
  • n

    exponent

  •  
  • q

    adsorbed amount of pesticide

  •  
  • t

    time

  •  
  • y

    data point value

  •  
  • average value of all data points

GREEK LETTERS

     
  • determination coefficient

SUBSCRIPT

     
  • eq

    equilibrium

  •  
  • F

    Freundlich

  •  
  • L

    Langmuir

  •  
  • max

    maximum

  •  
  • MP

    methyl parathion

  •  
  • PAC

    powdered activated carbon

  •  
  • pr

    prediction given by the model

  •  
  • TA

    Typha australis

INTRODUCTION

Pesticides are chemicals, which are commonly used in agriculture to protect crops from pest organisms. Methyl parathion (MP) (O, O-dimethyl O-(4-nitrophenyl) phosphorothioate) is an agricultural insecticide and acaricide. MP is an organophosphorus compound classified as highly toxic (class I) by the United States Environmental Protection Agency (USEPA) and extremely toxic (category Ia) by the World Health Organization (WHO) (Chávez- López et al. 2011). Some toxicological studies have showed that MP may appear as a pollutant in water sources and could threaten human health (Zhu et al. 2001).

There are many procedures available for MP removal from water, which include microbiological (Rani & Lalithakumari 1994), electrochemical (Alves et al. 2013), degradation (Pankaj & Parag 2012) and adsorption (Akhtar et al. 2007). In recent years, various natural adsorbents such as agricultural wastes including Rhizopus oryzae biomass (Chatterjee et al. 2010), bagasse fly ash (Gupta et al. 2002) and waste jute fiber carbon (Senthilkumaar et al. 2010) have been tried to achieve effective removal of pesticides. Some species of Typha are investigated for pollutants removal in aqueous media (Abdel-Ghani et al. 2009; Hegazy et al. 2011).

The focus of this research is to explore the possibility of using Typha australis, an abundant and available plant along the Senegal River, for removing MP from aqueous solution. The sorption of MP is found to be a spontaneous process (ΔG < 0) and, depending on used adsorbent, this process may be endothermic (Memon et al. 2008) or exothermic (Aktar et al. 2007). The effects of various experimental factors such as adsorbent dose, initial concentration, and contact time are studied using the batch technique. For this purpose, kinetic and isotherm studies have been carried out with the linear and non-linear methods.

MATERIALS AND METHODS

Collection, preparation and characterization

Biomass of Typha australis growing along the Senegal River was collected from the city of Rosso, Wilaya of Trarza, in Mauritania. Before use, Typha australis leaves were washed with ultrapure water to remove dirt. The biomass was then air dried for 10 days followed by drying in an oven at 70°C for 24 h (Abdel-Ghani et al. 2009). The dried biomass was ground, sieved (<100 μm) and stored in a dessicator before use.

The point of zero charge (pHpzc) of the Typha australis leaf powder adsorbent was carried out (Roudani et al. 2014) and the surface area was obtained using Sears method (Yadav et al. 2011).

For determination of the moisture content, 5 g of the material was weighed in a crucible. This was placed in the oven and heated for 5 h at a constant temperature of 105°C. The sample was then removed and put rapidly into a desiccator in order to prevent any moisture uptake from atmosphere. The difference in the mass constitutes the amount of moisture content of the adsorbent (Dada et al. 2013).

Concerning the determination of ash, this allows knowing the part of the mineral in the adsorbent used. 1 g of dry adsorbent was weighed and introduced into a crucible for calcination. The crucible was then placed in an oven at 600°C for 45 min. After cooling, the crucible was weighed again (Hamdaoui & Naffrechoux 2007).

The bulk density was determined using a picnometer (Moyo et al. 2013), and the surface acidic functional groups and basic sites of the Typha australis leaf powder were determined by the acid-base titration method proposed by Boehm (Evangelin et al. 2012).

Adsorbate preparation and analysis

All chemicals used in this study were of analytical reagent grade. A stock solution containing 1,000 mgL−1 of MP was prepared by dissolving 100 mg of MP in 100 mL of methanol. The MP molecule is about 9 Å radius with a quite heterogeneous surface charge distribution, as can be seen in Figure 1. MP solutions were prepared by diluting stock solution of MP to the desired concentrations in ultrapure water. All samples were filtered using a micro filter syringer and analyzed by High Performance Liquid Chromatography with a Diode-Array Detector (HPLC/DAD) using the ultimate 3000 system of Thermo Scientific with Chromeleon 7 Software. Ultra pure water (pH 6.8) and methanol (20:80 V/V) were used as a mobile phase at a flow rate of 0.5 mL min−1 at a selected wave length of 265 nm.

