In this work, the ability of granular activated carbon (GAC) to sorb metaldehyde was evaluated. The kinetic data could be described by an intra-particle diffusion model, which indicated that the porosity of the sorbent strongly influenced the rate of sorption. The analysis of the equilibrium sorption data revealed that ionic strength and temperature did not play any significant role in the metaldehyde uptake. The sorption isotherms were successfully predicted by the Freundlich model. The GAC used in this paper exhibited a higher affinity and sorption capacity for metaldehyde with respect to other GACs studied in previous works, probably as a result of its higher specific surface area and high point of zero charge.
INTRODUCTION
Metaldehyde is a cyclic tetramer of acetaldehyde, commonly used as molluscicide in agriculture and domestic gardening to control slugs, snails and other gastropods. Because it is an environmental contaminant, its current maximum application rate is fixed to 700 g metaldehyde/ha/calendar year in the UK (The Metaldehyde Stewardship Group 2013).
Since 2008, the UK Environmental Agency has been reporting that the metaldehyde level in drinking water exceeds European and UK limits of 0.1 μg L–1 (Busquets et al. 2014). This contaminant is inefficiently treated in water treatment plants. In fact, the maximum reported concentration of metaldehyde in UK water treated for drinking use is just above 1.03 μg L–1 (Busquets et al. 2014).
There are different methods designed for reducing the concentration of metaldehyde in water (Autin et al. 2012; Tao & Fletcher 2013, 2014; Busquets et al. 2014). An important method to reduce the concentration of metaldehyde is photocatalytic degradation using UV/TiO2 or UV/H2O2 systems. Autin et al. (2012) showed that both UV/H2O2 and UV/TiO2 are able to efficiently degrade metaldehyde in pure systems. However, in natural waters, the UV/TiO2 process is severely inhibited and the use of UV/H2O2 remains the only reliable option for metaldehyde removal, although both these processes are relatively expensive compared to the conventional processes.
Among other techniques proposed for metaldehyde removal from water, sorption is the most interesting one because it is generally cheap and easy to scale up. Tao & Fletcher (2013) studied the sorption of metaldehyde onto three different materials: (1) granular activated carbon (GAC): (2) non-functionalised hyper-cross-linked polymer; (3) ion-exchange resin. Their results indicated that the sorption kinetics of metaldehyde onto all tested materials was fast, reaching equilibrium within 8 hours. They also showed that ion-exchange resin exhibited the highest sorption capacity (1,807 mg g–1) and GAC was relatively inefficient in removing metaldehyde (71 mg g–1). In terms of operational costs, ion-exchange resins are generally slightly cheaper than GAC (Griffin 2009). However, on the other hand, GAC is less selective and hence more capable of adsorbing different types of pollutants.
The aim of this work was to investigate the sorption of metaldehyde onto a different commercial GAC by varying the amount of GAC, initial solute concentration, pH, ionic strength and temperature.
MATERIALS AND METHODS
Materials and reagents
Metaldehyde (CAS 9002-91-9) was supplied by Sigma-Aldrich; its main characteristics are reported in Table 1.
Main physicochemical characteristics of metaldehyde
Metaldehyde . | |
---|---|
Molecular formula | C8H16O4 |
Molecular mass | 176.21 g/mol |
Water solubility (25 °C) | 213 mg L–1a |
Appearance | White or colourless crystalline solid |
Toxicity | Moderately toxic, kidney and liver toxicanta |
Metaldehyde . | |
---|---|
Molecular formula | C8H16O4 |
Molecular mass | 176.21 g/mol |
Water solubility (25 °C) | 213 mg L–1a |
Appearance | White or colourless crystalline solid |
Toxicity | Moderately toxic, kidney and liver toxicanta |
aUniversity of Hertfordshire, Pesticide Properties Database-Metaldehyde, UK, 2012.
The cartridges, styrene-divinylbenzene (SDB1) used for the solid phase extraction (SPE) were provided by J. T. Baker (USA), while analytical (HPLC) grade methanol (CAS 67-56-1) and dichloromethane (CAS 75-09-2) were purchased from Fisher Scientific (UK).
The determination of the pH of the point of zero charge (pHpzc) of adsorbents was carried out by pH titration procedures (Rivera-Utrilla et al. 2001). Forty mL aliquots of 0.01 M NaCl solution were placed in 50 mL conical tubes and the pH was adjusted to a value between 2 and 10 by the addition of a few drops of 0.1 M HCL or 0.1 M NaOH solution. One hundred and twenty milligrams of GAC were added to each solution, and after 2 days the final pH was measured and plotted against the initial pH. The pH at which the curve pH final vs pH initial crosses the line pH final = pH initial represents the point of zero charge (PZC) of GAC.
