| A large amount of biosolids and ferric sludge is generated from municipal or industrial wastewater treatment.The proper disposal of two kinds of solid waste with high yield,moisture and heavy metals have become the bottleneck to further improve the quality and efficiency of wastewater treatment technology.To address this problem,numerous sludge treatment,disposal and recycling technologies have been developed and adopted,such as sludge concentration and dehydration,anaerobic fermentation,pyrolysis,etc.Thereinto,hydrothermal is a sludge thermal treatment technology with mild reaction conditions and strong adaptability to high moisture feedstock.Reduction,harmlessness and resource recovery of sludge via hydrothermal are current research hotspot.This study focuses on the sludge disposal and seeks a solution to solve the key problem of sludge reduction,harmless and recycling.The biosolids and ferric sludge have been synthesized the iron-based sludge hydrochar for a series of research,such as employing as Fenton-like heterogeneous catalyst,catalyst formation mechanism and catalytic performance,organic acid-assisted hydrothermal to achieve sludge reduction,heavy metal elimination and stabilization,solid fuel derived from excess sludge via hydrothermal.The main research contents and results are as follows:?1?Iron-based sludge hydrochar prepared via co-hydrothermal of biosolids and ferric sludge and its Fenton-like catalytic propertiesWhile magnetic biochar?MBC?derived from co-hydrothermal of biosolids and ferric sludge was employed as Fenton catalysts,the optimal hydrothermal conditions denoted as the Methylene blue?MB?decolorization were obtained as follows:residence time of 6 h,the temperature of200?,and ferric sludge to biosolids mixture ratio of 1:2.The resultant MBC has a diameter of around 200 nm,smaller pore diameter and specific surface area than traditional activated carbon,and higher degree of carbonization than the hydrochar derived from biosolids alone under the same conditions.The X-ray diffraction pattern showed that the iron oxide in MBC was paramagnetic Fe3O4.Based on Fourier transform infrared?FT-IR?results,it was found that biochar in Fe3O4 was combined with MBC in the form of Si-O-Fe chemical bond,and Fe3O4 resulted in that MBC had good magnetic property for magnetic recovery.MBC was employed as a catalyst to degrade MB in Fenton-like system.After 20 min of reaction,the degradation rate of MB reached 98%and the mineralization rate reached 43%.The catalytic performance of MBC can be maintained after 4 times of recycling.Depending on the analysis of gas chromatograph-mass spectrometer,the degradation pathway of MB in Fenton process was deduced.The results showed that MBC was a kind of fenton-like catalyst with excellent performance and potential for dyeing wastewater treatment.?2?The iron-based sludge hydrochar formation,its heterogeneous Fenton catalytic efficiency and mechanismTo elucidate the MBC formation mechanism,protein,carbohydrate and ferric iron were used as model compounds of biosolids and ferric sludge to synthesize the MBC via hydrothermal.Protein and carbohydrate could generate Maillard reaction products?MRPs?through Maillard reaction?MR?,which eliminated the spontaneous consumption of carbohydrate by inhibiting the caramelization of carbohydrates,and promoted more reducing hydroxyl groups participate in Fe3+reduction reaction.Based on parallel experimental data,it was found that carbohydrate and MRPs were both engaged in Fe3+reduction.Therefore,protein and carbohydrate in biosolids exerted synergistic effect on Fe3+reduction.Three-dimensional fluorescence spectra showed that MR led to the gradual humification of the model substrate,and this pathway facilitated the chelation activity of MRPs.Furthermore,the MRPs as ligands can chelate metal ions and provide site for Fe3O4 crystal growth.The MRPs?50–100?kDa?manifested the strongest relative reducing and chelation activity,and MRPs participated in the main ferric iron reduction and chelation reaction.MBC was employed as the Fenton-like catalyst to effectively degrade aniline and achieve the removal efficiency of77.9%.Meanwhile,the chemical oxygen demand?COD?of the aniline solution decreased from 56.0mg/L to 27.9 mg/L with 50.2%mineralization.According to bioluminescence inhibition experiments results,the toxicity of aniline solution treated by MBC-Fenton process was significantly reduced.Differing from·OH mainly present in Fenton-like system,·OH and O2–both contributed to the aniline degradation in MBC Fenton-like system,indicating biochar in MBC could induce Fe3O4 to shape various reactive oxygen species?ROS?rather than single ROS.Then,the catalyst was employed in the treatment of actual dyeing wastewater,COD and total organic carbon?TOC?of the wastewater were effectively removed,and the biodegradability of the wastewater was greatly improved,which was conducive to subsequent biological treatment in the wastewater treatment plant.?3?Reduction and stabilization of heavy metal in excess sludge via organic acid-assisted hydrothermalBased on the performance of several kinds of potential catalyst on sludge reduction by hydrothermal,the tartaric acid was selected as the optimal catalyst.Through variations of H/C and O/C in biosolids during hydrothermal,the organic acid-assisted hydrothermal?OAHT?can catalyze the decarboxylation,dehydration and demethylation of sludge.OAHT hydrolyzed the sludge and destroyed the structure of sludge which made it loose and porous.Consequently,the binding water escaped from the substrate and converted to free water.Meanwhile,the hydrophilic groups on the surface of sludge decreased with the hydrophobic groups increased.After OAHT treatment,Cr,Cd and As?100%in F3 state?were completely stable,while Ni and Cu were relatively stable?100%in F2/F3 state?.Tartaric acid could effectively chelate the heavy metals in the sludge and promote the immobilization of heavy metals,which significantly reducing the risk of environmental exposure.Futhermore,in OAHT system,the resultant biochar employed the unpaired electrons of itself to form environmental persistent free radicals,then generated 1O2,·OH and·O2-.The latter two ROS mineralized organic arsenic in the sludge and prevent its volatilization from sludge.?4?Biochar fuel derived from excess sludge by hydrothermalThis study focuses on physicochemical property and combustion characteristics of biomass fuel derived from excess sludge water via hydrothermal under different temperature,residence time.Among all produced hydrochar,the obtained hydrochar under hydrothermal condition of temperature 260?and residence time 4 h owned highest calorific value(HHVdaf=26.23 MJ/kg)with energy density at 1.43,and its fuel characteristics are similar to those of lignite.The hydrothermal process promoted the dehydration reaction and decarboxylation of sludge,which decreased the O/C and H/C values of hydrothermal carbon and increased the calorific value.The reduction of N in hydrochar could produce clean fuel with less NOx emission in the future energy application.During the hydrothermal process,some organic substances in the sludge were dissolved into hydrolysate.By analyzing the changes of pH value,protein concentration,carbohydrate concentration,total nitrogen?TN?,chemical oxygen demand?COD?and other indicators in hydrolysate,it was found that the substances released from the sludge into hydrolysate had a significant impact on the improvement of the calorific value of hydrochar.The pH value,COD,protein and carbohydrate concentration in hydrolysate were negatively correlated with the calorific value of hydrochar.In conclusion,it is speculated that the removal of the binding water in the sludge,the reduction of hydrophilic hydroxyl and carboxy-rich substances with low energy density via the hydrothermal,promoted the increase of the calorific value of hydrochar. |
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