An apparatus for generating and mixing gas bubbles into an aqueous solution, the apparatus comprising: a structure defining an interior fluid-flow chamber extending along a longitudinal axis between a liquid input end and a liquid output end, wherein the structure is characterized by: a gas injection portion located downstream from the liquid input end and upstream from the liquid output end and comprising an outer wall, a geometric structure surrounded by the outer wall and having a tip facing the liquid input end and a base facing the liquid output end, a hollow structure surrounded by the outer wall and extending in the downstream direction from the base of the geometric structure, and an inlet portion having a base, the gas injection portion defining: a first region of the interior fluid-flow chamber comprising a plurality of separate fluid-path lumens (212a, 212b) partially defined by a pair of wing structures (208a, 208b) extending between the outer surface of the hollow structure and the interior surface of the outer wall, and a gas injection lumen surrounded by the interior fluid-flow chamber and extending along a length of the gas injection portion, wherein the gas injection lumen is configured to inject gas from the interior of the gas injection lumen into the surrounding interior fluid-flow chamber, and the gas injection lumen includes a first portion defined by the interior of the hollow structure and a third portion that extends from the base of the inlet portion through one of the pair of wing structures and into the first portion of the gas injection lumen, and a mixing vane portion extending in the downstream direction from the gas injection portion and defining a second region of the interior fluid-flow chamber. a gas injection portion located downstream from the liquid input end and upstream from the liquid output end and comprising an outer wall, a geometric structure surrounded by the outer wall and having a tip facing the liquid input end and a base facing the liquid output end, a hollow structure surrounded by the outer wall and extending in the downstream direction from the base of the geometric structure, and an inlet portion having a base, the gas injection portion defining: a first region of the interior fluid-flow chamber comprising a plurality of separate fluid-path lumens (212a, 212b) partially defined by a pair of wing structures (208a, 208b) extending between the outer surface of the hollow structure and the interior surface of the outer wall, and a gas injection lumen surrounded by the interior fluid-flow chamber and extending along a length of the gas injection portion, wherein the gas injection lumen is configured to inject gas from the interior of the gas injection lumen into the surrounding interior fluid-flow chamber, and the gas injection lumen includes a first portion defined by the interior of the hollow structure and a third portion that extends from the base of the inlet portion through one of the pair of wing structures and into the first portion of the gas injection lumen, and a first region of the interior fluid-flow chamber comprising a plurality of separate fluid-path lumens (212a, 212b) partially defined by a pair of wing structures (208a, 208b) extending between the outer surface of the hollow structure and the interior surface of the outer wall, and a gas injection lumen surrounded by the interior fluid-flow chamber and extending along a length of the gas injection portion, wherein the gas injection lumen is configured to inject gas from the interior of the gas injection lumen into the surrounding interior fluid-flow chamber, and the gas injection lumen includes a first portion defined by the interior of the hollow structure and a third portion that extends from the base of the inlet portion through one of the pair of wing structures and into the first portion of the gas injection lumen, and a mixing vane portion extending in the downstream direction from the gas injection portion and defining a second region of the interior fluid-flow chamber.