Hydrothermal carbonization in the synthesis of sustainable porous carbon materials

Carbon materials are found versatile and applicable in wide range of applications. During the recent years research of carbon materials has focussed on the search of environmentally friendly, sustainable, renewable and low-cost starting material sources as well as simple cost-efficient synthesis techniques. As an alternative synthesis technique in the production of carbon materials hydrothermal carbonization (HTC) has shown a great potential. Depending on the application HTC can be performed as such or as a pretreatment technique. This technique allows synthesis of carbon materials i.e. hydrochars in closed vessel in the presence of water and self-generated pressure at relatively low temperatures (180-250 ˚C). As in many applications well developed porosity and heteroatom distribution are in a key role. Therefore in this study different techniques e.g. varying feedstock, templating and post-treatment in order to introduce these properties to the hydrochars structure were performed. Simple monosaccharides i.e. fructose or glucose and more complex compounds such as cellulose and sludge were performed as starting materials. Addition of secondary precursor e.g. thiophenecarboxaldehyde and ovalbumin was successfully exploited in order to alter heteroatom content. It was shown that well-developed porosity (SBET 550 m2/g) can be achieved via one-pot approach (i.e. exploitation of salt mixture) without conventionally used post-carbonization step. Nitrogen-enriched hydrochars indicated significant Pb(II) and Cr(VI) removal efficiency of 240 mg/g and 68 mg/g respectively. Sulphur addition into carbon network was not found to have enhancing effect on the adsorption of methylene blue or change acidity of the carbon material. However, these hydrochars were found to remove 99.9 % methylene blue and adsorption efficiency of these hydrochars remained over 90 % even after regeneration. In addition to water treatment application N-rich high temperature treated carbon materials were proven applicable as electrocatalyst and electrocatalyst support. Hydrothermal carbonization was shown to be workable technique for the production of carbon materials with variable physico-chemical properties and therefore hydrochars could be applied in several different applications e.g. as alternative low-cost adsorbent for pollutant removal from water.

Investigation of adsorption of dyes onto modified titanium dioxide

Titanium dioxide (TiO2) nanoparticles with different sizes and crystalloid structures produced by the thermal method and doped with silver iodide (AgI), nitrogen (N), sulphur (S) and carbon (C) were applied as adsorbents. The adsorption of Methyl Violet (MV), Methylene Blue (MB), Methyl Orange (MO) and Orange II on the surface of these particles was studied. The photocatalytic activity of some particles for the destruction of MV and Orange II was evaluated under sunlight and visible light. The equilibrium adsorption data were fitted to the Langmuir, Freundlich, Langmuir-Freundlich and Temkin isotherms. The equilibrium data show that TiO2 particles with larger sizes and doped with AgI, N, S and C have the highest adsorption capacity for the dyes. The kinetic data followed the pseudo-first order and pseudo-second order models, while desorption data fitted the zero order, first order and second order models. The highest adsorption rate constant was observed for the TiO2 with the highest anatase phase percentage. Factors such as anatase crystalloid structure, particle size and doping with AgI affect the photocatalytic activity significantly. Increasing the rutile phase percentage also decreases the tendency to desorption for N-TiO2 and S-TiO2. Adsorption was not found to be important in the photocatalytic decomposition of MV in an investigation with differently sized AgI-TiO2 nanoparticles. Nevertheless C-TiO2 was found to have higher adsorption activity onto Orange II, as the adsorption role of carbon approached synchronicity with the oxidation role.

Ultrasound-assisted electrochemical treatment of wastewaters containing organic pollutants by using novel Ti/Ta2O5-SnO2 electrodes

Advanced oxidation processes (AOPs) are modern methods using reactive hydroxyl radicals for the mineralization of organic pollutants into simple inorganic compounds, such as CO2 and H2O. Among AOPs electrochemical oxidation (EO) is a method suitable for coloured and turbid wastewaters. The degradation of pollutants occurs on electrocatalytic electrodes. The majority of electrodes contain in their structure either expensive materials (diamond and Pt-group metals) or are toxic for the environment compounds (Sb or Pb). One of the main disadvantages of electrochemical method is the polarization and contamination of electrodes due to the deposition of reaction products on their surface, which results in diminishing of the process efficiency. Ultrasound combined with the electrochemical degradation process eliminates electrode contamination because of the continuous mechanical cleaning effect produced by the formation and collapse of acoustic cavitation bubbles near to the electrode surface. Moreover, high frequency ultrasound generates hydroxyl radicals at water sonolysis. Ultrasound-assisted EO is a non-selective method for oxidation of different organic compounds with high degradation efficiencies. The aim of this research was to develop novel sustainable and cost-effective electrodes working as electrocatalysts and test their activity in electrocatalytic oxidation of organic compounds such as dyes and organic acids. Moreover, the goal of the research was to enhance the efficiency of electrocatalytic degradation processes by assisting it with ultrasound in order to eliminate the main drawbacks of a single electrochemical oxidation such as electrodes polarization and passivation. Novel Ti/Ta2O5-SnO2 electrodes were developed and found to be electrocatalytically active towards water (with 5% Ta content, 10 oxide film layers) and organic compounds oxidation (with 7.5% Ta content, 8 oxide film layers) and therefore these electrodes can be applicable in both environmental and energy fields. The synergetic effect of combined electrolysis and sonication was shown while conducting sonoelectrochemical (EO/US) degradation of methylene blue (MB) and formic acid (FA). Complete degradation of MB and FA was achieved after 45 and 120 min of EO/US process respectively in neutral media. Mineralization efficiency of FA over 95% was obtained after 2 h of degradation using high frequency ultrasound (381, 863, 1176 kHz) combined with 9.1 mA/cm2 current density. EO/US degradation of MB provided over 75% mineralization in 8 h. High degradation kinetic rates and mineralization efficiencies of model pollutants obtained in EO/US experiments provide the preconditions for further extrapolation of this treatment method to pilot scale studies with industrial wastewaters.