Montmorillonite–chitosan bionanocomposites as adsorbents of the herbicide clopyralid in aqueous solution and soil/water suspensions

Abstract

Montmorillonite (SWy-2)–chitosan bionanocomposites (SW–CH) were prepared following different methodologies, characterized, and assayed as adsorbents of the herbicide clopyralid (3,6-dichloropyridine-2-carboxylic acid) in aqueous solution and soil/water suspensions, to assess the potential of the materials to prevent and remediate soil and water contamination by anionic pesticides. The SW–CH bionanocomposites were good adsorbents for the herbicide at pH levels where both the anionic form of the herbicide (pKa=2.3) and the cationic form of CH (pKa=6.3) predominated. The performance of the SW–CH bionanocomposites as adsorbents of clopyralid depended on the amount and arrangement of chitosan in the samples. Clopyralid adsorption was rapid and mostly linear up to herbicide concentrations as high as 0.5mM. High salt concentrations (0.1M NaCl) promoted desorption of the adsorbed pesticide from SW–CH, strongly suggesting that adsorption of clopyralid occurred primarily through an ion exchange mechanism on positively charged CH sites at the montmorillonite surface. Amendment of an acidic soil (pH=4.5) with SW–CH at rates of 5% and 10% led to a significant increase in clopyralid adsorption, whereas this effect was negligible when SW–CH was added to an alkaline soil (pH=8.0), reflecting the absence of positively charged sites in SW–CH at high pH values. Montmorillonite–CH bionanocomposites can be useful as adsorbents for the removal and/or immobilization of anionic pesticides in soil and water under mild acidic conditions.