We report the applicability of an ultra-low cost, field-deployable, plastic fiber based smartphone spectrometer system to study dye adsorption from water. A custom cradle using the smartphone's built-in flash and camera was designed for visible absorbance spectroscopy without any need for external electrical components. We firstly investigated the performance of the smartphone spectrometer for methylene blue (MB) absorbance in water with a detection limit of 0.5 ppm, and compared it to that of a commercial spectrometer. We elaborated on the contribution of image formats to the calibration of the standard absorbance curve. We then studied the catalytic activity of electrospun polyacrylonitrile (PAN)/zeolite composite nanofibers by analyzing MB adsorption as a function of time. The results obtained in the proposed compact, cost-effective and high-performance platform can help transform measurement science for sustainable water management.