Figure 1

MP chemical structure and solvent accessible potential surface distribution based on Extended Huckel Charge calculation.

Figure 1

MP chemical structure and solvent accessible potential surface distribution based on Extended Huckel Charge calculation.

Batch experiments

MP batch adsorption using Typha australis as adsorbent was conducted in batch experiments.

In all sets of experiments, fixed concentrations of MP (5 mg L−1 and 10 mg L−1) were stirred (70 rpm) with varying adsorbent doses for different time periods. The effects of process conditions and contact time (5–60 min) were evaluated for Typha australis adsorbent. At the end of each experiment the stirred solution mixture was microfiltered, and the residual concentration of MP was determined chromatographically. The amount of MP adsorbed (mg g−1) at any time is computed using the following equation:  
formula
The percentage of the removal of MP concentration in solution is calculated using the equation:  
formula

where qe is the MP concentration in adsorbent (mg g−1), V is the volume of the solution (mL), Ci and Ce are the initial and equilibrium solution concentrations (mg L−1) and m is the mass of adsorbent (g). All experiments are conducted in triplicate and the mean values have been reported.

Kinetics and equilibrium adsorption modeling

The kinetic study was done for the Typha australis adsorbent in order to estimate the equilibrium time of adsorption and the best fitted pseudo-first order (PFO) and pseudo-second order (PSO) kinetic models. Different concentrations of MP (5 and 10 mgL−1) spiked in an ultrapure water sample (25 mL) were taken in a conical flask and an adsorbent mass of 0.5 g of Typha australis was added. The mixtures were stirred at 70 rpm at pH 6.8 for 60 min at ambient temperature. At the end of each experiment, the stirred solution mixture was microfiltered and the residual concentration of MP was analyzed by HPLC/DAD.

The linear and non-linear mathematical expressions of PFO and PSO models are summarized in Table 1.

Table 1

Mathematical models of the kinetic adsorption models

ModelEquationParameter and dimension
PFO  (non linear form) k1, k2: rate constant (min−1
(linear form) qe: solute adsorbed at equilibrium state (mg g−1
PSO  (non linear form) q: solute adsorbed at any time (mg g−1
(linear form) t: time (min) 
ModelEquationParameter and dimension
PFO  (non linear form) k1, k2: rate constant (min−1
(linear form) qe: solute adsorbed at equilibrium state (mg g−1
PSO  (non linear form) q: solute adsorbed at any time (mg g−1
(linear form) t: time (min) 

Batch isotherm experiments were conducted at different initial concentrations ranging from 5 to 100 mg L−1 taken in each conical flask. The adsorbent solution was stirred at ambient temperature at 70 rpm for a definite time period keeping initial pH at 6.8. At the end of each experiment, the stirred solution mixture was microfiltered and the residual concentration of MP was determined. The data were fitted to Langmuir and Freundlich isotherms to find the best fitted isotherm.

The linear and non-linear mathematical expressions of Langmuir and Freundlich models are summarized in Table 2.

Table 2

Mathematical equation of the isotherm models

ModelEquationLinear expressionParameter and dimension
Langmuir isotherm   qmax: maximum solute adsorbed at equilibrium state (mg g−1)
KL: Langmuir coefficient (mg L−1)−1 
Freundlich isotherm   KF: Freundlich coefficient (mg g−1).(mg L−1)−n
n: model exponent (–) 
ModelEquationLinear expressionParameter and dimension
Langmuir isotherm   qmax: maximum solute adsorbed at equilibrium state (mg g−1)
KL: Langmuir coefficient (mg L−1)−1 
Freundlich isotherm   KF: Freundlich coefficient (mg g−1).(mg L−1)−n
n: model exponent (–) 
Chi-square (χ2) analysis is used to fit experimental data with kinetic and isotherm using the Excel® solver. The χ2 value, relating the goodness of fit, is determined by:  
formula

So – the closer the value is to the number 100, the more perfect is the fit.

