Effects of pH and phosphate on cadmium adsorption onto goethite and a paddy soil : Experiments and NOM-CD model

Phosphate is one of the most frequently used fertilizers to promote the crop growth, whereas the impact of phosphate on the adsorption behaviors of cadmium (Cd) is complex. The objectives of this study were to reveal effects of pH, phosphate, and organic molecules on Cd adsorption onto soil (hydr)oxides, and to validate the reliability of the NOM-CD model in depicting effect of pH and phosphate on Cd solubility in soil systems. Materials and methods: In this study, effects of phosphate on Cd adsorption onto goethite and a paddy soil at pH 3–8 were studied by combing batch adsorption experiments with an advanced surface complexation model, i.e., natural organic matter-charge distribution (NOM-CD) model. In the NOM-CD model, the adsorption of ions onto oxides in the absence of NOM is calculated with the charge distribution and multi-site complexation (CD-MUSIC) model. Results and discussion: Adsorption of Cd onto goethite is dramatically increased with the addition of phosphate at pH 5–8, whereas effects of citric acid on Cd adsorption are weaker. The synergic co-adsorption of Cd and phosphate onto goethite, according to CD-MUSIC model, is mainly due to electrostatic attractions of these two ions instead of formation ternary surface complex. The NOM-CD model can basically predict variations of Cd solubility in the soil system, and it is observed only a slight increase of Cd solubility with phosphate addition under acidic conditions. It might be caused by competitive adsorption between NOM and phosphate to soil (hydr)oxides, which leads to a higher amount of Cd binding by dissolved organic matters in soil solution, thus inhibiting Cd adsorption. Conclusions: Electrostatic attraction dominates the co-adsorption of Cd and phosphate onto goethite, whereas the interactions between Cd-P-oxide-NOM result in negligible effects of phosphate on Cd solubility in the soil. Overall, this study revealed that the NOM-CD model could make predictions on the distribution of Cd at soil–water interface, and the outcome of this study can provide a more in-depth understanding of the factors controlling Cd solubility and mobility in contaminated soil and sediment environment. Graphical Abstract: [Figure not available: see fulltext.].