The presence of pesticides in the environment is of great concern due to their persistent nature and chronic adverse effect on human health and the environment. Water bodies are subject to pollution by organochlorine pesticides, especially in developing countries, where water pollution is a key sustainability challenge. Hence, activated carbon is considered a universal adsorbent for the removal of organochlorine pollutants from water. Activated carbon from Acatia etbaica was prepared using traditional kilns with low investment costs. Pesticides such as aldrin, dieldrin and DDT were selected for adsorption because of their common usage in agricultural and malaria control activities and may occur in high concentrations in surface waters that are used as drinking water sources. The effect of the adsorbent dose and initial concentration were investigated. To describe the equilibrium isotherms the experimental data were analyzed by the Langmuir and Freundlich isotherm models. The Freundlich model gave the best correlation with the experimental data. Activated carbon prepared from Acacia etbaica was found to be an effective and low-cost alternative for the removal of organochlorine pesticides from aqueous solutions. The preparation method allows the use of this material by local communities for effective remediation of pollution by pesticides.
Surface water bodies are subject to pollution by organochlorine pesticides, especially in developing countries, where the safety of surface water bodies is closely related to human health [
Organochlorine pesticides have been widely used around the world to boost agricultural crop yield and to control vector-borne diseases [
A number of technologies are available to control water pollution [
Activated carbon is considered a universal adsorbent for the removal of diverse types of pollutants from water [
Acacia etbaica (A. etbaica) belongs to the Fabaceae-Mimosoideae family of plants and is also known as arrad (in Arabic), mgunga (in Swahili) and seraw (in Tigrigna) (Agroforestry tree data base). Acacia etbaica occurs in dry bush land, thickets, semi-desert scrub and wooded grasslands. Countries where this crop is commonly known are Eritrea, Ethiopia, Kenya, Somalia, Sudan, Tanzania, and Uganda; however, it is also found elsewhere. Acacia etbaica is widely used as a source of firewood. The tree is also widely used to make the pillars and beams of earthen houses in northern Ethiopia [
The purpose of this work is not only to develop a low-cost method that can be used in remote communities, but also to evaluate the adsorption capacity of A. etbaica-based activated carbon in removing trace levels of organic pollutants from aqueous solution. None have been recorded on the adsorption of organochlorine pesticides using A. etbaica activated carbon and used to embark on this investigation. Organochlorine pesticides of aldrin, dieldrin and DDT were selected for this study, since they are a critical threat for local communities in developing countries. They are toxic and their application has been banned worldwide. Despite this, most are widely used in many developing countries for the control of mosquitos, harmful soil insects and plant pests [
The granulated activated carbons were selected from two raw materials: commercial activated carbon (CAC) (NORIT N.V, Amersfoort―The Netherlands) and activated carbon made from A. etbaica (AEAC) (locally made in Ethiopia). The commercial activated carbon was used as a reference in comparison with the local activated carbon. Acacia etbaica was obtained from local villages, where it is mainly used as energy source.
Dry wood logs of A. etbaica was cut into pieces, 50 - 100 cm size, and buried in earth-covered traditional kilns for weeks, where wood is cut and stacked before being covered in earth and carbonized. The kilns are practical with low-investment options for poor producers. The charcoal was ground in a high-speed rotary cutting mill and sieved into different mm sizes. Before the application of the charcoal to our research it was washed several times with distilled water to remove dust and some other residuals. The washed samples were dried at room temperature and packed in an air tight container.
For better understanding the surface properties, both commercial and locally made activated carbon was examined using scanning electron microscopy (Philips XL 30 FEG SEM, at 10 keV, The Netherlands).
For the determination of metal content, 0.1 g of adsorbent sample was mixed with 5 mL of 70% nitric acid in a plastic beaker and gently boiled for 15 minutes. When no more brown fumes of NO2 were observed, 5 mL of perchloric acid was added and gentle boiling continued until almost all material had dissolved. The mixture was then filtered and washed three times with distilled water. The filtrate and washings were diluted to 100 mL with de-ionized water. The solution obtained was analyzed for metal content using ICP-MS ('Thermo X Series 1, Thermo Fischer Scientific, Belgium). The methods of detection limits (MDLs) for ICP-MS used for the analysis of most elements and their limits of quantification was 1 µg/L with the relative standard deviation (RSD %) value 3.0%. T-test for the elemental composition of the adsorbents was performed using SAS 9.2.
