The first advantage of the proposed process consists in the decrease of the acid consumption related to the hydrothermal phase. Indeed, since the chrysotile fiber
concentrations in a CAC are approximately in the range of 8-16%w/w [37], the amounts
of calcium and magnesium to be dissolved are almost 30-35%w/w and 3-4%w/w
respectively. Consequently, by considering the stoichiometry of the reactions (2) and (3), the amount of sulfuric acid to be added during the hydrothermal treatment should decrease by 71-79% compared to what reported by Nam et al. [22], because the calcium present in the cement matrix could be completely dissolved by the VFAs produced during the DF pretreatment. Another important advantage of the proposed process
100 consists in the production of hydrogen during due to the DF process. Moreover, the effluents of the hydrothermal treatment, rich in VFAs, could undergo to an AD process, which can lead to the generation of methane. At this purpose, it has been demonstrated that coupling DF and AD processes could lead to a net energy yield ranging between the 250 kJ Lsol-1 and 300 kJ Lsol-1 [49]. Similarly, the effluents of the hydrothermal phase
could be allocated in a microbial fuel cell with a consequent production of electricity. In this case, the net energy production may be higher than that resulting from the final adoption of AD [50]. In this way, the energy generated during the two biological processes may be used to heat the solution during the hydrothermal phase, which should be carried out at 100 °C for 24 h [22].
Finally, both the production of VFAs and the production of energy could derive from the biodegradation of a carbon rich waste biomass, which could be represented by agro- food wastes, such as the molasses deriving from sugar refinery [51] and the exhaust cheese whey deriving from dairies [52]. Consequently, the proposed process, in line with the green economy principles, could be performed by the use of a non-marketable agro-food waste to decrease the costs related to the hydrothermal treatment of CAC.
5.4. Conclusions
The present study represents a preliminary investigation on an innovative combined system based on a DF and a hydrothermal treatment stages aimed to obtain an effective and cheaper CAC denaturation. Indeed, the DF step has been successfully applied to dissolve the cementitious phase of a composite, simulating a CAC, and to produce a
renewable source of energy, the bio-H2, supporting the hydrothermal phase. Indeed,
even if now it is not possible to make a balance between the produced and the consumed energy, the proposed process should be able to decrease the acid consumption and the energy costs during the hydrothermal phase.
In particular, Synthetic DF effluents were used to evaluate the best operative conditions for GFC dissolution. At this purpose, the results revealed a strong diffusional resistance to VFAs migration through GFC. Specifically, all the phenomena observed in this investigation could be explained by imposing the Knudsen diffusion coefficient into the integrated second Fick’s law.
The liquid DF effluents resulted effective for the dissolution of CAC simulating
conglomerates. In fact, in the case of 5.0‧103 ppm GFC load, the cement matrix was
101 this process resulted in a H2 production equal to 284 mmolH2 Lsol-1, which could be
used as energy source during the hydrothermal treatment, and in a final pH of the suspension equal to 4.71, which is close to the best conditions for the asbestos fiber denaturation. In particular, the adoption of different GFC loads resulted in lower hydrogen productions and/or dissolution efficiencies.
Compared to the other results reported in the literature, the H2 yields were low since
too acidic pH were reached. Nevertheless, the acidity of the solution was necessary for the dissolution of CAC cement matrix. On the other hand, if the effluents of the hydrothermal treatments, rich in VFAs and lactic acid, undergo to other biological processes, such as microbial fuel cells or anaerobic digestion, the energetic balance of the whole treatment could be further improved due to the final CH4 and H2 production.
The results obtained in this manuscript represent a starting point for future investigations on the proposed treatment train. Indeed, as future steps of this investigation, it will be carried out the dissolution of the cement matrix of a real CAC during the DF process and the subsequent hydrothermal treatment of these effluents, rich in asbestos fibers. Moreover, the adoption of real biodegradable wastes instead of glucose together with the treatment and valorization of the effluents deriving from the hydrothermal phase will be considered for scaled-up applications of the proposed system. After these mandatory steps, a life cycle assessment of the whole treatment train could be carried out.
102
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