RESULTS AND DISCUSSION

Characterization of Typha australis leaf powder adsorbent

The characterization of physical and chemical surface properties of adsorbent is one of the most important issues in an adsorption process because it evaluates its suitability for one or more of the application fields (Kumar et al. 2014).

The analysis of Typha australis leaf powder with physical and chemical properties is listed in Table 3.

Table 3

Characteristics of Typha australis adsorbent

ParametersMean
pHpzc 6.36 
Moisture (%) 3.9 
Bulk density (g mL−10.48 
Surface area (m2 g−10.91 
Ash (%) 9.9 
Loss of mass ignition (%) 16.9 
Total surface acidity (mequiv/g) 0.744 
Carboxyl (meq g−10.380 
Lactone (meq g−10.340 
Phenolic (meq g−10.024 
Total surface basicity (meq g−10.376 
Particle size (μm) <100 
ParametersMean
pHpzc 6.36 
Moisture (%) 3.9 
Bulk density (g mL−10.48 
Surface area (m2 g−10.91 
Ash (%) 9.9 
Loss of mass ignition (%) 16.9 
Total surface acidity (mequiv/g) 0.744 
Carboxyl (meq g−10.380 
Lactone (meq g−10.340 
Phenolic (meq g−10.024 
Total surface basicity (meq g−10.376 
Particle size (μm) <100 

To determine the surface functional groups of raw Typha leaf powder, its Fourier transform infrared (FTIR) spectrum is obtained as shown in Figure 2.

Figure 2

FTIR-ATR spectra of Typha australis.

Figure 2

FTIR-ATR spectra of Typha australis.

The broad band in the spectrum lies in the region of 3358.46 cm−1, due to –OH stretching mode and shows the presence of free and bonded hydroxyl groups of cellulose.

The small peak observed at 2,913 cm−1 denotes the presence of the stretching C–H vibration in the quinone group. Peaks at 1728.04 cm−1 (C = O stretching of COOH) and 1638.14 cm−1 are assigned to C = O (amide band/ OH). The band at 1519.64 cm−1 is assigned to C = C stretching vibration of cycloalkenes. The absorption peaks at 1376.62 cm−1 and 1245.86 cm−1 can be attributed to the presence of (CH2 and CH3) and –C–O stretching, respectively. The peak lying in the region of 1,037 cm−1 shows the presence of Si-O-Si linkages.

Effect of adsorbent mass

Biomass dosage is an important parameter in adsorption studies, as it gives the optimum dose at which maximum adsorption occurs.

The effect of the amount of adsorbent on the efficiency of adsorption was also studied. Variation of doses in the range 0.1–1 g at a fixed MP concentration (5 mg L−1) for MP removal by Typha australis is shown in Figure 3.

Figure 3

Effect of Typha australis dosage on MP removal percentage.

Figure 3

Effect of Typha australis dosage on MP removal percentage.

The results suggest that the increase in the dose of adsorbent results in an increase in adsorption, probably due to increase in the retention surface area. However, further increase after a certain dose does not improve the adsorption; perhaps due to the interference between binding sites of the adsorbent at different doses. The optimal Typha australis adsorbent dose obtained is 0.5 g.

Kinetic study

Contact time is an important issue in adsorption, which is mainly an equilibrium process, and determining the equilibrium time is of real importance. The effect of contact time on removal of MP (5 and 10 mgL−1) is shown in Figure 4. At equilibrium, 89.4% and 74% for 5 and 10 mg L−1, respectively are obtained with a contact time of 20 min for Typha australis.

Figure 4

Removal percentage of MP by Typha australis adsorbent.

Figure 4

Removal percentage of MP by Typha australis adsorbent.

The kinetic study is very important in the adsorption study, which gives an idea of adsorbate uptake rate and efficiency of adsorption. The mechanism of adsorption depends upon the physical and chemical characteristics of the adsorbent as well as the mass transfer process (Kumar et al. 2014).

The adsorption kinetics data are fitted to the PFO and PSO kinetic models to evaluate the adsorption mechanism of the adsorption process.

Constants k1 and qe for the PFO model could be calculated from the plot of ln (qe – qt) versus time t, and are presented in Table 4. The correlation coefficient, χ2 values for the PFO kinetic model are slightly lower at all initial MP concentrations. In addition, the PFO model gave a relatively large deviation between the calculated and experimental values of qe, indicating that the PFO model did not appropriately describe the adsorption process of MP Typha australis leaf powder adsorbent.

