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Original article
10 (
2_suppl
); S1523-S1529
doi:
10.1016/j.arabjc.2013.05.003

Removal of 4-chloro-2-methyl phenoxy acetic acid pesticide using coffee wastes from aqueous solution

,
 King Saud University, Chemistry Department, P.O. Box 11495, Riyadh 11452, Saudi Arabia

⁎Corresponding author. Tel.: +966 503469300. mzaben@ksu.edu.sa (M.I. Al-Zaben)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The objective of this study is to investigate the use of coffee waste (CW) to remove the 4-chloro-2-methyl phenoxy acetic acid (MCPA) from aqueous solutions. To prepare CW, it was first washed and boiled to remove color and impurities then it was air dried at room temperature for 48 h. Afterward the particle size distribution and zeta potential of the CW ground were determined. The porous texture of coffee was proved by scanning and transition electron microscopy. Batch adsorption tests were performed at 298 K. The effects of contact time, MCPA concentration, and pH were investigated. It was observed that the adsorption of MCPA by using CW is independent of the solution pH level. The Langmuir isotherm provided the best correlation for MCPA adsorption onto CW, showing that the adsorption was favorable. The Langmuir adsorption capacity was found to be 0.34 g/g. The second-order model provided the best description of MCPA adsorption onto CW when compared with the first order model. Infrared spectral studies revealed that acidic groups carboxyl and hydroxyl, are predominant contributors to MPCA adsorption by coffee.

Keywords

Coffee waste
Adsorbent
Wastewaters
Kinetic
MCPA
Zeta potential
1

1 Introduction

Increasing use of pesticides in agriculture and domestic activities for pest control is polluting our water resources day by day, forming a strong class of water pollutants, as they are sometimes non-biodegradable. Moreover, pesticides are carcinogenic in nature. Therefore, toxicity of pesticides and their degradation products is making these chemical substances a potential hazard by contaminating our environment. 4-Chloro-2-methyl phenoxy acetic acid (MCPA) belongs to a group of chemicals known as phenoxy compounds which are potentially toxic to humans. The herbicide is widely used to control broad leaf weeds and grasses in crops, and has been frequently detected in groundwater supplies. Thus, it is important to prevent the release of these pesticides into the environment.

Conventional methods have been reported to be more effective in removing less water soluble and easily degradable pesticides including filtration, adsorption, coagulation, and sedimentation (Inacio et al., 2001; Cardoso and Valim, 2006; Vergili and Barlas, 2009; Bruna et al., 2009; Ayar et al., 2008; Cho et al., 2006; Azouaou et al., 2010). Adsorption is considered quite an attractive method for removing pesticides from dilute solutions. Although, the use of commercially available activated carbon and zeolites for removing the pesticides is still very popular, it is very expensive (Terdkiatburana et al., 2008). Thus, there is a growing demand to find relatively efficient, low cost and easily available adsorbents for the adsorption of organic materials, particularly if the adsorbents were recycled waste. The study is oriented toward finding inexpensive plant based waste adsorbents such as: Tea waste (Amarasinghe and Williams, 2007) Coffee Waste (Kaikake et al., 2007), Sawdust (Sciban et al., 2007).

Tree Fern (Ho et al., 2004), Chitosan (Shafaei et al., 2007), Olive Oil waste (Doyurum and Celik, 2006; Azouaou et al., 2008; Perez-Marin et al., 2007), Orange Juicing process Waste (Prasanna Kumar et al., 2007), Orange Skin (Perez-Marin et al., 2007), Algae (Benhima et al., 2008) and Dried Plants (Ahmad and Rahman, 2011).

The polar nature of coffee waste can remove large quantities of metal ions from aqueous solutions since the chemical composition of the insoluble coffee waste cell’s wall is largely made up of lignin and some structural proteins (Azouaou et al., 2010), therefore metal (Azouaou et al., 2010; Utomo and Hunter, 2010; Oliveira et al., 2008) and organic matter removal should be feasible.

In Saudi Arabia, coffee is the traditional drink of most Saudi families. The normal practice after brewing coffee is to dispose of the waste. Therefore the aim of this study was to explore the feasibility of using coffee waste to remove MCPA from aqueous solution. The effect of pH, contact time, coffee mass and adsorption equilibrium were investigated.

