Use of compost for dye removal from textile wastewaters

Abstract

Finding low-cost adsorbents for organic dye removal from water is important for the development of clean technologies for the treatment of textile wastewater. In this sense, utilization of composted materials has not yet been studied in detail, although their use for the removal of other types of pollutants has been proved. In this thesis, we studied the performance and characteristics of the adsorption of three composts (municipal solid waste compost, pine bark compost and a green waste compost) towards different classes of dyes: Basic Blue 9 (methylene blue), Basic Violet 10 (rhodamine B), Acid Red 27, Reactive Violet 4, Direct Blue 71 and Direct Blue 151. Sorption kinetics and equilibrium were studied in batch and in fixed-bed column conditions; the influence of factors including solution pH, salinity and competition with other dyes were assessed. Moreover, the potential recycling of adsorbents was evaluated by studying desorption. The mechanism of the interaction between dye and compost was also investigated. Adsorption rates were relatively slow for Basic Violet 10, reaching equilibrium in 24 hours, and faster for the rest: around 5-6 hours for Reactive Violet 4 and Acid Red 27 and 2 hours for Direct Blue 151 and methylene blue. In general, dye sorption at the equilibrium was adequately described by the Langmuir model, what allows to estimate the maximum retention capacities for each dye by the composts. The dye adsorption capacity followed the sequence: Basic Blue 9 > Basic Violet 10 > Direct Blue 71 > Direct Blue 151 > Reactive Violet 4 > Acid Red 27, and the highest capacity among the adsorbents was obtained with pine bark compost. Sorption increased at acid pH in all cases, likely because the modification of charges of the dyes and higher electrostatic attraction. Increasing salinity also had a positive effect on sorption, which was attributed to a solute-aggregation mechanism in solution. Co-existence of several dyes in solution reduced their affinities in batch and column tests. The adsorbents could be effectively regenerated using ethanol, thus enabling reuse in the practical application of compost for textile dye removal. The study of the influence of these factors and the FTIR spectra of the adsorbents, before and after adsorption, showed that electrostatic forces cannot be the only or even the main mechanism for the interaction between dyes and compost. Considering the complex structure of the dye molecules, the contribution of other mechanisms such as hydrophobic-hydrophobic and dipole-dipole forces should be envisaged to explain dye sorption. Hydrophobic interaction between the organic dyes and organic matter-rich composts could explain the higher affinity of dyes toward pine bark compost. In conclusion, composts can be successfully applied for the removal of colorants from wastewater, although they would be more effective with basic cationic dyes than other types, due to more favorable electrostatic interaction with mostly negatively- charged composts.