Phosphate sorption by goethite and kaolinite‐goethite (k‐g) system as described by isotherms

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Abstract
Goethite and kaolinite are active sorbents in soils and sediments. Unfortunately measuring their phosphate sorption capacity is difficult, because of the relatively low proportion of these clay minerals in many natural mixtures and the difficulties involved in concentrating and separating them from accompanying minerals. Under these circumstances, synthetic samples are commonly used to study phosphate sorption. In this study, the objectives were to elucidate the Langmuir, Freundlich, Temkin, Gouy‐Chapman, and Sposito isotherms. The sorption of phosphate ions by goethite (a‐FeOH) and kaolinite‐goethite (k‐g) system depends on several factors including pH, specific surface, cation exchange capacity, time of sorption, surface charge, temperature and concentration. We investigated the effects of pH, time of sorption and concentration on the adsorption of phosphate on goethite and the k‐g system. Batch equilibrium phosphate adsorption experiments were conducted with pH, and initial concentration as changeable parameter in order to produce adsorption envelops. The Langmuir, Freundlich, Temkin, Gouy Chapman and Sposito isotherms were fitted to P sorption data. Each was found to describe phosphorus (P) sorption by goethite and k‐g system with comparable success, with the Sposito, Freundlich and Langmuir being slightly superior for kaolinite‐goethite (k‐g) system and Langmuir for goethite. The total amount of P sorbed by k‐g system and goethite after 103 days in equilibrium concentrations 0.0403–0.4435 mmol P L‐1 and pH 4.0–9.0 was found to range between 2.74–37.00 mmol kg‐1 and 22.97–106.93 mmol kg‐1, respectively.

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