Table 4

Linear and non linear kinetic model parameters

ModelCi (mg L−1)qexp (mg g−1)ParameterLinearχ2 (%)Non linearχ2 (%)
PFO 5 0.2235 K1 0.108 97.2 0.418 99.9 
qe 0.069 0.221 
PSO K2 6.30 99.9 5.21 99.9 
qe 0.226 0.228 
PFO 10 0.370 K1 0.035 99.2 0.470 99.9 
qe 0.17 0.367 
PSO K2 5.96 99.9 4.42 99.9 
qe 0.37 0.375 
ModelCi (mg L−1)qexp (mg g−1)ParameterLinearχ2 (%)Non linearχ2 (%)
PFO 5 0.2235 K1 0.108 97.2 0.418 99.9 
qe 0.069 0.221 
PSO K2 6.30 99.9 5.21 99.9 
qe 0.226 0.228 
PFO 10 0.370 K1 0.035 99.2 0.470 99.9 
qe 0.17 0.367 
PSO K2 5.96 99.9 4.42 99.9 
qe 0.37 0.375 

The PSO model predicted the adsorption behavior based on the agreement with chemisorption being the rate limiting step (Ho & McKay 1998). The linear plots of t/qt versus t are plotted. The constants k2 and qe could be calculated from the plots and are also shown in Table 4. The PSO kinetic model yielded a better fit than the PFO kinetic model as the correlation coefficient, χ2 values close to unity, indicating the applicability of the PSO kinetic model to depict the adsorption process of MP on Typha australis leaf powder adsorbent.

Figures 5 and 6 show the experimental equilibrium data and the predicted theoretical kinetics for the sorption of MP onto Typha australis for 5 and 10 mgL−1, respectively. The values of model parameters k1, k2 and χ2 are presented in Table 4.

Figure 5

PFO and PSO non linear for Typha australis adsorbent with initial MP concentration of 5 mg L−1.

Figure 5

PFO and PSO non linear for Typha australis adsorbent with initial MP concentration of 5 mg L−1.

Figure 6

PFO and PSO non linear for Typha australis adsorbent with initial MP concentration of 10 mg L−1.

Figure 6

PFO and PSO non linear for Typha australis adsorbent with initial MP concentration of 10 mg L−1.

The values of χ2 are compared between kinetic models, the PSO kinetic model shows a lower value than the PFO kinetic model at all initial MP concentrations. In addition, the qe calculated by the PSO kinetic model are close to those obtained from the experiments at all initial MP concentrations, indicating that the PFO kinetic model did not properly describe the adsorption process of MP on Typha australis adsorbent.

It could be concluded that the mechanism of adsorption is PSO reaction. A better fit to the PSO kinetic model suggested that the adsorption rate is dependent more on the availability of the adsorption sites rather than the MP concentration (Salman et al. 2011).

Adsorption isotherms

The adsorption isotherm gives an idea of the equilibrium behavior of an MP–Typha australis system. The Freundlich model is commonly used to describe the adsorption characteristics for a heterogeneous surface. The Freundlich constants KF and intensity n could be calculated from the slope and intercept of the linear plot of ln qe versus ln Ce, and the values are presented in Table 5. The adsorption capacity, KF, and intensity n for adsorption of MP on Typha australis adsorbent are 0.26 and 1.23, respectively. The value of n is greater than unity, indicating that the sorption of MP pesticide onto Typha australis adsorbent is favorable.

Table 5

Linear and non linear isotherm model parameters

ModelParameterLinearχ2 (%)Non linearχ2 (%)
Langmuir qm (mg g−1) 7.28 95.0 7.67 97.2 
KL (mg L−1)−1 0.030 0.028 
Freundlich n (−) 1.22 99.6 1.34 92.7 
KF (mg g−1).(mg L−1)−n 0.23 0.29 
ModelParameterLinearχ2 (%)Non linearχ2 (%)
Langmuir qm (mg g−1) 7.28 95.0 7.67 97.2 
KL (mg L−1)−1 0.030 0.028 
Freundlich n (−) 1.22 99.6 1.34 92.7 
KF (mg g−1).(mg L−1)−n 0.23 0.29 

