Zhuojun
Li
a,
Juntao
Li
b,
Zhengxiao
Guo
bcd and
Luiza C.
Campos
*a
aDepartment of Civil, Environmental and Geomatic Engineering, University College London, Gower Street, London WC1E 6BT, UK. E-mail: zhuojun.li.09@ucl.ac.uk; l.campos@ucl.ac.uk; Tel: +44(0)207 679 4162
bDepartment of Chemistry, University College London, Gower Street, London WC1E 6BT, UK. E-mail: juntao.li.16@ucl.ac.uk
cDepartment of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong. E-mail: zxguo@hku.hk
dHKU Zhejiang Institute of Research and Innovation, The University of Hong Kong, Qingshanhu Scitech City, Hangzhou, China
First published on 19th March 2020
Metaldehyde as a widely used pesticide has been detected in surface water and drinking water in the UK with concentrations higher than the EU and UK standard (0.1 μg L−1). Previous studies have shown that powdered activated carbon (PAC) can adsorb metaldehyde even with the presence of natural organic matter, suggesting a promising solution to the problem. This paper studies the adsorption of metaldehyde onto PAC using different water samples including synthetic water, natural surface water, and water samples taken at different treatment processes from a water treatment plant. Metaldehyde (5 μg L−1) was effectively removed by PAC (50 mg L−1) from all water samples in this study, regardless of the water quality (74.3% to 99.7%). A PAC dosage of 100 mg L−1 was considered appropriate to remove metaldehyde at 5 μg L−1 after the first treatment process of pre-ozone treatment with a maximum adsorption capacity (qm) of 0.25 μg mg−1 given by the data fitted to the Langmuir isotherm model. Removal of metaldehyde by PAC was found to be most effective when PAC was applied after the static flocculation treatment process (98.4%) with a qm of 0.29 μg mg−1. The low adsorption capacity of PAC for low initial concentrations of metaldehyde solution was observed due to the lower driving force for mass transfer in the process of adsorption and competition with water molecules for adsorption sites on PAC.
Water impactMetaldehyde is a persistent pesticide in surface water bodies which cannot be removed by traditional technologies in water treatment plants due to its physico-chemical properties. This research suggests that the application of powdered activated carbon after the ‘static flocculation’ treatment process in a water treatment plant can effectively remove metaldehyde. |
Granular activated carbon (GAC) filtration is the most widely used conventional water treatment method for removing organic pollutants, and has been proven to be effective in removing pharmaceutical and personal care products such as paracetamol, triclosan and caffeine.5,6 Chemical oxidation by ozone is another common treatment method that can be used to treat disinfection by-products (DBPs) such as trihalomethanes (THMs) and haloacetic acids (HAAs), as well as pesticides.7 However, oxidation by ozone and GAC are not effective in removing metaldehyde due to its physiochemical properties such as its stable ring structure and low logKow value, which suggests low sorption potential.8–10
There are many trending techniques regarding the removal of micropollutants from water, of which advanced oxidation processes (AOPs) are one. AOPs break down organic pollutants into benign sub-products by catalysed oxidation. For instance, Autin et al. have found that metaldehyde can be degraded by photocatalysis via UV/H2O2 and UV/TiO2.11 Another method is adsorption coupled with electrochemical organic destruction. The study of Nabeerasool et al. has shown that metaldehyde can be adsorbed and degraded by adsorbent Nyex™ with electrochemical regeneration without producing any wastes.12 However, these treatment methods require high energy input and are very difficult to apply in water treatment plants at the current stage. In fact, Autin et al. suggested that methods need to be established and tested to reduce the energy input before possible application at a large scale.11 Also, Nabeerasool et al. pointed out that large-scale continuous treatment and energy efficiency need to be determined before considering the viability of the method at the industrial scale.11,12
Recently, there has been a study of metaldehyde adsorption with the presence of serine, leucine, and resorcinol onto different filtration media including different sands, GAC, and biological activated carbon (BAC); results have shown that BAC is very effective for metaldehyde removal.13 This suggests that BAC filtration might be a feasible option for water treatment plants. However, there was observed desorption of metaldehyde from BAC back into the water, indicating that increased regeneration of BAC is required to maintain the effectiveness of metaldehyde removal which limits its economical application.13
Our previous research has shown that PAC, an alternative adsorbent to GAC, can remove metaldehyde very efficiently even in the presence of natural organic matter (NOM).14 In fact, it was reported that PAC has a much higher adsorption capacity (28 mg g−1) for metaldehyde14 compared to that of the GAC used by Busquets et al. which is 15 mg g−1 (ref. 15) and the BAC used by Rolph et al. which is 19 mg g−1.13 Busquets et al. explained that the adsorption of metaldehyde is affected by the pore size distribution of the carbon materials and a higher adsorption capacity was observed in microporous carbon materials.15 Since the pore size distribution of the PAC used in our previous research is dominated by micropores,14 the PAC may be an effective adsorbent for small molecular contaminants such as metaldehyde. Moreover, Rolph et al. argued that BAC could have low adsorption capacity for metaldehyde because of prior exposure and regeneration cycles.13 In comparison, the PAC is unused (virgin) when applied to water samples and its unoccupied adsorption sites are available for metaldehyde.
