Latvian Sheep Wool Fiber as a Cheap Natural Adsorbent for the Removal of Congo Red Dye from Wastewater

It is well known that sheep wool is an agricultural product consumed by the textile industry. However, a large part of this material is frequently wasted due to strict industrial requirements. Considering the cheapness, natural origin, and biodegradability of this product, it was proposed that the wool can be used as an adsorbent for isolation of harmful azo dyes from wastewater. Using genotoxic and mutagenic Congo Red (CR) dye as a model compound, different adsorptive properties of the non-irradiated sheep wool have been studied in an aqueous solution and simulated textile effluent. For the purpose of comparison, the surface of the wool was irradiated with gamma rays up to about 100 kGy absorbed dose and the performance of the obtained samples has been measured. The characterization of the wool surface was based on SEM, BET, FTIR, EPR, and determination of zero-point charge. The adsorptive performance of the samples was compared by constructing and analyzing adsorption isotherms and uptake kinetics curves. It was shown that the CR adsorption on all of the samples followed the Langmuir model and adsorption kinetics was described by the Elovich equation. The non-irradiated sheep wool demonstrated the highest value of saturation adsorption capacity for the CR of 5.7 µmol/g which is comparable to the capacities of a large group of unmodified natural adsorbents. Finally, it was shown that 20 mg of the non-irradiated sheep wool can be used repeatedly without cleaning/regeneration at least three times to isolate the CR from simulated effluent at a concentration of 25 µM.