Effects of water temperature on phosphate adsorption onto sediments in an agricultural drainage canal in a paddy-field district

The phosphorus (P) dynamics in an agricultural drainage canal are dependent on the adsorption characteristics of the underlying sediments. Water temperature is a basic factor affecting P adsorption on the sediments, and this study was aimed at understanding the effects of water temperature on P adsorption onto the sediments in an agricultural drainage canal in a paddy-field district. Phosphate (PO4-P) adsorption kinetics experiments were conducted at 5, 15, 25, and 35 °C to clarify temporal variations in the amount of PO4-P adsorption onto the sediments and the effect of water temperature on the PO4-P adsorption rate. We found that higher water temperatures enhanced the adsorption rate and the equilibrium concentration of PO4-P. Phosphate adsorption isotherm experiments were also conducted at 15, 25, 32, and 35 °C and three isotherm models were used to characterize adsorption behavior. The amount of PO4-P adsorbed onto the sediments increased with water temperature. However, there was a strong positive correlation between the zero-equilibrium P concentrations estimated using the linear model and the water temperature. These results suggest that sediments may be a greater source of PO4-P for the overlying water with the lower PO4-P concentrations when water temperatures in a drainage canal are high.