Considering the potential application of PAC in water treatment plants, this paper investigates the removal of metaldehyde at a low initial concentration (5 μg L−1) by PAC using different water samples including synthetic water, surface water, and water collected from different treatment stages at Walton-on-Thames Water Treatment Works (WTWTW). The working concentration of metaldehyde solution in this study was 5 μg L−1 since it is close to the detected concentration of metaldehyde in surface water (8 μg L−1) by the UK Environment Agency and representative of the metaldehyde concentration in raw water during the peak season.1,13,16 This study investigates the effect of PAC dosage and water quality on the removal of low concentration of metaldehyde from water. It also identifies the best treatment stage to dose PAC for removing metaldehyde at WTWTW, illustrating the potential application of PAC in a real water treatment plant to remove metaldehyde with consideration of possible regeneration of PAC at low temperature.
Fig. 1 Illustration of the main treatment stages at Walton-on-Thames Water Treatment Works with their approximated contact time. |
Water samples were collected at the end of each stage and securely sealed into 1 L plastic bottles by professional personnel from Thames Water. Water samples were then immediately transported to the Environmental Engineering Lab at UCL and stored in a fridge at 4 °C. All water samples were collected on the same day in September 2018 with an overall treated water output of 50 million litres. Considering the water treatment processes installed at WTWTW and the removal of PAC from the water (possibly towards the end of the treatment processes), PAC could potentially be applied after any treatment stage before ‘GAC adsorbers’. Therefore, this study did not consider the dosing of PAC after ‘GAC adsorbers’ since it would require additional installations to remove PAC, leading to extra cost.
Metaldehyde stock solution in methanol (100 mL of 500 mg L−1), metaldehyde calibration stock solution in DCM (100 mL of 500 mg L−1), and HA stock solution in MilliQ water (500 mL of 1000 mg L−1) were prepared the same way following our previous research.14,17 0.2 mL of metaldehyde stock solution was added to MilliQ water to make 100 mL of 1 mg L−1 metaldehyde in MilliQ water. From there, different volumes of metaldehyde in MilliQ water (100 mL of 1 mg L−1) were added to different water samples to prepare working metaldehyde solutions with concentrations from 1 to 50 μg L−1. PAC was added to the solution as slurry. PAC stock solution was prepared by adding 1.5 g of PAC into 50 mL of MilliQ water (30000 mg L−1). Prior to the addition of PAC, the slurry was well shaken to ensure uniformity.
Water characteristics including dissolved organic carbon (DOC), ions, dissolved oxygen (DO), pH, conductivity/total dissolved salts (TDS), turbidity, and UV absorbance at wavelength 254 (UV254) of different water samples were analysed using a Shimadzu total organic carbon analyser (TOC-L), Dionex ICS-1100, Jenway DO2 Meter 9200, pH/conductivity meter SevenMulti, Metter Toledo, HACH 2100AN IS turbidimeter (ISO method 7027), and Agilent Technologies Cary 60 UV-vis, respectively. These characteristics have been determined for the samples taken at different water sources without spiking metaldehyde (Table S2†), and they were measured in the Environmental Engineering Lab at UCL.