Kinetics and sorption of metaldehyde
Metaldehyde sorption was studied by batch method. Kinetic experiments were performed by adding 10 mg of GAC to 100 mL of metaldehyde solution (30 mg L–1). The samples were agitated in a shaker (30 rpm) at room temperature and, at pre-decided contact times, 0.2 mL aliquots of reacting solution were removed for gas chromatography–mass spectrometry (GC-MS) analysis.
Metaldehyde sorption at equilibrium was studied by contacting 1–10 mg of GAC with 10 mL of metaldehyde solution (1–30 mg L–1). The samples were stirred at 30 rpm until the attainment of equilibrium, and analyzed as described above. Sorption was studied as a function of temperature (5, 25, 45 °C), pH (2–8) and ionic strength (0–1 M). The pH of samples was adjusted to the desired value with a few drops of concentrated HCl or NaOH. KCl was used to alter the ionic strength of the solution.
GC-MS analysis
Before GC-MS analysis, aliquots collected from the samples were pre-concentrated using SPE according to the following procedure. A styrene-divinylbenzene (SDB1) cartridge was first activated, flushing 10 mL of methanol and then conditioned with 2 mL of Mill-Q water. Afterwards, 0.2 mL of sample was passed through the cartridge. The cartridge was then rinsed with 2 mL of Mill-Q water (to ensure that metaldehyde was entirely retained on the polymer) and dried by passing air through it for 40 min. Finally, the cartridge was flushed with 3 mL of dichloromethane; the fraction was collected in an appropriate glass tube and evaporated to 1 mL by nitrogen for GC-MS analysis. The GC-MS equipment used was a Perkin Elmer Clarus 500 which included an auto-injector, mass-spectrometer capable, a selective ion monitoring mode, and a column HP5-MS (30 m × 0.25 mm diameter, 0.25 μm film thickness).
The injection model was split-less and the temperatures of injector and detector were set at 100 and 180 °C, respectively. Helium was used as the carrier gas (1 mL min–1). The temperature programme for the oven was set at 100 °C and held for 1 min; then increased to 150 °C at a rate of 5 °C min–1 and held for 1 min.
RESULTS AND DISCUSSION
Sorption kinetics
A first attempt to model the sorption kinetic data was carried out using a pseudo-first and pseudo-second order equations.
Kinetic parameters for metaldehyde sorption as determined by the fitting procedure
Model . | qe (μg g–1) . | k1 (h–1) . | k2 (g μg–1h–1) . | Z (h–1) . | kD (μg g–1 h–0.5) . | I (μg g–1) . | R2 . |
---|---|---|---|---|---|---|---|
Pseudo-first order model | (1.45 ± 0.05) × 105 | 0.45 ± 0.05 | 0.870 | ||||
Pseudo-second order model | (1.49 ± 0.03) × 105 | (4.8 ± 0.5) × 10–6 | 0.950 | ||||
Vermeulen model | (1.48 ± 0.02) × 105 | 0.18 ± 0.01 | 0.975 | ||||
Weber–Morris model | (3.5 ± 0.2) × 104 | (3.0 ± 0.3) × 104 | 0.965 |
Model . | qe (μg g–1) . | k1 (h–1) . | k2 (g μg–1h–1) . | Z (h–1) . | kD (μg g–1 h–0.5) . | I (μg g–1) . | R2 . |
---|---|---|---|---|---|---|---|
Pseudo-first order model | (1.45 ± 0.05) × 105 | 0.45 ± 0.05 | 0.870 | ||||
Pseudo-second order model | (1.49 ± 0.03) × 105 | (4.8 ± 0.5) × 10–6 | 0.950 | ||||
Vermeulen model | (1.48 ± 0.02) × 105 | 0.18 ± 0.01 | 0.975 | ||||
Weber–Morris model | (3.5 ± 0.2) × 104 | (3.0 ± 0.3) × 104 | 0.965 |
(a) Pseudo-first order, (b) pseudo-second order and (c) Vermeulen kinetic models for metaldehyde sorption.
(a) Pseudo-first order, (b) pseudo-second order and (c) Vermeulen kinetic models for metaldehyde sorption.
In order to confirm that the pseudo-second order equation was appropriate for describing the experimental data, the linearized form of this model as a control tool was examined.
(a) and (b) Plots of metaldehyde kinetic sorption data: a) t/q vs t plot; b) q/t vs t plot.
(a) and (b) Plots of metaldehyde kinetic sorption data: a) t/q vs t plot; b) q/t vs t plot.
However, it has been demonstrated (Canzano et al. 2012) that the use of Equation (5) may lead to incorrect conclusions, especially when the sorption data are at equilibrium (or very close to) because in such cases the plot t/q vs t becomes linear irrespective of the sorption kinetics.
