No abstract

No abstract

No abstract

The presence of pesticides in water, soil, and air has been recognized as a major environmental problem. Only a small part of the applied amount of a pesticide is active. Most of them are distributed in the environment. Inactive pesticides remain attached to the soil or dust particles, are leached out, migrate into the groundwater, and are distributed by surface runoff. A certain quantity reaches the air and can diffuse over long distances [1]. There are few technologies for the removal of pesticides. Physical remediation, based on the process of adsorption, is one of the most widely applied methods for water purification due to its efficiency, capacity, and applicability on a large scale.In present work, we aimed to prepare and investigate unmodified and surfactant-modified fly ash-based zeolites and zeolite-carbon composites as adsorbents of four pesticides: 2Adsorbents were modified using cationic surfactant (hexadecyltrimethylammonium bromide (HDTMA-Br)). Structural and textural properties of adsorbents before and after modification was analysed using Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis/differential thermal analysis (TG/DTA). The influence of initial concentration, pH of the pesticide solution and kinetics of adsorption of pesticides were tested under static conditions. The concentration of each pesticide in water samples was analysed using high-performance liquid chromatography with a UV detector.The occurrence of new bands on the FTIR spectra and peaks related to the decomposition of surfactant on the thermal analysis graphs prove the effective modification. The amount of surfactant used for modification as equal for the 1 external cation exchange capacity (ECEC) of each zeolite or composite samples. It should result with the formation of a pseudosingle layer of surfactant [2]. Modification is limited only to the external surface of adsorbents because the surfactant's molecules are too large to enter the channels and modification occurs on the solid surface.The adsorption of pesticides was performed on modified and unmodified samples. 2,4-D and MCPA are acids and in aqueous solutions appear as anions. Therefore, HDTMA-modified adsorbents have higher adsorption capacities compared to unmodified ones. On the other hand, for carbendazim and simazine, unmodified zeolite-carbon composites seem to be more suitable. pH value of the initial solutions does not have an impact on the amount of pesticides adsorbed. This may be explained by the characteristic of the adsorbents, which exhibit good buffer properties. Moreover, for all pesticides, adsorption is a very fast process. During first 30 sec. of adsorption, up to 90% of pesticides are adsorbed, and equilibrium is reached during less than an hour. A detailed study of the adsorption of 2,4-D, MCPA, carbendazim and simazine is the topic of forthcoming works.

The presence of pesticides in drinking water sources is caused mainly by means of agricultural activities and the need to increase world food production which results in river and groundwater pollution, damaged ecosystems, and can negatively affect human health. Adsorption is one of the water treatment processes used for the removal from water. There has been a great deal of interest in the use of surfactant-modified zeolites and zeolite-carbon composites as adsorbents to prevent and remediate environmental contamination by pesticides. The introduction of organic cations through ion exchange reactions has been proposed to improve the adsorptive capacity of zeolites and zeolite-carbon composites for organic compounds, including hydrophobic pesticides.The present work aims to investigate the adsorption behaviour of our pesticides: 2-(2,4-dichlorophenoxy)acetic acid (2,4-D) and 2-(4-chloro-2-methylphenoxy)acetic acid (MCPA), 6-chloro-2-N,4-N-diethyl-1,3,5-triazine-2,4-diamine (simazine) and methyl N-(1H-benzimidazol-2-yl)carbamate (carbendazim) on fly-ash-based zeolites and zeolite-carbon composites. The adsorbents were modified using a cationic surfactant (hexadecyltrimethylammonium bromide (HDTMA-Br)) and a non-ionic surfactant (t-octylphenoxypolyethoxyethanol (Triton X-100, TX100)). Adsorption experiments involved the influence of initial concentration and co-adsorption of pesticides. Moreover, the stability of the adsorbents during the adsorption experiments was also examined. The concentration of each pesticide in water samples was analyzed using high-performance liquid chromatography with a UV detector.The results of the experiment clearly show that an increase in the concentration in the solution resulted in an increase in the adsorption capacity. At an initial concentration of 4 mg/L, the adsorption effectiveness of simazine and carbendazim is much higher than for any other concentration. For 2,4-D and MCPA adsorption efficiency increases more gradually. Simultaneous adsorption of MCPA, carbendazim, and simazine revealed that the adsorption of carbendazim and simazine from single-component solutions is higher than that of multicomponent solutions for all tested adsorbents. MCPA exhibit higher adsorption on HDTMA-modified adsorbents when adsorbed from a single-component solution, however, for unmodified adsorbents and those modified with Triton X-100 adsorption is more efficient if carbendazim and simazine are also present -adsorption on samples X-FA-T and X-C-T increases by more than 30%. Peaks in the XRD patterns of adsorbents before and after adsorption that correspond to the zeolite and their positions reveal no significant changes between samples before and after the adsorption of pesticides. This suggests that the adsorbents did not disintegrate during 24 hours of stirring with pesticide solution at pH 5, and in selected adsorption conditions, the adsorbents are stable.In summary, carbendazim and simazine are the most effectively absorbed in unmodified zeolite-carbon composite, but HDTMA-modified adsorb...