Investigation of the Sorption Mechanism of Selected Organophosphorus Pesticides on Danube Sediment

Research article

Tamara Apostolović1, Marijana Kragulj Isakovski1, Snežana Maletić1, Jelena Tričković1, Aleksandra Tubić1, Jelena Molnar Jazić1, Jasmina Agbaba1

 

1 University of Novi Sad, Faculty of Sciences, Department of Chemistry, Biochemistry and Environmental Protection, Trg Dositeja Obradovića 3, 21000 Novi Sad, R. Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

Sorption processes in equilibrium and non-equilibrium conditions have been thoroughly investigated over the past several years. In this paper, the sorption behaviour of two organophosphorus pesticides (chlorpyrifos-methyl and chlorfenvinphos) on Danube sediment was studied. For investigated pesticides, the retardation coefficients (Rd) were 6.2 for chlorfenvinphos and 15.5 for chlorpyrifos-methyl. In addition, chlorpyrifos-methyl showed a significant degree of biodegradation with λ = 4.15. Batch sorption experiments are in line with column experiments and showed that Danube sediment has higher sorption capacity for chlorpyrifos-methyl than for chlorfenvinphos. Both experimental approaches showed good agreement with the hydrophobicity of the investigated compounds. However, for final conclusions, further research should be focused on a more detailed characterization of sediment as well as an investigation of a larger set of organic compounds.

Keywords: transport, sorption, sediment, organophosphorus pesticides.

Introduction

 

Sorption of hydrophobic organic compounds on soil/sediment is the basic process that determines the transport and fate of these compounds in the aquatic environment. Sorption on solid phase can be investigated using two experimental techniques: static and dynamic experiments. Static experiments involve sorption under the conditions of the achieved sorption equilibrium, while dynamic experiments involve sorption in the non-equilibrium conditions that usually exist during the transport of organic compounds carried by water through the sediment column. The results obtained using dynamic experiments allow for a better estimation of the compounds’ mobility in porous media relative to the distribution coefficients obtained by static experiments in equilibrium conditions. Bi et al. (2009) showed that isotherms resulting from batch experiments had a somewhat higher degree of linearity and higher Kf values compared to the column experiments. The same group of authors have concluded that Freundlich isotherms cannot reveal mechanistically certain interactions in the sorption processes, but its exponent n can provide an insight into the potential types of sorptive processes (cation exchange, complexation). Alternatively, the discrepancy between the fitted parameters of the two methods may be attributed to the differences: mixing techniques applied in the two examined methods, reductions in soil particle spacing inside the column and differences in soil/water ratio between the batch and column experiments (Leovac Maćerak et al., 2019).

Nowadays, organophosphorus pesticides (OPPs) are intensively used, but their fate in the environment is still unknown. OPPs are the most widely spread group of insecticides which has been most commonly applied for the control of mosquitoes. Although they are less persistent and do not bioaccumulate in the environment, they attract the attention of the scientists due to high environmental toxicity (Yang et al., 2018). The metabolic degradation pathways of the OPPs occur through the oxidation or hydrolysis of the basic substance, where more toxic compounds may be formed. It should be noted that over 4 million tons of pesticides are being applied annually, with about 1% of that amount being accumulated (Kanagasubbulakshmi et al., 2018). OPPs are detected in the aquatic environment as a consequence of industrial waste deposition and leaching or through direct industry effluent discharge into surface waters. However, the most important pathway of surface water pollution by OPPs is the treatment of insects and plants followed by leaching of OPPs to and from soil into surface and groundwater (Kanagasubbulakshmi et al., 2018).

Taking this into account, the aim of the study was to investigate the equilibrium and non-equilibrium sorption of two selected OPPs (chlorpyrifos-methyl and chlorfenvinphos) with different hydrophobicity on the Danube river sediment, in order to assess the mobility of these compounds through the Danube sediment. Additionally, the obtained sorption affinity (Kd) was compared with retardation coefficients for the same compounds on the same sediments. It should be noted that the investigation of the organic compounds sorption behavior during their transport through the sandy material is significant due to the infiltration of the Danube water into the groundwater which could subsequently lead to pollutants such as OPPs reaching the wells for drinking water supply.

