Competitive Adsorption Of Phosphate And Fulvic/Humic Acid On Goethite/Water Interface

Phosphorus is an essential element for life, and phosphate is a kind of importantinorganic oxygen anion in the water. But excessive phosphate in water bodies can leadto significant eutrophication in surface water. Once into the water environment, it cannot be biodegradable and requires a series of physical and chemical effects to migrateand transform. It participates and interferes with various environmental chemicalprocess and material recycling process. Ultimately, it persists in the environment withone or more forms for a long time, resulting in permanent of potential hazards. Thecompetitive adsorption of natural organic matter and phosphate on metal hydrousoxide/water interface may strongly influence the solubility and mobility of phosphate,thereby affecting the migration, transformation and fate of phosphate in theenvironment. The competitive adsorption of phosphate and FA/HA on goethite/waterinterface was investigated by using batch experiments. The experimental conditionscovered pH, ionic strength, adsorption time and addition sequences of the adsorbates.The ATR-FTIR spectroscopy was used to research the adsorption mechanism ofphosphate and FA/HA on goethite.The experimental results demonstrate that acidic conditions are propitious to theadsorption of phosphate, FA and HA on goethite. The addition of FA or HA will makethe adsorption equilibrium time of phosphate increased from4h to48h. Theincreasing of the ionic strength has a small effect on the adsorption of phosphate andFA/HA. The presence of FA has large effect on phosphate adsorption to goethite,compared to phosphate-FA system, the presence of HA has smaller effect onadsorption of phosphate to goethite. The presence of phosphate has a larger effect onFA adsorption. The presence of phosphate led to a large decrease of the adsorptionamount of FA over the entire pH range. This is very different for the systems with HA.First of all, HA was more strongly bound in the absence of phosphate, and it remainedalso strongly bound in the lower pH when phosphate was added. The additionsequences of phosphate and FA/HA also have effect on competitive adsorption, previously added was beneficial to the adsorption of phosphate and FA/HA ongoethite. The addition sequences of the adsorbates have greater influence onphosphate-FA system. The kinetic data were analyzed using the pseudo first-order andpseudo second-order models and they were found to fit very well the pseudosecond-order kinetic model. The Langmuir and Freundlich models were used todescribe the adsorption isotherm data and it was found that the data conform toLangmuir isotherm model. The observations of ATR-FTIR spectra confirm that thepresence of adsorbed FA/HA led to a shift from the bidentate to monodentateadsorbed phosphate species especially at low pH, while FA/HA demonstratespredominant outer-sphere mode of adsorption in the absence or presence of phosphateat the goethite/water interface. Phosphate and FA/HA compete for the adsorption siteson the surface of goethite is the main reason of the competitive adsorption betweenthem. Electrostatic and some steric effects of FA/HA may also play an important rolein the behaviour of the competitive system.