Analytical grade chemicals were used during the experiment: aldrin, dieldrin, DDT and trifluralin (Sigma Aldrich, Belgium), methanol (VWR, Belgium), dichloromethane (Fisher Scientific, Belgium), anhydrous sodium sulphate (ACROS, Belgium), nitric acid (Fisher Scientific, Belgium), perchloric acid (Fisher Scientific, Belgium).
Test solutions of organochlorine pesticides were prepared by serial dilution of stock solutions using methanol (VWR, Belgium). The concentrations of pesticides added to the water samples ranged from 1 to 1000 µg/L, to simulate actual concentrations in surface water up to extreme circumstances [
The extracted pesticides were analyzed by gas chromatography (GC) on a Perkin Elmer Auto System XL and a Perkin Elmer electron capture detector (ECD). The column used was CP-Sil 8CB (Chromopack, WCOT Fused Silica, 50 m × 0.25 mm, ID = 0.4, Holland). All GC analyses were carried out at 260˚C, 250˚C and 275˚C for column, injector and detector, respectively, in a total run time of 40.5 min/sample. The mobile phase used was nitrogen at a flow rate of 60 mL/min.
Batch equilibrium adsorption experiments were performed using 100 mL of spiked pesticide aqueous solutions. After the adsorption process, the adsorbent was separated from the samples by filtering and pre-concentrated using liquid?liquid extraction. Two extractions with 35 mL of dichloromethane (Fisher Scientific) were carried out for each sample. To control for losses during extraction, 5 µL of 100 mg/L trifluralin (Sigma Aldrich) was added as an internal standard to each sample. The extracts were combined and dried with anhydrous sodium sulphate (ACROS). The extracts were concentrated to 1 - 2 mL by evaporation at 65˚C in a Kuderna-Danish flask. The samples were kept in a refrigerator at 4˚C until analysis.
We investigated the effects on adsorption of the organochlorine pesticides onto A. etbaica derived and commercial activated carbon of adsorption parameters such as particle size (0.25 - 2 mm), adsorbent dose (2.5, 5.0, 7.5, and 10.0 g/L), initial pesticides concentration (10, 25, 50, 100, 250 µg/L), contact time (120 min) were studied at room temperature and pH 7 in a batch mode of operation.
The amount of pesticide adsorbed per weight unit of activated carbon, qe, was calculated using the Equation (1):
where Co and Ce are the pesticide concentration measured before and after adsorption (µg/L), V is the volume of aqueous solution (L) and W is dry weight of the adsorbent (g). Two replicates per sample were done and the average results were used. By quantifying the pesticide concentration before and after adsorption, the efficiency of adsorption of pesticide by activated carbon was calculated by using Equation (2):
The column experiment was carried out using filter funnel columns (KU Leuven, C. G. B.) with an internal diameter of 2.5 mm and a bottom with a pore size less than 0.25 mm not to lose any adsorbent material. The columns were made of transparent glass, and had a height of 210 mm. These columns were filled with varied doses of 3, 5, 7, 9 and 11 g of 0.25 - 0.5 mm size AEAC adsorbents. Prior to column filtration, the turbidity of the adsorbent material were removed by flushing of distilled water through the adsorbent. Then, the column was used for sample filtration. In these experiments, a substantially lower concentration of pesticides 20 µg/L of each component was applied in order to approach realistic concentrations in surface waters. Each time 25 mL of water spiked with 20 µg/L of pesticides (aldrin, dieldrin and DDT) was flushed through the column and the amount of pesticides in the effluent determined. The small-scale column tests were performed in a laboratory set-up by adjusting a constant flow rate at room temperature. Through this the breakthrough curve of the AEAC was also determined.
