Pyrrhotite (Fe(1−x)S) is a non-stoichiometric structure where x varies from 0 to 0.125 and the non-stoichiometry is due to a system of ordered vacancies within the Fe lattice [13]. The grade of the pyrrhotite particles used in this study was very high (Fe + S > 95.15%), so the surface of the pyrrhotite particles could be considered as the surface of pure Fe(1−x)S. The surface of pyrrhotite is a system of ordered vacancies too. The vacancies of the surface of pyrrhotite show surface heterogeneity, and the order of vacancies shows surface homogeneity. Therefore P adsorption on pyrrhotite can be described using both Freundlich and Langmuir isotherm models. The n values of the Freundlich equation shown in Table 1 varied slightly from 5.13 to 5.60, revealing adsorption sites with low energetically heterogeneity of the pyrrhotite, and the surface homogeneity of the pyrrhotite. This can explain why P adsorption on pyrrhotite can be better described by Langmuir isotherm model.
The adsorption capacity of pyrrhotite for phosphate rose with increasing the temperature, and it was 1.15 mg/g at 29 °C according to the Langmuir isotherm. It is in the range of the adsorption capacities of other adsorbents, such as 0.398 mg/g of coal cinder [28], 1 mg/g of iron ochre [29], and 1.23 mg/g of melter slag [30].