In this study, attenuated total reflection (ATR)-Fourier transform infrared (FTIR) (Bruker's Platinum ATR) and scanning electron microscopy (SEM, a JSM-6701F FESEM) were employed to analyse the flocs (from water collected after ‘static flocculation’) and the used PAC from both water collected after ‘static flocculation’ (PAC-SF) and from the Regent's Park lake (PAC-RP). The flocs and the used PAC samples were collected by filtering sample solutions through a 0.45 μm membrane and dried at room temperature for 24 hours. These analyses provide further insight into the relationships of flocs, PAC, and suspended solids in natural water.
The percentage removal and the amount of metaldehyde adsorbed onto PAC were calculated using eqn (1) and (2), respectively, where C0 is the initial concentration of metaldehyde, Ce is the final concentration of metaldehyde after adsorption by PAC at equilibrium, V is the volume of metaldehyde solution, m is the mass of PAC, and qe has as unit of μg mg−1:14,19,20
(1) |
(2) |
As the PAC dosage increased, the percentage removal of metaldehyde was enhanced; this agrees with the research of Anupam et al. who found higher removal of chromium(VI) from aqueous solutions with increasing dosage of PAC.22 There was no removal of metaldehyde for PAC dosage ≤ 0.5 mg L−1 and metaldehyde could not be detected with PAC dosage ≥ 100 mg L−1. The modified SPE loading method allows metaldehyde to be detected by GC-MS after SPE with a concentration 500 times lower than the limit of quantification of metaldehyde (5 ppb), around 0.01 ppb.17 Therefore, it indicates that PAC dosage ≥ 100 mg L−1 could ensure the concentration of metaldehyde solution after treatment to be below 0.01 μg L−1, which would be within the EU and UK standard of 0.1 μg L−1. The dotted line with a PAC dosage of 50 mg L−1 (30 mg of PAC in 600 mL metaldehyde solution) was the selected PAC dosage for the next set of experiments to make sure that metaldehyde is detected after adsorption.
Fig. 4 Concentrations of metaldehyde solution before and after adsorption using 50 mg L−1 PAC in water samples taken from the different treatment stages. |
The removal of metaldehyde was expected to be higher at the later treatment stages such as after ‘CoCoDAF units’ and after ‘main ozone contactors’. This is because the water quality is expected to be better (i.e. with fewer pollutants, and hence less competitive adsorption) in the later treatment stages compared to earlier stages such as after ‘static flocculation’. However, the highest removal of metaldehyde (98.4%) was found in water collected after ‘static flocculation’, while 94.1% and 97.2% of metaldehyde were removed from water collected after ‘CoCoDAF units’ and after ‘main ozone contactors’, respectively. The existence of flocs in water collected after ‘static flocculation’ distinguishes this water sample from all others; it is therefore suggested that flocs might play an important role in assisting the adsorption of metaldehyde onto PAC. This agrees with the finding of Jiang et al. who showed that there is a high degree of removal of salicylic acid, ibuprofen and diclofenac from water using super PAC with the presence of flocs, due to neutralization of charge and possible adsorption on the flocs.23 Cook and Newcombe explained that flocs in the system may have an open structure which enables the adsorbate to diffuse easily to the PAC particle.24
Since the removal of metaldehyde was the highest using water collected after ‘static flocculation’, another set of experiments was performed using this water sample with PAC dosages ranging from 5 to 150 mg L−1 to determine the maximum adsorption capacity of PAC. The percentage removals of metaldehyde between the most effective PAC dosing stage (after ‘static flocculation’) and the least effective PAC dosing stage (after ‘pre-ozone contactors’) were compared (Fig. 5A). The removal of metaldehyde from water collected after ‘static flocculation’ was slightly higher than that from the water collected after ‘pre-ozone contactors’ at all PAC dosages but relatively more distinctive at 50 mg L−1. This suggests that as the PAC dosage increased, the removal of metaldehyde was more effective when PAC was dosed in water collected after ‘static flocculation’. This finding agrees with Zhou et al. who suggested that a higher dosage of PAC achieved a higher removal of DOC from water with the presence of flocs and Li et al. who found that there was a higher removal of chemical oxygen demand and lead ions using a higher PAC dosage from wastewater with the presence of flocs.25,26 This is explained by Serpa et al. and Jiang et al. in that there is adsorption on both PAC-embedded flocs and PAC.23,27 In this case, a higher dosage of PAC provides more adsorption sites for removing metaldehyde; therefore, a relatively more distinctive increase in percentage removal of metaldehyde from water collected after ‘static flocculation’ was observed at a PAC dosage of 50 mg L−1.