Adsorption isotherms are equilibrium relationships between the concentration of the adsorbed pesticides and their concentration retained in the solution at a given temperature. The adsorption isotherm experiments were conducted on the basis of batch experiments [
The Langmuir model is a non-linear model that suggests a monolayer uptake of the pesticides on a homogenous surface, having uniform energies of adsorption for all the binding sites without any interaction between the adsorbent molecules [
where Ce (µg/L) is the equilibrium concentration, Qo (µg/g) the monolayer capacity of the adsorbent, and b (L/µg) is the Langmuir adsorption constant. A plot of Ce/qe versus Ce gives Qo and b if the isotherm follows the Langmuir model.
The Freundlich isotherm is an empirical model that is based on adsorption on heterogeneous surface and active sites with different energy. The linearized Freundlich isotherm [
where qe is the amount of adsorbed analyte (µg/g), Ce is the equilibrium concentration of the adsorbate (µg/L) and KF (µg/g (L/µg)1/n and 1/n are Freundlich constants related to adsorption capacity of the adsorbents and surface heterogeneity. When log qe is plotted against log Ce and data are analyzed by linear regression, 1/n and KF constants can be determined from the slope and intercept, respectively [
Since adsorption is a surface phenomenon, the rate and extent of adsorption specific to a given adsorbent are influenced by the physico-chemical characteristics of the adsorbent such as surface area, pore size, surface chemistry and elemental composition [
Scanning electron microscopy (SEM) was used to observe the surface physical morphology of the adsorbents. SEM micrographs of AEAC and CAC are given in
The adsorbent dose is an important parameter because this parameter determines the capacity of adsorbent for a given adsorbate concentration and also determines sorbent-sorbate equilibrium of the system [
Type of adsorbent | Na | Mg | Si | K | Ca | V | Cr | Fe | Mn |
---|---|---|---|---|---|---|---|---|---|
AEAC | 15 ± 1 | 160 ± 18 | 631 ± 146 | 281 ± 32 | 5305 ± 495 | 25 ± 4 | 10 ± 4 | 118 ± 31 | 3 ± 0.7 |
CAC | 155 ± 13 | 850 ± 28 | 661 ± 331 | 94 ± 6 | 897 ± 54 | 26 ± 3 | 13 ± 8 | 220 ± 12 | 6 ± 0.4 |
p-value | 0.003 | <0.0001 | 0.893 | 0.008 | 0.004 | 0.734 | 0.660 | 0.019 | 0.005 |
Type of adsorbent | Co | Ni | Cu | Zn | Mo | Cd | Sn | Pb | As |
AEAC | 0.1 ± 0.02 | 2.8 ± 1 | 0.9 ± 0.07 | 3.9 ± 1 | 0.2 ± 0.03 | 0.1 ± 0.03 | 0.2 ± 0.04 | ||
CAC | 0.1 ± 0.01 | 1.1 ± 0.21 | 2.7 ± 1.9 | 6.0 ± 2 | 0.2 ± 0.05 | 0.2 ± 0.07 | 0.4 ± 0.2 | ||
p-value | 0.359 | 0.447 | 0.253 | 0.202 | 0.195 | - | 0.062 | 0.245 | - |
and pH 7. The percentage of pesticides adsorption with varying amounts of adsorbents (AEAC and CAC) is presented in
Adsorption of organochlorine pesticides (aldrin, dieldrin and DDT) from aqueous solution onto AEAC and CAC was measured for five different initial concentrations (10, 25, 50, 100, and 250 µg/L) at an adsorbent dosage of 10 g/L. The effect of the initial concentration of pesticides on the amount of adsorption is shown in
The Langmuir and Freundlich isotherm parameters for adsorption of organochlorine pesticides (Aldrin, Dieldrin and DDT) on AEAC and CAC are given in
with a high correlation coefficient for the locally prepared activated carbon. This result suggests the formation of multilayer coverage of the organochlorine pesticides (aldrin, dieldrin and DDT) at the surface of the two activated carbons, which is attributed to the heterogeneous active sites on the surface of AEAC and CAC.
