Study on the degradation and adsorption of some organic pesticides residues

Abstract

According to UN, a third of all nations are suffering from water scarcity and by 2025 two-thirds of the world population could be under stress conditions. Consequently, many countries have been directed to reuse its agricultural drainage water and control any possible contamination for its water resources. However, the uncontrolled pesticides use, especially in lower income countries, contaminates its limited water sources. In this study, two different treatment techniques for removing of pesticides from water have been studied, include: pesticides adsorption and degradation treatment. Adsorption of a mixture of 15 different pesticides from water by untreated and treated (using phosphoric acid) biochars and charcoal has been studied. The studied pesticides are: azinphos-Me, phosmet, boscalid, chlorfenvinphos, flutolanil, diazinon, carbaryl, malathion, imidacloprid, acetamiprid, propiconazole, flusilazole, triadimenol, atrazine, and oxamyl, with individual and total pesticide concentrations of 400 and 6000 µg/l respectively. Adsorptions of these pesticides mixture were used to simulate contaminated water by different agricultural pesticides. Since, agricultural drainage water and water effluent of pesticides factories are often contaminated by a mixture of pesticides. The choice of these pesticides was based on their current usage worldwide and in Egypt. Some factors affecting pesticide sorption (solution pH, pesticide concentration, and water/adsorbent ratio) were studied on untreated rice straw biochar, while the effect of adsorption contact time was studied on untreated and treated rice straw biochar. Solid–water distribution coefficients (Kd) were used to compare the pesticide adsorption abilities of the tested adsorbents. The phosphoric acid-treated and untreated biochar and charcoal were characterized by CHN-elemental analysis, ash content, and adsorbent pH in solution measurements, surface area (SBET), scanning electron microscopy (SEM), and Fourier-transform infrared (FTIR) spectroscopy. Pesticide residual concentration was analyzed using liquid chromatography– tandem mass spectrometry (LC–MS/MS).

It was found that, untreated rice straw and corn stover biochars possessed larger surface areas (SBET) and higher total pore volumes than untreated charcoal. Phosphoric acid treatment markedly increased the surface functional groups and aromatization for all of the treated adsorbents. It was found that, the treated biochars (TBRc and TBCn) had more surface functional groups and greater surface roughness than the treated charcoal (TCHc), indicating that the interaction between phosphoric acid and charcoal during the chemical treatment process is lower than during the reaction of phosphoric acid with biochar. This may result from the lower surface area of charcoal relative to biochar. Consequently, untreated biochar is a good replacement for charcoal in the production of treated (activated) carbonaceous adsorbents. In comparing the properties of the studied adsorbents, the surface area (SBET) was found to be more important factors in adsorption of pesticides from aqueous solution. It was suggested that pesticide with a high degree of hydrophobicity (Kow) shows higher adsorption ability on the studied adsorbents, while sorption of high polarity pesticides require longer adsorption contact times since water molecules competes for the polar functional groups of the sorbents.

The binding forces for most of the tested pesticides on the studied adsorbents may involve primary π-π dispersive interactions, van der Waals forces, and H-bonding (via the carbonyl group oxygen, the nitro group oxygen, and the highly polar bonds (C-F, P-S and PO). It was also proposed that, pesticides with lower partition coefficient (log P) and that with steric structure showed a less competitive sorption than others especially at lower contact times. With the exception of oxamyl, treated rice straw biochar effectively removed the studied pesticides from water in just 2 h at a buffered pH of 7. Oxamyl can be easily removed from unbuffered water through its degradation due to the alkaline effect of untreated biochar. The degradation treatment can only carried out for only one pesticide especially when we study also the structures of the degradation products. Boscalid has been chosen for the degradation treatment study. Where, boscalid is one of the most recently used fungicide in Egypt and worldwide and it has a high stability to photolysis and hydrolysis degradation in water. Different degradation conditions have been studied for boscalid degradation (in presence or absence of MeOH) includes: • Boscalid photocatalytic degradation using different concentrations of nitrogen doped titanium (NT) or undoped titanium (UT) under solar sun light. • Advanced oxidation process (AOP) of Boscalid by coexisting of persulfate with NT or UT. • Degradation of boscalid using persulfate by heating in water bath at 80 ºC. The prepared NT and UT were characterized by surface area (SBET), X-ray diffraction (XRD) and scanning electron microscope (SEM). It was found that, the particle size of NT particles was nearly half that of UT. So, surface area of NT (388) is higher than that of UT (247) and also the Vp of NT is higher than that for UT.

In presence of MeOH, It was observed that there is no degradation for boscalid with neither NT nor UT under sun light even after 3 h irradiation time. While, in absence of MeOH, there are a degradation for Bd by using NT or UT under sun light radiation with a higher degree by using NT, where NT has a higher surface and pore volume. Also, the degradation of Bd in absence of MeOH by PS or by a mixture of PS with NT or UT is high in absence of MeOH.