WATER TREATMENT RESIDUALS AGGREGATE SIZE INFLUENCES PHOSPHORUS SORPTION KINETICS AND P_MAX VALUES

Abstract

Drinking water treatment residuals (WTRs) are used as a soil amendment to minimize off-site P movement and increase a soil's phosphorus (P) sorption capacity. The aggregate size of WTRs may affect sorption kinetics and P sorption maxima (P_max) values. We hypothesize that finer-sized WTRs aggregates will have higher kinetic sorption rates and P_max values than coarser-size aggregates. The objectives were to determine WTRs aggregate size effects on kinetic rates of P sorption, on the magnitude of P_max values, and the time necessary to reach equilibrium with P. A WTR sample was ground and sieved into five aggregate size ranges (<0.5, 0.5 to 1.0, 1.0 to 2.0, 2.0 to 4.0, and >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 P_max values for each aggregate size range were calculated from the linear form of the Langmuir equation. The <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 < 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 >4.0-mm) had P_max values of <94 mg g⁻¹, whereas fine-sized (<1.0-mm) aggregates had values >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.