Adsorbent | AEAC | CAC | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
OPCs | Langmuir model | Freundlich model | Langmuir model | Freundlich model | ||||||||
Qo (µg/g) | b (L/µg) | R2 | KF | n | R2 | Qo (µg/g) | b (L/µg) | R2 | KF | n | R2 | |
Aldrin | −0.395 | −0.005 | 0.671 | 1.81 × 10−4 | 0.545 | 0.986 | −0.036 | −0.003 | 0.534 | 3.66 × 10−5 | 0.744 | 0.917 |
Dieldrin | −3.71 | −0.003 | 0.423 | 4.57 × 10−3 | 0.785 | 0.987 | −0.105 | −0.002 | 0.634 | 1.36 × 10−4 | 0.874 | 0.966 |
DDT | −0.489 | −0.005 | 0.535 | 1.99 × 10−4 | 0.518 | 0.981 | −0.002 | −0.002 | 0.765 | 1.25 × 10−4 | 0.823 | 0.975 |
The value of KF determines the adsorption capacity of adsorbent at equilibrium concentration in a solution [
The slope (1/n) for Freundlich’s model, which is used for assessing the adsorption intensity of a given substance from water phase adsorbent [
For the purposes of the removal of trace levels of organic pollutants from drinking water, the main concern is whether or not a treatment process can efficiently and economically remove the organochlorine pesticides to acceptable levels from water. Rapid small-scale column testing is an inexpensive, accelerated testing method that can be used to determine the adsorptive characteristics for large-scale adsorption using small column studies. The advantage of such rapid small-scale testing over pilot testing is the time savings [
Flushing distilled water through the column results in removal of the turbidity from AEAC. During flushing 0.2 L was found to be sufficient for AEAC to reduce turbidity to less than 5 NTU, which is in the range of international standards assumed acceptable [
Breakthrough of the three pesticides studied for a column was observed after filtering 2.5 L of water; the first pesticides to appear in the filtered water were aldrin and DDT. This means that roughly 1 kg of ACAC is needed to filter 10 L of contaminated water, corresponding small slices of the widely available A. etbaica wood. Although care should be taken about the adsorption capacity for other pesticides (and, potentially, other micropollutants) that may occur in surface waters, the method is postulated to be feasible and safe for production of potable water from sources contaminated with pesticides.
A distinct advantage of using AEAC as adsorbent is that it is cheaper than CAC available in Ethiopia which costs 373000.00 Ethiopian Birr/ton or US$ 20687.74/ton. The AEAC costs 3000.00 Ethiopian Birr/ton or US$ 166.39/ton, and is available at the local markets, so that extra costs for labor and transport are avoided. Another advantage is that this material can be locally made even in remote communities, at low cost as the materials do not have to be imported and local markets and production are stimulated.
The total cost of activated carbon adsorption also includes construction costs of the water purification unit, and regeneration of activated carbon. The former costs are similar for both types of activated carbon, whereas the regeneration costs depend on the runtime of an adsorption column when operated in continuous mode. Although this aspect was beyond the scope of the work presented here, it can be expected that regeneration costs are similar for the two types of activated carbon, given the fact that they have been shown to have a comparable performance in batch experiments. Therefore, the difference in cost is mainly related to the difference in cost of the activated carbon.
This study investigates the adsorption of organochlorine pesticides: aldrin, dieldrin and DDT onto AEAC and CAC. The extent of pesticide removal increased with decreasing initial concentration of pesticides and increased with increasing adsorbent mass. The adsorptions of pesticides on the studied adsorbents (AEAC and CAC) were influenced by the physico-chemical properties of pesticides and adsorbents such as molecular weight, log Kow, and hydrophobic characteristics of pesticides; and surface morphology of the adsorbents. The equilibrium data in this study were fitted with the Freundlich isotherm model. Our results indicate that the AEAC used in this investigation, which is freely, abundantly and locally available, could be used as an efficient and alternative adsorbent in water treatments. Finally, a cost comparison of AEAC and CAC leads to the conclusion that AEAC is a more effective and low-cost adsorbent that can be used for the removal of the studied pesticides and likely other impurities from water. The AEAC can also be produced and used by communities living in remote areas.
This study was financially supported by the Vlaamse Interuniversitaire Raad-University Development Cooperation (VLIR-UOS), Belgium in collaboration with Mekelle University, Ethiopia. The authors thank Mrs. Christine Wouters, Mrs. Michele Vanroelen and Mrs. Ruixin Zhang for their kind support in pesticide, metal and surface physical morphology (SEM) analysis.