| The removal of heavy metals has been a focus area of research over the recent years. Toxic heavy metals contamination is one of the most serious worldwide environmental problems. The removal of heavy metals is achieved through various techniques such as chemical precipitation, solvent extraction, ion-exchange, reverse osmosis and adsorption. The leading advantages of adsorption are low cost and relatively higher efficiency of heavy metals removal from aqueous solution. Land application of sludge is the most commonly used method around the world and is being considered as one of the most economical methods of sludge disposal, but the content of heavy metals is the major limitation of direct application of municipal sludge composting in agriculture. Therefore, the treatment of heavy metals in industrial wastewater and excess sludge is very worthy of research. Eucalyptus leaves are an important biomass resources and a very common forestry waste in Guangxi, in which the content of plant polyphenols is higher. The active groups in these substances can chelate heavy metals to form stable compounds. The extracted substance from eucalyptus leaves (ESEL) could not only effectively remove Cr(Ⅵ) from wastewater, but also significantly affect on bioavailability of Cr, Pb and Cd in sludge.In this study, tannin and other active materials were extracted from eucalyptus leaves. The effect factors of extraction process are temperature, leaves to solvent ratio and extracting time. The ESEL was immobilized with alginate sodium-calcium to form gel beads which were used as adsorbent to remove Cr(VI) from aqueous solution. Batch experiments of Cr(VI) adsorption were carried out by varying the parameters such as initial pH of solution, dosage of sorbent, temperature and contact time, initial Cr(Ⅵ) concentration and co-existence ions. The ESEL was added into the sludge for investigating the chemical speciation distribution and bioavailability of Cr, Pb and Cd.The conclusion could be drawn as follows.(1)The factors affecting the extraction process were extracting temperature, leaves to liquid ratio and extracting time. The maximum content of tannin was obtained at the temperature of60℃, leaves to liquid ratio of1:20and extraction time of60min.(2)The immobilized tannin beads (ITB) can remove Cr(Ⅵ) and total Cr effectively from aqueous solution. The results showed that almost100%of Cr(Ⅵ) was removed with the tannin dosage of492.8mg·L-1and contact time of3h at55℃under pH2.0when the initial Cr(Ⅵ) concentration was50mg·L-1. The co-existence cations (Pb2+, Zn2+, Cr3+and Cu2+) had positive effect on Cr(Ⅵ) removal. The presence of CO32-decreased the removal of Cr(Ⅵ) while the other anions (Cl-, SO42-and NO3-) had no effects. Kinetic studies showed that the experiment data fitted well to the pseudo second-order kinetic model. The Freundlich isotherm provided the best correlation for Cr(Ⅵ) adsorption. The negative change in free energy (ΔG0) and positive change in enthalpy (AH0) during the adsorption of Cr(VI) indicated that the adsorption was a spontaneous and endothermic process. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analysis revealed that Cr(Ⅵ) is reduced to Cr(Ⅲ) by ITB in the aqueous solution, and the reduced Cr(Ⅲ) forms complexes with carboxyl and phenolic hydroxyl.(3)The chemical speciation distribution of Cr, Pb and Cd was significantly affected after ESEL being added into the sludge. The bioavailability of Cr decreased and chemical speciation transformed from unstable state to stable state while Pb and Cd showed opposite. FTIR and SEM analysis revealed that phenolic hydroxyl in ESEL were predominant contributors and the structure and morphology of sludge were apparently changed.The present study demonstrated that the ESEL could be used to remove Cr(Ⅵ) and total Cr effectively from aqueous solution and to adjust the bioavailability of Cr, Pb and Cd in sludge. The research has provided the valuable theoretical basis for the treatment of heavy metals in industrial waste water and sludge. |
PDF Full Text Request