Catalyzes the hydrolysis of tetrathionate to generate elemental sulfur, thiosulfate and sulfate. H2O + tetrathionate = H(+) + sulfate + sulfur + thiosulfate Optimum pH is 1. No activity is detected at pH 6. Optimum temperature is 95 degrees Celsius. Monomer and homodimer; in equilibrium. Associated with the S-layer in the pseudo-periplasmic space. Expressed in tetrathionate-grown cells. Belongs to the tetrathionate hydrolase family.