Which filter material did you use for TiO2 separation from water? Maybe problem is in pore size of filter. If TiO2 is smaller than pores it will increase absorbance. Other problem is in case of paper filters. They can absorb arsenic from water.

It could be useful

(1) to make any test replicated

(2) to pass just pure water through the filter, and to check whether the filter does not release anything

(3) to pass a "non-treated" solution and check whether absorbance is not changed, to eliminate interactions with filter

(4) to pass a TiO2-containing solution (without As), see if something is changed after filtration

(5) what is the wavelength? Perhaps, check, if posssible, at another wavelength....

Yes I am doing photocatalytic arsenic elimination.