Development and application of eco-sorbents based on modified lignocellulosic biomass

[D. V. Bojić]

186 listova

Chemistry and Chemical Technology

The subject of this doctoral dissertation was the development of two new eco-sorbents, biosorbent and activated carbon, based on modified lignocellulosic precursor – shell of Lagenaria vulgaris plant, intended for the removal of cationic, weakly polar and non-polar pollutants from waste and natural polluted water. A literature review showed that there had not been any systematic evaluation of the application of similar sorbent based on L. vulgaris. Also, this plant, more popular because of its healing properties, grows well in a wide area in the whole world, is simple for cultivation and obtained in a high yield. Biosorbent was obtained by the chemical modification of L. vulgaris shell with concentrated sulfuric acid in a process called "cold carbonation". Activated carbon was synthesized in a two-stage thermal modification of the shell of this plant, by the process of thermal carbonization, and then activation by steam at high temperature. Textural analysis of the biosorbent surface showed that this material has, typically, a small specific surface area, which is not of crucial importance for its sorption characteristics. In contrast, activated carbon has very high specific surface area and porosity, because of the efficient activation of the material by the steam. On the surface and in the interior of the activated carbon particles micropores predominate, while the amount of mesopores is relatively small. The morphological analysis of biosorbent by scanning electron microscopy showed the presence of numerous macropores, channels and cavities, plant vessels that provide a large contact surface of biosorbent with the aqueous phase. This is of great importance for the efficient diffusion of sorbate molecules, especially when it comes to high initial concentrations. The same analysis showed that a typical particle of activated carbon also has numerous channels and macro pores, which provide good contact with the aqueous phase and the effective mass exchange. In the case of activated carbon, the macroporozity enables efficient contact with the aqueous phase and micropores inside the activated carbon particles. EDS analysis confirmed that biosorbent has a high amount of carbon and sulfur, which is a result of dehydration and sulfonation of biomass with sulfuric acid treatment. Elemental analysis of the active carbon surface confirms the presence of a large amount of carbon, compared to the precursor, which explains the changes that come in a two-stage synthesis process - thermal carbonization and activation of the steam. The chemically modified biosorbent was applied for the removal of cationic pollutants from water, such as Ni(II) ions and cationic dye methylene blue. Results of the effects of contact time on the sorption of cationic pollutants by biosorbent showed that during the first stage of treatment, over a period of 5 – 10 min, depending on the initial concentration, about 95 % of the sorbate was removed from water. This is followed by the slower phase that goes to equilibrium, which in the case of Ni(II) ions lasts 10 to 30 min, and in the case of methylene blue (MB) 20 to 90 min, for applied initial concentrations ranges. The effects of the initial pH, investigated in the ranges of 2 – 6 and 2 – 11, in the case of nickel and MP, respectively, show that this parameter has a relatively small effect on the uptake of cationic pollutants. Although the efficiency of the Ni(II) ions and MP removal by biosorbent in an acidic environment is somewhat lower than in a neutral one, it is still significantly higher as compared to other biosorbents, whose affinity for cations in such an environment is negligible. This influence of pH on the sorption of cations by biosorbent is a result of the strong acidic sulfonic group presence, which is completely dissociated under these conditions. The analysis of the effects of the pollutants initial concentration on the removal efficiency by biosorbent indicates that at relatively low initial concentrations (up to 100 mg dm–3 for Ni (II), or up to 200 mg dm–3 for MP) removal efficiency is very high (> 95 % ), but with a further increase of concentration, the efficiency decreases. It is assumed that at relatively low concentrations of adsorbates number of the free active centers on the biosorbent surface is much larger than the number of cations, which is why effective sorption takes place independently of the increase in the initial concentration. The effect of temperature was investigated in the range of 15 – 45°C. The results show that the sorption slightly increases with increasing the initial temperature, for both pollutants, probably due to increased diffusion efficiency. With a further increase of temperature, removal efficiency of Ni(II) starts to decrease due to desorption, while the sorption of MP does not change in these conditions. Most likely, in the case of MP, the crucial influence of temperature is that on the diffusion of large molecules of dye. The relatively low effect of temperature on the removal efficiency of cationic pollutants, confirms that the sorption by biosorbent is mainly based on ion exchange. The values of the Gibbs free energy are negative at all tested temperatures, indicating the spontaneous nature of the biosorption process. The value of the enthalpy change for the sorption on biosorbent is between the values of physisorption and chemisorption, which confirms that the dominant sorption mechanism is ion exchange. The sorption capacity of activated carbon were investigated with