Adsorption Kinetics and Stabilization Effects of Various Forms of Phosphorus on Schwertmannite

Schwertmannite (Sch) is an important Fe(III)-oxyhydroxysulfate in acidic mine drainage-polluted areas, and its stability depends on the surrounding environmental factors and previously bound elements. Phosphorus (P) is a ubiquitous element in Sch-rich environments and can adsorb onto Sch, affecting their fate. To better understand the adsorption behavior of different forms of P (inorganic phosphorus, iP; organic phosphorus, oP) on Sch, the adsorption kinetics of iP and oP on synthetic Sch were investigated; additionally, the stability changes of Sch were analyzed. The adsorption rates of iP and oP on Sch were different, and the adsorption rate of iP was higher than that of oP. The adsorption process conformed to a pseudo-second-order adsorption kinetics model, and the rate-controlling step was the rate of the adsorbate interaction with the adsorption site. The adsorption of iP and oP can improve the stability of Sch, even in a more alkaline environment. The adsorption of iP seemed to stabilize the Sch structure through a strong hydrogen bonding effect in addition to the surface passivation effect. In contrast, the adsorption of oP seemed to be stabilized mainly by the surface passivation effect and the weak hydrogen bonding effect. Therefore, the formation of Sch in an acidic environment immobilizes different forms of P and improves its own stability, which can prevent secondary environmental pollution.