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Paraquat Sorption as a Function of Particle Size in Natural Sediments¹

ABSTRACT

The distribution of sorbed paraquat (1,1'-dimethyl-4,4'-bypyiridinium ion) as a function of particle size (sand through clay) was determined on five natural sediments. Paraquat concentrations in individual size fractions varied as much as two orders of magnitude within a given sediment, showing a pronounced preference for the fine silt and clay fractions. Adsorption isotherms were measured for individual size separates on one sediment. Paraquat distributions in the whole sediment agreed well with those computed using the isotherm coefficients determined on individual size separates. Paraquat sorbs by ion exchange, and sorption partition coefficients showed a definite correlation with the cation exchange capacity (CEC) of individual size fractions. However, exchange sites in different fractions differed in their effectiveness in sorbing paraquat, with the fine silt and clay exchange sites being more effective than those of the larger separates.

In pure clay suspensions (hectorite and montmorillonite), sorbed paraquat did not distribute uniformly throughout the clay particle size range. The concentration peaked in the mid-size range (0.065-0.44 µm, equivalent Stokes radii) and decreased uniformly in the larger and smaller particle sizes. Peak concentrations were two to five times those of the less sorbing components. Because no corresponding trend was found in NH₄⁺-measured CEC's, exchange sites in the median clay particle size ranges seemed to be more "effective" in sorbing paraquat than the extreme size ranges.

UV spectra of paraquat in a hectorite suspension revealed that, after 4 hours of centrifuging at 20,000 rpm, >50% of the paraquat remaining in the suspended (water) phase was sorbed on particulates not removed by centrifugation. Therefore, for highly sorbed compounds such as paraquat, conventional phase separation techniques for measuring sorption may not distinguish sorbed vs. "free" compounds.

Key Words: hectorite • cation exchange • adsorption isotherms • particle-size fractionation • phase separation

¹ Contributed from work done at the Environ. Res. Lab., USEPA, Athens, GA 30605.

² Research Chemist and Research Soil Scientist, respectively.

Received for publication June 6, 1977.