| Steam gasification is a common technology for energy utilization of sewage sludge,which can convert organic matter in sludge into gas fuels such as H2 and CO.The previous research of our group confirmed that the Fenton/CaO pre-conditioning technology can significantly increase the H2 yield,promote the adsorption perforamce of gasification ash and reduce the generation of polluting gases such as NOx and SOx during the sludge steam gasification.In order to further investigate the effect of Fenton/CaO conditioning on the release of heavy metals in sludge steam gasification,arsenic?As?with large differences of valence and toxicity was selected as representative in this research,and its migration and transformation mechanisms were investigated.The key scientific problems to be solved include:?1?The lack of arsenic content and species analysis methods for sludge and its derivatives,?2?The content and species of arsenic that are unknown in Fenton/CaO conditioning sludge and related mechanisms to be studied,?3?The mechanisms of the effect of residual conditioning reagents on the migration and transformation behavior of arsenic during sludge steam gasification.To solve the above problems,the following research was carried out in this paper:An analysis method of arsenic content and species was established for different sludge samples and their derivatives.The method of microwave digestion?nitric acid-hydrofluoric acid-hydrogen peroxide digestion system?and atomic fluorescence spectrometry?AFS?can be used to determine the total amount of arsenic in different solid samples.The content of different species of arsenic can be measureed by high performance liquid chromatography-hydride generation-atomic fluorescence spectroscopy?HPLC-HG-AFS?combined with phosphoric acid extraction.The experimental results show that by optimizing the separation parameters,all the arsenic forms of trivalent arsenic?As?III??,dimethylarsenic?DMA?,monomethyl arsenic?MMA?and pentavalent arsenic?As?V??in the sludge can be successively separated within 9.0 min,which is using a mobile phase of 15 mmol/L diammonium hydrogen phosphate?NH4?2HPO4?pH adjusted to 6.0 with 10%?v/v?acetic acid solution?at the flow rate of 1.5 mL/min.In the Fenton/CaO composite conditioning sludge,arsenic was enriched in the solid products as iron-arsenic compounds.Fenton conditioning could oxidize As?III?in the sludge and convert part of DMA to MMA.Residual Fe?III?would hydrolyze to form FeO?OH?,which could enrich arsenic in the wet sludge cake.FeO?OH?has different adsorption capacities for different arsenic species,and the higher the degree of methylation,the lower the adsorption rate.CaO conditioning neutralized hydrogen ions and further promoted the formation of FeO?OH?with generating CaSO4·2H2O.In the thermal drying stage,the increase of temperature accelerated the oxidation of Fe?II?as well as As?III?.The presence of CaO suppressed the reverse conversion of MMA to DMA when dried at 393-453 K.During the steam gasification reaction,residual iron-containing and calcium-containing substances could inhibit the release of arsenic into the gas phase and change its forms in the solid phase product.Both Fenton conditioning and CaO conditioning could reduce the release of arsenic in the thermal reaction.And there was synergistic effect at 873 K and 1073 K,which can further enhance the effect of inhibiting arsenic release.DMA and MMA were easy to release following with volatile,so there was no methyl arsenic in the solid product of 873 K.In the devolatilization stage,the residual iron-containing substance was completely reduced to Fe?II?at 1073 K,and at the same time catalyzed the reduction of As?V?,so there was more As?III?in the pyrolysis char.The residual calcium-containing material acts as an adsorbent to increase the content of As?III?and As?V?in the solid product.When steam existed,the sulfate could oxidize As?III?to As?V?,so more than 90%of the arsenic in the gasification ash is As?V?.In addition to the partial volatilization of arsenic in the devolatilization,the species was converted to the amorphous iron/aluminum oxide bound state and the residual state.The presence of steam caused all forms become unstable,and the higher the temperature,the higher the degree of instability,even arsenic was not detected in the gasification ash of Fenton conditioning and Fenton/CaO conditioning sludge at 1273 K.Comparatively speaking,the best condition for controlling arsenic release is the steam gasification of Fenton/CaO composite conditioning sludge at 1073 K,which has the maximum of the residual rate of arsenic and the arsenic are mostly combined with amorphous iron/aluminum oxides in the form of As?V?.In summary,using Fenton/CaO composite conditioning technology,1073 K is a temperature point which is beneficial for sludge to control the release of arsenic and improve its stability in ash during steam gasification. |
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