2

2 Experimental

2.1

2.1 Materials

2.1.1

2.1.1 Coffee

The adsorbent used in this study is coffee waste collected from one of the Saudi houses. The coffee waste was washed and boiled seven times to remove adherence of chemicals such as metal ions and acid. The remaining solid material was dried at room temperature for 48 h. The dried coffee waste was crushed into fine particles and kept in desiccators for further experiments. The ground coffee is labeled CW. Particle size and zeta potential are important parameters for the characterization of the CW particles. Particle size and zeta potential (ζ) of coffee dispersion in deionized water were measured using zeta sizer, nano series of instruments from Malvern Instruments Ltd. (Malvern, Worcestershire, UK, nano, S, ZEN, 1600). The morphological analysis of the ground CW was performed by using a JEOL JSM-6360LV scanning electron microscope. TEM image was recorded on a JEOL JSM-6060LV transmission electron microscope to investigate CW microstructure.

2.1.2

2.1.2 Chemicals

Sulfuric acid 98% from BDH, NaCl (AR) from win lab. NaOH from winlab (GPR), analytical grade of 4-chloro-2-methylphenoxy acetic acid (MCPA) used in this study was purchased from Herbicide Selectomobeed (Saudi Arabia). The chemical structure is shown in Fig. 1.

Chemical formula of 2-methyl-4-chloro phenoxy acetic acid (MCPA).
Figure 1 Chemical formula of 2-methyl-4-chloro phenoxy acetic acid (MCPA).

2.2

2.2 Adsorption experiment

Adsorption of MCPA by CW sample was carried out in a batch reactor. A stock solution containing 0.825 g/l MCPA was prepared using deionized water. Different series of MCPA concentration ranging between 0.825 and 0.02 g/L were prepared by diluting the stock solutions. The concentration of MCPA remaining in the solution after shaking for 2 h (to ensure a balance) was analyzed using spectrophotometer, UV/Visible spectrophotometer Ultraspec 2000, Pharmacia Biotech at wavelength 196 nm. The amount of MCPA adsorbed was studied at different CW masses [0.2, 0.1, 0.01.and 0.02 and 0.004 g] at MCPA concentrations of 0.825 g/L. It was difficult to follow the adsorption process in 0.2, 0.1, 0.01.and 0.02 g of CW, because the remaining concentration of MCPA after equilibrium was zero. Therefore in this study, 0.004 g of CW was selected as suitable mass in all the following adsorption experiments. The influence of pH in the range of 2–8 was studied while keeping all other parameters constant (MCPA concentration = 0.825 g/l; shaking time = 1 h, temp. = 298 K). The pH of the MCPA solution was adjusted after adding the CW by using a diluted concentration of NaOH and H2SO4 solutions. Kinetic studies of adsorption were also carried out using 0.825 g/L MCPA.

The amount (qe) of MCPA adsorbed by CW during the series of batch adsorption was determined using the following equation:

(1)
q e = ( C i - C e ) V / m

  • Ci = initial MCPA concentration in solution (g/L).

  • Ce = MCPA concentration in solution (g/L) at equilibrium.

  • V = volume of initial MCPA solution used (L).

  • m = dry weight of CW used (g).

3

3 Results and discussion

3.1

3.1 Characterization of CW dispersion

Particle size was determined by dynamic light scattering measurement. Fig. 2 shows the histogram of size distribution of CW suspension. The figure also shows a narrow particle size distribution, with an average diameter of 37.84 nm. The result reveals the particles to be mono-dispersed (having the same size and shape). Fig. 3 shows the TEM image of CW. It shows the nano-particles to be symmetrical and spherical in shape, mono-dispersed in nature and well distributed without aggregation. Morphological analysis was performed by scanning electron microscopy (SEM) in order to determine CW surface structure. Fig. 4 shows the structure of CW and indicates that it has a porous and crystalline structure with a deep pore. CW dispersion exhibited a net negative zeta potential with the distribution peaks centered at −29.3 mV (Fig. 5) indicating that the CW particles have negative charge.

Histogram of particle size distribution of CW dispersion.
Figure 2 Histogram of particle size distribution of CW dispersion.
TEM images of CW.
Figure 3 TEM images of CW.
SEM image of CW.
Figure 4 SEM image of CW.
Zeta potential distributions for CW dispersion in water.
Figure 5 Zeta potential distributions for CW dispersion in water.

3.2

3.2 Effect of pH

pH of the solution is an important factor that controls the uptake of MCPA due to its impact on both the surface binding-sites of the CW and the MCPA molecule. Experimental results revealed that the removal of MCPA was almost constant in the pH range of 2–8 (Fig. 6). As we mentioned earlier the surface of the coffee carries a negative charge (as indicated from the zeta potential measurement). This charge does not change with the variation of pH of the CW dispersion. Therefore, it can be concluded that the CW charge does not depend on pH (pH independent surface). Consequently the MCPA molecules adsorbed through the hydrogen bond between OH on the CW surface and acidic group of MCPA molecules do not show electrostatic interaction. This result is considered an advantage in environments where MCPA can be removed by coffee waste at any pH.

