Glyphosate adsorption on synthetic allophanes and halloysite: Effects of pH and mineral properties

BackgroundGlyphosate (GLP) is a widely used herbicide with possible adverse effects on human health and the environment. In soils, GLP strongly adsorbs on clay-sized minerals, depending on pH, the amount of organic carbon, as well as the contents and properties of Al and Fe oxyhydroxides and clay minerals. Many clay-sized minerals have already been investigated regarding GLP adsorption behavior, but information on minerals commonly found in volcanic soils is still lacking. AimThe aim of this study was to investigate for the first time the pH-dependent adsorption of GLP on allophane and halloysite, typical minerals found in volcanic soils. MethodsGLP adsorption was studied in batch experiments at three pH values (5, 6, and 7). Synthetic allophanes with two different initial Al:Si ratios (1.4 and 1.8) and a halloysite were used as adsorbents. ResultsThe adsorption capacity (AC) increased with rising Al:Si ratio and decreasing pH. The AC of allophane was significantly higher than that of halloysite. GLP adsorption on allophane was larger than that reported for other clay minerals and Al and Fe oxyhydroxides, especially at low pH. The AC of halloysite was higher than reported for most other clay minerals. ConclusionDifferent mineral formation pathways in volcanic soils, notably the formation of halloysite versus allophanes, strongly affect the soils' retention capacity for GLP. The high AC of allophanes may induce the low mobility of GLP in allophane-containing soils. Long-term use of GLP may accumulate the herbicide in these soils with potential effects on biodiversity and ecosystem services.