# Reduction of Water Color in a Spinning Disc Reactor
## 1 Introduction  
Color is an indicator of general water quality that can significantly influence the quality of the final technological effluent, or the natural and artificial processes occurring in freshwater resources and also the technological processes supplied with it. Water color can be due to different organic and inorganic species present in small or high amounts in solid or dissolved forms, such as dyes, aquatic plants or other colored products among others. Synthetic reactive dyes (residual ones) found in different potential water resources are usually known to be persistent organic micropollutants, which can be immediately visualized by a change and/or intensification of water color, and their presence can be eliminated, or partially reduced, by different water treatments. Numerous practices based onion exchange (with synthetic ionic exchangers), advanced chemical oxidation, biodegradation, electrocoagulation, ultrafiltration or other membrane processes and adsorption [1; 2; 3; 4], among others, are used to remove residual dyes from water resources. Adsorption is one of the most useful techniques in the removal of reactive dyes from different effluents/water resources due to its numerous advantages such as its simple design, easy operation and the relatively simple regeneration of the synthetic adsorptive materials utilized, but its use in association with SD technology has not been intensively studied and applied.  
Previous experimental studies performed by the authors on real textile wastewaters containing 2% w. bentonite, within a rotating disc setup (SD), indicated good removal rates from 50% up to 60% for suspended solids and turbidity, within short time periods of 25 min, for different investigated values of effluent flow rate and disc rotational speed without any additional treatment [5; 6]. However, the color removal obtained in the same experimental setup and working conditions using synthetic-colored water samples revealed smaller values for most of the investigated operational parameters/variables within the applied SD technology. Thus, the main purpose of this study was to investigate the color removal efficiency within a spinning disc setup using different potential adsorptive materials added to the effluent (synthetic-colored water containing a reactive dye) prior to being fed to the disc. In addition, comparisons between regular mixing systems containing different materials tested as potential adsorbents and their use in a rotating disc setup were performed to assess whether the spinning disc technology improved the color removal from aqueous effluents. The selected materials chosen for water treatment in the SD setup were the double-layered hydroxide of a ZnAlLDH type, and also titanium oxide (TiO\({}_{2}\)) and aluminum oxide (Al\({}_{2}\)O\({}_{3}\)). These substances are known to display good photocatalytic activity [7; 8; 9] as a result of interplay among different influencing factors such as particle size and mobility, surface area, degree of aggregation, phase composition, adsorption of molecules from aqueous phase, interactions between adsorbed species and the nature of the solvent [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. However, in this study, only the adsorption performance of the above-mentioned selected materials was tested within the spinning disc laboratory experimental setup, for four different water flow rates and four different rotating disc speeds. The dye adsorption onto the tested adsorptive materials was highlighted by their presence in the FTIR spectra of specific peaks originating from the characteristic groups of the studied reactive dye. Additionally, for the studied double-layered hydroxide (ZnAlLDH) as an adsorptive material, at the end of the measurements, Fenton oxidation was applied in the SD system to control whether a higher removal of color degrees was attained. The association of adsorption with advanced oxidation (i.e., Fenton oxidation) seemed to be beneficial for increasing the water color reduction.