Effect of pH on the adsorption of MCPA onto coffee surface.
Figure 6 Effect of pH on the adsorption of MCPA onto coffee surface.

3.3

3.3 Kinetic study

In order to determine equilibration time for maximum MCPA adsorption, the adsorption of MCPA was studied as a function of contact time, ranging between 5 and 60 min by monitoring the amount of MCPA adsorbed by CW. The initial MCPA concentration was fixed at 0.825 g/L and the ambient temperature at 25 °C. It was found that the extraction rate of MCPA was extremely rapid in the first few minutes and the time required for equilibrium adsorption is 5 min since no change in the adsorbed amount is detected afterward as shown in Fig. 7. The variation in the extent of adsorption may be due to the fact that initially all sites on the coffee surface were vacant and the MCPA concentration was relatively high. Generally, when adsorption involves a surface reaction process, the initial adsorption is rapid. Then, lesser adsorption would follow, as the available adsorption site gradually decreases, which is consistent with former studies reported earlier (Cho et al., 2006; Aksu and Kabasakal, 2004; Cardoso and Valim, 2006). Due to such quick adsorption rate it can be inferred that chemisorption is predominant in this adsorption process, a MCPA–CW contact time of 2 h was selected in further adsorption experiments to be sure that equilibrium state has been reached.

Kinetic study of adsorption of MCPA at 298 k, initial MCPA concentration was 0.825 g/L, 0.004 g coffee.
Figure 7 Kinetic study of adsorption of MCPA at 298 k, initial MCPA concentration was 0.825 g/L, 0.004 g coffee.

The kinetics of adsorption was studied using two kinetic models: pseudo-first order and pseudo-second order models. The linear form of the pseudo-first order model used is that of Lagergren given by the following equation:

(2)
Ln ( q e - q t ) = ln qe - k 1 t where qt is the amount of adsorbate adsorbed at time t (mg/g), qe is the adsorption capacity in equilibrium (mg/g), k1 is the rate constant of pseudo-first-order model (min−1), and t is the time.

The pseudo-second-order equation is based on the sorption capacity of the solid phase and can be expressed using the equation used by (Ho et al., 2004) as

(3)
t / q t = 1 / ( k 2 q e 2 ) + t / q e where k2 is the rate constant of the pseudo-second order (g mg−1 min−1).

The plots of ln(qe − qt) versus (t) for the Lagergren-first-order model (not shown here) do not fit a pseudo-first-order kinetic model because of the solubility limitations of herbicides. The correlation coefficients (R2), rate constant (k1) and qe determined from the model are presented in Table 1. However, qe estimated by this model differs substantially from that measured experimentally (0.33 g/g), further suggesting that the adsorption of MCPA onto CW is not a first-order reaction (see Fig. 11).

Table 1 Parameters for adsorption of MCPA onto coffee derived from the pseudo-first and second-order kinetic models.
Pseudo-first order Second-pseudo-order
qe(g/g) K1(min−1) R2 qe(g/g) K2(g/g.min) R2
1.0086 0.6 0.4917 0.33 5.53 0.998
FTIR spectra of coffee grounds before adsorption (a), MCPA (b) and coffee after adsorption (c).
Figure 11 FTIR spectra of coffee grounds before adsorption (a), MCPA (b) and coffee after adsorption (c).

The pseudo-second-order rate constant k2 and equilibrium adsorption amount (qe) for MCPA were determined from the slopes and intercepts of plots of (t/q) vs. (t) (Fig. 8). The correlation coefficients (R2) for the pseudo-second-order kinetic were 0.998 and the calculated qe values were very close to the experimental values (Table 1). The high correlation coefficients and the agreement of calculated and experimental qe both demonstrated that the adsorption kinetics of MCPA onto CW followed the pseudo-second-order kinetic model. Therefore, the rate limiting step may be chemical sorption or chemisorption through share or exchange of electrons between sorbent and adsorbate. Other studies have used pseudo-second-order kinetics for sorption reactions, and some authors have reported that the kinetics of the sorption of MCPA on modified activated carbon and fertilizer and steel industry waste follows a pseudo-second-order reaction rate (Chingombe et al., 2006; Gupta et al., 2006).

