Synthesis Of Fe-rich Biochar Derived From Sewage Sludge Via Pyrolysis And Its Applications As Environmental Functional Materials

Municipal sludge,a by-product of the municipal sewage treatment plant,has a huge production,and is difficult to be dewatered.The composite conditioner of Fenton's reagent combined with red mud?referred to as Fenton-RM?according to our previous study has enhanced deep-dewatering of sludge,but the treatment and disposal of the obtained Fe-containing sludge cake is an urgent problem to be solved.In this paper,Fe-containing sludge cake combined with Fenton-RM system was used as RS material to prepare Fe-rich biochar by pyrolysis.Furthermore,the applications of the synthesized Fe-rich biochar were investigated as environmental functional materials.The main research contents and the corresponding achievements are presented as follows:1.Degradation of 4-chlorophenol by Fe-rich biochar derived from different sludge conditioning systemsThree types of Fe-rich biochar samples,referred to as Biochar-Fenton-RM,Biochar-Fenton and Biochar-RM,were derived from dewatered sludge cake conditioned with Fenton-RM,Fenton's reagent alone and red mud?RM?alone,respectively,by pyrolysis process at 800°C for 60 min.The catalytic degradation of 4-chlorophenol by these three types of Fe-rich biochar samples were compared.The results showed that the order of their degradation efficiency and rate of 4-chlorophenol were Biochar-Fenton-RM>Biochar-Fenton>Biochar-RM.The Fe-rich biochar prepared from pyrolysis of raw sludge with the addition of FeSO4·7H2O and red mud was also compared with Biochar-Fenton-RM to catalyze the degradation of 4-chlorophenol.It was found that the Fe-rich biochar obtained by in-situ introduction of iron source in the sludge conditioning stage exhibited better catalytic degradation performance of 4-chlorophenol.XRD characterization indicated that the Fe-containing phases in Biochar-Fenton-RM were more complicated,including Fe0.95C0.05,FeAl2O4,Fe3O4 and Fe⁰.2.Phase transformation of Fe-rich biochar during pyrolysis process and the mechanism of catalytic degradation 4-chlorophenolThe deep-dewatered Fe-containing sludge cake conditioned with Fenton-RM was used as raw material.Effects of temperature?200,400,600,800°C?and time?10,20,40,60 min?during pyrolysis process on the transformation of Fe-containing phases in Fe-rich biochar were studied.It was found that Fe mainly existed in the form of Fe2O3 at low temperature?200 and 400°C?.As the temperature was increased,the Fe2O3 was gradually reduced to low-valent Fe3O4,Fe0.95C0.05 and Fe⁰.Some of the Fe3O4 was transformed to FeAl2O4through the reaction with Al2O3.At 800°C,only Fe3O4 and FeAl2O4 phases were presented at 10,20,40 min.The Fe⁰ and Fe0.95C0.05 phases appeared until the time reached 60 min.Considering of the catalytic degradation results of 4-chlorophenol by sludge biochar prepared at different conditions,the Fe-rich sludge biochar was prepared at 800°C for 60min?Biochar-Fenton-RM?.The conditions for the catalytic degradation of 4-chlorophenol by Biochar-Fenton-RM were optimized.When the conditions were:pH=2,solid/liquid ratio=2 g/L,and initial 4-chlorophenol concentration=0.78 mM,the degradation efficiency of4-chlorophenol by Biochar-Fenton-RM could reach 100%and the COD removal rate was about 74%.The mechanism study showed that the catalytic degradation of 4-chlorophenol by Biochar-Fenton-RM included three stages:adsorption,homogeneous Fenton,and heterogeneous Fenton reactions,while the effect of adsorption was weaker.The homogeneous Fenton reaction was contributed with Fe²⁺,leached from Fe⁰ and Fe0.95C0.05 in sludge biochar under acidic conditions,to initiate H2O2 for the generation of hydroxyl radicals.The insoluble Fe3O4 and FeAl2O4 in Fe-rich biochar performed as catalyst in the heterogeneous Fenton reaction during degradation of 4-chlorophenol.Cyclic catalytic degradation experiments showed that Fe3O4 and FeAl2O4 were very stable during the cyclic processes.The degradation efficiency of 4-chlorophenol could still reach 100%at 120 min using the Fe-rich biochar after three cycles.3.Performance and mechanism of H₂S adsorption by Fe-rich biocharThe deep-dewatered Fe-containing sludge cake conditioned with Fenton-RM was used as RS material.At pyrolysis time of 60 min,the effect of temperature?400,600,800,950°C?on the H2S adsorption by prepared Fe-rich biochar were investigated.It was found that as the temperature was increased from 400°C to 800°C,the breakthrough time of H2S was gradually extended.However,as the temperature was further increased to 950°C,the breakthrough time of H2S was significantly shortened.Compared with the Fe-rich biochar obtained by pyrolysis of the raw sludge,the addition of inorganic minerals in the conditioning stage could promote the adsorption of H2S by Fe-rich biochar.H2S adsorption ability of Biochar-Fenton-RM was much better than that of Biochar-Fenton and Biochar-RM.The characterization results using XRD,XPS and BET showed that the adsorption of H2S by Biochar-Fenton-RM was a combination of physical adsorption and chemical reaction.The physical adsorption was mainly contributed by the abundant near micropore structure of Biochar-Fenton-RM.The chemical reaction was mainly due to the diffusion of S^2-from the external surface of Biochar-Fenton-RM to the internal to form FeS and other sulfides by ion exchange with O^2-during the adsorption process.This paper provides a new idea for the resource utilization of Fe-rich biochar prepared by pyrolysis of Fe-containing sludge.