These sprayers designed for bleach are great general clean up and for mold remediation.
If you are wanting to use a backpack sprayer for bleach, you want to ensure that you use a piston pump sprayer and not a diaphragm sprayer.
A piston pump sprayer has a sealed chamber that protects the unit from corrosion and damage.
In addition, you want to ensure that all O-rings are chemical resistant so that they will not break down and start leaking.
These O-ring seals are typically made of viron which is a synthetic and fluoropolymer elastomer. They are highly impermeable to many strong organic solvents that would destroy or permeate common O-rings.
It is very important that you buy a bleach-specific backpack sprayer and not just any sprayer.
or read our Best 4 Gallon Backpack Sprayer Review which includes a section on bleach and disinfectant sprayers.