Modified Carbonaceous Adsorbent For Heavy Metals And Dye Removal From The Aqueous Solution

The plenty of toxic substances (e.g., heavy metals and dyes) in the wastewater have resulted in increasingly serious environmental pollution problems. The main objective of this dissertation is to remove the heavy metals and the dye pollutants from the aqueous solution by adsorption. The adsorption performance of heavy metals for carbon adsorbents (biochar and activated carbon fiber) was improved by surface chemical modification. And the carbon fiber was modified with the photocatalyst to enhance the removal of rhodamine by the ways of adsorption-photocatalytic degradation. The materials before and after modification were characterized by different methods, including Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS), etc. The main contents and results are as follows:1. A chemical modified biochar with abundant amino groups was prepared using polyethylenimine (PEI) as a modification reagent, and used as an adsorbent for the removal of Cr(VI) from aqueous solution. The effects of pH, contact time, and initial concentration of Cr(Ⅵ) on the removal of Cr(Ⅵ) by the modified biochar were investigated. The mechanism between Cr(Ⅵ) and the functional groups on the surface of the PEI modified biochar was proposed. The results show that the modification significantly improve their adsorption performance for Cr(Ⅵ). Surface grafting PEI can be combined with heavy metal ions by chelating or electrostatic interactions. The adsorption of Cr(Ⅵ) by the modified biochar was obeyed pseudo-second-order kinetic model and Langmuir adsorption isotherm model, and the equilibrium sorption could be reached within1.5hours. The maximum adsorption capacity of PEI-alkali-biochar is about435.7mg/g at pH6.8, much higher than that before modification. Results also indicated that the removal of Cr(Ⅵ) by the PEI modified biochar depended on solution pH, and a low pH value was favorable for the Cr(Ⅵ) removal. The used adsorbents could be regenerated by1M of HCl solution, and the regeneration studies revealed that the adsorbent had a good stability during the adsorption-desorption cycles without significant decrease of the adsorption capacity.2. The chemical modified Polyvinyl Alcohol-Activated Carbon Fiber (PVA-ACF) was prepared using PEI as a modification reagent, and also used as an adsorbent for the removal of Cr(VI) from aqueous solution. The adsorption of Cr(Ⅵ) by the PEI-alkali-ACF agreed with the Langmuir model better than the Freundlich model. From the Langmuir adsorption model, the maximum adsorption capacity of PEI-alkali-ACF was calculated to be85.05mg/g. PEI-ACF were also studied as an adsorbent for the removal of Cu(Ⅱ), Zn(Ⅱ), Cd(Ⅱ), and Pb(Ⅱ) from aqueous solution through kinetic and regeneration experiments. The results indicate that there was no significant difference of the adsorption capacity between these heavy metals and Cr (Ⅵ). But, the adsorption equilibrium time increased significantly, the equilibrium sorption could be up to about6hours. And the priority order for the adsorption of these heavy metal is Pb(Ⅱ)> Cu(Ⅱ)> Zn(Ⅱ)> Cd(Ⅱ). The adsorption of Pb(Ⅱ), Cu(Ⅱ), and Zn(Ⅱ) by the modified ACF was obeyed pseudo-second-order kinetic model. In the study of three consecutive cycles of adsorption-desorption experiments, the whole mixed metal removal rate is less than20%in the second cycle, and the adsorption rates continue to drop to15%in the third cycle. The decrease of removal rate may result from the different adsorption mechanism of these four metals and Cr (Ⅵ) on PEI-ACF surface. HCl is not an ideal desorption agent for the four metal ions, it will make the surface of the adsorbent with excess positive charge, reduce the electrostatic attraction between metal ions and functional groups on the PEI-ACF, thus reducing the adsorption active sites.3. Two kinds of photocatalysts (TiO2and Ga2O3) modified with homemade PVA-ACF respectively (TiO2/ACF and Ga2O3/ACF) were used for adsorption and photodegradation of dye pollutants, rhodamine. The results of characterization indicate that TiO2and Ga2O3loaded on the surface of ACF with varying degrees changes its surface morphology and elemental composition. The kinetic adsorption analysis shows that the Ga2O3/ACF adsorption rate faster (about2hours) than TiO2/ACF (about10hours). In the study of consecutive cycles of adsorption-desorption experiments, Ga2O3/ACF shows a strong adsorption performance and stability for rhodamine, and the removal rate over90%after five cycles. But the photocatalytic performance needs further improvement. And shows higher photocatalytic performance than the former.