 

Materials and Methods

Characterization of the Sediment

Sampling of the alluvial sediment of the Danube River was carried out at the "Petrovaradinska ada" water source near Novi Sad (45°26'1.437"N, 19°86'6.364"E) according to methods ISO 10381-2:2002 and ISO 18512:2007. Sediment characterization implied determining the content of organic carbon (OC), organic matter (OM) and granulometric composition. Content of OC was determined using a TOC analyzer (liquiTOCII, Elementar, Germany) after treating the samples with a solution of acid to remove inorganic carbon, according to ISO 10694:1995 method. The granulometric composition was determined by a wet sieving method using a series of sieves (Sieve Shaker mod., PR 09, CISA), while the OM content was determined gravimetrically, after drying the sediment to a constant mass at 105°C and incineration at 550°C for 6 h.

 

Batch Experiments

Static, batch experiments were carried out in a synthetic matrix (background solution - 0.01 M CaCl2 and 100 mg/l NaN3 in distilled water) at room temperature (20 ± 2°C). The solid phase:liquid phase ratio was about 1:40 and 3:40 for chlorpyrifos-methyl and chlorfenvinphos, respectively, and the initial concentration of the OPPs was in range 100-1000 µg/l. The samples were subjected to continuous mixing for 48 h at 180 rpm, to ensure sorption-desorption equilibrium. Batch experiments were performed as followed: a certain mass of the sediment was measured into glass vials with PTFE screw caps, and 40 ml of background solution was added. Samples were spiked with certain volumes of either chlorpyrifos-methyl or chlorfenvinphos work solution, so that the initial concentrations ranged from 100 to 1000 µg/l. After subjecting the samples to 48 h of continuous mixing, the liquid and solid phase were separated by centrifuge. The remaining concentration of the OPPs in the liquid phase was determined after liquid-liquid extraction with hexane, by gas chromatography with mass spectrometry (GC/MS, Agilent 7890A/5975C, USA).

 

Column Experiments

Dynamic, column experiments were performed using a stainless steel column. The experiment was carried out in the following manner: a specific mass of the previously dried and ground Danube sediment was filled in the column from the upper side, while the background solution was fed from the bottom by a peristaltic pump. This way, the column was homogenously packed, without the presence of air and without the layering of different fractions of the sediment. All experiments were performed at room temperature (20 ± 2°C).

An aqueous solution of a tracer substance, thiourea, prepared in a background solution without NaN3 at a concentration of about 4 mg/l, was passed through the column. Eluates were collected in certain time intervals, in which the concentration of thiourea was determined on a UV spectrophotometer by measuring absorbance at a wavelength of 235 nm (UV-1800 Shimadzu, Japan). The tracer substance was used to determine the hydrodynamic properties of the packed column. The tracer substance was passed through the column until the concentration of thiourea in the eluate was equal to the initial concentration. After that, individual solutions of selected OPPs at a flow rate of about 6.35 ml/min were passed through the column, based on the measured filtration coefficient, in order to simulate the realistic conditions of hydraulic permeability during transport through the selected geosorbent. OPP solutions were prepared as follows: a certain volume of the tested OPP was added in 5 l of background solution with an automatic pipette so that the final concentration of the individual pesticide in the solution was about 50 μg/l. Eluates were collected at certain time intervals for 7 h. The concentration of the investigated OPPs in the eluates was determined by GC/MS analysis, preceded by liquid-liquid extraction of the aqueous solutions with hexane.

 

Data Analysis

For the fitting of the experimental data obtained by the batch experiments, two widely used models, Langmuir and Freundlich, were applied. The Freundlich model is an empirical equation assuming heterogeneous adsorptive energies on the adsorbent surface, whereas the Langmuir model is based on the assumption of a homogeneous adsorbent surface with identical adsorption sites (Liu et al., 2010). A conventional advective-dispersive equation (ADE) was used to describe the non-equilibrium sorption of OPPs during column experiments (Bertelkamp, 2015, Burke et al., 2013).