Fig. 5(B) shows the fittings of the Freundlich isotherm model and the Langmuir isotherm model for metaldehyde adsorbed onto PAC in water collected after ‘pre-ozone contactors’ and after ‘static flocculation’. These two models were used in this study since they are the two most common isotherm models which give the maximum adsorption capacity (qm) of PAC for metaldehyde, hence providing indicative information for the potential application of PAC at WTWTW. The Freundlich isotherm model assumes multilayer sorption28 and it is generally used for heterogeneous surfaces. This model predicts that an increase of the adsorbate in the liquid will lead to an increase of the adsorbate concentrations on the adsorbent.14,19Eqn (3) and (4) describe the Freundlich isotherm model, with 1/n being the heterogeneity factor (adsorption intensity) and KF being the Freundlich constant (adsorption capacity).19,29
qe = KFCe1/n | (3) |
(4) |
(5) |
(6) |
The percentage removals of metaldehyde in Regent's Park lake water, MilliQ water, and MilliQ water spiked with HA were compared with the percentage removal of metaldehyde in water samples collected from different treatment stages at WTWTW (Fig. 6), and the water characteristics of the different water samples before and after PAC adsorption are presented (Fig. S2†).
Metaldehyde was effectively removed from all water samples, suggesting that adsorption of metaldehyde onto the PAC used in this study was not significantly affected by water quality such as the presence of organic matter and negative ions (Fig. 6). The presence of humic acid did not affect the removal of metaldehyde by the PAC as well, which agrees with our previous research.14 Removal of metaldehyde from water collected from the Regent's Park lake was the lowest (74.3%). The most distinctive characteristic of this water sample is that it has the highest NPOC of 11.47 mg L−1 among all the water samples (Fig. S2†). It is almost twice as high compared to the water samples collected at WTWTW (6.01–6.7 mg L−1). However, the high NPOC in this water sample was expected because the Regent's Park lake is inhabited by aquatic animals. The PAC used in this study is characterized by abundant micropores and mesopores which is favoured by adsorption of metaldehyde.14,15 However, this characteristic would also allow other organic compounds with small molecules to adsorb onto this PAC. In fact, Altmann et al. argued that small dissolved organic matter constituents are more effectively reduced by adsorption onto PAC, compared to high molecular-weight organics such as humic substances.35 Therefore, the relatively lower removal of metaldehyde from water collected from the Regent's Park lake, compared to the other water samples, could result from its relatively high NPOC; this water sample possibly contains organic matter with smaller fractions than humic acid and can adsorb onto PAC,35 as its NPOC decreased from 11.47 to 8.37 mg L−1 after PAC treatment.
