Abstract: Traditional processes for the abatement of off gas pollutants, originated by soil remediation interventions, are mostly based on combustion and on granulated active coal (GAC) absorption.
Last technology is also the most frequently utilized in groundwater decontamination. Both the above technologies present drawbacks in terms of cost and of environmental concern.
In the case of combustion, a lot of thermal energy is involved (for chloroorganic compounds heating up to 1200°C is needed), whereas, in the case of GAC, characterized by low efficiency and reliability, the used carbon must be wasted or regenerated.
Among the emerging technologies, an important role is played by photocatalysis, which shows many advantages and some limitation, particularly related to the incomplete mineralization of pollutants.
There is a need, therefore, for more efficient and convenient processes for the degradation of gaseous organic pollutants in off gases coming from site remediation. This is particularly true in the case of chloroorganic pollutants.
In this purpose, we have studied an innovative approach for the degradation of pollutants originated by the association of a photo-catalytic process to a thermo-catalytic one, and adopting for both activities the same catalyst.
We have designed and realized an apparatus consisting of a stainless steel reactor equipped with quartz windows and with an alumina substrate covered with TiO2 catalyst, that can be separately or jointly elettrically heated and illuminated by a UV lamp.
The polluting substance introduced into the reactor was circulated by means of a pump and the degradation experiments were performed with three different procedures: (a) photocatalytic (PC) conditions (by UV irradiation of the catalyst); (b) thermocatalytic (TC) conditions (by heating the catalyst); (c) thermophotocatalytic (TPC) conditions (by heating and UV irradiating the catalyst).
The polluting compounds utilized as models were trichloroethylene (TCE) and monochlorobenzene (CB), and the degradation progress was monitored by on-line gas chromatographic analysis.
The experimental results are as follows:
there is an additive and synergic effect of the energetic sources (PC +TC) in the degradation of the polluter;
the thermal contribution to degradation has been tested as effective in the range 350-550°C.
with respect to the degradation of TCE, a remarkable advantage was shown by the TPC approach, as compared to the PC one (82 percent degradation under TPC condition, vs. 49 percent under PC condition, after one fourth of the overall reaction time); with respect to the degradation of CB, even better results in favour of TPC were obtained (81 percent degradation under TPC condition, vs. 36 percent degradation under PC condition, after one third of the overall reaction time);.
the most outstanding results given by TPC approach are related to its mineralising efficiency: (a) in TCE degradation, the recovery of CO2 was 71 percent under TPC, vs. 27 percent under PC condition; (b) in CB degradation, the recovery of CO2 was 98 percent under TPC, vs. 17 percent under PC condition.
In conclusion, our approach, although at a preliminary level, looks promising, as an improved technology for the abatement of off gas polluters in site reclamation technologies (soil vapor extraction, air stripping, thermal desorption).
Alessandro Selvaggi, Alberto David and Piergiorgio Zappelli
source : E.N.I
© 2001 Mena Report (www.menareport.com)