 

Results and Discussion

Characterization of Sediment and Physical Chemical Properties of OPPs

The OC and OM content of the Danube sediment was 0.24 and 1.02%, respectively. The grain size and distribution analysis showed that the sediment consisted mostly of sand (65.1% medium sized sand with grain diameters ranging 500-200 µm, and 24.5% fine sand with grain diameters ranging 200-63 µm). The content of silt (63-2 µm) and clay (<2 µm) was 3.67% and 5.47%, respectively. Based on the obtained results, it can be concluded that the investigated sediment is a sandy aquifer material with low OC content. A more detailed characterization of the investigated Danube sediment was given by Apostolović et al. (2018).

The main properties of the selected OPPs are presented in Table 1. It can be observed that the investigated compounds are hydrophobic organic compounds with logKOW 3.81 and 4.31 for chlorvenvinphos and chlorpyrifos-methyl, respectively.

 

Table 1: Characteristics of the selected OPPs.


Equilibrium Sorption of Organophosphorus Pesticides on Danube Sediment

Results of sorption isotherms were fitted by Freundlich and Langmuir sorption models (Figure 1A and B). The obtained sorption parameters are presented in Table 2. Determination coeficients, R2, showed that the sorption of OPPs on Danube sediment is better described by the Langmuir model. This indicates that the sorption process takes place in a monomolecular layer, as well as that there is a fixed number of active sorption sites on the surface of the geosorbent, which all show equal affinity for OPP sorption. Distribution coefficients (logKd), calculated from the sorption parameters obtained by modeling the experimental data with the Langmuir model, decreased with the increased equilibrium concentration of the aqueous solution, showing that at lower OPP concentrations, a higher degree of sorption was achieved, which gradually decreased as the concentration of OPPs in the solution increased, due to the saturation of the available active sites on the sediment.

 

Figure 1: Sorption isotherms fitted by Langmuir and Freundlich models for:
A) chlorpyrifos-methyl and B) chlorvenvinphos.

 

Further, lower logKd values can be observed for chlofenvinphos than chlorpyrifos-methyl. These results correlate with the hydrophobicity of the investigated OPPs, given that chlorpyrifos-methyl has a higher octanol-water distribution coefficient (logKOW=4.31) and is more likely to partition to the solid phase than the slightly more polar chlofenvinphos (logKOW =3.81). In addition, chlorfenvinphos has about 30 times higher water solubility, and thus shows a much higher affinity for the aqueous phase. The higher affinity of the investigated sediment for chlorpyrifos-methyl sorption is further shown by the maximum sorption capacities of the Danube sediment (qmax values) obtained by fitting the experimental data with the Langmuir model. The sediment showed about 3 times higher sorption capacity for chlorpyrifos-methyl than for chlorfenvinphos (Table 2). The influence of hydrophobicity on organophosphorus pesticide sorption in natural geosorbents has been suggested by other authors (Liu et al., 2018, 2019; Alfonso et al., 2017; Dare et al., 2017).

 

Table 2: Sorption parameters obtained using Freundlich and Langmuir sorption models.

 

Non-Equilibrium Sorption of Organophosphorus Pesticides on Danube Sediment

Further studies were thus conducted using column experiments with the aim of comparing the conclusions obtained using both experimental approaches. Additionally, column experiments were conducted to examine the transport of selected OPPs in order to assess the mobility of these compounds in the aquatic environment. Figure 2 presents the obtained breakthrough curves for chlorpyrifos-methyl and chlorfenvifos, as well as for a tracer substance, thiourea, whereas, Figure 3 shows the breakthrough curves of the investigated compounds modeled with ADE.

The breakthrough curve of thiourea as a tracer substance is symmetrical and sigmoidal, indicating the absence of physical non-equilibrium processes in the porous medium. The appearance of the tracer breakthrough curve is related to the physical non-equilibrium processes in the porous medium (Jellali et al., 2010), where the existence of such processes can be a direct consequence of the non-homogeneous packing of the column.

The obtained transport parameters for the investigated OPPs are presented in Table 3. In order to determine the retardation coefficient (Rd), the original dispersion coefficient obtained for thiourea was used in the case of chlorpyrifos-methyl (α=0.009 m), while for chlorphenviphos the dispersion coefficient was corrected and amounted to α = 0.045.