The Langmuir isotherm is valid for monolayer adsorption onto a surface containing a finite number of identical sites. The values of qm and KL are determined from the Langmuir plots and are presented in Table 5. A lower KL value of 0.030 indicates a high affinity of the MP for the Typha australis leaf powder adsorbent. The values of χ2 are compared, Langmuir isotherms are shown to have higher values than Freundlich isotherms. The adsorption capacity of Typha australis leaf powder is found to be 7.67 mg g−1. To compare the efficiency of Typha australis in removing aqueous MP, commercial powdered activated carbon (PAC) has been used and showed a capacity removal of 78 mgMP/gPAC. This result is comparable with the one found by Gupta et al. (2011), who reported a capacity removal of 88.9 mgMP/g of activated carbon prepared from waste rubber tires.

Figure 7 shows the experimental equilibrium data and the predicted theorical isotherms for the sorption of MP onto Typha australis leaf powder. The values of isotherm parameters with the non-linear isotherm that are studied in this work are shown in Table 5.

Figure 7

Langmuir and Freundlich non linear for Typha australis adsorbent.

Figure 7

Langmuir and Freundlich non linear for Typha australis adsorbent.

The values of χ2 are compared, Langmuir isotherm are shown to have higher values than Freundlich isotherms. The highest χ2 value further confirmed the suitability of Langmuir model in describing the equilibrium data, suggesting the existence of monolayer adsorption of MP onto Typha australis leaf powder.

In addition, the analysis of equilibrium data shows that it is not sufficient to use the coefficient of determination of the linear regression method for comparing the best-fitting isotherm. The non-linear curve fitting analysis method is found to be the more appropriate method to determine and confirm that the equilibrium data are best described by the Langmuir isotherm model.

Sensitivity analysis

A sensitivity analysis for the Langmuir and Freundlich models parameters using the contour curves was performed. The MODEST® (MODelisation ESTimation) software is used to carry out the estimation parameters, the sensitivity calculation as well as the contour plots (for the latter, MATLAB® software is used). The aim of parameter estimation is to find correct values for both Langmuir and Freundlich model parameters. The reliability and identifiability of the models' estimated parameters can be addressed using the objective function, fobj, which is the solution point of the least squares problem. By plotting the two-dimensional contour lines of fobj, one can study the identifiability of the problem. If the values of the objective function change rapidly in every direction from the peak point, the parameters are well defined (Figure 8(a)). Minimization of the objective function, fobj, can be performed with a number of different optimization routines. When analyzing the objective function contour plots in Figure 8(a), one can see that the global minimum point is found. The true values of these coefficients, qe and KL (Figure 8(a)) and (1/n) and KF (Figure 8(b)), are to a high degree of probability in the immediate vicinity of the computed minimum, because the value of the objective function increases relatively rapidly when moving from the minimum in the directions of the parameter axis. By plotting the contour lines with varying parameters intervals for the χ2 values of the fit, it was found that no crucial inter-correlations exist. So, the identifiability of the two parameters (Langmuir) proved to be good and precise.

Figure 8

Langmuir and Freundlich models sensitivity analysis.

Figure 8

Langmuir and Freundlich models sensitivity analysis.

CONCLUSION

The removal of MP pesticide from aqueous solution using Typha australis biomass collected from Senegal River banks as adsorbent has been investigated under different experimental conditions in batch process. In spite of its low accessible surface area, 0.91 m² g−1, the used biomass shows interesting pesticide uptake rate, more than 7 mg g−1. The linear and non-linear kinetic analysis with varying concentrations showed that the PSO model is applicable to Typha australis adsorbent used for MP removal, suggesting a chemisorption limiting rate step. Adsorption isotherm study showed that the Langmuir model well fitted the experimental data compared to the Freundlich model, suggesting the existence of monolayer adsorption of MP. These results indicated that Typha australis sorbent can be successfully used for the removal of MP from aqueous solutions.

ACKNOWLEDGMENTS

The authors express their gratitude to the Tunisian Higher Education and Scientific Research Ministry for the financial support. The authors wish also to thank Pr. Sonia Dridi-Dhaouadi from Institut Préparatoire aux Etudes d’Ingénieurs de Monastir, Tunisia, for advice and constructive discussions and revision.

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