Professor Stephen Allen

Research

Adsorbents, Adsorption and the Environment

Adsorption offers a cleaner technology, free from sludge handling problems and produces a high quality effluent. Over the last few decades, adsorption has gained importance as an effective purification and separation technique used in water and wastewater treatment. Adsorption is the process by which a solid adsorbent can attach a component dissolved in water to its surface and form an attachment via physical or chemical bonds, thus removing the component from the fluid phase. Adsorption is used extensively in industrial processes for many purposes of separation and purification. The removal of metals, coloured and colourless organic pollutants from industrial wastewater are considered an important application of adsorption processes using suitable adsorbents. The most frequently applied adsorbent for the removal of organic pollutants in wastewaters is currently activated carbon. However, activated carbon is an expensive material. There is growing interest in using low cost material for adsorption as alternatives to activated carbons. A wide variety of materials have been used in our research such as peat, lignite, diatomite, dolomite, kudzu, bone char, zeolites, peanut hulls and a range of other natural materials. A suitable adsorbent has to be meeting the following criteria:

1. high affinity and high adsorption capacity for the adsorbate;
2. safe and economically viable treatment;
3. tolerance for a wide range of wastewater parameters;
4. regeneration must be possible.

 

The use of alternative cheaper adsorbents is economically very attractive. Therefore, our research develops and evaluates such adsorbents. For example, dolomites and diatomites along with physically and chemically modified forms of these adsorbents. Furthermore an objective of the work is also to cover the fundamental principle of diffusion and adsorption into the adsorbents. The main emphasis is on the understanding of adsorption mechanisms and the modelling of adsorption systems. Adsorption of dyes and metals and the mechanisms of adsorption are related to the surface properties of the adsorbent, solution pH, particle size, initial dye concentration and thermodynamic parameters which are important in the adsorption process. These parameters are fundamental for the understanding of the chemistry and the mechanism of adsorption removal of these dyes from aqueous solution by the adsorbents.

The focus of our research is to modify the surface of the adsorbents and study the influence on the chemical and physical characteristics on the adsorption behaviour during adsorption. The general aims of the research can be summarised as follows:

a. Produce a set of modified adsorbents with different functional groups (i.e. different levels of acidity and basicity).

b. Assess the feasibility and efficiency of using natural adsorbents and the modified forms as adsorbents for the removal of pollutants from aqueous solution and from air.

c. Study the equilibrium adsorption isotherms for single and multicomponent systems.

d. Determine the physical and chemical properties (i.e. surface area, pore volume, acidity, basicity, surface charge density and pH_ZPC, etc) and the relationship between reactivity of the adsorbent and these properties.

e. Elucidate the role of adsorbent surface chemistry in the mechanism of dye adsorption.

f. Investigate the chemistry and the mechanism of adsorption and the type of adsorbent-adsorbate interactions accruing using various analytical techniques such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), atomic absorption (AA).

g. Study the adsorption kinetics and analyse the experimental data using different kinetic models at various experimental conditions such as different initial dye concentrations, particle size, mass of adsorbent, solution temperature, agitation speed and pH.

h. Study column adsorption using microcolumns and macrocolumns.

i. Develop mass transfer and diffusion models to describe the adsorption processes.

SEM Cu precipitate adsorbed to charred dolomite surface.

 

SEM of Zn precipitate adsorbed in porous structure of charred dolomite.

 

Constructed Wetlands and Reed beds

In recent years constructed wetlands and reed bed systems have emerged as one of the most popular wastewater treatment alternatives in Europe. Compared with other technologies such as activated sludge, tricking filter and biological reactors, reed beds have the advantages of aesthetical appearance, lower energy requirements and operational costs. Besides the function of water quality improvement, reed beds can perform a multitude of other functions such as biodiversity, hydrological and public use. Therefore reed bed systems are perceived as a ‘green’ technology by both industry and the public. In recent years our work has demonstrated that the efficacy of reed beds can be considerably improved by a novel operation technique of vertical flow with effluent recirculation. With the novel operation technique, suspended solids and soluble pollutants can now be extensively removed in reed beds from medium and strong effluents, e.g. domestic sewage, landfill leachate and agricultural waste slurry.

Investigate a new wastewater treatment system, tidal flow reed beds, for the treatment of high strength effluents with biochemical oxygen demand (BOD₅) and ammoniacal-nitrogen (NH₄-N), respectively. Determine the appropriate operating parameters for this system, e.g. frequency of 'tide', period of dosing and resting.

Study the mechanisms and kinetics for the removals of BOD₅, COD, Suspended Solids (SS), PO₄-P, NH₄-N, NO₂-N and NO₃-N in tidal flow reed beds.

Model the relationships between operating parameters, pollutant loadings and pollutant removal rates, and provide basic design criteria based on BOD₅, COD and NH₄-N removals.

 

 

 

Average NH₄-N removal rates

m² represents the surface area of each stage