If the test substance does not show sorption during transport, its breakthrough curve should be identical to the breakthrough curve of the tracer substance (Bi et al., 2009). However, by modeling the obtained experimental data using the ADE equation, it has been observed that both investigated pesticides exhibit a certain degree of nonlinear sorption (Table 3, Figure 3).

 

Figure 2: Breakthrough curve for the tracer substance and the investigated OPPs.

 

Figure 3: Fitted breakthrough curves for the tracer substance and the investigated OPPs.

 

Table 3: Parameters of transport for OPPs.

 

Under investigated experimental conditions, the obtained retardation coefficients, Rd, were 15.5 and 6.2 for chlorpyrifos-methyl and chlorfenivinphos, respectively. For the chlorpyrifos-methyl curve, the graph shows somewhat greater asymmetry compared to the curve obtained for chlorfenvinphos (Figure 3), which is in accordance with its lower degree of nonlinear sorption in relation to chlorpyrifos-methyl in non-equilibrium conditions.

By comparing Rd values with the logKOW values of the investigated compounds, it can be noticed that with the increase in the hydrophobicity of the molecules, Rd values also increase (Table 3). Thus, for the more hydrophobic chlorpyrifos-methyl an approximately 2.5 times higher retardation coefficient was obtained, which supports a greater non-linear sorption during transport through the sediment column. Additionally, the chlorpyrifos-methyl molecule showed a certain degree of biodegradation (Figure 3, Table 3) with biodegradation coefficient λ=4.15.

When the Rd values are normalized with the hydrophobicity of the investigated OPPs expressed through KOW, a Rd/KOW value was obtained. Based on the obtained ratios, it can be concluded that in addition to hydrophobic interactions, nonlinear sorption of chlorpyrifos-methyl can be the result of other specific interactions such as π-π, electron donor acceptor, hydrogen bond, etc. Literature data indicate the significance of specific interactions in the overall sorption mechanism of OPPs on the sediment organic matter (Liu et al., 2019; Yang et al., 2018).

Comparing the parameters obtained from the column experiments and the parameters of Langmuir model for the same compounds and the same sediment, the following can be concluded: the higher the sorption affinity, the higher retardation of the investigated OPPs in the column. It is important to note that both the sorption affinities obtained from the batch experiments and the retardation coefficients from the column experiments agreed well with the compound’s logKOW values. Danube sediment showed a good agreement between molecular hydrophobicity and sorption affinity, indicating that the increase in sorption affinities was consistent with the compounds hydrophobicity. Both experimental approaches showed that the hydrophobicity of a molecule provides a reasonable parameter for prediction of its sorption and transport behavior under the conditions investigated. Thus, Maraqa et al. (2011) who found that linear sorption distribution coefficient of phthalates could be reasonably predicted based on compound hydrophobicity and fraction of soil organic carbon. Also, authors Tričković et al. (2016) suggest the same conclusions.

 

Conclusion

The paper examines the sorption in equilibrium and non-equilibrium conditions, for two compounds from the group of OPPs on the sediment of the Danube River. Transport parameters show an increase in nonlinear sorption with the increase in hydrophobicity of the molecule. Further, the more hydrophobic chlorpyrifos-methyl showed a significantly higher degree of non-equilibrium sorption than chlorfenivinphos. In addition, during the transport of chlorpyrifos-methyl biodegradation occurs, while in the case of chlorfenvinphos biodegradation is not observed. Batch sorption experiments showed that the Danube sediment has about a 3 times higher sorption capacity for chlorpyrifos-methyl than for chlorfenvinphos. In addition, both experimental approaches showed that the hydrophobicity of OPPs is a significant parameter for predicting the behaviour of these pesticides in the environment. Further research should focus on the more detailed characterization of the Danube sediment as well as the testing of the transport of a larger set of organic molecules.

 

Acknowledgment

The research was funded by the Ministry of Education, Science and Technological Development of Republic of Serbia through project III43005. The authors also owe thanks for the kind support of the Provincial Secretariat for Higher Education and Scientific Research, Republic of Serbia, Autonomous Province of Vojvodina (142-451-2128/2019-02).


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