Sorption of pesticides by microplastics, charcoal, ash, and river sediments

PURPOSE: In addition to sediments, pesticides can be sorbed to other constituents present in rivers including ash, charcoal, and microplastics. Pesticide sorption by microplastics has been studied for hydrophobic compounds such as the legacy insecticide DDT (dichlorodiphenyltrichloroethane) but not for current-use herbicides. The purpose of this study was to investigate to what extent 2,4-dichlorophenoxyacetic acid (2,4-D) (weak acid), atrazine (weak base), and glyphosate (zwitterion) are sorbed by microplastics (i.e., fiber, polyethylene beads, polyvinyl chloride (PVC), and tire fragments) and other river constituents (i.e., ash, charcoal, suspended and bottom sediments). DDT was included in the study to provide reference data that could be compared to known literature values. METHODS: Batch equilibrium experiments were conducted following Guidelines 106 of the Organisation for Economic Co-operation and Development. Experiments utilized either a 1:100 solid/solution ratio with 0.1 g of a river constituent as the sorbent or a 1:5 solid/solution ratio with the sorbents consisted of 1.9 g bottom sediments mixed with 0.1 g of a river constituent. Background solutions included 0.01 M CaCl ₂ or 0.01 M KCl, deionized water, and river water. RESULT: Individual microplastics always sorbed >50% of DDT. Current-use herbicides had a weak affinity for microplastics (< 6%) except that a substantial amount of glyphosate was sorbed by PVC (32–36%) in 0.01M KCl and DI water. When river water was used as a background solution, rather than 0.01M KCl or deionized water, there was much less glyphosate sorption by PVC, ash, charcoal, and both sediments. This suggested that ions present in river water competed for sorption sites with glyphosate molecules. Across background solutions, sorption by sediments decreased in the order of DDT (91–95%) > glyphosate (36–88%) >atrazine (5–13%) >2,4-D (2–5%). Sorption of 2,4-D, atrazine, and DDT by ash and charcoal was always > 90% but < 35% for glyphosate. Relative to bottom sediments alone, the presence of ash or charcoal (5% by weight) with sediments significantly increased the sorption of 2,4-D, atrazine, and DDT. Microplastic additions (5% by weight) had no impact on all four pesticides’ sorption by sediments. CONCLUSION: Microplastics are not a strong sorbent for current-use herbicides, although there are exceptions such as glyphosate by PVC. Ions present in river water competed with glyphosate for sorption sites of river constituents. Hence, the types and concentrations of ions present in rivers might have some influence on the partitioning of glyphosate between the water column and solid phase, including glyphosate fate processes in rivers.