What is Solar Waste Detoxification?
Similar to solar water purification, solar waste detoxification uses the energy of the sun to purify harder wastes either by photochemical reactions or through thermal treatments.
The main photochemical processes that aid thermal treatment in solar detoxification include: photocatalytic oxidation using, for example, titanium dioxide (TiO2) as a catalyst; sensitised formation of singlet oxygen or hydroxyl radical using a dye-sensitiser; direct photochemistry where light is absorbed by a compound; and photo-Fenton chemistry with hydroxoiron (III) complexes.
Photocatalytic solar detoxification processes utilise solar radiation from various parts of the spectrum. Ultraviolet radiation is used to promote an oxidation reaction in photocatalytic reactions using a catalyst such as TiO2 in the presence of oxygen. The reactivity of singlet oxygen, irradiated with visible light in the presence of dissolved oxygen, is utilised in the dye-sensitiser processes. The reactive species formed can then react with contaminant molecules in the waste, affecting the treatment.
In thermal treatment, organic chemicals in soil and water, including dioxins, polychlorinated biphenyls (PCBs), and furans can be destroyed. Thermal treatment can also be used for inorganic wastes by melting eg filter dusts, and forming a glassy, inert material.
For efficient use of solar energy in thermal treatment, concentration of the solar radiation is necessary to achieve the high temperatures required for decomposition or destruction of the contaminants being treated. Solar radiation is reflected by mirrors (heliostats) and absorbed by a receiver reaching temperatures of up to 2300° K depending on the absorber.
Solar enhanced thermal treatment has been developed for solar detoxification of soils by Science Applications International Corporation (SAIC) and Energy and Environmental Research Corporation (EER) in the USA. The system is used for detoxification of organic contaminants from both soils and liquid wastes. No auxiliary fuel is required and it has been demonstrated to show an improvement in the destruction and removal efficiency (DRE) of organics including, PCBs, by a factor of 100 or more over conventional thermal technologies. High destruction efficiencies can be achieved at a temperature of 750° C, which is lower than the temperature required for thermal incineration.
