Influence of electrolyte composition and pH on glyphosate sorption by cow-dung amended soil

Abstract

This study investigated the pH-dependent relationship between glyphosate and soils before (NS) and after amendment with cow-dung, CD (SACD). Sorption isotherms were generated using batch equilibration techniques from CaCl₂, KCl, CaSO₄ and Ca(H₂PO₄)₂ background electrolytes. Linear partition coefficients decreased with increase in pH in the order: CaCl₂>KCl > CaSO₄>Ca(H₂PO₄)₂, which reflect both the effect of the inorganic anion in terms of competition and the cation in terms of bridging interactions. However, soil amendment with CD minimized the inhibitory effect of pH increase on glyphosate sorption. Variations in glyphosate sorption from CaCl₂ and Ca(H₂PO₄)₂ were attributed to fraction of hydrophilic contribution while variations in sorption from CaCl₂ and KCl were attributed to Ca-bridging and ranged from 22 to 70% across the two soils. Estimated hydrophobic partitioning ($K_{\mathrm{OC}}^{\mathrm{pH}}$) systematically decreased with increasing soil solution pH from 36 to 3% and 5 to <1% in NS and SACD, respectively. Greater sorption in NS over SACD was partly attributed to higher phosphate level in SACD which is preferentially sorbed over glyphosate. This study noted CD application as soil amendment for agriculture could impede glyphosate sorption and facilitate its off-site leaching and contamination of surface and groundwater.

Keywords:

• Glyphosate
• cow-dung
• sorption
• soil organic content
• partition coefficients
• soils pH