The conductivity, pH and TDS (Fig. S2†) increased for all the water samples after adsorption by PAC due to the point of zero charge of PAC being 7.35.14 In fact, the conductivity and TDS slightly increased after adsorption by PAC for almost all the water samples. This may be explained by the fact that there are inorganic impurities from the making of activated carbon such as salts of alkali and iron from coconut shells.36 These impurities could detach from activated carbon, dissolve in water, and therefore increase the conductivity and TDS of the water. It is suggested by Song et al. and Cooney et al. that PAC can be washed with weak acid and deionized water to remove inorganic impurities prior to application.37,38
The removal of NPOC varied from 2% for water collected after ‘static flocculation’ to 41.6% for water collected after ‘CoCoDAF units’ (Fig. S2†), and there was a significant difference in the NPOC level before and after PAC treatment for all the water samples (p < 0.05), except for water collected after ‘static flocculation’. It is worth noting that NPOC increased after PAC treatment for the metaldehyde solution prepared using MilliQ water and MilliQ water spiked with HA. This suggests that the addition of PAC increased the DOC content in the solutions prepared using MilliQ water. A control test by adding 30 mg of PAC into 600 mL of MilliQ water with 30 min of constant stirring was performed to confirm that NPOC increased from 0.158 mg L−1 to 1.086 mg L−1 after PAC treatment. This was caused by the release of DOC from the surface of PAC due to the high concentration gradient between PAC and MilliQ water. The relatively low removal of NPOC may also be explained by the fact that organic matter in most of these water samples has large molecules and does not favour adsorption by the PAC used.35
Considering the potential application of PAC in WTWTW, the optimal location found in the laboratory tests to dose PAC is suggested to be after ‘static flocculation’. This is not only because the highest removal of metaldehyde (98.4%) was found in the water sample taken after this treatment stage (Fig. 4 and 6), but also because the majority of PAC in water can potentially be removed in the subsequent treatment stages of ‘CoCoDAF units’ and ‘GAC adsorbers’. However, dosing PAC after ‘static flocculation’ may affect the performance of ‘CoCoDAF units’ since a large amount of suspended solids would be present in the system. If the suspended PAC in water cannot be completely removed by the ‘CoCoDAF units’, it will then be filtered and retained by the ‘GAC adsorber’, which suggests that more frequent backwashing of the ‘GAC adsorber’ may be required. For example, the appropriate PAC dosage to treat 600 mL of 5 μg L−1 metaldehyde solution was larger than or equal to 60 mg to ensure that the treated water would meet the UK and EU standard. Considering the daily output of WTWTW (50 to 135 million litres per day), 5 to 13.5 tons of PAC might be needed to treat water with a metaldehyde concentration of 5 μg L−1 every day. And this could have a potential impact on the CoCoDAF units depending on their solid loading capacity which is normally 4–15 kg dry solids per hour per m2.39 Therefore, although the optimum dosing position of PAC at WTWTW to remove metaldehyde was found to be after the ‘static flocculation’, it is suggested that the removal of PAC from water in WTWTW needs to be investigated in situ.
Water samples | q m (μg mg−1) | K L (L mg−1) | Experimental conditions |
---|---|---|---|
MilliQ | 28.3 | 88.3 | 5 to 500 mg of PAC into 500 mL of 1 mg L−1 metaldehyde solution for 2 hours14 |
Water collected after ‘pre-ozone contactors’ | 0.25 | 1860 | 0.05 to 150 mg of PAC to 600 mL of 5 μg L−1 metaldehyde solution for 30 min |
Water collected after ‘static flocculation’ | 0.29 | 4200 | 5 to 150 mg of PAC to 600 mL of 5 μg L−1 metaldehyde solution for 30 min |
From Table 1, the qm and KL of metaldehyde adsorption by PAC varied significantly between different experimental conditions with a high initial concentration of 1 mg L−1 and low initial concentration of 5 μg L−1.14 Therefore, it suggests that the adsorption mechanism of metaldehyde onto PAC is different for metaldehyde solutions with different initial concentrations. Hence, to compare with the qm and KL obtained by our previous study in Table 1, MilliQ water was used for the study of the effect of the initial concentration of metaldehyde solution on metaldehyde adsorption onto PAC.14 30 mg of PAC was added into 600 mL of metaldehyde solutions prepared using MilliQ water with different initial concentrations (1 μg L−1 to 1000 μg L−1) for a contact time of 30 min to investigate the effect of the initial concentration on metaldehyde adsorption in a single adsorption system (Fig. 7).
As the initial concentration increased, the percentage removal of metaldehyde decreased (Fig. 7A). A similar decrease in removal of Alizarin Red S dye by a mustard husk adsorbent with increasing initial concentration was observed.32 The UK and EU standard of 0.1 g L−1 can be met by adding 30 mg of PAC into 600 mL metaldehyde solution prepared using MilliQ water with an initial concentration ≤10 μg L−1.
