Commercial Dimethoate Pesticide Adsorption on Organic Soil: Experimental and Theoretical Investigations

This study presents a detailed experimental and modeling investigation into the adsorption of dimethoate (DMO) on agricultural organic soil, using both its pure form (DMO_p) and its commercial form (DMO_c), the latter being the one used by farmers. Experimental adsorption isotherms reveal that adjuvants in the commercial solution, particularly cyclohexanone (CYC), significantly enhance DMO adsorption. To comprehensively analyze these findings, four thermodynamic adsorption models (Langmuir, Freundlich, BET_n-finite, and BET_n-infinite) and three statistical models (STAT₁, STAT₂, and STAT₃) were employed. The BET_n-finite model suggests that DMO_p undergoes multilayer adsorption (n > 20), while DMO_c predominantly forms a monolayer (n = 0.98). Statistical models further indicate that DMO_p adsorps as aggregates, unlike DMO_c. CYC appears to act as a "disaggregant" in the commercial solution, promoting DMO adsorption. Density Functional Theory (DFT) calculations were used to assess the reactivity of the adsorbate molecules (DMO, CYC, XYL, and DMOCYC) and the adsorbents (two molecular structures of humic acid representing soil organic matter). Lower energy gaps (E_Gap) and hardness values (η) correlate with higher reactivity, with DMOCYC exhibiting the greatest reactivity (E_Gap = 3.41134 eV, η = 1.7067 eV), followed by DMO (E_Gap = 4.4385 eV, η = 2.2192 eV), in agreement with the experimental showing enhanced DMO adsorption in the presence of CYC. Additionally, the dipole moment (DM) and electrophilicity (ω) highlight the highly polar and electrophilic nature of DMOCYC compared to DMO. Among the humic acid structures, HA₂, which is richer in functional groups, shows higher reactivity (E_Gap = 3.6809 eV, η = 1.8404 eV) compared to HA₁ (E_Gap = 4.6997 eV, η = 2.3498 eV). DFT calculations of adsorbate-adsorbent binding energies confirm the cooperative role of CYC in enhancing DMO adsorption, with binding energies increasing from 5.23 kcal.mol⁻¹ for DMO_p to 10.09 kcal.mol⁻¹ for DMO_c. Hydrogen bonds are formed between the electrophilic hydrogen (LUMO) of the HA molecules and the nucleophilic oxygen (HOMO) of DMO.