WATER TREATMENT RESIDUALS AGGREGATE SIZE INFLUENCES PHOSPHORUS SORPTION KINETICS AND PMAX VALUES.

Drinking water treatment residuals (WTRs) are used as a soil amendment to minimize off-site P movement and increase a soilʹs phosphorus (P) sorption capacity. The aggregate size of WTRs may affect sorption kinetics and P sorption maxima (Pmax) values. We hypothesize that finer-sized WTRs aggregates will have higher kinetic sorption rates and Pmax values than coarser-size aggregates. The objectives were to determine WTRs aggregate size effects on kinetic rates of P sorption, on the magnitude of Pmax values, and the time necessary to reach equilibrium with P. A WTR sample was ground and sieved into five aggregate size ranges ((less than)0.5, 0.5 to 1.0, 1.0 to 2.0, 2.0 to 4.0, and (less than)4 mm). Phosphorus sorption isotherms for each aggregate size range were determined as a function of time (between 24 and 120 h). Reaction rate constants (k) were determined by using a first-order reaction equation and Pmax values for each aggregate size range were calculated from the linear form of the Langmuir equation. The (less than)0.5-mm WTRs aggregates had the highest k values, and the rates decreased with an increase in aggregate size. All isotherms showed that aggregate size ranges reached equilibrium between 72 and 96 h. There was a strong linear (r2 between 0.78 and 0.96) and significant (P (less than) 0.05) relationship between C (C = mean equilibrium P conc.) and C Q-1 (Q = P sorbed). Coarse-sized WTR aggregates (between 1.0 and (greater than)4.0-mm) had Pmax values of (less than094 mg g^-1, whereas fine-sized ((less than)1.0-mm) aggregates had values (greater than)98 mg g^-1. Aggregate size has an important influence on WTRs P sorption characteristics; therefore, it is recommended that aggregate size should be strongly considered when determining P isotherms or using residuals as a soil amendment to reduce non-point source P contamination of surface water bodies.