| The treatment and disposal of sewage sludge is one of hot issues in the field of water treatment.Traditional sludge treatment methods are unsustainable,which might cause risk of secondary pollution.Pyrolysis of sewage sludge and then applying the sludge-derived biochar,to repair soil and purify sewage,could be a sustainable way for resource utilization.The municipal sludge(RS)was activated by iron salt to obtain iron-activated sludge(MS).The sludge biochar(RB)and magnetic sludge biochar(MSB)samples were prepared by pyrolyzing RS and MS under N2 atmosphere,respectively.The physical and chemical properties of RB and MSB,their phosphorus fixation characteristic,and their efficiency and possible mechanism of phosphorus adsorption from water were studied by characterization analysis and sequencing batch tests.The main conclusions are as follows.(1)The results of characterization analysis showed that RS contains various elements with high content such as Al,Fe and P.The dehydration and decarboxylation reaction occurred during the process of RS pyrolysis,while the main reaction during the process of MS pyrolysis was dehydration.The crystallinity of metal compounds anchored to sludge-derived biochar was increased with the increase of pyrolytic temperature.The best MSB sample was obtained when the pyrolysis temperature was 250℃.The specific surface area and pore volume of MSB250 were 98.91 m2/g and 13.57 nm,respectively,and the specific surface area and pore volume of RB250 were 55.31 m2/g and 10.07 nm,respectively,which demonstrated the surface structure of sludge-derived biochar was improved by iron salt activation pretreatment.The amorphousα-FeOOH and β-FeOOH in RB and MSB samples were transformed into α-Fe2O3 and Fe3O4 manifesting more crystalline forms with the increase of pyrolysis temperature.The higher response intensity of Fe3O4 energy peak indicated that the MSB sample showed good ferromagnetism.The results of zeta potential test showed the zero charge point(pHPZC)of RB and MSB were less than 7.When the solution pH was>pHPZC,the sample was electronegative,and when the solution pH was<pHPZC,the sample surface was electrically positive.(2)Due to the simultaneous action of ferric salt activation pretreatment and pyrolysis,the MSB samples showed good ability of phosphorus fixation.The MSB250 sample prepared at 250℃ had the best phosphorus fixation performance among the obtained samples via MS pyrolysis.Both of the total phosphorus and total iron in the solution were below the detection limit after a 24-hour-reaction.The results of mechanism analysis revealed that the migration of phosphorus compounds released from MSB250 itself occurred in aqueous solution.Finally,the phosphorus compounds were fixed in the form of HPO42-through electrostatic attraction reaction with the functional group on the surface of MSB250.No leakage of iron could be attributed to the change of iron oxide crystal form at certain pyrolysis temperature.The results of influencing factors showed that the phosphorus release of MSB250 was significantly inhibited by a 24-hour-reaction when pH<7.Under weak acidic condition,Ca2+ improved the phosphorus fixation of MSB250 through the "metal bridge" effect and the reaction with other anions in solution.When pH≥ 7,MSB250 would release polymerized phosphorus(pyro-P)in major.Under the same condition,the release of orthophosphate(ortho-P)in MSB250 was also increased with the addition of Ca2+,and the maximum release capacity was 0.2 mg/g.(3)Phosphorus adsorption experiments on the MSB samples and control samples were carried out.The results showed that the MSB samples being rich in various iron oxides had better phosphorus adsorption efficiency.The adsorption kinetic fitting results of MS,MSB 150 and MSB250 showed that the pyrolysis temperature controlled the stability of iron oxides in the MSB samples,and was also a key control factor for phosphorus adsorption.The optimal preparation temperature of MSB was 250℃.The above results coupling of the analysis results of influencing factors proved that MSB250 had the highest phosphorus adsorption efficiency when the dosage was 50 mg/L at pH4,and the equilibrium adsorption capacity was 4.6 mg/g.The acidic conditions were beneficial to the phosphorus adsorption and removal of MSB250,but when the pH was lower than 3,the maximum iron leakage of MSB250 could reach to 1.48 mg/g.Adsorption kinetics analysis indicated that MSB250 removal methods were mainly chemical adsorption of phosphorus in the water.The fitting results of adsorption isotherms proved that the phosphorus adsorption of MSB250 was well followed the Freundlich model.The water temperature was also an essential factor affecting the phosphorus adsorption effect of MSB250.The optimal reaction temperature of MSB250 for phosphorus adsorption was 25℃.The results of the D-R model showed that the MSB250 was a multi-layer adsorption when the temperature was higher than 25℃,and the MSB250 was a single molecular-layer adsorption when the temperature was lower than 15℃.The results of adsorption thermodynamics showed that phosphorus adsorption on MSB250 was spontaneous and endothermic.The adsorption Mechanism showed that the surface-active functional groups(-OH,Fe-OH,-COOH,etc.)of MSB250 were related to the phosphorus removal process.There was an ion-exchange reaction between MSB250 and phosphorus in water,and the complexation was mainly based on the acid-base Lewis interaction,which could be concluded as the electrostatic attraction. |
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