The above equation was used to model the experimental kinetic data and the results are reported in Figure 2(c) and Table 2. It can be seen that Equation (8) gives a better fit compared to the pseudo-first and pseudo-second order models both in terms of R2 and parameter errors. This leads us to conclude that diffusion plays a major role in the sorption rate of metaldehyde onto GAC.
The results of our study indicate that the uptake of metaldehyde takes place slowly compared to that measured using a different commercial GAC (henceforth GAC-2; Tao & Fletcher 2013). For our GAC, the time required to reach equilibrium was about 24 h (Figure 2), whereas GAC-2 had a shorter (about 6 h) equilibration time. Having assumed that diffusion controls the rate of sorption, it is reasonable to ascribe the different behaviour of the two compared sorbents to their particle size as the rate of the process, in that case, should vary inversely with particle size (Boyd et al. 1947). Consistent with the results of the kinetic experiments, the GAC used in the present work has greater particle size (1–2 mm) than that of GAC-2 (0.4–0.8 mm).
Sorption isotherms
Sorption isotherms of metaldehyde determined according to the following models: (a) Langmuir model; (b) Freundlich model; (c) Langmuir–Freundlich model.
Sorption isotherms of metaldehyde determined according to the following models: (a) Langmuir model; (b) Freundlich model; (c) Langmuir–Freundlich model.
Having established that the Langmuir model is not suitable to represent the sorption data, the Langmuir–Freundlich model was used in order to gain information on the saturation level of the sorbent.
Thermodynamic parameters as determined by the fitting procedure
Model . | KL (L mg–1) . | KF (mg1–N g–1 LN) . | KLF (LN mg–N) . | N . | qmax (mg g–1) . | R2 . |
---|---|---|---|---|---|---|
Langmuir isotherm | 270 ± 30 | 220 ± 10 | 0.962 | |||
Freundlich isotherm | 1,800 ± 30 | 0.51 ± 0.10 | 0.989 | |||
Langmuir–Freundlich isotherm | (1.7 ± 0.4) × 10−3 | 0.69 ± 0.06 | (320 ± 70) | 0.980 |
Model . | KL (L mg–1) . | KF (mg1–N g–1 LN) . | KLF (LN mg–N) . | N . | qmax (mg g–1) . | R2 . |
---|---|---|---|---|---|---|
Langmuir isotherm | 270 ± 30 | 220 ± 10 | 0.962 | |||
Freundlich isotherm | 1,800 ± 30 | 0.51 ± 0.10 | 0.989 | |||
Langmuir–Freundlich isotherm | (1.7 ± 0.4) × 10−3 | 0.69 ± 0.06 | (320 ± 70) | 0.980 |
It is also interesting to note that the sorbent used here has a higher affinity for metaldehyde, as the initial slope of its isotherm is greater than that of the other GACs (Tao & Fletcher 2013; Busquets et al. 2014).
As mentioned in the introduction section, one of the main issues is the inefficiency of water treatment plants for reducing the metaldehyde concentration below the European limit of 0.1 μg L–1. Based on our results and considering that treatment plant effluents may contain up to 1.03 μg L–1 of metaldehyde (Busquets et al. 2014), it can be estimated that each gram of GAC would be sufficient to treat about 17,000 L of waste water in batch reactors so as to bring the concentration below the legal limit.
Effect of pH, ionic strength and initial concentration
Effect of the initial liquid phase concentration on metaldehyde sorption.
To understand the effect of ionic strength, the sorption performance of GAC in the presence of KCl was examined. The sorption isotherms (not shown), within the experimental errors, were quite similar in the range of concentration explored (0–1 M). Therefore, ionic strength has no influence on the sorption process.
Likewise, the temperature also has no detectable influence on the sorption isotherms (ΔH° ≈ 0), hence suggesting that sorption is physisorption (Colella et al. 2015).
CONCLUSIONS
In the present work, the sorption of metaldehyde onto GAC was investigated. It was found that GAC has an energetically heterogeneous surface. The rate of the sorption is diffusion-controlled. Among different models tested, the hybrid Freundlich isotherm was found to describe more adequately the sorption data at equilibrium, whereas the Vermeulen equation was found to be appropriate for modelling kinetic data, indicating that the process is rate limited by intraparticle diffusion. Metaldehyde uptake is likely promoted by electrostatic and/or H-bond interactions between the electronegative oxygen atoms of the molecule and the positively charged surface of the adsorbent. Ionic strength, pH and temperature have no significant influence on metaldehyde uptake.
ACKNOWLEDGEMENTS
The authors are grateful to Dr Judith Zhou and Dr Catherine Unsworth of the Department of Civil, Environmental and Geomatic Engineering at University College London, for their precious technical support. Part of this study was supported by the DST-UKIERI Thematic Partnerships (DST-2013-14/080).