Adsorption isotherm models with experimental data points are presented (Fig. 7B). Data were quite well fitted with the Freundlich model (R2 = 0.9894) and the Langmuir model (R2 = 0.9857). The 1/n obtained from the Freundlich model is 0.43 (n = 2.33), suggesting moderate affinity, similar to that of water collected after ‘pre-ozone contactors’ but suggesting a slightly more homogeneous adsorption system.19 The KF is 0.87 μg mg−1 (μg−1 L)1/n, higher than that of the water collected after ‘pre-ozone contactors’. However, KF as an indicator for adsorption capacity is still low in this case; as discussed in section 3.3, a KF as low as 0.87 cannot explain the experimental results of effective removal of metaldehyde. Hence, the Freundlich model is not suitable for explaining the adsorption mechanism. In terms of fitting with the Langmuir model, the maximum adsorption capacity (qm) is 13.16 μg mg−1 and the Langmuir constant (KL) is 0.07 L μg−1. Although they are lower than those of our previous study (28.3 μg mg−1 and 88.3 L mg−1), they could confirm the experimental results of effective removal of metaldehyde. Hence, the Langmuir model is considered a better model for adsorption of metaldehyde in MilliQ water onto PAC with low initial concentrations.
Since the qm of 13.16 μg mg−1 and KL of 70 L mg−1 are lower than those obtained in our previous study using MilliQ water,14 it suggests that the adsorption capacity of PAC is higher with a higher initial concentration of metaldehyde. This behaviour may be explained by the fact that the driving force to overcome the resistance to the mass transfer from the adsorbate in the aqueous phase to the solid phase on the adsorbent was initiated by the initial concentration of the adsorbate, and the increase in initial concentration would enhance the interaction between them.32 Therefore, the increase in initial concentration would increase the driving force of the concentration gradient and result in enhanced uptake of the adsorbate. This was also confirmed by the research of Gautam et al. who found that the adsorption capacity of activated carbon for metal ions increased with increasing initial concentration.40 The low adsorption capacity of PAC for metaldehyde at low initial concentrations can be explained from another aspect. The water molecule can be considered as a significant factor in explaining the mechanism of adsorption of metaldehyde by PAC as well. A low initial concentration of metaldehyde solution suggests that more water molecules are present in the system. The research of Ferino-Pérez et al. showed that water is like an intermediary between activated carbon and metaldehyde and it competes with metaldehyde for adsorption sites on PAC.41 Busquets et al. also argued that there is a possible competition between water and metaldehyde molecules for adsorption onto activated carbon.15 Therefore, less metaldehyde may be adsorbed onto PAC when the initial concentration of metaldehyde is low and there are more water molecules present in the system.
C metaldehyde in water after dosing used PAC | M metaldehyde desorbed back to water | |
---|---|---|
PAC dried at room temperature | 79.07 ± 7.17 μg L−1 | 47.44 ± 4.3 μg |
PAC dried in an oven at 60 °C | 6.41 ± 0.83 μg L−1 | 3.85 ± 0.5 μg |
From Table 2, it is suggested that a small amount of metaldehyde (i.e. 8.6 ± 0.8%) would desorb from PAC back to water which was due to the high concentration gradient of metaldehyde between MilliQ water and PAC. Desorption is very common for activated carbon and it is reported that desorption of metaldehyde from GAC back to water occurs when the inlet concentration of metaldehyde decreases.13,42 However, after heating the used PAC in the oven at 60 °C for 24 hours, the amount of metaldehyde found in water was 3.85 ± 0.5 μg which was only 0.7 ± 0.1% of the total 550.58 μg metaldehyde adsorbed, significantly less than the PAC dried at room temperature. This implies that the majority of adsorbed metaldehyde on PAC might have been thermally degraded in the oven at 60 °C. Therefore, although there was desorption of metaldehyde from PAC back to water, the degree of desorption was relatively small. Degradation of metaldehyde at 60 °C suggested that the used PAC for metaldehyde removal could be potentially regenerated at low temperature. Therefore, it is recommended to further investigate the regeneration of used PAC by thermal treatments using a wide range of temperatures. Also, it is suggested to repeat the tests using natural water to investigate the effects of combined adsorption and desorption of metaldehyde on PAC with the presence of other organic compounds and analyse the adsorption capacity of regenerated PAC regarding metaldehyde removal.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ew00962k |
This journal is © The Royal Society of Chemistry 2020 |