[Phosphate sorption characteristics onto sediments in the middle and lower reaches of the Yellow River].

The equilibrium phosphate concentration (EPC0) of the Yellow River bed sediments has been measured, which was used to predict whether bed sediments are acting as a source or sink of soluble reactive phosphate (SRP). The modified Langmuir isotherm equation was used to describe phosphate (P) sorption on the Yellow River sediments. The maximum P sorption capacity (PAC) and P-binding energy constant (k) were obtained by the modified Langmuir isotherm model. Native adsorbed exchangeable phosphorus (NAP), the EPC0, and partitioning coefficients (Kp) were subsequently calculated by the corresponding formulae. The influence of pH values and ion strength were evaluated. All the EPC0 s are higher than the P concentration in the overlying water, indicating a potential source of phosphate from the sediments. PAC is linearly related to the contents of TOC of the sediment. The sorption capacity of P increased rapidly with pH below 6.0, and then reached a plateau between pH 6.0 to 9.7, and finally maintained at a slightly higher level from pH 9.7 to 12.0.The adsorption of P by the sediment decreased with the increase in Ca2+ ionic strength.