With the use of cost-effective natural materials, biosorption is considered as an ecological tool that is applied worldwide for the remediation of pollution. In this study, we proposed Lemna gibba biomass (LGB), a lignocellulosic sorbent material, for the removal of two textile dyes, Direct Red 89 (DR-89) and Reactive Green 12 (RG-12). These azo dyes commonly used in dying operations of natural and synthetic fibres are the most important pollutants produced in textile industry effluents. For this purpose, batch biosorption experiments were carried out to assess the efficacy of LGB on dye treatment by evaluating the effect of contact time, biomass dosage, and initial dye concentration. The results indicated that the bioremoval efficiency of 5 mg L(-1) DR-89 and RG-12 reached approximately 100 % after 20 min of the exposure time; however, the maximum biosorption of 50 mg L(-1) DR-89 and 15 mg L(-1) RG-12 was determined to be about 60 and 47 %, respectively. Fourier transform infrared spectroscopy used to explain the sorption mechanism showed that the functional groups of carboxylic acid and hydroxyl played a major role in the retention of these pollutants on the biomass surface. The modelling results using Freundlich, Langmuir, Temkin, Elovich, and Dubini Radushkevich (D-R) isotherms demonstrated that the DR-89 biosorption process was better described with the Langmuir theory (R (2) =0.992) while the RG-12 biosorption process fitted well by the D-R isotherm equation (R (2) =0.988). The maximum biosorption capacity was found to be 20.0 and 115.5 mg g(-1) for DR-89 and RG-12, respectively, showing a higher ability of duckweed biomass for the bioremoval of the green dye. The thermodynamic study showed that the dye biosorption was a spontaneous and endothermic process. The efficacy of using duckweed biomass for the bioremoval of the two dyes was limited to concentrations ≤50 mg L(-1), indicating that L. gibba biomass may be suitable in the refining step of textile effluent treatment.

This study puts into light the capacity of Lemna gibba biomass (LGB) to be used as an effective biosorbent for the simultaneous removal of dyes from contaminated waters. The experiment was carried out at the laboratory scale and focuses on the single and binary biosorption of textile dyes. Direct Red 89 (DR-89) and Reactive Green 12 (RG-12)-two azo dyes-were treated with LGB which was selected as a biosorbent. The results showed that very acidic pH value (pH 1) was the optimal value for dye biosorption. When the aqueous system was charged with the two dyes (≥15 mg/L), the biosorption capacity of each individual dye decreased and the sorption capacity of LGB was reduced by 15%. The kinetic modeling results proved that the single biosorption of DR-89 and RG-12 was better accounted for by the pseudo-second-order (R 2 > 0.96). When the two organic pollutants were simultaneously present in the solution, the kinetic process well fitted the pseudo-second-order and pseudofirst-order models for removing, respectively, DR-89 and RG-12 from the mixture.