Method for treating hexavalent chromium-containing aqueous solution

1. A method for treating a hexavalent chromium-containing aqueous solution, comprising:

a step (a) of adding catalyst particles to the aqueous solution;

a step (b) of reducing hexavalent chromium by irradiating the aqueous solution with ultraviolet light having a wavelength in a range between 200 nanometers and 400 nanometers, both inclusive, while stirring the catalyst particles in the aqueous solution; and

a step (c) of stopping the stirring in the step (b) and separating the catalyst particles from the aqueous solution by sedimentation,

wherein each catalyst particle is composed only of a titanium dioxide particle having an average particle diameter in the range of 1 nm to 100 nm and a zeolite particle having a particle diameter of 1 μm to 10 μm, the titanium dioxide particle is adsorbed on an outer surface of the zeolite particle, the zeolite particle includes silica and alumina, and the catalyst particles are contained in the aqueous solution at a concentration in a range between 0.4 grams/liter and 16 grams/liter, both inclusive, and

a change in laser light transmittance of the aqueous solution from before sedimentation to after a sedimentation duration of 30 minutes in the step (c) is 20% or more.

2. The method according to claim 1 , comprising a step (d) of adding again the catalyst particles separated by sedimentation in the step (c) to the aqueous solution after the step (c),

wherein the step (b) and the step (c) are performed again after the step (d).

3. The method according to claim 1 , wherein

the catalyst particles are separated by sedimentation in a solid-liquid separation vessel including a filtration membrane element in the step (c),

the method further comprises a step (e) of producing treated water from the aqueous solution using the filtration membrane element, and

the filtration membrane element used in the step (e) is composed of a plate-shaped frame and sheets of filter paper made of resin and attached to both faces of the frame, and is arranged parallel to a direction in which the catalyst particles are sedimented.

4. The method according to claim 3 , comprising a step (f) of adding again the catalyst particles separated by sedimentation in the step (c) to the aqueous solution after the step (c), wherein the step (b), the step (c) and the step (e) are performed again after the step (f).

5. The method according to claim 1 , wherein the zeolite particle is a zeolite particle subjected to a process in which an alumina portion is dissolved by treatment with an acid aqueous solution to introduce an active site for direct adsorption of the titanium dioxide particle, and then the acid aqueous solution adhered to the surface of the zeolite particle is removed by washing with water.

6. The method according to claim 1 , wherein:

the step (b) is performed in a photo reactor,

the catalyst particles are transferred from the photo reactor to a solid-liquid separation vessel in the step (c), and

the catalyst particles are separated by sedimentation in a solid-liquid separation vessel in the step (c).