Comparative sorption of Organic Dyes using Xylocarpus Moluccensis and Rhizophora Mucronata Mangrove species from Kenyan Coastal Region

Abstract

Bark and stem samples from two mangrove species, Rhizophora mucronata (RM) and Xylocarpus moluccensis (XM); obtained from Kenyan coastal region, were investigated as potential low-cost adsorbents for the removal of toxic Crystal Violet (CV) and Malachite Green (MG) from wastewater mixtures. Adsorption efficacies of these adsorbents were compared for the two dyes and with literature values of recommended adsorbents. X. moluccenisis stem and stem-bark dye removal rate within the first 5 to 20 minutes increased from 75.9 ± 0.15% to 97.1 ± 0.15, and from 85.2 ± 0.16% to 96.7 ± 0.11% respectively while uptake increase from 85.2 ± 0.16% to 95.3 ± 0.20% and from 85.2 ± 0.47% to 95.3 ± 0.05% capacity was observed for R. mucronata stem and stem-bark. The optimum pH for the adsorption of CV and MG dye-was at pH 8 and pH 7 respectively. Significant equilibrium adsorption capacities, Qe (mg/g), with the stem-bark of the species giving highest capacities of 407.7 ± 0.03 mg/g for the adsorption of CV and 366.4 ± 0.07 mg/g, for MG dyes which translates up to 99.4 ± 0.15% dye removal. Equilibrium adsorption capacity increased with contact time, adsorbent dose and initial dye concentration but decreased with particle size, Ionic Strength and pH of the solution. Large correlation coefficient R2 values ranging from 0.7885 to 1.0000 noted implies adsorption occurred through monolayer formation for both species fitting Langmuir model while very low Freundlich constant, Kf , values of the range of 0.5000 to 2.2000 were observed. Kinetics studies showed that the equilibrium adsorption follow pseudo-second order kinetics with the corresponding regression coefficient, R2, in the range of 0.8788 to 1.0000 and interparticle diffusion was a factor that controlled adsorption process onto the two species. The results displayed in this study have demonstrated the effectiveness of R. mucronata (RM) and X. moluccensis (XM), in removing organic dyes from their aqueous wastewater mixtures.