organic pollutants, drug ranitidine and herbicide 2,4-dichlorophenoxy acetic acid. Activated carbon has shown high affinity for both investigated pollutants, which is why the sorption process was very fast, especially in the first phase. To achieve equilibrium, the sorption process took more time, bearing in mind that in the second phase the sorption took place on the inner surface of the activated carbon and micropores, where the diffusion is proportionally slower. The effect of pH indicates significant specificity, which is due to the nature of activated carbon, and investigated sorbate. In the case of ranitidine removal, efficiency is the lowest in a very acidic medium. At approximately neutral pH, the sorption of ranitidine is favored, with a certain decline in the alkaline medium. The reason for this pH effect can be the positive charge of protonated molecule at lower pH values, causing electrostatic repulsion with positively charged functional groups on the surface of the activated carbon, in accordance with the value pHpzc. In the case of 2,4-D, the highest removal efficiency is achieved in a strongly acidic medium. In these conditions, the herbicide is a neutral molecule, and thus it has the desirable hydrophobic properties and minimal solubility in water. The increase of pH leads to a slight reduction of the removal efficiency. Total variation of the removal efficiency in a wide range of pH was less than 10 %, indicating a relatively small influence of this parameter to the process. The effects of the initial concentration of pollutants show that the sorption at activated carbon occurs with the high efficiency over a wide area of concentration, because of high sorption capacity of activated carbon for ranitidine and 2,4-D, which is the result of a very developed surface and a large number of micropores. The effects of temperature on the sorption on activated carbon indicate a certain exothermicity of the process, which is consistent with the thermodynamic parameters, bearing in mind the negative values of ΔG and ΔH. Vibration energy of the molecule sorbed on the surface of activated carbon increases with temperature, and finally reaches a sufficient level to carry out desorption. The relatively low value of the enthalpy changes and the positive value of entropy changes, i.e. the increase of the degree of freedom, all indicate the physical interaction between the sorbate and sorbent. For all investigated temperatures, the change of Gibbs free energy is negative, and therefore it can be concluded that the sorption on the activated carbon is spontaneous in nature. The kinetic results of the sorption on biosorbent were fitted with four theoretical models, in order to determine the limiting stage of the process. Kinetic modeling points out that the results were the best fitted with the reaction model of pseudo-second-order, intraparticle diffusion model and Chrastil’s diffusion model. It is obvious that the process of sorption on the biosorbent is under the simultaneous control of the surface reaction, and diffusion through the boundary layer. Bearing in mind that it is a heterogeneous process, diffusion is a more significant limiting factor, because the process of ion exchange takes place relatively quickly. In the kinetics terms, sorption on activated carbon is also under the laws of the pseudosecond- order model and diffusion models. As regards the diffusion phenomena, in the case of activated carbon intraparticle diffusion is a more significant factor compared to boundary layer diffusion, having in mind high specific surface and the microporosity of activated carbon. Investigation of the equilibrium phenomena of sorption process on biosorbent shows that the experimental results can be best described by Lengmir and Temkin isotherms, which have a maximum value of R2, low relative deviation and values of the maximum sorption capacity closest to experimental. The reason for the mixed isothermal mode of sorption on biosorbent lies in the presence of energy differences between the active sites on the surface and those in the interior of the sorbent particles; and between the sulfonic group and oxygen functional groups. Sorption on active carbon, in isothermal sense, is a complex process that can be best described by Sips and Freundlich models. It is likely that sorption takes place largely by the physical binding of sorbate molecules to the surface of the activated carbon with intermolecular attraction forces, with a certain proportion of ion exchange as the accompanying mechanism. Research in this doctoral dissertation showed that by simple and affordable methods of chemical and thermal modification of inexpensive and available agro-waste materials sorbents with significantly improved sorption characteristics can be prepared. The developed materials have great potential for the removal of toxic metals and other cationic pollutants from water, as well as organic non-polar and weakly polar compounds, in a wide range of initial concentrations, pH and temperatures. High sorption capacities for investigated pollutants, a short time for achieving sorption equilibrium and a simple and inexpensive procedure for preparation, suggest that biosorbent and activated carbon can be competitive in the market of materials for water purification technologies

Lagenaria vulgaris, biosorbent, aktivni ugalj, sorpcija, teški metali, organski polutanti, pH sredine, kinetika, izoterma, termodinamika

Lagenaria vulgaris, biosorbent, activated carbon, sorption, heavy metals, organic pollutants, solution pH, kinetics, isotherm, thermodynamics

547.9 : 54.05

628 : 316.12

547.9:54.05(043.3)

Elektronska teza