Mixing impeller and treatment apparatus

A mixing impeller comprising:

• multiple blade pairs each having two blades, wherein each blade comprises a blade inner portion extending from a rotational shaft and a blade outer portion, wherein the blades included in the multiple blade pairs are attached around the rotational shaft extending in a lateral direction, so as to be positioned at a same point along the rotational shaft's length, the two blades of each blade pair each have a shape that is symmetric about a symmetry plane that is a plane perpendicular to the rotational shaft, and the two blades of each blade pair are formed so as to extend from the rotational shaft side toward an outer circumferential side, which is a side on the periphery of the impeller, and blade outer portions that are on the outer circumferential side are angled inwardly with respect to the blade inner portion, wherein inwardly is in a direction towards an area between the two blades of the blade pair, wherein, in the two blades of each blade pair, the blade outer portions that intersect the symmetry plane are provided with slit-like openings.