Mechanical activation of a natural calcite for enhanced orthophosphate sorption

We examined the effect of dry grinding on the properties of a calcite material and its sorption behaviour with respect to orthophosphate (OP). Limestone was ground in a planetary ball mill for 30 min using sample:ball mass ratio of 5:1 and grinding speed of 250–550 rpm. Specific surface area linearly developed from 7.2 m2 g-1 (initial material) to 15.6 m2 g-1 (550 rpm). Ground samples showed slightly lower content of calcite compared to the initial material (94.3–94.6 % vs 98.7 %), without evidence of an aragonite-like phase being formed, suggesting amorphization. Crystallite size in the samples linearly declined from 597 to 170 Å (initial material vs 550 rpm). Reactivity of limestone increased leading to higher dissolution with however only minor rise in the solution pH. Grinding distinctly enhanced maximum OP uptake from 14 % (initial material) to 60–92 % (250–550 rpm). Sorption kinetics accelerated with the grinding speed: reaction time needed to remove 50 % of the OP was reduced from 8 h (250 rpm) to only 5 h (550 rpm). This was due to increased surface area and enhanced reactivity. Mechanical processing of limestones by intense grinding is a promising solution to gain easily available, safe and relatively efficient calcite material for P control for environmental applications.