In this study, a polymeric adsorbent based on carboxymethyl tragacanth (CMT) grafted by poly acrylic acid-co-acrylamide (AAc-co-AAm) synthesized by radical polymerization for the first time was used to remove the fungicide penconazole (PEN) or Topas 20% from surface water. The parameters of solution pH, adsorption isotherm, and adsorption kinetics of PEN were studied by the synthetic adsorbent. The surface morphology and functional groups of CMT-g-poly (AAc-co-AAm) were confirmed by XRD, SEM and FT-IR techniques. Adsorption of PEN on CMT-g-poly (AAc-co-AAm) follows the Freundlich and pseudo-second-order models. The significant maximum adsorption capacity of the synthesized polymer was found to be 196.08 mg/g. The synthetic adsorbent had good reproducibility in PEN removal for up to 5 cycles. CMT-g-poly (AAc-co-AAm) is a cost-effective and non-toxic adsorbent for the decontamination of surface water from pesticides.
Introduction
Water is the most valuable and important substance neede by humans and its use and importance are shown in drinking, sanitary, agricultural, and industrial uses. The ever-increasing growth of the population, the improvement of the standard of living, and the development of urbanization are among the factors that cause an increase in water consumption and wastewater production and cause environmental pollution1. Meanwhile, pesticides, which are emerging contaminant, are among the most important and widely used poisons that are used in agriculture as insecticides to kill insects and arthropods, or as herbicides to fight weeds. The use of new technology in agriculture has led to the use of these materials to harvest more crops, however, the excessive use of pesticides has caused soil pollution and the entry of large amounts of these toxins into water sources2. Although fungicides have received less attention compared to other agricultural toxins, fungal diseases are considered a great threat to agricultural products. Fungicides are toxic to a ide range of organisms and are dangerous to aquatic life. Fungicides, like pyrethroid and organophosphate insecticides, are lipophilic. The effect of fungicides in the environment takes place directly or indirectly through another species that is affected by them3. Triazole fungicides are a group of heterocyclic compounds that have at least one five-membered ring of two carbon atoms and three nitrogen atoms and are widely used to prevent and treat various fungal diseases in agricultural products. These fungicides currently constitute 25 commercial agrochemicals worldwide. Also, they have good penetration and durability in soil and water, and their high consumption causes side effects related to endocrine glands in humans and animals4. PEN or (R, S)-1-[2-(2, 4-dichlorophenyl) pentyl]-1H-1, 2, 4-triazole is a fungicide from the triazole family that is used to control powdery mildew and other pathogenic ascomycetes, basidiomycetes, and Deuteromycetes. The chemical structure of PEN is show in Fig. 1. This fungicide is marketed under the brand name Topas 20% and is classified by the European Food Safety Organization as a dangerous substance for humans and the environment, so finding a way to remove it from the environment is very important5. In a study conducted by Nicoleta et al. to remove PEN from water using montmorillonite clay, the adsorption capacity of 6.33 mg/g of PEN by montmorillonite was obtained6. Considering that the studies conducted in the field of removal of PEN by surface adsorption method are limited, therefore we decided to use this method in the recent study. By using physical, chemical, and biological methods, pesticides can be removed from water sources. Membrane and adsorption processes are among the physical methods that rely on separation. The challenge of membrane fouling is the main limitation concerning the removal of pesticides with this method because it disrupts the performance of the membrane in separation7. The surface adsorption method o solid and porous substrates has a significant advantage over other water and wastewater treatment techniques due to its low cost, ease of use, production of fewer byproducts, and easy integration with other techniques for better efficiency8. The principles of the surface adsorption process are shown in Fig. 2. The mass transfer takes place when the solution contaminated with the pollutant comes into contact with the adsorbent. Pollutant-adsorbed species are selectively transported from the bulk solution and occupy the binding sites on the surface of the adsorbent. Based on the nature of the interaction between the adsorbent and the adsorbate, the phenomenon of surface adsorption is chemical or physical. Physical surface adsorption leads to the adsorption of several layers of contaminant on the adsorbent and is also endothermic and reversible, while chemical surface adsorption is a dense single-layer adsorption and is also an exothermic and irreversible process and is stronger than phyical adsorption9. Activated carbon, Biochar, montmorillonite clay, and hydrogels are the most important adsorbents used to remove pesticides from water and wastewater10,11,12. Hydrogel is a swellable, hydrophilic, and insoluble three-dimensional polymer network that is produced by the reaction of one or more monomers. These features distinguish hydrogels from other polymers. Based on the origin of the polymer, hydrogels are divided into two categories: natural and synthetic. Natural hydrogels have a long life, high water adsorption capacity, and high gel strength and have gradually replaced synthetic hydrogels13. Tragacanth Gum (TG) is one of the inexpensive natural polysaccharides that is obtained from the dried sap of the Astragalus, and due to the presence of hydroxyl, carboxylic, and epoxy functional groups, it can be used in the polymerization reaction with various reagents. This gum is non-toxic, biocompatible, and stable in a wide range of pH14. Hydrogels based on acrylic acid, ue to high swelling, high adsorption capacity, and high adsorption speed, have found wide applications in the field of preparation of polymer absorbents for water and wastewater treatment. The most common synthesis method for these hydrogels is the radical polymerization method. The degree of crosslinking is one of the most important factors in the adsorption capacity of these hydrogels. Various adsorption mechanisms have been proposed by these hydrogels, such as electrostatic interactions, hydrophobic interactions, ion exchange, and hydrogen bonding15. Also, acrylamide-based hydrogels are the most commonly used hydrogels and show a significant volume change in response to physical and chemical stimuli. These hydrogels are used to remove various contaminant16,17. In this study, for the first time, we used carboxymethyl tragacanth-based hydrogel grafted with poly (acrylic acid-co-acrylamide) to remove PEN from aqueous solutions and parameters of adsorbent contact time with the contaminat, contaminant concentration, and solution pH were evaluated.