Pseudo-second-order plot for MCPA on coffee at 298 k, initial MCPA concentration was 0.825 g/L, 0.004 g coffee.
Figure 8 Pseudo-second-order plot for MCPA on coffee at 298 k, initial MCPA concentration was 0.825 g/L, 0.004 g coffee.

3.4

3.4 Adsorption isotherm

The sorption isotherms (Fig. 9) were obtained by plotting the adsorbed amount (qe) versus the equilibrium concentration (Ce). The adsorption isotherms for MCPA were determined at natural pH which was of H-type according to Giles et al classification (Giles et al., 1960). This type of isotherm is typical of systems where MCPA molecules are strongly attracted by CW, reaching the saturation value at the isotherm ‘plateau’. This ‘plateau’ corresponds to the formation of a MCPA monolayer on the CW surface. The Langmuir isotherm was applied to the present study to estimate the adsorption capacity of CW. The linearized form of the Langmuir equation is represented by:

(4)
C e / q e = 1 / K Lqm + C e / q m where Ce is the equilibrium concentration of the metal in the solution (g/l), qe is the adsorbed amount of MCPA by unit weight of CW, qm is the maximum adsorbed amount (mg/g), and KL is the affinity constant, which represents the affinity of the surface for the adsorbent. The linear plots of (Ce/qe) vs. (Ce) for MCPA show that adsorption follows the Langmuir adsorption model (Fig. 10). qm and KL were determined from the slope and the intercept of the plot to be 0.34 g/g and 4.03 L/g, respectively. In order to predict the efficiency of the adsorption process, the dimensionless equilibrium parameter RL was determined using Eq. (5)
(5)
R L = 1 / ( 1 + K L C 0 ) where KL is the Langmuir constant and C0 corresponds to the highest value for the initial MCPA concentration (mg L−1). Known value of RL is related to the type of isotherm, which can indicate either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) adsorption. In this work the value of RL for the highest value for the initial MCPA concentration 0.825 g/L was found to be 0.23. These values indicate the adsorption of MCPA is favorable. The Freundlich adsorption isotherm was also applied for the adsorption of MCPA onto CW. The linear form of the Freundlich equation can be expressed as:
(6)
log q e = 1 / n log C e + log K f Linear plots of ln(Ce) vs. ln(qe) (not present here) show that the adsorption of MCPA onto CW does not follow the Freundlich isotherm model. Table 2 shows that the equilibrium data were well represented by the Langmuir isotherm model compared to the Freundlich. In order to determine which functional groups were responsible for MCPA adsorption onto CW, an FTIR analysis in the solid phase was performed for the CW sample, MCPA and CW after MCPA adsorption. The FTIR spectra show that several functional groups are available on the surface of CW for binding MPCA (Table 3). FTIR spectral analysis of CW after adsorption of MPCA (spectrum c) showed that the peak at 3467 cm−1 which corresponds to the OH group on the coffee waste surface has been shifted to 3453.33 cm−1 due to MCPA adsorption. These shifts were possibly attributed to the changes in counter ions associated with carboxylate and hydyroxylate anions. It can be suggested that the hydrogen bond is a predominant contributor to the adsorption of MPCA by CW based on pH and FTIR results.
Adsorption isotherms of MCPA onto coffee at 298 k.
Figure 9 Adsorption isotherms of MCPA onto coffee at 298 k.
Langmuir plot for MCPA adsorption onto coffee at 298 K.
Figure 10 Langmuir plot for MCPA adsorption onto coffee at 298 K.
Table 2 Langmuir and Freundlich parameters for adsorption of MCPA onto coffee surface.
Langmuir Freundlich
qe (g/g) KL (L/g) R2 RL n Kf R2
0.34 4.03 0.995 0.231 0.42 0.46 0.49
Table 3 Wavelengths corresponding to their respective functional groups in coffee waste:
Functional group –OH –CH –C⚌O –C⚌C –COO
Wavelength (cm−1) 3467 2925 1744 1653 1660

4

4 Conclusion

Equilibrium results demonstrated favorable adsorption and were better described by Langmuir. The maximum value of uptake capacity obtained for adsorption of MCPA by coffee waste (CW) was higher compared to the adsorbed amount of MCPA in the literature by other adsorbent wastes. The pH of the adsorption system did not present significant effects on the adsorption capacity. Adsorption kinetics was well described by the pseudo-second order kinetic model. The results presented in this study indicate that coffee waste (CW) presents great potential as an inexpensive and easily available alternative adsorbent for the removal of MCPA in wastewater treatments.

Acknowledgement

This project was supported by King Saud University, Deanship of Scientific Research College of Science Research Centre.

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